PRECIPITATION OF ULTRAFINE POWDERS OF ZIRCONIA POLYMORPHS AND THEIR CONVERSION TO BAZRO3, SRZRO3, CAZRO3 BY THE HYDROTHERMAL METHOD

Ultrafine powders of ZrO2 with a high degree of crystallinity and chemical purity are hydrothermally precipitated from aqueous solutions of impure zirconyl oxychloride or the acid extract of zircon (ZrSiO4)-frit at 2 to 8 MPa and 180 to 230 ° C. Monoclinic ZrO2 is produced from aqueous hydrochloric acid, whereas tetragonal ZrO2 is formed from the same medium when sulphate ions are present with [SO42-]/[Cl-] ≥ 0.08. If cation impurities such as Y3+ or Ce3+ are incorporated, the stability range of the tetragonal phase is extended to higher temperatures. Ultrafine powders are characterized by X-ray broadening methods, TEM and thermally as well as mechanically induced transformation characteristics. The tetragonal ZrO2 powder is constituted of polydomain crystallites with higher hydroxyl ion content than the monoclinic phase. Both series of powders convert to BaZrO3, SrZrO3 or CaZrO3 perovskites when suspended in the corresponding hydroxide solution at 190 to 480 °C and 2 to 100 M Pa. © 1990 Chapman and Hall Ltd.