Mechanism investigation on the solid–solid phase transition of CL-20 induced by water vapor

Energetic materials often possess different polymorphs that exhibit distinguishable performances. As a typical energetic material, hexanitrohexaazaisowurtzitane (CL-20 or HNIW) is one of the most powerful explosives nowadays. Phase transition of CL-20 induced by ubiquitous water vapor leading to an increase in sensitivity and a decrease in energy level is a key bottleneck that limits the widespread application of CL-20-based explosives. Herein, the solid-solid phase transition behavior of CL-20 induced by water vapor and the related mechanism have been investigated. The results show that CL-20 undergoes an irreversible ε to α phase transition at an initial temperature of 104 °C in the presence of water vapor, much lower than that induced by thermal stimulation alone. According to XRD results and phase transition kinetics analysis, a four-parameter model is established to describe the phase transition process as a function of time. Theoretical calculations further support the promoting effect of water molecules on the phase transition. Based on experimental and theoretical results, a possible mechanism of steam-induced solid-solid phase transition of CL-20 is proposed. This work will provide a theoretical basis for the reliable design of CL-20-based energetic materials and also for the study on polymorphic transition inhibition of organic crystals to obtain the preferred phase.