ABSTRACT
The crystal growth kinetics and morphology in germanium disulfide bulk glass and glass surface is described. The structural relaxation taking place below the glass transition is slow and the corresponding volumetric change is negligible. Therefore, it does not affect substantially the crystal growth process. The crystal growth rate of low temperature ß-GeS2 and high temperature α-GeS2 polymorphs in the bulk glass is comparable, being slightly decoupled from the shear viscosity below the glass transition. The crystal growth rate of ß-GeS2 in an amorphous thin film of the same composition is several orders of magnitude faster than that at the surface of bulk glass. This fast surface crystal growth is strongly decoupled from viscosity. Such behavior resembles the glass-to-crystal fast growth mode observed by several authors in some organic molecular glasses. Taking into account previously reported viscosity and heat capacity data, the crystal growth kinetics of both polymorphs can be quantitatively described by the 2D surface growth model for low and high supercooling. The nonisothermal differential scanning calorimetry experiments are analyzed, providing evidence of a complex nature of the overall crystallization process with apparent activation energy comparable to that obtained from isothermal microscopy measurement of crystal growth in the same temperature range.
ABSTRACT
Crystal growth, viscosity, and melting were studied in Ge2Sb2Se5 bulk samples. The crystals formed a compact layer on the surface of the sample and then continued to grow from the surface to the central part of the sample. The formed crystalline layer grew linearly with time, which suggests that the crystal growth is controlled by liquid-crystal interface kinetics. Combining the growth data with the measured viscosities and melting data, crystal growth could be described on the basis of standard crystal growth models. The screw dislocation growth model seems to be operative in describing the temperature dependence of the crystal growth rate in the studied material in a wide temperature range. A detailed discussion on the relation between the kinetic coefficient of crystal growth and viscosity (ukin â η(-ξ)) is presented. The activation energy of crystal growth was found to be higher than the activation energy of crystallization obtained from differential scanning calorimetry, which covers the whole nucleation-growth process. This difference is considered and explained under the experimental conditions.
