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.