Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J12: Focus Session: Thermoelectric and Coupled Phenomena |
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Sponsoring Units: DMP/DCOMP GERA FIAP Chair: Anindya Roy, John Hopkins University Room: 007C |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J12.00001: Colossal enhancement of the Seebeck coefficient in FeSb$_{2}$ driven by nearly ballistic phonons Hidefumi Takahashi, Ryuji Okazaki, Hiroki Taniguchi, Ichiro Terasaki An unusually large $S$ of $-45$ mV/K (at 10 K) was discovered in FeSb$_{2}$ single crystal, which prompted extensive investigations into its physical origin [A. Bentien $et$ $al$., EPL 80, 17008 (2007).] This compound has a small energy gap $\Delta \sim 5$ meV, which may be caused by strong correlations of Fe 3d-electrons, as observed with Kondo insulators, and the colossally large $S$ may be attributed to this unique band structure near the Fermi energy. However, the exceptional value of $S$ has not been clearly explained by electron correlations, suggesting an additional contribution such as the non-equilibrium phonon-drag effect [H. Takahashi $et$ $al$., JPSJ 80, 054708 (2011)., H. Takahashi $et$ $al$., PRB 84, 205215 (2011)., and H. Takahashi $et$ $al$., PRB 88, 165205 (2013).]. Here, we report on the direct investigation of this effect by measuring the transport properties of three samples with cross sections ranging from $250 \times 245$ $\mu$m$^{2}$ to $80\times 160$ $\mu$m$^{2}$. $S$ and $\kappa$ show a significant size effect, indicating that nearly ballistic phonons, which have a long mean free path relative to the sample dimensions, are responsible for the colossal $S$. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J12.00002: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J12.00003: Improved Thermoelectric Performance via Piezoelectric Interaction David Montgomery Presented are the initial findings of enhanced voltage output in a hybrid thermoelectric piezoelectric generator (TPEG). We constructed TPEG by integrating insulating layers of polyvinylidene fluoride (PVDF) piezoelectric films between flexible thin film p-type and n-type thermoelectrics. The piezoelectric bound surface charge modifies the thermoelectric properties of the semiconductor electrodes which facilitates an increase in voltage. The TPEG voltage output has three contributions: traditional thermoelectric and piezoelectric terms, and a unique coupling term. A combined thermoelectric and piezoelectric model can be used to quantify the expected coupling voltage as a function of stress and thermal gradient. The fabrication, placement, and configuration of this interface allows for different device designs and affects overall performance. Under easily achievable stress and thermal gradient this new coupling effect can increase voltage output by 20{\%}. Because of this piezoelectric modified thermoelectric effect these hybrid generators can out preform equivalent thermoelectric or piezoelectric generators. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J12.00004: The Role of Minority Carriers in Thermoelectrics: Why Half Heusler ZrNiSn is a good n-type but poor p-type Thermoelectric Invited Speaker: G. Jeffrey Snyder The bipolar excitation of minority carriers limits the maximum zT of a typical thermoelectric material. This is because the thermopower (absolute value of the Seebeck coefficient) of a typical heavily doped semiconductor rises with temperature until it reaches a maximum value, and then decreases due to the activation of minority carriers of opposite sign. The temperature of the thermopower roll-over is determined largely by the band gap which acts as the activation energy of the minority carriers. Julian Goldsmid and Jeff Sharp showed that the a simple relationship, Eg = 2 Sm Tm, between the maximum thermopower (Sm) the temperature where the maximum occurs (Tm) and the band gap (Eg, measured in eV) is a good approximation for many materials, particularly when both types of carriers have similar mobilities. The (Ti, Zr, Hf)NiSn half Heusler compounds, however, demonstrate the limits of this relationship. The Goldsmid-Sharp band-gap for n-type ZrNiSn is several times greater than that for p-type ZrNiSn that ultimately results in high thermopower at high temperature and therefore high zT for the n-type material but the p-type is not useful. We have explained this phenomena using optical band gap measurements and transport modeling. The greater mobility of the conduction band compared to the valence band suppresses the bipolar effect of the holes that enables the n-type material to retain high thermopower to high temperature. The models give quantitative guide to the accuracy of Goldsmid-Sharp band-gap by providing a correction factor that works well for Bi2Te3, and can be used to guide strategies for suppression of bipolar effects to increase the maximum zT. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J12.00005: The effect of thermoelectric contributions in switching dynamics and resistance drift of Phase Change Memory devices Egecan Cogulu, Ibrahim Cinar, Aisha Gokce, Barry Stipe, Jordan Katine, Gulen Aktas, Ozhan Ozatay Phase Change Memory (PCM) is a promising non-volatile data storage technology that allows for multiple-bit-per-cell operation due to its high contrast in the resistance levels between 0 and 1 logic states. To visualize the complex nature and the stability of the switching dynamics in PCM devices with or without an intermediate resistance state, 3D finite element simulations were carried out in cells with a single Ge2Sb2Te5(GST) layer incorporating temperature and phase dependent thermal and electrical conductivities as well as thermoelectric effects. We compare our results with the experimental data and with our previous simulations to understand the influence of the thermo-electric effect on the phase switching. In addition, we integrated drift equations into our multiphysics simulation to get a complete picture of structural relaxation in time in amorphous and mixed phases of the GST. We compare our results with experimental resistance drift measurements to calculate a decay rate for defect concentration. Our results yield a complete picture of switching dynamics and post-switching resistance drift phenomena on the microscopic scale. [Preview Abstract] |
(Author Not Attending)
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J12.00006: Thermopower of few-electron quantum dots with Kondo correlations Lvzhou Ye The thermopower of few-electron quantum dots is crucially influenced by on-dot electron-electron interactions, particularly in the presence of Kondo correlations. We present a comprehensive picture which elucidates the underlying relations between the thermopower and the spectral density function of two-level quantum dots. The effects of various electronic states, including the Kondo states originating from both spin and orbital degrees of freedom, are clearly unraveled. With these insights, we have exemplified an effective and viable way to control the sign of thermopower of Kondo-correlated quantum dots. This is realized by tuning the temperature and by selecting the appropriate level spacing and Coulomb repulsion strength. Such a physical picture is affirmed by accurate numerical data obtained with a hierarchical equations of motion approach. Our understandings and findings provide useful insights into controlling the direction of electric (heat) current through a quantum dot by applying a temperature (voltage) gradient across the two coupling leads. This may have important implications for novel thermoelectric applications of quantum dots. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J12.00007: Detection of Carrier Scattering Mechanism by Thermopower Shuang Tang, Mildred Dresselhaus We have developed a new method to detect the carrier scattering mechanism at different temperatures by measuring the maximum values of thermopower (Seebeck coefficient). The graphene system has been studied as a model example. The contribution of short-range interaction scattering, and long-range scattering has be inferred when the temperature varies from low temperature to room temperature. The approach to change the maximum values of thermopower (Seebeck coefficient) is also discussed. This method can not only be used in graphene, but in many novel systems, including MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, and black phosphorus, and other general systems. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J12.00008: Magnon drag thermopower and thermomagnetic properties of single-crystal iron Sarah Watzman, Hyungyu Jin, Joseph Heremans Lucassen et al. [1] demonstrate that magnon drag involves a spin-transfer mechanism closely related to the recently discovered spin-Seebeck effect. This talk will first present results of experiments mapping out the thermopower and magnetothermopower of single-crystal iron and prove that its thermopower is indeed dominated by magnon drag, as suggested by Blatt et al. in 1967 [2]. Measurements will then be presented on the magnetic field and temperature dependence of the full thermomagnetic tensor of iron's thermopower in the xxx, xyx, and xyz geometries (the first index gives the direction of the heat flux, the second the measured electric field, the third the applied magnetic field). Results of magneto-thermopower and Nernst coefficients will be reported for single-crystal samples oriented with x$=$[100]. The Nernst coefficients of elemental iron contain a contribution of a direct spin-transfer mechanism, which should be present in the absence of an interface between a ferromagnet and a normal metal. This mechanism could be put to use in high temperature ferromagnetic metallic thermoelectric alloys. 1. M. E. Lucassen et al., Appl. Phys. Lett. 99 262506 (2011) 2. F. J. Blatt et al., Phys. Rev. Lett. 18.11 (1967). [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J12.00009: Enhancement of thermoelectric performance in composite materials through locally-modulated doping Michael J. Adams, Hyungyu Jin, Joseph P. Heremans Composites of organic or inorganic constituents are often considered as a way to yield high thermoelectric figure of merit. The limit of this approach is set by the effective medium theory [1], which demonstrates formally that a composite of two materials A and B cannot have higher figure of merit than the highest of either A or B, in the absence of interaction between A and B. In this work, we show that this limit can be lifted by introducing into a host material a second phase that behaves differently vis-a-vis electrons than vis-a-vis phonons. This phase consists of electrically and thermally insulating islands of material that locally dope the semiconducting host. Doped material near the islands provides electrically conductive volumes for charge carriers. Phonons, unaffected by local doping, are scattered by the islands. Thermopower is less affected by the doped regions than electrical conductivity, by an intrinsic mathematical property of the effective medium theory [1]. We employ this concept in Bi$_{1-x}$Sb$_{x}$ alloys and in p-type (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ compounds, which are known as good thermoelectric materials at cryogenic and room temperatures, respectively. Experimental transport data and the local microscopic characterizations of the samples are presented. 1. D. J. Bergman and L. G. Fel, J. Appl. Phys. \textbf{85} 8205-8216, 1999 [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J12.00010: Next Generation Electrocaloric and Pyroelectric Materials for Solid State Electrothermal Interconversion S. Pamir Alpay, Joseph V. Mantese, Susan Trolier-McKinstry, Qiming Zhang, Roger W. Whatmore Thin film electrocaloric (EC) and pyroelectric (PE) electrothermal interconversion energy sources have recently emerged as viable means for primary and auxiliary solid state cooling and power generation. This emergence is a result of two significant developments: (1) advancements in the formation of high quality polymeric and ceramic thin films with figures of merit that project system level performance as a large percentage of Carnot efficiency, and (2) the ability of these newer materials to support larger electric fields which permit operation at higher voltage; thus making the power electronic architectures more favorable for thermal to electric interconversion. Current research targets to adequately address commercial device needs, include reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly free-standing element in the range of 1 - 10 microns in thickness. This article will describe the current state-of-the-art materials, thermodynamic cycles and device losses; pointing to potential lines of research that would lead to substantially better figures of merit for electrothermal interconversion. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J12.00011: Diameter Dependent Thermoelectric Properties of Individual SnTe Nanowires E.Z. Xu, Z. Li, J. Martinez, N. Sinitsyn, H. Htoon, N. Li, B. Swartzentruber, J. Hollingsworth, J. Wang, S.X. Zhang Tin telluride (SnTe), a newly discovered topological crystalline insulator, has recently been suggested to be a promising thermoelectric material. In this work, we report on a systematic study of the thermoelectric properties of individual single-crystalline SnTe nanowires with different diameters. Measurements of thermopower, electrical conductivity and thermal conductivity were carried out on the same nanowires over a temperature range of 25 - 300 K. While the electrical conductivity does not show a strong diameter dependence, we found that the thermopower increases by a factor of two when the nanowire diameter is decreased from 913 nm to 218 nm. The thermal conductivity of the measured NWs is lower than that of the bulk SnTe, which may be attributed to the enhanced phonon - surface boundary scattering and phonon-defect scattering. We further calculated the temperature dependent figure of merit ZT for each individual nanowire. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000). [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J12.00012: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J12.00013: Towards a high-current triode thermoelectronic generator Gerwin Hassink, Patrick Herlinger, Wolfgang Braun, Cyril Stephanos, Jurgen Smet, Jochen Mannhart Thermionic power generation obtains electrical power directly from a temperature gradient by thermionic emission from a hot electrode to a cold electrode. The space charge created by the emitted electrons, however, severely reduces the efficiency of such generators. Recently, a triode setup with supporting magnetic field has demonstrated to greatly reduce the space charge\footnote{Meir et al., J.Renew.Sust.Energ. 5, 043127(2013)}. Based on these results, further development has been started to reach higher output by, e.g., reducing the electrode spacing to 100 $\mu$m, and by increasing the electrode area. In addition, new gate electrode materials and geometries are investigated. The importance of the work function not only for the emitter and collector, but also for the gate, is clear from both the theory and experiment. Work function engineering through surface modification is discussed. [Preview Abstract] |
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