Thermal and structural characterization 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 crystallization of the low-temperature tetra gonal metastable phase of ZrO2, was analyzed using x-ray diffraction, high- resolution thermogravimetric analysis (TGA), nitrogen adsorption, and modul ated differential scanning calorimetry (MI)SQ. 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 x 10(-2) Mol of H2O /mol of ZrO2. That corresponds to about one -OH group per nm(2) being lost in that process. By using NMSC we determined that the change in enthalpy (D eltaH(global) = -15.49 kJ/mol of ZrO2) was the result of two parallel contr ibutions. One of them was reversible and endothermic (DeltaH(rev) = 0.11 kJ /mol of ZrO2), whereas the other was irreversible and exothermic (DeltaH(ir rev) = -15.60 kJ/mol of ZrO2). The variability and magnitude of the exother m, as well as the fact that the accompanying weight loss is so small, are c onsistent with a mechanism involving the formation of tetragonal nuclei, ra ther than global crystallization, and hence depend on the number of nuclei so formed.