Marc Comet

Tunable Generation and Adsorption of Energetic Compounds in the Vapor Phase at Trace Levels: A Tool for Testing and Developing Sensitive and Selective Substrates for Explosive Detection

Among various methods for landmine detection, as well as soil and water pollution monitoring, the detection of explosive compounds in air is becoming an important and inevitable challenge for homeland security applications, due to the threatening increase in terrorist explosive bombs used against civil populations. However, in the last case, there is a crucial need for the detection of vapor phase traces or subtraces (in the ppt range or even lower). A novel and innovative generator for explosive trace vapors was designed and developed. It allowed the generation of theoretical concentrations as low as 0.24 ppq for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in air according to Clapeyron equations. The accurate generation of explosive concentrations at subppt levels was verified for RDX and 2,4,6-trinitrotoluene (TNT) using a gas chromatograph coupled to an electron capture detector (GC-ECD). First, sensing material experiments were conducted on a nanostructured tungsten oxide. The sensing efficiency of this material determined as its adsorption capacity toward 54 ppb RDX was calculated to be five times higher than the sensing efficiency of a 54 ppb TNT vapor. The material sensing efficiency showed no dependence on the mass of material used. The results showed that the device allowed the calibration and discrimination between materials for highly sensitive and accurate sensing detection in air of low vapor pressure explosives such as TNT or RDX at subppb levels. The designed device and method showed promising features for nanosensing applications in the field of ultratrace explosive detection. The current perspectives are to decrease the testing scale and the detection levels to ppt or subppt concentration of explosives in air.

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.

Reactive characterization of nanothermites

Conventional thermal analysis techniques (TG and DSC) give valuable information on the activation energy and the reactivity of energetic materials such as organic explosives. Here, we discuss the use of these methods for characterizing nanothermites, energetic compositions made of metallic oxides and a fuel (often a reducing metal). The experimental limitations of these analysis techniques are identified. It is difficult to ignite nanothermites with slow heating rates as those used in DSC. This is due to the inorganic nature of the thermite components and because the reaction involves interparticular heat and matter transfers. In addition, during the progressive decomposition of nanothermites, there is no change in mass, so it cannot be observed by thermogravimetric analysis. The use of laser ignition to prime the abrupt combustion of nanothermite pellets allows determining the ignition energy and analyzing the propagation of the combustion front. It also provides qualitative data that can be used to understand the combustion mechanism and to correlate it to the microstructure of the nanothermites. By analyzing several examples, we will show that the coupling of high speed video to existing thermal analysis techniques could significantly extend their utilization range for the characterization of new energetic materials.

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.

Nanodiamond for tuning the properties of energetic composites.

Bismuth oxide (Bi2O3) particles were coated by detonation nanodiamonds. The resulting nanocomposite materials were mixed with an aluminum nanopowder (≈ 100 nm) to prepare nanothermites, with reduced sensitivity to friction and electrostatic discharge (ESD). The use of nanodiamond for this purpose is reported here for the first time. Their numerous qualities such as their small size, antifriction properties and thermal conductivity make them ideal candidates. Small amounts of detonation nanodiamonds allow obtaining impressive desensitization, making thus modified Bi2O3/Al nanothermite safe to handle. A composition containing around 1 wt.% of nanodiamond has a sensitivity threshold to friction superior to 100 N instead of 5 N for the thermite without nanodiamond. Furthermore, the sensitivity threshold to electrostatic discharge increases to 20 times when the nanodiamond content reaches 1.8 wt.%. The antifriction properties of nanodiamond limit the scratching of Bi2O3 surface by Al particles. The desensitization to ESD is observed for a sufficient coverage of the oxide particles (1.8 wt.% of ND), which restrains the effect of the melt dispersion mechanism of aluminum and prevents the mixing of the oxidizing and the reducing parts of the composites. A good reactivity of the thermite could be maintained for nanodiamond content up to 2.6 wt.%. The carburizing of aluminum coming on contact with nanodiamond during the thermite reaction could be evidenced by X-ray Diffraction and calorimetry measurements and also participates to the desensitization of the nanothermite. This kind of desensitization by using detonation nanodiamond can also be applied to other nanothermites having low sensitivity threshold to friction and ESD.

Two Parameter Model of Fuji Imaging Plate Response Function to Protons

Imaging plates are phosphor films routinely used in ultra high intensity laser experiments. They offer the possibilities of both imaging the beams of ionizing particles generated in the laser-matter interaction and characterizing their energy distribution. The response functions of the imaging plates are necessary to relate the detected signal intensity to the absolute flux of incoming particles. In this report we review our model of the response functions and discuss its parameters. We detail how we calibrated the parameters of the response functions to protons from absolute measurements. Their uncertainties are also presented.

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.

Nanothermites for space and defence applications

Thermites are energetic materials which are made of a metallic oxide mixed with a reducing metal1. The reactivity of classical thermites is moderate but it can be substantially improved when micron-sized particles are replaced by nanoparticles. In this paper, three examples of nanothermites are given in order to illustrate the contribution of these nanomaterials in the future spatial and defence applications: - the incidence of the size of the metallic oxide particles on the reactivity was illustrated by the case of WO3/Al nanothermites, - the correlation between the composition/structure of the oxide phase and the reactivity was achieved on AlxMoyOz/Al nanothermites, - the fabrication of Gas Generating Nanothermites was performed by adding military explosives in porous mineral oxides (Cr2O3; MnO2) used to fabricate nanothermites.