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Functional Inorganic Materials and Devices
Thermoelectric Transport Properties of CdxBiyGe1-x-yTe Alloys Juan Li, Wen Li, Zhonglin Bu, Xiao Wang, Bo Gao, Fen Xiong, Yue Chen, and Yanzhong Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15080 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018
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Thermoelectric Transport Properties of CdxBiyGe1-x-yTe Alloys Juan Li1, Wen Li1, *, Zhonglin Bu1, Xiao Wang1, Bo Gao1, Fen Xiong2, Yue Chen2, and Yanzhong Pei1, * 1Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji Univ., Shanghai 201804, China. 2 Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China. *Email:
[email protected] (WL);
[email protected] (YP)
Abstract: Band convergence has been proven as an effective approach for enhancing thermoelectric performance, particularly in p-type IV-VI semiconductors, where the superior electronic performance originates from the contributions of both L and band valleys when they converge to have a small energy offset. When alloying with cubic IV-VI semiconductors, CdTe has been found as an effective agent for achieving such a band convergence. This work focuses on the effect of CdTe-alloying on the thermoelectric transport properties of GeTe, where the carrier concentration can be tuned in a broad range through a Bi-doping on Ge site. It is found that CdTe-alloying indeed helps to converge the valence bands of GeTe in both low-T rhombohedral and high-T cubic phases for an increase in Seebeck coefficient with a decrease in mobility. In addition, the strong phonon scattering due to the existence of high-concentration Cd/Ge and Bi/Ge substitutions leads the lattice thermal conductivity to be reduced to as low as 0.6 W/m-K. These lead to an effectively increased average thermoelectric figure of merit (zTave~1.2) in 300-800 K, which is higher than that of many IV-VI materials with CdTe-alloying or alternatively with MnTe-, MgTe-, SrTe-, EuTe- or YbTe-alloying for a similar band convergence effect. Keywords: Thermoelectric, band convergence, lattice thermal conductivity, average zT performance largely originates from multiple valence bands with a small energy offset and with high valley degeneracies, namely 4 for L and 12 for bands30-31. Band convergence reduces the band offset thus enables an overall large number of transporting band valleys, which can usually be realized in PbTe32-37 and SnTe38-40 with MnTe-, CdTe-, MgTe-, SrTe-, EuTe- and YbTe-alloying. Moreover, these alloying activities also lead to an effective decrease in lattice thermal conductivity because of the strong phonon scattering by alloy defects introduced41-42. GeTe belongs to the same class of group IV-VI semiconductors and is the only one experiencing a phase transition (at ~720 K43) from cubic structure to rhombohedral structure at above room temperature. Its low-T rhombohedral structure can be considered as a directionally distorted cubic lattice along the [111] direction. It is reported recently that due to the rhombohedral distortion, the original 4 L hole pockets in high-T cubic structure splits into 3L+1Z ones and the 12 into 6 + 6 in the low-T rhombohedral structure. Additionally, valence becomes the dominant transporting band because of its higher band energy than that of L in the rhombohedral phase, while in all cubic IV-VI semiconductors (including cubic GeTe) the energy of L and valence bands arranges in an opposite way 30-31, 44-45. With a carrier concentration optimization44, 46, a reduction in lattice thermal conductivity by defects20-21, 47-48 and a fine control of rhombohedral distortion for maximizing valley degeneracy49-50, zT of GeTe has recently been improved significantly. All these works indicate that GeTe can be an extraordinarily performance thermoelectric material, once the valence bands are converged. In cases of GeTe’s analogue compounds PbTe and SnTe, formation of solid solutions with metal monotellurides such as MnTe32, 51-52, CdTe33, 39, MgTe34, 40, 53, SrTe54-55, EuTe37 and YbTe37 enables both a convergence of valence bands and an effective L-reduction for a significantly improved zT. This motivates the current work to realize similar effects in GeTe alloys and CdTe-alloying is focused on. Because of the high intrinsic hole concentration in GeTe-CdTe alloys, Bi-doping at Ge sites is further used for carrier concentration optimization. The DFT calculations indeed confirm the convergence between L and valence bands due to CdTe-alloying in both low-T rhombohedral and high-T cubic structures. This leads to an increase in the density-of-states mass due to CdTe-alloying, which ACS Paragon Plus Environment
Introduction Thermoelectric (TE) materials, enabling direct conversion between heat and electricity based on the Seebeck effect1, have attracted increasing attention as a solution to energy crisis. The conversion efficiency depends on the dimensionless figure of merit (zT) of thermoelectric materials, which is determined by zT=S2T/E L, where S, T, , E and L are Seebeck coefficient, absolute temperature, electrical resistivity, electronic and lattice components to thermal conductivity, respectively. The relatively low zT leads the majority of existing efforts to focus on zT-enhancement of thermoelectric materials. Since S, and E are strongly correlated, it is difficult to improve zT through manipulating the parameters individually. However, the optimal electrical performance of a thermoelectric material dependents primarily on the weighted mobility2-4, (m*/me)3/2, where m*=Nv2/3mb*, is nondegenerate mobility of carriers, Nv is number of degenerated bands and mb* is average band effective mass. By increasing the degeneracy of transporting band valleys, (Nv) 5, the electronic performance has been significantly increased in many materials including (Pb,Sn,Ge)Te5-7, Mg2Si8, CoSb39, AgSbTe210, La3Te411, Clathrates12, AB2C2 Zintls13, Mg3Sb214-15, Half-Heuslers16 and Bi2Te317-18. Alternatively minimizing the lattice thermal conductivity L, the only independent parameter determining zT, through strengthening the phonon scattering by various defects19, such as zero-dimensional point defects (including substitutional20-21, interstitial22, vacancy23), one-dimensional dislocations17, 24 and two-dimensional boundary interfaces25, has been demonstrated as an effective strategy as well for enhancing thermoelectric performance. Moreover, intrinsically low L, stemming from the complex crystal structure26, soft chemical bonds27 and strong lattice anharmonicity28, has also opened new possibilities for exploring novel thermoelectrics with a high zT. For power generation applications, high performance thermoelectric materials are typified by Half-Heuslers16, BixSb2-xTe317-18 alloys and PbTe29 alloys. Group IV-Telluride semiconductors, including PbTe, SnTe, GeTe and their alloys, have long been considered as one of the most important classes of thermoelectric materials. As reported in p-type PbTe, SnTe and cubic GeTe thermoelectrics, the high
ACS Applied Materials & Interfaces
Room temperature powder XRD patterns for Ge1-xCdxTe is consistent with the experimental results. In addition, (0≤x≤0.15) are shown in Fig. 1a. All the diffraction peaks x0.1. These results suggest the solubility of CdTe in improvement in the average thermoelectric figure of merit (zTave), GeTe is about 10%. Furthermore, SEM observations and EDS and the achieved zTave as high as ~1.2 within 300-800 K analysis are carried out on the samples with x=0.1 (Fig. 1c) and demonstrates GeTe-CdTe alloys as one of the top performanced 0.12 (Fig. 1d) to further confirm the phase compositions. Ge thermoelectrics among IV-VI semiconductors alloyed with the precipitates are observed in both samples, which is a common same (CdTe) or a similar agent (MnTe, MgTe, SrTe, EuTe or case in GeTe-based materials3-4, 48, 60-61, due to the low formation YbTe) for valence band convergence. Materials and Metods energy of Ge-vacancies63. CdTe impurities sized in microns are Polycrystalline samples of Ge1-xCdxTe (0≤x≤0.15) and observed in Ge0.88Cd0.12Te but not in Ge0.9Cd0.1Te, further Ge0.92-yCd0.08BiyTe (0≤y≤0.09) are synthesized by melting the confirming the solubility of ~10%. This is also well consistent stoichiometric amount of pure elements (>99.99%) in sealed with literature phase diagram of GeTe-CdTe64 system. ampoules at 1223 K for 6 hours, quenching in cold water and then Ge1-xCdxTe 300 K annealing at 963 K for 3 days. The obtained ingots were hand 88.55 b 6.010 a 88.50 crushed into fine powders for X-ray diffraction (XRD) with a 6.005 88.45 HAOYUAN DX-2700BH apparatus, which is equipped with a Cu 6.000 88.40 Kα radiation (λ1 = 1.54056 Å, λ2 = 1.54443 Å) and a PIXcel 5.995 88.35 detector. The optical reflectance was measured by a Fourier 88.30 5.990 88.25 Transform Infrared Spectroscopy (FTIR, Bruker Tensor II 5.985 88.20 equipped with a Diffuse Reflectance attachment) at room 5.980 88.15 58 temperature. Hot press was then followed by induction heating 5.975 88.10 20 30 40 50 60 70 80 90 0 2 4 6 8 10 12 14 at 873 K for 40 minutes under a uniaxial pressure of ~75 MPa. All X (%) 2 () d Ge0.88Cd0.12Te c Ge0.9Cd0.1Te consolidated samples are nearly full dense (>98% of the Cd Cd theoretical density), and ~12 mm in diameter and ~1.5 mm in thickness. Resistivity, Seebeck coefficient were simultaneously measured in a home-built apparatus on the hot-pressed pellet from 10m 10m 10m 300 to 800 K. The Seebeck coefficient was obtained from the 10m slope of the thermopower vs. temperature difference from 0~5 K59. Ge Te Ge Te The thermopower was measured by two Nb wires welded to the thermocouple tips, the hot and cold side temperatures were measured by two K-type thermocouples attached to the opposite edges of the pellet. Hall coefficient were measured by the Van der 10m 10m 10m 10m Pauw technique under a reversible magnetic field of 1.5 T. The Fig. 1. Room temperature powder XRD patterns (a), lattice thermal conductivity is determined via κ = dCpD, where D is the parameters and rhombohedral angles (b) for Ge1-xCdxTe. SEM images thermal diffusivity measured using a laser flash technique and corresponding EDS mappings for Ge0.9Cd0.1Te (c) and Ge0.88Cd0.12Te (Netzsch LFA457), d is the density determined by mass/volume, (d), indicating a solubility of 10% for CdTe in GeTe. Cp is the heat capacity determined by Dulong-Petit limit. The microstructure was characterized by a scanning electron microscope (SEM, Phenon-pro) equipped with an energy dispersive spectroscopy (EDS). Sound velocities were performed on pellet samples at room temperature, longitudinal and transverse sound velocities were determined using a pulse-receiver (Olympus-NDT) equipped with an oscilloscope (Keysight). Couplants were applied between the sample and the ultrasonic transducers. First-principles calculations were performed using Vienna ab initio simulation package (VASP)60. The generalized gradient approximation of Perdew, Burke and Ernzerhof61 was adopted as the exchange-correlation functional. The plane-wave cutoff energy was set to 450 eV and the electronic convergence criterion was 10-5 eV. One Ge atom in the 2×2×1 supercell, generated using a GeTe conventional unit cell, was substituted by one Cd atom to simulate the doped Ge11CdTe12. The Brillouin zone was sampled with a 7×7×5 k-point mesh. To perform band unfolding62, the atomic positions were relaxed with a fixed lattice. Spin-orbit coupling effects were considered. Results and Discussion ACS Paragon Plus Environment
x=0.15
x=0.12
x=0.10
a (Å)
angle (°)
CdTe
Intensity (a.u.)
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x=0.08
x=0.06
x=0.04 x=0.02 x=0
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respectively. Seebeck coefficient for all the samples is positive and is consistent with Hall measurements, indicating a p-type conduction. It is seen that Seebeck coefficient and resistivity for Ge1-xCdxTe increase continuously with increasing temperature, suggesting a degenerated semiconducting behavior. CdTe-alloying increases Seebeck coefficient and resistivity in the entire temperature range, which can be understood by the increased m* and reduced H (Fig. 3) respectively.
a 160 140
b 1.0
Ge1-xCdxTe
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80 x=0 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
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0.0
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x=0 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
6
Fig. 2 Effective band structures for low-T rhombohedral (a, b) and high-T cubic (c, d) GeTe without (a, c) and with (b, d) ~8.3% CdTe-alloying.
8.5E20 8.2E20 6.3E20 8.1E20 8.0E20 1.0E21
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The calculated band structures for GeTe with and without ~8.3% CdTe-alloying in both structures are shown in Fig. 2, an obvious reduced energy offset between L and valence bands is observed for rhombohedral GeTe due to alloying. Such an effect of band convergence is further confirmed by the increase in density-of-states effective mass (m*) with increasing CdTe-alloying concentration (Fig. 3a). The estimation of m* is based on a single parabolic band (SPB) model with an charge carrier scattering by acoustic scattering, which is confirmed by the T-1.5 dependence on Hall mobility (Fig. 3b). The band convergence achieved in this work is different from literature which is realized by manipulation of rhombohedral distortion and is mostly effective for rhombohedral phase6. Band convergence achieved here due to alignment of chemical orbitals induced by Ge/Cd substitution (Fig. 2) is affective on both rhombohedral and cubic phases. Although such a band convergence could lead to an enhancement in the electronic performance, but is partially compensated by the reduced Hall mobility (H, Fig. 3) due to CdTe-alloying.
0.2
1 0
300
400
500
600
T (K)
700
800
0.0
T (K)
Fig. 4. Temperature dependent Seebeck coefficient (a), resistivity (b), total and lattice thermal conductivities (c) and thermoelectric figure of merit (d) for Ge1-xCdxTe alloys (x≤0.1).
H (cm2/V-s)
H
d
Cd
H (cm2/V-s)
Temperature dependent thermal conductivity and its lattice component (L) are shown in Fig. 4c. L is estimated by subtracting the electronic component (E=LT/) from total thermal conductivity, where the Lorenz factor (L) is determined based on a single parabolic band (SPB) model with acoustic phonon scattering. Due to the additional phonon scattering introduced by the Ge/Cd substitution, L decreases with increasing x (Fig. 4c). A reduced L of 0.7 W/m-K is obtained at 800 K, leading to an increase in zT (Fig. 4d). The peak zT obtained at this point is not high for GeTe thermoelectrics, mainly because the carrier concentration (~81020 cm-3) in these alloys are significantly higher than optimum of 1~21020 cm-3 44. In order to estimate the full thermoelectric potential of Ge1-xCdxTe alloys, a proven efficient dopant Bi44 is utilized to reduce the carrier concentration. With an attempt to exclude the 70 b 100 a 2.5 -3/2 300 K T possible aggregations between impurities (Bi and Cd) forming 60 impurity phases, a slightly low concentration (8%) of 50 2.0 CdTe-alloying than its maximum (10%) is selected for the further 40 10 study of carrier concentration optimization. Room temperature Ge1-xCdxTe 30 x=0 8.5E20 powder XRD patterns for the synthesized Ge0.92-yCd0.08BiyTe 1.5 x=0.02 8.2E20 20 x=0.04 6.3E20 (y≤0.09) materials are shown in Fig. 5a. A BiTe impurity phase x=0.06 8.1E20 m* x=0.08 8.0E20 10 x=0.10 1.0E21 becomes observable when y increases to 9%, indicating a 0 1.0 1 0.00 0.02 0.04 0.06 0.08 0.10 300 400 500 600 700 800 solubility of ~8% for BiTe in Ge0.92Cd0.08Te. This is further T (K) x evidenced from the continuous increase in both lattice parameters and rhombohedral angles in materials with y up to 8% Fig. 3. Room temperature composition dependent density-of-states (Fig. 5b). The successful Bi/Ge substitution effectively reduces mass and Hall mobility (a), and temperature dependent Hall mobility (b) the carrier concentration to be as low as ~61019 cm-3 (Fig. 5c). for Ge1-xCdxTe (x≤0.1) indicating a dominant charge scattering by The decrease in carrier concentration due to Bi-doping leads to the acoustic phonons. reduction in photon energy that maximizes the infrared absorption Temperature dependent Seebeck coefficient and resistivity (plasma absorption Fig. 5d). for Ge1-xCdxTe (0≤x≤0.1) are shown in Fig. 4a and 4b, ACS Paragon Plus Environment md* (me)
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ACS Applied Materials & Interfaces
a
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300 K Ge1-xBixTe, Li, et.al
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1020
-3
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Fig. 6. Temperature dependent Seebeck coefficient (a), resistivity (b), Hall mobility (c), and carrier concentration dependent Hall mobility at 300 K (d) for Ge0.92-yCd0.08CdyTe (y≤0.08) alloys.
800
vL v
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CdxGe1-xTe Bi Cd Ge Te y 0.08 0.92-y
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Ge0.92-yCd0.08BiyTe
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GeTe y=0 5.2E20 y=0.02 6.21E21 y=0.035 3.2E20 y=0.04 2.62E20 y=0.04 2.18E20 y=0.05 2.32E20 y=0.055 2.07E20 y=0.07 1.41E20 y=0.08 6.3E19 Ge0.94Bi0.06Te 2.0E20
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1.2 t.al iu e Z. L T e, al Bi 0.04 g et. Mn 0.1 h en Z. Z Ge 0.86 T e, Sb 0.04 Mn 0.1
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Ge1+Te, Li, et.al
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Temperature dependent total thermal conductivity () and its lattice component (L) for Ge0.92-yCd0.08BiyTe are shown in Fig. 7a and 7b, respectively. Due to the further introduction of the point defects by Bi/Ge substitutions, a further decrease in the L is observed. The lowest L achieved here in only ~0.6 W/m-K in the entire temperature range, and this is approaching to the amorphous limit of GeTe according to the Cahill model65. To quantitatively understand the influences of Cd/Ge and Bi/Ge substitutions on L, composition dependent L at room temperature is shown in Fig. 7c, with a comparison to predictions based on the Debye-Callaway model6, 20, 66involving phonon-phonon and point defect scattering. Room temperature reduction in L can be well understood by the thermal model, indicating the existence of strong phonon scattering by point defects. The reduction in L is further found to be loosely related to the possible phonon softening effect in these materials with heavy-atom substitutions (Cd, Bi), since the measured sound velocities are nearly composition independent (Fig. 7d).
a
3
2
400
Fig. 7. Temperature dependent thermal conductivity (a) and lattice thermal conductivity (b), composition dependent lattice thermal conductivity (c) and sound velocities (d) at 300 K for Ge0.92-yCd0.08CdyTe alloys, with model predictions (solid curves) on lattice thermal conductivity.
2
50
0
0.6
Temperature dependent Seebeck coefficient and resistivity for Ge0.92-yCd0.08BiyTe (y≤0.08) are shown in Fig. 6a and 6b, respectively. Both Seebeck coefficient and resistivity increase with increasing Bi-doping in the entire temperature, resulting from the decrease in carrier concentration. The decrease in Seebeck coefficient and resistivity at high temperatures in low carrier concentration samples (y0.035), suggests an occurrence of bipolar conduction. It is found that both CdTe-alloying and Bi-doping do not change the scattering mechanism of the carriers, as shown in Fig. 6c. However, Bi-doping leads to a decrease in Hall mobility (Fig. 6c and 6d), which has also been observed in Bi-doped GeTe44. Therefore, the increase of the resistivity partially comes from the decrease of the Hall mobility.
0
300 K
0.0 0.1
L w/mk
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y= 0.0 7
10 12 14 16
d
2.0
y= 0.0 35 y= 0.0 55 y= 0.0 4
xCd+yBi (%)
T (K)
3.0 2.5
Fig. 5. Room temperature powder XRD patterns (a), lattice parameter and rhombohedral angles (b), room temperature. Composition dependent Hall carrier concentration (c) and normalized absorption versus photon energy at 300 K (d) for Ge0.92-yCd0.08CdyTe (y≤0.09).
a
c
y= 0.0 2 y= 0
Normalized Absorption
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GeTe-CdTe:Bi
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zT
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y=0.02 y=0
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L w/m-k
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GeTe y=0 y=0.02 y=0.035 y=0.04 y=0.04 y=0.05 y=0.055 y=0.07 y=0.08
6
6.00
a (Å)
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Ge
nH 1020 cm-3
Ge0.92-yCd0.08BiyTe
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BiTe
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a
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Ge0.92-yCd0.08BiyTe 300 K b
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Fig. 8. Temperature dependent figure of merit (zT) (a) and average zT (b) for Ge0.92-yCd0.08CdyTe (y≤0.08) alloys with a comparison to literature results for Ge1-xBixTe44, GeTe-MnTe67-69, PbTe-(Mg, Cd,Sr,Eu,Yb)Te29, 34-35, 37, 54, SnTe-(Cd, Mg, Mn)Te38-40. The effect of valence band convergence due to CdTe-alloying and
the optimization of carrier concentration by additional Bi-doping as well as a simultaneously enhanced phonon scattering, all contribute to the improvement in thermoelectric figure of merit ACS Paragon Plus Environment
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(zT) as shown in Fig. 8. The average zT, an critical parameter determining thermoelectric energy conversion efficiency70, is found to be as high as ~1.2 in the temperature range of 300-800 K. With a comparison to other high performance IV-VI based thermoelectric alloys with a similar effect of band convergence, Ge1-x-yCdxBiyTe shows one of the highest thermoelectric performance.
Thermoelectric Figure of Merit and Enhanced Mechanical Stability of P-Type Agsb1–Xznxte2. ACS Energy Lett. 2017, 2, 349-356. 1 (11) May, A. F.; Flage-Larsen, E.; Snyder, G. J. Electron and Phonon Scattering in 2 the High-Temperature Thermoelectric La 3 Te 4− Z M Z (M= Sb, Bi). Phys. Rev. 3 B 2010, 81, 125205. 4 (12) Wunderlich, W.; Amano, M.; Matsumura, Y. Electronic Band-Structure 5 Calculations of Ba8mexsi46-X Clathrates with Me= Mg, Pd, Ni, Au, Ag, Cu, Zn, 6 Al, Sn. J. Electron. Mater. 2014, 43, 1527-1532. 7 (13) Zhang, J.; Song, L.; Madsen, G. K.; Fischer, K. F.; Zhang, W.; Shi, X.; Iversen, B. Summary 8 B. Designing High-Performance Layered Thermoelectric Materials through CdTe-alloying is found to effectively converge the valence 9 Orbital Engineering. Nat. Commun. 2016, 7, 10892. 10 bands of GeTe in both low-T rhombohedral and high-T cubic (14) Ohno, S.; Imasato, K.; Anand, S.; Tamaki, H.; Kang, S. D.; Gorai, P.; Sato, H. K.; 11 phases. With a further aliovalent doping for carrier concentration Toberer, E. S.; Kanno, T.; Snyder, G. J. Phase Boundary Mapping to Obtain N 12 optimization, the thermoelectric performance of Ge1-x-yCdxBiyTe -Type Mg 3 Sb 2 -Based Thermoelectrics. Joule 2017, 2, 141-154. 13 alloys is greatly enhanced, which benefits from the simultaneous (15) Zhang, J.; Song, L.; Pedersen, S. H.; Yin, H.; Hung, L. T.; Iversen, B. B. Discovery 14 reduction in lattice thermal conductivity (of only 0.6 W/m-K at all of High-Performance Low-Cost N-Type Mg3sb2-Based Thermoelectric 15 temperatures) due to phonon scattering by alloy defects. The Materials with Multi-Valley Conduction Bands. Nat. Commun. 2017, 8, 13901. 16 achieved average zT of 1.2 demonstrates Ge1-x-yCdxBiyTe alloys as (16) Fu, C.; Bai, S.; Liu, Y.; Tang, Y.; Chen, L.; Zhao, X.; Zhu, T. Realizing High Figure 17 thermoelectrics of a great potential. of Merit in Heavy-Band P-Type Half-Heusler Thermoelectric Materials. Nat. 18 Commun. 2015, 6, 8144. 19 Acknowledgement (17) Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. 20 This work is supported by the National Natural Science J.; Shin, W. H.; Li, X. S.; Lee, Y. H.; Snyder, G. J.; Kim, S. W. Dense Dislocation 21 Foundation of China (Grant No. 11474219 and 51772215), Arrays Embedded in Grain Boundaries for High-Performance Bulk 22 National Key Research and Development Program of China Thermoelectrics. Science 2015, 348, 109-114. 23 (2018YFB0703600), Fundamental Research Funds for Science (18) Kim, H.-S.; Heinz, N. A.; Gibbs, Z. M.; Tang, Y.; Kang, S. D.; Snyder, G. J. High 24 and Technology Innovation Plan of Shanghai (18JC1414600), the Thermoelectric Performance in (Bi 0.25 Sb 0.75 ) 2 Te 3 Due to Band 25 Fok Ying Tung Education Foundation (Grant No. Convergence and Improved by Carrier Concentration Control. 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ACS Applied Materials & Interfaces
84, 2436-2438.
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ACS Applied Materials & Interfaces
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