Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M25: Focus Session: Thermoelectrics - Phonons and Heat Conduction I |
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Sponsoring Units: GERA DMP Chair: Andrew May Room: 503 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M25.00001: Phonon and magnon heat transport and drag effects Invited Speaker: Joseph P. Heremans Thermoelectric generators and coolers constitute today's solid-state energy converters. The two goals in thermoelectrics research are to enhance the thermopower while simultaneously maintaining a high electrical conductivity of the same material, and to minimize its lattice thermal conductivity without affecting its electronic properties. Up to now the lattice thermal conductivity has been minimized by using alloy scattering and, more recently, nanostructuring [1]. In the first part of the talk, a new approach to minimize the lattice thermal conductivity is described that affects phonon scattering much more than electron scattering. This can be done by selecting potential thermoelectric materials that have a very high anharmonicity, because this property governs phonon-phonon interaction probability. Several possible types of chemical bonds will be described that exhibit such high anharmonicity, and particular emphasis will be put on solids with highly-polarizable lone-pair electrons, such as the rock salt I-V-VI2 compounds (e.g. NaSbSe2). The second part of the talk will give an introduction to a completely new class of solid-state thermal energy converters based on spin transport. One configuration for such energy converters is based on the recently discovered spin-Seebeck effect (SSE). This quantity is expressed in the same units as the conventional thermopower, and we have recently shown that it can be of the same order of magnitude. The main advantage of SSE converters is that the problem of optimization is now distributed over two different materials, a ferromagnet in which a flux of magnetization is generated by a thermal gradient, and a normal metal where the flux of magnetization is converted into electrical power. The talk will focus on the basic physics behind the spin-Seebeck effect. Recent developments [2] will then be described based on phonon-drag of spin polarized electrons. This mechanism has made it possible to reach magnitudes of SSE that are comparable to the highest values of classical thermopower measured in semiconductors. This work is supported as part of the Revolutionary Materials for Solid State Energy Conversion (RMSSEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, by AFOSR MURI ``Cryogenic Peltier Cooling'' Contract {\#}FA9550-10-1-0533and by NSF-CBET-1133589. \\[4pt] [1] J. P. Heremans {\&} al., Nature Nanotechnology\textbf{ 8,} 471-473 (2013)\\[0pt] [2] C. M. Jaworski {\&} al., Nature, \textbf{487}, 210-213 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M25.00002: Phonon Scattering Mechanisms in Thermoelectrics Olivier Delaire, Jie Ma, Chen Li, Andrew May, David Singh, Georg Ehlers, Doug Abernathy, Brian Sales Improving our current microscopic understanding of thermal conductivity is needed to design more efficient thermoelectric materials. Thus, establishing a complete picture of phonon dispersions and mean-free-paths is crucial to provide a realistic microscopic characterization of phonon transport, against which theories can be tested. Thanks to recent advances in instrumentation, inelastic neutron scattering can map phonon dispersions and lifetimes across the entire Brillouin zone. As our studies illustrate, such measurements provide key insights about phonon scattering mechanisms, including phonon anharmonicity, electron-phonon coupling, and scattering by point defects or nanostructures. In addition, we perform first-principles simulations of atomic dynamics, including effects of anharmonicity and electron-phonon coupling, to quantitatively model the large experimental datasets. We present results from several studies of important thermoelectric materials [1,2], illustrating how this integrated approach can be used to reach a new level of microscopic understanding of thermal conductivity. [1] O. Delaire, J. Ma, K. Marty, et al. Nature Materials 10, 614 (2011). [2] J. Ma*, O. Delaire*, A. F. May et al. Nature Nanotechnology 8, 445 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M25.00003: Anisotropic Deviations from Fourier's Law in Si and MgO and the Importance of Temperature-Profile Extrema Richard Wilson, David Cahill Efforts to engineer thermal conductivity values by alloying, doping, or nanostructuring rely on a fundamental understanding of phonon-phonon and phonon-defect scattering. However, experimentally resolving phonon dynamics remains challenging. Recent studies demonstrate that time-domain thermoreflectance and frequency-domain thermoreflectance are sensitive to the mean-free-paths of heat-carrying phonons. The sensitivity of both techniques relies on the failure of Fourier theory when important length-scales of the temperature-profile become shorter than phonon mean-free-paths. However the correct interpretation of these experiments remains unclear. To address this issue, we characterize the relationship between the failure of Fourier's law, phonon mean-free-paths, important length-scales of the temperature-profile, and interfacial phonon scattering by performing extensive time-domain thermoreflectance experiments on Si, Si$_{\mathrm{0.99}}$Ge$_{\mathrm{0.01}}$, boron doped Si, and MgO crystals between 40 and 300 K. We find the failure of Fourier's law causes anisotropic thermal transport in Si and MgO despite cubic symmetry, and that in situations where Fourier's law fails, interfacial phonon scattering can affect the heat-current away from the interface. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M25.00004: Reducing the lattice thermal conductivity of the rocksalt I-V-VI$_{2}$ compounds Michele Nielsen, Sunphil Kim, Matthias Wuttig, Felix Lange, Joseph Heremans Reducing the lattice thermal conductivity is a crucial task in the optimization of the thermoelectric figure of merit. Recent theoretical calculations [1] have revealed the presence marginally stable acoustic phonons which have extremely large Gr\"{u}nesien parameters which result in a strong anharmonicity in heat-carrying acoustic phonon branches of select rocksalt I-V-VI2 compounds as a result of lone pair electrons on group V elements. Here, we present a new simple method of mapping Gr\"{u}nesien parameters, using readily available information on the ionization and the hybridization of the chemical bonds involved, and avoiding extensive numerical simulations. Additionally, we present current advances in doping on alkali based compounds which have inexpensive and non-toxic starting constituents. \\[4pt] [1] Michele D. Nielsen, Vidvuds Ozolins and Joseph P. Heremans, Lone pair electrons minimize lattice thermal conductivity, Energy Environ. Sci., 6, 570 -- 578 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M25.00005: Exploration of Phonon Behavior in PbTe from Ultrafast Time-Resolved Pump-Probe Measurements Mason Jiang, Matthias Hoffmann, Andrew May, Olivier Delaire, Brian Sales, Roberto Merlin, Ivana Savic, Eamonn Murray, Stephen Fahy, David Reis We report femtosecond-resolution measurements of phonon dynamics on photo-excited PbTe, an incipient ferroelectric. PbTe is a leading thermoelectric material with an unusually low thermal conductivity, which has been attributed to strongly anharmonic phonon interactions. In an attempt to understand in detail the nature of these interactions, we perform time-resolved pump-probe measurements using combinations of THz- and optical-based excitation and optical- and x-ray-based probes in variable temperature environments. Several interesting observations are highlighted. In IR pump/IR probe, anomalous oscillations are seen near 1.4 THz and just below 1 THz, with their relative amplitudes varying with temperature and pump fluence. The frequencies fall close to the Raman-forbidden TO phonon mode. Additionally, we use single-cycle THz pulses centered near 1 THz in an attempt to drive the TO mode. Probing with IR results in unexpected modulations with oscillatory behavior that last for a few picoseconds, fluctuate at a rate just below 1.4 THz, and grow in strength with decreasing temperature. This talk will discuss possible explanations for these effects and their impact on further understanding the relationship between anharmonicity and high temperature thermoelectric behavior. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M25.00006: Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics Chen Li, Olle Hellman, Jie Ma, Andrew May, Xin Chen, David Singh, Brian Sales, Huibo Cao, Andrew Christianson, Georg Ehlers, Olivier Delaire The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe show a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured energy spectra and their temperature dependence. The contributions to the complex features of the transverse-optic ferroelectric mode from phase-space for three-phonon scattering processes and the lattice instability are compared. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M25.00007: Fast full-spectrum phonon calculations for large lattices by Bloch mode synthesis Dimitri Krattiger, Mahmoud Hussein Computation of thermal properties from lattice dynamics models involves integration over all phonon modes in the system. These phonon modes can be found by solving an eigenvalue problem. For periodic nanostructures, the degrees of freedom in the system may number in the thousands or even millions, resulting in a very expensive computational problem. Bloch mode synthesis is a recently developed model reduction technique whereby the size of the eigenvalue problem is greatly decreased. Similar to sub-structuring techniques, this method separates the domain into interface and interior degrees of freedom, and performs a modal reduction on the interior. Meanwhile the interface is represented using a set of static constraint modes. In the current work, this reduction is modified by using dynamic constraint modes so that it can be applied at higher frequency ranges. This effectively breaks up the large eigenvalue problem into many small eigenvalue problems resulting in large computational savings. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M25.00008: Study on the Lattice Dynamics of the Argyrodite Ag$_{8}$GeTe$_{6}$ Dale Hitchcock, Emily Thompson, Jian He, Isaac Bredesen, Veelre Keppends, David Mandrus Ag$_{8}$GeTe$_{6}$~was initially studied as a super ionic-electronic mixed conductor in the 1970s, and more recently has attracted new interest for its thermoelectric performance. A key to the desirable thermoelectric performance of Ag$_{8}$GeTe$_{6}$ is its exceptionally low lattice thermal conductivity ($\sim$ 0.25W/m*K at 300K), which is intimately related to its structure, consecutive structural instabilities, and unusual lattice dynamics (e.g., anharmonicity). In this work, we have studied Ag$_{8}$GeTe$_{6}$ by means of thermal conductivity, electrical conductivity, Seebeck coefficient, Hall coefficient, magnetic susceptibility, resonant ultrasound spectroscopy (RUS), photoacoustic spectroscopy, and synchrotron x-ray diffraction at low temperatures in order to further understand the coexistence of mixed conduction and high thermoelectric performance at elevated temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M25.00009: First-principles study of anharmonic lattice dynamics and thermal conductivity of AgSbTe2 Yi Zhang, Paul Kent, Jihui Yang, Changfeng Chen We report on first-principles calculations of anharmonic lattice dynamics and thermal conductivity of AgSbTe2. We study the temperature dependence of phonon scattering and, in particular, examine the mechanism responsible for the low thermal conductivity of AgSbTe2, which holds the key to its potential thermoelectric applications. We perform systematic calculations and analysis to discuss the role of intrinsic anharmonic phonon-phonon scattering and strong phonon-nanodomain scattering in determining the phonon transport process in AgSbTe2. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M25.00010: Thermal conductivity of nano-structured materials Chandan Bera, Lasse Bjerg, Ankita Katre, Georg K.H. Madsen Manipulating the thermal properties of materials by nano-structuring is new successful route to improve the performance of thermoelectric materials. We present a new parameter free model to predict anharmonic scattering in bulk and nanoscale materials. Velocities and anharmonic scattering rates are calculated from the Gr\"uneisen parameter of the full phonon dispersions and used to calculate the lattice thermal conductivity using the phonon Boltzmann transport equation in the relaxation time approximation. We find good agreement with experiments for a range of materials. Furthermore, we show that our model, as opposed to simple models based on only the acoustic bands, finds the correct trend in the thermal conductivity of Mg2Si, Mg2Ge and Mg2Sn. We also examine thermal transport in more complex materials like Type-I Si clathrates and zinc-antimonides. Finally, discuss how nano-structure and disorder effect the thermal conductivity. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M25.00011: Phase Transition Enhanced Thermoelectric Performance in Copper Chalcogenides David Brown, Tristan Day, Kasper Borup, Sebastian Christensen, Bo Iversen, G. Jeffrey Snyder Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of \textit{zT} in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu$_{\mathrm{2}}$Se over a broad (360-410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of \textit{zT} (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the \textit{zT} of Cu$_{\mathrm{1.97}}$Ag$_{\mathrm{0.03}}$Se (almost 1.0 at 400K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high \textit{zT} and new green applications for thermoelectrics. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M25.00012: On Minority Carrier Scattering for Thermoelectrics Shanyu Wang, Jiong Yang, Trevor Toll, Jihui Yang Most of the past studies on thermoelectric materials have been focused on majority carriers and lattice phonons in heavily doped semiconductors. In this talk I will show that minority carriers, however, could have a significant impact on both electrical and thermal transport, especially at elevated temperatures. I will also describe means of improving thermoelectric performance of heavily doped semiconductors via selective minority carrier scattering. These results offer insights for understanding experimental findings and optimizing thermoelectric properties of narrow band-gap semiconductors. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M25.00013: Optical Band Gap and the Burstein Moss Shift in Doped PbTe Zachary Gibbs, Robert White, Jeffrey Snyder We will present an analysis of the room temperature Burstein Moss shift in Iodine doped PbTe. The shift explains the phenomena of a measured increase in the optical band gap as the carrier concentration increases. We quantify the magnitude of the effect and extract an estimate of the true band gap---which is observed to decrease with doping level (also known as band gap renormalization). The results imply that care be taken when using measured optical band gaps for comparison to thermoelectric transport data in doped samples because the true band gap can be quite different than the measured one. Temperature dependent optical measurements in IV-VI materials will also be discussed. [Preview Abstract] |
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