MEASUREMENT OF DIFFUSION-COEFFICIENTS OF ALKALI-METAL IONS IN POLYCRYSTALLINE-BETA''-AL2O3

The high temperature ion-exchange of a commercially prepared polycryst alline Na+-beta''/beta-Al2O3 is described. Time dependent weight-gain data for both K+ and Rb+ exchange is presented for temperatures betwee n 900 and 1200-degrees-C. The data are correlated with the solution of the diffusion equation to estimate the diffusion coefficients for the two cations as functions of temperature. The estimated values are wel l correlated by the Arrhenius expression although the calculated activ ation energies are considerably higher than might be expected from pre vious studies involving single crystals at lower temperatures. These h igh values, coupled with the change in the physical character of the m aterial indicate that the diffusion process might be limited by transp ort across the grain boundary.