Substitutional effect of Na₂O with K₂O on the viscosity and structure of CaO-SiO₂-CaF₂-based mold flux systems

The effect of the substitution of Na₂O with K₂O on the viscosity and structure of molten CaO-SiO₂-CaF₂-based mold fluxes containing alkali-oxides at high temperatures has been studied. The CaO/SiO₂ mass ratio (C/S) and CaF₂ were fixed at 0.8 and 10 mass pct., respectively. The total alkali-oxide was fixed at 20 mass pct. By systematically substituting the Na₂O with K₂O, the K₂O/(Na₂O + K₂O) mass ratio was modified between 0.0 and 1.0. Using the rotating spindle method to measure the viscosity at high temperatures, the viscosity was found to increase with higher K₂O/(Na₂O + K₂O). From the slope of the temperature dependence of the viscosity, an apparent activation energy was calculated and increased with higher K₂O/(Na₂O + K₂O), from 96 to 154 kJ/mol, due to the cation size effect on the resistance to shearing. Using Raman spectroscopy of as-quenched fluxes, the mole fraction of Q³ was found to increase, while the mole fractions of Q² and Q⁰ decreased with higher K₂O/(Na₂O + K₂O). The nonbridged oxygen per silicon cation (NBO/Si) decreased from 1.97 to 1.58 with increasing K₂O/(Na₂O + K₂O), suggesting greater complexity of the flux structure with higher K₂O/(Na₂O + K₂O), resulting in a higher viscosity.