AgSbTe2 nanoinclusion in Yb0.2Co4Sb12 for high performance thermoelectrics

Intermetallics 43 (2013) 79e84

Contents lists available at ScienceDirect

Intermetallics journal homepage: www.elsevier.com/locate/intermet

AgSbTe2 nanoinclusion in Yb0.2Co4Sb12 for high performance thermoelectrics Liangwei Fu a, Junyou Yang a, *, Ye Xiao a, Jiangying Peng b, **, Ming Liu a, Yubo Luo a, Gen Li a a b

State Key Laboratory of Material Processing and Die & Mound Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China School of Mechanical Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2013 Received in revised form 8 July 2013 Accepted 11 July 2013 Available online

Yb0.2Co4Sb12 based composites with AgSbTe2 nanoinclusion have been successfully prepared by a combined process of vacuum melting, pulverization and hot press sintering. XRD, FESEM, TEM and EDS were performed to characterize the microstructure and composition of the nanocomposites. It shows AgSbTe2 inclusions are about 50 nm in size and they distribute in the grain boundaries of Yb0.2Co4Sb12 filled skutterudite matrix. When the content of AgSbTe2 is less than 8wt%, Seebeck coefficient and electrical conductivity of the composites increase with increasing content of AgSbTe2, while the lattice thermal conductivity reduces with the increase of AgSbTe2 nanoinclusions. As a result, thermoelectric performance is improved with addition of AgSbTe2 nanoinclusion and a maximum ZT of 1.27 has been obtained for the sample of Yb0.2Co4Sb12/4wt%AgSbTe2 at about 300
C. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Composites B. Thermoelectric properties B. Thermal properties C. Powder metallurgy

1. Introduction High performance thermoelectric material has gained more and more attention for its potential application for power generation and electrical refrigeration [1]. The performance of a thermoelectric material is determined by its dimensionless figure of merit, ZT ¼ S2T/rk, where S, r and k are the Seebeck coefficient, electrical resistivity, and the thermal conductivity, respectively. Among the thermoelectric materials currently investigated, CoSb3-based skutterudite materials show very promising properties [2e4]. Binary skutterudite crystalizes in Im-3 space group, and a conventional unit cell of CoSb3 contains 32 atoms, in which Co atoms form eight tilt corner-shared octahedrons, and Sb atoms reside at the center of each octahedron. The eight corner-shared octahedrons produce a big void in the center of the unit cell, which could be occupied by rare earth (La [5], Ce [6], Yb [7]), alkaline earth metal (Ca [8], Sr [9], Ba [10]), alkali metal (Li [11], Na [12]) atoms to form the filled skutterudites. It is well known that the rattling of the filled atoms may result in significant reduction of the lattice thermal conductivity [13] and increase of ZT for skutterudite materials. The Yb atom which has heavy atomic mass and small ionic diameter * Corresponding author. Tel./fax: þ86 27 87558310. ** Corresponding author. E-mail addresses: [email protected], [email protected] (J. Yang), [email protected] (J. Peng). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.07.009

is a good filling atom. Nolas [7] has reported that the ZT value of the partially filled skutterudite Yb0.19Co4Sb12 reaches unity at 600e 750 K. Although the filling method has obtained great success in decreasing the lattice thermal conductivity, the thermal conductivity of filled skutterudites is still relatively high as compared with the other state-of-the-art thermoelectric materials, such as the Bi2Te3-based alloys [14e17]. Incorporating nanoscale inclusions into bulk materials to form nanocomposites is of great interest to obtain attractive thermoelectric properties [4,18e22]. The physics involved is to increase the density of interface which scatters heat-carrying phonons more effectively than charge carriers, and to increase Seebeck coefficient by energy filtering or quantum confinement effects [23]. Therefore thermoelectric nanocomposites have been paid much attention to in recent years. As is well known, for thermoelectric nanocomposites, the appropriate type, distribution and interface bonding [20] of the nanoscale secondary phase are the most important factors to obtain high thermoelectric properties. Up to now, several nanocomposite systems, such as CoSb3/CoSb3 [4], BayCo4Sb12/C60 [18], CoSb3/ZrO2 [19], Ba0.3Co4Sb12/Ag [20] and CoSb3/PbTe [21], Yb0.26Co4Sb12/GaSb [22], have been reported to further enhance the thermoelectric properties of skutterudites. AgSbTe2 compound, which has a very low thermal conductivity (about 0.6 W m1 K1) and a cubic rock salt structure [24], has been served as a common component of the best thermoelectric materialsdLAST [25] and TAGS [26] and plays a very important role in them. The combination of multiscale

80

L. Fu et al. / Intermetallics 43 (2013) 79e84

hierarchical microstructures [27] in bulk materials, i.e., in atomic level and in nanoscale level, will introduce more phonon scattering centers. Therefore, it is reasonable to expect that Yb filled skutterudites with evenly distributed AgSbTe2 nanoinclusions may have lower lattice thermal conductivity and better thermoelectric performance. However, less work has yet been reported on the composite of AgSbTe2 and filled skutterudites based materials. In this paper, a composite of AgSbTe2 nanoscale particle-dispersed Yb0.2Co4Sb12 was prepared by hot pressing, and a maximum ZT of 1.27 at 300
C was achieved. 2. Experimental procedure Firstly, Yb0.2Co4Sb12 and AgSbTe2 ingots were prepared via vacuum melting method, respectively. Stoichiometric amounts of Co powders (99.5%), Sb shot (99.99%), Yb ingot (99.9%) for Yb0.2Co4Sb12 and stoichiometric amounts of Ag powders（99.9%）, Sb shot (99.99%), Te powders (99.99%) for AgSbTe2 were mixed and sealed in evacuated quartz tubes, respectively. The quartz tube containing Yb0.2Co4Sb12 was heated slowly to 1050
C and held for 24 h, cooled to 650
C and held for another 4 days, and then cooled in the furnace to room temperature. The quartz tube containing AgSbTe2 was heated slowly to 850
C and held for 10 h, and then quenched in liquid-nitrogen. Subsequently, the AgSbTe2 ingot was crushed manually in an agate mortar and then milled in a planetary mill at 400 rpm for 5 h to get fine powders. After that, the solidified Yb0.2Co4Sb12 ingots were crushed and added with AgSbTe2 particles at different content of x (x stands for the weight percentage of AgSbTe2 nanoinclusion) and milled in a planetary mill at 300 rpm for 40 min. The obtained mixture was then hot pressed at 600
C under a pressure of 100 MPa for 2 h in vacuum circumstance. The relative density is about 95% of the theoretical value for all the samples. To make it simple, the Yb0.2Co4Sb12/x wt% AgSbTe2 composite will be written as SATx (x ¼ 2, 4, 8) in the following sections. The phase structure of the samples was characterized by X-ray powder diffraction in a PANalytical X’pert PRO diffractometer with Cu Ka radiation. The microstructure of the samples was observed with a field-emission scanning electron microscopy (FEI: Nano SEM 450) equipped with energy-dispersive X-ray spectroscopy (EDS) and a field-emission transmission electron microscope (FEI: Tecnai G2 20) equipped with EDS. The electrical resistivity and the Seebeck coefficient were measured in the temperature range of 25e400
C on a home build apparatus (Namicro-II) by using four-probe method and differential voltage/temperature techniques, respectively. The thermal conductivity (k) was calculated from the measured thermal diffusivity D, specific heat Cp, and density d using the expression k ¼ DCpd. D and Cp were measured by a laser flash method (Sinkuriko: TC-7000) in a vacuum in the temperature range of 25e400
C. The density d was measured by the Archimedes method. The Hall measurements were performed on the Quantum Design PPMS using a 5-probe configuration, with the magnetic field sweeping between 1.0 T to get a slope leading to the Hall coefficient RH. The carrier concentration n was estimated, using a simple band model, by the relation n ¼ 1/(RH/e), where e is the electron charge.

small volume fraction of AgSbTe2. Chubilleau et al. reported that some Te atoms dissolved into the CoSb3 matrix in the PbTe/CoSb3 nanocomposite during the sintering process [21], and the peaks in the XRD pattern of the skutterudite phase would change position with substitution of Sb with Te [28]. However, no change of the peak position of the skutterudite phase can be observed in the magnified XRD patterns shown in the inset of Fig. 1, indicating that the secondary phase AgSbTe2 is chemically compatible with the Yb0.2Co4Sb12 skutterudite matrix. The SEM fractographs of the samples with different content of AgSbTe2 are shown in Fig. 2. They are typical of the morphology of an intergranular brittle fracture. As shown in Fig. 2aee, different from the matrix sample, there are some nanoparticles, about 50 nm in size, homogeneously distributed on the granule surface of composite sample, and the content of the nanoparticles increases with the nominal content of AgSbTe2. As also can be seen in Fig. 2e, the inclusions seem to grow bigger and agglomerate. EDS composition analysis was performed and the result was shown in Fig. 2f. It indicates that the nanoparticles are rich of Ag and Te. Combined with the XRD patterns shown in Fig.1, these nanoparticles are identified as AgSbTe2. To characterize the secondary phase more meticulously, TEM observation was performed on the SAT4 sample and shown in Fig. 3. It can be seen that these particles which are about 50 nm in size are evenly distributed at the grain boundary of the skutterudite phase, which is consistent with the SEM observation. The nanoscale particles also appear at the triangle grain boundary, which is shown in the inset of Fig. 3b, and EDS results shown in Fig. 3b indicate they are rich of Ag and Te compared to the composition at the inner grain, also consistent with the SEM results. It is expected that the evenly distribution of AgSbTe2 nanoparticles will be beneficial to the electrical and thermal transport properties of the Yb0.2Co4Sb12/ AgSbTe2 composite.

3. Results and discussion

3.2. Thermoelectric properties

3.1. Structural characterization

The temperature dependence of the electrical conductivity, Seebeck coefficient, thermal conductivity and the ZT value of Yb0.2Co4Sb12/xwt% AgSbTe2 (x ¼ 0, 2, 4, 8) composites are presented in Fig. 4. As shown in Fig. 4a, the electrical conductivity of the matrix sample increases with the increase of temperature, which is similar to our previous results [29]. However, the electrical conductivity of the nanocomposites shows a negative temperature

XRD patterns of the hot pressed Yb0.2Co4Sb12/x%AgSbTe2 (x ¼ 0, 2, 4, 8) are showed in Fig. 1. Besides the main skutterudite phase, small peaks of cubic AgSbTe2 phase (JCPDS: 00-015-0540) are also detected in the SAT8 sample. The diffraction peaks of AgSbTe2 did not show in the patterns of SAT2 and SAT4 sample may due to the

Fig. 1. XRD patterns of the Yb0.2Co4Sb12/x%AgSbTe2 (x ¼ 0, 2, 4, 8) composites.

L. Fu et al. / Intermetallics 43 (2013) 79e84

81

Fig. 2. SEM images of the fractured surface (a: matrix; b: SAT2; c: SAT4; e: SAT8), (d) shows a magnified image of the circled area in the SAT4 sample, and (f) shows the EDS result conducted at the point labeled by a red cross in (e). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

dependence, indicating a heavily doped semiconductor behavior, and the electrical conductivity increases with increasing the AgSbTe2 content (x) when x is less than 8. To understand the effect of AgSbTe2 on electrical conductivity, Hall measurement was performed to all the samples and the results are listed in Table 1. As is known, AgSbTe2 is a p-type semiconductor, while Yb0.2Co4Sb12 is an n-type matrix; therefore the carrier concentration of the composite, shown in Table 1, decreases with the increase of the content of AgSbTe2 nanoparticles. On the other hand, the carrier mobility of the composites, which is different from the carrier concentration, increases by a factor of three with the addition of AgSbTe2 nanoinclusions. Since the hot press temperature of the composites is 600
C, which is just above the melt point of AgSbTe2 [30], and the liquid AgSbTe2 phase is beneficial to attain a well sintered sample, thus it can improve the carrier mobility. Furthermore, the evenly

distributed AgSbTe2 nanoinclusions, precipitated in the grain boundaries of the matrix in the subsequent cooling process, may improve the electrical connectivity in the grain boundaries, which is also beneficial to enhance the carrier mobility. A similar effect has been observed in Ba0.3Co4Sb12 with Ag nanoparticles [20]. When the amount of AgSbTe2 nanoinclusions is low enough (e.g. x ¼ 2, 4), the carrier mobility increase prevails over the decrease of carrier concentration, so the electrical conductivity of the composite increases with the increase of AgSbTe2. For the SAT8 sample, the great drop of carrier concentration takes over the control, therefore its electrical conductivity is lower than those of the SAT2 and SAT4 samples. Assuming a parabolic conduction band, the scattering mechanisms of the charge carriers are dominated by acoustic phonons and regardless of the effect of the nano inclusions, Seebeck

82

L. Fu et al. / Intermetallics 43 (2013) 79e84

Fig. 3. (a) TEM images of the SAT4 sample, (b) EDS results conducted at inner grain of grain c and the triangle grain boundary shown in the inset.

coefficient (S) and carrier concentration (n) can then be expressed in terms of the Fermi integrals Fj(h) [21,31]:


 k 2F1 ðhÞ h S ¼  B F0 ðhÞ e 4 2pm* kB T n ¼ pffiffiffiffi h2 p

(1)

where kB, e, h and h (h ¼ Ef/kBT) are the Boltzmann constant, the electron charge, the Plank constant and the reduced Fermi energy, respectively, and the Fermi integrals are given by

ZN Fj ¼ 0

!3=2 F1=2 ðhÞ

(2)

xj eðxhÞ þ 1

dx

(3)

where x ¼ E/kBT. The effective mass, m*, of the charge carrier was calculated and listed in Table 1. In general, the effective mass m* of filled skutterudites increases with increasing carrier concentration

Fig. 4. The temperature dependence of resistivity (a), Seebeck coefficient (b), thermal conductivity (c) and ZT value (d) of the nanocomposite with different contents of AgSbTe2 nanoinclusions.

L. Fu et al. / Intermetallics 43 (2013) 79e84

83

Table 1 Hall parameters, carrier effective mass and relative density of the studied samples at room temperature. Sample ID matrix SAT2 SAT4 SAT8

Nominal composition Yb0.2Co4Sb12 Yb0.2Co4Sb12/2wt%AgSbTe2 Yb0.2Co4Sb12/4wt%AgSbTe2 Yb0.2Co4Sb12/8wt%AgSbTe2

Carrier concentration, n (1019 cm3)

m (cm2/Vs)

Carrier effective mass, m* (m0)

Relative density, (%)

14.16 12.61 13.45 10.46

15.93 44.65 47.37 46.68

2.34 2.51 2.88 1.49

97.76 98.77 98.60 96.73

[12]. However, as shown in Table 1, except for the SAT8 sample, the effective mass of the filled skutterudites with AgSbTe2 nanoinclusions increases despite of the decrease of carrier concentration (n) in the nanocomposites, which is similar to the Yb0.26Co4Sb12 containing nano GaSb inclusions [22]. Therefore, some other scattering mechanisms, for example, scattering by the nanoinclusions, are also in working. Faleev et al. [32] have reported that at metal/ semiconductor interfaces, charge transfer between the metal and the semiconductor leads to band bending away from the interface. The band bending resulted interface potential barrier would scatter the electrons of low energy, which is beneficial to the enhancement of Seebeck coefficient. Referring to the calculated band structures in the literature [33,34], an energy barrier with a height of about 0.48 eV at the boundary of Yb filled skutterudite and AgSbTe2 nanoinclusion needs to be overcome by electrons. The electrons of low energy in nanocomposite are filtered by the potential barrier at the interface, thus leading to an increase of the Seebeck coefficient. The same results were also reported in the p-type nano GaSb inclusions in the Yb-filled skutterudite [22]. Furthermore, the carrier concentration of the composites decreases with the increase of AgSbTe2, therefore the composite samples have higher absolute value of Seebeck coefficient than the matrix, as shown in Fig. 4b. However, the absolute value of Seebeck coefficient and the effective mass m* of the SAT8 sample are smaller than those of the other samples. The large amount of AgSbTe2 particles and their aggregation in the sample should be ascribed to. Fig. 4c shows the temperature dependence of the thermal conductivity of the samples. The lattice thermal conductivity was obtained by subtracting the electronic thermal conductivity from the total thermal conductivity. The electronic thermal conductivity was calculated via the WiedemanneFranz law (ke ¼ L0sT) with Lorentz constant L0 ¼ 2.0  108 V2 K2 [22,29]. As shown in the inset of Fig. 4c, the lattice thermal conductivity decreases dramatically with the addition of AgSbTe2 nanoinclusions, indicating the strong phonon scattering induced by the nanoinclusions. Thus the total thermal conductivity is still suppressed when x is above 2, though the electronic thermal conductivity increases in the composite samples with an increase of the electrical conductivity. The aggregated AgSbTe2 inclusions should not be as effective as the dispersive ones in the SAT4 sample on scattering heat carrying phonons. However, the low thermal conductivity of AgSbTe2 (about 0.6 W m1 K1) inclusions serve as a non-ignorable factor according to the Bergman-Fel model [35]. Therefore, SAT8 sample has almost the same lattice thermal conductivity as that of the SAT4 sample. The dimensionless figure of merit (ZT) was calculated and shown in Fig. 4d. It can be seen that the samples with AgSbTe2 nanoinclusions have larger ZT values than the matrix sample in the whole temperature range due to their higher electrical properties and lower thermal conductivities. A maximum figure of merit of 1.27 is obtained for the SAT4 sample at 300
C, and this value is one of the best results in the skutterudite based composites [18,20,36e38]. 4. Conclusions In summary, Yb0.2Co4Sb12 based composites with AgSbTe2 nanoinclusions have been prepared successfully via a vacuum

Mobility,

melting-pulverization-hot press sintering process. AgSbTe2 nanoinclusions, which precipitate during the cooling process after hot press sintering from a liquid form, are about 50 nm in size and distribute evenly at the grain boundary of the Yb filled skutterudite matrix. With addition of AgSbTe2, both the electrical conductivity and the Seebeck coefficient of the composites are enhanced, and the lattice thermal conductivity is reduced due to the extra phonon scattering by the dispersive AgSbTe2 nanoinclusions. As a result, thermoelectric performance of the Yb filled skutterudite nanocomposite is improved and a maximum ZT of 1.27 has been obtained for Yb0.2Co4Sb12/4wt%AgSbTe2 sample at about 300
C. Acknowledgments This work is co-supported by National Natural Science Foundation of China (51072062, 51271084 and 51272080), National Basic Research Program of China (Grant No. 2013CB632500), Research Fund for the Doctoral Program of Higher Education, Ministry of Education of China (No. 20100142110016), the Natural Science Foundation of Hubei Province (2012FFB02215), and Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (No. 2013KF-3). The technical assistance from the Analytical and Testing Center of Huazhong University of Science & Technology is also gratefully acknowledged. References [1] Sales BC. Thermoelectric materials e smaller is cooler. Science 2002;295: 1248e9. [2] Su Xianli, Li Han, Wang Guoyu, Hang Chi, Xiaoyuan Zhou, Xinfeng Tang, et al. Structure and transport properties of double-doped CoSb2.75Ge0.25xTex (x¼0.125e0.20) with in situ nanostructure. Chemistry of Materials 2011;23: 2948e55. [3] Toprak MS, Stiewe C, Platzek D, Williams S, Bertini L, Muller E, et al. The impact of nanostructuring on the thermal conductivity of thermoelectric CoSb3. Advanced Functional Materials 2004;14:1189e96. [4] Mi JL, Zhao XB, Zhu TJ, Tu JP. Improved thermoelectric figure of merit in n-type CoSb3 based nanocomposites. Applied Physics Letters 2007;97:172116. [5] Nolas GS, Cohn JL, Slack GA. Effect of partial void filling on the lattice thermal conductivity of skutterudites. Physical Review B 1998;58:164e70. [6] Morelli DT, Meisner GP. Cerium filling and doping of cobalt triantimonide. Physical Review B 1997;56:7376e83. [7] Nolas GS, Kaeser M, Littleton IV RT, Tritt TM. High figure of merit in partially filled ytterbium skutterdite materials. Applied Physics Letters 2000;77: 1855e7. [8] Puyet M, Lenoir B, Dauscher A, Dehmas M, Stiewe C, Muller E. High temperature transport properties of partially filled CaxCo4Sb12 skutterudites. Journal of Applied Physics 2004;95:4852e5. [9] Zhao XY, Shi X, Chen LD, Zhang WQ, Zhang WB, Pei YZ. Synthesis and thermoelectric properties of Sr-filled skutterudite SrxCo4Sb12. Journal of Applied Physics 2006;99:053711. [10] Chen LD, Kawahara T, Tang XF, Goto T, Hirai T, Dyck JS, et al. Anomalous barium filling fraction and n-type thermoelectric performance of BaxCo4Sb12. Journal of Applied Physics 2001;90:1864e8. [11] Zhang Jianjun, Xu Bo, Wang Limin, Yu Dongli, Liu Zhongyuan, He Julong, et al. Great thermoelectric power factor enhancement of CoSb3 through the lightest metal element filling. Applied Physics Letters 2011;98:072109. [12] Pei YZ, Yang Jiong, Chen LD, Zhang W, Salvador JR, Yang Jihui. Improving thermoelectric performance of caged compounds through light-element filling. Applied Physics Letters 2009;95:0421001. [13] He Tao, Chen Jiazhong, Rosenfeld HD, Subramanian MA. Thermoelectric properties of indium-filled skutterudite. Chemistry of Materials 2006;18:759e62.

84

L. Fu et al. / Intermetallics 43 (2013) 79e84

[14] Poudel B, Hao Qing, Ma Yi, Lan Yucheng, Austin Minnich, Bo Yu, et al. Highthermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008;320:634e8. [15] Yang JY, Zhu W, Gao XH, Fan XA, Bao SQ, Duan XK. Electrochemical aspects of depositing Sb2Te3 compound on Au substrate by ECALE. Electrochimica Acta 2007;52:3035e9. [16] Yang JY, Chen RG, Fan XA, Bao SQ, Zhu. W. Thermoelectric properties of silver-doped n-type Bi2Te3-based material prepared by mechanical alloying and subsequent hot pressing. Journal of Alloys and Compounds 2006;407: 330e3. [17] Yang JY, Aizawa T, Yamamoto A, Ohta T. Effects of interface layer on thermoelectric properties of a pn junction prepared via the BMA-HP method. Materials Science and Engineering, B 2001;85:34e7. [18] Shi X, Chen LD, Bai SQ, Huang XY, Zhao XY, Yao Q, et al. Influence of fullerene dispersion on high temperature thermoelectric properties of BayCo4Sb12based composites. Journal of Applied Physics 2007;102:103709. [19] He Zeming, Stiewe C, Platzek D, Karpinski G, Muller E, Li Shanghua, et al. Effect of ceramic dispersion on thermoelectric properties of nano-ZrO2/CoSb3 composites. Journal of Applied Physics 2007;101:043707. [20] Zhou Xiaoyuan, Wang Guoyu, Zhang Long, Chi Hang, Su Xianli, Sakamoto J, et al. Enhanced thermoelectric properties of Ba-filled skutterudites by grain size reduction and Ag nanoparticle inclusion. Journal of Materials Chemistry 2012;22:2958e64. [21] Chubilleau C, Lenoir B, Dauscher A, Godart C. Low temperature thermoelectric properties of PbTedCoSb3 composites. Intermetallics 2012;22:47e54. [22] Xiong Zhen, Chen Xihong, Huang Xiangyang, Bai Shengqiang, Chen Lidong. High thermoelectric performance of Yb0.26Co4Sb12/yGaSb nanocomposites originating from scattering electrons of low energy. Acta Materialia 2010;58: 3995e4002. [23] Dresselhaus MS, Chen G, Tang MY, Yang R, Lee H, Wang D, et al. New directions for low-dimensional thermoelectric materials. Advanced Materials 2007;19:1043e53. [24] Wolfe R, Wernick JH, Haszko SE. Anomalous Hall effect in AgSbTe2. Journal of Applied Physics 1960;31:1959e64. [25] Hsu KF, Loo S, Guo Fu, Chen Wei, Dyck JS, Uher C, et al. Cubic AgPbmSbTe2þm: bulk thermoelectric materials with high figure of merit. Science 2004;303: 818e21.

[26] Yang SH, Zhu TJ, He J, Zhang SN, Zhao XB. Nanostructures in high-performance (GeTe)x (AgSbTe2)100x thermoelectric materials. Nanotechnology 2008;19:245707. [27] Biswas K, He Jiaqing, Blum ID, Wu CI, Hogan TP, Seidman DN, et al. Highperformance bulk thermoelectrics with all-scale hierarchical architectures. Nature 2012;489:414e8. [28] Duan Bo, Zhai Pengcheng, Liu Lisheng, Zhang Qingjie, Ruan Xuefeng. Synthesis and high temperature transport properties of Te-doped skutterudite compounds. Journal of Materials Science: Materials in Electronics 2012;23:1817e22. [29] Peng Jiangying, He Jian, Alboni PN, Tritt TM. Synthesis and thermoelectric properties of the double-filled skutterudite Yb0.2InyCo4Sb12. Journal of Electronic Materials 2009;38:981e4. [30] Armstrong RW, Faust JW, Tiller WA. A structural study of the compound AgSbTe2. Journal of Applied Physics 1960;31:1954e9. [31] Peng JY, Alboni PN, He J, Zhang B, Su Z, Holgate T, et al. Thermoelectric properties of (In, Yb) double-filled CoSb3 skutterudite. Journal of Applied Physics 2008;104:053710. [32] Faleev SV, Leonard F. Theory of enhancement of thermoelectric properties of materials with nanoinclusions. Physical Review B 2008;77:214304. [33] Zhou An, Liu Lisheng, Zhai Pengcheng, Zhao Wenyu, Zhang Qingjie. Electronic structure and transport properties of single and double filled CoSb3 with atoms Ba, Yb and In. Journal of Applied Physics 2011;109:113723. [34] Barabash SV, Ozolins V. Order, miscibility, and electronic structure of Ag(Bi, Sb)Te2 alloys and (Ag, Bi, Sb)Te precipitates in rocksalt matrix: a firstprinciples study. Physical Review B 2010;81:075212. [35] Xie WJ, He J, Zhu S, Su XL, Wang SY, Holgate T, et al. Simultaneously optimizing the independent thermoelectric properties in (Ti, Zr, Hf)(Co, Ni)Sb alloy by in situ forming InSb nanoinclusions. Acta Materialia 2010;58:4705e13. [36] Peng Jiangying, Liu Xiaoyan, Fu Liangwei, Xu Wei, Liu Qiongzhen, Yang Junyou. Synthesis and thermoelectric properties of In0.2þxCo4Sb12þx composite. Journal of Alloys and Compounds 2012;521:141e5. [37] Zhao XY, Shi X, Chen LD, Zhang WQ, Bai SQ, Pei YZ, et al. Synthesis of YbyCo4Sb12/Yb2O3 composites and their thermoelectric properties. Applied Physics Letters 2006;89:092121. [38] Mi JL, Zhao XB, Zhu TJ, Tu JP. Thermoelectric properties of Yb0.15Co4Sb12 based nanocomposites with CoSb3 nano-inclusion. Journal of Physics D: Applied Physics 2008;41:205403.