Effects of alkali metal cations and fluoride anions on viscosity and structure of CaO-SiO2-Al2O3 system

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 Q(3) structural units. Substitution of oxides with alkali metal fluo-rides 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 ther-mal 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.(C) 2022 The Author(s). Published by Elsevier B.V.