Jaroslav Barták

Electrical conductivity and crystallization kinetics in Te-Se glassy system

Non-isothermal measurements of dc conductivity were used to study crystallization in bulk glasses from the TexSe100−x system (x = 10, 20, and 30). In combination with infrared microscopy, it provides a very good qualitative picture of the complex crystallization processes, as the formation of conductive paths proceeds more or less separately for each involved process. In addition, due to the pronounced signal corresponding to the surface crystallization, the dc conductivity measurements seem to bear great potential for crystallization kinetic studies. Based on identified characteristic states corresponding to the particular crystallization mechanisms, the activation energies were calculated for surface and bulk processes and compared to the differential scanning calorimetry (DSC) results reported earlier. Furthermore, the conversion rate α was calculated from the conductivity data by using the Odolevsky equation, the obtained dependence is in a good agreement with the results from residual enthalpies meas...

Crystallization behaviour of Ge17Sb23Se60thin films

The crystallization behaviour of as-prepared and nucleated Ge17Sb23Se60 thin films was studied by means of differential scanning calorimetry, X-ray diffraction analysis and scanning electron microscopy. Detailed analysis of the non-isothermal crystal growth kinetics was performed; the apparent activation energy, kinetic model and value of the pre-exponential factor were determined. The kinetic behaviour was found to be surprisingly close to the ideal Johnson–Mehl–Avrami nucleation-growth process, with the only non-ideality being the prolonged peak end tail (which may be a specificity associated with certain thicknesses of thin layers). This corresponds to the initiation of crystal growth in agreement with the classical nucleation theory, with the amount of mechanical defects and strains being negligible. The value of the kinetic parameter suggests two-dimensional crystal growth, which is consistent with the idea of macroscopic crystallites growing in a sterically restricted thin layer. A similar conclusion can be made on the basis of direct microscopic observation of the crystallites’ morphology.

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...

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.

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.

Crystal growth in Se70Te30 thin films followed by SEM and in situ XRD

The isothermal crystal growth kinetics in Se70Te30 thin films was investigated using the microscopy and in situ X-ray diffraction (XRD) measurements. Plate-like crystals grew linearly with time which is the sign of liquid-crystal interface kinetics. In the studied temperature range, from 68 °C to 88 °C, crystal growth rates exhibit simple exponential behavior with an activation energy of crystal growth EG = 168 ± 12 kJ mol−1. The growth data obtained from the microscopy measurements were combined with viscosity data, melting parameters and the appropriate crystal growth model was assessed. The relation between the kinetic coefficient of crystal growth and viscosity (u∝η-ξ) is described in detail, and a correction of the standard growth model is suggested. The crystal growth data obtained from the in situ XRD measurements were described using the Johnson-Mehl-Avrami nucleation-growth model with the Avrami exponent m = 2.2 ± 0.2. The activation energy of the overall crystallization process EA was estimated ...

Pressure-Induced Phase Transitions in GeTe-Rich Ge–Sb–Te Alloys across the Rhombohedral-to-Cubic Transitions

We demonstrate that pressure-induced amorphization in Ge-Sb-Te alloys across the ferroelectric-paraelectric transition can be represented as a mixture of coherently distorted rhombohedral Ge8Sb2Te11 and randomly distorted cubic Ge4Sb2Te7 and high-temperature Ge8Sb2Te11 phases. While coherent distortion in Ge8Sb2Te11 does not prevent the crystalline state from collapsing into its amorphous counterpart in a similar manner to pure GeTe, the pressure-amorphized Ge8Sb2Te11 phase begins to revert to the crystalline cubic phase at ∼9 GPa in contrast to Ge4Sb2Te7, which remains amorphous under ambient conditions when gradually decompressed from 40 GPa. Moreover, experimentally, it was observed that pressure-induced amorphization in Ge8Sb2Te11 is a temperature-dependent process. Ge8Sb2Te11 transforms into the amorphous phase at ∼27.5 and 25.2 GPa at room temperature and 408 K, respectively, and completely amorphizes at 32 GPa at 408 K, while some crystalline texture could be seen until 38 GPa (the last measurement point) at room temperature. To understand the origins of the temperature dependence of the pressure-induced amorphization process, density functional theory calculations were performed for compositions along the (GeTe)x - (Sb2Te3)1-x tie line under large hydrostatic pressures. The calculated results agreed well with the experimental data.