How nucleation-growth kinetics is influenced by initial degree of material crystallinity

Abstract

Differential scanning calorimetry was used to investigate the effect of initial partial crystallinity on the non-isothermal nucleation-growth Johnson-Mehl-Avrami kinetics – selected Se-Te chalcogenide glasses were used as model systems. Different reproducible degrees of initial crystallinity were achieved by non-isothermal heating to a selected temperature. In case of the materials that primarily crystallize from mechanically induced defects, no change of the activation energy or kinetic mechanism was observed with increasing degree of initial crystallinity. On the other hand, material that showed complex crystallization behavior consisting from two competing overlapping crystal growth processes originating from volume-located nuclei and mechanical defects, respectively, exhibited marked shift in the dominance between the two kinetic mechanisms. In particular, with the increasing degree of initial crystallinity the defects-based crystallization mechanism started to dominate over the classical classic-nucleation-theory-based nucleation-growth crystallization mechanism. Increased amount of defects-based crystallization centers formed during the primary pre-crystallization appears to be the key factor for this change of the kinetic mechanism, rather than decreased activation energy (associated with the already existing crystal/glass interface) accelerating the actual crystal growth micro-mechanism.