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Thesis (Ph. D.)--University of Rochester. Department of Mechanical Engineering, 2016., The behavior of highly compressed material is of interest to planetary astrophysics, geophysics, inertial confinement fusion and stewardship science. This behavior is described by the high-pressure equation of state. Traditionally, experiments investigating the equation of state have used strong shock waves to compress the material, reaching high temperatures, pressures, and entropy. This is because shock waves are readily produced using a variety of drivers. However, the region of the thermodynamic plane accessible by a single shock is limited to the curve defined by the end states of a shock – the principal Hugoniot. Understanding the off-Hugoniot behavior is necessary to describing planetary interiors and reversible processes that compress a material without significantly increasing its entropy. The sound velocity and Grüneisen parameter of a compressed material provide information that, when coupled with the Hugoniot, can be used to model the behavior of a material off its principal Hugoniot. This work describes the first experimental measurements using a recently published technique to measure the sound velocity and Grüneisen parameter of shocked materials by tracking perturbations that propagate through the material. By relating the timing and amplitude of the perturbations to a standard material under identical drive conditions, the sound velocity and Grüneisen parameter can be referenced to the standard. These measurements demonstrate that the method is consistent with existing equation of state tables and previous data. To establish a standard for this method, a technique was developed to determine the sound velocity and Grüneisen parameter of α-quartz from an analytic model for the release isentrope. The near-impedance match release method was developed as an extension to impedance matching, allowing absolute sound velocity measurements in α-quartz. The sound velocity and Grüneisen parameter of fused silica, MgO, and polystyrene were investigated

Thesis (Ph. D.)--University of Rochester. Dept. of Mechanical Engineering, 2011., The compression of materials to high pressure can alter their optical properties in ways that provide insight into the resulting structural changes. Under strong shock compression, transparent insulators transform into conducting fluids as a result of pressure-induced reduction of the band gap and thermal promotion of electrons across that gap. A new ramp compression technique; direct-drive shaped ablation, is used to compress LiF to 800 GPa without generating shocks thereby producing high pressures at significantly lower temperatures than would be created by shock waves. In this study, ramp compressed lithium fluoride (LiF) is observed to remain transparent to 800 GPa, pressures seven times higher than previous shock compression experiments. The ramp compressed refractive index of LiF is measured at pressures up to 800 GPa and depends linearly on density over this range. This is the highest pressure refractive index measurement made to date. The linear dependence of the refractive index and density is examined using a single-oscillator model. This model indicates that the linear behavior is a result of monoatomic closure of the band gap. Extrapolation of these results indicates that the band gap closure (metallization) will be greater than 5,000 GPa, well above the Goldhammer-Herzfeld criterion for metallization (∼ 2,860 GPa). The high metallization pressure of LiF is attributed to its large band gap and isoelectronic counterparts that exhibit high metallization pressures. The high pressure transparency of LiF has technical utility as an optical window for materials studies since the transparency at high pressure allows in situ measurements of samples confined by that window. The observed transparency and measurement of LiF refractive index enables advancement of those experiments to higher pressure regimes.