Substitutional effect of Na 2 O with K 2 O on the viscosity and structure of CaO‐SiO 2 ‐CaF 2 ‐based mold flux systems

The effect of the substitution of Na 2 O with K 2 O on the viscosity and structure of molten CaO‐SiO 2 ‐CaF 2 ‐based mold fluxes containing alkali‐oxides at high temperatures has been studied. The CaO/SiO 2 mass ratio (C/S) and CaF 2 were fixed at 0.8 and 10 mass pct., respectively. The total alkali‐oxide was fixed at 20 mass pct. By systematically substituting the Na 2 O with K 2 O, the K 2 O/(Na 2 O + K 2 O) 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 K 2 O/(Na 2 O + K 2 O). From the slope of the temperature dependence of the viscosity, an apparent activation energy was calculated and increased with higher K 2 O/(Na 2 O + K 2 O), 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 Q 3 was found to increase, while the mole fractions of Q 2 and Q 0 decreased with higher K 2 O/(Na 2 O + K 2 O). The nonbridged oxygen per silicon cation (NBO/Si) decreased from 1.97 to 1.58 with increasing K 2 O/(Na 2 O + K 2 O), suggesting greater complexity of the flux structure with higher K 2 O/(Na 2 O + K 2 O), resulting in a higher viscosity.