Effect of alumina additions upon electrical properties of 8 mol.% yttria-stabilised zirconia

Solid oxide fuel cells (SOFCs) are energy converters that directly transform the chemical energy of combustible gases into electrochemical energy by oxidation. The design of SOFC, which has the highest volumetric power density, is a planar one in which the electrolyte, yttria-stabilised zirconia (YSZ), can be optimised by strengthening with small additions of alpha-Al3O3. Different commercial powders have different impurity contents and thus show different changes in ionic conductivity when alpha-Al2O3 is added. We describe the changes in oxide ion conductivity of Tosoh 8 mol.% YSZ that has been added to with a-alumina. AC impedance measurements show that small additions (similar to 1 wt.%) of Al2O3 can cause the ionic conductivity of Tosoh 8YSZ to increase due to a decrease in the grain boundary impedance which is observable at low to medium temperatures. Small wt.% additions of alpha-Al2O3 also cause an overall decrease in the high temperature impedance of 8YSZ. We have found that 10 wt.% alumina can be added to 8YSZ without any significant decrease in ionic conducting properties. Further additions of alumina cause a rapid decrease in conductivity due to the large volume percent of insulating alumina phases, which are present, and also due to the cracking of pellets that occurs on firing. We also report the improved stability of added-alumina 8YSZ to hydrothermal ageing. Hydrothermal ageing of unadded to 8YSZ, in an autoclave at 180 degrees C, can lead to a decrease in the conductivity at 1000 degrees C by as much as 40%. The drop in conductivity of 8YSZ can be limited to a decrease of only 10% by the addition of greater than 5 wt.% Al2O3. (C) 1999 Elsevier Science B.V. All rights reserved.