Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N43: Focus Session: Physics of Thermoelectric Materials and Phenomena III |
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Sponsoring Units: FIAP DMP Chair: Ctirad Uher, University of Michigan Room: Colorado Convention Center 506 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N43.00001: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N43.00002: Thermoelectric and Thermomagnetic Properties of Nanostructured Lead Chalcogenides. Suraj Joottu Thiagarajan, Troy Pyles, Ramachandra R. Revur, Vladimir Jovovic, Joseph D. West, Joseph P. Heremans, Suvankar Sengupta, J. Richard Schorr We report results of a study of the thermoelectric and thermomagnetic properties of nanostructured PbTe/PbSe alloys. By the four-coefficients method, the carrier density, effective mass, mobility and scattering parameter are obtained which could then be used for optimizing the material. The samples show higher thermopower than bulk. This enhancement is due to the energy filtering induced by change of the scattering parameter, which in turn, could be due to the electronic density of states altered by the quantum dot inclusions. This is a promising lead to high performance nanostructured thermoelectric materials manufacturable in bulk quantities. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N43.00003: Transport and magnetic properties of dilute rare-earth-PbSe alloys V. Jovovic, S. Joottu-Thiagarajan, J. West, J. P. Heremans, T. Story, Z. Golacki, W. Paszkowicz , V. Osinniy An increase in the density of states is predicted [1] to increase the thermoelectric (TE) figure of merit, and could be induced by doping TE materials with rare-earth elements. This was attempted here: the galvanomagnetic and thermomagnetic properties of dilute alloys of PbSe and Ce, Pr, Nd, Eu, Gd and Yb were measured from 80 to 380K; magnetic susceptibilities were measured from 4 to 120K. The density of states effective mass, the relaxation time, and the carrier density and mobility are calculated from measurements of the electrical conductivity and the Hall, Seebeck and transverse Nernst-Ettingshausen coefficients. The Eu, Gd, Nd and Yb-alloyed samples are paramagnetic; the concentrations of rare-earth atoms are determined from fitting a Curie-Weiss law. The magnetic behavior of the Ce and Pr-alloyed samples is different. Ce, Pr, Nd, Gd and Yb act as donors with efficiencies that will be reported. Alloying with divalent Eu does not affect carrier density but increases the energy gap. This work suggests that the 4f orbitals preserve their atomic-like localized character and exhibit only weak sp-f hybridization. 1 G. D. Mahan and J. O. Sofo, Proc. Natl. Acad. Sci. USA 93 7436 (1996) [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N43.00004: Synthesis \& Properties of Nano-Composite Thermoelectric Materials Invited Speaker: PbTe nanocrystals have been grown in our labs by chemical vapor deposition. These materials grow in size selective regions exhibiting very high yield and have size distributions of around 100 nm to 1000 nm. These nano-materials are incorporated into a bulk matrix, making a composite material in hopes of achieving a higher thermoelectric performance due to the increased phonon scattering that the nano-materials are expected to exhibit, as well as potential for enhancement of their Seebeck coefficient. Some of the advantages as well as the challenges will be discussed. These nanocomposites give a new level of potential control as a tuning parameter with which to vary the materials' thermoelectric properties. In addition, Bi$_{2}$Te$_{3}$, another state of the art thermoelectric material and skutterudites (CoSb$_{3})$ have been synthesized as nanomaterials using hydrothermal techniques. A brief discussion of the synthesis techniques, the characterization techniques and highlights of several systems of materials will be presented. \newline \newline In collaboration with Xiaohua Ji, Jian He, Bo Zhang, Nick Gothard, and Paola Alboni, Dept. of Physics, Clemson University. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N43.00005: Neutron scattering study of phonon dynamics on cage compounds C.H. Lee, H. Yoshizawa, I. Hase, H. Sugawara, M.A. Avila, T. Takabatake, H. Sato Filled skutterudite compounds have attracted great attention due to their potential as thermoelectric devices. In particular, their low lattice thermal conductivity is advantageous to achieve high thermoelectric performance. To improve the performance further, it is important to clarify the origin of their low lattice thermal conductivity. Previous studies suggest that the low thermal conductivity is a consequence of free vibration of rare-earth atoms in large lattice cages, which is so called rattling effect. To confirm the hypothesis, we have studied phonon dynamics of CeRu$_4$Sb$_{12}$ by neutron scattering using single crystal samples at JRR-3M reactor of JAERI in Tokai. As results, we have found optical phonons associated with large vibration of Ce atoms at relatively low energy of $E$=6meV, which show an anticrossing with acoustic phonons. According to the analysis based on a Born-von K'{a}rm'{a}n force model, the longitudinal force constants of the nearest Ce-Sb and Ce-Ru are both estimated to be 0.025 mdyn/A, while that of the nearest Ru-Sb shows a large value of 1.4 mdyn/A, indicating that the Ce atoms are bound very weakly with surrounding rigid RuSb$_6$-octahedron cages. We will discuss that the origin of the low lattice thermal conductivity can be intensive Umklapp scattering originating from low-lying optical phonons. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N43.00006: Superconductivity of Ba8Si46-xGax clathrates Yang Li, Ruihong Zhang, Ning Chen, Xingqiao Ma, Guohui Cao, Z.P. Luo, C.R. Hu, Joseph H. Ross, Jr. We have presented a combined experimental and theoretical study of the effect of Gallium substitution on the superconductivity of the type I clathrate Ba8Si46-xGax. In Ga-doped clathrates, the Ga state is found to be strongly hybridized with the cage conduction-band state. Ga substitution results in a shift toward to a lower energy, a decrease of density of states at Fermi level, a lowering of the carrier concentration and a breakage of integrity of the sp3 hybridized networks. These play key roles in the suppression of superconductivity. For Ba8Si40Ga6, the onset of the superconducting transition occurs at Tc=3.3 K. The investigation of the magnetic superconducting state shows that Ba8Si40Ga6 is a type II superconductor. The critical magnetic fields were measured to be Hc1=35 Oe and Hc2=8.5 kOe. Our estimate of the lectron-phonon coupling reveals that Ba8Si40Ga6 is a moderate phonon-mediated BCS superconductor. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N43.00007: Thermoelectric Properties of RE$_{3}$Ru$_{4}$Ge$_{13}$ compounds (RE = Y, Dy, Ho, Lu) D. Morelli, H. Kong, X. Shi, C. Uher Rare earth based compounds have been suggested as ideal thermoelectric materials due to the potential existence of sharp features in their electron density of states. One such series of compounds is of the form R$_{3}$Ru$_{4}$Ge$_{13}$. These materials crystallize in the cubic structure Pm3n and are known to exhibit a variety of interesting properties, including magnetic ordering, superconductivity, and anomalous semiconductor-like resistivity. These compounds can be considered as variants of the cubic structure compound RERuGe$_{3}$ (RE$_{4}$Ru$_{4}$Ge$_{12})$ in which one RE atom is replaced by a germanium atom. This ``extra'' Ge atom can reside on either the Ge or RE site, and the site disorder combined with the complex unit cell of these compounds suggests inherent low lattice thermal conductivity. In order to survey the potential of these materials as thermoelectrics we have synthesized several member compounds of this family. Results on Seebeck coefficient, resistivity, Hall coefficient, and thermal conductivity as a function of temperature will be reported. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N43.00008: The thermoelectric properties of Bi nanowires. Role of quantum size and surface effects. Tito Huber, Albina Nikolaeva, Dmitri Gitsu, Leonid Konopko, Michael Graf Because of the increased density of states arising from one-dimensional confinement, it is anticipated that bismuth quantum wires will exhibit superior thermoelectric properties. Recently, angle-resolved photoemission spectroscopy (ARPES) studies have shown that Bi supports surface states that have not been considered in current models of quantum confinement. Studies of the Fermi surface, employing the Shubnikov-de Haas (SdH) method, in arrays of 30- to 80-nm bismuth nanowires partially corroborates ARPES findings. Assuming diffusive conditions, the impact of the excess surface carriers on the thermopower is to effectively make it smaller than that of bulk Bi, in agreement with measurements reported in the literature. We will report the result of experiments designed to decrease the concentration of surface carriers.. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N43.00009: Thermoelectric properties of SiGe nanoparticle composites Ming Tang, Hohyun Lee, Asegun Henry, Ronggui Yang, Dezhi Wang, Jean-Pierre Fleurial, Pawan Gogna, Gang Chen, Zhifeng Ren, Mildred Dresselhaus Prior theoretical and experimental proof of principle studies on quantum well superlattice and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. We have shown that nanostructural composites exhibit nanostructures and properties that show great promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. We demonstrate that we can achieve (1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and (2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N43.00010: Synthesis and Characterization of Nanocomposite Chalcogenides Joshua Martin, W. Zhang, L. Chen, G.S. Nolas Recent results on thermoelectric properties of superlattice and nano-scale materials have energized the search for high-performance thermoelectric materials. Thermoelectric technology requires materials in large quantities therefore new approaches are needed to incorporate nano-scale enhancement into bulk materials. We present a technique to synthesize dense bulk nanocomposites reproducibly, and investigate their structural and transport properties. Transport properties demonstrate a strong dependence on porosity, highlighting the necessity of dense nanocomposites for thermoelectric applications. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N43.00011: Time-Resolved Diffraction Studies of Nanoscale Thermal Transport Matt Highland, Bryan Gundrum, Yee Kan Koh, Victor Elarde, James J. Coleman, David G. Cahill, Don Walko, Eric Landahl One of the major considerations in fabricating devices on ever smaller length scales is thermal management in nanometer sized structures. Studying thermal transport requires a temperature measurement accurate on short time scales and sensitive to temperature changes in nanoscale structures. Time Resolved X-ray Diffraction (TRXD) utilizes 100ps x-ray pulses as a probe of lattice expansion and ultrafast laser pulses as a pump for the measurement of optically excited materials. Thermal expansion due to laser heating can therefore be used to study thermal transport in thin films. Reported here are (TRXD) measurements of the temperature rise in (InAs)$_{x}$(GaAs)$_{1-x}$ thin films due to laser heating. These results are compared with continuum model predictions of temperature rise based on film parameters measured independently using Time Domain Thermal Reflectance. This comparison shows a continuum model is inadequate in predicting the thermal behavior these films on short time scales and is indicative of complex transport phenomenon. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N43.00012: Thermoelectric Properties of Heterostructure InAs/InP Nanowires Eric Hoffmann, Ann Persson, Henrick Nilsson, Linus Fr\"oberg, Lars Samuelson, Heiner Linke InAs nanowires with an embedded quantum dot defined by an InP double-barrier structure offer quantum confinement in all three spatial directions. Using a global gate, the Fermi energy can be tuned relative to the dot's density of states. Furthermore, at temperatures below $\sim $10 K, phonon states freeze out, decoupling phonons and electrons, and electron temperatures can be controlled independent of the lattice temperatures. Due to this control, nanowires at low temperatures lend themselves to detailed investigations of the dependence of thermoelectric effects on a strongly modulated density of states. This is of interest, because such systems have been predicted to be able to convert thermal energy to electrical energy at very high efficiency. We report on experiments where a temperature gradient in the electron gas is created along a single nanowire by heating the metallic lead at one end of the nanowire using an ac heating current. The resulting temperature gradient creates a thermovoltage across the nanowire whose sign and magnitude can be tuned by adjusting the Fermi energy relative to the discrete energy levels in the quantum dot. We find that the thermovoltage depends nonlinearly on the temperature differential at surprisingly small temperature gradients. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N43.00013: Round-robin measurement survey for Seebeck coefficient standard reference material Nathan Lowhorn, Winnie Wong-Ng, Makoto Otani, Martin Green, Thanh Tran Full characterization of a thermoelectric material requires measurement of the electrical resistivity, thermal conductivity, and Seebeck coefficient. While standard materials exist or have existed for the first two properties, a Seebeck coefficient standard material with moderate or high values does not exist. In an effort to expedite research efforts in this field, we have initiated a project to develop a Seebeck coefficient, or thermopower, standard reference material. To this end, we have conducted a round-robin measurement survey of candidate standard materials. Both rounds of the survey have been completed, and the results and methodology will be presented. [Preview Abstract] |
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