Multimegabar Pressures Research Articles

Convenient optical pressure gauge for multimegabar pressures calibrated to 300GPa

The accurate measurement of pressure by a straightforward and inexpensive optical procedure has been needed in the multimegabar region since static pressures over 216GPa, 361GPa, 420GPa and 560GPa were obtained in the diamond anvil cell. Here, a simple optical pressure gauge based on the Raman shift of the diamond at the center of a diamond tip at the diamond–sample interface is calibrated against a primary gauge (Pt isotherm at 300K from shock data) to 300GPa, thus enabling researchers who do not have a synchrotron to conveniently measure pressure with an optical scale from 50to300GPa.

High-pressure Raman spectroscopy of diamond anvils to 250GPa: Method for pressure determination in the multimegabar pressure range

The first-order Raman spectra of diamond anvils used in a gasketed high-pressure cell have been measured at pressure up to 250GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the culet face, is represented by a function of pressure in the sample chamber up to 250GPa. The dependence is almost independent on loading conditions. The application of the pressure dependence for pressure determination up to the multimegabars pressure region is proposed.

New condensed matter probes for diamond anvil cell technology

An overview of several new developments in diamond anvil cell techniques is presented. Thenovel techniques developed in our laboratory address fundamental properties of compressedmaterials, e.g. vibrational, electronic and spin excitations. We focus on the techniques madepossible by third-generation synchrotron x-ray sources and by the development of themagnetic susceptibility probes in diamond anvil cells. A brief list of topics is given below:Application of nuclear resonant inelastic x-ray scattering technique to themeasurements of the phonon density of states and elastic properties ofiron-containing materials at megabar pressures.Magnetic collapse and high-spin to low-spin transitions probed by the x-rayK β emission process in transition metal compounds.Superconductivity in multi-megabar pressure range studied by the magneticsusceptibility technique.Selected applications of these techniques will be covered, including elastic properties ofcompressed hcp-Fe, insulator–metal transition in FeO and superconductivity in Li.

Self-consistent, two-dimensional, magnetohydrodynamic simulations of magnetically driven flyer plates

The intense magnetic field generated by the 20 megaampere Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories is being used as a pressure source for material science studies. An application we have studied in great detail involves using the intense magnetic field to accelerate flyer plates (small metal disks) to very high velocities (>20 km/s) for use in shock loading experiments. We have used two-dimensional (2D) magnetohydrodynamic (MHD) simulation to investigate the physics of accelerating flyer plates using multi-megabar magnetic drive pressures. A typical shock physics load is comprised of conducting electrodes that are highly compressible at multi-megabar pressures. Electrode deformation that occurs during the rise time of the current pulse causes significant inductance increase, which reduces the peak current (drive pressure) relative to a static geometry. This important dynamic effect is modeled self-consistently by driving the MHD simulation with an accurate circuit model of Z. Self-consistent, 2D, MHD simulations are able to produce and predict time resolved velocity interferometry measurements when the drive circuit includes models of current losses and short circuiting in Z. Simulation results elucidate the phenomena contributing to the flyer velocity history, and show that electrical and hydrodynamic optimization of the load are necessary to minimize effects of time varying inductance. Details of the modeling, the physics, and comparisons with experiment are presented.

The equation of state of Bi and cross-checking of Au and Pt scales to megabarpressure

By means of x-ray diffraction experiments using a synchrotron radiationsource, the equation of state (EOS) of body-centred cubic Bi has beeninvestigated over a range of multimegabar pressure up to 222 GPa on thebasis of the Pt pressure scale and it is proposed as an internal pressurestandard over a range of megabar pressures. Pressure scales of the EOS of Auand Pt were cross-checked and the accuracy of the scales is discussed.

Impedance match equation of state experiments using indirectly laser-driven multimegabar shocks

Measurements of equation of state (EOS) points on the principal Hugoniots of Cu, Au, Pb and the plastics Parylene-C and brominated CH at multimegabar pressures have been made using the 1 TW HELEN laser at AWE. The aim was 1% accuracy in shock velocity measurement (3%–4% in pressure) in order to compare with data from gas-gun and nuclear underground test experiments and the theoretical EOS’s based on, or supported by, these data. Experiments comprised a hohlraum heated by two 500 J, 0.53 μm wavelength, 1 ns Gaussian laser pulses generating an x-ray flux which drove a shock into a target consisting of a base, with steps of a known EOS material and of the material of unknown EOS. Shock breakout from base and steps was detected by monitoring light emission from the target with optical streak cameras and shock velocities were derived from the transit times across the known-height steps.

Superconductivity in the chalcogens up to multimegabar pressures

Highly sensitive magnetic susceptibility techniques were used to measure the superconducting transition temperatures in S up to 231($\pm$5) GPa. S transforms to a superconductor with T$_c$ of 10 K and has a discontinuity in T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition. Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200 GPa and then slowly decreases with pressure to 15 K at 230 GPa. This trend in the pressure dependence parallels the behavior of the heavier members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K. Similiarities in the T_c dependences for S, Se, and Te, and the implications for oxygen are discussed.

In situ X-Ray study of thermal expansion and phase transition of iron at multimegabar pressure

The density of varepsilon-iron has been calculated at pressures and temperatures up to 300 GPa and 1300 K, respectively. We observe varepsilon to beta phase transition at pressures between 135 and 300 GPa and temperature above 1350 K; the pattern can be interpreted in terms of double hexagonal close-packed structure. The density calculated at high pressure and temperature (330-360 GPa and 5000-7000 K) closely matches with preliminary reference Earth model density, thereby imposing constraint on the composition of the Earth's inner core.

Use of focused ion beams for making tiny sample holes in gaskets for diamond anvil cells

To achieve multimegabar pressures in the diamond anvil cell, small diamond tips, 20 μm (or less) in diameter and high strength gasket materials are required. To prevent plastic instability it is therefore necessary to drill sample holes with diameters of 10 μm (or less) in extremely strong and tough materials such as tungsten. The present paper describes a technique for drilling such holes using focused ion beams. The superior roundness and surface finish of these holes is one of the reasons our group was able to reach pressures of 342 GPa on hydrogen, significantly higher than that reached by other researchers.

Finite-element modeling of diamond deformation at multimegabar pressures

Finite-element modeling calculations reveal the origin of the remarkably large elastic strains in diamond observed in recent experiments at multimegabar pressures. This approach provides a means to determine the pressure dependence of the yield strength of strong materials used in the gasket, and allows us to examine quantities that are not accessible experimentally such as the stress and strain relations in diamond. Stress tensor elements are obtained near the tip where large modifications in the optical properties of diamond have been observed.

In-situ x-ray study of the thermal expansion of iron at multimegabar pressure

The density of epsilon-iron was determined as a function of pressure, p, and temperature, T, at pressures up to 240 GPa and at temperatures ranging up to 1285 K by means of an externally electrically heated diamond anvil cell and in-situ x-ray powder diff

Excitation of ruby fluorescence at multimegabar pressures

We demonstrate the excitation of strong ruby fluorescence at multimegabar pressures by red incident radiation. We have studied the ruby R1 and N2 lines to 251 GPa and measured the R1 line intensity as a function of pump wavelength. This new method of exciting ruby fluorescence enables easy pressure determination at pressures beyond the 250 GPa limit previously predicted by Ruoff.

Pressure strengthening: A way to multimegabar static pressures.

This paper shows why a static pressure of several megabars can be reached in a diamond anvil cell using a molybdenum gasket whose yield strength at P=0 is only 0.04 Mbars. The elastic constants of bcc Mo at 0 K vs reduced volume and pressure were otained from first-principle, all-electron, density-functional calculations to V/${\mathit{V}}_{0}$=0.4 (P=11.6 Mbars) although the fcc phase was calculated to be more stable above 6 Mbars. These constants were used to obtain the Chua and Ruoff pressure scaling factor for the yield strength which combined with plasticity theory shows that a gasket of 2.6 \ensuremath{\mu}m final thickness can support 6 Mbars.

Equation of state and shear strength at multimegabar pressures: Magnesium oxide to 227 GPa.

The equation of state, elasticity, and shear strength of magnesium oxide were examined to 227 GPa using synchrotron x-ray diffraction in a diamond anvil cell. Static compression, ultrasonic elasticity, and shock data for MgO from ambient pressure to above 200 GPa can all be described by a single Birch-Murnaghan equation of state when the static shear strength, which is determined to be at least 11 GPa at 227 GPa, is taken into account. Our results show that there are significant changes in the degree and character of the elastic anisotropy of MgO at high pressure.

Production and metrology of 5 μm x-ray apertures for 100 keV diffraction studies in the diamond anvil cell

Microminiaturization of the diamond anvil tip is necessary in order to obtain multimegabar pressures. Because of this and the steep pressure gradients present tiny x-ray beams are required. This paper describes the production and metrology of 5 μm x-ray apertures for x-ray diffraction studies to 100 keV in the diamond anvil cell. It is expected that this design will serve as the basis for 2 μm apertures.

Phase transitions and equations of state at multimegabar pressures

Abstract Review of phase transitions and equations of state at multimegabar pressures (100–300 GPa) is presented. Energy dispersive x-ray diffraction techniques in conjunction with synchrotron radiation sources are used. Besides several transition metals, Pt to 282 GPa, Re to 251 GPa, W to 209 GPa, and Fe to 255 GPa, the special focus is on Group IVA elements and isoelectronic III-V compounds. At high pressure, the isoelectronic materials are isostructural and exhibit similar equation of state.

Synthetic diamonds for multimegabar pressures in the diamond anvil cell

Abstract Synthetic Type 1b yellow diamonds containing nitrogen in substitutional form and with extremely low birefringence were used as anvils for ultra high pressures in the diamond anvil cell. Pressures were measured by the ruby fluorescence technique to above 200 GPa. Using x-ray diffraction the maximum pressure was 210 GPa, while an x-ray based pressure of 245 GPa was achieved with natural diamonds with a somewhat more optimal geometry. Nitrogen platelets appear to be not essential for exceeding 200 GPa. The optical properties of synthetic diamond at ambient and megabar stresses will be discussed.

Multimegabar pressures using synthetic diamond anvils

Synthetic type 1B yellow diamonds containing nitrogen in single substitutional form, with extremely low birefringence, were used as anvils in a diamond anvil cell. The diamonds were well characterized by infrared and visible absorption spectra. Sensitive spectroscopic methods along with mechanical chopping techniques were used to record ruby fluorescence to 250–300 GPa, a pressure equal to or exceeding that attained with natural diamonds. Nitrogen platelets are not essential to attaining multimegabar pressures.

Pressure profiles at multimegabar pressures in a diamond anvil cell using x-ray diffraction

Pressure distributions in a diamond anvil cell with a rhenium gasket have been measured at various pressures up to 212±6 GPa using energy dispersive x-ray diffraction with a synchrotron source. Three sets of type IA yellow diamonds were used with bevels of 5°, 7°, and 10°. For the 7°-beveled tips, a 5-μm-diam collimated beam was used to a pressure of 206±6 GPa. In the other experiments, collimators of 10–30 μm were used. In the region of the 50-μm central flat, the pressure was essentially uniform. The effect of finite collimator size on the measurement of pressure profiles is also analyzed.

Statistical Mechanics of Light Elements at High Pressure. I. Theory and Results for Metallic Hydrogen with Simple Screening

view Abstract Citations (59) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Statistical Mechanics of Light Elements at High Pressure. I. Theory and Results for Metallic Hydrogen with Simple Screening Hubbard, W. B. ; Slattery, Wayne L. Abstract A numerical algorithm for calculating the thermodynamic state variables of light elements at multimegabar pressures and a wide range of temperatures is presented. The technique uses linear-response (dielectric function) theory to calculate the electron-density distribution as a function of ion configuration, and ensemble averages are evaluated by the use of Monte Carlo chains. A number of results are presented for liquid metallic hydrogen at densities of 2.679 and 2679 g and temperatures ranging from 313000 to 41700 K. Publication: The Astrophysical Journal Pub Date: August 1971 DOI: 10.1086/151068 Bibcode: 1971ApJ...168..131H full text sources ADS | Related Materials (2) Part 2: 1972ApJ...176..525H Part 3: 1973ApJ...181.1031S