The dissociative phase transition of fluid nitrogen at pressures in the range 30-110 GPa (0.3-1.1 Mbar) , temperatures in the range 4000-14 000 K, densities up to 3.5 gicm 3 , and internal energies up to 1 MJ/mol was investigated by shock compression. Equation-of-state, shocktemperature, and electrical-conductivity experimental data are presented and analyzed in detail.
Data from the Voyager II spacecraft showed that Uranus has a large magnetic field with geometry similar to an offset tilted dipole. To interpret the origin of the magnetic field, measurements were made of electrical conductivity and equation-of-state data of the planetary "ices" ammonia, methane, and "synthetic Uranus" at shock pressures and temperatures up to 75 gigapascals and 5000 K. These pressures and temperatures correspond to conditions at the depths at which the surface magnetic field is generated. Above 40 gigapascals the conductivities of synthetic Uranus, water, and ammonia plateau at about 20(ohm-cm)(-1), providing an upper limit for the electrical conductivity used in kinematic or dynamo calculations. The nature of materials at the extreme conditions in the interior is discussed.
Equation-of-state, temperature, and electrical-conductivity data were measured for a solution of water, ammonia, and isopropanol at shock pressures up to 200 GPa. The chemical composition is similar to that of the fluid mixture thought to be the major constituent of the giant planets Uranus and Neptune.
Radiative temperatures and electrical conductivities were measured for fluid nitrogen compressed dynamically to pressures of 18-90 GPa (180-900 kbar), temperatures of 4000-14000 K, and densities of 2-3 g/cm 3 . The data show a continuous phase transition above 30 GPa shock pressure and confirm that (BP/BT) V <0, as indicated previously by Hugoniot equation-of-state experiments. The first observation of shock-induced cooling is also reported. The data are interpreted in terms of molecular dissociation and the concentration of dissociated molecules is calculated as a function of density and temperature.PACS numbers: 64.70JaDiatomic molecules such as nitrogen and hydrogen are ideal systems for the testing of our understanding of condensed matter at extreme pressures, densities, and temperatures. Questions of greatest interest are the conditions required to metallize the insulator phase and the role played by dissociation to the monatomic state. We recently observed a phase transition in equation-of-state data for fluid nitrogen above 30 GPa (300 kbar) shock pressure, 2 g/cm 3 , and a calculated temperature of 6000 K. 1 By comparison of the shock-compression curves (principal Hugoniots) for liquid N2 and isoelectronic CO, the data were interpreted in terms of a continuous dissociative transition to the monatomic state. This is physically reasonable because at 30 GPa N2 is extremely dense, and the volume of monatomic nitrogen is much less than for the molecular state. The equation-of-state data included the first observation of double-shock points lying above the principal Hugoniot in pressure-volume space. These data showed that (BP/BE ) v < 0 and indicated that (BP/BT) V <0 in the phase transition region 1 since c v is always positive; that is, that as pressure increases above the Hugoniot the temperature should decrease at specific volumes in the transition region.In order to verify the observation of the phase transition and to verify that (BP/BT) V < 0, we have measured both the single-and double-shock temperatures of liquid nitrogen. The data confirm that at fixed volume the double-shock temperatures are lower at pressures above the principal Hugoniot. In addition, we have observed an even more remarkable result: the first observation of shock-induced cooling. Irreversible shock energy normally heats a material. However, if liquid N2 is first shockcompressed into the phase transition region, the specimen cools instantly (< 10 nsec) on reshock against a transparent AI2O3 or LiF window. Shock temperatures of liquid nitrogen were measured previously, but only up to 20 GPa. 2 In order to investigate possible metallization, we have also measured the electrical conductivity of shocked liquid nitrogen, which becomes quite large, reaching 50 (n cm)" 1 at 60 GPa and 12000 K. All these results are consistent with a continuous transition in which energy is absorbed in dissociation and ionization.The observation of a phase transition in shocked nitrogen led to a theoretical prediction that at 0 K solid N2 might transform to...
Electrical conductivities were measured for methane, benzene, and polybutene shock compressed to pressures in the range 20 to 60 GPa (600 kbar) and temperatures in the range 2000 to 4000 K achieved with a two-stage light-gas gun. The data for methane and benzene are interpreted simply in terms of chemical decomposition into diamondlike, defected C nanoparticles and fluid H2 and their relative abundances (C:H2), 1:2 for methane and 2:1 for benzene. The measured conductivities suggest that conduction flows predominately through the majority species, H2 for methane and C for benzene. These data also suggest that methane is in a range of shock pressures in which dissociation increases continuously from a system which is mostly methane to one which has a substantial concentration of H2. Thermal activation of benzene conductivities at 20–40 GPa is probably caused by thermal activation of nucleation, growth, and connectivity of diamondlike, defected C nanoparticles. At 40 GPa the concentration of these C nanoparticles reaches a critical density, such that further increase in density does not have a significant affect on the cross-sectional area of conduction and, thus, conductivity saturates. The electrical conductivity of polybutene (1:1) is very low. While the mechanism is unknown, one possibility is that the electronic bandgap of whatever species are present is large compared to the temperature. Electrical conductivity measurements are proposed as a way to determine the melting curve of diamondlike C nanoparticles at 100 GPa pressures.
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