The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

367 / 2019-02-28 09:54:48

Pressure-induced Phase Transitions of Energetic Material δ-HMX

Energetic Material; δ-HMX; High pressure; Phase transition

Plenary track > High Pressure Physics and Materials Science

HMX is an eight-membered heterocyclic ammonium nitrate high-energy explosive. Because of its high energy density, good thermal stability, high detonation performance and product cleanliness, HMX has been widely applied in military weapons, aerospace and modern industry. During detonation, HMX explosive will undergo the complex high temperature and high pressure circumstance before its complete decomposition or full reaction. The structural phase transition of HMX at extreme conditions is of great scientific significance for understanding the ignition reaction and initiation mechanism of high energetic explosive. There exist five crystalline forms of α-, β-, γ-, δ- and ε-HMX (which is different with the high pressure ε phase HMX) by controlling the crystallization rate in solution at ambient conditions. It confirmed that the “chair” structure of β-HMX molecules and the similar “boat” structure of α, δ-HMX. δ-HMX is the high temperature phase and formed at the solid melt interphase in deflagrating HMX, which has a hexagonal P6122 structure, c symmetry axis, and five independent elastic coefficients, and is considered to be stable at extreme conditions.
It has been studied that the thermal expansion behavior of four crystal forms of β, δ, γ and α of HMX by X-ray scattering experiments, and gave the order of HMX temperature-induced phase transition: β→δ; γ→δ and α→δ. It should be noted that in the conditions of mutual conversion between the various crystal forms of HMX, the temperature is often coupled with the pressure, and the phase transition conditions given by different research groups may have relatively large deviations. Apparently, further research efforts are required to clarify the controversial results. Here we report the pressure-induced phase transitions of δ-HMX under high pressure by using In situ Raman spectroscopy, mid-infrared spectroscopy and synchrotron x-ray diffraction. Five pressure-induced phase transitions were observed and confirmed their nature of conformational change. Phase transitions of δ-HMX crystal have been systematically studied by Raman, mid-infrared under high pressure up to 40 GPa. Two phase transitions are observed including δ→κ (1.6 GPa) and κ→κ (16 GPa) using neon gas as the pressure-transmitting medium. On releasing pressure to ambient pressure, the samples reverse partly to the δ phase. It is helpful to understand the initiation mechanism of high energetic explosive.