Made available in DSpace on 2014-12-17T14:54:58Z (GMT). No. of bitstreams: 1 ArrheniusVCO_TESE.pdf: 1241211 bytes, checksum: 636a162e62ee1658f0ee78ed1516228d (MD5) Previous issue date: 2011-03-03 / Eventually, violations of voltage limits at buses or admissible loadings of transmission lines and/or power transformers may occur by the power system operation. If violations are detected in the supervision process, corrective measures may be carried out in order to eliminate them or to reduce their intensity. Loading restriction is an extreme solution and should only be adopted as the last control action. Previous researches have shown that it is possible to control constraints in electrical systems by changing the network topology, using the technique named Corrective Switching, which requires no additional costs. In previous works, the proposed calculations for verifying the ability of a switching variant in eliminating an overload in a specific branch were based on network reduction or heuristic analysis. The purpose of this work is to develop analytical derivation of linear equations to estimate current changes in a specific branch (due to switching measures) by means of few calculations. For bus-bar coupling, derivations will be based on short-circuit theory and Relief Function methodology. For bus-bar splitting, a Relief Function will be derived based on a technique of equivalent circuit. Although systems of linear equations are used to substantiate deductions, its formal solution for each variant, in real time does not become necessary. A priority list of promising variants is then assigned for final check by an exact load flow calculation and a transient analysis using ATP Alternative Transient Program. At last, results obtained by simulation in networks with different features will be presented / Na opera??o de sistemas el?tricos verificam-se, eventualmente, viola??es de restri??es operativas, como sobrecargas em linhas de transmiss?o e transformadores de pot?ncia de subesta??es ou viola??es de limites de tens?es em barramentos. Caso o monitoramento dessas grandezas indique poss?veis problemas, ? necess?rio adotar medidas corretivas para elimin?los ou atenuar sua intensidade, devendo a restri??o de carga ser utilizada como ?ltimo recurso. Uma medida de controle bastante eficaz ? a t?cnica intitulada Chaveamentos Corretivos, capaz de influenciar o fluxo de pot?ncia de redes malhadas a partir de altera??es em suas topologias, sem, entretanto, produzir qualquer custo operacional adicional. Em trabalhos anteriores, o c?lculo desenvolvido para verificar a capacidade de uma determinada variante de chaveamento em eliminar a sobrecarga em um ramo espec?fico era realizado atrav?s de redu??o de redes ou atrav?s de an?lises heur?sticas. O presente trabalho objetiva a dedu??o anal?tica de equa??es para estimar a altera??o de corrente em um ramo espec?fico, ap?s a realiza??o de chaveamentos em subesta??es, mediante o emprego de poucos c?lculos, decorrentes de aproxima??es lineares. Para o acoplamento de barras de subesta??es, as dedu??es ser?o baseadas na teoria de curto-circuito e na metodologia da Fun??o de Al?vio. J? para a abertura de barras de subesta??es, ser? deduzida uma nova Fun??o de Al?vio, fundamentada na t?cnica de circuito equivalente. Embora sistemas de equa??es lineares sejam utilizados para fundamentar as dedu??es, sua solu??o formal para cada variante, em tempo real n?o ? necess?ria. A partir das estimativas realizadas, elabora-se uma lista de variantes priorit?rias, para teste final atrav?s de um c?lculo exato de fluxo de carga e de uma an?lise de transit?rios atrav?s do software ATP Alternative Transient Program. Por fim, apresentamse resultados obtidos atrav?s de simula??es em redes el?tricas de caracter?sticas distintas

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O objetivo geral dessa pesquisa é analisar o comportamento mecânico, com foco na tenacidade à tração, de corpos de provas de concreto reforçado com fibras de aço, por meio do ensaio de abertura por encunhamento, patenteado por Elmar K. Tschegg, comparando os resultados com os valores obtidos pelo método da ASTM C1609. Deseja-se ainda, avaliar a influência do teor de fibra de aço e da resistência à compressão na tenacidade do compósito. E também contribuir com a regulamentação e o controle na aplicação do material. O programa experimental foi composto de 112 corpos de prova (64 cilíndricos para caracterização, 24 prismáticos para ASTM C1609 e 24 cúbicos para encunhamento) que foram ensaiados no Laboratório de Estruturas da Faculdade de Engenharia Civil da Universidade Federal de Uberlândia. Todos os corpos de prova foram moldados e ensaiados com idade mínima de 60 dias. Após a realização dos ensaios e análise dos resultados, conclui-se que as fibras de aço conferem ao concreto um aumento na tenacidade à tração. Observou-se também o comportamento strain-hardening à tração do concreto com teor de fibras de 0,64%. O ensaio de abertura por encunhamento apresenta indicativos de reprodutibilidade e coerência, quando comparado com o teste ASTM C1609 (2012), ou seja, há uma correlação e confirma-se como um procedimento alternativo para o controle tecnológico do compósito. / The general objective of the research is to analyze the mechanical behavior, focusing on the tensile strength, of concrete reinforced concrete test bodies with steel fibers, by means of the Elmar K. Tschegg patented aperture test by comparing the results with The values obtained by the method of ASTM C1609. It is also desired to evaluate the influence of the steel fiber content and the compressive strength on the composite toughness. And also contribute with a regulation and control in the application of the material. The experimental program was composed of 112 test specimens (64 cylindrical characterization, 24 prismatic for ASTM C1609 and 24 cubic for encouragement) that were tested in the Structures Laboratory of the Faculty of Civil Engineering of the Federal University of Uberlândia. All specimens were molded and assayed at least 60 days old. After the tests and analysis of the results, it is concluded that the steel fiber gives the concrete an increase in tensile toughness. The strain-hardening behavior of the concrete with a fiber content of 0.64% was also observed. The Wedge Splitting Test presents indications of reproducibility and coherence, when compared with ASTM C1609 (2012), that is, there is a correlation and it is confirmed as an alternative procedure for the technological control of the composite. / Dissertação (Mestrado)

Ce travail de thèse traite de la production d’hydrogène par le procédé de photoélectrocatalyse en utilisant une photoanode à base de nanotubes de TiO2 verticalement alignés. L’utilisation du TiO2 étant limité pour des applications solaires en raison de son large gap, il est nécessaire de le modifier. Deux approches sont proposées pour modifier les nanotubes de TiO2 et leur permettre d’absorber la lumière visible. La première est une modification chimique du TiO2 par co-dopage cationique-anionique (Ta-N) ou (Nb-N). Les cations sont insérés durant la croissance des nanotubes grâce à une approche inédite, et l’azote est inséré durant le traitement thermique. Ceci a pour effet la formation d’orbitales hybrides qui entraîne une réduction du gap et une activité sous lumière visible, tout en permettant une stabilité de la structure. La seconde approche consiste à déposer des nanoparticules d’Ag sur la surface des nanotubes de TiO2. Grâce au contrôle de la morphologie des nanoparticules d’Ag, leur résonnance plasmonique permet de stimuler l’absorption du TiO2 et ainsi d’augmenter son rendement à la fois sous lumière UV et sous lumière visible. / This work is about the production of hydrogen by photoelectrocatalysis using a vertically aligned TiO2 nanotubes based photoanode. Utilization of TiO2 for solar applications is limited due to its large band gap, it has to be modified. Two approaches are proposed for the modification of the TiO2 nanotubes to make them absorb visible light. The first one is the chemical modification of the TiO2 by (Ta-N) or (Nb-N) cationic-anionic co-doping. Cations are inserted during the growth of the nanotubes by a novel approach, and nitrogen is inserted during heat treatment. This leads to the formation of hybrid orbitals resulting in a band gap reduction and of activity under visible light. The second approach consists of the deposition of Ag nanoparticles on the surface of the TiO2 nanotubes. Thanks to the control of the morphology of the Ag nanoparticles, their plasmonic resonance can enhance the absorption of TiO2 and thus increase its activity both under UV and visible light.

The design of complex structures demands the prediction of possible fracture-dominant failure processes, due to the existence of unavoidable preexistent flaws and other defects, as well as sharps and cracks. On one hand, the complexity of the structure and the presence of many defects to be accounted for in the modeling can become the computational effort impracticable. On the other hand, it is important to seek the development of a computational framework based on some numerical method to study these problems. A way to overcome the difficulties mentioned, therefore making feasible the analysis of complex structures with many cracks, flaws and other defects, consists of combining a representative mechanical modeling with an efficient numerical method. This is precisely the fundamental aim of this work. Firstly, the Splitting Method is used aiming to build a representative modeling. Secondly, the Generalized Finite Element Method (GFEM) is chosen as an efficient numerical method, in which enrichment strategies of the approximated solution using stress functions in particular can be explored. The GFEM framework also allows avoiding the excessive refinement of the mesh, which increases the computational effort in conventional finite element analysis. In the Splitting Method, a kind of decomposition method, the original problem is subdivided in local and global problems which are then combined by imposing null traction at the crack surfaces. In this work, the Splitting Method was completely programmed in Python language and its use extended to analyze crack propagation including fatigue crack growth. The generated code presents in addition to several features related to Fracture Mechanics concepts, as the computation of the stress intensity factor (mode I and II) trough J Integral. Some examples are presented to depict the propagation of the cracks in multisite damage structures. It is shown that for this kind of problems the enrichment strategy provided by GFEM is essential. Moreover, the final example demonstrates that the computational tool allows for investigation of different possible crack scenarios with a low cost analysis. One concludes about the representativeness and efficiency of the methodology hereby proposed. / O projeto de estruturas complexas demanda a previsão de possíveis processos de ruptura governados por fraturamento, devido à existência de inevitáveis defeitos pré-existentes, como entalhes e fissuras. Por um lado, a complexidade da estrutura e a presença de muitos defeitos a serem considerados no modelo podem tornar a análise inviável devido ao esforço computacional necessário. Por outro lado, é importante procurar desenvolver uma estrutura computacional baseada em métodos numéricos para estudar estes problemas. Um modo de superar as dificuldades mencionadas, portanto tornando possível a análise de estruturas complexas com muitas fissuras e outros defeitos, consiste em combinar um modelo mecânico que seja representativo com um método numérico eficiente. Este é precisamente o objetivo fundamental deste trabalho. Primeiramente, o Método da Partição é utilizado para a construção de um modelo representativo. Em segundo lugar, o Método dos Elementos Finitos Generalizados (GFEM) é empregado por ser um método numérico eficiente, no qual as estratégias de enriquecimento da solução aproximada usando funções de tensão, em particular, podem ser exploradas. A estrutura do GFEM também permite evitar o excessivo refinamento da malha, que aumenta o esforço computacional em análises convencionais nas quais se utiliza o método dos elementos finitos. No Método da Partição, um tipo de método de decomposição, o problema original é subdividido em problemas locais e globais que são então combinados impondo-se a nulidade do vetor de tensões na superfície da fissura. Neste trabalho, o Método da Partição foi completamente programado em linguagem Python® e sua utilização estendida para analisar a propagação de fissuras, incluindo-se a associação do crescimento com a resposta em fadiga. Além disso, o código gerado apresenta diversas características relacionadas aos conceitos da Mecânica da Fratura, como o cálculo do fator de intensidade de tensão (modos I e II) mediante a Integral J. Alguns exemplos são apresentados para ilustrar a propagação de fissuras em estruturas multi-fraturadas. Mostra-se que para este tipo de problemas a estratégia de enriquecimento fornecida pelo GFEM é essencial. Além disso, o exemplo final comprova que a ferramenta computacional permite a investigação de diferentes possíveis cenários de fissuras com uma análise de baixo custo. Conclui-se sobre a representatividade e eficiência da metodologia proposta.

A melhor maneira de se caracterizar o comportamento mecânico pós-fissuração dos concretos reforçados com fibras (CRF) para fins estruturais é por meio de ensaios que envolvem a ruptura do material à tração direta. Devido às dificuldades de execução destes, comumente se empregam ensaios de tração indireta, com destaque para os ensaios de flexão com sistema fechado. Entretanto, fatores limitantes, como a superestimação da capacidade resistente do compósito em testes de flexão, têm incentivado o desenvolvimento de outros métodos de ensaio mais fidedignos às características do CRF. Este é o caso do ensaio de compressão de corpo de prova com duplo corte em cunha, abreviado como DEWS (do inglês, Double Edge Wedge Splitting), em que o esforço de tração indireta não está associado a uma flecha, como nos ensaios de flexão, mas sim à própria fissuração da matriz de concreto. No DEWS há a possibilidade de avaliação direta da ortotropia do CRF e seu efeito na resistência pós-fissuração no estado limite de serviço (ELS) e último (ELU). Estudos anteriores sobre este ensaio utilizaram sistema aberto, mas se restringiram a avaliar apenas elevados teores de fibra, o que não traz dificuldades à caracterização da resistência residual no ELS. Este trabalho, contudo, buscou o estudo mais acurado a respeito do ensaio DEWS. Os programas experimentais desenvolvidos empregaram concretos reforçados com baixos teores de fibra de aço. Inicialmente, estudou-se a capacidade do ensaio DEWS em identificar o efeito da orientação das fibras na matriz na resistência pós-fissuração do CRF. Essa análise foi correlacionada com os resultados obtidos a partir de ensaios de flexão de prismas feitos como recomendado pela JSCE-SF4, assim, buscou-se avaliar se este teste promove uma superestimação da capacidade de reforço da fibra. Uma outra avaliação centrou-se no fator da taxa de carregamento de ensaio, realizando-se a metodologia DEWS e o teste de flexão (JSCE-SF4: 1984) sob quatro velocidades distintas, com intuito de verificar seu grau de influência no comportamento do material. A partir dos resultados obtidos, o ensaio DEWS mostrou-se aplicável na avaliação da ortotropia de corpos de prova em concretos reforçados com baixos teores de fibra de aço, mesmo com a utilização de sistema aberto e em velocidade de carregamento acima da proposta em literatura. A extensão da instabilidade pós-fissuração obtida a partir do ensaio DEWS foi menor que a encontrada para o ensaio de flexão de prismas. O estudo mostrou que a capacidade resistente pós-fissuração do CRF no ensaio de flexão pode ser até 300% maior do que a mesma resistência medida na direção transversal do prisma. / The best way to characterize the post-cracking behaviour of fibre reinforced concretes (FRC) for structural proposes is through direct tensile tests. As direct tensile tests are difficult to conduce, usually indirect tensile tests are employed, especially bending tests with closed-loop control. However, limiting factors, as the strength capacity overestimation of composites under flexural beams tests had motivated the development of another test procedure aiming to obtain responses closer to FRC basic characteristics. This is the case of Double Edge Wedge Splitting test (DEWS). In this test, the indirect tensile stress is not associated with a deflection - as is common in bending tests - but it is related with the cracking opening of concrete matrix. The DEWS test brings the possibility of direct evaluation of the FRC orthotropic behaviour and its effect in the post-crack strength at the serviceability (SLS) and ultimate (ULS) limit estates. Previous researches about DEWS test were done using open-loop control, but they were limited to higher fibre content. These do not provide difficulties about the residual strength characterization at the SLS. Thus, this study carried out an accurate experimental analysis about DEWS. This experimental programme used only fibre reinforced concrete with low steel fibre volume fractions. The first analysis was based in the DEWS test capacity for identification of the effect of predominant fibre orientation into matrix in the post-crack strength of the FRC. This analysis was linked with the flexural test made with prismatic specimens as recommended by JSCE-SF4 in order to evaluate if it promotes an overestimation of the fibre reinforcement capacity. The other evaluation focused on the test loading rate factor by doing DEWS test and bending test (JSCE-SF4:1984) under four different rates in order to verify its influence on the material behaviour. The results shown that DEWS test was capable to identify the fibre alignment into FRC specimens with low fibre content, even using open-loop control and increasing the load rate. The extension of the instability region on DEWS test response was lower than the provided by the flexural tests. The study also showed that the flexural test could overestimate the post-crack strength capacity of the FRC up 300% in relation to the transversal direction.

O trabalho trata da utilização de formulações não-convencionais de elementos finitos na obtenção de fatores de intensidade de tensão associados a múltiplas fissuras distribuídas num domínio bidimensional. A formulação do problema de múltiplas fissuras baseia-se numa abordagem de sobreposição proposta pelo Método da Partição (\"Splitting Method\"). Segundo essa abordagem a solução do problema pode ser encontrada a partir da sobreposição de três subproblemas combinados de tal forma que o fluxo de tensão resultante nas faces das fissuras seja nulo. O uso do Método dos Elementos Finitos (MEF) em sua forma convencional pode requerer um refinamento excessivo da rede nesse tipo de problema, aumentando o custo computacional da análise. Objetivando reduzir este custo, empregam-se duas formulações não-convencionais, de forma independente, num dos subproblemas, dito local: a formulação híbrido-Trefftz de tensão e o Método dos Elementos Finitos Generalizados (MEFG). Na formulação híbrido-Trefftz é adotado o recurso do enriquecimento seletivo mediante o refrno- p na aproximação dos campos de deslocamento no contorno do elemento. Já com relação ao MEFG, empregam-se funções polinomiais e a solução analítica da mecânica da fratura como funções enriquecedoras. Exemplos de simulação numérica são apresentados no sentido de comprovar que a utilização dessas formulações não-convencionais juntamente com o Método da Partição viabiliza a obtenção de resultados com boa aproximação com recurso a redes pouco refinadas, reduzindo significativamente o custo computacional de toda a análise. / This paper treats with the use of non-conventional finite element formulations to obtain the stress intensity factor of multiple cracks located in a two-dimensional domain. The formulation of the multiple cracks problem is based on an overlapping approach suggested by the Splitting Method. Accordingly, the solution of the problem can be achieved by dividing the problem in three steps, combined so that the resulting stress flux is zero on the cracks face. The use of the Finite Element Method (FEM) in its conventional formulation requires a mesh refinement in this kind of problem, then increasing the computational cost. Aiming to reduce this cost, two non-conventional formulations are used independently to solve the local problem: the Hybrid-Trefftz stress formulation and the Generalized Finite Elements Method (GFEM). The Hybrid-Trefftz formulation is applied with selective enrichment using p-refinement in the displacements field on the element boundaries. The GFEM employs polynomial functions and analytical solutions of the fracture mechanics as enrichment functions. Examples of numerical simulations are presented in order to show that non- conventional formulations and the Splitting Method can provide accurate results with coarse mesh, thus reducing the computational cost.

Variational methods and Partial Differential Equations (PDEs) have been extensively employed for the mathematical formulation of a myriad of problems describing physical phenomena such as heat propagation, thermodynamic transformations and many more. In imaging, PDEs following variational principles are often considered. In their general form these models combine a regularisation and a data fitting term, balancing one against the other appropriately. Total variation (TV) regularisation is often used due to its edgepreserving and smoothing properties. In this thesis, we focus on the design of TV-based models for several different applications. We start considering PDE models encoding higher-order derivatives to overcome wellknown TV reconstruction drawbacks. Due to their high differential order and nonlinear nature, the computation of the numerical solution of these equations is often challenging. In this thesis, we propose directional splitting techniques and use Newton-type methods that despite these numerical hurdles render reliable and efficient computational schemes. Next, we discuss the problem of choosing the appropriate data fitting term in the case when multiple noise statistics in the data are present due, for instance, to different acquisition and transmission problems. We propose a novel variational model which encodes appropriately and consistently the different noise distributions in this case. Balancing the effect of the regularisation against the data fitting is also crucial. For this sake, we consider a learning approach which estimates the optimal ratio between the two by using training sets of examples via bilevel optimisation. Numerically, we use a combination of SemiSmooth (SSN) and quasi-Newton methods to solve the problem efficiently. Finally, we consider TV-based models in the framework of graphs for image segmentation problems. Here, spectral properties combined with matrix completion techniques are needed to overcome the computational limitations due to the large amount of image data. Further, a semi-supervised technique for the measurement of the segmented region by means of the Hough transform is proposed.

Cette thèse s'intéresse à l'étude des problèmes de diagnostic des fautes sur les systèmes à événements discrets en utilisant les modèles réseau de Petri. Des techniques d'exploration incrémentale et à-la-volée sont développées pour combattre le problème de l'explosion de l'état lors de l'analyse de la diagnosticabilité. Dans le contexte atemporel, la diagnosticabilité de modèles RdP-L est abordée par l'analyse d'une série de problèmes K-diagnosticabilité. L'analyse de la diagnosticabilité est effectuée sur la base de deux modèles nommés respectivement FM-graph et FM-set tree qui sont développés à-la-volée. Un diagnostiqueur peut être dérivé à partir du FM-set tree pour le diagnostic en ligne. Dans le contexte temporel, les techniques de fractionnement des intervalles de temps sont élaborées pour développer représentation de l'espace d'état des RdP-LT pour laquelle des techniques d'analyse de la diagnosticabilité peuvent être utilisées. Sur cette base, les conditions nécessaires et suffisantes pour la diagnosticabilité de RdP-LT ont été déterminées. En pratique, l'analyse de la diagnosticabilité est effectuée sur la base de la construction à-la-volée d'une structure nommée ASG et qui contient des informations relatives à l'occurrence de fautes. D'une manière générale, l'analyse effectuée sur la base des techniques à-la-volée et incrémentale permet de construire et explorer seulement une partie de l'espace d'état, même lorsque le système est diagnosticable. Les résultats des simulations effectuées sur certains benchmarks montrent l'efficacité de ces techniques en termes de temps et de mémoire par rapport aux approches traditionnelles basées sur l'énumération des états / This PhD thesis deals with fault diagnosis of discrete event systems using Petri net models. Some on-the-fly and incremental techniques are developed to reduce the state explosion problem while analyzing diagnosability. In the untimed context, an algebraic representation for labeled Petri nets (LPNs) is developed for featuring system behavior. The diagnosability of LPN models is tackled by analyzing a series of K-diagnosability problems. Two models called respectively FM-graph and FM-set tree are developed and built on the fly to record the necessary information for diagnosability analysis. Finally, a diagnoser is derived from the FM-set tree for online diagnosis. In the timed context, time interval splitting techniques are developed in order to make it possible to generate a state representation of labeled time Petri net (LTPN) models, for which techniques from the untimed context can be used to analyze diagnosability. Based on this, necessary and sufficient conditions for the diagnosability of LTPN models are determined. Moreover, we provide the solution for the minimum delay ∆ that ensures diagnosability. From a practical point of view, diagnosability analysis is performed on the basis of on-the-fly building of a structure that we call ASG and which holds fault information about the LTPN states. Generally, using on-the-fly analysis and incremental technique makes it possible to build and investigate only a part of the state space, even in the case when the system is diagnosable. Simulation results obtained on some chosen benchmarks show the efficiency in terms of time and memory compared with the traditional approaches using state enumeration

In this dissertation the heat flux prediction capabilities of Residual Distribution (RD) schemes for hypersonic flow fields are investigated. Two canonical configurations are considered: the flat plate and the blunt body (cylinder) problems, with a preference for the last one. Both simple perfect gas and more complex thermo-chemical non-equilibrium (TCNEQ) thermodynamic models have been considered.<p><p>The unexpected results identified early in the investigation lead to a thorough analysis to identify the causes of the unphysical hypersonic heating.<p><p>The first step taken is the assessment of the quality of flow field and heat transfer predictions obtained with RD methods for subsonic configurations. The result is positive, both for flat plate and cylinder configurations, as RD schemes produce accurate flow solutions and heat flux predictions whenever no shock waves are present, irrespective of the gas model employed.<p><p>Subsonic results prove that hypersonic heating anomalies are a consequence of the presence of a shock wave in the domain and/or the way it is handled numerically.<p><p>Regarding hypersonic flows, the carbuncle instability is discarded first as the cause of the erroneous stagnation heating. The anomalies are shown next to be insensitive to the kind and level of dissipation introduced via the (quasi-)positive contribution P to blended B schemes. Additionally, insufficient mesh resolution locally over the region where the shock wave is captured numerically is found to be irrelevant.<p><p>Capturing the bow shock in a manner that total enthalpy is preserved immediately before and after the numerical shock wave is, on the contrary, important for correct heating prediction.<p><p>However, a carefully conceived shock capturing term is, by itself, not sufficient to guarantee correct heating predictions, since the LP scheme employed (be it stand-alone in a shock fitting context or combined into a blended scheme for a shock capturing computation) needs to be immune to spurious recirculations in the stagnation point. <p><p>Once the causes inducing the heating anomalies identified, hypersonic shocked flows in TCNEQ conditions are studied.<p><p>In order to alleviate the computational effort necessary to handle many species non-equilibrium (NEQ) models, the extension of an entropic (or symmetrizing) variables formulation RD to the nS species, two temperature TCNEQ model is accomplished, and the savings in computational time it allows are demonstrated.<p><p>The multi-dimensional generalization of Roe-like linearizations for the TCNEQ model is addressed next: a study on the existence conditions of the linearized state guaranteeing discrete conservation is conducted.<p><p>Finally, the new dissipative terms derived for perfect gas are adapted to work under TCNEQ conditions; the resulting numerical schemes are free of the temperature undershoot and Mach number overshoot problem afflicting standard CRD schemes. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Nous avons étudié, par RPE impulsionnelle, la cohérence quantique et des spins électroniques des ions de transition Mn2+, Co2+, Fe3+, et des complexes Fe3+/Cs+ et Fe3+/Na+, tous présents dans le ZnO monocristallin. Nous avons trouvé que l’anisotropie magnétique peut altérer la cohérence de la phase dynamique des qubits des spins électroniques. Nous avons mesuré une faible décohérence pour les spins d’ions Mn2+et Fe3+ dans ZnO, qui ont tous deux une faible anisotropie magnétique uniaxiale, tandis que les ions Co2+ isolés avec une très forte anisotropie magnétique uniaxiale, une décohérence rapide a été mis en évidence. Nous avons trouvé que les spins électroniques des complexes de type Fe3+/Cs+, ayant un tenseur d’anisotropie magnétique plus complexe que la simple anisotropie uniaxiale des ions Fe3+ isolés, possèdent presque le même temps de décohérence. Par la méthode des perturbations, nous avons mis en évidence théoriquement un terme supplémentaire à la phase habituelle de Berry, dû à l’anisotropie magnétique et qui existe dans tout système ayant un spin S>1/2. / We studied by pulsed EPR (p-EPR), the quantum coherence of electronic spins qubits of isolated transition metal ions of Mn2+, Co2+, Fe3+ and Fe3+/Cs+ as well as Fe3+/Na+ complexes, all found as traces in mono-crystalline ZnO. Indeed, we experimentally demonstrated that the magnetic anisotropy can alter the coherence of the dynamic phase of electronic spins qubits. We found a small decoherence for Mn2+ and Fe3+, spins having a small uniaxial magnetic anisotropy, and on the contrary, we found a very strong decoherence for Co2+ spins having a very strong uniaxial magnetic anisotropy. We found that the electronic spins of the Fe3+/Cs+ complex, having a more complex tensor magnetic anisotropy compared to the simplest uniaxial one of isolated Fe3+ spins in ZnO, have almost the same coherence time. By the perturbation method, we have found theoretically an additional term to the usual geometric Berry phase, due to the magnetic anisotropy which exists in any system having a spin S>1/2.