Facile Route to High-Performance SnTe-Based Thermoelectric

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Facile Route to High-Performance SnTe-Based Thermoelectric Materials: Synergistic Regulation of Electrical and Thermal Transport by In Situ Chemical Reactions Zhiwei Zhou,† Junyou Yang,*,† Qinghui Jiang,† Jiwu Xin,† Sihui Li,† Xiaochun Wang,† Xuesong Lin,† Ruisi Chen,† Abudl Basit,† and Qi Chen‡ State Key Laboratory of Materials Processing and Die & Mould Technology and ‡Wuhan National High Magnetic Field Center, Huazhong University of Science & Technology, Wuhan 430074, P. R China

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†

S Supporting Information *

ABSTRACT: In this work, a composite product of Mn-substituted SnTe, SnO2 nanoparticles, and MnTe-supersaturated precipitates has been fabricated by a simple in situ reaction between SnTe and MnO2 for the first time. Benefiting from the synergistic effect induced by the product of in situ reaction, a remarkable improvement in the thermoelectric performance has been achieved. On the one hand, Mn substitution in SnTe can effectively modify the band structure and enhance the electrical properties of SnTe; on the other hand, the thermal transport can also be dramatically suppressed by in situ reaction-derived multiscale phonon scattering by point defects, SnO2 nanoparticles, and supersaturated MnTe precipitates. Ultimately, a maximum ZT of ∼1.5 at 873 K has been achieved in the SnTe + 10 mol % MnO2 sample, which increases by 224% in comparison with the pristine SnTe, representing one of the best results ever reported for SnTe-based thermoelectric materials. bipolar conduction.8 In addition, the relatively high thermal conductivity also deteriorates its TE performance. Aiming at the excessive hole concentration, donor doping in intrinsic SnTe reported a ZT of 1.1 at 873 K by Zhou et al.9 The Seebeck coefficient can be enhanced through converging the light-hole band and heavy-hole band by some specific cation substitution (e.g., Cd,10 Hg,11 Mg,12 and Ca13). Low thermal conductivity can be achieved through the design of either interface structures at nanometer or mesoscopic length scales or by multiscale hierarchical architectures; therefore, some extrinsic secondary phases (e.g., CdS,10 Cu2Te,14 AgBiTe2,15 and SrTe16) have also been added to reduce the lattice thermal conductivity of SnTe and promote the TE performance of SnTe. By incorporating the individual substitution effect of Mn with the interstitial and precipitating effect of Cu in SnTe, Pei et al., reported a remarkable ZT improvement in Sn0.86Mn0.14Te(Cu2Te)0.05.14 Considering the interrelationship between σ, S and κ, it is very challenging to regulate one of the three parameters independently or sequentially for the sake of high ZT; hence, synergistic strategies shall be preferable and likely more efficient. Based on this scenario, a simple and unique in situ reaction strategy has been designed to tune the TE performance of SnTe for the first time. By the introduction of MnO2, an in situ chemical reaction occurs between the MnO2 additives and

1. INTRODUCTION Thermoelectric (TE) materials, which are very promising in harvesting thermal energy directly from car exhaust or other low-grade industrial waste heat, have been extensively investigated in recent decades.1,2 The main obstacle preventing TE power generation from large scale applications is the low conversion efficiency, which is closely related to the dimensionless figure of merit ZT of TE materials, given by ZT = σS2T/κ, where σ, S, and κ are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively.3,4 Because most of the waste heat in industry falls within the middle temperature range, TE materials with latent application in medium temperature are particularly attractive and have been widely studied in recent years.5 Among them, PbTe is one of the few TE materials that has been used in various applications due to its excellent TE performance, with ZT values steadily climbing to over 2, also in its derivatives.6 Unfortunately, it has been haunted by the growing environmental concern related to the high content of the ecoharmful Pb component. Another attractive TE compound, SnTe, has the same rocksalt crystal structure and similar band structure as PbTe and shows great potential for power generation in a similar temperature range.7 Currently, the TE performance of SnTe is still quite inferior compared to PbTe because of its intrinsic ultrahigh hole concentration (1020 to 1021 cm−3) and narrow band gap (0.18 eV at 300 K) with large energy separation (0.35 eV at 300 K) between the light-hole band and heavy-hole band, thus resulting in a low Seebeck coefficient and severe © XXXX American Chemical Society

Received: February 21, 2019 Revised: April 25, 2019 Published: April 27, 2019 A

DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. (a) Normal and (b) magnified XRD patterns of SnTe + x mol % MnO2 samples (x = 0, 4, 6, 8, 10, 12), (c) DSC curves of the original powder mixtures of SnTe + 10 mol % MnO2 and the hot-pressed SnTe + 10 mol % MnO2 sample. (d) In situ XRD patterns of the SnTe + 10 mol % MnO2 powder mixture at different temperatures. a measurement error on S and ρ of 5 and 2%, respectively. The Hall coefficient (RH) was measured with a HMS-5500 Hall measurement system. The thermal conductivity (κ) was calculated with the equation κ = DdCP, where the thermal diffusivity (D) was obtained from a NETZSCH LFA-427 Laser thermal conductivity instrument, with a measurement error of 2%. The density (d) was measured by the Archimedes method, and the heat capacity (CP) was derived from the reference data (Figure S3).17 2.4. Computational Details. Density functional theory (DFT) calculations of the pristine, stoichiometric SnTe, and Sn1−xMnxTealloyed were performed using the generalized gradient approximation with Perdew−Burke−Ernzerhof for the exchange−correlation functional and projector augmented wave in VASP.18,19 A plane-wave cutoff energy of 387.5 eV, k-point mesh of 4 × 4 × 4, as well as an energy conversion threshold of 10−6 eV per atom was applied. In order to qualitatively analyze the movements of the conduction bands and valence bands for the SnTe−Mn samples, two Sn atoms in a 3 × 3 × 3 supercell of Sn27Te27 were substituted with Mn atoms to simulate a doping concentration of about 7.4%, and the spin−orbit coupling effect has been included with an initial magnetic moment of 5 μB for the substituted Mn.

SnTe matrix, and the products of Mn-substituted SnTe and SnO2 nanoinclusions have been formed. As a result, the electrical transport properties have been enhanced due to the band modification by the substitution of Mn for Sn. Moreover, benefiting from multiscale phonon scattering by point defects, grain boundaries, dispersive SnO2 nanoparticles, and supersaturated MnTe precipitates, the thermal conductivity has also been greatly reduced. Ultimately, a maximum ZT of ∼1.5 at 873 K has been obtained in the SnTe sample doped with 10% MnO2, which increases almost by 224% in comparison with the undoped sample.

2. EXPERIMENTAL SECTION 2.1. Preparation. Polycrystalline SnTe alloys were synthesized by the high-temperature melting method. Commercial high-purity powder MnO2 (200 mesh, 99.99%, Aladdin, China) was mixed with SnTe powder prepared by mechanical alloying for 1 h to obtain bulk samples of SnTe + x mol % MnO2 (x = 0, 4, 6, 8, 10, 12) by hot press at 773 K under a pressure of 90 MPa for 2 h in the Ar atmosphere. The relative and absolute densities of samples are presented in Figure S1. 2.2. Characterization. The phase and structure of the obtained samples were characterized with Cu Kα (λ = 1.5406 Å) radiation in a PANalytical X’Pert PRO diffractometer, and the in situ X-ray diffraction (XRD) diffraction patterns were recorded with Cu Kα (λ = 1.5406 Å) radiation in a MAXima-X-7000 diffractometer. The XRD patterns (Figure S2) were analyzed with the JADE 6.0 software and fitted by the Rietveld refinement method using the GASA software. Thermal analysis was carried out with a differential scanning calorimeter (PerkinElmer Diamond DSC, Waltham, MA). The fracture morphology, microstructure, and chemical composition of the samples were characterized by a field-emission scanning electron microscope (NanoSEM 450) and a high-resolution transmission electron microscope (JEM-2100) equipped with energy-dispersive spectroscopy (EDS) in the bright-field mode at an accelerating voltage of 200 kV. 2.3. Measurement. The electrical data (S and ρ) of samples were collected simultaneously by a standard four-probe method and the slope of thermopower versus temperature gradients, respectively, with

3. RESULTS AND DISCUSSION 3.1. Phase Composition and Microstructure Characterization. Figure 1a,b shows the XRD patterns of the hotpressed SnTe + x mol % MnO2 (x = 0, 4, 6, 8, 10, 12) samples. As can be seen, except the rock-salt SnTe structure (PDF# 461210), SnO2 peaks also appear; the MnTe phase further shows up when the content of MnO2 x is over 8%, while no peak of MnO2 can be found in the XRD patterns. It is worth noting that only MnO2 powders were added into SnTe before hot pressing; why are there small amounts of SnO2 and MnTe in the hot-pressed samples? This is because the MnO2 additive reacted with the SnTe matrix during hot pressing, leading to the product mixture of Mn-substituted SnTe solid solution and SnO2 according to eq 1. SnTe + MnxO2x → Sn1 − xMnxTe + SnxO2x B

(1)

DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 2. Fracture morphology of the SnTe sample (a) and SnTe + 10% MnO2 sample (b), (c) enlarged image of the marked area in (b). TEM image of the SnTe + 10% MnO2 sample. (d) Low-magnification TEM image, HRTEM images of the MnTe precipitate (e) and SnO2 nanoinclusion (f).

Figure 3. (a) Temperature dependence of the electrical resistivity; (b) carrier concentration and mobility; (c−d) temperature dependence of the Seebeck coefficient and power factor of all samples.

As shown in Figure 2a, the pristine SnTe sample presents a relatively clean and smooth fracture surface, while many dispersed nanoparticles appear in the SnTe + 10 mol % MnO2 sample (Figure 2b,c), with a size of about 5−25 nm, more clearly shown in the transmission electron microscopy (TEM) image in Figure 2d. In order to further distinguish the nature of those nanoparticles (marked with white circles) and strip precipitates (marked with a white box), high-resolution TEM (HRTEM) was employed. The strip precipitates, and nanoinclusions are identified to be MnTe and SnO2, respectively, according to the corresponding high-resolution lattice image (Figure 2e,f), which is well consistent with the above XRD results (Figure 1). Moreover, the EDS results in Figure S4 also confirm the above statement. Conceivably, these SnO 2 nanoparticles and MnTe precipitates, embedded in the host material with disconnected heterojunction interfaces, will strongly scatter phonons thus effectively reducing the lattice thermal conductivity of the system.

This in situ reaction can be verified by the differential scanning calorimetry (DSC) curves and in situ XRD measurement as shown in Figure 1c,d. There is a clear exothermic peak at about 700 K in the DSC curve of the powder mixtures sample (red line), which corresponds to the in situ reaction between MnO2 and SnTe, while there is no peak in the DSC curve of the hot-pressed sample, indicating that the reaction was completed during the in situ process. The in situ high-temperature XRD patterns in Figure 1d also show that the disappearance of MnO2 additive accompanies the appearance of SnO2 once the temperature is over 723 K, which is in good agreement with the DSC results and further verifies the above reaction. Because of the limited solubility of MnTe in SnTe, which can be indicated by the variation of the lattice parameter as shown in Figure S3 and the solubility is about 9 at. % according to the studies by Tan20 and Wu,21 excessive MnTe precipitates and thus MnTe peaks appear in the XRD pattern of the sample with 10 at. % MnO2. C

DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 4. (a) Variation of the Lorenz number and effective mass with the MnO2 content added, (b) room-temperature Seebeck coefficients as a function of hole concentration, (c) calculated band structures of SnTe and ∼7.4 mol % Mn-doped SnTe, (d) schematic diagram of band structure evolution of SnTe with MnO2 addition.

Figure 5. Temperature dependence of thermal conductivity κ (a) and lattice thermal conductivity κl (b); (c) room-temperature lattice thermal conductivity as a function of MnO2 alloying fraction x for samples; (d) (κtot − κele) as a function of 1000/T for pure SnTe and SnTe + 10 mol % MnO2; the dashed line represents linear fitting from 300 to 900 K; the deviation of the thermal conductivity indicates a significant κbip; (e) κbip as a function of temperature; (f) temperature dependence of ZT values for the samples; the inset reports the recent progress in SnTe-based TE materials.

D

DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials 3.2. TE Properties. The electrical transport properties of the samples are collectively shown in Figure 3. The electrical resistivity (ρ) of the samples with addition of MnO2 (x = 4, 6, 8, 10, 12) is larger than that of the pristine SnTe, and it presents a typical metal or heavily doped semiconductor behavior, while the mobility (Figure 3b) systematically decreases with increasing MnO2, which dominates over the increase of carrier density and accounts for the augmented resistivity (Figure 3a). The gradual loss of μH should be attributed to the multiple scattering effect resulted from the in situ reaction as follows: (i) point defect scattering due to the large atomic radius difference between Sn and Mn, (ii) interfacial scattering caused by SnO2 nanoinclusions and Mnrich precipitates derived from the in situ reaction (Figure 2), (iii) enhanced effective mass of holes caused by Mn-induced valence band convergence (Figure 4a). The Seebeck coefficient and the electrical resistivity show a similar variation with temperature in all samples (Figure 3c). It is worth noting that the Seebeck coefficient, and accordingly the carrier concentration increase with the increase of the MnO2 content (Figure 3b). This is abnormal according to the single parabolic band model and can be explained by the two valence band character of SnTe,22−24 in which the Fermi level is pushed deeper into the valence band due to the higher presence of activated holes, leading to the heavy band taking part in the electrical transport process. Similar results were also discovered in Cd-doped SnTe10 and Mg-doped SnTe12 even though they are isovalent, with Sn being an electron acceptor in SnTe. Moreover, the positive dependence of the carrier concentration on the Seebeck coefficient is not highly consistent with Pisarenko’s theoretical plot in Figure 4b, where the Seebeck coefficients higher than those predicted by the theory also reflect the modification of the band structure by the in situ reaction between SnTe and MnO2. To understand the variation of the Seebeck coefficient more insightfully, DFT band structure calculations have been carried out, and the results are shown in Figure 4c. It can be seen that the introduction of Mn effectively brings the light-hole band and the heavy-hole band closer in energy and pushes the conduction minima at the L point to higher energies, resulting in a convergence of valence bands and a band gap enlargement. As schematically shown in Figure 4d, ΔEv decreases from 0.35 to 0.14 eV, while Eg increases from 0.16 to 0.24 eV, for pure SnTe and Sn25Mn2Te27, respectively. Because of the convergence of valence bands and the enlarged band gap by Mn alloying, the Seebeck coefficient has been effectively enhanced (Figure 3c), and the bipolar effect has been suppressed accordingly. Therefore, the power factor shows a remarkable enhancement reaching 2277 μW m−1 K−2 at 873 K for the SnTe + 10 mol % MnO2 sample, as shown in Figure 3d. Figure 5a,b presents the total thermal conductivity (κ) and lattice thermal conductivity (κl) of SnTe + x mol % MnO2 (x = 0, 4, 6, 8, 10, 12) samples, in which κl was obtained by extracting κe from κ; κel was calculated by Wiedemann−Franz law κe = LTσ

precipitates introduced by the in situ reaction between SnTe and MnO2, as demonstrated above. To further understand the effects of the in situ reaction for the low κl of the MnO2-added samples, κl was calculated by the Klemens−Drabble model26,27 and is shown in Figure 5c; the calculation parameters are listed in Table S1. The upward deviation from the experimental values of κl further verifies that other phonon scattering mechanisms such as SnO2 and MnTe nanoinclusions, besides the point defects of mass fluctuation in Sn1−xMnxTe, play a significant role in the reduction of κl. It is worthwhile noting that only pristine SnTe presents a visible upturn of κl at high temperature in Figure 5b; that is to say, the bipolar conduction has been suppressed in other samples with the addition of MnO2. In order to further verify this hypothesis, the relationship between κ and κe and κl has been analyzed. As it is well known, κ is composed of the electron thermal conductivity (κe), κl, and bipolar thermal conductivity (κbip). By only taking into consideration the predominant acoustic phonon scattering, κl should be approximately linearly related to T−1 at low temperature, according to the following equation28 κl ∝ M Ω1/3θ 3γ −2T −1

(3)

Here, M is the average mass per atom, Ω is the average atomic volume, θ is the Debye temperature, and γ is the Gruneisen parameter, whose respective values are listed in Table S1. As shown in Figure 5d, the plot of κ−κe versus 1/T for the SnTe + 10 mol % MnO2 sample follows an approximate linear relationship, while the pristine SnTe deviates from the linear relationship at high temperature. Accordingly, the bipolar thermal conductivities of two samples were derived and are shown in (Figure 5e). It can be observed that the bipolar conduction has been effectively suppressed in the SnTe + 10 mol % MnO2 sample. How to suppress the performance deterioration of TE materials in the intrinsic excitation region? Generally, bipolar conduction can be restrained by (i) increasing the majority of carrier concentration by heavy doping;29 (ii) widening the band gap;30 and (iii) adopting energy barrier filters by nanostructure engineering.31 In this work, the in situ reaction strategy could nearly play the above roles simultaneously. On one hand, Mn substitution in SnTe increases hole densities (majority carrier) and amplifies the band gap as discussed in the above section; on the other hand, some heterojunction interfaces of SnTe/SnO2 and SnTe/ MnTe serve as an energy barrier.32,33 Therefore, the bipolar conduction can be effectively suppressed in the samples with MnO2. Finally, the resulting ZT was calculated and is shown in Figure 5f. Benefiting from the synergistic effect induced by the in situ reaction, a remarkable improvement on the TE performance has been realized for SnTe, and a maximum ZT of ∼1.5 at 873 K has been achieved in the SnTe + 10 mol % MnO2 sample, which increases by 224% in comparison with pristine SnTe.

4. CONCLUSIONS In summary, an in situ chemical reaction between SnTe and MnO2 has been designed and a composite product of Mnsubstituted SnTe, SnO2 nanoparticles, and MnTe-supersaturated precipitates has been fabricated for the improvement of TE properties of SnTe for the first time. Benefiting from the synergistic effect induced by the in situ reaction, a remarkable improvement on the TE performance has been realized, and a

(2)

where L is the Lorenz number and estimated by the reduced Fermi energy and scattering parameter (Figure 4a).25 Both κ and κl decrease with the increasing MnO2 content, and the sharp reduction of κl should be attributed to the multiscale phonon scattering due to the high concentration of point defects in Sn1−xMnxTe, SnO2 nanoparticles and MnTe E

DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials maximum ZT of ∼1.5 at 873 K has been achieved in the SnTe + 10 mol % MnO2 sample. It increases by 224% in comparison with pristine SnTe and is also one of the best results ever reported for SnTe-based TE materials.

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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/acs.chemmater.9b00747. Calculated absolute and relative densities of SnTe + x mol % MnO2 samples; lattice parameter dependent on the content of the MnO2 additive; temperature dependence of the specific heat capacity; EDS results of the 10 mol % MnO2 sample corresponding to the spots marked in Figure S4a; detail calculation parameters; and Lorenz number and effective mass (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junyou Yang: 0000-0003-0849-1492 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is cofinanced by the National Natural Science Foundation of China (grant nos. 51572098, 51632006, 51772109 and 51872102), the Fundamental Research Funds for the Central Universities (no. 2018KFYXKJC002), the Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (no. 2016-KF-5), and Graduates’ Innovation Fund, Huazhong University of Science and Technology (no. 5003110006). The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged.

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DOI: 10.1021/acs.chemmater.9b00747 Chem. Mater. XXXX, XXX, XXX−XXX