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Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT
Materials Letters 63 (2009) 2139–2141
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT L. Deng a, H.A. Ma a, T.C. Su a, F.R. Yu b, Y.J. Tian b, Y.P. Jiang a, N. Dong a, S.Z. Zheng a, X. Jia a,⁎ a b
National Lab of Superhard Materials, Jinlin University, Changchun 130012, China National Lab of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
i n f o
Article history: Received 16 April 2009 Accepted 4 June 2009 Available online 10 June 2009 Keywords: Electrical properties HPHT Co4Sb12 − xTex X-ray techniques
a b s t r a c t Skutterudite compounds Co4Sb12 − xTex with bcc crystal structure were prepared by high pressure and high temperature (HTHP) method. The study explored chemical doping with Te at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system Co4Sb12 − xTex. The electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–710 K. We found that the presence of Te substantially decreased the electrical resistivity without any detrimental effect on the Seebeck coefficients, which improved the power factor. Among all the samples, Co4Sb11.5Te0.5 shows the highest power factor of 35.3 µw/(cmK2) at 710 K, and the maximum ZT value reaches 0.67 at 710 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CoSb3-based skutterudite materials have been extensively studied for thermoelectric (TE) materials since it was singled out in 1995 as a prime example as a potentially high ZT material from Slack's proposed “phonon glass electron crystal” concept [1–3]. Despite their favorable features such as high electron mobility and high Seebeck coefficient, which give skutterudites a high power factor, the undoped CoSb3-based skutterudites are disadvantaged by their inherently large thermal conductivity which lowers their ZT value [4,5]. The performance of a TE material is generally characterized by the dimensionless figure of merit, ZT =α2σT/κ, where α, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. CoSb3 and its related skutterudite compounds are expected to be the most promising TE materials. However, undoped pure CoSb3 can not be used in thermoelectric applications because of its high thermal conductivity. Most previous studies have proved that doping and/or filling [6,7] can reduce thermal conductivity and improve effectively thermoelectric properties. For example, Fe and Ni substituting for Co [8,9], and Te substituting for Sb [10] have been found to be active dopants. Liu [11] has reported that there was a maximal ZT value for CoSb2.85Te0.15 among substituting Te-doped CoSb3 compounds. Many techniques [8,9,12] have been used to synthesize skutterudite compounds, however, most methods have their limitations. Compared with other methods, HPHT method has many advantages, including the ability to tune rapidly and cleanly, typically without introducing disorder and phase separation, or other complicating factors. In our previous reports [13–15], this technology has been used to synthesize
⁎ Corresponding author. Fax: +86 431 85168858. E-mail address: [email protected] (X. Jia). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.008
other compounds successfully and obtained perfect results. In this work, the skutterudite compounds Co4Sb12 − xTex(x = 0, 0.1, 0.3, 0.5) were successfully synthesized by HPHT method, and the temperaturedependent thermoelectric properties were investigated in detail. 2. Experimental procedure Co powder (99.9%), Sb powder (99.9%) and Te powder (99.9%) were used as starting materials. These powders were weighed according to
Fig. 1. XRD patterns of CoSb3 and Co4Sb11.5 Te0.5 prepared by HPHT.
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L. Deng et al. / Materials Letters 63 (2009) 2139–2141
Fig. 2. SEM micrographs of the fractured surface for CoSb3 (a) and Co4Sb11.5Te0.5 (b).
the stoichiometry of Co4Sb12 − xTex (x = 0, 0.1, 0.3, 0.5), and then mixed in an agate mortar. The mixtures were shaped to a cylinder with about 3 mm thick and 10 mm in diameter by press. The cylinder samples were assembled for HPHT synthesis. The samples were prepared in a cubic anvil high pressure apparatus (SPD 6 × 1200) with a sample chamber of 23 mm on an edge at 900 K and 1.5 GPa. X-ray diffraction (XRD) measurements with Cu-Kα radiation were performed on an X-ray diffractometer (D/MAX-RA). Scanning electron microscopy (SEM) (JXA8200) was used to observe the fractured surface. The Seebeck coefficient and electrical conductivity were measured simultaneously by a ZEM-3 apparatus. The thermal conductivity κ was measured on a TC-7000 (ULVAC-RIKO Inc., Japan) Laser Flash Thermal Constants Measuring Apparatus.
3. Results and discussion Fig. 1 shows the XRD patterns of CoSb3 and Co4Sb11.5Te0.5. All peaks visible in the CoSb3 diffractogram can be indexed to the skutterudite crystal structure. For the Co4Sb11.5Te0.5 sample, several additional small peaks were observed, which were identified by filled arrows in Fig. 1. These peaks can be indexed to impurity phases such as Sb and CoSb2. The lattice parameters of CoSb3 and Co4Sb11.5Te0.5 prepared by HPHT are 9.0346 Å and 9.0514 Å respectively which are similar to the results of Liu [11]. Fig. 2(a) and (b) are the SEM micrographs for the fractured surfaces of CoSb3 and Co4Sb11.5Te0.5 respectively. The results show that Co4Sb11.5Te0.5 sample has smaller crystal grain sizes and more
Fig. 3. (a) Temperature dependence of Seebeck coefficient for Co4Sb12 − xTex; (b) Temperature dependence of electrical resistivity for Co4Sb12 − xTex; (c) Temperature dependence of power factor for Co4Sb1 − xTex.
L. Deng et al. / Materials Letters 63 (2009) 2139–2141
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significantly at high pressure and the excellent properties gained at high pressure can be partially kept to ambient pressure. The thermal conductivity (κ) is plotted versus temperature in Fig. 4(a). It can be seen that the thermal conductivities and the lattice thermal conductivities decrease with the increasing temperature. The thermal conductivity of Co4Sb11.5Te0.5 is smaller than that of CoSb3, especially at low temperature. This indicates that the smaller grain size and the more dispersed Te particles in the samples are more effective for the phonon scattering because the low energy phonon is apt to be scattered at the grain boundaries [19]. Fig. 4(b) shows the temperature dependence of the figure of merit (ZT) for Co4Sb11.5Te0.5 and CoSb3. The figure of merit (ZT) for Co4Sb11.5Te0.5 increases with temperature in the measured temperature range. The maximum value 0.67 was obtained at 710 K, which is much higher than that of CoSb3 (0.17), and also higher than the result of Christian Stiewe (0.65) [20]. The further work will be focus on the filling for CoSb3 based on doped with Te under HPHT. 4. Conclusion Fig. 4. (a) Temperature dependence of thermal conductivity for CoSb3 and Co4Sb11.5 Te0.5; (b) Temperature dependence of figure of merit (ZT) of CoSb3 and Co4Sb11.5 Te0.5.
abundant grain boundaries than those of CoSb3 sample, which may be helpful to decrease its thermal conductivity. The microstructure of the samples agrees with the advantages of high-pressure synthesis, these advantages include restraining disorder and introducing abundant grain boundaries, etc [16]. Fig. 3(a) shows the temperature and x dependence of the absolute Seebeck coefficient for Co4Sb12 − xTex. As expected, the higher the doping levels, the lower the Seebeck coefficients. The absolute Seebeck coefficient of CoSb3 gradually increases with increasing temperature and shows a maximum value at about 550 K, and thereafter it decreases. The absolute value of the Seebeck coefficient for the sample with x = 0.5 is generally larger than that for CoSb3, and its maximum absolute value attains to about 273.9 µV/K. Fig. 3(b) shows the temperature and x dependence of the electrical resistivity for Co4Sb12 − xTex. The temperature corresponding to the peak shifts toward the higher temperature as x is increased from 0.1 to 0.5 in Co4Sb12 − xTex. The electrical resistivity for Co4Sb11.5Te0.5 increases linearly with increasing temperature, and the minimum value of 0.85 × 10− 3 Ω cm was obtained at 301 K, which is even much lower than that of the same sample prepared at normal pressure (about 1.0 × 10− 3 Ω cm) [6]. According to the previous studies [17], Te atoms in the Sb sublattice serve as electron donors in CoSb3 − xTex. Each Te atom can provide 0.3–0.33 effective electrons to contribute to the electrical conduction. Power factor (PF) was calculated from the measured Seebeck coefficient and electrical resistivity as shown in Fig. 3(c). The Power factor for Co4Sb12 − xTex increases with increasing x. Co4Sb11.5Te0.5 shows the largest PF values both at room temperature and high temperature, owing to its lowest resistivity. The value of PF reaches 35.3 µw/(cmK2) at 710 K, quite high for a kind of n-type thermoelectric materials [18]. The above results agree with the advantages of high-pressure synthesis. The electrical transport properties can be improved
Skutterudite compounds Co4Sb12 − xTex were successfully synthesized by HPHT. Co4Sb11.5Te0.5 shows a typical behavior of degenerate semiconductors. A maximum ZT value of 0.67 was obtained for a heavily doped n-type sample at a temperature of 710 K. Skutterudite materials show good thermoelectric properties but efforts should now focus on lattice thermal conductivity reduction. Acknowledgments This work was financially supported by the National Science Foundation of China (50731006) (50801030), and Graduate Innovation Fund of Jilin University (20080219). References [1] Morelli DT, Meisner GP, Chen B, Hu S, Uher C. Phys Rev B 1997;56:7376–83. [2] Nolas GS, Slack GA, Morelli DT, Tritt TM, Ehrlich AC. J Appl Phys 1996;79:4002–5. [3] Tritt TM, Nolas GS, Slack GA, Ehrlich AC, Gillespie DJ, Cohn JL. J Appl Phys 1996;79:8412–8. [4] Morelli DT, Caillat T, Fleurial JP, Borshchevsky A, Vandersande J, Chen B, et al. Phys Rev B 1995;51:9622–8. [5] Nordström L, Singh DJ. Phys Rev B 1996;53:1103–8. [6] Shi X, Zhang W, Chen LD, Yang J, Uher C. Phys Rev B 2008;75:2352081–9. [7] Mi JL, Zhao XB, Zhu TJ, Tu JP. Mater Lett 2008;62:2363–5. [8] Peng JY, Yang J, Zhang TJ, Song XL, Chen YH. J Alloys Compd 2004;381:313–6. [9] Zhang X, Lu QM, Zhang JX, Wei Q, Liu DM, Liu YQ. J Alloys Compd 2007;457: 368–71. [10] Dudkin LD, Abrikosov NK. Sov Phys Solid State 1959;1:126–33. [11] Liu WS, Zhang BP, Li JF, Zhang HL, Zhao LD. J Appl Phys 2007;102:103717. [12] Yang L, Hng H, Cheng H, Sun T, Ma J. Mater Lett 2008;62:2483–5. [13] Su TC, Jia XP, Ma HA, Zang CY, Zhou L, Guo JG, et al. Mater Lett 2008;62:3269–71. [14] Su TC, Jia XP, Ma HA, Jiang YP, Dong N, Deng L, et al. J Alloys Compd 2009;468: 410–3. [15] Ma HA, Su TC, Zhu PW, Guo JG, Jia XP. J Alloys Compd 2008;454:415–8. [16] Zhu PW, Jia XP, Chen HY, Guo WL, Chen LX, Li DM, et al. Solid State Commun 2002;123:43–7. [17] Wojciechowski K, Toboa J, Leszczyski J. J Alloys Compd 2003;361:19–27. [18] Yasukawa M, Ioroi A, Ikeuchi K, Kono T. Mater Lett 2004;58:3536–9. [19] Wood C. Rep Prog Phys 1988;5:459–539. [20] Stiew C, Bertini L, Toprak M, Christensen M, Platzek D, Williams S, Gatti C, et al. J Appl Phys 2005;97:044317-1-7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT L. Deng a, H.A. Ma a, T.C. Su a, F.R. Yu b, Y.J. Tian b, Y.P. Jiang a, N. Dong a, S.Z. Zheng a, X. Jia a,⁎ a b
National Lab of Superhard Materials, Jinlin University, Changchun 130012, China National Lab of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
i n f o
Article history: Received 16 April 2009 Accepted 4 June 2009 Available online 10 June 2009 Keywords: Electrical properties HPHT Co4Sb12 − xTex X-ray techniques
a b s t r a c t Skutterudite compounds Co4Sb12 − xTex with bcc crystal structure were prepared by high pressure and high temperature (HTHP) method. The study explored chemical doping with Te at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system Co4Sb12 − xTex. The electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–710 K. We found that the presence of Te substantially decreased the electrical resistivity without any detrimental effect on the Seebeck coefficients, which improved the power factor. Among all the samples, Co4Sb11.5Te0.5 shows the highest power factor of 35.3 µw/(cmK2) at 710 K, and the maximum ZT value reaches 0.67 at 710 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CoSb3-based skutterudite materials have been extensively studied for thermoelectric (TE) materials since it was singled out in 1995 as a prime example as a potentially high ZT material from Slack's proposed “phonon glass electron crystal” concept [1–3]. Despite their favorable features such as high electron mobility and high Seebeck coefficient, which give skutterudites a high power factor, the undoped CoSb3-based skutterudites are disadvantaged by their inherently large thermal conductivity which lowers their ZT value [4,5]. The performance of a TE material is generally characterized by the dimensionless figure of merit, ZT =α2σT/κ, where α, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. CoSb3 and its related skutterudite compounds are expected to be the most promising TE materials. However, undoped pure CoSb3 can not be used in thermoelectric applications because of its high thermal conductivity. Most previous studies have proved that doping and/or filling [6,7] can reduce thermal conductivity and improve effectively thermoelectric properties. For example, Fe and Ni substituting for Co [8,9], and Te substituting for Sb [10] have been found to be active dopants. Liu [11] has reported that there was a maximal ZT value for CoSb2.85Te0.15 among substituting Te-doped CoSb3 compounds. Many techniques [8,9,12] have been used to synthesize skutterudite compounds, however, most methods have their limitations. Compared with other methods, HPHT method has many advantages, including the ability to tune rapidly and cleanly, typically without introducing disorder and phase separation, or other complicating factors. In our previous reports [13–15], this technology has been used to synthesize
⁎ Corresponding author. Fax: +86 431 85168858. E-mail address: [email protected] (X. Jia). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.008
other compounds successfully and obtained perfect results. In this work, the skutterudite compounds Co4Sb12 − xTex(x = 0, 0.1, 0.3, 0.5) were successfully synthesized by HPHT method, and the temperaturedependent thermoelectric properties were investigated in detail. 2. Experimental procedure Co powder (99.9%), Sb powder (99.9%) and Te powder (99.9%) were used as starting materials. These powders were weighed according to
Fig. 1. XRD patterns of CoSb3 and Co4Sb11.5 Te0.5 prepared by HPHT.
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Fig. 2. SEM micrographs of the fractured surface for CoSb3 (a) and Co4Sb11.5Te0.5 (b).
the stoichiometry of Co4Sb12 − xTex (x = 0, 0.1, 0.3, 0.5), and then mixed in an agate mortar. The mixtures were shaped to a cylinder with about 3 mm thick and 10 mm in diameter by press. The cylinder samples were assembled for HPHT synthesis. The samples were prepared in a cubic anvil high pressure apparatus (SPD 6 × 1200) with a sample chamber of 23 mm on an edge at 900 K and 1.5 GPa. X-ray diffraction (XRD) measurements with Cu-Kα radiation were performed on an X-ray diffractometer (D/MAX-RA). Scanning electron microscopy (SEM) (JXA8200) was used to observe the fractured surface. The Seebeck coefficient and electrical conductivity were measured simultaneously by a ZEM-3 apparatus. The thermal conductivity κ was measured on a TC-7000 (ULVAC-RIKO Inc., Japan) Laser Flash Thermal Constants Measuring Apparatus.
3. Results and discussion Fig. 1 shows the XRD patterns of CoSb3 and Co4Sb11.5Te0.5. All peaks visible in the CoSb3 diffractogram can be indexed to the skutterudite crystal structure. For the Co4Sb11.5Te0.5 sample, several additional small peaks were observed, which were identified by filled arrows in Fig. 1. These peaks can be indexed to impurity phases such as Sb and CoSb2. The lattice parameters of CoSb3 and Co4Sb11.5Te0.5 prepared by HPHT are 9.0346 Å and 9.0514 Å respectively which are similar to the results of Liu [11]. Fig. 2(a) and (b) are the SEM micrographs for the fractured surfaces of CoSb3 and Co4Sb11.5Te0.5 respectively. The results show that Co4Sb11.5Te0.5 sample has smaller crystal grain sizes and more
Fig. 3. (a) Temperature dependence of Seebeck coefficient for Co4Sb12 − xTex; (b) Temperature dependence of electrical resistivity for Co4Sb12 − xTex; (c) Temperature dependence of power factor for Co4Sb1 − xTex.
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significantly at high pressure and the excellent properties gained at high pressure can be partially kept to ambient pressure. The thermal conductivity (κ) is plotted versus temperature in Fig. 4(a). It can be seen that the thermal conductivities and the lattice thermal conductivities decrease with the increasing temperature. The thermal conductivity of Co4Sb11.5Te0.5 is smaller than that of CoSb3, especially at low temperature. This indicates that the smaller grain size and the more dispersed Te particles in the samples are more effective for the phonon scattering because the low energy phonon is apt to be scattered at the grain boundaries [19]. Fig. 4(b) shows the temperature dependence of the figure of merit (ZT) for Co4Sb11.5Te0.5 and CoSb3. The figure of merit (ZT) for Co4Sb11.5Te0.5 increases with temperature in the measured temperature range. The maximum value 0.67 was obtained at 710 K, which is much higher than that of CoSb3 (0.17), and also higher than the result of Christian Stiewe (0.65) [20]. The further work will be focus on the filling for CoSb3 based on doped with Te under HPHT. 4. Conclusion Fig. 4. (a) Temperature dependence of thermal conductivity for CoSb3 and Co4Sb11.5 Te0.5; (b) Temperature dependence of figure of merit (ZT) of CoSb3 and Co4Sb11.5 Te0.5.
abundant grain boundaries than those of CoSb3 sample, which may be helpful to decrease its thermal conductivity. The microstructure of the samples agrees with the advantages of high-pressure synthesis, these advantages include restraining disorder and introducing abundant grain boundaries, etc [16]. Fig. 3(a) shows the temperature and x dependence of the absolute Seebeck coefficient for Co4Sb12 − xTex. As expected, the higher the doping levels, the lower the Seebeck coefficients. The absolute Seebeck coefficient of CoSb3 gradually increases with increasing temperature and shows a maximum value at about 550 K, and thereafter it decreases. The absolute value of the Seebeck coefficient for the sample with x = 0.5 is generally larger than that for CoSb3, and its maximum absolute value attains to about 273.9 µV/K. Fig. 3(b) shows the temperature and x dependence of the electrical resistivity for Co4Sb12 − xTex. The temperature corresponding to the peak shifts toward the higher temperature as x is increased from 0.1 to 0.5 in Co4Sb12 − xTex. The electrical resistivity for Co4Sb11.5Te0.5 increases linearly with increasing temperature, and the minimum value of 0.85 × 10− 3 Ω cm was obtained at 301 K, which is even much lower than that of the same sample prepared at normal pressure (about 1.0 × 10− 3 Ω cm) [6]. According to the previous studies [17], Te atoms in the Sb sublattice serve as electron donors in CoSb3 − xTex. Each Te atom can provide 0.3–0.33 effective electrons to contribute to the electrical conduction. Power factor (PF) was calculated from the measured Seebeck coefficient and electrical resistivity as shown in Fig. 3(c). The Power factor for Co4Sb12 − xTex increases with increasing x. Co4Sb11.5Te0.5 shows the largest PF values both at room temperature and high temperature, owing to its lowest resistivity. The value of PF reaches 35.3 µw/(cmK2) at 710 K, quite high for a kind of n-type thermoelectric materials [18]. The above results agree with the advantages of high-pressure synthesis. The electrical transport properties can be improved
Skutterudite compounds Co4Sb12 − xTex were successfully synthesized by HPHT. Co4Sb11.5Te0.5 shows a typical behavior of degenerate semiconductors. A maximum ZT value of 0.67 was obtained for a heavily doped n-type sample at a temperature of 710 K. Skutterudite materials show good thermoelectric properties but efforts should now focus on lattice thermal conductivity reduction. Acknowledgments This work was financially supported by the National Science Foundation of China (50731006) (50801030), and Graduate Innovation Fund of Jilin University (20080219). References [1] Morelli DT, Meisner GP, Chen B, Hu S, Uher C. Phys Rev B 1997;56:7376–83. [2] Nolas GS, Slack GA, Morelli DT, Tritt TM, Ehrlich AC. J Appl Phys 1996;79:4002–5. [3] Tritt TM, Nolas GS, Slack GA, Ehrlich AC, Gillespie DJ, Cohn JL. J Appl Phys 1996;79:8412–8. [4] Morelli DT, Caillat T, Fleurial JP, Borshchevsky A, Vandersande J, Chen B, et al. Phys Rev B 1995;51:9622–8. [5] Nordström L, Singh DJ. Phys Rev B 1996;53:1103–8. [6] Shi X, Zhang W, Chen LD, Yang J, Uher C. Phys Rev B 2008;75:2352081–9. [7] Mi JL, Zhao XB, Zhu TJ, Tu JP. Mater Lett 2008;62:2363–5. [8] Peng JY, Yang J, Zhang TJ, Song XL, Chen YH. J Alloys Compd 2004;381:313–6. [9] Zhang X, Lu QM, Zhang JX, Wei Q, Liu DM, Liu YQ. J Alloys Compd 2007;457: 368–71. [10] Dudkin LD, Abrikosov NK. Sov Phys Solid State 1959;1:126–33. [11] Liu WS, Zhang BP, Li JF, Zhang HL, Zhao LD. J Appl Phys 2007;102:103717. [12] Yang L, Hng H, Cheng H, Sun T, Ma J. Mater Lett 2008;62:2483–5. [13] Su TC, Jia XP, Ma HA, Zang CY, Zhou L, Guo JG, et al. Mater Lett 2008;62:3269–71. [14] Su TC, Jia XP, Ma HA, Jiang YP, Dong N, Deng L, et al. J Alloys Compd 2009;468: 410–3. [15] Ma HA, Su TC, Zhu PW, Guo JG, Jia XP. J Alloys Compd 2008;454:415–8. [16] Zhu PW, Jia XP, Chen HY, Guo WL, Chen LX, Li DM, et al. Solid State Commun 2002;123:43–7. [17] Wojciechowski K, Toboa J, Leszczyski J. J Alloys Compd 2003;361:19–27. [18] Yasukawa M, Ioroi A, Ikeuchi K, Kono T. Mater Lett 2004;58:3536–9. [19] Wood C. Rep Prog Phys 1988;5:459–539. [20] Stiew C, Bertini L, Toprak M, Christensen M, Platzek D, Williams S, Gatti C, et al. J Appl Phys 2005;97:044317-1-7.