Experimental studies and modeling of viscosities of borate and borosilicate melts

Viscosity is one of the most important properties in the case of molten systems, in view of its direct effect on the kinetic conditions of the processes. Viscosity of molten oxides is a structure-related property. The objective of this work is to enhance our fundamental understanding of the viscous behavior of borate and borosilicate melts. In this research work, viscosities of unary B₂O₃, binary Na₂O-B₂O₃, and Na₂O-SiO₂ melts, and ternary Na₂O-SiO ₂-B₂O₃ glass melts are measured as a function of temperature and composition. The effects of temperature and composition on viscosity are studied here.; Based on the thermodynamic theory, a quantitative structure model for alkali borate melts is developed to describe the distribution of structural groups in the alkali borate melts as a function of compositions. The equilibrium relations between the activities of all the structural groups are quantitatively explicated over the entire composition range. A qualitative interpretation for borate anomaly in viscosity has been proposed, which regards the borate anomaly as a manifestation of the various antithetical effects caused by the introduction of alkali oxide into boric oxide. At any given temperature and composition, the structure's weakening effects of bond ionization in glass network and bond break by non-bridging oxygen are in equilibrium with the structure's strengthening effect of converting boron atoms from a threefold coordination state to a fourfold coordination state. It has been found that the viscosities of borate and borosilicate melts increase or decrease according to the changes in rigidity and spatial connectivity of the glass network structure, which is the function of temperature and composition.; Based on the present experimental data, a structure-based viscosity model has been proposed for Na₂O-SiO₂-B₂O ₃ ternary melts. This viscosity model relates viscosity of sodium borosilicate melts not only to the temperature and the composition, but also to the degree of polymerization of the melts, which is represented by the fraction of bridging oxygen in the melts. The calculated viscosities are in excellent agreement with the experimental data.