Shock Temperatures of Materials: Experiments and Applications to the High-Pressure Equation of State.
The experimental determination of temperatures in the high-pressure shocked state of condensed matter provides a useful supplement to equation-of-state models derived from Hugoniot measurements. An optical pyrometry technique has been developed to obtain temperature measurements during impact-driven shock wave experiments with solid and liquid samples at pressures near 100 GPa. Experimental results confirm that throughout moderate ranges of shock pressure amplitude, transparent dielectrics emit thermal radiation from the region of the shock front, with a spectrum which is characteristic of the Hugoniot state temperature. Shock temperatures in sodium chloride crystals have been measured in the pressure range 70-105 GPa. The observed temperatures, between 4000 and 8000 K, are in agreement with the results of earlier determinations and with calculations assuming the occurrence of shock-induced melting. Results of experiments to measure shock temperatures in metallic silver include a successful measurement of 5950 K at a pressure of 185 GPa. This result is consistent with the melting of silver under shock, with a melting pressure dependence described by the Lindemann criterion. Shock temperature measurements in silica (SiO(,2)) have produced anomalous results suggestive of melting occurring in the stishovite phase near 100 GPa pressure and 4700 K temperature. Experimental measurements with (alpha) -quartz and fused silica samples extend from pressures of approximately 60 GPa to 140 GPa, with shock temperatures between approximately 4500 K and 7000 K. The experimental data allow quantification of the thermodynamic relations among silica phases, including heats of transition and the Gruneisen parameter. Shock temperatures in single crystal forsterite (Mg(,2)SiO(,4)) between pressures of 150 GPa and 175 GPa range from 4500 K to 4950 K, a result which is consistent with occurrence of a polymorphic solid state transition accompanied by a substantial heat of transition ((TURN)1.5 MJ/kg). These results have potentially important implications for solid earth geophysics, and knowledge of the melting curves of candidate minerals of the earth's mantle provides some constraints on the geotherm. Hugoniot temperatures have been measured in liquid water between approximately 50 GPa and 80 GPa, with results ranging from 3500 K to 5400 K. The observed temperatures are well reproduced by theoretical calculations assuming a constant specific heat model, but further work is required to characterize fully the thermal variation of H(,2)O properties at high temperature. Compression measurements in pressure -volume states other than Hugoniot shock states and, in particular, in states of compression at constant entropy can provide both independent thermal equation-of-state information and the properties of high-density condensed phases inaccessible to shock wave experiments. Numerical calculations have been carried out for hypothetical experiments on water as well as carbon dioxide and liquid molecular hydrogen. The results of these calculations indicate that various experimental impact configurations may be employed to convert shock compression into isentropic compression, with pressures on the order of 100 GPa attained via impact velocities of a few kilometers per second. In the case of water, a net entropy production of a few percent of the Hugoniot entropy at the same pressure is predicted on the basis of these calculations.
- Pub Date:
- Physics: Condensed Matter