Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S25: Focus Session: Thermoelectrics - Phonons and Heat Conduction II |
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Sponsoring Units: DMP GERA FIAP Chair: Jonathan Malen, Carnagie Mellon University Room: 503 |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S25.00001: On Minority Carrier Scattering for Thermoelectrics Invited Speaker: 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.\\[4pt] In collaboration with Shanyu Wang, Jiong Yang, and Trevor Toll, Department of Materials Science and EngineeringUniversity of Washington, Seattle, WA, USA. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S25.00002: Low Cost Advanced Thermoelectric (TE) Technology for Automotive Waste Heat Recovery G.P. Meisner Low cost, fully integrated TE generators (TEGs) to recover waste heat from vehicle exhaust will reduce transportation sector energy consumption and emissions. TEGs will be the first application of high-temperature TE materials for high-volume use and establish new industrial sectors with scaled up production capability of TEG materials and components. We will create a potential supply chain for practical automotive TEGs and identify manufacturing and assembly processes for large scale production of TEG materials and components. Our work focusses on several innovative R{\&}D paths: (1) enhanced TE material performance by doping and compositional tuning, (2) optimized TE material fabrication and processing to reduce thermal conductivity and improve fracture strength, (3) high volume production for successful skutterudite commercialization, (4) new material, nanostructure, and nanoscale approaches to reduce thermal interface and electrical contact resistances, (5) innovative heat exchangers for high efficiency heat flows and optimum temperature profiles despite highly variable exhaust gas operating conditions, (6) new modeling and simulation tools, and (7) inexpensive materials for thermal insulation and coatings for TE encapsulation. Recent results will be presented. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S25.00003: High-Temperature, High-Concentration Solar Thermoelectric Generators Emily Warren, Lauryn Baranowski, Michele Olsen, Paul Ndione, Judy Netter, Alan Goodrich, Matthew Gray, Philip Parilla, David Ginley, Eric Toberer Solar thermoelectric generators (STEGs) powered with concentrated solar energy have potential for use as primary energy converters or as topping-cycles for more conventional concentrated solar power (CSP) technologies. Modeling based on current record modules from JPL suggests thermoelectric efficiencies of 18$\%$ could be experimentally expected with a temperature gradient of 1000 $-$ 100$^\circ$C. Integrating these state-of-the-art TEGs with a concentrating solar receiver requires simultaneous optimization of optical, thermal, and thermoelectric systems. This talk will discuss the modeling, design, and experimental testing of STEG devices under concentrated sunlight. We have developed a model that combines thermal circuit modeling with optical ray tracing to design selective absorber coatings and cavities to minimize radiation losses from the system. We have fabricated selective absorber coatings and demonstrated that these selective absorber films can minimize blackbody radiation losses at high temperature and are stable after thermal cycling to 1000$^\circ$C. On-sun testing of STEG devices and thermal simulators is ongoing and preliminary results will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S25.00004: Rattler behavior in As skutterudites and oxy-skutterudites Frank Bridges, Brad Car, Mikaela Hoffman-Stapleton, Trevor Keiber, Logan Sutton, M. Brian Maple We report EXAFS measurements for the series CeX$_4$As$_{12}$ (X = Fe, Ru, Os) and NdCu$_3$Ru$_4$O$_{12}$ as a function of temperature for most elements in the structure. In each case the rare earth atom is a ``rattler'' atom, with a low Einstein temperature while the skutterudite cage structure is relatively stiff. From temperature dependencies of the correlated Debye model for the cage atoms, one can estimate the effective spring constant for various atom pairs. We also find for the oxy-skutterudites that the planar CuO$_4$ sub-structure is very stiff, and likely vibrates as a rigid unit. We compare the behavior of the As-skutterudites with other skutterudites and with the oxy-skutterudites, and discuss in terms of the rigid cage model. The second neighbor pair Ce-X for the As-skutterudites is softer than expected while for the oxy-skutterudites the second neighbor Nd-Ru pair is stiffer than the nearest neighbor Nd-O pair. Models are need to explore this behavior. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S25.00005: Modeling correlated motion in thermoelectric skutterudite materials Trevor Keiber, Frank Bridges Filled skutterudite compounds, LnT$_{4}$X$_{12}$ (Ln=rare earth; T=Fe,Ru,Os; X=P,As,Sb), have previously been modeled using a rigid cage approximation for the ``rattling'' rare earth atom. The large thermal broadening with temperature of the rattler can be fit using an Einstein model. Recent measurements of the second neighbor Ln-T peaks show an unusually large thermal broadening suggesting motion of the cage of atoms. To incorporate these results we developed three and four mass spring models to give the acoustic and optical phonon mode spectra. For the simplest three mass model we identify the low energy optical mode as the rattling mode. This rattling mode is likely coupled to the acoustic mode, and responsible for the low thermal conductivity of the skutterudite compound. We extend this model to four atoms to describe the CuO$_{4}$ rings in oxy-skutterudites and the X$_{4}$ rings in LnT$_{4}$X$_{12}$. This talk provides a model for the experimental results of the previous presentation. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S25.00006: Effect of triple fillers on thermoelectric properties of p-type skutterudites Tulashi Dahal, Qing Jie, Zhifeng Ren Experiments were carried out to investigate the effect of triple fillers on the thermoelectric properties of p type skutterudites. We have synthesized the samples by hot pressing nano powders made by ball milling annealed ingots of Ca$_{\mathrm{x}}$Ce$_{\mathrm{y}}$Nd$_{\mathrm{y}}$Fe$_{3.5}$Co$_{0.5}$Sb$_{12}$. By tuning the concentration of Ca, Ce, and Nd, we have achieved a lower thermal conductivity $\sim$ 2 W m$^{-1}$ K$^{-1}$ at room temperature and $\sim$ 2.6 W m$^{-1}$ K$^{-1}$ at 530 $^{\circ}$C), leading to a peak ZT of about 1.1 at 475 $^{\circ}$C. The observed lower thermal conductivity can be attributed due to a broad range of phonon scattering due to multiple fillers. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S25.00007: Doping CoSb3 p-type with Al substitution for Sb Michael J. Adams, Michele D. Nielsen, Joseph P. Heremans Skutterudites such as CoSb$_{3}$ are compounds composed of group IX-B atoms (Co, Rh, and Ir) forming a simple cubic structure, and group V-A$_{3}$ pnictide atoms (primarily Sb and As) forming rings inside 6 of every 8 cubes. The remaining cubes remain empty. A common method for reducing thermal conductivity is to introduce impurity atoms such as rare-earths in the cubes that act as rattlers. P-type doping of CoSb$_{3}$ has led to some advances in zT, but the p-type material remains less performing than the n-type material due to the fact that the valence band, dominated by Sb levels, has a low effective mass. A promising method for improving p-type properties is to introduce an effective resonant level into the energy levels occupied by the light hole band, thereby increasing the Seebeck coefficient without strongly effecting other transport properties. A first attempt using Sn substitution was not successful. Here we try various concentrations of Al substituted at Sb sites to generate a resonant level. Material properties are measured and compared with a calculated Pisarenko relation, where thermopower is plotted as a function of hole concentration. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S25.00008: Systematic Studies on Anharmonicity of Rattling Phonons in Type I Clathrates by Low Temperature Heat Capacity Measurements Katsumi Tanigaki, Jiazhen Wu, Yoichi Tanabe, Satoshi Heguri, Hidekazu Shiimotani Clathrates are featured by cage-like polyhedral hosts mainly composed of the IV$^{th}$ group elements of Si, Ge, or Sn and alkali metal or alkaline-earth metal elements can be accommodated inside as a guest atom. One of the most intriguing issues in clathrates is their outstanding high thermoelectric performances thanks to the low thermal conductivity. Being irrespective of good electric conductivity $\sigma$, the guest atom motions provide a low-energy lying less-dispersive phonons and can greatly suppress thermal conductivity $\kappa$. This makes clathrates close to the concept of ``phonon glass electron crystal: PGEC'' and useful in thermoelectric materials from the viewpoint of the figure of merit. In the present study, we show that the local phonon anharmonicity indicated by the tunneling-term of the endohedral atoms ($\alpha$T) and the itinerant-electron term ($\gamma_{e}$T), both of which show T-linear dependences in specific heat C$_{p}$, can successfully be separated by employing single crystals with various carrier concentrations in a wide range of temperture experimennts. The factors affecting on the phonon anharmonicity as well as the strength of electron-phonon interactions will be discussed based on our recent experiments. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S25.00009: Computational Study of the Vibrational, Thermal and Transport Properties of the Type II Tin Clathrate Compounds Cs$_{\mathrm{x}}$Sn$_{136}$ (x $=$ 12, 16, 20) Dong Xue, Charles Myles The Type II clathrates A$_{\mathrm{x}}$B$_{136}$ (A $=$ alkali atom; B $=$ Si, Ge, Sn) are interesting because of their low lattice thermal conductivity and thermoelectric properties. Their low thermal conductivity is due to the low-frequency vibrational modes produced by the alkali guests in the host lattice cages. Heat transport theory predicts that such modes will scatter with the acoustic phonons of the host, reducing the lattice thermal conductivity. To understand this effect for the Type II Sn clathrates, we have performed a first principles computational study of the vibrational, thermal, and transport properties of Cs$_{\mathrm{x}}$Sn$_{136}$ (x $=$ 12,16,20). Our calculations use the VASP and PHONOPY codes to calculate the vibrational modes. We present results for the phonon modes, the heat capacity, and the Gruneisen parameter in Cs$_{\mathrm{x}}$Sn$_{136}$. Our calculated Cs modes are in the range 8-15 cm$^{-1}$ and we find that these frequencies decrease as x increases. Our results for the vibrational contribution to the heat capacity predict that it weakly depends on x, and that it increases smoothly with temperature T, approaching the Dulong-Petit value at T $=$ 600 K. Our calculations of the Gruneisen parameter predict that it has a weak x dependence, it increases with increasing T for 160 K $\le $ T $\le $ 300 K and it has a weak dependence on T for 300 K $\le $ T $\le $ 550 K. We have used these results to calculate the lattice thermal conductivity in Cs$_{\mathrm{x}}$Sn$_{136}$ within the kinetic theory approximation. The results of these calculations are also presented and discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S25.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S25.00011: Reduction of thermal conductivity in silicon phononic metamaterials William Jones, Axel Scherer, Slobodan Mitrovic We explore the limits of thermal conductivity reduction through phononic design of single crystal silicon membranes by direct measurement via thermal bridge method. Phononic metamaterials with nanoscale critical dimensions can modify the dispersion for heat carrying phonons via Brillouin-zone folding. In monolithic crystalline thin films, thermal conductivity can be further reduced by employing a superstructure of these patterned regions. We hypothesize that this approach can reduce the thermal conductivity due to phonon reflection. We also discuss the potential of these structures to enable a true phonon-glass, electron-crystal material and push the limits of thermoelectric cooling/heat conversion efficiency. We predict that the high electrical conductivity of doped single crystal silicon will endow this phononic metamaterial with a high ZT characteristic. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S25.00012: Thermal transport in nanostructured silicon membranes Sanghamitra Neogi, Luiz F.C. Pereira, Davide Donadio The recent focus in thermal management in nanostructures and energy harvesting using thermolectric devices has motivated the interest towards understanding the role of phononic thermal transport in these nanoscale materials. One way to obtain thermoelectric systems with improved efficiency is to engineer nanostructured semiconductors, so as to reduce the thermal conductivity of the crystalline materials while preserving their electronic properties [1]. Our study is driven towards understanding the nature of phononic thermal transport in nanostructured silicon membranes. We use harmonic lattice dynamics and classical molecular dynamics to compute the phonon transport properties in Si membranes, with thickness up to ~20 nm. We show that dimensionality reduction has a significant effect on the vibrational properties and leads to a 4-fold reduction in the thermal conductivity of the membranes. Combining dimensional reduction with surface modification, we obtain a reduction in the thermal conductivity of the membranes to a factor of 20 with respect to the bulk, implying a 20-fold enhancement of ZT at room temperature. Such figures make nanostructured silicon membranes viable materials for thermoelectric units. [1] M. S. Dresselhaus et al, Adv. Mater., 22, 3970 (2010). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S25.00013: Probing Large-Wavevector Phonons in Strain-Relief Patterned Silicon/Silicon Germanium Heterostructure Nanomembranes Kyle McElhinny, Gokul Gopalakrishnan, Don Savage, Max Lagally, Martin Holt, Paul Evans Freestanding Si/SiGe nanomembranes have promising thermal properties as a result of the ability to decouple the electronic and thermal transport. Challenges in fabrication of freestanding Si/SiGe nanomembranes arise due to buckling in reaction to stresses generated by the lattice mismatch between the Si and SiGe layers. This results in an equilibrium state where the elastic energy is minimized through a buckling distortion. We demonstrate that the strain and curvature of these membranes is reduced by nearly an order of magnitude through the strain-relief patterning of the buckled membrane. X-ray thermal diffuse scattering (TDS) studies of these membranes evaluate the effect of confinement on the phonon dispersion of the Si/SiGe heterostructure by probing the populations of acoustic phonons at wavevectors spanning the Brillouin zone. A comparison between the x-ray TDS intensity distributions of Si and Si/SiGe heterostructure nanomembranes demonstrates the importance of fabricating SiGe nanostructures with reduced strain and curvature. Results of these experiments show deviations in TDS intensity compared to bulk and Si nanomembranes. In Si nanomembranes these deviations have previously indicated a softening of 1-2 meV at large wavevectors. [Preview Abstract] |
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