| Summary: | The coupling effect of temperature and pressure plays a significant role in the behavior of energetic materials. Energetic materials present extremely complex behavior at high temperatures. Handling difficulties and safety risks have restricted detailed experimental studies on their decomposition mechanisms, especially for less-stable configurations of energetic materials. This has necessitated theoretical studies particularly under the coupling effect. A combinational strategy based on density functional tight-binding molecular dynamics (DFTB-MD) simulations and the density functional theory (DFT) was applied to study the effects of temperature and pressure on the initial decomposition and reaction activation barrier of α-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and δ-HMX. A comprehensive analysis of HMX in each system indicated that while the initial decomposition mechanism was not dependent on pressure at high temperatures, the initial reaction rate and activation barrier were highly dependent on pressure. The initial decomposition mechanisms of α-HMX at 1500 K and δ-HMX at 1200 K under pressures of 1–3 GPa involved unimolecular C–H bond breakage and N–NO2 homolysis, respectively. Pressure inhibited the decomposition of α-HMX, but accelerated that of δ-HMX. The decompositions of α-HMX and δ-HMX occurred via first-order reactions. The classical Arrhenius form indicated that pressure (1–3 GPa) increased and decreased the decomposition barriers of α-HMX at 1500 K and δ-HMX at 1200 K, respectively. These findings provide a new understanding of the thermal decomposition and reaction mechanisms of HMX under the coupling effect of temperature and varying pressures.
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