ALKALI SURFACE CONCENTRATION AT ELEVATED TEMPERATURES.

The alkali concentration in the surface monolayer of Na//2O multiplied by (times) 3SiO//2 and K//2O multiplied by (times) 3SiO//2 glasses was measured at elevated temperatures with ion scattering spectroscopy (ISS). At temperatures in and below the glass transformation range, the surface composition was time-dependent. In general, the alkali was depleted in the surface monolayer due to evaporation; the alkali depletion at the surface increased with decreasing temperature. These data could be mathematically modeled in terms of a surface reaction (desorption) term and a diffusion term. At temperatures above the transformation range, the alkali concentration in the surface reached an apparent steady-state value which was about 90% of the bulk concentration. It is suggested that surface tension, surface stress and convection may also influence the high temperature surface composition.