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.
Bibliographical noteFunding Information:
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1A2B2006609) and the third Stage of Brain Korea 21 Plus Project in 2018.
© 2019 The American Ceramic Society
All Science Journal Classification (ASJC) codes
- Ceramics and Composites
- Materials Chemistry