Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B29: Focus Session: Thermoelectrics I: Recent Concepts |
Hide Abstracts |
Sponsoring Units: DMP FIAP GERA Chair: Donald Morelli, Michigan State University Room: C123 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B29.00001: NMR and computational modeling of Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates Sergio Y. Rodriguez, Xiang Zheng, Joseph H. Ross, Jr. We report studies of Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates by NMR and modeling in order to better understand the electronic and dynamical properties. For samples exhibiting both the type-I and type-VIII clathrate structures, we have measured $^{71}$Ga quadrupole NMR lineshapes. We used {\it ab initio} modeling to extract structural information, providing an estimate of the site-occupation preferences in these alloys. For the type-I material, the resulting preferred structures are similar to those obtained in type-I Ba$_{8}$Al$_{16}$Ge$_{30}$, and we also find that the calculated formation energies show a preference for local structures that are similar to those of the BaAlGe analog. NMR Knight shifts at high temperatures are constant indicating metallic behavior. {\it Ab initio} results are in partial agreement with the local distribution of this metallic behavior. However the NMR lineshapes and $T_{1}$ relaxation time show low temperature changes in type-I samples attributed to atomic dynamics, which we connect to the prominent rattling behavior in this clathrate. Supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B29.00002: Direct verification of Ga-Ga bond avoidance in the clathrate Ba$_8$Ga$_{16}$Sn$_{30}$ from EXAFS studies Michael Kozina, F. Bridges, Y. Jiang, M. Avila, K. Suekuni, T. Takabatake Ba$_8$Ga$_{16}$Ge$_{30}$ and Ba$_8$Ga$_{16}$Sn$_{30}$ are important thermoelectric clathrate materials with low thermal conductivities. In these materials, the Ga/Ge or Ga/Sn atoms occupy three sites in the cage-like lattice, but the Ga are not randomly distributed. Experiments in the past have only been able to suggest that Ga-Ga bonds are not favored within the cage structure of many type I clathrates. Here we show definitive evidence that this is indeed the case for Ba$_8$Ga$_{16}$Sn$_{30}$. Using the EXAFS technique, we are able compare the backscattering functions for the first neighbors about Ga to the calculated functions for Ga-Ga and Ga-Sn bonds. The result is that only $\sim$15\% of the Ga nearest neighbors are Ga. Combining this result with diffraction data on occupational parameters, we propose one possible arrangement of Ga and Sn in the unit cell of Ba$_8$Ga$_{16}$Sn$_{30}$. Additionally, we find significant disorder in the Ga/Sn lattice; the Ga-Sn bond and Ga-Ga bonds are 0.07{\AA} and 0.2{\AA} (respectively) shorter than the average bond length, which must contribute to the smaller thermal conductivity. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B29.00003: Enhancing thermoelectric power factor with highly mismatched isoelectronic doping Joo-Hyoung Lee, Junqiao Wu, Jeffrey Grossman With a view to enhancing the thermoelectric Seebeck effect by introducing highly electronegativity mismatched impurities, we investigate the effect of O impurities on the thermoelectric properties of ZnSe from a combination of first-principles and analytic calculations. It is demonstrated that dilute amounts of O impurities introduce peaks in the density of states (DOS) above the conduction band minimum, and that the charge density near the DOS peaks is substantially attracted toward O atoms due to their high electronegativity. The impurity-induced peaks in the DOS result in an increase of the room-temperature Seebeck coefficient and power factor from those of O-free ZnSe by a factor of 30 and 180, respectively. Other impurities with different electronegativities such as N-F dual impurities and S are also examined and it is found that this effect is absent when the impurity electronegativity well matches the host that it substitutes. The results suggest that highly electronegativity-mismatched alloys can be designed for high performance thermoelectric applications. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:27PM |
B29.00004: Nanostructured Thermoelectrics Invited Speaker: The eternal quest to improve the figure of merit of thermoelectric devices, over wide ranges of temperature, has recently focused on nanostructured thermoelectrics. These include superlattices, quantum wires, and quantum dots. One clear benefit of nanostructure is to reduce the thermal conductivity. We review our calculations of heat flow which have been confirmed by recent experiments. We also discuss our theory of the thermoelectric properties of crystals of quantum dots. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B29.00005: Wave-particle conduction of thermal energy in thermoelectric materials Martin Maldovan Materials that can stop heat transfer are very important for increasing the efficiency of thermoelectric materials such as skutterudites, clatharates, superlattices, nanowires, and quantum dots. The transport of thermal energy, however, is a challenging phenomenon difficult to understand and control. Currently there is no theoretical model capable of accurately describing the flow of thermal energy in nanostructures and basic physical properties such as thermal conductivity can not be predicted. This is in part due to our lack of knowledge on a fundamental physical property: whether a phonon behaves as a particle or as a wave within a nanostructured material. We present physical mechanisms governing heat transfer in semiconductor nanostructures and a quantitative wave-particle model to calculate, predict, and explain experimental observations on thermal transport properties. Comparing theoretical and experimental results, we show that phonons with frequencies above 2THz behave as particles while those with frequencies below behave as waves. We also explain the existence of a minimum thermal conductivity and the nearly temperature-independent thermal conductivity in superlattices. This opens the opportunity for the design of nanostructured materials that can control heat in optoelectronic devices or thermoelectric materials. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B29.00006: Crystal Structure Effects on the Thermal Conductivity of Cu-Ge-Se Compounds Eric Skoug, Jeffrey Cain, Donald Morelli One approach to increasing the efficiency of a thermoelectric material is to decrease its thermal conductivity without degrading its electronic properties. Traditionally this has been accomplished, for instance, by forming solid solutions between compounds with similar crystal structures, or, more recently, by inducing nanostructure in the crystal lattice. These methods have proven effective in many cases; however discovering compounds with intrinsically low thermal conductivity provides a fundamental solution to the same problem. Here we describe our initial efforts in synthesis and characterization of compounds of the series Cu$_{2}$Ge$_{1+x}$Se$_{3}$, in which we observe a transition from orthorhombic to cubic symmetry at x = 0.55. The lattice thermal conductivity of the cubic phase is significantly lower than that of the orthorhombic phase, which we discuss here in relation to vacancies and anti-site defects. A simple valence argument is presented suggesting a change in the nominal valence of Ge as x approaches 1.0, which we speculate contributes to increased bond anharmonicity in the cubic-structure compounds. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B29.00007: Unusual electronic transport properties of the intermetallic compounds RuAl$_{2 }$and RuGa$_{2}$ V. Ponnambalam, K. Leikert, D.T. Morelli Semiconductor-like electrical transport has been reported for the title compounds RuAl$_{2}$ and RuGa$_{2.}$ Band structure calculations performed on these compounds suggest band gap opening or minimum density of states in the vicinity of Fermi level due to the hybridization of d- and sp-bands. The observed transport properties are rather unusual and attributed to band hybridization. Our primary objective is to study these transport properties in detail. We have prepared single phase compounds by a combination of arc melting and subsequent annealing at 950$^{\circ}$C - 750$^{\circ}$C. Measurements of the electrical resistivity, thermopower and Hall coefficient are being carried out below room temperature. The results of these experiments will be presented and discussed [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B29.00008: The influence of band structure on the thermoelectric performance of lanthanum telluride Andrew F. May, David J. Singh, Espen Flage-Larsen, G. Jeffrey Snyder The electrical transport properties of the high temperature, $n$-type thermoelectric material lanthanum telluride (La$_{3-x}$Te$_4$) will be discussed in detail. The influence of electronic structure on transport has been investigated via density functional calculations on La$_3$Te$_4$, which reveal light bands at the conduction band edge and heavy bands approximately 0.3 eV above this minimum. Multi-band transport explains the experimentally observed trend in Seebeck coefficient versus carrier density, which is qualitatively reproduced in the first principles calculations. The optimum doping level is found to correspond to a Fermi energy located near the heavy band minima. Finally, calculations on the single vacancy composition ($x=\frac{1}{4}$) will be utilized to discuss the influence of lanthanum vacancies on the electronic structure and the validity of the rigid band approximation. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B29.00009: Nanostructure and physical properties in novel thin film misfit layered chalcogenides M. Beekman, C. Heideman, Q. Lin, M. Smeller, M.D. Anderson, S. Tepfer, N. Nguyen, D.C. Johnson An expansive collection of novel misfit layered chalcogenide compounds has been prepared \textit{via} the method of modulated elemental reactants. Despite a lack of epitaxial relationships between the transition metal dichalcogenide and Group IV or rare earth chalcogenide components in these materials, X-ray diffraction and high resolution transmission electron microscopy studies show these nanostructured materials exhibit remarkably sharp and atomically smooth interfaces, indicating the potential for interfacial ``engineering.'' Ultra-low thermal conductivities are found to be characteristic, of interest in the context of new approaches to thermoelectric materials design. Results are presented from our ongoing program to understand the interplay between nanostructure and physical properties in these materials, in particular the electrical transport. Structural and compositional flexibility in these systems, which offers unique opportunities for understanding and realizing thermoelectric enhancement through nanostructure, will be discussed. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B29.00010: Validity of Wiedemann-Franz law in thermoelectric half Heusler compounds Mal-Soon Lee, S. D. Mahanti There is renewed interest in the field of thermoelectrics for power generation. Several promising thermoelectrics are half-Heusler narrow band gap semiconductors. The efficiency of thermoelectric energy conversion depends on the transport coefficients through the figure of merit $ZT=\sigma S^2T/\kappa $. For large \textit{ZT} , it is necessary to decrease the total thermal conductivity ($\kappa =\kappa _l +\kappa _{el} )$ as well as increase the Seebeck coefficient ($S)$ and the electrical conductivity ($\sigma )$To determine $\kappa _l $ experimentally, one usually subtracts the electronic thermal conductivity ($\kappa _{el} )$ from measured $\kappa $, using the Wiedemann-Franz law ($\kappa _{el} =L_0 \sigma T$, $L_0 =2.45\times 10^{-8}W\Omega /K^2)$. To examine the validity of this law in half-Heusler compounds, we have chosen HfCoS as an example. We have calculated the electronic transport coefficients by employing \textit{ab-initio} electronic structure method and the Boltzmann transport equation in HfCoSb. We calculate $\kappa _{el} $at constant current \textbf{J }($\kappa _{el,J} )$ and constant electric filed \textbf{E }($\kappa _{el,E} )$ where $\kappa _{el,J} =\kappa _{el,E} -T\sigma S^2$ which shows a significant deviation from values obtained with Wiedemann-Franz law. $\kappa _{el,J} $ is much smaller than $\kappa _{el,E} $at low carrier concentrations ($n)$ and/or at high temperatures ($T)$ and the ratio $\kappa _{el,J} /\kappa _{el,E} \to 1$ at high $n$ and/or low $T$. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B29.00011: Phonon Density of States in Equilibrium and Non-equilibrium Prepared Ytterbium and Cerium-Filled Skutterudites Ivo Dimitrov, Michael Manley, Qing Jie, Juan Zhou, Jorge Camacho, Georg Ehlers, Andrey Podlesnyak, Steven Shapiro, Qiang Li Filled skutterudites have been extensively studied due to their high thermoelectric figure of merit (ZT) and promise for applications in the 400 K -- 800 K range. They tend to exhibit higher ZT values than their binary counterparts, and a so-called ``rattling effect'' has been suggested to be a major factor that effectively reduces lattice thermal conductivity. However, this ``rattling'' scenario for the vibrational modes of a heavy element in a skutterudite cage remains controversial. We measured the phonon density of states (DOS) of Ce and Yb-filled skutterudites prepared by conventional equilibrium and non-equilibrium synthesis methods via inelastic neutron scattering. Nanoscaled substructures are commonly observed in the non-equilibrium samples as examined by high resolution TEM. Preliminary data analysis shows that the Yb filled antimonides exhibit well-defined localized modes in the 4-12 meV range and several unresolved optical modes in the 30-35 meV range, as compared with their Ce-filled counterparts. Differences in the 3-10 meV range in the DOS of the latter are believed to arise from non-equilibrium structures that developed during preparation. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B29.00012: Thermal transport in thermoelectric skutterudites: first-principles and phenomological approaches Dmitri Volja, Marco Fornari, Boris Kozinsky, Daehyun Wee, Nicola Marzari The possible application of pnictogen substituted ternary skutterudites in thermoelectric devices is currently limited due to their relatively large electrical resistivity. These materials, however, exhibit thermal conductivity of the order $0.7-2.2$ k/Wm$^{-1}$K$^{-1}$ at room temperature and may be amenable to optimization. In skutterudites at high temperatures the thermal transport is primarily dominated by anharmonic interactions of phonons. The precise evaluation of such interactions from first principles is currently a formidable task. We have analyzed the third order phonon-phonon scattering mechanisms, that arise from the anharmonicity of the interatomic potentials, and use a standard Boltzmann transport approach to derive the thermal conductivity. In our methodology we have combined first principles approaches and phenomenological interatomic potentials. We specifically studied the transport properties of $CoGe_{3/2}S_{3/2}$, $CoGe_{3/2}Te_{3/2}$ and $CoSn_ {3/2}Te_{3/2}$ and provide comparison to the parental binary CoSb$_3$ . We validate our approach by testing the approximations in simpler systems where full evaluation of anharmonic force constants from first- principles calculations is possible. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700