Cerâmicas porosas combinam baixa condutividade térmica com elevada estabilidade química, dimensional e refratariedade. Devido a isso, seu uso como isolante térmico em altas temperaturas (T >1000°C) tem se apresentado como uma importante solução para reduzir o consumo energético. Dentre as diversas matérias primas utilizáveis nessa aplicação, o hexaluminato de cálcio (CaAl12O19 ou CA6) é um novo tipo promissor de refratário leve. Entre suas características mais importantes, destacam-se o alto ponto de fusão (1875°C) e a dificuldade intrínseca para densificar, devido à morfologia dos cristais em forma de placas que permite manutenção da porosidade em altas temperaturas, e por longos tempos. Apesar do grande interesse tecnológico e dos diversos estudos relatando propriedades e aplicações, há poucos trabalhos descrevendo a evolução da microestrutura de estruturas porosas formadas \"in situ\" a partir de fontes de Al2O3 e CaO. Sabendo que essa rota de processamento permite significativa economia de energia e tempo, compreender os mecanismos e variáveis envolvidos torna-se um importante ponto de investigação. Neste trabalho, foram preparadas composições com diferentes proporções de Al2O3 e CaCO3 por meio de prensagem uniaxial. Após tratamento térmico (500-1500°C), as amostras foramcaracterizadas em relação à porosidade total, resistência à ruptura por flexão, módulo elástico, fases numeralógicas formadas, dilatação térmica linear e morfologia. Os principais fatores que afetaram a evolução da microestrutura porosa foram o teor de CaCO3 adicionado ao sistema, o tamanho médio dessas partículas e a temperatura final de tratamento térmico. Para todas as composições com CaCO3, verificou-se que a formação de poros após a decomposição do CaCO3 (650-720°C) não afetou o nível total de porosidade da estrutura. Entre 1300-1400°C ocorreu a formação de fases intermediárias (CaAl2O4 e CaAl4O7) com composição eutética e baixo ponto de fusão ao redor dos poros. Em 1500°C, essas fases líquidas adquiriram a composição do hexaluminato e se cristalizaram, gerando estruturas porosas e com razoável resistência mecânica. / Porous ceramics combine low thermal conductivity with high chemical and dimensional stability and refractoriness. Therefore, their use as insulators at high temperatures (T>1000°C) has emerged as an important solution for the reduction ofenergy consumption. Among the various raw materials to be used for such a purpose, calcium hexaluminate (CaAl12O19 or CA6) is a promising new type of lightweight refractory material. Some ofits most important features include high melting point (1875°C) and intrinsic difficulty to densify due to the morphology of its plate-like crystals that enables the maintenance of porosity at higher temperatures and for longer times. Despite the great technological interest it has drawn and the development of studies on its properties and applications, the literature reports few studies on the evolution of its microstructure formed in situ from Al2O3 and CaO sources. Because this processing route significantly saves energy and time, the mechanisms involved must be understood. This dissertation addresses the preparation of compositions with different proportions of Al2O3 and CaCO3 by uniaxial pressing. After heat treatment (500-1500°C), samples were characterized regarding total porosity, tensile strength by bending, elastic modulus, formed phases, dilatometric analyzis and scanning electron microscopy. The main factors that affected the evolution of the porous microstructure were the CaCO3 content added to the system, average size of the particles and the final temperature of the heat treatment. The formation of pores after the CaCO3 (650-720°C) decomposition did not affect the level of total porosity significantly. The formation of intermediate phases (CaAl2O4 and CaAl4O7) with eutectic composition and low melting point surrounding the pores was observed between 1300-1400°C. At 1500°C, such liquid phases displayed a hexaluminate composition and crystallized, which resulted in porous structures of reasonable strength.

Cerâmicas porosas combinam baixa condutividade térmica com elevada estabilidade química, dimensional e refratariedade. Devido a isso, seu uso como isolante térmico em altas temperaturas (T >1000°C) tem se apresentado como uma importante solução para reduzir o consumo energético. Dentre as diversas matérias primas utilizáveis nessa aplicação, o hexaluminato de cálcio (CaAl12O19 ou CA6) é um novo tipo promissor de refratário leve. Entre suas características mais importantes, destacam-se o alto ponto de fusão (1875°C) e a dificuldade intrínseca para densificar, devido à morfologia dos cristais em forma de placas que permite manutenção da porosidade em altas temperaturas, e por longos tempos. Apesar do grande interesse tecnológico e dos diversos estudos relatando propriedades e aplicações, há poucos trabalhos descrevendo a evolução da microestrutura de estruturas porosas formadas \"in situ\" a partir de fontes de Al2O3 e CaO. Sabendo que essa rota de processamento permite significativa economia de energia e tempo, compreender os mecanismos e variáveis envolvidos torna-se um importante ponto de investigação. Neste trabalho, foram preparadas composições com diferentes proporções de Al2O3 e CaCO3 por meio de prensagem uniaxial. Após tratamento térmico (500-1500°C), as amostras foramcaracterizadas em relação à porosidade total, resistência à ruptura por flexão, módulo elástico, fases numeralógicas formadas, dilatação térmica linear e morfologia. Os principais fatores que afetaram a evolução da microestrutura porosa foram o teor de CaCO3 adicionado ao sistema, o tamanho médio dessas partículas e a temperatura final de tratamento térmico. Para todas as composições com CaCO3, verificou-se que a formação de poros após a decomposição do CaCO3 (650-720°C) não afetou o nível total de porosidade da estrutura. Entre 1300-1400°C ocorreu a formação de fases intermediárias (CaAl2O4 e CaAl4O7) com composição eutética e baixo ponto de fusão ao redor dos poros. Em 1500°C, essas fases líquidas adquiriram a composição do hexaluminato e se cristalizaram, gerando estruturas porosas e com razoável resistência mecânica. / Porous ceramics combine low thermal conductivity with high chemical and dimensional stability and refractoriness. Therefore, their use as insulators at high temperatures (T>1000°C) has emerged as an important solution for the reduction ofenergy consumption. Among the various raw materials to be used for such a purpose, calcium hexaluminate (CaAl12O19 or CA6) is a promising new type of lightweight refractory material. Some ofits most important features include high melting point (1875°C) and intrinsic difficulty to densify due to the morphology of its plate-like crystals that enables the maintenance of porosity at higher temperatures and for longer times. Despite the great technological interest it has drawn and the development of studies on its properties and applications, the literature reports few studies on the evolution of its microstructure formed in situ from Al2O3 and CaO sources. Because this processing route significantly saves energy and time, the mechanisms involved must be understood. This dissertation addresses the preparation of compositions with different proportions of Al2O3 and CaCO3 by uniaxial pressing. After heat treatment (500-1500°C), samples were characterized regarding total porosity, tensile strength by bending, elastic modulus, formed phases, dilatometric analyzis and scanning electron microscopy. The main factors that affected the evolution of the porous microstructure were the CaCO3 content added to the system, average size of the particles and the final temperature of the heat treatment. The formation of pores after the CaCO3 (650-720°C) decomposition did not affect the level of total porosity significantly. The formation of intermediate phases (CaAl2O4 and CaAl4O7) with eutectic composition and low melting point surrounding the pores was observed between 1300-1400°C. At 1500°C, such liquid phases displayed a hexaluminate composition and crystallized, which resulted in porous structures of reasonable strength.

A study was conducted to investigate a novel route to low cost processing of alumina/calcium-hexaluminate (A/CA6) composites. The objectives of this study were to: (a) develop A/CA6 and ß-spodumene modified A/CA6 composites using an in-situ reaction sintering method and functionally-graded A/CA6 using an infiltration technique, and (b) evaluate the effects of CA6 platelets on the ensuing physical and mechanical properties. The study has revealed that the processing procedures played an important role in the microstructural development of A/CA6 composites. The microstructure-property relationships of these materials were found to be strongly influenced by the presence of CA6 phase.The A/CA6 composites were synthesised by in-situ reaction sintering of alumina powder and (0, 5, 15, 30, 50 and 100 wt%) CA6 precursor. The phase relations and development of this system were monitored using quantitative x-ray diffraction (XRD) and neutron diffraction (ND). Rietveld analysis which showed the CA6 content to increase in proportion with the increase of CA6 precursor added. The XRD study revealed that the CA and CA2 phases started to develop at approximately 1000 and 1100°C and transformed to CA6 phase at 1400T. Similarly, the dynamic high temperature ND study showed that the corresponding calcium aluminates phases commenced to develop at 1000°C and 1200°C and then eventually transformed to CA6 at 1400°C.The presence of the plate-like CA6 grains in the system was revealed by the back-scattered SEM imaging and confirmed by the Ca x-ray map. Although the presence of CA6 caused the reduction of hardness, the fracture toughness of A/CA6 composites were improved when compared with alumina. It was found that the presence of CA6 hindered the processes of sintering and densification in alumina matrix.The use of ß -spodumene had been investigated as a liquid-phase-sintering aid for ++ / the densification of A/CA6 composites. XRD, ND, differential thermal analysis (DTA), scanning electron microscopy (SEM) and Vickers indentation were used to characterise the effects of ß -spodumene on the phase relations, densification, microstructure and mechanical properties. The results showed that the presence of ß -spodumene had a profound influence on the phase relations, densification and microstructure of A/CA6 composites. Quantitative XRD and ND Rietveld analysis showed that the addition of > 2.5 wt% ß -spodumene caused the reduction of CA6 content due to the formation of ß -quartz solid solution. The reduction of porosity in the presence of ß -spodumene suggests that it may be used as an effective sintering aid for improving the densification of A/CA6 composites. However improvements in hardness and fracture toughness were not achieved probably due to the presence of large spherical pores as well as the formation of recrystallised ß -spodumene and ß -quartz solid solution.A functionally-graded alumina/calcium-hexaluminate (A/CA6) composite was successfully synthesised through infiltration of porous alumina preform with a solution containing calcium-acetate. The infiltration kinetics of liquid into porous alumina preform had also been investigated. It was found that the infiltration rate equation proposed by Washburn is most suitable for describing the effects of preform sintering temperature, viscosity and multiple infiltrations on the infiltration characteristics. The influence of applied pressure is consistent with the model proposed by Darcy, where the applied pressure enhances the infiltration rate behaviour. Key parameters for the optimum processing conditions of preforms for subsequent infiltration have also been identified.The graded composition character of the functionally-graded A/CA6 composites were characterised by XRD and synchrotron ++ / diffraction (SRD). Depth-profiling of compositions with XRD and SRD Rietveld refinement showed that the concentration of CA6 decreased with depth, while that of A1203 increased with depth. Both XRD and SRD results showed that CA and CA2 phases formed initially at 1000°C and 1300°C, respectively, but remained stable even at 1400°C, before eventually transformed to CA6 at 1650°C. These results are consistent with those of dynamic high temperature ND data.The graded microstructure was revealed by SEM back-scattered imaging whereby the content of CA6 platelets was most abundant near the surface and decreased with increasing depth towards the bulk. The presence of CA6 phase in the composite fire at 1400°C was also confirmed by the transmission electron microscopy (TEM) observation in conjunction with energy dispersive spectroscopy (EDS). The hardness results of the graded material showed that the graded-region was softer than the non-graded region as a result of the presence of softer CA6 phase in the former. However, the fracture toughness in the graded region was found to be higher than the non-graded region which might be attributed to the display of toughening processes such as crack deflection and grain bridging.

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.