INFLUENCE OF PH ON THE HYDROTHERMAL CRYSTALLIZATION KINETICS AND CRYSTAL-STRUCTURE OF ZRO2

Differential scanning calorimetry was used to measure the heat evolved as zirconia samples are heated at constant rate. The data are used to deduce the kinetics of zirconia crystallization during hydrothermal t reatments at pH 2, 7, 9.5 and 13. Hydrothermal crystallization was fou nd to proceed much more slowly in the neutral pH medium than in the ac idic or alkaline medium. The analogy between the rate of the hydrother mal crystallization of ZrO2 and solubility of zirconium hydroxide was discussed. The results of phase analysis showed that metastable cubic zirconia, c-ZrO2, could be produced by hydrothermal crystallization in the presence of NaOH as a stabilizing agent. The results of X-ray dif fraction showed that, even at low pH, monoclinic zirconia, m-ZrO2, was not the only product of the dissolution/precipitation mechanism of hy drothermal crystallization. It is concluded that in situ crystallizati on of amorphous zirconium hydroxide, if it occurs, is not a topotactic process. (C) 1997 Elsevier Science B.V.