Hydrothermal crystallization kinetics ofm-ZrO₂andt-ZrO₂

Abstract

The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions. It is shown that monoclinic ZrO2 precipitates at low pH whereas at high pH tetragonal ZrO2 crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xOy, 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 ZrO2 formed under hydrothermal conditions are presented.

Synthesis and Room‐Temperature Ultraviolet Photoluminescence Properties of Zirconia Nanowires

“…[27,28] Metastable t-ZrO 2 can be obtained by the thermal treatment of suitable starting materials (such as zirconium salts, zirconium alkoxides, or zirconium hydroxide). [27±30] In the case of gel-derived zirconia, particle size, [31] structural similarities between amorphous gels and crystalline phases, [32] and impurities [33] all have an influence on the existence of metastable phase t-ZrO 2 . The crystalline zirconia size of 30 nm or less is now believed to stabilize the tetragonal form at room temperature due to the contribution of surface energy [34] and size effects.…”

Zirconia: A Review of a Super Ceramic

“…This process yields fine crystalline or amorphous zirconium oxide. 38 A relationship between the crystal structure and the pH of the precipitation of hydrous zirconia was reported. 39 It was shown that the pH of the solution from which the gel precipitates determines the crystal phase of the calcined zirconia.…”

Structural and Morphological Control in the Preparation of High Surface Area Zirconia

“…Under strongly alkaline conditions caused by NaOH, only the tetragonal phase was formed while acidic conditions led to the monoclinic phase. Hydrothermal treatment of zirconyl nitrate and zirconyl chloride solutions showed the same dependence of phase and pH [72][73][74]. Differences in the solubility of hydrous zirconia at different pH was put forth to explain the formation of different crystallite phases.…”