Thermoelectric and Transport Properties of In-filled and Ni-doped CoSb 3 Skutterudites

Transcription

1 Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp Thermoelectric and Transport Properties of In-filled and Ni-doped CoSb 3 Skutterudites Jae-Yong Jung, Kwan-Ho Park and Il-Ho Kim Department of Material Science and Engineering/Regional Innovation Center for Sustainable Eco-Devices and Materials (RIC-ReSEM), Chungju National University, Chungju , Korea Soon-Mok Choi and Won-Seon Seo Energy Materials Laboratory, Green Ceramic Division, Korea Institute of Ceramic Engineering and Technology (KICET), Seoul , Korea (Received 5 August 2010, in final form 27 August 2010) In-filled and Ni-doped CoSb 3 (In zco 4 xni xsb 12) skutterudites were synthesized by encapsulated induction melting, and their thermoelectric and transport properties were examined at temperatures from 300 to 700 K. A single δ-phase was obtained successfully by subsequent heat treatment at 823 K for 120 h. In zco 4 xni xsb 12 was an n-type semiconductor at all temperatures examined, indicating that Ni atoms acted as electron donors by substituting for Co atoms. The thermal conductivity was reduced considerably by In filling and Ni doping due to an increase in phonon scattering and impurity scattering. The thermoelectric properties were improved due to the low thermal conductivity as a result of In filling and the optimum carrier concentration caused by Ni doping. PACS numbers: Jf, Pa Keywords: Skutterudite, Thermoelectric DOI: /jkps I. INTRODUCTION The main challenge in the field of thermoelectric research is to enhance the efficiency of thermoelectric materials and devices. In order to improve the performance of thermoelectric devices, the materials should have a thermal conductivity as low as that of glass and an electrical conductivity as high as that of a crystal. Since skutterudites can satisfy the PGEC (phonon glass and electron crystal) concept [1], many studies have been carried out on thermoelectric skutterudites [2-4]. They are expected to be the most promising thermoelectric materials for intermediate temperature applications. CoSb 3 -based skutterudites are of interest due to their excellent electrical transport properties and large Seebeck coefficients. Unfortunately, the thermal conductivity of binary CoSb 3 is too large for thermoelectric applications. CoSb 3 has two large voids (2a positions) in the skutterudite crystal structure (RCo 4 Sb 12 in terms of the half unit cell, where R stands for the void). Many studies have attempted to fill the voids with rattlers [5,6] and/or to dope with suitable impurities to reduce the thermal conductivity by introducing phonon scattering ihkim@cjnu.ac.kr; Fax: centers [7-11]. Filling the voids not only decreases the lattice thermal conductivity by reducing the phonon s mean free path but also changes the electronic properties [12,13]. Recently, indium has attracted considerable attention as a promising filling element in CoSb 3 -based skutterudites. He et al. [14] reported that the maximum dimensionless figure-of-merit (ZT max ) of the In 0.25 Co 4 Sb 12 compound reached 1.2 at 575 K. Other studies [15-19] of In-filled skutterudites suggested that such compounds have a high Seebeck coefficient and good thermoelectric performance. On the other hand, Dudkin and Adrikosov examined the effect of 13 different impurity elements on the electronic transport properties of CoSb 3 [20]. The substitution of dopants for Co or Sb can affect the electronic structure and electrical properties and cause a substantial change in the carrier mass. Furthermore, doping can affect the lattice thermal conductivity due to phonon scattering on the impurities [9]. Anno et al. [21] reported that Ni had a strong influence on the electronic properties of CoSb 3 due to electron-phonon interactions. In this study, In-filled and Ni-doped CoSb 3 (In z Co 4 x Ni x Sb 12 ) skutterudites were prepared, and the effects of filling and doping on the thermoelectric and the transport properties were examined.

2 -774- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 II. EXPERIMENTS AND DISCUSSION In-filled and Ni-doped CoSb 3 skutterudites (In z Co 4 x Ni x Sb 12 : z = 0.05, 0.15, 0.25; x = 0.1, 0.2) were synthesized by encapsulated induction melting. High purity Co (purity 99.95%), Sb (purity %), In (purity 99.99%), and Ni (purity 99.99%) were placed in an evacuated quartz ampoule and melted at an RF electrical power of 7 kw at 40 khz for 1 h. The ingots were annealed at 823 K for 120 h in vacuum to give sufficient time for In to fill the voids in the skutterudite structure and for dopant activation. Phase analysis was performed by using high resolution X-ray diffraction (HRXRD: Rigaku DMAX2500VPC) with Cu K radiation (40 kv, 200 ma). The annealed ingot was cut into rectangular shapes with dimensions of mm 3 for both the Seebeck coefficient and electrical conductivity measurements and was cut into disc shapes with dimensions of 10 mm (diameter) 1 mm (thickness) for the thermal conductivity and Hall effect measurements. The Hall effect measurements were carried out in a constant magnetic field (1 T) at a constant electric current (50 ma) at 300 K by using with a Keithley 7065 system. The Seebeck coefficient and the electrical conductivity were measured using the temperature differential and 4-probe methods, respectively, with Ulvac-Riko ZEM2-M8 equipment in a helium atmosphere. The thermal conductivity was evaluated from the thermal diffusivity, specific heat and density measurements using a laser flash Ulvac-Riko TC7000 system in a vacuum. The thermal diffusivity was measured by using a half-time laser pulse width correction and was analyzed by using a non-linear least-squares curve fitting. Reference data (0.23 J/gK) in the literature [22] were used for the specific heat. Figure 1 shows the XRD patterns of the In-filled/Nidoped CoSb 3 skutterudites. The sharp peaks in the diffraction pattern revealed that the materials were polycrystalline, and the calculated peak values matched well with the standard data (JCPDS file No ). The void radius of CoSb 3 is Å, and the atomic radius of In is 1.06 Å [14]. The In filler position is determined to be at the void position of (0, 0, 0) whereas the Co position is (0.25, 0.25, 0.25) and the Sb position is (0, 0.334, 0.157). The filling fraction limit of the filled CoSb 3 skutterudite was described by Shi et al. [23] on the basis of the density function method, keeping the thermodynamic stability of the filled skutterudite. If the filling fraction exceeds the limit, then the unit cell expands, and an unstable filled skutterudite, which leads to the filler interacting with host materials and secondary phases being formed, may occur. These secondary phases are thermodynamically more stable than the filled skutterudite [23]. In this study, phases were fully transformed to a single δ- CoSb 3 phase by subsequent heat treatment at 823 K for 120 h in a vacuum. No secondary phases were found. This confirms that the In is located at the rattler site Fig. 1. X-ray diffraction patterns of In zco 4 xni xsb 12 skutterudites prepared by encapsulated induction melting and post-annealing at 823 K for 120 h: (a) In 0.05Co 3.9 Ni 0.1 Sb 12, (b) In 0.15Co 3.9Ni 0.1Sb 12, (c) In 0.25Co 3.9Ni 0.1Sb 12, (d) In 0.05Co 3.8Ni 0.2Sb 12, (e) In 0.15Co 3.8Ni 0.2Sb 12, and (f) In 0.25Co 3.8Ni 0.2Sb 12. and that the In z Co 4 x Ni x Sb 12 skutterudite was thermodynamically stable, which means that In filling can reduce the lattice thermal conductivity. In addition, Ni ([Ar]3d 8 4s 2 ) substituting for Co ([Ar]3d 7 4s 2 ) is expected to generate excess electrons and to increase the electrical conductivity of n-type skutterudites. Table 1 lists the electronic transport properties of In z Co 4 x Ni x Sb 12 at 300 K. The Hall coefficients were negative for all specimens, which indicates that the In z Co 4 x Ni x Sb 12 skutterudites showed n-type conduction. The carrier concentration increased with increasing Ni doping and In filling content and ranged from to cm 3. The In filler and Ni dopant can affect the electronic structure of CoSb 3 and generate excess electrons. Therefore, the filling and the doping contents should be optimized for better thermoelectric performance. Akai et al. [13] has theoretically calculated and examined the carrier types and the positions of impurity atoms (fillers and dopants) in the unit cell of filled and doped (substituted) CoSb 3. They concluded that In atoms can fill the void sites and show the n-type conduction, but cannot occupy the Co sites or the Sb sites. Figure 2 presents the temperature dependence of the Seebeck coefficient of In z Co 4 x Ni x Sb 12 skutterudites. All In z Co 4 x Ni x Sb 12 compounds showed a negative Seebeck coefficient (n-type conduction) and became more negative, i.e., the absolute value increased, with increasing temperature. Ni doping and In filling reduced the absolute value of the Seebeck coefficient because the carrier concentration was increased as a result of donation of excess electrons to the conduction band. If a single type of carrier scattering is considered, the Seebeck coefficient (α) can be expressed as

3 Thermoelectric and Transport Properties of In-filled and Ni-doped CoSb 3 Skutterudites Jae-Yong Jung et al Table 1. Change in the electronic transport properties of In zco 4 xni xsb 12 with In filling and Ni doping content at 300 K. In content (z) Ni content (x) Hall coefficient, R H (cm 3 /C) Carrier concentration, n (cm 3 ) Hall mobility, µ H (cm 2 /Vs) Fig. 2. Temperature dependence of the Seebeck of In zco 4 xni xsb 12 skutterudites. Fig. 3. Temperature dependence of the electrical conductivity of In zco 4 xni xsb 12 skutterudites. [ (EC E F ) α = + 2k ] B, (1) et e where E C is the minimum of the conduction band, E F is the Fermi level, k B is the Boltzmann constant, e is the electronic charge and T is the absolute temperature [24]. It can be easily known that the Seebeck coefficient negatively decreases if the electron concentration and the Fermi level increases. Compared with other reported fillers [25-31] like Ba, Ca, Ce, Eu, La, Nd, and Sn, the In filler has a significant effect on the Seebeck coefficient. This may be due to an optimization of the concentration of the charge carrier, which also carries the heat. Figure 3 shows the temperature dependence of the electrical conductivity. The electrical conductivity of intrinsic CoSb 3 is very low (approximately Sm 1 ) at room temperature, but increases rapidly with increasing temperature, indicating a non-degenerate semiconducting behavior. However, in this study, the Infilled/Ni-doped CoSb 3 have high electrical conductivities of mid-10 4 to 10 5 Sm 1 at room temperature that changed with temperature. Therefore, In z Co 4 x Ni x Sb 12 skutterudites behave like degenerate semiconductors. This is due to the increases in the Fermi level and the electron concentration by adding In and Ni atoms. A similar result was reported by He et al. [14] at temperatures from 300 K to 600 K. As the In filling fraction (z) for In z Co 4 Sb 12 increases from 0.05 to 0.3, the electrical resistivity decreases. The In fillers significantly contribute to the reduction of the electrical resistivity by contributing excess electrons. Figure 4 shows the temperature dependence of the power factor (α 2 σ), which can be obtained from the measurements of the Seebeck coefficient (α) and the electrical conductivity (σ). In z Co 4 x Ni x Sb 12 skutterudites have power factors ranging from to Wm 1 K 2 at all temperatures examined. The power factor increased with increasing temperature mainly due to the large Seebeck coefficient. In particular, the power factor of the In 0.25 Co 3.9 Ni 0.1 Sb 12 specimen was > Wm 1 K 2 at temperatures from 300 K to 700 K, which was attributed to the large Seebeck coefficient and the high electrical conductivity. It is believed that In 0.25 Co 3.9 Ni 0.1 Sb 12 should be very useful for thermoelectric power generation. Figure 5 gives the temperature dependence of the thermal conductivity (κ). The thermal conductivity was evaluated from the density (d), the specific heat (C p ) and the thermal diffusivity (D) by using the relationship

4 -776- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 Fig. 4. Temperature dependence of the power factor of In zco 4 xni xsb 12 skutterudites. Fig. 6. Temperature dependence of the lattice thermal conductivity of In zco 4 xni xsb 12 skutterudites. Fig. 5. Temperature dependence of the thermal conductivity of In zco 4 xni xsb 12 skutterudites. κ = dc p D. (2) Intrinsic CoSb 3 has a much higher thermal conductivity, 11 Wm 1 K 1 at 300 K, which decreases with increasing temperature to 7.4 Wm 1 K 1 at 700 K [4]. The thermal conductivity was reduced drastically by In filling and Ni doping. In 0.25 Co 3.9 Ni 0.1 Sb 12 showed very low values, approximately Wm 1 K 1, at all temperatures examined. The lattice thermal conductivity (κ L ) was estimated by subtracting the electronic thermal conductivity (κ E ) from the total thermal conductivity (κ = κ E + κ L ) by using the Wiedemann-Franz relation, κ E = LσT, where L is the Lorenz number ( V 2 K 2 ). In this study, the value of L for a metal was used because the samples were degenerate like metals. Figure 6 shows the lattice thermal conductivity. The lattice contribution had a dominant effect on the thermal conductivity. The Fig. 7. Dimensionless-figure-of-merit of In zco 4 xni xsb 12 skutterudites. lattice thermal conductivity decreased with increasing In filling content. In fillers acted as rattlers and decreased the mean free path of phonons. A minimum κ L value of 1.3 Wm 1 K 1 was achieved for the In 0.25 Co 3.9 Ni 0.1 Sb 12 specimen. The thermal conductivity was proportional to the phonon mean free path because a phonon scattering increase gave rise to a decrease in the phonon mean free path. The filler and/or dopant atoms contributed to the electronic thermal conduction and the phonon scattering centers, which decreased the lattice thermal conductivity. Figure 7 shows the dimensionless figure-of-merit (ZT) given by Z = α2 σ κ ( m m e ) 3 2 µ κ L, (3) where m is the effective mass ofelectron, m e the mass of electron, and µ the electron mobility. The ZT in-

5 Thermoelectric and Transport Properties of In-filled and Ni-doped CoSb 3 Skutterudites Jae-Yong Jung et al creased with increasing temperature and was improved by In filling and Ni doping. The highest ZT value (0.94 at 700 K) was obtained from the In 0.25 Co 3.9 Ni 0.1 Sb 12 specimen. This is mainly due to the high power factor and the maintenance of a low thermal conductivity. III. CONCLUSION In z Co 4 x Ni x Sb 12 skutterudites were synthesized by encapsulated induction melting and subsequent isothermal annealing at 823 K for 120 h. All the specimens showed negative Hall and Seebeck coefficients at temperatures from 300 to 700 K. Ni doping and In filling reduced the absolute value of the Seebeck coefficient due to the increased carrier concentration from the donation of excess electrons to the conduction band. The In z Co 4 x Ni x Sb 12 skutterudites behaved like degenerate semiconductors, and the power factor increased with increasing temperature due mainly to the large Seebeck coefficient. The thermal conductivity was reduced by In filling and Ni doping. In 0.25 Co 3.9 Ni 0.1 Sb 12 showed very low values of approximately Wm 1 K 1 at all temperatures examined. The lattice thermal conductivity decreased with increasing In filling content. A minimum value of 1.3 Wm 1 K 1 was achieved for the In 0.25 Co 3.9 Ni 0.1 Sb 12 specimen. In 0.25 Co 3.9 Ni 0.1 Sb 12 showed the highest ZT = 0.94 at 700 K. ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Republic of Korea (2008EID11P ) and by the Regional Innovation Center (RIC) Program funded by the Ministry of Knowledge Economy, Republic of Korea and R&D Program funded by the Hyundai-Kia Motors. REFERENCES [1] G. A. Slack, CRC Handbook of Thermoelectrics (CRC Press, Boca Raton, 1995), p [2] T. Caillat, A. Borshchevsky and J.-P. Fleurial, J. Appl. Phys. 80, 4442 (1996). [3] S.-C. Ur and I.-H. Kim, J. Korean Phys. Soc. 53, 2415 (2008). [4] R. C. Mallik, J.-Y. Jung, V. D. Das, S.-C. Ur and I.-H. Kim, Solid State Commun. 141, 233 (2007). [5] H. Takizawa, K. Mimura, M. Ito, T. Sizuki and T. Endo, J. Alloys Compd. 282, 79 (1999). [6] G. S. Nolas, H. Takizawa, T. Endo, H. Sellinschegg and D. C. Johnson, Appl. Phys. Lett. 77, 52 (2000). [7] D. Mandrus, A. Migliori, T. W. Darling, M. F. Hundley, E. J. Peterson and J. D. Thompson, Phys. Rev. B. 52, 4926 (1995). [8] K. T. Wojciechowski, Mater. Res. Bull. 37, 2023 (2002). [9] K.-H. Park, H.-I. Jung, S.-C. Ur and I.-H. Kim, J. Korean Inst. Met. Mat. 45, 61 (2007). [10] M.-J. Kim, S.-C. Ur and I.-H. Kim, J. Korean Inst. Met. Mat. 45, 191 (2007). [11] X. Y. Li, L. D. Chen, J. F. Fan, W. B. Zhang, T. Kawahara and T. Hirai, J. Appl. Phys. 98, (2005). [12] K. Akai, H. Kurisu, T. Shimura and M. Matsuura, Proceedings of the 16th International Conference on Thermoelectrics (Dresden, Germany, IEEE, August ), p [13] K. Akai, H. Kurisu, T. Moriyama, S. Tamamoto and M. Matsuura, Proceedings of the 17th International Conference on Thermoelectrics (Nagoya, Japan, IEEE, May 24-28, 1998), p [14] T. He, J. Z. Chen, H. D. Rosenfeld and M. A. Subramanian, Chem. Mater. 18, 759 (2006). [15] J.-Y. Jung, S.-C. Ur and I.-H. Kim, Mater. Chem. Phys. 108, 431 (2008). [16] J. Y. Peng, P. N. Alboni, J. He, B. Zhang, Z. Su, T. Holgate, N. Gothard and T. M. Tritt, J. Appl. Phys. 104, (2008). [17] W. Y. Zhao, C. L. Dong, P. Wei, W. Guan, L. S. Liu, P. C. Zhai, X. F. Tang and Q. J. Zhang, J. Appl. Phys. 102, (2007). [18] J. Y. Peng, J. He, Z. Su, P. N. Alboni, S. Zhu and T. M. Tritt, J. Appl. Phys. 105, (2009). [19] H. Li, X. Tang, Q. Zhang and C. Uher, Appl. Phys. Lett. 94, (2009). [20] L. D. Dudkin and N. K. Abrikosov, Sov. Phys.-Sol. State 1, 126 (1959). [21] H. Anno, K. Matsubara, Y. Notohara, T. Sakakibara and H. Tashiro, J. Appl. Phys. 86, 3780 (1999). [22] E. Müller, C. Stiewe, D. M. Rowe and S. G. K. Williams, Thermoelectrics Handbook (CRC Press, Boca Raton, 2005), Chap. 26. [23] X. Shi, W. Zhang, L. D. Chen and J. Yang, J. Phys. Rev. Lett. 95, (2005). [24] R. C. Mallik, J.-Y. Jung and S.-C. Ur and I.-H. Kim, Met. Mater. Int. 14, 223 (2008). [25] J. S. Dyck, W. Chen, C. Uher, L. Chen, X. Tang and T. Hirai, J. Appl. Phys. 91, 3698 (2002). [26] M. Puyet, A. Dauscher, B. Lenoir, M. Dehmas, C. Stiewe, E. Müller and J. Hejtmanek, J. Appl. Phys. 97, (2005). [27] D. T. Morelli, G. P. Meisner, B. Chen, S. Hu and C. Uher, Phys. Rev. B 56, 7376 (1997). [28] G. A. Lamberton, Jr., S. Bhattacharya, R. T. Littleton IV, M. A. Kaeser, R. H. Tedstrom, T. M. Tritt, J. Yang and G. S. Nolas, Appl. Phys. Lett. 80, 598 (2002). [29] G. S. Nolas, J. L. Cohn and G. A. Slack, Phys. Rev. B 58, 164 (1998). [30] B. C. Sales, B. C. Chakoumakos and D. Mandrus, Phys. Rev. B 61, 2475 (2000). [31] R. C. Mallik, J.-Y. Jung, S.-C. Ur and I.-H. Kim, Met. Mater. Int. 14, 615 (2008).