XRPD study of phase transformations accompanied with grain growth in the alumina-zirconia system

Using the sol-gel technique followed by thermal annealing it is possible to obtain metal oxides with grain size from the lower to the higher nano range. Xerogels, formed by co-precipitation of Zr and Al ions at room temperature in a wide range of compositions, were fired for up to 3 h at constant temperature from 300 to 1500 ⁰C. Room temperature X-ray powder diffraction was the main tool for mapping of the structures and evaluating grain size and microstrain as function of time and temperature of formation. The grain size was cross-checked by HRSEM. Solid solutions with quasi-amorphous and tetragonal structures were found for compositions ranging from pure zirconia up to 25 mole-% alumina at low firing temperatures. On the alumina-rich side zirconia-like phases were mixed with alumina transi-tion phases. Full segregation of corundum occurred only from 1300 ⁰C up. Formation of quasi-amorphous and tetragonal-like zirconia and the spinel-like alumina phases was de-pendent on the grain size. It is suggested to use the grain size as a thermodynamic factor. Hence, phases conventionally regarded as metastable will be considered as stable phases confined to a range of grain size. This calls for designing new phase diagrams based on grain size and concentration