Transient Eutectics in Sintering of Sodium Beta Alumina

Abstract

Sodium beta alumina solid electrolytes for use in the sodium/sulphur battery1 have been prepared by a wide variety of methods. Two different powders can be used: component oxides or other precursors to give a reactive sintering2–4, or prereacted powders5 with or without additives6. Fast heat-up rates and zone sintering2 seem now universally accepted as the preferred sintering schedule. Additional improvements in ionic conductivity and mechanical strength can be obtained by post-annealing treatments4. Generally, for prereacted s alumina powders, such as the Alcoa powder XB-2 “superground”, heat-up rates are between 50–100°C min-1, and time- at-temperature (~1750–1800°C) is between 5 to 10 minutes. The high sintering temperatures are not an absolute necessity, as was recently demonstrated by Cannon and Chowdry6 who obtained 97% dense s-alumina from the submicron agglomerate fraction of preconverted XB-2 powder sintered at 1530°C. Sintering times then get prohibitively long for commercial application, however. The high rates of densification during zone sintering led to the postulate that a transient liquid phase might be active during the densification of sodium beta alumina7. Additionally, preconverted beta alumina (Alcoa XB-2 “superground”) mixed with a small amount of metastable NaA1O2-s alumina eutectic showed improved sinterability8,9 lending further support to the hypothesis that transient liquid phases are present at one point during the zone sintering process. In this paper we explore further if indeed a transient euteqtic liquid phase is responsible for the improved sinterability of preconverted sodium beta alumina powders when mixed with a small amount of metastable eutectic additive.