Ru Alloying Induced Enhanced Thermoelectric Performance in FeSi2


22 hours ago - In this study, we choose the Co-doped β-FeSi2 (Fe0.94Co0.06Si2), as the matrix and then prepare a series of Ru alloyed ...
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Functional Inorganic Materials and Devices

Ru Alloying Induced Enhanced Thermoelectric Performance in FeSi2-Based Compounds Xiaolong Du, Ping Hu, Tao Mao, Qingfeng Song, Pengfei Qiu, Xun Shi, and Lidong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10648 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Ru Alloying Induced Enhanced Thermoelectric Performance in FeSi2-Based Compounds Xiaolong Du,†,‡ Ping Hu,†,‡ Tao Mao,†,‡ Qingfeng Song,†,‡ Pengfei Qiu,⁎,† Xun Shi,⁎,† Lidong Chen† †State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

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

of Materials Science and Optoelectronics Engineering, University of Chinese Academy of

Sciences, Beijing 100049, China Email: [email protected]; [email protected]

Abstract β-FeSi2 has been considered as a promising material for thermoelectric applications, but its thermoelectric performance is greatly limited by the overhigh lattice thermal conductivity. In thermoelectrics, alloying element is an effective method to reduce the lattice thermal conductivity. In this study, we choose the Co-doped β-FeSi2 (Fe0.94Co0.06Si2), as the matrix and then prepare a series of Ru alloyed Fe0.94xRuxCo0.06Si2 (x

= 0, 0.005, 0.01, 0.02, and 0.05). X-ray characterizations show that all

samples crystallize in the β-FeSi2 structure. The elemental mappings detect an inhomogeneous Ru distribution in Fe0.89Ru0.05Co0.06Si2, which is attributed to the different Ru solution contents in ε-FeSi and α-FeSi2+δ before the formation of β-FeSi2 and the slow diffusion behavior of Ru during the annealing process. The Ru-alloying obviously reduces the lattice thermal conductivity via introducing the mass and strain field fluctuations to interrupt the phonon transports, while it has a weak effect on electrical transport properties. Finally, a maximum zT value of 0.33 at 900 K has been obtained for Fe0.89Ru0.05Co0.06Si2, which is about 27% higher than that for Fe0.94Co0.06Si2.

Keywords: thermoelectric, β-FeSi2, Ru alloying, element distribution, lattice thermal conductivity

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1. Introduction Thermoelectric (TE) technology has gained great attention in recent years because it can directly convert heat into electricity and vice versa. Due to the advantages of being free of moving parts, running quietly, and no greenhouse gasses emissions, TE technology shows the potential to be used in many applications such as waste heat harvesting, radioisotope TE power generation, and solid state Peltier refrigeration.1,2 The energy conversion efficiency of TE technology is governed by the material’s dimensionless TE figure of merit, zT = S2T/, where S, , T, and  are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. The  consists of the lattice thermal conductivity L and carrier thermal conductivity e. Good TE materials require both excellent electronic transport properties (often referred to high power factors PF = S2) and low L to realize high zT.3,4 In addition, the TE materials used for civil applications also require the features of economical and environment-friendly to realize the large-scale mass production. Investigation on semiconducting silicides is one of the hottest topics in TE community regarding the non-toxic and earth-abundant features of silicon.5 Typical semiconducting silicides are β-FeSi2,6,7 HMS (high manganese silicon),8,9 Mg2Si,10,11 and SiGe,12,13 etc.. Among them, β-FeSi2 is well-known by its strong oxidation resistance and good thermal stability, which allow it to work in atmospheric condition for a wide temperature range from 300 to 1200 K.14 β-FeSi2 crystallizes in orthogonal structure with the space group of Cmca.15 It has 48 atoms per Bravais unit cell with lattice constant a = 9.863 Å, b = 7.791 Å, and c = 7.833 Å. Both Fe and Si have two crystallographically inequivalent sites (FeI-8d, FeII-8f, SiI-16g, and SiII-16g), which are shown in Figure 1. Band structure calculation suggests that β-FeSi2 has an indirect band gap of 0.67-0.75 eV.16-18 Due to the intrinsically low  ( 10 -1 m-1 at 300 K) and high L ( 12 W m-1 K-1 at 300 K),19-27 the pristine β-FeSi2 shows a poor PF and low zT. Via doping external elements (e.g. Co,24, 28,29 Ni,20 Zr,25 Pt,23 P,30 B,31 Mn,22, 32 Cr,21 and Al33) to enhance the carrier concentration, the  can be enhanced by several orders of magnitude, leading to significantly improved PF and zT. For example, Tani

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et al. doped Pt into β-FeSi2 and obtained a maximum zT of 0.14 at 847 K.23 Chen et al. prepared Co-doped β-FeSi2, achieving a maximum zT of 0.25 at 960 K when the Codoping content is 0.06.28 Zhao et al. found that doping Mn in β-FeSi2 can turn the electrical transport properties into p-type conduction. A maximum zT of 0.17 at 873 K was obtained in Fe0.92Mn0.08Si2.32 However, the L values for these doped β-FeSi2 are still much higher than those for the state-of-the-art TE materials, which limit the further zT optimization of β-FeSi2. Making nanostructure or nanocomposite to strengthen the scattering to lowfrequency lattice phonons is a common strategy to reduce the L.34,35 Qu et al. prepared Fe0.98Co0.02Si2 with a fine grain size of about 300 nm by ball-milling methods. They found that the large amount of grain boundaries can significantly reduce the L to less than 4 W m-1 K-1 at 300 K, about half of that synthesized by traditional high-temperature solid state reaction.24, 29 In addition, the incorporation of nanosized second phases (e.g. oxide particles, SiGe particles, and Si particles) into β-FeSi2 also shows the ability to reduce L. Liu et al. achieved a uniform distribution of nano-SiGe particles in nanosized β-FeSi2 matrix by adopting a two-step sintering process. The L can be reduced by nearly 40% compared with the pristine β-FeSi2.36 In contrast to the scattering to lowfrequency lattice phonons, alloying the heavy element from the same family in the periodic table can introduce additional point defects to scatter the high-frequency lattice phonons, which has been widely used to reduce the L in many TE materials, such as half-Heusler alloys and Ⅳ-Ⅵ compounds.37-38 However, the related investigation in βFeSi2 system is very rare. The element Ru is in the same group with Fe in the periodic table. It has a heavier atomic mass and a larger atomic radius than Fe. Thus, it is expected that alloying Ru at the Fe-sites in β-FeSi2 can introduce huge mass and strain field fluctuations to strongly scatter high-frequency lattice phonons and to reduce L. In this study, we choose the well-optimized Co-doped β-FeSi2 (Fe0.94Co0.06Si2), as the matrix and then prepare a series of Ru-alloyed Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05) materials. It is found that alloying Ru at the Fe-sites scarcely modifies the electrical transport properties but it obviously reduces L. Finally, a maximum zT value of 0.33 at 900 K is ACS Paragon Plus Environment

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obtained in Fe0.89Ru0.05Co0.06Si2, which is about 27% higher than that for Fe0.94Co0.06Si2.

Figure 1. Crystal structure of β-FeSi2.

2. Experimental details High-purity elements, Fe (Alfa Aesar, 99.98%, granules), Ru (Alfa Aesar, 99.99%, powders), Co (Alfa Aesar, 99.99%, powders), and Si (Alfa Aesar, 99.98%, granules), were weighed out in the atomic ratio of Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). Ru and Co powders were pre-pressed into small pieces with the pressure of 10 MPa at room temperature in advance to prevent mass loss during melting process. The raw materials were arc-melted by three times in an argon atmosphere purified by melting titanium foam. The ingots were pulverized into fine powders using agate mortar and pestle. Then, the powders were sintered at 1223 K under a uniaxial pressure of 65 MPa for 10 minutes by Spark Plasma Sintering (Sumitomo, SPS-2040). The SPSsintered samples were sealed in silica tubes under vacuum and pre-annealed at 1423 K for 20 min, following with the annealing temperature reduced to 1173 K for another 48 h. Finally, the samples were naturally cooled to room temperature. The final bulks were about 10 mm in dimeter and 7 mm in height with high relative densities above 97%. In order to investigate the phase composition and microstructure evolution during the annealing process, the SPS-sintered samples were cut into several blocks with the dimension of 6×3×3 mm3 for the annealing experiments. After different annealing duration at assigned temperature, the blocks were quenched into ice water. Then, some blocks were pulverized into powders for the phase purity analysis while some of them were polished for the microstructure analysis.

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The X-ray diffraction analysis (XRD, Cu Kα,  = 1.5418 Å, Rigaku D/max 2550V) was performed with a step size of 0.02° in the 2θ range of 10-80°. Rietveld refinement was carried out using Fullprof software to obtain the lattice parameters. The sample morphologies and element distributions were determined by field emission scanning electron microscopy (FESEM, ZEISS Supra 55) equipped with energy dispersive Xray analysis (EDS, Oxford). The speeds of sound were obtained by using ultrasonic measurement system UMS-100 with shear wave transducers of 5 MHz and longitudinal wave transducers of 10 MHz. The Seebeck coefficient (S) and electrical resistivity (ρ) were simultaneously measured from room temperature to 900 K by the ordinary four probe DC method using ZEM-3 (Ulvac-Riko) under a sealed chamber with a small amount of helium gas. The thermal conductivity () at 300-900 K were calculated by using the relationship  = DCpd, where D was thermal diffusivity measured in an argon atmosphere by the laser flash method (LFA457, Netzsch). The heat capacity (Cp) was estimated by the Neumann-Kopp law according to the heat capacity of each component elements. The densities (d) were determined by the Archimedes method. The Hall coefficient (RH) were measured at room temperature by physical property measurement system (PPMS, Quantum Design). The Hall carrier concentration (nH) and Hall carrier mobility (μH) were estimated by nH = 1/RHe and μH = RHσ, respectively, where e is the elementary charge.

3. Results and discussion

Figure 2. (a) Room-temperature powder X-ray diffraction patterns of the assynthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). (b) The lattice

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parameters of as-synthesized Fe0.94-xRuxCo0.06Si2 as a function of Ru alloying content. The dashed lines are guides to the eyes.

Figure 2a shows the powder X-ray diffraction patterns of the as-synthesized Fe0.94xRuxCo0.06Si2

(x = 0, 0.005, 0.01, 0.02, and 0.05). All diffraction peaks can be well

indexed belonging to the orthorhombic structure of β-FeSi2 (Cmca, PDF#71-0642). The peak positions shift gradually to the lower angles with the increase of Ru content, indicating the expansion of the lattice. This is reasonable considering the larger atomic radius of Ru (1.34 Å) than that of Fe (1.26 Å).39 Figure 2b plots the lattice parameters obtained by the Rietveld refinement (Fullprof) performed on the powder X-ray diffraction pattern of Fe0.94-xRuxCo0.06Si2. The lattice parameters, a, b, and c, increase with increasing the Ru-alloying content, agreeing well with the Vegard’s law. This proves that Ru is successfully alloyed at the Fe-sites. The microstructures and EDS elemental mappings of the as-synthesized Fe0.93Ru0.01Co0.06Si2 and Fe0.89Ru0.05Co0.06Si2 are shown in Figure 3. All elements are homogeneously distributed inside the Fe0.93Ru0.01Co0.06Si2 matrix. No secondary phases are observed. But the case is different for Fe0.89Ru0.05Co0.06Si2, in which Co and Si are homogeneously distributed but Ru is not. It can be seen that Ru roughly has two different distribution regions. The average atomic percentage for the Ru at the Fe-sites in the bright area (Ru-rich region) is about 7.8%, while that in the gray area (Ru-poor region) is about 4.2%.

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Figure 3. Backscattered electron microscopy (BSE) images and elemental mappings for the as-synthesized (a) Fe0.93Ru0.01Co0.06Si2 and (b) Fe0.89Ru0.05Co0.06Si2.

The inhomogeneous distribution of Ru in Fe0.89Ru0.05Co0.06Si2 should be related with the different Ru solution contents in α-FeSi2+δ and ε-FeSi during the fabrication process of β-FeSi2. Figure 4a shows the microstructure of the SPS-sintered Fe0.89Ru0.05Co0.06Si2 without annealing. Because the sintering temperature (1223 K) is higher than the decomposition temperature of β-FeSi2 (1210 K), the sintered product exhibits a eutectoid structure composed of α-FeSi2+δ and ε-FeSi, corresponding to the gray and bright regions, respectively. Interestingly, Figure 4a shows that there are at least two different contrasts in each ε-FeSi and α-FeSi2+δ phase, indicating that the Ru distribution is inhomogeneous even in these two phases. However, after pre-annealing at 1423 K for just 5 minutes, the different contrasts (for example, the areas marked by the red arrows) in the ε-FeSi (bright regions) disappear (see Figure 4b). This suggests that the Ru distribution in ε-FeSi becomes homogenous. But the different contrasts (for example, the areas marked by the green arrows) in the α-FeSi2+δ (gray region) still exist. After annealing the sample for 10 minutes at 1423 K, the different contrasts in the αFeSi2+δ (gray region) disappear (see Figure 4c), indicating that the Ru distribution in αFeSi2+δ becomes homogenous. Thus, the Ru diffusion rate in ε-FeSi is faster than that in α-FeSi2+δ. With further prolonging the annealing duration to 20 minutes, the Ru distribution has no obvious change (see Figure 4d). Table 1 gives the actual chemical compositions characterized by the EDS analysis on the bright regions and gray regions in Figure 4d. The average atomic percentages of Ru at the Fe-sites in ε-FeSi and αFeSi2+δ are 7.5% and 4.3%, respectively. These values are comparable with those detected in Ru-rich and Ru-poor regions in as-synthesized β-FeSi2 shown in Figure 3b, which implies that Ru may diffuse very slow in the following solid reaction process at 1173 K.

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Figure 4. BSE images for the SPS-sintered Fe0.89Ru0.05Co0.06Si2 pre-annealed at 1423 K for (a) 0 min, (b) 5 min, (c) 10 min, and (d) 20 min. The red solid arrows point the Ru-rich and Ru-poor areas in ε-FeSi phase while the green solid arrows point the Rurich and Ru-poor areas in α-FeSi2+δ phase. Such Ru inhomogeneity inside the ε-FeSi or α-FeSi2+δ phase gradually disappears with prolonging the annealing duration. All characterizations were performed at the same region of the measured sample.

Table 1. The actual chemical compositions at the areas marked by the blue dashed lines in Figure 4d. The atomic percentage of Ru at the Fe-sites can be given by Ru/(Ru+Fe+Co).

To further illustrate the Ru diffusion behavior, we have characterized the evolution of the phase composition and microstructure on the same Fe0.89Ru0.05Co0.06Si2 sample re-annealed at 1173 K for different durations. According to the Fe-Si binary equilibrium phase diagram,40 the ingot after arc-melting should only consist of the α phase and ε

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phase (L→ α +ε). This is consistent with the X-ray diffraction pattern shown in Figure 5a for the ingot without annealing, in which only the peaks belonging to α phase and ε phase are detected. During the re-annealing process at 1173 K, β-FeSi2 forms via the solid reaction between ε-FeSi and α-FeSi2+δ. After annealing for 5 minutes, the X-ray diffraction peaks belonging to the β-FeSi2 start to appear. Their peak intensities increase with increasing the annealing duration. After annealing at 1173 K for 36 h, all the αFeSi2+δ and ε-FeSi phases are almost completely converted into the β-FeSi2. No diffraction peaks that belonging to the ε-FeSi and α-FeSi2+δ can be observed in the Xray diffraction pattern. Figure 5b-g shows the evolution of the microstructure corresponding to the X-ray diffraction patterns shown in Figure 5a. Interestingly, despite the significant change of phase composition, the Ru-rich regions in the final βFeSi2 (e.g. red dotted boxes in Figure 5g) are almost the same with those in the initial ε-FeSi phase (e.g. red dotted boxes in Figure 5b). This proves that Ru diffuses very slowly at 1173 K. The final distributions of Ru in Fe0.89Ru0.05Co0.06Si2 are mainly determined by the initial distributions of Ru in the respective α-FeSi2+δ and ε-FeSi.

Figure 5. (a) Room-temperature powder X-ray diffraction patterns of the SPS-sintered Fe0.89Ru0.05Co0.06Si2 re-annealed at 1173 K for different durations (0 min, 5 min, 45 min, 5 h, 15 h, and 36 h). BSE images for the SPS-sintered Fe0.89Ru0.05Co0.06Si2 re-annealed at 1173 K for (b) 0 min, (c) 5 min, (d) 45 min, (e) 5 h, (f) 15 h, and (g) 36 h, respectively. All the BSE images were obtained in the same area to facilitate the observation of the microstructure evolution.

Figure 6a and b show the temperature dependences of Seebeck coefficient (S) and electrical conductivity (σ) for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01,

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0.02, and 0.05). All samples exhibit negative S, indicating that electrons are the dominate carries. The pristine β-FeSi2 has very low σ in the order of 11 -1 m-1 at 300 K.24 Doping Co in FeSi2 can significantly enhance the σ by three orders of magnitude. Similar phenomenon has been also reported by Tani et al. and Kim et al..24, 27 Such σ enhancement should be attributed to the fact that Co has one more d electron than Fe. However, alloying Ru has weak influence on the electrical transport properties. All Ruincluded samples possess similar S and σ with Fe0.94Co0.06Si2. We have measured Hall data for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05) at room temperature. The calculated Hall carrier concentration (nH) and mobility (μH) are listed in Table 2. As expected, all Fe0.94-xRuxCo0.06Si2 samples have comparable nH and μH values. On one hand, alloying Ru in FeSi2 scarcely introduces extra carriers because Ru and Fe are in the same group in the periodic table. On the other hand, although Rualloying in Fe0.94Co0.06Si2 might introduce additional alloying scattering to interrupt the normal carrier transports, its effect should be weak considering the fact that the maximum Ru content in this work is only up to 0.05. Figure 6c shows the power factors (PFs) for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). The PF for Fe0.94Co0.06Si2 is 4.4 W cm-1 K-2 at 300 K and 12.1 W cm-1 K-2 at 900 K. Since alloying Ru scarcely modifies the S and σ, the Ru-alloyed samples show the similar PFs with the Fe0.94Co0.06Si2.

Figure 6. Temperature dependences of (a) electrical conductivity σ, (b) Seebeck coefficient S, (c) power factor PFs for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). Uncertainties for the measured σ and S are approximately ±5%.41

Table 2. Transport properties of the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005,

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0.01, 0.02, and 0.05) at room temperature.

Figure 7a displays the total thermal conductivity () as a function of temperature for the as-synthesized Fe0.94-xRuxCo0.06Si2. The  decreases with increasing temperature over the entire temperature range. Being different with the weak modification on S and σ, alloying Ru at the Fe-sites greatly reduces the . The  for Fe0.94Co0.06Si2 is 4.9 W m-1 K-1 at 300 K. It is reduced to 3.9 W m-1 K-1 for Fe0.89Ru0.05Co0.06Si2 at 300 K, about 21% reduction. The lattice thermal conductivity (L) of all samples are calculated by subtracting the electronic thermal conductivity (e) from the measured total . The e is estimated based on the Wiedeman-Franz law, e = LσT, where L is the Lorez number calculated using the empirical formula expressed by L = 1.5 + e-|S|/116.42 Figure 7b summarized the calculated L for all samples. The L is only slightly lower than the  for each sample, indicating that the  for the Fe0.94-xRuxCo0.06Si2 samples are dominated by the lattice part. Although doping Co in the pristine β-FeSi2 can significantly enhance the σ, the values are still much lower than those for the state-of-the-art TE materials such as Yb-filled skutterudite (YbxCo4Sb12),43 Cu-doped Bi0.5Sb1.5Te3 (CuxBi0.5Sb1.544

xTe3),

and Na-doped PbTe (Pb1-xNaxTe),45 of which the σ can reach up to 105 -1 m-1

at 300 K, one order of magnitude higher than the present Fe0.94-xRuxCo0.06Si2 samples. Thus, the Fe0.94-xRuxCo0.06Si2 samples still possess very low e values, as demonstrated in the inset in Figure 7b. In this way, the  reduction is mainly contributed by the decreased L with increasing the Ru-alloying content. This is consistent with our expectation that alloying the elements with heavy atomic mass and large atomic radius can introduce huge mass and strain field fluctuations to strengthen the scattering to phonons and reduce the L. ACS Paragon Plus Environment

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Figure 7. Temperature dependences of (a) total thermal conductivity  and (b) lattice thermal conductivity L for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). Uncertainty for the measured  is approximately ±5%.41 The inset in (b) demonstrates the temperature dependence of electronic thermal conductivity e. (c) The L values at 700 K as a function of the Ru content for Fe0.94-xRuxCo0.06Si2. The solid line is calculated based on Callaway model. (d) The mass fluctuation parameters (ΓM), strain fluctuation parameters (ΓS), and the total fluctuation parameters (Γ) as a function of the Ru content for Fe0.94-xRuxCo0.06Si2.

In order to further understand the role of element Ru on the thermal transports, the Callaway model is adopted by using Fe0.94Co0.06Si2 as the pure matrix.46 Assuming the dominated scattering mechanisms are Umklapp and point defect phonon scattering processes, the L of Ru-alloyed Fe0.94-xRuxCo0.06Si2 samples and that of the pure compound Fe0.94Co0.06Si2 (P L) above the Debye temperature (ϴD) can be expressed as: κ L tan -1 (u ) = κ Lp u

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(1)

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π 2θ D Ω p κL  2 hνavg

u2 =

(2)

where u represents the disorder scaling parameter,  is the average volume per atom, h, avg, and Γ correspond to the plank constant, the average sound velocity, and the experimental disorder scattering parameter, respectively. The scattering parameter Γ can be calculated by assuming Γ = ΓM+ΓS,47-48 where ΓM and ΓS are scattering parameter derived from mass and strain field fluctuation, respectively. ΓM and ΓS can be expressed as: 2

M   M1 -M 2   i=1 ci  Mi  fi1 fi 2  iM i  i     ΓM = n  i=1 ci

2

n

2

M   r1 - r 2   i=1 ci  Mi  fi1 fi 2εi  i r i     i  ΓS = n  i=1 ci

(3)

2

n

(4)

In general, there are different types of crystallographic sublattice in a structure. Thus, n represents the number of sublattice types and ci is the degeneracies of the respective sites. Assuming Co atoms construct an independent crystallographic sublattice, we can obtain n = 3, c1 = 0.94, c2 = 0.06, and c3 = 2 in Fe0.94Co0.06Si2. Besides, each sublattice might be occupied by several different kinds of atoms. The mass, radius, and fractional occupation of the kth atom in ith sublattice can be written as 𝑀𝑘𝑖, 𝑟𝑘𝑖, and 𝑓𝑘𝑖, respectively. In this way, the average atomic mass and radius on the ith sublattice can be expressed by 𝑀𝑖 = ∑𝑘𝑓𝑘𝑖𝑀𝑘𝑖 and 𝑟𝑖 = ∑𝑘𝑓𝑘𝑖𝑟𝑘𝑖. The average atomic mass of the compound can be calculated by 𝑀 = (∑𝑛𝑖= 1𝑐𝑖𝑀𝑖)/(∑𝑛𝑖= 1𝑐𝑖). 𝜀𝑖 is a fitting parameter. The scattering parameter Γ can be further written as: 2 2 2  ri1 - ri 2   1  M  x  0.94 - x   M i1 - M i2  =  i  + ε     i 3  M  0.94 0.94  M i   ri   

(5)

For the sample Fe0.94Co0.06Si2, the longitudinal sound velocity l is 7886 m s-1 while the shear sound velocity s is 4719 m s-1. Therefore, the average sound velocity

avg = 5222 m s-1 and ϴD = 669 K can be calculated via avg = [1/(33 l)+ 2/(33 s)]-1/3

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and ϴD = [(h/kB)(3N/4V)1/3avg], where kB is the Boltzmann constant, N is the atomic number per primitive unit cell and V is the unit cell volume. Figure 7c shows the predicated L at 700 K based on the Callaway model with 𝜀𝑖 = 198. The experimental

L data well fall on this calculated line, indicating that the mass and strain field fluctuations introduced by doping heavy element Ru is the primary reason for the reduced L. Likewise, this result also suggests that the phase boundaries between the Ru-rich and Ru-poor regions shown in Figure 3b would not introduce additional reduction on the L. The scattering parameters (ΓM and ΓS) as a function of Ru doping content are plotted in Figure 7d. Both ΓM and ΓS weigh large proportions in the Γ due to the significant differences in atomic mass and radius between the host Fe atoms and guest Ru atoms. Likewise, ΓS is larger than ΓM for each composition, which suggests that the strain fluctuation contributes more to the L reduction than the mass fluctuation in Fe0.94-xRuxCo0.06Si2. Figure 8 shows the temperature dependence of TE figure of merit zT for the assynthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). The Fe0.94Co0.06Si2 displays a maximum zT value of 0.26 at 900 K, which is comparable with those reported before.24,

28

Although alloying Ru in Fe0.94Co0.06Si2 scarcely modifies the PF, the

significant  reduction still leads to an obvious zT increment in the entire temperature range. The maximum zT reaches 0.33 at 900 K for Fe0.89Ru0.05Co0.06Si2, about 27% increment as compared with the Fe0.94Co0.06Si2. It should be noted that here only the high-frequency phonons are affected by alloying Ru. If the Fe0.89Ru0.05Co0.06Si2 is fabricated into the nanostructure to further interrupt the transport of low-frequency phonons, lower L would be expected to yield higher zT.

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Figure 8. Temperature dependence of TE figure of merit zT for the as-synthesized Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05).

4. Conclusions In summary, a series of Ru alloyed Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05) samples have been prepared and the effects of Ru alloying on the microstructure, phase composition, and thermoelectric performance have been systematically studied. Both the X-ray and EDS mapping confirm that Ru atoms can enter into the Fe sites without forming the impurity phase in the prepared samples, but the distribution of Ru is not homogeneous because Ru atoms scarcely diffuse during the annealing process at 1173 K. Alloying Ru at the Fe-sites scarcely modifies the electrical transport properties while it obviously reduces the L in the entire measured temperature range. Consequently, a maximum zT of 0.33 at 900 K for Fe0.89Ru0.05Co0.06Si2 is achieved, increased by 27% compared with the pristine Fe0.94Co0.06Si2.

Acknowledgements This work is supported by the National Key Research and Development Program of China (2018YFB0703600), the National Natural Science Foundation of China (51625205), the Key Research Program of Chinese Academy of Sciences (KFZD-SW-

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421), and Shanghai Rising-Star Program (19QA1410200). P.Q. thanks for the support by the Youth Innovation Promotion Association of CAS under Grant No. 2016232.

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