Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel

Abstract

The formation of metastable tetragonal zirconia nanophase by thermal treatment of a zirconia gel derived from zirconyl chloride has been studied by high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). HRTEM observations revealed that a fully crystallized sample consists of nanocrystals, around 13 nm in size. This nanocrystalline t-ZrO2 has practically the same crystal structure as that of the high-temperature tetragonal zirconia phase. The nonisothermal crystallization rate is very fast in as-prepared zirconia gel. DSC data at various heating rates can be described by a two-parameter model which predicts the crystallization kinetics in isothermal conditions very well. The Johnson–Mehl–Avrami (JMA) model can be used, however, in partially crystalline samples (crystallinity >30%) where the rate of crystallization process is considerably slower. The kinetic exponent of the JMA model (m = 1.0 ± 0.1) then corresponds to linear dependence of the crystallization rate as a function of the fraction crystallized.