Melhorias na eficiência energética motivam o desenvolvimento de isolamentos térmicos cada vez mais eficazes e duráveis. Cerâmicas porosas à base de mulita (Al6Si2O13 ou 3Al2O3.2SiO2) são ideais para essa aplicação devido à alta resistência à corrosão e à densificação. Apesar de rara sua forma mineral, esta é uma das fases mais importantes em cerâmicas tradicionais e avançadas, visto que pode ser obtida a partir de fontes de alumina e sílica, por meio de diversas rotas de processamentos. Sua formação in situ por sinterização reativa tem se destacado dentre os métodos de produção pelos bons resultados mecânicos e eficácia na formação e manutenção de porosidade, entretanto, ainda existem pontos a serem investigados como a influência do tamanho de partícula e porosidade inicial de suas matérias-primas nas propriedades finais das estruturas. Neste trabalho, foram produzidas peças porosas de alumina-mulita in situ a partir de suspensões de sílica coloidal com diferentes concentrações (30, 40 e 50 %) e hidróxidos de alumínio de diferentes granulometrias (fino, HAF e grosso, HAG) pré-calcinados em várias temperaturas (500-1500 °C), pelo processo de moldagem direta para aplicação como isolante térmico em temperaturas acima de 1000 °C. As amostras (verdes e tratadas termicamente - 1500 °C) foram submetidas à ensaios mecânicos (módulo elástico e resistência à ruptura por compressão e flexão), análise microestrutural (MEV e DRX) e de propriedades físicas (porosidade total, densidades e variação térmica dimensional). As aluminas de transição provenientes da calcinação agiram como agentes porogênicos no sistema e juntamente com a sílica coloidal (com funções simultâneas de fluido de mistura, agente ligante, aditivo de secagem e fonte de SiO2 amorfa) formaram estruturas com grande variação de propriedades. Estruturas com HAF apresentaram porosidade próxima a 40% e elevadas propriedades mecânicas, e com HAG foram obtidos níveis de porosidade acima de 50 % e baixas resistências. As composições mistas, contendo tanto HAF como HAG, obtiveram bons resultados mecânicos e porosidades acima de 50 %, mostrando-se bons candidatos para uso como isolamento térmico. Todos os sistemas contiveram a fase de mulita em maior quantidade coexistindo com alfa alumina (Coríndon). / Improvements in energy efficiency motivate the development of more effective and durable thermal insulation. Porous ceramics based on mullite (Al6Si2O13 or 3Al2O3.2SiO2) have great potential for this application due its high resistance to corrosion and densification. Although rare in its mineral form, this is one of the most important phases in traditional and advanced ceramics because it can be obtained from alumina and silica sources, through various processing routes. Among them, the solid-state in situ reactions by reactive sintering stands out for its good mechanical properties and efficiency in the formation and maintenance of pores. However, how particle size and initial porosity of its raw materials influences the final properties still requires investigation. In this work, porous ceramics of alumina-mullite were produced in situ from aqueous suspensions of colloidal silica with different concentrations (30, 40 and 50 %) and aluminum hydroxides of different grain sizes (fine, HAF and coarse, HAG) pre-calcined in several temperatures (500-1500 °C) by direct casting process for application as a thermal insulation at temperatures above 1000 °C. The samples (green and thermally treated - 1500 °C) were submitted to mechanical tests (elastic modulus and resistance to rupture by compression and flexural), microstructural analysis (SEM and XRD) and physical properties characterization (total porosity, densities and dimensional thermal variation). The transition aluminas acted as porogenic agents in the system and with the colloidal silica (with simultaneous functions of mixing fluid, binding agent, drying additive and source of amorphous SiO2), formed structures with great properties\' variation. Samples with HAF had porosity close to 40% and high mechanical properties, and with HAG, porosity levels above 50 % and low resistances were obtained. The mixed compositions, containing both HAF and HAG, obtained good mechanical results and porosities above 50 %, showing great potential to thermal insulation. All systems contained most of mullite coexisting with alpha alumina phase (Corundum).

O hexaluminato de cálcio (CaAl12O19 ou CA6) poroso é um material promissor para aplicações de isolamento térmico pois combina baixa condutividade térmica (~0,33 Wm-1K-1 a 1400 °C), resistência mecânica razoável (2 – 8 MPa), inércia química, boa refratariedade (Tf ~1830 °C) e alta resistência ao choque térmico. Existem várias rotas para se obter o CA6 por meio de reações em temperaturas acima de 1300 °C, usando diversas fontes de Al2O3 e CaO, assim como diferentes métodos de processamento. No entanto, embora suas propriedades físicas tenham sido avaliadas, dois pontos principais ainda requerem investigação: o impacto das características das matérias-primas no desenvolvimento da microestrutura de sistemas porosos formados in situ, e a evolução da microestrutura e propriedades de sistemas obtidos a partir de CA6 pré-formado. Neste trabalho, foram produzidas peças de CA6 in situ a partir de diferentes fontes de Al2O3 (alumina calcinada e hidróxido de alumínio) e carbonato de cálcio (CaCO3) de diferentes granulometrias, processados por prensagem uniaxial e moldagem direta de suspensões e submetidas a diferentes tratamentos térmicos. As amostras (verdes e secas e após tratamento térmico) foram submetidas à análise microestrutural (MEV e DRX) e dilatométrica, ensaios para determinação das propriedades físicas (porosidade total, distribuição de tamanho de poros e condutividade térmica) e propriedades mecânicas (resistência à ruptura por compressão e módulo elástico). Estruturas à base de CA6 formado in situ obtidas por prensagem e moldagem direta apresentaram elevada porosidade (até 71 %) e uma resistência à compressão acima de 10 MPa. Verificou-se que o processo de conformação determinou a porosidade à verde inicial da peça, enquanto o tamanho de partícula de alumina induziu a um crescimento de grão assimétrico (partícula grossa) ou à densificação da peça (partícula fina). Dois mecanismos antagonistas acontecem ao mesmo tempo na reação in situ: a reação expansiva da formação de aluminatos intermediários (efeito porogênico) e a densificação das partículas de Al2O3. As partículas de carbonato tiveram uma grande influência no tamanho final dos poros. O efeito porogênico do hidróxido de alumínio foi efetivo até um conteúdo máximo de 50 %vol. / Porous calcium hexaluminate (CaAl12O19 or CA6) is a promising material for thermal insulation applications because it combines low thermal conductivity (~0,33 Wm-1K-1 at 1400° C), reasonable mechanical strength (2 – 8 MPa), chemical inertia, good refractoriness (Tf ~1830 °C) and high resistance to thermal shock. There are several routes to obtain CA6 by reactions at temperatures above 1300 °C, using various sources of Al2O3 and CaO, as well as different processing methods. However, although its physical properties have been studied, two main points still require investigation: the impact of the characteristics of the raw materials on the development of the microstructure of in situ formed porous systems, and the evolution of the microstructure and properties of systems obtained from preformed CA6. In this study, in situ CA6 bodies were produced from different sources of Al2O3 (calcined alumina and aluminum hydroxide) and calcium carbonate (CaCO3) of different granulometries, processed by uniaxial pressing and direct molding of suspensions and thermally treated at different temperatures. The samples (green and heat treated ones) were submitted to microstructural analysis (SEM and XRD) and dilatometry, also tests to determine the physical properties (total porosity, Hg porosimetry and thermal conductivity) and mechanical properties (compression strength and elastic modulus). In situ formed CA6-based structures obtained by pressing and direct molding showed high porosity (up to 71%) and a compressive strength above 10 MPa. It was found that the conformation process determined the initial porosity of the green body, while particle size of alumina may induce asymmetric grain growth (coarse particle) or densification of the ceramic body (fine particle). Two antagonistic mechanisms occur at the same time in the in situ reaction: the expansive reaction of the formation of intermediate aluminates (porogenic effect) and the densification of Al2O3 particles. The carbonate particles had a great influence on the final pore size. The porogenic effect of aluminum hydroxide was effective up to a maximum content of 50% vol.

Ce projet se place dans le cadre du développement des nouvelles technologies de l'énergie respectueuses de l'environnement. Les piles à combustibles à oxydes solides (SOFC) permettent, pour les applications stationnaires, la génération de puissance de 1kW à 2MW avec un rendement électrique pouvant atteindre 70%. Elles fonctionnent à très hautes températures, typiquement entre 700-1000°C. La cellule d'une SOFC est constituée d'un électrolyte dense pris en sandwich entre deux électrodes poreuses (anode et cathode). Les électrodes poreuses, élaborées à partir de poudres céramiques, représentent un élément critique de l'assemblage. En effet, elles doivent être suffisamment poreuses pour optimiser à la fois la diffusion des gaz et les réactions électrochimiques. Cette nécessité est en contradiction avec l'exigence d'une bonne tenue mécanique. Cette contradiction doit pouvoir être résolue en proposant des microstructures d'électrodes poreuses hiérarchisées ou anisotropes. L'objectif de cette thèse est de montrer différentes voies possibles pour optimiser l'électrode en s'appuyant en particulier sur des simulations numériques et sur des caractérisations tomographiques. Les électrodes sont élaborées en utilisant deux protocoles différents conduisant à des porosités isotropes et anisotropes. Les échantillons anisotropes sont préparés en utilisant la méthode de moulage par congélation à partir de poudres YSZ/LSM, typiques de matériaux d'électrode. Cette méthode de fabrication conduit à une porosité hiérarchisée. La porosité totale est définie par le taux de chargement dans la barbotine initiale. La microporosité diminue avec la température de frittage et la taille des macropores est fonction de la vitesse de solidification. Les échantillons isotropes sont préparés en utilisant des agents porogènes avec des caractéristiques identiques aux échantillons anisotropes. Ces électrodes sont caractérisées par la technique d'Archimède pour déterminer les taux de porosités (macro et micro) et par microscopie à balayage pour connaître la taille des macroporosités. Des images tridimensionnelles des microstructures sont obtenues par FIB-SEM (Focused Ion Beam, 15µm³) et par nanotomographie-X (75µm³), avec des résolutions de 10nm et 75nm, respectivement. Le rendement énergétique d'une électrode dépend de différents paramètres : composition YSZ/LSM, taux de porosité, taille des particules, conductivités électronique/ionique et résistance électrochimique. Ces paramètres sont étudiés en utilisant des microstructures numériques associées à un réseau de résistance. Les simulations permettent de déterminer les facteurs qui contrôlent la conductivité effective. Ces microstructures numériques sont élaborées à l'échelle de la taille des particules en utilisant le code dp3D basé sur la méthode des éléments discrets (DEM) et développé au sein du laboratoire SIMaP. Nous montrons par exemple qu'en dessous d'une certaine épaisseur, la composition YSZ/LSM a très peu d'influence sur la conductivité effective. Une méthode a également été développée pour calculer cette conductivité effective à partir d'une image de FIB prenant en compte la résistance électrochimique aux points triples (gaz, YSZ, LSM). La tenue mécanique des différentes microstructures est testée en compression jusqu'à la rupture. En parallèle, des calculs sur image, couplés à la DEM sont effectués pour simuler les propriétés mécaniques. Nous comparons le comportement des microstructures homogènes (obtenues avec des agents porogènes) et celui des microstructures anisotropes. Les modules et les contraintes à rupture sont surestimés par les simulations. Qualitativement, les résultats expérimentaux et de simulation montrent des mécanismes de rupture cohérents entre eux. Par ailleurs, les modules et les contraintes à rupture sont différents entre les deux types d'échantillon (anisotrope et isotrope). Cette anisotropie peut être utilisée pour optimiser les propriétés mécaniques suivant une direction. / This project is involved in the development of new green power sources. Solid Oxide Fuel Cells (SOFCs) can achieve an output power of 1kW to 2MW and an energy conversion of up to 70%. Temperatures between 700 and 1000°C are required. A typical cell is made of an electrolyte sandwiched between two porous electrodes (anode and cathode). Porous electrodes are elaborated from ceramic powders and are critical components of the whole structure. These electrodes need to be porous enough to optimize gaz diffusion and electrochemical reactions. This requirement is antagonist to the need of a good mechanical strength. This conflict could be solved using hierarchical or anisotropic electrode microstructures. The aim of this thesis is to investigate possible ways to optimize an electrode. Numerical simulations and nanotomography characterizations are used for this purpose. Electrodes are elaborated using two different protocoles leading to anisotropic and isotropic porosities. Anisotropic samples are prepared by freeze-casting from a slurry of YSZ and LSM, which are typical materials for SOFCs. Freze-casting leads to a hierarchical porosity. The overall porosity is controlled by the loading of the slurry. The microporosity decreases with sintering temperature and the macropore size is function of the freezing rate. Isotropic samples are processed using pore formers. The size and the amount of pore formers are selected to match the characteristics of the anisotropic samples. These electrodes are characterized with Archimedes technique to determine the porosity, and with scanning electron microscope (SEM) to obtain the size of macropores. Three dimensional images of the microstructures are captured using focused ion beam (FIB-SEM tomography) technique (10nm} resolution) and using X-ray nanotomography (75nm} resolution). The overpotentials in an electrode depend on different parameters: composition of YSZ/LSM, porosity, particle sizes, electronic/ionic conductivities and electrochemical resistance. These parameters are studied on numerical microstructures coupled with a resistor network. These numerical microstructures have been generated at th scale of particles, using a numerical code based on the discrete element method (DEM). Simulations can be used to determine the limiting factor on the effective conductivity. For example, we show that the composition of YSZ/LSM in a sample matters little for electrodes below a certain thickness. A new method has also been developed to compute the effective conductivity from a FIB-SEM image taking into account the electrochemical resistance at the triple point boundaries between gaz, YSZ and LSM. The mechanical response of the elaborated microstructures are tested in compression up to the fracture. In parallel, DEM simulations are performed to simulate mechanical properties based on 3D images. The mechanical behaviours of homogeneous samples (with pore formers) and anisotropic samples are compared. The yield strength and stiffness are overestimated by simulations. Qualitatively, experimental results and simulations show consistent failure mecanisms. Moreover, the yield strength and stiffness are different in the two types of sample (anisotropic and isotropic). Such an anisotropy could be used to optimize mechanical properties in one direction.