Thermal decomposition mechanism of β-HMX under high Pressures via ReaxFF reactive molecular dynamics simulations

The thermal decomposition mechanisms of condensed phase β-HMX at various densities (ρ= 1.89, 2.11, 2.22, 2.46, 2.80, 3.20 g · cm ^-3) and at 2500 K were studied using ReaxFF reactive molecular dynamics simulations. The effects of pressure on the initial and secondary reaction rates, the main differences in the initial decomposition mechanisms between highly compressed and less compressed systems, as well as the reasons for these variations were analyzed. It was determined that the initial decomposition mechanisms of HMX were dependent on pressure (or density). At low densities (ρ< 2.80 g · cm ^-3), intramolecular reactions are dominant, these being N - NO ₂ bond dissociation, HONO elimination, and concerted ring fission by C - N bond scission. At high densities (ρ ≥ 2.80 g· cm ^-3), intramolecular reactions are well restrained, whereas intermolecular reactions are promoted, leading to the formation of small molecules, such as O ₂ and HO, and large molecular clusters. These changes in the initial decomposition mechanisms lead to different kinetic and energetic behaviors, as well as variations in the distribution of products. These results obtained through this work are significant in that they assist in understanding the chemical reactions involved in the initiation, reaction development, and detonation of energetic materials under extreme conditions.