Influence of temperature on multicomponent diffusion in calcium and sodium aluminosilicate melts

Abstract

The effect of temperature on multicomponent chemical diffusion in liquids of the quaternary system CaO – Na2O – Al2O3 – SiO2 has been studied. Diffusion-couple experiments were performed around a central composition of 64.5 wt % SiO2, 13.3 wt % Na2O, 10.8 wt % CaO, 11.4 wt % Al2O3. Experiments were performed for three temperatures far above the glass transition (1200, 1280 and 1360 °C), as well as 30°C above the glass transition. Strong multidiffusive effects were observed for all temperatures, with significant uphill diffusion of calcium, demonstrating that uphill diffusion happens close to the glass transition as well as at high temperature. For each temperature, we determined the diffusion matrix of the system and quantified its eigenvectors and eigenvalues, which correspond formally to (respectively) exchanges between ions, and associated diffusion coefficients. Little variation of the eigenvectors of the diffusion matrix was observed as a function of temperature, with a dominant eigenvector corresponding to the exchange of sodium with calcium, the two other eigenvectors corresponding to the exchange of calcium with network formers. For the temperature range 1200–1360 °C, the eigenvalues of the diffusion matrix have an Arrhenian temperature dependence, with an activation energy consistent with electrical conductivity for the exchange of sodium and calcium, and an activation energy consistent with viscosity for eigenvectors involving network formers. Multicomponent diffusion close to the glass transition is characterized by the same eigenvectors as at higher temperature, but some diffusion profiles are asymmetric due to strong viscosity contrasts resulting in concentration-dependent eigenvalues. Moreover, we observe some departure from Eyring relation close to the glass transition, with diffusion eigenvalues several orders of magnitude greater than the Eyring prediction.