Diffusion of transition metals (Co,Ni) and its effects on sol-gel derived ZrO2 polymorphic stabilities

Transition-metal-incorporated ZrO2 gel matrices have been prepared by an impregnation method. The polymorphic forms of the studied gels at room temperature were determined by Fourier-transform IR (FTIR) spectroscopy and X-ray diffraction (XRD) techniques, and the phase transformation behaviour of the gels was investigated with using differential thermal analysis (DTA) methods. For both undoped and doped ZrO2 gels, DTA reveals metastable tetragonal-monoclinic-tetragonal-cubic phase transformations in the heating process up to 1400 degrees C. A high-temperature tetragonal-monoclinic transition is also observed in the cooling process. For the metal-loaded ZrO2 gels, it is found that the diffusing metal cations stabilize the low-temperature tetragonal phase. However, for high-temperature (900 degrees C) calcined gels, both as-prepared and metal-stabilized tetragonal phases are reduced substantially, Correlations between metal diffusion and gel polymorphic stabilities are also demonstrated.