Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H28: Focus Session: Thermoelectric Materials: Transport Physics |
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Sponsoring Units: DMP FIAP Chair: David Cahill, University of Illinois Room: 330 |
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H28.00001: Thermoelectricity: A Bottom-up View Invited Speaker: It is well-known that the nature of electronic transport changes significantly as the length L of the active region of a device is reduced from millimeter down to nanometer dimensions. Historically our understanding of electrical resistance and conduction has progressed top-down: from large macroscopic conductors to small atomic scale conductors. Indeed thirty years ago it was common to argue about what, if anything, the concept of resistance meant on an atomic scale. Since then there has been significant progress in our understanding, spurred by actual experimental measurements made possible by the technology of miniaturization. However, despite this progress in understanding the flow of current on an atomic scale, the standard approach to the problem of electronic transport continues to be top-down and we have argued elsewhere that an alternative bottom-up viewpoint can be extremely illuminating [1]. In this talk we will briefly summarize this viewpoint and discuss the unique insights it provides into the subject of thermoelectricity and thermoelectric device design in general and into the possibilities of molecular thermoelectrics in particular. \\[4pt] [1] See for example, S.Datta, ``Nanolectronic Devices: A Unified View,'' to appear in The Oxford Handbook on Nanoscience and Nanotechnology: Frontiers and Advances, eds. A.V. Narlikar and Y.Y. Fu, volume 1, chapter 1, arXiv/0809.4460 [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H28.00002: Nanostructured materials design for thermoelectric applications Keivan Esfarjani, Mona Zebarjadi, Ali Shakouri Nanostructured materials have shown great promise for superior thermoelectric properties. Recently, our collaborators and us have been able to enhance thermoelectric properties of InGaAs by doping it with ErAs nanoparticles. Transport properties are dominated by scattering of electrons with nanoparticles, phonons and impurities. We can design the scattering potential of the former to maximize the power factor, P.is in fact an inverse problem, attempting to solve for the best nanoparticle scattering potential which maximizes P. Using a least square method, we find the potential which minimizes the difference between the actual scattering cross section and its target value. The target value is chosen so as to display energy filtering property. More generally, we also simply maximize the power factor with respect to the nanoparticle potential profile. A simple and fixed model is chosen for other scattering rates, as well as the dispersion relation for the bulk electrons.results between the two approaches will be compared in order to see the effect of electron filtering on the power factor enhancement. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H28.00003: Influence of Dimensionality on Thermoelectric Device Performance Raseong Kim, Supriyo Datta, Mark S. Lundstrom Significant improvements in the thermoelectric figure of merit have recently been demonstrated in low dimensional structures. These improvements have been largely due to the reduced lattice thermal conductivity, so the question of how much additional improvement is possible by engineering the electronic structure has become important. Our goal is to present a clear answer to this question using the Landauer formalism, which applies from the ballistic to diffusive limits. We first relate thermoelectric coefficients to the transmission and the number of conducting channels, M(E). The optimum M(E) is known to be a delta-function. We then compare thermoelectric coefficients in one, two, and three dimensions and show that the channels are utilized more effectively in lower dimensions. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). To realize the advantage of lower dimensionality, the packing density must be very high. The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H28.00004: Optimized vacuum thermionic energy conversion using diamond materials Joshua Smith, Griff Bilbro, Robert Nemanich The vacuum thermionic energy conversion device (TEC) has been an attractive alternative to other means of energy production for some time due to its potentially high efficiency operation, but practical devices have been difficult to develop as a result of the negative space charge effect. It is well known that a hydrogen termination layer on a diamond material induces a negative electron affinity (NEA). In this study we present a theoretical analysis showing it is possible to tune the parameters of a thermionic device featuring a doped diamond material as the emitter electrode to maximize the output power produced. For example, a TEC operating between $950K$ and $300K$ with an emitter negative electron affinity of $0.5eV$, a collector barrier height of $0.6eV$, Richardson's constant of both electrodes equal to $10A cm^{-2} K^{-2}$, emissivity of both electrode of $0.5$, and interelectrode spacing of $10\mu m$ will have a maximum output power of $1.5W cm^{-2}$ and efficiency of $20\%$ occurring at an emitter barrier height of $1.2eV$. The efficiency calculation includes electronic and blackbody heat transport across the device. The analysis establishes approaches to increase the efficiency to values greater than $20\%$. This work was funded by the Office of Naval Research through the TEC MURI Program. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H28.00005: Nanostructured Silicon as a Thermoelectric Material Invited Speaker: |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H28.00006: Design of thermoelectric composite materials for energy applications. Martin Maldovan, Edwin Thomas Energy supply is becoming a major world-wide problem as fossil energy supplies decrease while energy demands increase. Thermoelectric materials, which reversibly convert thermal and electrical energy, offer the prospect of power generation and cooling by means of the rational transport of electrons and phonons. In nanocomposite materials, both quantum and classical effects provide opportunities to control the transfer of electrons and phonons. The difficulty associated with thermoelectric materials is the need to couple and optimize a variety of physical properties in order to exhibit necessary efficiencies, which are determined by the figure of merit ZT. To exhibit large efficiencies, the best thermoelectric material should possess low thermal conductivity (similar to that of a glass) and high electrical conductivity (similar to that of a perfect crystal material). In this paper we study thermoelectric materials by preparing composite materials that can provide the desired coupled physical properties. Our research concentrates on predicting and designing thermoelectric material properties using theoretical and computational methodologies. We use currently available algorithms and numerical techniques to design thermoelectric materials with increased efficiencies. The ultimate goal of our research is to develop a basic understanding of the coupled physical properties in these materials and to create a framework that allows for the systematic design, optimization, and characterization of their thermal and electric properties. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H28.00007: Thermal conduction mechanisms in isotope-disordered boron nitride and carbon nanotubes Ivana Savic, Natalio Mingo, Derek Stewart We present first principles studies which determine dominant effects limiting the heat conduction in isotope-disordered boron nitride and carbon nanotubes [1]. Using an ab initio atomistic Green's function approach, we demonstrate that localization cannot be observed in the thermal conductivity measurements [1], and that diffusive scattering is the dominant mechanism which reduces the thermal conductivity [2]. We also give concrete predictions of the magnitude of the isotope effect on the thermal conductivities of carbon and boron nitride single-walled nanotubes [2]. We furthermore show that intershell scattering is not the main limiting mechanism for the heat flow through multi-walled boron nitride nanotubes [1], and that heat conduction restricted to a few shells leads to the low thermal conductivities experimentally measured [1]. We consequently successfully compare the results of our calculations [3] with the experimental measurements [1]. [1] C. W. Chang, A. M. Fennimore, A. Afanasiev, D. Okawa, T. Ikuno, H. Garcia, D. Li, A. Majumdar, A. Zettl, Phys. Rev. Lett. 2006, 97, 085901. [2] I. Savic, N. Mingo, D. A. Stewart, Phys. Rev. Lett. 2008, 101, 165502. [3] I. Savic, D. A. Stewart, N. Mingo, to be published. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H28.00008: Phonon relaxation times extracted from first principles thermal conductivity calculations D. A. Broido, A. Ward The lattice thermal conductivity of semiconductors, $\kappa _L $, is a key component in assessing a material's utility for thermoelectric applications. Calculations of $\kappa _L $ commonly employ phonon relaxation times, $\tau _{ph} $. Over the past few decades, a variety of mathematical forms have been used for these$\tau _{ph} $s to represent the phonon-phonon scattering [1], which dominates the behavior of $\kappa _L $ around and above room temperature. However, these forms have typically been obtained in a low frequency/low temperature approximation where umklapp scattering is weak and outside the thermal regime of interest for thermoelectrics. Recently we have developed a first principles approach that accurately calculates $\kappa _L $ using no adjustable parameters [2]. In this presentation, we use our \textit{ab initio} results for Si, Ge and diamond to examine the accuracy of the different models for $\tau _{ph} $, and we identify alternative models. [1] See for example, M. Asen-Palmer et al., Phys. Rev. B 56, 9431 (1997), and references therein. [2] D. A. Broido, M. Malorny, G. Birner, N. Mingo and D. A. Stewart, Appl. Phys. Lett. 91, 231922 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H28.00009: Intrinsic lattice thermal conductivity of diamond from first principles A. Ward, D. A. Broido, D. A. Stewart Predictive theoretical descriptions of the lattice thermal conductivity, $\kappa _L $, are essential in facilitating the design of high efficiency thermoelectric materials. In the thermal regime of interest for thermoelectrics, the $\kappa _L $ of high quality crystalline semiconductors is typically limited by phonon-phonon scattering due to the anharmonicity of the interatomic potential. We have calculated $\kappa _L $ for isotopically pure diamond, combining a first principles approach for the harmonic and anharmonic interatomic force constants with an iterative solution of the full phonon Boltzmann equation. Our adjustable parameter free calculation of $\kappa _L $ for diamond is in excellent agreement with measurements[1-3]. This provides further validation of our \textit{ab initio} approach previously used successfully for Si and Ge [4]. [1] D. G. Onn, et al.,Phys. Rev. Lett. 68, 2806 (1992). [2] L. Wei, et al., Phys. Rev. Lett. 70, 3764 (1993). [3] J. R. Olson, et al., Phys. Rev. B. 47, 14850 (1993). [4] D. A. Broido, et al., Appl. Phys. Lett. 91, 231922 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H28.00010: Thermal boundary resistance of closely-spaced Si/Ge interfaces from lattice dynamics calculations Eric Landry, Alan McGaughey An ability to accurately predict the thermal boundary resistance (TBR) of closely-spaced semiconductor interfaces will allow the design of superlattices with high values of the thermoelectric figure-of-merit. Here, the TBR and phonon transmission coefficients of closely-spaced Si/Ge interfaces are predicted using harmonic lattice dynamics calculations and the scattering boundary method. The atomic interactions are modeled using the Stillinger-Weber potential. The computational domain contains a thin germanium layer sandwiched between two semi-infinite extents of silicon, forming two closely-spaced interfaces. We also consider the opposite situation, where a silicon layer is placed between two large extents of germanium. Due to the harmonic approximation, the calculations are only valid when the phonon scattering is elastic. To examine the assumption of elastic scattering, we compare the lattice dynamics predictions to those obtained using molecular dynamics simulations and the direct method, which require no assumptions about the nature of the phonon transport. We conclude by discussing how the atomic force constants needed in the lattice dynamics calculations can be calculated from density functional theory. This novel approach will allow for the prediction of TBR for interfaces between semiconductors for which a suitable interatomic potential does not exist. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H28.00011: Prediction of phonon transport properties and thermal conductivities in superlattices by anharmonic lattice dynamics calculations Joseph Turney, Alan McGaughey, Cristina Amon Phonon transport in superlattices is investigated using anharmonic and quasi-harmonic lattice dynamics calculations. Within the lattice dynamics framework, we develop a method for predicting the properties of both coherent and incoherent phonons. The method is implemented for test systems consisting of Stillinger-Weber silicon-germanium superlattices. In these systems the mode dependent frequencies, heat capacities, group velocities, transmission coefficients, and relaxation times of the phonons are computed and used to predict the thermal conductivity. We relate changes in the superlattice structure (e.g., period length and interface roughness) to the predicted phonon properties and, for each structure, identify the phonon modes that dominate thermal transport. [Preview Abstract] |
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