Saad Nader

Nicolas Brosse
email: nicolas.brosse@univ-lorraine.fr

Malek Khadraoui

Cecilia Fuentealba

Isabelle Ziegler-Devin

Fabienne Quilès

Sofiane El

Kirat Chatel

Evelyne Mauret

A low-cost environmentally friendly approach to isolate lignin containing micro and nanofibrillated cellulose from Eucalyptus globulus bark by steam explosion

Keywords: lignin-containing microfibrils, Steam explosion, Eucalyptus globulus bark, lignin, Alkali treatment

Micro-and Nano-Fibrillated Cellulose (MNFCs) have gained an increasing attention due to their remarkable properties, but their production usually requires an intensive multi-step process. This study proposes to find a novel approach involving steam explosion for the production of lignin-containing micro-and nano-fibrillated cellulose (L-MNFCs) using Eucalyptus globulus bark as a new lignocellulosic feedstock. Eucalyptus globulus bark was first pre-treated in alkali conditions by steam explosion in alkaline conditions (200°C, 8 min) or by soda cooking in a rotating autoclave (170°C, 60 min), refined and then grinded until the formation of gels. The chemical composition (Ionic chromatography, FTIR-ATR) of the pulps and morphology of the products (Morfi Neo, Optical and Atomic Force Microscopies (AFM), suspension turbidity) have been studied. Nanopapers were produced from lignin-containing microfibrils to investigate the mechanical properties. Results obtained showed that steam explosion produced pulps with slightly higher lignin content (≈ 9 %), containing shorter fibers (≈ 400 µm) and higher amounts of fine elements (≈ 86 %) compared to soda cooking (≈ 5 %, ≈ 560 µm and 66 %, respectively). AFM images of SteamEx L-MNFC gels showed a web-like structure containing lignin nanoparticles.

Graphical abstract

INTRODUCTION:

Micro-and Nano-Fibrillated Cellulose (MNFCs) constitute a new bio-based, renewable, biodegradable and nontoxic material. MNFCs width varies between approximately 10 and 100 nm or even more depending on the raw material and the production process used, and their length can reach a few micrometers (TAPPI WI 3021). Owing to their high specific surface area and aspect ratio, MNFCs exhibit attractive physical and mechanical properties (165,166). As a result, MNFCs can be used in different applications, i.e. in paper reinforcement [START_REF] Ahola | Cellulose Nanofibrils-Adsorption with Poly(Amideamine) Epichlorohydrin Studied by QCM-D and Application as a Paper Strength Additive[END_REF][START_REF] Dufresne | Nanocellulose: From Nature to High Performance Tailored Materials[END_REF], nanocomposites [START_REF] Siro | Microfibrillated Cellulose and New Nanocomposite Materials[END_REF], packaging [START_REF] Lavoine | Microfibrillated Cellulose -Its Barrier Properties and Applications in Cellulosic Materials: A Review[END_REF], aerogels [START_REF] Sehaqui | High-Porosity Aerogels of High Specific Surface Area Prepared from Nanofibrillated Cellulose (NFC)[END_REF], bio-medical [START_REF] Lin | Surface Chemistry, Morphological Analysis and Properties of Cellulose Nanocrystals with Gradiented Sulfation Degrees[END_REF], cosmetics [START_REF] Ullah | Applications of Bacterial Cellulose in Food, Cosmetics and Drug Delivery[END_REF], etc. MNFCs are extracted from the wall of vegetal fibers and wood pulp was the first material used to produce them [START_REF] Herrick | Microfibrillated Cellulose: Morphology and Accessibility[END_REF].

Researchers focused on the production of fully bleached MNFCs. Meanwhile, in the last years, isolation of lignin-containing micro and nanofibrillated cellulose (L-MNFC) was also investigated: different lignocellulosic feedstocks have been experimented, such as banana, pineapple, jute [START_REF] Abraham | Extraction of Nanocellulose Fibrils from Lignocellulosic Fibres: A Novel Approach[END_REF], different wood species [START_REF] Park | Preparation and Characterization of Cellulose Nanofibrils with Varying Chemical Compositions[END_REF], Kraft wood pulp (Wang et al. 2012), etc. MNFCs are generally produced by combining different processes [START_REF] Nechyporchuk | Production of Cellulose Nanofibrils: A Review of Recent Advances[END_REF]: (1) homogenization [START_REF] Herrick | Microfibrillated Cellulose: Morphology and Accessibility[END_REF], grinding [START_REF] Gane | Pohl, Villach (AT)[END_REF], refining [START_REF] Henriksson | An Environmentally Friendly Method for Enzyme-Assisted Preparation of Microfibrillated Cellulose (MFC) Nanofibers[END_REF], extrusion [START_REF] Ho | Nanofibrillation of Pulp Fibers by Twin-Screw Extrusion[END_REF] being the most common studied processes, (2) chemical and biological pre-treatments such as carboxylation via TEMPO oxidation [START_REF] Bäckström | Effect of Ionic Form on Fibrillation and the Development of the Fibre Network Strength during the Refining of the Kraft Pulps[END_REF]. Depending on the applications, post-treatments can be used to modify MNFCs, i.e.

surface modification [START_REF] Missoum | Nanofibrillated Cellulose Surface Modification: A Review[END_REF]. The high-energy consumption of the whole processes remains the most important obstacle for MNFCs wide use. It has been shown that the energy consumption of homogenization process significantly decreases with pretreatments (from 70 MWh/t [START_REF] Eriksen | The Use of Microfibrillated Cellulose Produced from Kraft Pulp as Strength Enhancer in TMP Paper[END_REF]) to 2 MWh/t [START_REF] Lindström | Nanocellulose Research and Developments at Innventia[END_REF].

Steam explosion (SteamEx) is a thermomechanical process, which is known as one of the most efficient pre-treatments of lignocellulosic materials [START_REF] Sun | Hydrolysis of Lignocellulosic Materials for Ethanol Production: A Review q[END_REF]. A batch steam explosion facility is composed of a steam generator that supplies a reactor with vapor to heat the biomass under pressure for short times. A sudden release of pressure ejects the biomass from the reactor to the explosion tank. This process is constituted of two phases: (1) steam treatment where water vapor and high temperature acts together to initiate the depolymerization of hemicelluloses and degradation of lignin, (2) explosive defibration where the sudden depressurization provokes a rupture of the fiber cell wall due to shear and compression forces caused by the evaporation of the condensed water in the biomass [START_REF] Cherian | A Novel Method for the Synthesis of Cellulose Nanofibril Whiskers from Banana Fibers and Characterization[END_REF][START_REF] Jacquet | La steam explosion : application en tant que prétraitement de la matière lignocellulosique[END_REF][START_REF] Nechyporchuk | Production of Cellulose Nanofibrils: A Review of Recent Advances[END_REF]. An economic evaluation carried out by Avellar and Glasser in 1998 showed that SteamEx is a low-cost pretreatment, even at small scale. In this study, the cost of the steam explosion process was evaluated to 0.077 $/Kg dry basis (in 1998). As a result, this technology is currently developed at the industrial scale for the pretreatment of biomass for commodity and low-cost cellulosic bioethanol.

It has been reported that SteamEx significantly impacts the cell wall by converting a large part of hemicelluloses into soluble sugars [START_REF] Chadni | Steam Explosion Process for the Selective Extraction of Hemicelluloses Polymers from Spruce Sawdust[END_REF] and by degrading the lignin through aryl-ether bond hydrolysis [START_REF] Obame | Homolytic and Heterolytic Cleavage of β-Ether Linkages in Hardwood Lignin by Steam Explosion[END_REF]. During SteamEx treatment, the breakdown of lignin produces small fragments of melted lignin which may redeposit on the surface of the fibers. This lignin relocation has been extensively reported in the literature and microscopy images of lignin droplets have been published [START_REF] Takada | The Influence of Lignin Migration and Relocation during Steam Pretreatment on the Enzymatic Hydrolysis of Softwood and Corn Stover Biomass Substrates[END_REF][START_REF] Chandra | The Influence of Lignin on Steam Pretreatment and Mechanical Pulping of Poplar to Achieve High Sugar Recovery and Ease of Enzymatic Hydrolysis[END_REF]. In contrast to the other cell wall components, cellulose is relatively unaffected by the treatment. It has been shown that SteamEx primarily in alkali conditions [START_REF] Sauvageon | Toward the Cottonization of Hemp Fibers by Steam Explosion -Part 1: Defibration and Morphological Characterization[END_REF][START_REF] Sutka | Chemical and Physical Modification of Hemp Fibres by Steam Explosion Technology[END_REF] degrades the cell middle lamella allowing the isolation of cellulosic fibers and micro-fibrils from biomass. In a pioneering work, [START_REF] Kessler | STEAM EXPLOSION OF FLAX Ð A SUPERIOR TECHNIQUE FOR UPGRADING FIBRE VALUE[END_REF] reported the production of linen from soda impregnated flax fibers by SteamEx. More recently, in our group [START_REF] Sauvageon | Toward the Cottonization of Hemp Fibers by Steam Explosion -Part 1: Defibration and Morphological Characterization[END_REF], we described the cottonization of hemp fibers by alkali SteamEx and the production of individual fibers with a diameter of about 20 µm. Damaged fibers were also observed due to the conjugated effect of alkali degradation and steam explosion.

SteamEx has also been studied in various experimental conditions by other authors for the production of MNFCs from fibrous biomass. [START_REF] Cherian | Isolation of Nanocellulose from Pineapple Leaf Fibres by Steam Explosion[END_REF] produced MNFCs from pineapple leaves by steam treatment performed in an autoclave (0.13 MPa during one hour) followed by bleaching (NaClO, KMNO 4 ). Kaushik et Singh 2011 applied SteamEx (2 MPa, 4h) on wheat straw followed by bleaching (H 2 O 2 ), sonification and homogenization. [START_REF] Abraham | Extraction of Nanocellulose Fibrils from Lignocellulosic Fibres: A Novel Approach[END_REF] obtained MNFCs from three different biomasses (banana, jute and pineapple leaf fibers) by combining SteamEx to alkaline extraction, bleaching and acid hydrolysis. [START_REF] Tuzzin | Nanofibrillated Cellulose from Tobacco Industry Wastes[END_REF] used tobacco to extract MNFCs using SteamEx (1.2 MPa) followed by bleaching (NaClO, H 2 O 2 at 50°C) and grinding. Nevertheless, because of technical limitations, most of the described experiments were performed at relatively low steam pressure (around 0.1 -1.2 MPa) using an autoclave. In such conditions the explosion step may be quite less efficient because it is not well controlled and/or optimized. When SteamEx was applied in SteamEx apparatus, low pressures and long residence times have been used leading to a long energy consuming process. In addition, there were no consistency between mentioned pressures and cooking temperatures.

Economically, Eucalyptus globulus is one of the most important forest species in Chile.

Harvesting of Eucalyptus generates about 30% of residues (15-25 t/ha year), 10-12% of the residue volume corresponding to barks [START_REF] Sartori | Chemical Characterization of the Bark of Eucalyptus Urophylla Hybrids in View of Their Valorization in Biorefineries[END_REF]. There are only few publications on the valorization of Eucalyptus bark (EB) but, recently, this resource has been considered as a new raw material for the pulp and paper industry due to its high fiber content [START_REF] Miranda | INCORPORATION OF BARK AND TOPS IN EUCALYPTUS GLOBULUS WOOD PULPING[END_REF][START_REF] Romaní | Valorization of Eucalyptus Nitens Bark by Organosolv Pretreatment for the Production of Advanced Biofuels[END_REF]. [START_REF] Fuentealba | New Biobased composite material using bark fibres Eucalyptus[END_REF] have also demonstrated that the morphological and the physical properties of EB allow its use as an innovative insulation material.

To the best of our knowledge, no study has been conducted until now to produce L-MNFCs from Eucalyptus globulus bark. In addition, no comprehensive study has been conducted to thoroughly investigate the specific effect of the SteamEx treatment on MNFCs morphology and composition.

The aim of this study is to find a novel way to valorize Eucalyptus globulus barks considered as a harvesting residue. To this purpose, the feasibility of the production of lignin-containing micro and nanofibrillated cellulose (L-MNFC) from this new lignocellulosic feedstock has been tested. A combinative process including SteamEx as a pre-treatment has been used to examine the impact of the explosive decompression on the fiber's morphology (optical microscopy, Morfi Neo, AFM, turbidity and nanosized fraction) and the chemical composition. L-MNFCs gels has been produced and their quality index have been calculated.

Nanopapers were also produced and tested.

2 Materials and methods:

Materials:

Eucalyptus globulus barks (EB) used in this work were provided by Unidad de Desarollo Tecnológico (UDT) located in Coronel, Biobio, Chile. They were defibered, dried and dust was removed. Sodium hydroxide used for the pulping was purchased from VWR chemicals (Czech Republic).

L-MNFC Preparation:

EB were soaked, during one night at room temperature, in water (WS) or soda solution (SS, w:w = 1:100) to allow the fibers to swell and facilitate the penetration of chemicals during pulping.

SteamEx has been tested to study the effect of this pre-treatment on the production process. For this purpose, the treatment was carried out in a high pressure SteamEx apparatus. The equivalent of 100 g of barks was impregnated in 200 mL of NaOH solution (20% wt.) for one night. Barks were heated to reach 200°C in the reactor (real temperature, corresponding effective pressure of 1,45 MPa) by injecting vapor steam from a boiler (20 bar, heating temperature around 220°C) and heating was maintained for 8 minutes. A sudden decompression ejects barks to the explosion tank. The obtained exploded material was filtered and washed on a 40 µm mesh.

In order to get a reference, bark pulping was also performed by soda cooking (SC) in electrically heated rotating autoclaves. The equivalent of 100 g of dry barks and a sodium hydroxide solution (20% wt.) were placed in the autoclave (liquid/solid ratio = 6). The autoclave was hermetically closed and heated while rotating to reach 170°C. This temperature was maintained for 60 minutes. Autoclave was then cooled, and the pulp was filtered and washed on a 40 µm mesh.

The equivalent of 30 g dry pulp from soda cooking or SteamEx was then refined using a PFI mill until a drainage index of 70°SR (Schopper Riegler degree). Refining was accomplished until 6500-7500 and 8500 revolutions of the PFI mill disk for fibers produced by soda cooking and SteamEx, respectively.

A 2 % suspension of refined fibers was then grinded using a Masuko grinder (Model MKZA6-2, Disk model MKG-C 80, Masuko Sangyo Co., Ltd, Japan) as following: 10 passes at 0 (750 rpm), 10 passes at -5 (1200 rpm), 15 passes at -10 (1500 rpm), 5 passes at -15 (1700 rpm), 2 passes at -20 (1700 rpm). The whole process is summarized in figure 1.

Characterizations:

Measurements in this part were at least triplicated except for soxhlet extraction and turbidity (1 and 10 measurements, respectively).

Morphological properties:

MorFi NEO is an analyzer of fiber morphology. This technology is based on image analysis of a fiber suspension flowing through a measuring cell. An optical system acquires images, which are processed by a computer. In this work, MorFi NEO was used to determine the fiber length, fiber width and the percentage of fine elements of the suspensions obtained after refining and after Masuko grinding. Measurement time was fixed to 5 minutes and the adopted procedure considers a fiber as an element of length higher than 200 µm and fine elements as elements of length less than 200 µm. For this purpose, different suspensions were prepared by dispersing 0.3 g (dry weight) of material in 1L of water (15 g of L-MNFC gel in 1L water).

Macrosized fraction:

L-MNFC suspensions were diluted to 0.1% wt. (5 g of L-MNFCs gel in 95 g deionized water) in deionized water then stirred by ultraturax (IKA®T-25) on speed 5 for 10 minutes. Images of the suspension were taken by a digital camera (AxioCam MRc 5) placed on an optical microscope (Carl Zeiss Axio Vert.A1, Germany). Images were analyzed by ImageJ to calculate average size of particles.

Nanosized fraction (NF):

L-MNFC suspensions were diluted to 0.02% wt. (1 g of L-MNFCs gel in 99 g deionized water)

and stirred by ultraturrax (IKA®T-25) on speed 5 for 6 minutes. 100g of suspension were centrifugated for 15 minutes at 4°C (velocity = 1000 g) using a Sigma 3-18 KS Germany centrifuge. To calculate NF (1), concentrations before (Cb) and after (Ca) centrifugation (in the supernatant phase) were measured:

AFM:

L-MNFC were observed using a BioScope Resolve atomic force microscope (Bruker) in Peak Force Tapping mode using SNL-C tips. For each sample, images were obtained on three different areas. L-MNFC suspensions were diluted to (10 -5 % wt.) in deionized water. 200 µl of each suspension were deposited on freshly cleaved mica substrates and let to dry overnight at 30°C before imaging.

Turbidity:

The turbidity of L-MNFC suspensions (diluted to 0.1% wt. and stirred with ultraturrax) was measured with a turbidimeter (Aqualytic, AL-250T-IT, wave-length 860 nm). This test is based on the determination of the scattered light at an angle of 90° to the incident light; values are directly linked to the shape, size, concentration and refractive index of the suspended matter.

Physical and mechanical properties of nanopapers

Nanopapers were prepared with a handsheet former (Rapid Kothen, ISO 5269-2). The equivalent of 2g of dry L-MNFC (100 g of L-MNFC gel) was diluted to 0.5% in deionized water. The suspension was filtered on a 1 µm nylon sieve under vacuum to remove water.

The sheet was dried under vacuum at 85°C between two nylon sieves for 12 min.

Nanopapers were stored for at least 48 hours in a conditioned room at 23°C and 50% RH before characterization.

Tensile properties were measured with a vertical extensometer (Instron 5965), following the standard NF Q03-004. Tensile tests were performed at 10 mm/min. The dimensions of the samples were 10 cm for the length and 15 mm for the width. The porosity (P) was calculated as following, considering that nanopaper is constituted of cellulose only:

Where is the density (g/cm3) of the nanopaper and .

Simplified quality index:

In a previous work, [START_REF] Desmaisons | A New Quality Index for Benchmarking of Different Cellulose Nanofibrils[END_REF] developed an index that can be used as a quality control for MNFCs at the lab scale or industrial scale. This quality index was calculated according to the simplified formula:

with x 1 representing the nanosized fraction (%), x 2 the turbidity (NTU), x 3 the Young's modulus (GPa) and x 7 the macrosized fraction (µm 2 ).

Chemical composition:

Extraction of approximately 4 g of raw material or pulps was performed in a Soxhlet apparatus with a toluene/ethanol mixture (2/1 v/v) under reflux, for 8 h. The contents of extractives were determined from the mass of the solid residue after drying at 105°C, and reported as percent of that of the original sample.

In order to remove tannins and other polyphenols, alkaline lixiviation with 1% wt. NaOH was carried out in a stirred glass reactor under reflux using 1.0 g of the extractive-free material with a 1:50 solid: liquid ratio, at 100°C for 1h.

Klason lignin was determined on the extracted materials following an adapted method from National Renewable Energy Laboratory (NREL). Sulphuric acid (72%, 1.5 ml) was added to 0.175 g of the sample, and the mixture was placed in a water bath at 30°C for 1 h. Then, 42 mL of deionized water were added and the sample was hydrolyzed for 1 h at 120°C. The sample was vacuum filtered through a crucible and washed with ultrapure water until obtaining 100 mL of filtrate. Klason lignin was determined from the mass of the solid residue after drying at 105°C. The monomer sugar content in liquid fraction was analyzed using highperformance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) (ICS-3000 Dionex).

Infrared spectroscopy:

Fourier Transform Infrared (IR) spectra in Attenuated Total Reflexion mode (ATR) were recorded between 4000 and 700 cm -1 on a Bruker Vertex 70v spectrometer equipped with a KBr beam splitter and a deuterated triglycine sulphate (DTGS) thermal detector. The resolution of the single beam spectra was 4 cm -1 . A single-reflection diamond ATR accessory (Platinum ATR diamond, Bruker) was used for acquiring the spectra. The incidence angle was to ensure a good contact between fibers and the diamond crystal. Water vapor subtraction was performed when necessary. The base line was corrected at 3600, 2750, 1800 and 800 cm -1 . The spectra were normalized to one from region 1800-800 cm -1 .

3 Results and discussion:

The effect of SteamEx on the morphology and composition of lignocellulosic fibers has been extensively described in the literature. In a SteamEx treatment, the biomass is first impregnated and then treated with saturated steam at high pressure for a short period (only few minutes). During the cooking step, a hydrolytic breakdown of the lignocellulosic complex occurs impacting primarily non-cellulosic polymers (hemicelluloses and lignin). The second step consists of a quick depressurization causing water expansion and physical degradation of the fibrous material. As a result, this process leads to a significant modification of the cell wall structure, degrading the cell middle lamella and allowing the extraction of cellulosic fibers from biomass. One of the most studied applications of SteamEx is the production of a cellulose-rich residue bearing a higher enzyme accessibility for bioethanol production. In this study, SteamEx has been experimented as a pre-treatment s for L-MNFC gel production.

Alkaline conditions have been used to promote the defibration by removing lignin cement. In order to identify the impact of SteamEx on the fibers, soda cooking performed in electricallyheated rotating autoclaves without explosive decompression step was conducted for comparison.

Pulping yield and chemical composition:

Table 1 reports the pre-treatment and pulping conditions as well as the yield and the chemical composition of the different produced pulps.

In a surprising way, soda swelling had no visible effect on the chemical composition. It was expected that 1%NaOH impregnation will decrease the extractives content leading to a better response to treatments.

It appears that non-treated EB contains 15.1 % and 19.9±1.4 % toluene-EtOH and 1%-NaOH extractives, respectively. The lignin content is equal to 14.5±0.5 %. Cellulose and hemicellulose content are 34.03±0.3 and 14.7±0.0, respectively.

The pulping yield after soda pulping was 58% wt. and 52% wt. for fibers soaked in water and 1%-wt. soda, respectively. The data in table 1 also shows that Toluene-EtOH and 1%-NaOH extractives content decreased to about 8 % and 12 %. After pulping, hemicellulose and lignin contents are close to 9 % and 6.5 %. Meanwhile, obtained pulps contains about 50 % of cellulose.

Chemical composition was also analyzed after alkaline SteamEx. Data in table 1 shows that pulps were obtained with a yield of 40-44 % wt. Toluene-EtOH and 1% NaOH extractives content are close to 8 % and 10 %. Hemicellulose and lignin contents decreased to about 11 % and 9.0 %. Meanwhile cellulose content is 58.0-60.0 %.

Results thus show that alkaline SteamEx allowed the isolation of fibers with comparable chemical composition with those isolated with soda cooking method in autoclaves (SC). The advantages of SteamEx is that the cooking time is lower compared to SC (8 min and 60 min, respectively). Both approaches lead to hemicellulose hydrolysis and partial delignification.

Nevertheless, in the tested experimental conditions, the yield is higher for soda pulping even if delignification is greater. This observation is in accordance with the work of [START_REF] Lavoie | Production of Pulp from Salix Viminalis Energy Crops Using the FIRSST Process[END_REF] showing that the yield of kraft pulping was higher than that of SteamEx (34% and 30%, respectively).

NTEB, SC and SteamEx fibers were analyzed using IR-ATR to examine the changes in their chemical composition before and after the treatments (Figure 2). The bands at 1750 cm -1 (Kaushik and Singh 2011), present in the spectrum of NTEB, is assigned to the vibrations of the carboxyl and acetyl groups of hemicelluloses. The absence of this band in the SC and SteamEx fibers is in accordance with the low hemicelluloses content previously observed (Table 1) and can also be justified by saponification reactions of acetyl moieties. Spectra also show a drastic decrease in intensity of the bands at 1600 cm -1 and at 1510 cm -1 , and can be attributed to the C=C stretching from the aromatic Guaiacyl (G), Syringyl (S) ring of the lignin, respectively [START_REF] Carrillo | Comparative Study of Cellulosic Components Isolated from Different Eucalyptus Species[END_REF]. A decrease of the band at 1260 cm -1 (Kaushik and Singh 2011) assigned to C-O stretching from lignin and hemicelluloses is also observed. All these observations confirm the hemicelluloses hydrolysis and the delignification during the soda pulping and the SteamEx. In region 1200 -1000 cm -1 , where mainly C-C and C-O stretching vibrations absorb, the general feature of the massif resembles to those of commercial cellulose [START_REF] Richard | Infrared Spectroscopy and Multivariate Analysis to Appraise α-Cellulose Extracted from Wood for Stable Carbon Isotope Measurements[END_REF]. In addition, the anomeric band at 896 cm -1 , specific for βlinkages in cellulose, is clearly resolved in the spectra of the treated samples.

Refining:

Figure 3 shows the evolution of the Schopper Riegler degree of the obtained pulps as a function of the number of revolutions of the PFI mill.

We can observe from Fig. 3 that the four pulps do not display the same refining behavior.

WS-SC and SS-SC pulps (soda cooking in autoclaves) were refined more easily than SteamEx pulps, a Schopper Riegler degree of 70 being reached after ≈ 6000 revolutions and ≈ 8000 revolutions respectively. These different behaviors can be explained, at least partially, by the slight difference in lignin content: lignin makes the separation of hemicelluloses and cellulose in the lignocellulosic matrix more difficult [START_REF] Tuzzin | Nanofibrillated Cellulose from Tobacco Industry Wastes[END_REF], limits the swelling of fibers and thus negatively impacts the refining ability of the pulps.

Morphological properties:

Table 2 shows the data obtained from MorFi NEO analyzer. Before grinding, the refined pulps exhibited important morphological differences: compared to SC fibers, SteamEx fibers were shorter (≈ 560 µm and ≈ 400 µm, respectively) with higher fine elements contents (≈ 65 % and ≈ 85 %, in length respectively). A higher number of fine elements (length < 200 µm)

was also detected 5.3 -6.3 x 10 4 and 7.3 -6.3 x 10 4 for SC and SteamEx, respectively. These results prove a most intense degradation of the fibers with a production of shorter fragments for the SteamEx process. The mean total length was calculated for the refined samples. Results showed that the total length was ≈ 204-222 µm and ≈ 77-90 µm for SC and SteamEx, respectively which revealed that the remaining fibers after SteamEx were 2.5 times shorter than after SC. On the other hand, the width of the SteamEx refined fibers is greater (≈ 19.5 µm and ≈ 22.5 µm, for SC and SteamEx respectively). This may be due to the higher content of lignin for SteamEx (see table 1) leading to a lower separation of fibers.

Regarding the L-MNFC gels, grinding step had a levelling effect on the morphological properties of the fibers produced but SteamEx residual fibers were still shorter (≈ 300 µm compared to 360 µm for SC). As expected, the number of fibers (length > 200 µm) detected by MorFi decreased from ≈ 12.2 -13,5 x 10 3 in the refined pulps to ≈ 0.6 -3.0 x 10 3 in the gels because of the grinding process which enhances the fragmentation of fibers into finer elements. The fine element content reached 98 % (in length) and SteamEx gels exhibited the highest number of fine elements (21.7 -24.9 x 10 5 ) compared to SC gels (5.27 -5.85 x 10 5 ).

Finally, the difference in total mean length is also observed in the gels produced after grinding (≈ 33 -38 µm and ≈ 26 -31 µm for SC and SteamEx, respectively). These observations are in accordance with previously reported works : [START_REF] Luo | Effects of Steam Explosion on the Characteristics of Windmill Palm Fiber and Its Application to Fiberboard[END_REF] found that length and fine content of wind mill palm fibres were highly affected by SteamEx. [START_REF] Sauvageon | Toward the Cottonization of Hemp Fibers by Steam Explosion -Part 1: Defibration and Morphological Characterization[END_REF] reported that alkali SteamEx performed from hemp fibres at high severity produced short and damaged fibres. AFM images of the samples (Fig 4a,b and d) showed a web like morphology with some aggregates. The height of L-MNFCs, calculated from AFM Images, varied between 5 and 100 nm and no significant difference was observed between SC and SteamEx samples. For the SteamEx sample, dispersed globular particles are clearly visible (Fig. 4d). Based on literature data, they are most probably composed of lignin [START_REF] Vignon | Steam Explosion of Woody Hemp Ch Nevotte[END_REF][START_REF] Ehman | Influence of Initial Chemical Composition and Characteristics of Pulps on the Production and Properties of Lignocellulosic Nanofibers[END_REF][START_REF] Yuan | Lignin Containing Cellulose Nanofibers (LCNFs): Lignin Content-Morphology-Rheology Relationships[END_REF]. In fact, during SteamEx, lignin is depolymerized through aryl-ether scission producing small lignin fragments. The temperature of the SteamEx treatment being higher than the glass transition temperature of lignin, the lignin becomes mobile and migrates within the biomass sample. Due to its hydrophobic nature, lignin coalesces into spheres to minimize its contact surface with water. This lignin translocation has been observed by microscopic techniques in the form of droplets on the fiber surface. The determination of the macrosized fraction was overestimated for the L-MNFC suspensions obtained in our study, because of the broad size distribution of the samples which was an obstacle for this measurement. However, there was no significant difference between the mean values (≈ 50 µm) calculated for the different suspensions.

Figure 5 shows the turbidity and the nanosized fraction of the L-MNFC suspensions.

According to [START_REF] Nuopponen | Method for manufacturing nanofibrillated cellulose pulp and use of the pulp in paper manufacturing or in nanofibrillated cellulose composites[END_REF], with respect to turbidity, the lower the turbidity value, the more nano-sized material the sample should contain. Accordingly, the turbidity results obtained agree with the proportion of the nanometric fraction: SS-SteamEx had the highest proportion of nanometric fraction and the lowest turbidity while the WS-SteamEx sample, had the lowest proportion of nanometric fraction and the highest turbidity. This higher turbidity value can be also explained by the higher amount of aggregates in WS-SteamEx samples. The lowest proportion of nanosized fraction is also in accordance with this observation. However, the differences between the samples remain low considering the standard deviations.

Nanopapers

Nanopapers have been prepared from L-MNFC gels produced in this study. Table 3 gives their physical and mechanical properties. The Young's moduli, quite similar for all the nanopapers (comprised between 8.42 and 9.45 GPa) are quite good when compared, for instance, to 6.3 GPa for nanopapers produced from lignin containing cellulose nanofibrils (L-CNF) obtained from rice straw [START_REF] Taha | Evaluation of eco-friendly cellulose and lignocellulose nanofibers from rice straw using Multiple Quality Index[END_REF]. Nanopapers in this study were produced from lignin containing cellulose nanofibrils obtained from rice straw. Young's moduli values of nanopapers obtained in our study are quite comparable to values obtained by Rojo et al. 2015 (10 -12 GPa). In their study L-CNF with different lignin contents (2 -14%) were produced from Norway spruce using a combination of different mechanical processes. They showed that the produced nanopapers exhibited mechanical properties similar to fully bleached ones. The results also showed that the elongations at break for SteamEx L-MNFCs nanopapers are considerably higher than those obtained by soda cooking. This was confirmed by the tensile strength results which were higher for SteamEx. This could be due to the higher proportion of fine elements which results in a larger surface area favoring hydrogen bonding between the fibrils [START_REF] Kumar | Comparison of nano-and microfibrillated cellulose films[END_REF]. Indeed, density of the SteamEx nanopapers is slightly higher even if the differences remain small. In the same way, nanopaper porosity varied between 20 % (SteamEx) and 26 % (SC). No significative correlation was found with L-MNFCs composition in terms of cellulose, hemicelluloses and lignin. Interestingly, the relatively high lignin content remaining after SteamEx did not lead to the decrease of Young's moduli. This could be due to the relocation of the modified lignin during the explosive decompression previously discussed. In the literature, the effect of the presence of lignin in L-MNFC on the mechanical properties of nanopapers is still debated, the effect depending on the lignin content and the production process [START_REF] Rojo | Comprehensive Elucidation of the Effect of Residual Lignin on the Physical, Barrier, Mechanical and Surface Properties of Nanocellulose Films[END_REF][START_REF] Albornoz-Palma | Effect of Lignin and Hemicellulose on the Properties of Lignocellulose Nanofibril Suspensions[END_REF]

Quality index

Figure 6 shows the quality index of the L-MNFC gels. This simplified quality index has been defined as the sum of the marks of turbidity, nanosized fraction, macrosized fraction and Young's modulus (see 2.3.7). The initial objective of this work was to propose an index, based on relatively simple methods, and giving a quantitative evaluation of the quality of the nanofibrillated cellulose gels at the macro-and microscales. In addition, the use of this index is probably the most effective way to compare cellulose nanofibrils, in the absence of other quantitative approaches.

Quality indexes were quite similar for all the produced gels and varied between 54-56 and 52-57 for SC and SteamEx, respectively. This result was expected since there was no remarkable difference in the marks used in the calculations. The application of the same ultrafine grinding conditions to different pulps (soda cooking or SteamEx) resulted in the production of L-MNFCs with comparable properties. The lower value for the WS-SteamEx suspension is due to the highest turbidity value measured for this sample and to the lower proportion of nanometric fraction. For the best of our knowledge, this index has been used

to compare bleached pulps and it has not been developed yet for lignin containing pulps.

Indexes calculated in this work are comparable to values of enzymatic commercial MNFCs (quality index ≈ 60) [START_REF] Desmaisons | A New Quality Index for Benchmarking of Different Cellulose Nanofibrils[END_REF]. This result showed that the proposed process in this work leads to the production of good quality L-MNFCs without bleaching or enzymatic treatments.

Conclusion:

In this work, lignin-containing micro-and nano-fibrillated cellulose (L-MNFCs) were produced from Eucalyptus barks through a sequential process using steam explosion as pre-treatment.

Pulps were also obtained using soda cooking in rotating autoclaves to compare the different production processes. In the tested experimental conditions, the obtained results showed that the use of alkaline SteamEx allows to obtain pulps with comparable chemical composition to conventional pulping methods but with a significantly lower cooking time. L-MNFC exhibited web like morphology with fibrils width of 5-100 nm. AFM images showed that SteamEx L-MNFCs also contain lignin nanoparticles expelled during the explosive decompression. The increase in density due to fines and lignin compensated the less ideal fiber characteristics to achieve comparable properties. The results encourage the interest in using alkaline SteamEx pre-treatment as an alternative way to produce L-MNFCs comparable to L-MNFCs obtained by soda cooking, with respect to the quality index.

Figure 1 :

 1 Figure 1: Schematic of the L-MNFC production process used in our study.

  45° and the refraction index of the crystal was 2.4. The number of bidirectional double-sided interferogram scans was 200, which corresponds to a two-minute accumulation. All the interferograms were Fourier processed using the Mertz phase correction mode and a Blackman-Harris three-term apodization function. No ATR correction was performed. IR-ATR spectra are shown with an absorbance scale corresponding to log(R reference /R sample ), where R is the internal reflectance of the device. Recording of the spectra, data storage and data processing were performed using the Bruker OPUS 7.8 software. The measurements were performed at 21±1 °C in an air-conditioned room. The obtained fibers after alkaline SteamEx and soda cooking in rotating autoclaves were laid on the ATR crystal and gently pushed on it

Figure 2 :

 2 Figure 2: IR-ATR spectra of raw Eucalyptus globulus barks (dark), soda cooking in autoclave (red), and alkaline SteamEx (blue). Principal assignments are indicated. The spectra are normalized to one from region 1800-800 cm -1 . For clarity, spectra were shifted vertically. Key: n, stretching vibration; d, bending vibration.

Figure 3 :

 3 Figure 3: Evolution of Schopper Riegler degree for different pulps with the number of revolutions of the PFI mill disk. Key: WS, water swelling; SS, soda swelling; SC, soda cooking in autoclave.

Figure 4 :

 4 Figure 4: AFM height images of (a) WS-SC 10 µm x 10 µm, (b) WS-SC 3 µm x 3 µm, (c), WS-SteamEx 10 µm x 10 µm and (d) WS-SteamEx 3 µm x 3 µm. Lignin droplets are marked by red arrows in d. Key: WS, water swelling; SS, soda swelling; SC, soda cooking in autoclaves.

Figure 5 :

 5 Figure 5: Turbidity and nanosized fraction of different L-MNFC suspensions. Key: WS, water swelling; SS, soda swelling; SC, soda cooking.

Figure 6 :

 6 Figure 6: Quality index of L-MNFC suspensions. Key: WS, water swelling; SS, soda swelling; SC, soda cooking.

Table 1 :

 1 pulping conditions, yield and chemical composition of fibers obtained by alkaline SteamEx and soda cooking in rotating autoclaves313(values are given in wt% based on 100 g of pulp).

	314
							Chemical composition
	Sample	Pretreatment Temperature (°C) Time (min) Yield (%)	Toluene-EtOH extractives	1%-NaOH extractives	Lignin	Hemicelluloses Cellulose
	EB		No treatment			15,1	19,9 ± 1,4	14,5 ± 0,5	14,7 ± 0,0	34,3 ± 0,3
	WS-SC SS-SC	soda cooking	170	60	58 52	8,0 8,4	12,9 ± 0,5 11,7 ± 0,5	6,8 ± 0,4 6,3 ± 0,6	8,2 ± 1,7 10,9 ± 1,2	49,5 ± 5,1 52,2 ± 3,4
	WS-SteamEx SS-SteamEx	SteamEx	200	8	44 40	8,1 8,2	11,7 ± 0,5 8,0 ± 0,6	8,6 ± 0,4 9.0 ± 0,4	10,3 ± 0,4 11,8 ± 0,8	58,0 ± 1,3 60.0 ± 4,3

Table 2 :

 2

	Mechanical treatment	Sample	Fibers number (fibers/g)	Length (µm)	Width (µm)	Fines number (fines/g)	Fines content (%)	Fines length (µm)
		WS-SC	13.05x10 3	558 ± 2 19.8 ± 0.1	6.26x10 5	67	31
	Refining	SS-SC WS-SteamEx	12.74x10 3 13.51x10 3	565 ± 2 19.4 ± 0.1 399 ± 1 21.6 ± 0.2	5.26x10 5 6.88x10 5	64 84	32 32
		SS-SteamEx	12.27x10 3	400 ± 2 23.7 ± 0.2	7.33x10 5	88	31
		WS-SC	0.61x10 3	360 ± 3 23,6 ± 0.3	5.27x10 5	98	26
	Masuko	SS-SC	0.98x10 3	355 ± 3 23,0 ± 0.3	5.85x10 5	97	28
	grinding	WS-SteamEx	2.76x10 3	307 ± 2 21,7 ± 0.5	24.9x10 5	99	27
		SS-SteamEx	2.58x10 3	293 ± 4 22,0 ± 0.6	21.7x10 5	99	25

MorFi Analyzer results for refined pulps, and L-MNFC gels. Key: WS, water swelling; SS, soda swelling; SC, soda cooking in autoclaves.

  . The presence of lignin nanoparticles in SteamEx L-MNFC could have increased the nanopaper density. In accordance with Bian et al. 2018, higher lignin content in SteamEx fibers facilitates the compaction during paper formation, leading to higher paper density and lower porosity. In our study, the dispersion of lignin nanoparticles, observed by AFM, could have also filled the voids within the cellulose fibrils, thus impacting nanopaper density. Moreover, analyses of the morphological properties (MorFi) showed that the number of fine elements is higher for SteamEx L-MNFC gels, which may also lead to an

	increase in the density. According to Henrikkson et al. 2007, a less porous nanopaper results
	in a denser structure with higher mechanical properties. This trend was observed in our
	study for SteamEx L-MNFCs where lignin nanoparticles filled the voids leading to a denser
	nanopaper with a higher tensile strength.

Table 3 :

 3 Mechanical and physical properties of nanopapers prepared from L-MNFC gels.

	M/NFC suspension	Young's modulus (GPa)	Strain at break (%)	Grammage (g/m2)	Tensile strength (MPa)	Density (Kg/m3)	Porosity (%)
	WS-SC	8,67±0,09	1.6±0.1	65.21±0.44	78.4±0.56	1110±1.3	26
	SS-SC	9,45±0,05	2.2±0.1	64.34±0.57	93.1±0.69	1110±1.7	26
	WS-SteamEx	8,85±0,25	4.9±0.2	71.30±0.68	123±0.43	1220±1.7	19
	SS-SteamEx	8,42±0,08	4.0±0.1	65.38±0.45	107±0.63	1180±1.3	21

Acknowledgements

The authors gratefully acknowledge the financial support of Labex Tec21 and LAbex Arbre for the thesis funding. This work was also supported by the Franco-Chilean EcosSud Collaborative Program C18E05, ANID PIA/Apoyo CCTE AFB170007 of Universidad de Concepción. We thank the Spectroscopy and Microscopy of interfaces Service Facility (SMI) of LCPME where the IR-ATR and AFM measurements were performed (Université de Lorraine-CNRS-(www.lcpme.cnrs-nancy.fr); LCPME, UMR7564, 405 rue de Vandoeuvre 54600, France).

Authors contributions


K F Badawi

P Villain

Ph P.-O Goudeau

Renault

Measuring thin-film and multilayer elastic constants by coupling in situ tensile testing with x-ray diffraction

A direct determination of the Young's modulus and the Poisson's ratio in a 140 nm polycrystalline tungsten thin film deposited by ion-beam sputtering on a polyimide substrate has been performed by coupling x-ray diffraction measurements with in situ tensile testing. The method described in this letter to extract the Young's modulus of thin films from the evolution of the sin 2 ψ curves as a function of applied load only requires to know the substrate Young's modulus. The determination of the thin film Poisson's ratio can be realized without knowing any of the substrate elastic constants. In the case of the tungsten thin film, the obtained Young's modulus was close to the bulk material one whereas the Poisson's ratio was significantly larger than the bulk one.

For a few years, there has been an increasing interest about the mechanical properties of thin films.

Literature data show clearly that the elastic properties of metallic thin films and multilayers can differ significantly from the bulk metal ones 1,2,3 . In previous papers 4,5,6 , we presented a graphical method (called the "intersection method") to extract the Poisson's ratio in thin films or multilayers deposited on substrates from the evolution of the sin 2 ψ curves as a function of the applied strain.

The presented method needed the knowledge of the substrate Poisson's ratio.

In this paper, we describe a more accurate analytical method. It allows to determine the Poisson's ratio of a supported thin film without using any of the elastic constants of the substrate, thus the Poisson's ratio of the thin film can be obtained even if the substrate is unknown. Concerning the Young's modulus of the thin film, the only data to know is the Young's modulus of the substrate.

The method is based on the "sin 2 ψ method" which has already been extensively described elsewhere [START_REF] Noyan | Residual Stress Measurement by diffraction and Interpretation[END_REF][START_REF] Hauk | Structural and residual stress analysis by non destructive methods: evaluation, application, assessment[END_REF] . It consists in applying a uniaxial tensile force to the sample in situ in an x-ray diffractometer. The thin film elastic constants are determined by studying the evolution of the sin 2 ψ curves as a function of the applied load. The main assumption in the following calculations is the elastic and linear behavior of both the substrate and the thin film.

Using x-ray diffraction, the strain εφψ measured in the direction eφψ defined in the specimen coordinate system (e11, e22, e33) by the two Euler angles φ and ψ (Fig. 1) is given by

      θ θ =         = ε φψ φψ sin sin ln d d ln 0 0 (1)
where dφψ (resp. d0) is the (resp. unstrained) lattice plane spacing of the {hkl} planes, θφψ and θ0 the angular positions of the corresponding diffraction peaks through Bragg's law.

For polycrystalline specimen with a random crystalline orientation and negligible shear stress and stress gradient in the x-ray depth probed, the strain ε depends linearly upon sin 2 ψ, ψ being the angle between the normal to the diffracting planes and the sample surface normal. In particular for φ=0, ( )

33 2 33 11 , 0 sin ε + ψ ε - ε = ε ψ (2)
Then for an elastic isotropic material, Hooke's laws give the linear relationships between the strains and stresses via the Young's modulus and Poisson's ratio.

The tensile tester supporting the sample is placed at the center of the goniometer so that the loading direction corresponds to the e11 sample axis. Assuming a uniaxial applied stress state (σ22 Af = σ33 Af = 0), the stress σ11 Af applied to the thin film is related to the load F by

        + = σ f s s f Af 11 E E e e b F ( 3
)
where b is the sample width, ef (resp. es) the film (resp. substrate) thickness and Ef (resp. Es) the thin film (resp. substrate) Young's modulus. Then, combining relations (1), ( 2) and ( 3), we obtain the following equation for a given {hkl} plane family:

f 2 f 1 1 m sin P sin 1 ln + ψ =         θ ψ , with ( ) σ         ν + +         + ν + = σ + σ         ν + = rf 11 f f f f s s f rf 11 Af 11 f f f 1 E 1 b F e E e E 1 E 1 P
, and

      θ + σ + σ ν -         + ν - =       θ + σ + σ + σ + σ ν - = 0 rf 22 rf 11 f f f f s s f 0 rf 22 rf 11 Af 22 Af 11 f f f 1 sin 1 ln ) ( E b F e E e E sin 1 ln ) ( E m (4)
νf is the thin film Poisson's ratio; the A (resp. r) index refers to the applied (resp. residual) stresses.

Plotting P1 f versus the applied force F, we obtain a linear curve. Its slope is P *

        + ν + = f f s s f * e E e E 1 b 1 P . (5)
Similarly, the curve of m1 f versus F is linear; its slope m * is

        + ν - = f f s s f * e E e E b 1 m . ( 6
)
Then we can deduce the Young's modulus Ef of the thin film from the sum of P * and m * , only knowing the substrate Young's modulus Es Furthermore, the combination of the "intersection method" 4,5 and this analytical method also allows to extract the substrate Poisson's ratio νs:

        - + = s s * * f f e E ) m P ( b 1 e 1 E , (7)
( )

f 0 2 f f 0 2 f s sin 1 sin ψ - ν ψ - ν = ν (9)
where sin 2 ψ0 f is the abscissa of the intersection point of the thin film sin 2 ψ curves plotted for several loaded states. Here νf is the value deduced from Eq. ( 8).

Indeed, the hypothesis of uniaxial stress state induces a much smaller error than one can imagine at a first sight. In fact, the difference ∆ν=νf-νs between the Poisson's ratios of the thin film and the substrate induces in the film a transverse applied stress σ22 Af ≈ σ11 Af .∆ν while σ22 As ≈ 0.

Eq (3) becomes

        ν ∆ ν - + = σ ) 1 ( E E e e b F f f s s f Af 11 , ( 10
)
which results in [ ] ) 1 ( e E e E b 1 P f s s f f f * ν ∆ ν - + ν + = (11) and [ ] ) 1 ( e E e E b ) 1 ( m f s s f f f * ν ∆ ν - + ν ∆ + ν - = . ( 12
)
Finally the film modulus calculated under a biaxial stress state (Ef(bi)) can be estimated by

) uni ( f f s s * * f f ) bi ( f E ) 1 ( e E ) m P ( b 1 e 1 ) 1 ( E ν ∆ ν - =         - + ν ∆ ν - = (13)
where Ef(uni) is the film modulus calculated under uniaxial stress state (Eq.7). Consequently it is sufficient to first measure the Young's modulus under the uniaxial stress hypothesis and then correct the obtained value by means of Eq (13).

A 140 nm thick tungsten film was deposited on a 127.5 µm thick polyimide (Kapton  ) dogbone substrate by ion beam sputtering at room temperature. It was then submitted to an Ar 2+ ion irradiation (340 keV -7.10 14 ions/cm 2 ) to improve its crystalline quality [START_REF] Durand | [END_REF] .The in-plane sample dimensions were 8×3 mm 2 . Tungsten was chosen because of its isotropic mechanical behavior and its high x-ray scattering factor. The external load was applied by means of a 300 N Deben™ tensile module. This tensile tester is equipped with a 75 N load cell enabling the force measurement with a precision higher than 0.1 N; it can be easily fitted to most goniometers thanks to its small volume (90 × 60 × 30 mm 3 ) and low weight (350 g). Because of the low film thickness and small grain size (≥ 10 nm), X-ray diffraction measurements were performed using a four-circle goniometer on the H10 beam line at the French synchrotron radiation facility LURE (Orsay, France). A large wavelength (λ = 0.2248 nm) was chosen to analyze {211} family tungsten planes for each applied load.

The evolution of the sin 2 ψ curves as a function of the applied load is shown in Fig. 2. T0 corresponds to the unloaded state while T1, T2 and T3 are related to increasing loading states. As assumed, theses curves as linear. Their slope is directly related to the total stress in the film. The residual stresses are compressive; with increasing applied stress (from T1 up to T3), the total stress value decreases and becomes then tensile for T2 and T3. Table I presents the values of the applied force, the slope and the intercept of the least squared linear regression for each loaded state. Fig. 3 shows the evolution of (a) the slope P1 f and (b) the origin ordinate m1 f of the sin 2 ψ curves versus the applied force F. As predicted by Eq. ( 4), P1 f and m1 f depend linearly upon F. The slopes are respectively P * = 6.2324.10 -4 and m * = -1.4733.10 -4 . Having previously found by direct measurement the value of 5.17 GPa for the substrate Young's modulus, Eq. ( 7) leads to a value of 390 ± 40 GPa for the thin film Young's modulus (Ef), very close to the tungsten bulk value (388 GPa) [START_REF] Smithells | Metals Reference Book[END_REF] . The film Poisson's ratio deduced from Eq. ( 8) is νf = 0.310 ± 0.015, which is significantly larger than the bulk value (0.284).

It should be noted that, since tungsten is elastically isotropic, the measurement of Ef and νf allows to calculate the thin film stiffness constants C11, C44 and C12. The obtained values are C11=541 GPa, C44=149 GPa and C12=243 GPa, while the literature values for bulk tungsten are respectively 501, 151 and 198 GPa [START_REF] Smithells | Metals Reference Book[END_REF] . We can observe that an increase of νf (with a constant Ef) results in a decrease of C11 and C12 whereas C44 remains unchanged. This is an important result which shows the advantage of the method used in this study; an interpretation in terms of microstructure modification and interatomic potentials constitutes another work and is still under progress. Finally, we can extract the Poisson's ratio of the Kapton  substrate thanks to Eq. ( 9). As it can be seen on Fig. 1, all the sin 2 ψ curves present a common intersection point which abscissa (sin 2 ψ0 f ) is equal to 0.235. This leads to νs=0.312. We can then estimate the "biaxial correction" for the tungsten Young's modulus: according to Eq. ( 13), Ef(bi)/Ef(uni) = (1-νf.∆ν) = 1.0006. Consequently the error committed here when assuming a uniaxial applied stress state is less then 0.1%. Thus it is perfectly justified to extract the thin film elastic constants in a very simple way under the uniaxial hypothesis.

In conclusion, an experimental technique for the determination of the Young's modulus and

Poisson's ratio in thin films on substrates has been elaborated by combining x-ray diffraction strain measurements and in situ tensile testing. This method presents the following main advantages: (i) the unstrained lattice parameter of the film needs not to be known, (ii) no elastic constant of the substrate or the film is necessary to determine the Poisson's ratio of the film, and (iii) the only data needed to extract the Young's modulus of the film is the substrate Young's modulus. The precision will be improved thanks to an optimization of the sample dimensions. Currently we are engaged in the study of W sublayers in W/Cu multilayers to analyze the possible evolution of the W Young's modulus and Poisson's ratio when reducing the thickness period.

Figure captions

  and its Poisson's ratio by a simple combination of these two experimental data without any other information on the substrate nor on the film

Fig. 1 :

 1 Fig.1: Deben™ tensile tester with the sample coordinate (X1, X2, X3) and the x-ray measurement

Fig. 2 :

 2 Fig.2: Sin 2 ψ curves relative to the {211} planes of the tungsten thin film in the unloaded state (T0)

Fig. 3 :

 3 Fig.3: Slope P1 f (a) and intercept m1 f (b) of the sin 2 ψ curve versus the applied force F for four

Fig. 2 ,Fig. 3 (Fig. 3 (

 233 Fig. 2, K.F. Badawi et al., APL

TABLE I :

 I Slopes and intercepts of the sin 2 ψ curves relative to the tungsten {211} family planes for four force values.

	N° of the loading state	0	1	2	3
	Applied force F (N)	1.0	3.0	4.8	6.5
	Slope P1 f	-0.001986	-0.001001	0.000254	0.001408
	Intercept m1 f	0.142221	0.141993	0.141681	0.141424


P Villain

Ph Goudeau

P.-O Renault

K F Badawi

Size effect on intragranular elastic constants in thin tungsten films

The size effect on the elastic constants of nanocrystalline tungsten has been investigated in the case of W/Cu multilayers with two modulation wavelengths (3.1 and 24.0 nm). Tungsten Young's modulus and Poisson's ratio have been measured thanks to a technique coupling x-ray diffraction with in situ tensile testing. It is demonstrated that the in-grain thin film elastic constants are highly microstructure -sensitive: in the "3.1 nm" multilayer, tungsten Poisson's ratio is larger than the bulk one while it is smaller in the "24.0 nm" multilayer; a softening of tungsten Young's modulus is evidenced in the case of the specimen with the smallest period.

Theoretical studies concerning nanocrystalline materials 1,2 predict noticeable modifications of their elastic properties compared to the bulk ones due to a high volume proportion of surface and interface atoms, as well as a high point defect density. Grain boundaries, free surfaces and interfaces induce important alterations of cohesion energies. Numerous experimental studies have evidenced a very probable softening of elastic constants in nanostructured materials, in particular the C44 stiffness coefficient in multilayers with very low modulation wavelength (Λ ≤ 3 nm) 3,4,5 . In previous papers, we described an experimental technique combining in situ tensile testing and x-ray diffraction to extract the Poisson's ratio 6 and the Young's modulus 7 of supported thin metallic films and multilayers. This diffractometric method differs from all the other mechanical, vibration, and acoustic techniques since it allows to measure average in-grain strains, and thus intragranular elastic constants.

This letter deals with the study of the evolution of tungsten intragranular elastic constants in W/Cu multilayers as a function of the modulation wavelength Λ. This system was chosen for the following experimental and physical considerations: copper and tungsten are not miscible, tungsten presents a high atomic scattering factor and is elastically isotropic, a lot of structural studies have already been done on this system 8,9 .

The technique used here, based on the well known "sin 2 ψ method" [START_REF] Noyan | Residual Stress Measurement by diffraction and Interpretation[END_REF][START_REF] Hauk | Structural and residual stress analysis by non destructive methods: evaluation, application, assessment[END_REF] , consists of applying a uniaxial tensile force to the film/substrate system, in situ in an x-ray diffractometer. The formalism is very simple in the case of a polycrystalline elastically isotropic material such as tungsten: a linear relation is expected between the shift of each {hkl} peak position and sin 2 ψ, ψ being the angle between the normal to the diffracting planes and the normal to the sample surface. For each applied load value, the sin 2 ψ curves are determined for the tungsten layers. The slope P1, on the one hand, and the intercept m1, on the other hand, of the linear regressions on these experimental data is plotted as a function of the applied load F. Assuming an uniaxial applied stress state (similar Poisson's ratios for the film and the substrate), the mean Young's modulus EW of tungsten sublayers can be then calculated by means of the following relation:

        - - + = Cu Cu s s * * W W t E t E ) m P ( b 1 t 1 E (1)
where b is the sample width, ts the substrate thickness, tW (resp. tCu) the total thickness of the W (resp. Cu ) sublayers, Es (resp. ECu) the substrate (resp. copper) Young's modulus, P * (resp. m * ) the slope of the (P1, F) linear curve (resp. the (m1, F) curve).

The mean Poisson's ratio νW of tungsten sublayers is simply deduced from a combination of P * and m * :

* * * W m P m + - = ν . ( 2
)
It is worth noting that the determination of νW does not need any numerical value concerning geometrical nor mechanical properties of the various parts of the system. For the Young's modulus measurement, the substrate Young's modulus and the copper one must be known, as well as the respective thickness of the three constituents.

W/Cu multilayers were deposited onto a 127.5 µm thick polyimide (Kapton  ) dogbone substrate by ion beam sputtering at room temperature, tungsten being the first deposited layer. The in-plane sample dimensions were 8 × 3 mm 2 . The total thin film thickness ranged from 220 to 240 nm. Before performing the in situ tensile tests, several characterization experiments have been done.

X-ray reflectivity measurements were performed in our laboratory to deduce the modulation wavelength Λ (thickness of one W/Cu bilayer) from the small angle Bragg's peak positions. Atomic proportions of tungsten and copper were determined thanks to Electron Dispersive X-ray Spectroscopy (EDXS) measurements. The mean thickness of tungsten (eW) and copper (eCu)

individual sublayers was deduced from Λ and the atomic proportions, assuming the density of each material is equal to the bulk one 12 . All these values are reported in Table I for the two specimens studied here (Λ = 3.1 nm and Λ = 24.0 nm). It should be noticed that nanostructured copper and tungsten sublayers may be less dense than the bulk materials as the modulation wavelength Λ decreases. This effect implies small variations on the eCu and eW values when considering the density ratio of both elements in the thickness calculation and thus slightly influences the precision on the modulus measurement (Eq. ( 1)).

The initial microstructural and mechanical state was also characterized. Residual stresses (σXRD) and stress-free lattice parameters (a0) of the tungsten sublayers were analyzed by x-ray diffraction; results are given in Table I. It can be seen that tungsten is in a strongly compressive stress state; the σR value increases with Λ. In the small period multilayer, the stress-free lattice parameter is smaller than the bulk one (abulk = 0.3165 nm 12 ) whereas it is larger in the other specimen. This observation reveals important differences in the microstructure of these two specimens: a vacancy-type structure and a columnar grain morphology with nanometer-sized intergranular voids when the sublayers are the thinner, a densified structure with interstitial incorporation when the sublayer thickness increases . This can be related to the growth mode:

transition from an initial discrete-island stage towards a continuous film via an island coalescence process [START_REF] Floro | [END_REF] . Comparing the x-ray residual stresses with the residual stress values obtained by the substrate curvature method (σcurvature in Table I), it can be seen that the mean stresses in the whole thin films are considerably lower than the in-grain stresses in tungsten sublayers. This can be accounted for by the mechanical contribution of interfaces and copper layers in the total thin film stress: previous studies have demonstrated that copper layers are almost stress-free 14 .

The in situ tensile tests were performed at the French synchrotron radiation facility L.U.R.E.

(Orsay, France) on the H10 and DW22 beam lines where we took advantage of the high x-ray beam intensity and quality, as well as the large wavelength range accessible. For each specimen, several increasing strains were applied and x-ray diffraction measurements were realized in each case.

Table II presents examples of applied force values and corresponding in-grain elastic strains (ε11 A ) in the tungsten layers deduced from x-ray measurements 15 . The calculation of ε11 A allows to control that the applied strain remains in the elastic domain. In Fig. 1, the evolution of the slope P1 and the intercept m1 of the sin 2 ψ curves are plotted versus the mean applied stress σm = F/Stot, where Stot is the section area of the sample (Stot = (ts + tW + tCu) × b). The slopes of these curves are clearly different for the two multilayers. Linear regressions on the experimental data give the values of P * and m * for each specimen; we can then calculate tungsten Young's modulus and Poisson's ratio via Eq. ( 1) and Eq. ( 2), using the bulk value (130 GPa [START_REF] Simmons | Single Crystal Elastic Constants and Calculated Aggregate Properties: A Handbook[END_REF] ) for copper modulus. This is an important assumption in our calculation that could be avoided if x-ray diffraction measurements using sin 2 ψ method would be possible in copper sublayers. However, the mechanical contribution of copper is very small compared to the substrate one: in Eq. ( 1), the ECueCu term represents only 2% of the Eses term. Consequently, possible effects of nonisotropic textured copper, if they exist, may only have little influence on the results concerning tungsten. The obtained values for tungsten constants are reported in Table III. These results clearly demonstrate that the elastic constants are sensitive to the microstructure: in the "3.1 nm" multilayer, the Poisson's ratio (0.359) is larger than the bulk one (0.284) while it is smaller in the "24.0 nm" multilayer (0.221); concerning the Young's modulus, a noticeable softening is evidenced in the case of the small period multilayer (E = 338 GPa) while in the other one, tungsten modulus (E = 392 GPa) is equal to the bulk value (400 GPa) within the uncertainties. It should be kept in mind that the values obtained via x-ray diffraction measurements describe the intragranular elastic behavior of tungsten layers.

Assuming tungsten layers are elastically isotropic (like bulk tungsten), the stiffness coefficients Cij can simply be extracted from the measured E and ν 7 . The calculated values are given in Table III. For the "24.0 nm" multilayer, we can note a softening of C11 and C12, C44 being equivalent to the bulk value. For the "3.1 nm" multilayer, an unambiguous softening of C44 is evidenced, which is consistent with the results generally reported in the literature 3,4,5 . On the contrary, the increase of C11 and C12 appears like a particular phenomenon that needs to be confirmed by further investigations. The bulk modulus B= 1/3 (C11 + 2C12) can also be calculated and values are reported in Table III. For the small-modulation-wavelength sample, the increase of C11 and C12 results in a B value 33% higher than the pure bulk tungsten one, which is rather surprising (Young's modulus is 15% smaller than the bulk tungsten one). Although the unit-cell defects (evidenced by the stressfree-lattice parameter variations) are very different when the modulation wavelength changes, this result is more probably due to interface effects in the growth direction that prevail over the extrapolated mechanical properties.

In addition to elastic strain measurements, a cracking phenomena was observed in the case of the "3.1 nm" multilayer when the measured intragranular elastic strain in tungsten was about 0.45 %. The corresponding values found in the literature for bulk tungsten elastic limit range from 0.4 to 0.9 % 14 , depending on the microstructure. Ex situ atomic force microscopy observations confirmed the presence of a network of straight cracks perpendicular to the tensile direction and regularly spaced (20 to 30 µm). This cracking phenomena reveals a weak cohesion between grains in this structure, due to the growth mode. Like the softening of tungsten intragranular shear coefficient (C44), this effect contributes to the reduction of the mechanical performances of this multilayer.

In conclusion, intragranular elastic properties of nanocrystalline tungsten thin films have been studied using an x-ray diffraction method combined with tensile testing. Microstructure and size effects on tungsten in-grain elastic constants have been evidenced in the case of W/Cu multilayers deposited by ion beam sputtering on Kapton  substrates with two values of the modulation wavelength. The variations of elastic constants can be correlated with the microstructure, in particular the stress-free lattice parameter. All these results concerning the various elastic constants and their combinations undoubtedly indicate basic differences in the microstructures of the smalland large-period multilayers. Further investigations will aim to find out interpretations of these experimental results in terms of interatomic potentials and cohesion energies.

Figure caption

Fig. 1 :

 1 Fig.1: Evolution of (a) the slope P1, and (b) the intercept m1, of the sin 2 ψ curves relative to tungsten

Fig. 1 .

 1 Fig.1., P. Villain et al., APL

TABLE I :

 I Main geometric and microstructural parameters of the two studied multilayers. An estimate of the x-ray stress σXRD and stress-free lattice parameter a0 in W sublayers is done using W bulk elastic constants (see TableIII). The lattice parameter of bulk W is equal to 0.3165 nm 12 .

	Modulation wavelength Λ (nm)	3.1 ± 0.1	24.0 ± 0.1
	Number of bilayers	70	10
	Atomic composition (%) (EDXS) 44% Cu/56% W	52% Cu/48% W
	W layer thickness eW (nm)	2.0 ± 0.1	13.3 ± 0.1
	Cu layer thickness eCu (nm)	1.1 ± 0.1	10.7 ± 0.1
	σcurvature (GPa)	-0.3 ± 0.1	-1.7 ± 0.1
	σXRD (GPa) in W	-1.7 ± 0.3	-5.1 ± 0.1
	a0 (nm) in W	0.314	0.318

TABLE III :

 III Values of the elastic constants (E, ν) measured in the two W/Cu multilayers, and the Cij constants and bulk modulus B deduced from these experimental values. Bulk tungsten reference values are given for comparison.

	Modulation wavelength (nm)	3.1 ± 0.1	24.0 ± 0.1	Bulk
	Modulus E (GPa)	338 ± 65	392 ± 60	400 a
	Poisson's ratio ν	0.359 ± 0.014 0.221 ± 0.003	0.284 a
	C11 (GPa)	565	448	501 a
	C12 (GPa)	316	127	198 a
	C44 (GPa)	124	160	153 a
	Bulk modulus B (GPa)	400	234	300 a
	a See Ref. 16