Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al 2 O 3 compacts were doped with 0-350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al 2 O 3 at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, In Mg/Al = 3.80 - 2.63 x 10 4 /T. The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al 2 O 3 were probably not done in single-phase Al 2 O 3 solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A 2 O 3 may be correct. The effects of SiO 2 impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al 2 O 3 are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.