Crystallization kinetics and thermal stability in Se85-xTe15Sbx chalcogenide glasses

Crystallization process of Se_85-xTe₁₅Sb_x (x = 2.7, 7.5, 10 and 15 at %) chalcogenide glasses has been studied by using differential scanning calorimetry (DSC) with different heating rates. These glasses are found to have a double glasses transition and overlapped crystalline phases for Se₇₀Te₁₅Sb₁₅ glass while single glasses transition and single crystallization stage for other glasses. Glass transition temperature, T_g, onset crystallization temperature, T_c, and peak crystallization temperature, T_p, are found to be dependent on composition and heating rates. Values of various kinetic parameters such as activation energy of glass transition, E_g, activation energy of crystallization, E_c, Hurby number, H_r, thermal stability, S_p, rate constant, K_p, and Avrami exponent, n, are determined for the present systems. Results indicate that rate of crystallization is dependent on thermal stability and glass-forming ability. Crystallization mechanism occurs in two dimensions for studied compositions. Crystalline phases resulting from DSC and scanning electron microscopy have been identified by using X-ray diffraction.     

Study of crystallization kinetics of Se77.5Te15Sb7.5 glass using isoconversional models

The crystallization process of Se_77.5Te₁₅Sb_7.5 glass is studied by differential scanning calorimetry (DSC) technique under non-isothermal conditions at various heating rates. The crystallization parameters are deduced using different models. The validity of the Johnson–Mehl–Avrami (JMA) model to describe the crystallization process for the studied composition is investigated. Comparing experimental and calculated DSC curves indicate that the crystallization process of Se_77.5Te₁₅Sb_7.5 glass cannot satisfactorily be described by the JMA model. In general, simulation results indicate that the Sestak–Berggren model is more suitable to describe the crystallization kinetics. The crystalline phases are identified using the X-ray diffraction technique and scanning electron microscopy.     

A branching model for crystallization kinetics

In order to test whether the branching probability of crystallites plays an important role in determining the structure and the mutual interaction of growing bodies in a bulk crystallization system, mathematical model experiments for dendrite growth on a hexagonal lattice were performed. It was shown that when the branching frequency is high enough, a dendrite contour developed for each growing body from different primary nuclei, giving rise to a sharp boundary. As the branching frequency decreased, however, the boundary became blurred as the result of the interaction of growing units from different origins, suggesting the inadequacy of the Avrami-type kinetic model, i.e., the expanding-and-overlapping sphere model. For the latter case a calculation of kinetic equations was made (with simple assumptions) assuming growth of a one-dimensional unit which branches in succession. Crystallization curves similar to those derived from current theories were obtained without restriction on the constant (i.e., without integer constant).     

The crystallization kinetics studies of the two crystallization stages of As37.5Se37.5Ag25 glass using the model-fitting and model-free approaches

The kinetic transformation parameters of the overlapped crystalline peaks of As_37.5Se_37.5Ag₂₅ glass were examined through the model-fitting and model-free approaches. The analysis of the crystallization kinetic parameters of the studied glass utilizing the model-free approaches showed that the activation energy of crystallization is not constant but change with the conversion fraction. These results reflect the changes of the reaction mechanism and reveal that the free model is more realistic than model-fitting approach which assumes a constant mechanism. Furthermore, the present results showed that the conversion process of the first and second crystallization peak of As_37.5Se_37.5Ag₂₅ glass may be described by Avrami-Erofeev model An (g(χ) = [−ln(1−χ)]1/n) with n = 2 and 3, respectively. Comparing the experimental differential scanning calorimetry data with calculated ones indicated that Sestak-Berggren (SB) model is more realistic for describing the crystallization mechanism of both crystallization peaks of the studied composition.     

Thermal kinetics and short range order parameters of Se80X20 (X = Te,Sb) binary glasses

Bulk Se₈₀Te₂₀ and Se₈₀Sb₂₀ glasses were prepared using the melt–quench technique. Differential scanning calorimetry (DSC) curves measured at different heating rates (5 K/min≤α≤50 K/min) and X-ray diffraction (XRD) are used to characterize the as-quenched specimens. Based on the obtained results, the activation energy of glass transition and the activation energy of crystallization (E _g, E _c) of the Se₈₀Te₂₀ glass are (137.5, 105.1 kJ/mol) higher than the corresponding values of the Se₈₀Sb₂₀ glass (106.8, 71.2 kJ/mol). An integer n value (n=2) of the Se₈₀Te₂₀ glass indicates that only one crystallization mechanism is occurring while a non-integer exponent (n=1.79) in the Se₈₀Sb₂₀ glass means that two mechanisms are working simultaneously during the amorphous–crystalline transformations. The total structure factor, S(K), indicates the presence of the short-range order (SRO) and the absence of the medium-range order (MRO) inside the as-quenched alloys. In an opposite way to the activation energies, the values of the first peak position and the total coordination number (r ₁, η ₁), obtained from a Gaussian fit of the radial distribution function, of the Se₈₀Te₂₀ glass are (2.42 nm, 1.99 atom) lower than the corresponding values (2.55 nm, 2.36 atom) of the Se₈₀Sb₂₀ specimens.     

Optical and electrical properties of carbon nanotube-containing Se85Te10Ag5 glassy composites

The infrared optical transmission spectra of 3 and 5 wt.% of carbon nanotube (CNT)-containing Se₈₅Te₁₀Ag₅ glassy composites were recorded in the mid-IR region from 4000 to 400 cm^?1 using Fourier transform infrared spectroscopy (FTIR). The FTIR spectra manifested functional groups interacting the CNT surface and the Se₈₅Te₁₀Ag₅ glassy matrix. Photoluminescence (PL) spectra of the CNT-containing Se₈₅Te₁₀Ag₅ glassy composites exhibited PL emission peaks centred within the range 715–725 nm. The optical absorption spectra differed between pure Se₈₅Te₁₀Ag₅ glass and the CNT-containing Se₈₅Te₁₀Ag₅ glassy composites. The electrical conductivity of CNT-containing Se₈₅Te₁₀Ag₅ glassy composites was enhanced by four orders of magnitude with the CNT additive. This can be attributed to the formation of many conductive paths through the CNT–CNT internanotube percolation network.     

Elucidation of the crystallization kinetics for sodium-alumino-silicate glasses containing different amounts of manganese oxide

The glass samples [40SiO₂?+?5Al₂O₃?+?{55???x}Na₂O?+?xMnO₂] where x?=?0.05, 0.2, 0.4, 0.6, 0.8, and 1?mol% MnO₂ before and after being heat treated were subjected to X-ray diffraction. The diffraction lines provided clear evidence of the nucleation and growth, which are characteristic of sodium silicate phase. Crystallization studies were conducted using differential thermal analysis. Crystallization peak temperatures were identified and the transformed fractions were determined. Both the rate of growth, K ₀, and the activation energy, E, depend on the influence of manganese ions in the glass network as a modifier or as a former and the manganese content. The values of the Avrami parameter, n, were calculated using two methods and were in excellent agreement. The process of nucleation and growth rate depends on the manganese content.     

Amorphous to crystalline phase transition in glassy Se65Te20Ag15 alloy

In the present work, the amorphous to crystalline phase transition of chalcogenide glass Se₆₅Te₂₀Ag₁₅ has been studied using differential scanning calorimetric (DSC) measurements. The heating rate dependence of crystallization peaks has been used for the determination of activation energies of glass transition (E _g) and crystallization (E _c). Different non-isothermal methods have been used for this purpose. Other useful kinetic parameters such as the order parameter (n), the numerical factor of crystallization mechanism (m) and the frequency factor (K _o) of the rate constant (K) have been also determined.     

Behaviour of ultrasonic velocities in amorphous Se90Ge10 and Se85Ge15 alloys near their glass transition

Precise measurements of 10 MHz frequency longitudinal and shear wave velocities are reported in amorphous SeGe alloys near their glass transition temperature T _g . There is a sharp decrease of the velocities near T _g , but the reduction in velocities appears smaller than expected.     

Thermal stability and crystallization kinetics of Ge13In8Se79 chalcogenide glass

Ge-In-Se system, similar to many other chalcogenide glasses, has attracted much attention due to its interesting physical properties and applications. This article reports thermal analysis and estimation of activation energies of the glass transition and amorphous-crystallization transformation of Ge₁₃In₈Se₇₉ chalcogenide glass. The kinetic parameters were investigated under a non-isothermal condition at different heating rates (7–40?K/min) using differential scanning calorimetric (DSC) technique. The amorphous nature of the samples was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The activation energy was calculated from DSC data using five isoconversional methods: Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), Tang, Starink, and Vyazovkin. The results confirm that the activation energy of crystallization varies and depends on the degree of crystallization as well as temperature. It was also observed that the transformation from amorphous to the crystalline structure is complex involving different mechanisms of nucleation, diffusion, and growth.     

Effect of WO3 on the glass transition and crystallization kinetics of borotellurite glasses

Tellurite glasses of the system xWO₃–75TeO₂–(25 ? x)B₂O₃ (0 ≤ x ≤ 25 mol. %) were prepared and studied by differential thermal analysis to explore the effect of WO₃ on their glass transition and crystallization kinetics. The crystallization kinetics was studied under non-isothermal conditions using the formal theory of transformations for heterogeneous nucleation. The crystallization results were analyzed and both the activation energy of the crystallization process and the crystallization mechanism characterized. The phases into which the glass crystallizes were identified by X-ray diffraction. Diffractograms of the transformed material indicate the presence of microcrystallites of α-tellurite, Te_0.95W_0.05O_2.05, Te₂W and B₂O₃ in the amorphous matrix.     

The Cu and Te coordination environments in Cu-doped Ge–Te glasses

The structure of Ge₂₀Te₈₀, Ge₁₅Cu₈Te₇₇ and Ge₁₅Cu₅Te₈₀ glasses was investigated by diffraction techniques and extended X-ray absorption fine structure measurements. Large structural models were generated by fitting experimental data by the reverse Monte Carlo simulation technique. In Ge₂₀Te₈₀ glass, both Ge and Te obey the 8−N rule, and the structure is built up of GeTe₄ tetrahedra connected via Te–Te bonding or shared Te atoms connected to two Ge atoms. The coordination number of Te is significantly higher than 2 in Ge₁₅Cu₈Te₇₇. The average coordination number of Cu is 3.41±1 in this alloy. In Ge₁₅Cu₅Te₈₀ glass, Cu binds mostly to Te, while Cu–Cu bonding is significant in Ge₁₅Cu₈Te₇₇.     

Crystallization kinetics and thermal stability of Se98-x Zn2In x chalcogenide glasses

In this paper, we report on the non-isothermal crystallization kinetics of Se_{98 ?x} Zn₂In _x (0 ≤ x ≥ 10) chalcogenide glasses. The onset crystallization activation energy E _c, peak crystallization activation energy E _p and overall crystallization activation energy E have been determined by different approaches. The values of E _c, E _p and E have minima at a composition corresponding to 6 at% In. However the stability factor S and the crystallization rate constant stability factor K have a maximum and a minimum, respectively, at in the same composition. The nucleation and growth order parameter n, and the dimension order parameter m, are also determined and discussed for the present glasses.     

氟锆酸盐光纤玻璃中的析晶和分相现象

氟化物光纤玻璃中的夹杂物,分相颗粒和微晶颗粒等缺陷,造成光纤的散射损失,影响了实际制备工作.本文利用大视场光学显微镜,X射线衍射,透射电子显微镜和差热分析,研究了ZBLA氟化物玻璃中的分相和析晶现象,观察到了以成核生长机理进行的分相颗粒,析晶颗粒的成分是β-BaF_2·ZrF_4和α-BaF_2·2ZrF_4,通过DTA分析确定了这些相的析晶活化能.     

The effect of indium additive on crystallization kinetics and thermal stability of Se-Te-Sn chalcogenide glasses

The crystallization kinetics of bulk Se_90−xTe₅Sn₅In_x (x=0, 3, 6 and 9) multi-component chalcogenide glasses have been studied using differential scanning calorimetry (DSC) with heating rates 5, 10, 15 and 20 K/min under non-isothermal conditions. Values of various kinetic parameters of crystallization, such as onset crystallization temperature (T_c), peak crystallization temperature (T_p), activation energy of crystallization (E_c), rate constant (K_p), Hruby number (K_gl) and the order parameter (n) have been determined. It was found that activation energy of crystallization and rate constant (K_p) are minimum at 9 at% In. On the basis of the obtained experimental data the temperature difference (T_c−T_g) and K_gl increase with In concentration, which further indicates that 9 at% In glass is most thermally stable in the entire composition range of investigation.     

Role of Bi incorporation on glass transition kinetics in glassy Se78Te20Sn2 alloy

In this communication, we report the results of calorimetric measurements on the samples of recently synthesized multi-component glassy alloys of Se_78?xTe₂₀Sn₂Bi_x (0 ≤ x ≤ 6) system. For calorimetric study of glass transition kinetics, differential scanning calorimetry (DSC) technique has been used in non-isothermal mode. Peak glass transition temperature (T_g) is determined using the DSC scans. Kinetic parameters A and B of glass transition are determined using heating rate dependence of T_g. Activation energy of glass transition (E_g) has been calculated using Moynihan and Kissinger methods. Glass-forming ability and thermal stability are also determined using Hurby and Saad–Poulin relations, respectively.     

Nb添加对Cu-Zr非晶合金玻璃转变和晶化动力学的影响

采用铜模吸铸法制备出直径为2 mm的Cu_50.3Zr_49.7-xNb_x(x=0,2)大块非晶合金，利用示差扫描量热分析(DSC)研究了2at%Nb元素添加对Cu-Zr非晶合金玻璃转变动力学和晶化动力学的影响，发现含Nb合金具有较低的脆性指数，和较高的晶化激活能.这表明微量Nb的添加提高了该二元Cu基非晶合金过冷金属液相的热稳定性，从而有利于其非晶合金的形成. 关键词： Cu-Zr非晶合金 Nb添加 玻璃转变动力学 晶化动力学     

Annihilation kinetics for triplet excitations in organic glasses

The results of investigations of delayed luminescence decay are presented for disordered phenanthrene in the microsecond, millisecond, and second time ranges. At liquid nitrogen temperature, the highly non-exponential decay of both the phosphorescence and the annihilation delayed fluorescence is observed. This character of decay is caused by the relaxation of the electron-excitation energy in the system of energy-disordered centers when a quantity kT is much less than the width of the distribution for the excited-state energy. At the same time, the analysis of the time dependence for the triplet-triplet-annihilation rate exhibits two time intervals in which different kinetics is observed for the triplet excitations annihilation. In the microsecond range, the classical relationship between the phosphorescence and the delayed fluorescence takes place for the system under investigation; i.e., the triplet-triplet-annihilation rate is constant. At large times, the reaction-rate time dependence is described by the power law characteristic of inhomogeneous and low-dimensional systems. When the temperature increases, a transition to the classical behavior is observed throughout the entire time interval.     

Electrical and Galvanomagnetic Properties of Extruded Samples of Bi85Sb15 Solid Solutions with Pb and Te Impurities

Russian Physics Journal - The effect of tellurium impurities on the electrical conductivity (σ), Seebeck coefficient (α), and Hall coefficient R of extruded Bi85Sb15 + 0.001 аt.% Pb...