Phase transformation in the Al2O3 ZrO2 system

Co-precipitation methods have been used to produce 20 mol % Al2O3-80 mol % ZrO2 mixed oxides, from aqueous solutions of zirconium oxychloride and aluminium 3 chloride, followed by precipitation with ammonia. The resulting gel was calcined at increasing temperatures, and X-ray diffraction confirmed that the structure remained amorphous up to 750 degrees C and then crystallized as a single-phase cubic zirconia solid solution, but with a reduced unit-cell dimension. At higher temperatures, the unit-cell dimension increased and, above 950 degrees C, this phase started to transform to a tetragonal zirconia (t-ZrO2) phase, again of reduced cell dimensions compared with t-ZrO2, with simultaneous appearance of small amounts of theta-Al2O3. Above 1100 degrees C, the tetragonal phase transformed to monoclinic zirconia on cooling, and the amount of theta-Al2O3 increased. Above 1200 degrees C, the theta-Al2O3 transformed to the stable alpha Al2O3. These results confirm that aluminium acts as a stabilizing cation for zirconia up to (C) 1998 Chapman & Hall.