Size-dependent phase transition dynamics in HMX crystals: Mechanistic insights from defect-mediated transformation

• The PT process of SC HMX crystals was studied for the first time. • The mechanism of PT in SC HMX was revealed by both internal and surface defects . • This provides new insights into the PT process of HMX crystals. The size effects of energetic materials (EMs) particles has a profound influence on their solid-state phase transition (PT) and structural damage. In this study, the PT behavior of large-sized 1,3,5,7-tetranitro-1,3,5,7-tetrazole (HMX) was studied by a series of characterization methods. It has been demonstrated that both internal defects and surface damage of large-size HMX occur approximately 20 °C earlier than that of small-size HMX. The kinetic analysis demonstrates that the PT of large-size HMX is comprised of two distinct steps. The initial step is characterised by fragmentation, while the subsequent step represents the PT in its strict sense. This makes the PT process appear more clearer in comparison to the powder form of HMX. Additionally, it was determined that varying humidity levels exert an influence on the HMX phase recover. This provides fundamental support for a correct understanding of HMX solid-state PT.