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Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping
Journal Pre-proof Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping Fengkai Guo, Bo Cui, Muchun Guo, Jing Wang, Jian Cao, Wei Cai, Jiehe Sui PII:
S2542-5293(19)30136-1
DOI:
https://doi.org/10.1016/j.mtphys.2019.100156
Reference:
MTPHYS 100156
To appear in:
Materials Today Physics
Received Date: 12 October 2019 Revised Date:
26 October 2019
Accepted Date: 28 October 2019
Please cite this article as: F. Guo, B. Cui, M. Guo, J. Wang, J. Cao, W. Cai, J. Sui, Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping, Materials Today Physics, https:// doi.org/10.1016/j.mtphys.2019.100156. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping Fengkai Guo, Bo Cui, Muchun Guo, Jing Wang, Jian Cao, Wei Cai*, and Jiehe Sui* National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin, 150001, China *To whom correspondence should be addressed. E-mail: [email protected], [email protected] Keywords: SnTe, carrier concentration, nano-precipitate Abstract SnTe alloy is a kind of potential medium temperature thermoelectric materials to replace toxic PbTe. In this work, we demonstrate that both trivalent Ce doping and univalent Li doping can optimize the thermoelectric properties of SnTe by controlling carrier concentration in two directions, respectively. In addition, the dispersed high density nano-precipitates caused by Li doping are helpful to enhance the phonon scattering and then reduce the lattice thermal conductivity. Moreover, co-doping of Ce and Li promotes the peak ZT and average ZT of SnTe simultaneously. The ZT value reaches ~1 at 873 K and the ZTave reaches ~0.39 from 300 K to 873 K for Sn0.98Ce0.02Te(Li2Te)0.01.
Thermoelectric materials are considered to play an irreplaceable role in some special and harsh scenarios because of their ability to reversibly convert electrical and thermal energy without any moving part [1]. However, relatively low conversion efficiency has become a stumbling block for large-scale commercial applications of thermoelectric materials. The criteria for evaluating the thermoelectric performance is the figure of merit, ZT=S2σT/(κele+κlat), where S, σ, T, κele and κlat are the Seebeck coefficient, electrical conductivity, absolute temperature, electronic thermal conductivity and lattice thermal conductivity, respectively. The large-scale application of thermoelectric materials must also satisfy the environment-friendly condition. Thermoelectric PbTe with excellent performance is limited because of its main component, toxic Pb. Eco-friendly SnTe is expected to be a substitute for PbTe because of their same crystal structure (rock salt structure) and similar band structure (two valence bands). Howerer, the energy offset ∆E between the two bands (light band, L and heavy band, Σ ) of SnTe (~0.35 eV) [2] is much larger than that of PbTe (~0.17 eV) [3,4]. The heavy band contributes less to charge transport, resulting in smaller effective mass and lower Seebeck coefficient. On the other hand, a large number of cationic vacancies of SnTe results in an ultrahigh carrier concentration (1020 to 1021
1
cm-3) [5]. High carrier concentration not only leads to high σ and κele, but also low S, thereby leading to low ZT value of pure SnTe. Many efforts had been made to improve the performance of SnTe, including carrier concentration optimization (Self compensation/Bi/Sb/Ga/In) [6,7], band engineering (resonant state induced by Indium [8-10] and band convergence induced by Mn/Cd/Ca/Mg/Hg [11-18]) to enhance Seebeck coefficient,
and
enhancement
of
phonon
scattering (point
defects,
dislocations
[12],
nanoprecipitates [7], nanoscale pores [19], and so on) to reduce lattice thermal conductivity. According to the coupling relationship between material parameters, controlling carrier concentration is a basic and effective method to improve thermoelectric performance. Due to the volatilization of raw materials in the preparation process, the carrier concentration of undoped SnTe materials varies greatly with different sources of raw materials, preparation methods and operation procedures, and the ZT values of corresponding materials also vary significantly. The ZT values (300-873K) of pure SnTe polycrystalline samples from different reports [5,8,13-15,20-23] and this work were collected and illustrated in Figure 1. All the original data are recalculated using the same specific heat (Cp) [14] to eliminate the resulting deviation. The carrier concentration at room temperature ranges from ~9.30×1019 cm-3 to ~1.02×1021 cm-3, spanning two orders of magnitude. The peak ZT value at 873 K ranges from ~0.52 to ~0.78 and the latter is 50% higher than the former. The more important average ZT value (ZTave) between 300 K and 873 K range from ~0.14 to ~0.23, and the increasement is higher than 60%. From the macro point of view, excluding individual data, the optimum carrier concentration for the highest peak and average ZT values is between 1.5×1020 cm-3 and 2.5×1020 cm-3. In this work, trivalent cerium and monovalent lithium were used to adjust the carrier concentration in two direction, respectively. Cerium was doped into SnTe in the form of replacement atom, lithium was added in the form of Li2Te, hoping to form nano-precipitates to reduce the lattice thermal conductivity besides regulating carrier concentration. Finally, Ce and Li were doped cooperatively to optimize the carrier concentration of SnTe and enhance the electrical transport properties. High density nano-precipitates were introduced to enhance phonon scattering and reduce the lattice thermal conductivity. The peak ZT at 873 K reach ~1 and the ZTave from 300 K to 873 K was improved from ~0.19 to ~0.39, with an increasement of ~100%. The samples were synthesized by traditional melting and hot press sintering. The experimental details can be found in the supplementary materials. As shown in Figure S1, for Sn1-xCexTe (x=0, 0.005, 0.01, 0.02, 0.03) samples, all the X-ray diffraction patterns can be indexed to SnTe when x≤0.02. When the Ce content increases to 3%, Ce2Te3 phase was detected. While for the SnTe-
2
(Li2Te)y (y=0, 0.005, 0.01, 0.015, 0.02) samples, no secondary phase was found within the detection limit. Figure 2 shows the carrier concentration at 300 K of these two series of samples. When x is less than 0.02, the carrier concentration decreases linearly from ~1.4×1020 cm-3 to ~5.7×1019 cm-3 with the increase of Ce content, With the further increase of Ce content, the downward trend slows down, which corresponds to the appearance of the Ce2Te3 mentioned above. The carrier concentration increases rapidly and almost linearly with the increase of Li2Te content, and is not saturated when y reaches 0.02 (~6.3×1020 cm-3). These results indicate that Ce and Li are effective donor dopants and acceptor dopants in SnTe, respectively, and can be used to control carrier concentration in a wide range. The electrical transport properties including σ and S of Ce-doped samples and Li-doped samples were shown in Figures 3(a-d). The σ and S values increase and decrease with increasing Ce content (decreasing carrier concentration) in the whole temperature range, respectively. Interestingly, with increasing Li content, the S value increases monotonously before ~673 K, and then decrease. This abnormal behavior is related to the two valence bands of SnTe. Due to the rapidly increasing carrier concentration, the Fermi level drops gradually, leading to that the electron transport in the lower Σ band is activated, and then the effective mass increases, which eventually leads to the enhancement of Seebeck coefficient. However, in the high temperature region, the energy offset between the two valence bands decreases rapidly. For all samples, heavy bands are involved in charge transport, and the negative correlation between Seebeck coefficient and carrier concentration is restored. The total thermal conductivity κ and lattice thermal conductivity κlat are shown in Figures 3(e-h). κlat is calculated by the relationship κ = κlat + κele, where κele is calculated by the Wiedemann-Franz law κele = LσT (L is the Lorenz number, calculated approximately in a single parabolic band model with acoustic phonon scattering [24,25]). For the pristine SnTe, the calculated κlat increases after 573 K, which is due to the bipolar effect caused by low carrier concentration and narrow bandgap (~0.18eV) [26]. From the macro point of view, with increasing Ce content, the κlat value increases first and then decrease. Because of the existence of the secondary phase Ce2Te3, the sample x = 0.03 does not follow this rule. This phenomenon has also been reported in other works [11,14]. The intrinsic numerous vacancies in SnTe is a kind of effective phonon scattering mechanism. One possible explanation is that doping atoms will partially occupy the vacancies at first and reduce the lattice distortion and then the phonon scattering intensity to some extent, leading to the increase of lattice thermal conductivity. However, substitution atoms themselves are another phonon scattering mechanism. With the increase of doping amount, the number of substitution atoms increases drastically and the phonon scattering inversely increases gradually, leading to the reduction of 3
lattice thermal conductivity. Another strange phenomenon is that the lattice thermal conductivity of pure SnTe decreases faster than that of doped samples before 373 K, which can be attributed to the rapidly increasing equilibrium vacancy concentration with the increase of temperature. However, for the doped samples, we speculate that the foreign atoms disturbed the relationship between cation vacancy concentration and temperature and the specific mechanism needs further investigate. On the other hand, due to the decreased carrier concentration, the onset temperature of bipolar effect decrease from ~573 K for SnTe (n ≈ 1.40 ×1020 cm-3) to ~473 K for Sn0.98Ce0.02Te (n ≈ 0.57×1020 cm-3). For the Li-doped samples, the bipolar effect gradually disappears with the increase of Li content, which is caused by the sharp increase of carrier concentration. However, before the onset of bipolar effect, the lattice thermal conductivity of Li doped samples is higher than that of pure SnTe from a macro point of view. In order to clarify the cause of this abnormal phenomenon, transmission electron microscope (TEM) is employed to investigate the microstructure of SnTe-(Li2Te)0.015. Figures 4(a, c) show the bright field image on different directions. The sample displays different contrast on different directions, while numerous nano-inclusions can be observed in both of them. The contrast is more obvious in the high resolution transmission electron microscope (HRTEM) image as shown in Figures 4(b, d). The selected area electron diffraction (SAED) in the inset of Figure 3a displays two types of patterns, which belongs to [2 1 -1] axis of cubic SnTe and [3 2 -2] of hexagonal LiTe3, respectively. In the yellow circle, the strong spot belongs to SnTe and the weak one belongs to LiTe3. In addition, the strong spots of SnTe were stretched to some extent. These evidences mean that the coherent stress of the two phases is relatively large on this direction. The two types of patterns were observed again in Figure 4c. The strong spots belong to [111] axis of SnTe and the weak ones belong to [001] axis of LiTe3. Furthermore, the crystallographic orientation relationship between the SnTe matrix and the LiTe3 nano-precipitates could be deduced as follows, ∥
(1)
∥
(2)
According to the stoichiometric ratio of LiTe3, one Li atom coordinates three Te atoms. Furthermore, the precipitate concentration is extremely high as shown in the TEM result, so the matrix is Sn-rich and Te-poor in Li doped samples. This state is similar to the self-compensation SnTe, which means that the vacancy concentration in the matrix is greatly reduced [6]. Because the lattice distortion caused by Li replacing Sn is much smaller than that caused by Sn vacancy, the lattice thermal conductivity of Li doped samples is relatively higher than pure SnTe before the bipolar effect. Furthermore, the extremely dense and dispersed nano-precipitates (<10 nm) will 4
contribute additional strong phonon scattering [27] and result in the relatively low lattice thermal conductivity at high temperature. Specifically, the κlat value at 873 K falls below 0.7when y ≥ 0.015. The temperature-dependent figure of merit of Ce-doped samples and Li-doped samples are shown in Figures 5(a, b). For the Ce-doped samples, the average ZT value (ZTave) from 300 K to 873 K increases considerably, although the increase of peak ZT at 873 K is limited. However, for Li-doped samples, the peak ZT at 873 K increases to more than 0.8, but the enhancement of ZTave is almost negligible. Naturally, Ce and Li co-doped SnTe is expected to achieve both average and peak ZT values. Sn0.98Ce0.02Te was selected as the matrix because of its relatively high ZTave and no detectable secondary phase. On this basis, the content of Li2Te was adjusted to optimize the thermoelectric performance. The electrical and thermal transport properties for Sn0.98Ce0.02Te(Li2Te)z (z = 0, 0.005, 0.01, 0.015, 0.02) are shown in Figure S2, and the temperature-dependent ZT values are shown in Figure 5c. When z ≥ 0.01, the peak ZT at 873 K reaches ~1, while the Sn0.98Ce0.02Te-(Li2Te)0.01 sample possesses the highest ZTave of ~0.39 from 300 K to 873 K as shown in Figure 5d. The increasement of these two values is ~54% and ~100%, respectively, compared to that of pure SnTe. In summary, Ce-doped SnTe, Li-doped SnTe and Ce-Li co-doped SnTe are prepared and the corresponding thermoelectric properties were investigated. Cerium substituting Sn can effectively reduce the carrier concentration and significantly increase the ZTave. For lithium doping, besides adjusting the carrier concentration to optimize the electrical transport properties, high density nanoprecipitates were constructed to strongly scatter the phonons and then reduce the lattice thermal conductivity. As a result, the peak ZT is increased to ~0.8 at 873 K. Furthermore, cerium and lithum were employed to cooperatively improve both the peak ZT and average ZT. Finally, the peak ZT reaches ~1 for Sn0.98Ce0.02Te-(Li2Te)0.01, and the ZTave from 300 K to 873 K reaches ~0.39.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51771065, 51622101, and 51871082). Supplementary Materials See supplementary materials for experimental details and Figures S1-S2. Conflict of Interest The authors declare no conflict of interest. References [1] W. Liu, J. Hu, S. Zhang, M. Deng, C.-G. Han, Y. Liu, New trends, strategies and opportunities in thermoelectric materials: A perspective, Mater. Today Phys. 1 (2017) 50-60. [2] L.M. Rogers, Valence band of SnTe, J. Phys. D: Appl. Phys. 1 (1968) 845-852. 5
[3] R.S. Allgaier, Valence Bands in Lead Telluride, J. Appl. Phys. 32 (1961) 2185-2189. [4] H. Sitter, K. Lischka, H. Heinrich, Structure of the second valence band in PbTe, Phys. Rev. B 16 (1977) 680-687. [5] Z. Zhou, J. Yang, Q. Jiang, X. Lin, J. Xin, A. Basit, J. Hou, B. Sun, Enhanced Thermoelectric Performance of SnTe: High Efficient Cation - anion Co-doping, Hierarchical Microstructure and Electro-acoustic decoupling, Nano Energy 47 (2018) 81-88. [6] G. Tan, L.D. Zhao, F. Shi, J.W. Doak, S.H. Lo, H. Sun, C. Wolverton, V.P. Dravid, C. Uher, M.G. Kanatzidis, High thermoelectric performance of p-type SnTe via a synergistic band engineering and nanostructuring approach, J. Am. Chem. Soc. 136 (2014) 7006-7017. [7] L.D. Zhao, X. Zhang, H. Wu, G. Tan, Y. Pei, Y. Xiao, C. Chang, D. Wu, H. Chi, L. Zheng, Enhanced Thermoelectric Properties in the Counter-Doped SnTe System with Strained Endotaxial SrTe, J. Am. Chem. Soc. 138 (2016) 2366-2373. [8] G. Tan, W.G. Zeier, F. Shi, P. Wang, G.J. Snyder, V.P. Dravid, M.G. Kanatzidis, High Thermoelectric Performance SnTe–In2Te3 Solid Solutions Enabled by Resonant Levels and Strong Vacancy Phonon Scattering, Chem. Mater. 27 (2015) 7801-7811. [9] Q. Zhang, B. Liao, Y. Lan, K. Lukas, W. Liu, K. Esfarjani, C. Opeil, D. Broido, G. Chen, Z. Ren, High thermoelectric performance by resonant dopant indium in nanostructured SnTe, Proc. Natl. Acad. Sci. U.S.A. 110 (2013) 13261-13266. [10] M. Zhou, Z.M. Gibbs, H. Wang, Y. Han, L. Li, G.J. Snyder, Thermoelectric performance of co-doped SnTe with resonant levels, Appl. Phys. Lett. 109 (2016) 042102. [11] G. Tan, F. Shi, S. Hao, H. Chi, T.P. Bailey, L.D. Zhao, C. Uher, C. Wolverton, V.P. Dravid, M.G. Kanatzidis, Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe, J. Am. Chem. Soc. 137 (2015) 11507-11516. [12] F. Guo, B. Cui, Y. Liu, X. Meng, J. Cao, Y. Zhang, R. He, W. Liu, H. Wu, S.J. Pennycook, W. Cai, J. Sui, Thermoelectric SnTe with Band Convergence, Dense Dislocations, and Interstitials through Sn Self-Compensation and Mn Alloying, Small 14 (2018) e1802615. [13] W. Li, Z. Chen, S. Lin, Y. Chang, B. Ge, Y. Chen, Y. Pei, Band and scattering tuning for high performance thermoelectric Sn 1− x Mn x Te alloys, J. Materiomics 1 (2015) 307-315. [14] H. Wu, C. Chang, D. Feng, Y. Xiao, X. Zhang, Y. Pei, L. Zheng, D. Wu, S. Gong, Y. Chen, Synergistically optimized electrical and thermal transport properties of SnTe via alloying highsolubility MnTe, Energy Environ. Sci. 8 (2015) 3298-3312. [15] G. Tan, F. Shi, S. Hao, H. Chi, L.D. Zhao, C. Uher, C. Wolverton, V.P. Dravid, M.G. Kanatzidis, Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence, J. Am. Chem. Soc. 137 (2015) 5100-5112. [16] R.A.O.R. Al, N. Mecholsky, J.P. Hwang, W. Kim, J.S. Rhyee, D. Wee, M. Fornari, Band Degeneracy, Low Thermal Conductivity, and High Thermoelectric Figure of Merit in SnTeCaTe Alloys, Chem. Mater. 28 (2016) 376-384. [17] G. Tan, F. Shi, J.W. Doak, H. Sun, L.-D. Zhao, P. Wang, C. Uher, C. Wolverton, V.P. Dravid, M.G. Kanatzidis, Extraordinary role of Hg in enhancing the thermoelectric performance of ptype SnTe, Energy Environ. Sci. 8 (2015) 267-277. [18] J. Tang, Z. Yao, Z. Chen, S. Lin, X. Zhang, F. Xiong, W. Li, Y. Chen, Y. Pei, Maximization of transporting bands for high-performance SnTe alloy thermoelectrics, Mater. Today Phys. 9 (2019) 100091. [19] M. Hong, Y. Wang, S. Xu, X. Shi, L. Chen, J. Zou, Z.-G. Chen, Nanoscale pores plus precipitates rendering high-performance thermoelectric SnTe1-xSex with refined band structures, Nano Energy 60 (2019) 1-7. [20] L. Zhang, Q. Peng, C. Han, J. Wang, Z.-H. Ge, Q. Sun, Z.X. Cheng, Z. Li, S.X. Dou, Enhanced Thermoelectric Performance through Synergy of Resonance Levels and Valence Band Convergence via Q/In (Q = Mg, Ag, Bi) Co-Doping, J. Mater. Chem. A 6 (2018) 25072516. 6
[21] M. He, D. Feng, D. Wu, Y. Guan, J. He, Excellent thermoelectric performance achieved over broad temperature plateau in indium-doped SnTe-AgSbTe2 alloys, Appl. Phys. Lett. 112 (2018) 063902. [22] H. Wang, T. Wang, J. Hwang, W. Su, H. Kim, J. Zhai, X. Wang, C. Wang, W. Kim, Optimization of peak and average figures of merits for In & Se co-doped SnTe alloys, Inorg. Chem. Front. 5 (2018) 793-801. [23] D. Wang, X. Zhang, Y. Yu, L. Xie, J. Wang, G. Wang, J. He, Y. Zhou, q. Pang, J. Shao, L. Zhao, Enhancing thermoelectric performance of SnTe via stepwisely optimizing electrical and thermal transport properties, J. Alloy Compd. 773 (2019) 571-584. [24] H.J. Goldsmid, Introduction to Thermoelectricity, 2009. [25] J. Shuai, X.J. Tan, Q. Guo, J.T. Xu, A. Gellé, R. Gautier, J.F. Halet, F. Failamani, J. Jiang, T. Mori, Enhanced thermoelectric performance through crystal field engineering in transition metal–doped GeTe, Mater. Today Phys. 9 (2019) 100094. [26] M. Zhou, Z.M. Gibbs, H. Wang, Y. Han, C. Xin, L. Li, G.J. Snyder, Optimization of thermoelectric efficiency in SnTe: the case for the light band, Phys. Chem. Chem. Phys. 16 (2014) 20741-20748. [27] M.-S. Jeng, R. Yang, D. Song, G. Chen, Modeling the Thermal Conductivity and Phonon Transport in Nanoparticle Composites Using Monte Carlo Simulation, J. Heat Transfer 130 (2008) 042410.
Figure 1. Temperature-dependent figure of merit (ZT) for pristine SnTe of reports5,8,13-15,20-23 and this work. (a) Original data; (b) recalculated data with the same Cp14; (c) ZT vs room temperature carrier concentration; (d) ZTave from 300 K to 873 K. The number on the column is room temperature carrier concentration, in units of 1020cm-3. Figure 2. Room temperature carrier concentration of Sn1-xCexTe samples and SnTe-(Li2Te)y samples. Figure 3. Temperature-dependent electrical conductivity (a), (c), Seebeck coefficient (b), (d), total thermal conductivity (e), (g), and lattice thermal conductivity (f), (h) of Sn1-xCexTe samples and SnTe-(Li2Te)y samples, respectively. Figure 4. Microstructures of (SnTe) (Li2Te)0.015. (a) and (c) TEM images showing a high density of nano-precipitates on different direction. Inset: SAED patterns. (b), (d) HRTEM images of the red squares in (a) and (c). Figure 5. Temperature-dependent ZT of (a) Sn1-xCexTe samples, (b) SnTe-(Li2Te)y samples, and (c) Sn0.98Ce0.02Te-(Li2Te)z samples. (d) Comparison of ZT and ZTave of pure SnTe, Sn0.98Ce0.02Te, SnTe-(Li2Te)0.005, and Sn0.98Ce0.02Te-(Li2Te)0.01.
7
Figure 1
Figure 2
8
Figure 3 9
Figure 4
10
Figure 5
11
Both increasing and decreasing carrier concentration can improve thermoelectric properties of pure SnTe
Dense and dispersive nano-precipitates are introduced
ZT value and average ZT value are improved by cerium and lithium co-doping
Conflict of Interest Statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S2542-5293(19)30136-1
DOI:
https://doi.org/10.1016/j.mtphys.2019.100156
Reference:
MTPHYS 100156
To appear in:
Materials Today Physics
Received Date: 12 October 2019 Revised Date:
26 October 2019
Accepted Date: 28 October 2019
Please cite this article as: F. Guo, B. Cui, M. Guo, J. Wang, J. Cao, W. Cai, J. Sui, Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping, Materials Today Physics, https:// doi.org/10.1016/j.mtphys.2019.100156. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Enhanced thermoelectric performance of SnTe alloy with Ce and Li co-doping Fengkai Guo, Bo Cui, Muchun Guo, Jing Wang, Jian Cao, Wei Cai*, and Jiehe Sui* National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin, 150001, China *To whom correspondence should be addressed. E-mail: [email protected], [email protected] Keywords: SnTe, carrier concentration, nano-precipitate Abstract SnTe alloy is a kind of potential medium temperature thermoelectric materials to replace toxic PbTe. In this work, we demonstrate that both trivalent Ce doping and univalent Li doping can optimize the thermoelectric properties of SnTe by controlling carrier concentration in two directions, respectively. In addition, the dispersed high density nano-precipitates caused by Li doping are helpful to enhance the phonon scattering and then reduce the lattice thermal conductivity. Moreover, co-doping of Ce and Li promotes the peak ZT and average ZT of SnTe simultaneously. The ZT value reaches ~1 at 873 K and the ZTave reaches ~0.39 from 300 K to 873 K for Sn0.98Ce0.02Te(Li2Te)0.01.
Thermoelectric materials are considered to play an irreplaceable role in some special and harsh scenarios because of their ability to reversibly convert electrical and thermal energy without any moving part [1]. However, relatively low conversion efficiency has become a stumbling block for large-scale commercial applications of thermoelectric materials. The criteria for evaluating the thermoelectric performance is the figure of merit, ZT=S2σT/(κele+κlat), where S, σ, T, κele and κlat are the Seebeck coefficient, electrical conductivity, absolute temperature, electronic thermal conductivity and lattice thermal conductivity, respectively. The large-scale application of thermoelectric materials must also satisfy the environment-friendly condition. Thermoelectric PbTe with excellent performance is limited because of its main component, toxic Pb. Eco-friendly SnTe is expected to be a substitute for PbTe because of their same crystal structure (rock salt structure) and similar band structure (two valence bands). Howerer, the energy offset ∆E between the two bands (light band, L and heavy band, Σ ) of SnTe (~0.35 eV) [2] is much larger than that of PbTe (~0.17 eV) [3,4]. The heavy band contributes less to charge transport, resulting in smaller effective mass and lower Seebeck coefficient. On the other hand, a large number of cationic vacancies of SnTe results in an ultrahigh carrier concentration (1020 to 1021
1
cm-3) [5]. High carrier concentration not only leads to high σ and κele, but also low S, thereby leading to low ZT value of pure SnTe. Many efforts had been made to improve the performance of SnTe, including carrier concentration optimization (Self compensation/Bi/Sb/Ga/In) [6,7], band engineering (resonant state induced by Indium [8-10] and band convergence induced by Mn/Cd/Ca/Mg/Hg [11-18]) to enhance Seebeck coefficient,
and
enhancement
of
phonon
scattering (point
defects,
dislocations
[12],
nanoprecipitates [7], nanoscale pores [19], and so on) to reduce lattice thermal conductivity. According to the coupling relationship between material parameters, controlling carrier concentration is a basic and effective method to improve thermoelectric performance. Due to the volatilization of raw materials in the preparation process, the carrier concentration of undoped SnTe materials varies greatly with different sources of raw materials, preparation methods and operation procedures, and the ZT values of corresponding materials also vary significantly. The ZT values (300-873K) of pure SnTe polycrystalline samples from different reports [5,8,13-15,20-23] and this work were collected and illustrated in Figure 1. All the original data are recalculated using the same specific heat (Cp) [14] to eliminate the resulting deviation. The carrier concentration at room temperature ranges from ~9.30×1019 cm-3 to ~1.02×1021 cm-3, spanning two orders of magnitude. The peak ZT value at 873 K ranges from ~0.52 to ~0.78 and the latter is 50% higher than the former. The more important average ZT value (ZTave) between 300 K and 873 K range from ~0.14 to ~0.23, and the increasement is higher than 60%. From the macro point of view, excluding individual data, the optimum carrier concentration for the highest peak and average ZT values is between 1.5×1020 cm-3 and 2.5×1020 cm-3. In this work, trivalent cerium and monovalent lithium were used to adjust the carrier concentration in two direction, respectively. Cerium was doped into SnTe in the form of replacement atom, lithium was added in the form of Li2Te, hoping to form nano-precipitates to reduce the lattice thermal conductivity besides regulating carrier concentration. Finally, Ce and Li were doped cooperatively to optimize the carrier concentration of SnTe and enhance the electrical transport properties. High density nano-precipitates were introduced to enhance phonon scattering and reduce the lattice thermal conductivity. The peak ZT at 873 K reach ~1 and the ZTave from 300 K to 873 K was improved from ~0.19 to ~0.39, with an increasement of ~100%. The samples were synthesized by traditional melting and hot press sintering. The experimental details can be found in the supplementary materials. As shown in Figure S1, for Sn1-xCexTe (x=0, 0.005, 0.01, 0.02, 0.03) samples, all the X-ray diffraction patterns can be indexed to SnTe when x≤0.02. When the Ce content increases to 3%, Ce2Te3 phase was detected. While for the SnTe-
2
(Li2Te)y (y=0, 0.005, 0.01, 0.015, 0.02) samples, no secondary phase was found within the detection limit. Figure 2 shows the carrier concentration at 300 K of these two series of samples. When x is less than 0.02, the carrier concentration decreases linearly from ~1.4×1020 cm-3 to ~5.7×1019 cm-3 with the increase of Ce content, With the further increase of Ce content, the downward trend slows down, which corresponds to the appearance of the Ce2Te3 mentioned above. The carrier concentration increases rapidly and almost linearly with the increase of Li2Te content, and is not saturated when y reaches 0.02 (~6.3×1020 cm-3). These results indicate that Ce and Li are effective donor dopants and acceptor dopants in SnTe, respectively, and can be used to control carrier concentration in a wide range. The electrical transport properties including σ and S of Ce-doped samples and Li-doped samples were shown in Figures 3(a-d). The σ and S values increase and decrease with increasing Ce content (decreasing carrier concentration) in the whole temperature range, respectively. Interestingly, with increasing Li content, the S value increases monotonously before ~673 K, and then decrease. This abnormal behavior is related to the two valence bands of SnTe. Due to the rapidly increasing carrier concentration, the Fermi level drops gradually, leading to that the electron transport in the lower Σ band is activated, and then the effective mass increases, which eventually leads to the enhancement of Seebeck coefficient. However, in the high temperature region, the energy offset between the two valence bands decreases rapidly. For all samples, heavy bands are involved in charge transport, and the negative correlation between Seebeck coefficient and carrier concentration is restored. The total thermal conductivity κ and lattice thermal conductivity κlat are shown in Figures 3(e-h). κlat is calculated by the relationship κ = κlat + κele, where κele is calculated by the Wiedemann-Franz law κele = LσT (L is the Lorenz number, calculated approximately in a single parabolic band model with acoustic phonon scattering [24,25]). For the pristine SnTe, the calculated κlat increases after 573 K, which is due to the bipolar effect caused by low carrier concentration and narrow bandgap (~0.18eV) [26]. From the macro point of view, with increasing Ce content, the κlat value increases first and then decrease. Because of the existence of the secondary phase Ce2Te3, the sample x = 0.03 does not follow this rule. This phenomenon has also been reported in other works [11,14]. The intrinsic numerous vacancies in SnTe is a kind of effective phonon scattering mechanism. One possible explanation is that doping atoms will partially occupy the vacancies at first and reduce the lattice distortion and then the phonon scattering intensity to some extent, leading to the increase of lattice thermal conductivity. However, substitution atoms themselves are another phonon scattering mechanism. With the increase of doping amount, the number of substitution atoms increases drastically and the phonon scattering inversely increases gradually, leading to the reduction of 3
lattice thermal conductivity. Another strange phenomenon is that the lattice thermal conductivity of pure SnTe decreases faster than that of doped samples before 373 K, which can be attributed to the rapidly increasing equilibrium vacancy concentration with the increase of temperature. However, for the doped samples, we speculate that the foreign atoms disturbed the relationship between cation vacancy concentration and temperature and the specific mechanism needs further investigate. On the other hand, due to the decreased carrier concentration, the onset temperature of bipolar effect decrease from ~573 K for SnTe (n ≈ 1.40 ×1020 cm-3) to ~473 K for Sn0.98Ce0.02Te (n ≈ 0.57×1020 cm-3). For the Li-doped samples, the bipolar effect gradually disappears with the increase of Li content, which is caused by the sharp increase of carrier concentration. However, before the onset of bipolar effect, the lattice thermal conductivity of Li doped samples is higher than that of pure SnTe from a macro point of view. In order to clarify the cause of this abnormal phenomenon, transmission electron microscope (TEM) is employed to investigate the microstructure of SnTe-(Li2Te)0.015. Figures 4(a, c) show the bright field image on different directions. The sample displays different contrast on different directions, while numerous nano-inclusions can be observed in both of them. The contrast is more obvious in the high resolution transmission electron microscope (HRTEM) image as shown in Figures 4(b, d). The selected area electron diffraction (SAED) in the inset of Figure 3a displays two types of patterns, which belongs to [2 1 -1] axis of cubic SnTe and [3 2 -2] of hexagonal LiTe3, respectively. In the yellow circle, the strong spot belongs to SnTe and the weak one belongs to LiTe3. In addition, the strong spots of SnTe were stretched to some extent. These evidences mean that the coherent stress of the two phases is relatively large on this direction. The two types of patterns were observed again in Figure 4c. The strong spots belong to [111] axis of SnTe and the weak ones belong to [001] axis of LiTe3. Furthermore, the crystallographic orientation relationship between the SnTe matrix and the LiTe3 nano-precipitates could be deduced as follows, ∥
(1)
∥
(2)
According to the stoichiometric ratio of LiTe3, one Li atom coordinates three Te atoms. Furthermore, the precipitate concentration is extremely high as shown in the TEM result, so the matrix is Sn-rich and Te-poor in Li doped samples. This state is similar to the self-compensation SnTe, which means that the vacancy concentration in the matrix is greatly reduced [6]. Because the lattice distortion caused by Li replacing Sn is much smaller than that caused by Sn vacancy, the lattice thermal conductivity of Li doped samples is relatively higher than pure SnTe before the bipolar effect. Furthermore, the extremely dense and dispersed nano-precipitates (<10 nm) will 4
contribute additional strong phonon scattering [27] and result in the relatively low lattice thermal conductivity at high temperature. Specifically, the κlat value at 873 K falls below 0.7when y ≥ 0.015. The temperature-dependent figure of merit of Ce-doped samples and Li-doped samples are shown in Figures 5(a, b). For the Ce-doped samples, the average ZT value (ZTave) from 300 K to 873 K increases considerably, although the increase of peak ZT at 873 K is limited. However, for Li-doped samples, the peak ZT at 873 K increases to more than 0.8, but the enhancement of ZTave is almost negligible. Naturally, Ce and Li co-doped SnTe is expected to achieve both average and peak ZT values. Sn0.98Ce0.02Te was selected as the matrix because of its relatively high ZTave and no detectable secondary phase. On this basis, the content of Li2Te was adjusted to optimize the thermoelectric performance. The electrical and thermal transport properties for Sn0.98Ce0.02Te(Li2Te)z (z = 0, 0.005, 0.01, 0.015, 0.02) are shown in Figure S2, and the temperature-dependent ZT values are shown in Figure 5c. When z ≥ 0.01, the peak ZT at 873 K reaches ~1, while the Sn0.98Ce0.02Te-(Li2Te)0.01 sample possesses the highest ZTave of ~0.39 from 300 K to 873 K as shown in Figure 5d. The increasement of these two values is ~54% and ~100%, respectively, compared to that of pure SnTe. In summary, Ce-doped SnTe, Li-doped SnTe and Ce-Li co-doped SnTe are prepared and the corresponding thermoelectric properties were investigated. Cerium substituting Sn can effectively reduce the carrier concentration and significantly increase the ZTave. For lithium doping, besides adjusting the carrier concentration to optimize the electrical transport properties, high density nanoprecipitates were constructed to strongly scatter the phonons and then reduce the lattice thermal conductivity. As a result, the peak ZT is increased to ~0.8 at 873 K. Furthermore, cerium and lithum were employed to cooperatively improve both the peak ZT and average ZT. Finally, the peak ZT reaches ~1 for Sn0.98Ce0.02Te-(Li2Te)0.01, and the ZTave from 300 K to 873 K reaches ~0.39.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51771065, 51622101, and 51871082). Supplementary Materials See supplementary materials for experimental details and Figures S1-S2. Conflict of Interest The authors declare no conflict of interest. References [1] W. Liu, J. Hu, S. Zhang, M. Deng, C.-G. Han, Y. Liu, New trends, strategies and opportunities in thermoelectric materials: A perspective, Mater. Today Phys. 1 (2017) 50-60. [2] L.M. Rogers, Valence band of SnTe, J. Phys. D: Appl. Phys. 1 (1968) 845-852. 5
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Figure 1. Temperature-dependent figure of merit (ZT) for pristine SnTe of reports5,8,13-15,20-23 and this work. (a) Original data; (b) recalculated data with the same Cp14; (c) ZT vs room temperature carrier concentration; (d) ZTave from 300 K to 873 K. The number on the column is room temperature carrier concentration, in units of 1020cm-3. Figure 2. Room temperature carrier concentration of Sn1-xCexTe samples and SnTe-(Li2Te)y samples. Figure 3. Temperature-dependent electrical conductivity (a), (c), Seebeck coefficient (b), (d), total thermal conductivity (e), (g), and lattice thermal conductivity (f), (h) of Sn1-xCexTe samples and SnTe-(Li2Te)y samples, respectively. Figure 4. Microstructures of (SnTe) (Li2Te)0.015. (a) and (c) TEM images showing a high density of nano-precipitates on different direction. Inset: SAED patterns. (b), (d) HRTEM images of the red squares in (a) and (c). Figure 5. Temperature-dependent ZT of (a) Sn1-xCexTe samples, (b) SnTe-(Li2Te)y samples, and (c) Sn0.98Ce0.02Te-(Li2Te)z samples. (d) Comparison of ZT and ZTave of pure SnTe, Sn0.98Ce0.02Te, SnTe-(Li2Te)0.005, and Sn0.98Ce0.02Te-(Li2Te)0.01.
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Figure 1
Figure 2
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Figure 3 9
Figure 4
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Figure 5
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Both increasing and decreasing carrier concentration can improve thermoelectric properties of pure SnTe
Dense and dispersive nano-precipitates are introduced
ZT value and average ZT value are improved by cerium and lithium co-doping
Conflict of Interest Statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.