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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]

Isothermal and non-isothermal crystallisation behaviour of the GeTeSe chalcogenide glass was studied dependent on particle size and compared to the similar DSC study performed for the pure GeTe material. The effect of the Se ↔ Te substitution led to a decrease in the crystallisation process complexity in the case of powdered materials-the addition of Se led to domination of the slower nucleation-growth crystallisation mechanism over the fast autocatalytic growth from mechanically induced defects. The nature of the crystallisation mechanisms was confirmed by infrared microscopy and XRD. The addition of selenium into the GeTe matrix caused an enlargement of the identified crystallites; this, together with the overall deceleration of the crystallisation processes, indicates the suppression of nucleation processes. The presence of Se also greatly enhanced domination of the CNT-based three-dimensional growth of volume-located crystallites. The obtained findings provide a very good explanation for the markedly increased thermal stability of Se-doped Ge-Te infrared glasses. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the GeTe 4 infrared chalcogenide glass. The relaxation behavior was described in terms of the phenomenological Tool–Narayanaswamy–Moynihan (TNM) and Adam–Gibbs–Scherer (AGS) models. The direct curve-fitting procedure was found to provide the most accurate results under the assumption that high-quality DSC data are obtained and the correct pre-fitting treatment is applied. The recently developed novel evaluation of Δ h ⁎ from CR cycles and the improved simulation-comparative methodology were then found to provide the best results out of the tested non-fitting methods. [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]

Crystallization kinetics, studied under non-isothermal conditions in dependence on particle size, is reviewed for the Se-Te glassy system, with particular focus on occurring macroscopic crystallization mechanisms. The observed complexity of the crystallization processes is explained in terms of the two previously identified mechanisms-bulk crystallization originating from nuclei randomly distributed in the volume of each glass particle (formation of these nuclei is consistent with the classical nucleation theory) and crystallization from surface/volume active centers, dislocations, structural defects, and heterogeneities originating from the mechanical damage of the material (e.g., during grinding and milling). In addition, a new crystallization mechanism, previously unobserved in this system, was revealed for hyperquenched SeTe glass. The origin of this mechanism is suggested to be closely associated with the stress-induced defects contained in the highly strained structure of this glass. The strong compositional dependence of the occurrence of this mechanism then seems to be caused by the high cooperativity of the amorphous structure of the specific composition. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry (DSC) was used to study crystallization in GeSbSeTe glass under non-isothermal conditions. The crystallization kinetics was described in terms of the autocatalytic Šesták-Berggren model. An extensive discussion of all aspects of a full-scale kinetic study for a crystallization process was undertaken. In particular, the effect of Te ↔ Se substitution on the complexity of the crystallization process was analyzed. The addition of tellurium enhances bulk crystallization originating from volume nuclei at the expense of the surface/defects-based crystallization mechanism. Significantly higher activation energy in the case of the Te-doped material was attributed to the larger mass of the combined Se-Te chains and the larger spatial restrictions for their movement. On the other hand, the slightly lower crystallization temperature of the Te-doped glass corresponds to its higher tendency for crystallization. A supplemental X-ray diffraction study confirmed the findings obtained by DSC. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry was used to study crystallization behavior in selenium glass under non-isothermal conditions. In the current work, which is the second in a sequence of articles dealing with crystallization kinetics of complex processes, a novel method of modified Fraser-Suzuki deconvolution is applied. This method is complementary to that introduced in the preceding article-the advanced interpretation of characteristic kinetic functions. It is shown that together these two approaches provide a reliable and accurate kinetic analysis evaluation for complex and overlapping processes. Based on the determination of kinetic parameters depending on particle size, extensive information about the nature of the involved surface and bulk crystallization mechanisms is obtained. Based on this information, the deconvolution in terms of the Johnson-Mehl-Avrami process is performed, separating the involved mechanisms. The resulting concept not only describes very well, both qualitatively and quantitatively, the crystallization process in glassy selenium, but it is also capable of explaining all accessible data from literature on this topic. The complexity of the description, together with the general validity of the introduced approaches, appears to offer a valuable source of information for numerous material science applications. [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]

Differential scanning calorimetry measurements were used to study enthalpic structural relaxation in the GeSe glassy system. The examined compositional range was 0 ≤ y ≤ 15. Based on the results of both, fitting and non-fitting methods, the description of relaxation behavior was obtained in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Evolution of TNM parameters with increasing germanium content was interpreted within the conception of movements of occasionally linked Se polymeric chains continuously transiting toward rather independent positional changes within heavily inter-linked segments or clusters. As such, the interpretation represents a novel approach associating a phenomenological description with factual changes in molecular structure of the material. In addition, new non-fitting method is introduced, based on the examination of the overall shape of relaxation peak and its comparison to the simulated data. [ABSTRACT FROM AUTHOR]

Abstract: Enthalpic structural relaxation in the AsySe(100-y) glassy system was studied by means of differential scanning calorimetry (DSC). Glasses in the compositional range 0≤y≤15 were examined. Based on the curve-fitting results, the description of relaxation behavior was obtained in terms of the Tool–Narayanaswamy–Moynihan model. These results were further confirmed by a novel simulation-comparative non-fitting method. Evolution of TNM parameters with changing arsenic content was interpreted in terms of joint movements of occasionally linked Se polymeric chains continuously transiting towards rather independent positional changes within inter-linked segments or clusters. The interpretation represents a novel approach associating a phenomenological description with factual changes in the molecular structure of the material. [Copyright &y& Elsevier]

Crystallization kinetics of chosen compositions from Se-Te glassy system were studied under non-isothermal conditions depending on particle size using differential scanning calorimetry. The purpose of this article is to demonstrate the extent of information accessible by the modern kinetic analysis provided by the differential scanning calorimetry, and to suggest its importance and merit for the development of new, high-tech PCM materials. The crystallization kinetics was described in terms of the nucleation-growth Johnson-Mehl-Avrami model. Complexity of the crystallization process was in this 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 centers 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. Accent was laid on the merits resulting from interpretations of characteristic kinetic functions. A new criterion for quick determination of the dominating crystallization mechanism-surface or bulk-was introduced. [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: 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]

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]

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]

Abstract: Differential scanning calorimetry and dc conductivity measurements were used to study structural relaxation of Se70Te30 glass. A single set of Tool–Narayanaswamy–Moynihan (TNM) parameters was obtained from the curve-fitting procedure. The value of apparent activation energy Δh∗ was further confirmed by two non-fitting techniques. Results of the Δh∗ evaluation from the T g dependence on cooling rate are discussed in terms of how the T f determination might be influenced by the material’s structure type and by the interference of the crystallization process. [Copyright &y& Elsevier]

The effect of Ag-doping on the thermokinetic behavior (cold crystallization and structural relaxation kinetics, glass-forming ability) of the Ag y [(GeS 2 ) 50 (Sb 2 S 3 ) 50 ] (100 − y) glasses was studied by using differential scanning calorimetry, Raman spectroscopy and X-ray analysis – compositions up to y = 25 were included in the study. Structural relaxation kinetics was found to be only very weakly influenced by the Ag-doping; the following Tool-Narayanaswamy-Moynihan parameters were determined (within a 10% variation) for the majority of the glasses: activation energy 400 kJ mol − 1 , non-linearity parameter 0.72, non-exponentiality parameter 0.85. On the other hand, the glass transition temperature decreased significantly with increasing Ag content – the effect can be explained by dilution of the original glassy matrix. Combination of these findings with the corresponding Raman spectra has shown that it is the Ge-S structural units that carry the main portion of the relaxation motions. Regarding the crystallization behavior, the Ag-doping led to decreased glass-forming ability and increased tendency of the glasses towards crystallization. The Ag-rich compositions exhibited during heating well-developed crystallization peaks driven by the autocatalytic Šesták-Berggren kinetics. The X-ray analysis has shown that in case of the Ag-rich compositions Sb and argyrodite crystallites tend to form during the melt-quench. [ABSTRACT FROM AUTHOR]

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]

Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis were used to study crystallization in Ge 2 Sb 2 Se 4 Te glass under non-isothermal conditions as a function of the particle size. The crystallization kinetics was described in terms of the autocatalytic Šesták–Berggren model. An extensive discussion of all aspects of a full-scale kinetic study of a crystallization process was undertaken. Dominance of the crystallization process originating from mechanically induced strains and heterogeneities was confirmed. Substitution of Se by Te was found to enhance the manifestation of the bulk crystallization mechanisms (at the expense of surface crystallization). The XRD analysis showed significant dependence of the crystalline structural parameters on the crystallization conditions (initial particle size of the glassy grains and applied heating rate). Based on this information, a new microstructural crystallization mechanism, indistinguishable by DSC, was proposed. [ABSTRACT FROM AUTHOR]

• The pressure-assisted melt filling technique was used for (GeSe 2) x (Sb 2 Se 3) 100-x melts. • The complexity of the viscosity behavior was demonstrated by the disparity of fragility. • A very simple way of viscosity data extrapolation was introduced. Experimental viscosity data for (GeSe 2) x (Sb 2 Se 3) 100-x melts (x = 50, 60 and 70), obtained by deploying the pressure-assisted melt filling technique (PAMFT), are presented. These data were combined with previously published literature data and fitted over a broad temperature range including glass, undercooled melt, and melt regions by the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation. A detailed analysis of different fragility concepts for the studied glass-forming system as well as for other well-known chalcogenide glass formers is presented. On the basis of this extensive analysis, a simple procedure for carrying out viscosity data extrapolation was developed, and the validity of this procedure is discussed. [ABSTRACT FROM AUTHOR]