PHASE STABILITY OF EPSILON AND GAMMA HNIW (CL-20) AT HIGH-PRESSURE AND TEMPERATURE

Abstract

Hexanitrohexaazaisowurtzitane (CL‐20) is one of the few ingredients developed since World War II to be considered for transition to military use. Five polymorphs have been identified for CL‐20 by FTIR measurements (α, β, γ, ε, ζ). As CL‐20 is transitioned into munitions it will become necessary to predict its response under conditions of detonation, for performance evaluation. Such predictive modeling requires a phase diagram and basic thermodynamic properties of the various phases at high pressure and temperature. Therefore, the epsilon and gamma phases of CL‐20 at static high‐pressure and temperature were investigated using synchrotron angle‐dispersive x‐ray diffraction experiments. The samples were compressed and heated using diamond anvil cells (DAC). Pressures and temperatures achieved were around 5 GPa and 240 °C, respectively. The epsilon phase was stable to 6.3 GPa at ambient temperature. When heated at ambient pressure the epsilon phase was sustained to a temperature of 120 °C then underwent a transition to the gamma phase above 125 °C and then thermal decomposition occurred above 150 °C. Upon compression, the gamma phase underwent a phase transition at both ambient temperature and 140 °C. Pressure—volume data for the epsilon and gamma phase at ambient temperature and the epsilon phase at 75 °C were fit to the Birch‐Murnaghan formalism to obtain isothermal equations of state.