On the transformation kinetics of/ - to/ " alumina in the system AIzO3-Na20-Li20

Sodium/3,/3"-alumina ceramics are well-known fast ionic conductors of Na ion and are used in various electrochemical devices such as the Na/S battery [1]. /3"-Alumina phase has a higher ionic conductivity than/3-alumina [2], so the former is preferably used as a major phase in the solid electrolyte composition. Generally it is found that/3"-phase grows in syntaxy with /3-alumina [3], and during the production of solid electrolyte from polycrystalline phase mixture a post-sintering annealing is required to complete the conversion of /3to /3"-alumina phase [4]. A kinetic study of the conversion mechanism may lead to a better understanding of the efficient production of solid electrolyte. It is known that the/3"-phase is unstable [5] in the binary system Na20:A1203. MgO [6] and Li20 [7] are used to stabilize/3"-alumina phase and Li20 is found to be the most effective stabilizer [8]. The ternary phase (A1203-Na20-Li20) diagram is still to be properly evaluated. Hodge [5] studied the kinetics of/3"to/3-alumina transformation in the binary system Na20:A1203 and suggested an Al-ion diffusion-controlled growth of /3-phase at the expense of /3"-phase with very sluggish kinetics. The detailed mechanism of these phase transformations is still not well established. Transformation from /3to /3"-alumina may also follow the same mechanism as described by Hodge [5[ for the/3 --~/3" transformation. Kvachov et al. [9] stated that the /3 ~ /3" transformation may follow the reaction /3 + NaA102--~/3" (1)