Determination of the crystallization kinetic parameters of Ge22.5Te77.5 glass using model-free and model-fitting methods

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Ge22.5Te77.5 glass. The kinetic parameters of glassy Ge22.5Te77.5 under non-isothermal conditions are analyzed by the model-free and model-fitting approaches from a series of experiments at different constant heating rates (1–50 K/min). A strong heating rate dependence of the activation energy of crystallization was observed. The analysis of the present data shows that the activation energy of crystallization is not constant but varies with the degree of crystallization and hence with temperature. The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (two-dimensional growth) is responsible for the crystallization process for heating rates 1–30 K/min and two mechanisms (one- and two-dimensional growth) are working simultaneously during the amorphous–crystalline transformation of the Ge22.5Te77.5 glass for a heating rate 50 K/min. The reaction model that may describe crystallization process of the Ge22.5Te77.5 glass is Avrami–Erofeev model (g(α) = [−ln(1 − α)]1/n) with n = 2 for the heating range 1–30 K/min and n = 1.5 at a heating rate 50 K/min for the whole range of conversion crystallization fraction (α = 0.05–0.95). A good agreement between the experimental and the reconstructed (α − T) curves has been achieved. The transformation from amorphous to crystalline phase in glassy Ge22.5Te77.5 demonstrates complex multi-step involving several processes.