PREPARATION AND PROPERTIES EVALUATION OF ZIRCONIA-BASED AL2O3 COMPOSITES AS ELECTROLYTES FOR SOLID OXIDE FUEL-CELL SYSTEMS .3. MECHANICAL AND ELECTRICAL CHARACTERIZATION

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 degrees C the bending strength increased with Al2O3 content up to 10 wt % and then decreased, the measured value at 1000 degrees 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 degrees 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 ZrO2-8 mol % Y2O3/20 wt % Al2O3.