GLASSY MATERIALS - THERMODYNAMIC AND KINETIC QUANTITIES

Modern technology makes it possible to obtain different kinds of glass y material. In the as-prepared state, most of these show relaxation ef fects, glass transition and crystallization under heat treatment, with very similar phenomenological peculiarities. The thermodynamic treatm ent of glassy materials is complicated by the fact that the glassy sta te is not always a metastable state but is sometimes an unstable state . Evaluation of the entropy of the glass, with respect to the correspo nding stable crystalline phase at the same temperature and pressure, f rom heat capacity measurements is discussed. The ''ideal glass'' conce pt is introduced to calculate the Gibbs free energy of formation of th e glass. The residual entropy is presented as a measure of the glass d eviation from ideal behaviour. The treatment is extended to alloy glas s. The thermodynamic and kinetic parameters which control the crystall ization process are reviewed. Polymorphous and primary crystallization are considered, assuming either homogeneous or inhomogeneous nucleati on, followed by interface-controlled crystal growth.