The electrical conductivity of Al₂O₃ under shock-compression

摘要

Sapphire (Al₂O₃) crystals are used below 100?GPa as anvils and windows in dynamic-compression experiments because of their transparency and high density. Above 100?GPa shock pressures, sapphire becomes opaque and electrically conducting because of shock-induced defects. Such effects prevent temperature and dc conductivity measurements of materials compressed quasi-isentropically. Opacities and electrical conductivities at ~100?GPa are non-equilibrium, rather than thermodynamic parameters. We have performed electronic structure calculations as a guide in predicting and interpreting shock experiments and possibly to discover a window up to ~200?GPa. Our calculations indicate shocked sapphire does not metallize by band overlap at ~300?GPa, as suggested previously by measured non-equilibrium data. Shock-compressed Al₂O₃ melts to a metallic liquid at ~500?GPa and 10,000?K and its conductivity increases rapidly to ~2000?Ω?1cm?1 at ~900?GPa. At these high shock temperatures and pressures sapphire is in thermal equilibrium. Calculated conductivity of Al₂O₃ is similar to those measured for metallic fluid H, N, O, Rb, and Cs. Despite different materials, pressures and temperatures, and compression techniques, both experimental and theoretical, conductivities of all these poor metals reach a common end state typical of strong-scattering disordered materials.