Pressure-induced physical and chemical changes of non-conventional energetic materials: Nitrate, perchlorate and peroxide chemistries at high pressure and high temperature

Mihindra Dunuwille

Non-conventional explosives are widely used in terrorist activities, thus imposing a threat to homeland security. The simple combination of oxidizers with fuels is one popular category. Our goal is to explore and understand chemistry in selected oxidizers over a relevant pressure-temperature (P-T) regime of blast conditions, to get insights into fundamental chemistry associated with the selected energetic molecules and energetic processes at molecular level, for mitigation applications. We have investigated ammonium nitrate (AN), ammonium perchlorate (AP), and simple group IA peroxides under high-pressure high-temperature conditions, using confocal micro-Raman spectroscopy and synchrotron x-ray diffraction. AN studies conducted up to 50 GPa and 450 °C provide new constraints for its phase diagram. The spectral evidence confirms the presence of phase IV´ in a wide range of both thermal and chemical conditions. The strengthening of hydrogen bonding observed for phase IV´ influences the melt-decomposition line, showing an unusual turnover of decomposition temperature. We have proposed two models for decomposition, based on the phase from which it occurs. In addition, we observed an enhanced stability for phase IV in different chemical environments. AP studies conducted up to 50 GPa and 450 °C show spectral evidence for three new ordered phases (III, IV and VI) at various P-T conditions with varying degrees of hydrogen bonding. Decomposition occurs from phase V which we suggest to be initiated via a diffusion pathway. The proposed phase diagram shows a rich polymorphism and replacement of melt from a decomposition line. Both Li2O2 and Na2O2 studies show evidence for pressure-induced phase and chemical transitions, similar to H2O2. We observed comparatively higher transition pressures for both phase I → II and chemical decomposition transitions in Li2O2. This suggests the influence of the degree of intermolecular interactions on transition pressures. The orthorhombic Li2O2 phase II with enhanced stability is a structure with different packing. Unlike nitrates and perchlorates, alkali metal peroxides are the only type of oxidizers considered in this study in which hydrogen bonding is absent. Our results show that both the presence of hydrogen bonding and its degree have varying effects on their decomposition mechanisms and transition pressures and temperatures.

Pressure-induced physical and chemical changes of non-conventional energetic materials: Nitrate, perchlorate and peroxide chemistries at high pressure and high temperature

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