Kinetic aspects of heat capacity in the annealing of Ge20Te80glasses

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Abstract
Cp measurements for melt-quenching Ge20Te80 glass were made with an adiabatic calorimeter and a large sample that was roughly two thousand times heavier in comparison with the 5-10 mg sample used when measuring with a differential scanning calorimeter, in the neighbourhood of glass transition temperature Tg. The effect of annealing on Cp was well clarified in the experimental results of Cp; the intensity of the peaks for a 20 K interval around Tg increased with the increase in annealing time ta the maximum value of which is about 20 days. The kinetics of the structural relaxation towards an equilibrium liquid for the glassy Ge20Te80 need to be interpreted in terms of the thermodynamic parameters and fictive temperature Tf, where it is defined as the temperature at which the enthalpy has the equilibrium value. Under the conservative condition in the enthalpy change due to the glass transition the authors estimated the effect of annealing on enthalpy, entropy and Gibbs free energy using the isothermal evolution of Tf calculated from the data of Cp; the thermodynamic quantities involve no contribution from any irreversible processes. The Gibbs free energy change Delta Grev, taking the as-quenched glass state as the standard, showed the very small decrease with increasing ta. The structural process towards an equilibrium state corresponding to the undercooled liquid state was governed by the very slow kinetics associated with exothermic reaction and the decrease of entropy during annealing: the so-slow kinetics of the relaxation in the glasses is discussed in terms of the evolution of Gibbs free energy.

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