Thermal Decomposition Mechanism of 2,2′,4,4′,6,6′-Hexanitrostilbene by ReaxFF Reactive Molecular Dynamics Simulations

2,2′,4,4′,6,6′-Hexanitrostilbene (HNS) is an explosive with increased heat resistance, and its mechanism of thermal decomposition is of interest. In this paper, the decomposition processes of HNS at various temperatures (2500, 2750, 3000, 3250, and 3500 K) are calculated by large-scale reactive molecular dynamics simulations. The initial reactions and the evolution of clusters (whose molecular weight is larger than HNS) are analyzed. The reaction kinetics parameters are fitted. The results show that the main initial decomposition mechanisms of HNS are C–NO2 bond dissociation and nitro-nitrite (NO2–ONO) isomerization. During decomposition, O atoms are less likely to be released from the cluster than H and N atoms. Low temperatures tend to produce larger clusters, and clusters at higher temperatures tend to decompose. The thermal decomposition of HNS is a combination of single-molecule and bimolecular decomposition mechanisms. The dimerization reaction is clearly weakened, and the C–N bond cleavage is still the main initial reaction path with increase in temperature. Temperature has a great influence on the structure of the clusters. Single-step kinetics is a good approximation for the thermal decomposition of HNS.