Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y₂O₃ and 0 to 20 wt% Al₂O₃ were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al₂O₃ content. The bending strength of the composite at room temperature increased with increasing Al₂O₃ content. In the temperature range 500–1000 °C the bending strength increased with Al₂O₃ content up to 10 wt% and then decreased, the measured value at 1000 °C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K_IC) decreased with increasing Y₂O₃ content in the Al₂O₃-free zirconia materials. Al₂O₃ additions enhanced the fracture toughness and this was maximum (7 MPa m^1/2) for the composite ZrO₂-3 mol% Y₂O₃/10 wt% Al₂O₃. The electrical conductivity of cubic ZrO₂/Al₂O₃ composites decreased monotonically with Al₂O₃ content, but in tetragonal ZrO₂/Al₂O₃ composites hardly varied or apparently increased up to 10 wt% Al₂O₃. At 1000 °C the highest electrical conductivity was 0.30 S cm^–1 for ZrO₂-8 mol% Y₂O₃, and this decreased up to 0.10 S cm^–1 for the composite ZrO₂-8mol% Y₂O₃/20 wt% Al₂O₃.