Synergistic effect of bismuth and indium co-doping for high

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Functional Inorganic Materials and Devices

Synergistic effect of bismuth and indium co-doping for high thermoelectric performance of melt spinning SnTe alloys Huan Tan, Lijie Guo, Guiwen Wang, Hong Wu, Xingchen Shen, Bin Zhang, Xu Lu, Guoyu Wang, Xiao Zhang, and Xiaoyuan Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05880 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 8, 2019

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ACS Applied Materials & Interfaces

Synergistic effect of bismuth and indium co-doping for high thermoelectric performance of melt spinning SnTe alloys Huan Tan,a Lijie Guo,a Guiwen Wang,b Hong Wu,a,c,d Xingchen Shen, a,c,d Bin Zhang, b Xu Lu,a Guoyu Wang,*c,d Xiao Zhang,*b Xiaoyuan Zhou,*a,b aChongqing

Key Laboratory of Soft Condensed Matter Physics and Smart Materials，

College of Physics, Chongqing University, Chongqing 400044, P. R. China bAnalytical

and Testing Center of Chongqing University, Chongqing 401331, P. R. China

cChongqing

Institute of Green and Intelligent Technology, Chinese Academy of Science,

Chongqing 400714, P. R. China d

University of Chinese Academy of Sciences, Beijing 100190, P. R. China

*Corresponding

author. [email protected]; [email protected];

[email protected]

Abstract In this work, a non-equilibrium melt spinning technology combined with hot pressing was adopted for rapid synthesize of SnTe compounds in less than 1 hour. The refined microstructure generated by melt spinning significantly decrease the lattice thermal conductivity. Compared to the pristine SnTe sample prepared by traditional melting and long-term annealing, the melt-spun one reveals a 15% lower thermal conductivity ~ 6.8 W/m K at room temperature and a 10% higher zT ~ 0.65 at 900 K. To further improve the electrical transport properties of the SnTe system, elements of Bi and In are introduced. It was found that Bi and In co-doping can enhance Seebeck coefficients in a broad temperature range via optimizing carrier density and introducing resonant states. Point defects and nanoparticles introduced by Bi and In co-doping remarkably enhanced phonon scattering and decreased lattice thermal conductivities. Finally, a significant enhancement on the thermoelectric performance was achieved: a peak zT of 1.26 at 900 K, and an average zT of ~ 0.48 over the temperature range of 300~900 K are obtained in Sn0.9675Bi0.03In0.0025Te. This work demonstrates that melt-spinning combined with appropriate doping could be an effective strategy

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to improve the thermoelectric performance of SnTe-related samples. Keywords: SnTe, Melt spinning, thermal conductivity, resonant level, thermoelectric

1. Introduction Thermoelectric (TE) devices can convert heat directly into electricity or vice versa1-4 in a solid state, which make it a potential technique to solve the energy and environment-related issues. The potential of a material for thermoelectric application is governed by the thermoelectric dimensionless figure of merit zT, defined as zT = S2σT/κtot, where S is the Seebeck coefficient, σ is the electrical conductivity, and κtot is the total thermal conductivity (κtot = κlat + κe, the lattice and electronic contribution, respectively)5. It is explicit that a combination of high power factor (PF, S2σ) and low thermal conductivity is desired for good TE materials. Considering the coupling effect among parameters via Pisarenko relationship and Wiedemann-Franz law, zT cannot be enhanced by optimizing a single factor individually 6-8. Lead telluride (PbTe) has been well known as a thermoelectric material for its excellent performance at medium temperature9-12. However, the toxicity of Pb limits its potential applications. SnTe13-16 has recently been recognized as a promising isomorphic alternative for PbTe in real industry applications. Similar to PbTe, SnTe with rock-salt structure also has two valance bands, but the energy offset ΔΣ between L and Σ valence bands in SnTe (0.3 eV) is much larger than that in PbTe (0.18 eV)17. The lager ΔΣ in SnTe means that its Seebeck coefficient should be lower than PbTe since the heavy-hole band is harder to be involved in electron transport under normal circumstances. Indeed, the room-temperature Seebeck coefficient of SnTe is about 40 μV/K, when the hole concentration is around 1020 to 1021 cm-3. Compared with PbTe, the lattice thermal conductivity of SnTe is much higher due to the lighter atomic mass of Sn. In fact, low electrical transport properties and high thermal conductivity result in a mediocre zT ~ 0.2 at 723 K for pristine SnTe18. Recent investigations indicate that the successful strategies for PbTe, such as carrier concentration optimization19, valence band offset20, and lattice thermal conductivity reductions21 also work in SnTe system. For example, the carrier concentration in SnTe can be optimized by doping on Sn sites with Bi22, Sb23 and Mg24; the resonance state around Fermi level can be


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introduced by In doping 25. Specifically, Cd26, Mn27 or Ca28 alloying on Sn sites could increase the band degeneracy and reduce the lattice thermal conductivity simultaneously. Generally, reduction on the lattice thermal conductivity can be achieved by additional phonon scattering through point defects29 and nanostructures30. Notably, with the synergistic effective approaches, a record-high zT of 1.8 for Sn0.75+δGe0.05Mn0.2Te(Cu2Te)0.05 (δ=0.08) was successfully obtained by the simultaneously realization of valence band convergence, nanostructuring, and substantial/interstitial defects31. To date, most high thermoelectric performance SnTe compounds were prepared by the conventional solid solution method (SS). Melt spinning (MS) is a technique used for the rapid solidification of liquids. In contrast to the conventional SS method, the MS technique could generate hierarchical microstructure by rapid quenching. Such method can create refined nanostructures and even amorphous phases due to the ultrahigh cooling rate in the melt spinning process, yielding a highly reduced lattice thermal conductivity32-33. Moreover, the preparation time is greatly shortened by adopting a MS route as compared with the traditional SS processing34-35. It thus seems that the introduction of the MS technology is an excellent approach when attempting to achieve nanostructured TE materials in a very short time. Ibrahim et al.

36

reported the successful use of

melt spinning to synthesize SnTe for the first time. Driven by this motivation, we make use of the melt spinning and hot pressing technique to synthesize p-type SnTe compounds rapidly. In the MS process, different rotating speed (RS) of copper wheel were used, which is a crucial parameter for material nanostructures. The results show that thermoelectric performance of the MS-HP samples are better than the conventional SS samples. In this paper, we demonstrate the considerably enhanced thermoelectric performance can be realized in melt-spinning-synthetized Sn1-x-yBixInyTe system via the simultaneous achievement of improvement in electrical transport properties and decrease in lattice thermal conductivity. First, to reduce the high thermal conductivity of SnTe, the melt spinning technology is chosen and the copper wheel rotating speed is optimized. Second, Bi is selected as electron dopant to compensate the extremely high hole concentration and obtain a significant enhancement of the Seebeck coefficient over a broad temperature range. Third, to enhance the Seebeck coefficient at room temperature, resonant level is induced by In doping25. Moreover, our results show a strong reduction in the total thermal conductivity of co-doped SnTe by producing Bi nanoparticles formed in the grain boundary, improving the scattering of phonons



without disruption the electrical conductivity. Ultimately, a maximum zT of ~1.26 at 900 K was achieved in Sn0.9675Bi0.03In0.025Te, a 160% enhancement compared with the pristine SnTe prepared by traditional SS method.

2. Experiment 2.1 Sample Synthesis Stoichiometric amounts of elemental Sn, Bi, In and Te with 99.999% purity were weighed and mixed for the desired compositions of Sn1-x-yBixInyTe (x=0, 0.01, 0.03, 0.05, 0.07, y=0.0025, 0.005, 0.01), then loaded into a graphite tube with 0.3 mm-diameter nozzle for MS. And Different copper wheel rotating line speeds (RS) of 10, 15, 20 m/s were used for undoped SnTe, and the optimal speed of 15m/s was used for doped SnTe samples. Under the protection of argon, the admixture of each composition was melted at 1173 K for 10 min and then injected onto a rotating copper roller. The resulting ribbons were pulverized and hot-pressed at 903 K for 30 min in a graphite die under a stress of 70 MPa pressure in vacuum. The relative densities of all the consolidated samples were higher than 98%. The bar specimen with dimensions of 8.5 mm x 2.5 mm x 2.5 mm was cut for electrical properties measurements and the disk specimen with 10.0 mm x 1.5 mm for thermal conductivity measurements.

2.2 Sample Characterization Powder X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert apparatus with Cu Kα radiation at the voltage of 40 kV and current of 40 mA. The morphology of the products was investigated by scanning electron microscopy (SEM). Transmission electron microscopy (TEM) characterizations were performed on a probe-corrected FEI Titan G2 microscope at 300 kV. Samples for TEM characterizations were prepared by focused ion beam (FIB) technique with a liftout method from the bulk sample. High temperature electrical conductivity and Seebeck coefficient were measured on rectangular shaped samples with a commercial system (LINSEIS, LSR-3) under the protection of Helium. Thermal conductivity (κ) was calculated via the equation κ = ρDCp, where thermal diffusivity (D) was obtained by the laser flash technique microflash LFA system (NETZSCH, LFA 457) from 300 K to 900 K, specific heat (Cp) was determined from theoretical values of Dulong-Petit formula, and density (ρ) was estimated by Archimedes method. The relative


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densities are higher than 98% (See Table S1) for all the samples investigated in this study. The errors in the measurements were about 5%, 5% and 3% for electrical conductivity, Seebeck coefficient and thermal conductivity, respectively. The room temperature carrier concentration was measured in a homemade Hall apparatus under a magnetic field of ±1T.

2.3 Band Structure Calculation Our first principles computations based on density functional theory (DFT) utilize the Vienna ab initio Simulation Package (VASP)37, in which the projector augmented plane wave (PAW)38-39 method was used for the ion electron interaction. The exchange-correlation functional was defined by a generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)40. 2 × 2 × 2 Supercells of SnTe with 64 atoms inside were built for the purpose of randomly and uniformly replacing of Sn atoms by In atoms. The convergent standard of the total energies was 1 × 10-6 eV. The geometry was relaxed until the forces on the atoms were less than 0.01 eV Å-1. For the Brillouin zone integrations, the Monkhorst–Pack (MP) k-points scheme was adopted with 5 × 5 × 5 for the system. The density of states (DOS) was obtained using an energy cutoff of 500eV.

3. Results and Discussion 3.1 Optimizing Thermoelectric Performance in Pristine SnTe by Melt Spinning The room temperature XRD patterns for undoped SnTe with different copper wheel rotating speeds are shown in Figure S1. All peaks can be indexed to a single phase with cubic structure (space group Fm3m), indicating that all the samples are single-phased. Figure 1 shows Scanning Electron Microscopy (SEM) images for the free and contact surface of the ribbon with different rotating speeds. As is shown, the free face of the ribbon consists of micro-sized grains in addition to minority nano-sized grains, whereas no distinct grain structure is observed on the contact face. The Energy-dispersive X-ray spectroscopy (EDS) mapping data (Figure S2) indicates Sn and Te are homogeneously distributed in our MS samples and the line and area scan images (Figure S3) indicate Sn is abundant in these spots at the middle of the grains. The formation of different microstructure on the two surfaces of the ribbon is attributed to the different cooling rates during quenching. Contrast to the other two samples with different RS, the sample with RS of 15 m/s shows the most remarkable grain refinement. The refined microstructure generated by melt spinning can



help reduce grain size of hot-pressed samples and then contribute to the reduction of thermal conductivity though enhanced phonon scattering. The microstructure of fractured surface of undoped SnTe bulk samples after hot pressing are shown in Figure S4. The temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity of undoped SnTe samples with different RS are shown in Figure 2. Electrical conductivity of all the samples decreases with increasing temperature, indicating the behavior of a degenerated semiconductor. Recently Wu et al. reported SnTe synthesized by solid solution method (SS), and the Seebeck coefficient S increases from ~7.4 μV/K at 300 K to ~150 μV/K at 870 K. The electrical conductivity at room temperature is ~7643 S/cm, which mainly comes from its intrinsically high holes concentration, yielding a high total thermal conductivity of ~8 W/m K. And Zhang et al. demonstrated SnTe samples prepared by melting reaction followed by ball milling, and all samples exhibit low Seebeck coefficients range from 20 to 40 μV/K at room temperature. Meanwhile, Wang et al. reported that SnTe can be prepared by microwave-stimulated solvothermal method. The Seebeck coefficient increases with increasing temperature, reaching to 133 μV/K at 823 K in undoped SnTe. In contrast to the three different preparation processes described above, our pure SnTe prepared by melt spinning have larger Seebeck coefficients and lower electrical conductivities, resulting in remarkably enhanced power factors over the wide temperature range of 300-880 K. The lower electrical conductivities are possibly due to the slight sublimation of Te that occurs before the molten compound is ejected through the nozzle, which compensates the Sn vacancies and finally reduce the hole concentration. Moreover, the MS samples show distinctly lower thermal conductivities than those of SS samples in the entire measured temperature range. The minimum total thermal conductivity (κmin = 2.3 W/m K) is found in the MS sample with 15 m/s rotating speed, and the main reason is the reduction of the electronic thermal conductivity attribution and the refined grain size. As shown in Figure 2, the maximum zT for pristine SnTe is ~0.65 at 880 K found in the MS sample with 15 m/s rotating speed, higher than other three different synthetic method over the entire measurement temperature. Figure S5 shows the MS sample has a high average figure of merit zTave and thermoelectric conversion efficiency, almost twice as much as the microwave solvothermal sample41. In Figure 2, we also compared with the melt spinning samples with a similar hole concentration reported by Ibrahim et al., and the maximum zT value at high temperature are comparable to our data. Thus, it is believed that the MS technology is superior to other synthesis


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methods for compounds with intrinsic high carrier concentration, such as solid solution melting42, ball milling43, microwave solvothermal method41. Hence, the optimal RS is 15 m/s, and all the doped SnTe sample in this work are melt spun with this value. 3.2 Enhancing Performance of SnTe through Bismuth Doping To optimize the power factor of SnTe, Bi was chosen as electron dopant to reduce the extremely high hole concentration of ~1.8x1020 cm-3 at room temperature due to the Sn vacancies18. The room temperature powder X-ray diffraction (XRD) patterns for all the samples Sn1-xBixTe (x=0, 0.03, 0.05, 0.07) are shown in Figure 3a. All diffraction peaks can be indexed to the cubic SnTe structure, and no secondary phase was observed within the instrument detection limit.

The

calculated lattice parameter a for all the samples are shown in Figure 3b. It is clear that a increases with increasing Bi doping level, which is reasonable since the ionic radius of Bi3+ (0.96Å) is larger than that of Sn2+ (0.93 Å ). The temperature dependence of electrical conductivity (σ), Seebeck coefficient (S) and power factor (PF) for Sn1-xBixTe (x=0, 0.01, 0.03, 0.05, 0.07) samples is plotted in Figure 4. The electrical conductivity of all Sn1-xBixTe samples decreases steadily with increasing temperature up to 880 K, which is a normal behavior of degenerated semiconductor. The electrical conductivity decreases with the increasing Bi doping level at the same temperature, which can be mainly attributed to the decreased hole concentration (as shown in Table 1 and Figure 4d). The carrier mobility also decreases with increasing Bi doping level, indicating that the negative effect on carrier mobility via alloying overwhelms the positive effect via decreasing of hole concentration. Contrary to the behavior of the electrical conductivity, the room temperature Seebeck coefficient increases significantly with increasing content of Bi from ~20 μV/K to 82 μV/K for the Sn0.95Bi0.05Te, as shown in Figure 4b. The Seebeck coefficient of the doped SnTe samples are significantly enhanced over the entire temperature compared with the pristine one, but the upward trend is weakened at temperatures higher than 800 K, which may be the sign of bipolar effect. Benefited from the increased Seebeck coefficients upon doping, the power factors are enhanced as well relative to pristine SnTe. Among all the samples, Sn0.97Bi0.03Te shows the maximum power factor of 1.86 mW/m K2 at 790 K.



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Table 1. Room temperature carrier concentration and hall mobility of all samples Sample

Carrier concentration (1019 /cm3)

μ (cm2/Vs)

SnTe

10.01

446

Sn0.99Bi0.01Te

8.28

441

Sn0.97Bi0.03Te

7.17

415

Sn0.95Bi0.05Te

4.19

408

Sn0.93Bi0.07Te

2.87

380

Sn0.9675Bi0.03In0.0025Te

8.85

186

Sn0.965Bi0.03In0.005Te

8.98

128

Sn0.96Bi0.03In0.01Te

11.95

65

Figure 5 displays the total thermal conductivity and lattice thermal conductivity for Sn1-xBixTe (x=0, 0.01, 0.03, 0.05, 0.07) samples. The lattice thermal conductivity is calculated by subtracting the electronic part from total thermal conductivity. The electronic thermal conductivity can be estimated using Wiedemann-Franz law κe = LTσ, where L is the Lorenz number (shown in Figure S6), σ is the electrical conductivity, and T is the absolute temperature. In this work, L was estimated from a two-band model, which only considers bipolar effects within the L and Σ valence bands while the electron-hole bipolar effect is not considered in this Lorenz number calculation44. As shown in Figure 5, the total thermal conductivity undergoes a significant decrease with increasing Bi doping level, i.e. decreases from 6.8 W/m K for undoped SnTe to 2.3 W/m K for Sn0.93Bi0.07Te at room temperature, which may be attributed to the simultaneous decrease in both electrical thermal conductivity and lattice thermal conductivity. With increasing temperature, the lattice thermal conductivities show the same trends as the thermal conductivities, experiencing a decline from 2.5 W/m K for undoped SnTe to 0.9 W/m K for Sn0.93Bi0.07Te. Bismuth doping should be responsible for this. To better understand the point defect scattering mechanism including the mass difference and strain difference between Bi and Sn, the Callaway model45 is employed for lattice thermal conductivity analysis in Sn1-xBixTe compounds. Based on the Callaway model, lattice thermal conductivity κlat of Bi substituted samples can be expressed as46

 lat tan 1 u    lat 0 u u2 

 2 D  hva2

 lat 0 


(1)

(2)

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Where u, va, ΘD, Ω, h, and Γ are scaling parameter, Debye temperature, the mean acoustic velocity, the average volume per atom, the Planck constant, and the imperfection scaling parameter, respectively. And the mean acoustic velocity, va, could be given by47

 1  1 2  va    3  3    3  vl vt  

1 / 3

(3)

Here, the longitudinal (vl, 3250 m/s) and transverse (vt, 1750 m/s) sound velocities can be obtained from previous reports for the pure SnTe. The Debye temperature ΘD can be obtained as48 1/ 3

h  3N  D  va  B  4V 

(4)

Where N is the number of atoms in a unit cell and V is the unit-cell volume. The imperfection scaling parameter Γ can be calculated as Γ = Γm + Γs, where Γm is the scattering parameter caused by mass fluctuations and Γs is the scattering parameter from strain field fluctuations. For Sn1-xBixTe compounds, Γm and Γs can be given by49

1M m   2M

2

  M  M2   x(1  x) 1   M  

1M s   2M

2

 r r   x(1  x)  1 2   r  

2

(5) 2

M  M 1 x  M 2 (1  x)

(6)

(7)

1 1 M  M3 2 2

(8)

r  r1 x  r2 (1  x)

(9)

M 

Where M1, M2 and M3 are the atomic mass of Sn, Bi and Te, respectively, r1 and r2 are the atomic radius of Sn and Bi, and x is the Bi content in one molecular. Ɛ is the strain field factor, which can be estimated by the following equations:45, 50-51

2  6.4（1  v p )      9 1 vp 

2


(10)


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2

v  1  2 t   vl  vp  2  vt  2  2   vl 

(11)

3  1 v 

p     2  2  3v p 

(12)

Where γ, vp, vl and vt are the Gruneisen parameter, Poisson ratio, longitudinal sound velocity and transverse sound velocity, respectively. As listed in Table 2, the scaling parameter (u), lattice thermal conductivity from experiment (κ,expt) and calculation (κlat,calc) and imperfection scaling parameter (Γ) composed by mass fluctuation (Γm) and stain fluctuation (Γs) are obtained. The imperfection scaling parameters (Γ) increase as Bi content increases, consistent with the decreased lattice thermal conductivity as shown in our study. Meanwhile, with the increase of the doping concentration, the calculated lattice thermal conductivity is much higher than the measured one. Because we only consider U-process phonon scattering and point defect phonon scattering, the difference between the calculated lattice thermal conductivity and the measured one should be attributed to extra grain boundary scattering and phonon-defect point scattering, which is introduced by the melt spinning technology. The significantly enhanced power factor and reduced thermal conductivity result in an improvement of zT value, as shown in Figure 5. Sn0.97Bi0.03Te sample shows the highest zT value of ~ 0.9 at 880 K and the average zT value is 0.38 (from 300 K to 900 K). Table 2. Scaling Parameter (u), Lattice Thermal Conductivity from Experiment (κlat,expt) and Calculation (κlat,calc), and Imperfection Scaling Parameter (Γ) of Sn1-xBixTe (x=0.00, 0.01, 0.03, 0.05 and 0.07) Compounds Sample

u

κlat,expt

κlat,calc

Г

Гm

Гs

x=0.01

0.4060

2.30

2.35

0.0049

0.0026

0.0031

x=0.03

0.6968

1.35

2.16

0.0143

0.0076

0.0092

x=0.05

0.8911

0.93

2.02

0.0234

0.0123

0.0152

x=0.07

1.0440

1.32

1.91

0.0322

0.0166

0.0211

3.3 Introducing Resonant Levels to Modify Band Structures. Zhang et al. reported that In doping can enhance the room temperature Seebeck coefficient of SnTe by introducing the resonant state25. Inspired by this study, we doped In into Sn1-x-yBixInyTe


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(x=0.03, y=0.0025, 0.005, 0.01) to introduce resonant level, thereby enhancing the Seebeck coefficient at room temperature. As demonstrated in Figure 6a, asterisk signifies the presence of a trace amount of impurity phase of Bi at 2θ = 27.5°in the XRD patterns, suggesting In doping may reduce the solid solubility of Bi. As demonstrated in Figure 6b, the calculated lattice parameter a for samples Sn0.97-yBi0.03InyTe (y=0.0025, 0.005, 0.01) shows small fluctuations as function of In doping because In has similar ionic radii as Sn52-53. As shown in Figure 7a, the electric conductivity decreases with increasing temperature and In doping level. The Seebeck coefficient as a function of temperature for co-doped Sn1-x-yBixInyTe are illustrated in Figure 7b, and all Seebeck coefficients are positive and consistent with the signs of the hall carrier concentrations (Table 1). The carrier concentration increases with increasing In doping level, mainly due to the presence of Bi precipitation. The existence of two valence bands in SnTe demonstrate a unique Seebeck coefficient behavior as the variation of the carrier concentration. With increasing carrier concentration, there exists three different transport behavior. Specifically, light valence band, light valence band along with partial heavy valence band, and both light and heavy valence band13. In addition, the Bi as electron dopants (counter-doping) reduces the holes concentration, resulting in the increase of the Seebeck coefficient. As shown in Figure 7d, the wellestablished Pisarenko relation shows that the calculated Seebeck coefficient matches with the experimental values roughly. The Seebeck coefficient is fitted by a two-band model, a Kane band (SKB) for the light band and a parabolic band (SPB) for the heavy band44, respectively. It is worth noting that In and Bi co-doped SnTe presents significantly higher Seebeck coefficient at room temperature than that predicted by the Pisarenko plot as shown in Figure 7d. The calculated density of states of pure SnTe and In-doped SnTe are shown in Figure S7. For In-doped SnTe, the samples show much higher Seebeck coefficient than predicted by the Pisarenko relation, which was reported to arise from the resonant levels introduced by the In doping, as similar as that reported in Tl doped PbTe9. The Seebeck coefficient at 900 K of Sn0.9675Bi0.03In0.0025Te is enhanced as compared to the pristine SnTe, increasing from ~142 to 180 μV/K. As a result, the power factor of 2.01 mW/m K2 at 900 K is obtained as demonstrated in Figure 7c. For co-doped SnTe, the power factor enhancement is especially noticeable below 750 K. Clearly, co-doping is more favorable to enhance the electrical properties in Sn1-x-yBixInyTe samples. The temperature dependent κtot, κlat for Sn1-x-yBixInyTe samples as plotted in Figure 8a, indicates



that In doping does not affect the heat-carrying phonon transport of SnTe melt spinning samples. The lowest κlat is ~0.8 W/m K at 900 K for the Sn0.9675Bi0.03In0.0025Te sample, which is still higher than the theoretical minimum lattice thermal conductivity of 0.5 W/m K for SnTe system based on the Cahill model54. To investigate the mechanism for the low lattice thermal conductivity, the bulk sample with a nominal composition of Sn0.9675Bi0.03In0.0025Te is selected for microstructure analysis. Our TEM data as shown in Fig. 9 indicates micrometer-scaled grains along with some nanoparticles existed between the grain boundary. Energy-dispersive X-ray spectroscopy (EDS) mapping indicates the nano-sized particles are Bismuth, and all the other elements are homogeneously distributed in the sample. All these microstructures contribute to the stronger scattering of phonons, leading to the reduction of thermal conductivities. In fact, there still exists some space to further decrease κlat to reach the theoretical minimum value55. The zT for Sn1-x-yBixInyTe samples are shown in Figure 8. The maximum zT reaches 1.26 at 900 K for the Sn0.9675Bi0.03In0.0025Te sample, which is 138% higher than the pristine SnTe SS sample42. It is known that the zTave between 300 K and 900 K shown in Figure 10 is more important in terms of the actual power generation applications. For the Sn0.9675Bi0.03In0.0025Te sample, zTave reaches 0.48, nearly 100% higher than the pristine SnTe MS sample. According to the above zTave value, the theoretical efficiency of Sn0.9675Bi0.03In0.0025Te sample can reach 9% with the temperature range from 300 to 900 K. Therefore, melt spinning combined with hot pressing is a promising technology for synthesize high performance p-type SnTe-based compounds.

4. Conclusion High-performance SnTe samples, co-doped by Bi and In, were successfully synthesized within 1 hour by melt spinning technique combined with hot pressing. It is found that the Seebeck coefficient is enhanced by optimizing the carrier concentration over the broad temperature range by Bi doping, and inducing resonant levels at room temperature by In doping. Meanwhile, the thermal conductivity of SnTe system was greatly reduced through point defects introduced by alloying and Bi nanoparticles/microscale grains obtained by melt spinning technique. As a result, the Sn0.9675Bi0.03In0.0025Te melt spinning sample reaches a zT value up to ~1.26 at 900 K and a zTave of 0.48 between 300 K and 900 K, showing great potential application among lead-free thermoelectric


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materials. The rapid preparation processing coupled with the high thermoelectric performance make this synthesis technique promising for commercial applications of SnTe system compound.

Supporting Information Density data, XRD image, SEM image, calculated image, and additional experiment data.

Notes There are no conflicts of interest to declare.

Acknowledgements This work was financially supported in part by the National Natural Science Foundation of China (51772035, 11674040, 51472036), the Fundamental Research Funds for the Central Universities (106112017CDJQJ308821 and 2018CDYJSY0055). This work was also financially supported by Key Research Program of Frontier Sciences, CAS, Grant No. QYZDB-SSW-SLH016, the Project for Fundamental and Frontier Research in Chongqing (CSTC2017JCYJAX0388).

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Figure captions Figure 1. SEM images of (a, b, c) the free face and (d, e, f) contact face of MS-Ribbon for SnTe sample with the different rotating speeds (RS), which are represented by the units m/s.

Figure 2. High temperature thermoelectric transport properties of SnTe samples with different rotating speeds: (a) Electrical conductivity; (b) Seebeck coefficient; (c) Total thermal conductivity and lattice thermal conductivity; (d) zT. Data of SnTe-SS sample from Ref. 42 are also listed for comparison.


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Figure 3. (a) Powder X-ray diffraction patterns for bulk Sn1-xBixTe (x = 0, 0.01, 0.03, 0.05, 0.07) samples prepared by MS-HP. (b) Lattice parameters for Sn1-xBixTe (x = 0, 0.01, 0.03, 0.05, 0.07).

Figure 4. High temperature thermoelectric transport properties of Sn1-xBixTe (x = 0, 0.01, 0.03, 0.05, 0.07): (a) Electrical conductivity; (b) Seebeck coefficient; (c)Power factor; (d)The carrier concentration and carrier mobility.



Figure 5. Temperature dependence of (a) thermal conductivity and (b) zT for Sn1-xBixTe (x = 0, 0.01, 0.03, 0.05, 0.07).

Figure 6. (a) Powder X-ray diffraction patterns for Sn1-x-yBixInyTe (x=0.03, y = 0.0025, 0.005, 0.01) samples prepared by MS-HP. (b) Lattice parameters for Sn1-x-yBixInyTe (x = 0.03, y = 0.0025, 0.005, 0.01).


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Figure 7. High temperature thermoelectric transport properties of Sn1-x-yBixInyTe (x = 0.03, y = 0.0025, 0.005, 0.01): (a) Electrical conductivity; (b) Seebeck coefficient; (c)Power factor. (d) Carrier concentration dependence of Seebeck coefficient for SnTe samples at 300 K.

Figure 8. Temperature dependence of (a) thermal conductivity and (b) zT for Sn1-xyBixInyTe

(x = 0.03, y = 0.0025, 0.005, 0.01) samples.



Figure 9. TEM characterizations for the Sn0.9675Bi0.03In0.025Te sample: (a) HAADF image and (b-e) EDS maps of different elements: (b) Bi blue, (c) In red, (d) Sn green, and (e) Te yellow.

Figure 10. Average zT value and thermoelectric conversion efficiency for SnTe samples with different processing technology.


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Synergistic effect of bismuth and indium co-doping for high

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