Matter at such high P- T conditions is of increasing scientific interest for several reasons: (i) understanding the physics of warm dense matter, (ii) understanding the interiors of giant exoplanets now being discovered, and (iii) the need for alternative sources of commercial energy. The challenge with fluid metals in the warm dense matter regime is to understand the correlated-electron-ion physics of partially ionized fluids, and to develop the equation-of-state (EOS) and transport models. Basically, fluid metals under multi-TPa shock pressures can approach or attain warm dense matter (WDM) states in which the potential energy of electron-ion interactions is comparable to the kinetic energies of electrons ( i.e., T/ T F ≈ 1). The temperature of Al shock-compressed to ~1 TPa, for example, reaches ~62,000 K. We here call this common fit the universal Hugoniot of fluid metals (UHFM). No significant deviations of those data from the common linear fit has been observed. The Al, Cu, Fe, and Mo Hugoniot data show, surprisingly, that all lie on a common straight line in shock velocity ( U s) - particle velocity ( u p) space, as a preliminary study found 9. This pressure range is well below that where shell-structure effects become significant for the shape of the Hugoniot curve. In contrast, we found another systematic behavior of fluid metals from the Hugoniot data for the typical metals in the range between sub-TPa and ~10 TPa. Predicted shell-structure effects on Hugoniots at ultrahigh shock pressures and temperatures have yet to be observed definitively one possible reason is the large errors in measured extreme shock pressures and volumes. Experimental Hugoniot data at ultrahigh shock pressures have been measured by several investigators using shock waves generated by nuclear and chemical explosives, by a giant laser and by magnetic-field driven hypervelocity impact for comparison with theoretical calculations. These theoretical Hugoniots have complex shapes in pressure-volume space, which derive from partial ionization of the atomic shells at pressures above 10 TPa. Hugoniots of elemental metals, such as Al, Cu, Fe, and Mo, have been calculated at shock pressures even up to 10 4 TPa 8. The locus of states achieved by a series of shock jumps, each starting from a given initial density to a sequence of ever-increasing shock pressures, is called a Rankine-Hugoniot curve or simply a Hugoniot. Substantially higher pressures and temperatures are achieved and measured with single-shock compression than with multiple-shock compression 6, 7. Electrical conductivities of those ultra-condensed degenerate fluids reach a common value, i.e., the minimum metallic conductivity (MMC), that results from strong scattering of the conduction electrons with mean-free path lengths comparable to interatomic distances 2, 5. At that H density, the Fermi temperature is T F ≈ 220,000 K and the degeneracy factor is T/ T F ≈ 0.014. Metallization of fluid H occurs at ~3,000 K and 0.64 mol H/cm 3, essentially the density predicted by Wigner and Huntington, 0.62 mol H/cm 3 4. Hydrogen, nitrogen, and oxygen completed crossovers from fluid semiconductor to fluid metal at extreme pressures of 100–140 GPa (1.0–1.4 Mbar) and temperatures of a few thousand kelvins 1, 2, 3. Similar content being viewed by othersĪt the end of the twentieth century, novel materials were being made systematically through multiple shock (quasi-isentropic) compression. The systematic behaviors of warm dense fluid would be useful benchmarks for developing theoretical equation-of-state and transport models in the warm dense matter regime in determining computational predictions. These results suggest that most fluid compounds, e.g., strong planetary oxides, reach a common state on the universal Hugoniot of fluid metals (UHFM) with MMC at sufficiently extreme pressures and temperatures. Above 0.75 TPa, the GGG Hugoniot data approach/reach a universal linear line of fluid metals, and the optical reflectivity most likely reaches a constant value indicating that GGG undergoes a crossover from fluid semiconductor to poor metal with minimum metallic conductivity (MMC). Shock-compression equation-of-state data and optical reflectivities of the fluid dense oxide, Gd 3Ga 5O 12 (GGG), were measured at extremely high pressures up to 2.6 TPa (26 Mbar) generated by high-power laser irradiation and magnetically-driven hypervelocity impacts. Materials at high pressures and temperatures are of great current interest for warm dense matter physics, planetary sciences, and inertial fusion energy research.
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