More than 60mol% Zr4+ ion (ZrO2/(Al2O3+ZrO2)) is soluble into anhydrous Al2(SO4)3 crystal lattice. The apparent crystallite size decreases with increasing Zr4+ content, and the thermal decomposition temperature reduces at contents of zirconia >20mol%. Alumina/zirconia composite powders, in which an amorphous zirconia phase is dispersed in a matrix of fine η-alumina particles, are obtained by thermal decomposition of the sulfate solid solution. Tetragonal phase of zirconia crystallizes at 1100°C and the crystallinity increases with increasing the temperature of the heat-treatment. When mono-dispersed ultra fine particles of zirconia are added to η-alumina formed by thermal decomposition of pure anhydrous aluminum sulfate, the η→α phase transformation of alumina is suppressed, and the effect increases with increasing the amount of the additive. On the contrary, a small quantity of ZrO2<1mol% drastically suppresses the phase transformation of alumina formed by decomposition of sulfate solid solution. But, when ZrO2 content is more than 1mol%, the η→α phase transformation of alumina is accelerated. The acceleration effect becomes larger with increasing the content of ZrO2.
