A model to calculate the viscosity of silicate melts

Our recently developed model for the viscosity of borosilicate melts is extended to describe and predict the viscosities of melts containing alkali oxides. In addition to the two model parameters that are required for each B2O3–MO x melt, where MO x is a basic oxide, three more parameters are needed when MO x is an alkali oxide to account for the formation of clusters near the tetraborate composition. The additional parameters represent the size and Gibbs energy of formation of these clusters and their contribution to the activation energy of the viscous flow. A general algorithm for the calculation of the viscosity is presented which summarizes the application of the viscosity model to melts that can contain two network formers, SiO2 and B2O3, any basic oxide and amphoteric oxides exhibiting the Charge Compensation Effect such as Al2O3. The predictive ability of the model is tested on all ternary subsystems of the B2O3–Na2O–K2O–CaO–MgO–PbO–ZnO–Al2O3–SiO2 system containing both an alkali oxide and B2O3 for which experimental data are available and on several multicomponent glass-forming melts around commercial glass compositions.