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Dans le domaine de l'électronique de puissance, la gestion thermique de l'intégration des puces en silicium au sein du système global constitue un problème clé. La chaleur dissipée par les composants électriques est évacuée vers l’extérieur à travers un drain thermique, généralement en cuivre, qui est brasé sur le substrat céramique. Cette étude est consacrée à l'élaboration de drains thermiques adaptatifs en matériaux composites cuivre allié/fibres de carbone (FC) qui combinent une bonne conductivité thermique et un CTE proche de celui du substrat. Dans ce type de matériau, la liaison interfaciale renfort/matrice doit être forte afin d'optimiser le transfert des propriétés entre les deux composants. Le couple cuivre/carbone étant non réactif, un élément d’addition carburigène (Cr ou B) est ajouté à la matrice de cuivre afin de créer cette liaison chimique forte. Un matériau modèle a été réalisé par pulvérisation cathodique afin d’étudier la diffusion de l’élément d’addition au sein de la couche de cuivre vers la zone interfaciale et la formation d’un carbure métallique. Une méthode de chimie des solutions a ensuite été utilisée pour élaborer des poudres de cuivre allié de stœchiométrie donnée. Enfin, les matériaux composites (Cu-B/FC et Cu-Cr/FC) ont été élaborés par métallurgie des poudres et leurs propriétés thermiques et mécaniques corrélées à la microstructure et à la chimie des zones interfaciales / In the field of power electronics, thermal management of silicon chips plays a key role in our ability to increase their performance. Heat generated by the electronic components is dissipated through the heat sink, generally made of Copper that is brazed on to a ceramic substrate. This study focuses on the elaboration of adaptive heat sink material using Copper alloys/Carbon fibers (CF) composite materials which have a good thermal conductivity and a CTE close to the ceramic substrate. In this kind of material, it is necessary to have a strong matrix/reinforcement link in order to optimize transfer properties. Since there is no reaction between Copper and Carbon, a carbide element (Cr or B) is added to the Copper matrix to create this strong chemical bond. A model material has been elaborated by cathode sputtering in order to study the diffusion of the alloying element in the Copper layer and the metallic carbide formation in the interfacial zone. Copper alloy powders, with a given stoichiometry, have also been synthesized by a chemical method. Lastly, composite materials (Cu-B/CF and Cu-Cr/CF) have been elaborated by a powder metallurgy process and their thermal and mechanical properties correlated to the microstructure and the chemistry of the interfacial zones.

L’étude a pour objectif l’amélioration de la fiabilité des assemblages électroniques à travers la mise en œuvre de drains composites aluminium/fibres de carbone. Le travail a consisté à 1) modéliser, par la méthode des éléments finis, l’influence des propriétés thermiques et mécaniques du matériau de semelle sur l’assemblage életronique ; 2) élaborer (par frittage sous charge uniaxiale, frittage flash et extrusion à chaud) des matériaux composites aluminium/fibres de carbone ; et 3) lier les microstructures observées aux paramètres des procédés d’élaboration ainsi qu’aux propriétés thermiques et mécaniques mesurées. / This study has been done in order to improve power electronic devices reliability using aluminium/carbon fibres composites. This work has consisted in 1) determining, using finite elements method, the thermal and mechanical influence of the electronic base plate material; 2) elaborating (using hot pressing, spark plasma sintering and hot extrusion) aluminium/carbon fibres composites; and 3) linking the microstructures observed to the elaboration parameters and to the thermomechanical properties measured.

As crescentes exigências sobre o desempenho de sistemas de bombeamento multifásico combinadas aos aspectos relacionados com a maior disponibilidade operacional desses sistemas, bem como as futuras condições de funcionamento atingindo pressões perto de 150 bar, destacam a importância de desenvolver modelos matemáticos precisos para prever o comportamento do desempenho nestes equipamentos. Nesta tese foi aperfeiçoado o modelo termo-hidráulico de uma bomba multifásica de tipo duplo parafuso desenvolvido por Nakashima (2005) e foram incluídos os efeitos da abertura gradual da última câmara, recirculação de líquido entre a sucção e descarga, transferência de calor através do liner e expansão térmica. Uma vez fornecidos os dados geométricos da bomba e as suas condições de operação, é possível calcular os parâmetros de desempenho mais importantes, como: eficiência volumétrica, vazão de sucção e refluxo, potência consumida e distribuição de pressão e temperatura. As equações implementadas foram desenvolvidas a partir dos balanços de massa e energia nas câmaras, tendo em conta a geometria da bomba e a variação das fendas durante sua operação. As rotinas e métodos necessários para a sua solução numérica foram implementadas utilizando programação orientada a objetos (C++). Os resultados fornecidos pelo modelo aperfeiçoado foram comparados com dados experimentais da literatura e uma boa concordância foi encontrada na faixa de até 95 % FVG, nos casos estudados, para bombas com e sem tecnologia de recirculação. Devido à complexidade dos fenômenos físicos envolvidos durante a operação da bomba, o impacto de cada um dos efeitos incorporados nos cálculos do modelo foi avaliado e discutido individualmente. Assim, foi demonstrada a grande influencia da recirculação, da abertura gradual da câmara de descarga e da expansão térmica nos cálculos dos parâmetros de operação mais importantes da bomba. Além disso, a transferência de calor pode ser considerada desprezível, já que seu valor é baixo quando comparado com a potência fornecida pela bomba e, portanto, não influencia os balanços de energia que determinam os estados termodinâmicos das câmaras. No entanto, esse efeito é necessário para calcular a distribuição de temperatura da bomba e a expansão térmica nos parafusos e no liner. / The increasing requirements about the performance of multiphase pumping systems combined with those related to a higher operational availability of such systems, as well as future operating conditions with pressure increase at about 150 bar, highlights the importance of developing accurate mathematical models to predict the performance behavior of these equipments. In this thesis it was improved the thermo-hydraulic behavior of a twin screw multiphase pump developed by Nakashima (2005), and were included the effects of the gradual opening of the last chamber, fluid recirculation between suction and discharge of the pump, heat transfer though the liner, thermal expansion and different working fluids (water-air and oil-gas). Giving pump geometry and operational conditions, it is possible to calculate the most important pump parameters performance, such as, volumetric efficiency, suction flow, back-flow, power consumption and pressure and temperature distribution. The model equations were developed based on mass and energy balances in the chambers taking into account the pump geometry and the clearance variation due to operation. Its implementation was made in C++. The results obtained by the new model were compared with experimental data of the bibliography, and a good accuracy was found in it with values till 95% GVF for the studied cases, with and without recirculation technology. Due to the physical phenomenon complexity related with the pump operation, the impact of each effect in the model calculations was evaluated and discussed separately. So, it was demonstrated the importance of the recirculation, the gradual opening of the last chamber and the thermal expansion in the calculation of the most important pump operation parameters. However, the heat transfer can be neglected, because its value is very low when compared with the pump power supply, and therefore, it does not influence the energy balances that determine thermodynamic state in the chambers. However, this effect is necessary to calculate the temperature distribution along the pump and the thermal expansion in the screws and the liner.

Actuellement, la qualité de l’imagerie provenant de systèmes optiques spatiaux est limitée par la taille de leurs miroirs et la masse des structures supportant le miroir. Le développement de systèmes athermiques légers (un seul matériau) constitue le principal challenge dans l’amélioration de ces systèmes. De matériaux légers, résistants mécaniquement (E/ρ3 > 10, σf > 100 MPa) et stables thermiquement (< 2,0.e-6/K) doivent être développés. Dans ce cadre, notre travail porte sur l’élaboration de composites céramiques associant un matériau à coefficient de dilatation thermique (CTE) positif résistant mécaniquement (alumine ou zircone cériée) et un matériau à CTE très négatif (tungstate de zirconium ou β-eucryptite). L'étude du tungstate de zirconium a révélé plusieurs problèmes de décomposition et de réactions avec certaines matrices oxydes, menant à l’abandon de cet oxyde dans l’élaboration des composites. Dans le cas de la β-eucryptite, un phénomène de vermiculation a été mis en évidence, conduisant à la formation d’une porosité intragranulaire. L’optimisation des paramètres de frittage a permis de limiter cette porosité. L’étude du comportement thermique de la β-eucryptite confirme que son CTE très négatif provient principalement d’un phénomène de fissuration, généré par l’anisotropie de dilatation de sa maille cristalline. Cette fissuration est dépendante de la taille des grains mais également de la taille des agrégats de grains dans le cas des poudres. Ainsi, bien que le CTE intrinsèque de la β-eucryptite soit très faible (-0,4.e-6/K), son CTE extrinsèque peut atteindre des valeurs jusqu'à -10,9.e-6/K selon les conditions d’élaboration. Dans ce travail, deux stratégies d’élaboration de composites sont étudiées. Le premier cas consiste à diminuer le CTE des matrices oxydes à partir d’une poudre de β-eucryptite non microfissurée (-0,4.e-6/K) tandis que le second cas consiste à obtenir des matériaux à CTE très faible à partir d’une poudre de β-eucryptite microfissurée (-3,0.e-6/K). Lors de l’utilisation de la matrice en zircone cériée, le taux de dopage au cérium est optimisé afin de limiter la transformation de phase de la zircone. Cette transformation, induite par les contraintes de tension exercées par la β-eucryptite, affecte la linéarité du comportement thermique du composite. Dans les deux cas d’étude, les composites denses montrent une modification du CTE intrinsèque de la β-eucryptite passant de -0,4.e-6/K à plus de +3,2.e-6/K en raison des contraintes de compression appliquées par la matrice (alumine ou zircone cériée). La relaxation de ces contraintes nécessite une sous-densification des composites. A partir de ces observations, différents composites à CTE très faible sont élaborés. Toutefois, le sous-frittage des composites associé à la microfissuration de la β-eucryptite diminuent fortement les propriétés mécaniques des matériaux ainsi élaborés. / High resolution satellite imagery from space optical systems is mainly limited by the mirror size and the mass of structures supporting the mirror. Nowadays, the development of light athermal systems is the major challenge to improve these optical systems. So, light materials having good mechanical properties (E/ρ3 > 10, σf > 100 MPa) and thermal stability (< 2.0e-6/K) are required. Within this context, our project consists in processing new ceramic composites by combining positive thermal expansion coefficient (TEC) materials having good mechanical properties (alumina or ceria doped zirconia) and negative TEC materials (zirconium tungstate or β-eucryptite) The processing of zirconium tungstate-based materials showed several decomposition and chemical reactions with some oxide matrix leading to its giving up. In the case of β-eucryptite, vermicular phenomenon occurs during sintering leading to the formation of intragranular porosity. Sintering parameters optimization can limit this porosity. The study of the thermal behavior of pure β-eucryptite materials shows that the very negative TEC results from microcracking, generated by the TEC anisotropy of its crystal lattice. This microcracking depends on the grain size and the aggregate size in the case of powder materials. Despite the fact that the TEC of its lattice (called intrinsic TE C equals to -0.4e-6/K) is very low, its bulk (or extrinsic) TEC can reach values until -10.9e-6/K according to the processing conditions. In this work, two strategies for developing composites were studied. The first one consists in decreasing the matrix TEC using an uncracked β-eucryptite powder (-0.4e-6/K) while the second one consists in elaborating near zero TEC materials from a microcracked β-eucryptite powder (-3.0e-6/K). When ceria-doped zirconia is used, ceria content must be adjusted in order to limit zirconia phase transformation. This transformation is driven by tensile stresses induced by the β-eucryptite and modifies the composite thermal behavior linearity. In both studied cases, dense composites show a modification of the β-eucryptite intrinsic TEC from -0.4e-6/K to more than +3.2e-6/K as a consequence of compressive stresses applied by the oxide matrix. An uncompleted densification of composites is required to relax these stresses. Taking into account these observations, several very low TEC composites were elaborated. However, the uncompleted densification of composites and the β-eucryptite microcracking greatly decrease the mechanical properties of these materials.

This PhD thesis presents investigations of perovskite-related compounds in systems of interest for applications in components in solid oxide fuel cells. The compound compositions derive from substitutions in the parent compounds LaCoO3, LaCrO3 and SrFeO3. Novel phases La2Co1+z(MgxTi1-x)1-zO6 were synthesized and investigated with regard to structure, thermal expansion, electronic and magnetic properties. The study focused on the composition lines La2Co(MgxTi1-x)O6 (z=0), where the oxidation state of Co nominally changes from +2 (x=0.0) to +3 (x=0.5), and La2Co1+z(Mg0.5Ti0.5)1-zO6, with a varying fraction of Co3+ ions. XANES data show that the Co ions in the system have discrete oxidation states of +2 and +3. The TEC increases with increasing x due to an increasing contribution from spin state transitions of the Co3+ ions. Novel compounds La2Cr(M2/3Nb1/3)O6 with M=Mg, Ni, Cu were synthesized and characterized with respect to structure and magnetic properties. XRPD and NPD data indicate Pbnm symmetry; however, SAED patterns and HREM images indicate a P21/n symmetry for M=Mg, and Cu. The magnetic measurements results were rationalized using the Goodenough-Kanamori rules. Oxygen-deficient phases with x≥0.63 in SrxY1-xFeO3-δ and Sr0.75Y0.25Fe1-yMyO3-δ (M=Cr, Mn, Ni and y=0.2, 0.33, 0.5), were synthesized and characterized with respect to structure, oxygen content, thermogravimetry, TEC, conductivity and magnetic properties. Powder patterns of phases agree with cubic  perovskite structures. NPD data for x=0.75 reveal anisotropic displacement for the O atom, related to local effects from Fe3+/Fe4+ ions. SAED patterns for x=0.75 reveal the presence of an incommensurate modulation. The compounds start to lose oxygen in air at ~ 400°C. The TEC up to ~400°C for x=0.75 is ~10.5 ppm/K and increase to ~17.5 ppm/K at higher temperatures. The conductivity for x=0.91 is 164 S/cm at 400°C. Partial substitution of Fe by Cr, Mn or Ni does not increase the conductivity or decrease TEC.

Es werden Fragestellungen aus zwei aktuellen Problemkreisen der elektronisch hochkorrelierten Materialien untersucht. Dem unkonventionellen supraleitenden Zustand sowie dessen Wechselspiel mit magnetischen Effekten gelten die Arbeiten am Schwere-Fermionen-System UBe13 sowie der Dotierungsreihe (UTh)Be13. Sogenanntes Nicht-Fermiflüssigkeits-Verhalten steht im Zentrum der Untersuchungen an der Schwere-Fermionen-Verbindung CeNi2Ge2. Der Schwere-Fermionen-Supraleiter U1-xThxBe13 zeigt neben einem nichtmonotonen Verlauf der Übergangstemperatur in den supraleitenden Zustand Tc(x) einen zweiten Phasenübergang Tc2 &amp;lt; Tc im Konzentrationsbereich 0,019 &amp;lt; x &amp;lt; 0,0455. Als Ursache dieses Übergangs werden sowohl eine mit dem supraleitenden Zustand koexistierende magnetische Ordnung (Spindichtewelle) als auch eine Änderung des supraleitenden Zustands selber diskutiert. Hier konnte mittels dilatometrischer Untersuchungen gezeigt werden, dass der Phasenübergang bei Tc2 eine Vorläuferstruktur im Bereich x &amp;lt; 0,019 besitzt. Die aus diesem Ergebnis folgende Zuordnung charakteristischer Linien im T-x-Diagramm von U1-xThxBe13 schließt gängige Szenarien, die sich für T &amp;lt; Tc2 ausschließlich auf die Änderung des supraleitenden Zustands beziehen, aus. Manche Schwere-Fermionen-Verbindungen zeigen bis zu tiefsten Temperaturen keinen Übergang in einen kohärenten Fermiflüssigkeits-Zustand. Als Ursache dieses Nicht-Fermi-flüssigkeits-Verhaltens wird u.a die Ausbildung kritischer Spinfluktuationen diskutiert. Diese magnetischen Fluktuationen werden in Nähe eines quantenkritischen Punkts (QKP) erwartet, für den bei T=0 als Funktion eines Kontrollparameters ein magnetischer Phasenübergang auftritt. Die Vorhersagen des Konzepts eines &amp;quot;nearly antiferromagnetic Fermi liquid&amp;quot;, für die Temperaturabhängigkeiten verschiedener Messgrößen von Systemen nahe eines QKP können an der Verbindung CeNi2Ge2 überprüft werden. Während bei nicht allzu tiefen Temperaturen in der vorliegenden Arbeit eine Übereinstimmung mit den Vorhersagen gefunden wurde, muss die Anwendbarkeit des Konzepts für CeNi2Ge2 bei tiefsten Temperaturen in Frage gestellt werden.

O Tungstato de Zircônio (ZrW2O8) é um material que exibe Expansão Térmica Negativa (ETN), isotrópica em um amplo intervalo de temperatura (0,3 a 1050 K). Apesar de amplamente estudado, existem controvérsias acerca dos mecanismos microscópicos responsáveis por este comportamento. A fase cúbica deste composto, denominada a-ZrW2O8, já foi motivo de estudo através de simulações computacionais utilizando Potenciais Interatômicos (PI) e Dinâmica de Rede na Aproximação Quasi-Harmônica (DRQH). Nos dois PI distintos propostos na literatura conseguiu-se reproduzir a ETN da a-ZrW2O8, mas não a dependência com a temperatura do tensor de elasticidade. É partindo desta observação que este trabalho pretende avaliar o desempenho de PI existentes e de novos PI em simulações computacionais visando a descrição da dependência com a temperatura do tensor de elasticidade da a-ZrW2O8 entre 0 e 300 K. Utilizaram-se dados experimentais, tais como posições atômicas, parâmetros de rede e o tensor de elasticidade da a-ZrW2O8 em temperaturas entre 0 e 300 K e, em outra série de cálculos, a hipersuperfície de energia ab initio no limite atérmico para obter os parâmetros dos PI. Diferentes estratégias foram empregadas na busca pelos parâmetros dos PI incluindo minimização em linha, Newton-Raphson/BFGS e Algoritmo Genético (AG). Concluiu-se que não é possível descrever as propriedades estruturais e elásticas da a-ZrW2O8 em função da temperatura com PI simples e que esta incapacidade não está relacionada a qualquer limitação da DRQH ou dos parâmetros dos PI, mas à forma analítica dos PI empregados. Isto sugere que se deve ter cautela na interpretação de resultados obtidos com estes potencias já disponíveis na literatura. Como alternativas para a solução deste problema, pode-se considerar o uso de redes neurais para a representação da hipersuperfície de energia ab initio, o uso de PI mais sofisticados que levam em consideração a vizinhança atômica (bond order potentials) e, também, cálculo ab initio a T > 0, este último a um custo computacional muito mais elevado. / Submitted by Marcelo Teixeira (mvteixeira@ucs.br) on 2014-05-28T17:16:32Z No. of bitstreams: 1 Dissertacao Emiliano Chemello.pdf: 1343523 bytes, checksum: 46461698a2b6139def916307ab93478f (MD5) / Made available in DSpace on 2014-05-28T17:16:32Z (GMT). No. of bitstreams: 1 Dissertacao Emiliano Chemello.pdf: 1343523 bytes, checksum: 46461698a2b6139def916307ab93478f (MD5) / Zirconium tungstate (ZrW2O8) is a material that exhibits negative thermal expansion (NTE), over a wide temperature range (0.3 at 1050 K). Although thoroughly studied, controversies still remain concerning the microscopic mechanisms responsible for this behavior. The cubic phase of this compound, denominated a-ZrW2O8, was already the subject of study through computer simulations using interatomic potentials (IP) and lattice dynamics in quasiharmonic approximation (LDQH). In two different IPs proposed in the literature succeeded in reproducing the a-ZrW2O8 NTE, but not the dependence with temperature of the elasticity tensor. Starting from this observation, this work intends to evaluate of existent IPs and same proposed new IPs in computer simulations aiming the calculation of the tensor of elasticity for a-ZrW2O8 between 0 and 300 K. Experimental data (such as atomic positions, lattice parameters and the tensor of elasticity of a-ZrW2O8 at 0 and 300 K) and, in another series of calculations, the ab initio energy hypersurface in the athermic limit, were used to obtain the parameters of the IPs. Different strategies were used in the search for the parameters of IP, including line minimization, Newton-Raphson/BFGS and genetic algorithm (GA). At the end of an exhaustive search we were led to conclude that it is not possible to describe the structure and elastic properties of a-ZrW2O8 as a function of temperature with simple IPs and that this incapacity is not related the any limitation of LDQH or of the parameters of the IPs, but instead to the analytical form of the tested IPs. This suggests that same results obtained with IPs already available in the literature may be unreliable. As alternatives for the solution of this problem, it can be considered the use of a neural network for the representation of the ab initio energy hypersurface, the use of more sophisticated IPs than take into account the atomic neighborhood (bond order potentials) and even (with a computational cost much higher) ab initio calculations at T > 0.