Made available in DSpace on 2014-12-17T14:07:04Z (GMT). No. of bitstreams: 1 JulianaPN_DISSERT.pdf: 2953662 bytes, checksum: 47355bc7fed206f60c6c6b5d8565099b (MD5) Previous issue date: 2012-06-06 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Nickel alloys are frequently used in applications that require resistance at high temperatures associated with resistance to corrosion. Alloys of Ni-Si-C can be obtained by means of powder metallurgy in which powder mixtures are made of metallic nickel powders with additions of various alloying carriers for such were used in this study SiC, Si3N4 or Si metal with graphite. Carbonyl Ni powder with mean particle size of 11 mM were mixed with 3 wt% of SiC powders with an average particle size of 15, 30 and 50 μm and further samples were obtained containing 4 to 5% by mass of SiC with average particle size of 15 μm. Samples were also obtained by varying the carrier alloy, these being Si3N4 powder with graphite, with average particle size of 1.5 and 5 μm, respectively. As a metallic Si graphite with average particle size of 12.5 and 5 μm, respectively. The reference material used was nickel carbonyl sintered without adding carriers. Microstructural characterization of the alloys was made by optical microscopy and scanning electron microscopy with semi-quantitative chemical analysis. We determined the densities of the samples and measurement of microhardness. We studied the dissociation of carriers alloy after sintering at 1200 ? C for 60 minutes. Was evaluated also in the same sintering conditions, the influence of the variation of average particle size of the SiC carrier to the proportion of 3% by mass. Finally, we studied the influence of variation of the temperatures of sintering at 950, 1080 and 1200 ? C without landing and also with heights of 30, 60, 120 and 240 minutes for sintering where the temperature was 950 ?C. Dilatometry curves showed that the SiC sintered Ni favors more effectively than other carriers alloy analyzed. SiC with average particle size of 15 μm active sintering the alloy more effectively than other SiC used. However, with the chemical and morphological analyzes for all leagues, it was observed that there was dissociation of SiC and Si3N4, as well as diffusion of Si in Ni matrix and carbon cluster and dispersed in the matrix, which also occurred for the alloys with Si carriers and metallic graphite. So the league that was presented better results containing Si Ni with graphite metallic alloy as carriers, since this had dispersed graphite best in the league, reaching the microstructural model proposed, which is necessary for material characteristic of solid lubricant, so how we got the best results when the density and hardness of the alloy / Ligas de N?quel s?o freq?entemente utilizadas em aplica??es que requerem resist?ncia mec?nica a elevadas temperaturas associada ? resist?ncia ? corros?o. Ligas de Ni-Si-C podem ser obtidas por meio de metalurgia do p? em que s?o realizadas misturas de p?s de n?quel met?lico com adi??es de p?s de diferentes portadores de liga, para tal foram utilizados neste trabalho SiC, Si3N4 ou Si met?lico com grafita. P?s de Ni carbonila com tamanho m?dio de part?culas de 11 μm foram misturados a 3% em massa de p?s de SiC com tamanho m?dio de part?culas de 15, 30 e 50 μm e foram obtidas ainda amostras contendo 4 e 5% em massa de SiC com tamanho m?dio de part?culas de 15 μm. Tamb?m foram obtidas amostras variando-se o portador de liga, sendo estes p?s de Si3N4 com grafita, com tamanho m?dio de part?culas de 1,5 e 5 μm, respectivamente. Assim como Si met?lico com grafita, com tamanho m?dio de part?culas de 12,5 e 5 μm, respectivamente. O material de refer?ncia adotado foi n?quel carbonila sinterizado sem adi??o de portadores. A caracteriza??o microestrutural das ligas foi feita por microscopia ?ptica e eletr?nica de varredura com an?lise qu?mica semi-quantitativa. Foram determinadas as densidades das amostras e obtidas medidas de microdureza Vickers. Foi estudada a dissocia??o dos portadores de liga ap?s sinteriza??o em 1200 ?C durante 60 minutos. Foi avaliada, ainda, para as mesmas condi??es de sinteriza??o, a influencia da varia??o do tamanho m?dio de part?culas do portador SiC, para a propor??o de 3% em massa. Por fim, foi estudo a influencia da varia??o das temperaturas de sinteriza??o em 950, 1080 e 1200 ?C sem patamar e, ainda, com patamares de 30, 60, 120 e 240 minutos para sinteriza??o cuja temperatura foi de 950 ?C. As curvas de dilatometria mostraram que o SiC favorece a sinteriza??o do Ni de forma mais eficaz que os demais portadores de liga analisados. O SiC com tamanho m?dio de part?culas de 15 μm ativa a sinteriza??o da liga de forma mais eficaz que os demais SiC utilizados. Por?m, com as an?lises qu?mica e morfol?gica para todas as ligas, foi poss?vel observar que houve dissocia??o do SiC e do Si3N4, assim como difus?o do Si na matriz de Ni e carbono aglomerado e disperso na matriz, o que tamb?m ocorreu para as ligas com portadores Si met?lico e grafita. Portanto, a liga que apresentou melhores resultados foi de Ni contendo Si met?lico com grafita como portadores de liga, j? que esta apresentou grafita melhor dispersa na liga, atingindo o modelo microestrutural proposto, do qual ? necess?rio para material com caracter?stica de lubrificante s?lido, assim como obteve os melhores resultados quando a densidade e dureza da liga

Grâce à ses excellentes propriétés de résistance et de conductivité thermique ainsi qu'à sa stabilité mécanique et chimique à température ambiante et à haute température, le carbure de silicium (SiC) est un matériau de choix pour le gainage du combustible nucléaire ou les supports de catalyseurs. Cependant, une grande surface spécifique est souvent requise pour ce type d'applications. Cette étude propose deux approches de synthèse :(1) L'approche « Soft Templating ». La porosité et la structure des matériaux finaux sont définies par l'auto-assemblage supramoléculaire d'un agent de structure (SDA) dans un précurseur moléculaire de SiC. Des organogélateurs à faible masse moléculaire et un copolymère tri-bloc commercial sont considérés pour la synthèse de SiC méso-poreux.(2) L'approche « Hard Templating ». Des céramiques SiC sont synthétisées par nanomoulage de silices méso-poreuses par des polymères précéramiques. Ce procédé conserve la nanostructure du template solide et conduit à des SiC méso-poreux à forte surface spécifique.L'approche hard templating permet une bonne réplication du template solide mais la difficulté de cette méthode provient de l'étape d'élimination de ce même template. L'approche soft templating ne présente pas ce désavantage et peut, suivant le SDA utilisé, mener à des céramiques poreuses possédant des structures beaucoup plus variées. La complexité de cette approche réside dans l'étape de réplication du template. / Due to its excellent thermal resistance, mechanical and chemical stability both at room and elevated temperature, silicon carbide (SiC) is an attractive material for nuclear fuel cladding or catalyst substrates. Pore size control and high porosity are the key factors for such applications. Two approaches are studied during this PhD thesis:(1) The Soft Templating Approach. The porosity and the structure of the final materials are defined by the supramolecular self-assembly of a structure directing agent (SDA) into a molecular SiC precursor. Low molecular-mass organic gelators and a commercial tri-block copolymer are considered as SDA for the synthesis of mesoporous SiC materials.(2) The Hard Templating Approach. SiC materials are synthesized by preceramic polymer nanocasting into mesoporous silica. This process preserves the nanoscale structure of the solid template and leads to mesostructured SiC materials with a high specific surface area.The hard templating approach allows a good replication of the solid template but the difficulty of this method lies in the elimination step of this template. Meanwhile, soft templating approach does not have this drawback and may lead to porous ceramics with more varied structures depending on the SDA used. The complexity of this approach is the template replication step.

O carbeto de silício (SiC) apresenta várias propriedades extremamente interessantes para a fabricação de dispositivos eletrônicos submetidos a condições extremas como alta temperatura (300 a 600 °C), alta frequência e alta potência. Além disso, é o único semicondutor composto que, reagindo com o oxigênio, forma um óxido isolante estável, o SiO2. No entanto, as propriedades elétricas de estruturas de SiO2/SiC são degradadas pela alta concentração de estados eletricamente ativos na interface dielétrico/semicondutor. Tal característica representa uma barreira para a fabricação de dispositivos baseados nesse material. Nesta tese foram comparadas e analisadas as propriedades de estruturas SiO2/4H-SiC obtidas por diferentes processos de oxidação térmica. As estruturas resultantes foram caracterizadas por medidas de corrente-tensão, capacitância-tensão e condutância ac de alta frequência, espectroscopia de fotoelétrons induzidos por raios-X, análise por reação nuclear e microscopia de força atômica. O uso dessas técnicas analíticas visou a correlacionar o comportamento elétrico das estruturas obtidas com suas propriedades físico-químicas como, por exemplo, composição e estrutura química do óxido formado. Os resultados evidenciam diferenças específicas entre os ambientes de oxidação e temperaturas aos quais as amostras foram submetidas, com uma forte distinção entre 4H-SiC tipo-n e tipo-p. Em geral, amostras do substrato tipo-n apresentaram menores quantidades de defeitos na interface SiO2/SiC em comparação com as do tipo-p. Foram identificados comportamentos relacionados a defeitos no óxido, próximos à interface, responsáveis pela captura de portadores majoritários provenientes do semicondutor. Ficou evidente que alguns ambientes e temperaturas de oxidação beneficiam a interface em detrimento da qualidade do filme de óxido e vice-versa. Uma atmosfera de oxidação alternativa, utilizando H2O2 como agente oxidante, foi proposta. Tal processo mostrou-se eficaz na redução da quantidade de estados eletricamente ativos na interface em estruturas tipo-n através da conversão de compostos carbonados em SiO2 no filme dielétrico formado. / Silicon carbide (SiC) presents many advantageous properties for electronic devices designed to work under extreme conditions such as high-temperature (300 ~ 600 °C), high-frequency, and high-power. In addition, the formation of an insulating oxide layer (SiO2) by thermal oxidation is an attractive property for the microelectronics industry. Nevertheless, large densities of interface states at the SiO2/SiC interface degrade electrical properties of the resulting structure. Such states are responsible for undesirable effects which hamper the development of SiC-based devices. In this thesis, the properties of SiO2/4H-SiC structures obtained by distinct oxidation processes where analyzed and compared. The resulting structures where characterized by currentvoltage, high-frequency capacitance-voltage and ac conductance, X-ray photoelectron spectroscopy, nuclear reaction analysis, and atomic force microscopy. Such techniques were employed in order to correlate electrical and physico-chemical properties of the formed structures like composition and chemical bonding of the oxide layer. Results evidence differences among samples prepared under several oxidation atmospheres and temperatures, with a strong distinction among n- and p-type 4H-SiC. Overall, p-type samples presented larger values of interface states densities in comparison with their ntype counterparts. Near-interface traps in the oxide layer, responsible for capture of majority carriers from the semiconductor substrate, were identified. We could evidence that some oxidation conditions improve the bulk properties of the oxide layer, at the same time that they degrade the SiO2/SiC interface quality, and vice versa. An alternative oxidation process using H2O2 as oxidizing agent was proposed. Such process has shown to reduce the amount of electrically active defects at the interface in n-type samples by converting carbonaceous compounds in SiO2 in the formed dielectric layer.

Ce travail de thèse est consacré à la conception et à la fabrication de diodes JBS en carbure de silicium. Une première partie de ce travail a consisté à concevoir par simulation une protection périphérique de la diode la plus efficace possible en réduisant sa sensibilité à la technologie (charges dans l’oxyde et activation des dopants). L’impact de la géométrie de l’anode de la diode JBS sur le champ électrique maximum sous le contact Schottky en inverse et la résistance série de la diode à l’état passant a été étudié. Une nouvelle architecture de diode JBS, à base de tranchées implantées, a été proposée pour pallier les limitations liées aux faibles profondeurs d’implantation d’Al. Une deuxième partie de ce travail a concerné le développement de briques technologiques, indispensables à la fabrication de la diode JBS, tels que les contacts métalliques et la gravure. Enfin, la fabrication complète et la caractérisation électrique de diodes ont été réalisées afin de valider les éléments de conception et l’intégration des briques technologiques développées durant cette thèse. / This study was dedicated to the design and to the fabrication of SiC JBS diodes. The first part of this work includes the design of robust efficient edge termination of the diode with special care to its technology sensitivity. The impact of anode layout of JBS diode on the maximum electric field under Schottky contact and on the on-state resistance has been investigated. A new structure of JBS diode, trenched and implanted, has been proposed to overcome the low Al implantation depth. A second part of this work has been focused on the development of technological steps required for the fabrication of JBS diodes such as metal contact and SiC etching. Finally, full fabrication and electrical characterization of diodes have been carried out in order to validate the design and the integration of technological steps developed during this thesis work.

Le polytype cubique du carbure de silicium (3C-SiC) est un matériau très prometteur pour les applications MEMS. En plus de sa tenue mécanique et chimique, il peut être épitaxié sur des substrats Si de faible coût. De plus, l'hétéroépitaxie multiple, c’est-à-dire quand on empile plusieurs couches Si et 3C-SiC peut ouvrir des pistes pour de nouveaux dispositifs à base de 3C-SiC. Vue la complexité de développer de telles hétérostructures, nous avons procédé à l'amélioration de la qualité de chaque couche séparément. De plus, nous avons mené une étude approfondie sur la nature des défauts dans chaque couche. Après le développement de l'hétérostructure complète, nous avons procédé à la fabrication de microstructures à base de cet empilement. Nous présentons une méthode inédite pour former des membranes de 3C-SiC auto-supportées. Cette technique simplifie considérablement le procédé de fabrication de membranes tout en réduisant le temps de fabrication et le coût. En outre, elle aide à surmonter plusieurs problèmes techniques. / Due to its outstanding physico-chemical properties, the cubic polytype of silicon carbide (3C-SiC) gained significant interest in several fields. In particular, this material emerged as a potential candidate to replace Si in MEMS devices operating in harsh environment. The development of 3C-SiC/Si/3C-SiC heterostructures on top of Si substrate can pave the road towards original and novel MEMS devices profiting from the properties of the 3C-SiC. However, such epitaxial system suffers from wide range of defects characterizing each layer. Thus, we first tried to improve the quality of each layer in this heterostructure. This was achieved relying on two levers; (i) the optimization of the growth parameters of each layer and (ii) the understanding of the nature of defects present in each layer. These two key points combined together allowed an in-depth understanding of the limit of improvement of the overall quality of this heterostructure. After the development of the complete heterostructure, the fabrication of 3C-SiC microstructures was performed. Furthermore, we presented an unprecedented method to form free-standing 3C-SiC membranes in-situ during its growth stage. This novel technique is expected to markedly simplify the fabrication process of suspended membranes by reducing the fabrication time and cost.

Ce travail de thèse s'inscrit dans le cadre du projet CNES-PERSEUS. L’objectif principal est l’étude et la caractérisation d’un béton réfractaire à base de carbure de silicium, avec une taille maximale d'agrégats de 800 microns, dans un environnement de type propulsion hybride. Le col de la tuyère doit résister à un environnement très oxydant, produit par la combustion de polyéthylène solide et de protoxyde d’azote liquide, avec des températures statiques de gaz qui peuvent atteindre 2800K. L’étude est divisée en trois parties : une caractérisation thermomécanique du matériau jusqu’à 1500K ; une étude du comportement à l’oxydation en atmosphère standard, sous un flux solaire maximal de 15 MW/m2 ; des tests au banc avec un moteur hybride à l'ONERA, sur des tuyères conçues et réalisées au laboratoire ICA. Le frittage et la céramisation du microbéton engendrent une densification du matériau et le passage de liaisons de type hydrauliques à des liaisons de type covalentes et ioniques, avec augmentation du module d'élasticité et de la contrainte à la rupture à haute température. Ce matériau présente un comportement visco-élastique-plastique aux hautes températures : il reste majoritairement élastique linéaire jusqu'à la température de stabilisation du matériau, puis une composante viscoplastique apparaît, provoquée par la formation de phases liquides à partir de la matrice cimentaire. Les tests d’oxydation à haute température ont été menés au laboratoire PROMES-CNRS, sur une installation solaire de 2 kW, permettant d'appliquer à un flux maximal de 15 MW/m2. Des observations MEB, en microscopie optique et des analyses EDS ont été menées pour étudier les évolutions microstructurales et la cinétique d’oxydation du matériau. Les tests d’oxydation à 15 MW/m2 ont montré des vitesses d'érosion maximales de l'ordre de 5 microns/s pour une température de 2800 K. L'érosion est générée par l'oxydation active et par la sublimation du carbure de silicium. L'oxydation active se développe à partir de 2100 K, avec formation de SiO et CO gazeux. La sublimation du carbure de silicium, à partir de 2600-2700 K, entraine la formation de Si, Si2C et SiC2 gazeux. Les essais menés sur les tuyères montrent une bonne résistance du matériau après 20 secondes de tir. Une vitesse d'ablation moyenne proche de 60 microns/s a été observée au col de la tuyère. Le comportement thermo-élastique-ablatable a été modélisé tout d'abord sur la base d'une géométrie cylindrique multicouche, puis étendue au cas de la tuyère expérimentale testée au banc d'essai. / This research is part of the PERSEUS project, a space program concerning hybrid propulsion and supported by CNES. The main goal of this study is to characterize a silicon carbide based micro-concrete with a maximum aggregates size of 800 microns, in a hybrid propulsion environment. The nozzle throat has to resist to a highly oxidizing polyethylene/N2O hybrid environment, under temperatures ranging from room temperature up to 2800K. The study is divided in three main parts: the first one deals with the thermo-mechanical characterization of the materials up to 1500K, the second one with an investigation on the oxidation behaviour in a standard atmosphere, under a solar flux up to 15 MW/m2, the last part deals with the conception, the realization and the test of three nozzles in a hybrid rocket motor at ONERA. Elastic modulus was determined by resonant frequency method: results show an increase with the stabilisation temperature. Four points bending tests have shown a rupture tensile strength increasing with stabilization temperature, up to 1500 K. Sintering and ceramization process are primary causes of this phenomenon. Visco-plastic behaviour appears at 1400 K on a material staiblized at the same temperature, due to the formation of liquid phases in cement ternary system. High-temperature oxidation in air was carried out at PROMES-CNRS laboratory, on a 2 kW solar furnace, with a maximum solar flux of 15 MW/m2. Optical microscopy, SEM, EDS analyses were used to study the microstructure evolution and the mass loss kinetics, with a maximal erosion speed of 5 microns second. During theses tests, silicon carbide undergoes active oxidation at 2100 K, with production of SiO and CO smokes and ablation. SiC sublimation is observed since 2600-2700 K, with Si, Si2C and SiC2 vapour generation. Test performed on nozzle in hybrid rocket motors at ONERA, showed an average ablation speed at nozzle throat of 60 microns second, after 20 seconds of test. Thermo-elastic-ablative behaviour has been modelled using first composite cylinder geometry, and then it has been extended to the experimental nozzle geometry, tested on the test bench.

Le Carbure de Silicium (SiC) va permettre de repousser les limites des convertisseurs dans trois directions : tenue en tension élevée, haute température de fonctionnement et forte vitesse de commutation. Aujourd’hui, les premiers modules MOSFET SiC sont disponibles sur le marché et semblent prometteurs. L’objectif de ces travaux de thèse consiste plus particulièrement à mettre en œuvre des montages permettant de caractériser ces premiers modules de puissance MOSFET SiC en vue de les utiliser dans les convertisseurs ferroviaires. Le premier chapitre est consacré à l’état de l’art d’une chaîne de traction de Tramway. C’est ce type de chaîne de traction sur lequel se concentrent les études des premières implantations de composants en SiC. Le deuxième chapitre présente un état de l’art des composants semi-conducteurs de puissance en SiC. Il rappelle tout d’abord les propriétés du matériau et détaille ensuite différentes structures de composants en SiC. Le troisième chapitre concerne les modélisations et les simulations de modules de puissance MOSFET SiC au sein d’une cellule de commutation. Les phases de commutation de ces composants sont étudiées en détail, les influences de différents paramètres sont mises en évidence et des simulations multi-physiques permettent de concevoir les bancs d’essais nécessaires à la caractérisation. Le quatrième chapitre présente les résultats des caractérisations statiques et dynamiques de modules de puissance MOSFET SiC. Ces résultats d’essai sont comparés à des modules IGBT Si de même calibre. Le cinquième chapitre est consacré à la mise en œuvre d’un banc d’essai utilisant la « méthode d’opposition ». Celui-ci permet de comparer les modules IGBT Si et les MOSFET SiC en fonctionnement onduleur grâce à des mesures électriques et calorimétriques. Le sixième et dernier chapitre présente des conclusions et donne des perspectives d’utilisation des composants MOSFET SiC dans les convertisseurs ferroviaires. Différents projets visant à utiliser des MOSFET SiC sur des applications ferroviaires y sont présentés. / Silicon Carbide (SiC) technology is pushing the limits of switching devices in three directions: higher blocking voltage, higher operating temperature and higher switching speed. Nowadays, samples of Silicon Carbide (SiC) MOSFET modules are available on the market and seem promising. The aim of the thesis is to characterize these first power modules thanks to dedicated test beds in order to use them in railway converters. The first chapter focuses on the state of the art of Tramway traction chain. It is this type of traction chain which is the target application of these SiC components. The second chapter presents a state of the art of the SiC devices. First, we recall the material properties and then we detail different structures of SiC components. The third chapter concerns modelling and simulations of SiC MOSFET power modules within a commutation cell. The switching phases of these components are studied in detail and the influences of various parameters are highlighted. Multi-physicals simulations allow designing test benches necessary for the characterization. The fourth chapter presents the results of static and dynamic characterizations of SiC MOSFET power modules. The test results are compared with Silicon IGBT modules of the same rating. The fifth chapter is dedicated to the achievement of a test bench based on the "opposition method". This test bench allows comparing Si IGBT and SiC MOSFET modules in a voltage source inverter (VSI) operation by using electrical and calorimetric measurement methods. The sixth and last chapter presents conclusions and provides outlook for SiC MOSFET components in railway converters. Different projects targeting to use SiC MOSFET on railway applications are presented.

Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / Esforços constantes de pesquisa têm sido conduzidos na seleção de materiais, combinando propriedades de interesse (mecânicas, químicas, elétricas ou térmicas), versatilidade de uso, tempo de vida útil elevado e baixo custo de produção. A busca por materiais de elevado desempenho mecânico despertou grande interesse na pesquisa e desenvolvimento dos cerâmicos avançados com aplicações estruturais e funcionais, como o carbeto de silício. Entretanto, a porosidade ainda é vista como fator limitador do alto desempenho destes materiais visto que, acima de determinada porcentagem, reduz largamente sua resistência mecânica. Seu controle atualmente é realizado através de técnicas de alto custo, com a utilização de tomógrafos. Este trabalho buscou validar uma nova técnica, onde a porosidade foi avaliada através de processamento digital de imagens de microscopia ótica do material previamente lixado e polido em diversas profundidades, com controle dos parâmetros de lixamento e polimento. Esta nova metodologia mostrou-se apropriada e menos dispendiosa para a quantificação da porosidade do carbeto de silício, tendo sido validada para o estudo deste material. / Constant research efforts have been conducted in material selection, matching the properties of interest (mechanical, electrical, chemical and thermal), versatility of use, high lifetime and low cost of production. The search for materials with high mechanical performance aroused great interest in the research and development of advanced ceramic with structural and functional applications such as silicon carbide. However, the porosity is still seen as a limiting factor in high performance materials because, above a certain percentage, greatly reduces its mechanical strength. His control is currently performed using techniques of high cost, with the use of scanners. This study aimed to validate a new technique, where the porosity was evaluated by digital image processing of the optical microscopy in the material previously sanded and polished at various depths, with control of the material parameters. This new methodology was appropriate and less expensive to quantify the porosity of the silicon carbide and has been validated for the study of this material.

O carbeto de silício (SiC) apresenta várias propriedades extremamente interessantes para a fabricação de dispositivos eletrônicos submetidos a condições extremas como alta temperatura (300 a 600 °C), alta frequência e alta potência. Além disso, é o único semicondutor composto que, reagindo com o oxigênio, forma um óxido isolante estável, o SiO2. No entanto, as propriedades elétricas de estruturas de SiO2/SiC são degradadas pela alta concentração de estados eletricamente ativos na interface dielétrico/semicondutor. Tal característica representa uma barreira para a fabricação de dispositivos baseados nesse material. Nesta tese foram comparadas e analisadas as propriedades de estruturas SiO2/4H-SiC obtidas por diferentes processos de oxidação térmica. As estruturas resultantes foram caracterizadas por medidas de corrente-tensão, capacitância-tensão e condutância ac de alta frequência, espectroscopia de fotoelétrons induzidos por raios-X, análise por reação nuclear e microscopia de força atômica. O uso dessas técnicas analíticas visou a correlacionar o comportamento elétrico das estruturas obtidas com suas propriedades físico-químicas como, por exemplo, composição e estrutura química do óxido formado. Os resultados evidenciam diferenças específicas entre os ambientes de oxidação e temperaturas aos quais as amostras foram submetidas, com uma forte distinção entre 4H-SiC tipo-n e tipo-p. Em geral, amostras do substrato tipo-n apresentaram menores quantidades de defeitos na interface SiO2/SiC em comparação com as do tipo-p. Foram identificados comportamentos relacionados a defeitos no óxido, próximos à interface, responsáveis pela captura de portadores majoritários provenientes do semicondutor. Ficou evidente que alguns ambientes e temperaturas de oxidação beneficiam a interface em detrimento da qualidade do filme de óxido e vice-versa. Uma atmosfera de oxidação alternativa, utilizando H2O2 como agente oxidante, foi proposta. Tal processo mostrou-se eficaz na redução da quantidade de estados eletricamente ativos na interface em estruturas tipo-n através da conversão de compostos carbonados em SiO2 no filme dielétrico formado. / Silicon carbide (SiC) presents many advantageous properties for electronic devices designed to work under extreme conditions such as high-temperature (300 ~ 600 °C), high-frequency, and high-power. In addition, the formation of an insulating oxide layer (SiO2) by thermal oxidation is an attractive property for the microelectronics industry. Nevertheless, large densities of interface states at the SiO2/SiC interface degrade electrical properties of the resulting structure. Such states are responsible for undesirable effects which hamper the development of SiC-based devices. In this thesis, the properties of SiO2/4H-SiC structures obtained by distinct oxidation processes where analyzed and compared. The resulting structures where characterized by currentvoltage, high-frequency capacitance-voltage and ac conductance, X-ray photoelectron spectroscopy, nuclear reaction analysis, and atomic force microscopy. Such techniques were employed in order to correlate electrical and physico-chemical properties of the formed structures like composition and chemical bonding of the oxide layer. Results evidence differences among samples prepared under several oxidation atmospheres and temperatures, with a strong distinction among n- and p-type 4H-SiC. Overall, p-type samples presented larger values of interface states densities in comparison with their ntype counterparts. Near-interface traps in the oxide layer, responsible for capture of majority carriers from the semiconductor substrate, were identified. We could evidence that some oxidation conditions improve the bulk properties of the oxide layer, at the same time that they degrade the SiO2/SiC interface quality, and vice versa. An alternative oxidation process using H2O2 as oxidizing agent was proposed. Such process has shown to reduce the amount of electrically active defects at the interface in n-type samples by converting carbonaceous compounds in SiO2 in the formed dielectric layer.

Carbon nanotubes (CNTs) were successfully grown on the surface of carbon fibre reinforcements in carbon fibre architecture through in-situ catalytic chemical vapour deposition (CCVD). Success was also implemented on powders of oxides and non-oxides, including Y-TZP powder, ball milled alumina powder, alumina grits, silicon carbide powder. Preliminary results have been achieved to demonstrate the feasibility of making ceramic composites consisting of CNTs reinforcements.