Viscosity Behavior and Mixed Alkali Effect of Alkali Aluminosilicate Glass Melts

The viscosities of the four series of mixed alkali glass melts such as 25(Na2O+K2O)⋅75SiO2, 25(Na2O+K2O)⋅5Al2O3⋅70SiO2, 25(Na2O+K2O)⋅7.5Al2O3⋅67.5SiO2 and 25(Na2O+K2O)⋅10Al2O3⋅65SiO2 as well as their Tg were measured to elucidate a relation between property behaviors and mixed alkali effect models. The base mixed alkali glass melts (25R2O⋅75SiO2) showed a mixed alkali effect, namely viscosity minima at the composition K2O/R2O≈0.25, which become weaker as temperature increases. The activation energy for the viscous flow of this base glass melts showed also a minimum at this composition. However, the mixed alkali effect disappeared, as SiO2 is substituted by Al2O3. This implies that the nonlinearity disappears with decreasing nonbridging oxygen content. In the case of Tg no clear tendency with respect to the Al2O3 content was found.By comparison of the present work with the earlier work of resistivity, two distinct differences between the mixed alkali effects in viscosity and electrical conductivity were found. From these results, it was concluded that the mechanism for the mixed alkali effect in both properties should be treated independently.