Isentropic compression of cyclotetramethylene tetranitramine (HMX) single crystals to 50 GPa

Single crystals of cyclotetramethylene tetranitramine (HMX) were isentropically compressed perpendicular to (010) and (Oil) faces at the Sandia Z-Machine. A 50 GPa ramped magnetic pressure load of about 200 ns rise time loaded four specimens of each orientation. HMX specimens were from 300-600 μm thick. Velocity histories at the rear of each crystal were measured by Doppler velocimetry. Although a phase change in HMX at 27 GPa has been proposed based upon isothermal data, no evidence of this change is seen in our analyses between 5 and 50 GPa along the isentrope. Previous isentropic loading experiments on HMX had not shown evidence of a phase change either, but those experiments were complicated by the use of NaCl interferometer windows that have a phase change near the pressure of interest. The experiments described in this paper employed LiF interferometer windows that are known to be absent phase changes in the regime of application. Accurate determination of isentropic compressibility for HMX was not possible using data from these experiments owing to uncertainty in the very large compressibility at low pressures.