Synthesis and Thermoelectric Properties of Charge-Compensated

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Synthesis and Thermoelectric Properties of ChargeCompensated SyPdxCo4-xSb12 Skutterudites Shun Wan, Pengfei Qiu, Xiangyang Huang, Qingfeng Song, Shengqiang Bai, Xun Shi, and Lidong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15124 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Synthesis

and

Thermoelectric

Properties

of

Charge-

Compensated SyPdxCo4-xSb12 Skutterudites Shun Wan,

†,‡

Pengfei Qiu,

*,†

Xiangyang Huang,

†

Qingfeng Song, †,‡ Shengqiang Bai,

†

Xun

Shi*,† and Lidong Chen† †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China ‡

College of Materials Sciences and Opto-Electronic Technology, University of Chinese

Academy of Sciences, Beijing, 100049, China ABSTRACT: Recently, the electronegative elements (e.g. S, Se, Cl, and Br) filled skutterudites have attracted great attentions in thermoelectric community. Via doping some electron donors at the Sb sites, these electronegative elements can be filled into the voids of CoSb3 forming thermodynamically stable compounds, which greatly extend the scope of filled skutterudites. In this study, we show that doping appropriate elements at the Co sites can also stabilize the electronegative elements in the voids of CoSb3. A series of SyPdxCo4-xSb12 compounds were successfully fabricated by a traditional solid state reaction method combined with a spark plasma sintering technique. The phase composition, electrical and thermal transport properties were systematically characterized and the related mechanisms were deeply discussed. It is found that the charge compensation between Pd doping and S filling is the main reason for the formation of thermodynamically stable SyPdxCo4-xSb12 compounds. Filling S element in the voids of CoSb3 provides addition holes to reduce the carrier concentration, while scarcely affects the carrier mobility. However, doping Pd at the Co sites not only changes the carrier scattering mechanism but also deteriorates the carrier mobility. Low lattice thermal conductivities are observed in these SyPdxCo4-xSb12 compounds, which are attributed to the low resonant frequency of the S element. Finally, a maximal figure of merit of 0.85 is obtained for S0.05Pd0.25Co3.75Sb12 at 700 K.

KEYWORDS: Thermoelectric, Skutterudite, Charge-compensate, S filling, Pd doping

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1. INTRODUCTION Thermoelectric (TE) technology can realize the direct conversion between heat and electricity based on the Seebeck effect or Peltier effect.

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The conversion efficiency of a TE material is

determined by the dimensionless figure of merit zT = S2σT/κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity, and absolute temperature, respectively. One of the most important concepts used to design and develop high performance TE materials is phonon-glass electron-crystal (PGEC) proposed by Slack, who pointed out that a good TE material should conduct phonons like a glass and electrons like a crystal.5,6 CoSb3-based filled skutterudites are one family of typical TE materials well agreeing with the PGEC concept. CoSb3 has a caged-like body-centered-cubic crystal structure. In one unit cell, eight Co atoms occupy the 8c-sites and twenty-four Sb atoms occupy the 24g-sites to form [CoSb6] octahedra with the Co atom in the center. There exist two large lattice voids (1.89 Å in radius) at the 2a sites, which can be filled by the foreigner atoms. 7,8 Therefore, the chemical formula of CoSb3-based filled skutterudites can be written as RyCo4Sb12, where R is the filler and y is its filling fraction, respectively. Because the fillers have weak chemical bonds with the surrounding Sb atoms, they demonstrate unusual “rattling” behavior in the voids, which can strongly scatter the heat-carrying lattice phonons and suppress the lattice thermal conductivity without deteriorating the excellent electrical transports of CoSb3.9,10 Thus, high zTs around 1.0 have been achieved in single-filled CoSb3-based skutterudites.11 Moreover, choosing two or more types of fillers with different localized resonant frequencies can scatter a wider spectrum of heat-carrying lattice phonons and further suppress the lattice thermal conductivity. Correspondingly, higher zTs, about 1.3 - 1.4 and 1.7 - 2.0 have been achieved in double-filled and multiple-filled CoSb3-based skutterudites, respectively.12-15 Generally, the thermodynamically stable fillers in CoSb3 should satisfy χSb - χR > 0.80, where χSb and χR are the electronegativities of Sb atom and filler R, respectively.16 This simple rule has been successfully used to explain and predict most electropositive elements (such as rare-earth, alkaline-earth, and alkali metals) filled CoSb3-based compounds.10,17,18 However, when part Sb atoms are substituted by other atoms in the different column of the periodic table, this rule might


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become invalid. One typical example is the group 13 elements (Ga and In). Although Ga and In do not satisfy the rule χSb - χR > 0.80, the experiments found that they still can enter into the void of CoSb3. The simultaneously presence of part Ga/In atoms doping at the Sb sites is believed to be responsible for the stabilization of Ga/In atoms in the voids. These Ga/In atoms at the Sb sites provide extra holes to compensate the electrons generated by the ones at the void sites, which can reduce the energy of the whole system and then generate the thermodynamically stable Ga or In filled CoSb3 compounds. 19-21 Most recently, it is found that even the electronegative element S can be also stably filled into the voids of CoSb3 by simultaneously doping some Te at the Sb sites. 22 The S atoms in CoSb3 possess very low resonant frequencies in the range of 35 - 50 cm-1, comparable with Yb (42 cm-1),11 which has the lowest resonant frequency among all electropositive fillers. Thus, S filled CoSb3 skutterudites possess low thermal conductivities with the minimal value about 1.5 Wm-1K-1 at room temperature. A maximal zT of 1.5 has been achieved in S0.26Co4Sb11.11Te0.73, comparable with those electropositive elements filled skutterudites reported before.13,14 By using Te or Se doping at the Sb sites, some other electronegative elements (such as Se, Cl, and Br) filled CoSb3 compounds have been also successfully prepared, which greatly extend the scope of filled CoSb3 compounds. 22 Beyond introducing the dopants at the Sb sites, doping the electron donors (e.g. Ni, Pd, or Pt) at the Co sites might be also effective to stabilize the electronegative elements in the voids of CoSb3. The related investigation has been already performed by Duan et al., who successfully prepared a S filled CoSb3 compound with Ni doping at Co sites.22 However, so far, there is still lack of systemic and comprehensive study on the effect of these transition metals on the electrical and thermal transports of electronegative elements filled skutterudites. In this study, we have prepared a series of S filled CoSb3 compounds with Pd doping at the Co sites, SyPdxCo4xSb12

(x = 0.1, 0.15, 0.2, 0.25, and 0.3, y = 0, 0.025, 0.05, 0.1, and 0.15). The compound defects

and TE properties of these SyPdxCo4-xSb12 compounds have been systematically investigated. The evolutions of TE properties as a function of the S filling fraction and the Pd doping content have been deeply analyzed and the relevant mechanisms have been discussed. This study provides a full understanding on the electronegative elements filled CoSb3 with dopants at the Co sites.

2. EXPERIMENTAL SECTION


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Samples with the nominal chemical compositions SyPdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3, y = 0, 0.025, 0.05, 0.1, 0.15) were prepared via a traditional solid state reaction route. High purity raw materials of Co (powder, 99.9%), Pd (powder, 99.95%), Sb (powder, 99.9999%), and S (powder, 99.999%) were weighed out according to the stoichiometric proportions, and then mixed in an agate mortar by hand. The mixtures were cold pressed, and then sealed into quartz tubes under vacuum. The tubes were slowly heated up to 973 K and kept at that temperature for 72 h before furnace cooling to room temperature. The obtained samples were ground into fine powders by hand milling method, and subsequently the powders were sieved through a 200-mesh sieve. Dense bulk samples with a relatively high density (more than 97% of the theoretical density) were obtained by the Spark Plasma Sintering (SPS-2040, Sumitomo, Kyoto, Japan) technique at 873 K under an axis pressure of 40 MPa within 15 min. The heating rate was 100 K/min before 673 K and 50 K/min from 673 K to 873 K, respectively. The cooling rate was about 90 K/min.

The phase identification was employed by X-ray diffraction (XRD) characterization at room temperature by using a Rigaku Rint 2000 (Rigaku, Tokyo, Japan) with Cu Kα radiation. Scanning electron microscopy (SEM) using ZEISS Supra 55 (Carl Zeiss SMT, Oberkochen, Germany) equipped with energy dispersive X-ray spectrometer (EDS, Oxford Instrument, Oxford, UK) was performed to analyze the microstructure. The electrical conductivity (σ) and Seebeck coefficient (S) ranged from 300 K to 850 K for all samples were measured by using ZEM-3 (ULVACRIKO, Yokohama, Japan) apparatus under a helium atmosphere. The thermal diffusivity λ and specific heat Cp were measured by the laser flash method (LFA-427, Netzsch, Germany) and the differential scanning calorimetry method (DSC-200 F3, Netzsch, Germany), respectively. The density d was measured by using the Archimedes method. The total thermal conductivity κ can be calculated according to the relationship κ = dλCp. The Hall coefficient (RH) was measured in a Quantum Design Physical Property measurement system (PPMS, Quantum Design, San Diego, USA) by sweeping the magnetic field up to 3 T in both positive and negative directions from 5 K to 300 K. The Hall number is assumed to be 1 in the single carrier model. The Hall carrier concentration (n) was calculated by using the relation n = 1/RHe, where e is the elementary charge. The Hall carrier mobility (µH) was calculated by using the relation µH = RHσ. The low


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temperature electrical conductivity and thermal conductivity were also measured by PPMS from 5 K to 300 K. 3. RESULTS AND DISCUSSION 3.1 XRD & EDS analysis Figure 1a shows the powder XRD patterns for the SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples. All diffraction peaks can be identified as belonging to the skutterudite structure, suggesting that S and Pd enter into the lattice of CoSb3. Figure 1b shows the magnification of the XRD patterns at 28°-33° for the SyPd0.2Co3.8Sb12 samples. Considering the small atomic radius of S (1.04 Å) as compared with the large size of the void in CoSb3 (1.89 Å in radius), filling S might scarcely expand the lattice of CoSb3 like the other electropositive fillers. Thus, the (310) peak exhibits no obvious shift with increasing the S content. Take the S0.1Pd0.2Co3.8Sb12 sample as an example, Figure 2a shows the element distribution determined by EDS analysis. Each element is homogeneously distributed inside the matrix. No secondary phases are observed. However, for the samples with higher S nominal content, for instance S0.15Pd0.2Co3.8Sb12, some secondary phases are observed, which are identified as CoSbS and PdSb2 (see Figure 2b, Figure 2c). This suggests that S have a filling fraction limit in SyPd0.2Co3.8Sb12. This limit can be obtained from the relationship between the nominal S content and the actual S content inside the skutterudite matrix determined by the EDS analysis. As shown in Figure 1c and Table 1, the actual S content is almost the same with the nominal one when y < 0.1, but reaches a saturate value of 0.08 when y > 0.1. This indicates that the maximal S filling fraction in SyPd0.2Co3.8Sb12 is around 0.08. When the nominal S content y exceeds 0.08, S prefers to form the secondary phase such as CoSbS (see Figure 2c) instead of entering into the voids of CoSb3. In pure CoSb3, S can not be solely filled into the voids because the electronegativity difference between Sb and S (χSb - χS = -0.53) does not follow the selection rule (χSb - χR > 0.80) that is required for the thermodynamically stable filled CoSb3 compounds. Electron-donors must be simultaneously introduced at the [Co4Sb12] framework to reduce the energy raised by filling S in the voids. For the present SyPd0.2Co3.8Sb12 system, one Pd atom donates one electron while one S donates two holes. Thus, when y < 0.1, the electrons donated by Pd can completely compensate all the holes provided by S, leading to a low energy state to allow the formation of


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thermodynamically stable S filled CoSb3 compounds. When y ≥ 0.1, the nominal content of holes provided by S approaches or even exceeds the content of electrons donated by Pd, thus the system is energy unfavorable. Consequently, the extra S would form the secondary phases in order to lower the energy. Similar phenomenon has been also observed in Ga, In, and Se filled CoSb3-based skutterudites. 19,20,23 For example, when the content of Ga doped at the Sb sites is a half of that filled in the voids (y = 2x), the charge balance is reached in GayCo4Sb12-xGax and the energy of the system is the lowest. No secondary phases will be formed. However, when y ≠ 2x, the energy of the system is high and then the secondary phases (e.g. GaSb and Sb) will be formed in GayCo4Sb12-xGax (y ≠ 2x).19 Via fixing the S filling fraction y = 0.05, we also investigated the effect of Pd doping on the phase composition of SyPdxCo4-xSb12 compounds. Figure 3a-b present the powder XRD patterns for the S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples. As shown in Figure 3b, the (310) diffraction peak of these S0.05PdxCo4-xSb12 samples gradually shifts to the low angle direction with increasing the Pd-doping content from 0 to 0.25, suggesting that doping Pd can expand the lattice. This is reasonable because Pd (1.79 Å) has a larger atomic radius than Co (1.67 Å). When the Pd doping content x < 0.25, all diffraction peaks can be identified as belonging to the skutterudite structure. However, when x ≥ 0.25, some new diffraction peaks belonging to the secondary phases are observed, suggesting that Pd also has a doping limit at Co-site. Combining with the SEM analysis shown in Figure 2, these secondary phases are identified as PdSb2 and CoSbS. In Figure 3c, we also plot the actual Pd content determined by EDS as a function of the nominal Pd content. The actual Pd doping content is almost the same with the nominal one when x < 0.25, but it deviates off the nominal one when x reaches 0.25. The maximal actual Pd content in these S0.05PdxCo4-xSb12 samples is about 0.22, slightly higher than the Pd doping limit in pure CoSb3 (about 0.20) reported by Anno et al. 24 For comparison, the data for S0.1PdxCo4-xSb12 (x = 0.2, 0.25, 0.3) are also included in Figure 3c. Interestingly, the maximal actual Pd doping content in S0.1PdxCo4-xSb12 reaches 0.25. On one hand, the presence of Pd at the Co sites provides the possibility to fill S in the voids of CoSb3. On the other hand, the S atoms in the voids might also increase the Pd doping limit at the Co sites because they provide extra holes to compensate the electrons introduced by Pd dopants. Actually, the interaction between fillers and dopants is a very common phenomenon in electropositive elements filled CoSb3-based skutterudites. For example, the Fe doping limit in pure CoSb3 is 0.17. 25 However, if some electropositive elements


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(e.g. Ce) are filled in the voids of CoSb3 to compensate the holes introduced by the Fe dopants, the Fe doping limit can be greatly enhanced. Especially, when the Ce filling fraction approaches 1, all Co atoms can be completely substituted by Fe atoms. 26

Figure 1．． (a) Powder XRD patterns for all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples. (b) Magnification of the powder XRD patterns at 28°-33°. (c) Relationship between the nominal S content and the actual S content in the skutterduite matrix determined by EDS analysis.


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Figure 2 ． Microstructure and element distribution in (a) S0.1Pd0.2Co3.8Sb12, and (b) & (c) S0.15Pd0.2Co3.8Sb12. The secondary phases such as PdSb2 (b) and CoSbS (c) are detected in S0.15Pd0.2Co3.8Sb12.


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Figure 3．． (a) Powder XRD patterns for all S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples. (b) Magnification of the powder XRD patterns at 28°-33°. (c) Relationship between the nominal Pd content and the actual Pd content in skutterudite matrix determined by EDS analysis. The data of S0.1PdxCo4-xSb12 (x = 0.2, 0.25, 0.3) are included for comparison. The solid line represents the Pd doping limit in CoSb3 reported by Anno et al.24

3.2 Thermoelectric properties of SyPdxCo4-xSb12 3.2.1 Thermoelectric properties of SyPd0.2Co3.8Sb12 ( y = 0, 0.025, 0.05, 0.1, 0.15) The temperature dependences of electrical conductivity (σ) and Seebeck coefficient (S) for all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples ranged from 300 K to 850 K are presented in Figure 4a and Figure 4b. The σ values with a magnitude of 104 - 105 Sm-1 at room temperature are observed in these samples. σ for samples with y ≤ 0.05 decreases monotonously with increasing temperature in the whole temperature range, exhibiting a typical heavily doped semiconducting behavior. The σ values for S0.1Pd0.2Co3.8Sb12 and S0.15Pd0.2Co3.8Sb12 decrease monotonously before 700 K, and then increase from 700 K to 850 K, showing an intrinsic semiconducting behavior. With increasing the S filling fraction, σ gradually decreases in the entire measured temperature range. At room temperature, σ decreases from 1.25 × 105 Sm-1 when y = 0 to 6.05 × 104 Sm-1 when y = 0.15. As shown in Figure 4b, all samples display negative S values, implying that the majority of charge carriers are electrons. In fact, all the electronegative elements filled CoSb3 compounds reported so far are n-type materials despite that the electronegative elements are hole-donors. With increasing the S filling fraction, S gradually increases from -177 µVK-1 to -212 µVK-1 at room temperature. Based on the measured S values, the bandgap Eg can be estimated by using the formula27 Eg = 2eSmaxTmax

(1)

where Smax is the maximum S value, Tmax is the temperature when Smax occurs, and e is the electron charge. The estimated Eg data are listed in Table 1 with the values in the range of 0.26 0.33 eV, comparable with the pure CoSb3 and electropositive elements filled CoSb3-based skutterudites reported before.9,27 Due to the deterioration of the σ values, the power factors (PFs)


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decreases with increasing S content in the whole temperature range (see Figure 4c). For S0.1Pd0.2Co3.8Sb12, the PF is only 26 µWcm-1K-2 at 300 K, about 33 % reduction as compared with that for Pd0.2Co3.8Sb12. Figure 4d presents the temperature dependence of thermal conductivity (κ) for all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples ranged from 300 K to 850 K. κ for all samples firstly decrease with increasing temperature before reaching a minimum at around 600 K, and then increase with increasing temperature from 600 K to 850 K. Considering the similar variation of S shown in Figure 4b, such phenomenon is believed to be contributed by the bipolar effect, which is quite normal in narrow band gap semiconductors. When the electrons in valence band acquire enough energy to overcome the band gap Eg, they can be thermally excited to conduction band. The generation and recombination processes of the thermally excited electronhole pairs contribute to heat conduction by extracting an amount of energy at hot end and releasing them at cold end. With increasing the S filling fraction, κ monotonously decreases in the whole measured temperature range. At room temperature, κ is reduced from 5.6 WmK-1 in Pd0.2Co3.8Sb12 to 3.6 WmK-1 in S0.1Pd0.2Co3.8Sb12. At 850 K, the κ reduction is also quite significant. The κ of S0.1Pd0.2Co3.8Sb12 is only about 64% of Pd0.2Co3.8Sb12 at 850 K. The total κ in a solid is a sum of two contributions, termed the lattice thermal conductivity (κL) and the electronic thermal conductivity (κe). According to the Wiedemann-Franz law, κe is calculated by the relation κe = LσT, where L is the Lorenz number with an empirical value of 2.0 × 10-8 V2K-2 for skutterudite.9 Then, κL can be obtained by subtracting the carrier part (κe) from the total κ. As shown in Figure 4e, κL monotonously decreases with increasing the S filling fraction in the whole measured temperature range until y = 0.1. Combining the measured electrical and thermal properties, the dimensionless figure of merit (zT) for all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples are calculated and shown in Figure 4f. Although κ is significantly suppressed in Sfilled samples, the deteriorated electrical conductivity limits the enhancement of zT. A maximum zT of 0.78 is obtained for the sample with nominal composition S0.025Pd0.2Co3.8Sb12, which is increased 22% over the unfilled sample Pd0.2Co3.8Sb12.


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Figure 4. Temperature dependences of (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) power factor (PF), (d) thermal conductivity (κ), (e) lattice thermal conductivity (κL), and (f) the dimensionless figure of merit (zT) between 300 K and 850 K for all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) samples. The measurement errors for electrical conductivity, Seebeck coefficient, and thermal conductivity are estimated to be 5%, 3%, and 7%, respectively.


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3.2.2 Thermoelectric properties of S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) The temperature dependences of σ and S for all S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples ranged from 300 K to 850 K are presented in Figure 5a and Figure 5b. The negative S suggests that all samples are still electron-dominated n-type materials. Among all samples, S0.05Pd0.1Co3.9Sb12 shows the lowest σ, about 0.32 × 105 Sm-1 at room temperature. With increasing the Pd doping content, σ is significantly enhanced in the whole temperature range. For S0.05Pd0.3Co3.7Sb12, σ at room temperature reaches 1.05 × 105 Sm-1, about three times of that for S0.05Pd0.1Co3.9Sb12. Meanwhile, S is greatly decreased by increasing the Pd doping content. Especially, the decrement is more obvious at the temperature range from 300 K to 600 K. S for S0.05Pd0.3Co3.7Sb12 is -195 µVK-1 at room temperature, about 40% decrement as compared with that for S0.05Pd0.1Co3.9Sb12. By using Eq. (1), the band gaps Eg for these samples are also estimated with the values in the range of 0.28 - 0.32 eV. Combining the Eg values for SyPd0.2Co3.8Sb12 mentioned above, it can be concluded that Eg is almost independent on neither the S filling fraction nor the Pd doping content. Because of such narrow band gaps, the bipolar effect is also very obvious for these S0.05PdxCo4-xSb12 samples, leading to the nonmonotonic S variation as a function of the temperature as shown in Figure 5b. Above 700 K, the bipolar effect starts to dominate the electrical transports and thus S show complex variation with the Pd doping content. Due to the increased σ values, the PF values increase with the increase of Pd content in the entire temperature range (see Figure 5c). For S0.05Pd0.25Co3.75Sb12, the PF is raised to 36 µWcm-1K-2 at 300 K, about 51 % improvement as compared with that for S0.05Pd0.1Co3.9Sb12.

Figure 5d and 5e show the temperature dependence of thermal conductivity (κ) and lattice thermal conductivity (κL) for all S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25) samples ranged from 300 K to 850 K. Due to the bipolar effect, the κ for all samples also show the nonmonotonic variation as a function of the temperature with the minimal values appearing around 600 K. Interestingly, being different with the significant variations of σ and S, the κ decrement caused by increasing the Pd doping content is quite weak. For S0.05Pd0.25Co3.75Sb12, κ is 3.4 Wm-1K-1 at 300 K and 3.8 Wm-1K-1 at 850 K, only about 15% and 9% reduction as compared with those for


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S0.05Pd0.1Co3.9Sb12, respectively. The lattice thermal conductivity κL for these samples also monotonously decrease in the whole measured temperature range until x = 0.25. For S0.05Pd0.25Co3.75Sb12, the κL is 2.8 Wm-1K-1 at room temperature, about 26 % reduction as compared with that for S0.05Pd0.1Co3.9Sb12. Combining the measured electrical and thermal properties, the zTs for all S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples are calculated and shown in Figure 5f. As a result of the enhanced σ, zT is significantly improved with increasing the Pd doping content. A maximum zT of 0.85 at 700 K is obtained for the sample with nominal composition S0.05Pd0.25Co3.75Sb12, which is about three times of that for S0.05Pd0.1Co3.9Sb12.


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Figure 5．． Temperature dependences of (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) power factor (PF), (d) thermal conductivity (κ), (e) lattice thermal conductivity (κL), and (f) the dimensionless figure of merit (zT) between 300 K and 850 K for all S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples. The measurement errors for electrical conductivity, Seebeck coefficient, and thermal conductivity are estimated to be 5%, 3%, and 7%, respectively.


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3.3 Discussion of transport properties 3.3.1 Electrical transport properties In CoSb3, filling one S atom in the voids creates two extra holes, while substituting one Co atoms by Pd generates one extra electron. Thus, the different S/Pd content in these n-type SyPdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3, y = 0, 0.025, 0.05, 0.1, 0.15) samples would definitely lead to different carrier densities and consequently change the σ and S. The room temperature Hall carrier concentrations (n) as a function of the S filling fraction and the Pd doping content for these SyPdxCo4-xSb12 compounds were measured and shown in Figure 6a and Table 1. As we have expected, n almost linearly decreases with increasing the S filling fraction before reaching the maximal S filling fraction limit, while increases with increasing the Pd doping content before reaching the maximal Pd doping limit. Such n variations can well explain the measured electrical transport properties mentioned above, such as the increased σ with the increase of Pd doping content or the decreased σ with the increase of S filling fraction. Figure 6b shows the temperature dependence of the Hall carrier mobility (µH) for SyPdxCo4-xSb12 (x = 0.2, y = 0, 0.025, 0.05, 0.1; x = 0.25, y = 0.05) samples. µH for all samples deviate off the µH ~ T-3/2 relationship, suggesting that the carrier scattering is not only dominated by acoustic scattering but also simultaneously dominated by the ionized impurity scattering or neutral impurity scattering. Interestingly, when the Pd doping content is fixed at x = 0.2, µH of the SyPd0.2Co4Sb12 samples show almost the same temperature-dependence and numerical values with the S-free Pd0.2Co4Sb12 sample in the whole measured temperature range. This suggests that filling S in the voids of CoSb3 scarcely introduces additional scattering to deteriorate the carrier mobility, while only provides additional holes to change the carrier concentration. Such phenomenon is similar with those observed in electropositive elements filled skutterudites. 14,17,18 Table 1 Nominal and actual compositions, carrier concentration (n), and band gap (Eg) for all SyPdxCo4-xSb12 samples.

Nominal composition

Actual composition

n

Eg

(1020 cm-3)

(eV)


15


Page 16 of 30

Pd0.20Co3.80Sb12

Pd0.20(±0.03)Co3.80Sb12.01(±0.07)

2.05

0.26

S0.05Pd0.15Co3.85Sb12

S0.05(±0.01)Pd0.15(±0.01)Co3.85Sb12.03(±0.14)

0.792

0.28

S0.025Pd0.20Co3.80Sb12

S0.02(±0.01)Pd0.20(±0.02)Co3.80Sb12.06(±0.07)

1.58

0.33

S0.05Pd0.20Co3.80Sb12

S0.04(±0.01)Pd0.20(±0.01)Co3.80Sb11.99(±0.08)

1.36

0.30

S0.10Pd0.20Co3.80Sb12

S0.08(±0.01)Pd0.20(±0.02)Co3.80Sb12.04(±0.11)

1.02

0.31

S0.05Pd0.25Co3.75Sb12

S0.04(±0.01)Pd0.21(±0.02)Co3.75Sb11.92(±0.09)

2.07

0.32

S0.10Pd0.25Co3.75Sb12

S0.09(±0.01)Pd0.25(±0.02)Co3.75Sb12.05(±0.07)

1.12

0.29

S0.05Pd0.30Co3.70Sb12

S0.05(±0.01)Pd0.22(±0.02)Co3.70Sb11.88(±0.08)

2.18

0.33

S0.10Pd0.30Co3.70Sb12

S0.08(±0.02)Pd0.24(±0.02)Co3.70Sb11.94(±0.08)

1.21

0.34

Figure 6．． (a) Room temperature as a function of the S filling fraction and Pd doping content for SyPdxCo4-xSb12. (b) Temperature dependence of Hall mobility for SyPdxCo4-xSb12 from 5 K to 300 K. (c) Hall mobility and (d) Seebeck coefficient as a function of Hall carrier concentration


16

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for SyPdxCo4-xSb12. The data for some electropositive elements filled skutterudites are also included for comparison. The measurement error for carrier concentration is estimated to be 5%.

Being different with filling S in the voids, doping Pd at the Co sites significantly deteriorates the carrier mobility. These Pd dopants act as the ionized impurity scattering centers to affect the carrier transport, which is responsible for the observed Hall carrier mobilities deviating off the µH ~ T-3/2 relationship for all SyPdxCo4-xSb12 samples. With increasing the Pd doping content, the ionized impurity scattering is gradually strengthened. Thus, µH of S0.05Pd0.25Co3.75Sb12 is lower than those of all SyPd0.2Co3.8Sb12 samples in the whole measured temperature range. In order to further understand the Hall carrier mobilities of these S-Pd samples, the Matthiessen’s rule is used to fit the experimental measured µH. In the SPB model based on Fermi-Dirac statistics, the mobility for acoustic phonon scattering (µac) in the nondegenerate limit can be expressed as 29,30 =

() / ℏ (∗ )/ ( )/

,

(2)

where ℏ, vL, Eac, m*, and kB are the reduced Planck constant, longitudinal velocity of sound, band deformation potential, effective mass, and Boltzmann constant, respectively. vL is usually taken as 4.59 × 103 ms-1 for CoSb3-based skutterudites.24 The mobility by the ionized impurity scattering can be calculated by using the Brooks-Herring formula, which is given by 29,30 =

√ !" #$ (%& )/ '( √∗ )(*)

,

where f(x)=ln(x+1)-x/(x+1) and + =

(3) ,%∗ ( ) !" #$ -. ℏ

. ε0, εs, and NI are the vacuum dielectric

constants, relative dielectric constants, and ionized impurity concentration, respectively. εs is taken as 25.6 for CoSb3-based skutterudites.31 The mobility by the neutral impurity scattering is expressed by 29,30 ' =

∗

,/!" #$ '0 1

.

(4)


17


Page 18 of 30

where NN is the neutral impurity concentration. Using Matthiessen’s approximation, 32 the total mobility can be written as ,

2

,

= ∑5 2

4

.

(5)

Here Eac, NI, and NN are the fitting parameters. Good fits are found for all samples (see the dashed lines in Figure 6b). The fitting parameters are listed in Table 2. Eac for all SyPdxCo4-xSb12 samples are around 2.2 eV - 2.7 eV, comparable with the data reported before for skutterduites (such as 3.8 eV for Ni0.1Co3.9Sb12 33 and 1.8 - 2.2 eV for p-type CoSb3 34). For all SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1) samples, both the neutral impurity concentration NN and the ionized impurity concentration NI are quite similar, proving that filling S has neglectable effect on the carrier mobility. However, the NN and NI values are greatly improved when the Pd doping content is enhanced from 0.20 to 0.25. The NI value for S0.05Pd0.25Co3.75Sb12 is 9.76 × 1020 cm-3, about two times of that for S0.05Pd0.2Co3.8Sb12. This can well explain the significantly deteriorated µH in S0.05Pd0.25Co3.75Sb12 shown in Figure 6b. The presence of large amount of neutral impurities and ionized impurities in the SyPdxCo4xSb12

samples results in lower Hall carrier mobilities than those in the electropositive elements

filled skutterudites under the same carrier concentration (see Figure 6c). For example, when the carrier concentration is 1.36 × 1020 cm-3, µH for S0.05Pd0.2Co3.8Sb12 is 38.4 cm2V-1s-1, about 39% lower than that for Ba-filled skutterudite.35 In addition, for a degenerate semiconductor with linear electronic dispersion, S is given by 36,37 S ∝ (r + 1.5)n-1/3

(6)

where r is defined as the carrier scattering relaxation time power law parameter. It is -0.5 for electron-phonon scattering, 0 for neutral impurity scattering, and 1.5 for ionized impurity scattering. Because of the introduced strong ionized impurity scattering and neutral impurity scattering to carriers, doping Pd at Co sites definitely increases r and then improve S. This can explain why these SyPd0.2Co3.8Sb12 samples possess larger S than those electropositive elements filled skutterudites under the same carrier concentration (see Figure 6d). In addition, the enhanced S nearly fully compensates the negative effect of µH reduction on the electrical transports. Thus, as shown in Figure 7, under the same carrier concentration, these SyPdxCo4-


18

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xSb12

samples possess similar power factors (PF = S2σ) with those single electropositive

elements filled CoSb3 skutterudites at 300 K. However, due to the relative low carrier concentrations for these SyPdxCo4-xSb12 samples, their PFs are still quite low as compared with those best electropositive elements filled skutterudites.

14,17,35,38

Higher PFs can be expected in

SyPdxCo4-xSb12 if their carrier concentrations are improved to the optimized value.

Table 2 Room temperature electrical conductivity (σ), Seebeck coefficient (S), electron carrier concentration (n), Hall mobility (µH), effective mass (m*), band deformation potential (Eac), ionized impurity concentration (NI), and neutral impurity concentration (NN) for several SyPdxCo4-xSb12 compounds.

Pd0.2Co3.8Sb12

S0.025Pd0.2Co3.8Sb12

S0.05Pd0.2Co3.8Sb12

S0.1Pd0.2Co3.8Sb12

S0.05Pd0.25Co3.75Sb12

1.25

1.10

0.85

0.59

0.98

S (µVK )

-177

-186

-189

-212

-192

n (1020 cm-3)

2.05

1.58

1.36

1.05

2.07

2

µH (cm V s )

38.1

43.5

38.4

38.0

26.2

m* (me)

3.80

3.47

3.23

3.31

3.71

σ (105 m-1) -1

-1 -1

Eac (eV)

2.17

2.25

2.53

2.47

2.67

-3

NN ( 10 cm )

2.26

1.77

1.95

1.87

3.69

NI (1020 cm-3)

3.11

3.93

4.46

4.81

9.76

19


19


Page 20 of 30

Figure 7． Power factors at 300 K as a function of room temperature carrier concentration for SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1, 0.15) and S0.05PdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3) samples. For comparison, the data for the electropositive elements filled skutterudites taken from Ref. 14, 35, 38 are also included. The solid line shows a trend for both the calculated and measured data.

3.3.2 Thermal transport properties Figure 8a and Figure 8b show the κL values as a function of the S filling fraction and Pd doping content at 300 K and 850 K, respectively. Significant κL reduction is observed with increasing the S filling fraction in SyPd0.2Co3.8Sb12. Meanwhile, when fixing the S filling fraction y = 0.05, the S0.05PdxCo4-xSb12 sample with a larger Pd doping content also shows a lower κL. However, the effect of Pd doping on the κL reduction is much weaker than that of S filling. Actually, the κL reduction caused by filling S is even stronger than the κL reduction caused by filling many


20

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electropositive elements. Figure 8c shows the calculated room temperature κL as a function of the filling fraction for different kinds of single filled CoSb3. At a given filling fraction, these S-filled skutterudites possess much lower κL than those alkali metal and alkaline-earth metal filled skutterudites, and comparable κL with the rare earth elements filled skutterudites. When the S filling fraction is 0.025, the room temperature κL is already significantly reduced to 3.4 Wm-1K-1, only 70% of that for Pd0.2Co3.8Sb12. When the S filling fraction is 0.1 in Pd doped samples, κL is lowered to 2.2 Wm-1K-1. For comparison, the room-temperature κL data of two SyCo4Sb12-xTex (x = 0.19, y = 0.52; x = 0.26, y = 0.73) samples reported in Ref. 22 are also included in Figure 8c. Due to the higher S filling fraction in these Te-included samples, κL is further lowered to 1.5 Wm-1K-1 at room temperature, which is almost the lowest value reported in skutterudites. Generally, in a solid, the phonons with low frequencies dominate the heat conduction. Filling the foreigner elements in the voids of CoSb3 can scatter the phonons with the frequencies close to the “rattling” frequencies of fillers. Thus, the fillers with lower “rattling” frequency are preferred for the κL reduction. The calculation suggested that the “rattling” frequency of S is about 35 - 50 cm-1, which is comparable with those of rare-earth elements (such as 42 cm-1 for Yb), but quite lower than those of alkali metal and alkaline-earth elements (such as 142 cm-1 for K and 94 cm-1 for Ba).

11

Hence, although the S filling fraction is small, they can still greatly interrupt the

normal transport of the low-frequency phonons. Combining the scattering from the Pd dopants on the high-frequency phonons (see Figure 8d), very low κL values are obtained in these SyPdxCo4-xSb12 samples.

In order to further illustrate the role of S on the thermal transports, we modeled the low temperature κL data of SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1) samples using the Callaway model 35,40

67 = %8 9 (

AB /C *; < ) D+ : ? / ℏ => ( < @,)

(7)

where + = ℏω/FG H is the reduced phonon frequency, kB is the Boltzmann constant, ω is the phonon frequency, ℏ is the reduced Planck constant, θD is the Debye temperature, v is the sound velocity, and τC is the phonon-scattering relaxation time. The phonon-scattering relaxation time τC can be expressed by 35,40


21


9

A

V

B % τ@, J = 7 + Aω + BN Hexp R− T + C (V @V )

Page 22 of 30

(8)

"

where L is the grain size, ω0 is the resonance frequency, and the coefficients A, B, and C are the fitting parameters. The terms on the right side of Eq.(8) represent grain boundary, point defect, phonon-phonon umklapp, and phonon resonant scatterings, respectively. θD and v values for CoSb3-based skutterudites are 287 K and 2700 ms-1, respectively. 35 As shown in Figure 9a, good fits are found in the whole temperature range for all samples (see the dashed lines in Figure 9a). The fitting parameters are shown in Table 3. The derived resonant frequency for S is about 43 47 cm-1, comparable with that calculated by Duan et al.

22

The derived grain sizes (~ 2 µm) are

also comparable with the experimental values determined by the SEM characterization (See Figure S1) . Filling S in the voids of CoSb3 influences the thermal transports in two aspects. On one hand, these S fillers strengthen the point defect scattering to the phonons by introducing additional mass fluctuations and stress field fluctuations in the voids. In the Callaway model shown above, the prefactor A can be used to evaluate the magnitude of the point defect scattering. Figure 9b presents the prefactor A as a function of y(1-y) for all SyPd0.2Co3.8Sb12 (y = 0.025, 0.05, 0.1) samples. A gradually increases with increasing y(1-y), suggesting that the point defect scattering is indeed strengthened in the sample with a higher S filling fraction. On the other hand, the S fillers introduce extra localized vibrational modes to strongly scatter the lattice phonons. Its magnitude can be evaluated by the prefactor C. As shown in Figure 9c, C increases from 3.02 × 1033 s-3 for S0.025Pd0.2Co3.8Sb12 to 6.83 × 1033 s-3 for S0.1Pd0.2Co3.8Sb12, proving that the magnitude of phonon resonance scattering is strengthened with increasing the S filling fraction. For comparison, we also fitted the low-temperature κL data of SyCo4Sb12-xTex (x = 0.19, y = 0.52; x = 0.26, y = 0.73) samples reported in Ref. 22. Because of the higher S filling fractions and Te doping content, these Te-doped samples possess larger A and C than those of SyPdxCo4xSb12

samples prepared in this study. Especially, the parameter C almost linearly increases with

increasing the S filling fraction, which is reasonable because the presence of more S fillers in the voids would introduce stronger resonant scattering to the phonons. Thus, if the S filling fraction in SyPdxCo4-xSb12 samples can be further enhanced, higher zT can be expected due to the possibly further lowered κL.


22

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Figure 8．． Lattice thermal conductivities at 300 K and 850 K as a function of (a) the S filling fraction in SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1) samples and (b) the Pd doping content in SyPd0.2Co3.8Sb12 (y = 0.025, 0.05, 0.1) samples. (c) Lattice thermal conductivity as a function of the filling fraction for different kinds of single filled CoSb3 at 300 K. (d) Schematic diagram of phonon scattering in SyPdxCo4-xSb12.


23


Page 24 of 30

Figure 9 (a) Low-temperature lattice thermal conductivity for SyPd0.2Co3.8Sb12 (y = 0, 0.025, 0.05, 0.1) samples. The solid lines are the fitting results by using the Callaway model. (b) Plot of prefactor A as a function of y(1-y) for SyPd0.2Co3.8Sb12 (y = 0.025, 0.05, 0.1) samples. (c) Plot of prefactor C as a function of y for SyPd0.2Co3.8Sb12 (y = 0.025, 0.05, 0.1) samples. The solid lines are guide to the eyes.

Table 3 Composition and the fitted parameters of L, A, B, and C for several SyPdxCo4-xSb12 samples. Data of CoSb3 are taken from Ref. 40. Data of SyCo4Sb12-xTex fitted by using the experimental thermal conductivities taken from Ref. 22. compositon

L (µm) A(10-43 s3) B(10-18 s K-1) C(1033s-3) ω0 (cm-1)

CoSb3 (fitted in Ref. 36)

5.80

2.59

5.38

0

-

Pd0.2Co3.8Sb12

1.29

65.3

1.51

0

-

S0.025Pd0.2Co3.8Sb12

1.55

95.1

1.57

3.02

46.29

S0.05Pd0.2Co3.8Sb12

1.42

109.0

1.61

4.54

46.95

S0.1Pd0.2Co3.8Sb12

1.93

141.0

1.70

6.83

43.96

S0.19Co4Sb11.42Te0.52

2.04

228.0

1.73

13.02

44.96

S0.26Co4Sb11.11Te0.73

1.99

401.1

1.72

18.11

43.62

4. CONCLUSIONS In summary, a series of n-type SyPdxCo4-xSb12 (x = 0.1, 0.15, 0.2, 0.25, 0.3, y = 0, 0.025, 0.05, 0.1, 0.15) compounds has been successfully fabricated. Due to the charge compensation effect, a maximal S filling fraction of 0.08 is obtained in SyPd0.2Co3.8Sb12 skutterudites. All samples possess the similar band gap around 0.3 eV. The carrier concentration is significantly decreased with increasing the S filling fraction, while it is increased with increasing the Pd doping content. The roles of S and Pd on the carrier mobility are discussed by using the Matthiessen’s rule. Different with the neglectable effect of filling S, doping Pd significantly deteriorates the carrier


24

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mobility by increasing the neutral impurity concentration and the ionized impurity concentration. Meanwhile, it is found that SyPdxCo4-xSb12 compounds possess low lattice thermal conductivities with those heavy rare earth elements filled skutterudites. Further analysis suggests that the low resonance frequency of S in the voids of CoSb3 is responsible for the low lattice thermal conductivities. The optimal zT value is 0.85 at 700K for the sample with nominal composition S0.05Pd0.25Co3.75Sb12.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Xun Shi: 0000-0001-6011-1210 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (NSFC) under the No. 11234012 and 51625205, the Shanghai Government (Grant No.15JC1400301 and 16XD1403900), and Youth Innovation Promotion Association, CAS (Grant No. 2016232).

ASSOCIATED CONTENT

Supporting Information Available: Figure S1 (a) SEM images of the fracture surface of S0.05Pd0.2Co3.8Sb12 (b) Magnification of the circle area of the SEM images of S0.05Pd0.2Co3.8Sb12 in Figure S1 (a).


25


Page 26 of 30

REFERENCES

(1) Shi, X.; Chen, L. Thermoelectric Materials Step Up. Nat. Mater. 2016, 15, 691-692. (2) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105114. (3) Zhu, T.; Liu, Y.; Fu, C.; Heremans, J. P.; Snyder, J. G.; Zhao, X. Compromise and Synergy in High-Efficiency

Thermoelectric

Materials.

Adv.

Mater.

2017,

29.

DOI:

10.1002/adma.201702816. (4) Yang, J.; Yip, H. L.; Jen, A. K. Y. Rational Design of Advanced Thermoelectric Materials. Adv. Energy Mater. 2013, 3, 549-565. (5) Slack, G. A. CRC Handbook of Thermoelectrics, CRC, 1995, 34, 4-15. (6) Sales, B. C.; Mandrus, D.; Williams, R. K., Filled Skutterudite Antimonides: A New Class of Thermoelectric Materials. Science 1996, 272, 1325-1328. (7) Shi, X.; Chen, L.; Uher, C. Recent Advances in High-Performance Bulk Thermoelectric Materials. Int. Mater. Rev. 2016, 61, 379-415. (8) Fleurial, J. P.; Caillat,T. A.; Borshchevsky and Ieee. Skutterudites: An Update, Proceedings ICT 97 - 16th International Conference on Thermoelectrics, 1997, 1-11. (9) Nolas, G. S.; Morelli, D. T.; Tritt, T. M., Skutterudites: A Phonon-Glass-Electron Crystal Approach to Advanced Thermoelectric Energy Conversion Applications. Ann. Rev. Mater. 1999, 29, 89-116. (10) Yang, J.; Zhang, W.; Bai, S. Q.; Mei, Z.; Chen, L. D. Dual-Frequency Resonant Phonon Scattering in BaxRyCo4Sb12 (R = La, Ce, and Sr). Appl. Phys. Lett. 2007, 90, 192111. (11) Nolas, G. S.; Kaeser, M.; Littleton, R. T.; Tritt, T. M. High Figure of Merit in Partially Filled Ytterbium Skutterudite Materials. Appl. Phys. Lett. 2000, 77, 1855-1857. (12) 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 n-type BaxYbyCo4Sb12 Double-Filled Skutterudites. Appl. Phys. Lett. 2008, 92, 182101. (13) Zhao, W.; Wei, P.; Zhang, Q.; Dong, C.; Liu, L.; Tang, X. Enhanced Thermoelectric Performance in Barium and Indium Double-Filled Skutterudite Bulk Materials via Orbital Hybridization Induced by Indium Filler. J. Am. Chem. Soc. 2009, 131, 3713-3720.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Shi, X.; Yang, J.; Salvador, J. R.; Chi, M.; Cho, J. Y.; Wang, H.; Bai, S.; Yang, J.; Zhang, W.; Chen, L. Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports. J. Am. Chem. Soc. 2011, 133, 7837-7846. (15) Rogl, G.; Grytsiv, A.; Rogl, P.; Peranio, N.; Bauer, E.; Zehetbauer, M.; Eibl, O. n-Type Skutterudites (R,Ba,Yb)yCo4Sb12 (R = Sr, La, Mm, DD, SrMm, SrDD) Approaching ZT Approximate to 2.0. Acta Mater. 2014, 63, 30-43. (16) Shi, X.; Zhang, W.; Chen, L. D.; Yang, J. Filling Fraction Limit for Intrinsic Voids in Crystals: Doping in Skutterudites. Phys. Rev. Lett. 2005, 95, 185503. (17) Zhou, X. Y.; Wang, G. W.; Guo, L. J.; Chi, H.; Wang, G. Y.; Zhang, Q. F.; Chen, C.Q.; Thompson, T.; Jeff Sakamoto; Dravid, V. P.; Cao, G. Z., and Ctirad Uher. Hierarchically Structured TiO2 for Ba-Filled Skutterudite with Enhanced Thermoelectric Performance. J. Mater. Chem. A, 2014, 2, 20629-20635. (18) Pei, Y. Z.; Chen, L. D.; Zhang, W.; Shi, X.; Bai, S. Q.; Zhao, X. Y.; Mei, Z. G.; Li, X. Y. Synthesis and Thermoelectric Properties of KyCo4Sb12. Appl. Phys. Lett. 2006, 89, 221107. (19) Qiu, Y.; Xi, L.; Shi, X.; Qiu, P.; Zhang, W.; Chen, L.; Salvador, J. R.; Cho, J. Y.; Yang, J.; Chien, Y.c.; Chen, S.w.; Tang, Y.; Snyder, G. J. Charge-Compensated Compound Defects in Ga-Containing Thermoelectric Skutterudites. Adv. Funct. Mater. 2013, 23, 3194-3203. (20) 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. (21) Qiu, Y.; Xing, J.; Gao, X.; Xi, L.; Shi, X.; Gu, H.; Chen, L. Electrical Properties and Microcosmic Study on Compound Defects in Ga-Containing Thermoelectric Skutterudites. J. Mater. Chem. A 2014, 2, 10952-10959. (22) 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, P.; Tang, X.; Zhang, Q.; Yang, J. Electronegative Guests in CoSb3. Energy Environ. Sci. 2016, 9, 2090-2098. (23) Wan, S.; Huang, X.; Qiu, P.; Shi, X.; Chen, L. Compound Defects and Thermoelectric Properties of Self-Charge Compensated Skutterudites SeyCo4Sb12-xSex. Appl. Mat. Interfaces 2017, 9, 22713-22724.


27


Page 28 of 30

(24) Anno, H.; Matsubara, K.; Notohara, Y.; Sakakibara, T.; Tashiro, H., Effects of Doping on the Transport Properties of CoSb3. J. Appl. Phys. 1999, 86, 3780-3786. (25) Peng, J. Y.; Yang, J. Y.; Zhang, T. J.; Song, X. L.; Chen, Y. H., Preparation and Characterization of Fe Substituted CoSb3 Skutterudite by Mechanical Alloying and Annealing. J. Alloys Compd. 2004, 381, 313-316. (26) Chen, B. X.; Xu, J. H.; Uher, C.; Morelli, D. T.; Meisner, G. P.; Fleurial, J. P.; Caillat, T.; Borshchevsky, A. Low-temperature Transport Properties of the Filled Skutterudites CeFe4xCoxSb12.

Phys. Rev. B 1997, 55, 1476-1480.

(27) Qiu, P. F.; Liu, R. H.; Yang, J.; Shi, X.; Huang, X. Y.; Zhang, W.; Chen, L. D.; Yang, J.; Singh, D. J. Thermoelectric Properties of Ni-doped CeFe4Sb12 Skutterudites. J. Appl. Phys. 2012, 111, 023705. (28) Wang, S.; Yang, J.; Wu, L.; Wei, P.; Zhang, W.; Yang, J. On Intensifying Carrier Impurity Scattering to Enhance Thermoelectric Performance in Cr-Doped CeγCo4Sb12. Adv. Funct. Mater. 2015, 25, 6660-6670. (29) Li, S.S. Semiconductor Physical Electronics, Springer, 2006, 211-223. (30) Cheng, N.; Liu, R.; Bai, S.; Shi, X.; Chen, L. Enhanced Thermoelectric Performance in Cd Doped CuInTe2 Compounds. J. Appl. Phys. 2014, 115, 163705. (31) Kliche, G.; Lutz, H. D. Temperature-Dependence of the FIR Reflection Spectra of the Skutterudites CoAs3 and CoSb3. Infrared Physics Infrared phys. Techn. 1984, 24, 171-177. (32) Dube, G. P. The Electrical Resistance of A Metal at Low Temperatures and Matthiessen's Rule. Proceedings of the Cambridge Philosophical Society 1938, 34, 559-567. (33) Kajikawa, Y. Refined Analysis of the Transport Properties of Co1-xNixSb3 According to A Model Including A Deep Donor Level and the Second Lowest Valleys of the Conduction Band. J. Alloys Compd. 2015, 621, 170-178. (34) Kajikawa, Y., Analysis of High-Temperature Thermoelectric {roperties of p-type CoSb3 within A Two-Valence-Band and Two-Conduction-Band Model. J. Appl. Phys. 2014, 115, 203716. (35) Bai, S. Q.; Pei, Y. Z.; Chen, L. D.; Zhang, W. Q.; Zhao, X. Y.; Yang, J., Enhanced Thermoelectric Performance of Dual-Element-Filled Skutterudites BaxCeyCo4Sb12. Acta Mater. 2009, 57, 3135-3139. (36) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.;


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Yamanaka, S.; Snyder, G. J. Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States. Science 2008, 321, 554-557. (37) Shi, X.; Yang, J.; Bai, S.; Yang, J.; Wang, H.; Chi, M.; Salvador, J. R.; Zhang, W.; Chen, L.; Wong Ng, W., On the Design of High-Efficiency Thermoelectric Clathrates through a Systematic Cross-Substitution of Framework Elements. Adv. Funct. Mater. 2010, 20, 755763. (38) Li, Y.; Qiu, P.; Xiong, Z.; Chen, J.; Nunna, R.; Shi, X.; Chen, L., Electrical and Thermal Transport Properties of YbxCo4Sb12 Filled Skutterudites with Ultrahigh Carrier Concentrations. AIP Adv. 2015, 5, 117239. (39) Qiu, P. F.; Yang, J.; Liu, R. H.; Shi, X.; Huang, X. Y.; Snyder, G. J.; Zhang, W.; Chen, L. D. High-Temperature Electrical and Thermal Transport Properties of Fully Filled Skutterudites RFe4Sb12 (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb). J. Appl. Phys. 2011, 109, 023705. (40) Nolas, G. S.; Fowler, G.; Yang, J. Assessing the Role of Filler Atoms on the Thermal Conductivity of Filled Skutterudites. J. Appl. Phys. 2006, 100, 043705.

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Synthesis and Thermoelectric Properties of Charge-Compensated

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