Neste trabalho apresentamos um estudo teórico sistemático de propriedades eletrônicas e estruturais de compostos intermetálicos que cristalizam na mesma estrutura Cllb. Realizamos este estudo utilizando o método \"Full-Potential-Linearized-Augmented Plane-Wave\"(FP-LAPW) na versão computacional do código Wien97 para a obtenção da estrutura eletrônica e do Gradiente de Campo Elétrico no núcleo (GCE). Nossos resultados teóricos de GCE são comparados com medidas efetuadas através da técnica Time differential Perturbed Angular Correlation (TDPAC), realizadas com a impureza de Ta em compostos com Zr e Hf Através da comparação de resultados teóricos e experimentais investigamos a influência da impureza no GCE. As estruturas perfeitas aqui estudadas são também otimizadas através da variação de seus parâmetros de rede e internos. / Here we present the theoretical study of structural and electronic properties of some intermetallic compounds, belonging to the common crystallographic structure of C11b type. We have performed self-consistent electronic structure calculations using the full potential linearized-augmented-plane-wave (FP-LAPW) method as embodied in the Wien97 code and determined also the electric field gradient (EFG) values for the nonequivalent sites in these systems. The applied approach is currently one of the most accurate schemes to determine the electronic structure in ordered metallic solids. By comparison of the EFG inferred from measurements at the 181Ta probe with EFG calculations without probe, we investigate the influence of the probe in compounds with Hf and Zr. Here we also study the influence of the precise knowledge of the internal structural parameters on the EFG at both metals in the pure compounds.

Aplicamos a técnica da função de Green em conjunto com o modelo de força de valência para estudar algumas propriedades vibracionais de redes perturbadas. Apresentamos uma descrição teórica dos modos locais de vibração de vacâncias e anti-sítios em GaAs. Incluímos efeitos de relaxação simétrica para as vacâncias. Baseados em nossos resultados, podemos concluir que o modo vibracional respiratório medido em 227 cm-1 não é devido a vacância de arsênio, conforme fora anteriormente sugerido. Aplicamos o mesmo método para o estudo da variação de densidade de estados vibracionais da vacância em silício. Efeitos de relaxação simétrica e distorção tetragonal são incluídos. Tais resultados são aplicados ao cálculo da entropia de formação deste defeito em Si. A partir destes cálculos verificamos que a vacância pode ser o defeito nativo responsável pela alta entropia de auto-difusão obtida experimentalmente. Finalmente, empregamos o modelo de aglomerado molecular e o método de Hartree-Fock para estudar a estrutura eletrônica da vacância neutra em Si. Calculamos a energia total destes sistemas quando submetidos aos efeitos simultâneos de relaxação e distorção. Verificamos que este modelo fornece resultados equivalentes aos obtidos por métodos de função de Green autoconsistente para a relaxação e apresenta a vantagem adicional de conseguir calcular a energia de distorção. Discutimos as consequências destes resultados sobre a entropia de formação de vacância. / The Green\'s-function technique within the framework of a valence-force field Hamiltonian has been used to study vibrational properties of intrinsic defects in gallium arsenide and silicon. This theoretical approach has been employed to predict the local vibrational modes for vacancies and antisites in GaAs. Symmetrical relaxation was included for vacancies. Our results indicate that the arsenjc vacancy can not explain the breathing mode recently found in 227 cm -1 by Raman studies. The same procedure has been applied to study the changes induced by an isolated vacancy to the local vibrational density of states in silicon. In order to treat the dangling bonds reconstruction, relaxation and tetragonal distortion effects have been included. These calculations have led to the formation entropy of a single vacancy in Si. It has been found that the single vacancy may play an important role in self-diffusion at high temperatures for this material. We also have performed cluster total-energy calculations to study relaxation and distortion around a silicon vacancy. These calculations were carried out under the Hartree-Fock approximation. Our results show that the cluster model, besides providing relaxation energies with the same accuracy of self consistent Green\'s-function method, has the additional advantage of calculating distortion energies. Consequences of these total-energy calculations on formation entropy of this defect have also discussed.

Estudamos neste trabalho a estrutura eletrônica de defeitos localizados em silício, responsáveis pela introdução de níveis profundos na faixa proibida do semicondutor. Utilizamos para tanto modelos de aglomerados moleculares dentro do formalismo do Espalhamento Múltiplo com aproximação local X para o potencial de troca (MS-X). Um tratamento adequado para os orbitais de superfície foi usado. Os defeitos estudados foram de dois tipos: defeitos simples (monovacância Si:V e oxigênio substitucional Si:O) e pares de defeitos vizinhos na rede (divacância Si : V IND 2) e par de átomos de fósforo Si: P IND 2). Os defeitos foram estudados em diferentes estados de carga (Si:O e Si : O POT -\', Si: P IND 2 e Si: P IND 2 POT + Si: V POT 0 IND 2, Si: V IND 2 POT + e Si: V POT IND 2) e no caso da divacância e do centro P IND 2 POT + foram incluídos efeitos de polarização de spin. Encontramos para todos os defeitos estudados indícios da ocorrência de efeitos Jahn-Teller. Incluímos análises das distorções dos primeiros vizinhos ao defeito nos sistemas Si: O POT , P IND 2 POT + e V POT IND 2. O modelo adotado mostrou-se capaz de descrever satisfatoriamente a estrutura eletrônica dos defeitos estudados, fornecendo resultados quantitativos que podem ser comparados diretamente com a experiência. / In this work we studied the electronic structure of deep-level defects in silicon. To do this we use molecular cluster models within the formalism of the Multiple Scattering method, in the local density functional approximation (X). The surface orbitals of the cluster are treated in a convenient way. The defects studied here were of two kinds: simple defects (single vacancy Si:V and oxygen substitutional Si:0), and pairs of defects occupying neighbouring sites in the lattice (divacancy Si :V2 and the pair of phosphoru- atoms Si :P2 ). The defects were studied in different charge states (Si:O and Si:O-, Si:P2 and Si:P+, Si:V20, SI:V2-) and for the divacancy and the center Si:P2+ the calculations were carried out to the spin-polarized limit. For all defects studied we found evidence as to the possible occurrence of Jahn-Teller effects. Analysis of nearest-neighbours distortions were included for the systems Si:O- , Si:V2+ and Si:V2-. The electronic structure of the defects studied here was satisfactorily described by the model we adopted, and quantitative results are given, that can be compared straightforwardly with experimental results.

Desenvolveu-se um processo para cálculo de faixas de energia aliando a aproximação de Células Unitárias Gigantes (LUC) e o método da Onda Plana Aumentada (APW) não muffin-tin. Os cálculos foram realizados para o diamante (2 átomos por célula) e silício (com 2 e 16 átomos por célula), usando a teoria de grupos para resolver o problema nos pontos de alta simetria da zona de Brillouin. Para o silício, com 16 átomos por célula primitiva, resultados satisfatórios foram obtidos a partir da utilização de 48 SAPWs na expansão da função de onda, o que representa um número bastante razoável neste tipo de cálculo. Nos estágios finais de desenvolvimento, o processo encontra-se em condições de ser aplicado no estudo de semi-condutores com impurezas (ou defeitos), sem a necessidade de grandes adaptações. / A procedure to calculate energy bands, using the Large Unit Cells approximation (LUC) by non muffin-tin Augmented Plane Waves (APW), was developed. The calculations were performed for the diamond (with 2 atoms in the primitive cell) and the silicon (with 2 and 16 atoms in the cell). The group theory was used on the high symmetry points of the Brillouin zone. With only 48 symmetrized APWs, good results were obtained for silicon with 16 atoms in the primitive cell. This is a reasonable basis set for this sort of calculations. This procedure may be readily applied to study semiconductors with impurities or defects.

Desenvolveu-se um processo para cálculo de faixas de energia aliando a aproximação de Células Unitárias Gigantes (LUC) e o método da Onda Plana Aumentada (APW) não muffin-tin. Os cálculos foram realizados para o diamante (2 átomos por célula) e silício (com 2 e 16 átomos por célula), usando a teoria de grupos para resolver o problema nos pontos de alta simetria da zona de Brillouin. Para o silício, com 16 átomos por célula primitiva, resultados satisfatórios foram obtidos a partir da utilização de 48 SAPWs na expansão da função de onda, o que representa um número bastante razoável neste tipo de cálculo. Nos estágios finais de desenvolvimento, o processo encontra-se em condições de ser aplicado no estudo de semi-condutores com impurezas (ou defeitos), sem a necessidade de grandes adaptações. / A procedure to calculate energy bands, using the Large Unit Cells approximation (LUC) by non muffin-tin Augmented Plane Waves (APW), was developed. The calculations were performed for the diamond (with 2 atoms in the primitive cell) and the silicon (with 2 and 16 atoms in the cell). The group theory was used on the high symmetry points of the Brillouin zone. With only 48 symmetrized APWs, good results were obtained for silicon with 16 atoms in the primitive cell. This is a reasonable basis set for this sort of calculations. This procedure may be readily applied to study semiconductors with impurities or defects.

Improvements in process technology and design innovations have resulted in compact and cost effective digital baseband solutions. The radio part, however, has remained a bottleneck in terms of chip area and power consumption as the feature size of analog devices does not directly benefit from scaling. With the addition of yet more standards into emerging products, the requirements of future radios will extend over characteristic performance features into demands of programmable and reconfigurable hardware for radios covering multiple frequency bands. Hence, a guideline in the design of such radios is a large degree of hardware sharing. The thesis investigates the application of sigma-delta modulation to arising multistandard wireless radios. First, it reviews principles in wireless radios, such as selected modulation and access techniques. It also examines several communication standards of personal wireless radios as well as common receiver architectures for their implementation. This is followed by general considerations and background information about sigma-delta modulators. In the third and fourth chapter, implementations to two blocks of a wireless radio receiver system are suggested: An architecture for a frequency synthesizer and an implementation of an analog-to-digital converter. In the first contribution, the thesis develops a novel concept for frequency synthesis that is more suitable for multi-band, multi-standard radio architectures, achieving a large amount of hardware sharing among different wireless standards. As a second pillar, the thesis contributes with the design of a dual-standard sigma-delta modulator for data conversion within a radio receiver. Parts of the work concerning the dual-standard modulator are embodied in a granted swedish patent. / QC 20100830

<p>Measurements of partial discharges are commonly used to diagnose the insulation system in high voltage components. Traditionally a single xed frequency of the applied voltage is used for such measurements as in the Phase Resolved Partial Discharge Analysis (PRPDA) technique. With the Variable Frequency Phase Resolved Partial Discharge Analysis (VF-PRPDA) technique the frequency of the applied voltage is instead variable. This technique provides more information about the condition of the insulation than the PRPDA technique. To extract the extra information a physical understanding of the frequency dependence of partial discharges is necessary.</p><p>In this thesis partial discharges in cylindrical cavities in polycarbonate are measured using the VF-PRPDA technique in the frequency range 10 mHz { 100 Hz. It is studied how the cavity diameter and height inuence the frequency dependence of partial discharges. Insulated cavities are compared with cavities bounded by an electrode. It is shown that from measurements at variable applied frequency it is possible to distinguish between cavities of di erent dimensions and between insulated and metal bounded cavities.</p><p>A two-dimensional eld model of partial discharges in a cylindrical cavity is developed. The sequence of discharges in the cavity is simulated by use of the eld computation program FEMLABR. Discharges are modeled with a voltage and current dependent streamer conductivity and are simulated dynamically to obtain charge and current consistency. It is shown that the frequency dependence of partial discharges is signi - cantly inuenced by the statistical time lag and by the two dielectric time constants related to charge movements on the cavity surface and in the bulk insulation. Simulation results are used to interpret the frequency dependent partial discharge activity in a cylindrical cavity.</p>

Thesis (Ph.D.)--University of Wyoming, 2007. / Title from PDF title page (viewed on June 22, 2009). Includes bibliographical references.

No description available.

Diss. (sammanfattning) Uppsala : Uppsala universitet, 2007. / Härtill 6 uppsatser.