Glassy Se Research Articles

Calorimetric studies of thermal crystallization in glassy Se80−xTe20Snx (0⩽x⩽10) alloys

This paper reports a calorimetric study of the crystallization kinetics in glassy Se80−xTe20Snx (0⩽x⩽10) alloys in terms of kinetic parameters of crystallization (crystallization temperature Tc, activation energy of crystallization Ec and order parameter n) and thermodynamic parameters of crystallization (enthalpy of crystallization Hc and the entropy of crystallization Sc).

The behaviour of the As 20Sb 20Se 60 melt depending on the mode of cooling

Bulk chalcogenide glasses are usually prepared by equilibrium melt cooling. An equimolar stoichiometric (As 2Se 3) 50(Sb 2Se 3) 50 melt equilibrated at 850 °C was used and either quenched or slowly cooled. The basic thermoanalytical behaviour was studied and compared with the equilibrium phase diagram. Markedly different results were obtained. In the former case, the frozen state was completely glassy. On the base of DSC and Raman spectroscopy, the frozen melt structural units are close to the structural units in Sb 2Se 3, As 3Se 2 and glassy Se. In the latter case, one can conclude that the frozen state is mixture of both crystalline Sb 2Se 3 and the glass (instead of crystalline phase) close to the eutectic composition, i.e. As 35.2Sb 4.8Se 60.0. It means that at 850 °C the basic structural units of As 2Se 3 (trigonal pyramids AsSe 3/2) are no longer stable and decompose into As 3Se 2 and selenium. The cooling rate decides whether those products will be frozen in the structure (former case) or recombination of decomposition products take place (latter case).

Effect of some metallic additives (Ag, Cd, and Zn) on the crystallization kinetics of glassy Se 70Te 30 alloy

The present work reports the effect of some additives (Ag, Cd, and Zn) on the crystallization kinetics of glassy Se 70Te 30 alloy in terms of kinetic parameters of crystallization. Differential scanning calorimetry (DSC) technique is used for this purpose in non-isothermal mode. Various kinetic parameters (crystallization temperature T c , activation energy of crystallization E c , rate constant K, and order parameter n) are calculated using the non-isothermal methods.

Role of metallic Zn and In additives in the electrical and dielectric properties of Se_{0.85}Te_{0.15} glassy alloy

In this paper we report the effect of metallic Zn and In additives on the electrical and dielectric properties of Se_{0.85}Te_{0.15} glassy system. The temperature and frequency dependence of the dielectric parameters in glassy Se_{0.85}Te_{0.15}, Se_{0.75}Te_{0.15}In_{0.10} and Se_{0.75}Te_{0.15}Zn_{0.10} alloys are studied by measuring capacitance and dissipation factor in the frequency range (1 kHz--5 MHz) and temperature range (300--350 K). Direct current (dc) measurements were also performed in the aforesaid temperature range to assess the dc conduction losses. Results are interpreted in terms of a theoretical model which is based on two electron hopping over a potential barrier and is applicable in the present case. Dielectric constant \varepsilon^{\prime}, dielectric loss \varepsilon^{\prime\prime} and loss tangent \delta are found highly frequency and temperature dependent. Dependence of electrical and dielectric parameters on the additives has also been found in the present glassy system and has been discussed in terms of electronegativity difference between the elements used in making the aforesaid glassy system.

Dependence of activation energy and pre-exponential factor on audio frequency in glassy Se80−xTe20Snx alloys

In the present paper, we have studied the temperature and frequency dependence behaviours of ac conductivity in glassy system Se80−xTe20Snx (x=0, 2, 4, and 6). All the measurements have been done in the frequency range (1–500KHz) and in the temperature range from room temperature to glass transition temperature Tg. We have found that activation energy and pre-exponential factor both vary with audio frequency according to Meyer–Neldel's relation (MNR).We have investigated MNR by two different approaches. In first approach, the temperature dependence of ac conductivity is studied at four different audio frequencies without changing the composition of the glassy system. In the second approach, the composition itself varies at a particular audio frequency. The results are explained in terms of compensation effect in relaxation time and the multi-excitation character of the processes suggested by Yelon and Movaghar responsible for MNR in chalcogenide glasses.

Crystallization process analysis for Se0.95In0.05 and Se0.90In0.10 chalcogenide glasses using the contemporary isoconversional models

Abstract Carrying out crystallization studies for both Se0.95In0.05 and Se0.90In0.10 chalcogenide glasses under non-isothermal conditions at different heating rates, it was realized that a rate controlling process occurs where random nucleation of one- to two-dimensional growth is accompanied with the introduction of up to 10 at% In into glassy Se matrix. The crystallization kinetics together with its dimensionality has been studied using the four currently used isoconversional models (Kissinger–Akahira–Sunose, Ozawa–Flynn–Wall, Tang, and Starink). The activation energy of crystallization (Ec) has been determined using these indicated four models where a satisfactory concurrence is achieved. The value of Ec shows a decrease while increasing both the In-content as well as the extent of crystallization.

On the crystallization kinetics of In additive Se–Te chalcogenide glasses

The calorimetric parameters of glassy Se 85− x Te 15In x ( x = 0, 2, 6 and 10) alloys have been investigated using Differential Scanning Calorimetry (DSC) in non-isothermal conditions at different heating rates of 5, 10, 15 and 20 °C/min. It is observed that in these glasses, the glass transition temperature, the onset crystallization temperature and the peak temperature of crystallization are found to be dependent on the composition and heating rates. The glass transition activation energy and crystallization activation energy have been determined using different empirical approaches. The kinetic analysis of the crystallization peak has been performed using Matusita's model. The values of Avrami exponent ( n) and activation energy of crystallization ( E c) are evaluated. The validity of Matusita's model is ascertained by comparison with the results obtained by Kissinger model and Augis and Banett method as well as their approximations. The activation energy of crystallization is determined by analyzing the data using the isoconversional methods. The present study shows that the activation energy of crystallization varies with the degree of conversion and hence with temperature. Thermal stability and glass forming tendency have also been studied for different heating rates.

Role of Cu additive on the dielectric relaxation of Se75Te25and Se85Te15glassy alloys

The effect of Cu additive on the dielectric relaxation of two binary Se-Te glassy systems, comparing the properties of a-Se 75 Te 25 , a-Se 85 Te 15 and a-Se 75 Te 15 Cu 10 alloys has been reported. The temperature and frequency dependence of dielectric parameters in Glassy Se 75 Te 25 , Se 85 Te 15 and Se 75 Te 15 Cu 10 alloys are studied by measuring capacitance and dissipation factor in the frequency range (1 kHz-5 MHz) and temperature range (300-350 K). A Debye like relaxation of dielectric behavior has been observed. A comprehensive study on the relaxation mechanism revealed that the presence of grains and grain boundaries across the pallet thickness is the basic relaxation process. A detailed analysis shows that the observed dielectric loss is in agreement with the Guintini's theory of dielectric dispersion based on two electron hopping over a potential barrier and is applicable in the present case. Dielectric constant ( e ’), dielectric loss ( e ”), loss tangent ( δ ) and capacitive reactance ( X c ) are found highly frequency and temperature dependent. Dependence of these dielectric parameters over the metallic impurity Cu, has also been found in the present glassy system and has been discussed in terms of electronegativity difference between the elements used in making the aforesaid glassy system.

Further Meyer–Neldel Rule for Thermally Activated High Field Conduction in Thin Films of Glassy Se100−xSbx Alloys

In the present work, we report the applicability of the further Meyer–Neldel rule in thin films of glassy Se100−xSbx alloys for thermally activated high field conduction using two different methods. In first case, the temperature dependence of dc conductivity is studied at different electric fields without changing the composition of the glassy system, whereas, in the second case, the composition itself varies at a particular electric field. A strong correlation between prefactor σ00 and Meyer–Neldel energy in both cases is also observed. The observation of the correlation between prefactor σ00 and Meyer–Neldel energy can be described by multiple excitations stimulated by phonon energy as described by Yelon and Movaghar.

Further MN rule in thermally activated a.c. conduction of Se–Te–Sb chalcogenide glasses

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se 70Te 30− x Sb x (4 ≤ x ≤ 10) alloys. The observation of Further Meyer–Neldel rule in case of a.c. conductivity is reported. The observation of the correlation between Meyer–Neldel pre-factor σ 00 and Meyer–Neldel energy is explained by multiple excitations stimulated by optical phonon energy as described by Yelon and Movaghar.

Calorimetric studies of glass transition phenomenon in glassy Se80−xTe20Snx alloys

Glass transition kinetics has been studied in a glassy Se80−xTe20Snx (0 ⩽x ⩽10) system. Differential scanning calorimetry (DSC) runs were taken at four different heating rates (5, 10, 15 and 20 K min−1) for this purpose. The heating rate dependence of the glass transition temperature (Tg) has been studied using different non-isothermal methods. The activation energy of glass transition (Eg) has been evaluated using Kissinger's relation and the theory of Moynihan. The composition dependence of Tg and Eg has also been discussed.

Optical nonlinearities of Se-loaded zeolite (ZSM-5): A molded nanowire system

Nonlinear optical properties of Se-loaded ZSM-5 single crystals have been studied comparatively with those of glassy Se. Two-photon absorption coefficients and intensity-dependent refractive-index of the Se-zeolite are higher by three orders of magnitude than those of glassy Se. The enhancements can be ascribed to electron confinement in single Se chains.

Comments on the validity of Meyer-Neldel rule for AC conduction in glassy alloys

In a recent publication (Glass Phys. Chem., 2008, vol. 34, no. 1, pp. 42–46) the Meyer-Neldel rule was found to hold in the case of ac conductivity of glassy Se100 − x Te x (x = 10, 20, 30) alloys and a tentative explanation was forwarded. Here, we suggest an alternative explanation of this rule on the basis of a thermodynamical model that interrelates point defect parameters with bulk properties.

Effect of wide range of heating rate on the crystallization kinetic parameters of Se 77Te 20Sb 3 glass

The kinetic parameters of glassy Se 77Te 20Sb 3 under non-isothermal conditions are analyzed by the model-fitting and model-free approaches. Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Se 77Te 20Sb 3 glass using a wide range of heating rates (4–90 K/min). The analysis of the present data shows that the activation energy of crystallization is not constant but varies with the degree of conversion and hence with temperature. The reaction model that may describe the crystallization process of the Se 77Te 20Sb 3 glass is model A2 (Avrami–Erofeev with n = 2) for heating rates 4–30 K/min. While, the reaction model A3/2 (Avrami–Erofeev with n = 1.5) is a proper model that may describe crystallization process of the present glass for higher heating rates (35–90 K/min). The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (two-dimensional growth) is responsible for the crystallization process for heating rates (4–30 K/min) and two mechanisms (one- and two-dimensional growth) are working simultaneously during the amorphous–crystalline transformation of glassy Se 77Te 20Sb 3 for higher heating rates (35–90 K/min). A good agreement between the experimental and the reconstructed ( α– T) curves has been achieved. The transformation from amorphous to crystalline phase in glassy Se 77Te 20Sb 3 demonstrates complex multi-step involving several processes.

Further MN rule for thermally activated high field conduction in bulk samples of glassy Se 100− xSb x alloys

In the present paper, we report the observation of the Meyer–Neldel rule (MNR) in thermally activated high field conduction in bulk samples of glassy Se 100− x Sb x alloys. We have investigated Meyer–Neldel rule by two different approaches. In first approach, the temperature dependence of d.c. conductivity is studied at different electric fields without changing the composition of the glassy system. In the second approach, the composition itself varies at a particular electric field. We found that the conductivity obeys the Meyer–Neldel rule as the pre-exponential factor depends on activation energy. We also found a strong correlation between pre-factor σ 00 and Meyer–Neldel energy in both cases. The observation of the correlation between pre-factor σ 00 and Meyer–Neldel energy can be described by multiple excitations stimulated by phonon energy as described by Yelon and Movaghar.

Structural interpretation of specific heat measurements on glassy Se100− x Sb x alloys

Differential scanning calorimetry was used to study glassy Se100- x Sb x (x = 2, 4, 6, 8 and 10) alloys at a heating rate of 10°C min−1 under non-isothermal conditions. An extremely large increase in the specific heat, Cp , was observed at the glass-transition temperature. It was also found that the values of Cp below and above the glass-transition temperature, Cpg and Cpe , respectively, are highly composition dependent. This indicates that the Sb additive used in the present study influences the structure of the a-Se. The variation of Cp reveals local extrema in the Se–Sb glassy system at x = 4 and x = 8. The composition dependence of both Cpe and Cpg is explained in terms of the atomic structure.

Effect of Zn incorporation on the a.c. conductivity of glassy Se70Te30alloy

The present work reports the temperature and frequency dependence of a.c. conductivity in glassy Se 70 Te 30− x Zn x ( x = 0, 2, 4 and 6) alloys in the temperature range 300–500 K and frequency range 1 kHz. An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of the present samples can be successfully explained by correlated barrier hopping (CBH) model. The density of defect states has been determined using this model for all the glassy alloys. The results show that bipolaron hopping dominates over single-polaron hopping in this glassy system. This is explained in terms of lower values of the maximum barrier height for single-polaron hopping.

Composition dependence of the activation energy of the glass transition in some chalcogenide glasses

The activation energy (E t) of the glass transition process has been obtained for glassy Se80− x Te20M x (M = Ag, Cd, In, Sb) and Se100− x M x (M = In, Te, Sb) systems using differential scanning calorimetry. The composition dependence of E t is explained in terms of fragility index for the systems. A reversal in the trend of the E t is explained using topological models.

Kinetics of crystallization in glassy Se 70Te 30−xZn x using DSC technique

Calorimetric studies have been performed at different heating rates under non-isothermal conditions to determine the activation energy of crystallization ( E c) in glassy Se 70Te 30− x Zn x ( x=2, 4, 6, 8) using the well-known Kissinger's relation, Matusita–Sakka theory and the method of Augis and Bennett. The activation energy is found to be highly dependent on the Zn concentration. The rate of crystallization is found to be lowest for glassy Se 70Te 22Zn 8 alloy. The thermal stability ( T c− T g) is, however, found to be highest for this composition. This shows that the thermal stability is better in samples where the rate of crystallization is low.

Dielectric parameters in Se 70Te 30 and Se 70Te 28Zn 2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant ( ε′) and dielectric loss ( ε″) are studied in glassy Se 70Te 30 and Se 70Te 28Zn 2. The measurements have been made in the frequency range (8–500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case. No dielectric loss peak is observed in glassy Se 70Te 30. However, such loss peaks exist in the glassy Se 70Te 28Zn 2 in the above frequency and temperature range. The Cole–Cole diagrams have been used to determine some parameters such as the distribution parameter ( α), the macroscopic relaxation time ( τ 0), the molecular relaxation time ( τ) and the Gibb's free energy for relaxation (Δ F).