The influence of Si,B substitution and of the nature of network-modifying cations on the properties and structure of borosilicate glasses and melts

The influence of the Si,B substitution and of the nature of network-modifying cations on viscosities and heat capacities of ternary borosilicate melts has been investigated for two series of melts containing either 20 mol% Na2O or 33 mol% BaO. The measurements have been made near the glass transition and above the liquidus for viscosities, and from 300 to 1073 K for heat capacities. They have been used to determine the configurational entropies of the melts from Adam and Gibbs theory of relaxation processes. To complete these observations, the room-temperature Raman spectra of the glasses have also been recorded. The observed increase in the configurational heat capacity of melts with B2O3 content indicates the existence of strong temperature-induced structural changes induced by the presence of boron. Correlatively, the viscosity variations with composition can be related to the existence of distinct coordination environments for boron. As for the effects of barium and sodium, they are similar in silicate and borosilicate melts, and depend mainly on the field strength of the cation.