Te-doped CoSb3 skutterudites synthesized by high-pressure technique

Accepted Manuscript Structure and thermoelectric properties of Se- and Se/Te-doped CoSb3 skutterudites synthesized by high-pressure technique Jianying Dong, Kun Yang, Bo Xu, Long Zhang, Qian Zhang, Yongjun Tian PII:

S0925-8388(15)30070-0

DOI:

10.1016/j.jallcom.2015.05.171

Reference:

JALCOM 34293

To appear in:

Journal of Alloys and Compounds

Received Date: 16 April 2015 Revised Date:

12 May 2015

Accepted Date: 27 May 2015

Please cite this article as: J. Dong, K. Yang, B. Xu, L. Zhang, Q. Zhang, Y. Tian, Structure and thermoelectric properties of Se- and Se/Te-doped CoSb3 skutterudites synthesized by high-pressure technique, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.05.171. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structure and thermoelectric properties of Se- and Se/Te-doped CoSb3 skutterudites synthesized by high-pressure technique Jianying Dong, Kun Yang, Bo Xu, Long Zhang*, Qian Zhang, Yongjun Tian

Hebei 066004, China *

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State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao,

Author to whom correspondence should be addressed. Electronic mail: [email protected] (Long Zhang)

Abstract

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Se- and Te/Se-doped n-type CoSb3 skutterudites were synthesized by high-pressure synthesis (HPS) followed by spark plasma sintering. Se and Te substitutions on the Sb sites were verified by X-ray powder diffraction, energy

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dispersive spectrometry, and Raman spectroscopy. The experimental determined solubility of Se in CoSb3, was significantly enhanced by HPS technique, and reached a large value of 3%. Se doping shows a greater impact on the Seebeck coefficient than on the electrical resistivity because of the combined effects of the carrier transport performance, grain size, and pores. Meanwhile, the thermal conductivity was significantly suppressed after Se doping. The thermal conductivity of CoSb2.7Se0.3 is below 1.83 W/mK and is the lowest value for unfilled

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skutterudites to date. Extra doping of Te significantly increased the concentration and weighted mobility of the charge carrier, leading to an enhanced power factor. Moreover, the thermal conductivity was further reduced by Te co-doping due to the strong distortion of Sb4-ring in the framework of skutterudite. The highest ZT value of 1.29 is achieved at 780 K for CoSb2.8Te0.15Se0.05 with an appropriate Te/Se co-doping, which is the best ZT value

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for unfilled n-type skutterudites to the best of our knowledge. HPS is therefore a good choice to synthesize elemental filled and substituted (for Sb sites) skutterudites with increasing filling and doping levels for

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improving TE performance.

Keywords: Thermoelectric materials; Solid state reactions; High-pressure; Substitution; Skutterudite

ACCEPTED MANUSCRIPT 1. Introduction Thermoelectric (TE) materials have garnered significant attention for their ability to convert waste heat into electricity by the Seebeck effect or use direct current to cool through the Peltier effect. In addition, they do not need mechanical-parts and thereby have high reliability. TE properties are evaluated by the figure of merit ZT = S2T/(ρκ), where S, ρ, κ, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and

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temperature, respectively. A good TE material should have a large power factor (S2/ρ) arising from a high Seebeck coefficient and modest electrical resistivity, and low thermal conductivity. CoSb3-based skutterudites are a promising TE material used in waste heat recovery applications [1] because of their excellent TE properties

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[2-8] and mechanical robustness [9, 10]. The manipulable open structure and tunable electronic structure of CoSb3 make the feasibility to optimize TE performance by following the concept of phonon-glass electroncrystal proposed by Slack [11].

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Various approaches have been taken to improve the TE performance of CoSb3-based skutterudites by maintaining a large power factor while reducing the thermal conductivity. For example, filling atoms into the voids of skutterudites to scatter phonons [7, 12-18]; incorporating nanoparticles into matrix to scatter middleand high-frequency phonons [19-22]; and substituting the antimony 24g (Sb) sites with dopant atoms (IVA, VA, and VIA elements) to perturb the vibration of Sb4-ring and scatter high-frequency phonons [23-42]. In the case

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of Se substitution for Sb, the thermal conductivity was strongly suppressed because of (1) the mass and volume fluctuations caused by the presence of Se atoms in CoSb3 structure, (2) the scattering on grain boundaries of Serich secondary phase due to the limited solubility of Se in CoSb3 [24], and (3) preventing the growth of grain

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size [36]. Moreover, the high electrical resistivity of Se-doped CoSb3 can be optimized by TE co-doping [36, 38]. There have been several reports on Se and Te/Se doped CoSb3 [24, 36-38, 41, 42]. However, the total solubility

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of Se and Se-Te in CoSb3 is low [24, 36, 37], which may related to the large and non-hybridised s-like Se-DOS peak below the valence band unfavourable for chemical bonding, as revealed by a theoretical study [24]. ZT may be further improved if the doping level of Se and Te can be increased. High pressure synthesis (HPS) as a alternative technique can lower the reaction temperature and facilitate the synthesis of metastable phase by shifting the reaction equilibrium [43]. Our previous works highlighted high pressure as a fundamental thermodynamic variable in synthesizing elemental (e.g. Li [44], I [45], and Mg [46]) filled CoSb3, which may not be accessed under ambient pressure. Moreover, HPS may lead to larger Seebeck coefficient and increase power factor to some extent because the broadened energy band gap and electronic

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ACCEPTED MANUSCRIPT topological transition may be inherited from the high pressure after quenching the sample to ambient conditions [45, 47, 48]. The purpose of this work is to synthesize Se and Se-Te doped CoSb3 via HPS with increased dopant concentration, and to investigate how these dopants would affect the microstructure and TE performance. In this study, Se-doped n-type CoSb3-xSex (0≤x≤0.3) and Se-Te co-doped CoSb2.85-yTe0.15Sey (0≤y≤0.1) skutterudites

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were synthesized using HPS and their microstructures as well as high temperature TE transport properties were investigated. The Se and Te doping fractions for Sb 24g sites are successfully increased via HPS technique, leading to a significantly enhanced TE performance.

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2. Experimental procedures

Powders of high purity Co (99.8%), Sb (99.999%), Se (99.999%), and Te (99.999%) were mixed according to stoichiometric compositions CoSb3-xSex (x = 0.05, 0.1, 0.2, 0.3) and CoSb2.85-yTe0.15Sey (y = 0.025, 0.05, 0.1). The

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composition of CoSb2.85-yTe0.15Sey with y = 0.05 was prepared twice in order to confirm the reproducible process. The second one denotes as y = 0.05(R). The mixture was loaded into a steel mould, shaped with a cold press method, inserted into an h-BN crucible, and loaded into a high pressure apparatus for HPS. The first step of HPS was carried out at 1123 K and 5 GPa for 0.5 h. The resulting ingot was ground into powder under argon atmosphere, shaped with cold press, and loaded into the high pressure apparatus again for the second step of

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HPS (873 K and 5 GPa for 4 h). The obtained product was ground into powder and then sintered with spark plasma sintering (SPS, Dr. Sinter) at 873 K 50 MPa for 20 min into dense cylinder (φ10 mm × 10 mm), which was then cut into rectangular block (2 × 2 × 8 mm3) and disk (φ6 mm × 1 mm) for the electrical transport and

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thermal conductivity measurements, respectively. Note the samples with different shapes were cut in the way to ensure the measurements of these properties all along the direction perpendicular to SPS compression.

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X-ray diffraction (XRD) measurements were carried out with a Rigaku D/MAX/2500/PC (Cu Kα). Rietveld refinements were performed using the FULLPROF program to determine the lattice parameters [49]. Fracture images and actual compositions were taken with a Hitachi S-4800 II FESEM equipped with an energy dispersive spectrometry (EDS). Room temperature Raman scattering measurements were performed by using a Renishaw inVia system. Room temperature carrier concentrations for selected samples were calculated from Hall coefficient measurement (Physical Property Measurement System, Quantum Design). The electrical resistivity and Seebeck coefficient were measured with ZEM-3 (Ulvac-Riko) and the thermal conductivity was measured with TC-7000H (Ulvac-Riko). The uncertainty in each measurement is less than 5%. The densities were measured using the Archimedes method in ethanol, of which the repeatability was 99.5%.

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ACCEPTED MANUSCRIPT 3. Results and Discussion 3.1. Structure Characterizations Figure 1 shows XRD patterns for CoSb3-xSex and CoSb2.85-yTe0.15Sey samples after SPS densification at 873 K. The Bragg positions of skutterudites were marked. All the samples are primarily the skutterudite structure of Im3ത symmetry. For CoSb3-xSex, XRD profile (Fig. 1a) reveals single phase skutterudite for the sample where x =

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0.05. CoSbSe secondary phase is observed in other CoSb3-xSex samples and its quantity increases with increasing x. CoSbSe patterns are in agreement with the data available in PDF#39-0931, wherein the four highest peaks are at 2θ = 29.98, 33.49, 34.62, 46.08. A slight peak shift at 2θ = 46.08 to a lower angle is caused by the

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composition deviation. Another secondary phase of Co(Se0.46Sb0.54)2 rather than CoSbSe was determined from EDS for the sample where x = 0.3 (Fig. 2b). For CoSb2.85-yTe0.15Sey (Fig. 1b), pure skutterudite phase was observed for the samples where y = 0.025 and 0.05, whereas a weak impurity phase was observed for the sample

yTe0.15Sey.

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where y = 0.1. The insets of Fig. 1 show the magnified profiles at ~2θ = 70.5° for both CoSb3-xSex and CoSb2.85It is obvious that the diffraction peak gradually shifts to higher angle, indicating the contraction of the

unit cell, with increasing Se concentration. Such a contraction is expected because the atomic radius of Se is smaller than that of Sb. The trends are confirmed by the calculated lattice parameters (a) through Rietveld refinement listed in Table 1. a decreases monotonically with increasing Se concentration from 9.0322 Å to

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9.0230 Å for CoSb3-xSex and from 9.0524 Å to 9.0492 Å for CoSb2.85-yTe0.15Sey. The actual Se doping content for CoSb2.7Se0.3 is 0.12 (determined from EDS). It is noted that the lattice parameters of CoSb2.7Se0.3 and CoSb2.8Se0.2 are similar, indicating the Se doping level in CoSb3 may be saturated at 3% in this work, a much

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higher value than those reported previously [24, 36]. Also, the relative intensity of (211) diffraction peak (at 2θ = 24°) is independent of Se concentration, which is consistent with Wojciechowski’s conclusion and indicates Te

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and Se atoms are located at the crystal lattice sites (of Sb) rather than in the voids [50]. Figure 2 shows the back scattered SEM images for selected samples. It is clear that CoSb2.95Se0.05 (Fig. 2a) and CoSb2.8Te0.15Se0.05 (Fig. 2c) are homogeneous skutterudite phase, whereas the samples with higher Se content contains either plenty of Co(Sb0.46Se0.54)2 impurity phase (Fig. 2b, CoSb2.7Se0.3) or trace of CoSb1.94Te0.09 impurity phase (Fig. 2d, CoSb2.75Te0.15Se0.1). These results are consistent with XRD results. The chemical compositions determined from EDS (Table 1) confirm the successful substitution of Sb by Se and Te/Se. The doping levels of Se in our HPS specimens are significantly higher than those prepared at ambient pressure [36]. For example, the actual composition is CoSb2.825Te0.14Se0.0375 for the nominal CoSb2.8Te0.15Se0.05 sample in our study, while is CoSb2.832Te0.0675Se0.0175 for the nominal CoSb2.825Te0.15Se0.025 sample prepared at ambient pressure

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ACCEPTED MANUSCRIPT [36]. This comparison indicates HPS can effectively enhance substitutions for both Te and Se. The average grain size of CoSb2.9Se0.1 (Fig. 3a and b) and CoSb2.75Te0.15Se0.1 (Fig. 3c and d) is about 2 µm, which is smaller than that of pristine CoSb3 or Te-doped CoSb3 (about 5 µm) [36, 51]. The presence of Se can prevent the growth of grains and homogenize the grain size, which is in accord with previous reports on Te/Se doped CoSb3 [36, 41]. The pores seen in the SEM fracture images (Fig. 3) were generated by the vaporization of un-reacted Se, Te,

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and/or Sb. 3.2. Thermoelectric properties of CoSb3-xSex

The temperature dependent Seebeck coefficient (S), electrical resistivity (ρ), and power factor (S2/ρ) for

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CoSb3-xSex are shown in Fig. 4. The Seebeck coefficient (Fig. 4a) of all samples is negative over the measured temperature range, indicating electron-dominated transport. The absolute value of S increases with increasing temperature, reaches a maximum value (Smax) and then decreases due to the onset of intrinsic carrier activation. S

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monotonously decreases with increasing Se concentration. The magnitude of S for the sample where x = 0.05 falls rapidly at higher temperatures (> 500 K) after the onset of intrinsic carrier activation and is lower than that for x = 0.1 sample. Tmax corresponding to Smax shifts toward higher temperature with increasing Se concentration, indicating the electrons as a majority increases. The maximum S is -332 µV/K at 471 K for CoSb2.95Se0.05 and is

range of 0.31−0.34 eV.

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-258 µV/K at 593 K for CoSb2.7Se0.3. Energy gap (Eg = 2eSmaxTmax) [52] estimated from Smax and Tmax is in the

All samples show a negative temperature dependence of ρ (Fig. 4b), indicating a semiconductor transport behavior. ρ of CoSb3-xSex is larger than 5.8 mΩ cm. The difference of ρ for CoSb3-xSex with various x are within

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12% and the magnitude at higher temperature becomes smaller. The power factors (S2/ρ) calculated from S and ρ are shown in Fig. 4c. Because of the significant decrease of S and slight difference of ρ, the power factor

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decreases with increasing Se concentration. The monotonic trends for S and S2/ρ as a function of Se concentration for CoSb3-xSex at 600 K is obviously seen in Fig. 4d, whereas ρ is nearly Se-independent except the x = 0.05 one with a slightly reduced value. Total thermal conductivity κ and lattice thermal conductivity κl as a function of temperature for CoSb3-xSex are shown in Fig. 5a. κl (the inset of Fig. 5a) can be derived by subtracting electron contribution κe = LT/ρ from the total thermal conductivity, where L = 2.44×10-8 V2K-2 is a fully degenerate value of the Lorenz number and ρ is the electrical resistivity. The (lattice) thermal conductivity is intensively suppressed by Se doping. Comparing x = 0.3 with x = 0.05, the values of the former are 49% and 62% of the latter at low and high temperature, respectively. κ is below 1.83 W/m K for CoSb2.7Se0.3 over the measured temperature range. This is the lowest

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ACCEPTED MANUSCRIPT value for unfilled skutterudites to the best of our knowledge and is comparable to those filled skutterudites [8, 53]. κl for all samples initially decreases with temperature, reaches the minimum, and then increases with higher temperature. The temperature corresponding to the minimum κl shifts to higher value with increasing Se concentration, which is consistence with S mentioned above. The ZT values for CoSb3-xSex were calculated and shown in Fig. 5b. Although the thermal conductivity is very

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low, the poor electrical conductivity deteriorates figure of merit ZT. The highest ZT value of 0.42 is achieved at 640K for CoSb2.9Se0.1, which is higher than that of Sn-doped [34] but lower than that of Te-doped [54] CoSb3. Therefore, Te co-doping was carried out to optimize the electronic transport properties.

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3.3. Thermoelectric properties of CoSb2.85-yTe0.15Sey

Figure 6 shows the temperature dependent S, ρ, and S2/ρ for CoSb2.85-yTe0.15Sey. The Seebeck coefficient (Fig.

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6a) of all samples is negative over the measured temperature range in agreement with the Hall measurement that shows electron-dominated transport. The absolute value of S increases with increasing temperature. The magnitude of S increases dramatically with increasing y from 0.025 to 0.05, and decreases with higher y of 0.1. Such a variation is in consistent with the change of carrier concentration n (Table 1), the values of which are 4.34×1020 cm-3, -3.11×1020 cm-3, and -3.45×1020 cm-3 for y = 0.025, 0.05, and 0.1, respectively. S reaches the highest value of 238 µV/K at 760 K for CoSb2.8Te0.15Se0.05. The two y = 0.05 samples show nearly identical S

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over the measured temperature range. All samples show a positive temperature dependence of ρ (Fig. 6b), indicating a typical behavior of heavily doped semiconductors. ρ of the samples where y = 0.025 and 0.05 are close and much lower than y = 0.1, which is attributed to the combining effect of carrier concentration and

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mobility (Table 1). The power factor (Fig. 6c) of the sample where y = 0.05 is significantly enhanced because of the highest Seebeck coefficient and the lowest electrical resistivity achieved. The highest value of 4.34 mW mK-2 is obtained at 760 K for CoSb2.8Te0.15Se0.05. S, ρ, and S2/ρ versus Se concentration (y) are shown in Fig. 6d.

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The optimized composition achieves the best electrical transport properties.

κ and κl as a function of temperature are displayed in Fig. 7a for CoSb2.85-yTe0.15Sey. κ is almost temperature independent below 620 K, and slightly increases at higher temperature. κl initially decreases with temperature, reaches the minimum at around 620 K, and then increases with further increasing temperature. The (lattice) thermal conductivity for y = 0.025 and 0.05 are very close, and 20% (30%) larger than that for y = 0.1. The ZT values for CoSb2.85-yTe0.15Sey (y = 0.025, 0.05, 0.1) were calculated and shown in Fig. 7b. Owing to the superior S2/ρ and moderate κ, the highest ZT value of 1.29 is achieved at 780K for CoSb2.8Te0.15Se0.05. This is the best ZT value for unfilled n-type skutterudites to the best of our knowledge. Table 2 lists the compositions,

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ACCEPTED MANUSCRIPT synthesis technique, and maximum ZT of CoSb2.8Te0.15Se0.05 in comparison with those of Te/Se co-doped CoSb3 in literature. HPS demonstrates the benefits for increasing the doping level. The optimization of TE properties with appropriate Se-Te co-doping is achieved. 3.4. Discussion Substitution with Se on the Sb sites leads to changes of CoSb3 structure, such as the contraction of unit cell,

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pores, and reduced grain size. Moreover, the aliovalent substitution adds one electron to the system, thereby shifting the Fermi level towards the conduction band. TE transport properties are strongly associated with these factors. Generally the electron donor would promote n-type doping and reduce the Seebeck coefficient and

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electrical resistivity. However, the electrical resistivity of either CoSb3-xSex (Fig 4d) or CoSb2.85-yTe0.15Sey (Fig 6d) does not fully follow such a prediction, and shows a less dependence on charge carriers than the Seebeck coefficient does. We speculate this abnormality of the electrical resistivity is associated with the impurities,

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pores, and reduced grain size.

Assuming a simple single parabolic band model with acoustic phonon scattering as a predominant carrier scattering mechanism, which has been extensively accepted in CoSb3-based skutterudites, the Seebeck coefficient and carrier concentration can be related to each other through [24]:

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and

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S=−

where η is the reduced Fermi energy and Fj (η ) = ∫

∞

 2m*kBT  n = 4π   2  h 

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F1 2 (η ) ,

(1)

(2)

x j dx is a Fermi-Dirac integral of order j with x being the 1 + e x −η

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0

 kB  2 F1 (η ) − η  ,  e  F0 (η ) 

reduced carrier energy. The effective mass (m*) was estimated from the experimental data from Eq. (1) and Eq. (2), as listed in Table 1. The weighted mobility, U = µ(m*/me)3/2, is considered to be a primarily factor to determine the electronic performance of a TE material and has been used for selecting good TE compounds in the past [55]. Te/Se co-doped CoSb2.8Te0.15Se0.05 sample demonstrates a large weighted mobility (191 cm2V-1S-1), which is nearly twice that of CoSb2.825Te0.15Se0.025 (106 cm2V-1S-1) and three times that of CoSb2.75Te0.15Se0.1 (67 cm2V-1S-1). Compared with Se-doped CoSb2.9Se0.1 with a U of 38 cm2V-1S-1, Te/Se co-doped CoSb2.75Te0.15Se0.1 not only shows a much greater weighted mobility, but also possesses significantly enhanced carrier concentration, carrier mobility, and effective mass, suggesting Te substitution effectively modifies the band structure and

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ACCEPTED MANUSCRIPT flatten the bottom of the conduction band [54]. As a result, the power factor of Te/Se co-doped samples are dramatically enhanced compared with the corresponding Se-doped samples, as shown in Fig. 8a. These facts indicate that, by tuning Te/Se co-doping within a certain range, the electronic transport properties can be optimized. Similar results were previously observed in Te/Sn co-doped CoSb3 [30]. Based on the Debye-Callaway model, the phonon scattering process for an unfilled skutterudite involves

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grain-boundary, electron-phonon, phonon-phonon Umklapp, and point-defect scatterings [56]. The reduced grain size because of Se doping would increase the density of grain boundary, thus enhance the scattering of the lowfrequency phonons with a long free path and contribute to suppress κl. The electron-phonon interaction, which is

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closely related to the effective mass of charge carrier, may also play an important role to scatter low-frequency phonons, especially for a heavily doped CoSb3 with a relatively large m* [56]. The strong point-defect scattering due to Se doping is highly expected because of the significant mass, atomic radius, and electronegativity

xSex.

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difference between Se and Sb, which can be confirmed through analyzing the slope of the κl versus T for CoSb3Unlike grain-boundary and electron-phonon scatterings, which are not considered to affect the physical

trend at temperatures over Debye temperature (ca. 300 K), the point-defect scattering can suppress κl evidently at 300~600 [32]. The flattening of κl with respect to temperature (inset of Fig. 5a) becomes more obvious with

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increasing Se concentration, especially for the sample where x = 0.3. Therefore, the dominated phonon scattering mechanism changes from the phonon-phonon scattering to point-defect scattering with increasing Se concentration, indicating the important role of point-defect scattering on phonons in our samples. The upturn of

κl at high temperature can be attributed to the presence of electron-hole pairs excitated at high temperature [18].

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The temperature corresponding to the minimum κl shifts to higher temperature with increasing Se concentration, indicating a more severe degenerated semiconductor behaviour.

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Further suppression of κl by Te/Se co-doping is clearly demonstrated in Fig 8b by comparing CoSb2.8Te0.15Se0.05 and CoSb2.75Te0.15Se0.1 with CoSb2.95Se0.05 and CoSb2.9Se0.1. Extra Te doping augments the total doping level on Sb sites (see the EDS compositions listed in Table 1), induces more perturbations to the Sb4-ring in the framework of skutterudite, and scatters phonons more effectively than single Se-doping does. Note similar enhanced phonon scattering has been observed in Te/Ge co-doped CoSb3 previously [31]. The perturbation to Sb4-ring by Se and Te/Se substitutions was investigated by Raman spectroscopy. As shown in Fig. 9, the two Ag modes [57] located at 147 cm-1 from the longer Sb-Sb stretching vibration (Ag1) and at 179 cm-1 from the shorter Sb-Sb stretching vibration (Ag1) are broadened and flattened with increasing Se content for both CoSb3-xSex and CoSb2.85-yTe0.15Sey samples, indicating that Se substitution severely distorts the Sb4-rings, thereby

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ACCEPTED MANUSCRIPT lowering the symmetry of the ring. Also, extra doping of Te introduces greater perturbation to the ring structure of Sb, which is revealed by comparing Raman spectra from CoSb3-xSex and CoSb2.85-yTe0.15Sey with the same dose of Se.

4. Conclusions

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In this study, n-type CoSb3-xSex (x = 0.05, 0.1, 0.2, 0.3) and CoSb2.85-yTe0.15Sey (y = 0.025, 0.05, 0.1) skutterudites were synthesized with HPS method followed by SPS. HPS obviously increases the doping level compared with samples synthesized at ambient pressure. Se doping shows greater effect on the Seebeck

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coefficient than on the electrical resistivity due to the combined effects of the carrier transport performance, pores, and grain size. The thermal conductivity is significantly suppressed by Se doping, which goes below 1.83 W/m K for CoSb2.7Se0.3, the lowest value for unfilled skutterudites to the best of our knowledge and comparable

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to those filled skutterudites. Te/Se co-doping significantly increases the carrier concentration and weighted mobility, and further perturbs the stretching of Sb4-ring in CoSb3 framework, leading to a moderate Seebeck coefficient and lower electrical resistivity and thermal conductivity compared with Se single-doped samples. A high ZT value of 1.29 is achieved at 780K for CoSb2.8Te0.15Se0.05 through Te/Se co-doping, which records the

Acknowledgments

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highest ZT for unfilled n-type skutterudites to date.

This work was supported by the National Science Foundation of China (51201149, 51121061, and 51072175),

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the Natural Science Foundation for Distinguished Young Scholars of Hebei Province of China (E2014203150), the Key Basic Research Project of Hebei (14961013D), and the Research Fund for the Doctoral Program of

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Higher Education of China (20121333120007).

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ACCEPTED MANUSCRIPT Table 1 Nominal composition, EDS composition, room temperature lattice parameter (a), relative density (den.), Hall coefficient (RH), carrier concentration (n), carrier mobility (µ), effective mass (m*), and weighted mobility (U) for CoSb3-xSex and CoSb2.85-yTe0.15Sey.

(10 cm -

µ (cm2V-

m* (me) -

U (cm2V-

CoSb2.9Se0.1

-

9.0274(3) 97.2

-11.5

-0.54

8.75

2.67

38

CoSb2.8Se0.2

-

9.0228(3) 96.8

-

-

-

-

-

CoSb2.7Se0.3

CoSb2.875Se0.12

9.0230(3) 96.5

-

-

-

-

-

CoSb2.825Te0.15Se0.025

-

9.0524(2) 96.4

-1.45

-4.34

16.82

3.41

106

CoSb2.8Te0.15Se0.05

CoSb2.825Te0.14Se0.0375 9.0515(2) 97.7

-2.02

-3.11

26.28

3.76

191

CoSb2.75Te0.15Se0.1

CoSb2.85Te0.13Se0.065 9.0492(2) 95.9

-1.81

-3.45

11.53

3.24

67

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a (Å)

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RH (102 cm3c-

EDS composition

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CoSb2.975Se0.0425

Den. (%) 9.0322(2) 98.5

Nominal composition CoSb2.95Se0.05

Table 2 Compositions, synthesis technique, and maximum ZT of CoSb2.8Te0.15Se0.05 in this work in comparison with those of in literature. Synthesis

Measured composition

ZTmax.

Ref.

CoSb2.825Te0.15Se0.025

SA-SPS

CoSb2.832Te0.0675Se0.018

1.09

[36]

CoSb2.75Ge0.05Te0.175Se0.025

SA-SPS

-

0.99

[41]

Co0.975Ni0.025Sb2.875Te0.1Se0.025 SA-SPS

-

1.12

[42]

TE D

Nominal composition

SA-SPS

Co0.95Ni0.0475Sb2.8275Te0.0375Se0.04

0.83

[37]

CoSb2.8Te0.15Se0.05

HPS-SPS

CoSb2.825Te0.14Se0.0375

1.29

TW

EP

Co0.95Ni0.05Sb2.9Te0.05Se0.05

Abbreviation: SA, solid state reaction at ambient pressure; HPS, high pressure synthesis; SPS, spark plasma

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sintering; TW: this work.

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Figure 1. XRD profiles of CoSb3-xSex (x = 0.05, 0.1, 0.2, 0.3) (a) and CoSb2.85-yTe0.15Sey (y = 0.025, 0.05, 0.1) (b). The insets show the magnified (136) peak at about 2θ = 70.5° to display peak shift due to the contraction of the

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unit cell after Se doping.

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Figure 2. Back scattered images of the sample surface of CoSb2.95Se0.05 (a), CoSb2.7Se0.3 (b), CoSb2.8Te0.15Se0.05

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(c), and CoSb2.75Te0.15Se0.1 (d). A lot of Co(Sb0.46Se0.54)2 secondary phase can be obviously seen in CoSb2.7Se0.3

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sample (b), while the impurity in CoSb2.75Te0.15Se0.1 (d) is little.

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Figure 3. SEM images for CoSb2.9Se0.1 (a) and (b) and CoSb2.75Te0.15Se0.1 (c) and (d).

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(d), The Seebeck coefficient, electrical resistivity, and power factor as a function of x at 600 K.

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xSex.

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Figure 4. Temperature dependent Seebeck coefficient (a), electrical resistivity (b), and power factor (c) of CoSb3-

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Figure 5. Temperature dependent total thermal conductivity (a) and figure of merit ZT (b) of CoSb3-xSex. The

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inset to panel (a) plot temperature dependent lattice thermal conductivity.

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Figure 6. Temperature dependent Seebeck coefficient (a), electrical resistivity (b), and power factor (c) of

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CoSb2.85-yTe0.15Sey. (d), The Seebeck coefficient, electrical resistivity, and power factor as a function of y at 760

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Figure 7. Temperature dependent total thermal conductivity (a) and figure of merit ZT (b) of CoSb2.85-yTe0.15Sey.

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The inset to panel (a) plots temperature dependent lattice thermal conductivity.

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Figure 8. Influence of Te and Se doping on the power factor and lattice thermal conductivity as a function of

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temperature for selected samples.

Figure 9. Room temperature Raman spectra of selected CoSb3-xSex and CoSb2.85-yTe0.15Sey samples.

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ACCEPTED MANUSCRIPT Augmented Se and Te/Se doping levels were achieved via HPS technique.




Se doping reduced grain size and induced pores.




Se and Te/Se doping strongly distorted the Sb rings.




The lowest thermal conductivity among unfilled skutterudites was obtained.




ZT was significantly increased through Te/Se co-doping.

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