The size dependence of structural stability in nano-sized ZrO2 particles

The stable or metastable phase was found to be dependent on the size of ZrO2 nano particles. Amorphous hydrated ZrO2 powders prepared by precipitation were annealed at 1300 °C and maintained for different length of time followed by being quenched in ambient. The average particle size and phases were determined by X-ray diffraction cross-checked by transmission electron microscopy. The powders were found composed of monoclinic (m) phase when the particle size (d) is larger than 31 nm, whereas tetragonal (t) phase remains stable at room temperature when particle size is less than 14 nm. The mixture of the t and m phases is observed when average particle size locates between 14 and 31 nm. Thermodynamic calculation indicates that the surface free energy difference between tetragonal and monoclinic phases surpasses the volume chemical free energy difference between two phases at room temperature when the particle size of zirconia decreases below 13 nm. The lower surface energy of tetragonal phase results in the stability of tetragonal structure at room temperature, while tetragonal phase is normally stable at high temperature for coarse ceramics. The experimental results are in good agreement with theoretical prediction.