Stability of metastable tetragonal zirconia

This research was undertaken to determine the reason for the occurrence of metastable tatragonal zirconia (ZrO2). Metastable tetragonal ZrO2 was prepared by low temperature calcination of zirconium oxychloride octahydrate ZrO Cl2·8H2O. The tetragonal-to-monoclinic (t-m) transformation in pure metastable tetragonal ZrO2 powder was investigated with special reference to the isothermal annealing temperature, the variation of crystallite size and the annealing environment. The results indicate that the metastable tetragonal ZrO2 is stabilized thermodynamically by surface free energy and there is a so-called 'critical diameter (crystallite size)'. Moreover, thermodynamic calculation showed that the t-m transformation temperature depends on the critical crystallite size. (author)