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Thermoelectric properties in double-filled skutterudites InxNdyCo4Sb12
Solid State Communications 152 (2012) 2193–2196
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Thermoelectric properties in double-filled skutterudites InxNdyCo4Sb12 Guodong Tang a,n, Dewei Zhang b, Guang Chen a, Feng Xu a, Zhihe Wang b a b
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 15 December 2011 Received in revised form 30 August 2012 Accepted 6 October 2012 by M. Wang Available online 12 October 2012
Double-filled skutterudites InxNdyCo4Sb12 have been synthesized by the inductive melting method. The thermal conductivity of these compounds is significantly depressed as compared to that of unfilled CoSb3, while their Seebeck coefficient is remarkably enhanced. We explore simultaneously enhancing the power factor and thermoelectric figure of merit ZT through Nd and In double filling. The attained largest power factor 3.2 m Wm 1 K 2 (360 K) is comparable to Ba and In double-filled skutterudites which possess very high ZT values. ZT¼0.11 achieved in In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 at 360 K is about two times larger than that of In single-filled skutterudites. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Skutterudites D. Thermoelectric properties
1. Introduction Thermoelectricity has great potential in the field of cooling, heating, generating power, and recovering waste heat. The high performance of thermoelectric materials depends on the dimensionless figure of merit ZT¼S2sT/k, where T, s, S and k are the absolute temperature, electrical conductivity, Seebeck coefficient and thermal conductivity, respectively. The skutterudites as one of the most promising thermoelectric materials are being intensely pursued in hopes of developing more efficient thermoelectric materials [1–3]. One of the distinguishing characteristics of the filled skutterudites is the significant suppression of the lattice thermal conductivity because the guest atoms ‘‘rattle’’ inside these oversized cages, disrupting phonon transport [4]. The addition of filler atoms also have significant influence on the electrical transport properties such as carrier concentration, carrier mobility, and carrier effective mass [5,6]. So far, a broad variety of rare earth elements, alkaline earth elements and others as filling atoms have been tried in order to reduce their lattice thermal conductivity and improve their thermoelectric performance simultaneously [7–9]. As an effective approach, the double filling in particular has been a great deal of interest thanks to its induced dual-frequency resonant phonon scattering [10–15]. Among the vast published works on the filled skutterudites, In single-filled skutterudites were reported to have excellent thermoelectric properties [16]. In order to further improve the thermoelectric properties, double filling approach has been utilized in this study with Nd chosen as the other filler because of its small ionic radius and heavy mass as
n
Corresponding author. Tel.: þ86 25 83686402. E-mail addresses: [email protected] (G. Tang), [email protected] (Z. Wang).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.003
well as its apparently different chemical nature from In. The heavy element void-filling approach is an effective way to optimize thermoelectric properties of CoSb3 based skutterudites [17,18]. In the present work, we successfully introduce In and one of the heavy rare earth elements Nd into CoSb3 skutterudites by the inductive melting method. The effects of In and Nd double filling on thermoelectric properties are studied in detail. We have found that double filling with In and Nd is indeed efficient in improving thermoelectric performance, and especially power factor.
2. Experiment Polycrystalline InxNdyCo4Sb12 samples were synthesized from high-purity Co, In, Sb, and Nd. Preparation of the samples used in this investigation was performed by the inductive melting method. The obtained alloys were put into quartz tubes. The tubes were sealed under vacuum and transferred into a programmable furnace, which was annealed at 923 K for 144 h to homogenize the sample, and finally furnace-cooled to room temperature. X-ray diffractometry (Cu Ka) and electron probe micro-analysis (EPMA) were used to characterize the constituent phases and chemical composition of the samples. All the thermoelectric properties (Seebeck coefficient, thermal conductivity and electrical conductivity) were measured using a Quantum Design physical property measurement system.
3. Results and discussion X-ray powder diffraction (XRD) analysis of InxNdyCo4Sb12 shows that all samples consist entirely a single phase of CoSb3 skutterudite, as shown in Fig. 1. Rietveld refinements were performed on the
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Fig. 1. XRD patterns of InxNdyCo4Sb12. Fig. 2. Temperature dependence of Seebeck coefficient (S) for InxNdyCo4Sb12. Table 1 Nominal composition, actual composition (determined by EPMA) and lattice parameter (a (A1)) of the InxNdyCo4Sb12 skutterudites. Nominal composition
In0.15Nd0.05Co4Sb12 In0.25Nd0.05Co4Sb12 In0.15Nd0.1Co4Sb12 In0.25Nd0.1Co4Sb12
EPMA composition Co
Sb
In
Nd
˚ a (A)
4.00 4.00 4.00 4.00
12.16 11.87 12.12 11.93
0.09 0.18 0.10 0.16
0.03 0.03 0.05 0.06
9.0464 9.0460 9.0508 9.0476
double-filled specimens. The data were refined according to space group Im3. The results of these refinements are summarized in Table 1. Compared with CoSb3 [JCPDF-83-0055] and In0.09Co4Sb12 [19], the lattice constant values of double-filled samples increase. This lattice expansion has been observed for many filled CoSb3 skutterudites [16]. EPMA measurements were carried out at four different areas on the samples and the average composition is listed in Table 1. EPMA revealed that all four constituent elements were present in each grain of the materials investigated. EPMA results confirm that all the samples studied are composed of single phase skutterudite chemically represented by InxNdyCo4Sb12. In the following sections, the compositions determined by EPMA are used as the actual composition of the double-filled skutterudite compounds. Fig. 2 shows the temperature dependence of the Seebeck coefficient (S) for InxNdyCo4Sb12 bulk materials. All double-filled samples have negative S, suggesting that they are n-type semiconductors. The positive Seebeck coefficient of the reference binary CoSb3 [20] exhibiting p-type behavior is given in Fig. 4 for a comparison. The filling with In and Nd changes the sign of S in skutterudite. Filling of In, Lu and Nd into the voids of the skutterudite crystal structure provides electrons to the host. Additional donor-type levels will be introduced into the band gap of the host after such filling. The electrons in the shallow donor level can be easily excited, inducing a substantial increase in electron concentration [21,22]. Therefore, double-filled samples have negative Seebeck coefficient. As the temperature increases, the absolute values of S increase for all samples. The largest S is obtained in In0.09Nd0.03Co4Sb12.16 material. Recently, we reported that In0.13Lu0.05Co4.02Sb12 material has thermoelectric figure of merit ZT¼0.27 at 365 K [19]. S of the In0.09Co4Sb12 and In0.13Lu0.05Co4.02Sb12 is given in Fig. 2 for comparison. We find that S of In0.09Nd0.03Co4Sb12.16 is about twice as big as that of In0.13Lu0.05Co4.02Sb12 exhibiting a very respectable thermoelectric figure of merit. The large value of S is presumably due to the
Fig. 3. Temperature dependence of thermal conductivity (k) for InxNdyCo4Sb12. The inset shows the enlarged thermal conductivity of these samples.
enhanced electron effective mass of the conduction band in CoSb3 because of the introduction of In and Nd [13,23]. Fig. 3 shows the temperature dependence of thermal conductivity (k) for InxNdyCo4Sb12. The thermal conductivity of these double-filled skutterudites is much lower than that of their binary parent compounds CoSb3 reported in literature [13]. Partial filling establishes a random alloy mixture of filling atoms and vacancies enabling effective point-defect scattering. In addition, the large space for the filling atom in skutterudites can establish soft phonon modes and local or ‘rattling’ modes that lower lattice thermal conductivity [24]. Furthermore, the large difference in resonance frequencies broadens the range of normal phonon scattering in the double-filled skutterudites, resulting in lower kL [14,19]. Therefore, the thermal conductivity of InxNdyCo4Sb12 is significantly depressed as compared to that of unfilled CoSb3. To clearly show the thermal conductivity, we plot the enlarged thermal conductivity of these samples in the inset of Fig. 3. The thermal conductivity for In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06 Co4Sb11.93 are quite close over the entire temperature range. As compared to the reference In0.13Lu0.05Co4.02Sb12 and In0.09Co4Sb12 [19], InxNdyCo4Sb12 double-filled skutterudites have larger k.
G. Tang et al. / Solid State Communications 152 (2012) 2193–2196
Fig. 4 displays the temperature dependence of electrical conductivity (s) for the samples. s of InxNdyCo4Sb12 samples and In0.09Co4Sb12 increases with decreasing temperature, exhibiting metallic like behavior, which is in good agreement with the results reported in Ref. [6]. On the contrary, s gradually decreases as the temperature decreases for In0.13Lu0.05Co4.02Sb12 and CoSb3 compounds, characteristic of a semiconducting behavior. The positive Seebeck coefficient of the reference binary CoSb3 suggests holes are the majority carriers. The filling of In and Nd into the voids of the skutterudite structure provides extra electrons. Such filling introduces donor level into the band gap of the host and gives rise to an increase in electron concentration. The increased electrons in the newly introduced donor levels will counteract holes in the original acceptor levels. As a result, the major carriers in the samples are electrons. This is supported by the Seebeck coefficient results. In the case for the heavily In and Nd double-filled compounds, the donor levels broaden and shift toward the edge of the conduction band. Therefore, overlapping of the donor levels with the conduction band would occur, which could be responsible for the transition from semiconductor for CoSb3 to metallic like state for InxNdyCo4Sb12. From Fig. 4, we can see that In0.18Nd0.03Co4Sb11.87 has the highest s values among these materials. For In0.13Lu0.05Co4.02Sb12, the donor levels are under the conduction band as the doping levels are low [19]. Furthermore, XRD shows that excess Sb exists in In0.13Lu0.05Co4.02Sb12. Sb may play an important role in s of In0.13Lu0.05Co4.02Sb12. This may be the reason that this material shows lowtemperature semiconducting behavior [19]. We calculated the power factor (PF) according to the equation of PF ¼S2s. Power factor as a function of temperature is presented in Fig. 5. The PF of all materials increases with the temperature and then peaks at high temperature thanks to intrinsic excitation. The samples with low Seebeck coefficient have poor PF. In0.16Nd0.06Co4Sb11.93 has PF in excess of 3.2 m Wm 1 K 2 at 360 K due to its low resistivities and moderate Seebeck coefficient. The PF value of reference In0.09Co4Sb12 and In0.13Lu0.05Co4.02Sb12 [19] is given in Fig. 5 for comparison. Compared with the reference In0.13Lu0.05Co4.02Sb12, good samples in this study have excellent PF. The PF values are about four times bigger than that of In single-filled skutterudites [19,25], evincing that filling Nd in In filled skutterudites is effective way to improve their PF. The achieved largest PF 3.2 m Wm 1 K 2 is comparable to Ba and In double-filled skutterudites which possess very high ZT values [3,14].
Fig. 4. Temperature dependence of electrical conductivity (s) for InxNdyCo4Sb12.
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Fig. 5. Power factor (PF) as a function of temperature for InxNdyCo4Sb12.
Fig. 6. Temperature dependence of ZT for InxNdyCo4Sb12.
According to the measured thermal and electrical transport properties, thermoelectric figure of merit ZT values are calculated and shown in Fig. 6. ZT values increase with increasing temperature within the investigated temperature range. The maximum ZT¼0.11 achieved at 360 K for In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 materials is about twice as big as that of In0.09Co4Sb12 [19]. The improvement of ZT mainly results from the enhancement of Seebeck coefficient. It is worth noting that the ZT of In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 is bigger than that of In0.13Lu0.05Co4.02Sb12 at low temperature. We think that the obtained electrical transport properties are the main reason for this actual behavior. One is that InxNdyCo4Sb12 has higher s than In0.13Lu0.05Co4.02Sb12 at the low temperature. Furthermore, the power factor (PF) of InxNdyCo4Sb12 is larger than In0.13Lu0.05Co4.02Sb12, as shown in Fig. 5. Therefore, the good electrical transport properties of InxNdyCo4Sb12 at low temperature are responsible for this behavior. ZT values for In0.13Lu0.05Co4.02Sb12 are larger than the investigated samples as the temperature rises up to 285 K. This is mainly ascribed to its relatively low thermal conductivity and enhanced s in In0.13Lu0.05Co4.02Sb12. Our results suggest that In and Nd double filling is indeed efficient in improving thermoelectric performance, and especially PF.
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4. Conclusion Double-filled skutterudites InxNdyCo4Sb12 have been synthesized by the inductive melting method. The Seebeck coefficient for InxNdyCo4Sb12 compounds remarkably increases, which is presumably due to the enhanced electron effective mass of the conduction band in CoSb3 because of the introduction of Nd and In. Meanwhile, the electrical conductivity increases and the thermal conductivity decreases. As a result, In and Nd double filling results in much enhancement of power factor and ZT. In0.16Nd0.06Co4Sb11.93 has the largest power factor 3.2 m Wm 1 K 2 (360 K) which is comparable to Ba and In double-filled skutterudites with very high ZT. ZT value of 0.11 is attained in In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 materials at 360 K, which is about two times larger than that of In single-filled skutterudites.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC 11204134 and NSFC 51072077), Natural Science Foundation of Jiangsu Province (No. SBK2012404), Postdoctoral Science Foundation of China (No. 2012M511279) and Postdoctoral Science Foundation of Jiangsu Province (No. 1102068C). References [1] C. Zhou, D. Morelli, X.Y. Zhou, G.Y. Wang, C. Uher, Intermetallics 19 (2011) 1390. [2] T.M. Tritt, Science 283 (1999) 804.
[3] W.Y. Zhao, P. Wei, Q.J. Zhang, C.L. Dong, L.S. Liu, X.F. Tang, J. Am. Chem. Soc. 131 (2009) 3713. [4] D.T. Morelli, G.P. Meisner, J. Appl. Phys. 77 (1995) 3777. [5] W.Y. Zhao, C.L. Dong, P. Wei, W. Guan, L.S. Liu, P.C. Zhai, X.F. Tang, Q.J. Zhang, J. Appl. Phys. 102 (2007) 113708. [6] H. Li, X.F. Tang, Q.J. Zhang, C. Uher, Appl. Phys. Lett. 94 (2009) 102114. [7] V.L. Kuznetsov, L.A. Kuznetsova, D.M. Rowe, J. Phys.: Condens. Matter 15 (2003) 5035. [8] G.S. Nolas, G.A. Slack, D.T Morelli, T.M. Tritt, A.C. Ehrlich, J. Appl. Phys. 79 (1996) 4002. [9] D.T. Morelli, G.P. Meisner, B.X. Chen, S.Q. Hu, C. Uher, Phys. Rev. B 56 (1997) 7376. [10] J.R. Salvador, J. Yang, H. Wang, X. Shi, J. Appl. Phys. 107 (2010) 043705. [11] J.Y. Peng, J. He, Z. Su, P.N. Alboni, S. Zhu, T.M. Tritt, J. Appl. Phys. 105 (2009) 084907. [12] J. Yang, W. Zhang, S.Q. Bai, Z. Mei, L.D. Chen, Appl. Phys. Lett. 90 (2007) 192111. [13] D. Li, K. Yang, H.H. Hng, X.Y. Qin, J. Ma, J. Appl. Phys. 104 (2008) 103720. [14] X. Shi, H. Kong, C.P. Li, C. Uher, J. Yang, J.R. Salvador, H. Wang, L. Chen, W. Zhang, Appl. Phys. Lett. 92 (2008) 182101. [15] J.Y. Peng, P.N. Alboni, J. He, B. Zhang, Z. Su, T. Holgate, N. Gothard, T.M. Tritt, J. Appl. Phys. 104 (2008) 053710. [16] T. He, J.Z. Chen, H.D. Rosenfeld, M.A. Subramanian, Chem. Mater. 18 (2006) 759. [17] R.H. Liu, X.H. Chen, P.F. Qiu, J.F. Liu, J. Yang, X.Y. Huang, L.D. Chen, J. Appl. Phys. 109 (2011) 023719. [18] H. Li, X.F. Tang, Q.J. Zhang, C. Uher, Appl. Phys. Lett. 93 (2008) 252109. [19] G.D. Tang, Z.H. Wang, X.N. Xu, Y. He, L. Qiu, Y.W. Du, J. Electron. Mater. 40 (2011) 611. [20] M. Puyet, B. Lenoir, A. Dauscher, P. Pe´cheur, C. Bellouard, J. Tobola, J. Hejtmanek, Phys. Rev. B 73 (2006) 035126. [21] X.F. Tang, H. Li, Q.J. Zhang, M. Niino, T. Goto, J. Appl. Phys. 100 (2006) 123702. [22] D. Li, K. Yang, H.H. Hng, Q.Y. Yan, J. Ma, T.J. Zhu, X.B. Zhao, J. Phys. D: Appl. Phys 42 (2009) 105408. [23] X.Y. Zhao, X. Shi, L.D. Chen, W.Q. Zhang, W.B. Zhang, Y.Z. Pei, J. Appl. Phys. 99 (2006) 053711. [24] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2008) 105. [25] L. Deng, X.P. Jia, T.C. Su, S.Z. Zheng, X. Guo, K. Jie, H.A. Ma, Mater. Lett. 65 (2011) 2927.
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Thermoelectric properties in double-filled skutterudites InxNdyCo4Sb12 Guodong Tang a,n, Dewei Zhang b, Guang Chen a, Feng Xu a, Zhihe Wang b a b
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 15 December 2011 Received in revised form 30 August 2012 Accepted 6 October 2012 by M. Wang Available online 12 October 2012
Double-filled skutterudites InxNdyCo4Sb12 have been synthesized by the inductive melting method. The thermal conductivity of these compounds is significantly depressed as compared to that of unfilled CoSb3, while their Seebeck coefficient is remarkably enhanced. We explore simultaneously enhancing the power factor and thermoelectric figure of merit ZT through Nd and In double filling. The attained largest power factor 3.2 m Wm 1 K 2 (360 K) is comparable to Ba and In double-filled skutterudites which possess very high ZT values. ZT¼0.11 achieved in In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 at 360 K is about two times larger than that of In single-filled skutterudites. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Skutterudites D. Thermoelectric properties
1. Introduction Thermoelectricity has great potential in the field of cooling, heating, generating power, and recovering waste heat. The high performance of thermoelectric materials depends on the dimensionless figure of merit ZT¼S2sT/k, where T, s, S and k are the absolute temperature, electrical conductivity, Seebeck coefficient and thermal conductivity, respectively. The skutterudites as one of the most promising thermoelectric materials are being intensely pursued in hopes of developing more efficient thermoelectric materials [1–3]. One of the distinguishing characteristics of the filled skutterudites is the significant suppression of the lattice thermal conductivity because the guest atoms ‘‘rattle’’ inside these oversized cages, disrupting phonon transport [4]. The addition of filler atoms also have significant influence on the electrical transport properties such as carrier concentration, carrier mobility, and carrier effective mass [5,6]. So far, a broad variety of rare earth elements, alkaline earth elements and others as filling atoms have been tried in order to reduce their lattice thermal conductivity and improve their thermoelectric performance simultaneously [7–9]. As an effective approach, the double filling in particular has been a great deal of interest thanks to its induced dual-frequency resonant phonon scattering [10–15]. Among the vast published works on the filled skutterudites, In single-filled skutterudites were reported to have excellent thermoelectric properties [16]. In order to further improve the thermoelectric properties, double filling approach has been utilized in this study with Nd chosen as the other filler because of its small ionic radius and heavy mass as
n
Corresponding author. Tel.: þ86 25 83686402. E-mail addresses: [email protected] (G. Tang), [email protected] (Z. Wang).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.003
well as its apparently different chemical nature from In. The heavy element void-filling approach is an effective way to optimize thermoelectric properties of CoSb3 based skutterudites [17,18]. In the present work, we successfully introduce In and one of the heavy rare earth elements Nd into CoSb3 skutterudites by the inductive melting method. The effects of In and Nd double filling on thermoelectric properties are studied in detail. We have found that double filling with In and Nd is indeed efficient in improving thermoelectric performance, and especially power factor.
2. Experiment Polycrystalline InxNdyCo4Sb12 samples were synthesized from high-purity Co, In, Sb, and Nd. Preparation of the samples used in this investigation was performed by the inductive melting method. The obtained alloys were put into quartz tubes. The tubes were sealed under vacuum and transferred into a programmable furnace, which was annealed at 923 K for 144 h to homogenize the sample, and finally furnace-cooled to room temperature. X-ray diffractometry (Cu Ka) and electron probe micro-analysis (EPMA) were used to characterize the constituent phases and chemical composition of the samples. All the thermoelectric properties (Seebeck coefficient, thermal conductivity and electrical conductivity) were measured using a Quantum Design physical property measurement system.
3. Results and discussion X-ray powder diffraction (XRD) analysis of InxNdyCo4Sb12 shows that all samples consist entirely a single phase of CoSb3 skutterudite, as shown in Fig. 1. Rietveld refinements were performed on the
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G. Tang et al. / Solid State Communications 152 (2012) 2193–2196
Fig. 1. XRD patterns of InxNdyCo4Sb12. Fig. 2. Temperature dependence of Seebeck coefficient (S) for InxNdyCo4Sb12. Table 1 Nominal composition, actual composition (determined by EPMA) and lattice parameter (a (A1)) of the InxNdyCo4Sb12 skutterudites. Nominal composition
In0.15Nd0.05Co4Sb12 In0.25Nd0.05Co4Sb12 In0.15Nd0.1Co4Sb12 In0.25Nd0.1Co4Sb12
EPMA composition Co
Sb
In
Nd
˚ a (A)
4.00 4.00 4.00 4.00
12.16 11.87 12.12 11.93
0.09 0.18 0.10 0.16
0.03 0.03 0.05 0.06
9.0464 9.0460 9.0508 9.0476
double-filled specimens. The data were refined according to space group Im3. The results of these refinements are summarized in Table 1. Compared with CoSb3 [JCPDF-83-0055] and In0.09Co4Sb12 [19], the lattice constant values of double-filled samples increase. This lattice expansion has been observed for many filled CoSb3 skutterudites [16]. EPMA measurements were carried out at four different areas on the samples and the average composition is listed in Table 1. EPMA revealed that all four constituent elements were present in each grain of the materials investigated. EPMA results confirm that all the samples studied are composed of single phase skutterudite chemically represented by InxNdyCo4Sb12. In the following sections, the compositions determined by EPMA are used as the actual composition of the double-filled skutterudite compounds. Fig. 2 shows the temperature dependence of the Seebeck coefficient (S) for InxNdyCo4Sb12 bulk materials. All double-filled samples have negative S, suggesting that they are n-type semiconductors. The positive Seebeck coefficient of the reference binary CoSb3 [20] exhibiting p-type behavior is given in Fig. 4 for a comparison. The filling with In and Nd changes the sign of S in skutterudite. Filling of In, Lu and Nd into the voids of the skutterudite crystal structure provides electrons to the host. Additional donor-type levels will be introduced into the band gap of the host after such filling. The electrons in the shallow donor level can be easily excited, inducing a substantial increase in electron concentration [21,22]. Therefore, double-filled samples have negative Seebeck coefficient. As the temperature increases, the absolute values of S increase for all samples. The largest S is obtained in In0.09Nd0.03Co4Sb12.16 material. Recently, we reported that In0.13Lu0.05Co4.02Sb12 material has thermoelectric figure of merit ZT¼0.27 at 365 K [19]. S of the In0.09Co4Sb12 and In0.13Lu0.05Co4.02Sb12 is given in Fig. 2 for comparison. We find that S of In0.09Nd0.03Co4Sb12.16 is about twice as big as that of In0.13Lu0.05Co4.02Sb12 exhibiting a very respectable thermoelectric figure of merit. The large value of S is presumably due to the
Fig. 3. Temperature dependence of thermal conductivity (k) for InxNdyCo4Sb12. The inset shows the enlarged thermal conductivity of these samples.
enhanced electron effective mass of the conduction band in CoSb3 because of the introduction of In and Nd [13,23]. Fig. 3 shows the temperature dependence of thermal conductivity (k) for InxNdyCo4Sb12. The thermal conductivity of these double-filled skutterudites is much lower than that of their binary parent compounds CoSb3 reported in literature [13]. Partial filling establishes a random alloy mixture of filling atoms and vacancies enabling effective point-defect scattering. In addition, the large space for the filling atom in skutterudites can establish soft phonon modes and local or ‘rattling’ modes that lower lattice thermal conductivity [24]. Furthermore, the large difference in resonance frequencies broadens the range of normal phonon scattering in the double-filled skutterudites, resulting in lower kL [14,19]. Therefore, the thermal conductivity of InxNdyCo4Sb12 is significantly depressed as compared to that of unfilled CoSb3. To clearly show the thermal conductivity, we plot the enlarged thermal conductivity of these samples in the inset of Fig. 3. The thermal conductivity for In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06 Co4Sb11.93 are quite close over the entire temperature range. As compared to the reference In0.13Lu0.05Co4.02Sb12 and In0.09Co4Sb12 [19], InxNdyCo4Sb12 double-filled skutterudites have larger k.
G. Tang et al. / Solid State Communications 152 (2012) 2193–2196
Fig. 4 displays the temperature dependence of electrical conductivity (s) for the samples. s of InxNdyCo4Sb12 samples and In0.09Co4Sb12 increases with decreasing temperature, exhibiting metallic like behavior, which is in good agreement with the results reported in Ref. [6]. On the contrary, s gradually decreases as the temperature decreases for In0.13Lu0.05Co4.02Sb12 and CoSb3 compounds, characteristic of a semiconducting behavior. The positive Seebeck coefficient of the reference binary CoSb3 suggests holes are the majority carriers. The filling of In and Nd into the voids of the skutterudite structure provides extra electrons. Such filling introduces donor level into the band gap of the host and gives rise to an increase in electron concentration. The increased electrons in the newly introduced donor levels will counteract holes in the original acceptor levels. As a result, the major carriers in the samples are electrons. This is supported by the Seebeck coefficient results. In the case for the heavily In and Nd double-filled compounds, the donor levels broaden and shift toward the edge of the conduction band. Therefore, overlapping of the donor levels with the conduction band would occur, which could be responsible for the transition from semiconductor for CoSb3 to metallic like state for InxNdyCo4Sb12. From Fig. 4, we can see that In0.18Nd0.03Co4Sb11.87 has the highest s values among these materials. For In0.13Lu0.05Co4.02Sb12, the donor levels are under the conduction band as the doping levels are low [19]. Furthermore, XRD shows that excess Sb exists in In0.13Lu0.05Co4.02Sb12. Sb may play an important role in s of In0.13Lu0.05Co4.02Sb12. This may be the reason that this material shows lowtemperature semiconducting behavior [19]. We calculated the power factor (PF) according to the equation of PF ¼S2s. Power factor as a function of temperature is presented in Fig. 5. The PF of all materials increases with the temperature and then peaks at high temperature thanks to intrinsic excitation. The samples with low Seebeck coefficient have poor PF. In0.16Nd0.06Co4Sb11.93 has PF in excess of 3.2 m Wm 1 K 2 at 360 K due to its low resistivities and moderate Seebeck coefficient. The PF value of reference In0.09Co4Sb12 and In0.13Lu0.05Co4.02Sb12 [19] is given in Fig. 5 for comparison. Compared with the reference In0.13Lu0.05Co4.02Sb12, good samples in this study have excellent PF. The PF values are about four times bigger than that of In single-filled skutterudites [19,25], evincing that filling Nd in In filled skutterudites is effective way to improve their PF. The achieved largest PF 3.2 m Wm 1 K 2 is comparable to Ba and In double-filled skutterudites which possess very high ZT values [3,14].
Fig. 4. Temperature dependence of electrical conductivity (s) for InxNdyCo4Sb12.
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Fig. 5. Power factor (PF) as a function of temperature for InxNdyCo4Sb12.
Fig. 6. Temperature dependence of ZT for InxNdyCo4Sb12.
According to the measured thermal and electrical transport properties, thermoelectric figure of merit ZT values are calculated and shown in Fig. 6. ZT values increase with increasing temperature within the investigated temperature range. The maximum ZT¼0.11 achieved at 360 K for In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 materials is about twice as big as that of In0.09Co4Sb12 [19]. The improvement of ZT mainly results from the enhancement of Seebeck coefficient. It is worth noting that the ZT of In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 is bigger than that of In0.13Lu0.05Co4.02Sb12 at low temperature. We think that the obtained electrical transport properties are the main reason for this actual behavior. One is that InxNdyCo4Sb12 has higher s than In0.13Lu0.05Co4.02Sb12 at the low temperature. Furthermore, the power factor (PF) of InxNdyCo4Sb12 is larger than In0.13Lu0.05Co4.02Sb12, as shown in Fig. 5. Therefore, the good electrical transport properties of InxNdyCo4Sb12 at low temperature are responsible for this behavior. ZT values for In0.13Lu0.05Co4.02Sb12 are larger than the investigated samples as the temperature rises up to 285 K. This is mainly ascribed to its relatively low thermal conductivity and enhanced s in In0.13Lu0.05Co4.02Sb12. Our results suggest that In and Nd double filling is indeed efficient in improving thermoelectric performance, and especially PF.
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4. Conclusion Double-filled skutterudites InxNdyCo4Sb12 have been synthesized by the inductive melting method. The Seebeck coefficient for InxNdyCo4Sb12 compounds remarkably increases, which is presumably due to the enhanced electron effective mass of the conduction band in CoSb3 because of the introduction of Nd and In. Meanwhile, the electrical conductivity increases and the thermal conductivity decreases. As a result, In and Nd double filling results in much enhancement of power factor and ZT. In0.16Nd0.06Co4Sb11.93 has the largest power factor 3.2 m Wm 1 K 2 (360 K) which is comparable to Ba and In double-filled skutterudites with very high ZT. ZT value of 0.11 is attained in In0.09Nd0.03Co4Sb12.16 and In0.16Nd0.06Co4Sb11.93 materials at 360 K, which is about two times larger than that of In single-filled skutterudites.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC 11204134 and NSFC 51072077), Natural Science Foundation of Jiangsu Province (No. SBK2012404), Postdoctoral Science Foundation of China (No. 2012M511279) and Postdoctoral Science Foundation of Jiangsu Province (No. 1102068C). References [1] C. Zhou, D. Morelli, X.Y. Zhou, G.Y. Wang, C. Uher, Intermetallics 19 (2011) 1390. [2] T.M. Tritt, Science 283 (1999) 804.
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