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Correlation between structural and thermodynamic properties of some selenium based phase-change materials
Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Correlation between structural and thermodynamic properties of some selenium based phase-change materials Namrata Chandel a, b, Neeraj Mehta a, * a b
Glass Science Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Physics, Sunbeam College for women, Varanasi 221005, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous material Chalcogenide Differential scanning calorimetry Specific heat Structural relaxation Thermal stability
In this study, we prepared novel selenium rich multi-component glasses by incorporating In, Cd and Sb as foreign elements in an Sn containing Se–Te system in order to study their metal-induced effects on the thermal properties of the parent ternary glass. In particular, we determined the thermodynamic parameters of Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glassy semiconductors in a non-isothermal environment using the differential scanning calorimetry. Calorimetric measurements were obtained in the glass transition regions for Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glasses to determine their thermodynamic parameters such as the specific heat, enthalpy, and entropy during glass transition. We analyzed the variation in the specific heat before and after the heat capacity jump in these alloys. The metal-induced effects of foreign elements on the thermodynamic properties of the parent glass were also investigated in terms of the influence of the elemental specific heat of the added elemental metal as well as the thermal stability and glass-forming ability of the glasses.
1. Introduction In recent decades, there has been great progress in the development of new compositions of chalcogenide glasses. These compositions are potential candidates for applications in the design of optical systems, including optical memory applications, fibers for telecommunications, and lasers [1–7]. In these applications, the thermal and thermodynamic properties are important for determining the figure of merit of the optical glasses [8–11]. The determination of these properties for a glass-forming material requires measurements of a range of physical parameters using a combination of techniques. The specific heat and related thermodynamic properties can provide useful information about the glass structure, phonon energy, and heat diffusion coefficients [12–14]. Previous studies of the specific heat indicates that it is a significant parameter for providing information regarding the solubility, and thus it is directly related to the thermal transport properties of glass-forming melts after doping with ions [15]. Some studies have shown that this property play a key role in the structural modification of glasses in the glass transition range [14–16]. However, the variations in the specific heat values before and after thermal relaxation are highly diverse. Thus, specific heat measurements can be used to understand the glass transition behavior of chalcogenide glasses [8–16], but further
experimental studies are still required. Specific heat studies have been conducted for various two-component (i.e., binary) and three-component (i.e., ternary) glassy systems by investigating the effects of specific additives on the thermodynamic properties of binary and ternary glasses after varying their compositions. In the present study, we obtained calorimetric measurements for Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glasses after changing the additive element rather than varying the composition. 2. Experimental Ternary Se80Te18Sn2 glass and quaternary Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glass were prepared using the well-known quenching method, as described previously [17]. Details of the compositions are given in Table 1. The compositions of the as-prepared samples were analyzed using energy-dispersive X-ray spectroscopy (EDX). Fig. 1 shows the EDX patterns obtained for the glassy Se80Te8Sn2In10 and Se80Te8Sn2Sb10 alloys, which indicates that the uncertainties of the compositions of the asprepared samples were not significant i.e., the final compositions were almost the same as the initial concentrations introduced into the batch. Using X-ray diffraction (XRD), we confirmed that the final structures
* Corresponding author. E-mail address: [email protected] (N. Mehta). https://doi.org/10.1016/j.jpcs.2017.12.019 Received 23 June 2017; Received in revised form 10 December 2017; Accepted 11 December 2017 Available online 12 December 2017 0022-3697/© 2017 Elsevier Ltd. All rights reserved.
N. Chandel, N. Mehta
Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Table 1 Contents of glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. System.
Element
Atomic mass
Atomic %
Atomic mass %
Actual Weight (gm)
Se80Te18Sn2
Se Te Sn Se Te Sn Cd Se Te Sn In Se Te Sn Sb
78.96 127.6 118.71 78.96 127.6 118.71 112.41 78.96 127.6 118.71 114.82 78.96 127.6 118.71 121.75
80 18 2 80 8 2 10 80 8 2 10 80 8 2 10
6316.8 2296.8 237.42 6316.8 1020.8 237.42 1124 6316.8 1020.8 237.42 1148.2 6316.8 1020.8 237.42 1217.6
3.5684 1.2974 0.1341 3.6307 0.5867 0.1365 0.6461 3.6206 0.5851 0.1361 0.6581 3.5921 0.5805 0.1350 0.6924
Se80Te8Sn2Cd10
Se80Te8Sn2In10
Se80Te8Sn2Sb10
Fig. 2. XRD patterns obtained for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys.
The Q20 DSC instrument was calibrated using the calibration wizard over a temperature range of 273 K–673 K under standard conditions (a heating rate of 20 K/min for baseline and sapphire calibration runs, and a 10 mg indium sample (99.999% pure) was also crimped in aluminum pans and heated at 10 K/min). The Calibration Wizard is a calibration setup process in a step-by-step fashion, which allowed us to choose a Calibration method and then perform the calibration. The calibration was performed before obtaining the specific heat measurements. After T-zero and cell constant calibrations, the indium sample was reloaded and run. We carefully prepared and weighed 6–10 mg samples in standard aluminum pans. After crimping the pans containing the reference material, the pans were further flattened to improve the thermal contact, especially for the higher temperature melting standards (where a good thermal contact is particularly important). The melting heats and temperatures were calculated, where the experimental enthalpies were within 0.9% of the calibrated values using indium. The experimental heat capacities were within 1% of the calibration values using sapphire. 3. Theoretical basis DSC was used to monitor the thermally-induced phase transitions. DSC records the heat flow difference between the sample in the pan and a reference pan (which is usually empty) with identical temperature increases. Furthermore, we plotted this difference as a function of temperature. The temperature of both the sample and reference increased at a constant rate. The experiment was performed at constant pressure and thus the heat flow was comparable to the variations in enthalpy, as follows.
Fig. 1. EDX patterns obtained for the glassy Se80Te8Sn2In10 and Se80Te8Sn2Sb10 alloys.
of the as-prepared materials were glassy in nature (Fig. 2). The glasses were then ground to obtain fine powdered samples for differential scanning calorimetry (DSC) analyses. The advantages of DSC compared with other thermo-analytical techniques are as follows: (1) DSC is a more straightforward approach; (2) it only requires a finely ground powder; (3) it provides relatively better insights; and (4) unlike other methods, it does not depend on the size and shape of the sample. The thermal activities were determined using a DSC instrument (TA instrument USA; Model Q20), which had a temperature accuracy of ~ 0.1 K and a mean standard error of ~ 1 K in the measured value ranges (glass transition and crystallization temperatures). We heated 5–10 mg of sample at a rate of 20 K/min and the changes in the heat flow were recorded with respect to a blank pan. We obtained the measurements under almost the same conditions. We used the program provided with the DSC instrument to determine the dependence of the specific heat (Cp) on the temperature. Integrating the Cp curve using the DSC software provided the enthalpy value.
dH ¼ dt
dq dt p
(1)
The difference in the heat flow in both pans of the DSC cell is given by the following expression. Δ
dH dH dH ¼ dt dt sample dt reference
(2)
The absorption of heat occurs during endothermic reactions such as glass transitions and melting. Thus, the heat flow through the reference pan is smaller than that to the sample pan, i.e. Δ (dH/dt) > 0 for an endothermic reaction. By contrast, the liberation of heat occurs during the exothermic reactions such as crystallization. Thus, the heat flow to the sample pan is lower than that through the reference pan, i.e. Δ(dH/
114
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
dt) < 0 for an exothermic reaction. Above the baseline, if we take the integral under the DSC peak, then we obtain the net variation in the enthalpy for the solid state reaction considered, as follow. ∫
dH dt ¼ ΔHsample dt sample
4. Results and discussion DSC plots for the glassy alloys are shown in Fig. 4, where there are slight drifts near the glass transition and crystallization regimes in the samples. These drifts in the DSC results are expected due to the possibility of phase separation. In glass science and technology, observations of phase separation are generally reported [18–21], where they indicate an event due to mixing of a primarily homogeneous system (such as a liquid) into two or more finely mixed components or phases. The components are generally chemically or structurally different compared with the system under consideration. In some cases, the recognition of phase separation in a glassy material is not simple, where the difficulty is explained by only subtle changes in the diffraction pattern due to phase separation, whereas changes in the microstructure can be characterized readily because the electron densities of the separating phases fluctuate greatly. However, DSC is important for detecting phase separation in chalcogen-rich nonoxide glasses with well-defined dual endothermic and exothermic peaks [18–21]. Thus, the drift indicates a double-stage glass transition or crystallization. For the glasses investigated in this study, the peaks corresponding to double-stage glass transition/crystallization were not separated significantly, so it was difficult to analyze them separately. The glass transition peaks for the glassy Se80Te8Sn2Sb10 alloy at different heating rates are shown in Fig. 5, which indicates that the peak value of the glass transition temperature Tg shifted significantly as the heating rate increased. Thus, the rate of structural relaxation varied with the heating rate and it was related to the response in terms of the configurational changes in the glass transformation region. In present study, we used a state-of-the-art DSC instrument with access to advanced T-zero technology. In addition to the sample and reference temperature sensors, this technology introduces an independent T-zero sensor onto the DSC disk, which allows the calibration of the thermal lag character of both the cell and the pan encapsulating the sample. Thus, the DSC instrument generated sample data that compensated for the thermal lag due to the sample pan and the DSC cell. The reported temperature data were mechanically corrected for these thermal lags, thereby obtaining accurate peak temperatures [22]. The specific heat versus temperature is plotted in Fig. 6, which shows that Cp did not depend on the temperature before the glass transition region. However, it increased considerably with temperature near the glass transition temperature and reached a maximum at Tg. After Tg, the specific heat reached an off-set value. The off-set value of the specific heat was slightly higher compared with the on-set value. The on-set and off-set values of Cp are defined as the glass-specific heat Cpg and equilibrium-liquid specific heat Cpe, respectively. The rapid increase in the specific heat value for each alloy at the glass transition can be attributed to the anharmonic contribution [23,24]. The overshoot in the value of the specific heat (Cp) at the upper end of the “Cp jump” at the glass transition temperature was due to the relaxation effect. Previously, Moynihan reported that the time scale for structural relaxation is highly dependent on the temperature and the instant structure itself [25]. Therefore, the observed Cp peak at Tg can be attributed to the time required for the structural relaxation and the time scale of the experiment at this characteristic temperature. The difference (ΔCp) in the specific heat values for Cpe and Cpg was calculated for each glassy alloy, and the values of Cpe, Cpg, and ΔCp are given in Table 2, which shows that ΔCp was reduced after the addition of the fourth element (Cd, In, Sb) to the ternary Se80Te18Sn2 alloy. Thorpe et al. reported that glasses with lower jumps can be regarded as more inhomogeneous glass-forming liquids with a higher glass-forming ability (GFA) [26]. The GFA is calculated using the Hruby relationship [27]:
(3)
If we assume that the reference pan has a constant heat capacity represented by the peak over the temperature range, then ΔHreference cancels out when we consider the integral above the baseline. Thus, equation (2) is also applicable to the integral selected from the DCS plot of Δ(dH/dt). The heat capacity and its variation can be determined using the transfer in the baseline of the DSC thermogram. The expression for the heat capacity is: Cp ¼
dH dH dt 1 dH ¼ ¼ ; dT dt dT β dt
(4)
where β indicates the heating rate considered. In thermal differential analysis, the difference in the heat capacity of both pans can be expressed as follows. Δ Cp ¼ Cp sample Cp reference
(5)
For the actual DSC experiment, equation (5) has the following form: 1 dH Δ Cp ¼ K⋅ ⋅ ; β dt
(6)
where K is known as the cell constant which is determined by calibrating the heat capacity measurements for a standard material (sapphire). The specific heat is the main thermo-physical property of chalcogenides used for understanding the thermally-induced structural relaxation in a glassy system. In these materials, the specific heat exhibits an excellent response due to a mechanism where the component particles (atoms, ions, and/or molecules) are kinetically bonded. Thus, the temperature dependent specific heat provides an efficient assessment of glassy nature of a functional material. In the glass transition regime, a sudden change in the specific heat is a characteristic feature of glassy chalcogenides. The difference in Cp as the temperature increases is shown in Fig. 3 for a typical chalcogenide glass, which indicates that Cp is not highly temperature dependent before the beginning of the structural relaxation. However, after the structural relaxation commences, Cp increases considerably with the temperature until it reaches a peak value, after which Cp declines to an off-set value higher than the initial Cp value (before structural relaxation). This impulsive hop in the Cp value during structural relaxation is recognized as the contribution (anharmonic in nature) to the net specific heat.
GFA ¼
Tc Tg ; ðTm Tc Þ
(7)
where, the delay in the nucleation process is governed by the increase in
Fig. 3. Temperature dependence of the specific heat for a typical chalcogenide glass. 115
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Fig. 4. DSC scans of the multi-component glasses.
acquire supplementary qualitative information based on our observations. Further experiments could determine the precise nature of the metal-induced effects on the specific heat of chalcogenide glasses. The enthalpy (ΔHg) of structural relaxation was evaluated for each of the samples using the relationship: ΔHg ¼
KAg ; m
(8)
where K is the experimental constant employed for DSC in the present study. To determine the value of K, we computed the entire area for complete endothermic melting by highly pure zinc and we used the known melting enthalpy (~111.8 J/g) of this standard material. Ag is the area under the glass transition endothermic peak and m is the mass of the sample. The values of ΔHg determined for the samples are also given in Table 2, which clearly shows that the value of ΔHg increased after the addition of Cd, In and Sb to the parent glass. Due to the dependence of the physical stability of glassy materials on the molecular mobility, the glass transition enthalpy is a significant parameter for understanding the mobility of molecules in the glass state. Thus, the enthalpy is associated with the thermal stability of the glasses. In the present study, the stability of the parent glasses increased after the inclusion of Cd, In and Sb. The thermal stability was evaluated using the Saad and Poulin relationship [29]:
Fig. 5. Dependence of Tg on the heating rate for the glassy Se80Te8Sn2Sb10 alloy.
the values of (Tc – Tg) where small values of (Tm Tc) retard the growth process. The values of Tc, Tg, and Tm are given in Table 3. The GFA of each of the alloys was determined using the Hruby relationship [27] and the values are also shown in Table 3. Clearly, the GFA values of the quaternary alloys with lower Cp jumps (i.e., lower ΔCp values were higher compared with the parent ternary alloy with a higher Cp jump value i.e., higher ΔCp value). Our results are in good agreement with those reported by Thorpe et al. [26]. The value of the glass specific heat Cpg and equilibrium liquid specific heat Cpe were higher for the quaternary alloys Se80Te8Sn2M10 (M ¼ Cd and In) compared with the parent ternary glass, whereas their values were lower for M ¼ Sb. The additive elements (Cd, In, or Sb) were added to the Se–Te–Sn system at the cost of Te. The room temperature values of the molar specific heat CM for the additive elements comprising Cd (CM ¼ 26.02 J/mol K) and In (CM ¼ 26.74 J/mol K) were higher than that for CM in the case of Te (CM ¼ 25.73 J/mol K), but the molar specific heat CM for Sb (CM ¼ 25.23 J/mol K) was lower than that for CM in the case of Te [28], which probably explains the higher Cpg and Cpe values for the quaternary Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. Previous studies suggest that the specific heat (Cp) can be considered as the sum of two different contributions, i.e. Cp ¼ (Cp)P þ (Cp)E, where (Cp)P and (Cp)E are the phonon and electronic contributions, respectively. In glasses, the electronic contribution is much less compared with the phonon contribution. Thus, the anharmonic contribution to the specific heat is probably due to the phonon contribution. However, the results give above are quantitative and it is difficult to
TS ¼
ðTc To Þ Tc Tg ; Tg
(9)
where To represents the on-set crystallization temperature (see Table 3). We determined the thermal stability in terms of the Saad and Poulin stability criterion [29], and the thermal stability values are shown in Table 3. The degree of disorder in the amorphous structure can be inferred from the changes in entropy during the structural relaxation from a liquid state configuration to a local equilibrium. The change in entropy during the thermal relaxation was calculated as [30]: ΔS ¼
ΔHg ; Tg
(10)
where Tg is the glass transition temperature. The values of ΔHg and ΔS for the glasses are given in Table 1, which shows that both parameters increased significantly after the incorporation of Cd, In and Sb in the 116
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Fig. 6. Temperature dependence of the specific heat for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys.
DSC measurements. The glass transition enthalpy ΔHg and the entropy ΔS increased after the incorporation of Cd, In and Sb, thereby confirming that these additives dramatically affected the glass matrix of the ternary Se80Te18Sn2 alloy. The equilibrium liquid-specific heat Cpe and glassspecific heat Cpg were dependent on the composition.
Table 2 Thermodynamic parameters (the glass-specific heat Cpg, the equilibrium-liquid specific heat Cpe, their difference ΔCp, the enthalpy ΔHg of structural relaxation, and the change in entropy ΔS during the thermal relaxation) determined for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. Sample Se80Te18Sn2 Se80Te8Sn2Cd10 Se80Te8Sn2In10 Se80Te8Sn2Sb10
ΔS (J/g C)
Cpe (J/g C)
Cpg (J/g C)
ΔCp (J/g C)
ΔHg (J/ g)
0.405 0.895 0.701 0.401
0.560 0.911 0.730 0.490
0.155 0.016 0.029 0.089
113.7 183.1 379.4 326.2
1.44 3.10 5.40 4.20
Conflicts of interest We certify that we have NO affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Table 3 Glass-forming ability (GFA) and thermal stability (TS) of the glassy alloys.
Acknowledgments
Sample
Tg (K)
To (K)
Tc (K)
Tm (K)
GFA
TS (K)
Se80Te18Sn2 Se80Te8Sn2Cd10 Se80Te8Sn2In10 Se80Te8Sn2Sb10
348.6 329.2 341.0 347.3
351.5 345.5 353.5 395.8
367.7 376.7 421.9 435.5
527.7 511.3 514.0 509.2
0.12 0.35 0.88 1.20
0.9 4.5 16.2 10.1
NM is grateful to the Board of Research in Nuclear Sciences (BRNS), Mumbai, India for providing financial assistance through a DAE Research Award for Young Scientists (Scheme no. 2011/20/37P/02/BRNS). Appendix A. Supplementary data
parent Se80Te18Sn2 ternary glass. When Cd, In and Sb entered the matrix of the parent glass, they satisfied the requirements in terms of both the coordination and bond energy. Thus, we observed bond disorder and some degree of topological disorder, which are necessary conditions for the formation of amorphous solids. Thus, the increased entropy clearly indicated that the number of atomic arrays increased. Therefore, the internal energy due to the motion of atoms increased, so the barrier height between two adjacent metastable states decreased.
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5. Conclusions In this study, we investigated the effect of Cd, In and Sb additives on the structural relaxation of glassy Se80Te18Sn2 alloy by evaluating thermodynamic parameters (Cpe, Cpg, ΔHg and ΔS) based on state of the art 117
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Correlation between structural and thermodynamic properties of some selenium based phase-change materials Namrata Chandel a, b, Neeraj Mehta a, * a b
Glass Science Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Physics, Sunbeam College for women, Varanasi 221005, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous material Chalcogenide Differential scanning calorimetry Specific heat Structural relaxation Thermal stability
In this study, we prepared novel selenium rich multi-component glasses by incorporating In, Cd and Sb as foreign elements in an Sn containing Se–Te system in order to study their metal-induced effects on the thermal properties of the parent ternary glass. In particular, we determined the thermodynamic parameters of Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glassy semiconductors in a non-isothermal environment using the differential scanning calorimetry. Calorimetric measurements were obtained in the glass transition regions for Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glasses to determine their thermodynamic parameters such as the specific heat, enthalpy, and entropy during glass transition. We analyzed the variation in the specific heat before and after the heat capacity jump in these alloys. The metal-induced effects of foreign elements on the thermodynamic properties of the parent glass were also investigated in terms of the influence of the elemental specific heat of the added elemental metal as well as the thermal stability and glass-forming ability of the glasses.
1. Introduction In recent decades, there has been great progress in the development of new compositions of chalcogenide glasses. These compositions are potential candidates for applications in the design of optical systems, including optical memory applications, fibers for telecommunications, and lasers [1–7]. In these applications, the thermal and thermodynamic properties are important for determining the figure of merit of the optical glasses [8–11]. The determination of these properties for a glass-forming material requires measurements of a range of physical parameters using a combination of techniques. The specific heat and related thermodynamic properties can provide useful information about the glass structure, phonon energy, and heat diffusion coefficients [12–14]. Previous studies of the specific heat indicates that it is a significant parameter for providing information regarding the solubility, and thus it is directly related to the thermal transport properties of glass-forming melts after doping with ions [15]. Some studies have shown that this property play a key role in the structural modification of glasses in the glass transition range [14–16]. However, the variations in the specific heat values before and after thermal relaxation are highly diverse. Thus, specific heat measurements can be used to understand the glass transition behavior of chalcogenide glasses [8–16], but further
experimental studies are still required. Specific heat studies have been conducted for various two-component (i.e., binary) and three-component (i.e., ternary) glassy systems by investigating the effects of specific additives on the thermodynamic properties of binary and ternary glasses after varying their compositions. In the present study, we obtained calorimetric measurements for Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glasses after changing the additive element rather than varying the composition. 2. Experimental Ternary Se80Te18Sn2 glass and quaternary Se80Te8Sn2M10 (M ¼ Cd, In, Sb) glass were prepared using the well-known quenching method, as described previously [17]. Details of the compositions are given in Table 1. The compositions of the as-prepared samples were analyzed using energy-dispersive X-ray spectroscopy (EDX). Fig. 1 shows the EDX patterns obtained for the glassy Se80Te8Sn2In10 and Se80Te8Sn2Sb10 alloys, which indicates that the uncertainties of the compositions of the asprepared samples were not significant i.e., the final compositions were almost the same as the initial concentrations introduced into the batch. Using X-ray diffraction (XRD), we confirmed that the final structures
* Corresponding author. E-mail address: [email protected] (N. Mehta). https://doi.org/10.1016/j.jpcs.2017.12.019 Received 23 June 2017; Received in revised form 10 December 2017; Accepted 11 December 2017 Available online 12 December 2017 0022-3697/© 2017 Elsevier Ltd. All rights reserved.
N. Chandel, N. Mehta
Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Table 1 Contents of glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. System.
Element
Atomic mass
Atomic %
Atomic mass %
Actual Weight (gm)
Se80Te18Sn2
Se Te Sn Se Te Sn Cd Se Te Sn In Se Te Sn Sb
78.96 127.6 118.71 78.96 127.6 118.71 112.41 78.96 127.6 118.71 114.82 78.96 127.6 118.71 121.75
80 18 2 80 8 2 10 80 8 2 10 80 8 2 10
6316.8 2296.8 237.42 6316.8 1020.8 237.42 1124 6316.8 1020.8 237.42 1148.2 6316.8 1020.8 237.42 1217.6
3.5684 1.2974 0.1341 3.6307 0.5867 0.1365 0.6461 3.6206 0.5851 0.1361 0.6581 3.5921 0.5805 0.1350 0.6924
Se80Te8Sn2Cd10
Se80Te8Sn2In10
Se80Te8Sn2Sb10
Fig. 2. XRD patterns obtained for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys.
The Q20 DSC instrument was calibrated using the calibration wizard over a temperature range of 273 K–673 K under standard conditions (a heating rate of 20 K/min for baseline and sapphire calibration runs, and a 10 mg indium sample (99.999% pure) was also crimped in aluminum pans and heated at 10 K/min). The Calibration Wizard is a calibration setup process in a step-by-step fashion, which allowed us to choose a Calibration method and then perform the calibration. The calibration was performed before obtaining the specific heat measurements. After T-zero and cell constant calibrations, the indium sample was reloaded and run. We carefully prepared and weighed 6–10 mg samples in standard aluminum pans. After crimping the pans containing the reference material, the pans were further flattened to improve the thermal contact, especially for the higher temperature melting standards (where a good thermal contact is particularly important). The melting heats and temperatures were calculated, where the experimental enthalpies were within 0.9% of the calibrated values using indium. The experimental heat capacities were within 1% of the calibration values using sapphire. 3. Theoretical basis DSC was used to monitor the thermally-induced phase transitions. DSC records the heat flow difference between the sample in the pan and a reference pan (which is usually empty) with identical temperature increases. Furthermore, we plotted this difference as a function of temperature. The temperature of both the sample and reference increased at a constant rate. The experiment was performed at constant pressure and thus the heat flow was comparable to the variations in enthalpy, as follows.
Fig. 1. EDX patterns obtained for the glassy Se80Te8Sn2In10 and Se80Te8Sn2Sb10 alloys.
of the as-prepared materials were glassy in nature (Fig. 2). The glasses were then ground to obtain fine powdered samples for differential scanning calorimetry (DSC) analyses. The advantages of DSC compared with other thermo-analytical techniques are as follows: (1) DSC is a more straightforward approach; (2) it only requires a finely ground powder; (3) it provides relatively better insights; and (4) unlike other methods, it does not depend on the size and shape of the sample. The thermal activities were determined using a DSC instrument (TA instrument USA; Model Q20), which had a temperature accuracy of ~ 0.1 K and a mean standard error of ~ 1 K in the measured value ranges (glass transition and crystallization temperatures). We heated 5–10 mg of sample at a rate of 20 K/min and the changes in the heat flow were recorded with respect to a blank pan. We obtained the measurements under almost the same conditions. We used the program provided with the DSC instrument to determine the dependence of the specific heat (Cp) on the temperature. Integrating the Cp curve using the DSC software provided the enthalpy value.
dH ¼ dt
dq dt p
(1)
The difference in the heat flow in both pans of the DSC cell is given by the following expression. Δ
dH dH dH ¼ dt dt sample dt reference
(2)
The absorption of heat occurs during endothermic reactions such as glass transitions and melting. Thus, the heat flow through the reference pan is smaller than that to the sample pan, i.e. Δ (dH/dt) > 0 for an endothermic reaction. By contrast, the liberation of heat occurs during the exothermic reactions such as crystallization. Thus, the heat flow to the sample pan is lower than that through the reference pan, i.e. Δ(dH/
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
dt) < 0 for an exothermic reaction. Above the baseline, if we take the integral under the DSC peak, then we obtain the net variation in the enthalpy for the solid state reaction considered, as follow. ∫
dH dt ¼ ΔHsample dt sample
4. Results and discussion DSC plots for the glassy alloys are shown in Fig. 4, where there are slight drifts near the glass transition and crystallization regimes in the samples. These drifts in the DSC results are expected due to the possibility of phase separation. In glass science and technology, observations of phase separation are generally reported [18–21], where they indicate an event due to mixing of a primarily homogeneous system (such as a liquid) into two or more finely mixed components or phases. The components are generally chemically or structurally different compared with the system under consideration. In some cases, the recognition of phase separation in a glassy material is not simple, where the difficulty is explained by only subtle changes in the diffraction pattern due to phase separation, whereas changes in the microstructure can be characterized readily because the electron densities of the separating phases fluctuate greatly. However, DSC is important for detecting phase separation in chalcogen-rich nonoxide glasses with well-defined dual endothermic and exothermic peaks [18–21]. Thus, the drift indicates a double-stage glass transition or crystallization. For the glasses investigated in this study, the peaks corresponding to double-stage glass transition/crystallization were not separated significantly, so it was difficult to analyze them separately. The glass transition peaks for the glassy Se80Te8Sn2Sb10 alloy at different heating rates are shown in Fig. 5, which indicates that the peak value of the glass transition temperature Tg shifted significantly as the heating rate increased. Thus, the rate of structural relaxation varied with the heating rate and it was related to the response in terms of the configurational changes in the glass transformation region. In present study, we used a state-of-the-art DSC instrument with access to advanced T-zero technology. In addition to the sample and reference temperature sensors, this technology introduces an independent T-zero sensor onto the DSC disk, which allows the calibration of the thermal lag character of both the cell and the pan encapsulating the sample. Thus, the DSC instrument generated sample data that compensated for the thermal lag due to the sample pan and the DSC cell. The reported temperature data were mechanically corrected for these thermal lags, thereby obtaining accurate peak temperatures [22]. The specific heat versus temperature is plotted in Fig. 6, which shows that Cp did not depend on the temperature before the glass transition region. However, it increased considerably with temperature near the glass transition temperature and reached a maximum at Tg. After Tg, the specific heat reached an off-set value. The off-set value of the specific heat was slightly higher compared with the on-set value. The on-set and off-set values of Cp are defined as the glass-specific heat Cpg and equilibrium-liquid specific heat Cpe, respectively. The rapid increase in the specific heat value for each alloy at the glass transition can be attributed to the anharmonic contribution [23,24]. The overshoot in the value of the specific heat (Cp) at the upper end of the “Cp jump” at the glass transition temperature was due to the relaxation effect. Previously, Moynihan reported that the time scale for structural relaxation is highly dependent on the temperature and the instant structure itself [25]. Therefore, the observed Cp peak at Tg can be attributed to the time required for the structural relaxation and the time scale of the experiment at this characteristic temperature. The difference (ΔCp) in the specific heat values for Cpe and Cpg was calculated for each glassy alloy, and the values of Cpe, Cpg, and ΔCp are given in Table 2, which shows that ΔCp was reduced after the addition of the fourth element (Cd, In, Sb) to the ternary Se80Te18Sn2 alloy. Thorpe et al. reported that glasses with lower jumps can be regarded as more inhomogeneous glass-forming liquids with a higher glass-forming ability (GFA) [26]. The GFA is calculated using the Hruby relationship [27]:
(3)
If we assume that the reference pan has a constant heat capacity represented by the peak over the temperature range, then ΔHreference cancels out when we consider the integral above the baseline. Thus, equation (2) is also applicable to the integral selected from the DCS plot of Δ(dH/dt). The heat capacity and its variation can be determined using the transfer in the baseline of the DSC thermogram. The expression for the heat capacity is: Cp ¼
dH dH dt 1 dH ¼ ¼ ; dT dt dT β dt
(4)
where β indicates the heating rate considered. In thermal differential analysis, the difference in the heat capacity of both pans can be expressed as follows. Δ Cp ¼ Cp sample Cp reference
(5)
For the actual DSC experiment, equation (5) has the following form: 1 dH Δ Cp ¼ K⋅ ⋅ ; β dt
(6)
where K is known as the cell constant which is determined by calibrating the heat capacity measurements for a standard material (sapphire). The specific heat is the main thermo-physical property of chalcogenides used for understanding the thermally-induced structural relaxation in a glassy system. In these materials, the specific heat exhibits an excellent response due to a mechanism where the component particles (atoms, ions, and/or molecules) are kinetically bonded. Thus, the temperature dependent specific heat provides an efficient assessment of glassy nature of a functional material. In the glass transition regime, a sudden change in the specific heat is a characteristic feature of glassy chalcogenides. The difference in Cp as the temperature increases is shown in Fig. 3 for a typical chalcogenide glass, which indicates that Cp is not highly temperature dependent before the beginning of the structural relaxation. However, after the structural relaxation commences, Cp increases considerably with the temperature until it reaches a peak value, after which Cp declines to an off-set value higher than the initial Cp value (before structural relaxation). This impulsive hop in the Cp value during structural relaxation is recognized as the contribution (anharmonic in nature) to the net specific heat.
GFA ¼
Tc Tg ; ðTm Tc Þ
(7)
where, the delay in the nucleation process is governed by the increase in
Fig. 3. Temperature dependence of the specific heat for a typical chalcogenide glass. 115
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Fig. 4. DSC scans of the multi-component glasses.
acquire supplementary qualitative information based on our observations. Further experiments could determine the precise nature of the metal-induced effects on the specific heat of chalcogenide glasses. The enthalpy (ΔHg) of structural relaxation was evaluated for each of the samples using the relationship: ΔHg ¼
KAg ; m
(8)
where K is the experimental constant employed for DSC in the present study. To determine the value of K, we computed the entire area for complete endothermic melting by highly pure zinc and we used the known melting enthalpy (~111.8 J/g) of this standard material. Ag is the area under the glass transition endothermic peak and m is the mass of the sample. The values of ΔHg determined for the samples are also given in Table 2, which clearly shows that the value of ΔHg increased after the addition of Cd, In and Sb to the parent glass. Due to the dependence of the physical stability of glassy materials on the molecular mobility, the glass transition enthalpy is a significant parameter for understanding the mobility of molecules in the glass state. Thus, the enthalpy is associated with the thermal stability of the glasses. In the present study, the stability of the parent glasses increased after the inclusion of Cd, In and Sb. The thermal stability was evaluated using the Saad and Poulin relationship [29]:
Fig. 5. Dependence of Tg on the heating rate for the glassy Se80Te8Sn2Sb10 alloy.
the values of (Tc – Tg) where small values of (Tm Tc) retard the growth process. The values of Tc, Tg, and Tm are given in Table 3. The GFA of each of the alloys was determined using the Hruby relationship [27] and the values are also shown in Table 3. Clearly, the GFA values of the quaternary alloys with lower Cp jumps (i.e., lower ΔCp values were higher compared with the parent ternary alloy with a higher Cp jump value i.e., higher ΔCp value). Our results are in good agreement with those reported by Thorpe et al. [26]. The value of the glass specific heat Cpg and equilibrium liquid specific heat Cpe were higher for the quaternary alloys Se80Te8Sn2M10 (M ¼ Cd and In) compared with the parent ternary glass, whereas their values were lower for M ¼ Sb. The additive elements (Cd, In, or Sb) were added to the Se–Te–Sn system at the cost of Te. The room temperature values of the molar specific heat CM for the additive elements comprising Cd (CM ¼ 26.02 J/mol K) and In (CM ¼ 26.74 J/mol K) were higher than that for CM in the case of Te (CM ¼ 25.73 J/mol K), but the molar specific heat CM for Sb (CM ¼ 25.23 J/mol K) was lower than that for CM in the case of Te [28], which probably explains the higher Cpg and Cpe values for the quaternary Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. Previous studies suggest that the specific heat (Cp) can be considered as the sum of two different contributions, i.e. Cp ¼ (Cp)P þ (Cp)E, where (Cp)P and (Cp)E are the phonon and electronic contributions, respectively. In glasses, the electronic contribution is much less compared with the phonon contribution. Thus, the anharmonic contribution to the specific heat is probably due to the phonon contribution. However, the results give above are quantitative and it is difficult to
TS ¼
ðTc To Þ Tc Tg ; Tg
(9)
where To represents the on-set crystallization temperature (see Table 3). We determined the thermal stability in terms of the Saad and Poulin stability criterion [29], and the thermal stability values are shown in Table 3. The degree of disorder in the amorphous structure can be inferred from the changes in entropy during the structural relaxation from a liquid state configuration to a local equilibrium. The change in entropy during the thermal relaxation was calculated as [30]: ΔS ¼
ΔHg ; Tg
(10)
where Tg is the glass transition temperature. The values of ΔHg and ΔS for the glasses are given in Table 1, which shows that both parameters increased significantly after the incorporation of Cd, In and Sb in the 116
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Journal of Physics and Chemistry of Solids 115 (2018) 113–118
Fig. 6. Temperature dependence of the specific heat for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys.
DSC measurements. The glass transition enthalpy ΔHg and the entropy ΔS increased after the incorporation of Cd, In and Sb, thereby confirming that these additives dramatically affected the glass matrix of the ternary Se80Te18Sn2 alloy. The equilibrium liquid-specific heat Cpe and glassspecific heat Cpg were dependent on the composition.
Table 2 Thermodynamic parameters (the glass-specific heat Cpg, the equilibrium-liquid specific heat Cpe, their difference ΔCp, the enthalpy ΔHg of structural relaxation, and the change in entropy ΔS during the thermal relaxation) determined for the glassy Se80Te18Sn2 and Se80Te8Sn2M10 (M ¼ Cd, In, Sb) alloys. Sample Se80Te18Sn2 Se80Te8Sn2Cd10 Se80Te8Sn2In10 Se80Te8Sn2Sb10
ΔS (J/g C)
Cpe (J/g C)
Cpg (J/g C)
ΔCp (J/g C)
ΔHg (J/ g)
0.405 0.895 0.701 0.401
0.560 0.911 0.730 0.490
0.155 0.016 0.029 0.089
113.7 183.1 379.4 326.2
1.44 3.10 5.40 4.20
Conflicts of interest We certify that we have NO affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Table 3 Glass-forming ability (GFA) and thermal stability (TS) of the glassy alloys.
Acknowledgments
Sample
Tg (K)
To (K)
Tc (K)
Tm (K)
GFA
TS (K)
Se80Te18Sn2 Se80Te8Sn2Cd10 Se80Te8Sn2In10 Se80Te8Sn2Sb10
348.6 329.2 341.0 347.3
351.5 345.5 353.5 395.8
367.7 376.7 421.9 435.5
527.7 511.3 514.0 509.2
0.12 0.35 0.88 1.20
0.9 4.5 16.2 10.1
NM is grateful to the Board of Research in Nuclear Sciences (BRNS), Mumbai, India for providing financial assistance through a DAE Research Award for Young Scientists (Scheme no. 2011/20/37P/02/BRNS). Appendix A. Supplementary data
parent Se80Te18Sn2 ternary glass. When Cd, In and Sb entered the matrix of the parent glass, they satisfied the requirements in terms of both the coordination and bond energy. Thus, we observed bond disorder and some degree of topological disorder, which are necessary conditions for the formation of amorphous solids. Thus, the increased entropy clearly indicated that the number of atomic arrays increased. Therefore, the internal energy due to the motion of atoms increased, so the barrier height between two adjacent metastable states decreased.
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