Properties of LiF and Al_2O_3 to 240 GPa for Metal Shock Temperature Measurements

Abstract

Shock temperature experiments employing a six-channel pyrometer were conducted on 200, 500, and 1000 A thick films of Fe sandwiched between 3-mm-thick anvils of Al203 and LiF to measure the thermal diffusivity ratios Al_20_3/Fe and LiF / Fe at high temperatures and pressures. Temperature decays of 3000 ± 800 K in 250 ns were observed at Fe pressures of 194 - 303 GPa, which reflect the conduction of heat from the thin metal films into the anvil material. These results were achieved via experiments employing LiF anvils at conditions of 164 - 165 GPa and 4190 - 4220 K and Al_2O_3 anvils at conditions of 156 - 304 GPa and 1290 - 2740 K. Thermal modeling of interface temperature versus time yields best fit thermal diffusivity ratios of 4 - 19 ± 1 (Fe/anvil) over the pressure and temperature range of the experiments. Calculated thermal conductivities for Fe, using electron gas theory, of 111 - 181 W /mK are used to calculate thermal conductivities for the anvil materials ranging from 2 to 13 W /mK. Debye theory predicts higher values of 8 to 35 W /mK. Data from previous experiments on thick (≥l00μm) films of Fe and stainless steel are combined with our present results from experiments on thin (≤1000 A) films to infer a 5860 ± 390 K Hugoniot temperature for the onset of melting of iron at 243 GPa. Our results address the question of whether radiation observed in shock temperature experiments on metals originates from the metal at the metal/anvil interface or from the shocked anvil. We conclude that the photon flux from the shocked assemblies recorded in all experiments originates from the metal. Within the uncertainties of the shock temperature data, the uncertainties in shock temperatures resulting from the radiation from the anvils is negligible. This is in direct disagreement with the conclusions of previous work by Kondo.