Effect of Rare-Earth Oxides on Reaction Sintered Non-Stoichiometric Magnesium Aluminate Spinel

Non-stoichiometric spinel compositions with molar ratio of MgO:Al2O3 = 1:2 and 2:1, were prepared via single-stage reaction sintering technique at 1550 degrees-1600 degrees C using commercial grade oxide reactants. The effect of different rare-earth oxides, viz. CeO2, La2O3 and Y2O3 at 2 wt% level on the properties of different spinel batches was studied. Y2O3 was found to favour densification in both the spinel batches while CeO2 and La2O3 were found to favour densification only in the MgO-rich spinel compositions. Phase analysis revealed that Y2O3 containing Al2O3-rich spinel has a secondary phase with similar crystal structure to spinel (both cubic) and improved densification, while in the MgO-rich condition, Y2O3 remained as secondary phase and assisted in densification. CeO2 and La2O3 in Al2O3-rich samples formed secondary phases with different crystal structures and resulted in reduced densification. While in MgO-rich condition, CeO2 (cubic) did not react with Al2O3 and La2O3 formed a secondary phase with similar crystal structure as that of spinel and therefore assisted in densification. Addition of rare-earth oxides led to improvement in strength of MgO-rich spinels and maximum improvement in strength was observed with the addition of yttria.