Solid solution of Zr into anhydrous Al-2(SO4)(3) crystal lattice and eta ->alpha phase transformation of Al2O3 formed by thermal decomposition of thesulfate solid solution

More than 60 mol% Zr4+ ion (ZrO2/ (Al2O3 + ZrO2)) is soluble into anhydrous Al-2(SO4)(3) crystal lattice. The apparent crystallite size decreases with increasing Zr4+ content, and the thermal decomposition temperature reduces at contents of zirconia > 20 mol % , Alumina/zirconia composite powders, i n which an amorphous zirconia phase is dispersed in a matrix of fine eta-al umina particles, are obtained by thermal decomposition of the sulfate solid solution. Tetragonal phase of zirconia crystallizes at 1100 degrees C and the crystallinity increases with increasing the temperature of the heat-tre atment. When mono-dispersed ultra fine particles of zirconia are added to e ta-alumina formed by thermal decomposition of pure anhydrous aluminum sulfa te, the eta-->alpha phase transformation of alumina is suppressed, and the effect increases with increasing the amount of the additive. On the contrar y, a small quantity of ZrO2 < 1 mol % drastically suppresses the phase tran sformation of alumina formed by decomposition of sulfate solid solution. Bu t, when ZrO2 content is more than 1 mol %, the eta-->alpha phase transforma tion of alumina is accelerated, The acceleration effect becomes larger with increasing the content of ZrO2.