A GEOMETRIC AND ANALYTIC APPROACH TO THE PROCESS OF MELT VITRIFICATION .1. THERMODYNAMIC TREATMENT

By using simple geometric concepts, general thermodynamic dependences governing the process of vitrification are derived. The thermodynamic driving forces of crystallization, Delta G(f,c)(T), and stabilization, Delta G(st)(T), of melts and glasses are considered in the entire tem perature interval from temperatures below the grass transition tempera ture, T-g, to temperatures high above the melting point, T-m. It is sh own that at large deviations from T-g the temperature dependence of th e driving force for stabilization, Delta G(st)(T), is non-symmetric an d expressions derived by a Taylor expansion become a poor approximatio n. By using an appropriate thermodynamic model, a more general express ion for Delta G(st)(T) is obtained and the thermodynamic basis for the existence of superheated glasses is established. The thermodynamic in stability of glasses below and above T-g, i.e., of supercooled and sup erheated glasses is discussed.