Free Access
Issue
Eur. Phys. J. Appl. Phys.
Volume 90, Number 3, June 2020
Article Number 31101
Number of page(s) 5
Section Physics and Mechanics of Fluids, Microfluidics
DOI https://doi.org/10.1051/epjap/2020200099
Published online 14 July 2020

© EDP Sciences, 2020

1 Introduction

ChGs are one of the potential candidates for memory applications as rewriteable optical data storage because of phase changing nature. Nowadays, they are frequently utilized in non-volatile memory devices [13]. Since Cp is a noteworthy thermodynamic property which is found useful to acquire the information related to temperature-induced structural relaxation in ChGs [4,5], therefore it is essential to analyze this parameter in GFR. The measurements of heat capacity in ChGs report that the sudden variation in Cp in GTR is an inherent characteristic of ChGs. Thus, the study of structural relaxation has a technological aspect because of the variation in the several physical properties near the GTR [68].

A liquid is said to be in a quasi-equilibrium (meta-stable) state when the temperature T of the state lie between the melting temperature Tm and the glass transition temperature Tg (i.e., Tg < T < Tm ). This state is defined as the undercooled liquid state under which the time-scale for the relaxation of molecular movements is comparable to that of the experiment period in GTR. Thus, the diffusive motion of the liquid ceases and it falls into thermal equilibrium. At a temperature T (<Tg ), the glass is believed to be in a metastable owing to completely frozen molecular movements. Such a state is related to the factor of instability having dependence on the thermodynamic potentials between the corresponding non-crystalline and crystalline phases. Whenever external perturbation is absent, the amorphous material relaxes nearer to equilibrium. The structural relaxation is concerned about the alterations in the macroscopic parameters like Cp and ΔH, etc [914]. To use these materials in optical fibre application, it is essential to study the mechanical properties i.e., hardness, for understanding deformation and fracture mechanisms of bulk materials and their samples in thin-film form. There are several techniques that are utilized for the measurement of hardness, but we concern with the most reliable and common technique, Vickers hardness tester for the measurement of micro-hardness. Several attempts have been done by various research groups to determine the thermo-dynamical and thermo-mechanical properties on quaternary series but no one can report the investigation of these properties on Ge containing quaternary series [1519]. This paper reports the determination of specific heat value (above and below the glass transition temperature), hardness, and other thermo-mechanical and thermo-dynamical parameters for novel quaternary glassy Se78-xTe20Sn2Gex (x = 0, 2, 4, 6) alloys, to see the effect of Ge incorporation in Se78Te20Sn2 system. We have chosen the quaternary Se-Te-Sn-Ge system by keeping in mind the exciting and significant reports of Se-Te-Ge and Se-Ge-Sn systems [2024].

2 Experimental

The glassy alloys were obtained by employing the melt-quench method. The details of this technique are available in the literature [410]. The specific heat measurements of each sample have been done using differential scanning calorimeter (TA instrument, USA; Model: MDSC 2920) in which a certain amount (i.e., 5–10 mg) of materials are heated at a fixed heating rate (30 °C/min) as shown in Figure 1. The procedure used for the calculation of Cpe and Cpg is shown in Figure 2a and their values are given in Table 1.

thumbnail Fig. 1

Variation of Cp with temperature in present ChGs.

thumbnail Fig. 2

Plots showing the (a) calculation of Cpg and Cpe , (b) composition dependence of Hv and Vh , (c) variation of Hv and Vh with molar volume Vm , and (d) variation of Hv and Vh with compactness δ.

Table 1

Values of various thermo-dynamical and thermo-mechanical parameters.

3 Results and discussion

The glass transition enthalpy ΔH is proportional to the area under the curve of the endothermic peak appeared in GTR and it is determined by the following expression:(1)

Here, ξ represents the instrumental constant of the DSC unit employed. To determine ξ, we melt some benchmark materials in the DSC cell and then measure the entire area corresponding to the endothermic peak appeared as a signature of their melting. Generally, the indium and zinc elements of exceptional purity were used because of the values of their enthalpies of fusion are well-known. Also, A is the area under the curve of the endothermic peak appeared in GTR and m is the mass of the pan containing the powdered sample. The calculated values of ΔH are given in Table 1. The increment in ΔH can be explicated by considering the heat of the atomization HA of Se and Ge. Since Ge (HA  = 377 kJ/mol) has been incorporated at the cost of Se (HA  = 227 kJ/mol), therefore the average heat of atomization of quaternary alloys is higher than that of the parent ternary alloys.

For hardness measurements, we used Vickers hardness tester in which the Vickers hardness number Hv for each sample is calculated by applying load 100 gm for 20 seconds and measuring the average diagonal (d) of each indentation marks (as shown in Fig. 3) obtained by the penetration of diamond pyramidal indenter which included angle θ = 136° between opposite faces [16,19]. Hv is the ratio of applied load P (in kg) and the superficial area (in mm2) of the indentation as given in expression:(2)

The diagonals of each indentation were measured directly by using a micrometer eyepiece whose calibration was done with the help of an ERMA disc or manually using a state of the art software. The diagonal length of each indentation marks for different samples are given in Table 1. The other parameters like micro-voids volume (Vh ), the creation energy of micro-void (Eh ), and elasticity modulus (E) are determined by using the formulae:(3) (4) (5)

The derivations of these expressions are discussed in reference [18]. In the framework of the free volume concept, some researchers [2527] studied the interrelation between the thermal properties (e.g., Glass Transition Temperature, Thermal Expansion Coefficient) and thermo-mechanical properties (Poisson Ratio, micro-hardness, modulus of elasticity, etc). Using the correlation between the fluctuation free volume model and the model of soft atomic configurations, they derived the above three expressions.

We employed the Archimedes principle for calculating the density of each glassy alloy of the present system by using the following equation:(6)

Here, Wa , Wl ,, and ρl are the weight of the sample in air, liquid, and density of reference liquid (distilled water) respectively. The molar volume (Vm ) is determined by employing equation (7) and the compactness of samples by equation (8) [19,28] as follows:(7) (8)

Here, Mi , xi , and ρi are the molecular weight, atomic weight percentage, and density of ith element of the sample respectively. The calculated values of thermo-mechanical parameters are shown in Table 1.

From Table 1, one can see that the value of specific heat (Cpg and Cpe ) decreases with increasing Ge concentration in parent ternary alloy. However, the enthalpy of the system increases with Ge concentration. In ChGs, the accessible energy is inadequate to permit the molecules to relax cooperatively by coordinate their mobility. When the temperature is increased, the molecular species interact with each other and the system needs additional energy for their cooperative motions. This is observed as a jump in the Cp values in the glass transition region. However, such cooperative motions are frozen because the glass network of parent glass SeTeSn becomes more rigid after the addition of Ge. Probably, this is the reason for the lower values of Cpg and Cpe in quaternary SeTeSnGe glasses as compared to parent SeTeSn glass. This can also be understood in terms of atomic masses of Ge (72.6 g/mol) and Se (78.9 g/mol) atoms. The Ge atoms enter in the glass matrix at the cost of Se atoms and so the mean atomic mass of quaternary alloys is lesser than that of the parent ternary glass. Thus, less heat is required for the movements of the molecular species.

The density, elasticity (E), and micro-hardness are increased with Ge concentration. Consequently, we observe that the micro-void volume and molar volume are decreased with Ge concentration. The variations of micro-hardness and micro-void volume with coordination number <Z>, Molar volume Vm and compactness are shown in Figure 2(b,c,d), respectively. Thus, the micro-void volume follows the increasing trend which is just opposite to increasing trends of micro-hardness. It is interesting to note that the density increases while the molar volume decreases with the increasing Ge incorporation.

thumbnail Fig. 3

Micrographs showing indentation marks on the bulk samples of STSG system.

4 Conclusions

Thermal analysis shows a correlation between the mean atomic mass of the alloys and their Cpg and Cpe values. The values of the equilibrium specific heat Cpe and glass specific heat Cpg are decreased significantly after the incorporation of Ge. Further investigation reveals that the micro-hardness and density increase with Ge incorporation in parent ternary alloys. This is clarified in terms of the average heat of atomization. The compactness increases because of a decrease in micro-void volume followed by a decrease in molar volume. The enthalpy of the investigated sample also increases with Ge incorporation. The compositional dependence of the thermo-mechanical parameters shows a reversal in the trend at 4 at. wt.% of Ge.

Author contribution statement

Dipti Sharma and John C. MacDonald carried out the calorimetric experiment. Shiv Kumar Pal carried out the hardness experiment. Neeraj Mehta wrote the manuscript with support of Shiv Kumar Pal.

Acknowledgments

NM is grateful to the CSIR, New Delhi, India for providing financial assistance under a major project (Scheme No. 03(1453)/19/EMR-II).

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Cite this article as: Shiv Kumar Pal, Neeraj Mehta, John C. MacDonald, Dipti Sharma, Composition dependence of thermo-dynamical and thermo-mechanical properties in SeTeSnGe chalcogenide glasses (ChGs), Eur. Phys. J. Appl. Phys. 90, 31101 (2020)

All Tables

Table 1

Values of various thermo-dynamical and thermo-mechanical parameters.

All Figures

thumbnail Fig. 1

Variation of Cp with temperature in present ChGs.

In the text
thumbnail Fig. 2

Plots showing the (a) calculation of Cpg and Cpe , (b) composition dependence of Hv and Vh , (c) variation of Hv and Vh with molar volume Vm , and (d) variation of Hv and Vh with compactness δ.

In the text
thumbnail Fig. 3

Micrographs showing indentation marks on the bulk samples of STSG system.

In the text

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