SYNTHESIS AND CERAMIC PROCESSING OF ZIRCONIA ALUMINA COMPOSITES FOR APPLICATION AS SOLID OXIDE FUEL CELL ELECTROLYTES

CSZ. Os pós cerâmicos foram caracterizados por picnometria a hélio, difração de raios-X, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, termogravimetria, calorimetria diferencial exploratória, granulometria por espalhamento de laser e adsorção gasosa (BET). Por sua vez, a avaliação dos compactos sinterizados foi feita por técnicas de medidas de densidade, difração de raios-X, microscopia eletrônica de varredura, microscopia óptica, impressão Vickers e espectroscopia de impedância. A partir dos resultados obtidos, concluiu-se que a adição de alumina em zircônia estabilizada com 9 mol% de ítria na fase cúbica promove, nos pós cerâmicos, elevação da área superficial específica, desaglomeração e redução da cristalinidade dos óxidos, exigindo tratamentos térmicos mais energéticos para atingir elevada densidade. Em relação aos produtos sinterizados, verificou-se que a rota adotada confere homogeneidade microestrutural ao compósito, e que a adição de alumina restringe o crescimento dos grãos, eleva a dureza e tenacidade à fratura e contribui para o aumento da condutividade iônica total, apesar do decréscimo na condutividade dos grãos. Estes resultados mostraram que, para aplicação como eletrólito sólido, a adição de até 5 % de alumina, em massa, na matriz de zircônia estabilizada na fase cúbica eleva consideravelmente a tenacidade à fratura e a condutividade iônica do compósito, se comparada às cerâmicas de zircônia estabilizada com ítria na fase cúbica.The global warmness and the necessity to obtain clean energy from alternative methods than petroleum raises the importance of developing cleaner and more efficient systems of energy generation, among then, the solid oxide fuel cell (SOFC). Cubic stabilized zirconia (CSZ) has been the most studied material as electrolyte in SOFC, due to its ionic conductivity and great stability at operation conditions. However, its low fracture toughness difficults its application as a thin layer, what could lead to an improvement of cell efficiency. In this sense, the alumina addition in CSZ forms a composite, which can shift its mechanical properties, without compromising its electrical properties. In this work, coprecipitation synthesis route and ceramic processing of zirconia-alumina composites were studied, in order to establish optimum conditions to attain high density, homogeneous microstructure, and better mechanical properties than CSZ, without compromising ionic conductivity. For this purpose, composites containing up to 40 wt % of alumina, in a 9 mol % yttria-stabilized zirconia (9Y-CSZ) matrix were evaluated. In order to optimize the synthesis of the composites, a preliminary study of powder obtaining and processing were carried out, at compositions containing 20 wt % of alumina, in 9Y-CSZ. The ceramic powders were characterized by helium picnometry, X-ray diffraction, scanning electronic microscopy, transmission electronic microscopy, thermogravimetry, differential scanning calorimetry, granulometry by laser diffraction and gas adsorption (BET). The characterization of sinterized compacts were performed by X-ray diffraction, scanning electron microscopy, optical microscopy, density measurements, Vickers indentation and impedance spectroscopy. The obtained results show that the alumina adition, in the 9Y-CSZ matrix powders, raises the specific surface area,promotes deagglomeration of powders and elevates the oxides crystallization temperature, requiring higher energetic thermal treatments to attain high densities. In relation to the sintered products, it was confirmed the excellent homogeneity and crystallinity of microstructure provided by the chosen route, the restriction of grain growth by alumina addition, raise of hardness and fracture toughness, and higher ionic conductivity, even tough a lower bulk conductivity. These results indicate that the addition of 5 wt % alumina in CSZ matrix allows the application of this material as solid oxide fuel cell electrolytes, due to its better fracture toughness and ionic conductivity, compared to yttria-stabilized cubic zirconia ceramics