Bilel Grindi

Amor Benali

Cesar Magen

Guillaume Viau
email: gviau@insa-toulouse.fr

M-SrFe12O19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for permanent magnets: HAADF-STEM study and magnetic properties

Mixtures of M-type strontium hexaferrite (M-SrFe12O19) and ferrihydrite-like particles were prepared by a microwave-assisted hydrothermal process at 200 °C with heating rates in the range 40-50 °C.min -1 . The particles exhibited a platelet shape with a diameter comprised between 20 and 200 nm and a thickness between 2 and 5 nm. HAADF-STEM observations and EDS analysis were carried out for a better understanding of nucleation and growth process. EDS showed that most of the particles contained Sr and HAADF-STEM revealed that very thin particles with a hexaferrite core extending over less than a unit cell and with surface disorder crystallized along with well crystallized hexaferrite and defect free ferrihydrite particles. The symmetric multilayer structures (SRS) of the ultrathin particles suggested that the nucleation step of the hexaferrite particles involved clusters containing Sr atoms. In comparison with the M-SrFe12O19 micrometer sized platelets prepared with heating rate of 25 °C.min -1 , the mixtures of ultrathin hexaferriteand ferrihydrite-like particles combined after annealing a higher coercivity reaching 465 kA.m -1 thanks to the smaller initial particle size and a high magnetization reaching 65 A.m 2 .kg -1 thanks to a limited amount of hematite.

Introduction

Permanent magnets (PM) are essential in everyday life energy product ranging from large scale machines like high power wind mills to the smallest motors and actuators [1]. PM materials are dominated by two systems: the rare-earth based alloys (NdFeB and SmCo) and the barium and strontium hexaferrites. Despite they exhibit lower energy products than the rare-earth based magnets, the hexaferrite PMs are still used in lots of applications thanks to several advantages.

They are produced at very low cost, they are chemically stable and not sensitive to oxidation, and they are based on non-critical elements. Powerful PMs based on hexaferrites require a good control of the particle size in the submicrometer range and alignment of the magnetic easy axis after compaction of the powders. Several elaboration processes were developed for the synthesis of hexaferrites, among them co-precipitation followed by an annealing at an optimum temperature [2]. M-SrFe12O19 powders with high coercivity were produced by hydrothermal synthesis at 130°C followed by calcination at 850 °C [3]. Several chemical parameters were varied to obtain pure AFe12O19 (A = Ba, Sr) phases by hydrothermal synthesis and to control the particle size [4,5,6]. The formation of pure phases is obtained in alkaline medium by controlling the M/Fe molar ratio in the starting mixtures. Large excess of barium or strontium in the starting mixtures is necessary to avoid the crystallization of hematite or other undesired phase as side product [4,5]. The heating temperature and reaction time allowed to control the particle size: ultrafine nanoparticles with a disk-like shape were obtained below 170 °C while larger platelets were formed at higher temperatures [6]. The influence of sodium hydroxide concentration on the phase formation was also studied [7] and microwave-assisted hydrothermal methods were used to prepare pure BaFe12O19 thin platelets within very short reaction time [8]. Other solvents like ethylene glycol/deionized water mixtures were also used at 200 °C [9]. M-BaFe12O19 nanorods were synthesized by the reaction of α-FeOOH nanorods and BaCO3 at 220°C followed by a twosteps annealing [10]. Barium-doped iron oxide nanocubes were obtained by thermodecomposition process, using barium stearate as barium source, that can be converted easily to hexagonal barium ferrite [11]. In absence of surfactant or templating agent the hydrothermal syntheses in alkaline solution provide thin platelets with the crystallographic c axis perpendicular to the platelet. Thin platelets are not generally considered as the best shape to get high coercivity because of the competition between the magnetocrystalline anisotropy and the demagnetizing effects but present several advantages. Application of a magnetic field provokes the platelets orientation perpendicular to the applied field direction in agreement with a magnetocrystalline easy axis parallel the crystallographic c axis [12]. Furthermore, the self-assembly properties of platelets were also exploited in different studies. BaFe12O19 thin platelets were used as building blocks to form oriented films with highly anisotropic magnetic behavior by electrophoretic deposition [13].

Ferrofluids with very small BaFe12O19 platelets were obtained using oleic acid as stabilizing agent [14] and field dependent optical properties were described on colloidal solutions of SrFe12O19 plate-like nanoparticles [15]. Nanometre-sized BaFe11.5Sc0.5O19 ferromagnetic platelets synthesized by hydrothermal method and suspended in a nematic liquid crystal can constitute a new class of multiferroic materials [16]. Saura-Múzquiz et al. showed recently that compaction of hexaferrite platelets by Spark Plasma Sintering could give materials with a very high degree of alignment, even in absence of any applied magnetic field, and consequently provide high energy products [17]. Another interest of the nanoplatelets prepared by the hydrothermal methods is the possibility to grow another oxide at their surface to make "sandwich" platelets. Primc et al. exploited this opportunity to combine hard and soft magnetic materials in the same platelet [18]. Despite hexaferrite particles synthesis by hydrothermal methods is now well established, only few studies reported on the growth mechanism and thorough structural characterization of the solid intermediate phases before the high temperature annealing step. Based on X-ray diffraction patterns it was suggested that mixtures of goethite α-FeO(OH), hematite α-Fe2O3, hexaferrite SrFe12O19 and SrFeO2.5 could be intermediate phases of SrFe12O19 powders by hydrothermal synthesis [3]. Ultrathin BaFe12O19 nanoparticles prepared at lower temperature (150 °C) were analyzed using high resolution microscopy, X-ray absorption and Mössbauer spectroscopy by Makovec et al. [19]. The authors concluded that the particles crystallized with a strongly disordered hexaferrite structure. Granados-Miralles et al. described the synthesis of pure hexaferrite particles with ferrihydrite as intermediate phase [20]. The formation of pure hexaferrite at high temperature was obtained after a ripening process that involved the progressive dissolution of the ferrihydrite particles [20]. Interestingly, ultrathin hexaferrite platelets may exhibit composition and structure differing from bulk hexaferrite. BaFe12O19 and SrFe12O19 crystallize in the magnetoplumbite hexagonal structure (space group P63/mmc) that can be described as alternating stacking along the c-axis of R and S blocks, respectively (AFe6O11) 2-(A = Ba, Sr) and (Fe6O8) 2+ . The unit cell can be described as the RSR * S * stacking sequence along the c-direction, where the asterisk denotes the screw axis symmetry 63 along c.

Recently, B. Belec et al. isolated very thin nanoplatelets limited to the stacking sequence SR * S * RS paving the way to a chemical engineering of new layered nanostructures [21].

In this work, platelet shaped strontium-iron oxides were synthesized by a microwave-assisted hydrothermal method at 200°C and characterized by scanning transmission electron microscopy coupled with high angle annular dark field imaging (HAADF-STEM) performed on a probe corrected electron microscope equipped with a cold field emission gun. The microwave heating rate has a strong influence on the particle morphology and structure. The very small platelets prepared with the highest heating rates particles are the best precursors for high coercivity after annealing at 1000 °C. A special attention has been given to HAADF-STEM characterization of these ultrathin platelets for a better description of the nucleation/growth mechanism. An original SRS stacking is reported.

Experimental

Synthesis

Iron (III) nitrate nonahydrate Fe(NO3)3.9H2O (Strem Chemicals) and strontium nitrate Sr(NO3)3 (Alfa Aesar) were dissolved in demineralized water in various ratios. The obtained solution was afterwards added drop by drop, under argon atmosphere, in an aqueous sodium hydroxide NaOH (Sigma-Aldrich) solution. In all experiments, iron (III) concentration in the solution after mixing was fixed at 0.8 M, the molar ratio Fe/Sr at 3 and the pH of the starting mixture at 10. These solutions were subsequently transferred in a Teflon beaker EasyPrep reactor and heated at 200 °C for one hour in a microwave oven CEM Mars 6 (2.45 GHz, maximum power 650 W).

The heating rate was varied between 10 and 50 °C.min -1 .

After 1h heating at 200 °C the precipitate was cooled to room temperature, centrifuged, washed several times with diluted solution of acetic acid (0.1 M), demineralized water and ethanol. The resulting powder was dried at 50°C for 12 hours. The first washing with the diluted acetic acid solution aims to remove the undesired Sr-rich phases that precipitate with the SrFe12O19 due to the excess of strontium in the starting mixture. These particles were characterized by X-ray diffraction, electron microscopy, energy dispersive spectroscopy and magnetometry. The powders were annealed in air at 1000 °C and then characterized by X-ray diffraction, electron microscopy and magnetometry.

Characterizations

X-ray diffraction (XRD) pattern were recorded on a PANalytical Empyrean diffractometer in the 2θ range 10-90°, using CoKα radiation (λ= 1.789/1.792Å). The unit cell parameters were refined by full pattern matching simulation [22] with the FULLPROF program [START_REF]Computer Program FullProf and WinPLOTR[END_REF]. The raw particles morphology was studied by transmission electron microscope (TEM) at 100 kV with JEOL JEM1011 microscope. The particles were dispersed in ethanol and sonicated, one drop of the suspension was deposited on a carbon coated copper grid and the solvent was evaporated at room temperature under vacuum. Scanning transmission electron microscopy coupled with high angle annular dark field imaging (HAADF-STEM) observations were performed on a probe corrected JEOL JEM-ARM200F instrument equipped with a cold field emission gun. HAADF-STEM image simulations of thin platelets with thickness in the range 4-8 nm have been carried out using the multislice method with the QSTEM software package [START_REF] Kock | Determination of core structure periodicity and point defect density along dislocations[END_REF]. Annealed powders were observed by field emission gun scanning electron microscopy (FEG-SEM) using a JEOL JSM7800F microscope. Magnetic measurements were carried out using Quantum Design Physical Property Measurement System (PPMS) in Vibrating Sample Magnetometer (VSM) configuration. Magnetization loops, M(H), were recorded at 300K with applied field varying between ± 5T. The highest field value was generally not enough to reach the magnetization saturation of the powders. The saturation magnetization values, Ms, were determined by extrapolating the M(1/H) curves to 1/H = 0.

Results and Discussion

Microwave synthesis and annealing

Several powders were prepared by heating aqueous solutions of iron and strontium nitrate in for 1 h at 200 °C using a microwave oven. The heating rate (HR) was varied between 10 and 50 °C.min -1 while the other parameters like the Fe/Sr ratio and the pH of the solutions were fixed in all experiments (see experimental details). Transmission electron microscope (TEM) observations on powders prepared with different heating rates (HRs) showed platelet-like nanoparticles with a mean diameter that decreased when the HR was increased (Figure 1). Thin platelets with a micrometric diameter were obtained for HR up to 27 °C.min -1 (Figure 1a). The mean diameter decreased to 25 nm with an HR of 49 °C.min -1 (Figure 1d) and for intermediate HRs a broad diameter distribution was generally observed (Figure 2b,c). The thickness of the small platelets prepared with high HR was measured on dark field images and was found in the range of 2-5 nm (Figure 1e).

The powder X-ray diffraction (XRD) patterns were found to strongly depend on HR (Figure 2).

For the lower HRs the XRD patterns exhibit the strong lines of M-SrFe12O19 hexaferrite and weak lines of hematite (α-Fe2O3) (Figure 2a,b). Hematite weight fraction estimated by Rietveld analysis was 6 and 2 %, for HRs of 12 and 20 °C.min -1 , respectively. The hematite peaks were no more present for HR = 27 °C.min -1 and the XRD pattern could be indexed as pure M-SrFe12O19 phase (Figure 2c). For higher HR the intensity of the M-SrFe12O19 peaks progressively decreased and seven broad lines at the distances 2.538 Å, 2.447 Å, 2.227 Å, 1.963 Å, 1.710 Å, 1.497 Å and 1.465 Å appeared (Figure 2d,e). These seven lines fit well with the XRD pattern of ferrihydrite reported by Berquó et al. [START_REF] Berquó | [END_REF]. The ferrihydrite is a Fe(III) oxyhydroxide that crystallizes as a mixture of a defect free phase (f-phase) and a defective phase (d-phase) with small amount of hematite in variable proportion [26,27,28]. Both f-and d-phases present a hexagonal structure with parameters a = 2.955 Å and c = 9.370 Å but differs from the space groups, P-31c for the fphase and P3 for the d-phase [27]. The f-phase consists in a ABACA anionic packing in which the Fe atoms occupy the octahedral sites. The d-phase has a structure similar to the feroxyhite, δ-FeOOH, it is modelled as random sequences of ABA and ACA anionic layers with high degree of cation ordering [29]. Unit cells of f-phase and d-phase are given as supplementary information along with the strontium hexaferrite and hematite unit cells (Figure S1-S4). XRD pattern of ferrihydrite synthesized at low temperature generally exhibits six lines but a seventh one can be observed in case of good crystallinity [START_REF] Berquó | [END_REF]30] The saturation magnetizations (MS) of the grown powders prepared with different HR are reported in Table 1. The highest value, MS = 63 emu.g -1 , was obtained for the pure M-SrFe12O19 phase prepared with HR = 27 °C.min -1 . For HR = 12 °C.min -1 a lower MS value is explained by the presence of small amount of hematite that exhibits a magnetization close to zero. The very small MS values obtained with the highest HRs is related either to a large amount of ferrihydrite, which is antiferromagnetic at room temperature [27], and/or to an important amount of ultrathin hexaferrite particles as was described before by Primc et al. [6]. After annealing the XRD patterns were identified as mixtures of M-SrFe12O19 and hematite (Figure S5). The weight fractions of hematite determined by Rietveld analysis are reported in Table 1. is significantly higher when a mixture of ferrihydrite and strontium hexaferrite was obtained with the highest HRs, but never exceeded 16 % (Table 1). The saturation magnetization, MS, of the powders after annealing is very well fitted by the relationship Ms(A.m 2 .kg -1 ) = 75.5×(1-) (Figure 3) showing that the saturation magnetization of the in the literature for M-SrFe12O19 [START_REF] Kojima | Fundamental properties of hexagonal ferrites with magnetoplumbite structure[END_REF]. Annealing of the almost pure strontium hexaferrite powders with the lowest HR involved mainly a crystallinity improvement [START_REF] Grindi | [END_REF] while annealing of the powders prepared with high HR caused the disappearance of the ferrihydrite phase and the crystallization of the strontium hexaferrite along with hematite in small amount. The phase transformation of ferrihydrite into hematite at high temperature is well documented [33,34]. The limited content of hematite after annealing in the samples prepared with HR = 41 and 49 °C.min - 1 can be interpreted either by a limited content of ferriydrite in the raw samples and/or by the reaction of ferrihydrite particles with Sr-rich particles giving additional hexaferrite particles.

More details on the particle local composition inferred from electron micrsocopy are given in the next section.

The coercivity of the magnetization loops, HC, increased after annealing (Table 1) in agreement with the crystallization of M-SrFe12O19 which is a hard magnetic material. It is noteworthy that the highest values, HC = 455 -465 kA.m -1 , were obtained with the powders generated from the smallest particles, i.e. from the particles prepared with the highest HRs (Figure 4). The particle shape modification with annealing was examined by SEM (supplementary information, Figure S6). For the samples prepared with low HR the platelet shape with high aspect ratio was retained after annealing (Figure S6a-b). On the contrary, the nanometre size platelets obtained with the highest HRs transformed after annealing into isotropic particles with a mean size in the submicrometre range (Figure S6c-d). The smaller mean crystallite size and their rounded shape can explain the higher coercivity. Indeed, above a critical size the magnetic particles adopt multidomain configurations making easier the magnetization reversal. Moreover, the magnetocrystalline easy axis of the hexaferrite is the c axis. For very thin and large platelets, the magnetocrystalline anisotropy tends to align the magnetization along c while the demagnetizing effect tends to align the magnetization in plane. This competition makes easier the magnetization reversal, explaining the lower coercivity values of the large platelets. . The Fourier filtered HAADF image (Figure 5c) is very similar to the simulated image of a 4 nm thick M-SrFe12O19 platelet (Figure 5d). The intensity recorded along the 101 0 * direction presents the expected periodicity over 5 atomic columns (Figure 5e). In a rough approximation based on Rutherford scattering, HAADF-STEM image contrast of thin samples is proportional to the thickness and to Z 2 (Z being the atomic number). The power law observed experimentally is generally Z α with α slightly lower than 2.

Thus, as the oxygen atomic number is much lower than those of the metal atoms, the oxygen atom contribution to the HAADF images can be neglected. In the [0001] zone axis the columns observed along the 101 0 * direction contain successively 1 Sr + 4 Fe, 2 Fe, 4 Fe, 2 Fe, 1 Sr + 4

Fe atoms (Figure 5f-g and Figure S1 given in supplementary information). Considering that the M-SrFe12O19 platelet thickness is constant the relative intensities estimated in the Z 2 approximation are thus 100 % -32.6 % -65.2% -32.6% -100 %. The intensities scattered by the atomic columns between the points A and B follow the same sequence (Figure 5e) showing a good agreement between the experimental image and this estimation.

Besides the strontium hexaferrite particles a large number of platelets exhibited a diffraction pattern of a hexagonal structure in [0001] zone axis with a parameter a = 5.85 Å i.e. very close to the lattice parameter of M-SrFe12O19 (Figure 6). Nevertheless the main difference with the diffraction pattern of the strontium hexaferrite particles was the presence of six spots indexed as the 101 0 reflexions (Figure 6b) revealing an atomic pattern different from strontium hexaferrite. As a matter of fact, the filtered dark field images of different areas of the platelet showed strong local variations on the cation sub-lattice (Figure 6c-d). The crystallographic coherence of the atomic pattern was generally limited to 5 nm (Figure 6d). It is noteworthy that despite such a cation occupation disorder, the diffraction pattern of the platelet is typical of a single crystal particle which can be explained by a very good order of the anion sub-lattice.

Few platelets were oriented edgewise providing useful information on the atomic stacking along the c axis. The stacking of metal atom layers is clearly evidenced on the HAADF-STEM image of the 2.5 nm thick platelet presented on Figure 7. The Sr content measured by EDS in the central part of the platelet was found higher than the stoichiometry (Figure 7b). The very bright contrast of the central layer in the HAADF-STEM image is also in agreement with a Sr-rich atomic layer (Figure 7a). The contrast on the filtered image and the interlayer distances (Figure 7c) permitted to identify seven metal atom layers corresponding to the following stacking sequence along the c axis of the M-type strontium hexaferrite structure: Fe(5)/ Fe(3)-Fe(1)-Fe(3)/ Fe(5)/ Fe(4)-Sr+Fe(2)-Fe(4)/ Fe(5)/ Fe(3)-Fe(1)-Fe(3)/ Fe(5) (see more details on the Fe atoms labelling in the hexaferrite structure in supplementary information, Fig. S1). This stacking can be described as a SRS stacking of M-SrFe12O19 structure in 101 0 * zone axis (Figure 7d). On this stacking additional outer layers have grown with disorder. This finding may explain the differences on the HAADF images of sidewise platelets. A well crystallized core with a reduced extent along c and showing disorder in the surface shells could exhibit a contrast variation on the HAADF images like the one of Figure 6. On the contrary, the atomic pattern of Figure 5 requires well crystallized and thick enough platelets with hexaferrite structure. Another example of SRS stacking with surface disorder is given in supplementary information (Fig. S9).

A few particles did not contain any strontium and were identified as ferrihydrite particles. On Figure 8 is given an example of such a platelet viewed sidewise. The calculated FFT pattern can be indexed as the diffraction pattern of a hexagonal phase in [0001] zone axis with parameter a = 2.9 Å (Fig. 8b). This value is very close to the lattice parameter value of both the defect free and the defective ferrihydrite, a = 2.9550 Å [26,27,28]. The Fourier filtered image revealed a pattern with two cation columns by unit cell (Fig. 8c). The ABACA anionic packing in the defect free ferrihydrite gives the possibility to the Fe atoms to occupy two different columns (Fig. 8f,g) contrary to the defective ferrihydrite that exhibit only one Fe column per unit cell (see Figs. S2-3 in supplementary information). The intensity scattered by the atomic columns is not constant (Fig. 8e) showing a disorder on the metal atom occupancy in agreement with the structural model that was proposed from XRD and neutron diffraction. This metal atom pattern is similar to the simulated HAADF image of the defect free ferrihydrite deduced from this model (Fig. 9d). It is noteworthy that the simulated HAADF image exhibits more disorder than the experimental one.

One reason may be the rather good crystallinity of the ferrihydrite phase in comparison with the model that was proposed for 6 lines ferrihydrite powders elaborated at lower temperature and that includes the disorder on the metal atom occupancy [26,27]. This also may confirm the seven lines of the experimental XRD pattern that are a feature of good crystallinity. Another defect free ferrihydrite platelet viewed edgewise is also given as supplementary materials (Figure S11). Very few particles that did not contain strontium exhibited a diffraction pattern very similar to the defect free ferrihydrite particles in the (0001) zone axis but with a different atomic pattern (see supplementary information Figure S12). These particles were identified as defective ferrihydrite in the (0001) zone axis with the cell parameter a = 2.9 Å and an atomic pattern that contains a single Fe atom column per unit cell (Figure S11). However a hematite particle in (111) zone axis cannot be discarded since the projections of the Fe sub lattices of defective ferrihydrite in (0001) zone axis and of hematite in (111) zone axis are exactly the same (Figure S12).

Finally, EDS analysis showed also platelets exhibiting Sr content higher than the hexaferrite stoichiometry. On figure S12 (see supplementary information) is given an example of a platelet with atomic ratio Fe/Sr = 7. The fast Fourier transform (FFT) of the HAADF image showed a diffraction pattern similar to the hexaferrite one but a closer examination revealed an orthorhombic distortion (Fig. S12b). The atomic pattern on Fourier filtered HAADF image (Fig. S12c) also differed from the hexaferrite pattern (Fig. 5c). Intensity plotted along three pseudo 101 0 * directions revealed an intensity profile very different from the hexaferrite pattern for one direction while for the two others profiles were similar to the profile measured on the hexaferrite HAADF images with a periodicity over five atomic columns (Fig. S12c), in agreement with an orthorhombic distortion.

Discussion

Pure M-SrFe12O19 hexaferrite powders were prepared by microwave assisted hydrothermal method with an optimized HR of about 25 °C.min -1 . With higher HRs the powders were identified by XRD as mixtures of ferrihydrite and hexaferrite phases with M-type strontium hexaferrite lines that progressively vanished with increasing HR. The very small particle size and the cation disorder make complex the XRD pattern analysis. Electron microscopy in the HAADF-STEM mode coupled with local EDS analysis was used to have a deeper review of the ultra-small particles prepared with high heating rates. This study revealed a much more complex situation than simple mixtures of these two phases. Most of the particles contained strontium as inferred from EDS measurements discarding that pure ferrihydrite could be the major phase. Wellcrystallized hexaferrite, and defect free ferrihydrite were found in these powders but were in minority. The most frequently observed particles exhibited a hexagonal structure with parameter a very close to M-SrFe12O19 type but with strongly disordered cation sites occupancy on the Fe and Sr sites like the example of Figure 6. Platelets consisting in a well-crystallized M-type strontium hexaferrite core but extending over less than a unit cell along c-direction and with surface disorder, as was inferred from the edgewise platelet analysis, can be the main component of the powders. Finally, well crystallized particles with Sr content higher than the hexaferrite stoichiometry exhibiting a distorted hexagonal structure were also identified.

The low saturation magnetization of these samples can be explained by the disorder on metal atom occupancy in ultra-small hexaferrite-like particles and by the presence of ferrihydrite-like particles. Makovec et al. suggested in a previous study that strongly disordered particles with Mtype strontium hexaferrite-like structure may be paramagnetic or antiferromagnetic at room temperature because of disordered cation occupancy [19]. After annealing the crystallization of the small hexaferrite particles and the reaction between ferrihydrite and Sr-rich particles gave mainly the M-SrFe12O19 phase, increasing significantly the saturation magnetization and coercivity.

The high HC values obtained after annealing are mainly due to the small particle size in the microwave hydrothermal step. The particle mean size depends on the nucleation rate. By increasing HR in the microwave process the number of seeds was increased and the particle mean previous study on BaFe12O19-like ultrathin particles [21]. The platelets in that case were identified mainly as SRSRS stacking i.e. symmetrical from a S block. The SRSRSRS stacking was also observed but as minor product [21]. It suggests that the nucleation step of the ultrathin platelets depends on the composition of the starting mixture. In the present study the nuclation involved clusters containing Sr ions in agreement with an excess of strontium in the starting mixture.

Conclusion

The microwave-assisted hydrothermal process is a versatile method to synthesize in short reaction times pure M-type strontium hexaferrite powders or mixtures of strontium hexaferrite and ferrihydrite-like particles depending on the heating rate. Pure M-SrFe12O19 particles with platelet shape and mean diameter in the micrometer range were prepared by optimizing the heating rate at around 25 °C.min -1 . These particles exhibited a saturation magnetization close to the bulk after annealing at 1000 °C and a high coercivity. By increasing the microwave heating rate in the range 40-50 °C.min -1 the particle size and their aspect ratio were decreased. HAADF-STEM imaging and EDS analysis of the ultra-small particles showed well-crystallized nanoparticles with local variations in Sr-content. Atomic scale resolution analysis showed that, besides hexaferrite and ferrihydrite, particles crystallizing with a hexaferrite core extending over less than a unit cell and with surface disorder were the major population. The multilayer stacking SRS observed in several edgewise particles suggests that the nucleation step of these particles involved cluster containing Sr atoms. After annealing the powders exhibited a higher coercivity thanks to smaller particle size and a reasonable saturation magnetization thanks to a limited amount of hematite. The calculated FFT of figure S12b presents some similarities with the FFT pattern of a pure hexaferrite particle (see Fig. 5b of the article) but the spots do not form a regular hexagon showing an orthorhombic distortion with respect to the pure hexaferrite structure.

The comparison of the Fourier filtered HAADF image of the Sr-rich particle and of a typical hexaferrite particle shows that the atomic pattern are different (Fig. S12d-e). The scattering intensity along three pseudo 101 0 * directions (L1, L2 and L3) are plotted on Figure S12c. The intensity profile along L1 is very different from the profile of an hexaferrite particle (see for example Fig. 5 of the article). In contrast the intensity profiles along L2 and L3 were similar to the profile measured on the hexaferrite HAADF images with a periodicity over five atomic columns (Fig. S12c). This result confirm the orthorhombic distortion.

  . The peak positions measured experimentally on the pattern of Figures 2d-e are consistent with (100), (101), (102), (103), (104), (105) and (110) reflexions of a hexagonal phase with parameters a = 2.93 Å and c = 9.27 Å, thus very close to the ferrihydrite parameters. For the highest heating rate (Figure 2e), the broad hexaferrite lines are almost totally screened by the ferrihydrite bands.

Fig. 1 .Fig. 2 .

 12 Fig. 1. TEM images of powders obtained with different heating rate, (a) 27 °C.min -1 ; (b) 35 °C.min -1 ; (c) 41 °C.min -1 and (d) 49 °C.min -1 ; (e) HAADF-STEM images of particles prepared with a heating rate of 49 °C.min -1 .

  annealed powders is proportional to the weight fraction of M-SrFe12O19 as expected for mixtures of a ferromagnetic (strontium hexaferrite) and an antiferromagnetic phase (hematite). The saturation magnetization MS = 75.5 A.m 2 .kg -1 is in very good agreement with Ms value reported

Fig. 3 .Fig. 4 . 3 . 2 . 1 )

 34321 Fig. 3. Saturation magnetization of the powders synthesized with different heating rates before (○) and after annealing at 1000 °C for 1h (■). Inset: Saturation magnetization values vs weight fraction of hematite in annealed powders.

Fig. 5 .

 5 Fig. 5. (a) HAADF-STEM image of an hexaferrite nanoparticle; (b) Calculated FFT of the central part of the image indexed as a hexagonal phase in [0001] zone axis with quasi extinction of the {101 0} reflections; (c) Zoom on the Fourier filtered image; (d) HAADF-STEM image simulation of a 4 nm thick SrFe12O19 platelet in [0001] zone axis; (e) Intensity profile along the [011 0] * direction (dashed line on filtered image); (f) unit cell of the SrFe12O19 hexaferrite; (g) projection of the cationic columns in [0001] zone axis showing the three different kind of columns per unit cell, blue+grey = 4Fe +1Sr, magenta = 4 Fe, green = 2 Fe atoms.

Fig. 6 .Fig. 7 .

 67 Fig. 6. (a) HAADF-STEM image of a nanoparticle present in the powder prepared at 41 °C.min -1 ; (b) Calculated FFT indexed as a single hexagonal crystal in [0001] zone axis with parameter a = 5.85 Å ; (c) and (d) filtered images of down left part (c) and up right part (d) of the crystal showing different local atomic pattern.

Fig. 8 .

 8 Fig. 8. (a) HAADF-STEM image of a nanoparticle present in the powder prepared at 49 °C.min -1 ; (b) Calculated FFT of the central part of the HAADF image indexed as a hexagonal crystal in [0001] zone axis with parameter a = 2.9 Å; (c) Zoom on the Fourier filtered HAADF image; (d) HAADF-STEM image simulation of a 8 nm thick defect free ferrihydrite; (e) Intensity profile along the [11 00] * direction (dashed line on filtered image) ; (f) Unit cell of the defect-free ferrihydrite ; (g) Projection of the Fe columns in the (0001) plane.

  size was reduced. In case of edgewise platelets it has been possible to identify the SRS stacking along the c axis showing that the metal atom stacking is symmetrical starting from a central layer that contains the Sr atoms. This situation is different from what observed B. Belec et al. in a

Figure S5 .

 S5 Figure S5. XRD pattern of powders obtained after annealed at 1000 °C for one hour of sample synthesis with different heating rate (a) 12°C/min, (b) 27°C/min, (c) 35°C/min, (d) 41°C/min and (e) 49°C/min. Inset: zoom of the pattern in the 2θ range 37-41° revealing the presence of hematite (asterisk denotes the (104) reflexion of hematite).

Figure S6 .

 S6 Figure S6. SEM images of particles obtained with different heating rate annealed at 1000 °C, (a) 12°C/min, (b) 27°C/min, (c) 41°C/min and (d) 49°C/min.

Figure S7 . 1 .

 S71 Figure S7. HAADF-STEM images and EDS analyses of particles prepared with a heating rate of 41 °C.min - 1 . The Fe/Sr molar ratio measured on large areas is close to 12..

Figure S8 .

 S8 Figure S8. HAADF-STEM image and local EDS analysis of a platelet identified as SrFe12O19 hexaferrite in (0001) zone axis on the Fourier filtered image: (a) STEM-HAADF image and local Fe/Sr ratio measured by EDS ; (b) Fourier filtered images of the analyzed area showing the typical pattern of hexaferrite SrFe12O19 in (0001) zone axis.

Figure S9 .

 S9 Figure S9. (a) HAADF-STEM image of a 2.7-3 nm thick platelet viewed edge wise ; (b) Fourier filtered image of the red square area showing an atomic pattern similar to a hexaferrite core on [10-10] * zone axis.Like in the example given in Figure7of the main text, the crystallized part of the platelet consists in a S-R-S blocks stacking.

Figure S10 .

 S10 Figure S10. (a) HAADF-STEM image of a defect free ferrihydrite platelet viewed edgewise ; (b) Fourier filtered image of the yellow square ; (c) zoom on the filtered image (the image has been tilted for clarity); (d) Projection of the cationic columns for the defect free ferrihydrite structure in (1-210) zone axis showing a strong analogy with the experimental pattern.

Figure S11 .

 S11 Figure S11. (a) HAADF-STEM image and local EDS analysis of a platelet that does not contain Sr ; (b) Calculated Fourier transform of the white square indexed as a hexagonal structure in (0001) zone axis and with parameter a = 2.9 Å; (c) Zoom on the Fourier filtered image of the white square ; (d) HAADF-STEM image simulation of a 8 nm thick defective ferrihydrite particle in (0001) zone axis; (e) Unit cell of defective ferrihydrite; (f) Projection of the cationic columns of defective ferrihydrite in the (0001) zone axis; (g) Unit cell of hematite; (h) Projection of the cationic columns of hematite in the (111) zone axis.

Figure S12 .Figures S2 .

 S12S2 Figure S12. (a) HAADF-STEM image of Sr-rich nanoparticle (Fe/Sr ratio given by EDS analysis = 7) ; (b) Calculated FFT of the zone framed by the blue square; (c) Fourier filtered image of the blue square zone and intensity profiles along the lines L1, L2 and L3 ; (d) Projection of the orthorhombic cell of the (e) Typical Fourier filtered image of a M-SrFe12O19 hexaferrite particles with the projection of the hexagonal cell, as a matter of comparison.

Figures S3 .Figures

 S3 Figures S3.Crystallographicstructure of defective ferrihydrite according to according to Jansen et al. [Reference E. Jansen, A. Kyek, W. Schafer, U. Schwertmann, Appl. Phys. A, 2002, 74, S1004-S1006 ; COD file 9012761]. (a) Unit cell of defective ferrihydrite (d-phase): trigonal system, P3 space group (no. 143), unit cell dimensions a = 2.9550 Å, c = 9.3700 Å ; (b) Projection of the Fe atoms in the (0001) plane (Fe occupancy is 0.24).

Figure S5 .

 S5 Figure S5. XRD pattern of powders obtained after annealed at 1000 °C for one hour of sample synthesis with different heating rate (a) 12°C/min, (b) 27°C/min, (c) 35°C/min, (d) 41°C/min and (e) 49°C/min. Inset: zoom of the pattern in the 2θ range 37-41° revealing the presence of hematite (asterisk denotes the (104) reflexion of hematite).

Figure S6 .Figure S9 .

 S6S9 Figure S6. SEM images of particles obtained with different heating rate annealed at 1000 °C, (a) 12°C/min, (b) 27°C/min, (c) 41°C/min and (d) 49°C/min.

Figure S10 .

 S10 Figure S10. (a) HAADF-STEM image of a defect free ferrihydrite platelet viewed edgewise ; (b) Fourier filtered image of the yellow square ; (c) zoom on the filtered image (the image has been tilted for clarity); (d) Projection of the cationic columns for the defect free ferrihydrite structure in (1-210) zone axis showing a strong analogy with the experimental pattern.

Figure S11 . 13 Figure S12 .

 S1113S12 Figure S11. (a) HAADF-STEM image and local EDS analysis of a platelet that does not contain Sr ; (b) Calculated Fourier transform of the white square indexed as a hexagonal structure in (0001) zone axis and with parameter a = 2.9 Å; (c) Zoom on the Fourier filtered image of the white square ; (d) HAADF-STEM image simulation of a 8 nm thick defective ferrihydrite particle in (0001) zone axis; (e) Unit cell of defective ferrihydrite; (f) Projection of the cationic columns of defective ferrihydrite in the (0001) zone axis; (g) Unit cell of hematite; (h) Projection of the cationic columns of hematite in the (111) zone axis.

Table 1 .

 1 Synthesis conditions, mean diameter (Dm) and magnetic properties of particles prepared by microwave synthesis using different heating rates before annealing ; weight fraction of hematite and magnetic properties after annealing for 1 h at 1000 °C.

	Microwave		Before annealing	After annealing 1000 °C
	heating rate	Dm	MS	HC		MS	HC
	( °C.min -1 )	(nm)	(A.m 2 .kg -1 )	(kA.m -1 ) (% wt.)	(A.m 2 .kg -1 )	(kA.m -1 )
	12	1700	56	102	0.25	75	315
	27	1000	63	95	0	77	362
	35	25-500	39	150	4.3	73	398
	37	20-400	19	120	13.6	65	398
	41	18-100	13	100	13.8	64	465
	49	21-50	6	45	15.3	62	454

Acknowledgements

The authors would like to thank L. Datas and T. Hungria (Plateforme Castaing, Toulouse, France) for the HAADF-STEM experiments. C. M. and G. V. gratefully acknowledge the European Associated Laboratory (LEA)-TALEM2.

Supporting Information

Supplementary data associated with this article can be found in the online version


Brahim Mazian

Anne Bergeret

Jean-Charles Benezet

Luc Malhautier

Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering

Keywords:

The use of hemp fibres as reinforcements in polymer composites requires a thorough understanding of the hemp fibres transformation processes to obtain a constant quality. In this context, the upstream processing termed field retting is considered. Retting allows a subsequent fibre separation from the plant stems by degradation of cementing compounds by microorganisms. This operation depends on weather conditions and is currently empirically carried out in fields, so that a large variability in the hemp fibres quality (color, morphology, biochemical composition, thermal properties and mechanical properties) is resulting. Therefore, the present study aims to investigate the influence of different retting durations (up to 9 weeks) on hemp fibres properties when harvested at the beginning of flowering growth stage to survey their temporal dynamic. Various assessments were applied on fibres: color observations, morphological (optical microscope), surface (ESEM) and biochemical (gravimetry) analyses, spectrocolorimetric measurements (pectins content), thermogravimetric (TGA) analysis, and mechanical in tensile mode testings. The results reveal that increasing the field retting duration leads to a change of color characteristics from light green to grey due to the development of microbial communities (most probably fungal and bacteria) at the stem surface. A separation of the fibres bundle to elementary fibres occurs with the degradation of pectins during retting. An increase of thermal stability of the fibres is also observed. Both increase of cellulose fraction and crystallinity induce an enhancement in tensile properties.

Introduction

Hemp (Cannabis sativa L.) is an annual bast fibre plant from Asia and widely cultivated in many countries in Asia and Europe. Hemp bast fibres have traditionally been used for such purpose as the fabrication of textiles, and in recent years, are increasingly used as reinforcements in polymer composites due to their challenging performances including low-density [START_REF] Aziz | The effect of alkalization and fibre alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites: part 1-polyester resin matrix[END_REF], low cost [START_REF] Dittenber | Composites: part A Critical review of recent publications on use of natural composites in infrastructure[END_REF], good specific mechanical properties [START_REF] Bourmaud | Effects of thermo mechanical processing on the mechanical properties of biocomposite flax fibers evaluated by nanoindentation[END_REF][START_REF] Duval | Influence of the sampling area of the stem on the mechanical properties of hemp fibers[END_REF] and biodegradability [START_REF] Stamboulis | Effects of environmental conditions on mechanical and physical proprties of flax fibers[END_REF][START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites?[END_REF] compared to conventional fibres such as glass fibres [START_REF] Wambua | Natural fibres: can they replace glass in fibre reinforced plastics?[END_REF]. However, the huge variation of their biochemical, physical and mechanical properties [START_REF] Duval | Influence of the sampling area of the stem on the mechanical properties of hemp fibers[END_REF][START_REF] Placet | Diameter dependence of the apparent tensile modulus of hemp fibres: a morphological, structural or ultrastructural effect?[END_REF] makes this natural fibre limited to use in structural application when high consistency and homogeneity are required [START_REF] Stamboulis | Effects of environmental conditions on mechanical and physical proprties of flax fibers[END_REF]. The hemp fibres used for composites are primary and secondary fibres which are located between the epidermis and the xylem core and parallel to the longitudinal axis of the stem [START_REF] Schäfer | Effect of sowing date and plant density on the cell morphology of hemp (Cannabis sativa L.)[END_REF]. The hemp fibres are associated through their pectic middle lamella to form bundles of several dozens of fibres [START_REF] Chabbert | Structure and chemical composition of bast fibers isolated from developing hemp stem[END_REF]. For high performance composites, pectic middle lamella and other cementing compounds (e.g. wax) that bind the bast fibres and fibre bundles to other tissues must be removed in order to separate the fibres and to obtain individual fibres or smaller fibres bundles. This separation of the fibres bundles leads to improve interfacial bonding between fibres and matrix which enhances the mechanical performance of final composite [START_REF] Li | Determination of interfacial shear strength of white rot fungi treated hemp fibre reinforced polypropylene[END_REF][START_REF] Sisti | Evaluation of the retting process as a pre-treatement of vegetable fibers for the preparation of high-performance polymer biocomposites[END_REF]. In the hemp industry, traditional extraction and separation processes are commonly used to facilitate the separation of the fibres from stem and reduce fibre defects during decortication. These processes play a key role on the quality heterogeneity of hemp fibres, especially field retting treatment which consists to spread out hemp stems on the ground for a duration ranging from a few days to several weeks. When the plant stems are laid in the field, the microorganisms (fungi and bacteria) attack the stems producing a range of polysaccharide-degrading enzymes, especially pectinolytic which remove ⁎ Corresponding author.

E-mail address: Anne.Bergeret@mines-ales.fr (A. Bergeret). T

Sampling and weather conditions

After harvesting period, a weekly manual collection of the samples during retting was carried out. Table 1 gives the retting times and corresponding collected samples, from unretted fibres (R0) to 9 weeksretted fibres (R9). Stems collected were dried and stored in a cold (4 °C) room before being further processed.

Temperature and relative humidity have been constantly recorded in the field using three hygro-bouton sensors (Progesplus, Carquefou, France) that were put in contact with the soil. Daily average rainfall was obtained from Meteo France (weather station located at Méjannesle-Clap (GPS: N44.221944°, E4.344722°) in the vicinity of the Mas de la Valus). Daily weather conditions are exposed on Fig. 1. The weather was warm during retting period, except the last three weeks in September 2016. The maximum and minimum temperatures between the 1st week and the 6th week of retting period ranged from 38 to 55 °C and 15-20 °C, respectively, whereas during the last two weeks of the retting period, it decreased to 27-34 °C and 10 °C-20 °C, respectively. Rainfall and humidity were not homogeneously distributed during the retting period. There was little rainfall (11 mm) during the 1st and 4th weeks of retting period, while between the 7th week and the last sample collection, it was higher (50 mm). Rainfall generally increases the relative humidity from 55% to 90%.

The average length of the stems harvested at the beginning of flowering is 1.7 ± 0.1 m. In this present study, only the middle part of the stems with a length of 1 m was considered (Fig. 2) in order to limit the scatter of the results. Indeed other authors Liu et al. (2015b) and [START_REF] Duval | Influence of the sampling area of the stem on the mechanical properties of hemp fibers[END_REF] revealed that the location of the fibres on the stems (bottom, middle and top) influences the quality of fibres and the fibres issued from the middle part of the stems provide higher mechanical properties.

Experimental methods

Visual aspect

The color evaluation of the middle-stems and fibres along retting process was visually observed. The fibres were extracted using a razor blade and flat files.

Microscopy observations

The middle part of unretted (R0) and retted stems (R3, R5 and R9) was observed through scanning electron microscopy (ESEM) using an environmental scanning electron microscope Quanta FEG 200 (FEI, Merignac, France) with a magnification of ×500 and ×1000.

The influence of field retting durations on the morphology of fibres was also investigated using optical microscopy (Leica Laborlux 11 POL S). Prior to optical microscopy observation, the cross sections of the unretted (R0) and retted stems (R3, R5 and R9) were cut manually, commonly coated in an epoxy resin, and then subsequently polished to obtain smooth samples. Images were recorded using Archimed ® software in order to analyze qualitatively the evolution of bast fibres during field retting.

Biochemical analysis

Two methods were used, one based on solvent extractions to quantify cellulose, lignin and lipophilic extractives contents, the other based on spectrophotometry to evaluate the pectin content.

2.2.3.1. Solvent extractions. The biochemical composition of the fibres from unretted (R0) and retted hemp fibres (R3, R5 and R9) was the components in the middle lamella region surrounding the fibres [START_REF] Akin | Optimization for enzyme-retting of flax with pectate lyase[END_REF]Henriksson et al., 1997). Field retting is empirically carried out because of its very environmental-dependent conditions (temperature, humidity and duration) which results many problems of the inconsistency of the hemp fibres quality. For this reason, many efforts are nowadays directed for offering alternative methods of extraction of the fibres such as thermal and enzymatic treatment [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], steam explosion [START_REF] Vignon | Morphological characterization of steam-exploded hemp fibers and their utilization in polypropylene-based composites[END_REF] and chemical treatment [START_REF] Obendorf | Chemical and biological retting of kenaf fibers[END_REF]. These methods are well-controlled, but economic viability for industrial upscaling still remains difficult because they are expensive and need more energy or specific enzymes, contrary to the field retting that is cheaper and easier to be applied [START_REF] Tahir | Retting process of some bast plant fibers and its effect on fibre quality: a review[END_REF]. Therefore, field retting remains the most widely used in Europe for industrial production of bast fibres and thus must be understood in order to avoid the insufficient advancement of field retting which makes separation of the fibres difficult, or over retting that leads to a partial degradation of fibres by cellulose destructuring [START_REF] Sharma | Production of polysaccharide-degrading enzymes by saprophytic fungi from glyphosate-treated flax and their involvement in retting[END_REF].

Field retting is usually carried out at the maturity growth stage since the farmers could be retrieved the seed. However, this growth stage occurs in autumn when climatic conditions are not favorable for a better master of field retting progress. A recent study of Liu et al. (2015a) highlights that the field retting of the hemp fibres harvested at seed maturity during a colder and rainy season lead to a rapid change of the properties of the fibres. Indeed, the hemp fibres quality increases at the early stage (1 week) of the field retting and then rapidly decreases by extending the retting process, due to the accelerated degradation of cellulose by microorganisms. [START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF] compared unretted and 5 weeks-retted fibres during a very rainy retting period, and found that the prolongation of retting under these climatic conditions has caused over-retted fibres.Therefore,fullunderstanding of the evolution of the quality of hemp fibre during field retting process requires a study for a long period with a gradual development and activity of microorganisms. For this reason, the field retting of hemp fibres harvestedatthebeginningofflowering growth stage which coincides with summer conditions is investigated in this study.

The purpose of the present work was then to evaluate the effect of different field retting durations on the properties of the hemp fibres when harvested at the beginning of flowering growth stage i.e. to survey the temporal dynamic of hemp fibres composition, microstructure, thermal and mechanical properties during retting. The color change was analyzed by visual observations. The changing biochemical composition, pectin variation, crystalline structure was determined using successive solvent extractions, spectrocolorimetric and X-ray diffraction measurements, respectively. Thermal stability was studied by thermogravimetry analyses (TGA). Scanning electron and optical microscopy observations were carried out to examine the stem surface change and morphology of fibres. Finally, tensile tests were performed on manually extracted fibre bundles. In addition, this work will also contribute to the missing data on fibre harvested at beginning of flowering and retted for a long period under dry weather conditions.

Materials and methods

Raw material

Cultivation

The hemp (Cannabis sativa L., Cultivar Santhica 27) was grown at a rate of 30 kg/ha on May 6th, 2016 in the south of France around the Mas La Valus (GPS: N44.130673°, E4.315895°) by CIVAM Chanvre Gardois association (Bouquet, France). The hemp plants were manually pulled using a shear at the beginning of flowering (July 25th, 2016) and dew-retted in the field. Following the definition of the different hemp growth stages described by [START_REF] Mediavilla | Decimal code for growth stages of hemp (Cannabis sativa L.)[END_REF], the beginning of flowering period selected is corresponding to code 2101 (first opened staminate flowers). Plants harvested were turned regularly (once a week) on the ground to homogenize the retting of the stems. determined using successive solvent extractions based on different ASTM test methods as described on Fig. 3.

Prior to analyzing the chemical composition, fibres on stems were peeled and cut manually to obtain 10 g (weight of moisture free). Lipophilic extractives (waxes, fats, resins and some gums) were extracted with a solution of toluene/ethanol (2:1 w/w) for 8 h, the residue was filtered and dried in a desiccator at 105 °C before being weighted using an infrared balance (Precisa XM66, Precisa, Dietikon, Switzerland) at 105 °C according to ASTM D1107-56 method. A fraction of the fibres retrieved after this first extraction was treated with sulfuric acid in order to hydrolyze carbohydrates by leaving an insoluble residue which is determined as lignin according to ASTM D 1106-56 method. For this purpose, 1 g of fibres was mixed with 72% of sulfuric acid (H 2 SO 4 ) by shaking constantly for 2 h at room temperature. The mixture was then diluted by adding distilled water to obtain a H 2 SO 4 solution with a 3% concentration and boil for 4 h. The insoluble materials were filtered and dried at 105 °C. Another fraction of fibres retrieved after the first extraction was used for the determination of holocellulose and α-cellulose contents according to ASTM D 1104-56 and ASTM D 1103-60 methods, respectively. For this purpose, fibres were introduced in a solution made of 150 ml of distilled water, 0.2 ml of acetic acid and 1 g of sodium chlorite (NaClO 2 ). The mixture was heated at 75 °C and stirred constantly for 5 h, during which 0.2 ml of acetic acid and 1 g of sodium chlorite were added each hour. After 5 h, the mixture was placed in an ice-water bath until the temperature of the solution reached 10 °C. Distilled water was used to wash the residue until the color changed into yellow. The residue is the holocellulose fraction and was dried in an oven at 105 °C, then weighted using an infrared balance at 105 °C. Holocellulose was finally solubilized in sodium hydroxide solution (NaOH) and acetic acid to obtain α-cellulose. Ash content was determined after a dry oxidation of the fibres at 600 °C for 8 h according to ASTM D 1102-84 method.

2.2.3.2. Pectins analysis by spectrophotometer. Pectin content was determined using a colorimetric method developed by [START_REF] Blumenkrantz | New method for quantitative determination of uronic acids[END_REF]. This method consists of hydrolyzing pectics substances to galacturonic acid which is the main compound within pectins followed by a coloring step with a m-hydroxydiphenyl reagent at room temperature (Fig. 4).

Firstly, a standard assay was carried out using different quantities of commercial pectin (Coprale, Ales, France) between 0 and 8 μg (volume of 40 μl). 200 μl of concentrated sulfuric acid H 2 SO 4 (96%) were added to each well of microtiter containing 40 μl of pectin, and then the plate was incubated at 80°C for 1 h. After cooling at room temperature, the absorbance of the background samples was measured with a spectrophotometer (Thermo scientific Multiskan ® spectrum, Waltham, MA USA) at 540 nm. Then, 40 μl of m-hydroxydiphenyl reagent (100 μl of 100 mg of m-hydroxydiphenyl in 1 ml of dimethylsulfoxide mixed with 4.9 ml of 80% sulfuric acid) was added to each sample. A pink color appears and the highest absorbance of the samples is then measured at 540 nm. The content of galacturonic acid is obtained after subtraction of the background absorbance. A calibration curve is obtained between 0-8 μg of galacturonic acid.

Subsequently, to determine the galacturonic acid amount of samples, an extraction of the pectins from fibres was performed as described by [START_REF] Fila | In vitro evaluation of dew retting of flax by fungi from Southern Europe[END_REF]. Fibres were peeled and cut manually on stems to obtain 200 mg (weight of moisture free). Then lipid components were removed using a chloroform/methanol (1:1) solution by stirring constantly for 24 h at room temperature. After drying at 105 °C, pectins were then extracted with 200 ml of 5 gl -1 Na 2 EDTA for 2 h at 100 °C. The extraction product was then analyzed using the spectrophotometer calibration described above. The absorbance for unknown sample content is measured at 540 nm, and the calibration curve is used to find the concentration of the galacturonic acid.

X-Ray diffraction

Microstructure of unretted (R0) and retted fibres (R3, R5 and R9) was analyzed using X-ray (XRD, AXS D8 Advance Bruker) diffractometer equipped with Cu-Kα radiation (λ = 1.54 Å). Measurements were performed on cut and compressed fibres (disks of 25 mm in diameter and 2 mm in thickness). X-ray diffractograms were recorded from 5 to 70°(2θ, degrees) with a scanning speed of 0.01°/s. The crystalline order index (CI) was determined from X-ray diffractograms using a deconvolution method. Individual crystalline peaks were fitted using Origin ® software assuming Gaussian functions. Iterations were repeated until the maximum R 2 value is superior than 0.98. ( 101), ( 101) and (002) crystalline peak areas (Ic) and amorphous broad peak area (Iam) were used to calculate the crystallinity order index according to Eq. (1).

=

+ × CI Ic Ic Iam 100 2.2.5. Thermogravimetric analysis Thermogravimetric analyses of the unretted (R0) and retted hemp fibre (R3, R5 and R9) were carried out using a Perkin-Elmer Pyris-1 thermal analysis system. Samples of about 8 mg manually extracted fibres ( §2.2.3.) were heated in nitrogen from 30 °C to 700 °C at the rate of 10 °C/min. At least two samples from each retting time were performed. The mass change was recorded as a function of temperature. A differential thermogravimetric analysis (DTGA) curve was collected from the TGA analysis.

Tensile test of fibre bundle

Prior to tensile testing, the fibre bundles of the different retting durations (R0, R3, R5 and R9) were manually extracted from the stems after removing the epidermis using a razor blade. Fibre bundles were first positioned in one-part plastic tab with an UV-curing glue (DYMAX ultralight weld, DYMAX Europe GmbH, Wiesbaden, Germany). Then fibre bundles diameters are determined using an automated laser scanning device FDAS 765 (Diastron Ltd., Hampshire, UK). Fibres horizontally rotated within the laser beam at 21 slices along the fibre axis. The mean, the minimum and the maximum diameters for each slice are recorded in a dimensional report exported by UvWin ® 3.60 software. Thus, the mean diameters of 21 slices are averaged for each fibre bundle. The effective cross sectional area was calculated using the mean diameter of the fibre bundle assuming that the fibre had a circular shape. Fibre bundles were then mechanically tested under tensile mode at controlled temperature (23 °C) and relative humidity (48%) using Diastron LEX810 system (Diastron Ltd., Hampshire, UK). The samples are loaded at a constant crosshead displacement speed of 1.2 mm/min -1 and with 12 mm gauge length. A curve of force (Gram-Force) as function of displacement is obtained, and then the tensile strength, the Young's modulus and the failure strain were determined. Approximately 30 fibres were tested for each retting time (R0, R3, R5 and R9). The results were analyzed statistically using Analysis of variance ANOVA and Tukey multiple comparison test at significant level of 5%.

Results and discussion

Influence of field retting duration on the color of the hemp fibres

A visual assessment of the samples collected after harvesting and during different retting times was carried out. Fig. 5 shows the photographs of the stems and of the processed fibres issued from different retting durations. A significant change of the color was observed. The color varies between light green for unretted samples (R0), yellow for low retted samples (R1 to R3), yellow with the presence of grey fibres for medium retted samples (R4 and R5) and grey for highly retted samples (R9). As it can be seen on the stems surface, grey spots start to Fig. 7. Ash, lignin, hemicellulose, lipophilic extractives, hemicelluloses, cellulose (solvent extractions) and lignin (spectrophotometer) contents as a function of field retting times. appear after 3 weeks and then invade the surface stems at 9th week of the field retting period. The fibres have the same color than the stems from which they have been extracted. A similar color change was previously reported by [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] Pallesen | The quality of combine-harvested fibre flax for industrials purposes depends on the degree of retting[END_REF] after the field retting of flax fibres. ESEM images (Fig. 6) reveal that these grey spots are attributed to the development of microbial communities (fungal and bacteria [START_REF] Ribeiro | Microbial diversity observed during hemp retting[END_REF]) at the stem surface during field retting. The link between the increase of the grey and black color of the fibres during field retting and the activity of microorganisms especially fungi activity was also previously reported [START_REF] Akin | Color measurement of flax retted by various means[END_REF]. The microbial communities would start to colonize the stems surface after 3 weeks of retting (R3) and gradually cover the entire stems at the end of retting (R9). The large stem covering by microorganisms after 9 weeks of retting is probably due to more favorable conditions (higher rainfall (50 mm) after 7th of field retting period) which enhanced the development of the microbial biofilm.

Influence of field retting duration on the biochemical composition of hemp fibres

The biochemical composition of the hemp fibres was determined for unretted (R0) and retted hemp fibres (R3, R5 and R9). The main components of the hemp fibres are cellulose, hemicelluloses, lignin and pectins. As it can be seen in Fig. 7 the field retting treatment affects biochemical composition of the hemp fibres. A gradual and significant increase of the cellulose fraction from 63% to 77% after 9 weeks of the field retting was observed. Pectins are degraded at the early stage (7.8%-4.9%) and thereafter remained stable. Hemicelluloses are slightly degraded from 14.1% to 11.6% after nine week of retting. These variations during retting were found by different authors. Indeed, 2008) used the same extraction method as those of this study and found similar relative abundances for cellulose, and hemicelluloses for unretted hemp fibres. As concerns Liu et al. (2015a), these authors highlight an increase of cellulose content at early stage of field retting of hemp fibres with a decrease of the pectins and hemicelluloses. Fig. 7 shows also that minerals (ash) and lipophilic extractives were removed during retting. In contrast, the lignin content increases surprisingly during retting. Some authors (Liu et al., 2015a;[START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF] assumed that this apparent increase in lignin content is due to a lower degradation of lignin, which becomes concentrated when the other components (lipophilic extract, ash and carbohydrates) are removed at the same time. We have to be aware that the quantification of the mean components is relative and then could explain the increase of lignin quantity with retting. [START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF] suggested another assumption in relation with the Klason method used for lignin content determination. Indeed through this method, other phenolic or protein components that evolve during retting are probably also determined. The fibres extraction from the stems may also contribute in increasing of the lignin. The different structures of the stems (e.g. epidermis, xylem) become weaker with retting. Therefore, when bast fibres extracted from the retted stems, micro-residuals xylem (shives) bound to the fibres and could bias the real evolution of lignin during retting.

In addition to biochemical analyses, the cellulose organization of unretted and retted hemp fibres was characterized using X-ray diffraction. Fig. 8A shows X-ray diffractograms of hemp fibres collected at different retting times. The observed major crystalline peaks are located at 2θ diffraction angles of 14.8°, 16.2°and 22.6°, which correspond to diffraction planes of cellulose type I: (101), ( 101) and ( 002) respectively. It can be observed that the intensity of crystalline peaks is significantly increased with increasing of retting duration. In order to quantify these differences, the crystalline order index (CI) was determined as described in §2.2.3 (Fig. 8B).

The X-ray crystalline order index (CI) results are present in Table 2.

CI increased with the field retting duration from 53% for unretted fibres (R0) to 58%, 68% and 73% for R3, R5 and R9 retted fibres respectively. This result could be related to the degradation of non-cellulosic compounds during retting as shown previously in biochemical analysis. For unretted hemp fibre, the presence of a great amount of amorphous components such as hemicelluloses, lignin, and pectins between the cellulose micro-fibrils cause disoriented areas, which could undesirably influence the crystallinity of the cellulose micro-fibrils. On the other hand, the removal of amorphous materials during retting treatment leads to a better packing of the cellulose chains. An increase of cellulose fraction and cellulose crystallinity during field retting could impact positively the fibre mechanical properties [START_REF] Marrot | Analysis of the hemp fiber mechanical properties and their scattering (Fedora 17)[END_REF][START_REF] Placet | Diameter dependence of the apparent tensile modulus of hemp fibres: a morphological, structural or ultrastructural effect?[END_REF][START_REF] Alemdar | Biocomposites from wheat straw nanofibers morphology, thermal and mechanical properties[END_REF]. The change in biochemical composition and crystallinity of cellulose of the fibres during field retting is most probably related to the microorganisms activities [START_REF] Ribeiro | Microbial diversity observed during hemp retting[END_REF]. Both biofilm growth (ESEM investigations) and metabolic activity of microorganisms would lead to the removal of non-cellulosic materials which indirectly increase the percentage of cellulose in fibres (relative abundances).

Influence of field retting duration on the morphology of the hemp fibres

The optical microscope micrographs of cross-sections of unretted (R0) and retted hemp stem (R3, R5 and R9) is shown in Fig. 9. Images reveal the impact of the field retting duration on the morphology of the hemp fibres. The main parts of the hemp stem are central woody core and surrounding cortex which contains fibres bundle, epidermis, and cortical parenchyma. These two parts are linked by the cambium. Fig. 9A presents a cross section of the unretted hemp stem (R0) where the different parts of the stem are intact and well organized. The hemp fibres are combined in form of bundles as in living plants. In contrast, when the stems are field retted, their structure changed as can be seen in Fig. 9B, C and D. After 3 weeks of retting (Fig. 9B), the parenchyma cells interspersed among bast fibres and fibre bundles started to be removed resulting in a few open spaces between fibre bundles. Consequently, the fibre bundles began to be separated as well into smaller fibres bundles. As the field retting progressed, more separation of the fibre bundles into smaller fibre bundles and elementary fibres can be seen for 5 weeks retted hemp stems (R5) (Fig. 9C). In addition, void spaces are evident around the elementary fibres and the lumen of the elementary fibres can be also distinctly observed. For 9 weeks retted stem (R9) (Fig. 9D), the different parts of the stem were affected by the field retting. The epidermis layer and cambium are completely removed, the structure of central woody core is weaker and the bundles of fibres were considerably separated.

These significant changes on the morphology of the hemp fibres during field retting process are due to the action of microorganisms that gradually cover entire of the stems surface (ESEM observations). The microbial activity during field retting would permit the removal of intercellular substance and of material which is in agreement with the biochemical analyses and mainly with pectins and waxes degradation that leads to a separation i) of the bast fibre bundles from the central woody core and epidermis and ii) into smaller bundles and individual fibres. Further investigation into the temporal dynamics of the structure of microbial communities is then required to better understand biological mechanisms occurring during field retting. As expected, this result confirms the positive impact of field retting on the separation of the fibre bundles from the cortex tissues.

Influence of field retting duration on thermal stability of the hemp fibres

The thermal stability of the hemp fibres as a function of retting times was assessed on through TGA experiments. Separate constituents of plant fibres (cellulose and pectins) were first analyzed and compared to unretted hemp fibres (Fig. 10).

Thermographs of pectins and cellulose reveal a major peak at about 240 °C and 395 °C, respectively. The degradation temperature of cellulose is higher than the pectins due to the crystalline structure of cellulose. The cellulose structure contains strong intramolecular and intermolecular hydrogen bonding that requires high energies to be broken down [START_REF] Kabir | Effects of chemical treatments on hemp fibre structure[END_REF]. The unretted fibres mass loss curve highlights three mass losses, the first one at about 30-100 °C related to moisture removal, the second at about 260 °C attributed to decomposition of non-cellulosic components (pectins and hemicelluloses) and the third one at about 335 °C due to cellulose degradation. The peaks are partly superimposed on temperature scale, but can be clearly observed on DTGA curve. Similar range of degradation temperature of hemp fibres constituents was found in several previous studies [START_REF] Ghetti | Thermal analysis of biomass and corresponding Fig. 14. Scattergrams of measured diameters of unretted (R0) and retted fibres (R3, R5 and R9). The red crosses correspond to the means[END_REF][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][START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF].

The effect of field retting duration on the hemp fibre thermal stability is presented in Fig. 11. It can be observed that the thermal stability changed during field retting treatment. The increase of retting duration led to a gradual increase of the fibres thermal stability in the 200-400 °C temperature range corresponding to both pectins and hemicelluloses (shoulder peak) and to cellulose (main peak) decompositions. On one hand it can be observed that the intensity of the pectins and hemicelluloses peak decreases as a function of retting duration (Fig. 11B). This could be explained by the removal of portion of these non-cellulosic components during retting. However, this peak is not clearly visible or disappeared for the high-retted hemp fibres, indicating that non-cellulosic components were quasi totally removed. On the other hand an increase of field retting duration leads to a gradual increase of decomposition temperature of cellulose. is due to the removal of non-cellulosic components which induces a higher structural order of cellulose that requires a higher degradation temperature. This result is well correlated to biochemical and X-Ray diffraction analyses as shown in Fig. 12. This figure reveals that the degradation temperature of cellulose increases proportionally with the increment of crystallinity and cellulose fraction during retting. This indicates that thermal stability of the fibres can be enhanced with a long field retting duration.

3.5. Influence of field retting duration on the tensile properties of the fibre bundles Fig. 13 shows the results of the fibre bundle tensile test for unretted (R0) and retted hemp fibres (R3, R5 and R9). Since both failure stress and Young's modulus depend on the fibre diameter [START_REF] Charlet | Characteristics of Hermès flax fibres as a function of their location in the stem and properties of the derived unidirectional composites[END_REF][START_REF] Duval | Influence of the sampling area of the stem on the mechanical properties of hemp fibers[END_REF][START_REF] Placet | Diameter dependence of the apparent tensile modulus of hemp fibres: a morphological, structural or ultrastructural effect?[END_REF], the average diameter of fibre bundles for each retting time is also reported for comparison. The diameters of fibres selected for tensile test are between 100-230 μm (Fig. 14). The obtained results indicate that the retting duration influences the mechanical properties of hemp fibres. The tensile strength differs significantly (p < 0.01) between unretted fibres (R0) and retted fibres (R3, R5 and R9), and no significant difference (p > 0.05) is noticed between values of the fibres collected at different retting times (R3, R5 and R9). Indeed, the tensile strength of fibres increases gradually from 174 MPa for unretted fibres to 342 MPa for 5-weeks retted fibres, and then decrease slightly to 324 MPa after a prolonged field retting of 9 weeks (median values) (Fig. 13A). The Young's modulus of retted fibres (R3, R5 and R9) increases significantly (p < 0.05) to 12 GPa compared to that of unretted fibres (8 GPa) (Fig. 13B), however, no significant difference is observed (p > 0.05) between the Young's modulus of the fibres collected at different retting times (R3, R5 and R9). The strain at failure increases significantly (p < 0.01) from 2.4% for unretted fibres (R0) to 2.8%, 3.4%, 3.3% (median values), for retted hemp fibres R3, R5 and R9, respectively (Fig. 13C).

The change in tensile properties during field retting treatment can be explained by the results described in previous sections. A noticeable increase in both cellulose crystallinity and cellulose fraction during field retting could cause an increase of mechanical properties. This result is not surprising as it has been reported that both crystallinity and higher fraction of cellulose bring a better tensile performance [START_REF] Alemdar | Biocomposites from wheat straw nanofibers morphology, thermal and mechanical properties[END_REF][START_REF] Marrot | Analysis of the hemp fiber mechanical properties and their scattering (Fedora 17)[END_REF][START_REF] Placet | Diameter dependence of the apparent tensile modulus of hemp fibres: a morphological, structural or ultrastructural effect?[END_REF][START_REF] Reddy | Structure and properties of high quality natural cellulose fibers from cornstalks[END_REF].

Furthermore, the lower mechanical properties of unretted hemp fibres could be due to the presence of weak areas and a large amount of non-cellulosic substances such as pectins and hemicelluloses. This induces interfacial decohesion within fibres bundle. Another explication could be related to the manual extraction of the fibres that is not an easy step because of the presence of the cementing compounds (e.g. pectins) that bind the different parts of the stem. Therefore, hand extraction of fibres after removing epidermis using razor blade, may cause some micro-defects in the unretted hemp fibres. In contrast, when the stems are retted, as the cementing compounds are degraded, the generation of micro-defects in the fibres is limited and the manual fibres extraction is easier.

Mechanical properties of the fibres are governed by the retting duration which depends also on the weather conditions. In the present work, the field retting treatment of fibres harvested at the beginning of flowering was performed under summer weather conditions in the south of France. Therefore, in these conditions, results reveal that a long period of retting (5 weeks) is required for obtaining the highest mechanical properties. In contrast, Liu et al. (2015a) show that mechanical properties increase at the early stage of retting when the field retting period is carried out under a colder and homogeneous rainy weather condition, and then decrease rapidly due to the accelerated degradation of cellulose by microorganisms. [START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF] as well observed that a prolonged field retting until 5 weeks under rainy weather may lead to over-retted fibres. Hence, an extended of field retting duration under wet weather conditions must be avoided.

Conclusion

In this study, the influence of field retting duration on biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering was investigated. The retting period of fibres occurs under summer weather conditions.

A color change of fibres from light green (R0) to grey (R9) was observed either on stems or on fibre bundles extracted from stems. A colonization of microorganisms on stems surface has been highlighted from 3-weeks retting fibres by using ESEM. Removal of the cortical parenchyma, pectic middle lamella and other cementing compounds was given in evidence by both optical microscope (qualitative analysis) and biochemical measurements (quantitative analysis). The cellulose fraction increase (from 63% for R0 to 77% for R9) with a decrease of non-cellulosic component (from 7.81% to 4.19% for pectins for example) during retting. The increase of cellulose organization from 53% for R0 to 73% for R9 was characterized using X-ray diffraction. This increase of cellulose organization is related to degradation of non-cellulosic components during retting. TGA measurements, showed a gradual increase in thermal stability of fibres during field retting and a lower mass loss related to non-cellulosic components decomposition in relation with the removal of these constituents along retting. The tensile properties of fibre bundles increase during retting due to the increase of both cellulose crystallinity and the cellulose fraction.

All these results highlight that field retting can induce a significant change on hemp fibres properties, as well mechanical and thermal properties than fibre microstructure and microbiological medium. A 5 weeks field retting duration should be the best duration for higher mechanical properties of hemp fibres when harvested at beginning of flowering and retted in summer weather conditions. This approach also highlights that weather conditions are crucial for the implementation of the field retting process.

Further investigations will concern the evaluation of the field retting effect when fibres are harvested at different growth stages (beginning of flowering, end of flowering, and seed maturity) and with different weather conditions. In addition, the characterization of the structure of microbial communities should provide deep insights for understanding of biological mechanisms during retting. These future results should greatly determine easy and common indicators for improving the control of field retting process.

Fig. 1 .

 1 Fig. 1. Daily weather conditions (Minimum and maximum temperature, relative humidity and precipitation) during different retting durations in July to September 2016.

Fig. 2 .

 2 Fig. 2. Locations of the fibres on the stem (bottom, middle and top).

Fig. 3 .

 3 Fig. 3. Schema of successive solvent extractions used to analyze the biochemical composition of unretted and retted hemp fibres.

Fig. 4 .

 4 Fig. 4. Method used to analyze pectins content of unretted and retted fibres.

Fig. 5 .

 5 Fig. 5. Photographs of color change of the stems and fibres after different retting durations. (For interpretation of the references to colour, the reader is reffered to the web version of this article.)

Fig. 8 .

 8 Fig. 8. A) Diffractograms for unretted (R0) and retted hemp fibres (R3, R5 and R9) B) Example of curves of deconvolution method used for determining the crystalline order index (CI). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9 .

 9 Fig. 9. Optical microscope micrographs showing cross sections of unretted hemp stem R0 (A), 3 weeks retted hemp stem R3 (B); 5 weeks retted hemp stems R5 (C); 9 weeks retted hemp stem R9 (D) E: epidermis; PC: parenchyma cell; PFB: primary fibre bundles; SFB: secondary fibre bundles; ML: middle lamella; L: lumen; EF: elementary fibres; VS: void space; X: xylem.

Fig. 10 .

 10 Fig. 10. TGA (A) and DTGA (B) curves of unretted fibres (R0) compared to element constituents (commercial pectin and cellulose).

Fig. 11 .

 11 Fig. 11. TGA (A) and DTGA (B) curves of unretted fibres (R0) and retted fibres (R3, R5 and R9). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 12 .

 12 Fig. 12. Correlation between degradation temperature of cellulose (TGA), cellulose fraction (biochemical analysis) and cellulose crystallinity (DRX experiments).

Fig. 13 .

 13 Fig. 13. Box plots of tensile strength (A), Young's modulus (B), and ultimate strain (C) of unretted and retted fibres (R3, R5 and R9).

Table 1

 1 Name of samples collected during different retting times.

	Retting time (day)	0	7	14	21	28	35	42	49	56	63
	Samples name	R0	R1	R2	R3	R4	R5	-	-	-	R9

Acknowledgements

The authors would like to acknowledge the "CIVAM Chanvre Gardois association (Bouquet, France)" for making available for us a plot of land to conduct this study. The authors wish also to express sincere appreciation to Prof. Jean-Louis Fanlo, Dr. Mathilde Chaignaud, Dr. Stephane Cariou, and Dr. Sandrine Bayle for their assistance in this work.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not profit sector.