Kinetics of the β–δ Phase Transition in PBX 9501

The initial step in the thermal decomposition of HMX is the solid state phase transition from the centrosymmetric beta form to the noncentrosymmetric delta form. The symmetry change makes the phase transition amenable to the application of second harmonic generation (SHG) as a probe of transition kinetics. We have used SHG to study the temperature dependence of the kinetics for unconfined PBX9501 and HMX. Spatially resolved SHG measurements have shown a nucleation and growth mechanism for the solid state phase transition. We have measured the transition rate as a function of temperature in order to obtain the activation energy and entropy of transition, which determine the phase transition kinetics. Additionally, we have observed temperature dependent reversion of the delta phase to beta phase and have found that we can control the reversion rate by controlling the cooling.