THERMAL AND STRUCTURAL CHARACTERISATION OF THE ZrO2-x(OH)2x TO ZrO2 TRANSITION

The exothermic process that occurs around 700 K during calcination of ZrO2-x(OH)2x, associated with the crystallisation of the low-temperature tetragonal metastable phase of ZrO2, was analysed using XRD, high-resolution TGA, nitrogen adsorption, and modulated DSC. High-resolution TGA allowed determination of the water loss, resulting from condensation of OH- groups. The amount was 0.137 wt%, equivalent to 1.7 x 10 exp(-2) mol of H2O/mol of ZrO2. That corresponds to about one -OH group per nm2 being lost in that process. By using MDSC the change in enthalpy (DeltaHglobal = -15.49 kJ/mol of ZrO2) was the result of two parallel contributions, one reversible and endothermic (DeltaHrev = 0.11 kJ/mol of ZrO2), the other irreversible and exothermic (DeltaHirrev = -15.60 kJ/mol of ZrO2). The variability and magnitude of the exotherm, and as the fact that the accompanying weight loss is so small, are consistent with a mechanism involving the formation of tetragonal nuclei, rather than bulk crystallisation, and hence depend on the number of nuclei so formed. 15 refs.