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Study of the heating rate effect on the glass transition properties of (60 − x)V2O5–xSb2O3–40TeO2 oxide glasses using differential scanning calorimetry (DSC)
Measurement 44 (2011) 2049–2053
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Measurement journal homepage: www.elsevier.com/locate/measurement
Study of the heating rate effect on the glass transition properties of (60 x)V2O5–xSb2O3–40TeO2 oxide glasses using differential scanning calorimetry (DSC) Dariush Souri Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 30 July 2011 Accepted 17 August 2011 Available online 25 August 2011 Keywords: Glass transition temperature Crystallization temperature Differential scanning calorimetry (DSC) Oxide glasses Amorphous
a b s t r a c t The glass-transition temperature (Tg) and crystallization temperature (TCr) have been determined for the system (60 x)V2O5–xSb2O3–40TeO2 with 0 < x < 10 (in mol%) using differential scanning calorimetry (DSC) at heating rates u = 3, 6, 9 and 13 K/min. The effect of the heating rate and the Sb2O3 content on Tg is discussed. It was observed that the transition region shifts to higher temperatures when the measuring time is reduced (or, conversely, when the applied temperature rate is increased). Using differential scanning calorimetry, the compositional dependence of Tg has been determined and so, an empirical equation has been deduced which relates the glass-transition temperature with the Sb2O3 content. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Multicomponent oxide glasses were mainly studied for their optical, electrical and structural properties [1–17]. Among the oxide glasses, tellurite glasses are of technical interest because they have low melting point, high glass forming ability and no hygroscopic property which limit the application of phosphate glasses [18–23]. Study in structural features of glasses by DSC investigation is a suitable way to understand the behavior of glasses as a function of composition [18]. Investigation of the role of changing in glass composition on the thermal properties, the concentration of non-bridging oxygen, rigidity and packing of the structure of glass can help us to reach to optimized composition satisfying high thermal stability against thermal shocks for technological applications. The nature of glass transition is a fundamental issue in the condensed matter physics [19,24]. In spite of extensive research devoted to understand the phenomenon of glass transition, there is no satisfactory description of this
E-mail addresses: [email protected], [email protected] 0263-2241/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.08.005
phenomenon and more work is needed to overcome this difficulty. The differential scanning calorimetry (DSC) technique is widely used to investigate the glass transformation in glassy materials. The kinetics of the glass transition, as studied by the DSC method, is important in investigating the nature of the glass transformation process. The glass transition temperature, Tg, can be accurately determined by DSC measurements. Moreover, the kinetic aspect of the glass transition is evident from the strong dependence of Tg on the heating rate [19–23]. It can be observed that the transition region shifts to higher temperatures if the measuring time is reduced (or, conversely, if the applied temperature rate is increased) [25,26]. To the best of our knowledge, there are some papers on the calorimetric properties of TeO2-based glasses and then, tricomponent glass systems of the form AmOn–TeO2–V2O5 (AmOn is an another oxide) have been studied [13–17,27]; in this work, due to the importance of the binary TeO2–V2O5 glass, the calorimetric properties of V2O5– Sb2O3–TeO2 glasses are studied. Thus, the present paper reports the measurements of glass transition behavior for several V2O5–Sb2O3–TeO2 glasses. In the works have been done described in [2,3] the optical and electrical and some
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D. Souri / Measurement 44 (2011) 2049–2053
the melts for these compositions are relatively easy to form glass. The melts for the compositions containing either x > 10 crystallize rapidly during quenching. During the sample production, the melt was mixed every 5 min to prevent the separation of the three components. The melt was poured onto a polished steel block and immediately pressed by another polished steel block, where the blocks were kept at room temperature. All of the obtained bulk samples were annealed at 473 K for 2 h to eliminate the mechanical stresses resulting from the quenching [28,29]. The characterization of the glass systems was carried out by X-ray diffraction (XRD) studies using a Bruker diffractometer (AXS D8 Advance, Cu Ka, Germany). The density (q) of each sample was calculated by the Archimedes’s method using para-xylene as immersion liquid. Also, the glass transition temperature (Tg) of these samples were obtained using differential scanning calorimetry (NETZSCH DSC 200 F3, Germany); results of
structural properties of the present samples have been investigated, but there is not any report on their glass transition nature. In summary, the objectives of this work are: (1) to investigate the effect of heating rate on the glass transition temperature of the amorphous V2O5–Sb2O3– TeO2 samples, (2) to investigate the compositional dependence of the glass transition. 2. Experimental procedure Reagent grade (99.99%) chemicals V2O5, TeO2 and Sb2O3 were well mixed in prescribed composition in a mortar. Each glass batch (12 g) was melted in air in an alumina crucible in an electric furnace (ATBIN ALF15). Therefore, the ternary (60 x)V2O5–xSb2O3–40TeO2 glasses with 0 6 x 6 10 (in mol%), hereafter termed as 40TVSx, were prepared by standard melt quenching technique. This compositional range was chosen primarily for the reason that 2
0.5 0 -0.5 -1 -1.5
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Temperature (oC) Fig. 1. DSC curves of the 40TVS8 sample at different heating rates. (a) u = 3 K/min, (b) u = 6 K/min, (c) u = 9 K/min, (d) u = 10 K/min and (e) u = 13 K/min (for better clarity, plots are shown separately).
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3. Results and discussion 3.1. XRD patterns XRD characterization of 40TVSx samples has been carried out on different samples, confirming the amorphous nature of them; results have been reported in the work has been done described in [3]. 3.2. Thermal analysis The representative DSC outputs of the 40TVS8 at different heating rates are shown in Fig. 1. The obtained data for other samples are listed in Table 1. In the absence of thermal events, the position of the baseline in such a plot is proportional to the specific heat of the sample. The presence of an endothermic peak, superimposed on the baseline, indicates the occurrence of a heat-absorbing event such as glass transition or melting. On the other hand, an exothermic peak occurs as a result of some sort of heatreleasing event such as crystallization [30]. The glass transition temperature, Tg, as defined by the endothermic change in the DSC trace indicates a large change of viscosity, marking a transformation from amorphous solid phase to supercooled liquid state. Table 1 Glass transition temperature (Tg), crystallization temperature (TCr), and values of the constants A and n in Eq. (1) at the used heating rates (u = 3, 6, 9, 10, 13 K/min) for 40TVSx glasses. Glass
u (K/min)
Tg (°C)
TCr (°C)
n 103
A (K)
40TVS0
3 6 9 10 13
227.70 232.68 234.40 234.80 235.96
267.16 283.20 286.80 287.50 290.64
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
6 9 10 13
256.70 261.10 261.60 264.20
354 364.10 365.60 383
11.3 11.72 11.8 12
505.88 507.96 508.31 509.94
3 6 9 10 13
269.60 274.10 276.40 277.50 281.60
397.60 421.70 438.80 444.10 449.60
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
3 6 9 10 13
273.91 278.52 283 283.67 284.75
423.11 448.52 474.27 383.10 385.72
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
40TVS5
40TVS8
40TVS10
As the output of the DSC during heating is proportional to the heat capacity, it is a straightforward and convenient method of detecting the glass transition and investigating of its kinetics. For example, the heating-rate dependence of Tg can be investigated and used to determine the activation energy of the transition from glassy to liquid state [25,31–34]. In this work, the middle point of the endothermic trace was used to define Tg. Other definitions for Tg were used by different workers. For instance, Abu-Sehly et al. [32] and Avramov et al. [25] used different definitions of Tg that included the extrapolated onset, the inflection point and the maximum point of the endothermic trace. The exothermic peak temperature TCr is used to identify the crystallization process. Both TCr and Tg shift to higher temperatures with increasing heating rate. The heatingrate dependence of Tg is clearly seen in Fig. 1 and Table 1. The kinetic aspect of the glass transition is evident from the pronounced shift in Tg. It is worth observing that an order of magnitude increase in u causes a shift in Tg of about 5 K [35]. The general features of DSC thermograms for the 40TVS8 glass indicate a wide range between the glass-transition and the crystallization temperatures (see Fig. 1). These thermograms indicate that the glass-transition temperature varies with the rate of heating. It increases with increasing heating rate. The obtained DSC values for the glass-transition temperature Tg at different heating rates u for the other compositions show similar behavior. The variation of Tg with u for each composition shows a non-linear dependence as shown in Fig. 2. The curves in Fig. 2 indicate somewhat a more steep variation of Tg at a lower rate of heating. In other words, for the investigated system, the glass-transition temperature is more effective at a lower heating rate. This means that the dependence of Tg on u is more pronounced at the lower heating rate. On the other hand, the DSC data reported in Table 1 show that for the different compositions, the glass transition temperature increase with increasing antimony oxide content in all heating rates indicate similar behavior. This variation is shown in Fig. 3, where Tg is seen to increase with increasing x or with decreasing V-content for each heating rate. Furthermore, using the results of the work has been done described in [3] and results presented in Fig. 3, Tg data show that the
560
x=0 mol5
540
Tg (K)
density, XRD patterns and glass transition temperature have been previously reported [2,3]. For each DSC measurement, the sample was first heated at an arbitrary heating rate (normally 20 K/min) to a temperature that is 20–30 K higher than its glass transition temperature (Tg) and held there for 5 min to erase the previous thermal history (during cooling the melt) of the glass. After the isothermal hold, the sample was cooled at 6 K/min through the glass transition region to a temperature (100 °C) well below Tg, and then reheated at a rate (u) of 3, 6, 9 and 13 K/min to record the DSC curves.
x=5 mol% x=8 mol% x=10 mol%
520
500 3
5
7
9
11
13
15
17
(K/min) Fig. 2. Variation of glass transition temperature (Tg) versus heating rate (u), for 40TVSx glasses.
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510
550
508
A (Tg , K)
560
Tg (K)
540 =3 K/min
530
=6 K/min
520
=9 K/min
510
=10 K/min
A = 5.826 ln + 495.08 R2 = 0.9979
506 504 502
=13 K/min
500
500
1 0
3
6
9
1.4
12
1.8
2.2
2.6
Ln ( )
x: Sb 2O3 content (mol%) Fig. 3. Variation of Tg obtained from DSC with the antimony oxide content (x) for 40TVSx glasses, at different heating rates (u).
Fig. 5. Relation between the constant A and the heating rate u, for 40TVSx glasses.
12.2
6.33 =3 K/min 6.31
=6 K/min =9 K/min
Ln (Tg , K)
6.29
=10 K/min =13 K/min
6.27 6.25 6.23 6.21 -0.5
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
x 0.9 Fig. 4. Plot of ln Tg versus x0.9 at different heating rate u, for 40TVSx glasses.
n = 0.0031 2 + 11.479 R2 = 0.9983
12
n
glass transition temperature is sensitive to the Sb2O3 concentration. Increasing of Tg can be interpreted as increasing of the thermal stability of the glass. The thermal stability of the glass is a result of the glass structure; in other word, in this work, the change in Tg indicates a change related to the manner in which V2O5 and Sb2O3 get arranged in the glass. The thermally stable glasses will have close packed structure, while the unstable glasses will have loose packed structure [36]. Thus the addition of Sb2O3 increases the stability of the glass and the rigidity of the network, which is in agreement with the data of the glass density (a criterion of packing) (reported in [3]). It has been reported [3] that the data of density q increase with increase in Sb2O3 content. Since Sb2O3 has a high relative molecular mass, thus, it is an expected result. This means that the glass-transition temperature increases if the average coordination number increases. This may be due to the decrease in the number of V–V bonds and the increase of the V–Sb bonds as a result of the increasing of the Sb2O3 content (x) and the decrease of the V content. In other word, the cross-linking provided by Sb atoms increases for 40TVSx samples, which in turn affects the structure in a manner to increase the Tg. By plotting ln(Tg) versus x0.9, we find straight lines with different slopes, as shown in Fig. 4. It is found that the slope n of
11.8 11.6 11.4 0
40
80
[ (K/min)]
120
160
2
Fig. 6. Relation between the constant n and heating rate u, for 40TVSx glasses.
the straight lines depends on the heating rate u, that it has a positive value and that it increases with increasing u. This means that the rate of variation of Tg with the antimony oxide content x is larger for the lower heating rate. The dependence of n on the heating rate u is shown in Fig. 5. It can be concluded that the relation between Tg and x within the investigated ranges obeys an empirical equation of the Arrhenius form as follows:
T g ¼ A expðnx0:9 Þ
ð1Þ
where Tg is the glass-transition temperature (in K), which can be calculated at any heating rate and for any antimony oxide content within the measured range, A is a constant depending on u. Its value has been obtained by extrapolation of the linear relation in Fig. 6 to x0.9 = 0 (see also Table 1), n is a constant depending on u (see Table 1), and x is the antimony content (at mol%). The values of n and A can be derived at any desired heating rate within the measured range from Figs. 5 and 6, respectively; in Figs. 5 and 6 the variation of n and A can be estimated by using the equations n = (a0 u2) + b0 and A = alnu + b, respectively (as shown in the inset of these figures); a0 , b0 , a and b are constants, which have been illustrates in Figs. 5 and 6. Finally, one can conclude and determine the effect of heating rate on the glass transition temperature. 4. Conclusions Investigation of heating-rate dependence of the glass transition temperature in (60 x)V2O5–xSb2O3–40TeO2
D. Souri / Measurement 44 (2011) 2049–2053
glasses was carried out using DSC technique. The increase at the glass-transition temperature with antimony oxide content in the investigated system obeys an equation of the Arrhenius form. The increase of Tg with increasing Sb2O3 content may be due to an increase of the number of Sb–V bonds as a result of the increasing antimony oxide content. Tg depends on heating rate and increase with increase of it. The compositional dependence of Tg has been identified for the present samples.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
J.E. Stanworth, J. Soc. Glass Technol. 36 (1952) 217. D. Souri, J. Non-Cryst. Solids 356 (2010) 2181–2184. D. Souri, K. Shomalian, J. Non-Cryst. Solids 355 (2009) 1597–1601. D. Souri, S.A. Salehizadeh, J. Mater. Sci. 44 (2009) 5800–5805. D. Souri, J. Phys. D: Appl. Phys. 41 (2008) 105102. D. Souri, M. Elahi, Phys. Scr. 75 (2) (2007) 219–226. D. Souri, M. Elahi, M.S. Yazdanpanah, Cent. Eur. J. Phys. 6 (2) (2008) 306–310. B.V.R. Chowdari, P.P. Kumari, J. Phys. Chem. Solids 58 (3) (1997) 515–525. M. Pal, K. Hirota, Y. Tsujigami, H. Sakata, J. Phys. D: Appl. (2001) 459–464. B.K. Sharma, D.C. Dube, A. Mansingh, J. Non-Cryst. 65 (1984) 39–51. G.S. Murugan, Y. Ohishi, J. Non-Cryst. 341 (2004) 86–92. S. Jayaseelan, P. Muralidharan, M. Venkateswarlu, N. Satyanarayana, Mater. Sci. Eng. B 118 (2005) 136–143. R. El-Mallawany, A. Abousehly, E. Yousef, J. Mater. Sci. Lett. 19 (2000) 409–411. A. El-Adawy, R. El-Mallawany, J. Mater. Sci. Lett. 15 (1996) 2065–2067.
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[15] A. Abdel-Kader, R. El-Mallawany, M.M. Elkholy, J. Appl. Phys. 73 (1) (1993) 71–74. [16] R. El-Mallawany, Phys. Status Solidi (a) 177 (2000) 439–444. [17] M.A. Sidkey, R. El-Mallawany, A. Abousehly, Y.B. Saddeek, Glass Sci. Technol.: Glastechnische Berichte 75 (2002) 87–93. [18] G. Turky, M. Dawy, Mater. Chem. Phys. 77 (2002) 48–59. [19] G.W. Scherer, Relaxation in Glasses and Composites, John Wiley and Sons Inc., New York, 1986. [20] E. Gerlach, P. Grosse (Eds.), The Physics of Selenium and Tellurium, Springer, Berlin/New York, 1979. [21] S.G. Bishop, U. Strom, P.C. Taylor, Phys. Rev. Lett. 34 (1975) 1346. [22] J. Donohue, The Structure of the Elements, Wiley, New York, 1974. [23] C.T. Moynihan, A.J. Easteal, J. Wilder, J. Tucker, J. Phys. Chem. 78 (1974) 2673. [24] S.R. Elliott, Physics of Amorphous Materials, second ed., Longman Scientific & Technical, Essex, UK, 1990. [25] I. Avramov, G. Guinev, A.C.M. Rodrigues, J. Non-Cryst. Solids 271 (2000) 12–17. [26] D. Zhu, C.S. Ray, W. Zhou, D.E. Day, J. Mater. Sci. 39 (2004) 7351– 7357. [27] K. Sega, Y. Kuroda, H. Sakata, J. Mater. Sci. 33 (1998) 1303. [28] N. Lebrun, M. Levy, J.L. Soquet, Solid State Ionics 40 (41) (1990) 718– 722. [29] V.K. Dhawan, A. Mansingh, J. Non-Cryst. Solids 51 (1982) 87–103. [30] G.W. Smith, F.E. Pinkerton, J.J. Moleski, Thermochim. Acta 342 (1999) 31–39. [31] S. Vyazovkin, N. Sbirrazzuoli, I. Dranca, Macromol. Rapid Commun. 25 (2004) 1708. [32] A.A. Abu-Sehly, M. Abu El-Oyoun, A.A. Elabbar, Thermochem. Acta 472 (2008) 25–30. [33] D. Zhu, C.S. Ray, W. Zhou, D.E. Day, J. Non-Cryst. Solids 319 (2003) 247–256. [34] J. Rocherull’e, M. Matecki, Y. Delugeard, J. Non-Cryst. Solids 238 (1998) 51–56. [35] M. Lasocka, Mater. Sci. 23 (1976) 173. [36] R.N. Sinclair, A.C. Wrigth, B. Bachra, Y.B. Dimitriev, V.V. Dimitrov, M.G. Arnaudov, J. Non-Cryst. Solids 232–234 (1998) 38–43.
Contents lists available at SciVerse ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Study of the heating rate effect on the glass transition properties of (60 x)V2O5–xSb2O3–40TeO2 oxide glasses using differential scanning calorimetry (DSC) Dariush Souri Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 30 July 2011 Accepted 17 August 2011 Available online 25 August 2011 Keywords: Glass transition temperature Crystallization temperature Differential scanning calorimetry (DSC) Oxide glasses Amorphous
a b s t r a c t The glass-transition temperature (Tg) and crystallization temperature (TCr) have been determined for the system (60 x)V2O5–xSb2O3–40TeO2 with 0 < x < 10 (in mol%) using differential scanning calorimetry (DSC) at heating rates u = 3, 6, 9 and 13 K/min. The effect of the heating rate and the Sb2O3 content on Tg is discussed. It was observed that the transition region shifts to higher temperatures when the measuring time is reduced (or, conversely, when the applied temperature rate is increased). Using differential scanning calorimetry, the compositional dependence of Tg has been determined and so, an empirical equation has been deduced which relates the glass-transition temperature with the Sb2O3 content. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Multicomponent oxide glasses were mainly studied for their optical, electrical and structural properties [1–17]. Among the oxide glasses, tellurite glasses are of technical interest because they have low melting point, high glass forming ability and no hygroscopic property which limit the application of phosphate glasses [18–23]. Study in structural features of glasses by DSC investigation is a suitable way to understand the behavior of glasses as a function of composition [18]. Investigation of the role of changing in glass composition on the thermal properties, the concentration of non-bridging oxygen, rigidity and packing of the structure of glass can help us to reach to optimized composition satisfying high thermal stability against thermal shocks for technological applications. The nature of glass transition is a fundamental issue in the condensed matter physics [19,24]. In spite of extensive research devoted to understand the phenomenon of glass transition, there is no satisfactory description of this
E-mail addresses: [email protected], [email protected] 0263-2241/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.08.005
phenomenon and more work is needed to overcome this difficulty. The differential scanning calorimetry (DSC) technique is widely used to investigate the glass transformation in glassy materials. The kinetics of the glass transition, as studied by the DSC method, is important in investigating the nature of the glass transformation process. The glass transition temperature, Tg, can be accurately determined by DSC measurements. Moreover, the kinetic aspect of the glass transition is evident from the strong dependence of Tg on the heating rate [19–23]. It can be observed that the transition region shifts to higher temperatures if the measuring time is reduced (or, conversely, if the applied temperature rate is increased) [25,26]. To the best of our knowledge, there are some papers on the calorimetric properties of TeO2-based glasses and then, tricomponent glass systems of the form AmOn–TeO2–V2O5 (AmOn is an another oxide) have been studied [13–17,27]; in this work, due to the importance of the binary TeO2–V2O5 glass, the calorimetric properties of V2O5– Sb2O3–TeO2 glasses are studied. Thus, the present paper reports the measurements of glass transition behavior for several V2O5–Sb2O3–TeO2 glasses. In the works have been done described in [2,3] the optical and electrical and some
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D. Souri / Measurement 44 (2011) 2049–2053
the melts for these compositions are relatively easy to form glass. The melts for the compositions containing either x > 10 crystallize rapidly during quenching. During the sample production, the melt was mixed every 5 min to prevent the separation of the three components. The melt was poured onto a polished steel block and immediately pressed by another polished steel block, where the blocks were kept at room temperature. All of the obtained bulk samples were annealed at 473 K for 2 h to eliminate the mechanical stresses resulting from the quenching [28,29]. The characterization of the glass systems was carried out by X-ray diffraction (XRD) studies using a Bruker diffractometer (AXS D8 Advance, Cu Ka, Germany). The density (q) of each sample was calculated by the Archimedes’s method using para-xylene as immersion liquid. Also, the glass transition temperature (Tg) of these samples were obtained using differential scanning calorimetry (NETZSCH DSC 200 F3, Germany); results of
structural properties of the present samples have been investigated, but there is not any report on their glass transition nature. In summary, the objectives of this work are: (1) to investigate the effect of heating rate on the glass transition temperature of the amorphous V2O5–Sb2O3– TeO2 samples, (2) to investigate the compositional dependence of the glass transition. 2. Experimental procedure Reagent grade (99.99%) chemicals V2O5, TeO2 and Sb2O3 were well mixed in prescribed composition in a mortar. Each glass batch (12 g) was melted in air in an alumina crucible in an electric furnace (ATBIN ALF15). Therefore, the ternary (60 x)V2O5–xSb2O3–40TeO2 glasses with 0 6 x 6 10 (in mol%), hereafter termed as 40TVSx, were prepared by standard melt quenching technique. This compositional range was chosen primarily for the reason that 2
0.5 0 -0.5 -1 -1.5
3 2 1 0 -1 -2
-2 -2.5 150
(b)
4
1
Heat Flow (mW) Exo-Up
Heat Flow (mW) Exo-Up
5
(a)
1.5
200
250
300
350
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-3 150
500
200
250
300
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Temperature ( C) 0.5
12
Heat Flow (mW) Exo-Up
Heat Flow (mW) Exo-Up
8 6 4 2 0 -2 -4 150
(d)
0.4
(c)
10
0.3 0.2 0.1 0 -0.1 -0.2 -0.3
200
250
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400
450
-0.4 150
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Temperature (oC)
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6
(e)
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4 2 0 -2 -4 -6 150
200
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Temperature (oC) Fig. 1. DSC curves of the 40TVS8 sample at different heating rates. (a) u = 3 K/min, (b) u = 6 K/min, (c) u = 9 K/min, (d) u = 10 K/min and (e) u = 13 K/min (for better clarity, plots are shown separately).
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3. Results and discussion 3.1. XRD patterns XRD characterization of 40TVSx samples has been carried out on different samples, confirming the amorphous nature of them; results have been reported in the work has been done described in [3]. 3.2. Thermal analysis The representative DSC outputs of the 40TVS8 at different heating rates are shown in Fig. 1. The obtained data for other samples are listed in Table 1. In the absence of thermal events, the position of the baseline in such a plot is proportional to the specific heat of the sample. The presence of an endothermic peak, superimposed on the baseline, indicates the occurrence of a heat-absorbing event such as glass transition or melting. On the other hand, an exothermic peak occurs as a result of some sort of heatreleasing event such as crystallization [30]. The glass transition temperature, Tg, as defined by the endothermic change in the DSC trace indicates a large change of viscosity, marking a transformation from amorphous solid phase to supercooled liquid state. Table 1 Glass transition temperature (Tg), crystallization temperature (TCr), and values of the constants A and n in Eq. (1) at the used heating rates (u = 3, 6, 9, 10, 13 K/min) for 40TVSx glasses. Glass
u (K/min)
Tg (°C)
TCr (°C)
n 103
A (K)
40TVS0
3 6 9 10 13
227.70 232.68 234.40 234.80 235.96
267.16 283.20 286.80 287.50 290.64
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
6 9 10 13
256.70 261.10 261.60 264.20
354 364.10 365.60 383
11.3 11.72 11.8 12
505.88 507.96 508.31 509.94
3 6 9 10 13
269.60 274.10 276.40 277.50 281.60
397.60 421.70 438.80 444.10 449.60
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
3 6 9 10 13
273.91 278.52 283 283.67 284.75
423.11 448.52 474.27 383.10 385.72
11.5 11.3 11.72 11.8 12
501.30 505.88 507.96 508.31 509.94
40TVS5
40TVS8
40TVS10
As the output of the DSC during heating is proportional to the heat capacity, it is a straightforward and convenient method of detecting the glass transition and investigating of its kinetics. For example, the heating-rate dependence of Tg can be investigated and used to determine the activation energy of the transition from glassy to liquid state [25,31–34]. In this work, the middle point of the endothermic trace was used to define Tg. Other definitions for Tg were used by different workers. For instance, Abu-Sehly et al. [32] and Avramov et al. [25] used different definitions of Tg that included the extrapolated onset, the inflection point and the maximum point of the endothermic trace. The exothermic peak temperature TCr is used to identify the crystallization process. Both TCr and Tg shift to higher temperatures with increasing heating rate. The heatingrate dependence of Tg is clearly seen in Fig. 1 and Table 1. The kinetic aspect of the glass transition is evident from the pronounced shift in Tg. It is worth observing that an order of magnitude increase in u causes a shift in Tg of about 5 K [35]. The general features of DSC thermograms for the 40TVS8 glass indicate a wide range between the glass-transition and the crystallization temperatures (see Fig. 1). These thermograms indicate that the glass-transition temperature varies with the rate of heating. It increases with increasing heating rate. The obtained DSC values for the glass-transition temperature Tg at different heating rates u for the other compositions show similar behavior. The variation of Tg with u for each composition shows a non-linear dependence as shown in Fig. 2. The curves in Fig. 2 indicate somewhat a more steep variation of Tg at a lower rate of heating. In other words, for the investigated system, the glass-transition temperature is more effective at a lower heating rate. This means that the dependence of Tg on u is more pronounced at the lower heating rate. On the other hand, the DSC data reported in Table 1 show that for the different compositions, the glass transition temperature increase with increasing antimony oxide content in all heating rates indicate similar behavior. This variation is shown in Fig. 3, where Tg is seen to increase with increasing x or with decreasing V-content for each heating rate. Furthermore, using the results of the work has been done described in [3] and results presented in Fig. 3, Tg data show that the
560
x=0 mol5
540
Tg (K)
density, XRD patterns and glass transition temperature have been previously reported [2,3]. For each DSC measurement, the sample was first heated at an arbitrary heating rate (normally 20 K/min) to a temperature that is 20–30 K higher than its glass transition temperature (Tg) and held there for 5 min to erase the previous thermal history (during cooling the melt) of the glass. After the isothermal hold, the sample was cooled at 6 K/min through the glass transition region to a temperature (100 °C) well below Tg, and then reheated at a rate (u) of 3, 6, 9 and 13 K/min to record the DSC curves.
x=5 mol% x=8 mol% x=10 mol%
520
500 3
5
7
9
11
13
15
17
(K/min) Fig. 2. Variation of glass transition temperature (Tg) versus heating rate (u), for 40TVSx glasses.
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D. Souri / Measurement 44 (2011) 2049–2053
510
550
508
A (Tg , K)
560
Tg (K)
540 =3 K/min
530
=6 K/min
520
=9 K/min
510
=10 K/min
A = 5.826 ln + 495.08 R2 = 0.9979
506 504 502
=13 K/min
500
500
1 0
3
6
9
1.4
12
1.8
2.2
2.6
Ln ( )
x: Sb 2O3 content (mol%) Fig. 3. Variation of Tg obtained from DSC with the antimony oxide content (x) for 40TVSx glasses, at different heating rates (u).
Fig. 5. Relation between the constant A and the heating rate u, for 40TVSx glasses.
12.2
6.33 =3 K/min 6.31
=6 K/min =9 K/min
Ln (Tg , K)
6.29
=10 K/min =13 K/min
6.27 6.25 6.23 6.21 -0.5
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
x 0.9 Fig. 4. Plot of ln Tg versus x0.9 at different heating rate u, for 40TVSx glasses.
n = 0.0031 2 + 11.479 R2 = 0.9983
12
n
glass transition temperature is sensitive to the Sb2O3 concentration. Increasing of Tg can be interpreted as increasing of the thermal stability of the glass. The thermal stability of the glass is a result of the glass structure; in other word, in this work, the change in Tg indicates a change related to the manner in which V2O5 and Sb2O3 get arranged in the glass. The thermally stable glasses will have close packed structure, while the unstable glasses will have loose packed structure [36]. Thus the addition of Sb2O3 increases the stability of the glass and the rigidity of the network, which is in agreement with the data of the glass density (a criterion of packing) (reported in [3]). It has been reported [3] that the data of density q increase with increase in Sb2O3 content. Since Sb2O3 has a high relative molecular mass, thus, it is an expected result. This means that the glass-transition temperature increases if the average coordination number increases. This may be due to the decrease in the number of V–V bonds and the increase of the V–Sb bonds as a result of the increasing of the Sb2O3 content (x) and the decrease of the V content. In other word, the cross-linking provided by Sb atoms increases for 40TVSx samples, which in turn affects the structure in a manner to increase the Tg. By plotting ln(Tg) versus x0.9, we find straight lines with different slopes, as shown in Fig. 4. It is found that the slope n of
11.8 11.6 11.4 0
40
80
[ (K/min)]
120
160
2
Fig. 6. Relation between the constant n and heating rate u, for 40TVSx glasses.
the straight lines depends on the heating rate u, that it has a positive value and that it increases with increasing u. This means that the rate of variation of Tg with the antimony oxide content x is larger for the lower heating rate. The dependence of n on the heating rate u is shown in Fig. 5. It can be concluded that the relation between Tg and x within the investigated ranges obeys an empirical equation of the Arrhenius form as follows:
T g ¼ A expðnx0:9 Þ
ð1Þ
where Tg is the glass-transition temperature (in K), which can be calculated at any heating rate and for any antimony oxide content within the measured range, A is a constant depending on u. Its value has been obtained by extrapolation of the linear relation in Fig. 6 to x0.9 = 0 (see also Table 1), n is a constant depending on u (see Table 1), and x is the antimony content (at mol%). The values of n and A can be derived at any desired heating rate within the measured range from Figs. 5 and 6, respectively; in Figs. 5 and 6 the variation of n and A can be estimated by using the equations n = (a0 u2) + b0 and A = alnu + b, respectively (as shown in the inset of these figures); a0 , b0 , a and b are constants, which have been illustrates in Figs. 5 and 6. Finally, one can conclude and determine the effect of heating rate on the glass transition temperature. 4. Conclusions Investigation of heating-rate dependence of the glass transition temperature in (60 x)V2O5–xSb2O3–40TeO2
D. Souri / Measurement 44 (2011) 2049–2053
glasses was carried out using DSC technique. The increase at the glass-transition temperature with antimony oxide content in the investigated system obeys an equation of the Arrhenius form. The increase of Tg with increasing Sb2O3 content may be due to an increase of the number of Sb–V bonds as a result of the increasing antimony oxide content. Tg depends on heating rate and increase with increase of it. The compositional dependence of Tg has been identified for the present samples.
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