High Performance Î±-MgAgSb Thermoelectric Materials for Low

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High performance #-MgAgSb thermoelectric materials for low temperature power generation Pingjun Ying, Xiaohua Liu, Chenguang Fu, Xianqiang Yue, Hanhui Xie, Xinbing Zhao, Wenqing Zhang, and Tie-Jun Zhu Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 16 Jan 2015 Downloaded from http://pubs.acs.org on January 17, 2015

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Chemistry of Materials

High Performance α-MgAgSb Thermoelectric Materials for Low Temperature Power Generation Pingjun Ying1, Xiaohua Liu1, Chenguang Fu1, Xianqiang Yue1, Hanhui Xie1, Xinbing Zhao1, 2, Wenqing Zhang3 and Tiejun Zhu*1, 2. 1

State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China 2 Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou 310027, China 3 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Supporting Information Placeholder ABSTRACT: Bi2Te3 based alloys have long been the best and unique thermoelectric materials for power generation below 550K. Their substitutes with abundantly available elements are highly desirable due to the scarcity of Te element. In this work the band structure calculation of α-MgAgSb compound shows a narrow gap characteristic. Highly pure α-MgAgSb is obtained by a carefully controlled processing. The samples exhibit an intrinsically low thermal conductivity due to the unique crystal structure. A high zT of ~1.1 at 525 K is achieved in the In doped α-MgAgSb with the optimal carrier concentration of 8~9×1019 cm-3, comparable to that of Bi2Te3 based alloys. Considering the abundantly available constituent elements, the present results demonstrate that αMgAgSb is a promising candidate for low-temperature (RT-550K) power generation.

INTRODUCTION Thermoelectric (TE) technology, which can directly convert heat into electricity, has aroused increasing attention as one of promising solutions to current energy crisis and environmental issues1,2. The conversion efficiency of TE devices is gauged by the materials’ dimensionless figure of merit , zT=α2σT/(κe+κl), where α, σ, κe, κl and T are respectively the Seebeck coefficient, the electrical conductivity, the electrical and lattice components of thermal conductivity κtot and the absolute temperature3,4. The transport parameters α, σ and κe cannot be independently tuned due to the strong interdependence between them via the carrier concentration. In recent years, the two main strategies have been adopted to improve zT of well-established TE materials, such as Bi2Te3 based alloys, PbTe solid solutions and SiGe alloys: one is to increase the electrical power factor (α2σ) by optimizing carrier concentration5 or band engineering6, and the other is to reduce the lattice thermal conductivity κl through alloy scattering or boundary scattering7-9. Development of new materials is also one of central themes in TE community. Some high performance new TE materials have recently been identified, such as filled skutterudites10, Mg2Si1-xSn solid solutions11,12, half-Heulser alloys13, Yb14MnSb11 Zintl compounds14 and so on. However, these new TE materials have the maximum zT at relatively high temperatures above 550K, suitable for mid to high temperature power generation. There are few reports on new TE materials operating at low temperatures (RT-550K).

Bi2Te3 based alloys with zT ~1 are still the best TE materials near room temperature15,16,17, and have been used for solid state refrigeration15,18 and low temperature power generation below 550K. However, the scarcity and high cost of Te element limits their large-scale commercial applications. It is imperative to develop alternative high performance TE materials at low temperatures (RT-550K) with constitute elements relatively cheap and abundantly available in earth’s crust. MgAgSb has three different crystal structures19: a half_

heusler phase (space group F4 3m) at high temperatures (630~700K), a Cu2Sb-related phase (space group P4/nmm) at intermediate temperatures (560~630K), and a tetragonal _

structure α phase (space group I4 c2) with a=9.1761 Å and c=12.6960 Å at low temperatures (RT~560K). Melanie et al. recently tried to evaluate the TE application potential of low temperature α-MgAgSb phase19. However, the complicated phase transitions make it difficult to obtain pure α-MgAgSb phase. A high content of secondary phase Ag3Sb (~19%) was present in the α-MgAgSb and the composite sample showed a maximum zT of only ~0.5 at 425K. Zhao et al. recently used ordinary ball milling method to fabricate MgAgSb compounds and obtained improved zT20. However, ball milled Mg-based alloys are very flammable and prone to be oxidized. In this work, our electronic band calculations show that αMgAgSb has a narrow band gap while β-MgAgSb is metal (Supporting Information, Figure S1). Thus, the first step toward the overall evaluation of TE potential for MgAgSb sys-

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tem is to synthesize high quality low temperature α-MgAgSb with impurity phases eliminated, which is realized by a carefully controlled fabrication process. In doping is used to adjust carrier concentration and improve conductivity. A high zT of ~1.1 is achieved at 525K in In doped α-MgAgSb sample, showing the promising TE application of this system for low temperature power generation.

EXPERIMENTAL SECTION Synthesis. The In-doped MgAgSb1-xInx (x=0, 0.005, 0.01, 0.015, 0.02) samples were prepared from high-quality starting materials of Mg (99.9%, powder, Alfa Aesar), Ag (99.9%, powder, Alfa Aesar), Sb (99.999%, powder, Alfa Aesar) and In (99.99%, shot, Alfa Aesar) in carbon-coated silica tubes sealed under high vacuum. The samples are heated to 1273 K at 10K/min and held for 6 h and then air-quenched. The obtained ingots were grounded to powder. All the powder is sintered by a spark plasma sintering system (SPS-1050, Sumitomo Coal Mining Co.) under the axial compressive stress of 60 MPa in vacuum at 700 K for 6 min, resulting in a disk–shaped sample of φ12.6 × 2mm. Relative densities were ~93% of the theoretical values19. The obtained samples were sealed inside an evacuated quartz tube and annealed heated to 553 K at 5/minfor two weeks to get room-temperature α-MgAgSb phase. Characterization. The phase structure of obtained samples was characterized by a Rigaku D/MAX-2550PC X-ray diffraction (XRD) system using a Cu Kα radiation (λ = 1.5406 Å) in the range of 2θ = 20° - 80°. The standard diffraction of αMgAgSb was calculated using Material Studio. The compositional homogeneity of all samples was examined by electron probe microanalysis (EPMA, JEOL JXA-8100). The phase transformations of samples were observed by differential scanning calorimeter (DSC) (SI, Figure S2). Sample pellets were polished into coins of φ12.6 mm and 12 mm thickness for thermal diffusivity measurements. All the samples were coated with a single layer of graphite to minimize errors from the emissivity of the material. The thermal conductivity was calculated from κ = DρCp, where the density (ρ) was estimated by an Archimedes method, the thermal diffusivity coefficient (D) was measured using the laser flash method in a Netzsch LFA457 instrument, and the specific heat capacity (Cp) was derived using a Pyroceram standard (Pyroceram 9606) in the range 300 – 575 K (SI, Figure S3). The sintered pellets were then cut into bars with dimensions 12 mm × 3 mm × 2 mm, used for simultaneous measurement of the electrical conductivity and the Seebeck coefficient in the temperature range between 300 and 553 K on a computeraided apparatus in a vacuum using a DC four-probe method and differential voltage/temperature technique, respectively21. The Hall coefficient (RH) was measured at 300 K using a physical property measurement system (PPMS-9T, Quantum Design Inc. USA). The carrier concentration (pH) was calculated by pH = 1/eRH, where e is the unit charge. The carriers mobility (µH) was calculated by µH = σRH. Band Calculation. Electronic structure calculations were performed within density functional theory (DFT) using the projector augmented wave (PAW) method22,23 as implemented in the Vienna Ab-initio Simulation Package (VASP)24,25. We have used the local density approximation (LDA) for the exchange correlation potential26. The plane-wave energy cutoff

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was set 400 eV, and a Monkhorst-Pack k mesh 8×8×8 (120) was used for crystal structure optimization. The energy convergence criterion of 10-5 eV and a Hellmann-Feynman force convergence criterion of 10-3 eV/Å were used. Spin–orbit coupling is not considered.

RESULTS AND DISCUSSION Crystal and Band Structure. α-MgAgSb presents a new type structure composed of a distorted Mg-Sb rocksalt lattice with half of the Mg-Sb pseudocubes filled with Ag (Figure 1), similar to regular half-Heusler but the pseudocubes where Ag atoms are located are quite different from those in halfHeusler compounds27. The details of unit cells, space group and atomic positions have been given in the supporting information (SI, Table S1).

Figure 1. (a) Crystal structure and (b) primitive cell of αMgAgSb. The band structure of α-MgAgSb is shown in Figure 2(a). α-MgAgSb is a narrow band-gap semiconductor with the calculated indirect gap energy ~0.1 eV. The bottom of the conduction band is located at G-point, while the top of the valence band is at X-point, which has a higher valley degeneracy than G point.

Figure 2 (a) Calculated band structure of α-MgAgSb; (b) Calculated DOS and partial DOS of α-MgAgSb The density of states (DOS) of valence band edge is twice higher than that of conduction band edge shown in Figure 2(b). The high DOS of valence band can be expected to be beneficial for relatively high Seebeck coefficient at the suitable carrier concentrations, and makes α-MgAgSb promising p-type


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TE materials. To investigate the constitution of energy bands, the partial density of states for α-MgAgSb is displayed in Figure 2(b). Ag-4p and Sb-5p states mainly contribute to upper valence bands, while Mg-2p is responsible for the conduction bands edge. These calculated results provide a guide to enhance the thermoelectric conversion performance of αMgAgSb by the effect of doping. XRD and SEM Observations. The XRD patterns of αMgAgSb1-xInx (0≤x≤0.02) samples are shown in Figure 3(a). The most peaks of the samples could be indexed to the α_

MgAgSb with I4c2 space group. There is only a trace of impurity phase of Sb and Ag3Sb is occasionally observed. The calculated diffraction pattern of α-MgAgSb is consistent with previous results19. Figure 3(b) shows the back scattering electron image of the sintered α-MgAgSb0.99In0.01 sample. No obvious secondary phases can be observed in the matrix phase (grey) except occasionally found metallic Sb (the white spots, confirmed by EPMA). The mass fraction of impurities was determined to be 2.1% by Rietveld refinement (SI, Figure S4). The black spots in the image are confirmed to be pores. The previous work shows a high content of secondary phases in the α-MgAgSb samples fabricated by melting and hot pressing19, which contain 19% of Ag3Sb and 14% Sb hindering the overall and correct assessment of thermoelectric properties of αMgAgSb. We successfully obtain high purity of α-MgAgSb by elaborately controlled SPS processing in this work. The trace of impurity phases should have negligible effects on thermoelectric properties.

ably increased with increasing carrier concentration and the room temperature value increases from~5000 S.cm-1 at x=0 to ~100000 S.cm-1 at x=0.02, indicating that In doping on Sb site can increase the hole concentration and improve conductivity because In has less valence electrons than Sb. The σ follows a T-3/2 dependence before the intrinsic excitation, indicating that the acoustic phonon scattering is the dominant carrier scattering mechanism28. The Seebeck coefficient exhibits a contrary trend to the electrical conductivity, and decreases with increasing carrier concentration (Figure 4b). Table 1. Carrier concentration, carriers mobility and reduced chemical potential of all the samples at room temperature pH

µH

Label

Composition

(1019cm-3)

(cm2V-1S-1)

η

7E19

α-MgAgSb

7.3

48

0.6

8E19

α-MgAgSb0.995In0.005

8.0

50

0.8

9E19

α-MgAgSb0.99In0.01

9.1

54

0.9

10E19

α-MgAgSb0.985In0.015

10.1

55

1.1

11E19

α-MgAgSb0.98In0.02

11.6

55

1.4

Figure 4. Temperature dependences of (a) electrical conductivity and (b) Seebeck coefficient for In doped α-MgAgSb compounds. The solid lines in (b) is calculated by SPB model.

Figure 3 (a) XRD patterns and (b) back scattering electron image of α-MgAgSb0.99In0.01 sample. Thermoelectric Properties. Table 1 lists some room temperature physical properties of α-MgAgSb1-xInx (0≤x≤0.02), which indicates that the hole concentration increases with increasing In dopant content. The mobility exhibits a weak change and a little random trend with carrier concentration, possibly due to the different defect contents in samples. Figure 4 shows the electrical conductivity of α-MgAgSb1-xInx samples is remark-

The single parabolic band (SPB) model was used to analyze thermoelectric transport of α-MgAgSb, similar to the previously used for other TE materials12,13. The reduced chemical potential η=(EF-EV)/(kBT), calculated from the experimental Seebeck coefficient, is listed in Table 1. For doped samples, the chemical potential is only 0.03 ~ 0.04 eV away from the top of the valence band and hence only valance band edge around X point contributes to the carrier transport in the present carrier concentration range (see Figure 2a). The predicted Seebeck coefficient based on the SPB model is shown in Fig-



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ure 4b. A good agreement is displayed between the calculated curves and experimental data before the intrinsic excitation and the effective mass m*≈2.0me is derived. The effective mass remains almost unchanged with increasing In content and temperature, due to the minimal changes of the chemical potential.

Figure 5 Total thermal conductivity (a), lattice thermal conductivity (b), power factor (c), and figure of merit zT (d) of αMgAgSb1-xInx (0≤x≤0.02) Figure 5a shows that all the samples have the low thermal conductivity and the room temperature values are between 1.1 and 1.4 W/m.K, similar to the reported by Melanie et al.19. The Weidemann-Franz relationship permits an estimation of the electronic thermal conductivities, κe = LσT, where L is Lorenz number and can be calculated by the SPB model12,13. The cal-

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culated Lorenz constant for α-MgAgSb1-xInx is in the range of 1.6×10-8~1.8×10-8V2K-2. The electronic thermal conductivity κe of all the samples increases with carrier concentration and varies from 0.24 Wm-1K-1 to 0.54 Wm-1K-1 at room temperature. The lattice thermal conductivity κl was obtained by subtracting the κe from the total thermal conductivity κtot, and is plotted in Figure 5b. The lattice thermal conductivity of all samples is below 1.0 Wm-1K-1. Such low thermal conductivity is of great interest for this new TE material and the reason for it remains to be thoroughly studied. It may be attributed to its distorted lattice and the strong anharmonicity of interatomic bonds.29,30 Power factor (α2σ) values for In-doped αMgAgSb1-xInx samples are significantly higher than non-doped α-MgAgSb in Figure 5c, and the maximum value is 26 µWcm1 -2 K for the α-MgAgSb0.975In0.015 sample at 525 K.

Figure 6 Temperature dependences of (a) Seebeck coefficient and electrical conductivity, and (b) thermal conductivity and figure of merit zT for In doped α-MgAgSb compounds with similar carrier concentrations. The maximum zT value of ~1.1 was achieved at 525K for the α-MgAgSb0.99In0.01 sample, which is comparable to that of commercial Bi2Te3 based materials18. However, considering the constituent elements are abundantly available in earth’s crust, α-MgAgSb is more promising for large-scale commercial applications for low temperature power generation. The high TE performance of In doped α-MgAgSb can be ascribed to the intrinsically low thermal conductivity and optimized electrical properties. Because of significantly improve phase purity, the zT values in this work is much higher than the previously reported19. The TE properties of α-MgAgSb are very sensitive to the carrier concentration and the optimal carrier concentration is 8~9 × 1019 cm-3. In order to confirm that the high zT is repeatable, three α-MgAgSb samples with carrier concentration of ~8 × 1019 cm-3 were fabricated again using the same processing. Figure 6 shows their thermoelectric properties. The high zT was easily repeated and demonstrates


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the potential of α-MgAgSb as new low-temperature TE materials.

CONCLUSIONS In summary, bulk α-MgAgSb with improved phase purity has been obtained and its TE potential has been assessed. αMgAgSb possesses unique crystal structure and a narrow band gap, resulting in an intrinsically low thermal conductivity and promising electrical properties. Hole concentration of αMgAgSb can be optimized by In doping. A maximum zT of ~1.1 was achieved at 525 K for α-MgAgSb0.99In0.01 with the optimal carrier concentration of 8~9×1019cm-3, which is comparable to Bi2Te3 based alloys, the best low-temperature (RT550K) TE materials. Considering the abundantly available constituent elements, α-MgAgSb is great promising for lowtemperature (RT-550K) power generation.

ASSOCIATED CONTENT Supporting Information. Band structure of β-MgAgSb (Figure S1), thermal analysis (DSC) of MgAgSb (Figure S2), heat capacity of α-MgAgSb (Figure S3), structural parameters of α-MgAgSb (Table S1), experimental and calculated diffraction patterns and the difference profile of α-MgAgSb (Figure S4), repeated electrical proerty measurement of α-MgAgSb0.99In0.01(Figure S5).This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

[email protected]

ACKNOWLEDGMENT The work was supported by the National Basic Research Program of China (2013CB632503), the National Nature Science Foundation of China (51171171), the Program for New Century Excellent Talents in University (NCET-120495), the Program for Innovation Research Team in University of Ministry of Education of China (IRT13037), and the Ph.D Program Foundation of Ministry of Education of China (No.20120101110082)

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Table of Contents

α-MgAgSb possesses unique crystal structure and bulk α-MgAgSb with improved phase purity has been obtained. A maximum figure of merit zT of ~1.1 is achieved at 525 K for α-MgAgSb0.99In0.01 thermoelectric materials, comparable to Bi2Te3 based alloys, showing the bright potential for low-temperature (RT-550K) power generation.

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High Performance Î±-MgAgSb Thermoelectric Materials for Low

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