Pierre Gibot
email: pierre.gibot@isl.eu

Arnaud Bach
email: arnaud.bach@isl.eu

Loic Vidal
email: loic.vidal@uha.fr

Fabien Schnell
email: fabien.schnell@isl.eu

Roger Gadiou
email: roger.gadiou@uha.fr

Denis Spitzer
email: denis.spitzer@isl.eu

Safer and performing energetic materials based on polyaniline-doped nanocomposites

Keywords: Nanothermites, Polyaniline, Desensitisation, Energetic nanocomposites

Nanothermites, combining a fuel with an oxidizer at the nanoscale, represent a class of energetic material that has been attracting increasing attention over the past decade. This intensive interest is due to their tuneable pyrotechnic performance, making the materials promising candidates for ordnance applications. However, the extreme mechanical and electrostatic sensitivities of energetic composites make handling them hazardous. In this study, a realistic desensitisation method is suggested via the addition of polyaniline while maintaining an interesting combustion velocity in contrast to the literature values. This investigation claims a major scientific breakthrough in the preparation of safer energetic nanocomposites.

Introduction

Since 2006, the European laws on chemicals have reinforced the Regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) [1]. The aim of these laws is to improve the protection of human health and the environment by banning hazardous materials and products. Among all substances, lead and mercury are of particular focus, due to their well-known toxicity. In the field of energetic materials, lead-and mercury-based salts have been extensively used as primary explosives [START_REF] Matyas | Sensitivity to friction for primary explosives[END_REF][START_REF] Talawar | Primary explosives: electrostatic discharge initiation, additive effect and its relation to thermal and explosives characteristics[END_REF][START_REF] Ilyushin | Environmentally friendly energetic materials for initiation devices[END_REF]. Therefore, the replacement of these energetic materials is of outstanding importance. Energetic materials are undoubtedly used in military applications; however, pyrotechnic compositions or explosives in general are also frequently used in many civilian domains, such as fireworks, mining, the automotive industry, and the aerospace industry, strengthening the need to find alternative materials. The development of "green" energetic materials has consequently caused intense interest among the scientific community [START_REF] Talawar | Environmentally compatible next generation green energetic materials (GEMs)[END_REF][START_REF] Mehrkesh | Energetic Ionic Materials: How green are they? A comparative life cycle assessment study[END_REF][START_REF] Steinhauser | Green" pyrotechnics: a chemists' challenge[END_REF]. However, this novel research topic must not neglect the safety of energetic materials regarding production, handling and transport. For example, mechanical, electrostatic and thermal stresses can accidentally initiate dramatic or hazardous reactions in such compounds. For this reason, international standards have been established to govern the safe transport of energetic materials [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria, 4 th revised edition[END_REF]. Finally, the development of safer and green energetic materials that retain their reactive properties is a tremendous issue for the industry, with a focus on enabling new possibilities in civilian, military and space pyrotechnic applications.

A promising potential candidate to successfully address the aforementioned points could be a class of energetic composites named "thermites" or "Metastable Intermolecular Composites" (MICs) [START_REF] Dixon | [END_REF]. These energetic composite materials, featuring homogeneous physical mixtures of metal fuel (Al, Mg, …) and oxidiser (WO3, MoO3, …) at the microscale, undergo a highly exothermic redox reaction leading to temperatures above 2000 °C [START_REF] Fischer | Theoretical energy release of thermites, intermetallics, and combustible metals[END_REF]. Some of these materials may exhibit higher densities and energy amount in volume than traditional organic explosive molecules. In addition, if nanostructured powders are used, the pyrotechnic performance of such composites is greatly improved [START_REF] Piercy | Nanoscale Aluminum -Metal Oxide (Thermite) reactions for application in energetic materials[END_REF], such as an easier ignition and combustion velocities as high as 2500 m/s for Al/Bi2O3 [START_REF] Wang | The behavior of nanothermite reaction based on Bi2O3/Al[END_REF] or Al/CuO [START_REF] Shende | Nanoenergetic Composites of CuO Nanorods, Nanowires, and Al-Nanoparticles[END_REF] compositions. As a reminder, the explosive rates of the Hg-and Pb-based primary explosives are within the 4000 to 5000 m/s range. However, energetic nanocomposites are often more sensitive than their micron-sized counterparts. For example, this material exhibits an increased sensitivity to mechanical and electrostatic stimuli, making the handling of the material hazardous. In the literature, the desensitisation of nanothermites is an issue that can be successfully addressed by using mainly carbonaceous compounds or a Viton fluoropolymer as additives [START_REF] Poper | Controlling the electrostatic discharge ignition sensitivity of composite energetic materials using carbon nanotube additives[END_REF][START_REF] Collins | Ignition sensitivity and electrical conductivity of an aluminum fluoropolymer reactive material with carbon nanofillers[END_REF][START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Kappagantula | Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF][START_REF] Foley | Development of nanothermite composites with variable electrostatic discharge ignition thresholds[END_REF].

Unfortunately, by adding these desensitisers, the pyrotechnic properties of the energetic composites are often drastically altered [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF].

Here, we report the use of an organic conducting polymer as an additive within the Al/WO3 pyrotechnic formulation. Among the family of intrinsic conducting polymers, polyaniline (PAni) was selected due to its low cost and simple synthesis, environmental stability, tuneable electrical conductivity [START_REF] Bhadra | Progress in preparation, processing and applications of polyaniline[END_REF][START_REF] Ciric-Marjanovic | Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications[END_REF] and lubricating properties [START_REF] Song | Synthesis of polyaniline nanostructures in different lamellar liquids crystals and application to lubrication[END_REF]. The organic additive was synthesised for different aspect ratios and was added to the selected nanothermite, whose pyrotechnic performance was precisely characterised.

Experimental section

Chemical reagents were used "as received" and without any further purification. Aniline (C6H5NH2, 99%), potassium hydrogen diiodate (KH(IO3)2, 99.8%), sodium hypochlorite (NaOCl, available chlorine 4-4.99%), acetonitrile (CH3CN, 99%) and polyaniline (Emeraldine salt, Mw > 15,000 g.mol -1 ,  = 2-4 S.cm -1 ) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37%) was obtained from Prolabo. Aluminium (Al, Al-50-P, SSA = 37 m 2 .g -1 ) and tungsten trioxide (WO3, SSA = 13.5 m 2 .g -1 , 99.9%) nanopowders were provided by NovaCentrix (United States) and DK Nanomaterials (China), respectively.

Polyaniline nanofibre synthesis:

The protocol is inspired by the work of Rahy et al. [START_REF] Rahy | Synthesis of highly conductive polyaniline nanofibers[END_REF][START_REF] Rahy | Polyaniline nanofibers synthesis by co-use of ammonium peroxydisulfate and sodium hypochlorite[END_REF] and can be described as follows: 1 mL of aniline was mechanically stirred for 20 min in 100 ml HCl (1 M). Next, 100 ml of aqueous solution containing 0.4864 g of KH(IO3)2 was added to the aniline solution, and after 5 min, 5 mL of NaOCl was added dropwise to the previous solution while keeping it under vigorous stirring. After complete introduction, the stirring was stopped and the final solution was kept at room temperature for either 0.3 hours or 72 hours. Solid products were recovered by centrifugation and then washed with HCl (1 M), methanol and acetone. Consecutive overnight drying at 60 °C was performed to obtain the final powders. The "as-prepared" materials were labelled "PAni_SF" and "PAni_LF" for the samples obtained after a short (0.3 h) and a long ageing time (72 h), respectively. A commercial polyaniline reagent, PAni_Com, was also selected to carry out the investigation.

Nanothermite preparation:

The energetic mixtures were prepared by combining aluminium (Al), tungsten trioxide (WO3) and conducting polymer (PAni, 0-5wt %) nanopowders in predefined proportions in acetonitrile. Note that the Al-50-P powder is made of aluminium nanoparticles that exhibit a core-shell structure, i.e., an Al metallic core surrounded by an Al2O3 layer. The active Al content in the powdered fuel was determined to be equal to 57.9 wt %. To boost the energetic performance of the composites investigated, an excess of fuel, compared to the following stoichiometric chemical reaction (1), was considered.

2Al(s) + WO3(s) → Al2O3(s) + W(s) (1)
By taking the excess into account, the amount of active aluminium was calculated according to the equivalence ratio (Φ) described in Eq. ( 2) and fixed to a value of 1.4:

𝛷 = . . (2)
where F and O are the mass ratio of fuel (Al) to oxidiser (WO3). The subscripts exp. and st.

stand for the experimental and stoichiometric ratios.

The resulting dispersions (with or without additive) were homogenised for 1 hour by alternating stirring and ultrasonication (sonotrode, 300 W, 40 kHz, pulse of 0.25 s). Finally, the solvent was evaporated under 200 mbar and 80°C to obtain dry grey compositions. The diverse energetic composites were labelled "AW_PAni_xy". A, W, x, and y represent the fuel, the oxidiser, the nature of polyaniline (SF, LF, or com.), and the amount of polyaniline, respectively.

Characterisation techniques:

Fourier Transform Infrared (FT-IR) spectroscopic analysis of the polyaniline samples was conducted on a Bruker Tensor 27 spectrometer in transmission mode (KBr disks). The spectra were recorded within a 4000 -520 cm -1 range (resolution of 4 cm -1 ) by superposing 16 scans.

The elemental analyses (C, N, H and O) of the polyaniline samples were performed on a Thermo Finnigan Flash EA 1112 apparatus at 1050°C supported by a helium/oxygen flow.

Reaction products (CO2, H2O, and NOx) were separated on a chromatography column and quantified by using a thermal conductivity detector. The amount of chlorine was calculated with respect to 100% of the material. The morphology and microstructure of the different materials (polyaniline, oxidiser and fuel) were observed via Transmission and Scanning Electron Microscopy (TEM, SEM) by using a CM200 microscope (Philips, 200 kV) and a DSM 982 Gemini (Zeiss, 10 kV) microscope, respectively.

Sensitivity tests: for the impact, friction and electrostatic discharge sensitivity measurements, the threshold determined on the loose powder is the highest value, for which 6 reaction tests are related to a no-combustion result. The probability of obtaining no combustion at all is 98.4% at that threshold value. All of the experiments were performed in compliance with the NATO standards [START_REF]STANAG 4489. In: Standardization Agreement (STANAG) on Explosives, Impact Sensitivity Tests[END_REF][START_REF]STANAG 4487. In: Standardization Agreement (STANAG) on Explosives, Friction Sensitivity Tests[END_REF][START_REF]STANAG 4490. In: Standardization Agreement (STANAG) on Explosives, Electrostatic Discharge Sensitivity Tests[END_REF].

The impact sensitivity is determined by a Julius Peters BAM Fallhammer apparatus. During the experiment, the energetic material is placed between two steel cylinders. A hammer (1 or 5 kg) falls from various drop heights (max. of 1 m) onto the energetic material. The impact threshold value is given in joules (J). The friction sensitivity is determined by a BAM friction apparatus. Under the experimental conditions, the energetic material is placed between a ceramic plate and a peg: the force applied is a function of the weights fixed at specific locations on a lever arm. The friction threshold value is given in Newtons (N). The electrostatic discharge (ESD) sensitivity is measured by using an ESD 2008 testing device from OZM Research s.r.o., Czech Republic, with a discharge voltage of 4-10 kV and a distance of 1.0 mm between the electrodes. The ESD threshold value is given in joules (J).

Reactivity tests: a schematic of the equipment used to ignite the formulations and to record the combustion phenomena is shown in Fig. 1. Typically, nanothermite powders were loosely packed in cylindrical glass tubes with an inner diameter of 3 mm and a length of 150 mm. An electric primer was used for the ignition of the nanothermites, as proposed by Comet et al. [START_REF] Comet | Boron as Fuel for Ceramic Thermites[END_REF]. The combustion behaviours of the energetic compositions (with or w/o polyaniline) were recorded by using a Photrom FASTCAM high-speed camera (5×10 4 frames/s) placed perpendicularly to the glass tubes. The combustion velocities were calculated from a series of four tests. The apparent densities of the nanothermites tested were kept constant at approximately 10% (≈ loose powder) of their corresponding theoretical maximum densities (TMD) calculated from Eq. ( 3).

𝑇𝑀𝐷 = 1 ∑ (3)
where i is the considered chemical species and m and  are its mass and density, respectively.

Results and Discussion

Characterisation of the PAni conducting polymer

The samples recovered at the end of the synthesis procedure were a dark-green colour, and the yields were calculated to range from 21% to 47% for synthesis times ranging from 0.3 to 72 h.

This specific colour characterises the conducting phase of the PAni-named emeraldine salt, as opposed to the insulating leucoemeraldine and pernigraniline phases. The molecular structure of the emeraldine base is shown in Figure 2, where two chemical entities called benzenoid and quinoid coexist in equimolar amounts. The partial protonation of the quinoid functional group leads to the green emeraldine salt. The FTIR spectroscopic analysis of each "as-prepared" PAni sample is presented in Fig. 3.

The FTIR spectra of PAni-com, PAni_SF, and PAni_LF are consistent with the literature [START_REF] Rahy | Synthesis of highly conductive polyaniline nanofibers[END_REF][START_REF] Rahy | Polyaniline nanofibers synthesis by co-use of ammonium peroxydisulfate and sodium hypochlorite[END_REF] and are characterised by two active domains in the ranges of 1600-1100 cm -1 and 900-400 cm -1 . The vibration bands located at 1560 cm -1 and 1488 cm -1 were attributed to the stretching (ν) modes of the quinoid and benzenoid units, respectively. The latter entity confirms the successful synthesis of the conducting PAni polymer. The formation of such a molecular structure was further strengthened by the infrared band at 1250 cm -1 relating to the ν(C-N+) in the polaron lattice of the PAni polymer. The bands at 1300 cm -1 and 1146 cm -1 were identified as originating from the (C-N) stretching (ν) and (C-H) bending (δ) modes in a benzenoid and a quinoid unit, respectively. Finally, in the lowest wavenumber domain, the bands at 823 cm -1 and 504 cm -1 were assigned to the bending (δ) vibration of the C-H bond inside a 1,4-disubstituted aromatic cycle. The data relating to the elemental analyses performed on each of the three polymer samples are presented in Table 1. The amount of each chemical element is similar for the PAni_SF and PAni_LF samples, suggesting that the specific ageing time does not affect the chemical composition of the synthesised materials. The main difference between the commercial sample (PAni_com) and the synthesised ones is that the amount of oxygen is higher in the former case, which may be explained by the fact that the protonation of the commercial product is achieved by means of a sulfonic acid (RSO3H, where R is an alkyl group) according to the manufacturer's data. Hydrochloric acid (HCl) was used for PAni_SF and PAni_LF. Scanning and transmission electron microscopy (SEM and TEM) images of the PAni samples are presented in Figure 5. Whereas highly agglomerated 500-nm-sized sphere-like particles are observed for the commercial sample (PAni_com, Fig. 4A), the "as-synthesised" PAni polymers (PAni_SF and PAni_LF) exhibit a nanoscale-fibre morphology, as expected from the works of Rahy [START_REF] Rahy | Synthesis of highly conductive polyaniline nanofibers[END_REF][START_REF] Rahy | Polyaniline nanofibers synthesis by co-use of ammonium peroxydisulfate and sodium hypochlorite[END_REF]. Polymerisation times of 0.3 h (Fig. 4B) and 72 h (Fig. 4C) lead to the formation of agglomerates of short nanofibres (length < 0.5 µm) and more isolated long nanofibres (length = 1 -2 µm), respectively. The average diameter is close to 50 nm (Ø) and is independent of the polymerization time. first, a ternary system with the presence of PAni aggregates in a well-distributed Al/WO3 mixture obtained by adding commercial fibres (PAni_com, Fig. 6B) and short fibres (SF_PAni, Fig. 6C) of the conducting polymer; and second, a ternary system that is much more homogeneous with respect to the volume distribution of the components when adding long nanofibres of polyaniline (PAni_LF, Fig. 6D). Furthermore, the individual sizes of the PAni aggregates in Al/WO3/PAni_com are larger compared to those within the Al/WO3/PAni_SF formulations. This observation is consistent with the TEM analyses conducted on the single conducting polymer (Fig. 5), where agglomeration is detected for the PANi_com and the PAni_SF polymers.

%C

Sensitivities of PAni-doped Al/WO3 nanothermites

The electrical and mechanical sensitivities of the binary and ternary energetic formulations (nanothermites) are presented in With regard to the electrostatic discharge sensitivity (ESD), the use of the commercial conducting polyaniline or "as-synthesised" short nanofibres as desensitising additives does not change the sensitivity threshold of the binary Al/WO3 nanothermite, which is taken as reference. In this context, a constant value of 0.14 mJ was determined, which is well below the standard value (> 100 mJ). At this level, an accidental decomposition reaction can occur, for example, during handling by an operator, and consequently presents a considerable hazard. However, when long nanofibres of polyaniline (PAni_LF) were mixed with the Al/WO3 energetic formulation, a drastic change occurred, leading to an increase in the ESD thresholds from below 0.14 mJ to 120 mJ. As the electrical conductivity values of each PAni sample are similar, the behaviour of the Al/WO3/PAni_LF nanothermites toward the ESD sensitivity may be explained in terms of the preparation of the energetic formulations and the percolation theory. First, as previously demonstrated, the PAni_LF sample is much less agglomerated (Fig. 5) and better dispersed in the Al/WO3 energetic formulation than its PAni_com and PAni_SF counterparts (Fig. 6). This aspect is correlated with the fact that PAni_LF exhibits the highest aspect ratio of the nanofibres (length/diameter); thus, we think that the percolation of the polymer additive is effective in this case for a 5 wt% content (≈ 15 vol. %). This Al/WO3/PAni_LF nanothermite can be described as an Al/WO3 mixture embedded in a three-dimensional PAni conducting network. This conducting network is able to evacuate electrostatic charges through the composites, thus retaining little energy to heat the energetic formulation and to lead to its ignition. For the other two PAni samples (PAni_SF, PAni_com), a more or less high level of aggregation and smaller aspect ratios exclude the use of the percolation theory. Table 3 shows the influence of the mass content of long conducting nanofibres on the ESD sensitivity of a binary Al/WO3 nanothermite. It can be seen that at 3 wt %, the threshold value decreases until it is below 0.14 mJ, suggesting that the percolation is reached between 3-5 wt% by long PAni nanofibres (≈ 10 -15 vol.%). This result is consistent with the work of Steelman et al. [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF]. In that paper, 13 vol% of carbon nanotube (CNT) additive, exhibiting a similar morphology and size compared to the polyaniline nanofibres, was identified as the specific percolation threshold within the Al/CuO nanocomposites.

With regard to the friction sensitivity, the binary WO3/Al nanothermite is highly sensitive, with a threshold value of 8 N compared with the standard value of 80 N for the transport of explosive goods [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria, 4 th revised edition[END_REF]. The use of the conducting polyaniline, regardless of the form it takes (PAni_com, PAni_SF, and PAni_LF at 5 wt %), leads to a gradual increase in the sensitivity threshold value of the nanothermite up to a maximum of 360 N (i.e., largely above the standard value and therefore characterising the energetic formulation as totally insensitive).

The insensitive character of the different Al/WO3/polymer formulations might be explained by the lubricating properties of polyaniline [START_REF] Song | Synthesis of polyaniline nanostructures in different lamellar liquids crystals and application to lubrication[END_REF]. This mechanical behaviour might result from a low interchain cross-linking rate or weak interchain interactions (e.g., hydrogen bonding or π-π stacking), which allows the sliding of polymeric chains with respect to each other during mechanical stresses, thereby absorbing a part of the energy from stimuli. The non-absorbed energy is then insufficient to ignite the Al/WO3 energetic nanocomposite. However, as seen, the insensitivity increases as follows: PAni_com < PAni_SF < PAni_LF. The latter observation can be correlated with the homogeneity of the Al/WO3/conducting polymer energetic formulation (Fig. 6): the higher the homogenisation of the Al/WO3/polymer is, the lower the friction sensitivity becomes. Due to the higher effect observed with the long nanofibre polyaniline, the amount of mass of this polymer within the formulation was investigated (Table 3). By reducing the mass of long nanofibres inside the Al/WO3 nanothermite, a downward trend could be observed, together with a threshold value remaining above the standard value (84 N at 1 wt% vs. 80 N) and 10 times higher than the friction sensitivity of the PAni-undoped Al/WO3 nanothermite (8 N).

Regarding the sensitivity to impact, the Al/WO3 binary formulation can be considered to be insensitive, with a threshold value higher than 50 N, if we refer to the standard set at 2 J by the United Nations [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria, 4 th revised edition[END_REF]. The use of polyaniline as an additive leads to a systematic decrease in ignition energy down to a minimum value of 10 J for the shortest polyaniline nanofibres;

however, in all cases, the ternary nanothermites (Al/WO3 with PAni_com, PAni_SF, or PAni_LF) remain safe with respect to the standard, which can clearly be interpreted as a success. As reported in the literature [START_REF] Hardt | Shock initiation of thermite[END_REF][START_REF] Davis | Reactions in neat porous metal/metal and metal/metal oxide mixtures under shear induced plastic flow conditions[END_REF], the ignition of nanothermites can be activated by the formation of hot spots inside the investigated composite subjected to stress. Hot spots can be generated in three distinct ways:

. adiabatic compression of gases trapped inside the pores within the composite,

. chemical reactions due to particles that crash on the move, . interparticulate friction.

As the friction sensitivity of polyaniline-doped nanocomposites is low (i.e., > 190 N vs. 8 N for an Al/WO3 system) the last hypothesis can reasonably be discarded. Regarding the second hypothesis, it could be validated if the additive presented higher mechanical properties than the hardest components composing the conventional energetic mixtures (Al2O3 and WO3 ceramics); this is not expected in the present case, as the additive belongs to the polymer family. Consequently, the hot spot formation probably results from the adiabatic compression of gases inside the material during the impact stress. As the Al/WO3 energetic nanothermite is a very low gas-generating composite, only the gasification of the polyaniline particles can back up this hypothesis. From thermogravimetric analyses under oxidising atmosphere (not shown here), the combustion of polyaniline starts at 250°C and ends at 300°C and 600°C for the nanofibres (PAni_SF & PAni_LF) and the micron-sized spherical particles (PAni_com), respectively. The high agglomeration and low aspect ratio of the micron-sized particles might be responsible for the slower combustion kinetics. Because the gasification of the largest polyaniline particles is more difficult, the Al/WO3/PAni_com is less sensitive to impact (difficult hot spot formation) than its Al/WO3/PAni_SF and Al/WO3/PAni_LF counterparts (45 J vs. 10 J and 20 J, respectively).

By decreasing the mass content of the long conducting nanofibres (Table 3), the impact threshold increases to reach the value of the binary Al/WO3 nanothermite, thus confirming the previously given explanations.

Weight

Combustion performance of PAni-doped Al/WO3 nanothermites

In this section, the combustion performance of the Al/WO3/PAni_LF nanothermite (5 wt%) is compared with the conventional Al/WO3 nanothermite performance. It appears as the only ternary Al/WO3/conducting polymer energetic formulation that exhibits complete insensitivity to mechanical and electrical stresses, in accordance with the NATO standard (Table 3).

The combustion energies of the two energetic nanocomposites were measured by using a calorimetric bomb. The released energy of the Al/WO3 was determined to be close to 2815 J.g -1 , which is close to the theoretical data (2915 J.g -1 ) given by Fischer et al. [START_REF] Dixon | [END_REF]. With respect to the PAni-doped Al/WO3 nanocomposites, a lower value was found to be equal to 2536 J.g - 1 , suggesting a less powerful energetic character when the desensitising agent is added into the reference binary composites.

Figure 6 shows some selected video frames of the combustion of these two energetic composites. Combustion tests were performed in glass tubes filled with loose powders at loading densities calculated to be equal to 0.39 ± 0.02 g.cm -3 and 0.43 ± 0.02 g.cm -3 , corresponding to 8.2% and 9.1% of the TMD of the Al/WO3 and the Al/WO3/PAni_LF energetic nanocomposites, respectively. From a meticulous analysis of the video sequences, dark zones can be visualised (i.e., at the end of the combustion for the Al/WO3 nanothermite (0.5 ms) and directly from the ignition for the Al/WO3/PAni_LF nanothermite (0.4 ms)).

Whereas in the former case, these zones exhibit a classical decrease in temperature after the composition front has passed, in the latter case, these zones also suggest the spatial positioning of the conducting polymer in the ternary composition because the combustion of polyaniline requires energy and consequently accelerates cooling. From this consideration, a homogeneous distribution of PAni can be observed over the whole length of the tube (Figure 6 B, time: 0.5 ms). The progress of the combustion front filmed through the tube, for each energetic composite, is plotted as a function of time in Figure 7. Different profiles can be divided into three and four zones for the Al/WO3 and the Al/WO3/PAni_LF composites, respectively. The combustion regimes are multistage, and based on our experience and knowledge [START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF][START_REF] Gibot | Highly insensitive/reactive thermite prepared from Cr2O3 nanoparticles[END_REF][START_REF] Gibot | Synthesis of WO3 nanoparticles for superthermites by the template method from silica spheres[END_REF], an attempt to Regarding the Al/WO3 nanothermite, the calculated combustion velocity is slightly higher than the values reported by Prentice et al. (310 -490 m/s) [START_REF] Prentice | Combustion wave speeds of sol-gel synthesized tungsten trioxide and nano-aluminium: the effect of impurities on flame propagation[END_REF] for similar energetic composites. This difference can be explained by a confinement effect caused by our glass tubes, whereas in the literature, an open-channel testing apparatus was used. Regarding the Al/WO3/PAni_LF nanothermite, the increase in the combustion velocity within zone 3 can be interpreted as a pre-heating of the fuel, oxidiser and polymer nanoparticles (unreacted matter) caused by the combustion gaseous products (e.g., COx, H2O, NOx…) of the polyaniline component burned inside the third zone. Then, the ignition of the nanocomposites requires less energy to burn; this explains the acceleration of the displacement of the reaction front through the matter. Experiments are currently in progress to refine the understanding of the reaction mechanism that is implied in this polyaniline-based energetic nanocomposite.

Finally, note that the combustion velocity of the PAni-doped nanothermites is of the same order of magnitude as that of the conventional binary Al/WO3 nanothermite. This result represents a truly major scientific breakthrough in the preparation of safer and more reliable reactive nanocomposites because the studies conducted for desensitising energetic nanocomposites and reported in the literature have always led to a drastic loss of reactivity [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF].

Conclusion

In summary, a tremendous electrostatic desensitisation of a WO3/Al nanothermite while maintaining good pyrotechnic properties was achieved using high aspect ratio polyaniline nanofibres. The strong increase in the ESD threshold from 0.14 mJ to 120 mJ was obtained with a low mass content of 5 wt% additive. This value is six times higher than the estimated energy of a spark generated by an ungrounded human body (20 mJ). Moreover, the mechanical thresholds are higher than the standard values: a complete desensitisation to friction (> 360 N) is obtained while the impact sensitivity remains at a value that is 10 times higher than the international standard value (20 J vs. 2 J). Regarding the loose powder nanothermite, the combustion velocity is of the same order of magnitude as that of the binary nanothermite when using long PAni nanofibres (414 ± 35 m/s vs. 620 ± 5 m/s). This strong desensitisation (mechanical and electrostatic) coupled with good reactivity outperforms the achievements obtained so far in response to one of the most challenging issues for the development of safer nanothermites for civilian, military and aerospace applications.

Figure 1 :

 1 Figure 1: Schematic of the reactivity testing equipment used for nanothermites.

Figure 2 :

 2 Figure 2: Molecular structure of the conducting emeraldine salt form (X = anion).

Figure 3 :

 3 Figure 3: FTIR spectra of the PAni_com (A), PAni_SF (B) and PAni_LF (C) samples. The spectra were shifted to a better legibility.

Figure 4 :

 4 Figure 4: SEM and TEM images of the PAni_com (A), PAni_SF (B) and PAni_LF (C) conducting polymers.

Figure 5 :

 5 Figure 5: TEM images of the energetic systems of Al/WO3 (A), Al/WO3/PAni_com (B), Al/WO3/PAni_SF (C) and Al/WO3/PAni_LF (D).

Figure 6 :

 6 Figure 6: Sequences of combustion images of the Al/WO3 (A) and the Al/WO3/PAni_LF (B) energetic nanocomposites. The scale bar (in the inset of each first image) represents 10 mm, and the time scale ranges from 0 to 0.7 ms (the step size is 0.1 ms).

  explain them is described below. The first millimetres of the tubes (length < 5 mm) characterise the combustion of the Al/CuO igniting composition; consequently, no signals were intentionally taken into account. Within zone 1, a displacement of the reaction front was recorded as a function of time; this displacement represents the combustion of the Al/WO3 investigated thermite (w/o and with polyaniline). Nevertheless, at this stage, an influence of the CuO-based igniting composition (diffusion/conduction phenomena) cannot be reasonably neglected. The second zone is attributed to the self-combustion of the energetic formulations investigated and to combustion velocities of 620 ± 5 m/s and 256 ± 14 m/s, which were calculated for the Al/WO3 and the Al/WO3/PAni_LF nanocomposites, respectively. Finally, in the last millimetres of the glass tubes (zone 3), an acceleration of the combustion phenomenon is observed for the PAni-doped nanocomposites. The resulting combustion velocity was calculated to be close to 414 ± 35 m/s and was referenced as the actual combustion velocity of the Al/WO3/PAni_LF energetic formulation.

Figure 7 :

 7 Figure 7: Combustion profiles of the Al/WO3 (A) and the Al/WO3/PAni_LF (B) energetic nanocomposites.

Table 1 :

 1 Chemical composition of the different polyaniline samples by elemental analysis. The chlorine amount was calculated with respect to 100% of the material.

			%N	%H	%O	%S	%Cl
	PAni_com	61.15	6.31	5.65	15.77	7.99	/
	PAni_SF	52.33	10.39	4.21	10.91	/	22.16
	PAni_LF	53.59	9.48	3.88	10.09	/	22.96

Table 2 .

 2 The sensitivities were determined according to the protocol detailed in the Experimental Section.

		ESD (mJ)	Friction (N)	Impact (J)
	Standards	100	80	2
	Al/WO3	< 0.14	8	> 50
	Al/WO3/PAni_com	< 0.14	192	45
	Al/WO3/PAni_SF	0.14	288	10
	Al/WO3/PAni_LF	120	> 360	20

Table 2 :

 2

Sensitivity thresholds for Al/WO3 nanothermites mixed with the PAni polymer (5 wt%, com = commercial, SF = short nanofibres, LF = long nanofibres); ESD for ElectroStatic Discharge.

Table 3 :

 3 Sensitivity thresholds for an Al/WO3 nanothermite mixed with PAni long nanofibres (PAni_LF).

Acknowledgements

The authors gratefully acknowledge the French Defense procurement agency (Direction Générale de l'Armement (DGA)) and the French National Centre for Scientific Research (CNRS) for their financial support.


Pierre Gibot
email: pierre.gibot@isl.eu

Quentin Miesch

Arnaud Bach

Fabien Schnell

Roger Gadiou

Denis Spitzer

Mechanical desensitization of an Al/WO3 nanothermite by means of carbonaceous coatings derived from carbohydrates

Keywords: Nanothermites, Saccharides, Carbon, Desensitization, Energetic nanocomposites

Nanothermites show great developmental promise in the near future in civilian, military and aerospace applications due to their tuneable reactive properties (ignition delay time, combustion velocity and pressure release). However, the high mechanical sensitivities of some of these energetic nanocomposites can make transportation and handling them hazardous. Here, a mechanical desensitization (shock and friction) of an Al/WO3 nanothermite is successfully obtained by means of carbon adding through the pyrolysis of naturally occurring molecules (carbohydrates).

The combustion behaviour of the carbon-based energetic mixtures were also evaluated and a respectable reactivity has been evidenced.

Introduction

The mixtures of metal and oxide powders at the micro-scale, which are known as thermites, are interesting energetic composites for specific applications, such as in the ordnance domain. The reaction between both components is described as an exothermic oxidoreduction chemical reaction leading to a high adiabatic temperature [START_REF] Fischer | Theoretical energy release of thermites, intermetallics, and combustible metals[END_REF]. The amount of energy released during the reaction is considerable and can reach, in some cases, 20 kJ/cm 3 . The adiabatic flame temperatures are also very impressive with values ranging from 2000 to 3000 K. However, these traditional micro scale thermites may suffer from low performance, such as poor reactivities, high ignition delay times and low combustion velocities. These intrinsic reactive properties can be widely improved by the exclusive use of nanosized metallic and oxides powders since the chemical reactions are governed by diffusion/transport phenomena at the interphase of the reactants [START_REF] Pantoya | Combustion behavior of highly energetic thermites: Nano versus micron composites[END_REF]. The shorter the diffusion pathway is, the better the reactive properties. In such a configuration, the thermites, which are referred to as nanothermites or metastable intermolecular composite (MIC) materials, attract much attention [START_REF] Valliapan | Reactivity of aluminum nanopowders with metal oxides[END_REF][START_REF] Sanders | Reaction propagation of four nanoscale energetic composites (Al/MoO3, Al/WO3, Al/CuO and Bi2O3)[END_REF][START_REF] Puszynski | Processing and characterization of aluminum-based nanothermites[END_REF][START_REF] Sun | Dependence of size and size distribution on reactivity of aluminum nanoparticles in reactions with oxygen and MoO3[END_REF][START_REF] Dutro | The effect of stoichiometry on the combustion behavior of a nanoscale Al/MoO3 thermite[END_REF][START_REF] Pantoya | Effect of bulk density on reaction propagation in nanothermites and micron thermites[END_REF][START_REF] Comet | Phosphorous-based nanothermites: A new generation of energetic materials[END_REF][START_REF] Comet | Boron as fuel for ceramic thermites[END_REF]. These are very promising energetic composites and can advantageously substitute for explosives in many pyrotechnic applications due to their high available energy densities and reaction rates [START_REF] Dixon | Lead-free percussion primer mixes based on metastable interstitial composite (MIC) technology[END_REF][START_REF] Wang | The behavior of nanothermite reaction based on Bi2O3/Al[END_REF][START_REF] Shende | Nanoenergetic composites of CuO nanorods, nanowires, and Al-nanoparticles[END_REF] while being free of highly toxic metal (mercury and lead) [START_REF] Dixon | Lead-free percussion primer mixes based on metastable interstitial composite (MIC) technology[END_REF][START_REF] Wang | The behavior of nanothermite reaction based on Bi2O3/Al[END_REF][START_REF] Shende | Nanoenergetic composites of CuO nanorods, nanowires, and Al-nanoparticles[END_REF]. However, the downside of obtaining such reactive properties is that, in most cases, the sensitivities of the thermites to external stresses, such as mechanical ones, are highly increased [START_REF] Gibot | Synthesis of WO3 nanoparticles for superthermites by the template method from silica spheres[END_REF]. In addition, the extreme sensitivity of the nanothermites towards the electrostatic discharge does not need to be demonstrated. Subsequently, such behaviors can be harmful to the expected potential applications of these kinds of energetic composites since accidents can occur and the safety of the users cannot be assured. The development of desensitized nanothermites that maintain suitable reactivity is thus of primary importance. From an international point of view, the United Nations intergovernmental organization has established standards for the transport of explosive goods [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria[END_REF]. The energetic materials are classified with respect to their sensitivity thresholds relating to several stresses, such as impacts, friction, electrostatic discharge, temperature increases, magnetic fields and so on. According to these standards, the energetic compositions of the nanothermites are crucial to their future promise, as expected.

In the literature, a number of investigations have been conducted on mitigating the sensitivities of nanothermites. Aluminium metal was systematically chosen as the fuel and was mixed with different oxide ceramics (CuO, WO3, MoO3 and MnO2) [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Poper | Controlling the electrostatic discharge ignition sensitivity of composite energetic materials using carbon nanotube additives[END_REF][START_REF] Foley | Development of nanothermite composites with variable electrostatic discharge ignition thresholds[END_REF][START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF][START_REF] Collins | Synthesizing aluminium particles towards controlling electrostatic discharge ignition sensitivity[END_REF][START_REF] Weir | The role of aluminum particle size in electrostatic ignition sensitivity of composite energetic materials[END_REF][START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF] or a polymer (polytetrafluoroethylene, PTFE) [START_REF] Collins | Ignition sensitivity and electrical conductivity of an aluminum fluoropolymer reactive material with carbon nanofillers[END_REF][START_REF] Kappagantula | Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives[END_REF][START_REF] Kappagantula | Experimentally measured thermal transport properties aluminumpolytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives[END_REF]. Desensitization can occur due to two different approaches: either by modifying the aluminium particles (average particle size and passivizing alumina layer) [START_REF] Collins | Synthesizing aluminium particles towards controlling electrostatic discharge ignition sensitivity[END_REF][START_REF] Weir | The role of aluminum particle size in electrostatic ignition sensitivity of composite energetic materials[END_REF] or by adding an additive in the traditional binary fuel/oxidizer energetic composites [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Poper | Controlling the electrostatic discharge ignition sensitivity of composite energetic materials using carbon nanotube additives[END_REF][START_REF] Foley | Development of nanothermite composites with variable electrostatic discharge ignition thresholds[END_REF][START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF][START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF][START_REF] Collins | Ignition sensitivity and electrical conductivity of an aluminum fluoropolymer reactive material with carbon nanofillers[END_REF][START_REF] Kappagantula | Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives[END_REF][START_REF] Kappagantula | Experimentally measured thermal transport properties aluminumpolytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives[END_REF]. Regarding this second method, in a large majority of studies, synthetic carbons such as nanotubes, graphene and nanofibres were selected that provided more efficient electrostatic discharge desensitization [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Poper | Controlling the electrostatic discharge ignition sensitivity of composite energetic materials using carbon nanotube additives[END_REF][START_REF] Collins | Ignition sensitivity and electrical conductivity of an aluminum fluoropolymer reactive material with carbon nanofillers[END_REF][START_REF] Kappagantula | Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives[END_REF][START_REF] Kappagantula | Experimentally measured thermal transport properties aluminumpolytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF] without effect or mention regarding the friction sensitivity. Now, some nanothermites are very friction sensitive such as Al/CuO, Al/WO3 and Al/MnO2 with values of ranged from 8 to 9 N [START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF] that is widely below the 80 N standard required for the transportation of energetic materials on public roads [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria[END_REF]. Maintaining such level of friction sensitivity could restrict practical applications of nanothermites. Since then, the use of cheap amorphous carbon black (soot) was proposed within the Al/WO3 nanothermites to mitigate the friction sensitivity. Efficient mechanical desensitization (impact and friction) was observed with low amounts of carbon additives with a decreased combustion velocity [START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF]. More recently, two papers have shown the beneficial use of non-carbonaceous additives to mitigate the sensitivities of nanothermites. The first one examined and demonstrated the efficiency of a lubricating molybdenum sulphide material (MoS2) and hexadecane (C16H34) as friction desensitizing additives in an Al/MoO3 nanothermite, but no reactive properties were evaluated [START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF]. The second paper focused on the use of a polyaniline conducting polymer in an Al/WO3 nanothermite. The polymer proved to be a very promising additive for the friction and electrostatic discharge desensitizations at low amounts (5 wt. %) while maintaining good reactive performance for the investigated energetic formulation [START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF]. However, the performance of the nanothermite strongly depends on the homogenization/dispersion of the additive within the Al/WO3 energetic composites.

Generally speaking, the adding of additives within nanothermites leads to a mitigated reactive behaviour of the considered energetic systems (decrease of the combustion velocity, longer ignition delay time…) that can thereby limit or divert the thermites of their initial practical applications but in other side, it is important to keep in mind that using nanothermites free of any additives stay extremely hazardous (handling and transportation). Definitively, the nanothermite desensitization is an emerging research topic where numerous aspects must be explored (additive nature, combustion behaviour and potential applications).

Here, a carbon coating onto the tungsten (VI) oxide nanoparticle surface as a method to mechanically desensitize an Al/WO3 nanothermite is described. The carbon adding was achieved by a fast and simple process that carbonizes sucrose and cellulose molecules. These organic species (saccharides) are naturally occurring molecules, abundant and low cost, making them promising alternatives in the development of insensitive reactive energetic formulations. The performance of the nanothermites (mechanical sensitivities and reactivity), enriched or not enriched with carbon additives, were characterized.

Materials and Methods

Sucrose (C12H22O11, reagent grade), acetonitrile (CH3CN, 99%) and tungsten (VI) oxide (WO3, specific surface area (SSA) = 27 m 2 /g, 99.9%) were purchased from Sigma-Aldrich. Sulphuric acid (H2SO4, 98%) was obtained from Carl Roth. Cellulose microcrystalline ((C6H10O5)n, reagent grade) was provided by Alfa Aesar. Aluminium (Al, SSA = 37 m 2 /g, Al = 57.9 wt. %) nanopowder was purchased from NovaCentrix (United States). All chemical reagents were used as received.

Tungsten (VI) oxide/carbon composites synthesis.

The weight percent of carbon additives in the WO3/C composite was fixed at approximately 3 wt. % because, for lower values, tungsten (VI) oxide particle surfaces are not completely darkened, especially with the use of a cellulose precursor. For that, the carbonization reactions of sucrose (Eq. ( 1)) and cellulose (Eq. ( 2)) were considered.

C12H22O11(s) → 12C(s) + 11H2O(g) (1) (C6H10O5)n(s) → 6nC(s) + 5nH2O(g) (2)
The WO3/saccharide composites were assessed following two different approaches, depending on the saccharide molecule (sucrose or cellulose) that was used as the carbon source. With the sucrose molecule, the experimental approach can be described as follows. In a round-bottom flask (100 mL), 1 g of WO3 and 0.19 g of sucrose were poured in 50 mL of demineralized water and stirred and ultrasonicated for 30 min. Then, 100 µL of concentrated sulphuric acid (H2SO4) was added and the stirring was maintained for 30 min. The round-bottom flask was placed on a rotating evaporator (100 rpm) and the liquid phase was evaporated at 90 °C under 200 mbar of pressure until a dry powder was obtained. The product was heated up to 250 °C at 5 °C/min and maintained at this temperature for 2 hours to complete the drying. Concerning the preparation of the WO3/saccharide composites with the use of cellulose as the carbon source, the experimental method is as follows. 1 g of WO3 and 0.14 g of microcrystalline cellulose were mixed in a mortar. Then, 100 mg of this mixture was inserted in a 5 mL agate grinding jar that was equipped with an agate ball (5 mm, 100 mg) and placed on a mixer-mill apparatus (Retsch MM200). The mixture was then milled for 5 min at a frequency of 25 Hz. The sucrosebased composite was not submitted to this post-treatment since sucrose has high solubility in water and a high physical contact between components is expected after the solvent removal.

The resulting WO3/saccharide composites were heat-treated at 500 °C (5 °C/min., 2 hours) under an argon flow (100 mL/min) in order to assess the WO3/carbon composites. This onset temperature was chosen to be low in order to avoid interfacial reactions between the components leading to the synthesis of the under-oxygenated tungsten oxides (WO3-x) or tungsten carbide (WxC) phases [START_REF] Reddy | Nanostructured tungsten carbides by thermochemical processing[END_REF]. The experimental yields in the solid carbon phase were close to 20 and 24 wt. % for the sucrose and cellulose, respectively. These data are lower than the theoretical yields (42.1 and 44.5 wt. % according to Eq. ( 1)

and Eq. ( 2), respectively) mainly due to the formation of tars and gaseous carbonaceous species [START_REF] Inagaki | Materials Science and Engineering of Carbon: Fundamentals[END_REF][START_REF] Tang | Carbonization of cellulose fibers -Low temperature pyrolysis[END_REF].

Nanothermite preparation.

The energetic formulations were prepared by mixing the tungsten (VI) oxide and aluminium nanopowders in amounts defined according to the following stoichiometric chemical reaction (3).

2Al(s) + WO3(s) → Al2O3(s) + W(s) (3)
Therefore, an excess of aluminium was preferred in order to enhance the performance of the nanothermites [START_REF] Sanders | Reaction propagation of four nanoscale energetic composites (Al/MoO3, Al/WO3, Al/CuO and Bi2O3)[END_REF]. The excess active aluminium was adjusted by means of the equivalence ratio (Φ) described in Eq. ( 4) and was fixed at a value of 1.4 instead of 1:

Φ = ((F/O)exp.)/((F/O)st.) (4)
F/O is the mass ratio of fuel (Al) to oxidizer (WO3) and the subscripts exp. and st. stand for the experimental and stoichiometric ratios, respectively.

The aluminium and the tungsten (VI) oxide powders, which were covered by carbon, were poured into 60 mL of acetonitrile, sonicated and stirred for 1 hour in order to break the aggregates and to prepare homogenous mixtures. The solvent was evaporated under 200 mbar of pressure at 80 °C until dry greyish powders were obtained. The powders were then placed in an oven at 80 °C for 4 hours. The energetic compositions were labelled as Al/WO3/Cx, where x represents the nature of the biomolecules (suc for sucrose and cell for cellulose) that were used for the carbon adding within the nanothermite. An aluminotungstate nanothermite without a carbon additive was also prepared as a reference. The performance of the Al/WO3/Cx nanothermites was evaluated at once in terms of the sensitivity thresholds (impact and friction) and reactivity (combustion velocity). The impact sensitivity was determined by a Julius Peters BAM Fall hammer apparatus and the threshold value is given in joules (J). The friction sensitivity, whose value is in Newtons (N), was determined by a BAM friction apparatus. More details on the working principles of these apparatuses are mentioned in [START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF]. The different observed threshold values correspond to the highest value, for which 6 reaction tests are related to a no-combustion result. The probability of obtaining no combustion at all is 98.4% at that threshold value. All of the experiments were performed according to the standards that were established by NATO [START_REF] Nato | STANAG 4489. In: Standardization Agreement (STANAG) on Explosives, Impact Sensitivity Tests[END_REF][START_REF] Nato | STANAG 4487. In: Standardization Agreement (STANAG) on Explosives, Friction Sensitivity Tests[END_REF][START_REF] Nato | STANAG 4490. In: Standardization Agreement (STANAG) on Explosives, Electrostatic Discharge Sensitivity Tests[END_REF]. Reactivity tests are performed on the nanothermite powders by means of an optical flash ignition, which was inspired by the works of Wang et al. and Ohkura et al. [START_REF] Wang | Enhanced photothermal effect in Si nanowires[END_REF][START_REF] Ohkura | Flash ignition of Al nanoparticles: Mechanism and applications[END_REF]. The energy density that was delivered by our laboratory-made equipment, which was equipped with a xenon lamp, is 0.11 J/cm 2 . The heating temperature and the heating rate were determined to be higher than 673 K and 5.10 5 K/sec., respectively, by means of a nanocalorimeter. Typically, 10 mg of powdered energetic nanocomposites were placed on a glass substrate following a predefined contact surface of approximately 50 mm2 in order to expose the same amount of matter to the flash. The glass substrate was placed on the flash igniter device at a respective distance of 1 mm. All the combustion tests were performed in air on all unconfined powdered Al/WO3 nanothermites (with or w/o the carbon additive).

The combustion phenomena of the energetic formulations were recorded by using a Photron FASTCAM high-speed camera (10 3 frames/sec.) placed in front of the set-up. To precisely characterize the oxide/carbon multi-materials, the chemical composition of the carbon phases were determined by elemental analysis. products are mainly made of carbon with amounts that ranged from 86 to 82 wt. % with the use of sucrose and cellulose as reactants, respectively. The presence of the hydrogen and oxygen heteroatoms could be explained by the low temperature that was used for the pyrolysis, which causes the incomplete carbonization of carbohydrates. In the literature, temperatures close to 900 °C are classically used [START_REF] Reddy | Nanostructured tungsten carbides by thermochemical processing[END_REF]. We voluntarily chose a lower pyrolysis temperature to avoid the formation of tungsten derivatives (see Exp. Section -Paragraph 2.1), which could affect the combustion behaviour of the energetic formulations that are investigated. To sum up, the WO3/Csuc and WO3/Ccell composites are made of a carbonaceous phase that is globally similar in terms of its elemental composition with an oxygen amount ranging from 11 to 15 wt. %, respectively, which is to the detriment of the carbon amount. The SEM images of the Tungsten (VI) oxide/carbon composites are presented in Figure 3. Both composites, WO3/Csuc (Figure 3A) and WO3/Ccell (Figure 3B), are made of micron-sized particles with surfaces that appear to be very rough. After careful observation, this roughness corresponds to the highly agglomerated very small particles with very similar sizes to the pristine tungsten (VI) oxide particles that are used for the WO3/carbon composites' preparation (Figure S2). This hypothesis was confirmed by means of transmission electron microscopic analysis, as shown in Figure 4, where core-shell structures were observed for both WO3/carbon composites. However, the carbon coating that was achieved from the sucrose precursor seems to be less uniform and homogeneous on the WO3 nanoparticles' surface (Figure 4A) where large carbonaceous matter was sometimes found between the tungsten-based particles rather than the carbon-shell of the cellulose molecules (Figure 4B). After mixing with the aluminium nanoparticles in order to prepare the nanothermites according to the experimental section, the amounts of carbon were recalculated as 2.7 and 2.4 wt. % in the Al/WO3/Csuc and Al/WO3/Ccell energetic formulations, respectively (2.3 and 2.0 wt. %, respectively, when taking into account the alumina layer coating the aluminium nanoparticles). The microstructures of both aluminotungstate energetic formulations were also examined by scanning electron microscopy (SEM) and the corresponding images are shown in Figures 5A and5B for the nanothermites that are enriched with the carbonization products of sucrose and cellulose, respectively. For both Al/WO3/Cx mixtures, a similar result is obtained, namely, the observation of very fine particles on all examined samples, and when larger particles are seen, they correspond to an agglomeration of smaller ones, as previously observed for the WO3/carbon composites (Figure 3).

Results and Discussion

The aluminium nanoparticles seem to be well distributed within the two as-prepared nanothermites without leading to microstructural evolutions. The microscopic views of the ternary energetic systems are accompanied by the aluminium, carbon and tungsten elemental mappings (Figure 5 down). In a general way, both composites exhibit effective and successful volume distributions for their three chemical components (except for oxygen). For the tungsten element, some islets describing slightly richer domains in the transition metal can nevertheless be observed. As this trend is observed on the images describing the Al/WO3/Csuc and the Al/WO3/Ccell systems, both as-prepared nanothermites can then be said to exhibit similar homogeneities. The two Al/WO3 nanothermite mixtures that were prepared with carbon additives were used in their powder forms in order to assess their performances, such as their sensitivity thresholds for mechanical solicitations and their combustive behaviours.

Sensitivities of the Al/WO3/C nanothermites.

The mechanical sensitivities, i.e., impact and friction of the ternary energetic formulations are presented and compared to the binary Al/WO3 nanothermite in Table 2. The different sensitivity thresholds were determined according to the protocol detailed in [START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF]. Regarding the impact sensitivity, the aluminotungstate binary formulation can be considered to be insensitive with a threshold value of 42.2 N, which is significantly higher than the standard set at 2 J, as established by the United Nations [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria[END_REF]. The use of carbon as an additive leads to a systematic increase of this value with values of > 49.6 J and 47.1 J for the Al/WO3 nanothermites that enriched with carbon coming from sucrose and cellulose carbonization, respectively. Thus, the adding of carbon within an Al/WO3 energetic composite allows one to preserve and to strengthen the insensitivity of the formulation by increasing its impact sensitivity threshold [START_REF] Piercy | Nanoscale aluminum-metal oxide (thermite) reactions for application in energetic materials[END_REF].

Regarding the friction sensitivity, the Al/WO3 nanothermite is very sensitive with a threshold value of 8 N compared with the standard value of 80 N established by NATO [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria[END_REF]. The use of carbon, regardless of its origin (sucrose or cellulose), allows one to definitely raise the sensitivity threshold value of the aluminotungstate nanothermite ranging from 216 N (carbon sucrose) to 324 N (carbon cellulose), which ranks the carbon-based ternary nanothermites as moderately sensitive [START_REF] Piercy | Nanoscale aluminum-metal oxide (thermite) reactions for application in energetic materials[END_REF].

In a general point of view, regarding the mechanical sensitivities, the very low impact and friction sensitivities of the different Al/WO3/Cx formulations might be explained by the fact that the carbon adding, which leads to the preparation of either a core-shell structure and/or a homogeneous mixture of tungsten (VI) oxide and carbon for the WO3/Ccell and WO3/Csuc composites, respectively (Figures 3 and4), allows one to isolate the oxidizer of the reducing metal compound. Consequently, when the ternary energetic formulations are mechanically stressed (impact and friction tests), no hot spots are generated at the aluminium tungsten trioxide interphase because no direct contact exists between Al and WO3. The ignition of the thermites is then avoided at equivalent stress levels to the ones that are defined for the binary reference Al/WO3 nanothermite. Higher mechanical solicitations are thus required to burn the carbon-based energetic compositions.

Compared to the literature dedicated to the desensitization of the nanothermites by means of additives [START_REF] Steelman | Desensitizing nanopowders to electrostatic discharge ignition[END_REF][START_REF] Poper | Controlling the electrostatic discharge ignition sensitivity of composite energetic materials using carbon nanotube additives[END_REF][START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF][START_REF] Collins | Ignition sensitivity and electrical conductivity of an aluminum fluoropolymer reactive material with carbon nanofillers[END_REF][START_REF] Kappagantula | Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives[END_REF][START_REF] Kappagantula | Experimentally measured thermal transport properties aluminumpolytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives[END_REF], our results, which are focused on mechanical mitigation, can be compared to only a couple of works [START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF][START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF] since most of them are focused on electrostatic mitigation.

Moreover, sometimes the nature of the oxidizer component used for the nanothermite formulation is different (MoO3, MnO2…), which may lead to difficult direct comparisons [START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF]. However, due to the necessity to compare our results to the literature, it can be argued that the present results, in a general point of view, perform as well as or better than those in the literature. For instance, with respect to the impact sensitivity threshold, Siegert et al. obtained desensitization ranging from 24.5 to 44.2 J with 29 to 37 wt. % of carbon nanofibres added (depending on the selected method for introducing the additive within the Al/MnO2 nanothermite) [START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF], and Gibot et al. obtained a threshold value of 20 J with the addition of the polyaniline conductive polymer (5 wt. % within the Al/WO3 energetic system) [START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF]. Thus, the present results are similar to the results that were obtained by Bach within an Al/MoO3 nanothermite) [START_REF] Kelly | Formation of additive-containing nanothermites and modifications to their sensitivity[END_REF] and slightly lower and/or on the same order of magnitude as those of [START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF][START_REF] Siegert | Reduced-sensitivity Nanothermites containing manganese oxide filled carbon nanofibers[END_REF], namely, > 360 N, with an amount of additive that was almost twice as much as the present work. The camera flash ignition process has been carried out to investigate the combustion phenomena. The advantages of this method are that it does not depend on the environmental conditions (temperature, pressure, humidity…), it delivers constant thermal energy and it can be performed under air on unconfined powdered energetic systems. Then, all phenomena related to the evolution of the gas through carbon combustion should be avoided. Some selected video images of the combustion of the three investigated energetic nanocomposites are shown in Figure 6. In all cases, the energetic compositions completely combusted because no residual solid products (traces) were found on the glass substrate at the end of the tests. An intense white light is observed in the first frames of the recording videos and it takes on a yellow-orange tone at the end of the combustion, expressing a rapid increase in the temperature followed by cooling. This result suggests that the addition of a carbonaceous agent does not lead to a total loss of reactivity for the Al/WO3 nanothermite. Nevertheless, while a clapping sound was heard for the Al/WO3 nanothermite, the combustions of the carbon-doped systems are quieter. Most likely, the latter have a less powerful energetic character than the binary energetic nanocomposite. This mitigation of the combustion energy might be due to the energy consumed by the carbon additive for its own combustion (Eq. ( 5)), leading to a slight decrease of the total released energy according to the examined formulations.

2C(s) + O2(g) → 2CO(g) (5)
This trend is expected and has been already noted with the adding of other additives within a binary nanothermite [START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF][START_REF] Gibot | Safer and performing energetic materials based on polyaniline-doped nanocomposites[END_REF]. Finally, when observing the different sequences of the combustion images of the Al/WO3, Al/WO3/Csuc and Al/WO3/Ccell nanothermites (Figure 6), it might be that the combustion velocity is higher for the binary energetic system (Figure 6A) than for the ternary ones (Figures 6B andC). This hypothesis is based on the fact that for equivalent amounts of energetic matters (w or w/o carbon) that are ignited by means of an optical flash, the yellow-orange colour (synonymous with the cooling step of the energetic formulation) of the incandescent cloud located above the glass substrate is visible at a shorter duration time (~ 0.8 msec.) of the combustion of the binary Al/WO3 nanothermite than for the carbon-enriched Al/WO3 nanothermites (≥ 1.2 msec.). In other words, for the same amount of matter, the total combustion of the undoped mixtures is quicker than that for carbon-doped ones. To the same extent, the combustion velocities of both Al/WO3/carbon ternary nanothermites seem to be relatively similar. However, no more accurate details of the combustion velocity can be provided since it is difficult to evaluate the covered distance by the reaction front as a function of time. Currently, a reflection is conducted to optimize the in-air-combustion tests (without the confinement of nanothermites) and permit an accurate characterization of the combustion velocities of these carbon-doped energetic formulations.

Conclusions

In summary, an efficient friction desensitization of an Al/WO3 nanothermite while maintaining acceptable reactive properties was achieved using carbon as a desensitizing agent. The carbon additive was obtained by carbonizing the sucrose and cellulose molecules, which are naturally occurring molecules, abundant and low cost. With a carbon amount slightly lower than 3 wt. % within the nanothermite (with the carbon coming from the sucrose carbonization), the mechanical sensitivity threshold values are about >360 N and >49.6 J for the friction and impact tests, respectively, which are at least 4.5 and 25 times higher than the international standard values (80 N and 2 J, respectively). With regard to the reactive properties (ignition and combustion velocity), the carbondoped Al/WO3 nanothermites exhibit slightly mitigated properties compared to the Al/WO3 binary counterpart. Finally, this approach may be seen as providing the beginnings of an answer to the development of safer reactive nanothermites.

Supplementary Materials:

The following are available online at www.mdpi.com/xxx/s1, Figure S1: FTIR spectra of the different WO3/C composites. The pristine WO3 material is also reported. Figure S2: SEM view of the pristine WO3 material.

2. 3 .

 3 Characterization techniques.The chemical compositions (C and H) of the saccharide-derived carbon were performed by burning at 1050 °C in a helium/oxygen flow with a Thermo Finnigan Flash EA 1112 apparatus. The resulting gaseous products (CO2 and H2O) were separated on a chromatographic column and quantified by using a thermal conductivity detector. The amount of oxygen was calculated with respect to 100% of the material. The structures of the WO3/carbon composites were determined by using an Xray Diffractometer (XRD, D8 Advance, Bruker) operating at 40 kV -40 mA. The diffractograms were collected in the 2 theta range 10 -80° with a step size of 0.02° and a time by step of 0.1 sec. The weighted amounts of carbon in the WO3/carbon composites were determined by thermogravimetric analysis (SDT Q600 TA Instruments). The different composites were calcinated under air at between 25 -800 °C with a heating rate of 10 °C/min. A Scanning Electron Microscope (SEM, FEI Nova NanoSEM 450) working at 10 kV was used to observe the morphology of the WO3/carbon samples that were previously covered by a thin layer of gold (Desk II TSC Denton Vacuum).

3. 1 .Figure 1 (

 11 Figure 1 (B and C) shows the X-ray diffraction patterns of the different WO3/carbon composites that were synthesized via pyrolysis at 500 °C of the corresponding WO3/saccharide multi-materials.The diffraction peaks that are observed for all composites, with respect to the examined angular domain, can be attributed to the WO3 tungsten (VI) oxide phase (Figure1A). The oxide ceramic crystallizes in the monoclinic structure with a P21/n space group, as reported in the crystallographic card No 01-083-0950. Some of the Miller planes are given on the corresponding figure. No other diffraction peaks are detected, thereby confirming the amorphous character of the carbon component

Figure 1 :

 1 Figure 1: XRD patterns of the pure WO3 component (A) and the WO3/Csuc (B) and WO3/Ccell (C) composites synthesized via the pyrolysis of the corresponding WO3/saccharide multi-materials.

Figure 2 :

 2 Figure 2: Thermogravimetric curves of the WO3/Csuc and WO3/Ccell composites under air between 25-800 °C.

  The elemental mappings of the carbon and tungsten chemical elements corresponding to these WO3/carbon composites, which are established by means of energy dispersive X-ray (EDS) spectroscopy, are presented in Figures3A and 3B. Obviously, for both composites, the carbon and tungsten are evenly distributed across all microparticles and seem to be intimately mixed. The fact that carbon and tungsten can be observed at identical places suggests the successful coating of the carbon additive onto the tungsten (VI) oxide particles' surface.

Figure 3 :

 3 Figure 3: SEM views of the (A) WO3/Csuc and (B) WO3/Ccell composites. Elemental mapping analysis of the carbon (yellow plot) and tungsten (blue plot) are reported for each diffused electron image.

Figure 4 :

 4 Figure 4: TEM images of the (A) WO3/Csuc and (B) WO3/Ccell composites.

Figure 5 :

 5 Figure 5: SEM views of the (A) Al/WO3/Csuc and (B) Al/WO3/Ccell mixtures. The elemental mapping analysis of the aluminium (red plot), carbon (yellow plot) and tungsten (blue plot) are reported for each diffused electron image.

C 2019, 5 ,

 5 x FOR PEER REVIEW 9 of 12 et al. with a 5 wt. % of carbon black as the desensitization agent in the Al/WO3 nanothermites (impact sensitivity > 49.6 J)[START_REF] Bach | Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive[END_REF]. Regarding the friction sensitivity threshold values that were obtained in this work (216 and 324 N depending on the carbon source), again, the data are very competitive since they are clearly superior to the results of Kelly et al. (120 N with a 5 wt. % of MoS2 as an additive

3. 3 .

 3 Combustion of the Al/WO3/C nanothermites. The combustion properties of the Al/WO3/carbon nanothermites with added carbon coming from sucrose or cellulose carbonization are compared with the classical Al/WO3 nanothermite behaviour.

Figure 6 :

 6 Figure 6: Sequences of the combustion images of the (A) Al/WO3, (B) Al/WO3/Csuc and (C) Al/WO3/Ccell nanothermites. The first image of each series corresponds to the ignition of the energetic powders. The elapsed time between pictures is 200 µsec.

Table 1

 1

	C 2019, 5, x FOR PEER REVIEW

gathers the data relating to the saccharide molecules after pyrolysis at 500 °C under argon for 2 hours. As reported, the resulting

Table 1 .

 1 Chemical composition of the saccharide-derived carbon phases. The oxygen amount was calculated with respect to 100% of the material.

		C wt. %	H wt. %	O wt. %
	Csuc	85.7	3.2	11.1
	Ccell	82.0	3.3	14.7

Table 2 .

 2 Mechanical (impact and friction) sensitivity thresholds for the Al/WO3/C nanothermites (weight % of carbon of approximately 2.7 and 2.4 from sucrose and cellulose as the carbon sources, respectively).

		Impact (J)	Friction (N)
	Standards	2	80
	Al/WO3	42.2	8
	Al/WO3/Csuc	> 49.6	216
	Al/WO3/Ccell	47.1	324

Acknowledgments:

The authors acknowledge S. Adach (SRSMC, University of Lorraine, France) and L. Vidal (IS2M, University of Mulhouse, France) for the elemental chemical and the transmission electron microscopy analyses, respectively.

Author Contributions: P.G. suggested and guided this research, and wrote the paper. Q.M, A.B. and P.G. carried out the experimental work including materials syntheses, the diverse characterizations (TGA, XRD, sensitivities and combustion tests) and the corresponding data interpretation. F.S. performed the electron microscopy coupled to the energy-dispersive X-ray spectroscopy. R.G. and D.S. supervised closely the study, the interpretation of data and helped in the proofreading of the manuscript.