Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L28: Focus Session: Thermoelectric Materials: Tellurides |
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Sponsoring Units: DMP FIAP Chair: Stefan Zollner, IBM Room: 330 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L28.00001: Enhancement of the thermoelectric figure of merit by distortions of the dispersion relation Invited Speaker: A doubling of the thermoelectric figure of merit (zT) of p-type PbTe above 700 K has been recently demonstrated (1) in thallium-doped material. The effect comes about because an electronic energy level of the Tl atoms resonates with the valence band of PbTe. This creates an excess density of states, g(E), at a specific energy about 60 meV below the valence band edge, which in turn gives a thermoelectric power at that carrier concentration about three times higher than that of similarly-doped p-type PbTe. In this talk, we will review the mechanisms by which this distortion of the g(E) function, from the normal E$^{1/2}$ form valid for parabolic bands in three dimensions into a spike-like function, increases the thermoelectric power and thus zT. We further derive a set of criteria for the excess g(E) to improve zT. We will discuss the applicability of this approach to other electronic levels in PbTe first, and then describe the more general quest for such energy levels in other thermoelectric semiconductors. (1) J. P. Heremans et al., Science \textbf{321} 554 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L28.00002: Electronic Inhomogeneity in PbTe-based High Performance Thermoelectric Materials Observed by NMR E.M. Levin, K. Schmidt-Rohr, B.A. Cook, M.G. Kanatzidis Effects of composition and synthesis conditions on the local structure and charge carrier concentration in Ag$_{x}$Sb$_{y}$Pb$_{18}$Te$_{20}$ (LAST-18) thermoelectric (TE) materials have been studied by $^{125}$Te and $^{207}$Pb nuclear magnetic resonance (NMR) with magic-angle spinning. The high-resolution $^{125}$Te NMR spectra show that most Sb and Ag is not part of Sb$_{2}$Te$_{3}$, AgSbTe$_{2}$, or Ag$_{2}$Te inclusions.\textbf{ }Biexponential NMR spin-lattice (T$_{1})$ relaxation as well as Knight shifts of $^{125}$Te and $^{207}$Pb NMR signals show that many LAST-18 materials contain two phases of similar composition but with free electron concentrations that differ by more than an order of magnitude, i.e. these materials are electronically inhomogeneous. The NMR data were calibrated against Hall- and Seebeck-effect measurements to give the charge carrier concentrations in the two phases. This electronic inhomogeneity may result in the appearance of potential barriers inside TE materials, similar to those observed for semiconductor-semiconductor or metal-semiconductor junctions. Such barriers may affect thermopower, electrical, and thermal conductivity of TE materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L28.00003: Thermodynamic Properties of Pb and Ag-Sb Based Chalcogenides: A First-Principles Study Yi Zhang, Xuezhi Ke, Changfeng Chen, Jihui Yang, Paul R. C. Kent The Pb and Ag-Sb based chalcogenide compounds have received considerable interest for their potential applications in thermoelectric devices. Their low thermal conductivity plays a key role in producing the high figure of merit (ZT) that is critical for applications. We performed a series of first- principles calculations on several Pb and Ag-Sb based chalcogenide compounds to understand their lattice dynamics. The direct force method and density functional theory calculations were used to obtain the phonon dispersion and density of states. The phonon softening processes with the volume change were carefully evaluated. Moreover, we employed the quasiharmonic approximation to calculate the thermodynamic functions. The calculated results are in good agreement with available experimental data and provide insights for understanding the physical properties. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L28.00004: Link between changes in \textit{ZT} and microstructure in AgSbTe$_{2}$ Peter Sharma, Joshua Sugar, Douglas Medlin The best thermoelectric alloys have complex microstructures. For example, the LAST alloys, (AgSbTe$_{2})_{1-x}$(PbTe)$_{x}$, possess \textit{ZT}$\sim $1.5-2 but have a great variety of inclusions with different chemistry at different length scales. How does microstructure affect thermoelectric efficiency? Since the phase diagram of this and most quaternary alloys is poorly known, transport properties have not been systematically connected to microstructure. We are attacking this problem by studying the simple ternary alloy AgSbTe$_{2}$, a component of the LAST system, in order to show how thermoelectric transport changes with a known, controlled microstructure. AgSbTe$_{2}$ forms within the well-studied Ag$_{2}$Te-Sb$_{2}$Te$_{3}$ pseudobinary phase diagram. We have found that Sb-rich AgSbTe$_{2}$ is composed of Sb$_{2}$Te$_{3}$ precipitates embedded in a homogeneous rocksalt Ag$_{16}$Sb$_{30}$Te$_{54}$ matrix. The precipitates are plate-like and crystallographically aligned along their close packed planes parallel to that of the matrix. The size of these Sb$_{2}$Te$_{3}$ plates can be tuned from the nanometer to micron scale. In this work, the formation and growth of precipitates over a wide length scale is linked to changes in thermoelectric properties for the first time. This study is useful for understanding the complexity of LAST, or any bulk thermoelectric where second phase precipitation occurs. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L28.00005: Microstructure and Nucleation Mechanism for Nanoprecipitates in PbTe-AgSbTe$_2$ Xuezhi Ke, Changfeng Chen, Jihui Yang, Lijun Wu, Juan Zhou, Qian Li, Yimei Zhu, Paul R.C. Kent Many recent advances in thermoelectric (TE) materials are attributed to their nanoscale constituents. Determination of the nanocomposite structures has represented a major experimental and computational challenge and eluded previous attempts. Here we present the first atomically resolved structures of high performance TE material PbTe-AgSbTe$_2$ by transmission electron microscopy imaging and density functional theory calculations. The results establish an accurate structural characterization for PbTe-AgSbTe$_2$ and identify the interplay of electric dipolar interactions and strain fields as the driving mechanism for nanoprecipitate nucleation and aggregation, which provides key insights for understanding a broad class of complex nanocomposite materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L28.00006: Galvanomagnetic and Thermomagnetic Properties of Ag$_{(1-x)}$Na$_{x}$SbTe$_{2}$ Alloys Michele Nielsen, Vladimir Jovovic, Christopher Jaworski, Joseph Heremans Group I-V-VI$_{2}$ alloys have intrinsically low thermal conductivity$^{1}$ on the order of 0.65 W/mK due to Umklapp phonon-phonon scattering. Combined with the high valence band density of states in AbSbTe$_{2}$, this makes this material system ideal for thermoelectric applications up to 416 K, where AgSbTe$_{2}$ undergoes a crystallographic phase transition. The partial substitution of Na for Ag is expected to address this problem. We synthesize bulk Ag$_{(1-x)}$Na$_{x}$SbTe$_{2}$ alloys and measure the evolution of the phase transition as a function of Na concentration x. The thermoelectric and galvanomagnetic properties of the alloys are also studied: based on the measurement of resistivity, Seebeck, Nernst and Hall coefficients we calculate mobilities, Fermi energies and partial carrier concentrations of holes and electrons. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L28.00007: Electronic and Thermal Properties of Cubic Ge-Sb-Te Compounds Donald Morelli, Kevin Zhou The ternary rocksalt structure compound Ge$_{4}$SbTe$_{5}$ is unusual because most members of the Ge-Sb-Te family form along the tie-line of the binary compounds GeTe and Sb$_{2}$Te$_{3}$, and thus do not possess the 1:1 cation:anion ratio necessary to present themselves in a cubic structure. The TE properties of these (GeTe)$_{x}$(Sb$_{2}$Te$_{3})_{1-x}$ compounds, while interesting in their own right, are no better than those of commercially available materials. Ge$_{4}$SbTe$_{5}$ and its relatives, with equal numbers of atoms on the cation and anion sites, form stably in the cubic rocksalt structure. For TE applications a cubic compound is advantageous because there is no issue regarding anisotropy of the thermoelectric properties. We have fabricated bulk samples of Ge$_{4}$SbTe$_{5}$ and related compounds, characterized their crystal structure, and measured some of their thermal and electronic properties. Results of isoelectronic substitution of Se on the Te site and Sn on the Ge site will be reported. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L28.00008: Thermoelectric Figure-of-merit in Bulk p-type PbTe Bo Yu, Hui Wang, Bed Poudel, Kenneth McEnaney, Gang Chen, Zhifeng Ren Lead telluride and its related chalcogenide alloys have been well studied for decades. With various achievements in not only bulk by also in thin films, quantum dots, superlattices, nanowires, etc., they always come up as one of the best thermoelectric materials for middle-range temperature applications. Recently, thallium was reported as a good candidate for band structure engineering in p-type lead telluride ingot system to largely enhance the thermoelectric power factor and hence the dimensionless figure-of-merit (ZT). Here we used mechanical alloying as the approach for large-scale production and achieved ZT value of 1.05 at 300 \r{ }C. The details will be presented in this talk. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L28.00009: Modular assembly of binary nanocrystal composite solids for high-efficiency thermoelectric power generation Dong-Kyun Ko, Christopher Murray Despite the numerous advantages of semiconductor nanostructures, doping nanometer size crystals show difficulties not found in conventional bulk systems. Especially for thermoelectric applications, it is critical to control the number of carriers that are available in semiconductors in order to maximize the figure of merit. Here, we report modular assembly of binary composite nanocrystals, as an effective bottom-up design tool, to create a new family of artificial solids with a prescribed set of doping levels. Silver telluride (Ag2Te) nanocrystals, which can act as dopants, are introduced in lead telluride (PbTe) nanocrystal assemblies in order to modify the carrier concentration until an optimum power factor is realized. This study focuses on electronic and thermoelectric characterization of these binary composite solids. Hall measurement and field effect transistor characteristics were investigated in order to identify the carrier type, mobility, and concentration. Temperature dependence of low-bias conductivity was also characterized to gain a better understanding of electronic conduction. Finally, Seebeck voltage was measured with varying PbTe to Ag2Te nanocrystal concentration ratios in order to investigate the Seebeck coefficient as a function of carrier concentration. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L28.00010: Galvanomagnetic and thermomagnetic properties of thallium doped PbSnTe and PbSeTe Vladimir Jovovic, Joseph Heremans Thallium acts as a resonant level in PbTe, so that PbTe:Tl shows a significant improvement of thermoelectric properties due to an increase in thermopower as compared to that of similarly Na-doped PbTe [2]. Further improvements in zT are expected from a reduction of the thermal conductivity by alloy scattering in Pb$_{1-x-y}$ Tl$_{y}$Sn$_{x}$Te and Pb$_{1-y}$Tl$_{y}$Te$_{1-x}$Se$_{x}$ alloys. However, the band structure of PbTe is sensitive to alloying with Sn and Se, and thus the location of the Tl level with respect to the valence band can change with x. In this study, we investigate the effects that band structure modifications have on the enhancement of thermopower. Thermoelectric properties of Pb$_{1-x-y}$ Tl$_{y}$Sn$_{x}$Te and Pb$_{1-y}$Tl$_{y}$Te$_{1-x}$Se$_{x}$ alloys with y=0.01-0.04 and x=0-0.3 are measured in directions longitudinal and transverse to magnetic fields in the range of -1.5 to 1.5T. We report zero field values of electrical resistivity, thermopower, Hall coefficient and adiabatic Nernst-Ettinghausen coefficient as measured in temperature range 80-420K. From these we calculate carrier density and mobility and the density of states effective masses and Fermi energies. [2] J.P. Heremans et al., Science 321, 554 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L28.00011: Antimony: a Dual Donor in Lead Telluride Christopher Jaworski, Janusz Tobola, Joseph Heremans Band structure calculations indicate the formation of an antimony impurity level just above the Fermi level for Pb$_{1-x}$Sb$_{x}$Te and just below the Fermi level for PbSb$_{x}$Te$_{1-x}$.~ For experimental verification, we prepare bulk samples of Pb$_{1-x}$Sb$_{x}$Te and PbSb$_{x}$Te$_{1-x}$ (x = 0.25, 0.5, 1{\%}).~~ Electrical resistivity, Seebeck, Hall and transverse Nernst-Ettingshausen coefficients of the crystals have been measured in the temperature range 2-580 Kelvin. Thermal conductivity data was measured in the range 80-800 Kelvin.~We confirm the ability of antimony to take the place of a lead atom and dope PbTe n-type or take the place of a tellurium atom and dope PbTe p-type. Antimony, however, is not as efficient an acceptor in p-type material as it is a donor in n-type material. The Fermi levels are calculated using experimental data and will be reported here. Also, a phase transition is experimentally observed at 500 K in p-type PbSb$_{x}$Te$_{1-x}$.~ [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L28.00012: Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach Zhifeng Ren, Bed Poudel, Yi Ma, Yucheng Lan, Xiaowei Wang, Giri Joshi, Gaohua Zhu, Jian Yang, Bo Yu, Xiao Yan, Hui Wang, Dezhi Wang, Qing Hao, Hohyun Lee, Austin Minnich, Andrew Muto, Daryoosh Vashaee, Mildred Dresselhaus, Gang Chen The dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been successfully applied to a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient. [Preview Abstract] |
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