Daniela Brandová

Non-isothermal crystallization kinetics of GeTe4 infrared glass

Non-isothermal crystallization kinetics of the GeTe4 chalcogenide glass was studied in dependence on particle size. Complexity of the obtained DSC data was treated by means of the Fraser–Suzuki deconvolution, and the particular crystallization sub-processes were identified and described in terms of the JMA(2) and AC kinetic models. Bulk as-prepared GeTe4 samples, on the other hand, exhibited simple zero-order (F0) crystallization kinetics. The marked difference between the powder and bulk crystallization mechanisms was explained based on the mechanically induced defects and heterogeneities, which surrogate/accelerate the primary nucleation process. This concept also accounts for the remarkable stability of the studied telluride glass. Precipitation of Te followed by second-stage GeTe crystal growth was confirmed by XRD for all of the applied experimental conditions. The dominant influence of the nucleation process on the consequent crystallization kinetics is thereby implicated. Infrared microscopy was used to confirm the existence of the particular crystallization mechanism.

Crystal growth from mechanically induced defects

For the first time, infrared microscopy was used to directly confirm the presence of crystallites originating from mechanically induced defects. Differential scanning calorimetry and infrared microscopy were used to study crystallization behavior of the GeTe4 glass. Both non-isothermal and isothermal DSC crystallization data were deconvoluted by state-of-the-art methods, and the identified sub-processes were described in terms of appropriate kinetic models. While the bulk samples showed zero-order (F0) crystallization kinetics, the GeTe4 powders exhibited complex kinetic behavior describable by competing Johnson–Mehl–Avrami and autocatalytic crystallization mechanisms. The concept of crystallization from mechanically induced defects was used to explain the difference in the kinetic behavior of bulk and powdered materials. Microscopic observation of two types of crystallites being present in a partially crystallized powder grain corresponds well to the occurrence of two crystallization mechanisms identified for the DSC data. The zero-order crystallization kinetics found for the DSC data of bulk samples was verified by microscopic crystal growth rate measurements.

Crystallization kinetics of glassy materials: the ultimate kinetic complexity?

Four examples of complex crystallization kinetics changing with fundamental experimental conditions (temperature, heating rate) were introduced—formation of tetragonal ZrO2 in vanadium-doped zirconia catalyst, crystal growth in Y3Al5O12 (YAG) glass microspheres, multi-phase crystallization in (GeTe4)50(GaTe3)50 chalcogenide glass for far-infrared optics and formation of crystallites in selenium glass. In each case, a unique solution employing either mathematic or kinetic deconvolution was utilized to obtain full description of the crystal growth kinetics and its dependence on temperature/heating rate. The article discusses merits and flaws of each approach together with the general advantages and disadvantages of the mathematic and kinetic deconvolution procedures. In conclusion, the kinetic deconvolution can be used as an exploratory tool; suitability of the method for full kinetic description of the process is conditioned by relative constancy of apparent activation energy across the explored range of experimental conditions and by describability of the experimental data within a framework of some standard kinetic model. On the other hand, mathematic deconvolution is not limited by the above-listed conditions and cannot properly account for potential mutual dependences occurring between the particular sub-processes.

The effect of powder coarseness on crystallization kinetics of Ge11Ga11Te78 infrared glass

The effect of powder particle size on non-isothermal crystallization kinetics of Ge11Ga11Te78 glass was examined by differential scanning calorimetry. Multivariate kinetic analysis was used to deconvolute the complex crystallization data. The overall process was found to be composed of initial tellurium precipitation followed by simultaneous formation of Ga2Te5 and GeTe phases. While the Te precipitation followed the nucleation growth Johnson–Mehl–Avrami kinetics with the growth originating mainly from surface and mechanically induced defects, the formation of gallium and germanium tellurides proceeded autocatalytically with a significantly lower involvement of the defects-based crystal growth. The model-based part of the kinetic behavior was found to be uninfluenced by the change in powder coarseness—both crystallization sub-processes were described in terms of similar models with similar kinetic exponents for all particle size fractions. On the other hand, the model-free kinetic parameters (activation energies, pre-exponential factors and crystallization enthalpies) were found to be significantly changing with increasing particle size; the observed trends were explained in terms of the changing amounts of the two involved types of crystallization centers.