Coupled Ionic Diffusion in ulticomponent Silicate Glasses

Abstract

<p>A phenomenological scheme for ionic diffusion in multicomponent silicates based on self diffusion data and the Nernst-Planck equation has been formulated which is capable of predicting the concentration profiles of mobile ions in a diffusion couple with or without a fixed silicate anion step discontinuity at the boundary. It is demonstrated that the diffusion potential appearing within this formalism is due entirely to a distribution of self-generated dipoles and equivalently, that the well-known contradiction between the electroneutrality constraint on real charges and the non-zero field predicted within the phenomenological formalism vanishes when it is recognized that the charge to be entered into Poisson's equation is the dipole charge.</p> <p>The reported experimental diffusion profiles in K2O-SrO-SiO2 and Na2O-CaO-SiO2 glass couples have been compared with the predictions of the model and the agreement has been found to be good. Diffusion couples in the K2O-CaO-SiO2 system were investigated by microprobe analysis and the resulting concentration profiles were also found to be in good agreement with predictions. It has been demonstrated that while the quasi-binary (i.e., immobile silicate anions) approximation is satisfactory for the K2O-SrO-SiO2 system at low temperatures, it is unsatisfactory for all other systems considered at low and high temperatures.</p> <p>An extension of the phenomenological scheme to glass-metal diffusion couples with interfacial electrochemical reactions is given in Appendix A.</p>