Study of thermal instability of HMX crystalline polymorphs with and without molecular vacancies using reactive force field molecular dynamics

Abstract

The sensitivity of energetic materials can be identified by heat, friction, impact, shock, electrostatic charge, etc. A complexity of various factors may influence the sensitivity and among them are the crystal structure and the defects within the crystal very important. The explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) may exist in four crystalline polymorphs, denoted α, β, γ, and δ. It has been shown that β-HMX has the lowest impact sensitivity of them whereas δ-HMX has the highest, and that the density of the polymorphs decreases with increasing sensitivity. Knowledge and understanding of the relationship between polymorphism and chemical decomposition processes in crystalline HMX phases may enable improvement of existing energetic compounds or development of novel materials. Thermal instability may be strongly related to mechanical instability due to hot-spot mechanisms. In this work we apply reactive force field molecular dynamics (ReaxFF-MD) to study how the thermal stability of α- and β-HMX is affected by crystalline structure, molecular vacancies, and density. We have found decomposition temperatures of around 1550 K for both polymorphs, which is significantly higher than experimental results of around 550 K. However, by introducing molecular vacancies into the crystals the decomposition was calculated to occur at 400-600 K, in agreement with experimental values. The α polymorph was more sensitive than the β polymorph when vacancies were present. We also discovered that the β polymorph had better resistance against rupture of the N-NO2 bond at higher crystal densities. These results may hint on the different impact sensitivity of the two polymorphs and highlight the influence of crystal structure, crystal defects, density, and temperature on the sensitivity.