CRYSTALLIZATION AND PHASE-TRANSFORMATION PROCESS IN ZIRCONIA - AN INSITU HIGH-TEMPERATURE X-RAY-DIFFRACTION STUDY

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500-degrees-C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO2 phase, while the slowly precipitated precursor produced the tetragonal ZrO2 phase. The crystallization and phase transformations were followed by in situ high-temperature X-ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450-degrees-C. A tetragonal-to-monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275-degrees-C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal-to-monoclinic transformation for the rapidly precipitated ZrO2 sample. The crystallite size for the t-ZrO2 of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t --> m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal-to-monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.