RESUMO Materiais com estrutura do tipo perovskita têm sido extensamente investigados na tentativa de encontrar novos eletrólitos para Células a Combustível de Óxido Sólido que operem em baixas temperaturas. Dentre esses materiais destaca-se o LaAlO3 que, quando apropriadamente dopado, apresenta condutividade iônica considerável. Neste trabalho LaAlO3 puro e dopado com Sr, Ba e Ca foram estudados. O método utilizado para preparação dos pós foi o de reação no estado sólido. Análises microestrutural, estrutural e elétrica das amostras sinterizadas foram realizadas por microscopia eletrônica de varredura, difração de raios X e espectroscopia de impedância, respectivamente. Entre os diferentes tipos de dopantes, a amostra dopada com Sr foi a que apresentou maior condutividade, tanto do grão ( a 800°C) como total ( a 800°C). Em todas as amostras dopadas, a condutividade total é controlada pelo contorno de grão e as microestruturas são heterogêneas e o grau de heterogeneidade depende do dopante.

ABSTRACT Perovskite-type materials have been extensively investigated in the attempt to find new electrolyte materials for Solid Oxide Fuel Cell that work at low temperatures. Among these materials, LaAlO3 stands out for presenting considerable ionic conductivity when adequately doped. In this work, pure and Sr, Ba, and Ca-doped LaAlO3 were studied. The powders were prepared by solid state reaction. Microstructural, structural and electrical conductivity of sintered samples were analyzed by scanning electron microscopy, X-ray diffraction and impedance spectroscopy, respectively. Among the different kinds of dopants, Sr-doped sample presented the higher conductivity, for both the grain ( at 800°C) and total ( at 800°C) conductivity. In all doped samples, the total conductivity is controlled by the grain boundary, i.e., the grain boundary is more resistive than the grain, and the microstructures are heterogeneous and the degree of heterogeneity is determined by the dopant.

MgAl2O4 is a humidity sensing material, which high melting point requires high sintering temperatures. In this work, manganese oxide was used as a sintering aid to obtain MgAl2O4 samples sintered at 1350°C with a microstructure suitable for humidity sensing. Impedance spectroscopy analyses were performed at several levels of relative humidity, indicating that this correlation is possible within the range of 10-80% of relative humidity.

ABSTRACT GDC-YSZ bi-layer SOFC electrolytes are an interesting manner to protect GDC electrolytes of the low oxygen partial pressure present at anode-electrolyte interface. This oxygen partial pressure condition and high temperatures during the operation of SOFC permit GDC electrolyte reduction injuring the performance of SOFC. In this work, GDC-YSZ bi-layer electrolytes were prepared using double tape casting technique. Single layers of YSZ and GDC electrolytes were also prepared by the same technique for comparison. All samples were sintered at 1600 °C/2 h in air and analyzed by SEM and impedance spectroscopy technique. A good adhesion was established between the layer GDC and YSZ and no cracks were detected despite the differential firing shrinkage. The electrical conductivity of GDC-YSZ bi-layer sample at 350 °C was 7.98x10-5 S.cm-1 and for GDC and YSZ single layer samples electrical conductivities were 1.89x10-4 S.cm-1 and 4.71x10-5 S.cm-1, respectively. These results show that GDC-YSZ bi-layer electrolyte is superior to YSZ single layer electrolyte.

Expelling of the liquid phase during sintering of ZrO2-6.5 mol% Y2O3- 0.5 mol% Pr2O3 ceramic was observed as a result of grain coarsening. ZrO2- 7.0 mol% Y2O3 samples, without Pr2O3 addition, do not show this effect under the same sintering conditions. The expelling process is caused by surface tension forces and attracting van der Waals forces between the grains, coupled with the existence of two glass phases on the grain boundaries. The amount of expelled glass phase increases with grain growth, but saturates above 16 mm average grain size.