Philippe Poulin

Philippe Leclère

Jacques Cinquin

Philippe Bompard

Delong He

Anthony Dichiara

Jinkai Yuan

Youqin Lin

Jing Zhang

Hassan Harris

Farida Djebbari

Sokona Konaté

Keywords: Composites Polymère, Hybrides, Propriétés mécaniques, Propriétés électriques, Piézorésistivité Polymer composites, Hybrids, Mechanical properties, Electrical properteis, Piezoresistive

Bompard for the discussion and his suggestions in the three years, Sylviane Bourgeois for her help in the measurement system and françoise Garnier for SEM observation.

Finally, I would like to my family, especially my parents for their love and care over the past years.

Résumé

Hybrides nano /micrométriques de nanotubes de carbone (NTC) greffés sur microparticules d'alumina, microplaques de SiC ou nanoplaquettes de graphène (NPG) ont été utilisés comme renforts multifonctionnelles dans les composites à matrice polymère. Le NTCs utilisés étaient généralement sous forme de six branches symétriques et orthogonales sur microparticules sphériques d'Al 2 O 3 , mais d'une ou deux branches alignées verticalement sur les deux cotés de microplaques de SiC et de NPG. L'introudction des structures hybrides dans une matrice époxy permet d'améliorer la dispersion des NTC et l'interaction interfaciale entre les renforts et la matrice. Les propriétés mécaniques des composites ont été fortement améliorées avec une faible concentration de hybrides. La résistance électrique in situ des composites a affeint d'abord à sa valeur maximale et puis diminue avec la presence d'une déformation irréversible. Ce phenomena servé est complètement différent par rapport à ce des composites renforcé par NTC, c'est-à-dire, une augmentation monotone de la résistance jusqu'à leur rupture final. Les propriétés mécaniques et les comportements de self-sensing des composites dependent fortement de l'élancement de NTC de leur organisation et aussi des substrats. L'introduction des hybrides dans les composites renforcés par des fibres longues (verre) a demontré un grand potentiel pour développer des composites multi-échelles.

Les études réalisées sur la matrice époxy renforcée par les hybrides bien dispersés avec une faible fraction ont montré des améliorations importantes des propriétés de flexion à 3 points et des propriétés thermo-mécaniques. Les réseaux conducteurs formés par hybrides nano/micrométriques permettent de suivre in situ l'évolution de l'état de dégradation des composites à matrice époxy renforcés par des tissus de verre sous contrainte appliquée.

Acronyms and Symbols

General introduction

More recently, the multifunctional materials have been increasingly paid attention, which are tailored to achieve superior mechanical performance and specific functional requirements.

The future multifunctional materials are desired to possess combined the sensing, actuation, energy storage, and healing capabilities, etc. It has been found that the nanocomposites with polymer modified by nanoscale reinforcements could open up a wide range of opportunities for the multifunctional materials. Carbon nanotubes (CNTs) have drawn specific interest towards the development of new multifunctional materials due to their extraordinary intrinsic mechanical, thermal, electrical properties and high aspect ratios, which have wide applications from aerospace to electronics. Introducing a small amount of CNTs into the polymer matrix can improve the mechanical properties and enable the formation of electrical conductive networks in the composite, which make them to serve as structural reinforcement, conductive and piezosensitive materials, etc.

Besides, nanotubes have shown great promise for use in development of multifunctional multiscale laminated composites. It is known that the traditional fiber-reinforced polymer composites (FRPs) with excellent in-plane properties, such as, high tensile modulus and strength are important structural materials. However, their relatively weak out of-plane properties (matrix-dominated) still remain major obstacles. It has been demonstrated that the poor properties of the FRPs can be tailored at nanoscale by introduction of the CNTs. Due to their extremely small sizes, it is possible for CNTs to penetrate into the matrix-rich regions between the individual fibers and between the plies of the composites. The CNTs could be dispersed uniformly throughout the whole polymer matrix or directly deposited on the fiber surfaces. The multiscale hybrids with nanoscale CNTs as well as continuous microscale fiber have drawn significant interest in the field of advanced, high-performance materials. Besides, due to their exceptionally electrical properties, the formed conductive networks in the matrix enable CNTs to be utilized as sensors for in situ monitoring the deformation and damage of composites during loading.

However, the critical issues for preparing the multifunctional CNT-based polymer nanocomposites include the uniform dispersion of CNTs, control of CNT alignment, and strong CNT/matrix interfacial bonding for good load transfer, etc, which need to be solved.

Moreover, with regard to the CNT and fiber multiscale reinforcements reinforced composites, the dispersion of CNTs in the matrix are still urgent issues. Inadequate dispersion of CNTs in the matrix will introduce entangled aggregates, resulting in non-uniform infiltration of the fiber bundles and decreased mechanical properties. In addition, the nanotubes can result in substantially increased viscosity of the nanotube/polymer suspension, creating challenges for the infusion and wetting of the fibers. It is desirable that very low concentration of well dispersed CNTs could realize structural reinforcements and be used as damage sensor.

Based on the main problems of pristine CNTs, this thesis is focus on the multi-scale hybrid structures to be utilized as superior multifunctional reinforcements in the composites.

The hybrid structures with CNTs grown on microsized ceramic particles or two dimensional graphene were in situ synthesized by a simple one-step chemical vapor deposition (CVD) method, without any pre-patterned catalyst treatment. These hybrids could endow uniform CNT distribution and controllable CNT orientation and organization as well as the improved interfacial connection and load transfer between CNTs and matrix.

Firstly, three kinds of hybrids with CNTs grown on SiC microplates (CNT-SiC), Al 2 O 3 microparticles (CNT-Al 2 O 3 ) and graphene platelets (CNT-GNP) synthesized by CVD were used. The different CNT aspect ratios and organizations on the substrates were adjusted by the CVD conditions, such as growth temperature, time and hydrogen ratio, etc. The CNT-SiC, CNT-GNP and CNT-Al 2 O 3 reinforced epoxy nanocomposites were respectively fabricated using calendaring method. Compared to the randomly distributed CNTs, the more improved mechanical and electrical properties of the multiscale hybrids filled nanocomposites were studied in detail. Such as, with respect to neat epoxy, significantly enhanced mechanical properties in the CNT-GNP filled composites were achieved at ultralow hybrid loading (0.5

wt.%). The tensile modulus showed ~40% increase and the tensile strength was improved by ~36%. The value of percolation threshold (f c ) of the hybrids filled systems is much lower, for example, f c ~ 0.2 vol.% in the CNT-SiC composites. Besides, the epoxy composites with the hybrids could exhibit distinctive self-sensing behavior for in situ monitoring the onset of irreversibly permanent deformation. The in situ electrical resistance initially increases to its maximum value and then begins to decrease with the appearance of the residual strain and irreversible deformation, which is remarkably different from the randomly distributed CNTs filled composites only with monotonic increase of the resistance until catastrophic fracture.

In addition, the relationship between the hybrid structure (CNT aspect ratio, organization and substrate morphology) and composite mechanical and self-sensing properties has been well

established. For the underlying mechanisms of the self-sensing properties, it is found that the evolution of conductive networks resulted from the rotation of the substrates and hybrids as well as the reorientation of CNTs during loading plays an important role on the self-sensing behavior of the composites.

Secondly, the CNT-Al 2 O 3 hybrids which replace the pristine CNTs were used for matrix modification and in situ damage sensor in the glass fiber reinforced composites. The woven glass fabric/epoxy with CNT-Al 2 O 3 hybrids multiscale composite laminates were fabricated.

The composites incorporated with 0.5 wt.% CNT-Al 2 O 3 hybrids showed increment in the flexural modulus ~ 19% and flexural strength ~ 13% over that of the reference sample.

Similar, from the results of dynamic mechanical analysis, the samples exhibited considerable improvement of ~ 20% in the storage modulus at 50 o C and an increase of 15 o C in glass transition temperature compared to the control samples. As multifunctional reinforcements, the CNT-Al 2 O 3 hybrids could be utilized as in situ damage sensor to monitor the damage evolution of the glass fabric reinforced epoxy composites when subjected to uniaxial tensile loading. From the change of the in situ electrical resistances, the damage mechanisms in the composites were analyzed.

Chapter І CNTs for Multifunctional Reinforcements

Carbon nanotubes

Carbon nanotubes (CNTs) have attracted considerable attention in a wide range of scientific research fields since their discovery [1], which open the possibility for a new class of materials. Their intriguing nanostructure as well as combination of the extraordinary mechanical and electrical properties has aroused extensive interest to understand their fundamental nature and explore their potential applications.

CNT morphology

Carbon nanotubes could be visualized as a sheet of graphite that has been rolled into a cylinder [2]. The graphite layer could be formed as a 2-D sheet of carbon atoms arranged in a hexagonal array and each carbon atom has three nearest neighbors in this case. According to their arrangement, nanotubes can exist as either single-walled or multi-walled structures.

A single-walled nanotube (SWNT) can be seen as a seamless cylinder by rolling up a single such sheet with a typical diameter of 1-1.5 nm [3], whereas multi-walled nanotube (MWNT) is generally made of multiple layers of graphene sheets with inner diameters as small as those of the SWNTs rolled into a hollow core. But the ir outer diameters can vary from a few nanometers to tens of nanometers [4]. In general, the van der Waals interactions between sidewalls of the SWNTs result in close-packed bundles. MWNTs have an interlayer spacing of approximately 0.34 nm where the interaction between the individual shells is van der Waals bonding. Fig. 1-1a and b show the transmission electron micrograph (TEM) images of SWNT and MWNT, respectively.

The atomic structure of nanotubes is usually described in terms of the tube chirality, or helicity, which is assigned by a chiral angle θ, and the roll-up or chiral vector C (C=na 1 + ma 2 ) from a point of origin on a graphene sheet [5], as illustrated in Fig. 2. Here, n and m are the number of steps along the ziz-zag carbon bonds of the hexagonal lattice; a 1 and a 2 are unit vectors; and θ is the angle between the zigzag axis and roll-up vector, which determines the amount of ‗twist' of the tube. We can visualize cutting the graphite sheet along the dotted lines and rolling the tube so that the tip of the chiral vector touches its tail. The rolling direction of graphene sheet and the nanotube diameter will determine the nanotube chiralities which denote its type. According to the geometry of carbon bonds around the circumference of the nanotube, three types of microstructure can be realized, as shown in Fig. 12. If n=m and θ=30 o , the nanotube is known as -armchair.‖ In the case of m=0 and θ=0 o , the tube is called -zigzag‖. Othervise, for 0 o < θ < 30 o , the nanotube is called -chiral‖ [5]. Depending on their chirality, nanotubes can either be conducting or semiconducting [2]. All armchair nanotubes are generally metallic. For all other nanotubes, there are two possibilities. If n-m=3k (k is an integer), the tubes are expected to be metallic; otherwise, the tubes are semiconducting [5]. The properties of nanotubes depend on their diameter, nanostructure and atomic arrangement, i.e., the rolling form of the graphite sheets. [6][7][START_REF] Nardelli | Brittle and ductile behavior in carbon nanotubes[END_REF].

CNT properties

Due to their perfect sp 2 -hybridized in-plane bonds ( bonds) between adjacent carbon atoms and the regular crystal lattice, nanotubes possess remarkable mechanical properties i.e., high modulus, strength and flexibility, exceeding those of any existing materials. Numerous experimental and theoretical research been made to determine the mechanical properties of nanotubes. Salvetat et al. reported the CNT mechanical properties are largely independent on their chirality [START_REF] Salvetat | Elastic and shear moduli of single-walled carbon nanotube ropes[END_REF]. Though there is no consensus on the exact value of mechanical properties of CNTs, the theoretical and experimental results have shown unusual mechanical properties of CNTs with Young's modulus as high as about 1-1.2 TPa and tensile strength of GPa [START_REF] Salvetat | Mechanical properties of carbon nanotubes[END_REF].

Besides, nanotubes also exhibit excellent electrical properties ranging from metallic to semiconducting in a large band gap [START_REF] Ebbesen | Electrical conductivity of individual carbon nanotubes[END_REF], which are considered to depend the diameter and chirality of carbon atoms in the surface of nanotubes. The nanotubes have extraordinarily high current carrying capacity and can carry the highest current density as high as 10 9 A/cm 2 or more [START_REF] De Pablo | A simple, reliable technique for making electrical contact to multiwalled carbon nanotubes[END_REF][START_REF] Kaneto | Electrical conductivities of multi-wall carbon nano tubes[END_REF], which is almost one hundred times larger than typical metals. In addition, the superconductive behavior has also been found in SWNTs with carbon atoms arranged in certain way [START_REF] Bandaru | Electrical properties and applications of carbon nanotube structures[END_REF]. These superior electrical properties have generated substantial interest in utilizing CNTs in nanoelectronics, as shown in Fig. 123. Each atom on the surface of nanotubes is exposed to the environment, thus, any small changes from the chemical, thermal or mechanical environment could cause drastic changes in their electrical properties. As reported [START_REF] Tombler | Reversible electromechanical characteristics of carbon nanotubes under local-probe manipulation[END_REF][START_REF] Minot | Tuning carbon nanotube band gaps with strain[END_REF][START_REF] Cao | Electromechanical properties of metallic, quasimetallic, and semiconducting carbon nanotubes under stretching[END_REF][START_REF] Rochefort | Electrical and mec hanical properties of distorted carbon nanotubes[END_REF], the band structure of a nanotube is dramatically altered by mechanical strain and that the conductance of the CNTs can increase or decrease depending on the chirality of the nanotubes. Besides, nanotubes show extreme sensitivity towards changes of local chemical environment that stems from the susceptibility of their electronic structure to the interacting molecules [START_REF] Ong | A wireless, passive carbon nanotube-based gas sensor[END_REF][START_REF] Cho | Multiwall carbon nanotube gas sensor fabricated using thermomechanical structure[END_REF][START_REF] Kim | Thermal transport measurements of individual multiwalled nanotubes[END_REF][START_REF] Kawano | An electrothermal carbon nanotube gas sensor[END_REF][START_REF] Njuguna | S andwiched carbon nanotube film as strain sensor[END_REF][START_REF] Martel R A | Single-and multi-wall carbon nanotube field-effect transistors[END_REF]. Unlike the conventional piezoelectric materials, electromechanical coupling of carbon nanotubes results from phonon frequency shifts as a consequence of charge injection. This electromechanical property of nanotubes has attracted wide attention in their potential applications for gas, temperature and strain sensors.

Controlled synthesis of CNTs

The CNT morphology can be controlled by the process for CNT fabrication. There are three most common techniques for CNT synthesis, i.e., carbon arc-discharge, laser ablation and chemical vapor deposition (CVD).

Since MWNTs was discovered in the soot of the arc-discharge method by Iijima in 1991, this technique became a relatively simple method for preparing CNTs [1]. After that, Iijima et al. [START_REF] Iijima | Single-shell carbon nanotubes of 1-nm diameter[END_REF], and Bethune et al. [START_REF] Bethune | Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls[END_REF] also synthesized SWNTs using this method with an assistance of a metal catalyst in 1993. This method involves passing a direct current between two carbon electrodes in order to create an arc discharge at high temperature. Then, this high temperature arc discharge would melt and vaporize one of the carbon rods and the nanotubes would be collected on the other rod. Chemical refinement is necessary to obtain the pure CNTs by this method because there is soot and other forms of carbon apart from CNTs. The disadvantages of the arc discharge method lie in low purity of the resulting products and difficulty in controlling over arrangement relative to other methods.

In order to obtain pure CNTs with small diameter distribution, laser ablation technique was developed. Using this method, a significant progress was achieved on synthesis of bundles of aligned SWNTs with small diameter distribution by Smalley et al. in 1996 [START_REF] Philippe | An original growth mode of MWCNTs on alumina supported iron catalysts[END_REF].

A horizontal tube furnace with a target inside is pre-heated around 1200 o C under a controlled pressure using a flow of inert gas (Helium or Argon). This target consists of a composite, which is a mixture of graphite and metal catalysts such as Co or Ni. A laser is introduced onto the target for its vaporization. Then the plume from vaporization is collected at the cooler downstream end of the horizontal tube. However, this synthesis also produces soot and other carbon compounds apart from CNTs. The amount and type of catalysts, laser parameters (power and wavelength) and the environment parameters (temperature and pressure) will have influence on the quantity and quality of the products. Compared to the arc-discharge, this method has advantages in achieving high purity of CNTs and controlling the uniformity of CNT dimension, however, this technique is more expensive.

CVD is a more recent method for industrial synthesis of both MWNTs and SWNTs, which could easily control the dimension of resulting CNTs, such as diameter, length, aspect ratio, etc. Besides, the achieved products are high-yield and nearly completely graphitized without amorphous carbon. Catalytic growth of nanotubes using CVD method was first used by Yacaman et al. [START_REF] Jose-Yacaman | Catalytic growth of carbon microtubules with fullerene structure[END_REF]. The CVD process used for CNT growth is actually catalyst-assisted decomposition of hydrocarbons. A number of CVD reactor configurations such as horizontal, vertical and barrel have been reported to synthesize CNTs [START_REF] Choy | Chemical vapour deposition of coatings[END_REF]. The horizontal quartz tube reactor is a relatively simple system which is widely used in floating catalytic CVD. This process usually happens in a horizontal quartz tube, which is heated to a temperature ranging 500 to 1000 o C in inert carrier gases (argon and nitrogen). Gaseous or liquid carbon sources, such as methane, acetylene, ethylene, benzene, toluene and xylene, etc., are thermally decomposed, then the separated carbon molecules are attached on the surface of a metal catalyst (Fe, Ni, or Co, etc). The common combination of substrates and catalysts is silica and Fe catalysts for MWNTs, and silicon wafer and Fe/Mo catalysts for SWNTs. High temperature is especially needed for the SWNT production.

Recently, it is necessary to synthesize CNTs on a significant scale in order to improve their availability for the still growing number of potential applications, and to see their prices decrease on the market place. Among the three synthesis processes discussed above, both the arc discharge and laser ablation methods are demonstrated to rely on the evaporation of carbon atoms from solid graphite targets at temperature >3000 o C. Besides, the nanotubes are always tangled, which makes their purification difficult and greatly hinders their large scale applications. However, CVD is the mostly used method for CNT synthesis at moderate temperatures (500-900°C) [START_REF] Choy | Chemical vapour deposition of coatings[END_REF]. CVD synthesis appears to be the most promising route because of its cheapness, flexibility, high yield, homogeneity and selectivity. Especially, the vertically fluidized bed CVD is widely used for large-scale production of CNTs [START_REF] Hao | Agglomerated CNTs synthesized in a fluidized bed reactor: agglomerate structure and formation mechanism[END_REF][START_REF] Mora | Continuous production of single-walled carbon nanotubes using a supported floating catalyst[END_REF].

Today, bottlenecks are still remaining like the control of CNT morphology (orientation and organization) and the improvement of CNT distribution in different media, which are two urgent issues and need to be solved. As far as the CNT dispersion is concerned, the large scale controlled growth of vertically aligned CNTs may contribute to solve the problems encountered in this area. The growth of aligned MWNTs was first reported in 1996 [START_REF] Li | Large-scale synthesis of aligned carbon nanotubes[END_REF] on a flat substrate. Since then, the investigation in this field has been of great interest. More recently, the floating catalytic CVD as a simple one-step method has been successfully used to construct various nano/micrometer hybrid structures with CNTs selectively grown on the fibers or micrometer particles including nanosized metal or oxide particles, microsized ceramic particles, fullerenes and two dimensional graphene, such as, CNTs/SiO 2 [START_REF] Huang | Growing carbon nanotubes on patterned submicron-size SiO 2 spheres[END_REF],

CNTs/Al 2 O 3 [START_REF] Philippe | An original growth mode of MWCNTs on alumina supported iron catalysts[END_REF], CNTs/large ceramic spheres [START_REF] Zhang | Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres[END_REF] (Fig. 1234), CNTs/carbon fiber [START_REF] Qian | Hierarchical composites reinforced with carbon nanotube grafted fibers: the potential assessed at the single fiber level[END_REF][START_REF] Thostenson | Carbon nanotube/carbon fiber hybrid multiscale composites[END_REF][START_REF] Yamamoto | High-yield growth and morphology control of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites[END_REF][START_REF] Zhang | Hierarchical composites of carbon nanotubes on carbon fiber: Influence of growth condition on fiber tensile properties[END_REF] (Fig. 12345), CNTs/alumina fabric [START_REF] Garcia | Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ[END_REF] (Fig. 123456) and vertically aligned CNT arrays grown on the carbon fiber fabric [START_REF] An | Preparation of vertically aligned carbon nanotube arrays grown onto carbon fiber fabric and evaluating its wettability on effect of composite[END_REF] (Fig. 1234567). Fig. 1-5 SEM micrographs of carbon fibers, (a) before and (b) after nanotube growth. [START_REF] Thostenson | Carbon nanotube/carbon fiber hybrid multiscale composites[END_REF] It is worth noting that the CVD method could allow selective growth of CNTs on the given substrate in controllable ways, resulting in desirable CNT architectures with controlled orientation for various potential applications [START_REF] Dupuis | The catalyst in the CCVD of carbon nanotubes-a review[END_REF][START_REF] Philippe | An original growth mode of MWCNTs on alumina supported iron catalysts[END_REF]. The widely reported CNT structures include the forms of arrays of CNT carpets, pillars, forests, etc [42-44, 33, 45, 46]. However, owing to the catalyst particles, such as iron (Fe), may be easily diffused into the carbon substrate at high temperature (over 850 o C) to lose activity [START_REF] Zhu | Carbon nanotube growth on carbon fibers[END_REF], direct growth of CNTs onto carbon fiber commonly results in low density and inhomogeneity of CNTs with no orientation on the carbon fibers using CCVD [START_REF] Zhang | Hierarchical composites of carbon nanotubes on carbon fiber: Influence of growth condition on fiber tensile properties[END_REF]. An alumina [START_REF] Liu | Aligned multi-walled carbon nanotubes on different substrates by floating catalyst chemical vapor deposition: critical effects of buffer layer[END_REF], silicon [START_REF] Resende | Growth of carbon nanotube forests on carbon fibers with an amorphous silicon interface[END_REF] or silica [START_REF] Lv | Increasing the interfacial strength in carbon fiber/epoxy composites by controlling the orientation and length of carbon nanotubes grown on the fibers[END_REF] buffer layer may prevent the Fe catalyst diffusing into the carbon substrate and obtain vertically aligned CNT arrays grown onto the carbon fiber surface. As largely reported about the CNT growth on flat substrate, the diameter, length, area number density and organization of CNTs depend on both substrate properties, including their size, morphology, and the CVD synthesis parameters, such as carbon source, catalyst precursor, gas atmosphere, hydrogen ratio, temperature, growth time (see Fig. 12345678), gas flow, etc. However, many related research were generally obtained by exploring pre-patterned substrate or predefining catalyst particles into certain morphologies on the corresponding substrate. It is worth noting that the pattern procedures are generally limited to be applicable to the large flat surfaces. Obviously, these complex pretreatment techniques and strict application conditions greatly increase the difficulty and costs of elaborating aligned CNTs.

Besides, it is desirable to control CNT arrangements in the hybrid structures for achieving advanced multifunctional composite properties.

Critical issues in CNT/Polymer nanocomposites

According to the different applications purposes, polymer composites can be classified as structural and functional composites. More recently, the multifunctional composites with excellent mechanical properties as well as specific functional requirements have attracted keen interest [START_REF] Pandey | Carbon nanotube-based multifunctional polymer nanocomposites[END_REF]. Future multifunctional materials may possess integrated sensing, actuation, energy storage, and healing capabilities, much like biological systems. The extraordinary mechanical, electrical and thermal properties of carbon nanotubes make them candidates for the multifunctional fillers in the composites and endow them with many scientific and technological opportunities [START_REF] Thostenson E T, Ren | Advances in the science and technology of carbon nanotubes and their composites: a review[END_REF][START_REF] Thostenson | Nanocomposites in context[END_REF]. The modification of composite properties by integrating small amount of nanotubes is an enabling technology towards the development of new multifunctional material systems [START_REF] Gibson | A review of recent research on mechanics of multifunctional composite materials and structures[END_REF]. To date, more and more effort has been made to incorporate both SWNTs and MWNTs into the thermosetting polymers, such as, epoxy, polyimide (PI), polyurethane (PU), and phenol formaldehyde (PF) resins as well as the thermoplastic polymers, including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), nylon, and so on for the improved strength and conductivity. The mechanical properties combining with electrical, thermal, and many other properties have been widely investigated for a wide range of applications. However, in order to develop high performance CNT/polymer composites, the main problems and challenging tasks are in selecting CNT aspect ratio, controlling CNT alignment, creating a good dispersion and strong interface bonding of CNTs in the polymer matrix, forming a structural frame and electrical conducting path, attaining good load transfer from the matrix to the CNTs during loading, and increasing the electrical conductivity. In this part, these critical issues closely related to the composite properties will be introduced one by one.

CNT aspect ratio

A large amount of CNTs could be prepared by the arc discharge, laser ablation and CVD methods. But the obtained CNTs are not consistent in their intrinsic characteristics.

Aspect ratio (AR=l/d) as one of the most inherent properties is strongly dependent on the synthesis method used and other parameters involved. It is an important parameter that greatly influences the composite properties. The aspect ratio should be sufficiently large to maximize the load transfer between the nanotubes and matrix, thus achieving enhanced mechanical properties. The effect of CNT aspect ratio on the electrical and mechanical properties of the composites has been investigated. The composites with high MWNT aspect ratio demonstrate higher modulus and tensile strength than those with low aspect ratio MWNTs at the same filler concentration [START_REF] Wu | Relations between the aspect ratio of carbon nanotubes and the formation of percolation networks in biodegradable polylactide/carbon nanotube composites[END_REF][START_REF] Qian | Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites[END_REF]. Zhang et al. [START_REF] Zhang | The effect of carbon nanotube dimensions and dispersion on the fatigue behavior of epoxy nanocomposites[END_REF][START_REF] Zhang | Impact behaviour of polypropylene filled with multi-walled carbon nanotubes[END_REF] reported that the MWNTs with larger aspect ratio were more suited for the reduction of fatigue crack growth rate and exhibited higher toughening efficiency than the shorter ones. Sandler et al. [START_REF] Sandler | Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites[END_REF] concluded that it was easier for CNTs with extremely high aspect ratio to form electrically percolative networks along the tangled CNTs in the insulating polymer matrix, resulting in the ultra-low percolation thresholds as low as 0.0025 wt.%. Li et al. [START_REF] Li | Correlations between percolation threshold, dispersion state, and aspect ratio of carbo n nanotubes[END_REF] reported that there is a critical aspect ratio and with increasing the aspect ratio below a critical value, the percolation threshold of the composites decreased. Tucker and Liang [START_REF] Tucker | Stiffness predictions for unidirectional short-fiber composites: Review and evaluation[END_REF] summarized five different physical models, including Halpin-Tsai, and suggested that under strictly idealizing assumptions, the overall mechanical properties of composite materials significantly vary with the aspect ratio of the reinforced CNTs. However, these assumptions are difficult to experimentally achieve because CNTs coil and agglomerate to form bundles.

CNT dispersion

The potential of employing CNTs as reinforcements has been severely limited mainly because of the difficulties associated with dispersion of entangled CNTs during processing.

CNT dispersion is the foremost important issue in producing CNT/polymer nanocomposites.

Better dispersion not only makes more filler surface area available for bonding with polymer matrix, but also prevents the aggregated filler from acting as stress concentrator, which are detrimental to the composite performance. However, some challenges, such as CNT content in the composites, length and entanglement of CNTs as well as viscosity of matrix have to be overcome to obtain uniform CNT dispersion in nanocomposites. There were many reports [START_REF] Ma | Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites[END_REF][START_REF] Kosmidou | Structural, mechanical and electrical characterization of epoxy-amine/carbon black nanocomposites[END_REF] showing that there is a critical CNT content below which the strengthening effect for the composites increases with increasing CNT content. However, above this critical content, the mechanical properties of the composites decrease. These observations can be attributed to (i) the difficulties associated with uniform dispersion of CNTs at high CNT contents; (ii) lack of polymerization reactions that are adversely affected by the high CNT content. The latter effect becomes more pronounced when the functionalized CNTs are employed.

For CNT/polymer nanocomposites, dispersion has two aspects: (І) disentanglement of CNT bundles or agglomerates, which is nanoscopic dispersion and (ІІ) uniform distribution of individual CNTs or their agglomerates throughout the nanocomposites, which is more of a micro-and macroscopic dispersion. In general, several factors that influence the dispersion of CNTs in polymer matrix have to be considered in the preparation process. The nanometer scale of CNTs with large aspect ratio, surface area combined with high flexibility increase the more difficulty of their dispersion. Besides, the CNT entangling during growth process and the attraction between CNTs by van der Waals force make CNTs themselves tend to aggregate. These factors will prevent CNTs from being well dispersed within polymers.

Usually, in order to achieve a homogenous dispersion of CNTs in the matrix, many different methods have been focused on. Many mechanical techniques have been employed

including sonication, ball milling, calendering, stir, etc. Besides, there are two main kinds of surface treatment for CNTs to achieve well dispersed CNTs. One is surface functionalization: Some functional groups (amino, alkyl or hydroxyl, etc.), which can improve the interaction between CNTs and polymer matrix, are covalently bonded directly to the surface of CNTs [START_REF] Gojny | Surface modified multi-walled carbon nanotubes in CNT/epoxy-composites[END_REF]. The composite with a low CNT loading is fabricated by solution casting method or in situ polymerization method [START_REF] Cooper | Distribution and alignment of carbon nanotubes and nanofibrils in a polymer matrix[END_REF][START_REF] Dai | Properties of well aligned SWNT modified poly (methyl methacrylate) nanocomposites[END_REF]. Sometimes, the surfactant is also used to promote the dispersion of CNTs in the matrix [START_REF] Xie | Dispersion and alignment of carbon nanotubes in polymer matrix: a review[END_REF][START_REF] Cooper | Distribution and alignment of carbon nanotubes and nanofibrils in a polymer matrix[END_REF]. However, this method may disrupt the bonding between graphene sheets, and thus reduce the properties of functionalized CNTs in the final composites. Another method is to have CNTs physically coated by some surfactants, which is a non-covalent approach and may be a more facile and practical processing technique. In general, proper functionalization facilitates the CNT dispersion in the composites. However, the addition of functional groups resulted in disturbance of the graphitic layer and reduced the conductivity of the nanotubes. The reaction of the polymer chains with the surface groups of CNTs formed an electrically insulting polymer layer, which increased the contact resistance between individual CNTs, making the tunneling of electrons between nanotubes harder.

The technique employed for CNT dispersion can influence on the mechanical properties of CNT/polymer composites.

CNT orientation

Besides the dispersion of CNTs, their high orientation in matrix is another key issue for developing the CNT/polymer composites. It is desirable to align the nanotubes in a primary direction to take advantage of their anisotropic properties. With the limitations of randomly oriented CNTs based composites and the advance of the composite processing techniques, this topic has drawn much attention.

From geometric consideration, the difference between aligned and randomly orientated CNTs can result in significant changes in various properties of composites [START_REF] Kim | Engineered interfaces in fiber reinforced composites[END_REF]. The storage modulus of the polystyrene composite films containing random and oriented CNTs were 10% and 49% higher than the unreinforced bulk polymer, respectively [START_REF] Thostenson | Aligned multi-walled carbon nanotube-reinforced composites: processing and mechanical characterization[END_REF]. The alignment can be regarded as a special CNT dispersion. However, it is very difficult to align the SWNT in matrix homogeneously. Some methods such as electrospinning [START_REF] Sen | Preparation of single-walled carbon nanotube reinforced polystyrene and polyurethane nanofibers and membranes by electrospinning[END_REF][START_REF] Ko | Electrospinning of continuous carbon nanotube-filled nanofiber yarns[END_REF][START_REF] Hou | Electrospun polyacrylonitrile nanofibers containing a high concentration of well-aligned multiwall carbon nanotubes[END_REF], magnetic field inducing [START_REF] Choi | Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing[END_REF][START_REF] Kimura | Polymer composites of carbon nanotubes aligned by a magnetic field[END_REF], liquid crystal inducing [START_REF] Russell | Alignment of nematic liquid crystals using carbon nanotube films[END_REF][START_REF] Lynch | Organizing carbon nanotubes with liquid crystals[END_REF], shear flow [START_REF] Fan | Characterization of orientation state of carbon nanotubes in shear flow[END_REF][START_REF] Xu | Flow field induced steady alignment of oxidized multi-walled carbon nanotubes[END_REF], extrusion or injecting [START_REF] Xu | Flow field induced steady alignment of oxidized multi-walled carbon nanotubes[END_REF], melt spun [START_REF] Fornes | Morphology and properties of melt-spun polycarbonate fibers containing single-and multi-wall carbon nanotubes[END_REF][START_REF] Haggenmueller | Aligned single-wall carbon nanotubes in composites by melt processing methods[END_REF][START_REF] Park | Synthesis and Dispersion Characteristics of Multi-Walled Carbon Nanotube Composites with Poly (methyl methacrylate) Prepared by In-Situ Bulk Polymerization[END_REF], mechanical stretching methods [START_REF] Jin | Alignment of carbon nanotubes in a polymer matrix by mechanical stretching[END_REF][START_REF] Dai | Properties of well aligned SWNT modified poly (methyl methacrylate) nanocomposites[END_REF] and so on are used to align SWNT in the composite. The degree of CNT alignment in the composite can be governed by two factors: (i) CNT diameter and (ii) CNT content. A smaller diameter of CNT can enhance the degree of CNT alignment due to the greater extensional flow; and a higher CNT content decreases their alignment because of the CNT agglomeration and restrictions in motion from neighboring CNTs. While alignment is necessary to maximize the strength and modulus, it is not always beneficial because the aligned composites have very anisotropic mechanical properties, i.e., the mechanical properties along the alignment direction can be enhanced, whereas these properties are sacrificed along the direction perpendicular to this orientation.

Dai et al. [START_REF] Wang | The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites[END_REF] reported the SWNT/epoxy composite with good uniformity and alignment of SWNTs produced by solution casting and repeated stretching technique in semidried matrix.

Composite showed higher electrical conductivities and mechanical properties along the stretched direction than perpendicular to it, as shown in Fig. 1-12. In addition, the value of percolation threshold along the stretching direction is lower than that perpendicular to the SWNT orientation. Nanni et al. [START_REF] Nanni | Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET-MWCNT nanocomposites[END_REF] investigated the self-monitoring aligned MWNT loaded polyethylene terephthalate (PET) composites prepared via the twin-screw extrusion from a PET/MWCNT masterbatch. It was found that both the self-monitoring and mechanical performances are directly related to the direction of applied stress with respect to CNT orientation. Alignment is necessary to maximize the strength and modulus. However, it is not always beneficial because the aligned composites have very anisotropic mechanical properties. The mechanical properties along the alignment direction can be enhanced, whereas these properties are sacrificed along the direction perpendicular to this orientation.

Interfacial interaction between CNTs and matrix

Achieving strong CNT/polymer interfacial bonding which could provide effective load transfer is another critical challenge, especially for the CNT/polymer structural composites.

There are three main mechanisms of load transfer from the reinforcement to the matrix. The first is the weak van der Waals bonding between CNTs and the matrix, which is the main load transfer mechanism for the CNT/polymer composites. The second is micromechanical interlocking, which may be marginal in the CNT/polymer composites if the nanotubes have smooth surface. The third is chemical bonding between CNTs and matrix, which is not guaranteed in many cases.

Based on the above mechanisms, it is possible to enhance the load transfer from CNTs to the matrix by different treatments. Chemical and physical functionalization of CNTs has proven to enhance the interfacial adhesion. The van der Waals bonding can be increased by using CNTs with small size and close contact at the interface. From this point of view, individual SWNT well dispersed in a matrix is helpful. In the case of the micromechanical interlocking between CNTs and the molecular chains of matrix, we hope that CNTs are strong enough and inter-connected or long enough to block the movement of the polymer chains [START_REF] Zhang | Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres[END_REF][START_REF] Moniruzzaman | Increased flexural modulus and strength in SWNT/epoxy composites by a new fabrication method[END_REF]. The contribution of this mechanism may reach saturation at a relatively low CNT content [START_REF] Moniruzzaman | Increased flexural modulus and strength in SWNT/epoxy composites by a new fabrication method[END_REF]. It is unclear at this stage whether or not this critical content depends on the CNT orientation in the matrix. The chemical bonding between CNTs and matrices can be enhanced or created by surface treatments such as chemical and physical functionalization of CNTs. In order to achieve a strong interfacial bonding of CNTS and polymer, the CNTs are usually functionalized with some radical group chemically. It is well known that they can be functionalized easily with some radical groups such as alkyl, carboxyl and carbonyl in the purification process of pristine CNTs product using condensed sulfuric acid and nitric acid [1,2,6]. This covalent method can provide useful functional groups onto the CNT surface.

Non-covalent functionalization is an alternative method for tuning the interfacial properties of nanotubes. The suspension of CNTs in the presence of polymers leads to the wrapping of polymer around the CNTs to form supermolecular complexes of CNTs (Fig. 1-13a). Besides polymers, surfactants have also been employed to functionalize CNTs (Fig. 1-13b). The physical adsorption of surfactant on the CNT surface lowered the surface tension of CNT, effectively preventing the formation of aggregates. Furthermore, the surfactant-treated CNTs overcome the van der Waals attraction by electrostatic/steric repulsive forces. It was shown that CNT functionalization can enhance the modulus, strength as well as fracture resistance of the composites [START_REF] Koval | Effect of carbon nanotube functionalization on the structural and mechanical properties of polypropylene/MWCNT composites[END_REF][START_REF] Yuen | Preparation, morphology and properties of acid and amine modified multiwalled carbon nanotube/polyimide composite[END_REF][START_REF] Kim | Improvement of tensile properties of poly (methyl methacrylate) by dispersing multi-walled carbon nanotubes functionalized with poly (3-hexylthiophene)graft-poly (methyl methacrylate)[END_REF][START_REF] Sahoo | Effect of functionalized carbon nanotubes on molecular interaction and properties of polyurethane composites[END_REF]. Besides, proper functionalization facilitates the CNT dispersion and the formation of conducting networks in the composites, resulting in lowered percolation threshold [START_REF] Ma | Effects of silane functionalization on the properties of carbon nanotube/epoxy nanocomposites[END_REF]. However, some research reported that functionalized CNTs are detrimental to the composite electrical properties [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites[END_REF]. The addition of functional groups resulted in disturbance of the graphitic layer and reduced the conductivity of the nanotubes.

The functionalization significantly improved the affinity between the polymer and CNTs.

The reaction of the polymer with the CNT surface groups formed an electrically insulting polymer layer, which increased the contact resistance between individual nanotubes, making the tunneling of electrons between nanotubes harder. In addition, functionalizations using acids of a high concentration can severely damage and fragment CNTs into smaller pieces with decreased aspect ratios.

It needs to be emphasized that the introduction of the multiscale hybrid nanofillers with the hybridization of CNTs and 2D lamellar flakes is helpful for improving the interfacial interaction within the polymer matrix. Without any purification, the CNT-clay hybrids were directly used to fabricate nylon-6 composites via simple melt-compounding. The composites showed remarkably enhanced mechanical properties due to the homogeneous dispersion of na nohybrids and their strong interfacial interaction with the nylon matrix [START_REF] Zhang | Dramatically Enhanced Mechanical Performance of Nylon-6 Magnetic Composites with Nanostructured Hybrid One-Dimensional Carbon Nanotube-Two-Dimensional Clay Nanoplatelet Heterostructures[END_REF] (Fig. [1][2][3][4][5][6][7][START_REF] Nardelli | Brittle and ductile behavior in carbon nanotubes[END_REF][START_REF] Salvetat | Elastic and shear moduli of single-walled carbon nanotube ropes[END_REF][START_REF] Salvetat | Mechanical properties of carbon nanotubes[END_REF][START_REF] Ebbesen | Electrical conductivity of individual carbon nanotubes[END_REF][START_REF] De Pablo | A simple, reliable technique for making electrical contact to multiwalled carbon nanotubes[END_REF][START_REF] Kaneto | Electrical conductivities of multi-wall carbon nano tubes[END_REF][START_REF] Bandaru | Electrical properties and applications of carbon nanotube structures[END_REF]. fillers and matrix. [START_REF] Zhang | Dramatically Enhanced Mechanical Performance of Nylon-6 Magnetic Composites with Nanostructured Hybrid One-Dimensional Carbon Nanotube-Two-Dimensional Clay Nanoplatelet Heterostructures[END_REF] Chapter І

CNTs for Multifunctional Reinforcements -25 -

Other problems

For the electrically and thermally conductive CNT/polymer composites, good contact between individual CNTs is very important. Only when excellent contacts are established between the individual CNTs, the electrical and thermal signals could be well transmitted.

Simulation results indicated that the contact resistance of CNTs in composites played an important role in enhancing the conductivity of composites [START_REF] Buldum | Contact resistance between carbon nanotubes[END_REF][START_REF] Stadermann | Nanoscale study of conduction through carbon nanotube networks[END_REF]. This was confirmed by a recent study [START_REF] Ma | Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites[END_REF] in which when silver decorated CNTs were employed as conducting filler, the composites exhibited a significantly higher conductivity above the percolation threshold than those containing pristine CNTs. If the contact resistance between the individual CNTs is too high, the existing CNT paths will not endow better conductivity. Several main factors may influence the contact resistance between CNTs, such as the intrinsic transport properties of CNTs, the way of one CNT contacting with the others (overlap, end to end, one to one or one to more), the insulating layer coating the CNTs as well as its thickness. Based on these analyses, it is necessary to decrease the contact resistivity and improve transport properties between individual CNTs. For example, to improve their intrinsic transport properties, CNTs are treated under high temperature for improving their structural integrity and reducing the functional groups on the surface. The contact means between CNTs can be designed by adjusting the content and orientation of CNTs within a matrix. Huang et al. [START_REF] Huang | Aligned carbon nanotube composite films for thermal management[END_REF] proposed an ideal CNT structure, in which all the CNTs embedded in the matrix are aligned form one surface to the opposite side with all the CNT tips revealed on both surfaces. In addition, for the structural CNT/polymer composites, it is important to solve the slippage within the nanotubes.

Processing of CNT/Polymer nanocomposites

Because the structure of composites is formed during the synthesis and processing steps, knowledge of the processing-structure-property relations will enable the tailoring of material properties through the control of processing conditions. The effective utilization of CNTs in the composite is greatly dependent on their intrinsic morphology and dispersion state within the matrix. Besides, better CNT/polymer interfacial interaction will result in an efficient load transfer from CNTs to the polymer matrix. These important issues that affect the composite properties have been focused on by attempting different processing routes. The common fabrication methods which are available for the issues mentioned above will be summarized in this part.

Calendering (Three roll mill)

Calendering, which is commonly known as three roll mill, as shown in Fig. 1-15, is a dispersion method that employs shear force created by the rolls with different rotating speed to mix, disperse or homogenize viscous materials. The general configuration of the machine consists of three adjacent cylindrical rollers. The first and third rollers, called the feeding and apron rollers (n1 and n3 in Fig. 1-15), rotate in the same direction while the center roller rotates in the opposite direction. The narrow gaps (controllable from 500 to 5 μm) between the rollers, combined with the mismatch in angular velocity of the adjacent rollers, result in locally high shear forces in a short residence time. In order to create high shear rates, angular velocity of the center roll should be higher than that of feed roll (ω 2 >ω 1 ). One of the unique advantages of this technique is that the gap width between the rollers can be adjusted, thus it is easy to get a controllable and narrow size distribution of particles in viscous materials [START_REF] Viswanathan | Challenges and advances in nanocomposite processing techniques[END_REF][START_REF] Meng | Effects of acid-and diamine-modified MWNTs on the mechanical properties and crystallization behavior of polyamide 6[END_REF]. In some operations, the width of gaps can be decreased gradually to achieve the desired level of particle dispersion. This method has become a very promising approach to achieve relatively good CNT dispersion [START_REF] Thostenson | Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites[END_REF]. Thostenson et al. [START_REF] Chou | An assessment of the science and technology of carbon nanotube-based fibers and composites[END_REF] utilized a scalable calendering approach for achieving dispersion of CNTs through intense shear mixing. The degree of mixing of the resin and fillers is controlled by the spacing of the gap between neighboring cylinders, through which the mixture must pass. Fig. 1-16 shows the progressive development of nanocomposite structure during the calendering process.

However, there are several concerns should be focused on in using this technique for CNT dispersion: the feeding materials should be in viscous state when mixing with CNTs. In addition, the minimum gap between the rollers is about 1-5 μm, which is comparable to the length of CNTs, but is much larger than the diameter of individual CNTs.

Solution processing

Carbon nanotubes are generally dispersed in a certain solvent then mixed with polymer (solution) by vigorous stirring and/or sonication, i.e., mechanical mixing (high shear stirring), magnetic agitation or high energy sonication. Then, the CNT/polymer composites could be obtained by controlled evaporation of the solvent with or without vacuum conditions at a certain temperature. In this method, though prolonged high-energy sonication has the potential to introduce defects, ultrasonication process is considered to effectively aid nanotube dispersion in liquid media [START_REF] Koshio | A simple way to chemically react single-wall carbon nanotubes with organic materials using ultrasonication[END_REF]. In Addition, polymer-functionalized nanotubes [START_REF] Paiva | Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes[END_REF], surfactants [START_REF] Zhang | Poly (vinyl alcohol)/SWNT composite film[END_REF] and other chemical treatments [START_REF] Mitchell | Dispersion of functionalized carbon nanotubes in polystyrene[END_REF] are often employed.

With regard to the thermosetting matrices, the CNT/epoxy composites were fabricated by Windle et al. [START_REF] Shaffer | Fabrication and characterization of carbon nanotube/poly (vinyl alcohol) composites[END_REF] using the solution mixing and excellent electrical properties was achieved, resulting in a matrix conductivity around =10 -2 S/m with filler volume fractions as low as 0.1 vol.%. Ajayan et al. [START_REF] Schadler | Load transfer in carbon nanotube epoxy composites[END_REF] reported the MWNT/epoxy composites prepared by this method and the load transfer from nanotubes to the matrix was studied in both tension and compression testing. In our research, the MWNT were first dispersed in methanol solution under magnetic agitation then mixed with the epoxy matrix [START_REF] Coleman | Mechanical reinforcement of polymers using carbon nanotubes[END_REF]. The Young's modulus and the yield strength of the resulted MWNT/epoxy composites was doubled and quadrupled with respectively 1 and 4 wt.% nanotubes, compared to the pure resin matrix samples. Dang et al. [START_REF] Dang | Fundamentals, processes and applications of high-permittivity polymer-matrix composites[END_REF][START_REF] Dang | Giant Dielectric Permittivities in Functionalized Carbon-Nanotube/Electroactive-Polymer Nanocomposites[END_REF][START_REF] Wang | Carbon nanotube composites with high dielectric constant at low percolation threshold[END_REF] mainly focused on the CNT/PVDF composites with high permittivity and low percolation threshold prepared using this method, i.e., PVDF was firstly dissolved in DMF and then MWNT mixed in the PVDF solution.

For the thermoplastic matrices, the CNT based composites could also be prepared by the solution mixing. Since common solvents such as toluene, chloroform, tetrahydrofurane (THF), or dimethylformamide (DMF) are of high toxicity, a number of aqueous systems have been explored. Jin et al. have reported a similar method consisting of solution mixing, casting and drying [START_REF] Jin | Alignment of carbon nanotubes in a polymer matrix by mechanical stretching[END_REF]. Polyhydroxyaminoether (PHAE) was dissolved in the arc discharge grown CNTs in chloroform under sonication. In this work, the CNT orientation and degree of alignment was determined. Solution casting as an alternative processing method has been used to prepare the MWNT-based PS [START_REF] Paul | A low resistance boron-doped carbon nanotube-polystyrene composite[END_REF][START_REF] Safadi | Multiwalled carbon nanotube polymer composites: synthesis and characterization of thin films[END_REF], PVA [START_REF] Cadek | Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous polymer composites[END_REF][START_REF] Shaffer | Fabrication and characterization of carbon nanotube/poly (vinyl alcohol) composites[END_REF], ultrahigh molecular weight polyethylene (UHMWPE) [START_REF] Hirsch | Functionalization of single-walled carbon nanotubes[END_REF], and acrylonitrile butadiene styrene (ABS) [START_REF] Park | Dispersion and aspect ratio of carbon nanotubes in aqueous suspension and their relationship with electrical resistivity of carbon nanotube filled polymer composites[END_REF] composite films with homogeneous nanotube dispersions. Similarly, SWNT-based PP [START_REF] Grady | Nucleation of polypropylene crystallization by single-walled carbon nanotubes[END_REF],

PVA [START_REF] Paiva | Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes[END_REF], composite films have also been prepared. Since the casting/evaporation process results in the CNT reagglomeration within the composite film, Winey et al. proposed coagulation as an alternative approach for fabricating composites with individually dispersed CNTs [START_REF] Du | Coagulation method for preparing single-walled carbon nanotube/poly (methyl methacrylate) composites and their modulus, electrical conductivity, and thermal stability[END_REF]. After solution mixing, the CNT/PMMA suspension was dripped into a large excess of bad solvent (water) so as to induce instant precipitation of the polymer chains. The precipitating chains entrapped the carbon nanostructures and prevented them from bundling again. Oliva-Avilés et al. introduced the aligned MWNT/polysulfone (PSF) composite films which were prepared by solution casting under alternating electric field [START_REF] Oliva-Avilés | Electrical and piezoresistive properties of multi-walled carbon nanotube/polymer composite films aligned by an electric field[END_REF]. The electrical conductivity and piezoresistive response of electric-field-aligned and randomly oriented MWNT/PSF films are evaluated.

Melting blending

Melting blending has been a very practical method for the fabrication of CNT-based composites on the basis of the fact that thermoplastic polymers will soften when it is close to or above their melting point. Once the polymer was molten, the appropriate percentage of filler was added. Besides, this technique is suitable for polymers that could not easily be prepared using the solution processing due to their insolubility in common solvents. In general, the melt processing involves mixing nanoutbes with the polymer viscous fluid by using high shear forces to break down the CNT aggregates and to improve CNT dispersion.

The n the final composite samples with specific geometric shapes could be processed by several techniques, such as extrusion.

Due to their good dispersive and distributive mixing capability, twin screw extruders have been the most popular equipment for dispersing and processing CNT-based composites [START_REF] Lynch | Organizing carbon nanotubes with liquid crystals[END_REF]. In our previous studies, the conductive fillers [carbon black (CB), CNTs or the CB and

CNT mixture] were mixed with the PVDF and UHMWPE melts using the melting method by a lab-scale Haake rheometer equipped with a pair of high shear roller-type rotors [START_REF] Dang | Origin of remarkable positive temperature coefficient effect in the modified carbon black and carbon fiber cofillled polymer composites[END_REF][START_REF] Zha | High performance hybrid carbon fillers/binary-polymer nanocomposites with remarkably enhanced positive temperature coefficient effect of resistance[END_REF][START_REF] Xu | Enhanced dielectric properties and positive temperature coefficient effect in the binary polymer composites with surface modified carbon black[END_REF]. The preferential distribution of conductive fillers in the polymer matrix was observed and desirable positive temperature co-efficient (PTC) effect was achieved. In our group, the MWNT/PVDF nanocomposites were also fabricated by melt-blending the pristine MWNTs in PVDF matrix [START_REF] Yuan | Giant dielectric permittivity nanocomposites: realizing true potential of pristine carbon nanotubes in polyvinylidene fluoride matrix through an enhanced interfacial interaction[END_REF]. A giant dielectric permittivity (~ 3800) was observed when MWNT content is above the percolation threshold (f c ) of the system. However, a high value of f c (~10.4 vol.%) was found in the composites possibly due to a significant polymer layer wrapping on the nanotubes. Kenny et al. [START_REF] Manchado | Thermal and mechanical properties of single-walled carbon nanotubes-polypropylene composites prepared by melt processing[END_REF]

In situ polymerization process

In situ polymerization processes for fabricating functional composites have been focused on in some investigations. It could produce polymer grafted nanotubes, mixed with free polymer chains. Importantly, this method allows the preparation of composites with high CNT weight fraction. Park et al. [START_REF] Park | Dispersion of single wall carbon nanotubes by in situ polymerization under sonication[END_REF] prepared the SWNT/PI composites by an in situ polymerization process. This process enabled uniform dispersion of SWNTs in the PI matrix.

The electrical conductivity of the resultant SWNT/PI nanocomposite films were significantly improved by 10 orders of magnitude at a low SWNT loading (0.1 vol.%). Mechanical properties as well as thermal stability were also improved with the incorporation of the SWNTs. Feng et al. [START_REF] Feng | Well-aligned polyaniline/carbon-nanotube composite films grown by in-situ aniline polymerization[END_REF] reported that MWNTs encapsulated by polyaniline (PANI) of nanometer size have been synthesized by in-situ polymerization. Hu et al. [START_REF] Hu | Tunneling effect in a polymer/carbon nanotube nanocomposite strain sensor[END_REF] prepared the CNT/polymer strain sensor using in-situ polymerization. The numerical calculation and experimental measurement are used to understand the piezoresistivity of the composites.

Kang et al. [START_REF] Kang | Piezoresistive characteristics of single wall carbon nanotube/polyimide nanocomposites[END_REF] investigated the high piezoresistive characteristics of SWNT/PI composites from below to above the percolation threshold (~ 0.05 wt.%). The maximum piezoresistive stress coefficient of 1.52×10 -3 MPa -1 was found when f SWNT is just above f c . This coefficient value exceeds those of the metallic piezoresistive materials by two orders of magnitude (4.25×10 -5 MPa -1 for aluminum).

In situ polymerization processes allow for homogenous dispersion of functional fillers and therefore ensure strong interfacial interaction between the fillers and the polymer. As a result, the resulted composites display excellent properties. However, the narrow monomer alternative window impedes its wide application in various composites.

Electrospinning

Electrospinning technique as an electrostatic induced self-assembly process has been developed for decades. It was used to align CNTs in a variety of polymeric matrixes and produce CNT/polymer composite nanofiber in a continuous manner. The alignment of CNTs enhances the axial mechanical and physical properties of the filaments. As shown in Fig. 1-17, a high voltage is applied between an oppositely charged polymer fluid and a metallic collection screen. The fluid is contained in a glass syringe which has a capillary tip (spinneret). When the voltage reaches a critical value, the electric field overcomes the surface tension of the suspended polymer and a jet of ultra-fine fibers is produced. As the solvent evaporates, a mesh of nano to micro size fibers is accumulated on the collection screen. The fiber diameter and mesh thickness can be controlled through the variation of the electric field strength, polymer solution concentration and the duration of electrospinning.

Due to the sink flow and the high extension of the electrospun jet, it is expected to align CNTs during the electrospinning process. However, the distribution and alignment of the nanotubes in the nanofibers are strongly associated with the quality of the nanotube dispersion prepared before addition of the spinnable polymer solution. Generally, the well-dispersed CNTs were incorporated as individual elements mostly aligned along the nanofiber axis. Conversely, the irregular nanotubes were poorly aligned and appeared curled, twisted, and entangled. It is also indicated that the nanofiber diameter, the interaction between the spun polymer and nanotubes as well as the wetting ability are important factors affecting the alignment and distribution of the nanotubes. This was demonstrated by the difference in the alignment of SWNTs in polyacrylonitrile (PAN) and polylactic acid (PLA)

nanofibers [START_REF] Ko | Electrospinning of continuous carbon nanotube-filled nanofiber yarns[END_REF]. The investigations about the SWNTs/poly(ethylene oxide) (PEO) nanofibers by the electrospinning process exhibited that SWNTs were embedded in the PEO matrix in a more regular form because SWNTs are much smaller and uniform in shape and size, as compared to MWNTs. Besides, their stronger tendency to bundle up into coiled aggregates introduces a pronounced difficulty. As a result, more attention was given to the dispersion process, which is essential for successful alignment of the nanotubes by the electrospinning process. In summary, it should be noted that the techniques for CNT dispersion and alignment are not limited to the described above. Many of the recent studies are often based on the use of a combination of these techniques, such as ultrasonication plus ball milling [START_REF] Ma | In-situ amino functionalization of carbon nanotubes using ball milling[END_REF], and ultrasonication plus extrusion [START_REF] Moniruzzaman | Increased flexural modulus and strength in SWNT/epoxy composites by a new fabrication method[END_REF][START_REF] Isayev | Ultrasound assisted twin screw extrusion of polymer-nanocomposites containing carbon nanotubes[END_REF] [136] reviewed the challenges and opportunities in the multifunctional nanocomposite aerospace structures. Thostenson et al. [START_REF] Thostenson E T, Ren | Advances in the science and technology of carbon nanotubes and their composites: a review[END_REF] and Chou et al. [START_REF] Chou | An assessment of the science and technology of carbon nanotube-based fibers and composites[END_REF] reviewed recent advances related to the science and technology of carbon nanotubes and their composites. Li et al.

[137] surveyed the recent advances related to the use of carbon nanotubes and their composites as sensors and actuators.

CNT-polymer nanocomposite for structural materials

Among the most important structural functions that a system can provide are stiffness, strength, fracture toughness, ductility, fatigue strength, energy absorption, etc. In recent years, the composite materials with lightweight and simultaneous improvement in multiple structural functions are extremely needed. As discussed above, carbon nanotubes exhibit exceptional mechanical properties with Young's modulus as high as ~ 1 TPa and tensile strength of 50-200 GPa. The combination of the superior mechanical properties as well as the low density, high aspect ratio and high surface area make them ideal candidates for reinforcement in the composites. Both SWNTs and MWNTs have been utilized for reinforcing thermosetting and thermoplastic polymers. The CNT reinforced nanocomposites can be considered as a kind of particulate composites with the filler dimensions on nanometer scale and high aspect ratio. The addition of a small amount of CNTs in polymer composites has produced significant improvements in mechanical properties relative to the un-reinforced resin material.

Tai et al. [START_REF] Tai | Experimental study and theoretical analysis on the mechanical properties of SWNTs/phenolic composites[END_REF] examined the mechanical properties of SWNT/phenolic composites.

The change of Young's modulus and strength of the composites with nanotube percentage were investigated by perimental and Halpin-Tsai criterion, as shown in Fig. 123456789101112131415161718. With the addition of SWNT, Young's modulus and strength was increased. However, high loading of Halpin-Tsai equation. [START_REF] Tai | Experimental study and theoretical analysis on the mechanical properties of SWNTs/phenolic composites[END_REF] nanotubes causes insufficient impregnation of the resin within the SWNT networks, thus reducing the mechanical properties. Thostenson et al. [START_REF] Thostenson | Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites[END_REF] reported that the low CNT concentrations resulted in significant enhancements in fracture toughness and the overall fracture toughness were dependent on the dispersed and agglomerated structure of the composites, i.e., larger agglomerate was able to more effectively interact with crack front.

Grimmer and Dharan [START_REF] Grimmer | High-cycle fatigue of hybrid carbon nanotube/glass fiber/polymer composites[END_REF] demonstrated that incorporating of 1 wt.% CNTs improved the fatigue life of woven glass fiber/epoxy composites by 60%. The added CNTs inhibit the formation of large cracks and the crack bridging by the CNTs as well as nanotube pull-out from the matrix enhance energy absorption and inhibit damage propagation. Koratkar et al. [START_REF] Koratkar | Characterizing energy dissipation in single-walled carbon nanotube polycarbonate composites[END_REF] reported that the loss modulus of polycarbonate composites was improved more than 1000% by addition of 2 wt.% SWNTs. It was hypothesized that frictional sliding at the SWNT/PC interfaces was the reason for the enhanced energy dissipation.

In summary, the CNT/polymer composites could be used as structural materials for improved elastic modulus, strength, and fracture toughness and enhanced fatigue life in the glass/epoxy composites at low CNT loadings. However, if nanotube agglomerations are not well dispersed, the enhancement in mechanical properties of nanocomposites may not be significant and there may be even degradation. Surface functionalization of the nanotubes is a way to improve dispersion and strengthen nanotube-polymer interaction, but it may be detrimental to electrical properties of the nanocomposite. Besides, there is optimal nanotube concentration for improving the nanocomposite mechanical properties. In the case of higher nanotube content, the agglomerations and ends of nanotubes will act as stress concentrations and degrade mechanical properties of the nanocomposites. It is worth noting that, due to their distinctive architecture and well dispersed CNTs in the matrix, the CNT-mircoparticle nano/micro hybrids have been excellent reinforcements in the composites. In our research, compared to the SiC microparticles, the use of CNT-SiC hybrids in the epoxy matrix could provide more evident improvement in mechanical properties of the composites [START_REF] Ci | Novel micro/nanoscale hybrid reinforcement: multiwalled carbon nanotubes on SiC particles[END_REF]. Besides, as shown in Fig. 1-19, the CNT-clay hybrids were demonstrated to be ideal and excellent nanofillers for mechanical reinforcement for fabricating nylon-6 nanocomposites, due to their homogeneous dispersion and strong interfacial interaction with the matrix [START_REF] Zhang | Dramatically Enhanced Mechanical Performance of Nylon-6 Magnetic Composites with Nanostructured Hybrid One-Dimensional Carbon Nanotube-Two-Dimensional Clay Nanoplatelet Heterostructures[END_REF].

Electrical behavior of CNT-Polymer nanocomposites

In addition to serving as structural materials, the CNT/polymer composites have been considered to be electrically conducting materials used in various engineering applications including conductive adhesives, antistatic coatings and films, electromagnetic interference shielding, etc. Importantly, for aircraft structures, the conductive CNT/polymer composites are being substitute for the metallic conductors, which are too heavy and difficult to repair.

As show in Fig. 1-20, with the gradual increase of CNT content, the composites undergo an insulator-to-conductor transition. The critical CNT content is referred to the percolation threshold, where the electrical conductivity of the composites sharply jumps up several orders of magnitude due to the formation of conductive networks. Percolation theory is usually applied to explain the electrical behavior of the CNT/polymer composites. Below the percolation transition range, the conductive paths do not exist and their electrical properties are mainly dominated by the matrix. Above the percolation transition range, the continuous networks are formed and the conductivity of the composites shows a saturation plateau.

Fig. 1-20 Schematics of (A) percolation phenomenon and (B) the evolution of conducting networks in

composites with increasing conductive fillers. [START_REF] Dang | Fundamentals, processes and applications of high-permittivity polymer-matrix composites[END_REF] The investigation about electrical percolation threshold of the CNT/polymer composites has been a challenging and major concern. It turns out that small concentration of CNTs can lead to very large improvements in the electrical conductivity of the composite s. Moisala et al. [START_REF] Moisala | Thermal and electrical conductivity of single-and multi-walled carbon nanotube-epoxy composites[END_REF] identified the percolation threshold of 0.005 wt.% for MWNT/epoxy composites, and 0.05-0.23 wt.% for SWNT/epoxy composites. However, there is no apparent consensus on percolation threshold values of CNTs: their values reported for CNT/epoxy composites varied from 0.002 to more than 7 wt.% [START_REF] Li | Correlations between percolation threshold, dispersion state, and aspect ratio of carbo n nanotubes[END_REF][START_REF] Moisala | Thermal and electrical conductivity of single-and multi-walled carbon nanotube-epoxy composites[END_REF][START_REF] Sandler | Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites[END_REF]. This large variation indicates that the type of nanotubes (SWNT or MWNT), their surface functionalization, content, dispersion state, aspect ratio as well as the processing techniques used to produce the composites are important factors in determining the electrical properties of CNT/polymer composites. Gojny et al. [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites[END_REF] performed an extensive experimental research on the electrical conductivity of CNT/epoxy composites with relation to the contributing factors above. Li et al. [START_REF] Li | Correlations between percolation threshold, dispersion state, and aspect ratio of carbo n nanotubes[END_REF] produced CNT/epoxy nanocomposites with percolation thresholds varying in the range from 0.1 to above 1 wt.% by controlling the dispersion state and aspect ratio of CNTs. They noted that the critical factors determining the percolation threshold were: (i) aspect ratio of CNTs;

(ii) disentanglement of CNT agglomerates on the nanoscopic scale; and (iii) uniform distribution of individual CNTs or CNT agglomerate s on microscopic scale. There is a critical value of CNT aspect ratio, above which the percolation threshold was sensitive to dispersion state, while below which the percolation threshold increased rapidly with decreasing aspect ratio. The importance of waviness was confirmed by Li et al. [START_REF] Li | Effect of nanotube waviness on the electrical conductivity of carbon nanotube-based composites[END_REF], who used Monte Carlo simulations to show that the electrical conductivity of composites with wavy nanotubes is less than that of composites with straight nanotubes. Thostenson and Chou [START_REF] Thostenson | Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites[END_REF] concluded that the electrical resistivity are higher for the highly dispersed than for the partially agglomerated structure, particularly at lower fractions of CNTs.

Apart from the dc electrical properties, the nanotubes offer the potential to tailor the frequency-dependent electrical and dielectric properties of the composites. Dang et al. [START_REF] Dang | Fundamentals, processes and applications of high-permittivity polymer-matrix composites[END_REF] comprehensively investigated the varying ac electrical and dielectric properties of the CNTs filled PI and PVDF composites. For the CNT contents below the percolation threshold, the nanocomposites usually exhibit a typical dielectric behavior: the ac conductivity increases almost linearly as the increase of frequency. For CNT concentration above the percolation threshold, the ac conductivity is almost independent of frequency, suggesting an effective connection of the conducting networks in the composites. Arjmand at al. [START_REF] Arjmand | Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate[END_REF] studied the electromagnetic interference (EMI) shielding effectiveness of MWNT/PC nanocomposites.

It was found that the EMI shielding effectiveness increases linearly with MWNT content because the number of conductive networks and the number of free electrons increases with increasing MWNT content.

In summary, nanotubes are excellent reinforcements to prepare electrically conducting polymer composites with exceptionally low percolation thresholds. The electrical behaviors of the composites are greatly related to the CNT aspect ratio, dispersion state. As a result, the comprehensive research about the processing-structure-property relationship of composites is extremely needed. As our recently reported, the CNT-SiC multiscale hybrids significantly improve the dielectric permittivity of the CNT-SiC/PVDF composites with an extremely low CNT loading [START_REF] Yuan | High dielectric permittivity and low percolation threshold in polymer composites based on SiC-carbon nanotubes micro/nano hybrid[END_REF], as shown in Fig. 1-21. Thus, the CNT-microsubstrate hybrids are likely to have potential applications for improving the electrical properties of the composites.

Piezoresistivity of CNT-Polymer nanocomposites

As discussed in the previous part, once percolation threshold is reached, the connected CNT networks result in continuous electron transport paths across the composites. When the composites are subjected to an external load, the percolated CNT networks will be disrupted, resulting in an increase in electrical resistivity. The variation in resistivity is attributed to the following reasons: (1) the variation of contact configurations (percolation theory); (2) the increase of tunnelling distances among the CNTs with increase of strain (tunneling theory);

(3) piezoresistive behavior of individual CNT themselves.

Early research about the piezoresistivity involved free-standing CNT films or sheets, i.e., ‗buckypaper' bonded to the surface of various substrates (brass, aluminium, etc). When the load is applied on the substrates, the resistance between two electrodes attached to the CNT sheet was measured in situ [START_REF] Vemuru | Strain sensing using a multiwalled carbon nanotube film[END_REF]. Most of the related studies employed isotropic, randomly oriented CNT networks, and showed that the resistance increase linearly under tension and decreased linearly under compression. As recently reported, the piezoresistivity of the composites with CNTs added to the polymers including the general thermoplastics and thermosets, shape memory polymers, elastomers, and concrete have been investigated.

Kang et al. [START_REF] Kang | Introduction to carbon nanotube and nanofiber smart materials[END_REF] reported the strain sensing behavior of the SWNT-PMMA composites,

showing linear symmetric strain response trends in both compression and tension. Pham et al. [START_REF] Pham | Processing and modeling of conductive thermoplastic/carbon nanotube films for strain sensing[END_REF] reported the development of the MWNT-filled polymer composite films that can be used as strain sensors with tailored sensitivity. The electrical resistance of MWNT-PMMA films subjected to tensile strains was measured, and the potential applications of the films as strain sensors with a broad range of tunable sensitivity were studied. The largest sensitivity achieved was almost an order of magnitude greater than conventional resistance strain gages.

A semi-empirical model, based on the percolation theory, was developed to identify the relationship between the applied strain and sensitivity factor. Not only can the sensitivity be tailored over a broad, but also it can be increased significantly when CNT content is close to the percolation threshold. Zhang et al. [START_REF] Zhang | Carbon nanotube/polycarbonate composites as multifunctional strain sensors[END_REF] demonstrated the MWNT-PC composites as multifunctional strain sensors. The composites with 5 wt.% MWNT showed instantaneous electrical resistance response to linear and sinusoidal dynamic strain inputs and a sensitivity of ~3.5 times that of a typical strain gage. Billoti et al. [START_REF] Bilotti | Fabrication and property prediction of conductive and strain sensing TPU/CNT nanocomposite fibres[END_REF] investigated the good strain sensing abilities of the MWNT filled polyurethane (TPU) nanocomposite fibers fabricated via an extrusion process and the piezoresistivity can be tunable by controlling the extrusion temperature. Bautista-Quijanoa et al. [START_REF] Bautista-Quijano | Strain sensing capabilities of a piezoresistive MWCNT-polysulfone film[END_REF] reported the piezoresistive responses of the MWNT filled polysulfone (PSF) thin films. The films were bonded to aluminum specimens and evaluated as strain sensing elements during quasi-static and cycling tensile loading.

Excellent piezoresistive capabilities were found for the films with MWNT loadings as low as 0.5 wt.%. De la Vega, et al. [START_REF] De La Vega | Simultaneous global and local strain sensing in SWCNT-epoxy composites by Raman and impedance spectroscopy[END_REF] characterized the local and global stress response of SWNT-epoxy composites by simultaneous Raman spectroscopic and electrical measurement on the specimens subjected to various levels of strain. Both the Raman G-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at 1.5% strain. Anand at al. also [START_REF] Anand | Quasi-static and dynamic strain sensing using carbon nanotube/epoxy nanocomposite thin films[END_REF] studied the quasi-static and dynamic piezoresistivity of the CNT-epoxy composites. Wichmann et al.

[152] found that distinct piezoresistive behavior in the regime of elastic deformation of the MWNT/epoxy composites, which is in good agreement with prevalent theories about charge carrier transport mechanisms in isolator/conductor composites. Dang et al. [START_REF] Dang | Supersensitive linear piezoresistive property in carbon nanotubes/silicone rubber nanocomposites[END_REF] reported the high sensitivity and linear piezoresistive behavior of the MWNT filled methylvinyl silicone rubber (VMQ) nanocomposites. Zha et al. [START_REF] Shehzad | All-organic PANI-DBSA/PVDF dielectric composites with unique electrical properties[END_REF] investigated the effect of aspect ratio of MWNT on the piezoresistive behavior of thermoplastic elastomer (TPE) based composites in details. By introducing the effective MWNT aspect ratios which are length-dependent and diameter-dependent, it has been demonstrated that the piezoresistivity will decrease with the increase of effective aspect ratios. The piezoresistivity of the CNT-based nanocomposites exhibits a much higher sensitivity as compared to traditional strain gages. Future research in improving the repeatability of nanotube based strain sensors and the effect of CNT aspect ratio and orientation on the piezoresistive behavior is needed.

CNT-based multiscale hybrid composites

Conventional fiber-reinforced polymer composites (FRPs) have considered to be high strength structural materials and have been widely used in many fields including aerospace, automobile, marine and military due mainly to their high specific strength and stiffness, lower density and flexible tailoring, easy processability, etc [START_REF] Tang | A review of methods for improving the inter facial adhesion between carbon fiber and polymer matrix[END_REF]. Extensive research in the fiber-reinforced composites has achieved excellent in-plane properties (fiber-dominated), such as, high tensile modulus and strength. However, the relatively weak out of-plane properties (matrix-dominated) which control compression and delamination performance still remain major obstacles [START_REF] Hughes | The carbon fibre/epoxy interface-a review[END_REF]. It has been found that the factors most contributing to the weak out-of-plane properties are the brittle nature of matrix resins and weak interfacial adhesion of matrix and fiber, which relates to the wettability of fibers. Considerable effort towards solving the problems has been made, by introducing the through-the-thickness reinforcement (3D textiles) to change the architecture of the reinforcement [START_REF] Mouritz | Review of applications for advanced three-dimensional fibre textile composites[END_REF]. However, these modifications of the micro-structure of the reinforcement do not change the intrinsic interface and matrix properties. This can be done through surface modification of the fibers (for interface) and toughening of the matrix (for the both).

As discussed in the previous part, it was demonstrated that the addition of CNTs in the matrix can increase the toughness of the matrix and improve the interface properties of CNT-composites. Therefore, the poor properties of the traditional FRPs may be improved at nanoscale by introduction of multiscale reinforcements. That is, the nanotubes are dispersed uniformly throughout the polymer matrix, as shown in Fig. 1-18, or integrated by selectively depositing directly on the fiber surfaces (Fig. 1-22) [START_REF] Godara | Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes[END_REF]. The multiscale hybrid composites consist of nanoscale CNTs as well as continuous microscale (unidirectional or woven) fiber and the polymer matrix (macro) have drawn significant interest in the field of advanced, high-performance materials.

The small size of CNTs enables them to penetrate into the narrow matrix-rich regions between individual fibers in a bundle as well as between layers of laminated composites. It is important to understand the synergistic interactions of the multiscale nanotube/fiber hybrid system. At the single fiber level, the interaction of the nanoscale reinforcement is on the level of the fiber/matrix interface where selective local stiffening or strengthening of the polymer matrix surrounding the fiber could occur. At the bundle level, nanotubes could interact to share load in the transverse direction and also create electrically or thermally conductive pathways between or around the fiber reinforcements. At the microstructural level of the composite laminate, the nanotubes could provide reinforcement to improve interlaminar or through-thickness reinforcement and alter other matrix-dominated structural or functional properties. (b) grown on the surface of fibers. [START_REF] Vlasveld | Continuous fibre composites with a nanocomposite matrix: Improvement of flexural and compressive strength at elevated temperatures[END_REF] The nanoscale CNT holds the potential to tailor structural and functional properties and change the micro-scale damage mechanisms of through the fiber reinforced composites. The challenges associated with understanding the structure/property relations still remain in the CNT-based multiscale hybrid composites. In this part, we briefly review the critical issues and results associated with the development of these systems and highlight a multifunctional application of utilizing CNTs to in situ detect damage in composite materials.

Dispersed CNTs in the matrix

Dispersing CNTs into the polymer matrix for producing multiscale hybrid composites can be realized using conventional composite manufacturing process, such as prepregging, filament winding, pultrusion, or resin transfer molding (RTM). But the dispersion of CNTs in the polymer matrix has some processing challenges. Inadequate dispersion of CNTs in the matrix can result in agglomerations. Due to the very small inter-fiber spacing in the fiber reinforced composites, the CNT agglomerates are usually filtered-out by the fiber bundles, leading to the non-uniform infiltration of the fiber bundles. In addition, the dispersion of nanotubes also results in increased viscosity of the polymer suspension, creating difficulties for the infusion and wetting of the fibers. Thus, the addition of CNTs into the matrix is typically limited to relatively low concentrations and the improvement of CNT dispersion is extremely needed for the FRPs with CNT modified matrix.

Some promising enhancements of the matrix-dominated mechanical properties in the fiber composites have been observed by modifying the matrix with CNTs. Bekyarova et al. [START_REF] Bekyarova | Functionalized single-walled carbon nanotubes for carbon fiber-epoxy composites[END_REF] reported that the carboxylic acid functionalized SWNTs were dispersed in epoxy matrix and then infused into unidirectional carbon fibers using RTM, resulting in a 40% enhancement of the interlaminar shear strength at just 0.5 wt.% of CNTs. Yokozeki et al. [START_REF] Yokozeki | Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy[END_REF][START_REF] Yokozeki | Fracture toughness improvement of CFRP laminates by dispersion of cup-stacked carbon nanotubes[END_REF] reported that the interlaminar fracture toughness of carbon fiber reinforced composite laminates with cup stacked carbon nanotubes (CSCNTs) in epoxy matrix was improved up to 100%. Wichmann et al. [START_REF] Wichmann | Glass-fibre-reinforced composites with enhanced mechanical and electrical properties-benefits and limitations of a nanoparticle modified matrix[END_REF] found a 16.8% increase in the inter-laminar shear strength (ILSS) of the glass fiber/epoxy composites with the addition of 0.3 wt.% DWNTs in the epoxy matrix. However, there was no significant improvement in interlaminar fracture toughness of the multiscale hybrid composites.

Besides their structural properties, if the nanotubes are uniformly dispersed in the matrix and infused through the fiber bundles, the electrically percolating networks can be formed throughout the composites [START_REF] Wichmann | Glass-fibre-reinforced composites with enhanced mechanical and electrical properties-benefits and limitations of a nanoparticle modified matrix[END_REF][START_REF] Thostenson | Electrical anisotropy in multiscale nanotube/fiber hybrid composites[END_REF]. In this case, it can result in the improvement of electrical conductivity especially in the FRPs with insulating fibers, such as glass or advanced polymer fibers. Thostenson et al. [START_REF] Thostenson | Electrical anisotropy in multiscale nanotube/fiber hybrid composites[END_REF] fabricated the CNT/glass fiber/vinyl ester composites and measured the influence of both processing conditions and fiber orientation on the electrical properties of the composites at different CNT contents. Obvious anisotropy of the electrical properties in the longitudinal and transverse directions was observed, as shown in Fig. 1-23. There is approximately a one order of magnitude difference between the longitudinal and transverse directions. The theoretical modeling showed that the percolating pathways are closely related to the microstructure of the fiber composites which influences spanning cluster formation. It is probable that the spanning clusters will form in the regions between the fibers along longitudinal direction instead of around the fibers in the transverse direction. In addition, the clusters follow a longer mean path to go around the fibers in the transverse direction, resulting in higher overall resistances of the spanning clusters.

CNTs grown on surface of fibers

Unlike dispersing CNTs into the matrix, depositing nanotubes on the fiber surface could offer the possibility to achieve higher CNT concentrations but requires the development of new processing methods. The ways of fabricating the CNT-grafted fiber multiscale hybrids mainly include chemical reactions between modified fibers and CNTs [START_REF] He | Preparation of a carbon nanotube/carbon fiber multi-scale reinforcement by grafting multi-walled carbon nanotubes onto the fibers[END_REF][START_REF] Laachachi | A chemical method to graft carbon nanotubes onto a carbon fiber[END_REF], integration CNTs into fiber sizing [START_REF] Gao | Highly conductive polymer composites based on controlled agglomeration of carbon nanotubes[END_REF][START_REF] Warrier | The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix[END_REF][START_REF] Gao | A comparative study of damage sensing in fiber composites using uniformly and non-uniformly dispersed carbon nanotubes[END_REF], electrophoretic deposition [START_REF] Schaefer | Effects of electrophoretically deposited carbon nanofibers on the interface of single carbon fibers embedded in epoxy matrix[END_REF][START_REF] Bekyarova | Multiscale carbon nanotube-carbon fiber reinforcement for advanced epoxy composites[END_REF] and direct growth of CNTs onto fibers via catalytic chemical vapor deposition [START_REF] Qian | Carbon nanotube grafted carbon fibres: A study of wetting and fibre fragmentation[END_REF][START_REF] Zhang | Hierarchical composites of carbon nanotubes on carbon fiber: Influence of growth condition on fiber tensile properties[END_REF], as discussed above. Each of these techniques has its own potential benefits and drawbacks [START_REF] Qian | Carbon nanotube-based hierarchical composites: a review[END_REF]. Amongst them, CCVD appears to be a most frequently used way for its controllable diameter size [4] and orientation [START_REF] Sager | Effect of carbon nanotubes on the interfacial shear strength of T650 carbon fiber in an epoxy matrix[END_REF][START_REF] Lee | Control of growth orientation for carbon nanotubes[END_REF].

Gao et al. [START_REF] Gao | Highly conductive polymer composites based on controlled agglomeration of carbon nanotubes[END_REF] have utilized a fiber sizing approach for depositing uniformly dispersed CNTs onto glass fiber surfaces and investigated the electrical properties of the composites compared to composites with CNTs well dispersed throughout the matrix. The schematics in Fig. 1-24 show the CNT distribution in the two kinds of as-processed multiscale composites.

Using, CNTs are locally segregated near the fiber surface. Unlike the traditional calendering approach, with the fiber sizing method, the distribution of CNTs in the composite can be controlled and substantial amount of CNTs agglomerates on the glass fiber surface, which results in a 2-3 orders improvement in the composite electrical conductivity in the axial, transverse and through-thickness directions (Fig. . An et al. [START_REF] An | Preparation of vertically aligned carbon nanotube arrays grown onto carbon fiber fabric and evaluating its wettability on effect of composite[END_REF] synthesized vertically aligned CNT arrays onto the carbon fiber fabric using CCVD method. After CNTs growth, single fiber tensile tests indicated slight tensile strength degradation within 10% for all different lengths of fibers, while the fiber modulus has not been significantly damaged. Compared with the as-received carbon fibers, a nearly Veedu et al. [START_REF] Veedu | Multifunctional composites using reinforced laminae with carbon-nanotube forests[END_REF] demonstrated the growth of aligned CNT forests perpendicular to the surface of 2D woven SiC fabric cloth consisting of micron size SiC fibers using a CVD process shown in Fig. 1-26. The fabrics were then infiltrated with epoxy resin and stacked to form 3D composites. Significant improvements in interlaminar fracture toughness were achieved due to the presence of the CNT forests.

Garcia et al. [START_REF] Garcia | Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ[END_REF] reported that direct growth of aligned CNTs on the surface of advanced fibers in a woven fabric resulted in enhancement of multifunctional performance, 69% increase in the interlaminar shear strength and 10 6 (in-plane) and 10 8 (through-thickness)

increment in laminate-level electrical conductivity. A capillarity-driven mechanism is used to explain the observed effective and uniform wetting of the aligned CNTs in the interior of the laminate by unmodified thermoset polymer resins, as shown in Fig. 1-27. In summary, with regard to using CNT-grafted fiber as multiscale reinforcement for preparing the FRPs, there still exists some disagreement. Though these positive results have been reported, some researchers deduce that a poor interfacial adhesion probably appears due to the hydrophobicity of CNT grafting. Therefore, more deeply research in this field is needed in future.

CNT-based damage sensing

As discussed in the last part, the advanced fiber-reinforced polymer composites are now widely used in aircraft and other industries due to their high specific strength and modulus.

However, they are especially sensitive to the intrinsic damages with various scales including fiber/matrix interfacial debonding, transverse microcracking in the matrix, delamination at ply interfaces, and fiber fracture. Thus, during their service life, many structural components require inspection and evaluation to detect these damages that may lead to failure, and the development of techniques for damage detection is crucial to expanding their applications.

In the composite materials, the damage usually initiates in the polymer matrix in the form of microcracks or delaminations. Though the in-plane properties of composites are dominated principally by breakage of load-carrying fibers, the initiation of damage in the matrix leads to premature fracture and reduced durability. These matrix dominated damage mechanisms often occur at low strain or because of out-of-plane loading. Because of the micro-scale nature of damage in the composites, many non-destructive techniques are used to detect the initiation and accumulation of microcracks. Improved understanding of the damage mechanisms in composite materials is essential for enhanced methodologies for life prediction and also the development of in-service damage monitoring techniques.

For the carbon fiber reinforced composites, utilizing electrical resistance measurements have been established as one of the noninvasive ways for damage monitoring under static and dynamic loading conditions [START_REF] Schulte | Load and failure analyses of CFRP laminates by means of electrical resistivity measurements[END_REF][START_REF] Weber | Monitoring bending fatigue in carbon-fibre/epoxy composite strands: a comparison between mechanical and resistance techniques[END_REF][START_REF] Kupke | Non-destructive testing of FRP by dc and ac electrical methods[END_REF][START_REF] Schueler | Damage detection in CFRP by electrical conductivity mapping[END_REF]. Due to their inherent electrical conductivity, the breakage of load-carrying carbon fibers will result in the changes of the composite electrical resistance. But the electrical response of carbon fiber reinforced composites is dominated by fiber breakage and is less sensitive to the onset of matrix-dominated damage, where damage is first initiated. Besides, it is not applicable to the composites reinforced with fibers are non-conducting, such as glass or aramid fibers. Some researchers have attempted to locally modify the electrical properties of glass fiber yarn bundles via coating with carbon powder and utilizing them for damage sensing [START_REF] Muto | Hybrid composites with self-diagnosing function for preventing fatal fracture[END_REF]. Fiedler et al. [START_REF] Fiedler | Can carbon nanotubes be used to sense damage in composites?[END_REF] first introduced the concept of conductive modification with nanotubes as having potential for damage sensing.

As compared to traditional microscale fibrous reinforcements, carbon nanotubes have dimensions that are three orders of magnitude smaller. Due to their extremely small size, it is possible for nanotubes to penetrate the matrix-rich regions around the fibers and between the plies of the composite. Due to their exceptionally high current carrying capacity, CNTs are able to form electrically conductive networks in polymer matrix at very low volume contents.

The formed conductive networks in the matrix surrounding the fibers enables CNTs to be a distributed sensing network that can be utilized for sensing of deformation and damage in situ, as shown in Fig. 1-28 [START_REF] Thostenson | Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self healing[END_REF][START_REF] Li | Sensors and actuators based on carbon nanotubes and their composites: a review[END_REF][START_REF] Thostenson | Real-time in situ sensing of damage evolution in advanced fiber composites using carbon nanotube networks[END_REF]. When the cracks propagate in the composite, the conducting pathways are broken in the percolating network. The capability to sense damage is unique to nanostructured materials since a nano-scale conductor is necessary to sense the formation of a micro-sized crack. defined as a measure of damage. [START_REF] Gao | Sensing of Damage Mechanisms in Fiber-Reinforced Composites under Cyclic Loading using Carbon Nanotubes[END_REF] With increasing tensile load and number of cycles, different resistance changes are detected and the failure occurs as higher resistance change was detected. During the cyclic loading, the resistance change show more sensitivity to identify the crack location. Gao et al. [START_REF] Gao | Sensing of Damage Mechanisms in Fiber-Reinforced Composites under Cyclic Loading using Carbon Nanotubes[END_REF] investigated the damage mechanism under cyclic loading in cross-ply fiber reinforced composite laminates via the CNT sensors in the matrix. Fig. 1-30 shows a typical cycle of an incremental tensile test. At the beginning of a loading cycle, the re-opening of previously closed cracks accounts for the initial increase in resistance. The following linear portion of the curve is due to elastic deformation of the material. Then there are large resistance increases due to the formation of new cracks. Kim et al. [START_REF] Kim | Damage characterization of 3D braided composites using carbon nanotube-based in situ sensing[END_REF] analyzed the damage mechanisms of 3D braided composites were using a low fraction of CNTs dispersed in the resin matrix. Piezoresistive behavior was observed during the tensile testing and the electrical resistance varied with a discontinuous slope when the CNT conductive networks were broken-up due to the damage formation. Four characteristic strains were identified from the resistance change. Using X-ray CT, these characteristic stains were confirmed as critical strain levels, between which different damage mechanisms were involved.

Besides, the CNTs incorporated into the FRPs through surface treat or coat of the fibers, instead of dispersing them in the matrix, also could be used in situ sensors. Gao et al. [START_REF] Gao S L, Zhuang R C | Glass fibers with carbon nanotube networks as multifunctional sensors[END_REF] reported semiconductive MWNT-glass fibers with MWNTs deposited on the fiber surface.

The unidirectional composites containing MWNT-glass fibers exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold. And the fiber/polymer interphases was used as in-situ multifunctional sensors. Sureeyatanapas et al. [START_REF] Sureeyatanapas | SWNT composite coatings as a strain sensor on glass fibres in model epoxy composites[END_REF] prepared the glass fiber reinforced composites with SWNTs incorporated as a strain sensor on the fiber surface. Alexopoulos et al. [START_REF] Alexopoulos | Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers[END_REF] embedded CNT fibers to the glass fiber reinforced composites for structural monitoring of the materials. The CNT fibers were used as damage sensor in both tensile and compression loadings. Under high stress (or strain) level loadings, residual resistance measurements were observed after unloading. Accumulating damage in the composites was calculated and was correlated to the resistance change.

In summary, a number of studies have focused on tailoring mechanical and electrical properties of the CNT-based nanocomposites by controlling CNT dispersion, orientation, and CNT-matrix interface at the nanoscale. The insights gained from multifunctional properties of the CNT nanocomposites have opened up the potential to tailor structural and functional properties of the advanced fiber reinforced polymer composites using the nanoscale CNTs.

Introduction of small amount of CNTs can achieve electrically conductive networks in composites, which could serve as in situ damage sensor for structural monitoring. The CNTs can be well dispersed in the polymer matrix or grown directly on the fiber surfaces. However, with regard to the former, the uniform dispersion of CNTs in the matrix is extremely needed.

And for the later, the processing laminations and difficulties for the applications in industry scale as well as some controversy are the main issues.

Concluding remarks

The nanoscale structures with high aspect ratio as well as exceptional mechanical and multifunctional properties of CNTs have attracted extensive attention since their discovery.

CNTs have tremendous potential for applications in many scientific and technological fields.

This chapter provides a detailed overview on the structure and properties of CNTs and their utilization as multifunctional fillers in the polymer matrix to achieve improved mechanical, electrical and desirable piezoresistive properties of the resulting composites.

CNT aspect ratio is an important factor that influences the composite properties. An optimal aspect ratio is needed to provide an efficient load transfer from the nanotubes to matrix, and it is also helpful for the formation of conductive networks for improved mechanical and electrical properties. CNT orientation is another key issue for design the CNT/polymer composites. The desirable CNT orientation gives rise to CNT/polymer composites with higher electrical conductivity and mechanical reinforcement. The control of CNT alignment towards the pre-determined direction is important to take advantage of their anisotropic properties. Besides, there are two major interrelated issues that should be addressed, namely, dispersion of CNTs in the matrix and CNT/polymer interfacial adhesion.

Both the mechanical technique and surface treatment of CNTs are employed for improve their dispersion. The chemical and physical functionalizations of CNTs not only facilitate dispersion of CNTs, but also improve the interfacial interactions between the CNTs and polymer matrix. Various processing methods can be adopted to prepare the CNT/polymer nanocomposites. Based on the processing-structure-property relations of the composites, the issues can be realized by selection of the CNT synthesis and CNT/polymer preparation methods or controlling the conditions during process. Selection of a proper method or a combination of several methods has to be based on the desired properties of the composites.

The multifunctional CNT/polymer composites with the outstanding mechanical, electrical and piezoresistive properties have capability to serve as structural materials, and a platform for sensing, actuation, energy storage and so on. Besides, the fiber-reinforced composites modified with CNTs, known as multiscale composites as they are reinforced with microscale fibers and nanoscale nanotubes have drawn significant attention in the field of advanced high-performance materials. The multiscale reinforcements, containing fibers together with CNTs in the matrix or on the surface of the fibers can increase holds the potential to tailor structural and functional properties of the fiber reinforced composites. The electrically conductive networks of CNTs could serve as in situ sensor to detect the damage evolution in the composites.

More recently, it is worth noting that large scale controlled growth of aligned CNTs on microsubstrates could solve the problems above. The CCVD process has been successfully used to synthesize various nano/micrometer hybrid structures with CNTs selectively grown on fibers or micrometer particles including metal or oxide nanoparticles, microsized ceramic particles, fullerenes and two dimensional graphene. This hybrid structures show superior multifunctional properties in the composites, mainly due to the uniformly dispersed CNTs in the matrix as well as reinforced interfacial connection and charge transfer between CNTs and the matrix in composites. As a result, the nano/micro multiscale hybrids probably endow CNTs as multifunctional reinforcements for many potential applications in a wide range of fields including conductive and high-strength composites, strain sensors and actuators, etc.

In this thesis, the CNT-microparticle multiscale hybrids with various architectures will be used as novel multifunctional fillers in the composites, taking place of the conventional randomly orientated CNTs. Their mechanical, electrical, piezoresistive and damage sensing behavior in the nanocomposites or fiber-reinforced composites will be focused on.

Chapter ІІ CNT-Microparticle Hybrids Reinforced Epoxy Nanocomposites

Introduction

Carbon nanotubes with exceptional mechanical and electrical properties [START_REF] Salvetat | Mechanical properties of carbon nanotubes[END_REF][START_REF] Ebbesen | Electrical conductivity of individual carbon nanotubes[END_REF] have emerged as superior multifunctional nanofillers for the polymer composites. A significant improvement of composite properties would be achieved by incorporating a small amount of CNTs [START_REF] Thostenson E T, Ren | Advances in the science and technology of carbon nanotubes and their composites: a review[END_REF][START_REF] Thostenson | Nanocomposites in context[END_REF]. Such CNT-based polymer composites are ideal candidates for numerous engineering applications including automotive, aerospace and electronics. It is well known that the efficient stress transfer from CNTs to the matrix is prerequisite to attain required mechanical properties of the composites, which is greatly dependent on the CNT dispersion [START_REF] Ma | Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review[END_REF], orientation [START_REF] Wang | The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites[END_REF] and aspect ratio [START_REF] Wu | Relations between the aspect ratio of carbon nanotubes and the formation of percolation networks in biodegradable polylactide/carbon nanotube composites[END_REF], and the CNT/matrix interfacial properties [START_REF] Coleman | Small but strong: a review of the mechanical properties of carbon nanotube-polymer composites[END_REF].

It has been generally accepted that the uniform dispersion and larger aspect ratios of CNTs enabled enhanced mechanical properties in the composites. Graphene platelets (GNPs) with two-dimensional structure and high aspect ratio which are composed of several layers of graphite nanocrystals stacked together become ideal reinforcing and conducting fillers [START_REF] Viculis | Intercalation and exfoliation routes to graphite nanoplatelets[END_REF].

According to the up-to-date research, GNPs could offer more advantages towards highly improving mechanical properties of the composites [START_REF] Zhao | Enhanced mechanical properties of graphene-based poly (vinyl alcohol) composites[END_REF] because their planar structure and ultrahigh aspect ratio are expected to endow better stress transfer from GNPs to the matrix during loading. However, in many cases, the potential applications of GNPs are limited because and they tend to restack due to their large Van der Waals and strong π-π interactions [START_REF] Yang | Synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites[END_REF].

Besides, with regard to the structural composites based on CNTs, their reliability and long-term safety is a critical issue. For electrically conductive CNT/polymer composites, it is worth noting that their electrical resistance change makes them capable to in-situ monitor deformation or damage evolution under loading [START_REF] Li | Sensors and actuators based on carbon nanotubes and their composites: a review[END_REF]. For the composites with self-sensing capabilities, the formation of continuous conductive networks is needed. In this case, the percolation threshold is also related to the CNT dispersion, aspect ratio and orientation [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites[END_REF].

However, most of the available research on the self-sensing composites are mainly limited to randomly distributed CNTs as in-situ sensors [START_REF] Hu | Investigation on sensitivity of a polymer/carbon nanotube composite strain sensor[END_REF][START_REF] Zhang | Universal resistivity-strain dependence of carbon nanotube/polymer composites[END_REF]. In general, their in-situ electrical resistance monotonically increases under tensile loading till the catastrophic failure. More recently, the effect of CNT alignment on the self-sensing properties of the composites has also been addressed [START_REF] Nanni | Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET-MWCNT nanocomposites[END_REF]. The anisotropy of the aligned networks results in improved sensing capabilities. It could be concluded that the in-situ sensing behaviors are structure-dependent, which can be controlled by tailoring morphology of CNT conductive networks. As a result, the mechanical and self-sensing performances of the polymer composites depend on the intrinsic characteristics of carbon fillers including their morphology (dimension, aspect ratio, alignment, etc) as well as their dispersion state within the polymeric matrix.

Nowadays, the development of multifunctional CNT/polymer composites with excellent structural and self-sensing properties has been focused on. And the challenging tasks consist in attaining uniform CNT dispersion, optimal CNT aspect ratio and arrangement as well as a strong interface interaction between the CNTs and polymer matrix. Regarding the key issues mentioned above, the design of novel architecture based on the traditional fillers, their modification and the exploitation of other multifunctional fillers may be necessary to achieve outstanding mechanical and desirable self-sensing performance in the polymer composites.

The CNT-microparticle multiscale hybrids have been reported to possess certain advantages in achieving more controllable CNT dispersion and improved interfacial properties in the polymer matrix, compared to randomly distributed CNTs [START_REF] Zhang | Dramatically Enhanced Mechanical Performance of Nylon-6 Magnetic Composites with Nanostructured Hybrid One-Dimensional Carbon Nanotube-Two-Dimensional Clay Nanoplatelet Heterostructures[END_REF]. These hybrid architectures were obtained by self-organizing CNTs grown on micosubstrate surface, rather than simply mechanically mixed them. And the CNT aspect ratio can be controlled by varying the growth parameters [START_REF] He | Diameter-and length-dependent self-organizations of multi-walled carbon nanotubes on spherical alumina microparticles[END_REF]. These multi-scale hybrids with CNTs grown on SiC microplates or graphene platelets showed enhanced mechanical [START_REF] Ci | Novel micro/nanoscale hybrid reinforcement: multiwalled carbon nanotubes on SiC particles[END_REF], electrical properties [START_REF] Yuan | High dielectric permittivity and low percolation threshold in polymer composites based on SiC-carbon nanotubes micro/nano hybrid[END_REF] and novel self-sensing behaviors at low CNT concentrations. However, the related research on these multifunctional hybrids is still at an early stage and their underlying reinforcement mechanisms in the composites are not clear to date. Understanding the effect of the hybrid structure on the mechanical and self-sensing properties of the composites is extremely desired in order to fully take advantage of these nano/micro hybrids.

In this chapter, three kinds of hybrids with CNTs grown on SiC microplates (CNT-SiC), Al 2 O 3 microparticles (CNT-Al 2 O 3 ) and graphene nanoplatelets (CNT-GNP) were synthesized by chemical vapor deposition (CVD) process. The CNT aspect ratios and organizations of the hybrids can be adjusted by optimizing the CVD conditions. Then epoxy nanocomposites with the various hybrids were prepared by calendaring method. Mechanical, electrical and self-sensing properties of the nanocomposites were investigated in details. The underlying mechanisms associated to the composite properties with relation to the hybrid structure (CNT aspect ratio, orientation and substrate morphology) were analyzed.

Experimental

Raw materials for the hybrid and composite preparation

Micro-spherical alumina particles (μAl 2 O 3 , with 3~10 μm in size, 99.8% purity) (Fig.

2-1a

) were purchased from the Performance Ceramic Company (Peninsula, OH, USA). S iC microplates with the size of ~2 µm (Fig. 2-1b) were provided by Marion Technologies, France. Graphene nanoplatelets with high aspect ratio, i.e., 3~4 μm in length and ~30 nm in thickness (Fig. 2-1c) were obtained from Xiamen Knano Graphene Technology Co., Ltd.

GNPs are platelets like graphite nanocrystals with stacks of multi-layered graphene sheets and there are some wrinkles on their surfaces. This microstructure reveals that GNPs may be difficult to be uniformly dispersed in the matrix. The Al 2 O 3 , SiC and GNP were respectively employed as the substrates for the CNT growth. Commercial CNTs (randomly distributed), with the diameter of 30~50 nm and length about 20 μm as shown in Fig. 2-1d, supplied by Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences were used as-received.

The long and tortuous CNTs entangle with each other, which reveals that this kind of CNTs are difficult to be uniformly dispersed in the polymer matrix. Epoxy resin (1080S, Resoltech Ltd, France) and the curing agent (1084, Resoltech Ltd) was used as the matrix materials.

Preparation of the hybrids and composites

Hybrid preparation

Multi-walled nanotubes were in situ synthesized on three different substrates (Al 2 O 3 microspheres, SiC microplates and GNPs), respectively, by using CVD procedure without any pre-treatment. The acetylene (C 2 H 2 ) was selected as the carbon source and ferrocene Fe (C 5 H 5 ) 2 as catalyst precursor [START_REF] Ci | Novel micro/nanoscale hybrid reinforcement: multiwalled carbon nanotubes on SiC particles[END_REF][START_REF] He | Diameter-and length-dependent self-organizations of multi-walled carbon nanotubes on spherical alumina microparticles[END_REF][START_REF] Dichiara | Chemical Vapor Deposition Synthesis of Carbon Nanotube-Graphene Nanosheet Hybrids and Their Application in Polymer Composites[END_REF]. In brief, ferrocene (concentrated at 0.05 g ml -1 )

was dissolved in xylene solution to serve as catalyst precursor. Then, the mixture was fed by a syringe system and carried into the preheated stable reaction zone in the form of spray by carrier gas argon mixed with 10 ~ 40 % hydrogen. The total gas flow rate was fixed at 1 L min -1 . Acetylene (10 ml min -1 ) was injected as another carbon source. The CNT synthesis temperatures used were between 550-600 °C. The CNT growth time varied from 2 to 30 min in order to obtain CNTs of different lengths. After, the reactor was cooled down to room temperature under argon atmosphere (1 L min -1 ). The CNT morphology of the hybrids is strongly dependent upon the nature of substrates. That is, the well-aligned CNTs generally form into symmetric six-orthogonal branches on the spherical Al 2 O 3 particles, whereas, the CNTs perpendicularly aligned on the flat surfaces of SiC microplates as well as on the GNPs.

The CNT length and density of the hybrids can be tuned by the CVD conditions, for instance, the growth time and temperature.

Composite fabrication

The randomly distributed CNTs (commercial) or as-synthesized hybrids (CNT-Al 2 O 3 , CNT-SiC, CNT-GNP) were firstly mixed manually with the epoxy resin as per calculated weight ratio. Then to further improve the dispersion of the reinforcement, the mixture was passed through a three-roll mill (EXAKT, Germany), following a well-established protocol.

In this mixing process, the 1st and 3rd rollers rotate in the same direction whereas the 2nd roller placed in between rotates in the opposite direction thereby inducing high shearing in the mixture. In order to induce high shear force in the mixture, a varying gap setting between

Characterization and instruments

Thermogravimetric analysis

The thermogravimetric analysis (TGA) of the as-prepared hybrid fillers were conducted on a thermal analyzer (NETZSCH STA 449F3) in air atmosphere from 30 o C to 800 o C.

Calculation of the filler and matrix volume fraction

The corresponding filler and matrix volume fraction can be calculated using the density of the constituents as follows:

(1 )

Particle M Hybrid CNT W W       2-1 CNT M Hybrid CNT WW     2-2 / / / / particle Particle Particle CNT CNT Particle Particle W V W W W        2-3 / / / / CNT CNT CNT CNT CNT Particle Particle M M W V W W W       2-4
where W M , W Hybrid , W Particle and W CNT are the mass of the matrix, hybrid, particle substrate and CNT of the hybrids; V M , V Hybrid , V Particle and V CNT are the volume fraction of the matrix, hybrid, particle substrate and CNTs; ρ M , ρ particle and ρ CNT is the density of the matrix, particle substrate and CNT, respectively. Ф Hybrid is the weight fraction of the hybrid in the matrix.

φ CNT is the weight fraction of the CNT in the hybrids.

Micrographic analysis

SEM observations of the reinforcements and fracture surfaces of the nanocomposites were performed on ZEISS, LEO 1530 Gemini. For morphological observation, the obtained composite samples were frozen in liquid nitrogen then quickly fractured.

Mechanical characterization

Quasi-static and incremental cyclic tensile tests for the dumbbell-shape samples of the randomly distributed CNTs and hybrids reinforced epoxy composites were carried out on a micro-tensile machine (Instron 5544 with a 2 kN load cell) at a fixed displacement rate of 0.2mm/min. Toward determining the statistics of the modulus, strength and fracture strain of the composites, between three to five samples for each kind of composites are tested under the same condition. For the cyclic tensile testing, the specimens were loaded and un-loaded at the same rate with progressively increasing peak values of cyclic load.

Thermo-mechanical characterization

Thermo-mechanical properties of the samples were determined by dynamic mechanical

Electrical measurement

The alternating current (AC) conductivity of the composites was measured using a precision impedance analyzer (Agilent 4294A) over the frequency range from 10 2 to 10 7 Hz at room temperature. Prior to measurements, the silver paste electrodes were applied to both sides of the samples in order to reduce the contact resistance.

In situ sensing behavior

To determine the self-sensing behaviors of the composites, in situ electrical resistance measurements were performed for monitoring the deformation evolution of the composites.

A Keithley 2400 voltage-current meter was used to measure the in-plane volume electrical resistance of the specimens by sourcing a constant voltage and measuring the current using a two point technique. Prior to the electrical measurements, the silver paste electrodes were also applied on both sides of the samples. The schematic in Fig. 2-3 illustrates the testing system for the in situ sensing. Due to the nature of the electrically percolative specimens the electrical contact resistance is very small relative to the overall specimen resistance. The electrical resistance data were in situ recorded using data acquisition software LabView.

Morphologies of the nano/micro CNT-microparticle hybrids

SEM images in Fig. 234show the morphologies of the nano/micro CNT-microparticle hybrids. These hybrids were prepared using CVD synthesis. It can be seen that the CNTs on each substrate have high number density, but organize into varied architectures. As shown in Fig. 2-4a and b, the CNTs were vertically aligned on the flat surfaces of SiC microplates and the GNPs with a thickness less than 100 nm (see the high magnification image in Fig. 2-4h).

The CNT lengths on the two substrates were about 10~15 μm. However, the well-aligned nanotubes generally form into symmetric six-orthogonal branches on the spherical Al

Microstructure of the epoxy nanocomposites

SEM images of the fracture surfaces of epoxy nanocomposites are presented in Fig. 2345.

As shown in Fig. 2 CNTs. Thus, the introduction of these hybrids is expected to achieve the desired self-sensing and mechanical properties of the composites. In this case, the conductive fillers are very close to each other. When f is above f c , the greatly improved conductivities are observed and their values are almost independent of frequency within the low frequency range. In this case, the corresponding composites exhibit a DC conductivity behavior, since their conductivity does not change with the frequency. Such DC characteristic in the low frequency range is induced by few percolating paths formed through the polymer matrix. With regard to these systems, a moderate increase in conductivity is observed when the volume fraction of conductive filler is below the percolation threshold, while it becomes significant after the volume fraction is above the percolation threshold. The variation of conductivity shows a good agreement with the typical power law:

Electrical properties of the epoxy nanocomposites

  c t c f f as f f     2-5
Where is the conductivity of the composites, f is the volume fraction of conductive fillers, f c is defined as percolation threshold and t is the critical exponent in the conducting region which is dependent on the dimensionality of the conductive networks. In our research concerning the nano/micro hybrids filled epoxy systems, f represents the volume fraction of the CNTs instead of the hybrids. The best fits of the experimental conductivity data to the log-log plots of the power law could give the value of the percolation threshold and critical exponent in every system. In order to achieve the value of f c in the composites based on the hybrids, the fraction of CNTs in the hybrids is needed.

According to the Eqs. 2-1 ~ 4, the concentrations for each component in the as-prepared epoxy composites reinforced with the nano/micro hybrids can be calculated and the results are shown in Table 2-3. We can see that the percolation thresholds in the CNT-microparticle hybrids filled composites are much lower than the composites with the randomly distributed CNTs, which are primarily contributed to the unique architecture of the hybrids with various morphologies of the CNTs grown on the microparticle substrates. These nano/micro hybrids could endow high degree of CNTs, which efficiently increases its utilization ratio in forming conducting network at lower loading. With regard to the hybrids with the different substrate microparticles (SiC, GNP and Al 2 O 3 ) and the same CNT aspect ratios, it is observed that the re is not much difference the of percolation thresholds between the hybrids with different substrates. However, in the case of the CNT-Al 2 O 3 hybrids with the different CNT aspect ratios, it is found that the percolation threshold is dependent on the aspect ratios. With the increase of the CNT aspect ratio from 500 to 2000, its value consistently decreases, however, it begins to increase when the aspect ratio is up to 3200. The effect of aspect ratio on the percolation behavior and conductivity of the composites is illustrated in Fig. 234567.

Tensile properties of the epoxy nanocomposites

Tensile properties of the hybrids reinforced epoxy nanocomposites

In order to demonstrate the superiority of the nano/micro hybrids fillers on improving the mechanical performance of the composites, the multi-scale hybrids and traditional fillers reinforced epoxy nanocomposites were respectively investigated. As previously reported, the mechanical properties of epoxy composites with 0.5 wt.% filler (SiC microparticles with or without CNTs grown) were investigated [START_REF] Ci | Novel micro/nanoscale hybrid reinforcement: multiwalled carbon nanotubes on SiC particles[END_REF]. Compared to neat epoxy, there was no obvious improvement in mechanical properties of the SiC/epoxy composites and their fracture strain was greatly reduced. However, the CNT-SiC hybrids provide an evident improvement on the mechanical properties. The elastic modulus and tensile strength of their composites were significantly enhanced and the fracture strain was also higher than the SiC/epoxy composites.

The CNT-SiC hybrids could improve the interfacial properties between the SiC particles and epoxy matrix and the stress is efficiently transferred from the reinforcements to the matrix through the excellent interface during tensile loading. Therefore, the CNT-SiC hybrids have been considered to be one of the ideal candidates on improving the mechanical properties of the composites.

Graphene nanoplatelets composed of multi-graphene layers have emerged as promising fillers in the polymer composites due to their extremely high aspect ratio, unique graphitized plane structure and low manufacturing cost. The larger surface area can increase the contact area with the polymer matrix, maximizing the stress transfer from the polymer matrix to the fillers. Therefore, they are expected to exhibit better reinforcement than CNTs. However, the larger surface area between the planar nanosheets results in large Vander Waals forces and strong π-π interactions. The aggregation and inter-planer stacking of the nanosheets will limit the composite performance. Thus, their dispersion in the polymer is still a challenging task.

The idea of using hybrid filler comprised of GNPs and CNTs has already been explored. It was demonstrated that the improved composite performance can be achieved by combining the advantages of the GNPs and CNTs [START_REF] Yang | Synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites[END_REF]. Unlike the simple mixture of the GNPs and CNTs, it is required to achieve the essential connection between them, and especially the optimal CNT distribution and orientation in the matrix. Hence, the CNT-GNP hybrids with CNTs vertically grown on the opposite surfaces of the GNPs are expected to be utilized as reinforcements for the composites. Toward determining the statistics of the tensile modulus, UTS and fracture strain of the composites, between three to five samples for each kind of composite are tested under the same condition. The results of the tensile properties of baseline epoxy and their composites are summarized in Fig. 2-9b ~ d. The tensile modulus of the CNT-GNP/epoxy composite is up to about 3GPa and shows ~40% increase compared to that of pristine epoxy. However, there are only moderate enhancements of the tensile modulus for the CNTs/epoxy (~8%), GNPs/epoxy (~5%) and CNT+GNP/epoxy (~14%) nanocomposites. The variation tendency of the tensile strength in these composites is similar to that of tensile modulus. The tensile strength of the CNT-GNP/epoxy nanocomposites also acquires significant enhancement (+36%) compared to other kinds of composites. The lower improvement of the tensile modulus and tensile strength in the epoxy composites reinforced with CNTs, GNPs or CNT+GNP may be attributed to the waviness of pristine CNTs and the stacking behavior of GNPs arising from their large aspect ratio and van der Waals force. It is generally accepted that CNT orientation is a key parameter for the mechanical properties of the composites. The rotation of graphene substrate and the orientation of aligned CNT bundles gradually appear with the increase of strain, which potentially leads to the rearrangement of CNT networks.

This rearrangement behavior possibly inhibits the rapid growth of the emerging microcracks and some mechanical energy could be dissipated during this rearrange ment process. Besides, it could weaken the stress concentrations to a certain extent. All of these factors may account for the increase of toughness and fracture strain in the CNT-GNP/epoxy composites.

SEM images of the fracture surfaces of epoxy composites are presented in Fig. 2 GNPs. That is because the van der Waals forces resulted from the plane-to-plane contact of the adjacent GNPs may be stronger than that of CNTs. Besides, the GNP agglomerates lead to the formation of defects or flaws in the matrix, possibly limiting their reinforcement effect on the composite mechanical properties. Compared to the GNPs/epoxy composite, it seems that better dispersion of GNPs is obtained in the CNT+GNP mixture reinforced composite (Fig. 2-10c). Therefore, the incorporation of CNTs has great influence on the dispersion of GNPs in the matrix, which has been explained by the synergetic effect of the CNTs and GNPs. In the case of the CNT-GNP hybrids reinforced composite, the original architecture of the hybrids is retained (Fig. 2-10d). With the help of GNPs, the highly dispersed CNTs as well as uniform CNT networks are achieved in the matrix. These factors could lead to larger contact area and better interaction between CNTs and the matrix. Besides, the CNT bundles grown on the GNP surface could inhibit the plane-to-plane aggregations of GNPs and the GNP/matrix interfacial properties are also improved. As a result, it is expected that the mechanical properties of the composites could be significantly enhanced by addition of the CNT-GNP hybrids. Moreover, the slippage of the overlapped GNPs is also considered to appear during tensile loading. It is well known that a high degree of load transfer from the reinforcement to matrix is an important issue for the improvement of elastic modulus and strength. However, the adverse factors of CNTs and GNPs may restrain the stress transfer, limiting the reinforcement efficiency. Aside from the tensile modulus and strength, the fracture strain of the baseline epoxy and their composites is shown in Fig. 2-9d. The fracture strain of the CNT-GNP/epoxy composite is almost the same as that of neat epoxy, which is different from the CNTs, GNPs or CNT+GNP mixture reinforced composites with much lower fracture strain. In the case of poor dispersion of the reinforcements, the formation of nano or micro flaws results in local stress concentration in the matrix. The brittleness and ultimate failure of the composites isprobably caused by these voids and defects resulted from the GNP and CNT agglomerations. On the basis of the experimental results and analysis mentioned above, it could be inferred that the CNT-GNP/epoxy composite demonstrates better stiffness as well as toughness than other kinds of composites. The reinforcement effect is generally a complex issue involving the load transfer, stress concentration and defect distribution. Some explanations are proposed to reveal the underlying reinforcing mechanism of the CNT-GNP hybrids with distinctive architecture. First, the perfectly dispersed CNTs grown on GNPs could not only form uniform CNT networks but also inhibit the stacking and aggregation of GNPs. This three-dimensional hybrid structure leads to the larger surface contact and strong molecular coupling between CNTs and the matrix. It is possible that the excellent interfacial interaction between the CNT networks and surrounding polymer matr ix plays an important role on the effective stress transfer. Second, the aligned CNTs can be viewed to retain in the epoxy matrix at their actual lengths instead of curly state for optimizing their reinforcement efficiency. Third, the uniform dispersion of the CNTs and GNPs is not conducive to the formation of flaws, which is very important for stress transfer from the reinforcements to the matrix. Fourth, the orientation of the CNT grown on GNPs are much more difficult than the randomly orientated CNTs under low strain because of their stable junctions with graphene substrate and excellent molecular couplings with the polymer chains, which could give a good explanation to the higher elastic modulus in the CNT-GNP/epoxy composites.

Our experimental results of the tensile modulus are compared with the calculated values based on empirical formulas. The well-established Halpin-Tsai equation can be utilized to predict the tensile modulus of the unidirectional or randomly distributed fibers reinforced composites [START_REF] Zhao | Enhanced mechanical properties of graphene-based poly (vinyl alcohol) composites[END_REF]. In our calculation, CNT was considered as discontinuous fiber and GNP was assumed as effective rectangular solid fiber. Considering both random orientation and unidirectional dispersion of the CNTs and GNPs in the composites, the Halpin-Tsai equation could be rewritten as follows:

    M GN T NT T GN T NT T GN L NT L GN L GN NT L NT NT rand E V V V V V V V w l V d l E                          1 2 2 1 8 5 1 3 / 2 / 2 1 8 3 2-6 M GN L NT L GN L GN NT L NT NT paral E V V V w l V d l E                 1 ) 3 / 2 ( ) / ( 2 1 2-7     NT NT M NT M NT L d l E E E E / 2 / 1 /     2-8   2 ) / ( 1 /    M NT M NT T E E E E  2-9   ) 3 / 2 ( / 1 / w l E E E E GN M GN M GN L     2-10   2 ) / ( 1 /    M GN M GN T E E E E  2-11
In this modified Halpin-Tsai model, E rand and E paral represents the tensile modulus of the composites with randomly oriented and unidirectionally distributed fillers, respectively. E NT , E GN and E M are the tensile modulus of CNT, GNP and epoxy matrix. l NT , d NT , l GN and w is the average CNT length and diameter, GNP length and thickness, respectively. The values of these related parameters are listed in Table 2-4. The tensile modulus of the composites based on the two presumed dispersion of CNTs and GNPs are calculated and compared with our experimental data in Table 2345.

Table 2-4

The values of related parameters of CNT, GNP and epoxy matrix CNT-GNP/epoxy 3.11

E 4 > E paral3 > E rand3
The experimental values of the tensile modulus of the CNTs/epoxy, GNPs/epoxy and CNT+GNP/epoxy composites are generally lower than their values calculated using the Halpin-Tsai equation. However, the experimental tensile modulus of the CNT-GNP/epoxy composite is found to outperform that of the CNT+GNP/epoxy composite predicted using the Halpin-Tsai modeling. Hence, high reinforcing effectiveness is achieved by introduction of CNT-GNP hybrid as reinforcement. In this case, the uniform dispersion of CNTs and GNPs mainly contributes to the higher tensile modulus. In addition, it can be considered that better synergetic effect between CNTs and GNPs is established on improving the mechanical properties of the composites.

However, it is difficult to calculate the tensile modulus of the CNT-GNP hybrids based composites using this equation because the modulus of the hybrids is unknown. Based on the dispersed morphologies of the hybrids in the matrix, the CNT-SiC, CNT-GNP hybrids and the CNT-Al 2 O 3 (AR3200) can be considered as bundle like structures. The CNT-Al 2 O 3 hybrids with AR 500, 1200, 2000 can be regarded as the spherical particles. To predict the reinforcing effect of these hybrids, the following Guth -Gold-Smallwood (GGS) equation was put forward which deals mainly for the static and dynamic moduli:

[200-202] 2 1 0.67 1.62( ) m E E f f       2-12
where, E is tensile modulus of the composites, E m is the matrix's tensile modulus, f is the volume fraction of filler and α is a shape factor that allows the application of the equation to non-spherical fillers, particularly when the fillers are either platelet like structure or rod like structure. This equation is more commonly applied to understand the degree of dispersion of various nano-structured anisotropic fillers. The tensile modulus of the hybrids reinforced composites are calculated and compared with our experimental data in Table 2-6. It can be seen that the experimental values of the tensile modulus are generally higher than the values calculated using the GGS equation.

Effect of the hybrid content on tensile properties of the nanocomposites

Based on the discussions above, it has been well established the CNT-microparticle nano/micro hybrids could enable their utilization as high-performance structural additives for improving the mechanical performance of the composites. In this part, the influence of the hybrid content on the mechanical properties of epoxy composites will be investigated. In order to determine the statistics of the tensile modulus, tensile strength and fracture strain of the composites, five specimens of every kind of samples are tested under the same condition. the stress and strain before reaching the maximum stress, but no obvious yield point could be found in the curves. In order to have a better view of the reinforcement effect of the hybrids, the experimental results of the tensile properties of baseline epoxy and their composites as function of the CNT-GNP contents are summarized in Fig. 2-12b ~ d. When f CNT-GNP reaches 0.5 wt.%, the strengthening effect has been notable. The tensile modulus of the composites is nearly up to 3GPa, which reveals the greatest improvement (~ 40%) compared to that of pure epoxy. With further increasing the CNT-GNP content, the tensile modulus unfortunately begins to degrade. The tensile strength of the composites shows the similar trend, acquiring the highest enhancement of ~36% at f CNT-GNP =0.5 wt.% (Fig. 2-12c).

Tensile properties of the CNT-GNP/epoxy nanocomposites

In addition to the tensile strength and modulus, we also compared the ductility (fracture strain) and toughness (energy absorbed at failure) of the baseline epoxy and their composites.

As demonstrated in Fig. 2-12d, at low hybrid concentration, there is no obvious decrease of the fracture strain in the composites. In general, for the epoxy matrix, the 10 ~ 15% loss in ductility could be tolerated provided that other mechanical properties such as modulus and strength are enhanced. When the hybrid content is up to 1 wt.% or more, the fracture strain is found to be distinctly decreases. In this case, the CNT-GNP/epoxy composites demonstrate lower ductility than the baseline epoxy. Because of the non-linear stress-strain behavior, it is difficult to calculate the absorption energy from the typical tensile curves (Fig. 2-12a) during the fracture process. The total area under the stress-strain curves can be used to characterize the absorption energy before the emergence of facture, and then we can judge the composite toughness. From Fig. 2-12a, it can be seen that, as the CNT-GNP content increases from 0.2 wt.% to 0.5 wt.%, the areas under the stress-strain curves clearly increase, implying that the composite toughness is moderately improved owing to the introduction of CNT-GNP hybrids.

However, when f CNT-GNP is up to 1 wt.%, their toughness is obviously reduced. The reduced ductility may be caused by the stress concentration resulted from the hybrid fillers. This phenomenon typically occurs when hard fillers are incorporated into a brittle matrix. Besides, the higher hybrid content possibly leads to defects in the matrix, which serves as seed points for crack initiation and premature fracture. Similarly, the average tensile strength increases from ~ 45 MPa of the pure epoxy matrix to ~57 MPa, a rise scope of ~ 27%. Then further increase of the hybrid concentrations would cause the decrease in the tensile strength and modulus of the composites. At much higher hybrid content, the tensile properties of the CNT-SiC/epoxy composites are even lower than that of pure epoxy. This variation trend is almost the same as that of the CNT-GNP/epoxy system. Moreover, it is found that the fracture strain in this system is much lower than the pure epoxy with the addition of the CNT-SiC hybrids (Fig. 2-13d). And evident loss of the ductility and toughness is observed with the introduction of CNT-SiC hybrids. Chapter ІІ

Tensile properties of the CNT-SiC/epoxy nanocomposites

The CNT-Microparticle Hybrids Reinforced Epoxy Nanocomposites -85 - that the addition of CNTs with better CNT/polymer interfacial adhesion could increase the fracture energy because the crack propagation could be restrained by bringing up the crack faces of CNTs. Zhou et al. [START_REF] Zhao | Enhanced mechanical properties of graphene-based poly (vinyl alcohol) composites[END_REF] reported that the crack propagation changes direction as it crosses CNTs during failure process due to bridge effect which prevents crack opening. As a result, the crack initiation and propagation becomes more difficult in the system reinforced by CNTs than that without CNTs. It may be one of the reasons due to which the flexural strain to failure in the composites was increased. CNTs have a high aspect ratio and shows highly flexible elastic behavior during loading. Due to the strong interfacial bonding between this reinforcement and epoxy matrix, the composites exhibit higher absorption of energy. The increase in strain energy thus can be attributed to increase in strain to failure.

Tensile properties of the CNT-Al 2 O 3 /epoxy nanocomposites

Besides, it is worth noting that the CNT aspect ratio of the hybrids takes an important role on the mechanical properties of the composites. With increasing the aspect ratio, the required hybrid content is lowered to achieve better reinforcement effect. The general variation trend of mechanical properties of the composites with the hybrids (CNT-GNP, CNT-SiC and CNT-Al 2 O 3 ) can be explained as follows. The dispersion of CNTs and hybrids is more homogeneous at small concentrations. The uniformly dispersed fillers could restrict the mobility of polymer chains under tensile loading, resulting in improved tensile modulus and strength. However, concerning the decrease of mechanical properties at higher hybrid content, it is possibly contributed to the voids or other defects in the matrix.

Besides, in this case, the modification of the crystalline section of the matrix considerably decreases the plastic range, which causes a decrease in ductility and an increase in brittleness.

Moreover incorporation of higher loading of hybrids increased the viscosity of matrix which may have impeded the dispersion. Song et al. [START_REF] Song | Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites[END_REF] stated that poorly dispersed CNTs within epoxy have a higher viscosity than that of uniformly dispersed suspensions. In addition, high viscosity of the resin resulting from poor CNT dispersion may cause poor wetting and adhesion between CNTs and the matrix. Thus, it could be concluded that the tensile properties of the CNT-Al 2 O 3 /epoxy nanocomposites show the sensitivity on the dispersion state of the hybrids.

In summary, the CNT-microparticle multi-scale hybrids could be used reinforcements in the composites. It is most often the case that mechanical properties of the hybrids reinforced composites degrade or become saturated after a peak when the hybrid content exceeds a certain value. As a result, the optimal concentration of the hybrid is necessary to achieve the desirable reinforcement effect. The varying threshold hybrid contents also depend on their type. Besides, for the same hybrid or CNT concentration, the mechanical properties of the composites are greatly dependent on the microsubstrates and CNT aspect ratio of the hybrids.

This observation has a practical implication in that one can tailor the composite properties with an optimal hybrid content and desirable CNT aspect ratio and substrates depending on their applications with different property requirements.

Influence of the substrate and CNT aspect ratio of the hybrids on tensile properties of the composites

Concerning the CNT-GNP and CNT-SiC hybrids with the same CNT aspect ratio (1200), it is clearly seen that the substrates of the hybrids have some influence on the mechanical properties of the composites, as shown in Figs. 2-12 and 2-13. From the results indicated in Fig. 2-14, whatever the CNT aspect ratio, the most satisfactory reinforcement effect of the CNT-Al 2 O 3 hybrids can be achieved at their optimal loading in every system. However, the hybrid content in each system has obvious difference. In this section, the influence of the substrate and CNT aspect ratio of the hybrids on mechanical performance of the composites will be discussed. Here the optimal concentration of hybrids where the preferable composite properties are acquired is listed in Table 234567.

Table 2-7

The The reinforcing mechanism of these hybrids could be explained as follows. First, the distribution states of the hybrid fillers in the matrix could restraint the CNT agglomeration.

Second, their three-dimensional architectures give rise to larger contact surface and stronger molecular interaction between the CNTs and the matrix, resulting in an improved load transfer. Third, the alignment of CNTs in the hybrids retains their actual lengths in the matrix, enhancing their reinforcement efficiency. Fourth, the strong junction between the CNTs and the substrates makes CNT reorientation much more difficult under low strain, compared with the randomly distributed CNTs. This can explain the higher elastic modulus of the composites based on the hybrids. With the increase of aspect ratio from 500 to 2000, the experimental tensile modulus and strength show monotonically increasing trend. However, for the aspect ratio up to 3200, it is found that the mechanical properties of the composites unexpectedly decrease.

According to the simple Rule of Mixtures (ROM) for predicting the composite modulus and strength, the effective reinforcement modulus (E hy ) and strength ( hy ) of the hybrids in the composites could be given as:

m f m c hy E v E E E    2-12 f m m c hy v v      2-13
where, E c , E m , c , m , v m and v f are the composite tensile modulus, the matrix tensile modulus, the composite tensile strength, the matrix tensile strength, the matrix and the hybrid volume fraction, respectively. The results of E hy and hy for each system calculated from Eqs. 2-12 and 2-13 are also presented in Table 2-8 and 2-9.

It can be seen from Table 2-8 that the variation trend of E hy and hy with the hybrid substrate is similar to that of E c and c , respectively. For nearly the same weight ratio of CNTs with AR1200, the CNT-GNP hybrids offer the highest reinforcement effectiveness among the three hybrids. Their E hy is as high as ~330 GPa, which is about three and nine times of that of CNT-Al 2 O 3 and CNT-SiC, respectively. Their hy is up to ~ 6600 MPa, corresponding to almost two and three times of that of CNT-Al 2 O 3 and CNT-SiC, respectively. The more prominent enhancement of CNT-GNP hybrids lies in their superior mechanical properties, larger contact area with the matrix as well as the better synergistic effect of the perfectly dispersed GNPs and CNTs on the structural reinforcements. As shown in Table 2-9, the E hy and hy of the CNT-Al 2 O 3 hybrids increase and then decrease with the CNT aspect ratio, as do the E c and c . The highest E hy (320 GPa) and hy (10 4 MPa) are obtained at AR2000. When the CNT aspect ratio is relative small, the aligned CNTs can retain their actual lengths in the matrix instead of curved state. In this case, the reorientation of short CNTs grown on Al 2 O 3 is not easy to happen. However, with regard to AR2000, the reorientation of the aligned CNTs gradually appears with increasing the applied strain, which leads to the rearrangement of conductive networks. It has been demonstrated that the CNT reorientation is a key parameter for the mechanical properties of the composites, which weakens the stress concentration. And this behavior can inhibit the rapid growth of emerging microcracks and dissipate certain mechanical energy during tensile loading. However, in the case of AR3200, this nominal aspect ratio of CNTs is not the same as their actual one in the composites because of the waviness of some CNTs. Besides, many adjacent CNTs form the CNT bundles which may introduce some flaws in the matrix. These are the reasons why the incorporation of the CNT-Al 2 O 3 with too high CNT aspect ratios results in the decrease of the mechanical properties. Therefore, the hybrids with optimal aspect ratio and substrates are essential to achieve high performance composites.

Thermo-mechanical properties of the epoxy nanocomposites

Dynamic-mechanical properties of the composites were performed using a dynamic mechanical thermo-analyzer to study their mechanical properties as function of temperature.

This analysis could determine a variety of fundamental material parameters like the storage modulus, loss modulus, glass transition temperature (T g ) and damping behavior (ratio of loss/storage modulus). Three point bending mode was used on the specimens. The storage modulus (E′) and loss modulus (E′′) of the samples were measured at the oscillation frequency of 1 Hz and collected automatically at the temperature ranging from 20 o C to 160 o C at a constant heating rate of 3 o C/min. E′ represents the elastic behavior of materials and its value decreases with increasing temperature. In general, the E′ curve can be divided into three phases. The glassy phase usually occurs at low temperature. Then a transition from the glassy to rubbery phase is characterized by a narrow temperature region in which a sharp increase in macromolecular fragment mobility occurs. After that, it is the rubbery phase at the high temperature.

DMA analysis of the hybrids reinforced epoxy composites

As shown in Fig. 2 i.e., the corresponding enhancement is around 37%. This may be attributed to the stronger interfacial interaction between the CNTs of the hybrids and the polymer matrix. As shown in Fig. 2-16b, with the addition of reinforcements, we can clearly see that T g of the composites shifts to higher temperature, implying improved thermo-stabilities. The average T g obtained from the typical tan δ ~ temperature curves has been plotted in Fig. 2-16d. Particularly, T g was shifted from 82 o C for neat epoxy sample to 97 o C for 0.5 wt.% CNT-GNP concentration obtained from the peak of the glass transition temperature of the CNT-GNP/epoxy composites, and a maximum change of T g is achieved. The origin of such T g shifts has been attributed to the presence of so-called ‗interphase' polymer, which arises due to the interaction of the chains with the filler surface. In our system, as discussed above about the dispersion state of the hybrids, most of the aligned CNTs are wrapped by the host polymer, suggesting the existence of a very thin polymer layer embedded into the interspaces between any two well-aligned CNTs. This is equivalent to a wetting polymer-particle interface, which is proposed to possibly influence T g of the composites. The attractive interactions will restrict the mobility of the polymer chains. Percolation of this network of interphase polymer could then manifest the large T g shift of the bulk composite. This T g change also represents that the hybrids make strong interfacial bonding with the matrix, resulting in large free movement of the polymer chains to a large extent. It is well known that a relatively free movement of the polymer chains often affects the induce effect on T g of the composites. This improvement of T g is obviously due to the reduction of polymer chain mobility resulted from the increase of intermolecular interaction between CNTs and polymer chain molecules.

The influence of CNT aspect ratio on DMA behavior of the composites

In order to investigate the effect of CNT aspect ratio on thermomechanical properties of the CNT-Al Besides, the E′ and T g are dependent on the CNT aspect ratio of the hybrids. As shown in Fig. 2-17a andc, the storage modulus of the composites in the glassy and rubbery regions was increased upon incorporation of CNT-Al 2 O 3 hybrids with AR from 500 to 2000, and then begins to decrease with further increasing AR up to 3200. Likewise, the largest shift of glass transition temperature could be observed at AR=2000. And then the incremental changes in T g is reduced with further increase of AR. This variation trend is consistent with the results from tensile testing discussed above. And the similar mechanisms related to the hybrids can account for this phenomenon. For the CNTs with relatively smaller aspect ratio, the aligned CNTs hen the CNT aspect ratio is relative small, the aligned CNTs are more inclined to retain at their actual lengths in the matrix. And Al 2 O 3 microparticles are surrounded in the center by the CNTs. With increasing the CNT aspect ratio up to 3200 or more, some of the CNTs may exist as the curved state and Al 2 O 3 particles are wrapped at the end by CNTs and the hybrids exist like bundles in the composites. In this case, the nominal CNT aspect ratio is not the same as their actual one in the composites due to their waviness. Besides, this dispersion state more easily introduces the defects between the adjacent CNTs in the matrix.

The improvement in thermo-stability of the composites is closely related to the reduction in mobility of the matrix around the CNTs by interfacial interactions. Because this interaction induces different cross-linking regions in epoxy matrix which will eventually reduce the polymer chain motion, a strong shift of glass transition temperature in nanocomposites was observed. However, additionally, increases in the viscosity of the matrix by the addition of CNTs with too high aspect ratio could alter its flow behaviour both during impregnation as well as during the curing process, resulting in nanometer/micron scale porosities. This could lead to an increase in free volume of the polymer molecules in the matrix allowing additional mobility of molecules possibly contributing to the unexpected onset of decrease in T g of the CNT-Al 2 O 3 /epoxy composites at AR=3200.

Self-sensing behaviors of the epoxy na nocomposites

The CNT/polymer composites are increasingly being reviewed as a realistic alternative to conventional smart materials. Significant advances have been made in using CNT based polymer films as strain sensors. As compared to conventional sensors, higher sensitivity has been observed in these novel sensors. In general, the piezoresistivity observed in these strain sensors made from CNT/polymer composites can be mainly attributed to the variation of conductive networks with strain, such as loss of contact between the fillers, tunneling effect in neighbouring fillers and the conductivity change resulted from deformed CNTs. Re cently, great interest has been generated in building highly sensitive strain sensors with randomly oriented CNTs. Besides, some studies have been focused on the aligned CNTs in the matrix.

It was found that aligned CNT network resulted in improved electrical and piezoresistive sensing capabilities of the composites [START_REF] Nanni | Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET-MWCNT nanocomposites[END_REF]. In this part, the varying self-sensing behaviors of the composites filled with ran-CNTs and the hybrids (CNT-GNP, CNT-SiC and CNT-Al 2 O 3 )

will be comparatively investigated. The underlying mechanisms with relation to the substrates and CNT aspect ratios of the hybrids will be mainly discussed. The distinctive in situ sensing behaviors of the hybrids could offer great advantages for their potential applications. Here, the filler concentration in each system is close to its percolation threshold, as listed in Table 2-3. As shown by the dotted lines in Fig. 2-18a, the in-situ electrical resistance increases monotonically with the applied strain until catastrophic fracture of the composites. We can see from Fig. 2-18b that the self-sensing properties of the mixture of ran-CNTs and microparticles filled composites is almost the same as that shown in Fig. 2-18a. And this behavior is almost independent of the microparticle types. Besides, it should be noted that the in situ electrical resistance in the two systems increases in a fairly linear way at small strain and then exhibits a similar exponential behavior, increasing more significant per strain unit with the increase of strain. When the strain is near the fracture, a sudden increase of ∆R is observed. This phenomenon of the resistance-change is also independence of the ran-CNT content. different CNT aspect ratios. It is found that the response of the in-situ electrical resistance is strongly dependent on the CNT aspect ratio. In the case of AR500 and AR1200, the in-situ resistance exhibits a similar behavior to that shown in Fig. 2-18 with a consistent increase of electrical resistance until the ultimate failure of the composites. However, with increasing CNT aspect ratio up to 2000 or more, the self-sensing properties of the composites become similar to that shown in Fig. 2345678910111213141516171819.

Self-sensing behavior of the composites during quasi-static tensile tests

Self-sensing behavior of the epoxy nanocomposites with randomly distributed CNTs (ran-CNTs)

Self-sensing behavior of the CNT-GNP/epoxy and CNT-SiC

Self-sensing behavior of the epoxy composites during cyclic loading

In order to explore the underlying mechanisms behind these different self-monitoring behaviors aforementioned, the evolution of deformation occurred in the composites during tensile loading are investigated. Accordingly, the incremental cyclic loading tests of the composites are carried out. fairly linear behavior is detected for small deformations ( < c ), but the complete curve is clearly nonlinear. As it exceeds the critical strain ( c ), the resistance increases nonlinearly, gradually increasing the curve slope until the ultimate fracture. When the strain is close to the fracture, a sudden increase in ∆R occurs since the continuity of the CNT networks inside the matrix is disrupted. This nonlinear piezoresistive behavior in the ran-CNT/polymer composites under high deformation has been observed. Park et al. [START_REF] Park | Strain-dependent electrical resistance of multi-walled carbon nanotube/polymer composite films[END_REF] suggested that for deformations in the elastic range of the polymer matrix, CNTs are still in contact inside the Matrix, and thus the piezoresistive effects can be explained by common percolative theories.

Self-sensing behavior of randomly distributed CNTs/epoxy composites

However, for higher deformations (close to the elasto-plastic mechanical transition), CNT contacts decrease and the tunneling resistance may play a dominant role in the nonlinear behavior. According to Hu et al. [START_REF] Hu | Tunneling effect in a polymer/carbon nanotube nanocomposite strain sensor[END_REF][START_REF] Hu | Investigation on sensitivity of a polymer/carbon nanotube composite strain sensor[END_REF], increasing the tunneling resistance can improve piezoresistivity of the composites, which may be the reason for increased sensitivity (slope) observed in the large deformation zone. In our case, it is also possible that the large applied strain yields certain CNT reorientation inside the polymer.

Besides, an incremental cyclic loading has also been conducted as shown in Fig. 2-21b and c. After the fourth loading cycle (σ ≈ 49 MPa), some residual strain occurs when the specimen is unloaded to σ = 0, which can be attributed to the plastic deformation. The in situ electrical resistance increases and decreases monotonically with the cyclic stress and strain, however, an irreversible resistance change occurs after the fourth loading cycle which directly correlates with the emerging residual strain. That is, once the yield point where σ y ≈ 49 MPa is passed, the plastic deformation happened. However, from the in situ resistance change, there is not obvious inflexion region or other information to identify the elastic and plastic region. Chapter ІІ

The CNT-Microparticle Hybrids Reinforced Epoxy Nanocomposites -104 -

Self-sensing behavior of the CNT-GNP/epoxy nanocomposites

In attempt to further exploit CNT-GNP hybrid as novel in situ sensor, the electrical resistance response of the CNT-GNP/epoxy composite with 0.5 wt.% CNT-GNP hybrids is tracked in real-time when the specimens are subjected to quasi-static and incremental cyclic tensile loading. Between three to five samples are tested under the same condition. An example of the typical stress-strain curve and the in-situ electrical resistance response during quasi-static tensile loading is plotted in Fig. 2-22a. It is clearly seen that the in situ resistance increases to its maximum value and then begins to decrease at a critical strain ( c ). This self-sensing behavior is similar to that of the composites based on CNT-SiC hybrids [START_REF] Li | The use of vertically aligned carbon nanotubes grown on SiC for in situ sensing of elastic and plastic deformation in electrically percolative epoxy composites[END_REF].

When < c , the CNTs grown on the substrates are very difficult to orientate due to their strong junctions with the substrate and excellent molecular coupling with the polymer chains.

Then the matrix deformation makes the number of contact points between CNTs decrease.

Therefore, the formed conductive networks are broken up and the in situ electrical resistance increases. With the increase of applied strain ( > c ), it is helpful for the CNT orientation in the composites due to the formation of certain microvoids, shear bands and crazing. In this case, the rotation of the GNP substrates could also facilitate the rotation of the CNTs and hybrids. These rearrangement behaviors could make the separated conductive networks closer again, giving rise to the decrease of electrical resistance. In addition, the electrical tunneling resulted from the closing of some micro-cracks because of Poisson's contraction also contributes to the reformation of the conductive networks.

In order to further shed light on the deformation evolution in the composites, the incremental cyclic loading tests are also performed. Besides, during the first three cycles, the strain and in situ ΔR/R o always return to zero after each cycle. However, the slight deviation of the electrical resistance response to strain response could be observed during the fourth cycle. Upon unloading after the fourth cycle, the strain and ΔR/R o could not return to their original values, indicating that the residual strain, irreversible deformation and permanent resistance change has occurred during this cycle. It is noteworthy that the amplitude of strain in the fourth cycle ( max ) is very close to the critical strain ( c ) in Fig. 2

Self-sensing behavior of the CNT-SiC/epoxy nanocomposites

Fig. 2-23a presents the stress-strain curve (solid line) as well as in situ electrical resistance change (dotted line) in the CNT-SiC/epoxy composites during tensile loading. The resistance monotonically increases then begins to decrease when the strain passes a critica l value r , which obviously divides the dotted line into Region І and ІІ. The stress-strain curve (0~ r ) displays a typical linear-elastic behavior (see the inset), which is reconciled with the Hooke's Law. Traditionally, the elastic and plastic region could be id entified via the offset yield point [START_REF] Lim | Impact fracture behaviour of nylon 6-based ternary nanocomposites[END_REF]207]. A 0.2% residual strain has been taken to define the offset yield point (Y) and the corresponding y is very close to r . In order to further verify the yield point and the appearance of the permanently plastic deformation near yield point, the specimen is also subjected to tensile loading cycles with increasing maximum per cycle and unloaded to zero stress at the end of each cycle. After the fifth loading cycle (σ max =46 MPa), some residual strain occurs when the specimen is unloaded to σ=0 (Fig. 2-23b), which can be attributed to the permanently plastic deformation. As shown in Fig. 2-23c, the in situ electrical resistance increases and decreases monotonically with the cyclic stress and strain, however, an irreversible resistance change occurs after the fifth loading cycle when the specimen is unloaded, which directly correlates with the emerging residual strain. That is,once the yield point σ y ≈46 MPa is passed, the plastic deformation happened. Besides, as shown in the SEM image of Fig. 4a, some permanent micro-cracks in the matrix are initiated and observed at 4% strain. As a result, this in situ sensing behavior of our CNT-SiC hybrids in the composites could offer valuable information for identifying the elastic and plastic region. Chapter ІІ

The CNT-Microparticle Hybrids Reinforced Epoxy Nanocomposites -110 -

Self-sensing behavior of the CNT-Al 2 O 3 /epoxy nanocomposites

The tensile loading-unloading cycles of the CNT-Al 2 O 3 /epoxy composites (AR2000) is shown in Fig. 2-24b and c. The stress, strain and in-situ electrical resistance are plotted as function of time. During the first four cycles, the strain and in-situ ΔR/R o always return to zero after each cycle, indicating that no irreversible deformation occurs in the composites.

However, in the fifth cycle, the strain and ΔR/R o could not return to their original values after unloading. This indicates that the appearance of the residual strain and irreversible deformation as well as the permanent electrical resistance change at this strain level. Besides, the amplitude of ΔR/R o gradually increases with the loading-reloading number during the first five cycles, whereas, it begins to decrease in the sixth cycle. It is noteworthy that the amplitude of strain in the fifth cycle ( max ) is very close to the critical strain ( c ) shown in Fig.

Influence of the substrate and CNT aspect ratio of the hybrids on the self-sensing behavior of the composites

As shown in Fig. 2-25a, the composites based on the hybrids (CNT-SiC, CNT-GNP and CNT-SiC+CNT-Al 2 O 3 ) with the same CNT aspect ratio (AR1200) behave remarkably differently. These composites display a similar self-monitoring ability during tensile loading, that is, the in-situ electrical resistance monotonically increases to its maximum value and then begins to decrease. For the composites with the CNT-SiC+CNT-Al 2 O 3 mixture, the mass ratio of the two hybrids is set as 1:1. could not be seen from the optical image because the SiC microplate is much smaller compared to the CNTs. Therefore, it is possible for this kind of nano/micro hybrid to reform the conductive networks in the plastic region. This may explain the decrease of electrical resistance when it entered into the plastic region.

The self-sensing behavior of the composites is strongly dependent on the formed CNT conductive networks. Therefore, we try to explain the in-situ sensing behaviors mentioned by exploring the different evolutions of the conductive networks provided by the randomly distributed CNTs and the CNTs grown on the microsubstrates. The randomly distributed CNTs are generally supposed to more easily orientate in the polymer matrix compared to the CNTs grown on the microsubstrates. They are inclined to align along the stress direction during tensile loading, as shown in Fig. 2-26a. However, this rearrangement is not conducive to the reformation of conductive networks. Thus, the composites with randomly distributed CNTs show a monotonic increase of the in-situ electrical resistance until their ultimate fracture. However, the composites based on the CNT-SiC or CNT-GNP hybrids (AR1200) display a noticeable recovery of resistance when > c . Moreover, the electrical resistance at high strain becomes even lower than that at =0, as shown in Fig. 5b. When < c , the CNTs grown on the substrates are very difficult to orientate under low strain due to the strong junctions with the substrate and excellent molecular coupling with polymer chains. In this case, the broken of the formed conductive networks mainly happens. With the increase of applied stress, it may be helpful for the orientation of the CNTs and hybrids in the composites. As indicated previously, the SiC and GNP substrates are anisotropic. When > c , their rotation could facilitate the rotation of the CNTs and hybrids, due to the formation of certain microvoids, shear bands and crazing. Besides, the CNT-SiC and CNT-GNP hybrids show the asymmetric morphology in the composites, as shown in Fig. 2-26b. Then the rearrangement of the CNTs and hybrids can make the separated conductive networks closer again, giving rise to the decrease of electrical resistance. However, this does not happen in the case of the CNT-Al 2 O 3 /epoxy composites (AR1200). The in-situ electrical resistance continues to increase even at high strain ratio ( > c ). It is known that the morphology of Al 2 O 3 is spherical, and the CNT-Al 2 O 3 hybrids with relative small length exist in a symmetric form in the composites (Fig. 2-26c andd).

Therefore, the rotation of the CNTs and hybrids could not change their existing morphology in the composites. Besides the substrate morphologies, the CNT aspect ratio also has an important effect on the self-sensing behavior of the composites. As discussed in Fig. 2-25 concerning the self-sensing behaviors of the CNT-Al 2 O 3 /epoxy composites with different aspect ratios, when AR is equal to 500 and 1200, the in situ electrical resistance of the composites monotonically increases with the applied strain, whereas when AR is up to 2000 or more, it begins to decrease when > c . Asides from the rotation of the CNTs and hybrids resulted from that of the substrates, the reorientation of CNTs with high aspect ratio along the stress is much more evident than that of small ones under tensile loading, as shown in Fig. 2-26e. With the help of this orientation behavior, the conductive networks could easily reform during tensile loading. Moreover, for AR 3200, the Al 2 O 3 substrate seems to be wrapped by the CNTs in one end instead of in the center (Fig. 2-5g and Fig. 2-26f). These distribution states provide more chance for the reorientation of CNTs, which will contribute to the reformation of conductive networks when > c . Besides, most of the curved CNTs with higher aspect ratios may be stretched along the tensile loading, which also helps to reform the conductive networks. In addition, the variation tendency of percolation thresholds in CNT-Al 2 O 3 /epoxy composites could also be explained by the aspect ratio dependent CNT dispersion. With the increase of aspect ratio from 500 to 2000, the percolation threshold of the composites shows obvious downward trends, however, it begins to increase with further increasing aspect ratio to 3200. To some extent, the waviness of some CNTs in this system (AR3200) also decreases their actual length in the composites. Based on the above discussion, it can be concluded that the self-sensing behavior of the composites greatly depends on the evolution of conductive networks during tensile loading.

And this evolution is closely related to the CNT aspect ratio and reorientation as well as the microsubstrate geometry.

Concluding remarks

In order to develop the high performance CNT/polymer composites, the critical issues, such as, preparing the structure controllable CNTs (aspect ratio and orientation), creating uniform dispersion and desirable alignment of CNTs within the polymer matrix, improving load transfer from CNT to the matrix, etc, still exist and extremely need to be solved. Their (І) The CNT-microparticle multiscale hybrids could be used as high-performance structural additives for improving the composite mechanical properties. Such as, with respect to neat epoxy, significantly enhanced mechanical properties in the CNT-GNP filled composites were achieved at ultralow hybrid loading (0.5 wt.%). The tensile modulus showed ~40% increase and the tensile strength was improved by ~36%. But, it was found that the optimal concentration of the hybrids (f c ) is necessary to achieve the desirable reinforcement effect. It is most often the case that mechanical properties of the hybrids reinforced composites become saturated when the hybrid content exceeds a certain value. When f hybrid < f c , the tensile modulus and strength continuously increase with the embedding of the hybrids. As f hybrid approaches f c , the results indicate that they approximately reach the maximum values.

However, when f hybrid > f c , they begin to degrade with the hybrid loading. Besides, the value of percolation threshold of the hybrids filled systems is much lower, ~ 0.

Chapter ІІІ

The mutiscale glass fabric/epoxy composites

Introduction

Glass fiber-reinforced plastic laminated composites are widely used in many structural applications, such as wind energy, transportation, aerospace, etc, due to their high specific modulus and strength as well as significant weight reduction relative to metallic materials [START_REF] Hughes | The carbon fibre/epoxy interface-a review[END_REF]. The fiber plays an important role in the load carrying capacity of the composites along the fiber direction, endowing them with superior in-plane performance. The epoxy adhesives are usually incorporated to hold together the load-bearing fibers. These glassy networks offer high creep and corrosion resistance and good adhesion to the fibers, yet they are also highly brittle and insulating. Hence, the composites suffer from poor matrix-dominated mechanical properties, such as interlaminar toughness and transverse electrical and thermal conductivity, particularly in matrix-rich interlaminar regions between the fibers.

Many studies have focused on toughening the matrix of the composites to improve their mechanical robustness. Rubber-toughening through the addition of discrete rubber particles enhances toughness but degrades the thermomechanical properties of crosslinked polymer networks [START_REF] Bagheri | Rubber-toughened epoxies: A critical review[END_REF]. The addition of rigid thermoplastics is another method of toughening the matrix low degradation in its thermomechanical properties [START_REF] Li | Inhomogeneous toughening of carbon fiber/epoxy composite using electrospun polysulfone nanofibrous membranes by in situ phase separation[END_REF]. Even so, the new methods to improve toughness while maintaining or improving thermal properties and simultaneously addressing additional needs such as transverse electrical conductivity are also needed. In this point, nanofillers have the potential to toughen the matrix phase and offer multifunctionality for the enhancements of other properties. Besides, the out-of-plane or through thickness properties of the composites also depend on the fiber/matrix interface [START_REF] Tang | A review of methods for improving the inter facial adhesion between carbon fiber and polymer matrix[END_REF]. Generally, the decreased in-plane and out-of-plane performance of the composites are attributed to the breakage of load-carrying fibers and the matrix dominated cracks occurring at low strain. In order to improve the overall mechanical properties and long-term durability, modifying the fiber/matrix interface by nanofiller is a more achievable method. Even so, with the increase of their service time, many structural components require damage detection. As a result, it is imperative for the development of in situ damage sensor for their life prediction. Currently, electrical resistance measurement has been established as a nondestructive way for damage sensing [START_REF] Kupke | Non-destructive testing of FRP by dc and ac electrical methods[END_REF][START_REF] Schueler | Damage detection in CFRP by electrical conductivity mapping[END_REF], but it is not applicable for the glass fiber (non-conducting) reinforced composites. In this case, more interest has emerged in developing the electrically conductive matrix by introducing conductive fillers into the matrix. Then the conductive networks offer potential for in situ monitoring the damage evolution of the composites. It is expected that matrix modification via incorporation of multifunctional filler co uld provide feasibilities for attaining desirable mechanical and self-sensing properties in the fiber reinforced composites.

Carbon nanotubes with extraordinary strength and stiffness and high aspect ratio have emerged as potential candidates for the matrix modification. As inherently multifunctional nanofiller, CNTs could serve as a structural reinforcement, conductive additives and strain sensor at a nanoscale [START_REF] Gao | Sensing of Damage Mechanisms in Fiber-Reinforced Composites under Cyclic Loading using Carbon Nanotubes[END_REF][START_REF] Veedu | Multifunctional composites using reinforced laminae with carbon-nanotube forests[END_REF]. It was demonstrated that the addition of CNTs in the matrix improved the toughness of the matrix and the intrinsic interface properties of the composites.

Besides, due to the small scale of CNTs relative to that of the fiber reinforcement, CNTs can penetrate into matrix-rich regions among fibers to form electrically conductive networks.

The formed networks were utilized as sensitive sensors for monitoring the matrix-dominated damage. In recent years, significant attention has been paid to the multiscale, multifunctional structural composites containing conventional continuous fiber reinforcement together with the CNTs modified matrix [START_REF] Thostenson | Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self healing[END_REF]. Bekyarova et al. grew CNTs on carbon fiber surfaces for enhanced interactions, yielding tensile modulus and strength improvements of ~ 30% and 15% over the base composites, respectively [START_REF] Bekyarova | Multiscale carbon nanotube-carbon fiber reinforcement for advanced epoxy composites[END_REF]. Unfortunately, CNT growth from carbon fibers often results in non-uniform coverage or damage to the fibers, making this method difficult to optimize the composites.

However, the incorporation of CNTs in fibrous polymer composites still remains a challenging task. In view of the processability affected by the viscosity of the CNT-polymer mixtures, lower CNT content is required to simultaneously improve both mechanical and electrical properties of the composites. In this case, a prerequisite is the preferential CNT distribution in the matrix because more agglomerations of CNTs will severely reduce the matrix-dominated mechanical properties and increase the percolation threshold to some extent. In this respect, different approaches are selected, covering a wide range from stirring, high shear mixing to chemical functionalization. Additionally, interfacial adhesion between the matrix and CNTs is another critical issue. Strong polymer/CNT interaction results in efficient load transfer from the matrix to CNTs which may increase the mechanical performance. As previously reported [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites[END_REF], it was found that functionalization of CNTs could improve their degree dispersion and interfacial properties, whereas, unexpectedly increasing the percolation threshold and decreasing the electrical conductivity of the system. Besides, the CNT alignment is also one of the most parameters in influencing the mechanical and self-sensing properties of the composites.

With regard to the critical issues discussed above, the multiscale hybrids with CNTs grown on the surface of other microparticle has been introduced into the matrix, resulting in uniform CNT dispersion and improved interfacial properties [START_REF] Zhang | Dramatically Enhanced Mechanical Performance of Nylon-6 Magnetic Composites with Nanostructured Hybrid One-Dimensional Carbon Nanotube-Two-Dimensional Clay Nanoplatelet Heterostructures[END_REF]. The CNT organization can be tailored by varying the growth parameters. Moreover, the hybrids were proposed to be utilized as multifunctional fillers in the composites. As our recently reported, the CNT-SiC and CNT-graphene nanoplatelet hybrids with CNTs grown on the surfaces of SiC or GNP microparticles have respectively served as high-performance reinforcement [START_REF] Li | Carbon nanotube-graphene nanoplatelet hybrids as high-performance multifunctional reinforcements in epoxy composites[END_REF] and in situ strain sensor to detect the onset of irreversibly permanent deformation in the matrix [START_REF] Li | The use of vertically aligned carbon nanotubes grown on SiC for in situ sensing of elastic and plastic deformation in electrically percolative epoxy composites[END_REF].

To the best of our knowledge, the research on the glass fabric reinforced polymer composites incorporated with the CNT-microparticle multiscale hybrids has not yet been reported up to now. We hope that the hybrids become desirable alternative for matrix modification and in situ damage sensor in the fiber reinforced composites.

Epoxy is widely used as the polymer matrix for high performance laminated composites due to their good mechanical performance, processability, compatibility with most fibers,

Experimental

Materials

Randomly distributed CNTs (commercial) with a diameter of 30~50 nm and length ~20 μm supplied by Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences were used as-received (Fig. 2-1d). Micro-spherical alumina particles with 3~10 μm in size (99.8% in purity) were purchased from Performance Ceramic Company (Peninsula, OH, USA) (Fig.

2-1a

). Graphene nanoplatelets (GNPs) with high aspect ratio, i.e., 3~4 μm in length and ~30 nm in thickness were obtained from Xiamen Knano Graphene Technology Co., Ltd. The plain woven glass fiber fabric used as-received with a density of 206g/m 2 and warp 51%, weft 49% was manufactured by Hexcel Corporation. The CNT-microparticle nano/micro hybrids used were synthesized using CVD procedure in our lab. The detailed preparation process has been illustrated in the last part. Epoxy resin (1080S, Resoltech Ltd, France) and the curing agent (1084, Resoltech Ltd) was used as the polymer matrix.

Preparation of the composites

The multiscale composites were manufactured using the glass fabric and epoxy matrix doped with three kinds of particle reinforcements, such as, the ran-CNTs, ran-CNT+Al 2 O 3 mixture and CNT-Al 2 O 3 hybrids. The composite laminates without particle reinforcements were also fabricated for comparative purposes. The fabrication procedure of the composite laminates consists of two steps: (І) Preparation of the particle/epoxy suspensions and (ІІ)

Impregnation of the suspension into a mold containing the glass fiber performs.

Chapter ІІІ

The multiscale glass fabric/epoxy composites -125 -

Preparation of particle/epoxy suspensions

The present research concentrates on manufacturing of the initial epoxy suspension by dispersing 0.5 wt.% of particles in the epoxy matrix. The fraction of the ran-CNTs and Al 2 O 3 in the ran-CNT+Al 2 O 3 mixture is respectively the same as that of the CNT-Al 2 O 3 hybrids. In order to well disperse these particles, we manually added them into the neat resin and then added the resulting suspension to a lab scale three-roll mill for final high shear mixing. The gap distance between the three rollers and the roller spend was set according to the particle size. The detailed processing methods have been stated in the last part. The prepared epoxy suspensions were collected and then mixed with the added the harder with a mechanical stirrer for 10 min.

Fabrication of composite laminate

The glass fabric/epoxy composite laminates were fabricated by using an autoclave after typical manual hand layup and vacuum bagging of plies, then hot compression techniques. In our research, the volume percentage of the glass fiber fabric in the composite laminate is set about 50%. The glass fiber layers were properly stacked into four plies and the orientation of fiber within the glass fabric was kept constant without alternating warp direction. The epoxy suspensions prepared above was spread uniformly on each fabric layer. After the infusion, the preformed composites were put into a vacuum bag in the autoclave. After completing the degassing process, the laminates were cured under 3 bar at 60 o C for 2h in the autoclave.

Elevated temperature was selected 5 o C/min to reduce the curing period and production time.

After curing, the temperature of the hot press was gradually reduced at a rate of

Characterization and instruments

Micrographic analysis

Optical microscopy (OM) and Scanning Electron Microscope (SEM) observations were conducted to investigate the reinforcement dispersion and the local failure mechanisms in the fractured composites. The samples used for microscopic analysis were cut from the failure region of the fractured specimens and then polished using the grit abrasive wheel.

Mechanical characterization

3-1

The maximum stress at failure on the tension side of a flexural specimen was considered as the flexural strength of the material. The flexural strength in a 3-point flexural test was determined using Eq. 3-2:

Electrical conductivity measurement

The alternating current (ac) conductivity of the composites was measured using a precision impedance analyzer (Agilent 4294A) over the frequency range 10 2 ~10 6 Hz at room temperature. The measurements were performed by using the rectangle specimens with dimensions of 1.1×10×10 mm. Prior to the measurements, the silver paste electrodes were applied to both sides of the samples in order to reduce the contact resistance.

In situ sensing behavior

To determine the self-sensing behaviors of the composites, in situ electrical resistance measurements were performed for monitoring the deformation evolution of the composites.

A Keithley 2400 voltage-current meter was used to measure the in-plane volume electrical resistance of the specimens by sourcing a constant voltage and measuring the current using a four point technique. Prior to the electrical measurements, the silver paste electrodes were also applied on both sides of the samples. Due to the nature of the electrically percolative samples, the contact resistance is very small relative to their overall resistance. The electrical resistance data were in situ recorded using software LabView.

Morphologies of the reinforcements

The SEM morphology of the CNT-Al 2 O 3 hybrids is demonstrated in Fig. 3 The CNT grown on the substrates were with a length ranging from 20 to 25μm. The CNT length and density can be adjusted by the CVD conditions, such as growth temperature, time and hydrogen ratio. The morphology of the glass fabric as-received is shown in Fig. 3-3b. In this case, the nanotubes may break with low extraction lengths because most of them are coated by a matrix layer. It can be viewed that the dispersion states of the hybrids can ensure large contact area and good interfacial interaction between the matrix and CNTs. A good dispersion not only makes more filler surface area available, but also prevents aggregation of the filler acting as stress concentrators and the slippage of nanotubes during applied loading.

Therefore, it is expected that the mechanical properties of the composites could be improved by addition of the hybrids. In addition, the introduction of the special uniform CNT networks are expected to be utilized as in situ damage sensors in the multiscale glass fabric/epoxy composite laminates.

Electrical properties of the glass fabric/epoxy composites

The ac conductivity measurements of the composites were performed in the frequency range between 10 2 and 10 7 Hz. The ac conductivities of the glass fabric/epoxy composite laminates with and without the nanofillers as function of frequency in the through thickness and in-plane directions are shown in Fig. 3-5a and b, respectively. It can be clearly seen that the conductivities are enhanced in both in-plane and through-thickness directions, indicating the effect of matrix modification by incorporation of nanofillers on the electrical properties of the laminates. The resulting conductivities of the baseline laminates and the laminates reinforced with three kinds of nanofillers at f =10 3 Hz in the two directions are summarized in Fig. 3-5c. It is worth noting that the conductivities of the laminates containing 0.5 wt.% CNT-Al 2 O 3 hybrids are 3 ~ 4 orders of magnitude higher than that of the reference samples for both in-plane and through-thickness directions, up to values on the order of 10 -5 ~ 10 -6 S/m. This is attributed to the formation of a percolating conductive nanotube networks in the material. Moreover, the addition of CNT-Al 2 O 3 hybrids can easily bridge the insulating epoxy interlaminar region between plies. In this case, the well dispersed CNTs and the distinctive architecture formed by the multiscale hybrids play an important role in determining the significantly improved conductivity. However, as shown in Fig. 3-5c, for all the composites with or without nanofillers, the through-thickness conductivity is turned out to be lower than the in-plane value. The anisotropy observed is due to the differences between the resistivity of the epoxy and glass fibers and the geometry of the glass fibers which form a continuous path in the laminate plane whereas discontinuous path across ply interfaces. The conductive epoxy/CNT regions will be continuous in the in-plane direction at the layers of the fabric, not influencing by the insulating glass fiber. However, the conductive epoxy/CNT regions are interrupted by the glass fibers in the through thickness direction, resulting in a serial configura tion which is influenced more strongly by the low conductivity phase. The presence of insulating glass fiber reinforcements in plane precludes the formation of the conductive paths, which could explain the lower conductivities in the through-thickness direction.

The Rule of Mixture (ROM) is generally used to predict the properties of the composites made up of continuous and unidirectional fibers. According to ROM, the conductivity of the glass fiber/CNT-Al 2 O 3 /epoxy composites in the parallel directions to the fibers calculated using the equations given by:   In the initial stage (І), the resistance response with the strain is almost linear. At the onset of the loading, the incipient matrix-dominated damages such as the microcracks or flaws are initiated in the regions between the glass fiber and matrix. As demonstrated in Fig. 3-7a and Fig. 3-8a, the microcracks were found propagating along the fiber-matrix interface or through the glass fiber plies. Fig. 3-7b shows a microcrack passing through a hybrid. The formation of these microcracks causes the damage of the CNT conductive networks. In this stage, the crack density increases slightly due to the initiation of some microcracks (Fig.

1 para f f m f V V       3

3-6b

). With the gradual increase of the strain in the following stage, when 1 < < 2 , one can see that the electrical resistance begins to deviate from the initial linear resistance-strain curve and increases more evidently than in the previous stage. In this case, some of the matrix microcracks propagate which results in the interfacial debonding. The increased steep growth in this stage is attributed to the accumulation of the matrix-dominated damage (Fig. 3-6c), the debonding of fiber-epoxy interface (Fig. 3-6d) and the transverse crackings (Fig.

3-6e and g). These factors possibly result in more broken-up of the conducting paths in the composites. However, when it enters into the ІІІ stage ( > 2 ), the slope of the resistance change curve is decreased compared to the ІІ stage, suggesting the slight increase of the resistance. Besides the debonding of fiber-epoxy interface and transverse crackings, the fiber breakage along the direction of applied load and interply delamination will also take place, as shown in Fig. 3-6f and Fig. 3-8c. These factors further accelerate the broken of conductive networks. However, with regard to the decreased slope, it is worth noting that the Poisson contraction of the matrix possibly plays an important role on the evolution of the conductive networks, which possibly lead to the closing of the microcracks in the matrix. Then the two neighboring CNTs may be situated close enough to endow the tunneling effect, enabling the reformation of conductive networks. Moreover, the orientation of the CNTs grown on the Al 2 O 3 microparticles resulted from the increasing strain is also responsible for the formation of the conductive networks. As a result, during the ІІІ stage, there exist two kinds of factors which mainly influence the conductive networks in the matrix. One can give rise to the increasing resistance and the other often causes the decreasing resistance. But the former has much greater effect on the resistance change than the latter, which could be used to explain the consistently slight increase of the resistance. result in more increased viscosity, which is different from other fillers. This is helpful for the adhesion between the modified matrix and the glass fiber. Hussain et al. [START_REF] Hussain | Mechanical property improvement of carbon fiber reinforced epoxy composites by Al 2 O 3 filler dispersion[END_REF] proposed that the addition of nano-sized fillers to an epoxy matrix caused higher thermal residual stresses on the surface of the fibers, which could increase the fiber-matrix interfacial bonding. These factors play an important role in achieving efficient stress transfer from the matrix to reinforcement, leading to the improved mechanical properties. SEM images of the delaminated surface of the glass fabric reinforced epoxy composites were illustrated in Fig.

Mechanical properties of the glass fabric/epoxy composites

Flexural properties of the glass fabric/epoxy composites

3-11.

As shown in Fig. 3-11a, the composites made from neat epoxy exhibited remarkable river stripes of brittle fracture in the matrix, including some islands of matrix and the matrix among the fibers. The cracks propagated along the fiber-matrix interface, also reflecting the Chapter ІІІ

The multiscale glass fabric/epoxy composites -141 -weak fiber-matrix interfacial adhesion. Debonding between the fibers and the epoxy matrix due to weak interfacial bonding is also obviously observable. As shown in Fig. 3-11b~c, Fig.

3-7a and c, the composites with the CNT-Al 2 O 3 hybrids demonstrated a rougher matrix surface than those with the neat epoxy. Interconnecting CNT-Al 2 O 3 hybrids was observed between the neighboring glass fibers, greatly improving the matrix toughness. That is, the composite surfaces showed a higher interaction between the matrix and the fibers. These results indicate that the incorporation of nanofillers into the conventio nal fiber composites could create the anchor effect against the matrix cracking, which enhances their mechanical properties. For the composites made from the neat epoxy, it is clearly seen that there is fiber-matrix debonding on their fracture surface and their failure surface is brittle-like (Fig. 3-12a). The matrix detached from the fiber surface due to the weak adhesion with smooth fiber surface. The interfacial debonding of the matrix from the fiber can be regarded as the dominant mechanism for flexural fa ilure of composites without CNT-Al 2 O 3 hybrids. For the composites containing the CNT-Al 2 O 3 hybrids, one can see from Fig. 3-12b that better fiber-matrix bonding was observed from many locations of the fracture surfaces of the specimens and the cracks propagated through the breaking of the fibers but not along the fiber-matrix interface, suggesting the better adhesion between the fibers and the matrix. Besides, the glass fibers as continuous reinforcement tend to be broken rather than being pulled out, implying desirable ductile-like features in the modified matrix and ensuring the effective load transfer from the modified matrix to glass fibers.

A detailed investigation of the damages in the hybrids filled regions from the higher resolution SEM micrograph (Fig. 3-12c) revealed that the micro-cracks were stabilized by the bridging of the aligned CNT bundles, which can absorb a substantial amount of energy and stop or slow down the crack propagation, making the system tougher and stronger. We can see that several nanotubes bridged a micro-crack of several hundred nanometers. Due to the nanotube bridging, the micro-cracks were arrested and prevented from developing into larger and more harmful cracks. Though numerous micro-cracks were created in the epoxy matrix, the growth of the micro-cracks was arrested by the bridging or prevented by the hybrids. Thus, the CNT networks which serve as microcrack arrester enhance the composites toughness through the bridging mechanism in resin-rich region. The above mechanisms contributed to the enhancement of flexural modulus and strength. The SEM observations are in agreement with the experimental results of the mechanical properties. The introduction of the CNT-Al 2 O 3 hybrids seems to be responsible for the increase in the flexural performance of the multiscale glass fiber reinforced composites. show that the method used to incorporate the nanoreinforcements into the composite matrix has not diminished the structural integrity of the glass fiber composite for the tested CNT contents. Tensile properties are dominated by the continuous fiber when they are measured in the longitudinal direction. Therefore, the addition of the nanoreinforcement had produced defects during manufacturing, but its incorporation cannot change appreciably the properties with regard to those obtained for the neat resin composite material. Some authors have observed that it is difficult to achieve higher values of elastic modulus and even small drops have been measured with regard to the carbon fiber/epoxy composites reinforced with CNTs.

Tensile properties of the glass fabric/epoxy composites

For the plain-weave composites, the longitudinal and transverse yarns (warp and weft) pass over and under each other alternately. The longitudinal yarns are subject to uniform strains whereas the strains of transverse yarns are insignificant. The longitudinal yarns are under combined effect of bending and stretching due to the crimp. Based on the Rule of Mixture (ROM), the longitudinal modulus and transverse modulus of the yarns could be respectively calculated as follows [START_REF] Soykasap | Analysis of plain-weave composites[END_REF][START_REF] Hussain | Prediction of elactic properties of FRP composite lamina for longitudinal loading[END_REF]:

  1 II f f m f E V E E V    3-2   1 fm f m f f EE E V E V E   

3-3

Here 1 y E is the longitudinal modulus of the composites; 2 y E is the transverse modulus of the composites; V f is the volume fraction of the fiber; E m is the Young's modulus of the matrix; E 11 is the longitudinal modulus of the fiber. The underlying assumptions of these rules are:

the reinforcements are uniformly distributed in the matrix; perfect bonding between the reinforcements and matrix; the matrix and reinforcements behave as perfectly linear elastic materials. The transverse modulus of the composites can be described more accurately using the Halpin-Tsai equation:

  1 1 f m f V E E V      
3-4 In our research, E m is the Young's modulus of the epoxy matrix reinforced with CNT-Al 2 O 3 hybrids; V f is the volume fraction of glass fiber (V f =0.5); E f =70GPa. If the effect of weave on the modulus was neglected, the modulus of the composites can be calculated as follows:

11 ( ) / 2 C E E E    3-5
The experimental and predicted Young's modulus of the GF/epoxy composites with CNT-Al 2 O 3 hybrids (0.5 wt.%) are listed in Table 3-2. As shown in 3-2, the Young's modulus of the GF/CNT-Al 2 O 3 /epoxy multiscale composites is obviously larger than that of the CNT-Al 2 O 3 /epoxy composite matrix, showing the reinforcing effect of the glass fibers. Noticeably, the predicted Young's modulus of the composites is a little higher than the experimental values. Thus, the crimp of the yarns has less influence on the longitudinal modulus. However, it is lower than the predicted value of the longitudinal modulus.

11

E E  E 
Chapter ІІІ

The multiscale glass fabric/epoxy composites -147 -

Thermomechanical properties of the glass fabric reinforced epoxy composites

The influence of matrix modification by the reinforcements on the thermo-mechanical performance of the glass fiber/epoxy composites laminates was investigated via dynamic mechanical (DMA). The variation of the storage modulus and loss tangent of the composites with and without fillers as a function of temperature was plotted in Fig. 3-14a and b, respectively. The temperature corresponding to the peak value of tan δ is generally considered as glass transition temperature (T g ) of the composites, which is the characteristic temperature signifying the motion of polymer chains. As shown in Fig. 3-14a, by modifying the epoxy matrix, the storage modulus of the laminates was increased both in the glassy and rubbery regions in comparison to the control samples, but it demonstrates much stronger improvement in the glassy than in the rubbery region. The addition of low concentrations of CNT-Al 2 O 3 hybrids has improved the elastic properties of the composites at elevated temperatures. This behavior can be contributed to the enhanced CNTs dispersion in epoxy and improved interfacial interactions between CNTs and the epoxy matrix. This interfacial interaction reduces the mobility of the epoxy matrix around the CNTs and results in the observed increase in thermal stability. Besides, the T g was also found to shift toward higher temperature, suggesting improved thermo-stabilities. This behavior can be ascribed to the effects of interfacial interactions and entanglements between the fillers and epoxy matrix. In the glassy region, the presence of fillers will influence the alignment of the chain segment and molecular chains and thus restrict their movement, resulting in stronger amplitude variation of the elastic properties of the composites. However, above the glass transition temperature, the molecular chain shows intense movement and viscous state. In this case, some polymer molecular could not come into contact with the fillers, decreasing shear force between them. Thus, a relatively slight enhancement of storage modulus was observed in the rubbery region. Chapter ІІІ

The multiscale glass fabric/epoxy composites -149 -

The gain in thermo-stability could be explained by the fact that the augmentation of cross-linking density resulted from the addition of particles requires a higher temperature to provide the necessary thermal energy to induce the glass transition in the composites.

Moreover, the T g is considered to be sensitive to interfacial interactions between the matrix This is equivalent to a wetting polymer-particle interface, which is proposed to possibly influence the T g of the composites. Thus, the significant reinforcement of the mechanical properties is primarily contributed to the homogenous dispersion of the hybrids and the strong interfacial interaction between the fillers and matrix.

Concluding remarks

In this part, the hybrid multiscale composite laminates based on the plain woven glass fabric and particle reinforcements modified epoxy matrix were fabricated. The influence of particle reinforcements on improving the flexural and thermomechanical properties of the glass fabric/epoxy laminates was investigated with respect to the unmodified composites.

It is worth noting that the multiscale composite incorporated with 0. With the addition of 0.5 wt.% CNT-Al 2 O 3 hybrids, ac conductivities of the composites at f=10 3 Hz are up to values on the order of 10 -5 ~ 10 -6 S/m and 3 ~ 4 orders of magnitude higher than that of the reference samples for both in-plane and through-thickness directions, indicating the effect of matrix modification by the nanofillers on the electrical properties of the laminates. It is worth noting that, for all the composites with or without nanofillers, the through-thickness conductivity is turned out to be lower than that in the in-plane direction.

This anisotropy observed in the composites is due to the differences between the resistivity of the epoxy and glass fibers and the geometry of the glass fibers which form a continuous path in the laminate plane whereas discontinuous path across ply interfaces.

As multifunctional reinforcements, the CNT-Al 2 O 3 hybrids could be utilized as in situ damage sensor to monitor the damage evolution of the glass fabric/epoxy composites when they are subjected to uniaxial tensile loading. With the increase of strain, the in situ electrical resistance increases all the time until ultimate failure and it can be classified into three stages according to the different slopes of the resistance curve, which implies the different damage mechanisms in the three stages. When < 1 , the matrix-dominated damages such as the microcracks or flaws are initiated in the region between the glass fiber and matrix. When cracking and interfacial debonding contribute to the increased steep growth of the resistance curve. In the ІІІ stage, the interfacial debonding, interply delamination and fiber breakage along the strain direction will take place. However, Poisson contraction of the matrix and orientation of the CNT-Al 2 O 3 hybrids facilitate the reformation of the conductive networks, leading to the slight increase of the resistance in this stage.

General conclusions and perspectives

General conclusions

This thesis is focused on developing the CNT-microparticle multiscale hybrids with controllable CNT architecture to replace the pristine CNTs as superior multifunctional fillers in the composites. The mechanical, electrical and piezoresistive properties of these hybrids based epoxy nanocomposites and fiber-reinforced composite laminates were investigated in detail. The main results can be concluded as follows:

І. The CNT-microparticle hybrids reinforced epoxy nanocomposites

Three kinds of nano/micro multiscale hybrids with CNTs in situ grown on the SiC microplates, graphene nanoplatelets and Al 2 O 3 microparticles, i.e., CNT-SiC, CNT-GNP and CNT-Al 2 O 3 were used for preparing the epoxy nanocomposites using calendaring method.

The CNT-microparticle hybrids can serve as high-performance structural reinforcements.

With respect to the neat epoxy, significantly improved mechanical properties in the CNT-GNP filled nanocomposites were obtained at ultralow hybrid loading (0.5 wt.%). The tensile modulus exhibited up to 40% increase and the tensile strength was enhanced by ~36%. It was found that the optimal concentration of the hybrids (f c ) is necessary to achieve the desirable reinforcement effect.

Besides, the CNT-microparticle hybrids can be used as in situ sensors. The hybrids filled epoxy nanocomposites also exhibit distinctive self-sensing behaviors for in situ monitoring the onset of irreversibly permanent deformation. The in situ resistance initially increases to its maximum value and then begins to decrease with the appearance of residual strain and irreversible deformation, which is different from the randomly distributed CNTs filled composites only with monotonic increase of the resistance until their ultimate fracture.

The relationship between hybrid structure (CNT aspect ratio, orientation and substrate morphology) and the mechanical and self-sensing properties was investigated. For the same aspect ratio (1200), the in-situ electrical resistance of CNT-SiC/epoxy and CNT-GNP/epoxy composites increases when < c and then decreases when > c , which is different from that of the CNT-Al 2 O 3 /epoxy composites only with monotonic increase. Besides, the self-sensing and mechanical properties of the composites are also strongly dependent on the CNT aspect By introduction of 0.5 wt.% CNT-Al 2 O 3 hybrids, ac conductivities of the composites at f=10 3 Hz are up to values on the order of 10 -5 ~ 10 -6 S/m and 3 ~ 4 orders of magnitude higher than that of the reference composites in the in-plane and through-thickness directions.

It was found that the through-thickness conductivity is lower than the in-plane value.

The CNT-Al 2 O 3 multifunctional reinforcements can be utilized as in situ damage sensor to monitor damage evolution in the glass fabric reinforced laminates during tensile loading.

With the increase of strain, the in situ electrical resistance always increases until the ultimate failure. According to its different slopes, the electrical resistance curve can be classified into three stages, which implies the different damage mechanisms in the three stages. When < 1 (І), the matrix-dominated damages such as the microcracks or flaws are initiated in the region between the glass fiber and matrix. In the ІІ stage ( 1 < < 2 ), the increased steep growth of the curve are mainly contributed to the accumulation of the matrix-dominated damage, the fiber/matrix debonding and the transverse cracking. When enter into ІІІ stage ( > 2 ), besides the interfacial debonding and tans-ply cracking, the fiber breakage along the strain direction also take place. However, Poisson contraction of the epoxy matrix and the reorientation of the CNT-Al 2 O 3 hybrids possibly facilitate the reformation of the conductive networks, resulting in the slight increase of the resistance in this stage.

Perspectives

1. For the CNT-microparticle hybrids filled epoxy nanocomposites, the hybrid morphology (CNT aspect ratio, orientation and the substrate structure) has great effect on their in-situ sensing behaviors. However, establishing the correlation between the hybrid morphology and the sensitivity of the self-sensing composites are still interesting issues.

2. According to some existing models and empirical equations, the tensile modulus of the epoxy nanocomposites filled with the hybrids was calculated, which are generally lower than the experimental results. The modified equations used for calculation of the tensile modulus of the hybrids as reinforcements are needed.

3. The CNT-microparticle hybrids can be used as in situ damage sensor in the composites.

Besides, the measurements of the in situ resistance during loading, it is also necessary for the in situ observation of the crack propagation and the hybrid movement or orientation by microscopes. This is a challenging for the non-transparent composites with CNTs.

4. The introduction of the CNT-microparticle hybrid plays an important role on toughening the epoxy matrix. The improved mechanical properties were achieved in the multiscale glass fabric/epoxy composites with the hybrids. F urther research on the influence of the hybrids on the fiber/epoxy interface and how to improve the interfacial properties is also needed.

Fig. 1 - 1

 11 Fig. 1-1 TEM images of (a) SWNT and (b) MWNT.

Fig. 1 - 2

 12 Fig. 1-2 Schematic diagram showing how a hexagonal sheet of graphite is 'rolled' to form a carbon nanotube and illustrations of the three chiralities, A: 'armchair', B: 'zigzag' and C: 'chiral'.

Fig. 1 - 3

 13 Fig. 1-3 Schematic diagrams for nanotube sensors. (a) A single nanotube field-effect transistor; (b) A multiple nanotube interdigitated electrode device; (c) A multiple nanotube resonant-circuit sensor.

Fig. 1 - 4

 14 Fig. 1-4 Large amounts of VACNT array grown at 800o C on spheres with ethylene as carbon source for 2 h; (b) SEM of the VACNTs in (a).[START_REF] Zhang | Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres[END_REF]

Fig. 1 - 6 Fig. 1 - 7

 1617 Fig. 1-6 Illustration of the hybrid composite (A) Schematic illustration of the architecture composed of a cloth containing fiber tows, covered by CNTs. The two different plies are shown in two different colors;(B) Closer view of the interface cross-section between the two composite plies. The CNTs grown on the surface of each individual fiber.[START_REF] Garcia | Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ[END_REF]

Fig. 1 - 8

 18 Fig.1-8CNT morphology grown on the surface of carbon fiber (T650-35, unsized) as a function of growth time.[START_REF] Zhang | Hierarchical composites of carbon nanotubes on carbon fiber: Influence of growth condition on fiber tensile properties[END_REF]

Fig. 1 -Fig. 1 -

 11 Fig. 1-9 (A) Schematic of the procedure for the preparation of CNT-clay hybrids; [56] (B) Schematic of the CNT growth by liquid injection CVD.

Fig. 1 -

 1 Fig.1-11 Dispersion of CNTs in nanocomposites fabricated using different dispersion techniques (A : sonication in water bath; B: shear mixing; C: probe sonicator; D: Calendering).

Fig. 1 -

 1 Fig. 1-12 (a) DC volume conductivity vs. SWζT weight fraction of epoxy/SWζT composite; (b) Young's modulus vs. SWNT weight fraction of epoxy/SWNT composite. (A) Along the alignment of SWNT and (B)perpendicular to the alignment of SWNT.[START_REF] Wang | The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites[END_REF]

Fig. 1 -

 1 Fig. 1-13 Schematics of CNT functionalization using non-covalent methods (A) polymer wrapping; (B)surfactant adsorption.[START_REF] Hirsch | Functionalization of single-walled carbon nanotubes[END_REF]

Fig. 1 -

 1 Fig. 1-14 (A) Typical stress-strain curves of neat nylon-6 and CNT-clay hybrid composites (B) The microcrack linked by MWNTs sheathed with polymer showing the strong interfacial interaction between

Fig. 1 -

 1 Fig. 1-15 (a) Calendering (Three roll mill) machine used for particle dispersion; (b) Schematic of the general configuration and its working mechanism.

Fig. 1 -

 1 Fig. 1-16 ζanocomposite structure development after processing at different gap settings (a) 50 μm, (b) 20 μm, (c) 10 μm and (d) 5 μm [39].

Fig. 1 -

 1 Fig. 1-17 (a) Schematic of the electrospinning setup; (b) the obtained composite nanofiber.

Fig. 1 -

 1 Fig. 1-18 The experimental data of (a) Young's modulus and (b) tensile strength fitted by the modified

Fig. 1 -

 1 Fig. 1-19 SEM images of the fracture surface of the composites with (a) SiC microparticle; (b) CNT-SiC hybrids; (c) The typical stress-strain curves of the SiC and CNT-SiC hybrids reinforced composites.[START_REF] Ci | Novel micro/nanoscale hybrid reinforcement: multiwalled carbon nanotubes on SiC particles[END_REF]

Fig. 1 -

 1 Fig. 1-21 (a) SEM micrograph of the multi-directions SiC-CNT hybrids; (b) Dielectric permittivity and loss tangent of the SiC-CNT/PVDF composites as a function of f CNT , measured at 100 Hz and at roomtemperature.[START_REF] Yuan | High dielectric permittivity and low percolation threshold in polymer composites based on SiC-carbon nanotubes micro/nano hybrid[END_REF]

Fig. 1 -

 1 Fig. 1-22 Fiber reinforced multiscale hybrid polymer composites with CNTs (a) dispersed in the matrix;

Fig. 1 -

 1 Fig. 1-23Electrical anisotropy in carbon nanotube/fiber hybrid composites.[START_REF] Thostenson | Electrical anisotropy in multiscale nanotube/fiber hybrid composites[END_REF]

Fig. 1 -

 1 Fig. 1-24 Schematics of (a) CNTs evenly distributed in the matrix using the three-roll mill process; (b) CNT agglomeration on the surface of sized fibers; (c) Comparative study of electrical conductivity of three roll milled CNT/fiber/epoxy, fiber/sizing agent (sized once)/epoxy, fiber/sizing agent (sized twice)/epoxy, and fiber/epoxy composites.[START_REF] Gao | Highly conductive polymer composites based on controlled agglomeration of carbon nanotubes[END_REF]

  interfacial shear strength (IFSS) from 65 to 135 MPa has been identified by single fiber pull-out tests for the micro-droplet composite with CNT-grafted carbon fiber.Wardle et al.[START_REF] Garcia | Joining prepreg composite interfaces with aligned carbon nanotubes[END_REF] utilized vertically aligned CNT arrays transfer-printed the surfaces of prepreg plies to modify the interlaminar properties of carbon fiber reinforced composites.The improvement of both Mode I and Mode II fracture toughness was observed, which was attributed to the CNT bridging of interlaminar cracks (Fig.1-25).

Fig. 1 -

 1 Fig. 1-25 Illustration of the ideal hybrid interlaminar architecture: (A) VACNTs placed in between two plies of a laminated composite; and (B) close-up of the crack, showing VACNTs bridging the crackbetween the two plies. Illustrations are not to scale.[START_REF] Garcia | Joining prepreg composite interfaces with aligned carbon nanotubes[END_REF]

Fig. 1 -Fig. 1 -

 11 Fig. 1-26 Schematic diagram of the steps involved in the hierarchical nanomanufacturing of a laminated composite. (1) Aligned nanotubes grown on the fiber cloth; (2) Stacking of matrix-infiltrated CNT-grown fiber cloth; (3) Nanocomposite laminates fabrication by hand lay-up.[START_REF] Veedu | Multifunctional composites using reinforced laminae with carbon-nanotube forests[END_REF]

Fig. 1 -

 1 Fig. 1-28 (a) Three-dimensional model showing the penetration of nanotubes throughout a fiber array due to their relative scale;[START_REF] Li | Sensors and actuators based on carbon nanotubes and their composites: a review[END_REF] (b) Load-displacement and resistance curves for the 0/90 specimen.[START_REF] Thostenson | Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self healing[END_REF]

Fig. 1 -

 1 Fig.1-29 SEM images of (a) formation of microscale transverse cracks and (b) MWNTs protruding fromthe matrix in the region between two glass fibers.[START_REF] Thostenson | Real-time in situ sensing of damage evolution in advanced fiber composites using carbon nanotube networks[END_REF]

Fig. 2 - 1

 21 Fig. 2-1 SEM micrographs of (a) Al 2 O 3 microparticle, (b) SiC microplates, (c) GNP and (d) randomly distributed CNTs.

  the rolls was used according to the size of every kind of fillers. The induction of high shear forces allowed the separation of the entangled CNTs and their uniform dispersion in the epoxy matrix. The parameters of the three-roll mill used for preparing the composites are listed in Table2-1. After the calendering procedure, the suspension was collected and then the curing agent was added to the obtained suspension by stirring at mass ratio of 1/3 to the epoxy resin, according to the manufacturer's recommendations. The resulting mixture was degassed for 30 min to eliminate the entrapped air and then injected into the mold. The composites were cured at 60 o C under 10 MPa for 2 h, and then at 60 o C for 15h under atmospheric pressure. Dumbbell-shape samples with ~ 1 mm in thickness, ~50 mm in length and ~4 mm in gage width were obtained and edge-polished for the mechanical testing. The schematic in Fig.2-2 depicts the detailed procedure used for the composite preparation.

Fig. 2 - 2

 22 Fig.2-2 Schematic of the preparation process of the epoxy nanocomposites

  analysis (DMA). Dynamic mechanical analyzer (Netch 242C, Germany) was employed for the tests. The samples with dimensions ~ 60 mm in length and ~ 10 mm in width were tested under a three point bending mode from 30 o C to 180 o C at a heating rate of 3 o C /min and a frequency of 1.0 Hz in nitrogen atmosphere. The storage modulus (E'), loss modulus (E'') and loss tangent (tan ) were determined as a function of the temperature. Between three to five samples for each kind of the composites were tested. Their storage modulus and glass transition temperature achieved from the DMA curves are used to evaluate the viscoelastic and damping properties of the composites.

Fig. 2 - 3

 23 Fig.2-3 Schematic of the testing system for in situ sensing of the materials.

2 O 3 microparticles (Fig. 2 -

 232 4c ~ g), due to their specific crystallographic structures. It is worth underlining that the flake structure of SiC and GNP makes the CNTs mainly aligned on their two opposite sides, differing from the six-branch organization on the spherical Al 2 O 3 . The hybrids with varied CNT lengths were obtained by adjusting the CVD cond itions, such as the growth temperature, time and hydrogen ratio, etc. Therefore, the CNT distribution and arrangement patterns in the hybrids are dependent on the geometry and microstructure of the substrates. The basic information of the various hybrids used is listed in Table2-2. The aspect ratios (AR) of the CNTs are defined as the ratio of their average length (l) to the diameter (d), i.e., AR=l/d.

Fig. 2 - 4

 24 Fig.2-4 SEM micrographs of the various CNT-microparticle hybrids (a) and (b) CNT-SiC hybrids with CNT aspect ratio (AR1200); (c) ~ (f) CNT-Al 2 O 3 hybrids with different CNT aspect ratios; (c) AR500, (d) AR1200, (e) AR2000, (f) AR3200; (g) the mixture of CNT-SiC and CNT-Al 2 O 3 with CNT AR1200 (h) CNT-GNP hybrids with CNT AR1200.

  -5a, most of the tortuous CNTs are individually separated and randomly distributed within the epoxy matrix. Fig. 2-5b ~ h demonstrates the SEM micrographs of the fractured surfaces of the epoxy composites with different hybrids. It can be seen that the hybrids are well dispersed in the epoxy matrix and their original architectures are almost retained, which endows controlled CNT layout. By comparing Fig. 2-5d ~ g, one can see that the dispersion state of CNT-Al 2 O 3 hybrids in the composites depends on the CNT aspect ratio. In the case of relatively short CNTs, Al 2 O 3 microparticles are surrounded in the center by the CNTs and the hybrid structure is symmetric and isotropic (Fig. 2-5d ~ f). However, for too long CNTs, Al 2 O 3 particles are wrapped at the end by CNTs and the hybrids exist like bundles in the composites (Fig. 2-5g).

Fig. 2 - 5

 25 Fig.2-5 SEM micrographs of the fracture surfaces of epoxy composites based on different fillers (a) CNT/epoxy composites (b) and (c) CNT-SiC/epoxy composites with aspect ratio (1200); (d) ~ (g) CNT-Al 2 O 3 /epoxy composites with different aspect ratios, (d) AR500; (e) AR1200; (f) AR2000; (g) AR3200; (h) CNT-GNP/epoxy composites (AR1200).

Fig. 2 -

 2 Fig. 2-6 shows the frequency dependence of the ac conductivities measured at room temperature of the epoxy composites containing different volume fractions of the conductive fillers, which indicates the overall connectivity of conducting networks in all the composites.With regard to the randomly distributed CNTs or the hybrids filled epoxy nanocomposites, it is obvious that almost three different behaviors could be identified with increasing the filler content. When only a small amount of the conductive fillers is incorporated into the epoxy matrix, they are considered to be isolated to each other and the ac conductivities of the composites increased almost linearly as the frequency increases from 10 2 Hz to 10 7 Hz. That

Fig. 2 - 6

 26 Fig.2-6 Electrical properties of the epoxy composites based on different fillers: (a) randomly orientated CNTs; (b) CNT-SiC hybrids with CNT AR1200; (c) ~ (f) CNT-Al 2 O 3 hybrids with different CNT aspect ratios, (c) AR500; (d) AR1200; (e) AR2000; (f) AR3200; (g) CNT-GNP hybrids with CNT AR1200.

Fig. 2 - 7

 27 Fig.2-7 (a) The influence of CNT aspect ratio on the conductivity of the composites with different hybrid concentrations at f=10 3 Hz; (b) The influence of CNT aspect ratio on the conductivity of the composites with the same hybrid concentration (0.5 wt.%) at f=10 3 Hz.

Fig. 2 - 8

 28 Fig.2-8 Thermogravimetric analysis (TGA) of the various CNT-microparticle nano/micro hybrids.

Fig. 2 -

 2 Fig.2-9illustrates the typical stress-strain response of the baseline epoxy and their composites with the same concentration (0.5 wt.%) of CNTs, GNPs, CNT+GNP mixture and CNT-GNP hybrids under quasi-static tensile loading. Introducing such small amount of these reinforcements, the mechanical behavior of the composites apparently shows different levels of enhancing trend compared to that of baseline epoxy. It is obvious that the elastic modulus and ultimate tensile strength (UTS) of the CNT-GNP/epoxy nanocomposites are dramatically increased compared to the composites with other kinds of fillers.

Fig. 2 - 9

 29 Fig.2-9 Tensile properties of baseline epoxy and the epoxy composites with 0.5 wt.% reinforcements (a) Typical strain-stress curves; (b) Tensile modulus; (c) Ultimate tensile strength; (d) Fracture strain.

-10. Fig. 2 -

 2 [START_REF] Ebbesen | Electrical conductivity of individual carbon nanotubes[END_REF] schematically depicts the distribution of the reinforcements in the epoxy matrix. Most of the tortuous CNTs are individually separated and randomly distributed within the epoxy matrix (see Fig.2-10a). As shown in Fig.2-10b, some GNPs are non-uniformly dispersed and aggregated in the matrix. It indicates that CNTs are much easier to be dispersed than

Fig. 2 -

 2 Fig.2-10 SEM micrographs of the fractured surface of the epoxy composites with 0.5 wt.% reinforcements (a) CNTs; (b) GNPs; (c) CNT+GNP mixture.

Fig. 2 -

 2 Fig.2-11 Schematic images of the reinforcement dispersion in the epoxy composites (a) CNTs; (b) GNPs; (c) CNT+GNP mixture; (d) CNT-GNP hybrids.

Fig. 2 -

 2 Fig. 2-12 displays the influence of CNT-GNP hybrid concentration on the mechanical performances of epoxy nanocomposites. Fig. 2-12a depicts the typical stress-strain curves of the baseline epoxy and their composites with varying weight fractions of CNT-GNP hybrids under quasi-static tensile loading. The curves show evident non-linear relationship between

Fig. 2 -

 2 Fig.2-12 Tensile properties of baseline epoxy and the epoxy composites with varying weight fractions of CNT-GNP hybrids (AR1200), (a) Typical strain-stress curves; (b) Tensile modulus; (c) Tensile strength; (d) Fracture strain.

Fig. 2 -

 2 Fig.2-13 shows the influence of CNT-SiC concentration on the tensile properties of the CNT-SiC/epoxy nanocomposites. The typical stress-strain response of the pristine epoxy and their composites containing 0.2, 0.6, 0.7, 0.9, 1.0 wt.% CNT-SiC reinforcements are depicted in Fig.2-13a. The relationship between the tensile modulus, tensile strength, fracture strain of the composites and the hybrid concentration are plotted in Fig.2-13b ~ d. When f CNT-SiC < 0.7 wt.%, the strengthening effect is not remarkable at low CNT-SiC concentration. With increasing the CNT-SiC content, the tensile modulus and strength consistently increase.When f CNT-SiC is close to 0.7 wt.%, the reinforcement effect of the CNT-SiC hybrids reaches the maximum value. The average tensile modulus rises from ~ 2.16 GPa of neat epoxy to ~ 2.64 GPa of the 0.7 wt.% CNT-SiC reinforced epoxy composite, namely, ~ 22% increment.

Fig. 2 -

 2 Fig.2-13 Tensile properties of baseline epoxy and the epoxy composites with varying fractions of CNT-SiC hybrids (AR1200), (a) Typical strain-stress curves; (b) Tensile modulus; (c) Tensile strength; (d) Fracture strain.

Fig. 2 -

 2 Fig. 2-14 shows the influence of the concentration of CNT-Al 2 O 3 hybrids with different aspect ratios (500, 1200, 2000 and 3200) on mechanical properties of the epoxy composites.From the typical stress-strain curves, we can see that the whole variation trend of the tensile modulus and strength as function of the CNT-Al 2 O 3 content in the four systems is nearly the same as that of the CNT-GNP/epoxy and CNT-SiC/epoxy systems discussed above. The optimal loading of CNT-Al 2 O 3 hybrids is found to exist in every system. And it is very close to the percolation threshold f c mentioned above. Before f c , the tensile modulus and strength continuously increase with the embedding of the hybrids. As f hybrid approaches f c , the results indicate that they approximately reach the maximum values. However, when f hybrid > f c , they begin to degrade with the hybrid loading. Moreover, as shown in Fig.2-14, the composites based on the CNT-Al 2 O 3 hybrids (AR500) exhibit more favorable ductility and toughness, compared to the other three systems with larger CNT aspect ratios (AR1200, 2000, 3200). A slight increase in the strain to failure was also observed in the CNT-Al 2 O 3 (AR500) filled epoxy composites at 1 wt. % loading, compared to the neat epoxy. This phenomenon may be mainly contributed to their different dispersion states in the matrix, as illustrated in Fig.2-5d~ g.

Fig. 2 -

 2 Fig.2-14 Typical strain-stress curves of baseline epoxy and the epoxy composites reinforced with varying weight fractions of CNT-Al 2 O 3 hybrids with different CNT aspect ratios, (a) AR500; (b) AR1200; (c) AR2000; (d) AR3200.

Fig. 2 -

 2 Fig.2-15 Comparison of the mechanical properties of the CNT-Al 2 O 3 /epoxy composites with different CNT aspect ratios when ƒ hybrid = ƒ c (a) Typical strain-stress curves; (b) Tensile modulus; (c) Tensile strength.

  -16a and b, E′ and loss factor (tan ) were plotted as a function of temperature for baseline epoxy and their composites with the same weight fraction (0.5%) of CNTs, GNPs, CNT+GNP mixture and CNT-GNP hybrids. At low temperature all the samples show very high value of the elastic modulus, followed by one significant drop in the range of 60 o C to 100 o C due to the glass transition. In this temperature range the corresponding tan δ curves also display a peak, indicating the glass transition temperature of the materials. It is clearly seen that the value of E′ shows varying degrees of growth with the addition of these reinforcements. It is worth noting that, compared to other kinds of reinforcements, the introduction of CNT-GNP hybrids results in a more evident increment of E′ both in the glassy and rubbery region as well as significant shift of glass transition of the CNT-GNP/epoxy composites. As shown in Fig.2-16c, the value of E′ for pure epoxy at 40 o C is about 3.7 GPa which increases up to ~5.0 GPa for the CNT-GNP/epoxy composites,

Fig. 2 -

 2 Fig. 2-16 Thermomechanical analysis of the pure epoxy and epoxy composites from DMA results (a) Storage modulus vs. temperature; (b) Loss factor (tan ) vs. temperature; (c) Enhancement of the storage modulus compared to pure epoxy at 40 o C;(d) Glass transition temperature (T g ).

  2 O 3 hybrids reinforced epoxy composites. The variation of E′ and tan δ as a function of temperature in dynamic measurements is presented in Fig. 2-17a and b. Fig. 2-17c and d shows the mean E′ enhancement and glass transition temperature obtained from the DMA experiments. These results indicate that E′ of the CNT-Al 2 O 3 /epoxy composites are increased substantially relative to neat epoxy, and T g are shifted to higher temperature.

Fig. 2 -

 2 Fig. 2-17 Influence of CNT aspect ratio on the thermomechanical properties of the CNT-Al 2 O 3 /epoxy composites (a) Storage modulus vs. temperature; (b) Loss factor (tan ) vs. temperature; (c) Enhancement of the storage modulus compared to pure epoxy at 35 o C;(d) Glass transition temperature (T g ).

Fig. 2 -

 2 Fig. 2-18 ~ 2-20 demonstrates the typical stress-strain curves (solid lines) and the in-situ electrical resistance responses (dotted lines) of the epoxy composites with different fillers during quasi-static tensile loading until final fracture. Here, the filler concentration in each

Fig. 2 -Fig. 2 -

 22 Fig. 2-18a and b demonstrate the self-sensing behavior of the composites containing the ran-CNTs and mechanical mixture of ran-CNTs and microparticles (Al 2 O 3 , SiC and GNP), respectively.

Fig. 2 -Fig. 2 -

 22 Fig. 2-19 Self-sensing behavior of the composites containing different contents of the hybrid under quasi-static tensile loading, (a) CNT-GNP/epoxy; (b) CNT-SiC/epoxy composites.

Fig. 2 -

 2 Fig. 2-20 Self-sensing behavior of the composites containing different concentrations of the CNT-Al 2 O 3 hybrids with different CNT aspect ratios under quasi-static tensile loading; (a) AR500, (b) AR1200, (c) AR2000, (d) AR3200.

Fig. 2 -

 2 Fig. 2-21 shows the detailed research about characteristic stress ~ strain curve and the in situ sensing behavior of the composite containing 8 wt.% randomly distributed CNTs. A

Fig. 2 -

 2 Fig. 2-21 In situ sensing behavior of the epoxy composite incorporated with 8 wt.% of randomly distributed CNTs; (a) The stress-strain and electrical resistance-change response curves of the composite under quasi-static, (b) and (c) Incremental cyclic loading with transient stress, strain, and resistance response versus time.

Fig. 2 -

 2 22b and c shows an example of the transient stress, strain and electrical resistance response as function of time under cyclic tensile loading. During the first three loading and unloading cycles, the transient resistance change closely follows the response of applied stress and strain. The in situ resistance increases and decreases monotonically with the cyclic stress and strain. That is because the reversible deformation induces the break-up and intact reformation of the conductive networks with the open and closing of microcracks during tensile loading.

Fig. 2 -

 2 Fig. 2-22 In situ sensing behavior of the epoxy composites with 0.5 wt.% CNT-GNP hybrids: (a) Resistance change and the corresponding stress as function of strain under quasi-static loading, (a) Cyclic loading with transient stress, strain, and resistance response versus time; (b) Incremental cyclic loading and the corresponding resistance change.

  -22a where ΔR/R o begins to decrease. The subsequent cycles show more evident deviation of the resistance curve to the strain response with the appearance of larger permanent resistance change and residual strain. Additionally, the amplitude of ΔR/Ro begins to decrease in the fifth cycle. In order to gain an insight into the general tendency of resistance change with different applied stress, Fig.2-22cillustrates the stress and in situ resistance response of one sample undergoing cyclic loading with a progressively increasing step load. In the first fifth cycles, there is larger resistance change in the latter cycle as compared to the previous cycle upon re-loading and the resistance change is relatively stable in each cycle. In this case, the resistance increase in the following cycles may be attributed to the formation of new micocracks which break apart the electrically percolating networks in the matrix. However, the resistance change begins to decrease and becomes unstable from the sixth re-loading cycle. Besides, the permanent resistance change appears in the unloaded state after the sixth cycle. The stress in the sixth cycle ( max ) is very close to the stress corresponded to c in Fig.2-22a. Consequently, from these results one can see that the specimen experiences reversible elastic deformation during the first fifth cycle and the irreversible plastic deformation is initiated from the sixth cycle. The decrease of resistance change may be attributed to the reformation of new conductive networks induced by the orientation of CNT bundles and GNP substrate in the permanent deformation region.Based on the results above, we conclude that the onset of in situ resistance decrease can be used to identify the irreversible deformation onset in the CNT-GNP/epoxy composite, where the permanent resistance change and residual strain also begin to appear. The distinctive architecture of CNT-GNP hybrids endows possibility of the reformation of conductive networks with the emergence of irreversible defo rmation. As a result, the CNT-GNP hybrids as potential self-sensing materials could open up new opportunities in the field of smart sensing and structural health monitoring.

Fig. 2 -

 2 Fig. 2-23 In situ sensing behavior of the epoxy composite with 0.7 wt.% CNT-SiC hybrids; (a) The stress-strain and electrical resistance-change response curves of the composite under quasi-static, (b) and (c) Cyclic loading with transient stress, strain, and resistance response versus time.

Fig. 2 -

 2 Fig. 2-24 In situ sensing behavior of the epoxy composite with 0.5 wt.% CNT-Al 2 O 3 hybrids (AR2000); (a) The stress-strain and electrical resistance-change response curves of the composite under quasi-static, (b) and (c) Cyclic loading with transient stress, strain, and resistance response versus time.

2 -

 2 24a where the ΔR/R o begins to decrease. In addition, the critical strain values in each system are almost close to each other, as shown in Fig. 2-22a and Fig. 2-23a. Hence, one can say that the reversible elastic deformation mainly occurs in the composites when < c , and then the irreversible deformation or permanent damage is initiated when > c . The onset of irreversible deformation in the composite co uld be identified by the onset of the decrease of in-situ electrical resistance where the permanent resistance change begins to appear.

Fig. 2 -

 2 25b displays the self-sensing behavior of the CNT-Al 2 O 3 /epoxy composites with different CNT aspect ratios. It is found that the response of in-situ electrical resistance also depends on the CNT aspect ratio. In the case of micro-cracks because of Poisson's contraction. There is a good dispersion of the vertically aligned CNTs and two neighboring CNTs grown on SiC are not overlapping yet still situated closely enough to allow electrical tunneling, which is different from the case of randomly orientated CNTs. Besides, Poisson's ratio effect and the intense movement of the polymer chain may result in the rotation of some CNT bundles grown on SiC along the stress direction (Fig.2-26 and Fig.2-27). This rotation of CNT bundle is different from that of randomly orientated CNTs because only free ends of them could be rotated and the other ends are attached to SiC microplate. Besides, the possible reorientation of the SiC microplate

Fig. 2 -

 2 Fig.2-26 Schematic of the evolution of conductive networks provided by the fillers during tensile loading (a) randomly distributed CNTs; (b) CNT-SiC or CNT-GNP hybrids; (c) ~ (f) CNT-Al 2 O 3 hybrids with different aspect ratios: (c) AR500; (d) AR1200; (e) AR2000; and (f) AR3200. The dotted line represents the conductive networks.

Fig. 2 -

 2 Fig. 2-27 Optical images of CNT-SiC hybrids during tensile loading.

  hybridizations with other materials, like nanosized metal or oxide particles, microsized ceramic particles, fullerenes and two dimensional graphene could control CNT distribution and arrangements in polymer matrix and exhibit superior multifunctional properties of the composites than the pristine CNTs. In this chapter, three kinds of well-organized hybrid structures are respectively achieved by in situ grafting multi-walled carbon nanotubes on SiC microplates, graphene nanoplatelets, micro-spherical Al 2 O 3 during CVD synthesis process, without any pre-patterned catalyst treatment. The CNT organization and distribution patterns on the substrates is strongly dependent on their diameter, length and area number density by controlling CNT growth temperature, time and hydrogen ratio, etc. The CNT-SiC, CNT-GNP and CNT-Al 2 O 3 nano/micro hybrids reinforced epoxy composites were fabricated using calendaring method. Compared to the randomly orientated CNTs filled composites, the more improved mechanical, electrical and obviously different self-sensing properties of the multiscale hybrids filled composites were comprehensively investigated. The relationship between the composite properties and hybrid structure (CNT aspect ratio, orientation and substrate morphology) has been well established. The major conclusions are summarized as follows:

  2 vol.% in the CNT-SiC composites.(ІІ) The epoxy composites with CNT-GNP, SiC-CNT or CNT-Al 2 O 3 hybrids could exhibit distinctive self-sensing behavior for in situ monitoring the onset of irreversibly permanent deformation. The in situ electrical resistance initially increases to its maximum value and then begins to decrease with the appearance of residual strain and irreversible deformation, which is remarkably different from the randomly oriented CNTs filled composites only with monotonic increase of the resistance until their catastrophic fracture.(ІІІ) The mechanical and self-sensing properties of the composites are closely related to the substrate morphology of the hybrids. For the same aspect ratio (1200), the in-situ electrical resistances of CNT-SiC/epoxy and CNT-GNP/epoxy composites increase when < c and then decreases when > c , which is different from that of the CNT-Al 2 O 3 /epoxy composites with monotonic increase until their ultimate failure. Moreover, the CNT-GNP/epoxy composite shows the most significant improvement of the tensile modulus and tensile strength compared to the baseline epoxy. Besides, the self-sensing and mechanical properties of the composites also strongly depend on the CNT aspect ratio and organization. With increasing AR up to 2000 or more, the change of in-situ electrical resistance of CNT-Al 2 O 3 filled composites becomes similar to that of the CNT-SiC/epoxy and CNT-GNP/epoxy composites. The tensile modulus and strength of CNT-Al 2 O 3 /epoxy composites increase with increasing AR from 500 to 2000, but then decrease at AR=3200.(ІV) For the reinforcing mechanism of these hybrids, the uniform distribution states of the hybrids in the matrix can restraint CNT agglomeration. Their three dimensional architectures give rise to larger contact surface and stronger molecular interaction between the CNTs and the matrix, resulting in an improved load transfer. The alignment of CNTs in the hybrids retains their actual lengths in the matrix, enhancing their reinforc ement efficiency. Besides, the strong junction between the CNTs and the substra tes makes CNT reorientation much more difficult under low strain, compared to the randomly distributed CNTs. This could explain the higher elastic modulus of the composites based on the hybrids.With regard to the underlying mechanisms of self-sensing properties of the composites, it is found that the evolution of conductive networks resulted from the rotation of the substrates and hybrids as well as the reorientation of CNTs during tensile loading plays an important role on the self-sensing behavior of the composites. And the CNT reorientation, dispersion state and substrate also have significant influence on the composite mechanical properties. Proper selection of hybrid structure (CNT aspect ratio, organization and substrate) is critical for achieving desired mechanical and self-sensing performances in the nano/micrometer hybrids reinforced composites.

  chemical resistance, wear resistance and low cost. In our work, the CNT-Al 2 O 3 hybrids with well-aligned CNTs forming six-orthogonal branches on the spherical Al 2 O 3 microparticles were prepared by chemical vapor deposition (CVD) process. The multiscale glass fabric (GF) reinforced epoxy (EP) composites (GF/EP) using the particles [randomly distributed CNTs (ran-CNT), ran-CNT+Al 2 O 3 mixture and CNT-Al 2 O 3 hybrids] were fabricated, respectively. Dynamic mechanical analysis and three point bending were performed for a comparative research regarding the influence of reinforcements on the flexural and thermo-mechanical properties of the glass fiber/epoxy composites. In situ electrical resistance measurements were conducted to monitor the damage states and evolution of the continuous glass fiber reinforced composites. The potential of the CNT-Al 2 O 3 hybrids as structural reinforcement and in situ damage sensor in the composites were analyzed.

126 -Fig. 3 - 1

 12631 Fig.3-1 Schematic of the preparation process of the glass fiber/epoxy laminates.

  Mechanical tests of the glass fabric/epoxy composites were performed on a universal testing machine (Instron 5000) equipped with a 25 KN load cell for the quasi-static tensile tests, and with a 5 kN one for the flexural testing. From each composite panel, the tensile and flexural specimens were cut using garnet-abrasive-assisted water-jet. The tensile tests were performed according to ASTM D3039 standard and the specimen dimensions were 160 mm in overall length and 12.7 mm in gage width. Dimensions of the specimens for tensile testing are shown in Fig.3-2. Toward determining the statistics of the modulus, strength and fracture strain of the composites, between three to five samples for each kind of composites are tested under the same condition.

Fig. 3 - 2

 32 Fig.3-2 Schematic of the glass fabric/epoxy specimens for tensile testing.

  .3.3. Thermo-mechanical characterizationThermo-mechanical properties of the glass fabric/epoxy composite were determined using dynamic mechanical analysis (DMA). It was conducted using a dynamic mechanical analyzer (DMA 242c, Germany) under three-point bending mode from 30 o C to 180 o C at a heating rate of 3 o C /min and a frequency of 1.0 Hz in nitrogen atmosphere. The samples with dimensions ~ 60 mm in length and ~ 10 mm in width were tested. The storage modulus (E'), loss modulus (E'') and the loss tangent (tan ) were determined as a function of the temperature. Between three to five samples for each kind of composites were tested. The maximum point on the tan curve was chosen as the glass transition temperature (T g ) of the composites. The storage modulus and glass transition temperature achieved from the DMA curves were used to evaluate the viscoelastic and damping properties of the composites.

  -3a. It can be seen that the well-aligned CNTs generally form into symmetric six-orthogonal branches on Chapter ІІІ The multiscale glass fabric/epoxy composites -129 -the surface of the spherical Al 2 O 3 particles, due to their specific crystallographic structures.

Fig. 3 - 3

 33 Fig.3-3 Morphologies of the reinforcements in the composite laminates (a) SEM image of the CNT-Al 2 O 3 hybrids; (b) Optical images of the glass fabric.

Fig. 3 -

 3 Fig. 3-4 shows the distribution state of the CNT-Al 2 O 3 hybrids in the epoxy matrix. The SEM observation reveals that the hybrids are well dispersed in the epoxy matrix and their original architectures are almost retained, endowing controlled CNT layout. And the Al 2 O 3 microparticles are surrounded in the center by the CNTs, demonstrating symmetric hybrid structure. Besides, most of the CNTs retain aligned in the epoxy matrix at their actual lengths instead of curved states, confirmed by the SEM micrograph at high magnification (Fig. 3-4c).

Fig. 3 - 4

 34 Fig.3-4 SEM images of the dispersion of CNT-Al 2 O 3 hybrids in the epoxy matrix at lower magnification (a) and (b); aligned CNTs grown on Al 2 O 3 at high magnification level (c).

Fig. 3 - 5

 35 Fig.3-5 AC electrical conductivities of the glass fabric/epoxy composite laminates with and without

3. 5 .Fig. 3 -

 53 Fig.3-6a demonstrates the typical electrical resistance change as function of the applied strain for the composite laminates during a quasi-static tensile test until ultimate failure. Fig.3-6b shows the crack density as a function of the applied strain. When the strain reaches a certain value, the samples were unloaded. After polish, the crack number was counted under SEM. For the non-transparent composites, the in situ observation of the crack propagation is a challenging. As shown in Fig.3-6a, it is clearly seen that the resistance increases all the time and it can be classified into three stages according to the different slopes of the resistance curves, which implies the varying damage mechanisms existed in the three stages.The macroscopic damage mechanisms of the laminates generally include the matrix micro-cracking, localized warp fiber fracture, weft fiber pull-out, and delamination along the middle plies. SEM and optical micrographs of the damaged specimens are shown in Fig.3-7.

Fig. 3 -

 3 Fig. 3-8 schematically illustrates the damage propagation in different stages of the laminates.

Fig. 3 - 6

 36 Fig. 3-6 Piezoresistive behavior of the multiscale glass fabric reinforced epoxy composites with CNT-Al 2 O 3 hybrids.

Fig. 3 - 7 137 -Fig. 3 - 8

 3713738 Fig. 3-7 Damage evolution in the multiscale glass fabric/epoxy/CNT-Al 2 O 3 composites during tensile loading (a) ~ (f) SEM images; (g) Optical microscopy.

Fig. 3 - 9

 39 Fig. 3-9 Comparison the in situ sensing behaviors of the CNT-Al 2 O 3 /epoxy composite matrix and the glass fabric/CNT-Al 2 O 3 /epoxy composite laminate.

Fig. 3 -

 3 Fig. 3-10 demonstrates the typical stress-strain curves of the glass fabric reinforced neat epoxy and the modified epoxy matrix with the same concentration (0.5 wt.%) of ran-CNTs, ran-CNT+Al 2 O 3 and CNT-Al 2 O 3 hybrids under three-point bending test. In comparison to

Fig. 3 -

 3 Fig. 3-10 Flexural properties of the glass fabric/epoxy composites with and without nanofillers (a) typical stress-strain curves; (b) and (c) the obtained experimental results of flexural modulus and strength.

Fig. 3 -

 3 Fig.3-11 SEM images of the surfaces of the glass fabric/epoxy composites (a) without nanofillers; (b) with ran-CNTs; (c) with CNT-Al 2 O 3 hybrids; (d) the enlarged image of Fig. 3-9c showing the toughened epoxy matrix by the CNT-Al 2 O 3 hybrids.

Fig. 3 -

 3 Fig.3-12 SEM images of the fracture surface under tensile failure of the glass fabric reinforced pure epoxy matrix (a); and CNT-Al 2 O 3 modified epoxy matrix (b).

  Fig.3-13b and c. In general, the tensile modulus and strength of the studied composites was not greatly affected by the addition of nanoreiforcements at the experimental contents. Only

Fig. 3 -

 3 Fig.3-13 Tensile properties of the glass fabric/epoxy composite laminates with and without nanofillers (a) typical stress-strain curves; (b), (c) the obtained experimental results of flexural modulus and strength;

Fig. 3 -

 3 Fig.3-14 Thermomechanical analysis of the glass fabric/epoxy/CNT-Al 2 O 3 composite laminates from DMA results (a) Storage modulus vs. temperature; (b) Loss factor (tan ) vs. temperature; (c) Glass transition temperature g ).

  and fillers. It is worth noting that the introduction of CNT-Al 2 O 3 hybrids gave rise to a more remarkable increment of storage modulus. The mean storage modulus values at 50 o C and 150 o C from the DMA results of the glass fiber/epoxy composite laminates are summarized in Table 3-3. The storage modulus of the glass fabric/epoxy multiscale composites with 0.5 wt.% CNT-Al 2 O 3 hybrids increases dramatically to 19.8 GPa (at 50 o C) in the glassy region, which is 20% larger than that (16.5 GPa) of the reference samples. In addition, one can see that there is more noticeable shift of T g by addition of CNT-Al 2 O 3 hybrids, compared to other fillers. The T g was shifted from ~ 98 o C for control samples to about 113 o C for thesamples containing 0.5 wt.% CNT-Al 2 O 3 hybrids. The addition of 0.5 wt.% CNT-Al 2 O 3 significantly improved the T g of the multiscale composites up to 25 o C. These results also indicate that the hybrid fillers are uniformly dispersed throughout the polymer matrix and have strong interfacial adhesion with the surrounding matrix. As discussed above in Fig. 3-4c, we can see that the CNTs are wrapped by the host polymer, suggesting the existence of a very thin polymer layer embedded into the interspaces between any two well-aligned CNTs.

  5 wt.% CNT-Al 2 O 3 hybrids showed maximum increment in the flexural modulus ~ 19% and flexural strength ~ 13% over that of the reference sample. Similar, from the DMA results, the samples exhibited considerable improvement of ~ 20% in storage modulus at 50 o C and an increase of 15 o C in glass transition temperature compared to the control samples. In this case, the efficient stress transfer from the CNT-Al 2 O 3 hybrids modified epoxy matrix to the glass fiber resulted from the strong fiber/matrix interfacial interaction.

  ratio and organization. With increasing AR to 2000 or more, the change of in-situ electrical resistance of CNT-Al 2 O 3 filled composites becomes similar to that of the CNT-SiC/epoxy and CNT-GNP/epoxy composites. The tensile modulus and strength of CNT-Al 2 O 3 /epoxy composites increase with increasing AR from 500 to 2000, but then decrease at AR=3200. With regard to the reinforcing mechanism of these hybrids, the well dispersed CNT is one important reason. The three dimensional architectures of the hybrids give rise to larger contact surface and stronger CNT/matrix interaction, resulting in improved load transfer. The alignment of CNTs in the hybrids retains their actual lengths in the matrix, enhancing their reinforcement efficiency. The underlying mechanisms of the self-sensing properties can be contributed to the evolution of conductive networks resulted from the rotation of substrates and the reorientation of CNTs during tensile loading. Proper selection of hybrid structure (CNT aspect ratio, organization and substrate) is critical for achieving desired mechanical and self-sensing performances in the nano/micrometer hybrids reinforced composites.ІІ. The multiscale glass fabric/epoxy laminates with CNT-microparticle hybridsThe effect of CNT-microparticle reinforcements on the flexural and thermomechanical properties of the glass fabric/epoxy composite laminates was investigated. It was found that the composites incorporated with 0.5 wt.% CNT-Al 2 O 3 hybrids exhibit ~ 19% increment in the flexural modulus and about 13% enhancement in the flexural strength over the reference sample. The composites also show considerable improvement of ~ 20% in storage modulus at 50 o C and an increase of 15 o C in glass transition temperature compared to the control samples. In this case, the efficient stress transfer from the CNT-Al 2 O 3 hybrids modified matrix to the glass fiber resulted from the strong fiber/matrix interfacial interaction.

Table 1 -

 1 1 summarizes the flexural properties of CNT/epoxy composites fabricated from the same CNT type and content and the same epoxy matrix using different techniques for CNT dispersion. It is worth noting that the multi-scale hybrids with CNTs grown on carbon fibers or on different ceramic particles, as discussed in the previous section. In this case, CNTs can be homogeneously dispersed at microscale in the matrix with the help of these microscale carbon fibers or ceramic ‗vehicle' because the microscale reinforcements can be much easily handled.

Table 1 - 1

 11

	Neat epoxy	3.43 ± 0.01 (+0.00%)	140.0 ± 2.3 (+0.00%)
	Sonication in water bath	3.41 ± 0.04 (-0.58%)	144.1 ± 1.8 (+2.93%)
	Shear mixing	3.36 ± 0.09 (-2.08%)	140.4 ± 2.4 (+0.29%)
	Probe sonication

Effect of CNT dispersion on the flexural properties of CNT/epoxy nanocomposites.[74]

CNT dispersion technique

Flexural modulus (GPa) Flexural strength (MPa) 3.47 ± 0.06 (+1.17%) 142.7 ± 1.7 (+1.93%) Calendering 3.65 ± 0.02 (+6.41%) 145.2 ± 0.82 (+3.71%)

. Multifunctional applications of CNT/Polymer composites

	materials are an emerging field, many studies are being made to devise new processing
	methods that can produce nanocomposites with unique structures and properties for specialty
	end applications.
	1.4It is known that nanostructure composites could produce and enhance multifunctionality
	in ways that the conventional composites could not. Multifunctional materials are necessarily
	composite materials. The extraordinary electric and thermal properties combined with their
	specific modulus and strength as well as their high aspect ratio has made nanotubes
	outstanding candidates for multifunctional nanofillers in composites, which have stimulated
	the development of CNT/polymer composites with the capability to serve as structural
	reinforcements, and a material platform for sensing, actuation, energy storage, electrostatic
	discharging, electromagnetic interference shielding, etc. The research paid attention on the
	CNT/polymer multifunctional materials has increased markedly in recent years. Baur et al.
	. In addition, Many factors, such as physical (solid
	or liquid) and chemical (thermoplastic or thermoset) states of polymer matrix, dimensions
	and content of CNTs to be added, alternative of the fabrication processes, should be taken
	into account when selecting a proper technique for CNT dispersion. There is no almighty
	tool to achieve perfect dispersion of different CNTs in various types of polymer matrices. In
	addition, to obtain CNT/polymer nanocomposites with a very high CNT content or for some
	specific applications, new methods have been developed including densification, spinning of
	coagulant, layer-by-layer deposition and pulverization. It should be noted that as composite

Table 2 - 1

 21 The parameters of three-roll mill for preparing the composites

	Epoxy composites	Rotation speed of rollers (r/min)	Rotation time (min)	Roll gap (μm)
	CNT+SiC (AR1200)

Table 2 - 2

 22 Basic information of the randomly distributed CNTs (rand-CNTs), the CNTs grown on the particles and

	the particle substrates
	Materials	CNT length (μm)/ diameter (nm)	Average CNT aspect ratio	CNT density (g/cm 3 )	Substrate density (g/cm 3 )
	rand-CNTs	(10~15	a )/(~10	a )	1200
	1 CNT-Al 2 O 3	(4~6)/(~10)		500
	2 CNT-Al 2 O 3	(10~15)/(~10)	1200	1.78	3.1
	3 CNT-Al 2 O 3	(18~22)/(~10)	2000
	4 CNT-Al 2 O 3	(30~35)/(~10)	3200
	CNT-SiC	(10~15)/(~10)	1200	1.78	3.9
	CNT-GNP	(10~15)/(~10)	1200	1.78	2.25

a a According to the values provided by the companies.

Table 2 - 3

 23 Percolation thresholds of the CNTs in the epoxy composites based on different hybrid fillers

	Materials	f hybrid (wt.%)	f CNT (wt.%)	f CNT (vol.%)
	Randomly distributed CNTs		7.92	5.05
	CNT-SiC (AR1200)	0.7	0.21	0.16
	CNT-Al 2 O 3 (AR500)	1.95	0.27	0.16
	CNT-Al 2 O 3 (AR1200)	0.96	0.24	0.15
	CNT-Al 2 O 3 (AR2000)	0.52	0.12	0.08
	CNT-Al 2 O 3 (AR3200)	0.40	0.25	0.16
	CNT-GNP (AR1200)	0.50	0.24	0.15

Table 2 -5 Tensile modulus (E) as calculated using Halpin-Tsai equation and measured from tensile tests

 2

	Systems	E rand (GPa) by Halpin-Tsai	E paral (GPa) by Halpin-Tsai	E (GPa) experimental data	Comparison
	CNTs/epoxy	2.71	3.51	2.36	E 1 < E rand1 < E paral1
	GNPs/epoxy	2.32	2.52	2.26	E 2 <E rand2 < E paral2
	CNT+GNP/epoxy	2.51	2.99	2.45	E 3 <E rand3 < E paral3

Table 2 -6 Tensile modulus of the hybrids reinforced composites as calculated using the GGS equation and measured from tensile tests

 2

	Composites with hybrids	α	f hybrid (vol.%)	Calculated E (GPa)	Experimental E (GPa)
	CNT-GNP (1200)	4	0.15	2.21	3.1
	CNT-SiC (1200)	3	0.13	2.21	2.3
	CNT-Al 2 O 3 (500)	1	0.667	2.21	2.4
	CNT-Al 2 O 3 (1200)	1	0.368	2.20	2.6
	CNT-Al 2 O 3 (2000)	1	0.185	2.20	2.8
	CNT-Al 2 O 3 (3200)	8	0.179	2.22	2.5

  concentrations of CNT-Al 2 O 3 hybrids in the epoxy composites with different CNT aspect ratios

	a Materials	Ф hybrid (wt.%)	Ф CNT (wt.%)	Ф matrix (vol.%)	Ф hybrid (vol.%)
	AR500-Al 2 O 3	2.0	0.28	99.3	0.667
	AR1200-Al 2 O 3	1.0	0.25	99.6	0.368
	AR2000-Al 2 O 3	0.5	0.13	99.8	0.185
	AR3200-Al 2 O 3	0.4	0.25	99.8	0.197
	AR1200-SiC	0.67	0.26	99.7	0.13
	AR1200-GNP	0.5	0.28	99.7	0.15
	The mechanical properties of the CNT-Al 2 O 3 /epoxy nanocomposites with different CNT
	aspect ratios are compared in Fig. 2-15. Table 2-8 summarizes the experimental results of the
	mechanical properties of the composites containing the hybrids with the same CNT aspect

a The epoxy composites based on the different hybrids will be identified as CNT AR-substrate. ratio (1200). Table

2-

9 displays the experimental results of the CNT-Al 2 O 3 /epoxy composites with different CNT aspect ratios (500, 1200, 2000 and 3200). The neat epoxy matrix tensile modulus and strength are E m = 2.2±0.1 GPa and m =50.0±1.6 MPa. By introducing such small amount of the hybrids, the composites show obvious enhancements of mechanical properties compared to the baseline epoxy.

Table 2 -8 Comparison of the mechanical properties of the CNT-Al 2 O 3 /epoxy composites with different CNT aspect ratios

 2

	Materials	Tensile modulus (GPa)	Effective reinforcing modulus (GPa)	Tensile strength (MPa)	Effective reinforcing strength (MPa)
	epoxy	2.2±0.1		50±2
	AR500-Al 2 O 3	2.4±0.1	32±9	57±2	1099±240
	AR1200-Al 2 O 3	2.6±0.1	111±19	61±2	2955±462
	AR2000-Al 2 O 3	2.8±0.1	326±32	69±2	10143±918
	AR3200-Al 2 O 3	2.5±0.1	154±25	61±2	5476±1014

Table 2 -9 Comparison of the mechanical properties of the epoxy composites based on different hybrids with the same CNT aspect ratio

 2

	Materials	Tensile modulus (GPa)	Effective reinforcing modulus (GPa)	Tensile strength (MPa)	Effective reinforcing strength (MPa)
	AR1200-SiC	2.3±0.1	35±19	56±2	1995±518
	AR1200-Al 2 O 3	2.6±0.1	111±19	61±2	2955±462
	AR1200-GNP	3.1±0.1	328±29	68±2	6679±652
	As shown in

Table 2 -

 2 8, the mechanical properties of the composites are dependent o n substrates of the hybrids. The tensile modulus and tensile strength in the CNT-GNP/epoxy composites are dramatically increased compared to the other hybrids reinforced composites.

	Their tensile modulus is up to 3GPa, corresponding to about 40 % increase compared to that
	of the pristine epoxy. However, only moderate enhancements of the tensile modulus are
	achieved in the other composites, ~4 % for CNT-SiC/epoxy, ~18 % for CNT-Al 2 O 3 /epoxy
	and ~9 % for CNT-SiC+CNT-Al 2 O 3 /epoxy. As presented in Table 2-9, the tensile modulus

and tensile strength of the CNT-Al 2 O 3 /epoxy composites strongly depend on the CNT aspect ratios.

  -1 fraction of the fiber; f is the conductivity of the glass fiber; m is the conductivity of the CNT-Al 2 O 3 /epoxy composite matrix. The experimental results obtained from Fig.3-5cand the predicted values are compared in Table3-1. As demonstrated in Table3-1, there is not obvious difference between the experimental values of in-plane and calculated results of para .The conductivity of the glass fiber/CNT-Al 2 O 3 /epoxy multiscale composites is dependent on that of the CNT-Al 2 O 3 /epoxy matrix.

	Table 3-1
	m	5.7×10	-6 S/m
	f	1.0×10	-13 S/m
	in-plane (Experimental)	5.97×10	-6 S/m
	para (Calculation)	2.85×10	-6 S/m

Table 3 - 2

 32

	E CNT-Al2O3/epoxy	2.6 (±0.1) Gpa
	E c (Experimental)	23.4 (±0.7) Gpa
		36.3 GPa
	(ROM)	5.0 GPa
	(Halpin-Tsai)	8.9 GPa
	E c (Predicated)	20.6 GPa

Table 3 -3

 3

	Temperature	Baseline	Storage Modulus (GPa) CNT CNT+Al 2 O 3	CNT-Al 2 O 3
	50 o C	16.5	17.2	18.0	19.8
	150 o C	8.2	8.8	9.2	9.3

< <

, the accumulation of matrix-dominated damages and the emergence of the transverse

Acknowledgements

Most of all, I would like to express my most sincere appreciation to my supervisor Mr Jinbo BAI. During the three years, he gave patient guidance, many encouragement and valuable opinions for my thesis.

I very appreciate China Scholarsip Council (CSC) which is the financial support for this research. I also thank the Service de l'Education Ambassade de la République Populaire de Chine en République Française for their many help.

AR500 and AR1200, the in-situ resistance exhibits a similar behavior to that of the ran-CNTs filled composites. However, with increasing aspect ratio up to 2000 or more, the self-sensing properties of the composites become similar to that shown in Fig. 2-25a.

Fig. 2-25 Self-sensing behaviors of the epoxy composites with (a) hybrid fillers with the same aspect ratio (AR1200), (b) CNT-Al 2 O 3 hybrids with different aspect ratios.

Before tensile loading, the vertically aligned CNTs well dispersed in the matrix and the conductive networks are formed (Fig. 2-26). In the elastic region, the elastic deformation makes the number of contact points between CNTs on different SiC microplates decrease (Fig. 2-26). As a result, the in situ electrical resistance increases in this region. When the plastic deformation occurs, there is reformation of the conductive networks after their break-ups, which can be explained by the electrical tunneling resulted from the closing of the