Thermal and structural characterization of the ZrO2− x(OH)2 xto ZrO2 transition

The exothermic process that occurs around 700 K during calcination of ZrO 2− x (OH) 2 x , associated with the crystallization of the low-temperature tetragonal metastable phase of ZrO 2 , 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 H 2 O/mol of ZrO 2 . That corresponds to about one −OH group per nm 2 being lost in that process. By using MDSC we determined that the change in enthalpy (∆ H global = −15.49 kJ/mol of ZrO 2 ) was the result of two parallel contributions. One of them was reversible and endothermic (∆ H rev = 0.11 kJ/mol of ZrO 2 ), whereas the other was irreversible and exothermic (∆ H i rrev = −15.60 kJ/mol of ZrO 2 ). 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.