MICROSTRUCTURE CONDUCTIVITY RELATIONSHIP IN THE SCANDIA ZIRCONIA SYSTEM

Ageing behaviour has been studied in the scandia-zirconia system (7.8 mol% Sc2O3 content) at 1000-degrees-C and below with four-probe dc conductivity measurements and impedance spectroscopy in conjunction with X-ray diffraction and transmission electron microscopy. In the literature, for similar compositions, the decrease in the conductivity, with time has been attributed to the precipitation of a low conducting ordered rhombohedral beta-phase (Sc2Zr7O17). This study shows that in homogeneously prepared samples of 7.8 mol% Sc2O3-ZrO2, the conductivity deterioration is inexorably tied to the formation and subsequent decomposition of a high temperature tetragonal phase, designated as the t'-phase. This phase is formed by diffusionless transformation from the cubic phase on cooling from the sintering temperature. Some local ordering (formation of gamma or delta-phase) was also detected in the microstructure. The beta-phase is not observed at this composition in as-sintered or annealed specimens in agreement with the proposed phase diagrams and is formed only if incomplete reaction has occurred between SC2O3 and ZrO2 during fabrication.