The elastic constants of Mg(1-x)AlxB2 have been calculated in the regime 0<x<0.25. The calculations show that the ratio, B/G, between the bulk- and the shear-modulus stays well below the empirical ductility limit, 1.75, for all concentrations, indicating that the introduction of Al will not change the brittle behaviour of the material considerably. Furthermore, the tetragonal elastic constant C’ has been calculated for the transition metal alloys Fe-Co, Mo-Tc and W-Re, showing that if a suitable tuning of the alloying is made, these materials have a vanishingly low C'. Thermal expansion calculations of the 4d transition metals have also been performed, showing good agreement with experiment with the exception of Nb and Mo. The calculated phonon dispersions of the 4d metals all give reasonable agreement with experiment. First principles calculations of the thermal expansion of hcp Ti have been performed, showing that this element has a negative thermal expansion along the c-axis which is linked to the closeness of the Fermi level to an electronic topological transition. Calculations of the EOS of fcc Au give support to the suggestion that the ruby pressure scale might underestimate pressures with ~10 GPa at pressures ~150 GPa. The high temperature bcc phase of the group IV metals has been calculated with the novel self-consistent ab-initio dynamical (SCAILD) method. The results show good agreement with experiment, and the free energy resolution of < 1 meV suggests that this method might be suitable for calculating free energy differences between different crystallographic phases as a function of temperature.

Acoustic plane wave intearctions with a rough seabed with a continuously varying density and sound speed in a fluid-like sediment layer overlying an elastic basement is considered in this thesis. The acoustic properties in the sediment layer possess an exponential type of variation in density and one of the three classes of sound speed profiles, which are constant, k^2-linear, or inverse-square variations. Analytical solutions for the Helmholtz equation in the sediment layer, combined with a formulation based upon boundary perturbation theory, facilitate numerical implementation for the solution of coherent field. The coherent reflection coefficients corresponding to the aformentioned density and sound speed profiles for various frequencies, roughness parameters, basement stiffness, are numerically generated and analyzed. Physical interpretations are provided for various results. This simple model characterizes three important features of an realistic sea floor, including seabed roughness, sediment inhomogenieties, and basement shear property,%Two dimensions is considered in the seafloor environment and the random roughness is belong to one dimension space.% , therefore, provides a canonical model for the study of seabed acoustics. The variation of the acoustic properties takes such a form that it is not only geologically realistic, but also renders analytical solutions for the Helmholtz equation, thus facilitating the formulation of the problem. The computational algorithm for the spatial spectrum of the scattered field due to random seabed has been developed based upon a boundary perturbation method. %About scattering field, only one time reflection from the sediment is taked account of, because the higher numerical order is, the lower scattering energy exist.% The results have shown that, while the coherent field mainly depends upon the gross structure of the rough seabed represented by the RMS roughness, the scattered field heavily depends upon the details of the roughness structure specialized by the roughness power spectrum and the spatial correlation length of the rough surface. The dependence of the spatial spectrum on the sediment stratification is also carefully examined.

No description available.

Worldwide interest in gas production from shale formations has rapidly increased in recent years, mostly by the successful development of gas shales in North America. The Haynesville Shale is a productive gas shale resource play located in Texas and Louisiana. It produces primarily through enhanced exposure to the reservoir and improved permeability resulting from horizontal drilling and hydraulic fracturing. Accordingly, it is important to estimate the reservoir properties that influence the elastic and geomechanical properties from seismic data. This thesis estimates pore shapes, which affect the transport, elastic, and geomechancial properties, from wellbore seismic velocity in the Haynesville Shale. The approach for this work is to compare computed velocities from an appropriate rock physics model to measured velocities from well log data. In particular, the self-consistent approximation was used to calculate the model-based velocities. The Backus average was used to upscale the high-frequency well log data to the low-frequency seismic scale. Comparisons of calculated velocities from the self-consistent model to upscaled Backus-averaged velocities (at 20 Hz and 50 Hz) with a convergence of 0.5% made it possible to estimate pore aspect ratios as a function of depth. The first of two primary foci of this approach was to estimate pore shapes when a single fluid was emplaced in all the pores. This allowed for understanding pore shapes while minimizing the effects of pore fluids. Secondly, the effects of pore fluid properties were studied by comparing velocities for both patchy and uniform fluid saturation. These correspond to heterogeneous and homogeneous fluid mixing, respectively. Implementation of these fluid mixtures was to model them directly within the self-consistent approximation and by modeling dry-rock velocities, followed by standard Gassmann fluid substitution. P-wave velocities calculated by the self-consistent model for patchy saturation cases had larger values than those from Gassmann fluid substitution, but S-wave velocities were very similar. Pore aspect ratios for variable fluid properties were also calculated by both the self-consistent model and Gassmann fluid substitution. Pore aspect ratios determined for the patchy saturation cases were the smallest, and those for the uniform saturation cases were the largest. Pore aspect ratios calculated by Gassmann fluid substitution were larger because the velocity is inversely related to the aspect ratio in this particular modeling procedure. Estimates of pore aspect ratios for uniform saturation were 0.051 to 0.319 with the average of 0.171 from the velocity modeling using the self-consistent model. For patchy saturation, the aspect ratios were 0.035 to 0.296 with a mean of 0.145. These estimated pore aspect ratios from the patchy saturation case within the self-consistent model are considered the most reasonable set of values I determined. This is because the most likely in-situ fluid distribution is heterogeneous due to the extremely low permeability of the Haynesville Shale. Estimated pore aspect ratios using this modeling help us to understand elastic properties of the Haynesville Shale. In addition, this may help to find zones that correspond to optimal locations for fracturing the shale while considering brittleness and in-situ stress of the formation. / text

The objectives of this dissertation were to find a principal domain of promising and technologically feasible reactor physics characteristics for a multi-purpose, modular-sized, lead-cooled, fast neutron spectrum reactor fueled with an advanced uranium-transuranic-nitride fuel and to determine the principal limitations for the design of an autonomous long-term multi-purpose fast reactor (ALM-FR) within the principal reactor physics characteristic domain. The objectives were accomplished by producing a conceptual design for an ALM-FR and by analysis of the potential ALM-FR performance characteristics. The ALM-FR design developed in this dissertation is based on the concept of a secure transportable autonomous reactor for hydrogen production (STAR-H2) and represents further refinement of the STAR-H2 concept towards an economical, proliferation-resistant, sustainable, multi-purpose nuclear energy system. The development of the ALM-FR design has been performed considering this reactor within the frame of the concept of a self-consistent nuclear energy system (SCNES) that satisfies virtually all of the requirements for future nuclear energy systems: efficient energy production, safety, self-feeding, non-proliferation, and radionuclide burning. The analysis takes into consideration a wide range of reactor design aspects including selection of technologically feasible fuels and structural materials, core configuration optimization, dynamics and safety of long-term operation on one fuel loading, and nuclear material non-proliferation. Plutonium and higher actinides are considered as essential components of an advanced fuel that maintains long-term operation. Flexibility of the ALM-FR with respect to fuel compositions is demonstrated acknowledging the principal limitations of the long-term burning of plutonium and higher actinides. To ensure consistency and accuracy, the modeling has been performed using state-of-the-art computer codes developed at Argonne National Laboratory. As a result of the computational analysis performed in this work, the ALM-FR design provides for the possibility of continuous operation during about 40 years on one fuel loading containing mixture of depleted uranium with plutonium and higher actinides. All reactor physics characteristics of the ALM-FR are kept within technological limits ensuring safety of ultra-long autonomous operation. The results obtained provide for identification of physical features of the ALM-FR that significantly influence flexibility of the design and its applications. The special emphasis is given to existing limitations on the utilization of higher actinides as a fuel component.

Cette thèse est principalement consacrée à l'étude des inhomogénéités de taille nanométrique dans les systèmes magnétiques désordonnés ou dilués. La présence d'inhomogénéités, souvent mise en évidence dans de nombreux matériaux, donne lieu à des propriétés physiques intéressantes et inattendues. La possibilité de ferromagnétisme à l'ambiante dans certains matéraux a généré un grand enthousiasme en vue d'application dans la spintronique. Cependant, d'un point de vue fondamental la physique de ces systèmes reste peu explorée et mal comprise. Dans ce manuscrit, on se propose de fournir une étude théorique complète et détaillée des effets des inhomogenéités de tailles nanométriques sur les propriétés magnétiques dans les systèmes dilués. Tout d'abord, on montre que l'approche RPA locale autocohérente est l'outil le plus adapté et fiable pour un traitement approprié du désordre et de la percolation. Nous avons implémenté cet outil et étudié dans un premier temps, les propriétés magnétiques dynamiques d'un modèle Heisenberg dilué (couplages premiers voisins) sur un reseau cubique simple. Nous avons reproduit précisémment la disparition de l'ordre à longue portée au seuil de percolation et comparé ce travail à des études précédentes. Dans le cadre d'un Hamiltonien minimal (modèle $V$-$J$) nous avons ensuite étudié en détails les propriétés magnétiques de (Ga,Mn)As (température critique, excitations magnétiques, stiffness,..). Nous avons obtenu de très bon accords avec les calculs textit{ab initio} et les résulats expérimentaux. Finalement, nous avons étudié les effets des inhomogénéités dans les sytèmes dilués. Nous avons montré, qu'inclure des inhomogenéités pourrait s'averer être une voie très efficace et prometteuse pour dépasser l'ambiante dans de nombreux matériaux. Nous avons pu obtenir une augmentation colossale de la température critique dans certains cas comparée à celle des systèmes dilués homogènes. Nous avons atteint une augmentation de 1600% dans certains cas. Nous avons également analysé les effets des inhomogénéités sur les courbes d'aimantations, elles sont inhabituelles et peu conventionelles dans ces systèmes. Les spectres d'excitations magnétiques sont très complexes, avec des structures très riches, et présentent de nombreux modes discrets à haute energie. De plus, nos calculs ont montré que la ``spin-stiffness" est fortement supprimé par l'introduction d'inhomogénéités. Il reste encore de nombreuses voies à explorer, ce travail devrait servir de base à de futures études théoriques et expérimentales des systèmes inhomogènes. / This thesis is mainly devoted to the study of nanoscale inhomogeneities in diluted and disordered magnetic systems. The presence of inhomogeneities was detected experimentally in several disordered systems which in turn gave rise to various interesting and unexpected properties. In particular, the possibility of room-temperature ferromagnetism generated a huge thrust in these inhomogeneous materials for potential spintronics applications. However, a proper theoretical understanding of the underlying physics was a longstanding debate. In this manuscript we provide a detailed theoretical account of the effects of these nanoscale inhomogeneities on the magnetic properties of diluted systems. First we show the importance of disorder effects in these systems, and the need to treat them in an appropriate manner. The self-consistent local RPA (SC-LRPA) theory, based on finite temperature Green's function, is found to be the most reliable and accurate tool for this. We have successfully implemented the SC-LRPA to study the dynamical magnetic properties of the 3D nearest-neighbor diluted Heisenberg model. The percolation threshold is found to be reproduced exactly in comparison with previous existing studies. Following this, we discuss the essential role of a minimal model approach to study diluted magnetic systems. The one-band $V$-$J$ model, has been used to calculate the Curie temperature and the spin excitation spectrum in (Ga,Mn)As. An excellent agreement is obtained with first principles based calculations as well as experiments. Finally we propose an innovative path to room-temperature ferromagnetism in these materials, by nanoscale cluster inclusion. We find a colossal increase in $T_C$ of up to 1600% compared to the homogeneous case in certain cases. Also the spontaneous magnetization is found to exhibit anomalous non-mean-field like behavior in the presence of inhomogeneities. In addition we observe a complex nature of the magnon excitation spectrum with prominent features appearing at high energies, which is drastically different from the homogeneous case. Our study interestingly reveals a strong suppression of the spin-stiffness in these inhomogeneous systems. The results indicate toward the strong complexities associated with the interplay/competition between several typical length scales. We believe this work would strongly motivate detailed experimental as well as theoretical studies in this direction in the near future.

No description available.

Thermal oxidation of silicon belongs to the most decisive steps in microelectronic fabrication because it allows creating electrically insulating areas which enclose electrically conductive devices and device areas, respectively. Deal and Grove developed the first model (DG-model) for the thermal oxidation of silicon describing the oxide thickness versus oxidation time relationship with very good agreement for oxide thicknesses of more than 23 nm. Their approach named as general relationship is the basis of many similar investigations. However, measurement results show that the DG-model does not apply to very thin oxides in the range of a few nm. Additionally, it is inherently not self-consistent. The aim of this paper is to develop a self-consistent model that is based on the continuity equation instead of Fick’s law as the DG-model is. As literature data show, the relationship between silicon oxide thickness and oxidation time is governed—down to oxide thicknesses of just a few nm—by a power-of-time law. Given by the time-independent surface concentration of oxidants at the oxide surface, Fickian diffusion seems to be neglectable for oxidant migration. The oxidant flux has been revealed to be carried by non-Fickian flux processes depending on sites being able to lodge dopants (oxidants), the so-called DOCC-sites, as well as on the dopant jump rate.

While the spin trapping properties of thiols have been investigated through EPR analysis and kinetics studies, few groups have studied these properties using strictly computational methods. In particular, α-lipoic acid (ALA) and its reduced form, dihydrolipoic acid (DHLA), one of the strongest endogenously produced antioxidants, show potential for being effective, naturally occurring spin traps for the trapping of reactive oxygen species. This research covers electronic structure calculations of ALA, DHLA, and their corresponding hydroxyl radical spin adducts, performed at the cc-pVDZ/B3LYP/DFT level of theory. The effects on DHLA introduced by other radicals such as ·OOH, ·OCH3, and ·OOCH3 are reported. Explicit solvation was carried out using open-source molecular packing software and was studied using MOPAC PM6 semi-empirical geometry optimizations. Complete Basis Set (CBS) limit extrapolations were performed using cc-pVXZ (X = D, T, Q) Dunning basis sets under the DFT/B3LYP level of theory, and results are compared to the literature.

<div> Challenges in the semiconductor industry have resulted in the discovery of a plethora of promising materials and devices such as the III-Vs (InGaAs, GaSb, GaN/InGaN) and 2D materials (Transition-metal dichalcogenides [TMDs]) with wide-ranging applications from logic devices, optoelectronics to biomedical devices. Performance of these devices suffer significantly from scattering processes such as polar-optical phonons (POP), charged impurities and remote phonon scattering. These scattering mechanisms are long-ranged, and a quantitative description of such devices require non-local scattering calculations that are computationally expensive. Though there have been extensive studies on coherent transport in these materials, simulations are scarce with scattering and virtually non-existent with non-local scattering. </div><div> </div><div>In this work, these scattering mechanisms with full non-locality are treated rigorously within the Non-Equilibrium Green's function (NEGF) formalism. Impact of non-locality on charge transport is assessed for GaSb/InAs nanowire TFETs highlighting the underestimation of scattering with local approximations. Phonon, impurity scattering, and structural disorders lead to exponentially decaying density of states known as Urbach tails/band tails. Impact of such scattering mechanisms on the band tail is studied in detail for several bulk and confined III-V devices (GaAs, InAs, GaSb and GaN) showing good agreement with existing experimental data. A systematic study of the dependence of Urbach tails with dielectric environment (oxides, charged impurities) is performed for single and multilayered 2D TMDs (MoS2, WS2 and WSe2) providing guideline values for researchers. </div><div><br></div><div>Often, empirical local approximations (ELA) are used in the literature to capture these non-local scattering processes. A comparison against ELA highlight the need for non-local scattering. A physics-based local approximation model is developed that captures the essential physics and is computationally feasible.</div>