Neil C. Holmes

The equation of state of platinum to 660 GPa (6.6 Mbar)

Platinum metal was shock compressed to 660 GPa using a two‐stage light‐gas gun to qualify this material as an ultrahigh‐pressure standard for both dynamic and static experiments. The shock velocity data are consistent with most of the previously measured low‐pressure data, and an overall linear us−up relationship is found over the range 32–660 GPa. As a part of this work, we have also extended the Hugoniot of the tantalum standard we use to 560 GPa; we have included these data into a new linear fit of the tantalum Hugoniot between 55–560 GPa. We also present the results of a first‐principles theoretical treatment of compressed platinum. The fcc phase is predicted to remain stable to beyond 550 GPa. In addition, we have calculated the 300‐K pressure‐volume isotherm and the Hugoniot. The latter is in excellent agreement with experimental results and qualifies the former to at least 10% accuracy.

Melting of iron at the physical conditions of the Earth's core.

Seismological data can yield physical properties of the Earths core, such as its size and seismic anisotropy. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions. We show that melting starts at 225 ± 3 GPa (5,100 ± 500 K) and is complete at 260 ± 3 GPa (6,100 ± 500 K), both on the Hugoniot curve—the locus of shock-compressed states. This new melting pressure is lower than previously reported, and we find no evidence for a previously reported solid–solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).

Equation of state of Al, Cu, Mo, and Pb at shock pressures up to 2.4 TPa (24 Mbar)

Equation‐of‐state data and corresponding first‐principles theory for the metals Al, Cu, Mo, and Pb are reported over the shock pressure range 0.4–2.4 TPa (4–24 Mbar). Strong shock waves were generated by nuclear explosions and a two‐stage light‐gas gun. The experimental data occur in the hot liquid‐metal regime, where condensed‐matter theory applies but with unusually large thermal components to the equation of state.

Equation-of-state, shock-temperature, and electrical-conductivity data of dense fluid nitrogen in the region of the dissociative phase transition

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 g/cm3, and internal energies up to 1 MJ/mol was investigated by shock compression. Equation‐of‐state, shock‐temperature, and electrical‐conductivity experimental data are presented and analyzed in detail.

Anisotropic shock sensitivity and detonation temperature of pentaerythritol tetranitrate single crystal

Shock temperatures of pentaerythritol tetranitrate (PETN) single crystals have been measured by using a nanosecond time-resolved spectropyrometric system operated at six discrete wavelengths between 350 and 700 nm. The results show that the shock sensitivity of PETN is strongly dependent on the crystal orientation: Sensitive along the shock propagation normal to the (110) plane, but highly insensitive normal to the (100) plane. The detonation temperature of PETN is, however, independent from the crystal orientation and is determined to be 4140 (±70) K. The time-resolved data yielding the detonation velocity 8.28 (±0.10) mm/μs can be interpreted in the context of a modified thermal explosion model.

Temperature measurements of shock-compressed liquid hydrogen: implications for the interior of Jupiter

Shock temperatures of hydrogen up to 5200 kelvin were measured optically at pressures up to 83 gigapascals (830 kilobars). At highest pressures, the measured temperatures are substantially lower than predicted. These lower temperatures are caused by a continuous dissociative phase transition above 20 gigapascals. Because hydrogen is in thermal equilibrium in shock-compression experiments, the theory derived from the shock data can be applied to Jupiter. The planets molecular envelope is cooler and has much less temperature variation than previously believed. The continuous dissociative phase transition suggests that there is no sharp boundary between Jupiters molecular mantle and its metallic core. A possible convectively quiescent boundary layer might induce an additional layer in the molecular region, as has been predicted.

Equation of state of shock-compressed liquids: Carbon dioxide and air

Equation‐of‐state data were measured for liquid carbon dioxide and air shock‐compressed to pressures in the range 28–71 GPa (280–710 kbar) using a two‐stage light‐gas gun. The experimental methods are described. The data indicate that shock‐compressed liquid CO2 decomposes at pressures above 34 GPa. Liquid air dissociates above a comparable shock pressure, as does liquid nitrogen. Theoretical intermolecular potentials are derived for CO2 from the data. The calculated shock temperature for the onset of CO2 decomposition is 4500 K at a volume of 17 cm3/mol.

Equation of state measurements of hydrogen isotopes on Nova

High intensity lasers can be used to perform measurements of materials at extremely high pressures if certain experimental issues can be overcome. We have addressed those issues and used the Nova laser to shock-compress liquid deuterium and obtain measurements of density and pressure on the principal Hugoniot at pressures from 300 kbar to more than 2 Mbar. The data are compared with a number of equation of state models. The data indicate that the effect of molecular dissociation of the deuterium into a monatomic phase may have a significant impact on the equation of state near 1 Mbar.

Equation of state of beryllium at shock pressures of 0.4–1.1 TPa (4–11 Mbar)

High-pressure shock Hugoniot data were measured for Be using strong shock waves generated by underground nuclear explosions. These data and a preliminary theoretical analysis are reported.

High-pressure tailored compression: Controlled thermodynamic paths

We have recently carried out exploratory dynamic experiments where the samples were subjected to prescribed thermodynamic paths. In typical dynamic compression experiments, the samples are thermodynamically limited to the principal Hugoniot or quasi-isentrope. With recent developments in a functionally graded material impactor, we can prescribe and shape the applied pressure profile with similarly shaped, nonmonotonic impedance profile in the impactor. Previously inaccessible thermodynamic states beyond the quasi-isentropes and Hugoniot can now be reached in dynamic experiments with these impactors. In the light gas gun experiments on copper reported here, we recorded the particle velocities of the Cu–LiF interfaces and have employed hydrodynamic simulations to relate them to the thermodynamic phase diagram. Peak pressures for these experiments are on the order of megabars, and the time scales range from nanoseconds to several microseconds. The strain rates of these quasi-isentropic experiments are approx...