Hydrothermal crystallization kinetics ofm-ZrO₂andt-ZrO₂

The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO₂have been determined at various pH conditions. It is shown that monoclinic ZrO₂precipitates at low pH whereas at high pH tetragonal ZrO₂crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)_xO_y, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO₂: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO₂formed under hydrothermal conditions are presented.