High Temperature Thermoelectric Properties of Yb ... - ACS Publications

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High Temperature Thermoelectric Properties of Yb14MnSb11 Prepared from Reaction of MnSb with the Elements Jason H. Grebenkemper,† Yufei Hu,† Dashiel Barrett,† Pawan Gogna,‡ Chen-Kuo Huang,‡ Sabah K. Bux,‡ and Susan M. Kauzlarich*,† †

Department of Chemistry, One Shields Avenue, University of California, Davis, California 95616, United States Thermal Energy Conversion Technologies Group, Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive, MS 277-207, Pasadena, California 91109, United States

‡

S Supporting Information *

ABSTRACT: Compounds of the Yb14MnSb11 structure type are the highest efficiency bulk p-type materials for high temperature thermoelectric applications, with reported figures of merit (ZTs) as high as ∼1.3 at 1275 K. Further optimization of ZT for this structure type is possible with the development of a simple synthetic route. However, this has been difficult to achieve because of the small amount of Mn required compared with Yb and Sb. A simple synthetic route for Yb14MnSb11 has been developed utilizing a combination of ball milling and annealing to produce phase-pure material followed by spark plasma sintering for consolidation. The materials have been characterized by powder X-ray diffraction before and after spark plasma sintering. The stoichiometric reaction of Yb, Sb, and MnSb provides phase-pure powder by X-ray diffraction. Upon cycling to temperatures greater than 1272 K, Yb14MnSb11 shows the presence of Yb11Sb10. Additional samples with 5% and 10% excess Mn were also investigated. Adding 5−10% excess Mn does not change the low temperature properties and improves the high temperature ZT, resulting in a ZT of 1.1−1.2 at 1000 K for Yb14Mn1.05Sb11, 30−40% improvement over that of the Sn flux reaction. The increase in ZT is attributed to optimization of the carrier concentration. These results provide a reliable method of bulk synthesis of this Zintl phase and open the way for discovery of new compounds with potential for even higher ZT.

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consisting of A2+ cations, MPn49− tetrahedra, Pn37− anion, and isolated Pn3− anions. Yb14MnSb11 has low thermal conductivity due to its complex structure and large unit cell.9 The carrier concentration is 1.1(3) × 1021 carriers/cm3, which is higher than optimal for obtaining the best thermoelectric efficiency.4 The thermoelectric efficiency, thus, could be improved by lowering the carrier concentration, such as through small substitutions for the three different elements. Improvements in thermoelectric properties have already been observed with small elemental substitutions on the various sites, including La and Tm for Yb;10,11 Zn, Al, and Mg for Mn;12−14 and Te for Sb.15 Sn-flux methods have been shown to be an excellent medium for discovering new compounds and providing high-quality crystals for property measurements.16,17 However, the Sn-flux synthesis for Yb14MnSb11 is not ideal for producing large amounts of material for manufacturing thermoelectric devices. In part, this is because large quantities of materials are needed for device applications, and producing sufficient amounts of

INTRODUCTION Discovering new materials for thermoelectric applications such as direct heat to electrical energy conversion has been a major goal in energy research for the past decade.1−3 There have been great strides with new materials showing high dimensionless figures of merit, ZT = S2T/κρ, where S is the Seebeck coefficient, T the temperature, κ is thermal conductivity, and ρ electrical resistivity. These properties are interrelated, and therefore, optimization of ZT is challenging. In recent years, a number of new materials and composites have been shown to have high ZT, paving the way for more efficient devices.1−3 One of these materials is Yb14MnSb11, which has been shown to be the best bulk p-type thermoelectric material for high temperature applications with a ZT ∼ 1 at 1200 K; this material is under development by the Jet Propulsion Laboratory (JPL) for their next generation radioisotope thermoelectric generator (RTG).4,5 Until recently, Yb14MnSb11 has primarily been synthesized using the Sn-flux method.6,7 The structure of Yb14MnSb11 is shown in Figure 1 and is isostructural with the Zintl phase Ca14AlSb11.8 This structure type has been described in detail and has the general formula A14MPn11 (A = Ca, Sr, Ba, Eu, Yb; M = Mg, Nb, Mn, Zn, Cd; Pn = P, As, Sb, Bi). In brief, the structure can be described as © XXXX American Chemical Society

Received: June 25, 2015 Revised: August 3, 2015

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DOI: 10.1021/acs.chemmater.5b02446 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

with the highest ZT.21 The synthesis and thermoelectric properties of Yb14MnSb11 will be presented and compared with previously published Sn flux prepared samples and the JPL ATEC material. This new synthetic route allows for precise control of stoichiometry and will expand the chemistry of this structure type.

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EXPERIMENTAL SECTION

Reagents. Dendritic Yb (99.99%, Metall Rare Earth Limited), Mn pieces (99.95%, Alfa Aesar), and Sb shot (99.999%, Alfa Aesar) were used for the following syntheses. All elements were handled in an argon-filled drybox with water levels 1 have lower electrical resistivity. Because all three of the samples measured above 1100 K have similar carrier concentrations, it is possible that with increasing amounts of Mn, the samples are better sintered

Figure 7. High temperature heat capacity for JPL ATEC Yb14MnSb11 compared with the linear equation from Cox et al.13 determined as an average for Yb14Mn1−xAlxSb11 crystals grown from Sn flux and the Dulong−Petit value, CDP = 3kB/atom.

Dulong−Petit law. This heat capacity is lower and less noisy than that collected for the work on the Yb14Mn1−xAlxSb11 system, which was measured on Sn-flux grown crystals and corrected for the presence of Sn.13 In this paper, the heat capacity for the JPL ATEC sample was used to determine thermal conductivity because the ATEC sample is free of Sn impurities, shows less noise, and is closer to the value predicted by the Dulong−Petit law.34,35 Thermoelectric Properties. Yb14MnSb11 produced from Yb filings and MnSb showed comparable if not slightly improved thermoelectric properties over materials produced by Sn flux. Figure 8 shows enhanced Seebeck coefficients compared with the published Sn flux data,4 with all samples showing very similar Seebeck coefficients at low temperatures, whereas the samples measured at UC Davis deviate slightly from those measured at JPL at higher temperatures (>700 K). Samples measured at UC Davis reach a maximum of about 220 μV/K just below 1100 K. It is possible that the slightly higher Seebeck for these measurements are due to a coldfinger effect from the four-probe measurements.36−38 In a four-probe measurement, the expected error is about ±5.5% between measurements,37 whereas experiments have shown that the coldfinger effect leads to a Seebeck coefficient 12% higher than that obtained from a two-probe measurement at 626 °C, with E


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previously published values, consistent with the higher Seebeck and resistivity at high temperatures. The majority of samples display enhanced ZT over the material made by Sn flux, as shown in Figure 11, attributed to

Figure 9. Electrical resistivity vs temperature for Yb14MnxSb11, prepared with x = 1.0, 1.05, 1.10, compared with Yb14MnSb11 Sn flux and JPL ATEC. Samples are indicated by Mnx−A, where A indicates the reaction batch and −JPL indicates the measurement was performed at JPL.

Figure 11. Figure of merit ZT vs temperature for Yb14MnxSb11, prepared with x = 1.0, 1.05, 1.10 compared with Yb14MnSb11 Sn flux and JPL ATEC. Samples are indicated by Mnx−A, where A indicates the reaction batch and −JPL indicates the measurement was performed at JPL.

and thereby have lower electrical resistivity. Although the Sn flux material has similar resistivity to Mn1−B and Mn1.1 below 800 K, at higher temperatures it has significantly lower resistivity than all other measured materials. Figure 10 shows the thermal conductivity for these samples compared with Yb14MnSb11 prepared by flux. The thermal

Figure 10. Thermal conductivity vs temperature for Yb14MnxSb11, prepared with x = 1.0, 1.05, 1.10 compared with Yb14MnSb11 Sn flux and JPL ATEC. Samples are indicated by Mnx−A, where A indicates the reaction batch and −JPL indicates the measurement was performed at JPL. The points and solid line show total thermal conductivity, whereas the broken lines show lattice thermal conductivity.

higher Seebeck coefficients and control of carrier concentration. Samples measured at UCD display higher ZT than those measured at JPL, as a result of the slightly higher Seebeck coefficients of samples measured on the LSR-3. The coldfinger effect38 for the four-probe geometry is important at high temperatures and may contribute to the slightly higher Seebeck values. Because the Seebeck coefficient is squared, the small difference in Seebeck translates into a significant difference in ZT values. The differences between the Yb14MnxSb11 x = 1.0 samples (Mn1−A and Mn1−B) may be attributed to small differences in mixing of the starting materials batch to batch. The PXRD of the sample Mn1−C after measurements revealed the sample to contain 20% Yb11Sb10 (Supporting Information STable 1, Mn1−C−TE). The high temperature resistivity and Seebeck coefficient of Mn1−C both display a turnover that is not observed in the Sn-flux data. The sample with 10% excess Mn, Mn1.1, does not show this turnover in the high temperature data. Of the three samples measured at temperature above 1100 K, Mn1.05 possesses the highest ZT, both measured on the LSR-3 at UCD and at JPL. The improvement over Sn flux is greater than the assumed maximum error of ±20% in ZT.34

diffusivity data for the flux material4 has been treated in the same fashion as the other materials presented here. Although there is some variation between Yb14MnSb11 samples, they are all fairly consistent with each other and the Sn flux material, falling within the reported ±17% scatter of thermal diffusivity data at 475 K.34 Sample Mn1−A, which also showed high electrical resistivity exhibits the lowest thermal conductivity when compared with the other two Yb14MnSb11 samples, Mn1−B, Mn1−C. Yb14MnxSb11 prepared with x = 1.0, 1.05, 1.10, measured above 1100 K show consistent results with the high temperature data consistent with the electrical resistivity. Mn1−C shows the bend over in both Seebeck, resistivity above 1100 K, whereas the thermal conductivity increases whereas Mn1.05 and Mn1.10 thermal conductivity data parallel

CONCLUSIONS Through a combination of ball milling and annealing, Yb14MnSb11 can be produced with minimal waste and in high purity, resulting a product with an enhanced ZT compared to that produced by Sn flux. This synthesis is scalable and provides the opportunity to make new compositions that are unobtainable through flux methods. It is important to ensure thorough uniform mixing by using finely divided Yb (in this case, filed Yb), and Mn as part of a precursor binary (MnSb). Yb11Sb10 is formed after heating samples to 1273 K and has the most significant effect on the properties above that temperature. The formation of Yb11Sb10 depends on exposed surface area of the samples during heating rather than the manner in which it was produced. Small amounts of excess Mn increase the ZT of the material but do not appear to eliminate the production of Yb11Sb10 during high temperature cycling.

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Yb14MnSb11 with 5% excess Mn results in the highest ZT as a result of both optimized carrier concentration and low electrical resistivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02446. Additional X-ray diffraction patterns, Rietveld refinements, microprobe images, and raw thermal diffusivity data. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Financial support from NEUP is gratefully acknowledged. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This work was supported by the NASA Science Mission Directorate’s Radioisotope Power Systems. We thank Thierry Caillat for the ATEC sample and data.

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High Temperature Thermoelectric Properties of Yb ... - ACS Publications

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