DENSE POLYCRYSTALLINE STRONTIUM BETA''-ALUMINA AND BETA-ALUMINA - SYNTHESIS, XRD CHARACTERIZATION AND THERMAL-STABILITY

Dense polycrystalline ceramic tubes of strontium beta"- and beta-alumina were prepared by ion exchange of sintered tubes of Li2 O-stabilized Na beta"-alumina and of Na beta-alumina at 585 +/- 5-degrees-C in a molten eutectic mixture of SrCl2-Sr(NO3)2. X-ray powder diffraction patterns are presented. The thermal stability of the materials was studied at temperatures up to 1550-degrees-C. Both Sr beta"- and beta-alumina were found to be more stable than other M2+ substituted isomorphs. Sr beta"-alumina appears to be stable to at least 1100-degrees-C, but its stability decreases with an increasing beta-phase content in the starting material. It transforms first to a beta-alumina phase (< 1250-degrees-C) and then gradually to a magnetoplumbite phase. Unstabilized Sr beta-alumina starts to transform to a magnetoplumbite structure above 1000-degrees-C.