A model to calculate the viscosity of silicate melts: Part V: Borosilicate melts containing alkali metals

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 B ₂O ₃-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, SiO ₂ and B ₂O ₃, any basic oxide and amphoteric oxides exhibiting the Charge Compensation Effect such as Al ₂O ₃. The predictive ability of the model is tested on all ternary subsystems of the B ₂O ₃ - Na ₂O-K ₂O-CaO-MgO-PbO-ZnO- Al ₂O ₃-SiO ₂ system containing both an alkali oxide and B ₂O ₃ for which experimental data are available and on several multicomponent glass-forming melts around commercial glass compositions.