High temperature conductivity of potassium-β″-alumina
Abstract Potassium β″-alumina single crystals have been reported by several groups to have higher ionic conductivity than sodium β″-alumina crystals at room temperature, and similar conductivities are obtained at temperatures up to 600–700 K. Potassium β″-alumina ceramics have been reported to have significantly poorer consuctivities than those of sodium β″-alumina ceramics, but conductivity measurements at temperatures above 625 K have not been reported. In this study, K + -β″-alumina ceramics were prepared from Na + -β″-alumina ceramic using a modified version of the exchange reaction with KCl vapor reported by Crosbie and Tennenhouse, and the conductivity has been measured in K vapor at temperatures up to 1223 K, using the method of Cole, Weber and Hunt. The results indicate reasonable agreement with earlier data on K + -β″-alumina ceramic measured in a liquid K cell, but show that the K + -β″-alumina ceramic's conductivity approaches that of Na + -β″-alumina ceramic at higher temperatures, being within a factor of four at 700 K and 60% of the conductivity of Na + -β″-alumina at T > 1000 K. Both four-probe dc conductivity and four probe ac impedance measurements were used to characterize the conductivity. A rather abrupt change in the grain boundary resistance suggesting a possible phase change in the intergranular material, potassium aluminate, is seen in the ac impedance behavior.