Enthalpic relaxation and the glass transition

Enthalpic relaxation was investigated on the As2Se3 bulk glass with perfectly defined thermal history by differential scanning calorimetry (DSC). The relaxation enthalpy change, DeltaH, the parameters of the Tool-Narayanaswamy-Moynihan (TNM) relaxation model and the parameter of non-exponentiality, beta, were evaluated. It was found that beta is both temperature and time dependent. The value of enthalpy change necessary to reach a metastable equilibrium is less than that assumed on the base of a linear extrapolation of H(T) of a supercooled liquid. The glass transition was studied by a new stepwise DSC technique on the bulk As2Se3 and As2S3 glasses. The glass transition was found to be a superposition of two parts: a reversible or thermodynamic component, reflecting temperature changes of vibrational amplitudes, and an irreversible or kinetic process, so-called enthalpy relaxation. The value of the glass transition temperature, T-g, determined from the thermodynamic part of the glass transition was found to be independent on both the heating/cooling rate and the thermal history of glass. The T-g depends only on the chemical composition of the glass and thus it could be regarded as a material constant. Therefore the heating/cooling rate dependence of T-g, known from DSC or DTA measurements, is caused by a kinetic process with relaxation time dependent on structure and temperature.