Viscosity, structure and mixing in (Ca, Na) silicate melts

Configurational entropy has been linked with the structure of Na2O–CaO–SiO2 melts, based on combined viscosimetry and Raman spectroscopic investigations. From viscosity measurements at low and high temperatures, we have obtained the configurational entropy, Sconf (using log η = Ae + Be / TSconf, where η is the viscosity, T the temperature and Ae, Be two constants). Using Raman spectroscopy, we obtained structural information from the Q speciation and from the variation of the boson peak with chemical composition.A rapid decrease in the viscosity at low temperature was observed in Ca-silicate melts with addition of Na2O. At high temperature, the viscosity is almost the same for the Ca- and Na-silicate liquids. The configurational entropy calculated from the viscosity measurements for Ca/Na mixing shows a non-ideal variation, which can be interpreted in term of non-random distribution of Na and Ca in the silicate network. The addition of Na2O to the Ca-silicate melts produces a decrease of the fragility of the liquid and an increase of the Q3/Q2 ratio observed with Raman spectroscopy.