HEAT-CAPACITIES OF TELLURITES AND RESTRUCTURING THERMODYNAMICS ON GLASS-FORMATION

An increased heat capacity of a supercooled liquid, compared to its su perliquidus melt, has been measured in situ in some glass-forming tell urite systems by differential scanning calorimetry (DSC). complementin g earlier work on fluorozirconates. This increased heat capacity confi rms that restructuring occurs in the supercooled Liquid regime, in goo d agreement with structural investigations. The thermodynamics of rest ructuring decrease the free energy of the supercooled liquid, and henc e diminish the thermodynamic driving force for crystallization, as eva luated using the classical nucleation approach. Glass formation thus r esults from a combination of restructuring thermodynamics and kinetics . Thermodynamic aspects of lass formation, in fragile, intermediate an d strong systems, are systematized by free energy versus temperature d iagrams within the supercooled liquid regime. A common basis for glass formation is revealed, that is, a glass-forming liquid has a tendency to retain its high-temperature liquid structure for some temperature range below the liquidus. Major structural change will not take place until much lower temperatures, and culminates in the glass transition. This viewpoint suggests that glass formation is decisively controlled by physical and chemical properties in the high-temperature liquid. ( C) 1998 Elsevier Science B.V. All rights reserved.