Abstract
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.
Highlights
Sapphire (Al2O3) crystals are used below 100 GPa as anvils and windows in dynamic-compression experiments because of their transparency and high density
Heterogeneous, non-equilibrium shock-induced defects in strong sapphire cause c-cut sapphire to become opaque at shock pressures in the range 100 to 130 GPa5, which prevents measurements of thermal emission temperatures and optical spectroscopies of Metallic fluid hydrogen (MFH) and other novel materials
Because of the importance of optical experiments, one of our goals is to identify transparent anvil/windows that remain transparent under dynamic pressures above 150 GPa
Summary
Sapphire (Al2O3) crystals are used below 100 GPa as anvils and windows in dynamic-compression experiments because of their transparency and high density. 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 quasiisentropically. 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. Heterogeneous, non-equilibrium shock-induced defects in strong sapphire cause c-cut sapphire to become opaque at shock pressures in the range 100 to 130 GPa5, which prevents measurements of thermal emission temperatures and optical spectroscopies of MFH and other novel materials. Investigating effects of shock propagation in directions other than parallel to the c axis of the rhombohedral structure might lead to improved transparency in shocked sapphire
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