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The effect of long-term room-temperature degradation (up to 6 months) on Se 70 Te 30 crystallization kinetics was investigated by differential scanning calorimetry in dependence on experimental conditions of the measurement—applied heating rate and particle size of the powdered material. In case of very fine powders massive amorphous-to-crystalline degradation occurred even at room temperature, indicating that the small powder grains have fully defects-imbued inner structure that accelerates crystal growth. The kinetic data obtained for the consequent crystallization of the remaining glassy matrix showed that the defects-based Johnson-Mehl-Avrami crystallization is under quasi-equilibrium conditions further accelerated towards autocatalytic mechanism. Coarse powders, on the other hand, exhibited primary degradation only in a surface layer of the powder grains. The main effect of the coarse powders aging was found to be associated with marked enhancement of crystal growth originating from volume-located nuclei—either a sub-T g nucleation occurred during aging or some pre-existing nuclei were activated during this time (possibly via mechanical stresses arising from structural relaxation processes). [ABSTRACT FROM AUTHOR]

The effect of partial crystallinity on SeTe crystallization kinetics was investigated in dependence on experimental conditions of the measurement (applied heating rate and particle size of the powdered material). The whole range from completely glassy to almost fully crystalline Se-Te material was examined in this regard; the reproducible initial degree of crystallinity was achieved by preceding non-isothermal heating to a certain selected temperature. With the increasing degree of initial crystallinity, the defects-based crystallization mechanisms begin to dominate over the classical-nucleation-theory-based nucleation-growth crystallization mechanism, with the apparent activation energy and Johnson-Mehl-Avrami kinetic exponent limiting the values characteristic for the crystallization of fine powders (~140 kJ mol and 1.4, respectively). The kinetic analysis based on the advanced interpretation of the characteristic kinetic functions provided detailed description of the changes associated with the complex crystallization process consisting of the competing surface- and volume-located growth mechanisms occurring in the grains of finite size. [ABSTRACT FROM AUTHOR]

Non-isothermal crystallization kinetics of the GeTe 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 GeTe 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. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry (DSC) was used to study the crystallization behaviour in a selenium–tellurium (Se–Te) glassy system under isothermal conditions. The particle-size-dependent isothermal crystallization kinetics were described in terms of the Johnson–Mehl–Avrami (JMA) nucleation-growth model. The complexity of the crystallization process was found to be represented by overlapping competing surface and bulk nucleation-growth mechanisms. Based on the deconvolution in terms of the JMA model, the particular crystal growth processes were identified, and the interpretation of their physical origins was performed. Presence of the A and B types of spherulitic crystallites was confirmed; the temperature range of the transition in-between these two spherulitic forms was determined to be similar as for pure selenium. Increasing tellurium content was found to cause an increase of the apparent activation energy of the overall crystallization process as well as a gradual increase of the dominance of the CNT (classical nucleation theory)-based volume-located crystal growth over the surface crystallization, which originates from mechanically induced heterogeneities. [ABSTRACT FROM AUTHOR]

The SeTe alloy readily forms a stable chalcogenide glass that has widely documented applications. In this work, the nucleation behavior of SeTe was studied by infrared microscopy as well as by differential scanning calorimetry. Nuclei densities were directly estimated by image analysis of micrographs of samples following nucleation-development heat treatments, and they were subsequently compared with indirect quantitative estimation by DSC. Both the methods yielded comparable results in the non-intersecting region of the nucleation-growth curves while diverging in the nucleation-growth domain. The rates of nucleation could be adequately described by the classical nucleation theory. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study crystallization behavior in selenium glass under isothermal conditions. In the current work, which is the third in a sequence of articles dealing with the crystallization kinetics of complex processes, the isothermal crystallization kinetics was described in terms of the Johnson-Mehl-Avrami nucleation-growth model. The study was performed in dependence on particle size so that the advanced interpretation of characteristic kinetic functions could be employed. The complexity of the crystallization process was found to be represented by overlapping competing surface and bulk nucleation-growth mechanisms. Based on this information, the deconvolution in terms of the Johnson-Mehl-Avrami process was performed, separating the mechanisms involved. High consistency of the resulting kinetic parameters confirms the accuracy and physical meaningfulness of the deconvolution procedure. The resulting concept not only describes the isothermal crystallization process in glassy selenium very well, both qualitatively and quantitatively, but is also capable of explaining all accessible data from the literature on this topic. In addition, comparisons of the present and literature data clearly imply that the nucleation processes play a major role in the isothermal crystallization of amorphous selenium. [ABSTRACT FROM AUTHOR]

The crystal growth kinetics of germanium disulfide in undercooled melts has been studied by optical microscopy under isothermal conditions. The linear growth kinetics of GeS has been observed in the temperature range 672 ≤ T ≤ 711 K in thin film samples. The activation energy of crystal growth assuming Arrhenius behavior has been determined as E = 166 ± 8 kJ mol for thin film samples. From the dependence of reduced growth rate on undercooling, the interface driven 2-D surface nucleated model was estimated. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study nucleation behavior in As 2 Se 3 glass, dependent on particle size. The nucleation process was examined for a series of different coarse powders; the nucleation rate was estimated from the proportion of the crystalline material fraction. The enthalpy of fusion was utilized in this respect, and a correlation between Δ H m and Δ H c was confirmed. Two mechanisms of nucleus formation were found: classical heterogeneous nucleation (following CNT) and so-called “activation” of mechanically-induced defects. The latter appears to represent rapid formation of crystallization centers from a damaged glassy structure, where complete saturation occurs for fine powders in the range of 195–235 °C. A high amount of mechanical defects, on the other hand, was found to partially suppress the CNT nucleation process. [ABSTRACT FROM AUTHOR]

Highlights: [•] Non-isothermal crystallization kinetics of As2Se3 glass was studied by DSC in dependence on particle size. [•] Two differently quenched glass batches were prepared. [•] Crystallization kinetics was described in terms of the Johnson–Mehl–Avrami model. [•] Increased number of mechanical defects effectively increases the number of primary nucleation centers. [•] Stress-induced defects originating from glass-formation surrogate the secondary nucleation process. [Copyright &y& Elsevier]

The constant heating rate crystallization of As 2Se 3 undercooled melt was measured by the differential scanning calorimetry. The nonlinear regression analysis of conversion temperature dependence was performed in frame of the classical nucleation theory supposing the normal 3D growth. The parameters of temperature dependence of nucleation rate and growth rate were determined by three step process. First the simple parabolic model was used to estimate the maximum and width of nucleation/growth rate temperature dependence. Then the obtained parabolic curves were fitted by the theoretical ones. In the third step, the obtained parameters were used as zero estimates for nonlinear regression analysis of experimental data. The results obtained by using conversion degree α were compared with the results obtained by using the −ln(1 −  α) transformed function. Although both treatments give comparable results the use of −ln(1 −  α) input data is preferred due to better numerical stability of nonlinear regression treatment. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study crystallization behavior in selenium glass under non-isothermal conditions. The crystallization kinetics were described in terms of the Johnson-Mehl-Avrami nucleation-growth model; activation energies and kinetic parameter m were determined. The study was performed in dependence with particle size, so that a novel approach to the evaluation of crystallization kinetics-the advanced interpretation of characteristic kinetic functions-could be employed. Extensive discussion of all aspects of a full-scale kinetic study for a complex crystallization process was performed within the framework of the introduced conception. The complexity of the crystallization process was found to be represented by very closely overlapping consecutive competing surface and bulk nucleation-growth mechanisms. Mutual interactions of both mechanisms as well as all other observed effects were explained in terms of thermal gradients, surface crystallization centers arising from the sample preparation treatments and a changing amount of volume nuclei originating from the combination of the pre-nucleation period and the actual glass preparation phase. The main objective of the study is to demonstrate the extent of so-far neglected information hidden in the characteristic kinetic functions and introduce a convenient tool for its acquisition. [ABSTRACT FROM AUTHOR]

Abstract: Differential scanning calorimetry (DSC) measurements were used to study crystallization in the Se70Te30 glass under non-isothermal conditions. The crystallization kinetics was described in terms of the nucleation-growth Johnson–Mehl–Avrami and autocatalytic Šesták–Berggren models. An extensive discussion of all aspects of a full-scale kinetic study for a complex crystallization process was performed. Number of suggestions regarding the experimental part (sample and glass preparation, temperature programs, data acquisition, etc.) was introduced to maximize precision and reproducibility of the experimental data. Complexity of the crystallization process was in this particularly described case represented by very closely overlapping consecutive competing surface and bulk nucleation-growth mechanisms. Mutual interactions of both mechanisms as well as all other observed effects were explained in terms of thermal gradients, surface crystallization centres arising from the sample preparation treatments and changing amount of volume nuclei originating from the combination of pre-nucleation period and the very glass preparation phase. Advanced error analysis was performed for each step of the kinetic study. Objective of the presented study was to demonstrate extensity of information the differential scanning calorimetry is able to provide and, furthermore, to show how a thorough kinetic analysis may lead to reliable, valid and detailed description of complex processes as well as to interpretations of any observable trend occurring in experimental data. [Copyright &y& Elsevier]

The Te-rich Ge 20 Se y Te 80−y infrared chalcogenide glasses (for y up to 8 at.%) were studied by means of DSC, XRD, Raman spectroscopy and infrared microscopy. Thorough non-isothermal thermo-kinetic characterization of the glass transition, crystallization and melting phenomena was performed in dependence on the particle size. The Tool–Narayanaswamy–Moynihan model was successfully used to describe the structural relaxation processes; the compositional evolution of the relaxation parameters was then explained in terms of the structural changes and movements of the characteristic structural units detected by Raman spectroscopy. The nucleation-growth Johnson-Mehl-Avrami model and empirical Autocatalytic model were used to describe the complex kinetics of the DSC crystallization data. Based on the XRD and microscopic analyses the following crystallization mechanisms were revealed: initial nucleation-growth precipitation of hexagonal Te (surface-located) followed by a bulk-located autocatalytic growth of rhombohedral GeTe; in case of higher Se contents an additional formation of the Ge-Te-Se crystalline phase occurred at high temperatures. Higher glass-stability determined for increasing Se content can be associated with the partial inhibition of the crystallization processes, the initial Te precipitation remains, however, further unaffected once Se content reaches ∼4 at.%. [ABSTRACT FROM AUTHOR]

Abstract: Differential scanning calorimetry measurements were used to study crystallization in the SeyTe(1−y) glassy system under non-isothermal conditions. The examined compositions were y=0.1, 0.2 and 0.3. The crystallization kinetics was described in terms of the Johnson–Mehl–Avrami nucleation–growth model. It was found that with the addition of tellurium into the selenium matrix the apparent activation energy of the crystallization process monotonically increases and, more importantly, the parameter of the JMA model m decreases. This change in the crystallization mechanism was interpreted as a gradual crossover from surface to bulk process. Both processes are competitive and proceed simultaneously in glasses with low tellurium content, while in the case of increased Te content the surface crystallization mechanism recedes. The transition between mechanisms was discussed in terms of changes in molecular structure of the material. [Copyright &y& Elsevier]

• Conventional approaches for determination of nuclei density are time-consuming. • A method for rapid determination of nucleation rates using fast DSC is described. • Nucleation rates in a Se 90 Te 10 chalcogenide glass were deduced from melting peaks. • Results at low growth temperatures are comparable to that of microscopy data. Fast scanning calorimetry has been used to study isothermal nucleation rates in a Se 90 Te 10 chalcogenide glass. Calorimetry scans at 2000 °C/s, sufficient to suppress crystallization of glass, were used on samples subjected to combined nucleation-growth treatments. Nuclei densities were obtained from the analysis of melting-peaks using the Johnson–Mehl–Avrami–Yerofeeyev–Kolmogorov equation. The estimated densities and rates of nuclei agree with results obtained from microscopic studies and conventional differential scanning calorimetry. [ABSTRACT FROM AUTHOR]

The project involves determination of the activation energies and physical models for thermolysis of BCHMX and its PBXs. The initial decomposition pathways were also proposed on the basis of molecular dynamic simulation. The goal is to find the mutual relationships among the physical models, decomposition pathways, and the impact sensitivities for BCHMX and its PBXs. It has been shown that the physical model of the first step of BCHMX thermolysis is close to first order and the second step is governed by a first order autocatalytic model, which turns to “2D or 3D Nucleation and Growth” models under the effect of polymeric binders probably due to their hindrances on topochemical reaction of BCHMX. Simulation results show that the scission of N-NO 2 is the initial step for BCHMX pyrolysis, followed by HONO and HNO eliminations, where the latter is due to nitro-nitrite rearrangement. Under the effect of hydrocarbon polymers, the HONO/HON elimination and collapse of ring structure of BCHMX occur earlier without changing the time for N-NO 2 scission, which might be the reason why those polymers have little effect on the thermal stability of BCHMX, while they could make it decompose almost in a single complex step. [ABSTRACT FROM AUTHOR]

The rates of isothermal nucleation and growth in chalcogenide glass thin films of (GeS2)0.9(Sb2S3)0.1 were studied using optical microscopy. Nucleation data was obtained from double stage heat treatment, followed by image analysis of polarized optical micrographs. The applicability of classical nucleation theory (CNT) to the nucleation data has been discussed. The growth data, corrected for viscosity suggest interface controlled 2D nucleated growth of β-GeS2 crystals. The isothermal curves of nucleation and growth determined by CNT and 2D nucleated growth model, respectively were found to be weakly intersecting. The nucleation curve had maxima at 350°C, which is below the glass transition temperature, while the growth curve had a peak at 439°C. [ABSTRACT FROM AUTHOR]

Abstract: The crystal growth kinetics of antimony trisulfide in (GeS2)0.1(Sb2S3)0.9 glass has been studied by microscopy and DSC. The linear crystal growth kinetics has been confirmed in the temperature range 492⩽ T ⩽515K (E G =405±7kJmol−1). The applicability of standard growth models has been assessed. From the crystal growth rate corrected for viscosity plotted as a function of undercooling it has been found that the most probable mechanism is interface controlled 2D nucleated growth. The non-isothermal DSC data, corresponding to the bulk sample, can be described by the Johnson–Mehl–Avrami equation. [Copyright &y& Elsevier]