Effects of alkali metal cations and fluoride anions on viscosity and structure of CaO-SiO₂-Al₂O₃system

Alkali metal oxides (R2O) in calcium-aluminosilicate-based melts were substituted with alkali metal fluorides (RF) while maintaining a constant concentration of the alkali metal cation to investigate the effects of the alkali metal cations and fluoride anions on the viscosities and structures of high-temperature melts. Substitution with larger alkali metal cations, e.g., Li+, Na+, and K+, increased the viscosity of the melts and enhanced the degree of polymerization of the Q3 structural units. Substitution of oxides with alkali metal fluorides in the range of 0-10 mol% reduced the viscosity, but enhanced the polymerization of the structural units. F1s binding energies estimated through X-ray photoelectron spectroscopy indicated that the F- anions coordinated with the Ca2+ cations. The occupation of the network-modifying Ca2+ cations by the F- anions increased the overall degree of polymerization of the melt structures, but the melt viscosities were slightly reduced because NBO-Ca2+-NBO links transformed to NBO-Ca2+-F-, where NBO corresponds to the non-bridging oxygen. Isothermal and non-isothermal viscosity measurements indicated the viscosities were lower under non-isothermal conditions because of the high excess thermal energy of the melts and the presence of large alkali metal cations with low mobility. These factors were found to increase the difference between the isothermal and non-isothermal viscosity measurements.