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Kinetics of thermally induced processes in Ag doped As40Se30Te30 chalcogenide glass
Abstract
The processes of glass-transition and crystallization of chalcogenide glass As40Se30Te30 with 5 at.% silver were analyzed using differential scanning calorimetry. The values of glass-transition temperatures and activation energy were determined. Two crystallization processes were also detected and three-dimensional growth. Using non-isoconversional models the activation energies for both processes amounted to 112(2) kJ/mol and 97(2) kJ/mol. Isoconversional models were used to track changes in activation energy. The presence of Te significantly affects the thermal parameters as well as the structure of the glass while the presence of Ag does not significantly change the degree of connectivity of the As40Se30Te30 glass matrix.
... Notably, in the sample with 3 at.% of Ag, two distinct crystallization peaks are clearly observed. Similar observations were made for the composition with 5 at.% of Ag [36]. Additionally, in the composition with 7 at.% of Ag, two peaks are also observed at certain applied heating rates. ...
... Figure 9 shows the dependence of the parameter B, while Fig. 10 shows a graphical representation of E g as a function of the silver content. The values of the parameters T g , B and E g for composition with 5 at.% of Ag were previously determined [36] and are included in the graphs for comparison [E g = 160(3) kJ mol −1 , B = 7.8 (6)]. ...
... Combined with the above-mentioned XRD analysis, it can be concluded that this peak corresponds to matrix crystallization, while the first peak corresponds to a structural unit with silver. This effect was also observed in the composition with 5 at.% [36]. ...
The investigation of the glass transition process and crystallization, along with the analysis of the crystallization kinetics, is conducted on chalcogenide As40Se30Te30 glass with different concentrations of silver (0, 1, 3, 7, 9, 13, and 17 at.%). The differential scanning calorimetry technique is employed at different heating rates to examine these phenomena. It is demonstrated that the glass undergoes minimal structural changes during the glass transition process. Glass transition temperatures as well as apparent activation energies are determined. Measurements indicated that in certain examined compositions, thermally induced crystallization manifests as a complex process wherein multiple structural units crystallize. In compositions containing 3 and 7 at.% of Ag, two separate crystallization processes are observed. It was shown that the commonly used Johnson–Mehl–Avrami model does not describe crystallization processes of the investigated glass well enough. Additional analysis was conducted using the Sestak-Breggren kinetic model. The apparent activation energies were determined using Kissinger, Mahadevan, and Augis-Bennett models, falling within the range from 93 to 128 kJ mol−1. To track changes in activation energy during the crystallization process itself, isoconversional models such as Vyzovkin, Kissinger–Akahira–Sunose, and Ozawa-Flynn-Wall were employed. It is observed that the introduction of silver contributes to the stabilization of the chalcogenide matrix. Concurrently, the activation energies exhibit a decreasing trend with minor fluctuations. The presence of a singular crystallization peak attributed to the AgAsSe2 structural unit in samples containing over 9 at.% of Ag is a notable feature that holds promise for the further application of the investigated glass.
The glass transition and non-isothermal crystallization behavior of Ge20Se70Sn10 glass prepared by the melt-quenching technique was investigated using differential scanning calorimetry (DSC) at continuous different heating rates. The structure and surface morphology of as-prepared and annealed samples were characterized using X-ray diffraction (XRD) and scanning electron microscopy. The as-prepared samples showed the amorphous glassy nature, while the annealed ones are polycrystalline. Furthermore, XRD phase analysis allowed us to find the SnSe2, GeSe2, Ge4Se9 and Sn0.5 SeGe0.5 phases in the annealed samples. According to the value of Avrami index (n), the crystallization process of studied composition has more than one crystal growth mechanism. In addition, the results of DSC showed that the investigated glass has only a single glass transition and double crystallization stages. Furthermore, the activation energy of transition as well as the crystallization has been determined based on different approximation methods. In addition, the experimental DSC data of the first and second crystallization peak were compared with that calculated with the Johnson–Mehl–Avrami and Sestak–Berggren SB(M, N) model. The results revealed that the SB(M, N) model is more suitable for describing the crystallization kinetics of studied glass.
The article “The Faraday Isolator, Detailed Balance and the Second Law” [Journal of Applied Mathematics and Physics, Vol. 5, No. 4, April 2017 PP. 889-899] erroneously assumes that in a Faraday isolator, thermal radiation from a black body/polarizer com-bination entering the rotator is polarized and therefore can be acted upon by the rotator, resulting in the rectification of heat flow. In fact, this thermal radiation is unpolarized rendering the rotator inoperative. Hence a Faraday isolator cannot rectify thermal ra-diation.
The crystal growth and nucleation in glasses in the lithium silicate system have been
investigated. Phase separation in ultimately homogenized glasses of the lithium silicate
system x Li2O⋅(100 − x )SiO2 (where x = 23.4, 26.0, 29.1, and 33.5 mol% Li2O) has
been studied. The glasses of these compositions have been homogenized using the
previously established special temperature-time conditions, which make it possible to
provide a maximum dehydration and removal of bubbles from the glass melt. The
parameters of nucleation and growth of phase separated in homogeneities and homogeneous
crystal nucleation have been determined. The absolute values of the stationary
nucleation rates I st of lithium disilicate crystals in the 23.4Li2O⋅76.6SiO2,
26Li2O⋅74SiO2 and 29.1Li2O⋅70.9SiO2 glasses with the compositions lying in the metastable
phase separation region have been compared with the corresponding rates I st
for the glass of the stoichiometric lithium disilicate composition 33.51Li2O⋅66.5SiO2.
It has been found that the crystal growth rate has a tendency toward a monotonic increase
with an increase in the temperature, whereas the dependences of the crystal
growth rate on the time of low temperature heat treatment exhibit an oscillatory behavior
with a monotonic decrease in the absolute value of oscillations. The character
of crystallization in glasses with the compositions lying in the phase separation region
of the Li2O-SiO2 system is compared with that in the glass of the stoichiometric
lithium disilicate composition. The conclusion has been made that the phase separation
weakly affects the nucleation parameters of the lithium disilicate and has a
strong effect on the crystal growth.
Thermal properties of glasses from the system Agx(As40S30Se30)100-x for x = 0, 0.5, 1, 3, and 5 at.% were
investigated by differential scanning calorimetry. The DSC curves were obtained under non-isothermal conditions which allowed determination of the glass transition temperature Tg (onset temperature), crystallization temperature Tp (corresponding to the crystallization peak maximum), melting temperature Tm, crystallization enthalpy Hc, and melting enthalpy Hm. The DSC curves obtained at the same heating rate were analyzed in order to study the variation of glass transition temperature with Ag concentration. Observed Tg shift toward higher values, with increase in the heating rate, is in agreement with the Lasocka equation. Samples with 3 at.% and 5 at.% Ag were further thermally treated at different heating rates with the aim of analyzing kinetic processes of crystallization. The Moynihan and Kissinger models were used to calculate the activation energy of glass transition and activation energy of crystallization. For the samples that showed the crystallization processes an assessment of the thermal stability was done based on different criteria.
We are reporting the linear and nonlinear optical properties of Se-based quaternary chalcogenide Se–Sn–(Bi,Te) thin films. Thin films of bulk chalcogenide glasses, prepared by melt quenching method are deposited on glass substrate using thermal evaporation technique. The optical behavior of studied chalcogenide glass systems is investigated using transmission spectra in the spectral range of 400–2500 nm. The glasses exhibit considerable optical nonlinearities which are estimated using linear optical parameters. Linear refractive index has been calculated using well-known Swanepoel method. Wemple-DiDomenico (WDD) parameters are also reported for the investigated glasses. Optical band gap is determined using Tauc extrapolation method and is observed to increase with Sn content. The formulation proposed by Fournier and Snitzer is used to determine the nonlinear behavior of the refractive index. It is observed that n 2 increases linearly with increasing n. The values of n 2 are compared with pure silica and the results are 100–600 orders higher. The third-order susceptibility χ(3) is also reported in this paper. Two-photon absorption coefficient β2 is determined using optical band gap data. A strong dependence of β2 and n 2 is observed on normalized photon energy () for a fixed excitation wavelength (1064 nm).
The particle size effects on the kinetic parameters of two overlapping crystallization peaks of Se85Te10Sb5 chalcogenide glass were studied using differential scanning calorimetry (DSC) under experimental and predicted isothermal conditions. The crystallization kinetics parameters of the two peaks were separated based on the application of a multi-peak Gaussian function using the advanced thermokinetics software package (AKTS). The nonisothermal methods of Friedman, Kissinger–Akahira–Sunose and the minimization, in addition to the predicted isothermal method, were used to investigate the variation of the effective activation energy with the extent of crystallization and, hence, with temperature. The local Avrami exponent as a function of the crystalline volume fraction was obtained at a constant heating rate or temperature for nonisothermal and isothermal processes. Its value was found to vary with particle size. The crystallization process of the two peaks was found to follow the Avrami–Erofeev reaction model.
The glass transition temperature dependence on a heating rate was investigated by differential scanning calorimetry for chalcogenide glasses from the Agx (As2(S0.5Se0.5)3)100−x system for x = 0, 0.5, 2 and 3 at.%. According to this dependence, apparent glass transition activation energies Eg were calculated for differently defined glass transition temperatures (onset Tg1, midpoint Tg2, endpoint Tg3 and endset Tg4). Also, Eg was analysed using isoconversional (model free) method. The results showed that the activation energy is not constant during the investigated process and vary with the extent of transformation from the glassy to the supercooled phase, pointing out that a complicated, multi-step process occurs. Eg decreases from about 340 to 200 kJ mol⁻¹ in the composition with 2 at.% of silver. The values of the fragility index showed that these glasses do not exhibit large configuration changes during the glass transition process. Influence of silver content on investigated parameters was discussed. Apparent glass transition activation energy, variability parameter and fragility index showed a significant change in their values for the composition with 3 at.% of silver. Application of the isoconversion model for determination of apparent glass transition activation energy has showed that this parameter (at extent of conversion of 0.5) decreases from about 260 kJ mol⁻¹ for x = 0, 0.5, 2 at.% to about 185 kJ mol⁻¹ for x = 3 at.% of silver content. Likewise, fragility parameter drops from ~ 28.5 for x = 0, 0.5, 2 at.% to 21.5 x = 3 at.% of silver.
Chalcogenide glasses of (As50Se50)100−xAgx (0 ≤ x ≤ 25) were prepared using the melt quenching technique under non-isothermal conditions. Differential scanning calorimetry curves measured at different heating rates (5 ≤ β ≤ 40 K min⁻¹) are used to characterize the as-quenched samples. The thermal stability was monitored through the calculation of the temperature difference Tc − Tg, stability parameter S and crystallization rate factor Kp. The glass-forming ability (GFA) was investigated on the basis of Hurby parameter Hr which is a strong indicator of GFA. In addition, the activation energy of glass transition Et, activation energy of crystallization Ec and Avrami exponent n of the studied compositions were determined. The mechanism of crystallization was found to be a combination of two- and three-dimensional crystal growth.
Chalcogenide phase-change materials (PCMs), such as Ge-Sb-Te alloys, have shown outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, recently, in non-volatile resistive memories. The latter, known as PCM memories or phase-change random access memories (PCRAMs), are the most promising candidates among emerging non-volatile memory (NVM) technologies to replace the current FLASH memories at CMOS technology nodes under 28 nm. Chalcogenide PCMs exhibit fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties leading to a unique set of features for PCRAMs, such as fast programming, good cyclability, high scalability, multi-level storage capability, and good data retention. Nevertheless, PCM memory technology has to overcome several challenges to definitively invade the NVM market. In this review paper, we examine the main technological challenges that PCM memory technology must face and we illustrate how new memory architecture, innovative deposition methods, and PCM composition optimization can contribute to further improvements of this technology. In particular, we examine how to lower the programming currents and increase data retention. Scaling down PCM memories for large-scale integration means the incorporation of the PCM into more and more confined structures and raises materials science issues in order to understand interface and size effects on crystallization. Other materials science issues are related to the stability and ageing of the amorphous state of PCMs. The stability of the amorphous phase, which determines data retention in memory devices, can be increased by doping the PCM. Ageing of the amorphous phase leads to a large increase of the resistivity with time (resistance drift), which has up to now hindered the development of ultra-high multi-level storage devices. A review of the current understanding of all these issues is provided from a materials science point of view.
The non-isothermal method for estimating the kinetic parameters of crystallization for the phase change memory (PCM) materials was discussed. This method was applied to the perspective PCM material of Ge2Sb2Te5 with different Bi contents (0, 0.5, 1, 3 mass%) for defining the kinetic triplet. Rutherford backscattering spectroscopy and X-ray diffraction were used to carry out elemental and phase analysis of the deposited films. Differential scanning calorimetry at eight different heating rates was used to investigate kinetics of thermally induced transformations in materials. Dependences of activation energies of crystallization (E
a) on the degree of conversion were estimated by model-free Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, Tang and Starink methods. The obtained values of E
a were quite close for all of these methods. The reaction models of the phase transitions were derived with using of the model-fitting Coats–Redfern method. In order to find pre-exponential factor A at progressive conversion values, we used values of E
a already estimated by the model-free isoconversional method. It was established that the crystallization processes in thin films investigated are most likely describes by the second and third-order reactions models. Obtained kinetic triplet allowed predicting transition and storage times of the PCM cells. It was found that thin films of Ge2Sb2Te5 + 0.5 mass% Bi composition can provide the switching time of the phase change memory cell less than 1 ns. At the same time, at room temperature this material has a maximum storage time among the studied compositions.
In differential thermal analysis, the temperature at which the maximum deflection is observed varies with heating rate for certain types of reactions. An expression can be derived relating this variation with the kinetics of the reaction. By making a number of differential thermal patterns at different heating rates, the kinetic constants can be obtained directly from the differential thermal data. Measurements of the variation of peak temperature with heating rate have been made for several minerals of the kaolin group, the values of the kinetic constants determined, and these values compared with corresponding values obtained for both the same samples and similar material by conventional isothermal techniques. Some factors affecting the results are discussed.
Effect of Ag doping on the crystallization kinetics of amorphous Se80.5Bi1.5Te18−yAgy (for y = 0, 1.0, 1.5, and 2.0 at.%) glassy alloys has been studied by differential scanning calorimetry (DSC). The DSC curves recorded at four different heating rates are analyzed to determine the transition temperature, activation energy, thermal stability, glass forming ability, and dimensionality of growth during phase transformation. Present study shows that the thermal stability and the glass-forming ability increase with an increase in the Ag content which is in agreement with the earlier studies. Our results show that Se80.5Bi1.5Te16Ag2 composition is thermally more stable and has a little tendency to crystallize in comparison to other compositions under study. The increase in thermal stability with increasing Ag concentration is attributed to an increase in the cohesive energy.
Several isothermal experiments are generally needed to determine the parameters of the Avrami equation which describe most of the heterogeneous solid state reactions. Differential scanning calorimeters are suitable for such experiments. While most differential thermal analysis (DTA) apparatus cover a wider temperature range than DSC apparatus they cannot be used to perform isothermal determinations. However, Kissinger has already shown how activation energy and frequency factor can be calculated from DTA experiments for the case of homogeneous reactions following first order kinetics. We derive in this paper an extension of the Kissinger method and show its applicability to heterogeneous reactions described by an Avrami expression. The new method will allow the study of the kinetics of metallic reactions at the higher temperature range obtainable with DTA. The transformation kinetics of the metastable equiatomic tin-nickel alloy are given as an example.
The theory of the kinetics of phase change is developed with the experimentally supported assumptions that the new phase is nucleated by germ nuclei which already exist in the old phase, and whose number can be altered by previous treatment. The density of germ nuclei diminishes through activation of some of them to become growth nuclei for grains of the new phase, and ingestion of others by these growing grains. The quantitative relations between the density of germ nuclei, growth nuclei, and transformed volume are derived and expressed in terms of a characteristic time scale for any given substance and process. The geometry and kinetics of a crystal aggregate are studied from this point of view, and it is shown that there is strong evidence of the existence, for any given substance, of an isokinetic range of temperatures and concentrations in which the characteristic kinetics of phase change remains the same. The determination of phase reaction kinetics is shown to depend upon the solution of a functional equation of a certain type. Some of the general properties of temperature‐time and transformation‐time curves, respectively, are described and explained.
Following upon the general theory in Part I, a considerable simplification is here introduced in the treatment of the case where the grain centers of the new phase are randomly distributed. Also, the kinetics of the main types of crystalline growth, such as result in polyhedral, plate‐like and lineal grains, are studied. A relation between the actual transformed volume V and a related extended volume V1 ex is derived upon statistical considerations. A rough approximation to this relation is shown to lead, under the proper conditions, to the empirical formula of Austin and Rickett. The exact relation is used to reduce the entire problem to the determination of V1 ex, in terms of which all other quantities are expressed. The approximate treatment of the beginning of transformation in the isokinetic range is shown to lead to the empirical formula of Krainer and to account quantitatively for certain relations observed in recrystallization phenomena. It is shown that the predicted shapes for isothermal transformation‐time curves correspond well with the experimental data.
The theory of the preceding papers is generalized and the notation simplified. A cluster of molecules in a stable phase surrounded by an unstable phase is itself unstable until a critical size is reached, though for statistical reasons a distribution of such clusters may exist. Beyond the critical size, the cluster tends to grow steadily. The designation ``nuclei'' or ``grains'' is used according as the clusters are below or above the critical size. It is shown that a comprehensive description of the phenomena of phase change may be summarized in Phase Change, Grain Number and Microstructure Formulas or Diagrams, giving, respectively, the transformed volume, grain, and microstructure densities as a function of time, temperature, and other variables. To facilitate the deduction of formulas for these densities the related densities of the ``extended'' grain population are introduced. The extended population is that system of interpenetrating volumes that would obtain if the grains granulated and grew through each other without mutual interference. The extended densities are much more readily derivable from an analysis of the fundamental processes of granulation and growth. It is shown that, under very general circumstances, the densities of the actual grain population may be expressed simply in terms of the extended population.
The effects of the kinetics of reactions of the type solid → solid + gas on the corresponding differential thermal analysis pattern are explored. Curves of reaction rate vs. temperature for constant heating rates constructed by analytical methods are used to demonstrate the effect of varying order of reaction. The information so obtained is used to analyze the differential thermal patterns of magnesite, calcite, brucite, kaolinite, and halloysite. The results of the differential thermal study agree with results obtained isothermally except in some specific cases.
An advantageous algorithm is proposed for correcting the Doyle approximation for the popular Ozawa-Flynn-Wall analysis to make a model-free evaluation of the activation energy from a series of non-isothermal measurements carried out at different heating rates. In this way a definite improvement in the corrective factor is reached by an approximation formula. The maximum approximation error is 3 × 10−4. Evidence concerning this new variant is provided by means of simulated data.
Results of differential scanning calorimetry of high purity As40Se60−xTex (x=0–40) chalcogenide glasses are reported. The characteristic temperatures were studied under nonisothermal conditions. The activation energy of the glass transition (Eg), the activation energy of crystallization (Ec), the Avrami exponent (n), the frequency factor (Ko) and the crystallization criteria of these glasses were determined. The present investigation indicates that the crystallization of high purity As40Se60−xTex (x=0–40) depends from glass composition and impurity content and occurs in a single stage with one- or two-dimensional crystal growth. The glass having the minimal tendency to crystallization is As40Se40Te20.
A simple and satisfactorily accurate solution of the exponential integral in the nonisothermal kinetic equation for linear heating is proposed:
\mathop \smallint 0T e - E/RT dT = \fracRT2 E + 2RTe - E/RT \mathop \smallint \limits_0^T e^{ - E/RT} dT = \frac{{RT^2 }}{{E + 2RT}}e^{ - E/RT}
X-ray absorption spectroscopy (EXAFS and XANES) was used to determined the local structure of AgAsSe glasses that belong to the (Ag2Se)x(AsSe)1−x line of the phase diagram. Each of the three atoms has been excited. At the arsenic K edge, the one-shell fit shows the existence of AsSe3 units for all compositions and an invariant Debye-Waller factor when silver content increases. At the selenium K edge, the fit shows that the total number of first neighbours around the selenium atom varies from nearly three (as in the Ag3AsSe3 crystalline compound) for the silver-enriched glass (x = 0.43) to two (as in the As4Se4 glass or crystalline phase) for the silver-poor glasses. The radial distribution function for the silver K edge at room temperature shows a unique contribution related to the AgSe pairs. However, at low temperature (35 K) a second peak appears. The qualitative interpretation of the XANES part of the absorption spectra is in good agreement with the present structural investigation. Based on these results a tentative model is given.
The glass transition temperature Tg of eutectic splat-quenched TeGe alloy was measured on a Perkin-Elmer DSC-2 unit over a wide range of scanning rates β from 1.25 to 80 deg/min. The results are described by a linear plot. Tg = A + B lg β with parameters A = 386 K and B = 14.88 over the whole experimental range of β. The physical meaning of the Tg values measured for glassy splats by DSC is discussed. The possibility of using the slope B of the Tg (lgβ) plot for comparison of glasses obtained in different ways is suggested.
Results of thermal analysis performed at different heating rates on eight glasses of the As-Sb-Se system, with compositions represented by (As, Sb)40Se60 and AsxSb15Se85 − x are reported and discussed.The values of the glass transition temperature (Tg), the crystallisation temperature (Tc) and the peak temperature of crystallisation (Tp) are found to be independent of particle size. From the heating rate dependence of Tg and Tp values of the activation energy for glass transition (Et) and the activation energy for crystallisation (Ec) are evaluated and their composition dependence discussed.The crystallisation data are examined in terms of recent analyses developed for non-isothermal crystallisation studies to arrive at Ec and details of the crystallisation mechanism. The results indicate bulk nucleation and crystallisation with two and three dimensional growth respectively for the AsxSb15Se85 − x and (As, Sb)40Se60 glasses.
Amorphous thin films of Se80−xTe20Pbx (0<x<2) have been prepared by a thermal evaporation technique. Dark and photoconductivity of the samples was measured at different temperatures. The value of the DC activation energy ΔEa and the optical band gap Egopt for Se80Te20 was found to be ∼0.67 and ∼1.68 eV, respectively. The addition of a small amount of Pb (0.6 at.%) to the Se–Te system decreases ΔEa (∼0.3 eV) and Egopt (∼1.39 eV) considerably. However, further addition of Pb (up to 2 at.%) does not cause much change in these parameters. The photoconductivity is found to increase with increasing temperature and light intensity. The square-root dependence of photocurrent on the illumination intensity indicates that the recombination process is bimolecular. The photosensitivity and the differential lifetime are also reported for Se80−xTe20Pbx (0<x<2) thin films.
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