Using zirconium oxychloride solution as precursor, monoclinic zirconia crystallites withnarrow distribution of nanosize were obtained in the hydrothermal reaction. However, when thereaction was in weak acidic medium or base medium, whether directly using the colloidal precipi-tate prepared from zirconium salt solutions with base solution as precursor added, or using theprecipitate after filtrating, washing and drying treatments as precursor, the product of the hydro-thermal reaction was the mixture of both monoclinic and tetragonal polymorphs. As the pH of themedium rises, the content of tetragonal phase in the product, the morphologies and size of thecrystallites all change. There are three types of formation mechanisms under hydrothermal condi-tion, which can be called as saturation-precipitation mechanism in homogeneous solution, dissolu-tion-crystallization mechanism and in-situ crystallization mechanism, respectively. The formationmechanism of crystallites varies with different hydrothermal conditions, such as the states of theprecursor and the pH of the medium, which lead to changes in the phases, morphologies andsizes of the resulting crystallites.
