published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

A novel approach to study electron thermal transport in dense plasmas was successfully implemented to measure the temperature-dependent conductivity and test the currently available dense plasma model by Lee and More. Intense, femtosecond laser pulses with energy up to 7 mJ per pulse were used to heat free-standing 170-370 nm aluminum foils. We carried-out a new approach to study the plasma transport properties of electron and thermal conduction. In this new approach, rather than probing the front (laser-heated) surface, probing was done on the back surface of a thicker metallic foil heated by a thermal conduction wave generated from a laser-heated front surface. Frequency-domain interferometry with chirped probe pulses allowed us to simultaneously measure the time-dependence of the optical reflectivity and phase-shift in a single shot with subpicosecond resolution. In addition, solid heating was observed to be dominated by the thermal conduction wave prior to the shock-wave breakout at the back surface when laser energy was directly deposited in a thin metallic foil. As a result we were able to estimate the optical conductivity of a dense aluminum plasma in the range of 0.1 – 1.5 eV. The optical parameters were calculated using the output of a hydrodynamic simulation along with the published models of bound electron contributions to the conductivity and were found to be in reasonable agreement with the measurement. We found that the Lee and More model of a dense plasma’s conductivity predicts the real and imaginary part of the measured optical conductivity to within 20%. The simulation results were then used to examine the temperature dependence of the conductivity for 170 and 230 nm aluminum foils heated with the 2-5 mJ pulses. In all cases the same conductivity was obtained, though the arrival of the heat wave and subsequent shock waves varied with the choice of intensity and target thickness. This consistency in the data gave us good confidence in the validity of this technique for deriving conductivity as a function of temperature. / text

Dynamic abnormal grain growth (DAGG) is a phenomenon that produces abnormal grain growth at elevated temperatures during plastic deformation. It is distinct from classically studied static abnormal grain growth phenomena in that it only occurs during plastic deformation. Previous investigations of DAGG in a Mo sheet material produced using powder metallurgy techniques observed DAGG grains to grow more rapidly near the sheet surface than near the sheet center. This phenomenon is explored in the present study. A Mo sheet material produced using arc melting techniques is also studied to determine the morphology of DAGG grains. A preference for growth near the sheet center is observed in this material. The through-thickness variations in texture and grain size for both the arc-melted and powder-metallurgy Mo sheet materials are investigated. The preference for growth near the surface in the powder-metallurgy material is due to a through-thickness variation in grain size, with smaller grains near the surface and larger grains near the center. The preference for DAGG grain growth at the center of the arc-melted sheet material is because of very large grains that grow near the sheet surface. These large grains may be the product of multiple abnormal grains occurring near the sheet surface because of texture variation through the sheet thickness. Regardless, the DAGG grain cannot consume these large grains and leaves them as island grains decorating the region near the sheet surface. These results suggest that DAGG is driven primarily by grain boundary curvature. Microstructures that include DAGG grains are investigated with electron backscatter diffraction (EBSD). A new method to evaluate geometrically necessary dislocation densities using EBSD data is derived. DAGG grains are relatively undeformed compared to the polycrystalline microstructure. DAGG grains are not oriented either favorably or unfavorably for slip. Results of the analysis of the grain boundaries between DAGG grains and normal grains do not indicate any special character preference for these grain boundaries. / text

Polarization plays a significant role in the physical and chemical properties of water, thus polarizable water models have been extensively evolved and studied in the past several decades. In this dissertation, two polarizable water models have been extended, and some physical properties in gas phase and condensed phase were studied and analyzed. It was verified that the out-of-plane polarization effect is of great importance in some physical properties. Besides, we proved that there are some connections between these two models, although they were derived from different methodologies. The first polarizable water model we developed was a combination of charge response kernel (CRK) method and polarizable point dipole (PPD) method. In the CRK method, a CRK matrix is defined as the second order derivative of energy with respect to the external potential at atomic sites. It is applied to represent the intensity of charge response to external environment. While in the PPD method, the polarizability tensor which is the second order derivative of energy with respect to external field at the same site, is introduced to characterize the variation of dipole moment in the presence of external perturbation. In our method, we proved that although the CRK matrix of three-site water model has 9 element, it only carries two independent variables, and these two variables only rely on the water geometry and the in-plane polarizability. Thus besides the CRK matrix located on each atomic site, an additional polarizability residing on oxygen atom specifically inducing dipole moment along the direction perpendicular to the water plane was added in our model. With the addition of the out-of-plane description, some physical properties were much enhanced. In the second polarizable water model we extended, electronegativity equalization (EE) method was employed. In this three-site water model, atomic electronegativity and hardness matrix were the first and second order derivative of energy with respect to the partial charge on atomic sites, respectively. In this method, electronegativity differs among different atom types, and the off-diagonal elements in hardness matrix are related on not only atom types but also distances among the corresponding atoms. Accordingly, the intramolecular water deformation can be included. Thus flexible polarizable water model is accessible. With flexibility, this water model is more realistic. Our model validated that more flexible parameterization and geometry could improve the physical performance. At last, we connected the second polarizable water model with the first one. Although the two polarizable models were derived from different methodologies, we proved that under one simple approximation, corresponding CRK matrix can be achieved from hardness matrix. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy