Optical emission, shock-induced opacity, temperatures, and melting of Gd_3Ga_5O_(12) single crystals shock-compressed from 41 to 290 GPa

摘要

Strong oxides at high shock pressures have broad crossovers from elastic solids at ambient to failure by plastic deformation, to heterogeneous deformation to weak solids, to fluid-like solids that equilibrate thermally in a few ns, to melting and, at sufficiently high shock pressures and temperatures, to metallic fluid oxides. This sequence of crossovers in single-crystal cubic Gd_3Ga_5O_(12) (Gd-Ga Garnet-GGG) has been diagnosed by fast emission spectroscopy using a 16-channel optical pyrometer in the spectral range 400-800 nm with bandwidths per channel of 10 nm, a writing time of ～1000 ns and time resolution of 3 ns. Spectra were measured at shock pressures from 40 to 290 GPa (100GPa= 1 Mbar) with corresponding gray-body temperatures from 3000 to 8000 K. Experimental lifetimes were a few 100 ns. Below 130 GPa, emission is heterogeneous and measured temperatures are indicative of melting temperatures in grain boundary regions rather than bulk temperatures. At 130 GPa and 2200 K, GGG equilibrates thermally and homogeneously in a thin opaque shock front. This crossover has a characteristic spectral signature in going from partially transmitting shock-heated material behind the shock front to an opaque shock front. Opacity is caused by optical scattering and absorption of light generated by fast compression. GGG melts at ～5000K in a two-phase region at shock pressures in the range 200 GPa to 217 GPa. Hugoniot equation-of-state data were measured by a Doppler Pin SystemDPS with ps time resolution and are generally consistent with previous data. Extrapolation of previous electrical conductivity measurements indicates that GGG becomes a poor metal at a shock pressure above ～400 GPa. Because the shock impedance of GGG is higher than that of Al_2O_3 used previously to make metallic fluid H (MFH), the use of GGG to make MFH will achieve higher pressures and lower temperatures than use of Al_2O_3. However, maximum dynamic pressures at which emission temperatures of fluid hydrogen made by shock reverberation between GGG anvils could be measured remains limited to ～130 GPa, as for Al_2O_3 anvils.