Enhanced ZT of InxCo4Sb12âInSb Nanocomposites Fabricated by

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Enhanced ZT of InxCo4Sb12-InSb Nanocomposites Fabricated by Hydrothermal Synthesis Combined with Solid-Vapor Reaction: A Signature of Phonon-Glass and Electron-Crystal Materials Ahmad Gharleghi, Peng-Chun Hung, Fei-Hung Lin, and Chia-Jyi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09026 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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ACS Applied Materials & Interfaces

Enhanced ZT of InxCo4Sb12-InSb Nanocomposites Fabricated by Hydrothermal Synthesis Combined with Solid-Vapor Reaction: A Signature of Phonon-Glass and Electron-Crystal Materials Ahmad Gharleghi, Peng-Chun Hung, Fei-Hung Lin, and Chia-Jyi Liu* Department of Physics, National Changhua University of Education, Changhua 500, Taiwan. *e-mail: [email protected]

Abstract A rapid route of synthesizing pristine Co4Sb12 at relatively low temperature was previously developed. However, filling the voids using the same procedure is not successful. We develop a new route to fabricate In-filled cobalt skutterudites with InSb nanoinclusions InxCo4Sb12-(InSb)y via solid-vapor reaction between hydrothermally synthesized Co4Sb12 powder and the indium chunk. The nanocomposites are characterized using powder x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS). With the success of partial filling of In into the voids and InSb nanoinclusions, the power factor of the InxCo4Sb12-(InSb)y nanocomposites is significantly enhanced and the thermal conductivity is lowered as compared with the pristine Co4Sb12. As a result, ZT with

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its highest value of 1.0 is attained for the hierarchical structured In0.04Co4Sb12-(InSb)0.05 nanocomposite at 575 K. The attained ZT value is among the highest ever reported value at T ≤575 K for In-filled cobalt skutterudites.

Keywords: hydrothermal synthesis, solid-vapor reaction, filled skutterudite, nanocomposites, phonon-glass and electron-crystal thermoelectric material.

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1. Introduction Developing high-efficiency thermoelectric materials for applications in generating electric power from low-grade heat released into the environment has recently received much attention. The thermoelectric efficiency of a material depends on its dimensionless figure of merit, which is expressed as ZT = σS2T/κ.1 Large thermopower (Seebeck coefficient) S and electrical conductivity σ in conjunction with low thermal conductivity κ are the properties that characterize a good thermoelectric material. However, materials with high thermal conductivity and large power factor σS2 are favorable for achieving high efficiency in the case of thermoelectric coolers.2 Fabricating materials with ZT higher than available values to achieve application goals in generating the electrical energy has received major ongoing research challenges.3-14 In this regard, various strategies such as tuning the electronic structure via introducing extrinsic carrier through doping and/or alloying are adopted to enhance the power factor σS2.3,5,6,12,14,15 Furthermore, electron energy filtering and band offset minimization strategies have been developed for enhancing the power factor of the nanostructured thermoelectrics.3,6,10,16,17 In addition, decreasing the lattice component of the total thermal conductivity via broad-based phonons scattering from hierarchical nanostructures is progressively investigated.18-22 Recently, fabrication of microcomposites embedded with nanoinclusions is found to be a practical strategy for enhancing ZT.7,17,23-27

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Nanocomposites proven to be capable of enhancing the power factor by tuning the electronic properties and simultaneously suppressing the lattice thermal conductivity by progressing all-scale hierarchical phonon scattering.24,25 Hence, fabricating nanocomposites could be one of the straightforward strategies towards producing phonon-glass electron-crystal (PGEC) thermoelectric materials.28,29 Intrinsically possessing two Sb-icosahedral voids per unit cell, the Co8Sb24 skutterudite is amongst the potential thermoelectric materials capable for exhibiting PGEC properties.29,30 However, the relatively high lattice thermal conductivity (> 9 Wm-1K-1 around room temperature) degrades the thermoelectric performance of the pristine bulk cobalt skutterudite.30 Formation of ternary analogues or solid solutions and filling the intrinsic voids could be effective strategies for reducing the thermal conductivity of skutterudite.31-34 Selectively filling the Sb-voids by various elements enhances the power factor via modifying the charge carriers concentration and to some extent decreases the lattice thermal conductivity via rattling-assisted phonon scattering.35-39 The concentration of Sb-voids rattler-elements is limited and differs for different filler ions R in RxCo4Sb12.38-43 For instance, this solubility limit is reported to be x≅0.22 for the indium filler.40,44 We have previously developed a rapid procedure of synthesizing pristine Co4Sb12 at relatively low temperature.45 However, filling the voids using the same procedure is not successful. In this study, we report fabrication of InxCo4Sb12-(InSb)y nanocomposites via

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solid-vapor reaction between hydrothermally synthesized Co4Sb12 powder and the indium chunk. The material characterization is carried out using powder x-ray diffraction, field-emission scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy and inductively coupled plasma mass spectrometry (ICP-MS). With partial filling of In into the voids and InSb nanoinclusions, the power factor of the InxCo4Sb12-(InSb)y nanocomposites is significantly enhanced and the thermal conductivity is lowered as compared with the pristine Co4Sb12, which leads to ZT ≅ 1.0 for the In0.04Co4Sb12-(InSb)0.05 nanocomposite at 575 K.

2. Experimental Details Co4Sb12 powders were synthesized using hydrothermal synthesis. The ratio of Sb powders to CoCl2·6H2O is 3 to 1. The synthetic procedure was described in details elsewhere.45 Similar procedure was carried out except the synthesis temperature is 260°C. The hydrothermally synthesized powders were dried and then heated at 580°C in an evacuated Pyrex ampoule for 5 h.45,46 The phase purity was examined using a Shimadzu XRD-6000 diffractometer using Fe Kα radiation. The solid-vapor reaction method was then adopted to fill indium into the Sb-icosahedron voids of Co4Sb12. In a typical reaction, hydrothermally synthesized and compacted Co4Sb12 together with a certain amount of indium ingots were loaded into an evacuated quartz ampoule (10-5 -10-6 Torr) and heated at 580°C for 72 h. For comparing the transport properties, the pristine Co4Sb12 was also compacted and heated in the same condition for 72 h. The resulting pristine Co4Sb12 and indium-filled Co4Sb12 was then ground 5



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for XRD phase characterization. The ground powders were cold-pressed at 16.9 MPa into a cuboid. The cuboid was then loaded into a Pyrex tube, which was then evacuated (10-5 -10-6 Torr) and sealed. The evacuated and sealed ampoule were then sintered at 580°C for 5 h. The simultaneous measurements of electrical resistivity and thermopower of the cuboid were performed from 300 to 700 K. The steady-state technique was used for measuring the thermopower of the as-fabricated samples. A Keithley 2182 nanovoltmeter was used for measuring the thermally generated Seebeck voltage across the sample. A type E differential thermocouple connected to a Keithley 2000 multimeter was used to measure the temperature difference between hot and cold ends of the sample. The measurement details for electrical resistivity and thermopower is described elsewhere.47 The Hot Disk transient plane source technique was used to measure thermal conductivity of the samples.46,48 In this technique, the Hot Disk dual spiral sensor acts as heat source for generating temperature gradient in the pelleted samples and as resistance thermometer. The uncertainty for the electrical resistivity, thermopower and thermal conductivity in these measurements were about ±3, ±4, and ±5%, respectively. Hall effect data were obtained using the van der Pauw method under an applied magnetic field of 0.55 T (ECOPIA: HMS-3000). The morphology of the materials was investigated using a JEOL JSM-6700F field emission scanning electron microscope (FE-SEM). Elemental analyses of the materials were carried out using energy-dispersive x-ray spectroscopy (EDX) and inductively coupled plasma mass spectrometry (ICP-MS; PE-SCIEX ELAN 6100 DRC). Microstructure observation was performed using high-resolution transmission electron microscopy (HRTEM) (JEM-2100F). 6


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The Archimedes’ method was used to measure the bulk density of the samples.

3. Results and Discussion Fig. 1 displays the powder XRD patterns of as-fabricated InxCo4Sb12 with x = 0.0, 0.09 and 0.42. The indium content is determined using ICP-MS. For the pristine Co4Sb12 (x = 0.0), there exists a tiny amount of Sb (demonstrated by ¤ symbol) in addition to the skutterudite phase. Formation of InSb impurity is commonly observed in fabricating partially-filled InxCo4Sb12.36,44,49 The symbol * near 2θ ≈ 30° indicates the maximum reflection of InSb (PDF # 00-006-0208) for indium-filled samples. Apart from the maximum reflection of InSb, the XRD patterns can be indexed based on the skutterudite structure with space group Im3. The lattice constant of the cubic skutterudites is refined and listed in Table 1. It can be readily seen that the lattice constant a varies in different samples, indicating different amount of indium filling the Sb-icosahedron voids of Co4Sb12. Because of the coexistence of InxCo4Sb12 and InSb, the analytical results of ICP-MS do not indicate the actual amount of indium filling the voids. However, we correlate the refined lattice constants with those reported in reference 38 and estimate the indium content residing in the voids.44 The filling content of indium is estimated to be 0.04 and 0.06 for x = 0.09 and 0.42, respectively. To simplify notation from here on we will use InxCo4Sb12-(InSb)y to indicate the estimated amount (x) of indium filled

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to Sb-voids and the amount (y) of InSb. Fig. 2 displays the FE-SEM morphology of fractured surface of InxCo4Sb12-(InSb)y. It is worth noting that all the samples are fabricated through the same sintering conditions. The micrographs of pristine Co4Sb12 in Figs. 2a-b exhibit a collection of multiform particles with sizes ranging from nanometer to micrometer. Figs. 2c-d and Figs. 2e-f are the micrographs of InxCo4Sb12-(InSb)y nanocomposites with (x,y) = (0.04,0.05) and (0.06,0.36). It can be readily seen that the (0.06,0.36) sample has more nanosized particulates than the (0.04,0.05) sample. The EDX analyses performed on the nanocomposites suggest that the nanosized particulates are InSb. Fig. 3a shows the EDX spectra of an area of a square with side length of 6.7 micron (inset SEM image) that reveals the molar ratio of In and Co4Sb12 is 0.17. Fig. 3b is the EDX spectra of a much smaller area of about 1 micron (inset SEM image) that covers mainly the nanosized particulate and partial portion of micron-sized particulate. The EDX result reveals that the molar ratio of In and Co4Sb12 is 0.55. These results suggest the nanosized particulates are InSb when considering the coexistence of InxCo4Sb12 and InSb in the nanocomposites. The size of InSb particles is estimated between 30 to 50 nm according to SEM micrographs of Figs. 2d and 2e. Fig. 4 shows the HRTEM images of In0.04Co4Sb12-(InSb)0.05 nanocomposite. The selected area electron diffraction (SAED) pattern shown in Fig. 4b reveals the single crystal

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nature of the sample. The Miller indices are labelled based on the cubic skutterudite structure (space group Im3). Various size of nanoparticles with the area enclosed by contour lines shown in Fig. 4c could be attributed to InSb nanograins. Figs. 4d-f show the high-resolution HRTEM images taken along the [001] zone axis. The lattice fringes have spacing of 0.677 nm and 0.295 nm, which correspond to (110) and (013) lattice planes, respectively, of the Co4Sb12 structure. Particle size distribution could increase the packing density of a bulk composed of multi-sized particles due to van der Waals force as compared with packing mono-sized particles.50 The packing density of InxCo4Sb12 compound could be estimated from the measured bulk density of InxCo4Sb12-(InSb)y nanocomposites. The variation of packing density of InxCo4Sb12 due to the in-situ formation of nano-sized InSb particles is calculated using the following equation

=

+

,

(1)

where and are the weight percentages of the InxCo4Sb12 and InSb, respectively, the measured density of InxCo4Sb12-(InSb)y nanocomposites, the packing density of the InxCo4Sb12 skutterudite, and the density of InSb. After substituting the bulk density of

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5.78 g cm-3 for InSb into Eq. 1, we find that the packing density of InxCo4Sb12 is ~84 and ~87 % of the theoretical density for x = 0.04 and 0.06, respectively (Table 1). It should be noted that if the density of nanosized InSb is less than the theoretical density of 5.78 g cm-3, the packing density of InxCo4Sb12 would be underestimated in our calculation. Table 1 presents the room-temperature Hall mobility and carrier concentration of InxCo4Sb12-(InSb)y nanocomposites. The Hall measurements confirm that the electrons are the majority carriers for all the samples. Compared with the indium-free CoSb3 sample, both the electron concentration and mobility of InxCo4Sb12-(InSb)y nanocomposites are simultaneously increased (Table 1). The Hall mobility of (x,y)=(0.06,0.36) nanocomposite is 140 cm2/s·V, which is 6 times higher than that of indium-free Co4Sb12. The measured mobility of (x,y)=(0.06,0.36) nanocomposite is enhanced as compared with InxCo4Sb12-InSb composites fabricated using other methods such as spark plasma sintering (SPS),52 hot press (HP) sintering,53 combination of melting, annealing and SPS,54 melting,

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combination of

solvothermal and melting processes.56 Fig. 5 shows the room-temperature mobility and electron concentration of our InxCo4Sb12-(InSb)y nanocomposites in comparison with similar InxCo4Sb12-InSb composites fabricated by other methods. In addition, the trend line of filled CoSb3 that was obtained in ref. 57 is also displayed for further comparison. According to Fig. 5, one could readily see that the in-situ formed nanosized InSb phase is capable of facilitating

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the mobility enhancement of charge carriers within InxCo4Sb12-InSb. It is noted that InSb has high carrier mobility on the order of 104 cm2/s·V at room temperature.58,59 The high carrier mobility of InSb might facilitate the mobility enhancement of InxCo4Sb12-(InSb)y nanocomposites, in particularly for In0.06Co4Sb12-(InSb)0.36. Hence, the InSb grains might act as passing channels for electron transport.7 Fig. 6 shows the temperature dependence of electrical conductivity of Co4Sb12 and InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0.04,0.05) and (0.06,0.36) between 300 and 700 K. Apparently, the electrical conductivity increases upon increasing the indium content. The room-temperature electrical conductivity of the (x,y) = (0.04,0.05) and (0.06,0.36) nanocomposites is respectively 22 times and 48 times larger than that of pristine Co4Sb12. The unfilled Co4Sb12 displays a nonmetal-like temperature dependence of electrical conductivity. The temperature dependence of electrical conductivity of the (0.06,0.36) nanocomposite shows a metal-like behavior between 300 and 700 K; the and (0.04,0.05) nanocomposite exhibits a metal-like temperature dependence of electrical conductivity between 300 and 525 K and switches to a nonmetal-like behavior between 525 and 700 K. These results are consistent with other reports for similar systems.35,38,44 As shown in Table 1, the enhanced electrical conductivity of InxCo4Sb12-(InSb)y nanocomposites arise from both the increase of concentration and mobility of carriers.

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Fig. 7 shows the thermopower of Co4Sb12 and InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0.04,0.05) and (0.06,0.36) as a function of temperature between 300 and 700 K. The room-temperature absolute thermopower of InxCo4Sb12-(InSb)y nanocomposites decreases with increasing indium content, which is in agreement with other reports.35,38,39,44 The thermopower of pristine Co4Sb12 shows an upturn behavior, which is attributed to the early onset of bipolar mechanism and is commonly observed in n-type pristine Co4Sb12.35,39,42,45-47 It can be readily seen that the bipolar mechanism is compromised by partial indium filling and InSb nanoinclusions. The thermopower of the (0.04,0.05) and (0.06,0.36) nanocomposites tends to level off above 525 K within the investigated temperature range. Similar tendency is observed in the temperature dependence of electrical conductivity, which is analogous to other reports for In-filled Co4Sb12 system.56 Fig. 8 displays the power factor of the Co4Sb12 and InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0.04,0.05) and (0.06,0.36). Compared with the maximum power factor of pristine Co4Sb12 (~3.30 µWcm-1K-2 at 425 K), the power factor of the InxCo4Sb12-(InSb)y nanocomposites is noticeably increased. As shown in Fig. 8, the magnitudes of power factor for both the (x,y)= (0.04,0.05) and (0.06,0.36) nanocomposites are relatively large throughout the investigated temperature range of 300 to 700 K. The (0.04,0.05) and (0.06,0.36) nanocomposites exhibit a maximum power factor of ~ 25.0

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µWcm-1K-2 at 650 K and 24.7 µWcm-1K-2 at 700 K, respectively, which are nearly 8 times greater than that of the pristine Co4Sb12. It should be noted that the power factor of the (0.06,0.36) nanocomposite is between 23.4 and 24.7 µWcm-1K-2 for 450 ≤ ≤ 700 K. The nearly constant power factor is favorable for thermoelectric applications. Fig. 9 shows the total thermal conductivity of Co4Sb12 and InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0.04,0.05) and (0.06,0.36) as a function of temperature between 300 and 575 K. Due to the In fillers and InSb nanoinclusions, the room-temperature thermal conductivity is noticeably reduced (over 44 %) in comparison with that of the pristine Co4Sb12. The measured room-temperature thermal conductivity of Co4Sb12 is ~3.75 Wm-1K-1, while it is lessened to ~1.94 Wm-1K-1 and 2.1 Wm-1K-1 for the (0.04,0.05) and (0.06,0.36) nanocomposites, respectively. Due to the hierarchically structured porous nature (a wide range of particle sizes and porous), the thermal conductivity of the pristine Co4Sb12 is significantly smaller than that (> 9 Wm-1K-1 at 300 K) of the pristine Co4Sb12 fabricated via solid state reactions.29,30,39,42,54 Furthermore, the thermal conductivity of all the samples reduces with increasing temperature and reaches 2.41, 1.34, and 1.59 Wm-1K-1 at 575 K for (x,y)=(0,0),(0.04,0.05) and (0.06,0.36), respectively. The microstructural observation shows that the InxCo4Sb12-(InSb)y nanocomposites are also hierarchically structured materials due to formation of multi-size particles. The hierarchical structure acts to disrupt heat transport via

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multi-scale broad-band phonon scattering. Besides, both the indium rattlers and InSb nanograins (30 to 50 nm) further lessen the thermal conductivity of InxCo4Sb12. The former scatters high frequency phonons36,38,39,49,54 and the latter scatters mid-to-long wavelength phonons.18,25 Apparently, the thermal conductivity of the (0.06,0.36) nanocomposite is larger than that of the (0.04,0.05) nanocomposite (Fig. 9). This could be due to the larger electrical conductivity of the former than the latter. Fig. 10 shows the lattice thermal conductivities ( ) of InxCo4Sb12-(InSb)y which are estimated by means of the Wiedemann-Franz law with the assumption that the relaxation times for electrical and thermal processes are identical. The obtained values of are significantly decreased for InxCo4Sb12-(InSb)y nanocomposites as compared with indium-free CoSb3. The small difference between and the total thermal conductivity for all the samples indicates that the lattice phonon scattering possesses the major contribution to the thermal conduction in InxCo4Sb12-(InSb)y within the investigated temperature range. Besides, the curves of (0.04,0.05) and (0.06,0.36) nanocomposites display similar temperature dependence with comparable values. The value of is 2.38, 1.13 and 1.19 Wm-1K-1 for InxCo4Sb12-(InSb)y with (x, y) = (0.0,0.0), (0.04,0.05) and (0.06,0.36) at 575 K, respectively. These results show that the lattice thermal conductivity of InxCo4Sb12-(InSb)y nanocomposites in the present study is remarkably low as compared with of similar

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compositions, which are fabricated using different procedures.36,38,44,49,54 However, it should be noted that the of our InxCo4Sb12-(InSb)y nanocomposites is larger than that of In0.20Ce0.20Co4Sb12-InSb with double fillers.60 The remarkably low lattice thermal conductivity of the preset InxCo4Sb12-(InSb)y nanocomposites demonstrates that phonon-glass materials can be attained using our fabrication method. Fig. 11 illustrates ZT of Co4Sb12 and InxCo4Sb12-(InSb)y nanocomposites with (x,y) = (0.04,0.05) and (0.06,0.36) as a function of temperature between 300 and 575 K. It can be readily seen that the ZT of InxCo4Sb12-(InSb)y nanocomposites is markedly enhanced as compared with the pristine Co4Sb12. In addition, the ZT of InxCo4Sb12-(InSb)y nanocomposites increases monotonically with temperature and reached about 1.0 and 0.8 at 575 K for (x,y) = (0.04,0.05) and (0.06,0.36) nanocomposites, respectively. The attained ZT ≅ 1.0 is among the highest ever reported value for cobalt skutterudites compound at T ≤575 K containing single filler of indium36,38,39,44,49 and InSb nanoinclusions, fabricated using other procedures.52,55,56 Fig. 12 shows the highest ZT magnitudes of filled CoSb3 system containing single36,38,39,44,49,52,61 or multi-fillers,55,57,60 from some other reports in comparison with our best ZT values corresponding to the In0.04Co4Sb12-(InSb)0.05 nanocomposite.

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4. Conclusions The partially In-filled nanocomposites with InSb nanoinclusions InxCo4Sb12-(InSb)y are fabricated via hydrothermal synthesis combined with solid-vapor reaction. The indium content is quantitatively determined using ICP analyses. The amount of filled indium inside Sb-voids of Co4Sb12 structure is estimated using the refined lattice constants, which are then compared with reported data of InxCo4Sb12. Significant enhancement of electrical conductivity of InxCo4Sb12-(InSb)y can be attributed to simultaneous increase of electron carrier concentration and mobility as compared with the pristine Co4Sb12. The enhanced mobility of the In0.06Co4Sb12-(InSb)0.36 nanocomposites seems to correlate with the high mobility of InSb. The commonly observed bipolar effects on the thermopower in n-type pristine Co4Sb12 are compromised by partial indium filling and InSb nanoinclusions. As a result, the power factor of the InxCo4Sb12-(InSb)y nanocomposites is about 8 times higher than that of the pristine Co4Sb12. With enhancement of the power factor together with low thermal conductivity, ZT ≅ 1.0 is attained for the In0.04Co4Sb12-(InSb)0.05 nanocomposite at 575 K. Our results show that the hierarchical structure of InxCo4Sb12-(InSb)y nanocomposites display significant contribution in enhancing transport properties of InxCo4Sb12.

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Acknowledgments

This work was supported by Ministry of Science and Technology of Taiwan under the Grant No. 104-2112-M-018-002-MY3.

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(18) Wang, H.; Bahk, J.-H.; Kang, C.; Hwang, J.; Kim, K.; Kim, J.; Burke, P.; Bowers, J. E.; Gossard, A. C.; Shakouri, A.; Kim, W. Right Sizes of Nano- and Microstructures for High-Performance and Rigid Bulk Thermoelectrics. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10949-10954. (19) Tan, G.; Zhao, L.-D.; Shi, F.; Doak, J. W.; Lo, S.-H.; Sun, H.; Wolverton, C.; Dravid, V. P.; Uher, C.; Kanatzidis, M. G. High Thermoelectric Performance of p-Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach. J. Am. Chem. Soc. 2014, 136, 7006-7017. (20) Tan, G.; Liu, W.; Wang, S.; Yan, Y.; Li, H.; Tang, X.; Uher, C. Rapid Preparation of CeFe4Sb12 Skutterudite by Melt Spinning: Rich Nanostructures and High Thermoelectric Performance. J. Mater. Chem. A 2013, 1,12657. (21) Tan, G.; Shi, F.; Sun, H.; Zhao, L.-D.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. SnTe– AgBiTe2 as an Efficient Thermoelectric Material with Low Thermal Conductivity. J. Mater. Chem. A 2014, 2, 20849. (22) Tan, G.; Zheng, Y.; Tang, X. High Thermoelectric Performance of Nonequilibrium Synthesized CeFe4Sb12 Composite with Multi-scaled Nanostructures. Appl. Phys. Lett. 2013, 103, 183904.

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(23) Aminorroaya Yamini, S.; Wang, H.; Gibbs, Z. M.; Pei, Y.; Mitchell, D. R. G.; Dou, S.-X.; Snyder, G. J. Thermoelectric Performance of Tellurium-Reduced Quaternary p-Type Lead-Chalcogenide Composites. Acta Mater. 2014, 80, 365-372. (24) Zhao, L. D.; Wu, H. J.; Hao, S. Q.; Wu, C. I.; Zhou, X. Y.; Biswas, K.; He, J. Q.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravidc, V. P.; Kanatzidis, M. G. All-Scale Hierarchical Thermoelectrics: MgTe in PbTe Facilitates Valence Band Convergence and Suppresses Bipolar Thermal Transport for High Performance. Energy Environ. Sci. 2013, 6, 3346-3355. (25) Zhao, L.-D.; Hao, S.; Lo, S.-H.; Wu, C.-I.; Zhou, X.; Lee, Y.; Li, H.; Biswas, K.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures. J. Am. Chem. Soc. 2013, 135, 7364-7370. (26) Feng, B.; Xie, J.; Cao, G.; Zhu, T.; Zhao, X. Enhanced Thermoelectric Properties of p-Type CoSb3/Graphene Nanocomposite. J. Mater. Chem. A 2013, 1, 13111. (27) Kim, W.; Zide, J.; Gossard, A.; Klenov, D.; Stemmer, S.; Shakouri, A.; Majumdar, A. Thermal Conductivity Reduction and Thermoelectric Figure of Merit Increase by Embedding Nanoparticles in Crystalline Semiconductors. Phys. Rev. Lett. 2006, 96, 045901.

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(29) Nolas, G. S.; Morelli, D. T.; Tritt, T. M. Skutterudites: A Phonon-Glass-Electron Crystal Approach to Advanced Thermoelectric Energy Conversion Applications. Annu. Rev. Mater. Sci. 1999, 29, 89-116. (30) Sales, B. C.; Mandrus, D.; Chakoumakos, B. C.; Keppens, V.; Thompson, J. R. Filled Skutterudite Antimonides: Electron Crystals and Phonon Glasses. Phys. Rev. B 1997, 56, 15081-15089. (31) Tan, G.; Liu, W.; Chi, H.; Su, X.; Wang, S.; Yan, Y.; Tang, X.; Wong-Ng, W.; Uher, C. Realization of High Thermoelectric Performance in p-Type Unfilled Ternary Skutterudites FeSb2+xTe1-x via Band Structure Modification and Significant Point Defect Scattering. Acta Mater. 2013, 61, 7693-7704. (32) Tan, G.; Wang, S.; Tang, X. High Thermoelectric Figure of Merit of p-Type Ternary Unfilled Skutterudite FeSb2Te via Ge Doping. Sci. Adv. Mater. 2013, 5, 1974-1982. (33) Tan, G.; Zheng, Y.; Yan, Y.; Tang, X. Preparation and Thermoelectric Properties of p-Type Filled Skutterudites CeyFe4-xNixSb12. J. Alloys. Compd. 2014, 584, 216-221. (34) Duan, B.; Yang, J.; Salvador, J. R.; He, Y.; Zhao, B.; Wang, S.; Wei, P.; Ohuchi, F. S.; Zhang, W.; Hermann, R. P.; Gourdon, O.; Mao, S. X.; Cheng, Y.; Wang, C.; Liu, J.; Zhai,

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P.; Tang, X.; Zhang, Q.; Yang, J. Electronegative Guests in CoSb3. Energy Environ. Sci. 2016, 9, 2090-2098. (35) Eilertsen, J.; Subramanian, M. A.; Kruzic, J. J. Fracture Toughness of Co4Sb12 and In0.1Co4Sb12 Thermoelectric Skutterudites Evaluated by Three Methods. J. Alloys. Compd. 2013, 552, 492-498. (36) Visnow, E.; Heinrich, C. P.; Schmitz, A.; de Boor, J.; Leidich, P.; Klobes, B.; Hermann, R. P.; Müller, W. E.; Tremel, W. On the True Indium Content of In-Filled Skutterudites. Inorg. Chem. 2015, 54, 7818-7827. (37) Shi, X.; Kong, H.; Li, C.-P.; Uher, C.; Yang, J.; Salvador, J. R.; Wang, H.; Chen, L.; Zhang, W. Low Thermal Conductivity and High Thermoelectric Figure of Merit in nType BaxYbyCo4Sb12 Double-Filled Skutterudites. Appl. Phys. Lett. 2008, 92, 182101. (38) Tang, Y.; Qiu, Y.; Xi, L.; Shi, X.; Zhang, W.; Chen, L.; Tseng, S.-M.; Chen, S.-W.; Snyder, G. J. Phase Diagram of In-Co-Sb System and Thermoelectric Properties of In-Containing Skutterudites. Energy Environ. Sci. 2014, 7, 812-819. (39) Mallik, R. C.; Jung, J.-Y.; Ur, S.-C.; Kim, I.-H. Thermoelectric Properties of InzCo4Sb12 Skutterudites. Met. Mater. Int. 2008, 14, 223-228. (40) Grytsiv, A.; Rogl, P.; Michor, H.; Bauer, E.; Giester, G. InyCo4Sb12 Skutterudite: Phase Equilibria and Crystal Structure. J. Electron. Mater. 2013, 42, 2940-2952.

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Co4-xNixSb12

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Polyol

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Synthesis

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Thermoelectric Performance in In1-xGaxSb Originating from the Scattering of Point Defects and Nanoinclusion. J. Mater. Chem. 2011, 21, 12398-12401. (60) Li, H.; Tang, X.; Zhang, Q.; Uher, C. High Performance InxCeyCo4Sb12 Thermoelectric Materials with in Situ Forming Nanostructured InSb Phase. Appl. Phys. Lett. 2009, 94, 102114. (61) Wang, S.; Salvador, J. R.; Yang, J.; Wei, P.; Duan, B.; Yang, J. High-Performance n-Type YbxCo4Sb12: from Partially Filled Skutterudites towards Composite Thermoelectrics. NPG Asia Mater. 2016, 8, e285.

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Table 1. Structural and electronic properties obtained at room-temperature for InxCo4Sb12-(InSb)y

nanocomposites;

the

compounds

were

fabricated

via

hydrothermal synthesis combined with solid-vapor reaction method. Property Lattice Constant, a

Unit

x=0.00, y=0.00 x=0.04, y=0.05

x=0.06, y=0.36

Å

9.0354(3)

9.0395(2)

9.0424(3)

g/cm3

6.41

6.62

6.48

%

83.9

86.6

86.8

Electron Concentration, n

1019/cm3

0.30

4.48

4.93

Mobility

cm2/V·s

23

39

140

Measured Bulk density Relative Packing Density of InxCo4Sb12

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Figure Captions Fig. 1. Powder XRD patterns of InxCo4Sb12-(InSb)y composites with (x,y) = (0,0), (0.04,0.05), (0.06,0.36). The (hkl) index denotes the reflection planes of the Co4Sb12 skutterudite. The most intense peak of the Sb impurity occurring near 2θ ≈ 36° is indicated by ¤ symbol for the pristine (x,y) = (0,0) sample. The reflection peaks of InSb are indicated by asterisks.

Fig. 2. FE-SEM micrographs of fractured Co4Sb12 (a and b), In0.04Co4Sb12-(InSb)0.05 nanocomposite (c and d) and In0.06Co4Sb12-(InSb)0.36 nanocomposite (e and f); the in-situ produced nanosized particles in Fig. (d, e, f) are confirmed as InSb compound using EDX.

Fig. 3. The EDX spectra from the fractured surface of In0.06Co4Sb12-(InSb)0.36 nanocomposite; (a) the EDX spectra of an area of a square with side length of 6.7 micron (inset SEM image); (b) the EDX spectra of a much smaller area of about 1 micron (inset SEM image) that covers mainly the nanosized particulate and partial portion of micron-sized particulate.

Fig. 4. TEM images showing the microstructures of In0.04Co4Sb12-(InSb)0.05 nanocomposites 29



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fabricated using hydrothermal synthesis followed by solid-vapor reaction. (a) Low-magnification TEM image showing a large aggregate; (b) SAED pattern from (a) showing single-crystal diffraction spots labeled with (hkl) indices of skutterudite structure; (c) Low-magnification TEM image from (a) showing various size of nanoparticles with the area enclosed by contour lines; (d) HRTEM image taken along [001] zone axis; (e) The plans distortions and grain matches illustrated by parallel lines and arrows; (f) HRTEM image showing lattice fringes with different lattice spacing of (110) and (013) crystalline planes with the corresponding d-spacing of about 0.677 nm and 0.295 nm, respectively.

Fig. 5. The room-temperature Hall mobility as a function of electron concentrations of InxCo4Sb12-(InSb)y nanocomposites with (x,y)= (0.04,0.05) and (0.06,0.36) and some of similar InxCo4Sb12-InSb compounds fabricated by other methods. The solid line is the trend line of the filled CoSb3.

Fig. 6. Temperature dependence of electrical conductivity for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700 K.

Fig. 7. Temperature dependence of thermopower for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700K.

Fig. 8. Temperature dependence of the power factor for InxCo4Sb12-(InSb)y nanocomposites 30


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with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700 K.

Fig. 9. Temperature dependence of the thermal conductivity for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K.

Fig. 10. Temperature dependence of the lattice thermal conductivity ( ) for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K.

Fig. 11. Temperature dependence of ZT for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K. The nanocomposites are fabricated using hydrothermal synthesis combined with the solid-vapor reaction. Fig. 12. Comparison of ZT of filled CoSb3 system containing single or multi-fillers from some other reports with that of In0.04Co4Sb12-(InSb)0.05 nanocomposite from the present work.

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Table of Contents Graphic

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Fig. Table of Contents Graphic Fig. Table of Contents Graphic 414x346mm (96 x 96 DPI)



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Fig. 1. Powder XRD patterns of InxCo4Sb12-(InSb)y composites with (x,y) = (0,0), (0.04,0.05), (0.06,0.36). The (hkl) index denotes the reflection planes of the Co4Sb12 skutterudite. The most intense peak of the Sb impurity occurring near 2θ ≈ 36° is indicated by ¤ symbol for the pristine (x,y) = (0,0) sample. The reflection peaks of InSb are indicated by asterisks. Fig. 1. XRD 259x182mm (150 x 150 DPI)


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Fig. 2. FE-SEM micrographs of fractured Co4Sb12 (a and b), In0.04Co4Sb12-(InSb)0.05 nanocomposite (c and d) and In0.06Co4Sb12-(InSb)0.36 nanocomposite (e and f); the in-situ produced nanosized particles in Fig. (d, e, f) are confirmed as InSb compound using EDX. Fig. 2. FE-SEM 180x234mm (96 x 96 DPI)



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Fig. 3. The EDX spectra from the fractured surface of In0.06Co4Sb12-(InSb)0.36 nanocomposite; (a) the EDX spectra of an area of a square with side length of 6.7 micron (inset SEM image); (b) the EDX spectra of a much smaller area of about 1 micron (inset SEM image) that covers mainly the nanosized particulate and partial portion of micron-sized particulate. Fig. 3. EDX Spectra 124x164mm (96 x 96 DPI)


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Fig. 4. TEM images showing the microstructures of In0.04Co4Sb12-(InSb)0.05 nanocomposites fabricated using hydrothermal synthesis followed by solid-vapor reaction. (a) Low-magnification TEM image showing a large aggregate; (b) SAED pattern from (a) showing single-crystal diffraction spots labeled with (hkl) indices of skutterudite structure; (c) Low-magnification TEM image from (a) showing various size of nanoparticles with the area enclosed by contour lines; (d) HRTEM image taken along [001] zone axis; (e) The plans distortions and grain matches illustrated by parallel lines and arrows; (f) HRTEM image showing lattice fringes with different lattice spacing of (110) and (013) crystalline planes with the corresponding d-spacing of about 0.677 nm and 0.295 nm, respectively. Fig. 4. TEM 611x420mm (96 x 96 DPI)



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Fig. 5. The room-temperature Hall mobility as a function of electron concentrations of InxCo4Sb12-(InSb)y nanocomposites with (x,y)= (0.04,0.05) and (0.06,0.36) and some of similar InxCo4Sb12-InSb compounds fabricated by other methods. The solid line is the trend line of the filled CoSb3. Fig. 5. Hall Mobility vs Elect 289x223mm (150 x 150 DPI)


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Fig. 6. Temperature dependence of electrical conductivity for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700 K. Fig. 6. Electrical Conductivit 404x315mm (96 x 96 DPI)



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Fig. 7. Temperature dependence of thermopower for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700K. Fig. 7. Thermopower 404x315mm (96 x 96 DPI)


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Fig. 8. Temperature dependence of the power factor for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 700 K. Fig. 8. Power Factor 404x315mm (96 x 96 DPI)



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Fig. 9. Temperature dependence of the thermal conductivity for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K. Fig. 9. Thermal Conductivity 404x315mm (96 x 96 DPI)


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Fig. 10. Temperature dependence of the lattice thermal conductivity〖 (κ〗_L) for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K. Fig. 10. Lattice Thermal Condu 289x223mm (150 x 150 DPI)



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Fig. 11. Temperature dependence of ZT for InxCo4Sb12-(InSb)y nanocomposites with (x,y)=(0,0), (0.04,0.05) and (0.06,0.36) between 300 and 575 K. The nanocomposites are fabricated using hydrothermal synthesis combined with the solid-vapor reaction. Fig. 11. , ZT 289x225mm (150 x 150 DPI)


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Fig. 12. Comparison of ZT of filled CoSb3 system containing single or multi-fillers from some other reports with that of In0.04Co4Sb12-(InSb)0.05 nanocomposite from the present work. Fig. 12. Comparison of ZT 289x223mm (150 x 150 DPI)


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