Drying Effect on Thermal Behavior and Structural Modifications of Hydrous Zirconia Gel

Adopting four different drying modalities on three single stocks of hydrous zirconia gel, 12 samples of pure zirconia xerogels were obtained. In particular, the following drying modalities were adopted: drying in air at room temperature, drying in air oven at 60° or 110°C, and, finally, drying by freeze‐drying. The resulting xerogels showed, on heating, very different reactivity, affecting the structural phase transformations of pure zirconia. The kinetics feature could be the key to explain the formation mechanism of the metastable phases of zirconia at room temperature. The experimental results show, in fact, that the metastable phase is favored by a fast crystallization rate, whereas the stable one, by a slow crystallization rate. These findings could resolve the conflicting results on the metastability of zirconia reported in the literature.