Thermodynamic and viscous behaviour of undercooled liquids

The Gibbs free energy difference (Delta G) between the undercooled liquid and the corresponding equilibrium solid phases and the viscosity (eta) of the undercooled liquid are key variables in the metastable solidification of glass-forming melts. Novel expressions have been developed for Delta G recently by the authors using the hole theory of liquids as well as by expanding the free energies of liquid and solid phases in the form of the Taylor series. These expressions have also been used to derive the temperature-dependence of the viscosity of the undercooled melts. The present article summarises the procedures for obtaining the expressions for Delta G; their application in obtaining the temperature-dependence of the viscosity and relaxation phenomenon are also discussed. The procedure for the estimation of the ideal glass transition temperature has been reviewed together with its importance in the glass-forming ability of materials. Glass formation has been discussed in terms of minimum volumes of constituent atoms, concentration-concentration fluctuations, driving force for nucleation and phase diagram features. The glass-forming ability of materials is also explained in terms of the ratio of specific heat difference to the entropy difference between liquid and solid phases at the melting point. Finally an attempt has been made to review the procedure for the estimation of the heat of crystallisation in terms of the viscosity of the melt.