The effect of field reversals on refractive index profiles and stress in electric field assisted K+-Na+ ion-exchanged soda-lime glass

Soda-lime glass samples have been subjected to both single and double field reversals during electric field assisted K+-Na+ ion exchanges in the temperature range 350-450 degrees C. Changes in refractive index profiles, chemical profiles and surface stress were observed during the exchanges and a comparison has been made with previous studies. It is shown that large reductions in surface stress seen after single field reversals, which have previously been attributed primarily to a stress relaxation mechanism, are mainly caused by the removal of a large number of K+ ions. For example, at 395 degrees C K+ ion removal accounts for 80-95% of the stress reduction. It is shown that the drift-diffusion model of ion transport, using concentration-independent self-diffusion coefficients, cannot consistently model this large ion removal rate. A new model is developed which is based on drift-diffusion transport with concentration-dependent self-diffusion coefficients. The effects of double field reversals are also investigated. It is suggested that the glass may undergo an irreversible change associated with K+ ion removal.