Modelling the ion exchange process in glass: Phenomenological approaches and perspectives

A great deal of work has been devoted in the last decades to the study of the ion exchange process modelling, in order to tailor the physical properties of the modified layers, e.g. refractive index or stress profiles. The first models were based on simple ionic diffusion equations, while in the last years it became clear that the glass undergoes local restructuring during the ion exchange even at temperatures lower than the glass transition one. The aim of this work is to review the approaches that lead to the introduction of self-diffusion coefficients depending on the concentration of the migrating cations, and to outline a physical approach in selecting a phenomenological expression for the coefficients. In particular, this approach refers to models developed for describing the ionic diffusion in mixed-alkali glasses.