Viscosity of K2O-B2O3 Melts

A structure based model has been developed for the prediction of viscosities of K20-B20, melts. These melts exhibit an anomalous behavior of viscosity with continuous variation in B203 content. A maximum and a minimum are observed at low temperatures, which disappear on raising the temperature above the softening range. The addition of K20 to the borate melts causes two opposing phenomena: (1) depolymerization effects and the related breakdown of network structure resulting in the formation of singly bonded oxygens, which weakens the structure and hence causes a decreases in viscosity, and (2) the formation of four coordinated borons (BO4 tetrahedra) which strengthen the structure leading to an increase in the viscosity. The structural changes taking place in these melts on varying the temperature and the alkali content have been reviewed and are incorporated in developing this model. The predicted values are in excellent agreement with the experimental values over the whole temperature and composition range.