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The effect of gamma irradiation on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass
ARTICLE IN PRESS Radiation Physics and Chemistry 79 (2010) 104–108
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
The effect of gamma irradiation on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass Omar A. Lafi, Mousa M.A. Imran Materials Science Laboratory, Department of Applied Sciences, Prince Abdullah Bin Ghazi Faculty of Science and Information Technology, Al-Balqa’ Applied University, Al-Salt 19117, Jordan
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 May 2009 Accepted 6 August 2009
Se96Sn4 chalcogenide glass was prepared by melt quenching technique and exposed, at room temperature, to different doses of 4, 8, 12, 24 and 33 kGy of high-energy 60Co gamma irradiation. Differential scanning calorimeter (DSC) was used under non-isothermal condition to determine the glass transition temperature Tg, onset Tc and peak Tp temperatures of crystallization, of un-irradiated and g-irradiated samples, at four different heating rates. The variation of Tg with heating rates was utilized to calculate the glass transition activation energy Et for un-irradiated and g-irradiated glass, using the methods suggested by Kissinger and Moynihan. Based on the obtained values of the characteristic temperatures Tg, Tc and Tp, thermal stability was monitored through the calculation of the S parameter and the crystallization rate factor /KpS for irradiated and un-irradiated glass. Results reveal that, as g-dose increases Tg increases up to 12 kGy then decreases at higher doses but remains more than that of un-irradiated glass. Meanwhile, both Et and /KpS attain their minimum values at the same dose of 12 kGy and the glass is thermally stable at this particular dose. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Chalcogenide glasses Gamma irradiation Glass transition temperature Thermal stability
1. Introduction During the last two decades, several investigations have dealt with the influence of laser, electron, gamma and neutron irradiation on the physical properties of chalcogenide glasses (Shpotyuk, 1995; Shpotyuk et al., 2000, 2007; Imran et al., 2002; Lucas et al., 2006; El-Zahed, 2001; Balitska et al., 2003; El-Sayed, 2004; Amin and Spyrou, 2005; Salem et al., 2007; Abu El-Fadl et al., 2007; Xia et al., 2008). According to these studies, the structure of a glass is modified under the influence of irradiation giving rise to changes in thermal, optical, and electrical properties without any atomic transmutations or surface damages that are usually produced by particle irradiation. Many chalcogenide glasses, whose properties can be tailoredmade, are used as main component in optoelectronics and photovoltaic devices to perform certain applications. Such devices may be used in an environment where temperature is fluctuating, which affect the properties of the used glass. It is well known that (Clavaguera-Mora, 1995) as-prepared glasses exist in an unstable state with characteristic thermal parameters such as enthalpy, glass transition temperature and entropy that are usually evolved with time. In fact, temperature fluctuation may transfer asprepared glass to metastable state with different thermal
Corresponding author. Tel.: +962 777 993459; fax: +962 5 3530462.
E-mail addresses: [email protected], [email protected] (M.M. Imran). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.08.005
parameters (Imran et al., 2008). Therefore, as-prepared glass must be relaxed before its final usage. This can be accomplished, by natural storage for long period of time (sometimes several years) or annealing at different temperatures for different annealing times or by high-energy irradiation of a glass, in a process termed as structural relaxation or physical ageing. However, physical ageing induced by high-energy gamma irradiation is very fast and the changes in thermal parameters are uniform through the whole bulk of the glass (Golovchak et al., 2006). To monitor the aforementioned changes, differential scanning calorimeter (DSC) is a very useful technique, as it is usually used to determine the transformation temperatures in chalcogenide glasses, such as glass transition Tg, crystallization Tc and melting Tm temperatures. These characteristic temperatures are further used to evaluate thermal stability before and after g-irradiation. Structural investigation of chalcogenide glasses is important in determining the transport mechanism and thermal stability. This is because the glassy state is essentially a metastable one and hence it inherently possesses the possibility of transforming into a more stable crystalline state. The most promising properties of chalcogenide glasses have been found to deteriorate drastically during crystallization. Thus, understanding the glass transition mechanism and thermal stability is important to determine their effective working limits for a specific technological application before crystallization takes place (Imran et al., 2001; Lafi et al., 2007). On the other hand, transport properties of chalcogenide
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
2. Experimental details Se96Sn4 bulk glass was prepared by melt quenching technique as described elsewhere (Imran et al., 2008). Disc shaped samples (pellets) of diameter 13 mm and different thicknesses ranging from 0.45 to 0.5 mm were prepared by grinding the bulk glass to a fine powder and compressing it in a die at a load of 6 t. The pellets irradiated with gamma rays obtained from a Co60 source (available at Jordan Atomic Energy Commission) with mean energy of gamma-quanta of 1.25 MeV (Co60 is a gamma emitter at 1332.50 and 1173.24 keV) at temperature of 30 1C. The duration of gamma irradiation is such that the overall accumulated dose was 4, 8, 12, 24, and 33 kGy. About (1070.5) mg of the g-irradiated powder sample was capsulated in aluminum pan and subjected to the differential scanning calorimeter (Perkin Elmer DSC-7) at different heating rates (5, 10, 15 and 20 K/min). The temperature precision of this equipment is 70.1 K with an average standard error of about 1 K in the measured values. The DSC equipment was calibrated prior to measurements, using high purity standards Pb, Sn and In with well-known melting points. For the sake of accuracy and to ensure the reproducibility of the characteristic temperatures Tg, Tc and Tp of the DSC curves, three measurements were conducted for each heating rate of gamma irradiated and un-irradiated glass under identical conditions. The experimental data points were taken as the average value of the three supposedly identical measurements. All measurements were referenced to an empty aluminum pan and the temperature range covered in DSC is from room temperature to 250 1C.
3. Results and discussion Typical DSC curves of un-irradiated and g-irradiated Se96Sn4 glass at a heating rate of 20 K/min are shown in Fig. 1, as an example. Similar DSC curves were also obtained for the other heating rates. Two characteristic phenomena (endothermic and exothermic peaks) are evident in the DSC curves in the temperature range of investigation. The first one corresponds to the glass transition region and the other to the crystallization region. The glass transition temperature Tg, onset and peak crystallization temperatures Tc and Tp, respectively, were determined from these DSC curves, for both before and after
Tp Exo
0kGy 4kGy 8kGy 12kGy 24kGy 33kGy
Tg
Heat Flow
glasses strongly depend on the nature and degree of short-range order, which is a characteristic structure of these glasses. The objective of the present work is to investigate the effect of g-irradiation on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass using differential scanning calorimeter. In this respect, the dependence of the glass transition temperature on heating rates is used for calculation of the glass transition activation energy Et of un-irradiated and g-irradiated, at different doses of 4, 8, 12, 24 and 33 kGy, Se96Sn4 glass using the methods suggested by Kissinger (1957) and Moynihan et al. (1974). Thermal stability is investigated through the calculation of thermal stability parameter S (Saad and Poulin, 1987) and the average value of the crystallization rate factor /KpS (Gao and Wang, 1986). It is reported (Imran et al., 2000) that a glass having minimum activation energy have higher probability to jump to a metastable state of lower internal energy and hence is considered most stable. In view of this the aforementioned parameters S, /KpS, and Et are considered as good indicators of thermal stability of a glass. Results indicate that the change in the glass transition temperature Tg is maximum at a dose of 12 kGy and thermal stability indicators /KpS and Et attain their minimum values, while the S parameter is maximum, at this particular dose.
105
Tc Endo 75
100
125
150
175
200
225
250
Temperature (°C) Fig. 1. Non-isothermal DSC curves (at a heating rate of 20 K/min) for Se96Sn4 glass irradiated by different doses of gamma irradiation.
Table 1 Glass transition temperature Tg at 5 K/min, average value of crystallization rate factor /KpS, and glass transition activation energy (Et) for Se96Sn4 glass before and after g-irradiation. Doses (kGy)
0 4 8 12 24 33
Tg (5 K/min) (1C)
90.2 94.0 94.7 96.2 92.7 91.2
/KpS (s)1
0.055 0.092 0.090 0.065 0.088 0.088
Et (kJ/mole) Moynihan
Kissinger
249711 212715 197721 114714 152713 141710
242711 206715 190721 108714 145713 135710
g-irradiation. Both temperatures Tg and Tc have been defined as the temperature corresponds to the intersection of the two linear portions adjoining the transition elbow of the DSC traces in the endothermic and exothermic direction, respectively. The values of Tg for un-irradiated and g-irradiated Se96Sn4 glass at a heating rate of 5 K/min, as obtained from the DSC curves are listed in Table 1. From this table, a small increase (1–6 K) in the glass transition temperature (for the heating rate 5 K/min) is observed after girradiation. The glass transition temperature for the other heating rates gives nearly the same indication. Since chalcogen elements are normally two-fold coordinated, the structural configurations which can be formed are limited essentially to rings (Se8) or chains (–Se–Se–Se–). These configurations being held together in the structure by Van der Waals interactions (Thiruvikraman, 2006). According to Schotmiller et al. (1970) in glassy selenium about 40% of the atoms have a ring structure and 60% of the atoms are bonded as polymeric chains. The additive elements to Se will dissolve into polymeric chains rather than into the Se ring structure and form a cross-link structure. It is known that (Khan et al., 2002) the glass transition temperature Tg increases with increasing chain length and decreases with increasing ring concentration. It is evident that Tg increases with increasing gamma dose and attains its maximum value at a dose of 12 kGy after which it decreases but remains above that of un-irradiated glass. The post-irradiation effect in Se96Sn4 glass is due to straightening/aligning of Se chains in Se-rich glasses. According to Golovchak et al. (2007) such a
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
glassy material consists of Se chain segments in its structure and the inner Se atoms of a chain segment will have two different states within their chain. Since a glass is thermodynamically instable, a group of atoms (from different chains segments) will transfer to a thermodynamically more stable state in a process termed as structural relaxation. This, however, initiate similar processes in Se chains everywhere in the glass and are facilitated due to straightening of Se chains, which occupy a volume less than that of Se8 rings. This in turn accounts for the observed increase in the glass transition temperature Tg upon g-irradiation of the glass. It is well known that, in the calorimetric studies, the glass transition temperature Tg of glassy alloys varies with the heating rate b (Lasocka, 1976; Laramagnac et al., 1981; Imran et al., 2001; Abdel-Rahim et al., 2002). From the dependence of the glass transition temperature Tg on the heating rate, one can find the glass transition activation energy Et. The glass transition activation energy is the amount of energy, which is absorbed by a group of atoms in the glassy region so that a jump from one metastable state to another is possible (Imran et al., 2000). This activation energy is involved in the molecular motion and rearrangements of atoms around the glass transition temperature. When the sample is reheated in the DSC furnace, the atoms undergo infrequent transitions between the local potential minima separated by different energy barriers in the configuration space where each local minimum represents a different structure. The most stable local minimum in the glassy region has lower internal energy. Accordingly, the atoms in a glass having minimum activation energy have higher probability to jump to the metastable (or local minimum) state of lower internal energy and hence are the most stable (Imran et al., 2000). The glass transition activation energy Et can be calculated using the following equation (Moynihan et al., 1974): dðln bÞ Et ¼ R dð1=Tg Þ
ð1Þ
where R is the universal gas constant ( ¼ 8.314 JK1 mol1). It is clear from this equation that the plot of ln b vs. 1/Tg should be straight line and the activation energy involved in the process can be calculated from the slope of the resulting straight line. Fig. 2 shows the plot of ln b vs. 1000/Tg for Se96Sn4 glass irradiated at a dose 12 kGy. From the slope of the resulting straight line the glass transition activation energy is obtained and is listed in Table 1. The obtained values of Et for the other doses of gamma irradiated and
3.2 3.0
-8.8 -9.0 -9.2 ln [β / (Tg)2]
106
-9.4 -9.6 -9.8
24kGy Et = (145 ± 13) kJ/mol
-10.0 -10.2 2.66 2.67 2.68 2.69 2.70 2.71 2.72 2.73 2.74 2.75 1000/Tg Fig. 3. Plot of ln[b/(Tg)2] vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 24 kGy.
also for un-irradiated glass were deduced from the slope of similar plots and are given in the same table. Kissinger equation (Kissinger, 1957) is basically derived for phase transformation from amorphous to crystalline phase, it may also be valid for glass to amorphous transformation, the same equation can be used to evaluate the value of Et. For this purpose Kissinger equation takes the following form: lnðb=Tg2 Þ ¼ Et =RTg þ constant
ð2Þ
This equation is used to calculate Et by plotting ln(b/Tg2) vs. 1/Tg. Fig. 3 shows the plot of ln[b/(Tg)2] vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 24 kGy. From this figure, and similar figures for other doses, the values of Et for both cases are obtained and listed in Table 1. From this table it is clear that the values of Et obtained using the two methods of Moynihan and Kissinger, are in good agreement. It is also evident that Et decreases with increasing g-ray doses and attains its minimum value at 12 kGy after which it increases as the dose increases. This indicates that thermal stability is optimum at a dose of 12 kGy and irradiation over this threshold dose is not going to improve the stability of the glass. To confirm this result, thermal stability parameter S has been calculated using the following equation (Saad and Poulin, 1987): S ¼ ðTc Tg ÞðTp Tc Þ=Tg
ð3Þ
2.8
lnβ
2.6 2.4 2.2 2.0 1.8
12kGy Et = (114 ± 14) kJ/mol
1.6 1.4 2.64
2.66
2.68
2.70
2.72
2.74
2.76
1000/Tg Fig. 2. Plot of ln b vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 12 kGy.
The higher value of this parameter reflects the greater thermal stability of the glass. The obtained values of the S parameter were plotted as shown in Fig. 4 for all doses and at different heating rates (5, 10, 15 and 20 K/min). The maximum value of S, for all heating rates, occurs at a dose of 12 kGy, which again confirm that the optimum stability among gamma irradiated glasses occurs at this particular dose. It is also possible to investigate thermal stability from crystallization kinetics point of view where it can be defined as the resistance to crystallization and is given in terms of the crystallization reaction rate factor Kp. The crystallization rate constant corresponding to the temperature (Tp) at which the crystallization rate is maximum, can be calculated from the analysis of DSC curves using the following condition (Gao and Wang, 1986):
bEc =RKp Tp2 ¼ 1
ð4Þ
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
107
Acknowledgements
7.5 7.0
Authors are grateful to Al-Balqa Applied University for providing all necessary facilities during this research work. Thanks are also due to Dr. Ma’moun Makahleh and Mrs. Sukiena Jarrar, Jordan Atomic Energy Commission, for glass irradiation using Co60 gamma source.
6.5 6.0 5.5 S
5.0
References
4.5 4.0
5K/min 10K/min 15K/min 20K/min
3.5 3.0 2.5 -4
0
4
8
12
16 20 Dose
24
28
32
36
Fig. 4. Dose dependence of S parameter at different heating rates.
where the symbols carry their usual meaning used throughout the text and Ec is the activation energy of crystallization calculated in accordance with Kissinger equation (Kissinger, 1957). The values of /KpS, calculated by averaging the values of Kp found for each exothermic peak at the different heating rates, are listed in Table 1. It is reported (Abdel-Rahim et al., 2005) that the glass with maximum stability will have minimum rate of crystallization. However, among all doses of gamma irradiation, the glass irradiated by 12 kGy has minimum crystallization rate at Tp which requires longest time to be fully crystallized and hence it has the greatest stability against devitrification. This further supports our earlier arguments regarding optimum thermal stability, at a dose of 12 kGy of gamma irradiation, on the basis of glass transition activation energy and the S parameter results that mentioned above.
4. Conclusion The effect of different gamma doses on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass was studied, under non-isothermal condition, using differential scanning calorimeter (DSC) and the following conclusions were drawn:
The structural relaxation in Se96Sn4 can be induced by gamma
irradiation. This is evident from the obtained values of the glass transition temperature Tg and the glass transition activation energy Et. The glass thermal stability was also improved upon gamma irradiation and the calculated values of thermal stability indicators, the S parameter and the crystallization rate factor /KpS, are in support of this argument. As Se96Sn4 glass shows large relaxation effects upon gamma irradiation, therefore such glass is fragile because strong glass has very little ability to relax and shows almost no detectable change in the relaxation parameters. Since the relaxation parameters Tg and Et shows their optimum values at a dose of 12 kGy, therefore, if this particular Se–Sn glass is to be used in a certain applications where stability with temperature and time during use is required, it is suggested to accelerates the relaxation process by gamma irradiation at a dose of 12 kGy.
Abdel-Rahim, M.A., Abdel-Latief, A.Y., Soltan, A.S., Abu-El-Oyoun, M., 2002. Crystallization kinetics of overlapping phases in Cu6Ge14Te80 chalcogenide glasses. Physica B 322, 252–261. Abdel-Rahim, M.A., Hafiz, M.M., Shamekh, A.M., 2005. A study of crystallization kinetics of some Ge–Se–In glasses. Physica B 369, 143–154. Abu El-Fadl, A., Hafiz, M.M., Wakaad, M.M., Aashour, A.S., 2007. Influence of g-radiation on the optical parameters of Ag10Te90 thin films. Radiat. Phys. Chem. 76, 61–66. Amin, G.A.M., Spyrou, N.M., 2005. Study of gamma-radiation-induced optical effects in Ge–Se–Cd for possible industrial dosimetric applications. Radiat. Phys. Chem. 72, 419–422. Balitska, V., Golovchak, R., Kovalskiy, A., Skordeva, E., Shpotyuk, O.I., 2003. Effect of 60 Co g-irradiation on the optical properties of As–Ge–S glasses. J. Non-Cryst. Solids 326&327, 130–134. Clavaguera-Mora, M.T., 1995. Glassy materials: thermodynamic and kinetic quantities. J. Alloys Compd. 220, 197–205. El-Sayed, S.M., 2004. Electron beam and gamma irradiation effects on amorphous chalcogenide SbSe2.5 films. Nucl. Instrum. Methods B 225 (4), 535–543. El-Zahed, H., 2001. The effect of g-doses on the optical band gap of AgInSe2 films. J. Phys. Chem. Solids 62, 641–646. Gao, Y.Q., Wang, W., 1986. On the activation energy of crystallization in metallic glasses. J. Non-Cryst. Solids 81, 129–134. Golovchak, R.Ya., Kozdras, A., Gorecki, Cz., Shpotyuk, O.I., 2006. Gamma-irradiation-induced physical ageing in As–Se glasses. J. Non-Cryst. Solids 352, 4960–4963. Golovchak, R., Shpotyuk, O., Kozdras, A., Bureau, B., Vlcek, M., Ganjoo, A., Jain, H., 2007. Atomistic model of physical ageing in Se-rich As–Se glasses. Philos. Mag. 87 (28), 4323–4334. Imran, M.M.A., Al-Hamarneh, I.F., Awaddallah, M.I., Al-Ewaisi, M.A., 2008. Physical ageing in Se94Sn6 glass induced by gamma irradiation. Physica B 403, 2639–2642. Imran, M.M.A., Bhandari, D., Saxena, N.S., 2001. Kinetic studies of bulk Ge22Se78xBix (x ¼ 0, 4 and 8) semiconducting glasses. J. Therm. Anal. Calorim. 65, 257–274. Imran, M.M.A., Saxena, N.S., Bhandari, D., Husain, M., 2000. Glass transition phenomena, crystallization kinetics and enthalpy released in binary Se100xInx (x ¼ 2, 4 and 10) semiconducting glasses. Phys. Status Solidi (a) 181, 357–368. Imran, M.M.A., Saxena, N.S., Vijay, Y.K., Vijayvergiya, R., Maharjan, N.B., Husain, M., 2002. Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation. J. Non-Cryst. Solids 298, 53–59. Khan, S.A., Zulfequar, M., Husain, M., 2002. On the crystallization kinetics of amorphous Se80In20xPbx. Solid State Commun. 123, 463–468. Kissinger, H.E., 1957. Reaction kinetics in differential thermal analysis. Anal. Chem. 29 (11), 1702–1706. Lafi, O.A., Imran, M.M.A., Abdullah, M.K., 2007. Glass transition activation energy, glass forming ability and thermal stability of Se90In10xSnx (x ¼ 2, 4, 6, and 8) chalcogenide glasses. Physica B 395, 69–75. Laramagnac, J.P., Grenet, J., Michon, P., 1981. Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films. J. Non-Cryst. Solids 45, 157–168. Lasocka, M., 1976. The effect of scanning rate on glass transition temperature of splat-cooled Te85Ge15. Mater. Sci. Eng. 2-3, 173–177. Moynihan, C.T., Easteal, A.J., Wilder, J., 1974. Dependence of the glass transition temperature on heating and cooling rate. J. Phys. Chem. 78 (26), 2673–2677. Lucas, P., King, E.A., Horner, A.D., Johnson, B.R., Sundaram, S.K., 2006. Photostructural relaxation in As–Se–S glasses: effect of network fragility. J. NonCryst. Solids 352, 2067–2072. Saad, M., Poulin, M., 1987. Glass forming ability criterion. Mater. Sci. Forum 19–20, 11–18. Salem, A.M., Marzouk, S.Y., Moustafa, S.H., Selim, M.S., 2007. Effect of g-irradiation on the optical properties of a-(Sb0.1Ge0.3Se0.6) and Ag5(Sb0.1Ge0.3Se0.6)95 thin film material. Nucl. Instrum. Methods B 262, 225–230. Schotmiller, J., Tabak, M., Lucovsky, G., Ward, A., 1970. The effects of valency on transport properties in vitreous binary alloys of selenium. J. Non-Cryst. Solids 4, 80–96. Shpotyuk, O.I., Golovchak, R.Ya., Jain, H., Kozdras, A., 2007. Radiation-induced physical ageing of structure of arsenic–selenide glasses. J. Phys. Chem. Solids 68, 901–905.
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Shpotyuk, O.I., Golovchak, R.Ya., Kavetsky, T.S., Kovalskiy, A.B., Vakiv, M.M., 2000. Radiation-optical effect in glassy Ge–As(Sb)–S systems. Nucl. Instrum. Methods B 166–167, 517–520. Shpotyuk, O.I., 1995. Amorphous chalcogenide semiconductors for doisimetry of high-energy ionizing radiation. Radiat. Phys. Chem. 46, 1279–1282.
Thiruvikraman, P.K., 2006. Rings, chains and planes: variation of Tg with composition in chalcogenide glasses. Bull. Mater. Sci. 29 (4), 371–374. Xia, F., Baccaro, S., Wang, H., Hua, W., Zeng, H., Zhang, X., Chen, G., 2008. g-ray irradiation induced multiple effect on Ge–As–Se chalcogenide glasses. J. NonCryst. Solids 354, 1365–1368.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
The effect of gamma irradiation on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass Omar A. Lafi, Mousa M.A. Imran Materials Science Laboratory, Department of Applied Sciences, Prince Abdullah Bin Ghazi Faculty of Science and Information Technology, Al-Balqa’ Applied University, Al-Salt 19117, Jordan
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 May 2009 Accepted 6 August 2009
Se96Sn4 chalcogenide glass was prepared by melt quenching technique and exposed, at room temperature, to different doses of 4, 8, 12, 24 and 33 kGy of high-energy 60Co gamma irradiation. Differential scanning calorimeter (DSC) was used under non-isothermal condition to determine the glass transition temperature Tg, onset Tc and peak Tp temperatures of crystallization, of un-irradiated and g-irradiated samples, at four different heating rates. The variation of Tg with heating rates was utilized to calculate the glass transition activation energy Et for un-irradiated and g-irradiated glass, using the methods suggested by Kissinger and Moynihan. Based on the obtained values of the characteristic temperatures Tg, Tc and Tp, thermal stability was monitored through the calculation of the S parameter and the crystallization rate factor /KpS for irradiated and un-irradiated glass. Results reveal that, as g-dose increases Tg increases up to 12 kGy then decreases at higher doses but remains more than that of un-irradiated glass. Meanwhile, both Et and /KpS attain their minimum values at the same dose of 12 kGy and the glass is thermally stable at this particular dose. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Chalcogenide glasses Gamma irradiation Glass transition temperature Thermal stability
1. Introduction During the last two decades, several investigations have dealt with the influence of laser, electron, gamma and neutron irradiation on the physical properties of chalcogenide glasses (Shpotyuk, 1995; Shpotyuk et al., 2000, 2007; Imran et al., 2002; Lucas et al., 2006; El-Zahed, 2001; Balitska et al., 2003; El-Sayed, 2004; Amin and Spyrou, 2005; Salem et al., 2007; Abu El-Fadl et al., 2007; Xia et al., 2008). According to these studies, the structure of a glass is modified under the influence of irradiation giving rise to changes in thermal, optical, and electrical properties without any atomic transmutations or surface damages that are usually produced by particle irradiation. Many chalcogenide glasses, whose properties can be tailoredmade, are used as main component in optoelectronics and photovoltaic devices to perform certain applications. Such devices may be used in an environment where temperature is fluctuating, which affect the properties of the used glass. It is well known that (Clavaguera-Mora, 1995) as-prepared glasses exist in an unstable state with characteristic thermal parameters such as enthalpy, glass transition temperature and entropy that are usually evolved with time. In fact, temperature fluctuation may transfer asprepared glass to metastable state with different thermal
Corresponding author. Tel.: +962 777 993459; fax: +962 5 3530462.
E-mail addresses: [email protected], [email protected] (M.M. Imran). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.08.005
parameters (Imran et al., 2008). Therefore, as-prepared glass must be relaxed before its final usage. This can be accomplished, by natural storage for long period of time (sometimes several years) or annealing at different temperatures for different annealing times or by high-energy irradiation of a glass, in a process termed as structural relaxation or physical ageing. However, physical ageing induced by high-energy gamma irradiation is very fast and the changes in thermal parameters are uniform through the whole bulk of the glass (Golovchak et al., 2006). To monitor the aforementioned changes, differential scanning calorimeter (DSC) is a very useful technique, as it is usually used to determine the transformation temperatures in chalcogenide glasses, such as glass transition Tg, crystallization Tc and melting Tm temperatures. These characteristic temperatures are further used to evaluate thermal stability before and after g-irradiation. Structural investigation of chalcogenide glasses is important in determining the transport mechanism and thermal stability. This is because the glassy state is essentially a metastable one and hence it inherently possesses the possibility of transforming into a more stable crystalline state. The most promising properties of chalcogenide glasses have been found to deteriorate drastically during crystallization. Thus, understanding the glass transition mechanism and thermal stability is important to determine their effective working limits for a specific technological application before crystallization takes place (Imran et al., 2001; Lafi et al., 2007). On the other hand, transport properties of chalcogenide
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
2. Experimental details Se96Sn4 bulk glass was prepared by melt quenching technique as described elsewhere (Imran et al., 2008). Disc shaped samples (pellets) of diameter 13 mm and different thicknesses ranging from 0.45 to 0.5 mm were prepared by grinding the bulk glass to a fine powder and compressing it in a die at a load of 6 t. The pellets irradiated with gamma rays obtained from a Co60 source (available at Jordan Atomic Energy Commission) with mean energy of gamma-quanta of 1.25 MeV (Co60 is a gamma emitter at 1332.50 and 1173.24 keV) at temperature of 30 1C. The duration of gamma irradiation is such that the overall accumulated dose was 4, 8, 12, 24, and 33 kGy. About (1070.5) mg of the g-irradiated powder sample was capsulated in aluminum pan and subjected to the differential scanning calorimeter (Perkin Elmer DSC-7) at different heating rates (5, 10, 15 and 20 K/min). The temperature precision of this equipment is 70.1 K with an average standard error of about 1 K in the measured values. The DSC equipment was calibrated prior to measurements, using high purity standards Pb, Sn and In with well-known melting points. For the sake of accuracy and to ensure the reproducibility of the characteristic temperatures Tg, Tc and Tp of the DSC curves, three measurements were conducted for each heating rate of gamma irradiated and un-irradiated glass under identical conditions. The experimental data points were taken as the average value of the three supposedly identical measurements. All measurements were referenced to an empty aluminum pan and the temperature range covered in DSC is from room temperature to 250 1C.
3. Results and discussion Typical DSC curves of un-irradiated and g-irradiated Se96Sn4 glass at a heating rate of 20 K/min are shown in Fig. 1, as an example. Similar DSC curves were also obtained for the other heating rates. Two characteristic phenomena (endothermic and exothermic peaks) are evident in the DSC curves in the temperature range of investigation. The first one corresponds to the glass transition region and the other to the crystallization region. The glass transition temperature Tg, onset and peak crystallization temperatures Tc and Tp, respectively, were determined from these DSC curves, for both before and after
Tp Exo
0kGy 4kGy 8kGy 12kGy 24kGy 33kGy
Tg
Heat Flow
glasses strongly depend on the nature and degree of short-range order, which is a characteristic structure of these glasses. The objective of the present work is to investigate the effect of g-irradiation on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass using differential scanning calorimeter. In this respect, the dependence of the glass transition temperature on heating rates is used for calculation of the glass transition activation energy Et of un-irradiated and g-irradiated, at different doses of 4, 8, 12, 24 and 33 kGy, Se96Sn4 glass using the methods suggested by Kissinger (1957) and Moynihan et al. (1974). Thermal stability is investigated through the calculation of thermal stability parameter S (Saad and Poulin, 1987) and the average value of the crystallization rate factor /KpS (Gao and Wang, 1986). It is reported (Imran et al., 2000) that a glass having minimum activation energy have higher probability to jump to a metastable state of lower internal energy and hence is considered most stable. In view of this the aforementioned parameters S, /KpS, and Et are considered as good indicators of thermal stability of a glass. Results indicate that the change in the glass transition temperature Tg is maximum at a dose of 12 kGy and thermal stability indicators /KpS and Et attain their minimum values, while the S parameter is maximum, at this particular dose.
105
Tc Endo 75
100
125
150
175
200
225
250
Temperature (°C) Fig. 1. Non-isothermal DSC curves (at a heating rate of 20 K/min) for Se96Sn4 glass irradiated by different doses of gamma irradiation.
Table 1 Glass transition temperature Tg at 5 K/min, average value of crystallization rate factor /KpS, and glass transition activation energy (Et) for Se96Sn4 glass before and after g-irradiation. Doses (kGy)
0 4 8 12 24 33
Tg (5 K/min) (1C)
90.2 94.0 94.7 96.2 92.7 91.2
/KpS (s)1
0.055 0.092 0.090 0.065 0.088 0.088
Et (kJ/mole) Moynihan
Kissinger
249711 212715 197721 114714 152713 141710
242711 206715 190721 108714 145713 135710
g-irradiation. Both temperatures Tg and Tc have been defined as the temperature corresponds to the intersection of the two linear portions adjoining the transition elbow of the DSC traces in the endothermic and exothermic direction, respectively. The values of Tg for un-irradiated and g-irradiated Se96Sn4 glass at a heating rate of 5 K/min, as obtained from the DSC curves are listed in Table 1. From this table, a small increase (1–6 K) in the glass transition temperature (for the heating rate 5 K/min) is observed after girradiation. The glass transition temperature for the other heating rates gives nearly the same indication. Since chalcogen elements are normally two-fold coordinated, the structural configurations which can be formed are limited essentially to rings (Se8) or chains (–Se–Se–Se–). These configurations being held together in the structure by Van der Waals interactions (Thiruvikraman, 2006). According to Schotmiller et al. (1970) in glassy selenium about 40% of the atoms have a ring structure and 60% of the atoms are bonded as polymeric chains. The additive elements to Se will dissolve into polymeric chains rather than into the Se ring structure and form a cross-link structure. It is known that (Khan et al., 2002) the glass transition temperature Tg increases with increasing chain length and decreases with increasing ring concentration. It is evident that Tg increases with increasing gamma dose and attains its maximum value at a dose of 12 kGy after which it decreases but remains above that of un-irradiated glass. The post-irradiation effect in Se96Sn4 glass is due to straightening/aligning of Se chains in Se-rich glasses. According to Golovchak et al. (2007) such a
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
glassy material consists of Se chain segments in its structure and the inner Se atoms of a chain segment will have two different states within their chain. Since a glass is thermodynamically instable, a group of atoms (from different chains segments) will transfer to a thermodynamically more stable state in a process termed as structural relaxation. This, however, initiate similar processes in Se chains everywhere in the glass and are facilitated due to straightening of Se chains, which occupy a volume less than that of Se8 rings. This in turn accounts for the observed increase in the glass transition temperature Tg upon g-irradiation of the glass. It is well known that, in the calorimetric studies, the glass transition temperature Tg of glassy alloys varies with the heating rate b (Lasocka, 1976; Laramagnac et al., 1981; Imran et al., 2001; Abdel-Rahim et al., 2002). From the dependence of the glass transition temperature Tg on the heating rate, one can find the glass transition activation energy Et. The glass transition activation energy is the amount of energy, which is absorbed by a group of atoms in the glassy region so that a jump from one metastable state to another is possible (Imran et al., 2000). This activation energy is involved in the molecular motion and rearrangements of atoms around the glass transition temperature. When the sample is reheated in the DSC furnace, the atoms undergo infrequent transitions between the local potential minima separated by different energy barriers in the configuration space where each local minimum represents a different structure. The most stable local minimum in the glassy region has lower internal energy. Accordingly, the atoms in a glass having minimum activation energy have higher probability to jump to the metastable (or local minimum) state of lower internal energy and hence are the most stable (Imran et al., 2000). The glass transition activation energy Et can be calculated using the following equation (Moynihan et al., 1974): dðln bÞ Et ¼ R dð1=Tg Þ
ð1Þ
where R is the universal gas constant ( ¼ 8.314 JK1 mol1). It is clear from this equation that the plot of ln b vs. 1/Tg should be straight line and the activation energy involved in the process can be calculated from the slope of the resulting straight line. Fig. 2 shows the plot of ln b vs. 1000/Tg for Se96Sn4 glass irradiated at a dose 12 kGy. From the slope of the resulting straight line the glass transition activation energy is obtained and is listed in Table 1. The obtained values of Et for the other doses of gamma irradiated and
3.2 3.0
-8.8 -9.0 -9.2 ln [β / (Tg)2]
106
-9.4 -9.6 -9.8
24kGy Et = (145 ± 13) kJ/mol
-10.0 -10.2 2.66 2.67 2.68 2.69 2.70 2.71 2.72 2.73 2.74 2.75 1000/Tg Fig. 3. Plot of ln[b/(Tg)2] vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 24 kGy.
also for un-irradiated glass were deduced from the slope of similar plots and are given in the same table. Kissinger equation (Kissinger, 1957) is basically derived for phase transformation from amorphous to crystalline phase, it may also be valid for glass to amorphous transformation, the same equation can be used to evaluate the value of Et. For this purpose Kissinger equation takes the following form: lnðb=Tg2 Þ ¼ Et =RTg þ constant
ð2Þ
This equation is used to calculate Et by plotting ln(b/Tg2) vs. 1/Tg. Fig. 3 shows the plot of ln[b/(Tg)2] vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 24 kGy. From this figure, and similar figures for other doses, the values of Et for both cases are obtained and listed in Table 1. From this table it is clear that the values of Et obtained using the two methods of Moynihan and Kissinger, are in good agreement. It is also evident that Et decreases with increasing g-ray doses and attains its minimum value at 12 kGy after which it increases as the dose increases. This indicates that thermal stability is optimum at a dose of 12 kGy and irradiation over this threshold dose is not going to improve the stability of the glass. To confirm this result, thermal stability parameter S has been calculated using the following equation (Saad and Poulin, 1987): S ¼ ðTc Tg ÞðTp Tc Þ=Tg
ð3Þ
2.8
lnβ
2.6 2.4 2.2 2.0 1.8
12kGy Et = (114 ± 14) kJ/mol
1.6 1.4 2.64
2.66
2.68
2.70
2.72
2.74
2.76
1000/Tg Fig. 2. Plot of ln b vs. 1000/Tg for Se96Sn4 glass irradiated at a dose of 12 kGy.
The higher value of this parameter reflects the greater thermal stability of the glass. The obtained values of the S parameter were plotted as shown in Fig. 4 for all doses and at different heating rates (5, 10, 15 and 20 K/min). The maximum value of S, for all heating rates, occurs at a dose of 12 kGy, which again confirm that the optimum stability among gamma irradiated glasses occurs at this particular dose. It is also possible to investigate thermal stability from crystallization kinetics point of view where it can be defined as the resistance to crystallization and is given in terms of the crystallization reaction rate factor Kp. The crystallization rate constant corresponding to the temperature (Tp) at which the crystallization rate is maximum, can be calculated from the analysis of DSC curves using the following condition (Gao and Wang, 1986):
bEc =RKp Tp2 ¼ 1
ð4Þ
ARTICLE IN PRESS O.A. Lafi, M.M.A. Imran / Radiation Physics and Chemistry 79 (2010) 104–108
107
Acknowledgements
7.5 7.0
Authors are grateful to Al-Balqa Applied University for providing all necessary facilities during this research work. Thanks are also due to Dr. Ma’moun Makahleh and Mrs. Sukiena Jarrar, Jordan Atomic Energy Commission, for glass irradiation using Co60 gamma source.
6.5 6.0 5.5 S
5.0
References
4.5 4.0
5K/min 10K/min 15K/min 20K/min
3.5 3.0 2.5 -4
0
4
8
12
16 20 Dose
24
28
32
36
Fig. 4. Dose dependence of S parameter at different heating rates.
where the symbols carry their usual meaning used throughout the text and Ec is the activation energy of crystallization calculated in accordance with Kissinger equation (Kissinger, 1957). The values of /KpS, calculated by averaging the values of Kp found for each exothermic peak at the different heating rates, are listed in Table 1. It is reported (Abdel-Rahim et al., 2005) that the glass with maximum stability will have minimum rate of crystallization. However, among all doses of gamma irradiation, the glass irradiated by 12 kGy has minimum crystallization rate at Tp which requires longest time to be fully crystallized and hence it has the greatest stability against devitrification. This further supports our earlier arguments regarding optimum thermal stability, at a dose of 12 kGy of gamma irradiation, on the basis of glass transition activation energy and the S parameter results that mentioned above.
4. Conclusion The effect of different gamma doses on glass transition temperature and thermal stability of Se96Sn4 chalcogenide glass was studied, under non-isothermal condition, using differential scanning calorimeter (DSC) and the following conclusions were drawn:
The structural relaxation in Se96Sn4 can be induced by gamma
irradiation. This is evident from the obtained values of the glass transition temperature Tg and the glass transition activation energy Et. The glass thermal stability was also improved upon gamma irradiation and the calculated values of thermal stability indicators, the S parameter and the crystallization rate factor /KpS, are in support of this argument. As Se96Sn4 glass shows large relaxation effects upon gamma irradiation, therefore such glass is fragile because strong glass has very little ability to relax and shows almost no detectable change in the relaxation parameters. Since the relaxation parameters Tg and Et shows their optimum values at a dose of 12 kGy, therefore, if this particular Se–Sn glass is to be used in a certain applications where stability with temperature and time during use is required, it is suggested to accelerates the relaxation process by gamma irradiation at a dose of 12 kGy.
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