Coefficient of Thermal Expansion of the Beta and Delta Polymorphs of HMX

Dimensional changes related to temperature cycling of the β and δ polymorphs of HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) are important for a variety of applications. The coefficient of thermal expansion (CTE) of β and δ phases are measured over a temperature range of −20 °C to 215 °C by thermo‐mechanical analysis (TMA). Dimensional changes associated with the phase transition were also measured, and the time‐temperature dependence of the dimensional change is consistent with phase transition kinetics measured earlier by differential scanning calorimetry (DSC). One HMX sample measured by TMA during its initial heating and again three days later during a second heating showed the β‐to‐δ phase transition a second time, thereby indicating back conversion from δ‐to‐β phase HMX during those three days. DSC was used to measure kinetics of the δ‐to‐β back conversion. The most successful approach was to first heat the material to create the δ phase, then after a given period at room temperature, measure the heat absorbed during a second pass through the β‐to‐δ phase transition. Back conversion at room temperature follows nucleation‐growth kinetics.