Molecular Dynamics (MD) and Molecular Statics (MS) simulations have always proven to be powerful tools to study material behavior at the lowest length scale. However, one of the greatest challenges in material modelling is the effective upscaling of relevant material properties from atomic scale to continuum scale. One such challenge is how best to capture the role of twinning in higher scale plasticity. Capturing twin nucleation in fullield crystal plasticity is a long-standing problem in materials science. The challenge resides mainly in the biased regional lattice transformation associated with twin formation in defiance of its obedience to a threshold stress law which could be fulfilled in regions where twinning is deferred. Hence, determining a favorable site for nucleation of a twin variant remains a daunting task. This dissertation is an attempt to understand twin nucleation in hexagonal close packed (HCP) metals using a technique called Nudged Elastic Band (NEB) method. NEB calculations are performed between two stable configurations and use a number of intermediate images to describe the transition. Results of calculations demonstrate that the role of stress and atomic structure in twin nucleation could be understood in terms of the minimum energy path, energy barrier, and relaxed energy. This method allows to gauge the transformations that the lattice could experience when evolving from a given thermodynamic state to a final state of a predefined twin embryo under various boundary conditions. This allows direct comparisons between various cases with respect to twin nucleation and hence provides a measure of the material plasticity. Such results are crucial in higher scale modelling of the material.

This thesis is a work of literary non-fiction. It is divided into six sections. The first, "A Sense of Place," and the last, "Pulling a Geographical," are collage essays which establish and then reiterate major themes in the work: movement, work, geography, climate, people, and education--some curricular, some not. The middle four chapters--"Daybreak" through "Nightshift"--are a mix of related essays and poetry, grouped according to theme, and together form a memoir. / Graduation date: 1991

Thesis (Ph. D.)--University of Nebraska, 1935. / Bibliography: leaves 132-138.

Thesis (M.A.)--University of Nebraska (Lincoln campus)--1927. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Nebraska--Lincoln, 1997. / Includes bibliographical references.

Thesis (M.A.)--University of Nebraska (Lincoln campus)--1927. / Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--University of Nebraska, 1935. / Bibliography: leaves 132-138.

Thesis (Ph. D.)--University of Nebraska, 1935. / Bibliography: leaves 132-138.

En esta tesis se ha estudiado la intercalación de Li en dos nanoestructuras de titanatos, TiO2(B) y H2Ti3O7, siendo ambos materiales candidatos para ser aplicados en baterías de ión-Li. En el caso del primer material estudiado, se realizaron estudios sobre el voltaje de intercalación del Li en las superficies ultrafinas (100) y (001) y en el sistema “bulk”, mientras que en el segundo material se estudió sólamente la intercalación en el “bulk”. En general, se obtuvieron los parámetros para las estructuras y las curvas DOS de los sistemas puros e intercalados. Se estudió la diferencia de densidad de carga y se realizó el análisis de Bader para estudiar las cargas del sistema. Se realizaron estudios vibracionales, obteniendo las phDOS, y el espectro Raman en el caso del estudio sobre TiO2(B). Se realizaron estudios de la difusión de Li en los dos materiales, y las propiedades termodinámicas de los dos sistemas “bulk” han sido estudiadas, para así poder tener una comprensión más realista para la aplicación de estos sistemas. En el desarrollo de la presente tesis, todos los cálculos fueron realizados en el marco de la Teoría del Funcional de la Densidad con la metodología de Hubbard (DFT+U). Los cálculos de optimización de geometría, estructura electrónica, estudios de carga, espectros Raman y difusión, entre otros, se obtuvieron mediante el código VASP. Los estudios de enlace se realizaron mediante el código DDEC6. Cabe destacar que el postprocesamiento para la obtención de las propiedades vibracionales y termodinámicas fueron realizadas con el código libre Phonopy. Los cálculos fueron comparados con estudios teóricos previos de DFT y con resultados experimentales, cuando había disponibles. / In this thesis, the Li intercalation of two titanates nanostructures were studied, TiO2(B) and H2Ti3O7, being both materials candidates to be applied in Li-ion batteries. In the case of the first material studied, studies about voltage intercalation of Li in the ultrathin surfaces (100) and (001) and in the “bulk” systems were performed, while in the second material, only the intercalation in the “bulk” was studied. In general, the parameters obtained for the structures and the DOS curves of the pure and intercalated systems were obtained. The charge density difference was studied and the Bader analysis was performed in order to study the charges of the system. Vibrational studies were performed, obtaining the phDOS and the Raman spectra in the case of the TiO2(B) study. Li diffusion studies in both materials were performed, and the thermodynamic properties of both “bulk” systems have been studied to have a more realistic comprehension for the application of these systems. In the development of the present thesis, all of the calculations were performed with Density Functional Theory with the Hubbard methodology (DFT+U). The calculations of optimization of geometry, electronic structure, charge studies, Raman spectra and diffusion, among others, were obtained with the VASP code. The bond studies were performed with the DDEC6 code. It is important to mention that the postprocessing to obtain the vibrational and thermodynamics properties was performed with the open source package Phonopy. The calculations were compared with previous DFT theoretical studies and experimental results, when they were available.

LD2668 .T4 1966 E47 / Master of Science