Jana Shánělová

Structural relaxation of amorphous Ge38S62 studied by length dilatometry and calorimetry

The structural relaxation of Ge38S62 glass has been studied by length dilatometry and calorimetry. The Tool-Narayanaswamy-Moynihan model was applied on obtained data of structural relaxation and parameters of this model were determined: Δh*= 483±2 kJ mol-1, ln(A/s)= -81±1, β= 0.7±0.1 and x=0.6±0.1. Both dilatometric and calorimetric relaxation data were compared on the basis of the fictive relaxation rate. It was found that the relaxation rates are very similar and well correspond to the prediction of phenomenological model.

Crystal Growth Kinetics and Viscous Behavior in Ge2Sb2Se5 Undercooled Melt

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 ∝ 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.

Crystallization Kinetics in Amorphous and Glassy Materials

Glassy materials lack the periodic atomic arrangements typical for crystals. They are by definition prepared by cooling a viscous glass-forming liquid fast enough to avoid crystallization. This way of preparation has been known for millennia and is used for the fabrication of conventional glassy products from such as windows panels and glass containers to more sophisticated materials such as bulk optical glasses for cameras and optical fibers that interconnect computer networks with recording devices, transmitting, and finally bringing the external world to our homes. Figure 14.1 shows the specific volume or enthalpy as a function of temperature for a typical glass-forming liquid.

Viscosity Measurements Applied to Chalcogenide Glass-Forming Systems

Viscosity is an important physical parameter which determines the flow of material. The knowledge of viscous behaviour is important for example for the process of the material production. In the case of glasses and their undercooled melts, viscosity influences also the processes of structural relaxation and crystallization. Structural relaxation is in fact a very slow structural rearrangement of glass. This process can be realized through viscous flow and therefore is influenced by it. Crystallization process which may occur in undercooled melts is also influenced by the diffusion coefficient in the glassy matrix and therefore by its viscosity. This chapter tries to summarize the available viscosity data for chalcogenides and the basic measuring methods which are mostly often used to determine them.

Structural Relaxation in Amorphous Materials

In the glass transition region the molecular rearrangements slow down considerably. Then the macroscopic properties of the amorphous material changes on a scale of minutes and they can easily be observed. If a such material is equilibrated at temperature T o in the glass transition region and then suddenly cooled to temperature T, the volume will change as shown in Fig. 1. The approach of the structure towards equilibrium as a response to this temperature jump is called structural relaxation and it has been studied extensively, both for practical reasons and for a better theoretical understanding of the glass transition phenomena [1,2].