Petr Koštál

Modified stepwise method for determining heat capacity by DSC

The methods of heat capacity data determination from differential scanning calorimetry measurements are described. The negative effects increasing uncertainty of heat capacity determination are mentioned. Modified stepwise method was described and verified using molybdenum, copper, and gold standards. Modified stepwise method provides better accuracy of Cp values compared to continuous and stepwise method.

Crystallization behavior in Se90Te10 and Se80Te20 thin films

Isothermal crystal growth kinetics in Se90Te10 and Se80Te20 thin films was studied by microscopy and in situ X-ray diffraction (XRD) measurements. The spherulite-like crystals grew linearly with time. In a narrow temperature range of between 65 and 85 °C, crystal growth rates exhibit simple exponential behavior with activation energies EG = 193 ± 4 kJ mol−1 for Se90Te10 and EG = 195 ± 4 kJ mol−1 for Se80Te20. The crystal growth in both compositions is controlled by liquid-crystal interface kinetics and can be described by a screw dislocation growth model. From the XRD data, the crystallization fraction was estimated. The crystallization data were described by Johnson-Mehl-Avrami (JMA) model with Avrami exponents m = 1.4 ± 0.3 for Se90Te10 and m = 1.6 ± 0.4 for Se80Te20. Activation energies were estimated from the temperature dependence of rate constant evaluated from the JMA model. The activation energies of nucleation-growth process were found to be Ec = 184 ± 21 kJ mol−1 for Se90Te10 and Ec = 179 ± 7 kJ...

Viscosity of Se–Te glass-forming system

Abstract This work is focused on viscosity of chalcogenide system Se–Te. Viscosity values of four glass-forming compositions in this system (containing 5, 10, 20 and 30 at.% of tellurium) were measured by penetration and parallel-plate method in the region of undercooled melt and glass. These results are combined with literature data for melt region. The kinetic fragility and viscosity glass transition temperature values obtained by different methods are calculated from measured data.

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.

Impact of particle size reduction on glaze-melting behaviour

The effect of milling on thermal behaviour of two types of industrial ceramic glazes was investigated by heating microscopy. The Vogel–Fulcher–Tamman equation was used for viscosity calculation of the glaze samples, and the impact of particle size reduction on calculated data has been analysed. The employed model provides a good fit to the general tendencies of viscosity change during melting and can be applied for a comparative description of glaze-melting behaviour. Observed correlations between the viscosity, the contact angle, milling time, glazing temperature and the specular reflection of the glazed tiles help to reveal the optimal physical characteristics for glaze manufacturing and to achieve the desired gloss and smoothness of glazed surface. On its turn, variation of particle size distribution highly affects thermal behaviour of ceramic glazes, and thus, the milling procedure provides the possibility of manipulation of glaze-melting behaviour during heating and allows to change the manufacturing heating cycle when required.

Extended Study on Crystal Growth and Viscosity in Ge–Sb–Se Bulk Glasses and Thin Films

Crystal growth rates in Ge18Sb28Se54 bulk glass and thin film were measured using optical and scanning electron microscopy under isothermal conditions. The studied temperature region was 255-346 °C and 254-286 °C for bulk glass and thin film, respectively. The compact crystalline layer growing from the surface into the amorphous core was formed in bulk glasses and no bulk crystallization was observed. In the case of thin films, needle-shape crystals were formed. The crystalline layer and needle-shape crystals grew linearly with time that corresponds to a crystal growth controlled by the crystal-liquid interface kinetics. In the narrow temperature range, crystal growth rates exhibit simple exponential behavior, so the activation energies of crystal growth for the studied temperature regions were estimated (EG = 294 ± 6 kJ/mol for bulk glass and EG = 224 ± 12 kJ/mol for thin film). Viscosity of Ge18Sb28Se54 material was measured in the region of the undercooled melt and glass. The extrapolation of viscosity data into the immeasurable, but important, temperature range is discussed. The experimental growth data were combined with melting and viscosity data and the appropriate growth models were proposed to describe crystal growth in a wide temperature region. The standard crystal growth models are based on a simple proportionality of the crystal growth rate to the viscosity (u ∝ η-1). This simple proportionality holds for the bulk material. Nevertheless, in the thin films the decoupling of the crystal growth rate from the inverse viscosity occurs, and the standard kinetic growth models need to be corrected. Such corrections provide better description of experimental data and more realistic value of the parameter describing the mean interatomic distance in the crystal-liquid interface layer, where the crystal growth takes place.

Up-Conversion in Er3+-Doped Ge25Ga5Sb5S65 Chalcogenide Glass for Enhancement of Silicon Solar Cell Efficiency

Er³⁺-doped Ge₂₅Ga₅Sb₅S₆₅ chalcogenide glass was studied as a potential material for up-conversion application to enhancement of silicon solar cell efficiency. Thermal properties were studied via DSC measurement, optical properties via variable angle spectroscopic ellipsometry and photoluminescence measurement. The host chalcogenide glass shows good thermal, chemical and physical properties. The band gap energy is ~2.23 eV which allows to emit green and red luminescent light from ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 4f-4f transitions of Er³⁺ ions under 800 nm pumping with Ti-sapphire tunable laser.

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.