Multicomponent diffusion in sodium borosilicate glasses

Multicomponent chemical diffusion has been investigated in the SiO2-Na2O-B2O3 system for melts with an average composition (mol.%) of 68SiO2-18B2O3-14Na2O. Three diffusion couples were studied at 5 different temperatures between 700 °C and 1100 °C. The extended form of Fick's second law was used to fit the data, derive the diffusion matrix at each temperature, and quantify eigenvectors and eigenvalues. The results reveal diffusive mechanisms in chemical space (i.e. eigenvectors) that appear constant over the temperature range studied. The principal eigenvector is characterized by an exchange between silicon and sodium, while the secondary eigenvector corresponds to an exchange between silicon and boron made possible by sodium. Eigenvalues vary considerably with temperature, but do not follow an Arrhenian law. This behavior has been attributed to the structural changes of the borosilicate network with temperature. The diffusion data are then compared with viscosity and ionic conductivity measurements, revealing close links between viscosity and chemical diffusion.