Fabien Schnell

Understanding ultrafine nanodiamond formation using nanostructured explosives

The detonation process is able to build new materials with a bottom-up approach. Diamond, the hardest material on earth, can be synthesized in this way. This unconventional synthesis route is possible due to the presence of carbon inside the high-explosive molecules: firing high-explosive mixtures with a negative oxygen balance in a non-oxidative environment leads to the formation of nanodiamond particles. Trinitrotoluene (TNT) and hexogen (RDX) are the explosives primarily used to synthesize nanodiamonds. Here we show that the use of nanostructured explosive charges leads to the formation of smaller detonation nanodiamonds, and it also provides new understanding of nanodiamond formation-mechanisms. The discontinuity of the explosive at the nanoscale level plays the key role in modifying the diamond particle size, and therefore varying the size with microstructured charges is impossible.

Continuous engineering of nano-cocrystals for medical and energetic applications

Cocrystals, solid mixtures of different molecules on molecular scale, are supposed to be tailor made materials with improved employability compared to their pristine individual components in domains such as medicine and explosives. In medicine, cocrystals are obtained by crystallization of active pharmaceutical ingredients with precisely chosen coformers to design medicaments that demonstrate enhanced stability, high solubility, and therefore high bioavailability and optimized drug up-take. Nanoscaling may further advance these characteristica compared to their micronsized counterparts – because of a larger surface to volume ratio of nanoparticles. In the field of energetic materials, cocrystals offer the opportunity to design smart explosives, combining high reactivity with significantly reduced sensitivity, nowadays essential for a safe manipulation and handling. Furthermore, cocrystals are used in ferroelectrics, non-linear material response and electronic organics. However, state of the art batch processes produce low volume of cocrystals of variable quality and only have produced micronsized cocrystals so far, no nano-cocrystals. Here we demonstrate the continuous preparation of pharmaceutical and energetic micro- and nano-cocrystals using the Spray Flash Evaporation process. Our laboratory scale pilot plant continuously prepared up to 8 grams per hour of Caffeine/Oxalic acid 2:1, Caffeine/Glutaric acid 1:1, TNT/CL-20 1:1 and HMX/Cl-20 1:2 nano- and submicronsized cocrystals.

Sulfates‐Based Nanothermites: An Expanding Horizon for Metastable Interstitial Composites

Metal sulfates (Ba, Bi, Ca, Cu, Mg, Mn, Na, Zn, Zr) were used as oxidizers in reactive compositions with Al nanopowder. These new kinds of nanothermites have outstandingly high reaction heats (4-6 kJ g(-1) ) compared to conventional Al/metal oxides (1.5-4.8 kJ g(-1) ) and also have good combustion velocities (200-840 m s(-1) vs 100-2500 m s(-1) ). These compositions are extremely insensitive to friction making their preparation and handling easy and safe. The sulfate hydration water increases the reaction heats and has a significant effect on the sensitivity to impact and to electrostatic discharge. The reaction of Al with water is easier to initiate than the one with sulfate which leads to two possible decomposition modes for samples exposed to an open flame. The pyrotechnical properties observed with sulfates have also been found for other sulfur oxygenates (SO3 (2-) , S2 O3 (2-) , S2 O8 (2-) ) which opens up new horizons in the domain of metastable interstitial composites.

Energetic nanocomposites for detonation initiation in high explosives without primary explosives

The mixing of aluminum nanoparticles with a metal containing oxidizer (here, WO3 or Bi2(SO4)3) gives reactive materials called nanothermites. In this research, nanothermites were combined with high explosive nanoparticles (RDX) to prepare energetic nanocomposites. These smart nanomaterials have higher performances and are much less hazardous than primary explosives. Their flame propagation velocity can be tuned from 0.2 to 3.5 km/s, through their explosive content. They were used to initiate the detonation of a high explosive, the pentaerythritol tetranitrate. The pyrotechnic transduction of combustion into detonation was achieved with short length systems (<2 cm) and small amounts of energetic nanocomposites (∼100 mg) in semi-confined systems.

Chip Calorimetry for the Sensitive Identification of Hexogen and Pentrite from Their Decomposition inside Copper Oxide Nanoparticles

Smart detection systems for explosive sensors are designed both to detect explosives in the air at trace level and identify the threat for a specific response. Following this need we have succeeded in using microthermal analysis to sensitively identify and discriminate between RDX and PETN explosive vapors at trace level. Once the explosive vapor is trapped in a porous material, heating the material at a fast rate of 3000 K/s up to 350 °C will result in a thermal pattern specifically corresponding to the explosive and its interaction with the porous material. The explosive signatures obtained make it possible to simultaneously identify the presence and the nature of the explosive vapor in just a few milliseconds. Therefore, this also allows the development of multitarget devices using porous material for capturing the vapor combined with microthermal analysis for fast detection and identification. So far it is the first time that chip calorimetry has been used to characterize and identify explosives in vapor state.

Nanostructuring of carbon materials by means of a calcium phosphate template

Nanostructured porous carbon powders were elaborated by means of a hard-template approach, using environmentally friendly materials such as sucrose and Ca3(PO4)2 tricalcium phosphate hydrate as carbon and template sources, respectively. While the naturally occurring carbohydrate is widely used for carbon materials synthesis, the tricalcium phosphate was never suggested as template despite its efficiency as shown in the present study. The resulting carbon materials were characterized by elemental analysis, Raman spectroscopy, nitrogen adsorption and electron microscopies. Porous carbon powders with disordered hierarchical porous structure, exhibiting tunable textural properties (specific surface areas, pore volume…), were thus synthesized. For instance, with a template/carbon precursor weight ratio of 1, the specific surface area and pore volume were significantly increased compared to the counterpart which was elaborated without a Ca3(PO4)2 template. The ease of implementation coupled to the low cost of different reagents make the present process potentially competitive for synthesizing porous carbon powders.

Aluminum nanopowder: A substance to be handled with care

Aluminum nanopowders are increasingly used in various areas of research in materials and physical chemistry. Their unconventional properties are still little understood and make their handling sometimes quite hazardous. In this article, we report the case of apparently benign mixtures of Al with sulfuric acid (H2SO4), which violently explode when they are exposed to a flame. The explosions of 100mg samples were observed by high speed video (60000fr/s). These experiments have showed a three-step mechanism, in which the primary hydrogen combustion ignites and disperses the nano-Al/H2SO4 paste in clusters with high velocities (∼100m/s). The combustion of the paste increases the hydrogen release and initiates the explosion of the H2/air mixture, which propagates at high velocities (760-1060m/s). This effect was not observed with micron-sized Al powders, and it is a good illustration of new hazards with nano-Al. Extreme caution is hence recommended to chemists who handle such materials.

Safer and performing energetic materials based on polyaniline-doped nanocomposites

ABSTRACT 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 desensitization 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.