Yuri V Kolokolov

Stanislav L Koschinsky
email: koschinsky@orel.ru

Kondo H Adjallah
email: kondo.adjallah@utt.fr

Bifurcation Approach to the Emergency Situation Monitoring of Pulse Energy Conversion Systems

The new approach to the forecasting and detection of the emergency situations in pulse energy conversion systems based on the bifurcation approach was proposed. The essence of the proposed approach is in the on-line monitoring of "dangerous" bifurcation boundaries in system's parameter space. It is proposed to identify "dangerous" bifurcation situation (emergency) on-line on the basis of state vector analysis.

Introduction

Increase of specific energy consumption in so-called "responsible technological processes" stipulates high demands to the reliability and safety of energy conversion and delivering systems. On the other hand the necessity to increase a technological and economical efficiency of energy conversion processes stimulates wide use of pulse energy conversion systems (PECS). The complicated physical nature of the PECS and their operation under conditions of substantial changes in internal and external parameters make prerequisites for the appearance of emergency in their dynamics. The use of agreed-upon approaches to the fault detection and isolation (FDI) for such systems is hindered by some reasons.

A. The high dimension of the state apace of the mathematical model (MM) which describe the PECS and incomplete observability of the state vector make difficulties the use of the methods of FDI which are based upon the analysis of the state vector of the system. Again the high order of nonlinear models complicates their realization in real time and causes contradictions between the main criteria of the forecasting efficiency, the adequacy degree and the forecasting time.

B. The substantial nonlinearity of the PECS, which is stipulated by their operation principle, causes the appearance of both the quantitative and qualitative (bifurcation) changes in the dynamics of such systems.

C. The high dimension of the parameter space and a great number of various bifurcation boundaries in the parameter space causes indeterminacy in realization of either scenarios of the development of the dynamics in the PECS when their internal parameters or the operating conditions are changed.

The above-mentioned problems are typical of a different classes of nonlinear systems of various nature and they stimulate the development of radically new approaches to the forecasting and detection of emergency in such systems.

Description of the Proposed Approach

The proposed approach to the construction of the system for the forecasting and detection of emergency situations in the dynamics of the PECS is based upon the use of bifurcation analysis and the theory of nonlinear observers. The approach involves the following steps.

A. The structural analysis of the PECS that is carrying out to reveal some structural peculiarities which are characteristic of the dynamics of the PECS. The result of this analysis is the decomposition of the PECS into some simpler structural elements, which are studied separately. If it is not possible, an attempt is made to substantially decrease the order of MM by the separation of the most important parts and links (which determine the dynamics) and elimination less significant ones [START_REF] Haken | Advanced synergetic. Instability hierarchies of self-organizing systems and devices[END_REF].

B. The detection of the main (potentially "expected") scenarios of the development of the dynamics of the PECS and the detection of the interconnection between the detected scenarios and the structural peculiarities of the PECS.

C. The detection of the "dangerous" bifurcation boundaries in the parameter space of the PECS, that is, the location of the boundaries whose intersection leads to emergency, and the formation of quantitative criteria for the detection of either bifurcation boundaries by the use of the state vector of the system. D. The construction of the FDI system by using the nonlinear observers. In this case any emergency is detected on the basis of the criteria stated in item C by the use of the state vector of the system which was partially reconstructed by the observer.

The problem of the forecasting adequacy of any emergency in the dynamics of the PECS by using the proposed approach is directly connected with the evaluation of the adequacy of simplifications which are used during their analysis (see item A) and during the synthesis of the observers (see item D).

In order to ensure the adequacy degree and forecasting time (for specific aims) it is necessary:

-to study all permissible simplifications in the mathematical models of the PECS depending on the purposes of forecasting and available material resources;

-to place the limits for use of simplifications depending also on the purposes of forecasting.

As an example let us consider the electromechanical PECS (automated d.c. electric drive) whose functional scheme is shown in Fig. 1. This PECS is an automatic system for regulating the current in the armature of a d.c. electric motor and may be represented structurally in the form of three coupled oscillators. The representation of the electromechanical energy conversion systems as some coupled oscillators is sufficiently well known for the researchers of such systems [START_REF] Nayfeh | Application of bifurcation theory to subsynchronous resonance in power systems[END_REF][START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF].

The mechanical subsystem (the first oscillator) is an equivalent rotating mass, which is reduced to the shaft of the electric motor, and is nonlinearly coupled to the electrical subsystem of the PECS (the second oscillator). There is a passive LC-filter (the third oscillator) between the power source and the electrical subsystem of the PECS. Moreover, the LC-filter and the PECS are coupled through a discrete function of coupling (so-called coupling of the "pulse pumping" type [START_REF] Haken | Advanced synergetic. Instability hierarchies of self-organizing systems and devices[END_REF]).

The mechanical subsystem of the PECS and the input filter may be considered as linear oscillating elements to a first approximation. Moreover, for relatively powerful drives (more than dozens of kW) the resonance frequency of the mechanical subsystem is much less than the modulation frequency of the electrical subsystem. Therefore, when analyzing such systems the known simplification may be the method of dividing motions into fast motions (in the electrical subsystem) and low motions (in the mechanical subsystems) [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF].In this case the equations of the mechanical subsystem may be eliminated from the dynamic MM of the PECS, and the mechanical subsystem itself may be accounted for by the parameter ω -the rotation frequency of the motor shaft. While making a bifurcation analysis the ω is considered as varying parameter. The resonance frequency of the input filter on the contrary may be comparable with the modulation frequency of the electrical subsystem of the PECS. Consequently the interaction between the input filter and the electrical subsystem of the PECS determines largely the dynamics of the PECS on the whole [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF].

Finally, the mathematical model of the PECS under consideration is a system of five differential equations with the discontinuous right-hand part [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF]. The components of the state vector X are: the current fed from the power source i c ; the voltage at the capacitor of the input filter U c ; the armature current i r and the magnetic flux Ô of the electric motor and the variable describing the PI-element of the regulator.

The instants of switching of the switch element for the PECS with pulse-width modulation is described by a scalar commutation function of the state vector X and time t [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF][START_REF] Yu | On the bifurcation of stationary motions in impulse systems of automatic control[END_REF][START_REF] Yu | Experimental Study of the Adequacy of Bifurcation Approach to Emergencies Forecasting in Dynamics of Pulse Energy Conversion Systems[END_REF]. The parameters of the mathematical model correspond to the electric motor drive of a tram or a trolley bus having the equivalent power of the electric motors of 160 kW. The control frequency of the PECS f 0 is equal to 400 Hz.

At this design step of the PECS it is assumed that its stationary motion should be periodic oscillations with the frequency f 0 , but experimentally and on models it is determined that the variations in the parameters of the PECS may result in the appearance of more complicated motions in it, subharmonic (with frequency of f 0 /m), quasiperiodic and chaotic (see for example [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF]). Fig. 2 shows two parametric bifurcation diagram of stationary motions (from [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF]). The rotation frequency of the motor shaft, ω and the resonance frequency of the input filter, f r are considered in Fig. 2 as varying parameters. In the diagram the domains, where the periodic motions with the frequency f 0 /m exist, are denoted by P m and the domains, where quasiperiodic or chaotic motions exist, are marked by Q and Ch respectively.

As it is readily noticed in the diagram of Fig. 2, the domains of periodicity alternate with the domains of quasiperiodicity (at variation in f r ). It is explained by the interaction of two oscillators with rationally not coupled frequencies (of the input filter and the electrical subsystem of the PECS). Thus, the main type of stationary motion in the PECS under consideration are quasiperiodic oscillations which are characterized by two-dimensional torus in the state space. In this case, the periodic motions in the domains P m may be considered as the motions which are synchronized on the torus.

The "stepwise" change in system's state during corresponded bifurcation boundary intersection can be considered as the criteria to separation of "dangerous" bifurcation boundaries in Fig. 2. The essence of bifurcation boundaries separation with "dangerous" and "non-dangerous" may be illustrated with the consideration of specific period doubling bifurcation boundary (the boundary between P 1 and P 2 in Fig. 2). Let us consider that the systems dynamics is described by stroboscopic mapping X k+1 =F(X k ) [START_REF] Yu | The regularities of the development of the nonlinear dynamics of the control systems with pulse-width modulation[END_REF][START_REF] Yu | On the bifurcation of stationary motions in impulse systems of automatic control[END_REF][START_REF] Yu | Experimental Study of the Adequacy of Bifurcation Approach to Emergencies Forecasting in Dynamics of Pulse Energy Conversion Systems[END_REF]. Here X k =X(kT) is a state vector of the system, k=0,1,2,…, T=1/f 0 . If one looks upon discrete sequence X k , k = 0,1,2,... the motion with period equals to the 1/f 0 , corresponds to the equation X k = F(X k ). Similarly, the equation X k =F(F(X k )), X k ≠F(X k ) corresponds to the motion with period doubled to the 1/f 0 (see Fig. 3).

.

X(t) X(t) kT kT (k+1)T (k+1)T t t
Fig. 3. Illustration of period doubling phenomenon. Now suppose, the doubling of the period in the system in question is the result of change of some parameter α. As shown in [START_REF] Yu | On the bifurcation of stationary motions in impulse systems of automatic control[END_REF] in application to the PECS with PWM the above mentioned doubling of period of stationary motion may occur by two ways:

-so-called "smooth" doubling (supercritical bifurcation of period doubling); -"rigorous" doubling (subcritical bifurcation of period doubling). These both cases are illustrated by bifurcation diagrams in Fig. 4 a,b. On the ordinate X k values are plotted which determine stationary motions with a specific value of α. In Fig. 4 the stable motions are shown by solid lines and unstable motion by dashed lines. The point of bifurcation of period doubling of stationary motion is denoted as α * . Are the situations in Fig. 4a and Fig4b emergency from the point of view of operation of the system? In the first case the change of stationary motion in the system occurs "smoothly" and therefore it may be detected and compensated some how. This is not a type of "dangerous" bifurcation boundary. In the second case a "stepwise" change of stationary motion is observed and this is an emergency conditions undoubtedly. Moreover, the presence of hysteresis domain in Fig. 4b creates preconditions for phenomenon of alternation (in a definite time interval) of stationary motion, for example, by impact of external noise. This bifurcation boundary is "dangerous". Similarly we can characterize the bifurcation boundary between the domain of periodic motion existence P 1 and the domain of chaos Ch as a "dangerous" boundary.

The correlation function both with the static error signal are proposed to consider as a qualitative criteria of on-line monitoring of "dangerous" bifurcation boundaries intersection (an emergency situation) in PECS. The static error signal in common case is proposed to be reconstructed with the observer. With the help of this criteria one can easily detect emergency situation in the PECS' dynamics.

Conclusions

In practice, the use of the results of the bifurcation analysis for the purposes of FDI is complicated by a number of mathematical problems.

1. To obtain a pattern of division of the parameter space is such a process which requires very large computing expenditures. This process can not be realized in real time.

2. For "direct" use of the results of the bifurcation analysis for FDI it is necessary to solve the problem of parametric identification of the PECS but to do it is practically impossible.

Hence the authors of the paper offer an approach to FDI for the PECS which uses not the pattern of division of the parameter space into the domains with different types of motions, but the mathematical criteria based upon the use of the state vector of the PECS which describe the bifurcation boundaries of the domain having the given motion. The nonobserved components of the state vector of the PECS is proposed to restore by nonlinear observers. At present the problems of the formation of mathematical criteria which describe the bifurcation boundaries of the PECS on the basis of its state vector and the problems of the formation of nonlinear observers of the PECS are not completely solved and determine the directions of further investigations.

Fig. 1 .

 1 Fig. 1. Functional scheme of the considered electromechanical system

Fig. 2 .

 2 Fig. 2. Two parametric bifurcation diagram of stationary motions

Fig. 4 .

 4 Fig.4. Bifurcation diagrams of stationary motions


Brahim Mazian

Anne Bergeret
email: anne.bergeret@mines-ales.fr

Jean-Charles Benezet

Luc Malhautier

Impact of field retting and accelerated retting performed in a lab-scale pilot unit on the properties of hemp fibres/polypropylene biocomposites

Keywords:

Lignocellulosic fibres such as hemp fibres have emerged as an attractive alternative to nonrenewable fibers in the reinforcements in polymer composites. These fibres are subjected to pretreatment prior fibre extraction. In hemp industry, the retting is the first treatment applied to the plant after harvesting in order to separate the fibres from the central woody part of the stem. This treatment is needed to be more understood because of its importance for the development of high-performance hemp biocomposites. In this study, the influence of field retting treatment and lab-scale retting treatment of hemp fibres harvested at the end of flowering (EF) and at the seed maturity (SM) respectively, on hemp fibres /polypropylene biocomposites was investigated. The results highlight that, regardless the harvest period (initial state of the fibres) and type of retting, the thermal stability of biocomposite increased gradually with retting duration due to the increase of thermal stability of the fibres. The decomposition temperature of the fibres harvested at EF and SM increased from 335 °C and 352 °C, respectively to 350 °C and 368 °C for fibres that retted during five weeks. Tensile strength and Young's modulus of the biocomposites reinforced with fibres harvested at EF increase gradually until reaching a maximum (5 weeks) (46.0 ± 1.9 MPa and 4072 ± 196 MPa respectively), and then tends to decrease with a prolonged field retting (42.9 ± 1.6 MPa and 3627 ± 188 MPa respectively). In contrast, for biocomposites reinforced with fibres harvested at SM and retted in accelerated conditions, the tensile strength and Young's modulus decreased rapidly from 44.9 ± 2.2 MPa and 3732 ± 291 MPa for unretted fibres to 39.9 ± 2.6 MPa and 3327 ± 183 MPa for five week retted fibres.

Introduction

Due to environmental consideration and economic factors, plant fibres reinforced polymer composites (biocomposites) have acquired a remarkable interest over last decades. They are developed in many sectors, such as in building (decking), naval, automobile (interior equipment, cowls), and sports (surfboard) as an alternative of conventional fibers (e.g. glass or carbon) reinforced composites materials [START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites?[END_REF][START_REF] Pappu | Manufacturing and characterization of sustainable hybrid composites using sisal and hemp fibres as reinforcement of poly (lactic acid) via injection moulding[END_REF][START_REF] Muthuraj | Sustainable thermal insulation biocomposites from rice husk, wheat husk, wood fibers and textile waste fibers: elaboration and performances evaluation[END_REF]. Besides their excellent mechanical properties and their lightening structures [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Bourmaud | Towards the design of high-performance plant fibre composites: how can we best define the diversity and specificities of plant cell walls?[END_REF], these plant fibres reinforced composites, can reduce the environmental impact [START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites?[END_REF], can be recycled while keeping the original properties [START_REF] Bourmaud | Investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites[END_REF], and their processing do not damage the processing equipment as natural fibres causes less surface abrasion. However, the incompatibility of these fibres with the polymer matrix and the variation of their quality in term of the morphology, chemical composition, physical and mechanical properties greatly reduce their high use as reinforcement in polymers for structural application [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Bourmaud | Towards the design of high-performance plant fibre composites: how can we best define the diversity and specificities of plant cell walls?[END_REF].

Hemp plant fibres are one of the species that are used as reinforcement in polymers. These fibres are located at the periphery of the hemp stems in form of bundles containing several elementary fibres that are assembled together by an interface of amorphous polysaccharides (cementing compounds e.g. pectins) [START_REF] Crônier | Structure and chemical composition of bast fibers isolated from developing hemp stem[END_REF]. Each unitary fibre (or elementary fibre) is composed of concentric cylindrical walls and has a complex structure that contains the microfibrils of cellulose as basic units and matrix of amorphous polysaccharide including pectins, hemicelluloses and lignin [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Bourmaud | Towards the design of high-performance plant fibre composites: how can we best define the diversity and specificities of plant cell walls?[END_REF][START_REF] Väisänen | Effect of steam treatment on the chemical composition of hemp (Cannabis sativa L.) and identification of the extracted carbohydrates and other compounds[END_REF]. The variation of amount of hemp fibres substances has an impact on their intrinsic mechanical properties [START_REF] Bourmaud | Relationships between micro-fibrillar angle, mechanical properties and biochemical composition of flax fibers[END_REF][START_REF] Marrot | Analysis of the hemp fiber mechanical properties and their scattering (Fedora 17)[END_REF] that play a key role in mechanical performance of final composite. In addition, the presence of some substances such as pectins and

• The plants harvested at the end of flowering were left in the field for retting (Fig. 2A).

• The plants harvested at the seed maturity were retted using two designed lab-scale pilot units (Fig. 2B). Two equivalent lab-scale pilot units were used for performing the retting in controlled conditions, especially in terms of humidity and air speed. These labscale pilot units consist of two parts: i) the upper part that is transparent in polymethyl methacrylate (PMMA). The dimensions of this part are 1 m (length) x1 m (width) x45 cm (height). ii) the lower part is in polyvinyl chloride (PVC) with dimensions of 1 m (length) x1 m (width) x25 cm (height), filled with gravel / clay balls and containing 15 cm of soil retrieved directly from the field were the hemp was sown. The ventilation of this lab-scale pilot unit is ensured by suction of the air through a perforated surface. The air speed sweeping the surface of the soil is representative of a calm time, 0.2 m.s -1 . Intermittent watering of the soil and of the stems dropped and lined on the soil surface is preformed 4 times for 24 h to maintain the relative humidity at around 75%.

Weather conditions

During field retting of the stems harvested at the end of flowering, the monitoring of the temperature and the relative humidity was realized using the hygro-buttons sensors (Progesplus, Carquefou, France) which are put in contact with the soil in the field. In addition, daily average precipitation data were acquired from Météo France (meteorological station Méjannes-le-Clap). For retting in the lab-scale pilot units, these parameters were followed using thermo-hygrometer Kimo.

Fig. 3A shows the weather conditions during field retting of plants harvested at the end of flowering. During this field retting, the weather was hot, except two last weeks of treatment. Indeed, the average maximum and minimum temperature between the 1 st week and the 9th week are 40.2 °C and 17.5 °C, respectively, while during two last weeks of this period, they decreased to 26.3 °C and 7.1 °C, respectively. As concern the retting conditions for the stems harvested at SM and retted in the lab-scale pilot unit (Fig. 3B), the temperature and relative humidity are homogeneous. The relative humidity and the temperature are around of 75% and between 10-20 °C, respectively, throughout treatment.

Matrix

The polymer used as a matrix for manufacturing of composite materials is a polypropylene (PP, grade H733-07) with a melt flow rate of 7.5 g/10 min (230 °C, 2.16 kg) provided by Braskem (Brazil). In order to improve the compatibility between the matrix and the reinforcements, 1 wt% maleic anhydride grafted polypropylene (MA-g-PP, Orevac CA100) with 1 wt% grafting rate purchased from Arkema (France), was added as coupling agent. A hemp fibres loading of 30 wt% was chosen.

Composite material processing

The fibres and the coupling agent (MA-g-PP) have been dried before processing for 24 h at 60 °C in a vacuum to eliminate residual water. Granules of PP and MA-g-PP were mixed with hemp fibres using a twinscrew micro-compounder (DSM Xplore, microcompounder, Geleen, The Netherlands, 15cm3) with the temperatures profile of 180 °C (feeder) -180 °C (middle screw) -200 °C (die) and a screw speed of 80 rpm.

The extruded pellets were dried for 3 days at 60 °C, then injected molded using IM15 Zamak Mercator machine (Skawina, Poland) with a barrel temperature of 195 °C. Molded samples are tensile dogbones specimens corresponding to the ISO 527-2 type-1BA to obtain ISO ½ dogbone specimens. Table 1 gives the different biocomposites for each selected growth stage. An additional biocomposite was elaborated without MA-g-PP in the case of hemp fibres harvested at SM period and unretted, so called later R0PPSMref.

Experimental methods

Biocomposite thermal stability

The thermal stability of biocomposites was analyzed using a Perkin-Elmer Pyris-1 thermal analysis system. Samples of about 10 mg were waxes, that bind the fibres together, can prevent the hydroxyl groups of cellulose from interacting with polar polymer matrix. Therefore, removing these substances and separating the fibres to obtain individual fibres or smaller fibres bundles, are needed to improve interfacial bonding between fibres and matrix in order to enhance the mechanical performance of final composite [START_REF] El Mechtali | Mechanical and thermal properties of polypropylene reinforced with almond shells particles: impact of chemical treatments[END_REF][START_REF] Li | Analysis of green hemp fibre reinforced composites using bag retting and white rot fungal treatments[END_REF][START_REF] Martin | Influence of the degree of retting of flax fibers on the tensile properties of single fibers and short fiber/polypropylene composites[END_REF][START_REF] Sisti | Evaluation of the retting process as a pre-treatment of vegetable fibers for the preparation of high-performance polymer biocomposites[END_REF].

For this aim, many pre-treatments of the fibres have been developed in order to improve the intrinsic properties of the fibre and the adhesion between the fibre and matrix: i) Chemical treatment, using silane, alkaline, acetylation ethylenediaminetetraacetic acid (EDTA) and ethylene diamine tetra (methylene phosphonic acid) (EDTMPA) increases the compatibility between fibres and matrix [START_REF] Bledzki | The effects of acetylation on properties of flax fibre and its polypropylene composites[END_REF][START_REF] Cantero | Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites[END_REF][START_REF] Islam | Influence of alkali fiber treatment and fiber processing on the mechanical properties of hemp/epoxy composites[END_REF][START_REF] Liu | Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites[END_REF], ii) Enzymatic treatment, (e.g. endo-polygalacturonase, hemicellulases) [START_REF] Liu | Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites[END_REF][START_REF] Nykter | Effects of thermal and enzymatic treatments and harvesting time on the microbial quality and chemical composition of fibre hemp (Cannabis sativa L.)[END_REF] inducing a cleaner fibre's surface, helps to improve the mechanical properties of fibre reinforced composites and iii) steam explosion [START_REF] Kukle | The effect of steam explosion streatment on technical hemp fibers[END_REF][START_REF] Thomsen | Effects of chemical-physical pre-treatment processes on hemp fibres for reinforcement of composites and for textiles[END_REF][START_REF] Vignon | Morphological characterization of steam-exploded hemp fibers and their utilization in polypropylene-based composites[END_REF]. Although these methods are reported in literature, the most widely used pretreatment in the hemp industry is field retting (also known as dew-retting) thanks to its low cost and ease of use (application) [START_REF] Sisti | Retting Process as a Pretreatment of Natural Fibers for the Development of Polymer Composites[END_REF]. The field retting treatment consists of cutting and spreading out the hemp plants on the field after harvesting, so that the microorganisms (fungi and bacteria) colonize the stems [START_REF] Djemiel | Characterization of bacterial and fungal community dynamics by high-throughput sequencing (HTS) metabarcoding during flax dew-retting[END_REF][START_REF] Ribeiro | Microbial diversity observed during hemp retting[END_REF] producing a range of polysaccharide-degrading enzymes, especially pectinolytic which remove the components in the middle lamella (pectic substance) and allow the cortex fibres to be progressively separated [START_REF] Akin | Optimization for enzyme-retting of flax with pectate lyase[END_REF][START_REF] Henriksson | Identification and retting efficiencies of fungi isolated from dewretted flax in the United States and europe[END_REF]. This treatment is weather-dependent and is performed in empirical way in the field which leads to a variation of the hemp fibres quality in term of division state of the fibres, chemical, thermal and mechanical properties [START_REF] Goudenhooft | Evolution of flax cell wall ultrastructure and mechanical properties during the retting step[END_REF][START_REF] Mazian | Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering[END_REF][START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF][START_REF] Réquilé | Peeling experiments for hemp retting characterization targeting biocomposites[END_REF]. Indeed, field retting can be considered as an advantage or an inconvenience depending of its degree. Therefore, it is necessary to control this treatment in order to avoid under or over retting.

Therefore, the objective of this work is to assess the influence of two types of retting (a field retting on one side and an accelerated retting performed in lab-scale pilot units on the other side), the retting duration (up to 11 weeks) and the harvesting period (end of flowering (EF) and seed maturity (SM)) on the properties of a polypropylene/30 wt% hemp fibres. Thermal stability and microstructure were surveyed using thermogravimetry analyses (TGA) and differential scanning calorimetry (DSC), respectively. Scanning electron microscopy observations were carried out on the tensile fracture surfaces after mechanical test.

Materials and methods

Raw materials

Hemp cultivation and sampling

Hemp (Cannabis sativa L., Cultivar 'Santhica 27') was sown at a rate of 30 kg/ha on 10th May, 2017 in the south of France (N 44.130673°, E 4.315895°) by CIVAM Chanvre Gardois (Le Bouquet, France). The hemp plants were harvested manually at the end of flowering (EF) (21th august, 2017), and at the seed maturity (SM) (26th September, 2017) (Fig. 1). The stems lengths at EF and SM are 1.56 m and 1.58 m, respectively.

Retting conditions

After each harvest period, the plants were subjected to specific retting treatments. heated in nitrogen from 30 °C to 700 °C at the rate of 10 °C.min -1 . The mass variation was recorded as a function of temperature. A differential thermogravimetric analysis (DTGA) curve was collected from the TGA analysis.

Mechanical characterization of biocomposites

Before mechanical test, biocomposites were conditioned for 3 days at 23 ± 2 °C and 50 ± 10% RH. Then, tensile test characterization was performed in the same hygrothermal conditions on ISO ½ dog-bone samples. A Zwick TH 0101 apparatus machine equipped with an extensometer Zwick "clip-on" for the determination of the Young's modulus was used. The displacement speed was set to 1 mm.min -1 for the tensile modulus measurements, and 20 mm.min -1 for the break property measurements. The tests were carried out at least four times for each specimen to evaluate the reproducibility.

Microstructure characterization of biocomposites

The crystallinity behavior of biocomposites was assessed through differential scanning calorimetry (DSC, Perkin Elmer) using a Q100 TA Instruments® equipped with a cooling attachment, under a nitrogen atmosphere. The measurements were performed under nitrogen atmosphere at a 20 mL.min -1 flow. Two heating steps from 30 °C to 220 °C at 10 °C/min with an intermediate cooling step at 10 °C/min were carried out. Samples of about 10 mg were analyzed in standard aluminum DSC pans. Melting enthalpy was determined from the 1 st and 2nd heating steps.

The crystallinity ratio was calculated according to the equation below:

= × × χ ΔH W ΔH 100 C1 m1 m100%
(1)

= × × χ ΔH W ΔH 100 C2 m2 m100%
(2)

Where ΔH m1 and ΔH m2 are the melting enthalpy obtained from the 1 st and 2nd heating steps, respectively. W is the PP content by weight and ΔH m100 is the estimated melting enthalpy of a fully crystalline PP (209 J.g-1) [START_REF] Sperling | Introduction to Physical Polymer Science[END_REF].

Microscopic observations

After the mechanical tests, microscopic observations of the tensile fracture surfaces were carried out. The samples were sputter-coated with a thin layer of gold, and then analyzed through environmental scanning electron microscope (ESEM) Quanta FEG 200 (FEI Co.)

Results and discussion

Color of stems after different types of retting

The visual assessment of the stems and fibres color has been carried out during field retting and lab-scale retting of the stems harvested at the end of flowering and seed maturity, respectively. It can be observed in the photograph (Fig. 4), that the color of stems and fibres changes during plant growth from light green to yellow. This color variation is related to decomposition of the chlorophyll that is responsible of green color and the partial retention of carotenoids which emit yellow color [START_REF] Hörtensteiner | The loss of green color during chlorophyll degradation-a prerequisite to prevent cell death?[END_REF][START_REF] Matile | Biochemistry of Indian summer: physiology of autumnal leaf coloration[END_REF][START_REF] Merzlyak | Why and what for the leaves are yellow in autumn? On the interpretation of optical spectra of senescing leaves (Acerplatanoides L.)[END_REF].

During field retting of hemp stems harvested at EF, the color of fibres and stems change gradually and slowly from light green for unretted samples (R0EF), yellow for low retted samples (R1EF to R3EF), yellow with the presence of grey and black spot for medium retted samples (R4EF and R5EF) and black for highly retted samples (R9EF and R11EF). In contrast, for the samples of SM, the color changes rapidly from yellow (R0SM) to black after one week of retting. This indicates that the retting in the lab-scale pilot was fast due to the homogeneous distribution of temperature and relative humidity which are favorable conditions for a rapid development of microorganisms at the stems surface [START_REF] Bleuze | Tracking the dynamics of hemp dew retting under controlled environmental conditions[END_REF][START_REF] Mazian | Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering[END_REF].

Evolution of biocomposite thermal stability with hemp fibre retting

Thermogravimetric analysis (TGA) was carried out in order to assess the impact of retting on the thermal performance of biocomposite. Fig. 5 presents the curves of TGA and corresponding DTGA respectively of neat PP and of biocomposites, with fibres harvested at the EF period and then field retted, and with fibres harvested at SM period and artificially retted in the lab-scale pilot unit.

The thermal degradation curve of neat PP gives evidence of a single weight-loss region located at 460 °C. However, the biocomposites degradation occurs in three stages process, where the first stage at about 200-280 °C is associated to the degradation of non-cellulosic components (pectins and hemicelluloses), the second stage at 340-380 °C is attributed to the decomposition of major component of fibres (cellulose) and the third stage at around 460-470 °C is related to the decomposition of polymer matrix.

As concerns the biocomposites with fibres harvested at EF period and field retted (Fig. 5A), TGA curves show different thermal behaviors according to retting duration. From the derivative of the weight loss as a function of temperature (Fig. 5B), it can be seen on one hand that the increase of retting duration leads to an increase of thermal stability in the 200-400 °C temperature range corresponding to both non-cellulosic (pectins and hemicelluloses) and cellulose decompositions. On the other hand, it can be observed that, the portion of the pectins and hemicelluloses gradually decreased (peak intensity at around 220-280 °C) with the retting duration owing to the microorganisms activity [START_REF] Bleuze | Tracking the dynamics of hemp dew retting under controlled environmental conditions[END_REF][START_REF] Mazian | Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering[END_REF] which leads to an increase of cellulose degradation temperature until reaching the same temperature of the degradation of pure cellulose. Indeed, the removal of non-cellulosic component induces a higher structural order of cellulose that contains strong intramolecular and molecular hydrogen bonds that requires a higher degradation temperature to be brokendown [START_REF] Kabir | Effects of chemical treatments on hemp fibre structure[END_REF][START_REF] Ouajai | Composition, structure and thermal degradation of hemp cellulose after chemical treatments[END_REF]. The cellulose degradation temperature shifts from 335 °C for R0PPEF to 342 °C for R3PPEF, 350 °C for R5PPEF and then to 360 °C for R11PPEF.

As concerns the biocomposites with the fibres harvested at SM period and retted in the lab-scale pilot unit (Fig. 5C andD), the same behavior is observed. The degradation temperature of the fibres increases with retting due to an increase of cellulose degradation temperature increases from 352 °C for R0PPSM, to 364 °C for R3PPSM and 368 °C for R5PPSM. This variation led to an improvement of thermal stability of biocomposite.

Besides the thermal stability changes with retting duration, these results also allowed to highlight that the initial state according to plant growth is not similar. The fibres harvested at the seed maturity had fewer portions of non-cellulosic components and present higher temperature degradation of cellulose (352 °C for R0PPSM) compared to the fibres harvested at end of flowering (335 °C for R0PPEF). This means that at the seed maturity, the cellulose fraction of fibres is higher than that of fibres harvested at end of flowering. This initial difference is remaining all along the retting process (342 °C for R3PPEF compared to 364 °C for R3PPSM and 350 °C for R5PPEF compared to 368 °C for R5PPSM).

Change in biocomposite microstructure with hemp fibre retting

The crystallinity degree and the melting temperature for neat PP and the biocomposites were determined from the first and the second heating ramps (Fig. 6 and Table 2). The melting enthalpy measured at first heat ramp is linked to the initial crystallinity which depends on the thermal background of the sample i.e. the processing steps. In the second ramp, the thermal background of the sample does not furthermore significantly influence the crystallization and retting treatment could only have an impact on the melting enthalpy measured in this ramp. Nevertheless, a comparison of crystallinity ratio of neat PP and biocomposite shows clearly higher values for the biocomposite. This observation explained that the fibres act also as a nucleating agent for the crystallization and the partial crystalline growth of PP, as already shown in the literature [START_REF] Amash | Morphology and properties of isotropic and oriented samples of cellulose fibre-polypropylene composites[END_REF][START_REF] Arbelaiz | Thermal and crystallization studies of short flax fibre reinforced polypropylene matrix composites: effect of treatments[END_REF].

Regarding the effect of retting on the crystallinity ratio, no trend could be observed either for composites with retted fibres harvested at EF or for composites with retted fibres harvested at SM. Indeed, for the first run, the crystallinity ratio of R0PPEF, R3PPEF, R5PPEF and R11PPEF are 49%, 44%, 45% and 45% respectively. The crystallinity ratio of R0PPSM, R1PPSM, R3PPSM, and R5PPSM are 43%, 43%, 41% and 46%, respectively. This variation of the crystallinity could be related to the roughness change of fibres and processing impact during compounding of the fibres.

Results show also that crystallinity ratio is significantly higher in the second run than first one. Indeed, it varies from 54% for R0PPEF, to 49% for R3PPEF, 51% for R5PPEF, 48% for R11PPEF. As concern the composites reinforced with fibres retted in lab-scale pilot unit, their crystallinity ratio are 48% for R0PPSM, 48% for R1PPSM, 44% for R3PPSM and 51% R5PPSM. The results reveal also that the crystallinity ratio of composite reinforced with the fibres harvested at the end of flowering (R0PPEF) is slightly higher than that of composite with fibres harvested at the seed maturity (R0PPSM).

Evolution of biocomposite mechanical properties with the hemp retting

Tensile properties (Figs. 7 and8; Table 3) have been analyzed for neat PP and biocomposites. Whatever the retting duration, the type of retting and the harvest period, the addition of hemp fibers to PP always has a positive effect, conferring a mechanical reinforcement to the polymer matrix with an increase in tensile strength and Young's modulus and a decrease in elongation at break. This highlights the capacity of hemp fibres reinforcement as largely exposed in literature (Badji et al., 2018;[START_REF] Martin | Influence of the degree of retting of flax fibers on the tensile properties of single fibers and short fiber/polypropylene composites[END_REF].

As concerns the influence of MA-g-PP coupling agent, Fig. 8 reveals that the addition of the coupling agent has also a significant positive effect on the mechanical performance of biocomposite with an increase in tensile strength and Young's modulus and a decrease in elongation at break by comparing R0PPSMref (without MA-g-PP) and R0PPSM (with MA-g-PP) as already presented in literature [START_REF] Bourmaud | Investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites[END_REF][START_REF] Pracella | Functionalization, compatibilization and properties of polypropylene composites with Hemp fibres[END_REF][START_REF] Sullins | Hemp fiber reinforced polypropylene composites: the effects of material treatments[END_REF].

Regarding the fibres harvested at the end of flowering (Fig. 7), results show that the mechanical properties of the biocomposites increased slightly at the first stage of field retting (up to 5 weeks) and then tend to decrease with an extended field retting (11 weeks). The Young's modulus (Fig. 7B) increased gradually from 3772 ± 265 MPa for R0PPEF to 4072 ± 196 MPa for R5PPEF, and then decreased to 3627 ± 188 MPa for R11PPEF. A similar trend is observed for the tensile strength (Fig. 7A), with a slight increase from 44.8 ± 2.7 MPa (mean value) for R0PPEF to 46.2 ± 1.5 MPa for R3PPEF, and then a decrease with a prolonged retting duration to 42.9 ± 1.6 MPa for R11PPEF. The strain at break (Fig. 7C) tends to decrease slightly with increasing of retting duration from 6.2 ± 1.0 %, (R0PPEF) to 5.8 ± 0.4 %, (R3PPEF) and 5.6 ± 0.2 % (R5PPEF).

As concerns the fibres harvested at seed maturity and retted under controlled conditions in the lab-scale pilot unit (Fig. 8), results show that the Young's modulus remain stable after one week of retting (∼3700 MPa for R0PPSM and R1PPSM), then reduced to 3599 ± 163 MPa and 3327 ± 183 MPa after 3 weeks and 5 weeks of retting (R3PPSM and R5PPSM), respectively. The tensile strength decreased gradually during retting, from 44.8 ± 2.2 MPa for R0PPSM, to 43.4 ± 2.0 MPa, 41.6 ± 2.6 MPa, and 39.9 ± 2.6 MPa for R1PPSM, R3PPSM R5PPSM, respectively. No significant change of the strain at break is observed with increasing of retting duration since it varies from 5.8 ± 0.5 % (R0PPSM) to 5.6 ± 0.4 %, 5.4 ± 0.7 %, and 5.3 ± 0.6 % for R1PPSM, R3PPSM and R5PPSM, respectively.

These results indicate clearly that mechanical properties of biocomposites differ according to the harvest period and the type of retting. Indeed, the mechanical properties of biocomposites with the fibres harvested at SM decreased rapidly after one week of controlled retting, whereas for the biocomposites with the fibres harvested at EF, they decreased only at the end of retting. This could be explained by the retting types performed for each harvest period. Indeed, the stems harvested at EF were field retted under dry weather conditions which leads to a slow variation of fibres characteristics, while the stems harvested at SM and retted under wet conditions in laboratory result a rapid change of the fibres characteristics. In our previous study in agreement with literature, it has been demonstrated that intrinsic properties of the fibres (e.g. fibres mechanical properties) change during retting. The first stage of retting treatment results in an improvement of the mechanical properties of fibres due to the removal of non-cellulosic compounds which leads to an increase of the cellulose fraction and of the cellulose crystallinity [START_REF] Mazian | Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering[END_REF].

However, when the fibres are highly retted (over-retted), their structure changes by a high degradation of non-cellulosic compounds or even by microbial degradation of cellulose [START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF]. Therefore, in this case, the mechanical properties of the fibres decreased which results in lower tensile properties of the composites. The behavior of mechanical properties of the biocomposites during retting pursue to mechanical properties of the fibres.

It can be concluded that the mechanical properties of the biocomposite are governed by the quality of hemp fibre which depends also on retting duration and the weather conditions. In dry weather conditions, a long period of retting (5 weeks) is required for obtaining the highest mechanical properties. In contrast, under wet conditions and homogeneous distribution of humidity (75%), the prolonged field retting must be avoided.

Effect of retting on fibre/matrix interfacial adhesion

Fig. 9A andB show the fracture cross-sections of PP/unretted hemp fibre biocomposite without MA-g-PP and with MA-g-PP/respectively. Fig. 9C, D and E correspond to fracture cross-sections of PP/field retted hemp fibres biocomposites with different retting durations, i.e. 3, 5 and 11 weeks respectively.

Without MA-g-PP, micro spaces (voids) can be observed at the interfacial zone, indicating a poor adhesion between fibres and matrix. In contrast, by adding MA-g-PP, a better adhesion seems to be involved (ex. Fig. 9B). These observations are in agreement with the results of [START_REF] Pracella | Functionalization, compatibilization and properties of polypropylene composites with Hemp fibres[END_REF] and [START_REF] Bourmaud | Investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites[END_REF].

As concerns the influence of retting on the adhesion of the fibres through the matrix, Fig. 9B, C, D andE show a significant adhesion of the fibres to the matrix and no pull out phenomena. This good adhesion of the fibres through the matrix could be not only related to coupling agent used, but also to degradation of interfibrillar cementing components (pectins) during retting. [START_REF] Li | Analysis of green hemp fibre reinforced composites using bag retting and white rot fungal treatments[END_REF], have reported that removal of non-cellulosic compounds leads to a separation of the fibres which bring a better adhesion between fibre and matrix because of higher surface contact and increased rugosity and an increase of mechanical properties of final material.

Conclusion

The present study revealed the influence of the retting on PP/hemp fibres biocomposites properties. Whatever the harvest period and retting type, the thermal stability of the biocomposites improves with retting duration due the enhancement of thermal stability of fibres. Indeed, the increase of retting duration induces a removal of non-cellulosic materials (pectins and hemicellulose) leading to a progressive increase of the degradation temperature of cellulose that is a major compound of the fibre. The fibres harvested at SM exhibit a better thermal stability compared to that of the fibres harvested at EF, due to the high cellulose fraction present in the fibres at plant maturity. The retting has no effect on the crystallinity of biocomposites, since no trend has been clearly observed.

In the case of field retted fibres, the tensile properties of biocomposites (tensile strength and Young's modulus) increased during retting to a threshold value (after 5 weeks) and decreased at the end of retting. As concerns the accelerated retted fibres in the lab-scale pilot unit, the tensile properties decreased rapidly with increasing of retting duration. The removal of non-cellulosic materials and increasing of cellulose fraction at beginning of retting bring better tensile performances of the biocomposite. However, increasing of retting duration leads to a high degradation of non-cellulosic materials, or even to the cellulose degradation which result in lower tensile properties of biocomposites. Knowledge of retting mechanisms will allow to a significant contribution in optimizing the biocomposites properties.

Fig. 1 .

 1 Fig. 1. Hemp stem length during growth.

Fig. 2 .

 2 Fig.2. A) Field retting (left: non-retted stems; middle: 3 weeks retted stems; right: 9 weeks retted stems). B) Lab-scale pilot units used for a controlled and accelerated retting.

Fig. 3 .

 3 Fig. 3. Daily weather conditions (minimum and maximum temperature, relative humidity and precipitation) (A) during field retting of stems harvested at EF and (B) during artificially/controlled retted stems harvested at SM in lab-scale pilot unit.

Fig. 4 .

 4 Fig. 4. Color change during field retting of the stems harvested at EF and during lab-scale retting of the stem harvested at SM. Corresponding color change of the fibres.

Fig. 5 .Fig. 6 .

 56 Fig. 5. TGA (A) and DTGA (B) curves of neat PP and biocomposites with fibres harvested at EF (R0PPEF) and field retted at different times (R3PPEF, R5PPEF and R9PPEF). TGA (C) and DTGA (D) curves of neat PP and biocomposites with fibres harvested at SM (R0PPSM) and field retted at different times (R3PPSM, R5PPSM and R9PPSM).

Fig. 7 .

 7 Fig. 7. Results of tensile strength (A), Young's modulus (B), and elongation at break (C) for neat PP and the biocomposites with fibres harvested at end of flowering (R0PPEF) and field retted at different retting durations (R3PPEF, R5PPEF and R11PPEF). The back crosses and red line correspond to the means and medians, respectively.

Fig. 8 .

 8 Fig.8. Results of tensile strength (A), Young's modulus (B), and elongation at break (C) for neat PP and biocomposites with fibres harvested at seed maturity (R0PPSM with MA-g-PP and R0PPSMref without MA-g-PP) and retted in the lab-scale pilot unit at different retting durations (R1PPSM, R3PPSM, R5PPSM). The back crosses and red line correspond to the means and medians, respectively.

Fig. 9 .

 9 Fig. 9. Tensile fracture cross-sections of A) PP/unretted hemp fibres biocomposite without MA-g-PP, B) PP/unretted hemp fibres biocomposite with MA-g-PP, C) PP/ field retted hemp fibre biocomposite with different retting durations: 3 weeks, D) 5 weeks and E) 11 weeks.

Table 1

 1 The performed biocomposites.

	Retting time (day)	0	7	21	35	77
	Retting time (week)	0	1	3	5	1 1
	Samples name	R0PPEF	-	R3PPEF	R5PPEF	R11PPEF
		R0PPSM	R1PPSM	R3PPSM	R5PPSM	-

Table 3

 3 Results of tensile strength, Young's modulus, and elongation at break for neat PP and the biocomposites.

		Young's modulus (MPa)	Tensile strength (MPa)	Elongation (%)
	PP	1859 ± 51	36.6 ± 0.4	8.6 ± 0.4
	R0PPEF	3772 ± 265	44.8 ± 2.7	6.2 ± 1.0
	R3PPEF	3818 ± 129	46.2 ± 1.5	6.2 ± 0.2
	R5PPEF	4072 ± 196	46.0 ± 1.9	5.8 ± 0.4
	R11PPEF	3627 ± 188	42.9 ± 1.6	5.6 ± 0.2
	R0PPSMref	3409 ± 472	38.3 ± 0.5	5.9 ± 0.4
	R0PPSM	3732 ± 291	44.8 ± 2.2	5.8 ± 0.5
	R1PPSM	3729 ± 186	43.4 ± 2.0	5.6 ± 0.4
	R3PPSM	3599 ± 163	41.6 ± 2.6	5.4 ± 0.7
	R5PPSM	3327 ± 183	39.9 ± 2.6	5.3 ± 0.6

Acknowledgements

The authors would like thank the "CIVAM Chanvre Gardois association (Le Bouquet, France)" for their collaboration to conduct this study.

Declaration of Competing Interest

The authors declare that he has no conflict of interest


A Tallaire

L Mayer

O Brinza

M A Pinault-Thaury

T Debuisschert

J Achard

Highly photostable NV centre ensembles in CVD diamond produced by using N2O as the doping gas

High density Nitrogen-Vacancy (NV) centre ensembles incorporated in plasma assisted CVD diamond are crucial to the development of more efficient sensing devices that use the properties of these luminescent defects. Achieving high NV doping with N2 as the dopant gas source during diamond growth is however plagued by the formation of macroscopic and point defects that quench luminescence. Moreover, such NVs are found to exhibit poor photostability under high laser powers. Although this effect can be harnessed to locally and durably switch off NV luminescence for data storage, it is usually undesirable for most applications. In this work the use of N2O as an alternative doping source is proposed. Much higher amounts of the doping gas can be added without significantly generating defects which allows the incorporation of perfectly photostable and higher density NV ensembles. This effect is believed to be related to the lower dissociation energy of the N2O molecule together with the beneficial effect of the presence of a low and controlled amount of oxygen near the growing surface.

Point defects in diamond and in particular the nitrogen-vacancy (NV) centre are among the most studied solid-state systems for applications in quantum technologies that promise to bring significant breakthroughs in computing, simulation, communication and sensing. NV's electronic spin state can indeed be manipulated with ms coherence time at room temperature. 1,2 For example, shifting of the NV centre's spin states with an applied magnetic field can be optically detected under resonant microwave radiation leading to promising magnetic sensors 3,4 or spectrum analysers 5,6 combining both a high spatial resolution and an extremely high sensitivity. One powerful approach to achieve this, relies on NV centre ensembles located in a layer near or at the surface since the signal to noise ratio and hence the magnetic sensitivity increases with NNV 1/2 where NNV is the number of luminescent defects involved in the measurement 7 . Creating a high density of NVs with controlled spatial distribution while preserving their good coherence properties is thus an important challenge.

NV centres in diamond can be created by implantation of N + ions followed by annealing above a temperature at which vacancies are mobile and can combine with nitrogen in the crystal. 8 In this case high NV densities can be locally achieved (up to 10 18 cm -3 ) and accurate positioning on the surface or indepth by changing the implantation energy is possible. However, implanting close to the surface (10 nm range) requires low implantation energy which results in a low creation yield of NVs. In addition, this technique may result in a reduced coherence time due to the generation of other defects during implantation that cannot be completely annealed out. 9 On the other hand native NV centres can be uniformly embedded into diamond layers during Chemical Vapour Deposition (CVD) leading to much longer coherence times. The CVD technique is versatile and allows stacking layers with different isotopic purity and thickness opening the way to complex structures. 10 By combining nitrogen doped CVD layers and He + bombardment after growth to generate vacancies, 1×10 17 NVs/cm 3 (0.6 ppm) have recently been obtained. 11 So far the highest densities were reported for type Ib High Pressure High Temperature (HPHT) diamonds that were proton irradiated leading to 2×10 18 NVs/cm 3 (11 ppm) but at the expense of significant line broadening. 12 Reaching high NV concentrations by using in situ doping of CVD films alone remains in fact quite difficult but would be desirable in order to create a flexible platform that aims at exploiting NV centre properties for magnetometry. N2 is the most common intentional dopant for creating NVs in diamond. At concentrations as low as parts per million (ppm), nitrogen impurities in the gas phase are known to dramatically accelerate CVD diamond growth rates. 13,14 Possible explanations are the lowering of the energy barrier that needs to be overcome to incorporate C atoms in the lattice or nucleate the next diamond layer, or the reduction in the diffusion of carbon-containing species that improves their anchoring at the surface. 15,16 There is however no consensus on the chemical species responsible for this catalytic effect. Radicals such as CN, NH or atomic nitrogen (N) have all been monitored in the plasma by optical emission spectroscopy. 17,18 N2 is one of the most stable molecules with triple nitrogen-nitrogen bounds that require 9.8 eV for breaking apart. Despite the use of high plasma power densities (pressure > 150 mbar and microwave power > 2 kW) only a moderate fraction of about 0.5 % of N2 is actually dissociated. In such conditions nitrogen doping efficiency is poor with most of it being substitutional to a carbon atom (Ns). 19 NV centres typically represent a low fraction of 10 -2 to 10 -3 of the total nitrogen content in the crystal. 20 Due to the stability of N2 molecules and issues related to the presence of large amounts of nitrogen in the gas phase, directly incorporating high NV densities is very challenging. In this paper we report on the use of N2O as an alternative doping source and found that much larger amounts can be added to the gas phase while keeping good morphologies, leading to higher NV densities that possess improved properties and photostability.

The effect of large N2 additions to the gas phase during growth is particularly obvious when a high thickness of several hundreds of micrometres of N-doped material is required as illustrated in Fig. 1. With a few tens of ppm of nitrogen, NV 0 and NV -emissions at 575 and 637 nm respectively appear as illustrated on the photoluminescence (PL) spectra of Fig. 1a using a 488 nm laser as the excitation line. Their intensity normalized to the diamond Raman peak however does not scale linearly with the amount of nitrogen added and exhibits saturation (or quenching) for concentrations around 100 ppm under our growth conditions. 21 Besides, for thick freestanding layers, an increasing absorption in the UV extending to the visible range is observed as N2 concentration is increased 22 (Fig. 1b). With 100 ppm of N2 in the gas phase the diamond grows brownish possibly because of the presence of vacancy clusters, 23 while an undoped crystal of similar thickness is almost uncoloured (Fig. 1e). It should be noted that the brown colour can be partly removed by a HPHT annealing (typically 7 GPa, 2200 °C) due to a rearrangement of point and aggregated impurities that are causing this colouration. 24 Above a few tens of ppm, N2 also promotes the formation of extended or unepitaxial defects during growth. 25,26 A polycrystalline rim is for example formed on the thick diamond crystal presented in Fig. 1d (40 ppm N2) as compared to an undoped film (Fig. 1c). PL images were obtained using near band-edge UV light with a DiamondView TM . Green luminescence from the HPHT substrate (H3 centres) can be seen in the centre together with a red colouration indicative of NV emission and a bright blue luminescence from the polycrystalline defects at the edges. On the other hand, the undoped sample shows only dim blue luminescence and clean edges. It should be mentioned that for much higher levels of nitrogen (> 500 ppm) and under our high power conditions, it is almost impossible to grow a thick single crystalline layer. Because of these defects that prevent growth or quench luminescence, obtaining films with a high NV density using N2 as a doping gas is particularly difficult.

We now detail efforts in growing thinner layers of about 10 µm with NV ensembles using the highest possible N2 concentration before reaching saturation (100 ppm under our conditions). Previously, we have shown that reducing the growth temperature is an efficient way to increase NV luminescence. 27 Therefore the structure presented in Fig. 2a was prepared with the top layer grown with 100 ppm of N2 and a temperature reduced from 890° C to 810 °C in order to promote the formation of NV centres (Sample A). After growth, luminescence was observed with a scanning confocal microscope that uses a 532 nm laser (Fig. 2b and set-up in Fig. S1 of supporting information file). Inserting an appropriate pinhole in a secondary image plane allows collecting the luminescence from a limited volume of the sample (typ. 1 µm 3 ) and drastically removing the background signal. By comparing with the PL intensity measured from a single NV centre, NV density could be estimated to roughly 10 ppb (i.e. 10 15 NVs/cm 3 ) which is a fairly high value leading to bright emission. Optically Detected Magnetic Resonance (ODMR) spectroscopy was also carried out. It aims at detecting the Zeeman shifts of the spin levels under an applied magnetic field only by optical means. Four pairs of resonances corresponding to a random orientation of NV centres along the 4 different <111> axes are visible (Fig. 2d). This is expected for growth on a (100)-oriented substrate contrary to growth on (110), (113) or (111) diamonds that lead to preferential orientation of 50, 73 and 97 % respectively. 28,29,30 Each resonance shows a characteristic triplet due to additional hyperfine splitting of the spin energy levels due to interaction with the nuclear spin of 14 N. 31 Their full width at half maximum (FWHM) was found to be 0.58 MHz (Fig. 2e). This is a low value, comparable to that obtained with single NV centres in high quality CVD diamond with continuous ESR spectroscopy 32 and consistent with NV centres having good coherence properties.

The photostability of NV centres was then evaluated by scanning the sample with a higher laser power of 5 mW. Surprisingly this led to a strong (nearly 50%) and long lasting bleaching of PL emission (Fig. 2c). One possible mechanism is a change in the charge state due to the loss of one or several electrons from the emitting NV -centres that might be recaptured by adjacent defects. 33 This trapping was fully reversible since a scanning with an intermediate laser power of 0.5 µW was sufficient to generate photo carriers and fully reconvert NV centres to their negative charge state, thus recovering the initial PL intensity (see Fig. S2 in supplementary information). In a recent paper, Jayakumar et al. have explored the dynamics of NV photoionization using type Ib HPHT diamonds that contain large amounts of nitrogen. 34 Luminescent patterns could be written and erased using excitation at different wavelengths (green and red). They were able to model their time evolution by considering the diffusion of photo-carriers that convert NV centres back to their negatively charge state. In our case a similar behaviour was observed but with a single wavelength at different excitation powers. Similarly to Dhomkar et al. 35 we used this property to realize a simple data storage experiment by writing a luminescent pattern into the diamond (see Fig. S2 in supplementary information). It was confirmed that the created patterns were still visible even after 24 hour time interval. The precise nature of these defects is yet to be investigated but a possible candidate is the nitrogen-vacancy-hydrogen (NVH) defect which is present at relatively high concentrations in nitrogen-doped CVD diamonds. Khan et al. have for example reported long-lasting charge transfer between NVH and surrounding defects leading to photochromism reversible through heating or UV exposure. 22 This ability to switch on and off luminescent regions of the crystal on demand might be of interest to create memories based on this material or to locally generate arrays of NV centres. If close enough, those centres could be coupled via the dipolar magnetic interaction and be used as a quantum register. Nevertheless for the purpose of creating dense NV ensembles for magnetometry applications, this low photostability is an important drawback.

The addition of N2 together with appropriate growth conditions can lead to relatively dense ensembles of NV centres. However further increasing NV density is particularly challenging due to the appearance of defects that compromise the photostability of luminescent centres. As an alternative we propose the use of nitrous oxide (N2O) as a doping source during growth. N2O is a non-toxic nonflammable gas. It is a strong oxidizer used in the semiconductor industry available with a high purity for the deposition of silicon dioxide from SiH4/N2O mixtures. 36 The energy required to dissociate this molecule in the plasma (4.6 eV) is much lower than N2 leading to a higher amount of atomic nitrogen at the growing surface together with atomic oxygen. Thin films similar to sample A were grown but with 500 ppm and 2000 ppm of N2O at 900 °C (samples B and C) for the top layer. An additional sample (sample D) was prepared with 500 ppm N2O at 800 °C to maximize NV density.

The laser microscope images of Fig. 3a-c show that all samples have a smooth morphology without any unepitaxial defects indicating that the presence of relatively high amounts of N2O did not significantly impact the growth in contrast with what is observed when using N2. Bright emission from NV centres is confirmed on the spectra of Fig. 3d obtained with a 473 nm excitation line. From the intensity of PL images acquired with a confocal scanning microscope (not shown here), it was possible to estimate NV densities to 15, 40 and 70 ppb of NV -for samples B, C and D respectively. These concentrations are significantly higher than those obtained for sample A (10 ppb) for which only a limited amount of N2 could be added in order to preserve the surface morphology. This is favourable to the development of devices that exploit luminescence from NV ensembles such as magnetic sensors and imagers. 31 It should be added that by submitting the samples to an additional post-treatment such as He or electron bombardment, to convert part of Ns into NV -, much higher densities could be achieved. Four pairs of resonances were found in the ODMR spectra indicating that NV centres were randomly oriented as expected for (100)-orientation. The FWHM of the characteristic triplet (Fig. 3a-c) was estimated to around 0.9, 1.6 and 1.4 MHz for samples B, C and D respectively. This increase in the inhomogeneous linewidth roughly scales with the NV density although slightly thinner resonances are observed for the sample with highest doping. Widening of the peaks is usually indicative of a higher stress in the crystal or higher spin bath density. Therefore to improve the performance of magnetic devices based on NV centres, one needs to find a compromise between a higher NV density and longer coherence time of the ensembles.

In order to assess the nitrogen doping efficiency with different dopant sources, we performed Secondary Ion Mass Spectroscopy (SIMS) analysis on sample A (100 ppm N2) and another diamond layer (sample E) grown under similar conditions as B (2000 ppm N2O and high temperature). Details of the analysis conditions and depth profiles are presented in the supplementary information file (Fig. S3). Tab. 1 shows the measured and calculated values. Although the nitrogen doping efficiency is an order of magnitude higher with N2, the proportion of NV to the total N amount is improved with N2O. It is possible that the presence of oxygen limits the incorporation of N in the crystal. However much higher amounts of this gas can be added without deteriorating the crystalline quality thus allowing reaching higher NV densities. The most interesting finding though is the high photostability of NV ensembles produced by doping with N2O during growth. Fig. 3e shows PL emission under low and high laser scanning powers (250 nW and 5 mW) for sample C. No change in NV emission intensity was observed in contrast with sample A that showed strong bleaching of the luminescence. This result was confirmed for the other 2 samples (B and D). Defects that contribute to the change in NV charge state through trapping and detrapping of carriers have therefore a much lower density here. It is unclear what the reason for this improvement was but the different chemical species produced near the growth surface could lead to a different mechanism of nitrogen incorporation in the crystal. In particular the presence of atomic oxygen through the dissociation of N2O could have a beneficial effect by etching away unwanted defects as already reported for intentional additions of O2. 37 Using N2O is indeed a very efficient way to bring a low amount of oxygen near the growth surface together with the nitrogen dopant.

In summary, we have explored the possibility to grow CVD diamond layers that contain a high density of NV ensembles on a (100) orientation. The amount of N2 that can be added to the gas phase is limited to a few tens or hundreds of ppm before the crystalline quality of the films is degraded leading to quenching of NV luminescence and defect formation. Despite the relatively high NV density obtained (10 ppb) the photostability is low, and under a high laser power, bleaching occurs. On the other hand the use of N2O as an alternative dopant gas source leads to much higher NV densities (up to 70 ppb) without impacting crystalline quality. Although inhomogeneous linewidth increases together with nitrogen concentration, the NV ensembles produced show perfect photostability. This result is possibly attributed to the higher dissociation efficiency of N2O in the plasma and the presence of a low amount of oxygen near the surface. The use of this alternative dopant could thus be significant for the purpose of creating highly NV-doped diamond layers with improved properties for applications in magnetic imaging and sensing. It would for example be relevant to couple this doping technique with growth on (111) or (113) substrates in order to achieve preferential orientations or to use an isotopically purified carbon source to further reduce linewidths.

Supplementary Material

Information is given about the growth procedure for the nitrogen-doped CVD diamond samples as well as the photoluminescence and ODMR set-ups used for estimating the NV density in the films. The emission stability of NV centres under various green laser powers is described leading to the possibility of writing and erasing luminescent patterns on the diamond surface. The SIMS analysis procedure is provided together with the depth profiles of samples A and E.

Tab. 1 .

 1 Concentration of N and NV measured in samples grown with different dopant gas sources.

Fig. 1 .Fig . 2 .

 12 Fig. 1. Effect of the addition of N2 on the growth of thick CVD diamond films. (a) Room temperature Raman/PL spectra (488 nm laser) of thick films (around 1 mm) grown with 20, 40 and 100 ppm of N2. Spectra were normalized to the Raman peak (labelled as R) and vertically shifted for clarity. (b) UV-Vis absorption of the same freestanding CVD films showing an increasing absorption at higher nitrogen doping. (c) and (d) laser optical and PL images (DiamondView TM ) of thick CVD films on their substrates grown with 0 and 40 ppm of N2. (e) Optical images of freestanding thick CVD crystals grown without and with 100 ppm of N2 for which brown colouration is visible. Scale bars are 1 mm.

Fig. 3 .

 3 Fig. 3. (a)-(c) Laser microscope images of samples B, C and D grown with N2O addition and zoom into one of the hyperfine structures of the ODMR spectra showing the triplet and its linewidth. (d) Room temperature Raman/PL spectra (473 nm laser) for samples A, B C and D. Spectra were normalized to the Raman peak (labelled as R) and vertically shifted for clarity. The broader baseline around 520 nm originates from luminescence of the HPHT substrate underneath. (e) PL images obtained on sample C with a confocal scanning microscope (laser 532 nm) showing the perfect photostability of NV centres under higher laser powers.

Acknowledgements

This work has been funded by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no 611143 (DIADEMS).