Abstract
Ageing behaviour has been studied in the scandia-zirconia system (7.8 mol%Sc 2O 3 content) at 1000°C and below with fourprobe 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 β-phase (Sc 2Zr 7O 17). This study shows that in homogeneously prepared samples of 7.8 mol%Sc 2O 3-ZrO 2, 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 γ- or σ-phase) was also detected in the microstructure. The β-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 Sc 2O 3 and ZrO 2 during fabrication.
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