GLASSY MATERIALS - THERMODYNAMIC AND KINETIC QUANTITIES

Modern technology makes it possible to obtain different kinds of glassy material. In the as-prepared state, most of these show relaxation effects, glass transition and crystallization under heat treatment, with very similar phenomenological peculiarities. The thermodynamic treatment of glassy materials is complicated by the fact that the glassy state is not always a metastable state but is sometimes an unstable state. Evaluation of the entropy of the glass, with respect to the corresponding stable crystalline phase at the same temperature and pressure, from heat capacity measurements is discussed. The ''ideal glass'' concept is introduced to calculate the Gibbs free energy of formation of the glass. The residual entropy is presented as a measure of the glass deviation from ideal behaviour. The treatment is extended to alloy glass. The thermodynamic and kinetic parameters which control the crystallization process are reviewed. Polymorphous and primary crystallization are considered, assuming either homogeneous or inhomogeneous nucleation, followed by interface-controlled crystal growth.