Atomic-level insight into the thermal decomposition of HMX/TEX mixed explosives via reactive molecular dynamics

The thermal decomposition of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) and 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclododecane (TEX) mixed explosives, along with pure HMX, was investigated using reactive molecular dynamics at temperatures between 2000 and 3500 K. The study aimed to evaluate the effect of the caged explosive TEX on HMX decomposition and clarify the reaction mechanism of the mixture. The results showed that the primary initial decomposition step remains cleavage of the nitro group, while decomposition of TEX increased the NO₂ concentration in the system. NO₂ released from TEX combined with H atoms from HMX to form nitrite intermediates, which subsequently decompose into NO, HNO, and OH. Meanwhile, HMX and its decomposition products further reacted with OH and nitrogen oxides such as NO and HNO to yield final products including H₂O and N₂. The addition of TEX also introduced H atoms predominantly form H₂ rather than combine with N atoms to generate NH₃, leading to higher H₂ production in the mixture compared to pure HMX. The activation energies (Eₐ) for the initial and intermediate stages of decomposition in the mixed system were determined to be 87.06 kJ/mol and 100.72 kJ/mol, respectively-significantly lower than those of pure HMX. These findings confirmed that TEX promotes the decomposition of HMX and reduces the thermal stability of the mixed system.

Molecular dynamics simulations for the HMX/TEX mixed system were performed using the ReaxFF module within the LAMMPS software package, with the ReaxFF/lg force field employed to model interatomic interactions and chemical reactivity.