Creep properties of high dense La9.33Si6O26 electrolyte for SOFCs

High density La9.33Si6O26 polycrystals were fabricated by conventional and spark plasma sintering starting from nanopowders synthesized by freeze-drying. The materials exhibit a homogeneous microstructure formed by equiaxed grains with average sizes of 1.1 mu m and 0.2 mu m-diameter depending on the sintering route. Compressive mechanical tests were performed in air at constant strain rate between 900 and 1300 degrees C. A gradual brittle-to-ductile transition was found with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region, characterized by a stress exponent n = 1 for the conventional sintered (large-grained) material and n = 2 for the spark plasma sintered (fine-grained) material; in both cases, the activation energy for creep was 360 kJ/mol. Effective cation diffusivities have been derived from mechanical data by comparison with appropriate models. The creep properties of lanthanum silicates are reported here for the first time.