Thermal and structural characterization of the ZrO_2−x(OH)_2xto ZrO₂ transition

The exothermic process that occurs around 700 K during calcination of ZrO_2−x(OH)_2x, associated with the crystallization of the low-temperature tetragonal metastable phase of ZrO₂, was analyzed using x-ray diffraction, high-resolution thermogravimetric analysis (TGA), nitrogen adsorption, and modulated differential scanning calorimetry (MDSC). High-resolution TGA allowed us to determine the water loss, resulting from condensation of OH⁻ groups. The amount was 0.137 wt% in our case, equivalent to 1.7 × 10⁻² mol of H₂O/mol of ZrO₂. That corresponds to about one −OH group per nm² being lost in that process. By using MDSC we determined that the change in enthalpy (∆H_global = −15.49 kJ/mol of ZrO₂) was the result of two parallel contributions. One of them was reversible and endothermic (∆H^rev = 0.11 kJ/mol of ZrO₂), whereas the other was irreversible and exothermic (∆Hi^rrev = −15.60 kJ/mol of ZrO₂). The variability and magnitude of the exotherm, as well as the fact that the accompanying weight loss is so small, are consistent with a mechanism involving the formation of tetragonal nuclei, rather than global crystallization, and hence depend on the number of nuclei so formed.