SYNTHESIS OF DENSE BETA, BETA''-ALUMINA CERAMICS BY REACTION SINTERING OF NA2O-CONTAINING ALUMINA COMPACT

Reaction sintering in the Na2O-MgO-Al2O3 system was studied with alpha-alumina green compacts containing Na2O and MgO in the pores of a controlled structure to produce beta, beta"-alumina polycrystalline at low temperatures. HCOONa and (CH3COO)2Mg were infiltrated into the alumina powder compacts (average particle size 0.2-mu-m) with relative densities of 51 to 66% at the compositions of Na2O/MgO/Al2O3 = 1/0.10/4.0-7.6. The alumina powder compacts were formed by filtration of the aqueous colloidal suspensions through gypsum molds. Phase change from alpha to beta, beta"-alumina proceeded fast at low temperatures in the samples with high Na2O/Al2O3 ratios. However, no significant densification occurred below 1500-degrees-C owing to the volume increase of powder compacts associated with the phase change of alpha to beta, beta"-alumina. At 1600-degrees-C, the green compacts were rapidly densified to relative densities above 99%. This densification was related to the liquid phase sintering based on the partial decomposition of beta"-alumina. The dense beta, beta"-alumina polycrystalline consisted of needle-like grains of 2-5-mu-m in length and 1-2-mu-m in width.