Ion Exchange in Sodium Borosilicate Glasses

The kinetics of K⁺-Na⁺ exchange in sodium borosilicate glasses containing 20 mol% Na₂O were studied as a function of glass composition and exchange time and temperature. The distribution of K in the glass after exchange in molten KNO₃ was determined using an electron microprobe. In those specimens which were not chemically attacked by the molten salt, the K profiles were consistent with Fickean behavior. The calculated interdiffusion coefficients varied with local composition in each specimen. This composition dependence and its variation with temperature could be fitted approximately to a mixed-alkali model using the Nernst-Planck relation for diffusive fluxes. However, some deviation from this behavior, presumably as a result of the generation of microscopic stresses around an exchange site, was observed. The dependence of the apparent activation energy, , on the B/Si ratio was analogous to that of the activation energy for ionic conduction in these glasses: decreased with increasing fraction of nonbridging oxygen. This result disagrees with the presumed behavior in sodium aluminosilicate glasses.