Timothy P. Hogan Michigan State University
Jihui Yang General Motors R&D Center
Ryoji Funahashi National Institute of Advanced Industrial Science and Technology
Terry Tritt Clemson University
U1: Nanocomposites I
Monday PM, November 26, 2007
Room 311 (Hynes)
11:15 AM - **U1.1
Low-dimensional and Nano-composite Thermoelectric Materials.
Lidong Chen 1 Show Abstract
1 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai China
Recent resurgence in thermoelectric (TE) materials research has led to a significant improvement of TE performance and development of some new TE materials designs and concepts. This review provides a summary of some effective techniques for improving TE performance through multi-level microstructure control. A proven approach to elevate figure of merit is via formation of nanocomposites, in which nanophases are dispersed at grain boundaries or within grain. Acting as energy filter or scattering center, nanophases contribute to the increase of Seebeck coefficient and the reduction of thermal conductivity without much degradation of electrical conductivity. For anisotropic TE materials, textured microstructure favors to enhance TE performance along the certain direction. In addition, TE thin film assembled from one-dimensional nanotructured materials is presented as a promising TE film.
11:45 AM - U1.2
Alkali Metal Hydrothermal Treatment ---- Fabricate a Beneficial Interface on p-type Bi2Te3 Thermoelectric Materials.
Xiaohua Ji 1 , Jian He 1 , Zhe Su 1 , Nick Gothard 1 , Terry Tritt 1 Show Abstract
1 Department of Physics and Astronomy, Clemson University, Clemson, South Carolina, United States
Bi2Te3 based alloys have been known as one of the best room-temperature thermoelectric (TE) materials for decades. However, the thermoelectric performance of Bi2Te3 based alloys is sensitive to the inherent microstructure of the material: the pulverized system often exhibits an inferior figure of merit than that of the aligned ingot, due primarily to the inter-grain boundary scattering. So an important question may arise as to whether one can somehow fabricate a beneficial inter-grain boundary. In order to address this question, p-type Bi2Te3 is employed as a test-system in present work. Pulverized Bi0.4Sb1.6Te3 (p-type) powders with selected sizes were put into the autoclave and then hydrothermally treated, where the solution of various alkali metal (Li, Na, K, Rb, Cs) compounds were being used as reaction medium. After the treatment, the as-processed powders were removed, washed and dried, followed by hot pressing into pellet for further TE property measurements. The TE properties were found to be significantly improved as compared to the untreated reference sample. Extensive characterizations including X-ray/electron diffraction, TGA analysis, Raman / Fourier Transform Infrared Spectroscopy, electron microscopy, Rutherford back-scattering and Energy dispersive X-ray analysis were performed on the treated sample. The results revealed that a surface layer (from 10 nm to up to micron in thickness) exhibiting a combined crystalline/amorphous feature was formed on the original bare particles. This layer is believed to be the key factor in the improvement of TE properties of the p-type Bi0.4Sb1.6Te3 material. The synthesis technique will be discussed in detail while some results on the microscopic analysis and TE properties will be presented briefly.
12:00 PM - U1.3
Synthesis and Characterization of Novel Thermoelectric Nanomaterials.
Xiaofeng Qiu 1 , Ian Steward 2 , Jeffrey Dyck 2 , Clemens Burda 1 Show Abstract
1 Chemistry, Case Western Reserve University, Cleveland, Ohio, United States, 2 Physics , John Carroll University, University Heights, Ohio, United States
12:15 PM - U1.4
Thermoelectric Properties of Bi2Te3-based Nanocomposites.
Nick Gothard 1 , X. Ji 1 , J. He 1 , T. Tritt 1 Show Abstract
1 , Clemson University, Clemson, South Carolina, United States
Nanocomposites have been produced by incorporating thermoelectric nanoparticles into a matrix of bulk Bi2Te3 material via a hot pressing process. These nanocomposites have been examined by SEM and X-ray powder diffraction. The effects of the incorporation of a variety of nanoparticles upon the resulting thermoelectric properties such as the thermopower, electrical resistivity, thermal conductivity, etc., have been studied in these composites at room temperature and below. The details of the synthesis along with results of the microscopic analysis and thermoelectric properties will be discussed. The potential for improving the figure of merit within the Bi2Te3 system by this technique is considered.
12:30 PM - U1.5
Synthesis and Thermoelectric Properties of Lead Chalcogenide Nanocomposites.
Joshua Martin 1 , Stevce Stefanoski 1 , Lidong Chen 2 , George Nolas 1 Show Abstract
1 Physics, University of South Florida, Tampa, Florida, United States, 2 , Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 China
Lead chalcogenide dimensional nanocomposites were prepared by densifying nanocrystals synthesized employing an alkaline aqueous solution-phase reaction. The nanocrystal synthesis procedure resulted in high product yields of over 2 g per batch. These nanocrystals were then subjected to Spark Plasma Sintering (SPS) for densification. Transport properties were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements, indicating a strong sensitivity to stoichiometry, surface oxygen adsorption, and porosity. The results for these lead chalcogenide nanocomposites were compared to bulk polycrystalline lead chalcogenides with similar carrier concentrations.
12:45 PM - U1.6
Thermoelectric Properties of Semiconducting Silicide Nanowires.
Song Jin 1 , Jeannine Szczezh 1 , Feng Zhou 2 , Li Shi 3 2 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering Program, Texas Materials Institute, , Materials Science and Engineering Program, Texas Materials Institute, , Austin, Texas, United States, 3 Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas, United States
Semiconducting silicides (e.g. CrSi2, β-FeSi2, MnSi1.8, Mg2Si) are promising thermoelectric materials. In addition to their respectable thermoelectric figure-of-merit (ZT up to 0.8), silicides have the advantages of low cost, excellent thermal stability and mechanical strength, and outstanding oxidation resistance, making them suitable for high temperature applications. We have developed general synthetic approaches to high quality single crystal nanowires of silicides to investigate their potential enhancement of thermoelectric properties due to the reduced nanoscale dimension and to explore their applications in thermoelectrics. We will specifically focus on the synthesis and structural characterization of nanowires of chromium disilicide (CrSi2) prepared via a chemical vapor transport (CVT) method. This method complements the more versatile chemical vapor deposition (CVD) of metal carbonyl-silyl single source organometallic precursors we have developed. Structural characterization using electron microscopy, powder X-ray diffraction, and energy dispersive spectroscopy shows that these nanowires are hexagonal CrSi2 single-crystal structures along the <001> growth axis, with diameters ranging from 20−300 nm and length up to 100 μm. The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual CrSi2 nanowires were characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. The obtained thermoelectric figure of merit of the nanowires was close to 0.1 and comparable to the bulk values. This combined Seebeck coefficient and electrical conductivity measurements also provide an effective approach to probing the Fermi level, carrier concentration and mobility in nanowires. We will also discuss our recent results of silicide nanowires of complex Novotny chimney ladder phases and our progress in using individual nanostructures combined well-defined structural characterization to conclusively investigate the complex thermoelectric behaviors of these silicide materials.
U2: Nanocomposites II and Theory
S.D. (Bhanu) Mahanti
Monday PM, November 26, 2007
Room 311 (Hynes)
2:30 PM - **U2.1
New Opportunities in Existing Thermoelectric Materials: Interface Engineering in Pulverized p-Bi2Te3 System.
Jian He 1 , Xiaohua Ji 1 , Zhe Su 1 , Nick Gothard 1 , Terry Tritt 1 Show Abstract
1 Physics, Clemson University, Clemson, South Carolina, United States
Grain boundary scattering provides an avenue by which to effectively lower the thermal conductivity in pulverized thermoelectric materials, however, the “bare” inter-grain boundary often simultaneously degrades the electrical conductivity and thermopower. Thus a controlled inter-grain boundary would be very beneficial in order to improve the thermoelectric performance of the system. But the question is how to engineer such a boundary.In this talk we present a proof-of-principle investigation on the pulverized p-Bi2Te3 (Bi0.4Sb1.6Te3) system by means of electrical resistivity, thermopower, thermal conductivity, specific heat, Hall coefficient, Raman/Infrared spectroscopy, X-ray/electron diffraction, electron microscopy and compositional analysis. Utilizing the alkaline hydrothermal treatment and nano-coating techniques recently developed at Clemson, we fabricate a thin layer on the surface of fine p-Bi2Te3 grains. The interface layer, ~ few tens nm thick and formed right at the inter-grain boundary in a hotpress-densified sample, enabled us to “decouple” and individually optimize the various thermoelectric properties. As a result, the hydrothermally treated and pulverized sample possessed ZT values comparable to those of a commercial ingot but with a better so called “compatibility factor” as well as better mechanical properties. In view of the concept of material design, this process helps achieve a new level of control as a tuning parameter with which to optimize the figure of merit ZT and compatibility factor. In principle, this strategy can be readily applied to other existing thermoelectric materials. This presentation will focus on the resulting thermoelectric properties and microscopic analysis and the synthesis techniques will be discussed in detail elsewhere.
3:00 PM - U2.2
Effect of In-Situ Hydrogen Annealing on the Thermoelectric Properties of Individual Bismuth Telluride Nanowires.
Anastassios Mavrokefalos 1 , Michael Pettes 1 , Li Shi 1 , Wei Wang 2 , Xiaoguang Li 2 Show Abstract
1 Mechanical Engineering, University of Texas at Austin, Austin, Texas, United States, 2 Department of Physics, University of Science and Technology of China, Hefei China
Several theoretical studies suggested that Bi-based and III-V nanowire structures may possess enhanced thermoelectric figure of merit, ZT. It was found in our earlier measurements employing a suspended microdevice that the thermoelectric properties of individual bismuth telluride, InSb, and CrSi2 nanowires are largely influenced by the crystalline quality, chemical composition and surface roughness of the nanowires. In addition, a major problem for thermoelectric measurements of individual nanowires especially bismuth telluride nanowires is the presence of a stable native oxide that prohibits electrical contact to be made directly to the nanowires. Focused electron or ion beam induced deposition of Pt on the nanowire was used in our previous work to make electrical contact to the nanowire. Care was needed to prevent the nanowire from being contaminated by ions present during the Pt deposition process. Furthermore, it has been suggested that the presence of the surface oxide or surface contamination can result in high surface charge state densities that can dominate the intrinsic transport properties of nanowire and thin film thermoelectric materials. In fact, it was found that annealing in a hydrogen environment can significantly enhance the thermoelectric properties of bismuth telluride films.In this work, we investigate the effect of in situ hydrogen annealing on the thermoelectric properties of individual bismuth telluride nanowires. The thermoelectric measurement method is based on an improved design of our microfabricated suspended device. The current measurement does not require Pt deposition on the nanowire for making electrical contact. Instead, it was found that ohmic contact between the nanowire and the underlying pre-patterned Pt electrodes on the suspended devices can be made by annealing the nanowire at about 480 K while hydrogen is flown into the evacuated sample space of a cryostat. Our measurement results show that that the thermal and electrical conductances and ZT of the nanowires are increased upon hydrogen annealing. In addition, both the contact thermal and electrical resistances are eliminated from the measured thermal conductivity, electrical conductivity, or Seebeck coefficient by using a unique four-probe thermoelectric measurement method. Transmission Electron Microscopy (TEM) and Energy Dispersion Spectroscopy (EDS) measurements are performed on the same nanowires assembled on the suspended device so as to correlate the structural characteristics to the measured thermoelectric properties of the nanowires. High resolution TEM results reveal highly crystalline structure of the hydrogen-annealed bismuth telluride nanowires.
3:15 PM - U2.3
Synthesis and Thermoelectric Properties of High-purity Single-crystal InSb Nanowires.
Feng Zhou 1 , Jae Hun Seol 2 , Yong Lee 2 , Li Shi 2 1 , Qi Laura Ye 3 Show Abstract
1 Texas Materials Institute, University of Texas at Austin, Austin, Texas, United States, 2 Mechanical Engineering, University of Texas at Austin, Austin, Texas, United States, 3 , NASA Ames Research Center, Moffett Field, California, United States
Indium antimonide (InSb) is a narrow bandgap semiconductor with one of the smallest effective mass values among semiconductors and very high mobility. It is commonly used in infrared detectors and magnetic field sensors. The thermoelectric properties of bulk InSb crystals have been characterized by Yamaguchi et al. in the 10-723 K temperature range, with highest figure of merit (ZT) of 0.6 found at 673 K . A theoretical calculation by Mingo has predicted that quantum confinement of electrons and diffuse phonon-surface scattering in InSb nanowires can result in enhanced ZT compared to the bulk value [2, 3]. Ye et al. has developed a vapor-liquid-solid (VLS) method to synthesize single crystal InSb nanowires. In an previous measurement, we observed that the obtained VLS InSb nanowires possess higher electrical conductivity and lower Seebeck coefficient than bulk crystals, most probably due to tellurium or oxygen impurities in the nanowire . In this work the impurity concentration in the VLS InSb nanowires is minimized by using pure InSb wafers as source materials for VLS growth in high vacuum environment. The crystal structure and chemical composition of the obtained nanowires are analyzed using High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDAX). Temperature-dependant thermopower and electrical conductance are probed using a nanofabricated device where a top gate and substrate back gate voltage can be used to tune the Fermi level of the system via the field effect. Experiments are conducted to investigate the effect of in situ hydrogen annealing and surface passivation on the thermoelectric properties of the InSb nanowires. S. Yamaguchi, T. Matsumoto, J. Yamazaki, N. Kaiwa, and A. Yamamoto, "Thermoelectric properties and figure of merit of a Te-doped InSb bulk single crystal," Applied Physics Letters, vol. 87, Nov 2005.N. Mingo, "Thermoelectric figure of merit and maximum power factor in III--V semiconductor nanowires," Applied Physics Letters, vol. 84, pp. 2652-2654, 2004.N. Mingo, "Thermoelectric figure of merit and maximum power factor in III-V semiconductor nanowires (vol 84, pg 2652, 2004)," Applied Physics Letters, vol. 88, pp. -, Apr 3 2006.J. H. Seol, A. L. Moore, S. K. Saha, F. Zhou, L. Shi, Q. L. Ye, R. Scheffler, N. Mingo, and T. Yamada, "Measurement and analysis of thermopower and electrical conductivity of an indium antimonide nanowire from a vapor-liquid-solid method," Journal of Applied Physics, vol. 101, pp. 023706-6, 2007.
3:30 PM - **U2.4
Using Nano-composites to Enhance ZT.
Mildred Dresselhaus 1 Show Abstract
1 EECS and Physics, MIT, Cambridge, Massachusetts, United States
The concept of using self assembled nano-composites to enhance the thermoelectric figure of merit relative to bulk materials is presented in general terms. Specific application is made to the Si-Ge system for use at high temperature for space vehicle propulsion applications. The scientific advantages of the nano-composite approachfor the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and an easy method for the of scale-up of nanostructured building blocks into bulk quantities of material for device fabrication.
4:00 PM - U2:Nano2amp;Theory
4:30 PM - **U2.5
David Singh 1 Show Abstract
1 Materials Science and Technlogy Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
NaxCoO2 is a fascinating material displaying a complex phase diagram and showing unprecedented properties. These include superconductivity upon hydration, apparent proximity to magnetic quantum critical points and high thermoelectric performance at metallic carrier densities. This high thermoelectric performance is remarkable as it was long held that neither oxides nor high carrier density metals could be good thermoelectrics. Furthermore, since the discovery of the thermoelectric properties of NaxCoO2 a decade ago, there are still no other examples of high performance thermoelectric oxides. Here, the problem of oxide thermoelectricity is discussed starting with NaxCoO2 within the context of electronic structure calculations and Boltzmann transport theory. These calculations suggest that there may well be other oxide thermoelectrics and suggest directions for identifying them. Some candidate materials are discussed.
5:00 PM - U2.6
An Investigation of Sodium Ordering in NaxCoO2 (x ≧ 0.50) by Density Functional Theory.
Ying Meng 1 , Yoyo Hinuma 1 , Gerbrand Ceder 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Understanding the remarkable thermoelectric properties and interesting electronic/magnetic phenomena in P2-NaxCoO2 requires a detailed understanding of the structures at various sodium concentrations where sodium and vacancy order at finite temperature, which can couple strongly with the electronic structure. Using first principles electronic structure methods within the GGA and GGA+U approximations we find that Na ordering is determined by a competition between Na site energies difference and Na-Na repulsion. In addition the Co-Na interlayer interaction when charge localization occurs can facilitate the lock-in of certain ordering patterns of Na-Vacancy at simple fraction fillings. We will compare and contrast the stable ordering schemes obtained in this work with available experimental observations. Our work shows excellent agreement with experiments, in contrast to the previous DFT studies, in addition we predict a series of new ground states at concentration 0.60 to 0.71.
5:15 PM - U2.7
Large Thermoelectric Power Generated by the van Hove Singularity of Two-dimensional Triangular Lattice.
Tsunehiro Takeuchi 1 Show Abstract
1 EcoTopia Science Institute, Nagoya University, Nagoya Japan
Recently, layered cobalt oxides, such as NaxCoO2, Bi2Sr2Co2O9, and Ca3Co4O9, were found to simultaneously possess large thermoelectric power and metallic electrical conduction. Those properties are two of the three necessities of practical thermoelectric materials. The layered cobalt oxides, therefore, were widely considered as one of the promising candidates for the thermoelectric materials in the next generation. Surprisingly, the carrier concentration of these materials were distributed over 10^(21)~10^(22) cm^(-3), which values are considered as those of metallic phases which generally possess very small thermoelectric power less than 10 μV/K. Thus a large number of attentions have been focused on the mechanism leading to the large thermoelectric power exceeding 100 μV/K with the large carrier concentration. In our resent studies, we performed high-resolution angle resolved photoemission spectroscopy measurements on these layered cobalt oxides and investigated the energy-momentum dispersion near the Fermi level (EF) in detail. By using the experimentally determined electronic structure and the Bloch-Boltzmann theory, we found that the large thermoelectric power with metallic conduction of the layered cobalt oxides was brought about by the Boltzmann-type electrical conduction with a unique spectral conductivity characterized by the large peak just below EF in associated with the van Hove singularity (vHs) of the two-dimensional system. More recently, however, we realized that even though a large peak in the electronic density of states is generated by the vHs, the layered copper oxides of two-dimensionally spanned square lattice have a very small peak in the spectral conductivity and consequently possess relatively small magnitude in their thermoelectric power. In order to investigate the origin of the large vHs peak in the spectral conductivity of the layered cobalt oxides, we employed, in this study, simple tight-binding simulations for several systems of different symmetry and calculated spectral conductivity within the rough assumption of a constant mean free path. As a result of the simulations, the very important role of the two-dimensionally spanned triangular lattice leading to a large peak in the spectral conductivity and consequently to a large magnitude in thermoelectric power was clearly revealed. In the presentation, the results obtained by the simulations will be explained in detail together with some typical examples of the two-dimensional triangular lattice possessing a large thermoelectric power.
5:30 PM - U2.8
Defect Clustering and Nanostructure Formation in PbTe-based Bulk Thermoelectrics.
Khang Hoang 1 , Subhendra Mahanti 1 Show Abstract
1 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan, United States
Lead chalcogenides (PbTe, PbSe, and PbS) are IV-VI narrow band-gap semiconductors whose studies over several decades has been motivated by their importance in infrared detectors, light-emitting devices, infrared lasers, photovoltaics, and high temperature thermoelectrics. PbTe in particular is the end-compound of several high performance high temperature thermoelectrics such as AgPbmSbTe2+m  and Na1-xPbmSbyTe2+m . These quaternaries are found to have enhanced thermoelectric figure of merit (compared to PbTe), due to the nanostructuring, which helps in lowering the lattice thermal conductivity and possibly enhancing the thermopower [1-2]. In this work we systematically study defect clustering in PbTe using density functional theory and supercell models. The defects being considered are (but not limited to) Na, K, Ag, Sb, Bi, and vacancies. Our energetic studies show that many defect pairs are stabilized when the two defects in a pair are either the nearest or the next-nearest neighbors in the PbTe lattice, which can help explain the nanostructuring found in various PbTe-based systems. Comparisons with similar defect cluster calculations in SnTe and GeTe will be made. We also study the electronic structure as a function of the relative distance between the two defects in a pair. Work partially supported by ONR-MURI.1. K.-F. Hsu et al., Science 303, 818 (2004).2. P. Poudeu et al., Angew. Chem. Int. Ed. 45, 3835 (2006).
5:45 PM - U2.9
Disorder Creates Band Gap in (Pb,Sn)Te Alloys.
Xing Gao 1 , Murray Daw 1 Show Abstract
1 Dept. of Physics & Astronomy, Clemson University, Clemson, South Carolina, United States
The PbTe, SnTe and their alloys are one of the commercial thermoelectric materials nowadays. The efforts to enhance its figure of merit have been extensively undertaking by doping rare-earth atoms, creating nano-structures in it, and so on. Furthermore, the electronic structure of Sn-doped PbTe is fundamentally interesting because of the so-called band inversion in its two end members, PbTe and SnTe [1,2]. Although, the electronic structure of these two compounds and their analogies have been intensively studied by first-principles calculations, to our best knowledge, there are no direct first-principles calculations of the band gap evolution through the full range of alloying. We report a study of the electronic structure of this material through the full range of alloy content, combining SQS  and LDA. Our results show that disorder plays an important role in the electronic structure of this alloy. The calculated results by taking account of the short-range disorder are in good agreement with experimental results.  Dimmock, Melngailis, and Strauss, Phys. Rev. Lett. 16, 1193 (1966). Tung, and Cohen, Phys. Rev. 180, 823 (1969). Wei and Zunger, Phys. Rev. B 55, 13605 (1997).This work is supported by DOE-EPSCoR.
U3: Poster Session
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - U3.10
Lanthanum Telluride: A Refractory Thermoelectric Material by Mechanical Alloying.
Andrew May 1 , Jeffrey Snyder 1 , Jean-Pierre Fleurial 2 Show Abstract
1 , California Institute of Technology, Pasadena, California, United States, 2 , Jet Propulsion Laboratory, Pasadena, California, United States
Lanthanum telluride demonstrates significant potential as an n-type material for high temperature thermoelectric application. The phase of interest is the cubic Th3P4 structure, which exists for compositions La3-xTe4 with 0≤x≤⅓. Mechanical alloying is utilized to synthesize the refractory compound at room temperature. The TE properties of various compositions (0<x<0.3) are examin