Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-018-7309-5 (0123456789().,-volV)(0123456789().,-volV)

Effect of antimony on glass transition and thermal stability of Se782xTe18Sn2Sbx (x = 0, 2, 4 and 6 at.%) multicomponent glassy alloys Vandita Rao1 • N. Chandel2 • N. Mehta2 • D. K. Dwivedi1 Received: 13 July 2017 / Accepted: 16 April 2018  Akade´miai Kiado´, Budapest, Hungary 2018

Abstract Multicomponent glassy alloys Se78-xTe18Sn2Sbx (x = 0, 2, 4 and 6) have been synthesized using melt quench technique. The prepared samples have been characterized by X-ray diffraction technique and differential scanning calorimetry (DSC). Glass transition kinetics of Se78-xTe18Sn2Sbx (x = 0, 2, 4 and 6 at.%) glassy alloys has been examined using DSC. DSC runs have been recorded at different heating rates (5, 10, 15 and 20 K min-1) for each sample under investigation. Heating rate dependence of glass transition temperature (Tg) has been studied using Lasocka empirical relation. The activation energy of glass transition has been evaluated using Kissinger and Moynihan’s relation. The effect of antimony concentration on glass transition temperature and activation energy has been investigated in the prepared samples. Glass-forming ability and thermal stability of Se78-xTe18Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys have been monitored through the evaluation of thermal stability using Dietzal relation, Hurby parameter, and Saad and Poulin parameter. The abovementioned parameters are found to be compositionally dependent, which indicates that among the studied glass samples the stability is maximum for Sb at 2% content. Keywords Chalcogenide glass  DSC  Glass transition temperature  Activation energy  Thermal stability

Introduction Chalcogenide glasses (sulfur, selenium, tellurium and their alloys) have drawn much attention of researchers in the recent past because of their unique semiconductor characteristics. They have shown interesting properties that depend on the change of chemical composition, which allows their materials to be used in infrared laser power transmission, sensors, imaging and spectroscopy, and active and passive optical fibers [1]. Chalcogenide glasses are being reinvestigated nowadays to estimate the new technological applications, besides their current applications in memory devices, xerography, photoreceptors, & D. K. Dwivedi [email protected] 1

Amorphous Semiconductor Researcher Lab, Department of Applied Sciences, M. M. M. University of Technology, Gorakhpur 273010, India

2

Department of Physics, Institute of Sciences, Banaras Hindu University, Varanasi 221005, India

medical and thermal imaging, laser fiber techniques, solar cells, etc. [2, 3]. Chalcogenide glassy materials are IR transparent, have high refractive indices and low phonon energy, and moreover easy to fabricate, which makes such material a potential candidate to be employed in various kinds of solid state electronic devices [4–8]. Amorphous Se suffers from thermal instability, aging effect and low electrical conductivity due to structural relaxation and phase transformation, since it has low glass transition temperature [9]. The addition of Te–Se exhibits many advantages because the Se–Te binary alloys possess higher glass transition and crystallization temperature, higher photosensitivity, higher hardness, higher stability and lower aging effect in comparison to pure Se [10]. Also Se–Te binary alloys are found to be a potential candidate to be used as phase change material. This property allows this system to be used in optical memory devices and photonics [11]. Addition of Sn (semi-metallic elements) improves the property of the Se–Te binary system [12] and thus leads to widening the scope of application of new materials. When

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V. Rao et al.

Experimental A glassy alloy of Se78-xTe18Sn2Sbx (x = 0, 2, 4 and 6) was prepared by the melt quench technique. To synthesize the amorphous materials, high purity specimens (99.999% pure) were used. 5 N (99.999%) pure constituent elements (Se, Te Sn, and Sb) in appropriate quantities were weighed using an electronic balance (LIBROR, AGE-120) with the least count of 10-4 gm and sealed in an evacuated quartz ampoule at 10-5 torr, to avoid oxidation. The sealed

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ampoule containing the material was heated in a furnace at a rate of 100 C h-1 and was maintained at 900 C for 14 h. To obtain homogeneous glassy materials, the ampoules were constantly rocked during the heating process with the help of a rotating ceramic rod to which the ampoule was attached. The sealed ampoules with molten materials were rapidly quenched into the ice-cooled water to obtain bulk glasses. The ingots of glassy materials were taken out from ampoules by breaking them. The prepared glassy material was ground to make a fine powder for DSC studies. The structure of the prepared samples was investigated by X-ray diffraction analysis with Cu Ka target and Ni filter. The study of glass transition of the prepared samples was performed under non-isothermal measurements using DSC (Shimadzu DTA-50). These calorimetric measurements were performed mainly in a linear scanning mode using a sample of about 10 mg in an open alumina crucible under various heating rates (b) of 5, 10, 15 and 20 K min-1. The accuracy of the instrument temperature was ± 0.01 mW. Nitrogen gas was used as purge gas in the DSC measurements.

Results and discussions Structural analysis Figure 1 reveals the XRD patterns for Se74Te20Sn2Sb4 and Se72Te20Sn2Sb6 glassy alloys. The absence of sharp structural peaks in the diffraction pattern of the prepared glassy alloy confirms their amorphous nature. Similar

Se72Te20Sn2Sb6 Intensity/a.u.

Sn is added to Se–Te binary alloys, the glass properties of new glass so produced are improved [13]. It has been reported that the glass-forming region of Sndoped binary and ternary chalcogenide glasses is very narrow, which corresponds to a small concentration of Sn in the proposed alloys [14]. It has been reported that the thermal stability of Se-rich chalcogenide glasses increases with Sn additives [15, 16]. The thermal stability and glass-forming ability have been reported to increase appreciably when Bi, Pb and Ag are added to Se–Te–Sn glassy systems [17–19]. From literature survey, it is evident that appreciable attention has been paid to Se-based binary and ternary glasses, whereas very less effort has been made to study the synthesis and investigation of Se-based quaternary glasses which may further extend the utility of these glasses. In recent years, it has been reported that metallic additives cause an appreciable effect on the physical property of glassy network [20–22]. In the present work, Sb has been chosen as a chemical modifier in the Se–Te–Sn ternary system, which may further broaden the glassforming area and also create configurationally and compositionally disordered system with respect to ternary alloys that will provide a better understanding of the glass transition kinetics and thermal stability of the Se–Te–Sn– Sb system. From ongoing discussions, it can be concluded that glass transition, chemical stability, and glass stability are the important properties of glassy materials to be investigated for their phase change optical storage media. Keeping the aforesaid discussion in mind, the present work has been carried out to study the glass transition kinetics and thermal stability of Se78-xTe18Sn2Sbx (x = 0, 2, 4 and 6 at.%) glassy alloys. Sb was taken as the additive element at the cost of Se. Calorimetric studies were performed under non-isothermal conditions at different heating rates. The activation energy of glass transition was examined. The role of Sb incorporation in the glass transition mechanism and activation energy has been reported. The thermal stability of the proposed quaternary glassy alloys was analyzed using different parameters.

Se74Te20Sn2Sb4

20

40

60

80

Degree/2θ

Fig. 1 XRD pattern of Se74Te20Sn2Sb4 and Se72Te20Sn2Sb6 glassy alloys

Effect of antimony on glass transition and thermal stability of Se78-xTe18Sn2Sbx…

patterns were obtained for the rest of the prepared samples, which are not shown here.

Thermal analysis The DSC experiments were carried out at 5, 10, 15 and 20 K min-1 different heating rates to evaluate the phase transition and the level of thermal stability. Figure 2 shows the DSC curve for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys at 10 K min-1 heating rate. Similar patterns were obtained at rest heating rates (5, 15 and 20 K min-1), which are not shown here. The obtained DSC curves in Fig. 2 are characterized by three important peaks (i.e., two endothermic and one exothermic). Furthermore, the thermographs can be categorized into three main parts. The first endothermic (negative) peak corresponds to the Tg. This characteristic temperature shows a huge change of viscosity making a transition from amorphous solid phase to supercooled liquid state. The second characteristic peak, i.e., exothermic peak is associated with the crystallization process. This exothermic peak has three peculiar temperature points: the initial point of these peaks is termed as the onset crystallization temperature (To), the maxima of the peaks are labeled as the peak crystallization temperature (Tp) and the end point of an exothermic peak is designated as complete crystallization temperature (Tc). Along with the higher temperature side in the DSC trace, there is another endothermic peak which is assigned to the melting temperature of the crystallized phase. The obtained characteristic transition temperatures Tg, To, Tp, Tc, and Tm for the studied compositions at 5, 10, 15 and 20 K min-1 heating rates are reported in Table 1. x=6

Se78–xTe20Sn2Sbx

x=4 x=2

Tg ¼ To ðbÞy :

400

500

600

Temp/K

Fig. 2 DSC curves at 10 K min-1 for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys

ð1Þ

Equation (1) is an empirical relation in which (b)y is a multiplier and y is power of b. The value of Tg at b = 1 was evaluated theoretically. Here, y is given by y =log [(Tg)10/ (Tg)1]. (Tg)1 and (Tg)10 are the glass transition temperatures Tg at b = 1 and 10 C min-1, respectively. Equation (1) provides another proof for the increase of Tg with an increase in heating rates.

Heating rate dependence of glass transition temperature It has been observed that during the process of continuous heating, the characteristic temperatures are shifted toward a higher degree with an increase in heating rate, which is in accordance with Lasocka relation [26]. Such behavior is observed because the rate of change of heat is slower as compared to that of change in temperature; therefore, the temperature at which heat is supplied to the sample is increased by increasing the heating rate [27]. To study the heating rate dependence on Tg, Lasocka suggested an empirical relationship which is as follows: Tg ¼ A þ Bln(bÞ;

Endo

Exo

Heat flow/mW

x=0

It is clear from Table 1 that all characteristic temperatures are increased with heating rates as well as Sb content. The increase in heating rate results in an increase of potential barrier height, which is ultimately related to amorphous to crystalline phase transition, and may be responsible for increasing the characteristic temperature with heating rate [23]. The occurrence of high-intensity exothermic peak may be used to identify fast crystallization processes from heterogeneous growing sites [24]. The variations of Tg with different concentrations of Sb in Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys at different heating rates are shown in Fig. 3. This nature can be described on the basis of structural changes due to the incorporation of Sb atoms in the Se–Te–Sn glassy system. The incorporation of Sb atoms in the Se–Te–Sn glassy system enhances the efficient bond energies between the Se and Se bonds. Furthermore, the change in Tg follows the power law nature as demonstrated by the relation [25]:

ð2Þ

where A and B are constants. This relation is based on the results for Te0.85Cu0.15 glass. The value of the constant provides an estimate for glass transition temperature at a heating rate of 1 K min-1. The value of the constant B indicates the temperature that is 0.693 times Tg, where the sample was scanned at a heating rate of 10 K min-1 [28]. However, B indicates the response time taken by the system for the changes in configuration in the glass transformation region. This is because B is also associated with the cooling rate of the melt; the lower the cooling rate of

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V. Rao et al. Table 1 Different characteristic temperatures for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys at various heating rates

Samples Se78Te20Sn2

Se76Te20Sn2Sb2

Se74Te20Sn2Sb4

Se72Te20Sn2Sb6

370

5 K min–1 10 K min–1 15 K min–1 20 K min–1

365

Heating rates/K min-1

To/K

Tp/K

Tc/K

Tm/K

5

349.64

376.47

400.87

430.87

533.84

10

353.17

380.66

408.81

440.70

533.84

15

355.89

383.16

416.26

442.32

533.84

20

356.76

386.91

419.43

445.41

533.84

5

352.36

386.07

407.96

438.23

535.34

10

357.77

391.58

416.19

444.36

535.34

15

360.05

393.46

422.18

458.69

535.34

20

363.43

397.45

427.41

475.29

535.34

5

354.74

410.13

431.32

454.68

536.11

10

358.14

420.67

443.79

465.31

536.11

15

359.55

428.71

450.17

483.57

536.11

20

361.78

432.95

457.18

493.33

536.11

5

356.89

416.22

436.44

461.17

537.17

10

359.38

420.22

441.75

485.51

537.17

15 20

363.14 364.26

428.81 430.38

450.06 453.44

488.10 491.68

537.17 537.17

Se78–xTe20Sn2Sbx

366 Se78Te20Sn2 Se76Te20Sn2Sb2 Se74Te20Sn2Sb4 Se72Te20Sn2Sb6

363 360

360

Tg/K

Tg/K

Tg/K

355

357

350

354 0

2

4

6

x/at.%

Fig. 3 Variation of Tg with different concentrations of Sb in Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys at different heating rates (5, 10, 15 and 20 K min-1)

the melt, the lower is the value of B. The variation of Tg with heating rate (5, 10, 15 and 20 K min-1) is shown in Fig. 4. The slopes of this denote the value of B and intercepts denote the value of A. The obtained values of the constants A and B from Fig. 4 are listed in Table 2. These results are confirmation of the fact that these glasses can sustain various structural changes.

Evaluation of activation energy of glass transition The activation energy is associated with the molecular motions and rearrangement of the atoms around the glass transition temperature, i.e., activation energy of glass

123

351 1.5

1.8

2.1

2.4

2.7

3.0

In β

Fig. 4 Variation of Tg with lnb for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys Table 2 Values of A and B for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys

Samples

A

B

Se78Te20Sn2

342.74

4.78

Se76Te20Sn2Sb2

348.37

5.72

Se74Te20Sn2Sb4 Se72Te20Sn2Sb6

354.21 360.43

6.45 7.20

transition is the minimum required energy which is absorbed by a group of atoms in the glassy region, so they jump from one metastable state to another [8]. Moynihan et al. [29] developed a relationship to calculate

Effect of antimony on glass transition and thermal stability of Se78-xTe18Sn2Sbx…

lnðbÞ ¼ constant 

Eg ; RTg

ð3Þ

where R is a universal constant. The slope of the plots of lnb versus 1000/Tg for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys should be straight lines as shown in Fig. 5. The obtained values of Eg are given in Table 3. The Kissinger’s relation [30] is another approach to calculate the activation energy of glass transition. Basically, this equation is derived for the calculation of the activation energy of crystallization [31–33] but it has been found that the same is equally applicable for the activation energy of glass transition [34]. The Kissinger relation is as follows: ! Eg b ln 2 ¼ constant  : ð4Þ Tg RTg A plot of ln(b/T2g) versus 1000/Tg for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys should be a straight line as displayed in Fig. 6. The values of the glass transition activation energy have been reported in Table 3. From Table 3, it is evident that the glass transition activation energies calculated from Moynihan’s and Kissinger’s relation, having the same trends of variation with different compositions, are quite similar and also the values obtained are nearly the same. It is worth mentioning that the values of Eg increases with increasing percentage of Sb in the Se–Te–Sn glassy system. This behavior can be understood on the basis of the heat of atomization (HS).

Se78Te20Sn2 Se76Te20Sn2Sb2 Se74Te20Sn2Sb4 Se72Te20Sn2Sb6

2.8

Table 3 Activation energy of glass transition (Eg) for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys

Moynihan’s method

Kissinger’s method

Se78Te20Sn2

186.99

166.45

Se76Te20Sn2Sb2

192.32

185.33

Se74Te20Sn2Sb4 Se72Te20Sn2Sb6

207.81 213.53

203.64 211.30

Se78Te20Sn2 Se76Te20Sn2Sb2 Se74Te20Sn2Sb4 Se72Te20Sn2Sb6 –9.0

–9.6

–10.2 2.72

In β

1.4 2.61

2.70

2.79

2.88

1000/Tg/K–1

Fig. 5 Variation of lnb versus 1000/Tg for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys

2.76

2.80

2.84

1000/Tg/K–1

Fig. 6 Variations of ln(b/T2g) versus 1000/Tg for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy alloys

The heat of atomization is defined as the change in enthalpy, which is required to separate all the atoms of a chemical compound in such a way that the bonds of a compound are broken and constituent atoms of a compound are available as individual atoms. For quaternary glassy system, the heat of atomization can be calculated using the following relation: HS ¼

2.1

Eg/KJ mole-1

Samples

In β Tg–2

the activation energy of glass transition Eg,which is as follows:

aðHS ÞSe þ bðHS ÞTe þ cðHS ÞSn þ dðHS ÞSb ; aþbþcþd

ð5Þ

where (HS)Se, (HS)Te, (HS)Sn and (HS)Sb are heat of atomization of Se, Te, Sn and Sb, whose values are 227, 197, 302 and 262 kJ mol-1, respectively. The values of HS for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems are listed in Table 4. From Table 4, it is evident that the values of HS of quaternary glassy systems are higher than those of ternary systems. This is the reason behind increasing the value of Eg in the studied glassy systems. The variation of Eg with compositions is shown in Fig. 7.

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V. Rao et al. Table 4 Values of heat of atomization for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems Samples

Heat of atomization/KJ mole-1

Se78Te20Sn2

222.50

Se76Te20Sn2Sb2

223.20

Se74Te20Sn2Sb4

223.90

Se72Te20Sn2Sb6

224.60

The fragility indices of the Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems are listed in Table 5. It is evident that the fragility index increases with increase in Sb content in the studied samples. It is noticeable that the composition dependence of the fragility index is much similar to that of the activation energy of glass transition for the present samples. It is clear that the studied glass becomes more fragile and its tendency to structural arrangement increases with increase in non-directional inter-atomic bonds.

Table 5 Fragility index of Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems

Thermal stability

Samples

Fragility index

Se78Te20Sn2

0.229

Se76Te20Sn2Sb2

0.233

Se74Te20Sn2Sb4

0.252

Se72Te20Sn2Sb6

0.258

On the basis of calorimetric measurements using DSC, the GFA and thermal stability were examined. Ovshinsky has reported two types of switching phenomena on the basis of the physical properties of these amorphous semiconductors. The first is the Ovonic threshold switching (OTS) and the second is Ovonic memory switching (OMS). These two types of electrical switching have been discovered in 1968, but still attract a great amount of scientific interest. A glass with bad glass-forming ability and low thermal stability indicates memory switching. On the other hand, threshold switching is found in those glasses which have good glassforming ability (GFA) and thermal stability (TS). GFA and TS are the important issues in the synthesis of the phase change memory for OTS and OMS [38]. The glass-forming ability (GFA) has been examined on the parameter reduced glass transition (Trg), introduced by Zanotto [39, 40] and given by:

220

Moynihan Kissinger

Eg/KJ mole–1

Se78–xTe20Sn2Sbx

200

Trg ¼ 180

0

2

4

6

Atomic % of Sb

Fig. 7 Variation of Eg with x (0, 2, 4 and 6) for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems

Fragility index Fragility index characterizes and quantifies the peculiar non-Arrhenius transport behavior of glassy materials as they approach the ergodicity-breaking glass transition [35, 36]. The fragility index can be calculated using the following relation [37]:   Eg F¼ : ð6Þ Tg ln 10

123

Tg : Tm

ð7Þ

The values of Trg are found to be an order of 2/3 for all studied glassy alloy compositions at various b values, which show a good glass-forming tendency [41]. The obtained values of glass-forming ability (GFA) for different concentration of Sb in Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems are listed in Table 6. It is found that addition of Sb results in broadening of GFA as shown in Fig. 8. Turnbull [42] gives more explanation in the case of larger value than 2/3 of Trg which is reported. Table 6 Different thermal stability parameters of Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems Samples

DT

Hr

S

Trg

Se78Te20Sn2

55.64

0.44

2.19

0.6615

Se76Te20Sn2Sb2

85.65

0.92

4.03

0.6683

Se74Te20Sn2Sb4

85.42

0.49

2.32

0.6680

Se72Te20Sn2Sb6

82.37

0.86

3.58

0.6690

Effect of antimony on glass transition and thermal stability of Se78-xTe18Sn2Sbx…

All the different thermal stability parameters are given in Table 6. From the table, it is evident that that thermal stability decreases with increase in the Sb content in the ternary glassy alloy. Sb at 2% has the highest DT value, highest Hruby number, and highest S parameter. Thus, among all prepared glassy alloys, Se76Te20Sn2Sb2 glassy alloy is the most stable.

0.670

Se78–xTe20Sn2Sbx

0.669 0.668 0.667

GFA

0.666 0.665 0.664

Conclusions

0.663 0.662 0.661 0

2

4

6

x/at.%

Fig. 8 Variation of glass-forming ability with x (0, 2, 4 and 6) for Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems

Since Tg provides much information on the rigidity and strength of the glassy system but does not possess any information about TS, different approaches are used to investigate the thermal stability. The first and basic approach is examined by Dietzal [43], which is DT = Tc - Tg. A huge DT value may show that the undercooled liquid can remain stable in a large temperature range without crystallization, thus leading to a larger GFA of the alloy. On the other hand, Hruby [44, 45] has introduced a parameter Hr, which includes both the nucleation and growth aspects of phase transition as an indicator of GFA and is given by: Hr ¼

Tc  Tg ; Tm  Tc

ð8Þ

where Tm is the melting temperature. A larger value of (Tc- Tg) delays the nucleation process and a small value of (Tm- Tc) retards the growth process. If HrB 0.1, the glass is usually hard to prepare. A good glass former has values of Hr C 0.4 [46]. Saad and Poulin [47] have introduced the simple elements allowing stability evaluation as follows: S¼

ðTc  Tg ÞðTp  Tc Þ : Tg

ð9Þ

From literature survey, we found that Saad and Poulin formula is the most reliable experimental criterion for determination of thermal stability, where we used directly the kinetic temperatures measured experimentally as compared to values of activation energies calculated on the basis of kinetic theories. That is why we have used it in the present study.

All the characteristic temperatures (Tg, To, Tp, Tc and Tm) were obtained from DSC curves. The heating rate dependence on glass transition temperature of an Se–Te–Sn–Sb quaternary system was calculated by Lasocka empirical relation, which displays a strong dependence on the Sb content. The increasing value of activation energy for glass transition can be explained using the heat of atomization, which increases on the addition of Sb. The increase in Eg indicates that the probability of the system toward devitrification decreases on Sb alloying. The glass-forming ability and thermal stability of Se78-xTe20Sn2Sbx (x = 0, 2, 4 and 6) glassy systems were calculated by different approaches based on peculiar temperatures. It is observed that all the thermal stability parameters (DT, Hr and S) decrease with increase in Sb percentage. The results show that the thermal stability is maximum for 2 at.% of Sb content.

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