Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W29: Focus Session: Thermoelectrics V: III-V's & Nanostructures |
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Sponsoring Units: DMP FIAP GERA Chair: Lilia Woods, University of South Florida Room: C123 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W29.00001: Thermal conductivity of ``nanoparticle in alloy'' materials from a first principles approach David Broido, Natalio Mingo, Derek Stewart It has been demonstrated that nanoparticles embedded in alloys can produce large reductions in lattice thermal conductivity with corresponding increases in the thermoelectric figure of merit [1,2]. Here we present an\textit{ ab initio} approach to calculate the lattice thermal conductivity of an SiGe alloy host containing embedded nanoparticles. This approach is based on density functional perturbation theory and employs a virtual crystal approximation for the alloy and a relaxation time approximation for anharmonic, alloy disorder, and nanoparticle scattering. We apply the method to nanoparticles with a range of different chemical compositions, concentrations and sizes embedded in the SiGe alloy. We compare our first principles based results to those from previous calculations [2] based on the debye approximation. \\[4pt] [1] W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A. Shakouri, and A. Majumdar, Phys. Rev. Lett. 96, 045901 (2006). \\[0pt] [2] N. Mingo, D. Hauser, N. P. Kobayashi, M. Plissonnier and A. Shakouri, Nano Letters 9, 711 (2009); S. Wang and N. Mingo, Appl. Phys. Lett. 94, 203109 (2009). [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W29.00002: Thermoelectric materials with embedded nanoparticles- An effective medium approach Mona Zebarjadi, Keivan Esfarjani, Zhixi Bian, Ali Shakouri The effect of adding spherical nano-particles inside a host matrix is investigated using the coherent potential approximation. A parabolic band structure is assumed for the host matrix and it is shown that nano-particles can modify the effective mass by up to 20\% when their volume fraction is about 10\%. The effective band-structure can be fitted by the standard non-parabolic relation, resulting in the negative non-parabolic coefficients in the case of barrier type nano-particles. Interesting peaks have been observed in the group velocity curve versus energy when the nano-particles are deep wells and their volume fraction is more than few percent. We show that using a high volume fraction ($\sim$5\%) of relatively small ($\sim$1nm) uniform size nano-particles the power factor can be enhanced significantly especially at low temperatures. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W29.00003: Optimization of the Thermoelectric Effect in GaN-based Materials Ian Ferguson, Na Lu III-Nitride, primarily InGaN, based solar cells have gained importance owing to the band gap range from 0.7 eV to 3.4 eV covering most of the solar spectrum. However, to harvest further energy benefits other properties these materials such as the thermoelectric effects need to be investigated. Current research demonstrates that the electrical conductivity decreases and the Seebeck coefficient increases for GaN up to $\sim $ 1000K. The maximum power factor (S$^{2}\lambda )$ for GaN is observed at 373K which is a typical temperature for solar cell operation. This paper will report a systematic study of GaInN-based materials to optimize the TE effect in these materials. This will be achieved by varying the indium concentration, the intelligent control dislocations and the incorporation of transition metals (Mn, Fe, Cu, etc.) to decouple $\sigma $/$\lambda $. For example, alloy disorder effectively scatters high frequency phonons, while low frequency phonons which are responsible for heat transfer through the material respond to boundary scattering. Consequently fine-grained materials exhibit a lesser thermal conductivity in comparison with single crystal structures. Growth of these materials will be performed using standard epitaxy techniques used in solar cell manufacturing. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:27PM |
W29.00004: Investigation of ErAs and ErSb doped semiconductors for enhanced thermoelectric Invited Speaker: |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W29.00005: High current density diamond based electron emitters for vacuum thermionic energy conversion Franz Koeck, Robert Nemanich Vacuum thermionic energy conversion utilizes thermionic emission to release electrons from an emitter into vacuum and collection at a counter-electrode. In our approach for an efficient thermionic emitter a multi-layer diamond thin film structure was synthesized by plasma-assisted CVD on a metallic substrate with controlled surface roughness including a nanodiamond pretreatment step. Introduction of nitrogen during ultra-nanocrystalline diamond (UNCD) film growth resulted in a low resistivity interstitial layer significantly enhancing emission current density which can be related to the Richardson constant. The top layer of polycrystalline nitrogen doped diamond was exposed to a hydrogen plasma inducing negative electron affinity characteristic presenting a low effective emitter work function $<$ 1.3 eV. Thermionic emission from this material commences at temperatures as low as 260$^{o}$C and observes the law of Richardson -- Dushman. From a data fit a significant Richardsons constant $>$ 2 A/cm$^{2}$ K$^{2}$ was extracted and at a temperature of 500$^{o}$C a thermionic emission current $>$ 5 mA was measured. This may well be the highest current density reported from a thermionic emitter operating at the moderate temperature of 500$^{o}$C. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W29.00006: Photon Enhanced Thermionic Emission for Solar Concentrator Systems Jared Schwede, Igor Bargatin, Dan Riley, Brian Hardin, Roger Howe, Nick Melosh, Zhi-Xun Shen Photon Enhanced Thermionic Emission (PETE) is a newly proposed method of solar energy harvesting which combines quantum and thermal processes into a single electricity generating mechanism. The proposed PETE device can be thought of as a synthesis of a photovoltaic (PV) cell and thermionic converter, and the process is expected to overcome some of the challenges which limit either of its intellectual antecedents. Because PETE can harvest the energy of sub-bandgap photons and recover heat produced by thermalization and recombination, possible PETE conversion efficiencies exceed the theoretical limits of single junction PV cells. A PETE converter operates most efficiently at high temperatures, which would allow the waste heat of the device to be used to power a secondary thermal cycle. Principles of PETE operation and limiting efficiencies are described. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W29.00007: Thermoelectric Properties of MOVPE Grown AlInN, Lattice-Matched to GaN Jing Zhang, Hua Tong, Guangyu Liu, Juan Herbsommer, Gensheng Huang, Nelson Tansu In this work, we investigate experimentally the growth and thermoelectric properties, i.e., thermal conductivity, Seebeck coefficient, and electrical conductivity, of n-type wurtzite high quality Al$_{x}$In$_{1-x}$N, grown on GaN template on sapphire substrate by MOVPE, in-plane lattice-matched to GaN. The thermal conductivity is measured by 3$\omega $ method differential technique for thin films. The thermal conductivity value of Al$_{0.83}$In$_{0.17}$N is measured as 5.7 W/(mK). The Seebeck coefficient is calculated as the ratio of measured voltage difference and temperature difference when a temperature gradient is created in the sample. The absolute Seebeck coefficient value of Al$_{0.83}$In$_{0.17}$N is measured as 6.2$\times $10$^{-4}$ V/K. The sheet resistivity of lattice-matched Al$_{0.83}$In$_{0.17}$N is measured using Van der Pauw scheme and the electric conductivity is acquired accordingly to be 2.9$\times $10$^{4}$ /($\Omega $.m). The Z*T value of Al$_{0.83}$In$_{0.17}$N obtained is above 0.2 at room temperature. The results indicate AlInN based alloys are good candidates for thermoelectric devices. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W29.00008: Enhancement of thermoelectric efficiency in arrays of InAs/GaAs quantum dots Vladimir Fomin, Peter Kratzer We investigate the effect of the electron miniband energy spectrum of periodic 1D stacks of self-assembled InAs/GaAs quantum dots (QDs) on their electronic transport characteristics. The electron minibands are calculated within tight-binding and Kronig-Penney models. An explicit evaluation of the transport relaxation time in minibands is provided employing the Boltzmann transport equation. The transport relaxation time reveals a significant dispersion as a function of the wave vector in the stacking direction. From the numerical analysis of the electric and thermal conductivities, the Seebeck coefficient and the figure-of-merit, we conclude that a 1D stack of QDs achieves a geometry-controlled enhanced efficiency as a thermoelectric converter in certain windows of the donor concentration. Engineering the electronic factors in the figure-of-merit requires a fine-tuning of both the geometrical parameters of the stack and the doping. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W29.00009: Quantum-enhanced performance of nanoscale thermoelectrics Justin Bergfield, Charles Stafford In a recent paper, we predicted that the linear thermoelectric response of a nanoscale junction is strongly enhanced by quantum interference in the vicinity of a transmission node.\footnote{J.~P.~Bergfield and C.~A.~Stafford, Nano Letters {\bf 9}, 3072 (2009).} In this talk, we use our nonequilibrium many-body transport theory\footnote{J.\ P.\ Bergfield and C.\ A.Stafford, Phys.\ Rev.\ B {\bf 79}, 245125 (2009)} to investigate the performance of thermoelectric devices based on single-molecule junctions, determining the thermodynamic efficiency and power at finite bias. By comparing the linear and nonlinear device characteristics, the applicability of the dimensionless thermoelectric figure-of-merit $ZT$ to predict device performance at the nanoscale is tested. Finally, we report on a class of high-impedance nanoscale devices which possess additional quantum-enhancement, and exhibit high thermodynamic efficiency and $ZT>10$ limited only by the coherence length. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W29.00010: Optimization of Thermoelectric transport in Solution Processed Nanocrystalline/Conducting Polymer Composites Kevin C. See, Joseph Feser, Jeffrey Urban, Rachel A. Segalman In order to increase the broad applicability of thermoelectrics for cooling as well as energy conversion, both efficiency and materials cost must be reduced. Soluble conducting polymers are an attractive material due to their low cost and ease of processing, however most highly conductive systems suffer from low thermopowers. Previous work has shown the potential for nanostructured systems to overcome the performance limitations of bulk materials, enabling improvements in the thermoelectric figure of merit\textit{, ZT}. Here we have synthesized novel composites composed of both inorganic nanostructures and highly conducting polymers. Films cast from solution have stable room temperature thermoelectric power factors exceeding 50 $\mu $W/mK$^{2}$ and thermal conductivity values near 0.2 W/mK. This combination of tunable power factor and low-$\kappa $ provide a platform for developing all-solution processed high-ZT materials. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W29.00011: Synthesis and thermoelectric properties of nanosize CoSb$_{3}$ skutterudite by solvothermal route Wenzhi Li, Latha Kumari, Zhifeng Ren Skutterudite CoSb$_{3}$ material has remarkable thermoelectric property and great potential applications for thermoelectric power generation and solid state cooling. In this work, we report the synthesis of the skutterudite CoSb$_{3}$ nanoparticles by solvothermal method with or without surfactants. Pure cubic phase of CoSb$_{3}$ has been synthesized with or without the surfactants; however, the size and shape of the CoSb$_{3}$ nanoparticles are affected by the surfactants. The small particles have an optical band gap of around 3.44 eV and a broad photoluminescence emission band with maximum at 409 nm. Thermoelectric measurements were performed on the nanosize CoSb$_{3}$ materials. A maximum power factor of 1.3$\times $10$^{-4}$Wm$^{-1}$K$^{-2}$, low thermal conductivity of 1.5 Wm$^{-1}$K$^{-1}$ and thermoelectric figure of merit (ZT) of 0.06 was obtained at 720 K. The skutterudite CoSb$_{3}$ nanostructures may be used to develop high efficiency thermoelectric devices. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W29.00012: Direct measurement of the thin-film thermoelectric figure of merit at high temperatures Tela Favaloro, James Christofferson, Ali Shakouri, Gehong Zeng, John Bowers, Hong Lu, Arthur Gossard Thin-film and nanostructured materials offer the potential to selectively engineer material properties and improve the thermoelectric figure of merit. Embedded nanostructures can reduce the thermal conductivity and also enhance the power factor via energy-dependent scattering and hot electron filtering. Here, we utilize the transient Harman technique for high temperature characterization of 50 micron thick n-type InGaAs thin films with embedded ErAs nanoparticles to directly obtain the cross-plane thermoelectric figure of merit. These materials have shown to be promising for thermoelectric energy conversion as they have power factors similar to BiTe at room temperature and significantly higher at increased temperatures. To perform transient Harman characterization of thin films, we fabricated novel device structures to reduce parasitic electrical resistance and thermal leakage. We accurately extract the electrical and thermal signals at high temperatures by applying high speed packaging. The surface temperature profile is obtained using high temperature thermoreflectance imaging and is used to identify major parasitics and acquire thermoelectric parameters of the thin film. [Preview Abstract] |
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