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Abstract: Scandia-stabilised zirconia (ScSZ) electrolyte materials exhibit the highest conductivity of all the zirconia systems, but the degradation in the ScSZ system has only been investigated yet for samples of 7.8, 8.0 and 11.0 mol% Sc2O3. Therefore, ScSZ electrolytes with scandia contents varying from 7 to 13 mol% were synthesised and their electrical conductivity and phase composition was determined as a function of annealing time at 1000 °C. A conductivity degradation occurs in aged specimens with less than 10 mol% Sc2O3, which were cubic at room temperature. On the other hand, the conductivity of the specimens with higher scandia content was stable after annealing. These specimens were rhombohedral at room temperature, but transform into the cubic phase between 400 and 600 °C. This phase transition is accompanied by a volume change ΔV of 0.15%. To suppress the phase change, 1 mol% Al2O3 was added to specimens with 7–13 mol% scandia. They showed no relevant difference in conductivity and phase to the specimens without alumina. In addition, 10ScSZ samples were doped with higher amounts of alumina up to 5.5 mol% (Al-10ScSZ), and titania up to 6 mol% (Ti-10ScSZ). The cubic phase stabilisation was not obtained with the addition of 5.5 mol% Al2O3, but already with the addition of 2 mol% TiO2. On the other hand, the conductivity drop of both the Al- and the Ti-10ScSZ electrolytes with increasing alumina or titania content, makes them uninteresting for applications in solid oxide fuel cells (SOFC). Finally an interpretation for the conductivity degradation for stabilised zirconia electrolyte is given. [Copyright &y& Elsevier]

Abstract: Annealing at 1000 °C reduces the electrical conductivity of 8 mol% yttria-stabilised zirconia (8YSZ), known as a good performing material for Solid Oxide Fuel Cells (SOFC) electrolytes. This decrease in electrical conductivity by ageing for 1000 h was not observed for doped zirconia with higher yttria contents. High-resolution XRD, neutron-diffraction and STEM were performed to get information of possible changes in the microstructure. Both, the electrical measurements and structural investigations of yttria-stabilised zirconia have been correlated. It is explained that the degradation was not caused by the phase change that occurs in samples with lower dopant content. The ageing effect of the electrical properties is interpreted by the presence of different defect associates present before annealing in all the doped zirconia specimens, or rather the transformation within these associates. This transformation also affects the observed structural change. A confirmation of this model of defect associates transformation is given in part II, wherein the scandia-stabilised system has been studied. [Copyright &y& Elsevier]