Entropy-Dominated Dissipation in Sapphire Shock-Compressed up to 400 GPa (4 Mbar)

TL;DR

This study investigates how sapphire, a key Earth material, undergoes entropy-dominated dissipation under shock compression up to 400 GPa, revealing disordering without significant heating until amorphization occurs.

Contribution

It provides new experimental wave profile data and theoretical insights into sapphire's behavior under extreme shock pressures, highlighting entropy's role in dissipation.

Findings

Sapphire disorders without significant heating up to ~400 GPa

Shock rise times depend on crystallographic orientation

Sapphire becomes amorphous above 400 GPa

Abstract

Sapphire (single-crystal Al2O3) is a representative Earth material and is used as a window and/or anvil in shock experiments. Pressure, for example, at the core-mantle boundary is about 130 gigapascals (GPa). Defects induced by 100-GPa shock waves cause sapphire to become opaque, which precludes measuring temperature with thermal radiance. We have measured wave profiles of sapphire crystals with several crystallographic orientations at shock pressures of 16, 23, and 86 GPa. At 23 GPa plastic-shock rise times are generally quite long (~100 ns) and their values depend sensitively on the direction of shock propagation in the crystal lattice. The long rise times are probably caused by the high strength of inter-atomic interactions in the ordered three-dimensional sapphire lattice. Our wave profiles and recent theoretical and laser-driven experimental results imply that sapphire disorders without significant shock heating up to about 400 GPa, above which Al2O3 is amorphous and must heat. This picture suggests that the characteristic shape of shock compression curves of many Earth materials at 100 GPa pressures is caused by a combination of entropy and temperature.