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Energetic Nanoparticles and Nanomaterials for Future Defense Applications

Energetic Nanoparticles and Nanomaterials for Future Defense Applications The integration of nanostructured materials in defense systems is expected to improve their performance in terms of power, safety, and reliability. That is why considerable research effort has been undertaken by major military powers worldwide in this domain. The first important step was to develop the production capacities of organic explosives in the state of fine powders with submicron to nanosized particle size distributions. The Spray Flash Evaporation (SFE) process, which is a unique method for producing such materials, was developed at industrial scale. Explosive nanopowders obtained by this process were subsequently mixed with nanosized pyrotechnic compositions such as nanothermites, to prepare hybrid detonating materials able to replace lead-based primary explosives. Composite propellants can also be prepared by SFE which allows mixing their components in a single step with better homogeneity. The ultimate challenge is to move from powder to object, in order to integrate energetic nanomaterials in operational systems. Although the research in this last domain is still in its infancy, several ways of preparation of objects from nanothermites have been recently reported in scientific literature. The focus will be on two examples studied in our laboratory. The first one is the preparation of nanothermites in the state of solid, porous foams; the second one is the use of nanothermites for coating grains of propulsive powder to change their combustion properties. . . . Keywords Nanomaterials Explosives Nanothermites Pyrotechnics Introduction overcome was to have particles with size distributions below 1 μm. For this purpose, a revolutionary process called the In the domain of energetic materials, the research has Spray Flash Evaporation (SFE) process for nanostructuring mainly focused for one century and a half on the synthesis explosives and the components of energetic compositions of new molecules. The most glaring exception to this was developed over the last decade [2, 3]. The second trend was the invention of dynamite by Alfred Nobel, challenge was to use these elementary bricks to prepare hybrid who stabilized nitroglycerine into different porous composite nanomaterials, by mixing the explosive nanomaterials and thus became the first scientist to have nanopowders with nanothermites. These new detonating com- a materials approach of explosives [1]. positions, which were called NSTEX (NanoStructured The era of nanomaterials has open new horizons to the Thermites and Explosives), have unconventional properties science of explosives and pyrotechnic compositions and is in comparison to those of classical explosives and pyrotechnic deeply changing the ways to think and to do in this field. compositions, which make them promising candidates for re- The future energetic substances will be Bsmart materials^ with placing primary explosives containing heavy metals such as high performances, high safety and reliability of use, and min- lead or cobalt [4]. The last challenge is to move from imized impact on environment. The first challenge to nanopowders to objects, in order to integrate energetic nanomaterials in real systems produced at industrial scale. This specific aspect will be first discussed through the exam- * Marc Comet ple of the chemical synthesis of nanothermites in the form of marc.comet@isl.eu combustible foams from aluminum nanopowder and ortho- phosphoric acid (H PO )[5, 6]. A second example dealing 3 4 with the coating of a propulsive powder by a nanothermite NS3E laboratory (UMR 3208 ISL/CNRS/Unistra), French-German research institute of Saint-Louis (ISL), 5 rue du Général Cassagnou, will be given [7]. BP 70034, 68301 Saint-Louis Cedex, France 1 Page 2 of 6 Hum Factors Mech Eng Def Saf 3 Preparation of Fine Explosive Powders single step [19]. A co-crystallized structure (1:2 mol/mol) was obtained from HMX and CL-20, which confirmed that The preparation of high explosives in the state of fine powders molecular recognition occurs in short times [20]. These exam- (0.05–1 μm) is difficult owing to the specific properties of ples show the outstanding versatility of this process. these compounds. First, most of organic explosives are mole- Composite LOVA propellants were also prepared by SFE cules which cannot be polymerized: it is therefore impossible from solutions of nitrocellulose, a plasticizer (2,4-dinitro-2,4- to prepare them by a sol-gel approach, unless using a gelling diazahexan: DNDA6) and hexogen, in ethyl acetate [21]. In substance as template [8, 9]. For this reason, explosive this domain, SFE makes possible the preparation of propulsive nanopowders prepared by this technique are never chemically powders with a better homogeneity, in a single step. pure. The milling technology is hazardous to carry out with explosives for obvious safety reasons. Samples must be pre- pared by operating in successive small batches. Furthermore, Use of Fine Explosive Powders for Preparing in a milling process, large particles are broken in smaller ones, Hybrid Energetic Nanocomposite Materials which are afterward agglomerated in larger aggregates by the pressure exerted by the impact of milling beads. An intrinsic Fine explosive powders prepared by SFE process were mixed problem of the milling technique is the contamination of sam- with nanothermites to obtain hybrid energetic nanocomposite ples by the matter coming from different sources: beads, materials, which were called NSTEX (NanoStructured grinding bowl, and liquid used to desensitize the energetic Thermites and Explosives). This concept originates from the material during the process. The anti-solvent method consists research of Comet et al., who have proposed to solidify to induce the fast precipitation of an explosive dissolved in a hexogen in a porous matrix of chromium (III) oxide firstly solvent, by gradually pouring the solution into an anti-solvent synthesized by the combustion of ammonium dichromate, of the explosive [10, 11]. This technique generally leads to and then to use the resulting RDX@Cr O material as gas 2 3 micron-sized powders, with inclusions of solvent. The spray generating oxidizer in aluminothermic compositions to im- techniques are the most efficient for producing explosives in prove their reactivity [22]. In the most classical sense, fine powders. They differ by the technique used to evaporate nanothermites are energetic materials made up of a metallic the solvent. In spray-drying, the aerosol is dried progressively oxide such as Fe O ,MnO ,WO ,MoO ,CuO, andBi O in 2 3 2 3 3 2 3 by a hot gas flow [12, 13]. The particles formed have spherical the form of submicron-sized powder and of an aluminum shapes and typical size of 1 μm. The rapid expansion of su- nanopowder with typical particle size distribution ranging percritical solution (RESS process) lead to fine powders with from 50 to 120 nm [23]. The thermochemical properties of submicron-sized to nanosized particles. The main drawback the most representative thermite compositions were reported of this technique is the use of important amounts of supercrit- by Fischer and Grubelich [24]. In an extended meaning, ical fluid (CO ) for preparing limited quantities of sample [14, nanothermites are any combustible compositions containing 15]. None of these techniques can be used for preparing ex- a significant amount of metallic species, used as fuels or/and plosive fine powders at industrial scale. oxidizers, in form of fine powders. In the field of The SFE process is the first technique that allows preparing nanothermites, oxygenated metallic salts are more and more fine and pure explosive powders, in amounts of typically used as oxidizers instead of metallic oxides to improve reac- 100 g per hour. The underlying principle is to spray in a cham- tivity [25–29]. ber maintained under dynamic vacuum an aerosol of pre- The nanothermite part of NSTEX is prepared by dispersing heated solvent containing one or several explosives dissolved its components in a liquid phase (e.g., acetonitrile) by ultra- in it. The ultrafast evaporation of the solvent induces the rapid sonic agitation. The mixture is recovered by evaporating the crystallization of the explosive into small particles which do liquid under reduced pressure. Nanothermites should be han- not have time to grow, and have typical sizes ranging from dled with precaution, owing to their particularly high sensitiv- 50 nm to less than 1 μm. The most used solvent for preparing ity, especially to electrostatic discharge (ESD). explosive nanopowder by SFE is acetone, whose physico- The formulation of a NSTEX is achieved by the dry mixing chemical properties are particularly well suited for this tech- of the nanothermite with the fine explosive powder. This del- nique. To date, trinitrotoluene (TNT), pentaerythritol icate operation is carried out by a vortex mixing and a manual tetranitrate (PETN), hexogen (RDX), octogen (HMX), crushing of aggregates with a spatula, which are repeated till hexanitrohexaazaisowurtzitane (CL-20), ammonium obtaining a material with a good homogeneity. No liquid dinitramide (ADN) were prepared by SFE (Fig. 1)[2, 16–18]. should be used for mixing the components of NSTEX, in One of the advantages of SFE process relies in the possi- order to avoid the dissolution and the growth of the explosive bility to produce fine powders with different particle morphol- particles. ogy. For instance, core-shell particles of hexolite, in which The main NSTEX feature is their ability to detonate by a RDX is the core and TNT is the shell, were prepared in a transition from deflagration to detonation (TDD). This :1 (2019) Hum Factors Mech Eng Def Saf 39)01(2 :1 Page 3 of 6 1 Fig. 1 Macroscopic aspect of some explosive nanopowders prepared by SFE: nano-PETN, nano-RDX and nano-HMX/CL- 20 from left to right phenomenon is only observed in loose or slightly pressed the most used primary explosive in detonators: It is synthe- NSTEX powders. The high power delivered by the fast com- sized by the Curtius’ method from soluble lead salts. On the bustion of the nanothermite activates the detonation of the fine other hand, the detonation of lead azide produces small lead explosive powder, which occurs in short distances (10– particles which are dispersed in environment. The persistence 20 mm). The detonation wave then propagates in the of the pollution by lead and its devastating effects on human NSTEX charge: Its propagation velocity depends both on health, has led European Chemical Agency to put a series of the density of the explosive in the composition and on the lead compounds on the list of the Substances of Very High resistance opposed by the nanothermite particles. In other Concern (REACH regulation). It was therefore necessary to words, the velocity can be adjusted through the ratio develop Bgreener^ compositions for replacing traditional pri- nanothermite/explosive and the density of the NSTEX. The mary explosives. From this standpoint, NSTEX are particular- detonation wave produced by the reaction of a NSTEX (e.g., ly promising, as they can be prepared from benign compounds nano-Al/nano-WO /nano-RDX:12.4/27.6/60.0 wt.%) can be such as potassium or calcium sulfates [30], aluminum and transmitted to a PETN secondary charge. This shock to deto- pentaerythritol tetranitrate. nation transition is only observed when the detonation veloc- ity of the NSTEX is higher than 3 km/s [4]. Figure 2 illustrates the functioning of an experimental detonating device, in a Fabrication of Macroscopic Materials transparent tube, observed with a high speed camera operating from Nanoscopic Powders at 840000 fr/s. The system comprises of a layer of ignition nanothermite (5 mm), a NSTEX in loose powder state The extremely fast flame propagation velocities (FPV) is only (20 mm), a compacted layer of the same NSTEX (5 mm) observed in nanothermites, in the state of loose powders. The and finally a 1 g PETN charge (20 mm). pressing of nanothermites decreases their internal porosity, The detonation characteristics of NSTEX make them suit- which considerably slows down the FPV. This effect was ob- able for the replacement of primary explosives which are salts served on Al/WO nanothermites by Prentice et al. [31]and of heavy metals such as lead or cobalt. Lead azide (PbN )is on Al/CuO nanothermites by Apperson et al. [32]It is attrib- uted to the fact that the convection of hot gases is made more difficult in denser materials. Moreover, the compression of aluminothermic compositions could slow down the fast spreading of the combustion by the melt dispersion mecha- nism (MDM) of aluminum [33, 34]. On the other hand, the mixtures of nanopowders of different natures are metastable from a physical point of view, because the phases tend to separate along time by the effect of gravity. The settling changes the local stoichiometry, which alters the pyrotechnics properties of the composition. Having a high porosity is essential to keep a fast FPV in nanothermites and the most prominent challenge in the forth- coming years will be to develop efficient processes for shap- ing nanothermite powders into solid, porous objects with good mechanical properties. Various approaches have been ex- plored for this purpose: Tillotson et al. synthesized Fig. 2 Curve representing the distance traveled by the flame front nanothermites as aerogel monoliths [9], Yan et al. used the according to time, in an experimental device (length = 50 mm): burning electrospinning technique with an energetic polymer as binder of the nano-Al/nano-CuO nanothermite used for ignition (1), deflagration to prepare nanothermite mats [35] and Yang et al. simply used of a NSTEX composition (2), detonation of the NSTEX (3), acceleration filtration to shape nanothermites in membranes [36]. One of of the detonation wave in a denser NSTEX layer (4), transmission and detonation of a PETN charge (5) the most promising ways was reported by Comet et al. who 1 Page 4 of 6 Hum Factors Mech Eng Def Saf 3 transformed an aluminothermic composition into solid, highly Vectan A1 powder flakes by a layer of a sulfate-based porous objects, by a chemical foaming process [5, 6]. nanothermite (Al/Na SO ) with a thickness of 50 μm. The 2 4 The principle of the foaming process is based on the reac- addition of only some percents (10 wt.%) of nanothermite tion of aluminum nanopowder which is introduced in excess totally changed the ignition and the combustion regime of in an aluminothermic mixture (Al/WO ) with an aqueous so- the propulsive powder. The effect observed was tuned by lution of orthophosphoric acid (H PO ). The reaction occurs mixing coated and uncoated Vectan A1 grains in different 3 4 in two exothermic stages: The acid solution first reacts with mass ratios [7]. In these energetic systems, the density of the oxide shell of aluminum nanoparticles; then, it oxidizes the nanothermite layer and the thermal conductivity of the sub- aluminum core of particles. Water vapor and hydrogen are strate on which it is coated are crucial parameters. Sullivan respectively released by these reactions: In escaping through et al. have deposited Al/CuO nanothermite dense layers by the reaction medium, these gases create the final porosity of electrophoresis on platinum electrodes in miniaturized sys- the foam. On the other hand, aluminum phosphate (AlPO ) tems. However, the propagation velocity is smaller than which is produced by the reaction acts as a strong binder 50 m/s owing too high density of the nanothermite deposits (cement) towards the nanoparticles from which the (≈ 29% of the TMD) [38]. nanothermite is formed. It is worth noting that phosphates and sulfates, which are considered as inert compounds from a pyrotechnic point of view, behave as oxidizers at high tem- Conclusions peratures, especially when they are mixed with aluminum nanopowders [5, 30]. Special caution is required to handle The use of nanomaterials in the science of pyrotechnics and the pastes prepared from concentrated inorganic acids such explosives will lead to important breakthroughs in this do- as H PO and H SO and fine aluminum powders, due the main. Future energetic materials will be greener and safer 3 4 2 4 high risk of hydrogen explosion [37]. The induction time of [39]. Moreover, they will have intrinsic high performance, the foaming reaction and the maximal temperature of the me- owing to the fact that they will be designed in such a way to dium can be controlled through the concentration of the aque- perfectly match with precise needs. Although this trendy re- ous orthophosphoric acid solution (Fig. 3). search area has its roots in past, black powder and dynamites Nanothermites foams have the appearance of concrete are indeed historical examples of energetic nanomaterials, pieces; they are quite insensitive to impact, friction and elec- much academic and applicative research remains to be done trostatic discharge (ESD). They can be sawn or drilled without to integrate nanomaterials in operational systems. activating their combustion. Once ignited, nanothermite The first step was to have the components of energetic foams combust violently, with a blinding fireball and the re- systems in the state of nanopowders. The research carried lease of abundant fumes. The reaction mechanism consists, out over the past two decades has led to the commercialization first in the reduction of aluminum phosphate in phosphorus, of fine and stable aluminum nanopowders, which are a major which in turns burns in contact with atmospheric oxygen. component of most of energetic nanomaterials. On the other Nanothermites in the state of loose powders can be stabi- hand, the Spray Flash Evaporation (SFE) process developed lized in thin layers deposited on the surface of the grains of a in our laboratory has been used for preparing numerous high propulsive powder. For instance, Berthe et al. have coated explosives in the state of fine or ultrafine nanopowders in quantities which are still compatible with industrial applications. The fine explosive powders prepared by the SFE process, were mixed with nanothermites to prepare a new family of detonating substances, which were called Nanostructured Thermites and Explosives (NSTEX). The detonation velocity of NSTEX can be set at a desired value, by playing on their composition and on their porosity. NSTEX have similar prop- erties as those of primary explosives, which are molecules comprising toxic metals, such as lead or cobalt. Furthermore, NSTEX can be prepared from benign com- pounds from a toxicological standpoint, which makes them promising candidates for replacing classical primary explosives. The last challenge to overcome for the integration of Fig. 3 Curves representing the induction time of the foaming reaction nanomaterials in pyrotechnic systems is the transformation (squares) and the maximum temperature reached during the reaction (circles) depending on the orthophosphoric acid concentration of loose nanopowders, which are metastable in nature, into :1 (2019) Hum Factors Mech Eng Def Saf 39)01(2 :1 Page 5 of 6 1 13. Qiu H, Stepanov V, Di Stasio AR, Surapaneni A, Lee WY (2015) porous, solid and stable objects. 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Propellants Explos Pyrotech 44:18–36. doi: https://doi.org/ 071909. https://doi.org/10.1063/1.2335362 10.1002/prep.201800095 :1 (2019) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Human Factors and Mechanical Engineering for Defense and Safety Springer Journals

Energetic Nanoparticles and Nanomaterials for Future Defense Applications

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Engineering; Mechanical Engineering; Structural Materials; Textile Engineering; Security Science and Technology
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Abstract

The integration of nanostructured materials in defense systems is expected to improve their performance in terms of power, safety, and reliability. That is why considerable research effort has been undertaken by major military powers worldwide in this domain. The first important step was to develop the production capacities of organic explosives in the state of fine powders with submicron to nanosized particle size distributions. The Spray Flash Evaporation (SFE) process, which is a unique method for producing such materials, was developed at industrial scale. Explosive nanopowders obtained by this process were subsequently mixed with nanosized pyrotechnic compositions such as nanothermites, to prepare hybrid detonating materials able to replace lead-based primary explosives. Composite propellants can also be prepared by SFE which allows mixing their components in a single step with better homogeneity. The ultimate challenge is to move from powder to object, in order to integrate energetic nanomaterials in operational systems. Although the research in this last domain is still in its infancy, several ways of preparation of objects from nanothermites have been recently reported in scientific literature. The focus will be on two examples studied in our laboratory. The first one is the preparation of nanothermites in the state of solid, porous foams; the second one is the use of nanothermites for coating grains of propulsive powder to change their combustion properties. . . . Keywords Nanomaterials Explosives Nanothermites Pyrotechnics Introduction overcome was to have particles with size distributions below 1 μm. For this purpose, a revolutionary process called the In the domain of energetic materials, the research has Spray Flash Evaporation (SFE) process for nanostructuring mainly focused for one century and a half on the synthesis explosives and the components of energetic compositions of new molecules. The most glaring exception to this was developed over the last decade [2, 3]. The second trend was the invention of dynamite by Alfred Nobel, challenge was to use these elementary bricks to prepare hybrid who stabilized nitroglycerine into different porous composite nanomaterials, by mixing the explosive nanomaterials and thus became the first scientist to have nanopowders with nanothermites. These new detonating com- a materials approach of explosives [1]. positions, which were called NSTEX (NanoStructured The era of nanomaterials has open new horizons to the Thermites and Explosives), have unconventional properties science of explosives and pyrotechnic compositions and is in comparison to those of classical explosives and pyrotechnic deeply changing the ways to think and to do in this field. compositions, which make them promising candidates for re- The future energetic substances will be Bsmart materials^ with placing primary explosives containing heavy metals such as high performances, high safety and reliability of use, and min- lead or cobalt [4]. The last challenge is to move from imized impact on environment. The first challenge to nanopowders to objects, in order to integrate energetic nanomaterials in real systems produced at industrial scale. This specific aspect will be first discussed through the exam- * Marc Comet ple of the chemical synthesis of nanothermites in the form of marc.comet@isl.eu combustible foams from aluminum nanopowder and ortho- phosphoric acid (H PO )[5, 6]. A second example dealing 3 4 with the coating of a propulsive powder by a nanothermite NS3E laboratory (UMR 3208 ISL/CNRS/Unistra), French-German research institute of Saint-Louis (ISL), 5 rue du Général Cassagnou, will be given [7]. BP 70034, 68301 Saint-Louis Cedex, France 1 Page 2 of 6 Hum Factors Mech Eng Def Saf 3 Preparation of Fine Explosive Powders single step [19]. A co-crystallized structure (1:2 mol/mol) was obtained from HMX and CL-20, which confirmed that The preparation of high explosives in the state of fine powders molecular recognition occurs in short times [20]. These exam- (0.05–1 μm) is difficult owing to the specific properties of ples show the outstanding versatility of this process. these compounds. First, most of organic explosives are mole- Composite LOVA propellants were also prepared by SFE cules which cannot be polymerized: it is therefore impossible from solutions of nitrocellulose, a plasticizer (2,4-dinitro-2,4- to prepare them by a sol-gel approach, unless using a gelling diazahexan: DNDA6) and hexogen, in ethyl acetate [21]. In substance as template [8, 9]. For this reason, explosive this domain, SFE makes possible the preparation of propulsive nanopowders prepared by this technique are never chemically powders with a better homogeneity, in a single step. pure. The milling technology is hazardous to carry out with explosives for obvious safety reasons. Samples must be pre- pared by operating in successive small batches. Furthermore, Use of Fine Explosive Powders for Preparing in a milling process, large particles are broken in smaller ones, Hybrid Energetic Nanocomposite Materials which are afterward agglomerated in larger aggregates by the pressure exerted by the impact of milling beads. An intrinsic Fine explosive powders prepared by SFE process were mixed problem of the milling technique is the contamination of sam- with nanothermites to obtain hybrid energetic nanocomposite ples by the matter coming from different sources: beads, materials, which were called NSTEX (NanoStructured grinding bowl, and liquid used to desensitize the energetic Thermites and Explosives). This concept originates from the material during the process. The anti-solvent method consists research of Comet et al., who have proposed to solidify to induce the fast precipitation of an explosive dissolved in a hexogen in a porous matrix of chromium (III) oxide firstly solvent, by gradually pouring the solution into an anti-solvent synthesized by the combustion of ammonium dichromate, of the explosive [10, 11]. This technique generally leads to and then to use the resulting RDX@Cr O material as gas 2 3 micron-sized powders, with inclusions of solvent. The spray generating oxidizer in aluminothermic compositions to im- techniques are the most efficient for producing explosives in prove their reactivity [22]. In the most classical sense, fine powders. They differ by the technique used to evaporate nanothermites are energetic materials made up of a metallic the solvent. In spray-drying, the aerosol is dried progressively oxide such as Fe O ,MnO ,WO ,MoO ,CuO, andBi O in 2 3 2 3 3 2 3 by a hot gas flow [12, 13]. The particles formed have spherical the form of submicron-sized powder and of an aluminum shapes and typical size of 1 μm. The rapid expansion of su- nanopowder with typical particle size distribution ranging percritical solution (RESS process) lead to fine powders with from 50 to 120 nm [23]. The thermochemical properties of submicron-sized to nanosized particles. The main drawback the most representative thermite compositions were reported of this technique is the use of important amounts of supercrit- by Fischer and Grubelich [24]. In an extended meaning, ical fluid (CO ) for preparing limited quantities of sample [14, nanothermites are any combustible compositions containing 15]. None of these techniques can be used for preparing ex- a significant amount of metallic species, used as fuels or/and plosive fine powders at industrial scale. oxidizers, in form of fine powders. In the field of The SFE process is the first technique that allows preparing nanothermites, oxygenated metallic salts are more and more fine and pure explosive powders, in amounts of typically used as oxidizers instead of metallic oxides to improve reac- 100 g per hour. The underlying principle is to spray in a cham- tivity [25–29]. ber maintained under dynamic vacuum an aerosol of pre- The nanothermite part of NSTEX is prepared by dispersing heated solvent containing one or several explosives dissolved its components in a liquid phase (e.g., acetonitrile) by ultra- in it. The ultrafast evaporation of the solvent induces the rapid sonic agitation. The mixture is recovered by evaporating the crystallization of the explosive into small particles which do liquid under reduced pressure. Nanothermites should be han- not have time to grow, and have typical sizes ranging from dled with precaution, owing to their particularly high sensitiv- 50 nm to less than 1 μm. The most used solvent for preparing ity, especially to electrostatic discharge (ESD). explosive nanopowder by SFE is acetone, whose physico- The formulation of a NSTEX is achieved by the dry mixing chemical properties are particularly well suited for this tech- of the nanothermite with the fine explosive powder. This del- nique. To date, trinitrotoluene (TNT), pentaerythritol icate operation is carried out by a vortex mixing and a manual tetranitrate (PETN), hexogen (RDX), octogen (HMX), crushing of aggregates with a spatula, which are repeated till hexanitrohexaazaisowurtzitane (CL-20), ammonium obtaining a material with a good homogeneity. No liquid dinitramide (ADN) were prepared by SFE (Fig. 1)[2, 16–18]. should be used for mixing the components of NSTEX, in One of the advantages of SFE process relies in the possi- order to avoid the dissolution and the growth of the explosive bility to produce fine powders with different particle morphol- particles. ogy. For instance, core-shell particles of hexolite, in which The main NSTEX feature is their ability to detonate by a RDX is the core and TNT is the shell, were prepared in a transition from deflagration to detonation (TDD). This :1 (2019) Hum Factors Mech Eng Def Saf 39)01(2 :1 Page 3 of 6 1 Fig. 1 Macroscopic aspect of some explosive nanopowders prepared by SFE: nano-PETN, nano-RDX and nano-HMX/CL- 20 from left to right phenomenon is only observed in loose or slightly pressed the most used primary explosive in detonators: It is synthe- NSTEX powders. The high power delivered by the fast com- sized by the Curtius’ method from soluble lead salts. On the bustion of the nanothermite activates the detonation of the fine other hand, the detonation of lead azide produces small lead explosive powder, which occurs in short distances (10– particles which are dispersed in environment. The persistence 20 mm). The detonation wave then propagates in the of the pollution by lead and its devastating effects on human NSTEX charge: Its propagation velocity depends both on health, has led European Chemical Agency to put a series of the density of the explosive in the composition and on the lead compounds on the list of the Substances of Very High resistance opposed by the nanothermite particles. In other Concern (REACH regulation). It was therefore necessary to words, the velocity can be adjusted through the ratio develop Bgreener^ compositions for replacing traditional pri- nanothermite/explosive and the density of the NSTEX. The mary explosives. From this standpoint, NSTEX are particular- detonation wave produced by the reaction of a NSTEX (e.g., ly promising, as they can be prepared from benign compounds nano-Al/nano-WO /nano-RDX:12.4/27.6/60.0 wt.%) can be such as potassium or calcium sulfates [30], aluminum and transmitted to a PETN secondary charge. This shock to deto- pentaerythritol tetranitrate. nation transition is only observed when the detonation veloc- ity of the NSTEX is higher than 3 km/s [4]. Figure 2 illustrates the functioning of an experimental detonating device, in a Fabrication of Macroscopic Materials transparent tube, observed with a high speed camera operating from Nanoscopic Powders at 840000 fr/s. The system comprises of a layer of ignition nanothermite (5 mm), a NSTEX in loose powder state The extremely fast flame propagation velocities (FPV) is only (20 mm), a compacted layer of the same NSTEX (5 mm) observed in nanothermites, in the state of loose powders. The and finally a 1 g PETN charge (20 mm). pressing of nanothermites decreases their internal porosity, The detonation characteristics of NSTEX make them suit- which considerably slows down the FPV. This effect was ob- able for the replacement of primary explosives which are salts served on Al/WO nanothermites by Prentice et al. [31]and of heavy metals such as lead or cobalt. Lead azide (PbN )is on Al/CuO nanothermites by Apperson et al. [32]It is attrib- uted to the fact that the convection of hot gases is made more difficult in denser materials. Moreover, the compression of aluminothermic compositions could slow down the fast spreading of the combustion by the melt dispersion mecha- nism (MDM) of aluminum [33, 34]. On the other hand, the mixtures of nanopowders of different natures are metastable from a physical point of view, because the phases tend to separate along time by the effect of gravity. The settling changes the local stoichiometry, which alters the pyrotechnics properties of the composition. Having a high porosity is essential to keep a fast FPV in nanothermites and the most prominent challenge in the forth- coming years will be to develop efficient processes for shap- ing nanothermite powders into solid, porous objects with good mechanical properties. Various approaches have been ex- plored for this purpose: Tillotson et al. synthesized Fig. 2 Curve representing the distance traveled by the flame front nanothermites as aerogel monoliths [9], Yan et al. used the according to time, in an experimental device (length = 50 mm): burning electrospinning technique with an energetic polymer as binder of the nano-Al/nano-CuO nanothermite used for ignition (1), deflagration to prepare nanothermite mats [35] and Yang et al. simply used of a NSTEX composition (2), detonation of the NSTEX (3), acceleration filtration to shape nanothermites in membranes [36]. One of of the detonation wave in a denser NSTEX layer (4), transmission and detonation of a PETN charge (5) the most promising ways was reported by Comet et al. who 1 Page 4 of 6 Hum Factors Mech Eng Def Saf 3 transformed an aluminothermic composition into solid, highly Vectan A1 powder flakes by a layer of a sulfate-based porous objects, by a chemical foaming process [5, 6]. nanothermite (Al/Na SO ) with a thickness of 50 μm. The 2 4 The principle of the foaming process is based on the reac- addition of only some percents (10 wt.%) of nanothermite tion of aluminum nanopowder which is introduced in excess totally changed the ignition and the combustion regime of in an aluminothermic mixture (Al/WO ) with an aqueous so- the propulsive powder. The effect observed was tuned by lution of orthophosphoric acid (H PO ). The reaction occurs mixing coated and uncoated Vectan A1 grains in different 3 4 in two exothermic stages: The acid solution first reacts with mass ratios [7]. In these energetic systems, the density of the oxide shell of aluminum nanoparticles; then, it oxidizes the nanothermite layer and the thermal conductivity of the sub- aluminum core of particles. Water vapor and hydrogen are strate on which it is coated are crucial parameters. Sullivan respectively released by these reactions: In escaping through et al. have deposited Al/CuO nanothermite dense layers by the reaction medium, these gases create the final porosity of electrophoresis on platinum electrodes in miniaturized sys- the foam. On the other hand, aluminum phosphate (AlPO ) tems. However, the propagation velocity is smaller than which is produced by the reaction acts as a strong binder 50 m/s owing too high density of the nanothermite deposits (cement) towards the nanoparticles from which the (≈ 29% of the TMD) [38]. nanothermite is formed. It is worth noting that phosphates and sulfates, which are considered as inert compounds from a pyrotechnic point of view, behave as oxidizers at high tem- Conclusions peratures, especially when they are mixed with aluminum nanopowders [5, 30]. Special caution is required to handle The use of nanomaterials in the science of pyrotechnics and the pastes prepared from concentrated inorganic acids such explosives will lead to important breakthroughs in this do- as H PO and H SO and fine aluminum powders, due the main. Future energetic materials will be greener and safer 3 4 2 4 high risk of hydrogen explosion [37]. The induction time of [39]. Moreover, they will have intrinsic high performance, the foaming reaction and the maximal temperature of the me- owing to the fact that they will be designed in such a way to dium can be controlled through the concentration of the aque- perfectly match with precise needs. Although this trendy re- ous orthophosphoric acid solution (Fig. 3). search area has its roots in past, black powder and dynamites Nanothermites foams have the appearance of concrete are indeed historical examples of energetic nanomaterials, pieces; they are quite insensitive to impact, friction and elec- much academic and applicative research remains to be done trostatic discharge (ESD). They can be sawn or drilled without to integrate nanomaterials in operational systems. activating their combustion. Once ignited, nanothermite The first step was to have the components of energetic foams combust violently, with a blinding fireball and the re- systems in the state of nanopowders. The research carried lease of abundant fumes. The reaction mechanism consists, out over the past two decades has led to the commercialization first in the reduction of aluminum phosphate in phosphorus, of fine and stable aluminum nanopowders, which are a major which in turns burns in contact with atmospheric oxygen. component of most of energetic nanomaterials. On the other Nanothermites in the state of loose powders can be stabi- hand, the Spray Flash Evaporation (SFE) process developed lized in thin layers deposited on the surface of the grains of a in our laboratory has been used for preparing numerous high propulsive powder. For instance, Berthe et al. have coated explosives in the state of fine or ultrafine nanopowders in quantities which are still compatible with industrial applications. The fine explosive powders prepared by the SFE process, were mixed with nanothermites to prepare a new family of detonating substances, which were called Nanostructured Thermites and Explosives (NSTEX). The detonation velocity of NSTEX can be set at a desired value, by playing on their composition and on their porosity. NSTEX have similar prop- erties as those of primary explosives, which are molecules comprising toxic metals, such as lead or cobalt. Furthermore, NSTEX can be prepared from benign com- pounds from a toxicological standpoint, which makes them promising candidates for replacing classical primary explosives. The last challenge to overcome for the integration of Fig. 3 Curves representing the induction time of the foaming reaction nanomaterials in pyrotechnic systems is the transformation (squares) and the maximum temperature reached during the reaction (circles) depending on the orthophosphoric acid concentration of loose nanopowders, which are metastable in nature, into :1 (2019) Hum Factors Mech Eng Def Saf 39)01(2 :1 Page 5 of 6 1 13. Qiu H, Stepanov V, Di Stasio AR, Surapaneni A, Lee WY (2015) porous, solid and stable objects. 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Propellants Explos Pyrotech 37: Commons Attribution 4.0 International License (http:// 699–706. https://doi.org/10.1002/prep.201100139 creativecommons.org/licenses/by/4.0/), which permits unrestricted use, 16. Klaumünzer M, Hübner J, Spitzer D (2016) Production of energetic distribution, and reproduction in any medium, provided you give appro- nanomaterials by Spray Flash Evaporation. World Acad Sci Eng priate credit to the original author(s) and the source, provide a link to the Technol 10:1191–1195 Creative Commons license, and indicate if changes were made. 17. Séve A, Pichot V, Schnell F, Spitzer D (2017) Trinitrotoluene nanostructuring by Spray Flash Evaporation process. Propellants Explos Pyrotech 42:1051–1056. https://doi.org/10.1002/prep. Publisher’sNote Springer Nature remains neutral with regard to jurisdic- tional claims in published maps and institutional affiliations. 18. Risse B, Schnell F, Spitzer D (2014) Synthesis and desensitization of nano-β-HMX. 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Human Factors and Mechanical Engineering for Defense and SafetySpringer Journals

Published: Feb 8, 2019

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