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Thermoelectric transport properties of nickel-doped Co4−xNixSb11.6Te0.2Se0.2 skutterudites
Physica B 425 (2013) 34–37
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Thermoelectric transport properties of nickel-doped Co4−xNixSb11.6Te0.2Se0.2 skutterudites Chenglong Xu a, Bo Duan a,n, Shijie Ding a, Pengcheng Zhai a,b, Peng Li b a b
Department of Engineering Structure and Mechanics, Wuhan University of Technology, Wuhan 430070, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 16 May 2013 Accepted 18 May 2013 Available online 31 May 2013
In this paper, skutterudite compounds Co4−xNixSb11.6Te0.2Se0.2 (x ¼ 0, 0.05, 0.1 and 0.2) were successfully prepared by the solid state reaction and spark plasma sintering methods. The component phases and the chemical composition of all samples were characterized by the X-ray diffraction and the electron microprobe analysis, respectively. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the 300–800 K temperature range. It is found that suitable amount of Ni dopant in the Co4−xNixSb11.6Te0.2Se0.2 system not only resulted in an improvement of the electrical conductivity but also a drop of the lattice thermal conductivity. The maximum dimensionless figure of merit of 0.83 was obtained at 800 K for sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, representing a 23% improvement compared with that of the Ni-free sample. & 2013 Elsevier B.V. All rights reserved.
Keywords: Multiple-doping Thermoelectric properties Skutterudite
1. Introduction CoSb3-based skutterudite compounds were considered one of the most promising medium-temperature thermoelectric materials for solar thermal energy conversion and industrial waste heat recovery [1,2], and many experiments on it have been carried out in search of a highly efficient thermoelectric skutterudite compound. The performance of a thermoelectric material is gauged by its dimensionless figure of merit, defined as ZT ¼α2sT/κ, where T, α, s and κ are the absolute temperature, the Seebeck coefficient, the electrical conductivity and the thermal conductivity, respectively. It is found that the thermoelectric properties of CoSb3 skutterudite can be significantly improved when doped with suitable elements, which will result in an improvement of the electrical properties and a decline of the thermal conductivity. There are a number of researches reporting about ternary/ multiple CoSb3-based skutterudite compounds that substitute Ni, Fe, Pd, Pt for Co, or Sn, Te, Se for Sb [3–10]. Research findings indicate that appropriate Ni-doping in CoSb3-based skutterudites lowers both the electrical resistance and the lattice thermal conductivity at the same time, especially in the case of Co1−xNixSb3, CaxCo4−yNiySb12, and Ba0.3Co4−xNixSb12 systems [4,5,11,12], with all the results suggesting that Ni-doping is an attractive approach to optimize pure and filled skutterudites. In our previous study [7,13], Te and Se co-doped skutterudite materials Co4Sb12−x−yTexSey have been synthesized by the solid
n
Corresponding author. Tel.: +86 1582 7413 511; fax: +86 278 7867 199. E-mail address: [email protected] (B. Duan).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.05.026
state reaction method, and the thermoelectric properties of them were significantly improved compared with those of undoped CoSb3 skutterudites, but it maintains relatively low electrical conductivity and high thermal conductivity. So far, many Co-site or Sb-site doped skutterudites have been reported, but there are few reports [9,10] on Co-site and Sb-site co-doped skutterudites. In this paper, we prepared a series of tri-doped skutterudite compounds in order to explore the influence of Ni-doping in Co4−x NixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2). The effects of doping fraction on the electronic properties and thermoelectric properties are investigated systematically. 2. Experimental procedure The skutterudite compounds were prepared by the solid state reaction method. Powders of Co (99.9%), Ni (99.999%), Sb (99.999%), Te (99.999%) and Se (99.999%) were blended in stoichiometric ratios of Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) and then loaded into the carbon crucible. The carbon crucible was sealed in vacuum and heated slowly up to 953 K with the heating rate fixed at 0.5 K min−1 and held at 953 K for 6000 min. The obtained materials were ground to powder again before being loaded into the graphite dies, which were then consolidated by a spark plasma sintering (SPS) process at 903 K for 7 min under the pressure of 40 MPa. The phase and microstructure of the SPS samples were characterized by the X-ray diffraction (XRD, D/MAX-RB, Cu Kα) and the electron microprobe analysis (EPMA). The Hall coefficient (RH) was measured by the Physical Properties Measurement System (PPMS
C. Xu et al. / Physica B 425 (2013) 34–37
35
Quantum Design), the carrier concentration (n) and carrier mobility (μΗ) were calculated from n ¼1/eRH and μΗ ¼sRH (s is the electrical conductivity). The Seebeck coefficient and the electrical resistivity were measured by using a measuring system (Sinkuriko, ZEM-3), the standard four-probe method, at the 300–800 K temperature range in an argon atmosphere. The thermal conductivity was evaluated by the combination of the density, the specific heat and the thermal diffusivity. The density (d) was measured by the Archimedes method at room temperature, the specific heat by the differential scanning calorimeter (TA: DSC Q20), and the thermal diffusivity by the laser-flash technique (Netzsch LFA 457) in an Ar flowing atmosphere from 300 K to 800 K.
3. Results and discussion Fig. 1 shows the XRD patterns of powder samples Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2). The XRD patterns are highly consistent with that of the CoSb3 skutterudite phase, with no obvious impurity phase. Together with the back-scattered electron images of the sample surface after SPS, as shown in Fig. 2, it demonstrates that a homogeneous skutterudite phase is obtained in all samples. The chemical compositions determined by the EPMA are displayed in Table 1. As is found for Sb, Te and Se dopant elements, the actual doping fractions are lower than the nominal composition, which is presumably caused by the sublimation owing to their high vapor pressure during the reaction process [14]. Fig. 3 shows the Seebeck coefficient of Co4−xNixSb11.6Te0.2Se0.2 skutterudites. It can be seen that all the samples show n-type conduction. The absolute value of Seebeck coefficient decreases with the increasing Ni doping fraction, due to the increase of carrier concentration [4], and the highest absolute value is obtained as 299 μV K−1 at 590 K in specimen Co4Sb11.6Te0.2Se0.2. The absolute value of Seebeck coefficient of all the samples increases with the temperature rising first and then decreases, indicating the onset of intrinsic conduction [8]. With the increase of Ni content, the peak of the absolute value grows alongside the rising temperature, which is mainly because the extrinsic electron concentration increases while the intrinsic hole decreases due to the cross-gap excitation [6,8]. Fig. 4 displays the variation of the electrical conductivity with temperature in the system of Co4−xNixSb11.6Te0.2Se0.2. The electrical conductivity of all the samples shows similar temperature dependence at low temperature, and it changes only slightly when
Fig. 1. XRD patterns of Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1, and 0.2) samples.
Fig. 2. Back scattered images of the sample surface of Co4−xNixSb11.6Te0.2Se0.2 (x ¼0 and 0.2).
temperature increases from 300 K to 650 K. However, as the temperature exceeds 650 K, the electrical conductivity of Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05 and 0.1) samples increases notably with the rising temperature, behaving like a semiconductor. The electrical conductivity increases obviously with the increasing content of Ni, especially for the Co3.8Ni0.2Sb11.6Te0.2Se0.2 sample, and this results from an increase of the carrier concentration (Table 1) on account of the introduction of extra electrons to the structure by Ni substitution. The maximum value of 6.83 104 Sm−1 is obtained at 300 K for the sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, which is an increase of 122% over that of Co4Sb11.6Te0.2Se0.2 at the same temperature. Fig. 5 presents the temperature dependence of the thermal conductivity (κ) and lattice thermal conductivity (κL) for Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples. As shown in Fig. 5(a), the thermal conductivity of all the samples are significantly decreased compared with that of the undoped CoSb3 skutterudites, as displayed in the inset [15]. The thermal conductivity decreases markedly at temperature before 680 K and then increases with the growing temperature because of the bipolar conduction effect [6]. Fig. 5(b) shows the temperature dependence of the lattice thermal conductivity κL, which is calculated according to the formula: κL ¼κ−κc, where κc is calculated by the Wiedemann– Franz law κc ¼LsT (L is Lorenz number 2 10−8 V2 K−2) [16]. The data of Se/Te single-doped Co4Sb11.8Se0.2 [15] and Co4Sb11.7Te0.3 [17] samples are also presented for comparison. It is obvious that the lattice thermal conductivity of the Ni, Te and Se co-doped skutterudites is much lower than those of single-doped skutterudites in the entire temperature range, mainly because of the
36
C. Xu et al. / Physica B 425 (2013) 34–37
Table 1 Summary of composition, carrier concentration n, carrier mobility μΗ, lattice thermal conductivity κL for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples at room temperature. Nominal composition
EPMA composition
n (1020 cm−3)
μH (cm2 V−1 s−1)
κL (W m−1 K−1)
Co4Sb11.6Te0.2Se0.2 Co3.95Ni0.05Sb11.6Te0.2Se0.2 Co3.9Ni0.1Sb11.6Te0.2Se0.2 Co3.8Ni0.2Sb11.6Te0.2Se0.2
Co4Sb11.33Te0.16Se0.16 Co3.95Ni0.05Sb11.30Te0.16Se0.15 Co3.90Ni0.09Sb11.32Te0.16Se0.16 Co3.80Ni0.19Sb11.31Te0.15Se0.16
−1.22 −1.38 −1.50 −3.16
15.82 16.08 15.76 13.52
3.86 3.64 3.33 3.26
Fig. 3. Temperature dependence of Seebeck coefficient for Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) samples.
Fig. 5. Temperature dependence of thermal conductivity (a) and lattice thermal conductivity and (b) for Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) samples. Fig. 4. Temperature dependence of electrical conductivity for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples.
enhanced point-defect scattering due to the larger electronegativity, mass and volume fluctuations between Sb, Te and Se. The lattice thermal conductivity decreases with the increasing content of Ni, and sample Co3.8Ni0.2Sb11.6Te0.2Se0.2 achieves the lowest lattice thermal conductivity of 1.85 W m−1 K−1 at 775 K, a drop of 25% in comparison with that of Co4Sb11.6Te0.2Se0.2 at the same temperature. The reduction should be attributed to the enhanced point-defect scattering, and more importantly maybe, to the electron–phonon scattering [18] due to the minor difference of atomic masses between Ni (58.7) and Co (58.9) according to the following formula scattering parameter A [19,20]: A¼
Ω0 ΔM 2 xð1−xÞ M 4πυ3
ð1Þ
where Ω0, υ, x, ΔM and M are the volume of the unit cell, the sound velocity, the fraction of guest atoms, the atomic mass difference between the guest and host, and the average mass of the cell, respectively. The dimensionless figure of merit (ZT) of Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) is presented in Fig. 6. The ZT values of Ni–Te–Se co-doped samples are considerably improved compared with that of the Ni-free sample, especially at the high temperature range. As mentioned above, the considerable improvement is mainly affected by the increased electrical conductivity and the decreased lattice thermal conductivity. The maximum ZT was obtained as ∼0.83 at 800 K for the sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, representing a 23% improvement compared with the highest ZT of Ni-free sample Co4Sb11.6Te0.2Se0.2 prepared by the same method.
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Acknowledgments This work is financially supported by the Development Program of China (No. 2012AA051104), the National Basic Research Program of China (No. 2013CB632505), the National Natural Science Foundation of China (Nos. 10832008 and 51272198) and the China Postdoctoral Science Foundation (No. 2013M531752). References
Fig. 6. Temperature dependence of ZT for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples.
4. Conclusions Tri-doped skutterudites Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) have been synthesized by the solid state reaction method and the SPS process. The XRD results, together with backscattered electron images of the sample surface, indicate that single doped skutterudite phase was obtained. It is found that codoping with Ni, Te and Se has a synergic beneficial effect on the skutterudite systems. The suitable amount of Ni dopant in the system of Co4−xNixSb11.6Te0.2Se0.2 not only resulted in an improvement of the electrical conductivity but also a decrease of the thermal conductivity. Co3.8Ni0.2Sb11.6Te0.2Se0.2 showed the highest ZT of 0.83 at 800 K. Better performance of Co4−xNixSb12−y−zTeySez material could be achieved by optimizing the proportion of Ni, Te and Se elements in the composition.
[1] G.S. Nolas, J.L. Cohn, G.A. Slack, Phys. Rev. B 58 (1998) 164. [2] M. Xie, D.M. Gruen, J. Phys. Chem. B 114 (2010) 14339. [3] X.F. Tang, L.D. Chen, J. Wang, Q.J. Zhang, T. Goto, T. Hirai, J. Alloys Compd. 394 (2005) 259. [4] H. Kitagawa, M. Wakatsuki, H. Nagaoka, H. Noguchi, Y. Isoda, K. Hasezaki, Y. Noda, J. Phys. Chem. Solids 66 (2005) 1635. [5] S. Katsuyama, M. Watanabe, M. Kuroki, T. Maehata, M. Ito, J. Appl. Phys. 93 (2003) 2758. [6] W.S. Liu, B.P. Zhang, J.F. Li, H.L. Zhang, L.D. Zhao, J. Appl. Phys. 102 (2007) 103717. [7] B. Duan, P.C. Zhai, L.S. Liu, Q.J. Zhang, Mater. Res. Bull. 47 (2012) 1670. [8] W.S. Liu, B.P. Zhang, L.D. Zhao, J.F. Li, Chem. Mater. 20 (2008) 7526. [9] K.H. Park, I.H. Kim, Electron. Mater. Lett. 7 (2011) 39. [10] C. Stiewe, L. Bertini, M. Toprak, M. Christensen, D. Platzek, S. Williams, C. Gatti, E. Muller, B.B. Iversen, M. Muhammed, M. Rowe, J. Appl. Phys. 97 (2005) 044317. [11] M. Puyet, B. Lenoir, A. Dauscher, C. Candolfi, J. Hejtmanek, C. Stiewe, E. Muller, Appl. Phys. Lett. 101 (2012) 222105. [12] J.S. Dyck, W.D. Chen, C. Uher, L. Chen, X.F. Tang, T. Hirai, J. Appl. Phys. 91 (2002) 3698. [13] B. Duan, P.C. Zhai, L.S. Liu, Q.J. Zhang, X.F. Ruan, J. Solid State Chem. 193 (2012) 8. [14] X.Y. Zhao, X.Y. Li, L.D. Chen, Y.Z. Pei, S.Q. Bai, X. Shi, T. Goto, Jpn. J. Appl. Phys. 47 (2008) 7470. [15] B. Duan, P.C. Zhai, L.S. Liu, Q.J. Zhang, X.F. Ruan, J. Mater. Sci. Mater. Electron. 23 (2012) 1817. [16] B.C. Sales, D. Mandrus, B.C. Chakoumakos, V. Keppens, Phys. Rev. B Condens. Matter 56 (1997) 15081. [17] X.Y. Li, L.D. Chen, J.F. Fan, W.B. Zhang, T. Kawahara, T. Hirai, J. Appl. Phys. 98 (2005) 083702. [18] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Phys. Rev. B 65 (2002) 094115. [19] B. Abeles, Phys. Rev. 131 (1963) 1906. [20] G.P. Meisner, D.T. Morelli, S. Hu, J. Yang, C. Uher, Phys. Rev. Lett. 80 (1998) 3551.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Thermoelectric transport properties of nickel-doped Co4−xNixSb11.6Te0.2Se0.2 skutterudites Chenglong Xu a, Bo Duan a,n, Shijie Ding a, Pengcheng Zhai a,b, Peng Li b a b
Department of Engineering Structure and Mechanics, Wuhan University of Technology, Wuhan 430070, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 16 May 2013 Accepted 18 May 2013 Available online 31 May 2013
In this paper, skutterudite compounds Co4−xNixSb11.6Te0.2Se0.2 (x ¼ 0, 0.05, 0.1 and 0.2) were successfully prepared by the solid state reaction and spark plasma sintering methods. The component phases and the chemical composition of all samples were characterized by the X-ray diffraction and the electron microprobe analysis, respectively. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the 300–800 K temperature range. It is found that suitable amount of Ni dopant in the Co4−xNixSb11.6Te0.2Se0.2 system not only resulted in an improvement of the electrical conductivity but also a drop of the lattice thermal conductivity. The maximum dimensionless figure of merit of 0.83 was obtained at 800 K for sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, representing a 23% improvement compared with that of the Ni-free sample. & 2013 Elsevier B.V. All rights reserved.
Keywords: Multiple-doping Thermoelectric properties Skutterudite
1. Introduction CoSb3-based skutterudite compounds were considered one of the most promising medium-temperature thermoelectric materials for solar thermal energy conversion and industrial waste heat recovery [1,2], and many experiments on it have been carried out in search of a highly efficient thermoelectric skutterudite compound. The performance of a thermoelectric material is gauged by its dimensionless figure of merit, defined as ZT ¼α2sT/κ, where T, α, s and κ are the absolute temperature, the Seebeck coefficient, the electrical conductivity and the thermal conductivity, respectively. It is found that the thermoelectric properties of CoSb3 skutterudite can be significantly improved when doped with suitable elements, which will result in an improvement of the electrical properties and a decline of the thermal conductivity. There are a number of researches reporting about ternary/ multiple CoSb3-based skutterudite compounds that substitute Ni, Fe, Pd, Pt for Co, or Sn, Te, Se for Sb [3–10]. Research findings indicate that appropriate Ni-doping in CoSb3-based skutterudites lowers both the electrical resistance and the lattice thermal conductivity at the same time, especially in the case of Co1−xNixSb3, CaxCo4−yNiySb12, and Ba0.3Co4−xNixSb12 systems [4,5,11,12], with all the results suggesting that Ni-doping is an attractive approach to optimize pure and filled skutterudites. In our previous study [7,13], Te and Se co-doped skutterudite materials Co4Sb12−x−yTexSey have been synthesized by the solid
n
Corresponding author. Tel.: +86 1582 7413 511; fax: +86 278 7867 199. E-mail address: [email protected] (B. Duan).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.05.026
state reaction method, and the thermoelectric properties of them were significantly improved compared with those of undoped CoSb3 skutterudites, but it maintains relatively low electrical conductivity and high thermal conductivity. So far, many Co-site or Sb-site doped skutterudites have been reported, but there are few reports [9,10] on Co-site and Sb-site co-doped skutterudites. In this paper, we prepared a series of tri-doped skutterudite compounds in order to explore the influence of Ni-doping in Co4−x NixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2). The effects of doping fraction on the electronic properties and thermoelectric properties are investigated systematically. 2. Experimental procedure The skutterudite compounds were prepared by the solid state reaction method. Powders of Co (99.9%), Ni (99.999%), Sb (99.999%), Te (99.999%) and Se (99.999%) were blended in stoichiometric ratios of Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) and then loaded into the carbon crucible. The carbon crucible was sealed in vacuum and heated slowly up to 953 K with the heating rate fixed at 0.5 K min−1 and held at 953 K for 6000 min. The obtained materials were ground to powder again before being loaded into the graphite dies, which were then consolidated by a spark plasma sintering (SPS) process at 903 K for 7 min under the pressure of 40 MPa. The phase and microstructure of the SPS samples were characterized by the X-ray diffraction (XRD, D/MAX-RB, Cu Kα) and the electron microprobe analysis (EPMA). The Hall coefficient (RH) was measured by the Physical Properties Measurement System (PPMS
C. Xu et al. / Physica B 425 (2013) 34–37
35
Quantum Design), the carrier concentration (n) and carrier mobility (μΗ) were calculated from n ¼1/eRH and μΗ ¼sRH (s is the electrical conductivity). The Seebeck coefficient and the electrical resistivity were measured by using a measuring system (Sinkuriko, ZEM-3), the standard four-probe method, at the 300–800 K temperature range in an argon atmosphere. The thermal conductivity was evaluated by the combination of the density, the specific heat and the thermal diffusivity. The density (d) was measured by the Archimedes method at room temperature, the specific heat by the differential scanning calorimeter (TA: DSC Q20), and the thermal diffusivity by the laser-flash technique (Netzsch LFA 457) in an Ar flowing atmosphere from 300 K to 800 K.
3. Results and discussion Fig. 1 shows the XRD patterns of powder samples Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2). The XRD patterns are highly consistent with that of the CoSb3 skutterudite phase, with no obvious impurity phase. Together with the back-scattered electron images of the sample surface after SPS, as shown in Fig. 2, it demonstrates that a homogeneous skutterudite phase is obtained in all samples. The chemical compositions determined by the EPMA are displayed in Table 1. As is found for Sb, Te and Se dopant elements, the actual doping fractions are lower than the nominal composition, which is presumably caused by the sublimation owing to their high vapor pressure during the reaction process [14]. Fig. 3 shows the Seebeck coefficient of Co4−xNixSb11.6Te0.2Se0.2 skutterudites. It can be seen that all the samples show n-type conduction. The absolute value of Seebeck coefficient decreases with the increasing Ni doping fraction, due to the increase of carrier concentration [4], and the highest absolute value is obtained as 299 μV K−1 at 590 K in specimen Co4Sb11.6Te0.2Se0.2. The absolute value of Seebeck coefficient of all the samples increases with the temperature rising first and then decreases, indicating the onset of intrinsic conduction [8]. With the increase of Ni content, the peak of the absolute value grows alongside the rising temperature, which is mainly because the extrinsic electron concentration increases while the intrinsic hole decreases due to the cross-gap excitation [6,8]. Fig. 4 displays the variation of the electrical conductivity with temperature in the system of Co4−xNixSb11.6Te0.2Se0.2. The electrical conductivity of all the samples shows similar temperature dependence at low temperature, and it changes only slightly when
Fig. 1. XRD patterns of Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1, and 0.2) samples.
Fig. 2. Back scattered images of the sample surface of Co4−xNixSb11.6Te0.2Se0.2 (x ¼0 and 0.2).
temperature increases from 300 K to 650 K. However, as the temperature exceeds 650 K, the electrical conductivity of Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05 and 0.1) samples increases notably with the rising temperature, behaving like a semiconductor. The electrical conductivity increases obviously with the increasing content of Ni, especially for the Co3.8Ni0.2Sb11.6Te0.2Se0.2 sample, and this results from an increase of the carrier concentration (Table 1) on account of the introduction of extra electrons to the structure by Ni substitution. The maximum value of 6.83 104 Sm−1 is obtained at 300 K for the sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, which is an increase of 122% over that of Co4Sb11.6Te0.2Se0.2 at the same temperature. Fig. 5 presents the temperature dependence of the thermal conductivity (κ) and lattice thermal conductivity (κL) for Co4−x NixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples. As shown in Fig. 5(a), the thermal conductivity of all the samples are significantly decreased compared with that of the undoped CoSb3 skutterudites, as displayed in the inset [15]. The thermal conductivity decreases markedly at temperature before 680 K and then increases with the growing temperature because of the bipolar conduction effect [6]. Fig. 5(b) shows the temperature dependence of the lattice thermal conductivity κL, which is calculated according to the formula: κL ¼κ−κc, where κc is calculated by the Wiedemann– Franz law κc ¼LsT (L is Lorenz number 2 10−8 V2 K−2) [16]. The data of Se/Te single-doped Co4Sb11.8Se0.2 [15] and Co4Sb11.7Te0.3 [17] samples are also presented for comparison. It is obvious that the lattice thermal conductivity of the Ni, Te and Se co-doped skutterudites is much lower than those of single-doped skutterudites in the entire temperature range, mainly because of the
36
C. Xu et al. / Physica B 425 (2013) 34–37
Table 1 Summary of composition, carrier concentration n, carrier mobility μΗ, lattice thermal conductivity κL for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples at room temperature. Nominal composition
EPMA composition
n (1020 cm−3)
μH (cm2 V−1 s−1)
κL (W m−1 K−1)
Co4Sb11.6Te0.2Se0.2 Co3.95Ni0.05Sb11.6Te0.2Se0.2 Co3.9Ni0.1Sb11.6Te0.2Se0.2 Co3.8Ni0.2Sb11.6Te0.2Se0.2
Co4Sb11.33Te0.16Se0.16 Co3.95Ni0.05Sb11.30Te0.16Se0.15 Co3.90Ni0.09Sb11.32Te0.16Se0.16 Co3.80Ni0.19Sb11.31Te0.15Se0.16
−1.22 −1.38 −1.50 −3.16
15.82 16.08 15.76 13.52
3.86 3.64 3.33 3.26
Fig. 3. Temperature dependence of Seebeck coefficient for Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) samples.
Fig. 5. Temperature dependence of thermal conductivity (a) and lattice thermal conductivity and (b) for Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) samples. Fig. 4. Temperature dependence of electrical conductivity for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples.
enhanced point-defect scattering due to the larger electronegativity, mass and volume fluctuations between Sb, Te and Se. The lattice thermal conductivity decreases with the increasing content of Ni, and sample Co3.8Ni0.2Sb11.6Te0.2Se0.2 achieves the lowest lattice thermal conductivity of 1.85 W m−1 K−1 at 775 K, a drop of 25% in comparison with that of Co4Sb11.6Te0.2Se0.2 at the same temperature. The reduction should be attributed to the enhanced point-defect scattering, and more importantly maybe, to the electron–phonon scattering [18] due to the minor difference of atomic masses between Ni (58.7) and Co (58.9) according to the following formula scattering parameter A [19,20]: A¼
Ω0 ΔM 2 xð1−xÞ M 4πυ3
ð1Þ
where Ω0, υ, x, ΔM and M are the volume of the unit cell, the sound velocity, the fraction of guest atoms, the atomic mass difference between the guest and host, and the average mass of the cell, respectively. The dimensionless figure of merit (ZT) of Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) is presented in Fig. 6. The ZT values of Ni–Te–Se co-doped samples are considerably improved compared with that of the Ni-free sample, especially at the high temperature range. As mentioned above, the considerable improvement is mainly affected by the increased electrical conductivity and the decreased lattice thermal conductivity. The maximum ZT was obtained as ∼0.83 at 800 K for the sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, representing a 23% improvement compared with the highest ZT of Ni-free sample Co4Sb11.6Te0.2Se0.2 prepared by the same method.
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Acknowledgments This work is financially supported by the Development Program of China (No. 2012AA051104), the National Basic Research Program of China (No. 2013CB632505), the National Natural Science Foundation of China (Nos. 10832008 and 51272198) and the China Postdoctoral Science Foundation (No. 2013M531752). References
Fig. 6. Temperature dependence of ZT for Co4−xNixSb11.6Te0.2Se0.2 (x ¼0, 0.05, 0.1 and 0.2) samples.
4. Conclusions Tri-doped skutterudites Co4−xNixSb11.6Te0.2Se0.2 (x¼ 0, 0.05, 0.1 and 0.2) have been synthesized by the solid state reaction method and the SPS process. The XRD results, together with backscattered electron images of the sample surface, indicate that single doped skutterudite phase was obtained. It is found that codoping with Ni, Te and Se has a synergic beneficial effect on the skutterudite systems. The suitable amount of Ni dopant in the system of Co4−xNixSb11.6Te0.2Se0.2 not only resulted in an improvement of the electrical conductivity but also a decrease of the thermal conductivity. Co3.8Ni0.2Sb11.6Te0.2Se0.2 showed the highest ZT of 0.83 at 800 K. Better performance of Co4−xNixSb12−y−zTeySez material could be achieved by optimizing the proportion of Ni, Te and Se elements in the composition.
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