Synergistic Effect of Bismuth and Indium Codoping for High

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23337−23345

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Synergistic Effect of Bismuth and Indium Codoping 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*,†,‡

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†

Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart Materials, College of Physics, Chongqing University, Chongqing 400044, P. R. China ‡ Analytical and Testing Center of Chongqing University, Chongqing 401331, P. R. China § Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science, Chongqing 400714, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: In this work, a nonequilibrium melt spinning (MS) technology combined with hot pressing was adopted for rapid synthesizing of SnTe compounds in less than 1 h. The refined microstructure generated by MS significantly decreases 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 of ∼6.8 W/m K at room temperature and a 10% higher zT of ∼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 codoping 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 codoping 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 MS combined with appropriate doping could be an effective strategy to improve the thermoelectric performance of SnTe-related samples. KEYWORDS: SnTe, melt spinning, thermal conductivity, resonant level, thermoelectric

1. INTRODUCTION

Lead telluride (PbTe) has been well known as a TE material for its excellent performance at medium temperature.9−12 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 a 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 that of PbTe because the heavy-hole band is harder to be involved in electron transport under normal circumstances. Indeed, the room-

Thermoelectric (TE) devices can convert heat directly into electricity or vice versa1−4 in a solid state, which make them a potential technique to solve the energy and environmentrelated issues. The potential of a material for TE applications is governed by the TE 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 © 2019 American Chemical Society

Received: April 3, 2019 Accepted: June 7, 2019 Published: June 7, 2019 23337

DOI: 10.1021/acsami.9b05880 ACS Appl. Mater. Interfaces 2019, 11, 23337−23345

Research Article

ACS Applied Materials & Interfaces 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 because of 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 SnTe.18 Recent investigations indicate that the successful strategies for PbTe, such as carrier concentration optimization,19 valence band offset,20 and lattice thermal conductivity reductions,21 also work in the SnTe system. For example, the carrier concentration in SnTe can be optimized by doping on Sn sites with Bi,22 Sb,23 and Mg;24 the resonance state around the Fermi level can be introduced by In doping.25 Specifically, Cd,26 Mn,27 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 nanostructures.30 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 simultaneous realization of valence band convergence, nanostructuring, and substantial/interstitial defects.31 To date, most high TE performance of SnTe compounds was prepared by the conventional solid solution (SS) method. 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 a hierarchical microstructure by rapid quenching. Such method can create refined nanostructures and even amorphous phases because of the ultrahigh cooling rate in the MS process, yielding a highly reduced lattice thermal conductivity.32,33 Moreover, the preparation time is greatly shortened by adopting a MS route as compared with the traditional SS processing.34,35 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 MS to synthesize SnTe for the first time. Driven by this motivation, we make use of the MS and hot pressing technique to synthesize p-type SnTe compounds rapidly. In the MS process, different rotating speeds (RSs) of a copper wheel were used, which is a crucial parameter for material nanostructures. The results show that TE performance of the MS-Hot pressing samples is better than that of the conventional SS samples. In this paper, we demonstrate that the considerably enhanced TE performance can be realized in the 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 MS technology is chosen and the copper wheel rotating speed is optimized. Second, Bi is selected as an electron dopant to compensate the extremely high hole concentration and to obtain a significant enhancement of the Seebeck coefficient over a broad temperature range. Third, to enhance the Seebeck coefficient at room temperature, the resonant level is induced by In doping.25 Moreover, our results show a strong reduction in the total thermal conductivity of codoped SnTe by producing Bi nanoparticles formed in the grain boundary, improving the scattering of phonons without disrupting 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 the 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, and 0.07; y = 0.0025, 0.005, and 0.01) and then loaded into a graphite tube with 0.3 mm diameter nozzle for MS. In addition, Different copper wheel rotating line speeds (RSs) of 10, 15, and 20 m/s were used for undoped SnTe, and the optimal speed of 15 m/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 of the consolidated samples were higher than 98%. The bar specimen with dimensions of 8.5 mm × 2.5 mm × 2.5 mm was cut for electrical properties’ measurements and the disk specimen with 10.0 mm × 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 a focused ion beam technique with a lift-out 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 to 900 K, specific heat (Cp) was determined from theoretical values of the Dulong−Petit formula, and density (ρ) was estimated by the Archimedes method. The relative densities are higher than 98% (see Table S1) for all of 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 home-made Hall apparatus under a magnetic field of ±1 T. 2.3. Band Structure Calculation. Our first-principles computations based on density functional theory utilize the Vienna Ab initio Simulation Package (VASP),37 in which the projector augmented plane wave38,39 method was used for the ion electron interaction. The exchange−correlation functional was defined by a generalized gradient approximation 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 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 k-points scheme was adopted with 5 × 5 × 5 for the system. The density of states (DOS) were obtained using an energy cutoff of 500 eV. 23338


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ACS Applied Materials & Interfaces

3. RESULTS AND DISCUSSION 3.1. Optimizing TE Performance in Pristine SnTe by MS. The room-temperature XRD patterns for undoped SnTe with different copper wheel rotating line speeds are shown in Figure S1. All peaks can be indexed to a single phase with a cubic structure (space group Fm3m), indicating that all of the samples are single-phased. Figure 1 shows SEM images for the

enhanced phonon scattering. The microstructure of the fractured surface of undoped SnTe bulk samples after hot pressing is shown in Figure S4. The temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity of undoped SnTe samples with different RSs are shown in Figure 2. The electrical conductivity of all of the samples decreases with increasing temperature, indicating the behavior of a degenerated semiconductor. Recently, Wu et al. reported SnTe synthesized by the SS method, 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 hole concentration, yielding a high total thermal conductivity of ∼8 W/m K. In addition, 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 the 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 MS has larger Seebeck coefficients and lower electrical conductivities, resulting in remarkably enhanced PFs 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 reduces 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

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

free and contact surface of the ribbon with different rotating line speeds. As is shown, the free face of the ribbon consists of microsized grains in addition to minority nanosized grains, whereas no distinct grain structure is observed on the contact face. The energy-dispersive X-ray spectroscopy (EDXS) mapping data (Figure S2) indicate that Sn and Te are homogeneously distributed in our MS samples, and the line and area scan images (Figure S3) indicate that Sn is abundant in these spots at the middle of the grains. The formation of different microstructures on the two surfaces of the ribbon is attributed to the different cooling rates during quenching. In contrast to the other two samples with different RSs, the sample with a RS of 15 m/s shows the most remarkable grain refinement. The refined microstructure generated by MS can help reduce the grain size of hot-pressed samples and then contribute to the reduction of thermal conductivity through

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; and (d) zT. Data of the SnTe-SS sample from ref 42 are also listed for comparison. 23339


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

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

temperature dependence of electrical conductivity (σ), Seebeck coefficient (S), and PF for Sn1−xBixTe (x = 0, 0.01, 0.03, 0.05, and 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 the 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 the increasing Bi doping level, indicating that the negative effect on carrier mobility via alloying overwhelms the positive effect via

synthetic methods over the entire measurement temperature. Figure S5 shows that the MS sample has a high average figure of merit zTave and TE conversion efficiency, almost twice as much as the microwave solvothermal sample.41 In Figure 2, we also compared with the MS samples with a similar hole concentration reported by Ibrahim et al. and the maximum zT value at high temperature is comparable to our data. Thus, it is believed that the MS technology is superior to other synthesis methods for compounds with intrinsic high carrier concentration, such as SS melting,42 ball milling,43 and microwave solvothermal method.41 Hence, the optimal RS is 15 m/s, and all of 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 PF of SnTe, Bi was chosen as an electron dopant to reduce the extremely high hole concentration of ∼1.8 × 1020 cm−3 at room temperature because of the Sn vacancies.18 The room-temperature powder XRD patterns for all of the samples Sn1−xBixTe (x = 0, 0.03, 0.05, and 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 of the samples is shown in Figure 3b. It is clear that a increases with increasing the Bi doping level, which is reasonable because the ionic radius of Bi3+ (0.96 Å) is larger than that of Sn2+ (0.93 Å). The

Table 1. Room-Temperature Carrier Concentration and Hall Mobility of All Samples

23340

sample

carrier concentration (1019/cm3)

μ (cm2/V s)

SnTe Sn0.99Bi0.01Te Sn0.97Bi0.03Te Sn0.95Bi0.05Te Sn0.93Bi0.07Te Sn0.9675Bi0.03In0.0025Te Sn0.965Bi0.03In0.005Te Sn0.96Bi0.03In0.01Te

10.01 8.28 7.17 4.19 2.87 8.85 8.98 11.95

446 441 415 408 380 186 128 65


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

In addition, the mean acoustic velocity, va, could be given by47 Ä ÉÑ −1/3 ij 1 ÅÅÅ 1 2 ÑÑÑÑyzz Å j Å va = jjj ÅÅ 3 + 3 ÑÑzzz j 3 ÅÅÅ v l vt ÑÑÑÖz{ (3) k Ç

decreasing the hole concentration. On contrary to the behavior of the electrical conductivity, the room-temperature Seebeck coefficient increases significantly with increasing content of Bi from ∼20 to 82 μV/K for the Sn0.95Bi0.05Te, as shown in Figure 4b. The Seebeck coefficient of the doped SnTe samples is 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 PFs are enhanced as well relative to pristine SnTe. Among all of the samples, Sn0.97Bi0.03Te shows the maximum PF of 1.86 mW/m K2 at 790 K. Figure 5 displays the total thermal conductivity and lattice thermal conductivity for Sn1−xBixTe (x = 0, 0.01, 0.03, 0.05, and 0.07) samples. The lattice thermal conductivity is calculated by subtracting the electronic part from the 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, whereas the electron−hole bipolar effect is not considered in this Lorenz number calculation.44 As shown in Figure 5, the total thermal conductivity undergoes a significant decrease with increasing Bi doping level, that is, 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 the lattice thermal conductivity analysis in Sn1−xBixTe compounds. On the basis of the Callaway model, lattice thermal conductivity κlat of Bi substituted samples can be expressed as46 κlat tan−1(u) = u κlat0

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 Ä É1/3 h ÅÅ 3N ÑÑÑÑ ΘD = ÅÅÅÅ Ñ va κB ÅÅÇ 4πV ÑÑÑÖ (4) where N is the number of atoms in a unit cell and V is the unitcell 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

π ΘDΩ

κlat0 Γ

(6)

M̅ = M1x + M 2(1 − x)

(7)

1 1 M̅ + M3 2 2

(8)

M̿ =

r ̅ = r1x + r2(1 − x)

(9)

where M1, M2, and M3 are the atomic masses of Sn, Bi, and Te, respectively, r1 and r2 are the atomic radii of Sn and Bi, respectively, and x is the Bi content in one molecular. ε is the strain field factor, which can be estimated by the following equations45,50,51 2 ij 6.4γ(1 + vp) yzzz ε = jjjj z 9 j 1 − vp zz k {

(1)

(2)

where u, va, ΘD, Ω, h, and Γ are the scaling parameter, Debye temperature, the mean acoustic velocity, the average volume per atom, the Planck constant, and the imperfection scaling parameter, respectively.

γ=

hva

(5)

2

vp =

2

2 1 ij M̅ yz i r − r2 yz jjj zzz x(1 − x)εjjj 1 zz 2 k M̿ { k r̅ {

Γs =

2

u2 =

2 i M − M 2 yz 1 ij M̅ yz jjj zzz x(1 − x)jjj 1 zz 2 k M̿ { M̅ k { 2

Γm =

23341

2

(10)

1 − 2(vt /v l)2 2 − 2(vt /v l)2

3 jijj 1 − vp zyzz j z 2 jj 2 − 3vp zz k {

(11)

(12) DOI: 10.1021/acsami.9b05880 ACS Appl. Mater. Interfaces 2019, 11, 23337−23345

Research Article


shows small fluctuations as a function of In doping because In has ionic radii similar to Sn.52,53 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 is 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 because of the presence of Bi precipitation. The existence of two valence bands in SnTe demonstrates a unique Seebeck coefficient behavior as the variation of the carrier concentration. With increasing carrier concentration, there exists three different transport behaviors, specifically, light valence band, light valence band along with partial heavy valence band, and both light and heavy valence band.13 In addition, the Bi as electron dopants (counter-doping) reduces the hole concentration, resulting in the increase of the Seebeck coefficient. As shown in Figure 7d, the well-established 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 for the light band and a parabolic band for the heavy band.44 It is worth noting that In and Bi codoped SnTe presents significantly higher Seebeck coefficient at room temperature than that predicted by the Pisarenko plot, as shown in Figure 7d. The calculated DOS of pure SnTe and In-doped SnTe are shown in Figure S7. For Indoped SnTe, the samples show much higher Seebeck coefficient than that predicted by the Pisarenko relation, which was reported to arise from the resonant levels introduced by the In doping, as similar to that reported in Tl-doped PbTe.9 The Seebeck coefficient at 900 K of Sn0.9675Bi0.03In0.0025Te is enhanced as compared to that of the pristine SnTe, increasing from ∼142 to 180 μV/K. As a result, the PF of 2.01 mW/m K2 at 900 K is obtained as demonstrated in Figure 7c. For co-doped SnTe, the PF enhancement is especially noticeable below 750 K. Clearly, codoping 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 MS 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 the SnTe system based on the Cahill model.54 To investigate the mechanism for the low lattice thermal conductivity, the bulk sample with a nominal composition of

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 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 x x x x

= = = =

0.01 0.03 0.05 0.07

u

κlat,expt

κlat,calc

Γ

Γm

Γs

0.4060 0.6968 0.8911 1.0440

2.30 1.35 0.93 1.32

2.35 2.16 2.02 1.91

0.0049 0.0143 0.0234 0.0322

0.0026 0.0076 0.0123 0.0166

0.0031 0.0092 0.0152 0.0211

parameter (u), lattice thermal conductivity from experiment (κexpt) and calculation (κlat,calc), and imperfection scaling parameter (Γ) composed of mass fluctuation (Γm) and stain fluctuation (Γs) are obtained. The imperfection scaling parameters (Γ) increase as the 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 Uprocess 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 MS technology. The significantly enhanced PF and reduced thermal conductivity result in an improvement of zT value, as shown in Figure 5. The 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 to 900 K). 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 state.25 Inspired by this study, we doped In into Sn1−x−yBixInyTe (x = 0.03; y = 0.0025, 0.005, and 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 that 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, and 0.01)

Figure 6. (a) Powder XRD patterns for Sn1−x−yBixInyTe (x = 0.03; y = 0.0025, 0.005, and 0.01) samples prepared by MS-HP. (b) Lattice parameters for Sn1−x−yBixInyTe (x = 0.03; y = 0.0025, 0.005, and 0.01). 23342


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Figure 7. High-temperature thermoelectric transport properties of Sn1−x−yBixInyTe (x = 0.03; y = 0.0025, 0.005, and 0.01): (a) electrical conductivity; (b) Seebeck coefficient; and (c) PF. (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, and 0.01) samples.

Sn0.9675Bi0.03In0.0025Te is selected for microstructure analysis. Our TEM data as shown in Figure 9 indicate micrometerscaled grains along with some nanoparticles existed between

the grain boundaries. EDXS mapping indicates the nanosized particles are bismuth, and all of the other elements are homogeneously distributed in the sample. All of 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 value.55 The zT for Sn1−x−yBixInyTe samples is 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 sample.42 It is known that the zTave between 300 and 900 K shown in Figure 10 is more important in terms

Figure 9. TEM characterizations for the Sn0.9675Bi0.03In0.025Te sample: (a) HAADF image and (b−e) EDXS 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 technologies. 23343


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(3) Zhou, X.; Yan, Y.; Lu, X.; Zhu, H.; Han, X.; Chen, G.; Ren, Z. Routes for high-performance thermoelectric materials. Mater. Today 2018, 21, 974−988. (4) Chang, C.; Zhao, L.-D. Anharmoncity and low thermal conductivity in thermoelectrics. Mater. Today Phys. 2018, 4, 50−57. (5) Tritt, T. M.; Subramanian, M. A. Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View. MRS Bull. 2011, 31, 188−198. (6) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and old concepts in thermoelectric materials. Angew. Chem. 2009, 48, 8616− 8639. (7) Peng, K.; Lu, X.; Zhan, H.; Hui, S.; Tang, X.; Wang, G.; Dai, J.; Uher, C.; Wang, G.; Zhou, X. Broad temperature plateau for high ZTs in heavily doped p-type SnSe single crystals. Energy Environ. Sci. 2016, 9, 454−460. (8) Wu, H.; Lu, X.; Wang, G.; Peng, K.; Chi, H.; Zhang, B.; Chen, Y.; Li, C.; Yan, Y.; Guo, L.; Uher, C.; Zhou, X.; Han, X. SodiumDoped Tin Sulfide Single Crystal: A Nontoxic Earth-Abundant Material with High Thermoelectric Performance. Adv. Energy Mater. 2018, 8, 1800087. (9) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 2008, 321, 554−557. (10) Xiao, Y.; Zhao, L.-D. Charge and phonon transport in PbTebased thermoelectric materials. npj Quantum Mater. 2018, 3, 55. (11) Tan, G.; Zhang, X.; Hao, S.; Chi, H.; Bailey, T. P.; Su, X.; Uher, C.; Dravid, V. P.; Wolverton, C.; Kanatzidis, M. G. Enhanced Densityof-States Effective Mass and Strained Endotaxial Nanostructures in Sb-Doped Pb0.97Cd0.03Te Thermoelectric Alloys. ACS Appl. Mater. Interfaces 2019, 11, 9197−9204. (12) Tan, G.; Hao, S.; Cai, S.; Bailey, T. P.; Luo, Z.; Hadar, I.; Uher, C.; Dravid, V. P.; Wolverton, C.; Kanatzidis, M. G. All-Scale Hierarchically Structured p-Type PbSe Alloys with High Thermoelectric Performance Enabled by Improved Band Degeneracy. J. Am. Chem. Soc. 2019, 141, 4480−4486. (13) Zhao, L.-D.; Zhang, X.; Wu, H.; Tan, G.; Pei, Y.; Xiao, Y.; Chang, C.; Wu, D.; Chi, H.; Zheng, L.; Gong, S.; Uher, C.; He, J.; Kanatzidis, M. G. Enhanced Thermoelectric Properties in the Counter-Doped SnTe System with Strained Endotaxial SrTe. J. Am. Chem. Soc. 2016, 138, 2366−2373. (14) Moshwan, R.; Yang, L.; Zou, J.; Chen, Z.-G. Eco-Friendly SnTe Thermoelectric Materials: Progress and Future Challenges. Adv. Funct. Mater. 2017, 27, 1703278. (15) Moshwan, R.; Shi, X.-L.; Liu, W.-D.; Yang, L.; Wang, Y.; Hong, M.; Auchterlonie, G.; Zou, J.; Chen, Z.-G. High Thermoelectric Performance in Sintered Octahedron-Shaped Sn(CdIn)xTe1+2x Microcrystals. ACS Appl. Mater. Interfaces 2018, 10, 38944−38952. (16) Hong, M.; Chen, Z. G.; Pei, Y.; Yang, L.; Zou, J. Limit of zT enhancement in rocksalt structured chalcogenides by band convergence. Phys. Rev. B 2016, 94, 161201. (17) Rogers, L. M. Valence Band Structure of Snte. J. Phys. D: Appl. Phys. 1968, 1, 845. (18) Brebrick, R. F.; Strauss, A. J. Anomalous Thermoelectric Power as Evidence for Two-Valence Bands in SnTe. Phys. Rev. 1963, 131, 104−110. (19) Pei, Y.; Gibbs, Z. M.; Gloskovskii, A.; Balke, B.; Zeier, W. G.; Snyder, G. J. Optimum Carrier Concentration in n-Type PbTe Thermoelectrics. Adv. Energy Mater. 2014, 4, 1400486. (20) Zhang, Q.; Cao, F.; Liu, W.; Lukas, K.; Yu, B.; Chen, S.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z. Heavy Doping and Band Engineering by Potassium to Improve the Thermoelectric Figure of Merit in p-Type PbTe, PbSe, and PbTe1−ySey. J. Am. Chem. Soc. 2012, 134, 10031−10038. (21) Sootsman, J. R.; Pcionek, R. J.; Kong, H.; Uher, C.; Kanatzidis, M. G. Strong Reduction of Thermal Conductivity in Nanostructured PbTe Prepared by Matrix Encapsulation. Chem. Mater. 2006, 18, 4993−4995.

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 the Sn0.9675Bi0.03In0.0025Te sample can reach 9% with the temperature range from 300 to 900 K. Therefore, MS combined with hot pressing is a promising technology for synthesize highperformance p-type SnTe-based compounds.

4. CONCLUSIONS High-performance SnTe samples, codoped by Bi and In, were successfully synthesized within 1 h by the MS 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 the SnTe system was greatly reduced through point defects introduced by alloying and Bi nanoparticles/microscale grains obtained by the MS technique. As a result, the Sn0.9675Bi0.03In0.0025Te MS sample reaches a zT value up to ∼1.26 at 900 K and a zTave of 0.48 between 300 and 900 K, showing great potential application among lead-free TE materials. The rapid preparation processing coupled with the high TE performance makes this synthesis technique promising for commercial applications of the SnTe system compound.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05880.

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Density data, XRD image, SEM image, calculated image, and additional experimental data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.W.). *E-mail: [email protected] (Xiao Zhang). *E-mail: [email protected] (Xiaoyuan Zhou). ORCID

Xiaoyuan Zhou: 0000-0002-0930-1278 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by the National Natural Science Foundation of China (51772035, 11674040, and 51472036) and 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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