Session Q04: Materials under ramp compression

Dynamic Compression of Planetary Materials

Dynamic compression is a powerful means of studying the physical and chemical properties of materials under extreme conditions on time scales between nano- and micro-seconds. Dynamic compression allows for the study of a broad range of time-dependent condensed matter phenomena, such as structural phase transformations, inelastic deformation, and fracture. Furthermore, dynamic compression has been used to constrain the equation of state of materials to conditions that traditional static-compression techniques cannot reach. Recently, newly developed ramp-loading techniques using high-powered lasers and pulsed-power systems provide access to new regimes of pressure-temperature space. Ramp compression combined with in situ X-ray diffraction allows observation of the structural behavior, phase transitions, and kinetics of planetary materials at extreme conditions.

This talk will summarize our recent studies at the Omega laser and Z facility. At Omega, laser-driven ramp-compression experiments with in situ X-ray diffraction were performed to explore the structural behavior and phase transitions in germanium dioxide, GeO₂. For GeO₂ which serves as an analog for SiO₂, we have examined its crystal structure up to 882 GPa. X-ray diffraction results show that GeO₂ adopts the HP-PdF₂-type structure under ramp loading from 154 GPa to 440 GPa. Above 440 GPa, we observe evidence for a post-HP-PdF₂ phase in GeO₂. The best candidate for this new phase is the cotunnite-type structure. These results offer a test of theoretical calculations as well as insights into the possible high-pressure behavior of SiO₂. At the Z-facility, shock-compression experiments were performed on iron-bearing bridgmanite (Mg_0.92,Fe_0.08)SiO₃ in the range of ~450 GPa to constrain its melting point on the Hugoniot. Our Hugoniot sound velocity measurements will allow for interpreting the melting point of Mg-rich silicates. These experiments will offer insights into impact processes relevant to planetary formation and evolution.   

Structural Competitiveness in Ramp-Compressed Sodium

At high-energy-density conditions, a new realm of quantum behavior emerges including electron localization, structural complexity, core-electron chemistry, and more. Sodium (Na) behaves particularly bizarre at these conditions because of its very high compressibility. Normally a shiny ideal metal, Na transforms to a topological insulator at 200 GPa. This topologically insulating phase (hP4) is due to the valence electrons occupying interstitial positions of its crystalline lattice rather than the orbitals centered on ionic cores. Using lasers as high-pressure drivers, we report the structural and electronic properties of Na at the most extreme compressions yet studied. X-ray diffraction measurements to 480 GPa and 2000 K reveal unexpected new phases. Simultaneous reflectivity measurements suggest a dramatic drop in the conductivity of both the solid and fluid phases. These data together with ab initio evolutionary structure searches reveal a rich structural competitiveness than extends to greater than 300 GPa and thousands of degrees Kelvin. Recent experiments on ramp-compressed sodium at the National Ignition Facility will be discussed.   

Toward Accessing the Solid Metallic State of Hydrogen via Ramp Compression of Solid parahydrogen

Producing solid metallic hydrogen in a laboratory has been recognized as one of the grand challenges of physics¹. Theoretical work suggests that crystalline metallic hydrogen possesses a number of remarkable properties such as room-temperature superconductivity. The investigation of this material is also of fundamental interest for planetary science since metallic hydrogen is the primary constituent of gas giants.

            Recent studies² at the National Ignition Facility observed an insulator–metal transition during ramp compression of liquid hydrogen. To extend these experiments into the crystalline metallic regime, similar experiments with solid parahydrogen were conducted. With an initial temperature of 10 K, this target has minimal initial entropy, limiting the temperature increase during compression. The drive, consisting of an initial picket and a ramp, was tailored to a near-isentropic compression using the hydrocode HYADES. Pressure and density of the target were reconstructed from VISAR data³, while the temperature was measured using streaked optical pyrometry. Experimental results of a recent pilot study will be discussed.

Effect of artificially implanted helium bubbles on material strength in the high energy density regime

Understanding plastic deformation dynamics of materials under extreme conditions is of high interest to a number of fields, including meteor impact dynamics and advanced inertial confinement fusion. We infer the strength of samples at pressures up to 8 Mbar, strain rates of ~10⁷ s^-1, and high strains (> 30%) by measuring the growth of Rayleigh-Taylor instabilities (RTI) under ramped compression. We are now studying the dynamic response of materials that are aged by the radioactive alpha decay process. One consequence of alpha radiation is the formation of helium bubbles.  We fabricate lead samples that are artificially implanted with helium bubbles to study the effect of those bubbles on strength. We conducted side-by-side comparisons of pure versus helium-implanted lead samples using the NIF laser facility. Relative strength effects in the high energy density regime are rather different from those exhibited by irradiated materials in conventional stress-strain testing. Initial simulation results help to rationalize the experimental observations, with a porosity-mechanics-based model being used for the material containing helium bubbles.