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

Configurational entropy has been linked with the structure of Na(2)O-CaO-SiO(2) melts, based on combined viscosimetry and Raman spectroscopic investigations. From viscosity measurements at low and high temperatures, we have obtained the configurational entropy, S(conf) (using log eta=A(e) + B(e)/TS(conf), where eta is the viscosity, T the temperature and A(e), B(e) 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 Na(2)O. 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 Na(2)O to the Ca-silicate melts produces a decrease of the fragility of the liquid and an increase of the Q(3)/Q(2) ratio observed with Raman spectroscopy (c) 2006 Elsevier B.V. All rights reserved.