Interstitial Defects Improving Thermoelectric SnTe in Addition to

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Interstitial Defects Improving Thermoelectric SnTe in Addition to Band Convergence Linglang Zheng, Wen Li, Siqi Lin, Juan Li, Zhiwei Chen, and Yanzhong Pei* Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China S Supporting Information *

ABSTRACT: SnTe, as a top Pb-free alternative to PbTe, attracts extensive attention for thermoelectric applications. For thermoelectric performance enhancement, successful electronic strategies are typified by band convergence and resonant doping, while effective thermal strategies include nanostructuring and, recently, interstitial defects. This work demonstrates that phonon scattering by interstitial defects, as a nearly immune strategy integratable to band convergence, independently reduces the lattice thermal conductivity to the amorphous limit. This leads to a zT as high as 1.4 in Sn1−yMgyTe(Cu2Te)x, where Cu2Te acts as the source of interstitial defects while MgTe converges the valence bands. Evolutionarily, either Cu2Te or MgTe enables a ∼100% zT-enhancement as compared with that of pristine SnTe, while an overall ∼200% enhancement is successfully realized when both exist. This work not only improves SnTe as an eco-friendly alterative to thermoelecric PbTe but also demonstrates an approach potentially applicable for improving thermoelectrics. applications.45−47 Although a peak zT of ∼1.8 or higher is achievable,21,28,29,48 the toxicity of Pb makes it a big concern for a large-scale application. Because of similar two-valence band structure and the same crystal structure as PbTe, SnTe has attracted increasing attention for thermoelectric application as a Pb-free alternative to PbTe. Following the strategy of band convergence demonstrated in PbTe,27−29,48,49 where substitution of Pb by isovalent impurities is used to reduce the energy offset between the two valence bands, a significantly enhanced zT has been achieved in SnTe solid solutions27−36 as well. The similarity among those high zT SnTe solid solutions relies on their relatively high lattice thermal conductivity (κL) as compared with that of PbTe. 27−29,48,49 This leads to straightforward attempts on a further zT-enhancement by introducing additional scattering of phonons. Among these efforts, nanostructuring is the main focus and indeed leads to an excitingly low κL of ∼0.6 W/m·K (at 850 K),40 which results in a high zT (∼1.3 or slightly higher) in composite materials with an effect of either band convergence or resonant doping.34,35,38 The achieved lattice thermal conductivity still remains higher than the amorphous limit (0.4 W/m·K) of SnTe, and these high zT materials usually include Cd or Hg,34,35,40 unfortunately.

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hermoelectric materials directly convert heat into electricity based on the Seebeck effect. Without hazardous emissions or moving parts, it is considered as a sustainable and clean technology for generating electricity. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit, zT = σS2T/(κE + κL), where σ, S, T, κE, and κL are the electrical conductivity, Seebeck coefficient, absolute temperature, and electronic and lattice components of the thermal conductivity, respectively. Performance enhancement of thermoelectric materials remains as the key challenge for their large-scale applications. Because of a strong coupling between σ, S, and κE, most thermoelectric research activities have been focused on the reduction in lattice thermal conductivity for enhancing zT, which achieves great success in various materials. These strategies include lattice anharmonicity;1,2 liquidlike lattice;3,4 rattling impurities;5 nanostructures;6−10 dislocations;11,12 low sound velocity;13 and point defects including substitutions,14,15 vacancies,16−18 and interstitials19 in various materials. Recently, concepts for engineering20 the band structure, including band convergence,20,21 nestification,22 resonant doping,23,24 effective mass,25 and deformation potential coefficient,26 have also been shown to enable somewhat decoupled electronic properties among σ, S, and κE for zT enhancements. These are demonstrated in various materials, such as PbTe,27−30 SnTe,23,31−40 Mg2Si,41,42 half-Heusler,43,44 and elemental Te,22 for successfully enhancing the power factor (S2σ). Since the 1950s, lead telluride, PbTe, as a thermoelectric material has been extensively studied for military and space © 2017 American Chemical Society

Received: December 9, 2016 Accepted: February 1, 2017 Published: February 2, 2017 563

DOI: 10.1021/acsenergylett.6b00671 ACS Energy Lett. 2017, 2, 563−568

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ACS Energy Letters Among the SnTe alloys with all eco-friendly constituent elements, SnTe−MgTe solid solutions enable one of the highest zT,32 which is believed to be due to both its effectively converged valence bands and its high concentration of MgTe. It is known that formation of a solid solution would introduce point defects for scattering phonons and therefore reducing lattice thermal conductivity. Mg substitutional point defects scatter phonons and lead to a low lattice thermal conductivity of 0.8 W/m·K. A further reduction in lattice thermal conductivity of SnTe−MgTe solid solutions, especially without involving toxic elements, would ideally promote its potential as an alternative32,34−36,38−40,50 to PbTe. The scattering of phonon by point defects mainly comes from the mass and strain fluctuations between the host and guest atoms. It is shown that introducing interstitial or vacancy point defects9,16,19,51 would maximize both of the fluctuations and therefore the phonon scattering. Alloying SnTe with Cu2Te has been recently reported to successfully introduce interstitial Cu defects preferentially located at (1/4, 1/4, 1/4) sites and therefore lead to an extremely low lattice thermal conductivity of 0.5 W/m·K.19 This is one of the lowest lattice thermal conductivity reported in SnTe-based materials to date. Importantly, the introduction of point defects in SnTe− Cu2Te solid solutions does not introduce any observable detrimental effects on the electronic transport properties. This motivates the current work to further improve the thermoelectric performance of SnTe−MgTe alloys with a known band convergence effect, by an additional thermal strategy of Cu2Te for reducing the lattice thermal conductivity. The lattice thermal conductivity (κL) is found to be reduced independently, meaning no observable detrimental effects on electronic properties. The achieved κL as low as 0.5 W/m·K, which is approaching the amorphous limit of SnTe, enables a further 50% zT-enhancement up to 1.4 in a chemical composition with all eco-friendly elements. This combination of strategies should be applicable in SnTe and similar thermoelectrics. According to existing work on SnTe solid solutions27−36 with a valence band convergence effect, the degree of the convergence of the valence bands in SnTe usually increases with increasing concentration of the MgTe. Previous study32 indicates that MgTe remains soluble in SnTe with a concentration as high as 12%, which is one of the highest among the known SnTe alloys enabling a valence band convergence. This study confirms that the X-ray diffraction (XRD) results of Sn0.91Mg0.12Te show no impurity peaks other than SnTe (Figure 1a). The lattice parameter change is found to be only about 0.3% for SnTe alloyed with 12% MgTe; therefore, the occupancy of Cu due to the lattice strain of Mg/ Sn substitution is assumed to be negligible in this work. This alloy composition, having a high concentration of MgTe to ensure sufficiently converged valence bands, is chosen for a further zT-enhancements by introducing Cu2Te. Because it is possible that the existence of MgTe reduces the solubility of Cu2Te (with a solubility up to 6% in pristine SnTe19,52), another series of materials with a fixed Cu2Te concentration of 5% but a variable concentration of MgTe (5−12%) are investigated as well. This enables a clear evolution on both the phase compositions and thermoelectric properties. The powder XRD results for both Sn0.91Mg0.12Te(Cu2Te)x and Sn1.03−yMgyTe(Cu2Te)0.05 samples at room temperature are shown in Figure 1. The characteristic peaks can be indexed to the cubic structure of SnTe. However, impurity peaks of Cu2Te

Figure 1. XRD patterns for Sn0.91Mg0.12Te(Cu2Te)x (a) and Sn1.03−yMgyTe(Cu2Te)0.05 (b) and SEM images for the samples with x = 0.05 (c) and y = 0.05 (d), showing that Cu2Te (dark spots) precipitates out while most of the MgTe remains soluble.

are detected when x = 0.05 (Figure 1a) and y ≥ 0.05 (Figure 1b). This is consistent with the scanning electron microscopy −energy disperse spectroscopy (SEM/EDS) observations at room temperature, where Cu2Te precipitates (dark spots) can be observed in the samples with x = 0.05 (Figure 1c) and y = 0.05 (Figure 1d). The size of Cu2Te precipitates is in a micrometer scale. The EDS analyses for the matrix phase in the sample with x = 0.05 (Figure 1c) show the concentration of MgTe to be ∼12%, while the concentration of Cu2Te is ∼3%. A high concentration of Cu2Te (∼5%) can be obtained only when the MgTe content is low (< ∼5%) in (Sn1.03−yMgyTe)(Cu2Te)0.05 (Figure 1d). This suggests that Cu2Te does not reduce the solubility of MgTe, but MgTe does reduce the solubility of Cu2Te, at room temperature. In other words, MgTe is much more preferentially soluble in SnTe than Cu2Te. However, the solubility of Cu2Te in SnTe is known to increase with increasing temperature,19,52 leading to an expectation that more Cu2Te becomes soluble at high temperatures in both series of materials studied in this work. The preferentially soluble MgTe as observed in this work indeed enables a band convergence as effective as that reported previous work on MgTe−SnTe alloys.32 This is first evident from the temperature-dependent Hall coefficient (RH ) measurements, which are shown in Figure 2a for Sn0.91Mn0.12Te(Cu2Te)x. As a sign of band convergence, a lower temperature that maximizes RH can be observed because of the redistribution of carriers between the light and heavy bands53 with significantly different mobilities. This is often observed in IV−VI thermoelectrics29,31,54 as an indication of band convergence. The temperature peaking R H in Sn0.91Mn0.12Te(Cu2Te)x (∼600 K) is much lower than that in pristine SnTe (∼750 K), suggesting a reduced energy offset between the light and heavy valence bands.31,55,56 It is seen that the Cu2Te does not lead to an observable change in the temperature peaking RH for Sn0.91Mn0.12Te(Cu2Te)x, which is consistent with the case in SnTe−Cu2Te alloys.31 This further indicates that the concentration of soluble MgTe is about the 564

DOI: 10.1021/acsenergylett.6b00671 ACS Energy Lett. 2017, 2, 563−568

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Figure 2. Temperature-dependent Hall coefficient (a, b) and Hall mobility (c, d) for Sn 0.91 Mg 0.12 Te(Cu 2 Te) x (a, c) and Sn1.03−yMgyTe(Cu2Te)0.05 (b, d), indicating a convergence of the two valence bands and a dominant scattering of carriers by acoustic phonons.

Figure 3. Hall carrier concentration-dependent Seebeck coefficient (a, b) and Hall mobility (c, d) at 300 K (a, c) and 723 K (b, d) for Sn0.91Mg0.12Te(Cu2Te)x and Sn1.03−yMgyTe(Cu2Te)0.05, with a comparison to the literature modeling57 and experimental results for pristine SnTe57 and Sn0.85Mn0.15Te.31

same for all the Sn0.91Mn0.12Te(Cu2Te)x samples, which is consistent with the phase composition as discussed above (Figure 1). The soluble content of MgTe and its effect on converging the valence bands can be further confirmed in Sn1.03−yMgyTe(Cu2Te)0.05, where the temperature peaking RH gradually decreases with increasing y (Figure 2b). However, coexistence of MgTe and Cu2Te does not change the dominant scattering of charge carriers by acoustic phonons, as indicated in Figure 2c,d. The Hall carrier concentration (nH) dependent Seebeck coefficient further supports the effect of band convergence in both series of materials. The significantly enhanced Seebeck coefficient (symbols), as compared with that of pristine SnTe (curves, Pisarenko line57) at a given nH, can be seen at both low (Figure 3a) and high (Figure 3b) temperatures in all these materials. It is seen that S-enhancement overall increases with increasing MgTe content, meaning a reduced energy offset (convergence) between the light and heavy valence bands. As a result, an increased population of lower-mobility holes residing in the lower-energy valence band (heavy band) is expected to reduce the overall Hall mobility, which is shown in Figure 3c,d. It should be noted that the Hall mobility reduction would also be partially due to the introduction of point defects and precipitate boundaries. Temperature-dependent Seebeck coefficient, resistivity, thermal conductivity, and figure of merit for Sn0.91Mg0.12Te(Cu2Te)x are shown in Figure 4. The samples are notated with both room-temperature nH and x. The nH of all the samples ranges from 1.5 to 3.0 × 1020 cm−3, corresponding to a degenerate semiconducting behavior for SnTe. The difference in Seebeck coefficient and resistivity can be understood by the decrease in carrier concentration. In addition to the introduced phonon scattering by substitutional point defects due to alloying with MgTe, existence of Cu2Te further enables a significant reduction in thermal conductivity (Figure 4c), especially for the lattice component. The lattice thermal conductivity is estimated by

Figure 4. Temperature-dependent Seebeck coefficient (a), resistivity (b), total and lattice thermal conductivity(c), and thermoelectric figure of merit zT (d) for Sn0.91Mg0.12Te(Cu2Te)x, with a comparison to that of literature Sn0.93Mg0.1Te32 and SnTe.19

subtracting the electronic contribution via the Wiedemann− Franz law (κe = LT/ρ) from the total thermal conductivity (κ), where L is the Lorenz factor determined by a single Kane band (SKB) model with an acoustic phonon scattering.58 Both κ and κL decrease with increasing Cu2Te concentration and temperature. Because the dominant source for the observed significant reduction in the lattice thermal conductivity (κL) in SnTe− Cu2Te alloys is known to be the interstitial Cu,19 which actually leads to one of the lowest κL ever reported in SnTe, the same 565

DOI: 10.1021/acsenergylett.6b00671 ACS Energy Lett. 2017, 2, 563−568

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In summary, given that either band convergence due to MgTe or interstitial defects due to Cu2Te enables a ∼100% zTenhancement as compared with that of pristine SnTe, this work indeed realizes a ∼200% net enhancement in zT when both exist and importantly in a composition without toxic elements. The integration of interstitial defects with band convergence, of which individual effect on zT-enhancement is nearly immune to each other, successfully enables taking full advantage of both for improving thermoelectric SnTe. This work not only improves SnTe as an eco-friendly alternative to thermoelectric PbTe but also evokes a synergic approach that is highly effective for advancing thermoelectrics.

reason is expected to hold for the reduction observed in this work as well. This is further supported by the factor that the solubility of Cu2Te increases with increasing temperature in SnTe,19,52 which should lead to an increased concentration of interstitial Cu for a strong phonon scattering at high temperatures in the samples with a large x (> ∼3%) studied here. According to the SEM observations (Figure 1), a precipitation of Cu2Te is found. However, the relatively large size of micrometers for these precipitates is usually not expected to introduce a strong phonon scattering because of boundary interfaces. Most importantly, the samples with x > 3% indeed show a lattice thermal conductivity quite comparable with that of SnTe alloyed with 5% Cu2Te19 in the entire temperature range, all of which show a κL approaching the amorphous limit (0.4 W/m·K19,59) of SnTe. Without Cu2Te, the obtained zT in the entire temperature range is consistent with that of previously reported Sn0.93Mg0.1Te,32 the composition of which is the closest to the current work. With Cu2Te, a thermoelectric figure of merit, zT, as high as 1.4 is achievable (Figure 4d). More quantitatively, alloying with 12% MgTe enables a ∼100% zT-enhancement as compared with that of pristine SnTe, while an additional involvement of 5% Cu2Te leads a further ∼50% zT-enhancement. The high zT is found to be reproducible (Figure S1), and the material is stable (Figure S2). Temperature-dependent transport properties of the other series of materials, Sn1.03−yMgyTe(Cu2Te)0.05 (Figure 5) further

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EXPERIMENTAL METHODS Polycrystalline (Sn0.91Mg0.12Te) (Cu2Te)x (x ≤ 0.05) and (Sn1.03−yMnyTe) (Cu2Te)0.05 (y ≤ 0.12) were synthesized by melting stoichiometric high-purity elements (>99.99%) at 1123 K for 6 h, quenching in cold water, and annealing at 950 K for 3 days. A 3% excess of Sn is used to locate an optimal carrier concentration for this material.40 The obtained ingots were ground for hot pressing and powder X-ray diffraction. Pellet samples were obtained by an induction heating hot press system at 950 K for 30 min under a uniaxial pressure of ∼70 MPa. The microstructure and composition are characterized by a SEM with EDS. The pellet samples for transport property measurements were ∼12.0 mm in diameter and ∼1.5 mm in thickness, and the density (d) is 98% or greater. Thermal diffusivity (D, Figure S3) was measured using a laser flash technique with the Netzsch LFA457 system; the heat capacity was determined from the measured values of Blachnik and Igel60 by Cp(kB/ atom) = (3.07 + 0.00047(T/K − 300)) for lead chalcogenides25 and tin telluride31 (Figure S4). This equation is obtained by fitting the experimental data of Blachnik and Igel within an uncertainty of 2% for lead chalcogenides and SnTe.60 The thermal conductivity was calculated by κ = dCpD, where d is the density measured by the mass/volume of the pellet. The resistivity and Hall coefficient were measured using the van der Pauw technique under a reversible magnetic field of 1.5 T. The details on the estimation of Hall factor can be found elsewhere.25 For comparison, the Seebeck coefficient and resistivity for Sn0.91Mg0.12Te(Cu2Te)0.05 were also measured using an ULVAC ZEM-3 system (Figure S5). The Seebeck coefficient was obtained from the slope of the thermopower versus temperature gradient within 0−5 K. All of the measurements are carried out during heating and cooling. The electrical transport properties (S, σ, and κ) were measured simultaneously during both heating and cooling. The measurement details have been given elsewhere.31 Transport property measurements (S, σ, and κ) were carried out under vacuum from room temperature to 927 K, with the uncertainty of each being ∼5%.

Figure 5. Temperature-dependent Seebeck coefficient (a), resistivity (b), total and lattice thermal conductivity(c), and thermoelectric figure of merit zT (d) for Sn1.03−yMgyTe(Cu2Te)0.05, with a comparison to those of literature SnTe−Cu2Te alloys.19

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ensures the evolution for the observed high zT. Band convergence gradually increases Seebeck coefficient and resistivity. A 5% Cu2Te successfully ensures an extremely low lattice thermal conductivity in the majority of the samples, indicating interstitial Cu is indeed the dominant source for scattering the phonons. Similarly, a peak zT of ∼1.4 is achievable, but is limited only in samples with a high concentration of MgTe that ensures sufficiently converged valence bands.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00671. Temperature-dependent Seebeck coefficient, resistivity, thermal conductivity, thermal diffusivity, heat capacity, and zT (PDF) 566

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yanzhong Pei: 0000-0003-1612-3294 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the National Natural Science Foundation of China (Grant Nos. 51422208, 11474219), the national Recruitment Program of Global Youth Experts (1000 Plan), and the fundamental research funds for the central universities. The authors thank Professor Xun Shi and Lidong Chen from the Shanghai Institute of Ceramics, CAS for their support on Seebeck coefficient and resistivity measurements using ZEM-3 for comparison.

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ACS Energy Letters

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DOI: 10.1021/acsenergylett.6b00671 ACS Energy Lett. 2017, 2, 563−568