A study of complex effects of alumina addition on conductivity of stabilised zirconia

When zirconia is used as electrolyte material in intermediate temperature solid oxide fuel cells, the influence of impurities on conductivity becomes more significant than at high temperature. It is, therefore, important to understand the effect of impurities and if possible remove them. SiO2 is an impurity known to be present in zirconia and to have a negative effect on conductivity. Since the early 1980s, several groups have reported Al2O3 to act as a scavenger of SiO2 in zirconia. However, conflicting results have been obtained. Both increases and decreases in grain boundary conductivity have been found along with a decrease in grain interior conductivity. The effect of Al2O3 seems to be dependent on the ratio between Al2O3 and SiO2 in the material, and on the concentration and the distribution of Al2O3 in zirconia. Here, the results described in literature are reviewed and compared with new results obtained on Sc0.16Y0.04Zr0.80O1.9 and Tosoh TZ8Y. The samples were investigated with respect to conductivity (impedance spectroscopy) and microstructure (scanning and transmission electron microscopy) in order not only to find a trend but also to understand the mechanism of Al2O3 addition.