Entropy-dominated dissipation in sapphire shock-compressed up to 400 GPa (4 Mbar)

Abstract

Sapphire (single-crystal Al2O3) is an Earth material and a window in shock experiments. Pressure at the core-mantle boundary is ~130 GPa. Sapphire becomes opaque at 100-GPa shock pressures, which precludes measuring temperature with thermal radiance. Ruby is sapphire with a few 0.1% Cr. We have measured wave profiles of sapphire crystals with seven crystallographic orientations at shock pressures of 16, 23 and 86 GPa. At 23 GPa plastic-shock rise times are 1-300 ns, depend sensitively on the direction of shock propagation, and are totally contrary to conventional expectations. Long rise times are probably caused by strong inter-atomic interactions. Our wave profiles and published experiments and theory imply sapphire disorders without significant shock heating up to ~400 GPa, above which Al2O3 is amorphous and must heat. The characteristic shape of shock compression curves of many Earth materials at 100 GPa pressures is probably caused by a combination of entropy and temperature. Optimal shock windows are probably m- and s-cut crystals. Implications for the ruby scale, shock synthesis, and the ~$6B National Ignition Facility (NIF) are discussed.