Journal of the Korean Physical Society, Vol. 64, No. 1, January 2014, pp. 84∼88
Synthesis and Thermoelectric Properties of Cez Fe4−x Cox Sb12 Skutterudites Kwan-Ho Park, Soonil Lee and Won-Seon Seo Energy and Environmental Materials Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea
Il-Ho Kim∗ Department of Materials Science and Engineering, Korea National University of Transportation, Chungju 380-702, Korea (Received 17 September 2013) P-type Ce-filled skutterudites Cez Fe4−x Cox Sb12 (z = 0.3-1, x = 1-3) were prepared, and their thermoelectric properties were examined at temperatures between 500 K and 800 K. Specimens were synthesized by encapsulated melting at 1323 K for 10 h and were hot pressed after annealing at 873 K for 24 h. Stable skutterudite phases could be obtained by the incorporation of Ce filling and charge compensation with Co for Fe. The electrical conductivity decreased slightly with increasing temperature and Co content. The Seebeck coefficient had positive values (p-type conductivity), increased with increasing temperature until a specific temperature, and then decreased thereafter. The thermal conductivity decreased through the further lattice scattering induced by substituting Co at Fe sites and by Ce filling. The thermoelectric performance of Fe-rich skutterudites was superior to that of Co-rich skutterudites. A dimensionless figure of merit of ZTmax = 0.7 was achieved at 823 K for CeFe4 Sb12 . PACS numbers: 72.15.Jf, 72.20.Pa Keywords: Thermoelectric, Skutterudite, Ce filling, Charge compensation DOI: 10.3938/jkps.64.84
I. INTRODUCTION
= 1.4) and triple filling (ZT = 1.7) [22–25]. However, p-type filled skutterudites have relatively low ZT values of less than 1.0. Therefore, better p-type filled skutterudites that are compatible with n-type filled skutterudites need to be developed [17–21]. The improvement of the thermoelectric performance of single-filled or multi-filled n-type skutterudites by lowering the lattice thermal conductivity has been very effective. This implies that it would be possible for a p-type filled skutterudite to be designed with an optimal composition through a systematic understanding of the skutterudite. RM4 X12 (R: alkaline, alkaline-earth, rare earth element, M: Fe, Ru, Os, Co, Rh, Ir, X: P, As, Sb) can be used as a typical formula for a filled skutterudite. Research on filled skutterudites has focused on RCo4 Sb12 -based materials for n-type skutterudites, and RFe4 Sb12 -based materials for p-type skutterudites. The rare-earth-filled RFe4 Sb12 skutterudite has a tilted octahedral structure with Fe atoms at the center in the form of a slightly distorted rectangular ring and two dodecahedral cages surrounded by Sb atoms. [Fe4 Sb12 ]4− is an unstable structure because it only has 68 outermost electrons, but the incorporation of R4+ filler ions into the cage makes R4+ [Fe4 Sb12 ]4− stable. In this study, Ce-filled skutterudites were prepared, and their phases and thermoelectric properties were examined by changing the filling fraction and the charge com-
Superior thermoelectric materials should satisfy the PGEC (phonon glass and electron crystal) concept suggested by Slack [1], for which the application of skutterudites is well suited [2–5]. The rattling effect of filler atoms in the skutterudite structure can reduce the lattice thermal conductivity due to phonon scattering. The fillers are located at the voids of the skutterudite lattice, are loosely bonded with the host atoms, and can absorb heat through thermal vibrations [6]. Therefore, filling a skutterudite can bring about a drastic reduction in its thermal conductivity without a significant decrease in its electrical conductivity. Recently, many studies have examined the extent of the thermal conductivity reduction at a specific temperature in filled skutterudites by controlling the filler atoms, the filling fraction, the atomic displacement, and the frequency [7–21]. Filled skutterudites are superior thermoelectric materials due to their high electrical conductivity and low lattice thermal conductivity. N-type filled skutterudites have exhibited high ZT (dimensionless thermoelectric figure of merit) values with single or double filling (ZT ∗ E-mail:
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Synthesis and Thermoelectric Properties of Cez Fe4−x Cox Sb12 Skutterudites· · · – Kwan-Ho Park et al.
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pensation.
II. EXPERIMENTS AND DISCUSSION Ce-filled Cez Fe4−x Cox Sb12 skutterudites were synthesized by encapsulated melting and were consolidated by hot pressing. Elemental Ce (purity 99.9%), Fe (purity 99.95%), Co (purity 99.95%), and Sb (purity 99.999%) were melted at 1323 K in an encapsulated quartz ampoule that was internally coated with carbon, after which the melts were quenched in water. The quenched ingots were annealed at 873 K for 100 h to homogenize and transform them into skutterudite phases. These annealed ingots were crushed to a particles size of less than 75 µm. The powders were then subjected hot pressing in a graphite die with an internal diameter of 10 mm at 873 K under a pressure of 70 MPa for 1 h in a vacuum. The phases of the sintered samples were examined by using X-ray diffraction (XRD: Bruker D8 Advance) with Cu Kα radiation (40 kV, 40 mA). The diffraction patterns were measured in the θ-2θ mode (10 to 90◦ 2θ) with a step size of 0.02◦ , a scan speed of 3 ◦ /min, and a wavelength of 0.15405 nm. The sintered bodies were cut to 3 × 3 × 10 mm3 for measurements of the Seebeck coefficient and the electrical conductivity and were cut to 10 mm (diameter) × 1 mm (length) for thermal conductivity measurements. The Seebeck coefficient and the electrical conductivity were measured using the temperature differential and the four-probe methods (Ulvac-Riko ZEM3), respectively. The thermal conductivity was evaluated through the measuring the thermal diffusivity, specific heat and density by using the laser flash method (UlvacRiko TC9000H). The Hall coefficient, carrier concentration, and mobility were examined at room temperature in a constant magnetic field (1 T) at a constant electric current (50 mA). Figure 1 presents the X-ray diffraction patterns of Cez Fe4−x Cox Sb12 skutterudites. All specimens were transformed into skutterudite phases after annealing. Ce3+∼4+ ion filling into voids and charge compensation with Co for Fe made the skutterudite phase stable through excess electron donation. Table 1 summarizes the electrical transport properties of Cez Fe4−x Cox Sb12 at room temperature. The Hall coefficients had positive values, indicating that the majority charge carriers were holes. The carrier concentration decreased with increasing Co content due to the charge compensation resulting from the substitution of Co for Fe. The electrical conductivity (σ) can be written as [15]
σ=
ne2 τ = neµ m∗
(1)
where n is the carrier concentration, e is the electronic charge, τ is the relaxation time of the carrier, m∗ is the
Fig. 1. (Color online) XRD patterns of Cez Fe4−x Cox Sb12 skutterudites: (a) CeFe4 Sb12 , (b) Ce0.9 Fe3 CoSb12 , (c) Ce0.6 Fe2 Co2 Sb12 and (d) Ce0.3 FeCo3 Sb12 .
Fig. 2. Temperature dependence of the electrical conductivity of Cez Fe4−x Cox Sb12 .
effective mass of the carrier and µ is the carrier mobility. The carrier density is related to the reduced Fermi energy by n = 2(
2πm∗ kT 3 ) 2 exp(η) h2
(2)
where η = EF /kT is the reduced Fermi energy, k is the Boltzmann constant and T is the temperature in Kelvin. Figure 2 indicates the temperature dependence of the electrical conductivity of Cez Fe4−x Cox Sb12 . Overall, Ferich skutterudites showed higher electrical conductivities compared with Co-rich skutterudites. The electrical conductivity decreased with increasing substitution of Co for Fe, which was related to a reduction in the carrier concentration by charge compensation. The electrical conductivities of all specimens decreased slightly with increasing temperature, which indicated that the
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Table 1. Transport properties of Cez Fe4−x Cox Sb12 skutterudites at room temperature. Composition CeFe4 Sb12 Ce0.9 Fe3 CoSb12 Ce0.6 Fe2 Co2 Sb12 Ce0.3 FeCo3 Sb12
Hall coefficient [cm3 C−1 ] 1.75 × 10−3 3.42 × 10−3 1.40 × 10−2 1.03 × 10−1
Mobility [cm2 V−1 s−1 ] 3.18 2.86 7.69 38.1
Fig. 3. Temperature dependence of the Seebeck coefficient of Cez Fe4−x Cox Sb12 .
Cez Fe4−x Cox Sb12 skutterudites were degenerate semiconductors. Figure 3 shows the temperature dependence of the Seebeck coefficient of Cez Fe4−x Cox Sb12 . The Seebeck coefficient (α) is expressed as [15] 5 k α = ± [η − (r + )] e 2
(3)
where r is the scattering parameter, and we assumed that the carrier relaxation time could be expressed in terms of the carrier energy as τ = τo Er . η is the reduced potential energy of the carrier, and (r + 5/2) is the reduced kinetic energy that is transported by the current. In this study, all specimens had positive values, showing p-type conduction. The Seebeck coefficient increased with increasing temperature until a particular temperature and then decreased thereafter. This was due to bipolar conduction resulting from intrinsic excitation. The intrinsic transition temperature was shifted to a lower value with increasing amount of Co substitution. In general, a lower band-gap energy of a material shifts the intrinsic transition temperature to a lower value. Overall, the Seebeck coefficient of the Fe-rich skutterudite was lower than that of the Co-rich skutterudite. This is due to the change in the carrier concentration caused by the replacement of
Carrier concentration [cm−3 ] 3.57 × 1021 1.82 × 1021 4.46 × 1020 6.05 × 1019
Fig. 4. Temperature dependence of the power factor of Cez Fe4−x Cox Sb12 .
Fe by Co, which is in good agreement with the decrease in the carrier concentration (Table 1) and the change in the electrical conductivity (Fig. 2). Figure 4 presents the temperature dependence of the power factor of Cez Fe4−x Cox Sb12 . The power factor increased to a saturated level with increasing temperature. Fe-rich skutterudites showed higher power factor values compared with Co-rich skutterudites. The highest power factor of 2.7 mW/mK2 was achieved in the range of 723 K to 823 K with CeFe4 Sb12 . The Ce-filled Fe-rich skutterudite was superior to the Ce-filled Co-rich skutterudite with regard to thermoelectric power generation at high temperatures. Figure 5 shows the temperature dependence of the thermal conductivity of Cez Fe4−x Cox Sb12 . The thermal conductivity of the Fe-rich skutterudite was higher than that of the Co-rich skutterudite. The thermal conductivity was reduced by substituting Co for Fe due to the phonon scattering from the solid solution and the change in carrier concentration induced by the filling and substitution. The thermal conductivity (κ) is the sum of the lattice thermal conductivity (κL ) due to phonons and the electronic thermal conductivity (κE ) due to carriers, and it is expressed as [15]
Synthesis and Thermoelectric Properties of Cez Fe4−x Cox Sb12 Skutterudites· · · – Kwan-Ho Park et al.
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Fig. 6. Temperature dependence of the dimensionless figure of merit of Cez Fe4−x Cox Sb12 .
skutterudite is about 4 W/mK and that its lattice thermal conductivity is about 3 W/mK [27], Ce filling and Co substitution for Fe significantly reduced the thermal conductivity. The dimensionless thermoelectric figure of merit (ZT) is given by [28]
ZT =
Fig. 5. Temperature dependence of (a) the thermal conductivity and (b) the lattice thermal conductivity of Cez Fe4−x Cox Sb12 .
κ = dCp D = κL + κE
(4)
where d is the density, Cp is the specific heat and D is the thermal diffusivity. Both components can be separated using the Wiedemann-Franz law [26]
κE =
π2 k 2 ( ) σT = LσT 3 e
(5)
where the Lorenz number is assumed to be a constant (L = 2.45 × 10−8 V2 K−2 ) for the evaluation. The lattice thermal conductivities of all samples except Ce0.3 FeCo3 Sb12 showed low values less than 2 W/mK at all temperatures examined, as shown in Fig. 5(b). The thermal conductivity of binary CoSb3 skutterudite is about 8 W/mK, and its lattice thermal conductivity is about 7.5 W/mK [27]. Considering that the thermal conductivity of the unfilled Fe-doped CoSb3
m∗ 3/2 µT 5/2 α2 σT ∼( ) κ me κL
(6)
where me is the mass of an electron. Therefore, a superior thermoelectric material should have a large Seebeck coefficient (large effective mass of carrier), high electrical conductivity (low carrier scattering) and low thermal conductivity (high phonon scattering) simultaneously. Figure 6 presents the temperature dependence of the dimensionless figure of merit (ZT) of Cez Fe4−x Cox Sb12 . The values of the ZT for all the specimens showed a maximum value at a certain temperature and then decreased. Except for Ce0.3 FeCo3 Sb12 , the ZT exhibited similar values at temperatures lower than 623 K. However, in the case of CeFe4 Sb12 , the ZT values increased with increasing temperature to a maximum of ZT = 0.7 at 823 K. Ce-filled skutterudites did not need the charge compensation of Co for Fe, or should have been substituted with x < 1. The Fe-rich skutterudite had superior thermoelectric performance due to their having a higher power factor and a lower thermal conductivity compared with the Co-rich skutterudite.
III. CONCLUSION Cez Fe4−x Cox Sb12 skutterudites were synthesized by encapsulated melting/annealing and were consolidated
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by hot pressing. The phase changes and the thermoelectric properties were investigated with changing Ce filling fraction and with changing charge compensation by substituting Co for Fe. The electrical conductivities of all specimens decreased slightly with increasing temperature, which indicated that the Cez Fe4−x Cox Sb12 skutterudites were degenerate semiconductors. The Seebeck coefficient showed positive values, increased with increasing temperature until a particular temperature, and decreased thereafter. The intrinsic transition temperature was shifted to a lower value with increasing amount of Co substitution. The thermal conductivity decreased through further lattice scattering caused by substituting Co for Fe and by Ce filling. The power factor and the dimensionless figure of merit of the Fe-rich skutterudite were superior to those of the Co-rich skutterudite. The Ce-filled Fe-rich skutterudite is more suitable for thermoelectric power generation at high temperatures.
ACKNOWLEDGMENTS This work was supported by the Defense Acquisition Program Administration (DAPA) and by the Agency for Defense Development (ADD) under contract No. UC120037GD, Republic of Korea.
REFERENCES [1] G. A. Slack, Handbook of Thermoelectrics, edited by D. M. Rowe (CRC, Boca Raton, 1995), p. 407. [2] T. H. An, C. Park, S. M. Choi, W. S. Seo, I. H. Kim and S. U. Kim, J. Kor. Phys. Soc. 60, 1717 (2012). [3] I. H. Kim, S. M. Choi, W. S. Seo and D. I. Cheong, J. Kor. Phys. Soc. 61, 1376 (2012). [4] M. S. Song, S. M. Choi, W. S. Seo, J. Moon and K. W. Jang, J. Kor. Phys. Soc. 60,1735 (2012). [5] K. H. Park, S. W. You, S. C. Ur and I. H. Kim, J. Kor. Phys. Soc. 60, 1485 (2012). [6] V. Keppens, D. Mandrus, B. C. Sales, B. C. Chakoumakos, P. Dai, R. Coldea, M. B. Maple, D. A. Gajewski, E. J. Freeman and S. Bennington, Nature 395, 876 (1998). [7] J. W. Kaiser and W. Jeitschko, J. Alloys Compd. 291, 66 (1999).
[8] B. Bourgoin, D. B’erardan, E. Alleno, C. Godart, O. Rouleau and E. Leroy, J. Alloys Compd. 399, 47 (2005). [9] E. Alleno, D. Be’rardan, C. Godart, M. Puyet, B. Lenoir, R. Lackner, E. Bauer, L. Girard and D. Ravot, Phys. B 383, 103 (2006). [10] Y. G. Yan, W. Wong-Ng, J. A. Kaduk, G. J. Tan, W. J. Xie and X. F. Tang, Appl. Phys. Lett. 98, 142106 (2011). [11] Q. Jie, J. Zhou, I. K. Dimitrov, C. P. Li, C. Uher, H. Wang, W. D. Porter and Q. Li, Mater. Res. Soc. Symp. Proc. 1267, DD03-03 (2010). [12] G. J. Tan, S. Y. Wang, Y. G. Yan, H. Li and X. F. Tang, J. Electron. Mater. 41, 6 (2012). [13] J. Zhou, Q. Jie and Q. Li, Mater. Res. Soc. Symp. Proc. 1267, DD05-25 (2010). [14] G. Tan, S. Wang, X. Tang, H. Li and C. Uher, J. Sol. Stat. Chem. 196, 203 (2012). [15] J. Y. Jung, K. H. Park and I. H. Kim, J. Kor. Phys. Soc. 58, 1421 (2011). [16] P. F. Qiu, R. H. Liu, J. Yang, X. Shi, X. Y. Huang, W. Zhang, L. D. Chen, J. Yang and D. J. Singh, J. Appl. Phys. 111, 023705 (2012). [17] B. C. Sales, D. Mandrus and R. K. Williams, Sci. 272, 1325 (1996). [18] M. D. Hornbostel, E. J. Hyer, J. H. Edvalson and D. C. Johnson, Inorg. Chem. 36, 4270 (1997). [19] B. C. Sales, D. Mandrus, B. C. Chakoumakos, V. Keppens and J. R. Thompson, Phys. Rev. 56, 15081 (1997). [20] D. M. Rowe, V. L. Kuznetsov and L. A. Kuznetsova, Proc. the 17th Intl. Conf. Thermoelectrics, IEEE (1998), p. 323. [21] P. F. Qiu, J. Yang, R. H. Liu, X. Shi, X. Y. Huang, G. J. Snyder, W. Zhang and L. D. Chen, J. Appl. Phys. 109, 063713 (2011). [22] X. Shi, J. Yang, J. R. Salvador, M. Chi, J. Y. Cho, H. Wang, S. Bai, J. Yang, W. Zhang and L. Chen, J. Am. Chem. Soc. 133, 7837 (2011). [23] W. Zhao, Q. Zhang, C. Dong, L. Liu and X. Tang, J. Am. Chem. Soc. 131, 3713 (2009). [24] J. Graff, S. Zhu, T. Holgate, J. Peng, J. He and T. M. Tritt, J. Electron. Mater. 40, 5 (2011). [25] C. Uher, C. P. Li and S. Ballikaya, J. Electron Mater. 39, 9 (2010). [26] C. Kittel, Introduction to Solid State Physics, 6th ed. (John Wiely & Sons, 1986), p. 152. [27] I. H. Kim and S.C. Ur, Mater. Lett. 61, 2446 (2007). [28] S. W. You and I. H. Kim, Curr. Appl. Phys. 11, S392 (2011).
