Phase Change of Yttrium-Containing Hydrous-Zirconia Fine Particles

Yttrium-containing monoclinic hydrous-zirconia fine particles were heated from 300 to 700°C to investigate the phase change to 2.8mol% Y2O3-doped tetragonal ZrO2. Both ZrOCl2⋅8H2O and YCl3⋅6H2O were dissolved into aqueous solutions and then boiled to make the hydrous-zirconia particles. The hydrous-zirconia particles containing Y(OH)3 were prepared by dropping aqueous ammonia into the boiled solutions, while hydrous-zirconia particles containing YClx(OH)y⋅nH2O were obtained by spray-drying of the boiled solutions. The monoclinic-to-tetragonal phase change of hydrous-zirconias containing Y(OH)3 was observed at 400°C by X-ray diffraction. The hydrous-zirconias containing YClx(OH)y⋅nH2O transformed to tetragonal phase at 500°C. The results of chemical analysis indicated that the atomic ratio of Cl/Y decreased with increasing heat-treatment temperature. At 500°C, this atomic ratio became less than 1. These experimental results suggested that the hydrous-zirconias containing YClx(OH)y⋅nH2O transformed to tetragonal phase through the decomposition reaction of YClx(OH)y⋅nH2O. Therefore, it is concluded that as the decomposition of YClx(OH)y⋅nH2O affects phase change, the temperature of phase change of hydrous-zirconias containing YClx(OH)y⋅nH2O becomes higher than those containing Y(OH)3. X-ray diffraction and BET specific surface area measurements revealed that for hydrous-zirconia containing Y(OH)3, the primary particle of small size with monoclinic phase was transformable to tetragonal phase at lower temperature comparing with that of large size. The hydrous-zirconias containing YClx(OH)y⋅nH2O with smaller particle size also showed the similar tendency. This tendency can be explained by the increase of the points contacted with yttrium and the shortening of diffusion length in the hydrous-zirconia particle with small size.