Ion exchange studies in beta''-alumina

Hg {dollar}betasp{lcub}primeprime{rcub}{dollar}-alumina was prepared from both single crystal and polycrystalline Na and Ag {dollar}betasp{lcub}primeprime{rcub}{dollar}-alumina using conventional molten salt immersion and reflux ion exchange, which was developed in this research. In the reflux method, the sample was continually bathed in pure HgCl{dollar}sb2{dollar}, which yielded much higher Hg(II) exchange than using conventional immersion. Beginning with single crystal Ag {dollar}betasp{lcub}primeprime{rcub}{dollar}-alumina, about 91 mole percent of the Ag(I) was replaced by Hg(II) after seven days of reflux exchange compared to 15 mole percent exchange with conventional immersion exchange.; The electrical conductivity of single crystal and polycrystalline Ag-Hg and Na-Hg {dollar}betasp{lcub}primeprime{rcub}{dollar}-aluminas with varying Hg contents was measured. The conductivity of pure Hg {dollar}betasp{lcub}primeprime{rcub}{dollar}-alumina, extrapolated from conductivity isotherms of single crystal Ag-Hg {dollar}betasp{lcub}primeprime{rcub}{dollar}-alumina, was calculated as {dollar}sigma{dollar}T = 2.98 {dollar}times{dollar} 10{dollar}sp6{dollar}exp ({dollar}-{dollar}0.956eV/kT) ({dollar}Omegacdot{dollar}cm){dollar}sp{lcub}-1{rcub}{dollar}K. The Hg tracer diffusion coefficient was calculated from these conductivity data using the Nernst-Einstein equation, then combined with Na and Ag diffusion data to calculate Na-Hg and Ag-Hg interdiffusion coefficients using the Nernst-Planck equation. These calculated interdiffusion coefficients were in reasonable agreement with those extracted from the reflux exchange data, indicating that interdiffusion rate limited the exchange.; In high-temperature Na to K vapor phase exchange the rate limiting step in this study was transport of NaCl, formed during exchange, from the gas phase to the KCl melt. We proposed an exchange model in which liquid KCl covered the tube walls, and controlled the transport of NaCl from the gas phase to the KCl melt. In this model, the majority of NaCl molecules did not diffuse directly to the KCl melt through the gas phase, but rather spent most of the time in the KCl-NaCl liquid on the tube walls. They moved through the gas phase to the KCl melt in a series of condensation-evaporation steps.