Phase mapping of multi-component oxides derived from sol-gel precursors

A sol-gel coprecipitation technique was used to prepare a wide range of compositions in two systems: MgO-TiO₂ and Al₂O ₃-ZrO₂. Nanocrystalline, metastable phases were found in both systems after firing in temperatures up to 900°C for several hours. Above 1000°C, the metastable phases gradually transformed to the structures expected by the equilibrium phase diagrams. In the MgO-TiO₂ system, a single phase with structure similar to the H.T. inverse spinel (qandilite) was formed from xerogels 500°C below the eutectoid point. Furthermore, this phase tolerated a wide range of compositions, ranging from Mg/Ti = 1.2 to 2.0. In the Ti-rich side, orthorhombic (karrooite-like) solid solutions were found from xerogels at compositions ranging from Mg/Ti = 0.5 to 0.63. The stability of these phases was tested up to 1500°C for a period of 3 h. It was found that at maximum deviation from stoichiometry, the spinel-like structure is stable up to 700°C and the karrooite-like non-stoichiometric phases are stable up to 600°C. Annealed xerogels prepared by sol-gel coprecipitation from the alcoholates of Zr and Al showed a wide solubility range for alumina in nanocrystalline zirconia, up to about 26 mol-%, accompanied by stabilization of the high-temperature tetragonal and cubic zirconia phases. The phases found in this research do not exist in the equilibrium phase diagrams at the same compositions and temperatures.