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Heat capacity measurements were performed for Se, Se90Te10, Se80Te20, and Se70Te30 materials in the 230-630 K temperature range. Both glassy and crystalline Cp dependences were found to be identical within the experimental error. The compositional dependence of the N-type undercooled liquid Cp evolution was explained on the basis of free-volume theory; vibrational and chemical contributions to heat capacity were found to be roughly similar for all Se-Te compositions. The thermal behavior in the Se-Te chalcogenide system was thoroughly studied: glass transition, cold crystallization, and melting were investigated in dependence on composition and various experimental conditions (heating rate, particle size, and pre-nucleation period). The kinetics of the structural relaxation and crystallization processes are described in terms of the Tool-Narayanaswamy-Moynihan and Johnson- Mehl-Avrami models. The complexity of these processes is thoroughly discussed with regard to the compositionally determined changes of molecular structures. The discussion is conducted in terms of the mutual interplay between the thermodynamics and kinetics in this system. [ABSTRACT FROM AUTHOR]

Kinetic fragility was determined for a number of compositions from the Ge–Se, As–Se and Te–Se chalcogenide glassy systems. Values of fragility indices were evaluated from both viscosity and enthalpy relaxation data. Comparison of the viscosity and enthalpy fragilities shows marked decoupling in cases of low-Ge/As-content compositions and the whole Te–Se glassy system. A possible explanation regarding the increased viscosity fragilities involves the influence of dynamic structural heterogeneity (density fluctuations and clustering of structural elements) on shear viscosity in the glass transition region. In particular, the observed decoupling seems to occur in case of the materials exhibiting moderate-to-high activation energy of relaxation processes and large structural cooperativity. The origin of the fragility decoupling may lie either in different molecular movements being involved in the two processes (viscous flow versus structural relaxation) or in their different manifestation in terms of volume and enthalpy behavior. [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]

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. [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]

The original Kissinger, Friedman, and modified Kissinger-Akahira-Sunose (KAS) methodologies were used to evaluate the apparent activation energy of theoretically simulated complex processes consisting of two overlapping independent JMA-like (Johnson-Mehl-Avrami-like) subprocesses. Three overlay series were studied, each representing one of the basic conceptual types of peak overlap. It was shown that, in the case of complex processes, both the tested isoconversion methods (KAS and Friedman) provide good qualitative information about the activation energies of the involved overlapping signals. However, from the quantitative point of view, the data are not easy to interpret, and deconvolution procedures are necessary for meaningful results to be obtained. On the other hand, in most cases, the apparent activation energy determined by the original Kissinger equation for the overlapped dominant peak corresponded very well to the true values determined for the sole processes. This suggests large robustness of the Kissinger methodology that even nowadays may be considered very advantageous and utilized in kinetic analysis of complex processes. [ABSTRACT FROM AUTHOR]

The applicability of the Kissinger equation for the evaluation of apparent activation energy corresponding to glass transition kinetics is examined. Theoretically simulated data based on the generally accepted Tool-Narayanaswamy-Moynihan model were used to represent relevant cases of structural relaxation behavior. The values of the apparent activation energy determined by the Kissinger equation were, despite the linearity of the dependencies, in major disagreement with the original values of ∆ h used for the simulation of the source data. Furthermore, a large dependence of the ∆ h evaluation (performed using the Kissinger equation) on the thermal history of the glass was found. The latter represents an unacceptable systematic error in the methodology, implying the incorrectness of the Kissinger equation usage for the evaluation of 'glass transition activation energy'. This study addresses the currently widespread (incorrect) usage of the Kissinger equation for the above-mentioned purpose. [ABSTRACT FROM AUTHOR]

The concept of generalized time $$ \theta = \int {{ \exp }\left( { - E_{\text{a}} /RT} \right){\text{d}}t} $$ in non-isothermal kinetics was introduced by Ozawa in 1965, together with the well-known isoconversional plot, i.e., Ozawa plot. The generalized time is the key concept to tie the kinetic data under varying temperature to the kinetic relationship at a constant temperature. It is well known that many processes studied by thermal analysis and calorimetry reveal a complex nature. Therefore, the generalized time concept seems to be very useful for the description of the change in the rate behavior depending on the fractional conversion. Using the concept of θ, three kinds of experimental master plots can be formalized in differential, integral, and multiplied forms. Among others, combination of the differential and multiplied master plots, y( α) = (d α/d θ) and z( α) = (d α/d θ) θ, show a high performance to discriminate the kinetic model based on the maxima condition of y( α*) and z( α*). The α* − α* kinetic plot is a useful tool to visualize the complexity of the kinetic process and to determine the most suitable kinetic model. The usefulness of α* − α* kinetic plot and the YZ-master plots is illustrated as exemplified by the kinetic analyses of complex crystallization processes of the as-prepared, thermally and mechanically treated amorphous zirconia. [ABSTRACT FROM AUTHOR]

A modified peak-deconvolution procedure for complex crystallization processes was introduced. The method is based on the constrained curve-fitting technique using the Fraser-Suzuki (FS) function, where the FS asymmetry parameter a correlates with the value of the Johnson-Mehl-Avrami (JMA) kinetic parameter m. The correlation was verified for an extensive number of theoretically simulated JMA curves; in addition, the dependencies of the a parameter on other kinetic variables ( E, A, q) were quantified. The suggested deconvolution procedure was tested on two glassy systems with different overlay degree of the involved overlapping surface and bulk crystallization processes. In both cases, the kinetic analysis of deconvoluted data provided reasonable, consistent and accurate results. However, certain level of knowledge and experience was needed in order to correctly recognize and consequently account for all deviations from the theoretical behavior caused by thermal gradients or imperfections of the data acquisition process. As the input data for the fitting procedure can be in any form equivalent to the dα/d T temperature dependence, the method seems to be highly universal and may be applied to data obtained by various TA techniques. [ABSTRACT FROM AUTHOR]

SbS crystal growth kinetics in (GeS)(SbS) thin films ( x = 0.4 and 0.5) have been investigated through this study by optical microscopy in the temperature range of 575-623 K. Relative complex crystalline structures composed of submicrometer-thin SbS crystal fibers develop linearly with time. The data on temperature dependence of crystal growth rate exhibit an exponential behavior. Corresponding activation energies were found to be E = 279 ± 7 kJ mol for x = 0.4 and E = 255 ± 5 kJ mol for x = 0.5. These values are similar to activation energies of crystal growth in bulk glasses of the same compositions. The crystal growth is controlled by liquid-crystal interface kinetics. It seems that the 2D surface-nucleated growth is operative in this particular case. The calculated crystal growth rate for this model is in good agreement with experimental data. The crystal growth kinetic characteristic is similar for both the bulk glass and thin film for x = 0.4 composition. However, it differs considerably for x = 0.5 composition. Thermodynamic and kinetic aspects of crystal growth are discussed in terms of Jackson's theory of liquid-crystal interface. [ABSTRACT FROM AUTHOR]

Abstract: Differential scanning calorimetry was used to study crystallization in the Ge2Sb2Se5 glass under non-isothermal conditions. The crystallization kinetics was described in terms of autocatalytic Šesták–Berggren model. An extensive discussion of all aspects of a full-scale kinetic study for a crystallization process was performed. Number of suggestions regarding the experimental part and importance of the particle size influence study was introduced in order to maximize precision, reproducibility and predicative potential of the experimental data. Complexity of the crystallization process was identified to be represented by closely overlapping consecutive competing surface and bulk mechanisms. Mutual interactions of both mechanisms as well as all other observed effects were discussed in relation to the ability of nowadays theoretical models to accurately and correctly describe the experimental data. Advanced error analysis was performed for each step of the kinetic study. [Copyright &y& Elsevier]

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]

Thermomechanical analysis (TMA) can be used as a sensitive tool to follow crystallization behavior in non-crystalline materials. Newly developed method is based on slowing down of sample deformation caused by viscous flow above the glass transition due to macroscopic crystal growth. It is shown that a typical TMA sigmoidal curve reasonably well corresponds to direct measurement of crystal growth kinetics by means of optical microscopy. The method has been used to study crystallization kinetics in GeS glass. The TMA measurement is able to detect earlier stages of crystallization than obtained by differential scanning calorimetry measurement. The activation energy obtained from the shift of extrapolated end of TMA curve with heating rate ( E = 263 ± 7 kJ mol) is similar to the activation energy of α-GeS crystal growth in GeS glass ( E = 247 ± 23 kJ mol) obtained from direct optical microscopy measurements. [ABSTRACT FROM AUTHOR]

Abstract: A novel method to study crystallization behavior in non-crystalline materials has been developed. The method is based on slowing down of sample deformation by viscous flow above the glass transition due to macroscopic crystal growth. This process might be detected by thermomechanical analysis (TMA). The influences of sample length, applied force and heating rate have been examined. Newly developed method has been used to study crystallization in (GeS2)0.3(Sb2S3)0.7 glass. It was found that a typical TMA measurement reasonably well corresponds to previously reported crystal growth kinetics by means of optical microscopy. The activation energy obtained from the shift of extrapolated TMA onset temperature with heating rate (E =286±8kJmol−1) is very similar to the activation energy of Sb2S3 crystal growth in (GeS2)0.3(Sb2S3)0.7 glass (E G =288±7kJmol−1) from optical microscopy measurements. The TMA curve does not match with non-isothermal DSC curve taken at comparable experimental conditions. It seems that TMA measurement reflects early stages of crystal growth that are not associated with thermal effects measured by DSC. [Copyright &y& Elsevier]

Abstract: Volume and enthalpy relaxation studies of amorphous Se have been performed in the glass transition region by mercury dilatometry and differential scanning calorimetry. For simple temperature jump experiments, as well as for more complex thermal history the volume and enthalpy relaxation data can be described by a single set of kinetic parameters for Tool-Naraynaswamy-Moynihan (TNM) model [Δh∗ /R =42.8kK, ln(ATNM /s)=−133]. Slightly different non-linearity and non-exponentiality parameter were found for volume [x =0.42, β =0.58] and enthalpy [x =0.52, β =0.65] relaxation data. Similar results were obtained also for Adam-Gibbs-Scherer (AGS) model. The activation energy of viscous flow in the glass transition range is identical with the effective activation energy for relaxation process. A self-consistent data evaluation shows that at moderate departure from equilibrium, volume and enthalpy in amorphous selenium relax in the same way as expressed by TNM and AGS models. Both volume and enthalpy change can be interpreted within the same fictive temperature concept. [Copyright &y& Elsevier]

Kinetic processes (cold crystallization and structural relaxation) occurring in the Ge 20 Te 76 Se 4 infrared bulk glass were studied by means of differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The crystallization behaviour was described in terms of the Autocatalytic and Johnson–Mehl–Avrami kinetic models. The crystallization activation energies evaluated by the two techniques were: 164 ± 7 kJ mol − 1 (DSC) and 153 ± 8 kJ mol − 1 (TMA). The agreement between the two values was explained in terms of the marked glass stability and glass-forming ability of the Ge 20 Te 76 Se 4 glass, where the crystal growth proceeds at rather low viscosities. Regarding the process complexity, increasing heating rate led to a decrease of the dimensionality of the formed crystallites (3D → 2D). Structural relaxation kinetics was described in terms of the Tool–Narayanaswamy–Moynihan model. Values of the parameters of the TNM model were interpreted with respect to the molecular structures determined by Raman spectroscopy. The Se ↔ Te substitution appears to lead to larger interconnection between the GeTe 4 tetrahedral chains rather than to significant changes in the intra-chain bonding. Good agreement between the activation energies of the relaxation process determined by the two experimental techniques was again obtained: 501 ± 10 kJ mol − 1 (DSC) and 490 ± 34 kJ mol − 1 (TMA). [ABSTRACT FROM AUTHOR]

The isothermal crystal growth in Se100-xTex bulk glasses (x = 10, 20, and 30) was studied directly using infrared microscopy. The crystals grew spherulitically and linearly in the course of time, which is typical for crystal growth controlled by liquid-crystal interface kinetics. An operative growth model was found using a combination of growth and viscosity data, and using two different approaches for calculations of the Gibbs free energy change between the undercooled melt and crystalline phase. The study shows that the exact knowledge of the Gibbs free energy change calculated from both, heat capacities, and the simple approximation proposed by Turnbull, can provide comparable results regarding determination of an operative crystal growth model. A detailed discussion about the relationship between the kinetic coefficient of crystal growth rate and viscosity (ukin ∝ η-ξ) is presented. Moreover, the activation energies of crystal growth were found to be higher than the activation energies of the overall crystallization process obtained by differential scanning calorimetry. The relation between these two quantities is considered under the experimental conditions. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study the non-isothermal crystallization behavior of selenium–tellurium thin films within the 0–30 at.% Te compositional range. The non-isothermal crystallization kinetics were described in terms of the Johnson–Mehl–Avrami nucleation-growth model. The apparent activation energy of crystallization was found to exhibit a step-like compositional behavior, with E changing from ~ 115 to ~ 145 kJ·mol − 1 in the 10–15 at.% Te range. Two-dimensional growth of crystallites, consistent with the idea of sterically restricted crystallization in a thin layer, was confirmed for all data. However, in the case of the Se 70 Te 30 thin film, indications of three-dimensional crystal growth were found at high heating rates. This corresponds to the previously reported behavior of Se–Te chalcogenide matrices, where the addition of tellurium leads to the formation of smaller, volume-located crystallites. [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]

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the (GeTe 4 ) y (GaTe 3 ) 1 − y infrared chalcogenide glasses for the compositional range y = 0.4 − 1.0. The relaxation behavior was described in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Direct curve-fitting procedure was used to determine the values of TNM parameters; these results were successfully correlated with data provided by the non-fitting methodology based on the evaluation from constant-ratio (CR) cycles. The addition of GaTe 3 into the GeTe 4 matrix led to a moderate decrease of activation energy of the relaxation process and to a large increase of the relaxation linearity – the structural relaxation became driven solely by temperature. Interpretation of the compositional evolution of the TNM parameters was used to verify structural information provided by Raman spectroscopy and molecular dynamics simulations: the initial GaTe 3 addition causes dilution of the GeTe 4 tetrahedral network, which carries the main portion of the relaxation motions; further increase of GaTe 3 content then leads to an increase of connectivity outside the GeTe 4 sub-networks due to the threefold coordinated Ga atoms bonding with the lone-pair electrons of Te dimers and short chains. [ABSTRACT FROM AUTHOR]

Cold crystallization and structural relaxation kinetic processes occurring in the Ge 11 Ga 11 Te 78 infrared bulk glass were studied using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The complex crystallization behaviour was described as overlapping surface nucleation-growth Te precipitation and volume-located autocatalytic formation of GeTe and Ga 2 Te 5 crystallites. Similar crystallization activation energies were obtained by DSC and TMA. Direct comparison of the DSC and TMA data reveals that only a narrow temperature window of approx. 20 °C exists between the start of the glass softening and first formation of Te crystallites, which makes knowledge of crystallization kinetics crucial for the industrial glass processing and real-life commercial applications. Structural relaxation kinetics was described in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Good agreement between the relaxation activation energies determined by DSC and TMA was obtained. Values of the parameters of the TNM model were interpreted with respect to the molecular structures determined by Raman spectroscopy and reverse Monte Carlo simulations. The stabilizing role of Ga was explained by its preferred bonding to the non-bonding p electrons of Te. [ABSTRACT FROM AUTHOR]