Quantitative links between microscopic properties and viscosity of liquidsin the system SiO2-Na2O

The Adam-Gibbs theory of structural relaxation is presented in terms of the microscopic parameters controlling viscous flow; Delta mu, the energy barr ier hindering rearrangement of a single silicate monomer, and z(*) the numb er of monomers that must simultaneously overcome their barrier in order for a change in configuration to occur. Independent viscosity, calorimetric an d spectroscopic data for glasses and melts in the system SiO2-Na2O are used to show that z(*) at the glass transition has a value of approximately 10 for all of the compositions considered and that microscopic energy barriers are on the order of several tens of kJ per mole of silicon. Furthermore, t he variation of configurational entropy of glasses in the system SiO2-Na2O has been determined from the viscosity and calorimetric data. It is conclud ed that a stable melt complex with a stoichiometry close to Na2Si3O7 exists and that medium range order plays an important role in determining the con figurational entropies, although the details are unclear. Overall, a self-c onsistent picture of the quantitative links between microscopic and macrosc opic properties is determined for this simple system, providing an encourag ing first step towards a fully generalisable predictive model for silicate melt viscosity. (C) 2001 Elsevier Science B.V. All rights reserved.