As cerâmicas a base de carbeto de silício, SiC, tem muitas aplicações na engenharia devido às suas excelentes propriedade mecânicas, térmicas e químicas. Para a produção de cerâmicas de SiC com propriedades específicas, seu processamento deve ser escolhido de maneira a se produzirem microestruturas adequadas para cada aplicação. Para isso, na maioria das vezes, essas cerâmicas são sinterizadas via fase líquida, usando como aditivos formadores dessa fase misturas de óxidos de alumínio e algumas terras raras. Esta dissertação tem dois objetivos principais: implantar o método SEVNB no Departamento de Engenharia de Materiais (DEMAR), na Escola de Engenharia de Lorena (EEL), para medida de KIC, propriedades mecânicas que, muitas vezes, limita aplicações das cerâmicas, e introduzir mais dois óxidos de terras raras para formação da fase líquida na sinterização do SiC, os óxidos de disprósio e itérbio. Após a mistura e secagem das matérias primas, os pós foram prensados com dupla ação de pistões e isostaticamente, e em seguida foram sinterizados em atmosfera de argônio utilizando seis ciclos diferentes para cada sistema de aditivos. Após sinterização as amostras foram caracterizadas e entalhadas com lâmina de barbear utilizando pastas diamantadas de 6 e 0,25 ?m e flexionadas em 4 pontos. Os resultados mostraram uma maior densificação para o sistema de aditivos Al2O3/ Yb2O3, de aproximadamente 97% da densidade teórica, prensados a 400 MPa e sinterizados na temperatura de 1950ºC por 1 hora, com formação de ?-SiC, DyAG e YbAG em todas as temperaturas estudadas. A máquina fabricada para entalhamento das amostras se mostrou eficiente, produzindo raios de entalhes atendendo a norma ISSO 23146:2008. Quanto as propriedades mecânicas, o sistema com itérbia possui os maiores valores de módulo de elasticidade e dureza, porém menor tenacidade à fratura em relação a cada ciclo de sinterização, resultado que sugere a influência do raio catiônico das terras raras formadoras da fase secundária nas cerâmicas a base de SiC. / The silicon carbide, SiC, has many applications in engineering due to its excellent mechanical, thermal and chemical properties. For the production of SiC ceramics with specific properties, the processing should be chosen to produce microstructures suitable for each application. For this reason, in most cases, these ceramics are liquid phase sintered using additives such as formers of this phase mixed oxides of aluminum and certain rare-earths. This dissertation has two main objectives: to implement the method SEVNB at Department of Materials Engineering (DEMAR), Engineering School of Lorena (EEL), to measure KIC, mechanical propertie, which often limits the ceramic\'s applications, and introduce two more rare-earth oxides for the formation of liquid phase sintering SiC, dysprosium and ytterbium oxide . After mixing and drying, the powders were pressed with a double acting piston and isostatically, and then sintered in an argon atmosphere using six different cycles for each additive system. After sintered, samples were characterized and V-notched using a razor blade and diamond pastes of 6 and 0.25 ?m and four-point flexured. The results showed a higher densification for the system Al2O3 / Yb2O3, approximately 97% , pressed at 400 MPa and sintered at a temperature of 1950 ° C for 1 hour, with formation of ?-SiC, DyAG and YbAG at all temperatures studied. The machine used to notched the samples proved to be effective, producing notch root according to the standard ISO 23146:2008. As for the mechanical properties, the system with ytterbium oxide has higher values of elastic modulus and hardness, but lower fracture toughness for each sintering cycle studied, result that suggests the influence of the rare-earth\'s cationic radius that forming the secondary phase in the SiC.

Ce travail porte sur la caractérisation optique d'échantillons de 4H-SiC. Les échantillons étudiés ont été répartis en deux groupes : type-n et type-p. La croissance des épitaxies a été réalisée par CVD technique utilisant horizontal, paroi chaude, chauffée par résistance, en utilisant de l'hydrogène comme gaz porteur silane et/propane en tant que précurseurs de Si/C respectivement. Pour atteindre différents dopages : N2 pour le n-type et TMA pour de type p ont été utilisés. Les échantillons ont été étudiés par photoluminescence en basse température, micro-Raman and spectroscopies de masse d'ions secondaires. Pour les échantillons de type-p mesures d'effet Hall ont été utilisés pour déterminer la concentration de porteurs. Avec l'aide de ces techniques, il a été possible de déterminer le niveau de dopage dans une très large gamme pour les deux types. Les deux spectroscopies : Raman et LTPL peut donner des informations sur la concentration, polytype, qualité du cristal et concentration de porteurs, mais seulement LTPL fournit des informations sur la compensation et est indispensable de définir la polarité. Pour les échantillons faiblement dopés les meilleures façons de déterminer le niveau de dopage semble être des mesures LTPL. Pour les échantillons fortement dopés on a remarqué l'avantage de Raman, qui permet de déterminer la concentration en porteurs jusqu'à 10^20cm-3. Enfin en utilisant les mesures électriques et de Fano-paremeters obtenus grâce à micro-Raman, nous avons fait la courbe d'étalonnage pour type p 4H-SiC. / The main topic of this thesis is the optical characterization of 4H-SiC samples. The samples were divided in 2 groups: type-n doped with nitrogen and type-p doped with aluminum. Samples were grown by CVD method performed in a horizontal, hot wall, resistively heated, using hydrogen as a carrier gas and silane/propane as Si/C precursors respectively. To achieve different doping N2 for n-type and TMA for p-type were used. The samples were studied by three different spectroscopies techniques: low temperature photoluminescence, micro-Raman and secondary ion mass spectroscopies. For p-type samples Hall effect measurements were used to determine carrier concentration. With the help of this techniques it was possible to determine doping level in a very large range for both types. Both LTPL and Raman spectroscopy can give information about the polytype, crystal quality and carrier concentration but only LTPL provides information about compensation and is indispensable to define the polarity. For low doped samples since the LOPC & FTA modes of Raman spectra do not exhibit any significant changes the best ways seems to be LTPL measurements. For the highest doped samples notice the advantage of Raman which allows to determine the carrier concentration up to 10^20cm-3. Finally due to electrical measurements and fano-paremeters obtained by micro-Raman spectra we made calibration curve for p –type 4H-SiC.

In modern high temperature nuclear reactors, silicon carbide (SiC) is used as the main diffusion barrier for the fission products in coated fuel spheres called TRISO particles. In the TRISO particle, pyrolytic carbon and SiC layers retain most of the important fission products like xenon, krypton and cesium effectively at temperatures up to 1000 oC. Previous studies have shown that 400 oC to 600 oC implantation of heavy ions into single crystal 6H-SiC causes the SiC to remain crystalline with many point defects and dislocation loops (damage). The release of Xe at annealing temperatures above 1400 oC is governed by the normal volume diffusion without any hindrance of trapping effects. In this study two phenomena in single crystal 6H-SiC implanted by 360 keV Xenon ions were studied using Rutherford Backscattering Spectroscopy (RBS) and channeling. Radiation damage and its annealing behavior at annealing temperatures ranging from 1000 oC to 1500 oC, and the diffusion of xenon in 6H-SiC at these annealing temperatures were investigated. 360keV xenon ions were implanted into a single crystalline wafer (6H-SiC) at 600 oC with a fluence of 1 × 1016 cm-2. The sample was vacuum annealed in a computer control Webb 77 graphite furnace. Depth profiles were obtained by Rutherford backscattering spectrometry (RBS). The same set-up was used to investigate radiation damage of the 6H-SiC sample by channeling spectroscopy. Isochronal annealing was performed at temperatures ranging from 1000 to 1500 °C in steps of 100 oC for 5 hours. Channeling revealed that the 6H-SiC sample retained most of its crystal structure when xenon was implanted at 600 °C. Annealing of the radiation damage took place when the sample was heat treated at temperatures ranging from 1000 oC to 1500 oC. The damage peak almost disappears at 1500 oC but the virgin spectrum was not achieved. This happened because of dechanneling due to extended defects like dislocations remaining in the implanted region. RBS profiles showed that no diffusion of the Xe occurred when the sample was annealed at temperatures from 1000 oC to 1400 oC. A slight shift of the xenon peak position towards the surface after annealing at 1400 °C was observed for 600 oC implantation. After annealing at 1500o C, a shift toward the surface accompanied by a broadening of the Xe peak indicating that diffusion took place. This diffusion was not accompanied by a loss of xenon from the SiC surface. The shift towards the surface is due to thermal etching of the SiC at 1400-1500 °C. Modern high temperature gas-cooled reactors operate at temperatures above 600 oC in the range of 750 oC to 950 oC. Consequently, our results indicate that the volume diffusion of Xenon in SiC is not significant in SiC coated fuel particles. / Dissertation (MSc)--University of Pretoria, 2014. / gm2014 / Physics / unrestricted

La croissance localisée de SiC dopé p par un mécanisme Vapeur-Liquide-Solide (VLS) a été effectuée sur substrats SiC-4H (0001) 8°off et diamant (100). Pour ce faire, des motifs constitués d'un empilement silicium-aluminium sont fondus puis alimentés en propane. Dans le cas de l'homoépitaxie de SiC-4H, il a été démontré que la quantité limitée de phase liquide initiale entraine une évolution constante des paramètres de croissance en raison de l'appauvrissement graduel en silicium. Il est toutefois possible de trouver des conditions de croissance satisfaisantes (alliage contenant 40 at% Si, 1100 °C) résultant en un dépôt conforme sur l'ensemble des motifs avec une morphologie step-bunchée. A partir de tels dépôts, des contacts ohmiques de très faible résistivité (jusqu'à 1,3.10-6 Ω.cm2) ont été mesurés et des diodes PiN ont été fabriquées et caractérisées. Dans le cas de la croissance de SiC sur diamant, la forte réactivité entre l'alliage Si-Al liquide et le substrat diamant conduit à la formation d'un dépôt dense et polycristallin de SiC-3C par un mécanisme de dissolution-précipitation. Nous avons montré que la formation préalable d'une couche tampon nanométrique de SiC par siliciuration du substrat de diamant (réaction solide-solide entre une couche de Si et le diamant) permet d'obtenir une croissance épitaxiale de SiC-3C en ilots, avec les relations [110] SiC // [110] diamant et (100) SiC // (100) diamant. Il n'a cependant pas été possible de former une couche complète et épitaxiale de SiC sur diamant par VLS localisée. Nous avons toutefois montré que cela est réalisable par dépôt chimique en phase vapeur (CVD) en utilisant la même étape de siliciuration / The localized growth of p-doped SiC by Vapor-Liquid-Solid (VLS) mechanism was made on (0001) 8°off 4H-SiC and (100) diamond substrates. A silicon-aluminium stacking, localized on top of the substrate, is used after melting as the liquid phase for the growth, carbon being brought by the propane of the gas phase. Regarding the homoepitaxial growth of 4H-SiC, the limited amount of liquid phase leads to a significant consumption of silicon during the growth which is responsible for a continuous variation of the growth parameters. Satisfying growth conditions can therefore be found (40 at% Si alloy, 1100 °C) leading to the formation of a step-bunched layer on the initial Si-Al patterns. Very Low resistivity ohmic contacts (as low as 1.3x10-6 Ω.cm2) and PiN diodes were successfully fabricated from these deposits. Regarding the SiC growth on diamond, the high reactivity between the Si-Al liquid alloy and the diamond substrate leads to the polycrystalline growth of 3C-SiC by a dissolution-precipitation mechanism. It is thus necessary to use a SiC buffer layer in order to achieve an epitaxial growth. This buffer layer, grown by a solid-solid reaction between silicon (deposited by CVD) and the diamond called silicidation, favors the epitaxial growth of 3C-SiC ([110] SiC // [110] diamond and (100) SiC // (100) diamond) during the later VLS growth. Considering the 3D growth mechanism that takes place the formation of a single-crystalline layer from these epitaxial islands seems difficult. Such single-crystalline layer can be achieved using chemical vapor deposition (CVD) after the silicidation step of the diamond substrates

Defect-induced ferromagnetism is attracting intensive research interest. It not only challenges the traditional opinions about ferromagnetism, but also has some potential applications in spin-electronics. SiC is a new candidate for the investigation of defect-induced ferromagnetism after graphitic materials and oxides due to its high material purity and crystalline quality. In this thesis, we made a comprehensive investigation on the structural and magnetic properties of ion implanted and neutron irradiated SiC sample. In combination with X-ray absorption spectroscopy and first-principles calculations, we try to understand the mechanism in a microscopic picture. For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. We qualitatively explain the magnetic properties as a result of the intrinsic clustering tendency of defects. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling. Thus, we provide a direct correlation between the collective magnetic phenomena and the specific electrons/orbitals. With the aim to verify if the defect-induced magnetization can be increased by orders of magnitude, i.e., if a sample containing defects through its bulk volume can persist ferromagnetic coupling, we applied neutron irradiation to introduce defects into SiC. Besides a weak ferromagnetic contribution, we observe a strong paramagnetism, scaling up with the neutron fluence. The ferromagnetic contribution induced by neutron irradiation only occurs in a narrow fluence window or after annealing. It seems non-realistic to make the bulk specimens ferromagnetic by introducing defects. Instead, we speculate that defect-induced ferromagnetism rather locally appears in particular regions, like surface/interface/grain boundaries. A comparable investigation on neutron irradiated graphite supports the same conclusion.

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Los materiales compuestos de matriz cerámica basados en carburo de silicio (SiC) presentan excelentes propiedades mecánicas y gran estabilidad estructural a temperaturas superiores a los 1000°C. Por otro lado, el iridio presenta excelentes propiedades a bajas y altas temperaturas y tiene la ventaja de ser altamente resistente a la corrosión. El principal objetivo de esta Tesis Doctoral es sintetizar materiales compuestos basados en SiC e Ir mediante infiltración reactiva con el fin de lograr una sinergia entre las propiedades individuales de los materiales constituyentes. Para lograr una comprensión total del método de procesamiento por infiltración reactiva en estos materiales, la Tesis fue organizada en 7 capítulos. En el capítulo 1 se desarrolla el estado del arte de los materiales compuestos. En el capítulo 2 se explican los materiales y métodos empleados en el seguimiento de la reacción de infiltración, las técnicas de caracterización y los ensayos termomecánicos. En el capítulo 3 se realiza un estudio fundamental de la mojadura de aleaciones de Si-Ir con creciente concentración de Ir sobre sustratos de carbono y SiC. En el capítulo 4 se estudia la reacción del estado sólido y los fenómenos de interfase del Ir puro sobre sustratos de SiC. En el capítulo 5 se evalúa el efecto de la atmósfera de procesamiento en los materiales compuestos finales. En el capítulo 6 se sintetizan los materiales compuestos SiC/Si-IrSi3, donde se estudia el método de infiltración reactiva gracias a la técnica de gota yacente y se evalúan las propiedades finales del material producido. Finalmente, en el capítulo 7 se resumen las principales conclusiones generadas en la memoria. En general, este estudio es una contribución importante para el desarrollo de materiales avanzados que puedan ser utilizados en ambientes altamente oxidantes y/o que requieran alta resistencia a la corrosión (como, por ejemplo, aplicaciones electrónicas). En conclusión, en esta Tesis Doctoral, se ofrece un estudio sistemático y fundamental de la infiltración reactiva para la síntesis de materiales compuestos basados en SiC e Ir. Queda demostrado que los materiales compuestos SiC/Si-IrSi3 surgen como candidatos ideales para aplicaciones estructurales de alta temperatura debido a sus excelentes propiedades termomecánicas y su prometedora resistencia a la oxidación.

L’introduction des systèmes électriques notamment des dispositifs d’électronique de puissance « embarqués » dans le domaine de transport aérien, requiert une forte compacité et une forte intégration des systèmes de conversions électriques ainsi des fonctionnalités électroniques a des hautes t ambiantes dépassant les 200°C.Mais pour répondre à ces besoins, le silicium a atteint ses limites d'où le recours à des nouveaux matériaux semi-conducteurs comme ceux à grand gap.Dans le cadre de ses recherches, Thales, en collaboration avec le laboratoire SATIE , mènent des investigations sur les performances des semi-conducteurs en technologie sic pour développer des convertisseurs aéronautiques de nouvelle génération.Ces travaux de thèse s'inscrivent dans le cadre d'une étude des potentialités des premiers composants SiC disponibles sur le marché. Cette étude permet de comparer les principales performances de ces dispositifs ainsi que les contraintes spécifiques induites lors de leur intégration dans les convertisseurs aéronautiques. / The potential of wide band gap transistors based on silicon carbide are remarkable and open new prospects for improvement (high efficiency, high breakdown voltage, high switching frequency and high operating temperature ...). This leads to volume and weight reduction for future converters. In order to improve new generation of power converters for future needs, Thales, in cooperation with SATIE and LTN IFSTTAR laboratories, performs investigations on the performances, advantages and disadvantages of SiC technology semiconductors. This comparative study will assess the strengths and weaknesses as well as the stresses induced during integration into aircraft converters of commercially available devices.

Dans l’industrie aéronautique, tant au niveau des constructeurs que des équipementiers, les actionneurs électriques sont de plus en plus utilisés. De ce fait, l’apparition de convertisseurs de puissance à découpage basés sur les nouveaux composants à grand gap en carbure de silicium (MOSFET, JFET état on ou off) engendre de plus en plus des perturbations électriques conduites (BF et HF) et rayonnées (HF) au sein des systèmes, ils sont de plus sources de pertes. L’objectif de cette thèse est donc de mettre en place une approche de modélisation des différents éléments d’un convertisseur DC-DC qui puisse servir à mettre en œuvre un processus d’optimisation afin que soit pris en compte dès la phase de conception les contraintes liées à la CEM et à la thermique. Dans un premier temps, une étude de la physique des semi-conducteurs et en particulier celle du carbure de silicium (SiC) a été réalisée afin de développer une approche de modélisation des MOSFET et diode de puissance SiC adaptée à ces composants. Les modèles ainsi développés sont des modèles fins qui prennent en compte les effets liés à la thermique. Ces modèles ont donc dans un premier temps servi à la réalisation de simulations afin d’évaluer tant sur le plan thermique qu’électromagnétique, la validité de l’approche de modélisation. Par la suite afin de prendre en compte les contraintes CEM et thermique dès la phase de conception, une approche de dimensionnement par optimisation a été mise en œuvre. A cause de sa rapidité d’exécution, le choix a été porté sur un algorithme d’optimisation déterministe. Ce choix a donc imposé le développement préalable de modèles de dimensionnent et d’optimisation analytiques. L’ensemble de ces modèles a été utilisé dans l’environnement logiciel d’optimisation CADES et des résultats concernant les masses optimales de convertisseur pour différentes fréquences de commutation ont été obtenus. L’analyse des résultats d’optimisation a permis dans la dernière partie de se rendre compte que si l’évolution de la masse du dissipateur en fonction de la fréquence de commutation est semblable à ce que l’on peut prédire (l’augmentation de la fréquence de commutation entraîne l’augmentation des pertes et donc l’augmentation de la masse du dissipateur), celle des passifs et du filtre CEM ne peut être facilement anticipée. Il existe une compétition entre la masse du filtre et celle des passifs, car en effet une fréquence de commutation élevée suppose en première approximation des éléments passifs de lissage moins importants, mais des problèmes CEM beaucoup plus importants. Pour compléter les études menées l’approche préalablement mise en œuvre pour des composants SiC a été transposée à des composants silicium pour des besoins de comparaison. / In the aeronautics industry, both manufacturers and OEMs, electric actuators are increasingly used. As a result, the appearance of switching power converters based on new large-gap silicon carbide components (MOSFET, JFET state on or off) increasingly generates electrical (LF and HF) and radiated perturbations (HF) within the systems, they are also sources of losses.The objective of this thesis is therefore to set up a modeling approach for the various elements of a DC-DC converter that can be used to implement an optimization process so that the design phase is taken into account constraints related to EMC and thermal.First, a study of the physics of semiconductors and in particular that of silicon carbide (SiC) was carried out in order to develop a modeling approach of the MOSFET and SiC power diode adapted to these components. The models thus developed are fine models that take into account the effects related to thermal. These models were therefore used initially to perform simulations in order to evaluate both thermally and electromagnetically the validity of the modeling approach.Subsequently, in order to take into account the EMC and thermal constraints from the design phase, an optimization dimensioning approach was implemented. Because of their speed of execution, the choice was made on a deterministic optimization algorithm. This choice therefore imposed the prior development of models of analytical dimensioning and optimization. All of these models have been used in the CADES optimization software environment and results regarding optimal converter masses for different switching frequencies have been obtained.Analysis of the optimization results in the last part of the study revealed that if the evolution of the dissipator mass as a function of the switching frequency is similar to what can be predicted the switching frequency causes the losses to increase and therefore the dissipator mass increases), that of the passive and the EMC filter can not be easily anticipated. There is a competition between the mass of the filter and that of the passive, since a high switching frequency presupposes, as a first approximation, the passive elements of lesser smoothing, but EMC problems are much greater.To complete the studies carried out, the previously implemented approach for SiC components has been transposed to silicon components for comparison purposes.

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