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Observation of phase separation in some Se–Te–Sn chalcogenide glasses
Physica B 406 (2011) 1053–1059
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Physica B journal homepage: www.elsevier.com/locate/physb
Observation of phase separation in some Se–Te–Sn chalcogenide glasses Fouad Abdel-Wahab a,b,1 a b
Department of Physics, Faculty of Science, South Valley University, Aswan, Egypt Department of Physics, Faculty of Science, Taif University, P.O. Box 888, Al Taif, Saudi Arabia
a r t i c l e in f o
abstract
Article history: Received 13 May 2010 Received in revised form 28 September 2010 Accepted 29 September 2010 Available online 16 October 2010
Differential scanning calorimeter (DSC) and X-ray diffraction (XRD) techniques were employed here to investigate the glass transition behavior and crystallization kinetics of Se80 x Te20Snx (x ¼0.0, 2.5 and 5) alloys, which were prepared by the conventional melt quenching method. Two exothermic peaks have been observed in the DSC scans for the samples that contain Sn. Three crystalline phases (Se7.68Te0.32, SnSe and SnTe) were classified after heat treating the Se77.5 Te20Sn2.5 glass at temperature corresponding to the second crystallization peaks for 3 h. All the characteristic temperatures such as glass transition temperature (Tg), crystallization temperature (Tc) and crystallization peak temperatures (Tp) were found to depend on both the heating rate and the composition. This dependence has been used to deduce the activation energy of the glass transition (Eg), the activation energy of crystallization (Ec), the Avrami exponent (n), thermal stability and the fragility index (Fi). & 2010 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide glasses Non-isothermal Activation energy of crystallization Glass transition Fragility Thermal stability
1. Introduction Chalcogenide glasses have been investigated extensively over the past few decades because of their remarkable fundamental properties as well as their wide technological applications. Binary glassy alloys of the Se–Te system have acquired several advantages more than pure amorphous Se. These advantages appeared at their higher photosensitivity [1,2], great hardens [3,4] and moderate crystallization temperature [5]. To enhance thermal stability and glass-forming ability (GFA) different metals are included as glass modifiers, such as In [6], Sb [7], Pb [8], Ge [9] and Bi [10]. In the present work, Sn has been added as third element in binary Se–Te system to see the effect of Sn incorporation on crystallization kinetics. The reason for selection of Sn as chemical modifier in Se– Te system is based on its attractive and important applications in chalcogenide glasses. Also the addition of Sn may expand the glassforming area and create compositional and configurational disorder. It should be mentioned that tin–selenide glass systems showed numerous applications as an efficient solar cell material [11], memory switching devices [12,13] and holographic recording system [14]. It should be noted that the investigation of crystallization behavior of any glass is of great importance to initiate its thermal stability and glass-forming ability (GFA) and eventually to determine the suitable range of operating temperatures for a specific technological application before crystallization takes place.
E-mail address: [email protected] Permanent address. Department of Physics, Faculty of Science, South Valley University, Aswan, Egypt. 1
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.048
In the present paper a detailed study of non-isothermal transformation crystallization kinetics, thermal stability and glass-forming ability (GFA) of Se80 xTe20Snx (x ¼0, 2.5 and 5) chalcogenide glass system are presented. We report double stage crystallization in the present glass system that contains Sn. The effect of composition on crystallization mechanism is studied, and some aspects of the crystallization mechanism are also discussed.
2. Experimental Bulk Se80 xTe20 Snx (x ¼0, 2.5 and 5) chalcogenide glasses were prepared by the conventional melt quenching technique using high purity elemental Se, Te and Sn. About 6 g of each batch was transferred into a quartz ampoule of about 9 cm length and its internal diameter was about 8 mm. Subsequently, these ampoules were evacuated up to a pressure of 10 5 Torr. The samples were kept inside a controlled muffle furnace at 950 1C for 12 h with frequent rocking to ensure complete homogeneity of the melts. The quenching was then done in ice water. The crystallization behavior in non-isothermal conditions was investigated using a differential scanning calorimeter (TA 2010 DSC) instrument. About 15 mg of each sample in powdered form was sealed in standard aluminum pan in a dry nitrogen atmosphere. Non-isothermal DSC curves were obtained with selected heating rates of 5, 10, 15, 20, 25 and 35 K/min in the range 323–623 K. The values of the glass transition temperature (Tg), the onset temperature of crystallization (Tc), the peak temperature of crystallization (Tp) and the melting temperature (Tm) were determined.
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F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
35K/min 25K/min
20K/min ΔT
15K/min
10K/min Endothermic
Typical DSC thermograms of Se80Te20, Se77.5Te20Sn2.5 and Se75Te20Sn5 alloys at six different heating rates are shown in Figs. 1, 2 and 3, respectively. The characteristic phenomena (endothermic and exothermic peaks) are evidenced in the obtained DSC curves in the applied temperature range. Regarding all the obtained DSC thermograms in the mentioned figures, it is obvious that all these curves can be divided into three main regions. The first one represents the glass transition region characterized by an endothermic reaction at the glass transition temperature (Tg). The second region is associated with the crystallization process specified by exothermic peaks. In this part, the glass sample Se80Te20 exhibits only one exothermic peak while for Se77.5Te20Sn2.5 and Se75Te20Sn5 glasses two exothermic crystallization peaks can be easily observed. The third part is the melting region represented by endothermic reactions on the DSC curves. As shown in Figs. 2 and 3, there are two overlapped crystallization exothermic peaks for the studied glass that contains Sn. To identify the crystalline phases, the glassy Te20Se75Sn5 sample was annealed at TP2 for 2 h. Fig. 4 shows the XRD patterns for the annealed sample (Se75Te20Sn5). Three crystalline phases of the type Se7.68Te0.32, SnSe and SnTe are observed in the obtained XRD pattern. It should be mentioned that these peaks have also been observed [15] in other Se–Te–Sn chalcogenide glass system.
Exothermic
3. Results and discussion
5K/min
350
400
450 Temperature [K]
500
550
Fig. 2. DSC thermograms of glassy Se77.5Te20Sn2.5 alloy at different heating rates.
The glass transition temperature (Tg) represents the strength and rigidity of a glass network. It can be observed and determined from the endothermic reaction results from the breaking of the network and it is defined as the onset of the endothermic DSC occurrence. The dependence of Tg on the heating rate has been analyzed using three approaches. The first approach is the following empirical relation: Tg ¼ Aþ Bln ðaÞ
Exothermic
3.1. Glass transition
35K/min
25K/min
20K/min
where A and B are constants. The values of A represent the glass transition temperature for a heating rate of 1 K/min. It was found
ΔT
ð1Þ
15K/min 10K/min
Endothermic
35K/min Exothermic
5K/min
25K/min
350
20K/min
400
450 Temperature [K]
500
550
ΔT
Fig. 3. DSC thermograms of glassy Se75Te20Sn5 alloy at different heating rates.
15K/min
that this equation holds very well for the present system. The estimated values of A and B are listed in Table 2. The second approach is the evaluation of the activation energy for the glass transition (Eg) from the dependence of Tg on the heating rate using the Kissinger formula [16]: ! Tg2 Eg ln þ constant ð2Þ ¼ a RTg
Endothermic
10K/min 5K/min
350
400
450 Temperature [K]
500
550
Fig. 1. DSC thermograms of glassy Se80Te20 alloy at different heating rates.
where Eg is the activation energy of glass transition and R the gas constant. Fig. 5 shows ln(T2g /a) versus 1000/Tg plots for the present glasses, from which the activation energy for glass transition (Eg)
F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
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Table 1 Values of the parameters A and B in Eq. (4), Eg in Eq. (5), C and a0 in Eq. (6) for Se80 xTe20Snx. Sample
A
B
Eg (kJ)
C (K 1)
a0 (K/min 1)
Se80Te20 Se77.5Te20Sn2.5 Se75Te20Sn5
327.36 337.06 348.7
5.88 6.21 6.66
159.587 19.2 160.53 7 19.3 159.997 19.3
3.22 10 5 2.84 10 5 3.34 10 5
5.24 1015 7.81 1015 4.44 1012
3.5 Fig. 4. XRD patterns for the annealed sample of Se75Te20Sn5.
ln(α)
e 0 Se 80T 2
Sn 2.5 Te 20 Se 77.5
2.5
Sn 5 Te 20 Se 75
3.0
2.0 10.0 1.5 0.0008
0.0009
0.0010 (1/Tg-1/Tm)
0.0011
9.5
Se8 T 0 e2 0
Se7
9.0
7.5 Te S 20 n 2.5
Se7 T 5 e2 Sn 0 5
2
ln (Tg/α)
Fig. 6. Plot of ln a against (1/Tg 1/Tm) for estimation of some parameters mentioned in Eq. (6) for the present system.
8.5
2.700
2.775
2.850 1000/Tg
2.925
Fig. 5. Plot of ln(T2g /a) against 1000/Tg for evaluation of glass transition activation energy for the present system.
was evaluated and is also listed in Table 1. The values of the activation energy lie in the range generally observed for chalcogenide glasses [16–18] and are almost constant. The third approach based on the free volume theory, in which the relation between Tg and the cooling rate takes the following expression [19,20]:
a ¼ a0 exp½1=Cð1=Tg 1=Tm Þ
ð3Þ
where a0 and C are constants. Fig. 6 shows the relation between ln a and (1/Tg 1/Tm). The estimated values of a0 and C are represented in Table 1 also. The obtained values of C for all the studied glasses
appeared to be mostly constant while the values of a0 showed drastic changes. These results agree well with those obtained previously by Pradeep et al. [21] for pure Se as well as for binary As2Se3 and As2S3 chalcogenide glasses. Also, it was found from the DSC results that Tg increases as the heating rate was increased. Such increase in Tg was found by about 16% and 19% for Se–Te and Se–Te–Sn glasses, respectively. This trend can be attributed to the differences in the rates of cooling from the melt during preparation and the cooling rate during DSC analysis, where it is known that the samples when prepared by cooling from molten state (very high rate of cooling) accompanied by new configurational energy states or degrees of freedom, but during calorimetric analysis, the samples were reheated with different heating rates. These rates are obviously different from those of cooling during preparation. The greater is the difference, the greater is the structural difference between the glass and the reheated solid. The faster the reheating rate, the ‘longer’ the glass solid can remain as glass, leading to a higher glass transition temperature. It is well known that Tg depend on many factors such as band gap, bond energy cohesive energy and average heat of atomization. The structure of Se–Te system prepared by the milt-quenching method is represented as a combination of Se8 member rings, Se3Te mixed ring and Se–Te chain. A strong covalent bond [22] exists between the atoms in the ring, whereas between the chain only the van der Waals forces are dominant. With the increase in Sn content, the glassy matrix becomes heavily crosslinked. The Se–Se bonds (bond energy 183 kJ/mol) will be replaced by Sn–Se bonds, which have a higher bond energy (224 kJ/mol). Hence the cohesive energy of the system increases with increase in Sn content. This results in the increase in the Tg and increase in rigidity of Se80 x Te20Snx glasses.
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3.2. Crystallization kinetics and activation energy
The heat flow (g) evolved during crystallization is given by [23,24]
DSC thermograms have been analyzed to find the activation energy of amorphous–crystalline transformation (Ec), the Avrami exponent (n) and the frequency factor (K0) for each glass samples. Different approaches have been used in the interpretation of the experimental crystallization. Crystallization (amorphous–crystalline transformation) in chalcogenide glasses can be investigated by isothermal and nonisothermal methods. In the non-isothermal method, the sample is heated at a fixed rate and the physical quantity is recorded as a function of time, temperature or both. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) represent popular methods for studying non-isothermal transformation kinetics.
g ¼ DH
11.0
x=2.5
x=5
x=5 x=2.5 x=0
10.5
ln (T2p/α)
10.0
dw dT
ð4Þ
where DH is the heat of crystallization and w the crystallization fraction. This fraction can be expressed as a function of time according to the Johnson–Mehl–Avrami (JMA) transformation equation [17,18]:
wðtÞ ¼ 1exp½ðKtÞn
ð5Þ
where n is the Avrami index and K the reaction rate constant, which is given by K ¼ K0 expðEc =RTÞ
ð6Þ
where K0 is the frequency factor and Ec the effective activation energy. Hence, it can be stated that Eqs. (2) and (3) provide the basis for nearly all the experimental treatments in DSC. The first approach is to evaluate the activation energy of crystallization (Ec) from the variation in peak position of the crystallization temperature (Tp) when applying different heating rates. This method has been undertaken by the Kissinger equation [16]: ln Tp2 =a ¼ Ec =RTp þconstant ð7Þ Fig. 7 shows the plots of ln(T2p/a) against 1000/Tp of the three studied samples for both the first crystallization peak (open symbol) and the second crystallization peak (closed symbols). The slopes of the plots give the activation energies (Ec) for the different studied compositions. The values of the calculated activation energies are given in Table 2. The second method is to calculate the activation energy (Ec) and the frequency factor (K0) using the method proposed by Augis– Bennett’s [25] applying the following equation: lnðTp =aÞ ¼ ðEc =RTp Þ þ ln K0
ð8Þ
9.5
The plots of ln a against 1000/Tp in Fig. 7 gives a straight line, and their slopes give the activation energy (Ec; Fig. 8). Other method is to calculate the activation energy of crystallization (Ec) from the variation in the onset of the crystallization peak (Tc) with the heating rate, using the Ozawa [26] equation:
9.0
ln a ¼ ðEc =RTc Þ þconstant
The activation energies for the present system can also be deduced from the slopes of the plots of ln a against 1000/Tp. The values are also given in Table 1. Furthermore, the activation energies of amorphous–crystalline transformation of the present system could also be calculated using the Matusita formula [27]. For non-isothermal crystallization, the volume fraction of crystals precipitated in a glass heated at a uniform rate (a) is directly related to the activation energy (Ec) of the amorphous–crystalline transformation applying the following expression:
8.5
2.0
2.1
2.2
2.3
2.4
2.5
ð9Þ
2.6
1000/Tp [K-1] Fig. 7. Plots of ln (T2p /a) versus 1000/Tp: the closed symbols represent the first peak and open symbols for second Se80 xTe20Snx chalcogenide glasses.
1:052mEc ln lnð1wÞ ¼ n ln a þconstant RT
ð10Þ
Table 2 Activation energies of the crystallization (Ec), frequency factor of crystallization and the Avrami exponent (n) of Se80 xTe20Snx glasses. Sample
Activation energy, Ec (kJ) Kissinger
Augis–Bennett
Matusita-Sakka
K0 (s 1)
n
Se80Te20 Se77.5Te20Sn2.5 Se75Te20Sn5
First peak
128.237 3.8 73.03 7 2.7 105 7 2.9
131.497 3.7 76.66 7 2.76 109 7 2.9
134.747 3.7 80.31 7 2.75 113.2 7 2.9
1.27 1016 4.40 107 5.37 1010
1.85o no 2.5 2.6o no 3.7 2.6o no 3.1
Se77.5Te20Sn2.5 Se75Te20Sn5
Second peak
390.9 7 35.9 288.967 8.7
395.1 7 36 293.02 7 8.7
399.2 7 36 297.1 7 8.7
5.59 1029 1.01 1040
2.4o no 3.5 2.3o no 4.5
F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
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4.5 -1
4.0
3.5 ln(-ln(1-χ))
ln (Tp/α)
-2
-3
3.0
-4
2.5
2.0
2.1
2.2
2.3
2.4
2.5
2.6
35
2.7
15
25 20
5
10
1000/Tp [K-1] -5
Fig. 8. Plots of ln (Tp/a) versus 1000/Tp: the closed symbols represents the first peak and open symbols for second Se80 xTe20Snx chalcogenide glasses.
2.3
2.4
2.5
1000/T K-1
30 25 20
15 10
5
0
ln(-ln(1-χ))
where m and n are constants, which depend on the morphology of the crystal growth. In order to determine m, a relation between ln[ ln (1 w)] and 1000/T at different heating rates was plotted, where straight lines were obtained over the used temperature range. Fig. 9a and b shows such plots for the first and the second crystallization peaks, respectively, of the Se77.5Te20Sn2.5 glass system. From the slope of each straight line, the mEc value was calculated. Since the values of Ec were determined previously using three different approaches, the average value could be obtained. Avrami exponent n has been evaluated from the slopes of the straight lines of ln[ ln(1 w)] and ln a relation at a fixed temperature. Fig. 9 shows such a plot for the two peaks of the Se80 xTe20Snx chalcogenide glass. The values of n are listed in Table 1. It can be noted that the calculated values of n are not integers, which means that the crystallization occurs with different mechanisms. This result is confirmed by the presence of more than one crystalline phase as indicated in the diffraction patterns of the XRD, as shown in Fig. 4. Note that the value of n 4 for the second peak indicates that the crystal growth could be possibly carried out in three dimensions with heterogeneous and homogeneous nucleation, that is the growth of SnSe and SnTe can take place in bulk forms. While for the first peak due to Se7.68Te0.32, the obtained n value indicates that the growth is carried out in only two dimensions. The activation energy (Ec), which was determined from thermal analysis, is considered to be the sum of the nucleation activation energy (En) and the crystal growth activation energy (EG) [28–32]. It has been pointed out [33] that in non-isothermal measurements,
2.2
-1
-2
-3 1.96
1.98
2.00
2.02
2.04
2.06
1000/T K-1 Fig. 9. Plot of ln[ln(1 w)] versus 1000/T for Se77.5Te20Sn2.5 glass at different heating rates (a) for first peak and (b) second peak.
due to a fast increase in temperature and large discrepancies in the latent heats of nucleation and growth, the crystallization exotherm characterizes the growth of the crystalline phase from the amorphous matrix, nucleation calorimetrically unobservable at temperatures below the crystallization exotherm. Therefore, the obtained values of Ec can be taken to represent the activation energy of growth EG of the present system.
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F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
Table 3 Characteristic parameters for the thermal stability and glass fragility of the Se80 xTe20Snx chalcogenide glasses. Heating rate
Se80Te20
Se77.5Te20Sn2.5
Thermal parameters
5
10
15
20
25
35
First phase Hr H/ S Fi
0.221 0.109 0.936 35.38
0.244 0.116 0.878 24.46
0.246 0.116 1.060 20.68
0.252 0.115 1.128 18.57
0.268 0.120 1.057 17.21
0.289 0.125 1.076 15.5
First phase Hr H/ S Fi
0.406 0.152 2.889 34.54
0.458 0.163 3.826 23.89
0.472 0.165 4.502 20.16
0.499 0.171 4.628 18.12
0.544 0.180 5.105 16.79
0.565 0.184 6.124 15.12
3.004 0.396 2.980
2.946 0.387 2.916
2.886 0.382 2.888
2.792 0.379 2.878
2.803 0.377 2.861
2.956 0.380 2.405
0.864 0.228 2.019 33.23
1.055 0.248 1.665 22.99
1.100 0.249 2.131 19.37
1.153 0.254 2.335 17.43
1.193 0.257 2.601 16.14
1.288 0.264 2.926 14.53
1.829 0.319 2.578
1.954 0.320 2.251
1.995 0.317 2.052
2.068 0.319 1.682
2.111 0.320 1.390
Second phase Hr H/ S Se75Te20Sn5
First phase Hr H/ S Fi Second phase Hr H/ S
Tammann Fulcher equation) and are declared as fragile glassforming liquids.
3.3. Glass-forming ability, thermal stability and glass fragility Glass-forming ability (GFA) is a fundamental issue in the study of chalcogenide glasses, because it determines the degree of utilization of the investigated materials in various applications. Hruby [34] has introduced a parameter Hr, which can be used as the index of the GFA: Hr ¼
Tc Tg Tm Tc
ð11Þ
On the other hand, thermal stability was investigated by Saad and Poulain [35], and they introduced two parameters, which can be used as an indication of thermal stability of a glass. These parameters are H= ¼
S¼
Tc Tg Tg
ðTp Tc ÞTc Tg Tg
ð12Þ
ð13Þ
The values, Hr, H/ and S are all given in Table 3 as function of heating rate and composition. It can be noted from this table that the the addition of Sn to the Se–Te binary glass increases the stability of the studied glasses. On the basis of the obtained Tg and Eg data, the fragility index m is calculated from the following equation [36]: Eg Fi ¼ RTg ln a
4. Conclusions Calorimetric measurements were performed on Se80 xTe20Snx (x¼0, 2.5 and 5) chalcogenide glasses. DSC thermograms of the studied glasses show that each composition has one glass transition and double crystallization stage except the Sn free sample that has only one crystallization stage. All characteristic temperatures Tg, Tc and Tp increase with the increase in the heating rate and Sn content. The average value of Ec for the second crystallization stage is higher than that for the first crystallization phase. According to the Avrami theory of nucleation, the calculated values of the kinetic exponent n suggest surface growth for the first stage, Se77.5Te20Sn2.5 and bulk crystallization for the second stage. The crystalline phases developed in the sample annealed at temperature corresponding to the crystallization peak were identified by XRD to be Se7.68Te0.32, SnTe and SnTe phases.
Acknowledgments The author wishes to thank Professor A.G. Mostafa for useful discussion and comments.
ð14Þ
Note that a low value of Fi (Fi E16) [37] defined the strong glassforming liquids, while a high value of Fi (Fi E200) [38] represents the fragile glass-forming liquid. The values of Fi for the present system at different heating rates are presented in Table 3. The results show that this system was created from the strong glass-forming liquids. On the other hand, the classification of glass-formation dependence on their viscosities [39], it is known that liquids that exhibit an Arrhenius temperature dependence on viscosity are defined as strong glass-forming liquids and these exhibit a non-Arrhenius behavior (described by a Vogel
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Observation of phase separation in some Se–Te–Sn chalcogenide glasses Fouad Abdel-Wahab a,b,1 a b
Department of Physics, Faculty of Science, South Valley University, Aswan, Egypt Department of Physics, Faculty of Science, Taif University, P.O. Box 888, Al Taif, Saudi Arabia
a r t i c l e in f o
abstract
Article history: Received 13 May 2010 Received in revised form 28 September 2010 Accepted 29 September 2010 Available online 16 October 2010
Differential scanning calorimeter (DSC) and X-ray diffraction (XRD) techniques were employed here to investigate the glass transition behavior and crystallization kinetics of Se80 x Te20Snx (x ¼0.0, 2.5 and 5) alloys, which were prepared by the conventional melt quenching method. Two exothermic peaks have been observed in the DSC scans for the samples that contain Sn. Three crystalline phases (Se7.68Te0.32, SnSe and SnTe) were classified after heat treating the Se77.5 Te20Sn2.5 glass at temperature corresponding to the second crystallization peaks for 3 h. All the characteristic temperatures such as glass transition temperature (Tg), crystallization temperature (Tc) and crystallization peak temperatures (Tp) were found to depend on both the heating rate and the composition. This dependence has been used to deduce the activation energy of the glass transition (Eg), the activation energy of crystallization (Ec), the Avrami exponent (n), thermal stability and the fragility index (Fi). & 2010 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide glasses Non-isothermal Activation energy of crystallization Glass transition Fragility Thermal stability
1. Introduction Chalcogenide glasses have been investigated extensively over the past few decades because of their remarkable fundamental properties as well as their wide technological applications. Binary glassy alloys of the Se–Te system have acquired several advantages more than pure amorphous Se. These advantages appeared at their higher photosensitivity [1,2], great hardens [3,4] and moderate crystallization temperature [5]. To enhance thermal stability and glass-forming ability (GFA) different metals are included as glass modifiers, such as In [6], Sb [7], Pb [8], Ge [9] and Bi [10]. In the present work, Sn has been added as third element in binary Se–Te system to see the effect of Sn incorporation on crystallization kinetics. The reason for selection of Sn as chemical modifier in Se– Te system is based on its attractive and important applications in chalcogenide glasses. Also the addition of Sn may expand the glassforming area and create compositional and configurational disorder. It should be mentioned that tin–selenide glass systems showed numerous applications as an efficient solar cell material [11], memory switching devices [12,13] and holographic recording system [14]. It should be noted that the investigation of crystallization behavior of any glass is of great importance to initiate its thermal stability and glass-forming ability (GFA) and eventually to determine the suitable range of operating temperatures for a specific technological application before crystallization takes place.
E-mail address: [email protected] Permanent address. Department of Physics, Faculty of Science, South Valley University, Aswan, Egypt. 1
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.048
In the present paper a detailed study of non-isothermal transformation crystallization kinetics, thermal stability and glass-forming ability (GFA) of Se80 xTe20Snx (x ¼0, 2.5 and 5) chalcogenide glass system are presented. We report double stage crystallization in the present glass system that contains Sn. The effect of composition on crystallization mechanism is studied, and some aspects of the crystallization mechanism are also discussed.
2. Experimental Bulk Se80 xTe20 Snx (x ¼0, 2.5 and 5) chalcogenide glasses were prepared by the conventional melt quenching technique using high purity elemental Se, Te and Sn. About 6 g of each batch was transferred into a quartz ampoule of about 9 cm length and its internal diameter was about 8 mm. Subsequently, these ampoules were evacuated up to a pressure of 10 5 Torr. The samples were kept inside a controlled muffle furnace at 950 1C for 12 h with frequent rocking to ensure complete homogeneity of the melts. The quenching was then done in ice water. The crystallization behavior in non-isothermal conditions was investigated using a differential scanning calorimeter (TA 2010 DSC) instrument. About 15 mg of each sample in powdered form was sealed in standard aluminum pan in a dry nitrogen atmosphere. Non-isothermal DSC curves were obtained with selected heating rates of 5, 10, 15, 20, 25 and 35 K/min in the range 323–623 K. The values of the glass transition temperature (Tg), the onset temperature of crystallization (Tc), the peak temperature of crystallization (Tp) and the melting temperature (Tm) were determined.
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F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
35K/min 25K/min
20K/min ΔT
15K/min
10K/min Endothermic
Typical DSC thermograms of Se80Te20, Se77.5Te20Sn2.5 and Se75Te20Sn5 alloys at six different heating rates are shown in Figs. 1, 2 and 3, respectively. The characteristic phenomena (endothermic and exothermic peaks) are evidenced in the obtained DSC curves in the applied temperature range. Regarding all the obtained DSC thermograms in the mentioned figures, it is obvious that all these curves can be divided into three main regions. The first one represents the glass transition region characterized by an endothermic reaction at the glass transition temperature (Tg). The second region is associated with the crystallization process specified by exothermic peaks. In this part, the glass sample Se80Te20 exhibits only one exothermic peak while for Se77.5Te20Sn2.5 and Se75Te20Sn5 glasses two exothermic crystallization peaks can be easily observed. The third part is the melting region represented by endothermic reactions on the DSC curves. As shown in Figs. 2 and 3, there are two overlapped crystallization exothermic peaks for the studied glass that contains Sn. To identify the crystalline phases, the glassy Te20Se75Sn5 sample was annealed at TP2 for 2 h. Fig. 4 shows the XRD patterns for the annealed sample (Se75Te20Sn5). Three crystalline phases of the type Se7.68Te0.32, SnSe and SnTe are observed in the obtained XRD pattern. It should be mentioned that these peaks have also been observed [15] in other Se–Te–Sn chalcogenide glass system.
Exothermic
3. Results and discussion
5K/min
350
400
450 Temperature [K]
500
550
Fig. 2. DSC thermograms of glassy Se77.5Te20Sn2.5 alloy at different heating rates.
The glass transition temperature (Tg) represents the strength and rigidity of a glass network. It can be observed and determined from the endothermic reaction results from the breaking of the network and it is defined as the onset of the endothermic DSC occurrence. The dependence of Tg on the heating rate has been analyzed using three approaches. The first approach is the following empirical relation: Tg ¼ Aþ Bln ðaÞ
Exothermic
3.1. Glass transition
35K/min
25K/min
20K/min
where A and B are constants. The values of A represent the glass transition temperature for a heating rate of 1 K/min. It was found
ΔT
ð1Þ
15K/min 10K/min
Endothermic
35K/min Exothermic
5K/min
25K/min
350
20K/min
400
450 Temperature [K]
500
550
ΔT
Fig. 3. DSC thermograms of glassy Se75Te20Sn5 alloy at different heating rates.
15K/min
that this equation holds very well for the present system. The estimated values of A and B are listed in Table 2. The second approach is the evaluation of the activation energy for the glass transition (Eg) from the dependence of Tg on the heating rate using the Kissinger formula [16]: ! Tg2 Eg ln þ constant ð2Þ ¼ a RTg
Endothermic
10K/min 5K/min
350
400
450 Temperature [K]
500
550
Fig. 1. DSC thermograms of glassy Se80Te20 alloy at different heating rates.
where Eg is the activation energy of glass transition and R the gas constant. Fig. 5 shows ln(T2g /a) versus 1000/Tg plots for the present glasses, from which the activation energy for glass transition (Eg)
F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
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Table 1 Values of the parameters A and B in Eq. (4), Eg in Eq. (5), C and a0 in Eq. (6) for Se80 xTe20Snx. Sample
A
B
Eg (kJ)
C (K 1)
a0 (K/min 1)
Se80Te20 Se77.5Te20Sn2.5 Se75Te20Sn5
327.36 337.06 348.7
5.88 6.21 6.66
159.587 19.2 160.53 7 19.3 159.997 19.3
3.22 10 5 2.84 10 5 3.34 10 5
5.24 1015 7.81 1015 4.44 1012
3.5 Fig. 4. XRD patterns for the annealed sample of Se75Te20Sn5.
ln(α)
e 0 Se 80T 2
Sn 2.5 Te 20 Se 77.5
2.5
Sn 5 Te 20 Se 75
3.0
2.0 10.0 1.5 0.0008
0.0009
0.0010 (1/Tg-1/Tm)
0.0011
9.5
Se8 T 0 e2 0
Se7
9.0
7.5 Te S 20 n 2.5
Se7 T 5 e2 Sn 0 5
2
ln (Tg/α)
Fig. 6. Plot of ln a against (1/Tg 1/Tm) for estimation of some parameters mentioned in Eq. (6) for the present system.
8.5
2.700
2.775
2.850 1000/Tg
2.925
Fig. 5. Plot of ln(T2g /a) against 1000/Tg for evaluation of glass transition activation energy for the present system.
was evaluated and is also listed in Table 1. The values of the activation energy lie in the range generally observed for chalcogenide glasses [16–18] and are almost constant. The third approach based on the free volume theory, in which the relation between Tg and the cooling rate takes the following expression [19,20]:
a ¼ a0 exp½1=Cð1=Tg 1=Tm Þ
ð3Þ
where a0 and C are constants. Fig. 6 shows the relation between ln a and (1/Tg 1/Tm). The estimated values of a0 and C are represented in Table 1 also. The obtained values of C for all the studied glasses
appeared to be mostly constant while the values of a0 showed drastic changes. These results agree well with those obtained previously by Pradeep et al. [21] for pure Se as well as for binary As2Se3 and As2S3 chalcogenide glasses. Also, it was found from the DSC results that Tg increases as the heating rate was increased. Such increase in Tg was found by about 16% and 19% for Se–Te and Se–Te–Sn glasses, respectively. This trend can be attributed to the differences in the rates of cooling from the melt during preparation and the cooling rate during DSC analysis, where it is known that the samples when prepared by cooling from molten state (very high rate of cooling) accompanied by new configurational energy states or degrees of freedom, but during calorimetric analysis, the samples were reheated with different heating rates. These rates are obviously different from those of cooling during preparation. The greater is the difference, the greater is the structural difference between the glass and the reheated solid. The faster the reheating rate, the ‘longer’ the glass solid can remain as glass, leading to a higher glass transition temperature. It is well known that Tg depend on many factors such as band gap, bond energy cohesive energy and average heat of atomization. The structure of Se–Te system prepared by the milt-quenching method is represented as a combination of Se8 member rings, Se3Te mixed ring and Se–Te chain. A strong covalent bond [22] exists between the atoms in the ring, whereas between the chain only the van der Waals forces are dominant. With the increase in Sn content, the glassy matrix becomes heavily crosslinked. The Se–Se bonds (bond energy 183 kJ/mol) will be replaced by Sn–Se bonds, which have a higher bond energy (224 kJ/mol). Hence the cohesive energy of the system increases with increase in Sn content. This results in the increase in the Tg and increase in rigidity of Se80 x Te20Snx glasses.
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3.2. Crystallization kinetics and activation energy
The heat flow (g) evolved during crystallization is given by [23,24]
DSC thermograms have been analyzed to find the activation energy of amorphous–crystalline transformation (Ec), the Avrami exponent (n) and the frequency factor (K0) for each glass samples. Different approaches have been used in the interpretation of the experimental crystallization. Crystallization (amorphous–crystalline transformation) in chalcogenide glasses can be investigated by isothermal and nonisothermal methods. In the non-isothermal method, the sample is heated at a fixed rate and the physical quantity is recorded as a function of time, temperature or both. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) represent popular methods for studying non-isothermal transformation kinetics.
g ¼ DH
11.0
x=2.5
x=5
x=5 x=2.5 x=0
10.5
ln (T2p/α)
10.0
dw dT
ð4Þ
where DH is the heat of crystallization and w the crystallization fraction. This fraction can be expressed as a function of time according to the Johnson–Mehl–Avrami (JMA) transformation equation [17,18]:
wðtÞ ¼ 1exp½ðKtÞn
ð5Þ
where n is the Avrami index and K the reaction rate constant, which is given by K ¼ K0 expðEc =RTÞ
ð6Þ
where K0 is the frequency factor and Ec the effective activation energy. Hence, it can be stated that Eqs. (2) and (3) provide the basis for nearly all the experimental treatments in DSC. The first approach is to evaluate the activation energy of crystallization (Ec) from the variation in peak position of the crystallization temperature (Tp) when applying different heating rates. This method has been undertaken by the Kissinger equation [16]: ln Tp2 =a ¼ Ec =RTp þconstant ð7Þ Fig. 7 shows the plots of ln(T2p/a) against 1000/Tp of the three studied samples for both the first crystallization peak (open symbol) and the second crystallization peak (closed symbols). The slopes of the plots give the activation energies (Ec) for the different studied compositions. The values of the calculated activation energies are given in Table 2. The second method is to calculate the activation energy (Ec) and the frequency factor (K0) using the method proposed by Augis– Bennett’s [25] applying the following equation: lnðTp =aÞ ¼ ðEc =RTp Þ þ ln K0
ð8Þ
9.5
The plots of ln a against 1000/Tp in Fig. 7 gives a straight line, and their slopes give the activation energy (Ec; Fig. 8). Other method is to calculate the activation energy of crystallization (Ec) from the variation in the onset of the crystallization peak (Tc) with the heating rate, using the Ozawa [26] equation:
9.0
ln a ¼ ðEc =RTc Þ þconstant
The activation energies for the present system can also be deduced from the slopes of the plots of ln a against 1000/Tp. The values are also given in Table 1. Furthermore, the activation energies of amorphous–crystalline transformation of the present system could also be calculated using the Matusita formula [27]. For non-isothermal crystallization, the volume fraction of crystals precipitated in a glass heated at a uniform rate (a) is directly related to the activation energy (Ec) of the amorphous–crystalline transformation applying the following expression:
8.5
2.0
2.1
2.2
2.3
2.4
2.5
ð9Þ
2.6
1000/Tp [K-1] Fig. 7. Plots of ln (T2p /a) versus 1000/Tp: the closed symbols represent the first peak and open symbols for second Se80 xTe20Snx chalcogenide glasses.
1:052mEc ln lnð1wÞ ¼ n ln a þconstant RT
ð10Þ
Table 2 Activation energies of the crystallization (Ec), frequency factor of crystallization and the Avrami exponent (n) of Se80 xTe20Snx glasses. Sample
Activation energy, Ec (kJ) Kissinger
Augis–Bennett
Matusita-Sakka
K0 (s 1)
n
Se80Te20 Se77.5Te20Sn2.5 Se75Te20Sn5
First peak
128.237 3.8 73.03 7 2.7 105 7 2.9
131.497 3.7 76.66 7 2.76 109 7 2.9
134.747 3.7 80.31 7 2.75 113.2 7 2.9
1.27 1016 4.40 107 5.37 1010
1.85o no 2.5 2.6o no 3.7 2.6o no 3.1
Se77.5Te20Sn2.5 Se75Te20Sn5
Second peak
390.9 7 35.9 288.967 8.7
395.1 7 36 293.02 7 8.7
399.2 7 36 297.1 7 8.7
5.59 1029 1.01 1040
2.4o no 3.5 2.3o no 4.5
F. Abdel-Wahab / Physica B 406 (2011) 1053–1059
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4.5 -1
4.0
3.5 ln(-ln(1-χ))
ln (Tp/α)
-2
-3
3.0
-4
2.5
2.0
2.1
2.2
2.3
2.4
2.5
2.6
35
2.7
15
25 20
5
10
1000/Tp [K-1] -5
Fig. 8. Plots of ln (Tp/a) versus 1000/Tp: the closed symbols represents the first peak and open symbols for second Se80 xTe20Snx chalcogenide glasses.
2.3
2.4
2.5
1000/T K-1
30 25 20
15 10
5
0
ln(-ln(1-χ))
where m and n are constants, which depend on the morphology of the crystal growth. In order to determine m, a relation between ln[ ln (1 w)] and 1000/T at different heating rates was plotted, where straight lines were obtained over the used temperature range. Fig. 9a and b shows such plots for the first and the second crystallization peaks, respectively, of the Se77.5Te20Sn2.5 glass system. From the slope of each straight line, the mEc value was calculated. Since the values of Ec were determined previously using three different approaches, the average value could be obtained. Avrami exponent n has been evaluated from the slopes of the straight lines of ln[ ln(1 w)] and ln a relation at a fixed temperature. Fig. 9 shows such a plot for the two peaks of the Se80 xTe20Snx chalcogenide glass. The values of n are listed in Table 1. It can be noted that the calculated values of n are not integers, which means that the crystallization occurs with different mechanisms. This result is confirmed by the presence of more than one crystalline phase as indicated in the diffraction patterns of the XRD, as shown in Fig. 4. Note that the value of n 4 for the second peak indicates that the crystal growth could be possibly carried out in three dimensions with heterogeneous and homogeneous nucleation, that is the growth of SnSe and SnTe can take place in bulk forms. While for the first peak due to Se7.68Te0.32, the obtained n value indicates that the growth is carried out in only two dimensions. The activation energy (Ec), which was determined from thermal analysis, is considered to be the sum of the nucleation activation energy (En) and the crystal growth activation energy (EG) [28–32]. It has been pointed out [33] that in non-isothermal measurements,
2.2
-1
-2
-3 1.96
1.98
2.00
2.02
2.04
2.06
1000/T K-1 Fig. 9. Plot of ln[ln(1 w)] versus 1000/T for Se77.5Te20Sn2.5 glass at different heating rates (a) for first peak and (b) second peak.
due to a fast increase in temperature and large discrepancies in the latent heats of nucleation and growth, the crystallization exotherm characterizes the growth of the crystalline phase from the amorphous matrix, nucleation calorimetrically unobservable at temperatures below the crystallization exotherm. Therefore, the obtained values of Ec can be taken to represent the activation energy of growth EG of the present system.
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Table 3 Characteristic parameters for the thermal stability and glass fragility of the Se80 xTe20Snx chalcogenide glasses. Heating rate
Se80Te20
Se77.5Te20Sn2.5
Thermal parameters
5
10
15
20
25
35
First phase Hr H/ S Fi
0.221 0.109 0.936 35.38
0.244 0.116 0.878 24.46
0.246 0.116 1.060 20.68
0.252 0.115 1.128 18.57
0.268 0.120 1.057 17.21
0.289 0.125 1.076 15.5
First phase Hr H/ S Fi
0.406 0.152 2.889 34.54
0.458 0.163 3.826 23.89
0.472 0.165 4.502 20.16
0.499 0.171 4.628 18.12
0.544 0.180 5.105 16.79
0.565 0.184 6.124 15.12
3.004 0.396 2.980
2.946 0.387 2.916
2.886 0.382 2.888
2.792 0.379 2.878
2.803 0.377 2.861
2.956 0.380 2.405
0.864 0.228 2.019 33.23
1.055 0.248 1.665 22.99
1.100 0.249 2.131 19.37
1.153 0.254 2.335 17.43
1.193 0.257 2.601 16.14
1.288 0.264 2.926 14.53
1.829 0.319 2.578
1.954 0.320 2.251
1.995 0.317 2.052
2.068 0.319 1.682
2.111 0.320 1.390
Second phase Hr H/ S Se75Te20Sn5
First phase Hr H/ S Fi Second phase Hr H/ S
Tammann Fulcher equation) and are declared as fragile glassforming liquids.
3.3. Glass-forming ability, thermal stability and glass fragility Glass-forming ability (GFA) is a fundamental issue in the study of chalcogenide glasses, because it determines the degree of utilization of the investigated materials in various applications. Hruby [34] has introduced a parameter Hr, which can be used as the index of the GFA: Hr ¼
Tc Tg Tm Tc
ð11Þ
On the other hand, thermal stability was investigated by Saad and Poulain [35], and they introduced two parameters, which can be used as an indication of thermal stability of a glass. These parameters are H= ¼
S¼
Tc Tg Tg
ðTp Tc ÞTc Tg Tg
ð12Þ
ð13Þ
The values, Hr, H/ and S are all given in Table 3 as function of heating rate and composition. It can be noted from this table that the the addition of Sn to the Se–Te binary glass increases the stability of the studied glasses. On the basis of the obtained Tg and Eg data, the fragility index m is calculated from the following equation [36]: Eg Fi ¼ RTg ln a
4. Conclusions Calorimetric measurements were performed on Se80 xTe20Snx (x¼0, 2.5 and 5) chalcogenide glasses. DSC thermograms of the studied glasses show that each composition has one glass transition and double crystallization stage except the Sn free sample that has only one crystallization stage. All characteristic temperatures Tg, Tc and Tp increase with the increase in the heating rate and Sn content. The average value of Ec for the second crystallization stage is higher than that for the first crystallization phase. According to the Avrami theory of nucleation, the calculated values of the kinetic exponent n suggest surface growth for the first stage, Se77.5Te20Sn2.5 and bulk crystallization for the second stage. The crystalline phases developed in the sample annealed at temperature corresponding to the crystallization peak were identified by XRD to be Se7.68Te0.32, SnTe and SnTe phases.
Acknowledgments The author wishes to thank Professor A.G. Mostafa for useful discussion and comments.
ð14Þ
Note that a low value of Fi (Fi E16) [37] defined the strong glassforming liquids, while a high value of Fi (Fi E200) [38] represents the fragile glass-forming liquid. The values of Fi for the present system at different heating rates are presented in Table 3. The results show that this system was created from the strong glass-forming liquids. On the other hand, the classification of glass-formation dependence on their viscosities [39], it is known that liquids that exhibit an Arrhenius temperature dependence on viscosity are defined as strong glass-forming liquids and these exhibit a non-Arrhenius behavior (described by a Vogel
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