Viscosity and Fragility of Alkaline-Earth Sodium Boroaluminosilicate Liquids

Understanding the composition and temperature dependence of viscosity of silicate liquids is of the highest importance not only for geological processes but also for production of industrial glass. In this work, we have determined the temperature dependence of equilibrium liquid viscosity of 36 alkaline‐earth sodium boroaluminosilicate liquids as a function of the Si/Al ratio and the type of alkaline‐earth oxide (MgO, CaO, SrO, or BaO). We demonstrate that the isokom temperature at 1012 Pa s (i.e., the glass transition temperature) generally increases with increasing Si/Al ratio, whereas the isokom temperatures at 104 and 101.5 Pa s exhibit a decrease with increasing [Al2O3] in the peraluminous regime. The isokom temperatures decrease with increasing alkaline‐earth size in the peralkaline regime, whereas they increase with increasing alkaline‐earth size in the peraluminous regime. The liquid fragility index m exhibits a minimum value at an intermediate Si/Al ratio, with the position of the minimum increasing to a higher value of [Al2O3] with increasing alkaline‐earth size. We have discussed our findings in terms of the underlying structural and topological changes as a function of composition and temperature.