Jihui Yang General Motors R&D Center
George S. Nolas University of South Florida
Kunihito Koumoto Nagoya University
Yuri Grin Max-Planck-Institute for Chemical Physics of Solids
Office of Naval Research
N1: Applications and Devices
Tuesday PM, April 14, 2009
Room 2010 (Moscone West)
10:00 AM - **N1.1
Thermoelectrics for High Temperatures: A Survey of State of the Art.
Harald Boettner 1 Show Abstract
1 TES, Fraunhofer Institute for Physical Measurement Techniques, Freiburg Germany
Energy is a scarce resource. Nevertheless, heat can be found escaping unused wherever you look. Around 60 percent of all fossil primary energy is converted into unused waste heat. Thermogenerators (TEGs) are known to be able to use those otherwise forever lost treasures of our earth. This makes thermoelectric generators useful assistants in the process known as energy harvesting. Converting heat into electrical energy using car waste heat will be not only a vision was demonstrated by the preliminary system presented by e.g. BMW during summer 2008. Fuel economy improvement of 5-8% for highway driving was reported by BMW.To enable this technology for exploiting waste heat and thus contribute to a more efficient utilization of natural resources, thermoelectric materials and standardized so called high temperature modules for high temperature differences – 500°C or even more – are a prerequisite. They must be easily accessible like today’s Bi2Te3-based standard modules. To achieve this goal much effort is under way worldwide.Here a survey of the state of the art published development of high temperature materials is presented and the development in comparison to the well known situation in the 90th of the last century will be presented. An attempt will be made to assess state of art concerning thermoelectric properties, technical level, and possible potential for standard device technology. Also a first assessment based on the current commodity prices of elements of some important thermoelectric compounds will be made.For some compounds a survey will be given concerning advantages and drawbacks. Only if highly efficient, cost-effective TEGs for high temperatures will be commonly available, waste heat in automobiles or in large-scale industrial plants, such as furnaces and refuse incinerators can be converted into usable energy. Then thermoelectrics, as a recycler of “nomadic” energy supplies, could play an important role in complementing renewable energies as a complementary technology.
10:30 AM - N1.2
Fabrication and Testing of Skutterudite-based, Thermoelectric Devices for Power Generation.
Jeffrey Sakamoto 1 Show Abstract
1 CHEMS, MSU, East Lansing , Michigan, United States
MSU and JPL are engaged in a collaborative effort to develop the next generation thermoelectric power generators for terrestrial waste heat recovery. This project is sponsored by the DOE EERE Program to improve fuel efficiency of commercial and passenger vehicles. The emphasis has primarily been on thermoelectric technology employing Skutterudite-based technology, which also includes segmenting with heritage materials such as TAGS and Bi2Te3. This work entails scaled-synthesis of n and p type Skutterudite powders, mass-production of metallized legs, module-bonding technology and thermoelectric subassembly integration into prototypical generators. Two thermoelectric generators are under consideration. First, a traditional design is under consideration whereby heat is extracted from the periphery of an exhaust component. The second design is enabled by aerogel-based thermal insulation, which integrates the thermoelectric elements inside an exhaust component giving direct access to high temperature exhaust gas. Finally, preliminary power output validation for >10Watt Skutterudite subassemblies will be presented.
10:45 AM - N1.3
>10% Single-Stage and >13% Two-Stage Conversion Devices for Increased Fuel Efficiency in Portable Generator Systems
Chris Caylor 1 , Rama Venkatasubramanian 1 , Paul Dev 2 , Chris Howells 3 , Selma Matthews 3 Show Abstract
1 , RTI International, Research Triangle Park, North Carolina, United States, 2 , D-STAR Engineering, Shelton, Connecticut, United States, 3 , U.S. Army CERDEC, Ft. Belvoir, Virginia, United States
The development of single- and multi-stage thermoelectric devices for high conversion efficiencies will be reported. Single-stage devices consisting of n-PbTe and p-TAGS elements have demonstrated >10% conversion efficiencies with Thot ~ 450°C and Tcold ~ 25°C. Data from experiments using a Q-meter test apparatus will show a matrix of power and conversion efficiencies with Thot from 400°C to 500°C and Tcold from 25°C to 150°C to cover a broad range of thermal management scenarios on the cold-side of the device. For example, >10% conversion efficiency is achieved for Thot ~ 450°C and Tcold ~ 25°C, as stated above, however, >10% is achieved with cold-side temperature as high as ~75°C with Thot ~500°C with maximum efficiency of >11% with Thot ~500°C and Tcold ~ 25°C. In addition to the single-stage results, two-stage devices consisting of the same n-PbTe and p-TAGS elements combined in a cascade structure with bulk Bi2Te3-based devices will be discussed with conversion efficiencies >13% verified with the Q-meter test apparatus. We will also report on early integration with Bi2Te3-based thin-film superlattice devices as the bottom stage of the two-stage device. These devices are being developed to convert waste heat present in the exhaust of portable diesel generators for the Army under a SERDP (Strategic Environmental Research and Development Program) funded effort. D-STAR engineering has characterized a 3kW generator and shown that temperatures up to ~470°C are present in the exhaust. With this information and D-STAR’s heat exchanger design, we are attempting to develop a two-stage thermoelectric converter to enable 10% fuel efficiency gain over normal operation. The engineers at U.S. Army CERDEC are helping in the evaluation of the converter system and the testing of the power generation unit. Early studies of this integration with heat-exchanger and system-level tests will be presented.Acknowledgements: SERDP Contract No: W912HQ-08-C-0013
11:30 AM - **N1.4
Thermoelectric Generators Made with Novel Lead Telluride Based Materials.
Timothy Hogan 1 , Chun Wu 1 , Jonathan D'Angelo 1 , Nuraddin Matchanov 1 , Muhammad Farhan 1 , Muhammad Khan 1 , Fei Ren 2 , Brad Hall 2 , Jennifer Ni 2 , Eldon Case 2 , Ed Timm 3 , Harold Schock 3 , Joe Sootsman 4 , Steven Girard 4 , Mercouri Kanatzidis 4 Show Abstract
1 Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan, United States, 2 Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States, 3 Mechanical Engineering Department, Michigan State University, East Lansing, Michigan, United States, 4 Chemistry Department, Northwestern University, Evanston, Illinois, United States
Thermoelectric modules made from novel lead telluride based materials have been fabricated and tested. Open circuit voltages measured on modules of Ag1-xPbmSbTe2+m (n-type) legs and Ag(PbSn)mSbTe2+m (p-type) legs have shown good agreement with expected values based on temperature dependent material properties. Systematic improvements in the parasitic electrical losses have resulted in increased short circuit electrical current output from these modules to over 5.5 (A) for a hot side temperature of 870K, and a cold side of 300K. Modules based on segmented legs using Bi2Te3-xSex and BixSb2-xTe3 materials for the colder sides will also be presented. Common thermoelectrics are narrow bandgap semiconductors which are brittle, and mechanical characterization of these materials is essential for the design of robust modules. Through powder processing and hot pressing techniques, the fracture strength of these materials has been increased by as much as a factor of 3. Investigations of contacts for reducing the electrical parasitic resistances will be presented along with the latest advancements in the fabrication and characteristics of modules based on these and other novel high ZT materials developed in our group.
12:00 PM - N1.5
Fabrication of Power Generating Couples Using Advanced Thermoelectric Materials.
Erik Brandon 1 , Thierry Caillat 1 , Su Chi 1 , Richard Ewell 1 , Samad Firdosy 1 , Billy Li 1 , Bill Nesmith 1 , Jong-Ah Paik 1 , Vilupanur Ravi 1 2 Show Abstract
1 Materials and Device Technologies Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States, 2 Department of Chemical and Materials Engineering, California State Polytechnic University, Pomona, California, United States
A next generation thermoelectric power converter for use in deep space missions is currently under development. This development effort is driven by increasingly ambitious science goals for future NASA robotic missions, combined with a decreasing availability of plutonium fuel. The goal of this current effort is to demonstrate improved system specific power and efficiency relative to the General Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) used in previous missions (such as Galileo, Ulysses, Cassini and New Horizons to Pluto) as well as the Multi-mission Radioisotope Thermoelectric Generator (MMRTG) planned for use in upcoming missions. The specific system performance goals are an 8-10% thermal-to-electric conversion efficiency and a 6-8 W/kg specific power. The key to meeting these system performance goals is the planned use of power generating thermocouples incorporating advanced thermoelectric materials that are capable of operating with a maximum zT near a hot side temperature of 1000°C for up to 14 years of service. Several promising p-type and n-type semiconductor materials are currently under development, for incorporation into thermocouples. One candidate p-type material under investigation is the compound Yb14MnSb11, which exhibits a Zintl structure type and electrical properties on the borderline between a semiconductor and a metal. Although Yb14MnSb11 has a favorable zT of >1 near 1000°C, this compound exhibits a high sublimation rate and a pronounced tendency to form oxides. The lack of a well defined phase diagram for this ternary compound combined with the limited availability of reactivity data make the design of appropriate bonding technologies challenging. Furthermore, its coefficient of thermal expansion is high relative to widely used n-type materials such as silicon germanium alloys, making their joining to a common hot side member difficult. To enable the incorporation of Yb14MnSb11 into thermocouples requires a better understanding of its reactivity combined with unique approaches to the thermocouple design and fabrication process. In addition, new nano-structured p-type and n-type silicon germanium alloys are currently under development, exhibiting lower thermal conductivities, simpler synthetic routes and more homogenous compositions than previous materials. The challenges of integrating these new materials into power generating thermocouples will be reviewed in this talk.
12:15 PM - N1.6
Short Time Transient Behavior of SiGe-based Microrefrigerators.
Ezzahri Younes 1 , James Christofferson 1 , Kerry Maize 1 , Ali Shakouri 1 Show Abstract
1 Electrical Engineering, UCSC, Santa Cruz, California, United States
We use Thermoreflectance Thermal Imaging technique to study the transient cooling of SiGe-based microrefrigerators. Thermal imaging with submicron spatial resolution, 0.1C temperature resolution and 300 nanosecond temporal resolution is achieved. Transient temperature profiles of SiGe-based superlattice microrefrigerator devices of different sizes are obtained. The dynamic behavior of these microrefrigerators, show an interplay between Peltier and Joule effects. Peltier cooling appears first with a time constant of about 10-20 microseconds, then Joule heating in the device starts taking over with a time constant of about 100-150 microseconds. The experimental results agree very well with the theoretical predictions based on Thermal Quadrupoles Method. The difference in the two time constants can be explained considering the thermal resistance and capacitance of the thin film. In addition this shows that the Joule heating at the top metal/semiconductor interface does not dominate the microrefrigertor performance. If this was the case, we would have obtained the same time constants for Peltier and Joule effects. Experimental results show that under high current values, pulse-operation the microrefrigerator device can provide cooling for about 30 microseconds, even though steady state measurements show heating. Temperature distribution on the metal leads connected to the microrefrigerator’s cold junction show the interplay between Joule heating in the metal as well as heat conduction to the substrate. Modeling is used to study the effect of different physical and geometrical parameters of the device on its transient cooling. 3D geometry of heat and current flow in the device plays an important role. One of the goals is to maximize cooling over the shortest time scales.
12:30 PM - N1.7
Materials for Solar Thermoelectric Generators.
Reja Amatya 1 , Rajeev Ram 1 Show Abstract
1 EECS, MIT, Cambridge, Massachusetts, United States
Solar thermoelectric generators employing light concentrators and high ZT thermoelectric materials are an attractive alternative to solar photovoltaics for micro-power applications. We have shown that cost savings of 30% can be realized over comparable photovoltaic technology. In this paper we report on optimization of thermoelectric materials for this important energy application.Specifically, we present a new thermodynamic analysis of solar thermoelectrics that is experimentally verified for a subset of materials. Solar TE generator performance is modeled and measured using mature thermoelectric materials (micro-alloy Bi2Te3 & SiGe) as well as novel nanocomposite materials ([PbTe, ErAs:InGaAs]). In addition, we explore the optimal choice of material for this application. Solar TE requires: 1.a fixed input power density with low-cost light concentrators providing 10,000 W/m2,2.a high optimal temperature (set in-part by thermodynamic balance between concentrator and TE hot-side) of 800-900 C, and3.a wide-operating temperature over various hours in the day and seasons in the year.For example, a TE element with a thermal conductivity of 4.13 W/m/K and an aspect ratio (cross-section area/length) of 0.01m, with a cold-side heat transfer coefficient of 1.5 W/cm2/K gives a hot-side temperature of 1200K. A ZT = 0.45 at the absorber temperature of 1200K gives the system efficiency of 3.8%.
12:45 PM - N1.8
High Power Density Thermoelectric Modules for Thermal Energy Harvesting
John Posthill 1 , Jerry Fleming 2 Show Abstract
1 Center for Solid State Energetics, RTI International, Research Triangle Park, North Carolina, United States, 2 Secure Computing and Communications Group, Luna Innovations Incorporated, Roanoke, Virginia, United States
The potential for energy harvesting from waste heat is becoming an increasingly important topic for both military and commercial systems. The integration of thermoelectric devices to perform this task can be challenging and must compete for space and area with other important functionality in the system. Ideally, incorporating a new capability (such as energy harvesting) into a material or structure with little-to-no adverse impact on other properties is desired. This has led to the concept and study of multifunctional materials.We have conducted an initial investigation and comparison between commercial, bulk Bi2Te3-based Peltier thermoelectric devices with thin-film Bi2Te3-based thermoelectric modules. The metrics we have used for this comparison are: (1) power density based on area (PDa in units of mW/cm2) and (2) power density based on volume (PDv in units of W/cm3). Power results were obtained as a function of externally applied temperature difference (ΔT) in the range from 5K to 150K, with the subsequent power densities calculated from the area of the hot side header and the module thickness. The point of comparison between modules was chosen to be ΔT = 25K.The thin film modules with a packing fraction 18.1% and appropriately thinned headers demonstrated PDa = 184 mW/cm2 and PDv = 2.84 W/cm3 at ΔT = 25K. As ΔT was raised to 150K, PDa = 6,530 mW/cm2 and PDv = 100 W/cm3 were measured, thereby demonstrating the expected ΔT2 dependency. We are unaware of higher power density results than these from a single-stage Bi2Te3-based thermoelectric module.Comparative results varied because a number of commercial thermoelectric modules were tested, though smaller commercial modules of area 1cm2 or less were chosen. Packing fractions as well as the module heights varied based (presumably) on commercial considerations, not necessarily on maximizing power density. Nevertheless, we found at a ΔT = 25K for the smallest bulk module chosen, the thin film module exceeded commercial bulk modules by a factor of 6.8 for PDa and by a factor of 21.8 for PDv.These results will also be compared with other thin film thermoelectric results in the literature. Direction for future increases in thermal energy harvesting power density will be discussed as well as comparative thermoelectric module efficiency as a function of ΔT.
Jihui Yang General Motors R&D Center
George S. Nolas University of South Florida
Kunihito Koumoto Nagoya University
Yuri Grin Max-Planck-Institute for Chemical Physics of Solids
N4: Nanocomposites and Nanostructured Materials II
Wednesday AM, April 15, 2009
Room 2010 (Moscone West)
9:30 AM - N4.1
Understanding Electrical Transport and the Large Power Factor Enhancements in Co-Nanostructured PbTe.
Joseph Sootsman 1 , Vladimir Jovovic 2 , Christopher Jaworski 2 , Joseph Heremans 2 , Jiaqing He 3 , Vinayak Dravid 3 , Mercouri Kanatzidis 1 Show Abstract
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Department of Mechanical Engineering, Ohio State University, Columbus, Ohio, United States, 3 Department of Materials Science, Northwestern University, Evanston, Illinois, United States
Thermoelectric materials hold promise in waste heat to electrical power generation and could play a role in future energy management if their efficiency can be improved. Primarily, recent enhancements in thermoelectric figure of merit have come from reduction in the thermal conductivity in nanostructured materials. However, we recently reported the synthesis and characterization of PbTe nanostructured with both Pb and Sb precipitates which has an increased figure of merit. The combination of these precipitates caused unexpected changes in the electron mobility resulting in higher than expected power factors at high temperature. These enhanced power factors coupled with a reduced thermal conductivity resulted in ZT values approaching 1.4 at 675K. Additional transport measurements to determine the scattering parameter, high temperature transmission electron microscopy, and three-dimensional atom probe tomography results will be reported with the hope of elucidating the mechanism of this novel transport behavior.
9:45 AM - N4.2
Tailoring Thermoelectric Properties of Segregated-network Polymer Nanocomposites for Thermoelectric Energy Conversion.
Dasaroyong Kim 1 , Yeonseok Kim 1 , Jaime Grunlan 1 , Choongho Yu 1 Show Abstract
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States
Polymers are intrinsically poor thermal conductors, which are ideal for thermoelectrics, but low electrical conductivity and thermopower have excluded them as feasible candidates as thermoelectric materials in the past. However, recent progresses in polymer technology, particularly nanomaterial-polymer composites can bring them into degenerate semiconductor or metallic regimes by incorporating a small amount of conductive fillers. Here, we demonstrate that such polymer nanocomposites can be viable for light-weight and economical thermoelectrics by using a segregated network approach for the nanocomposite synthesis. In the case of 20 wt% CNT polymer composite films made of PVAc matrix, a thermoelectric figure of merit was measured to be 0.006 at room temperature. The thermoelectric properties were further improved by replacing PVAc with electrically conductive polymers including Poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) doped with dimethyl sulfoxide (DMSO) The influence on thermoelectric properties from filler concentration, matrix materials, and stabilizer materials are also discussed.
10:00 AM - **N4.3
Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach.
Zhifeng Ren 1 , Bed Poudel 2 1 , Yi Ma 1 2 , Yucheng Lan 1 , Xiaowei Wang 1 , Giri Joshi 1 , Gaohua Zhu 1 , Jian Yang 1 , Bo Yu 1 , Xiao Yan 1 , Dezhi Wang 1 , Qing Hao 3 , Hohyun Lee 3 , Austin Minnich 3 , Andrew Muto 3 , Daryoosh Vashaee 3 , Xiaoyuan Chen 3 , Gang Chen 3 , Junming Liu 4 , Mildred Dresselhaus 5 Show Abstract
1 Physics, Boston College, Chestnut Hill, Massachusetts, United States, 2 , GMZ Energy Inc., Newton, Massachusetts, United States, 3 Department of Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 4 Department of Physics, Nanjing University, Nanjing China, 5 Department of Physics, MIT, Cambridge, Massachusetts, United States
The dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been demonstrated in a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient.
10:30 AM - **N4.4
Nanosized Granular Boundaries in Polycrystalline Pb0.75Sn0.25Te: An Innovative Approach to Enhancing the Thermoelectric Figure of Merit.
Jian He 1 , Xiaohua Ji 1 , Bo Zhang 1 , Zhe Su 1 , Tim Holgate 1 , Terry Tritt 1 Show Abstract
1 Physics and Astronomy, Clemson University, Clemson, South Carolina, United States
Most of the recent advances in enhancing the dimensionless thermoelectric figure of merit ZT are linked to the thermal conductivity reduction. As the heat-carrying phonons are widely dissipated in the energy and momentum space, the structural complexity on the multiple length scales, especially at those comparable to the phonon wavelength, is necessary for the thermal conductivity reduction desired. In this work we utilized an Alkali metal (Na, K) hydrothermal treatment followed by hotpressing procedure to facilitate the formation of a nanosized granular grain boundary in the polycrystalline Pb0.75Sn0.25Te system. The presence of this rough grain boundary, whose mean roughness height and roughness period are on the order of several nanometers, results in a significant lattice thermal conductivity reduction without appreciably affecting the thermopower or the electrical resistivity. As a result, a ZT ~ 0.50 (0.38) has been attained in the Na (K)-processed sample at ~ 425 K (475 K), as compared to the ZT ~ 0.34 at ~ 490 K in the bulk reference sample. The present work provides a novel avenue by which the lattice thermal conductivity of a polycrystalline system can be decoupled from the electronic properties via controlling the micro-morphology of the grain boundary.
11:30 AM - **N4.5
Metal-semiconductor Nanocomposites for Thermoelectric Energy Conversion.
Ali Shakouri 1 Show Abstract
1 , Univ. of California Santa Cruz, Santa Cruz, California, United States
We will review the potential of metal-semiconductor multilayers and embedded nanoparticles for thermoelectric energy conversion. Theoretical calculations of the electrical conductivity, Seebeck coefficient and the electronic contribution to thermal conductivity using Boltzmann equation is presented. Corrections due to superlattice minibands and nanoparticle scattering show the role of hot electron energy filtering. Finite particle sizes in couple of nanometer range require the use of partial wave technique in estimating the scattering cross sections. In addition, coherent potential approximation is used to include multiple scatterings at high concentrations of nanoparticles. Theory and experiment are compared in the case of rare-earth ErAs nanoparciles in InGaAlAs semiconductor matrix. Experimental in-plane electrical conductivity and Seebeck coefficient are obtained in 300-800K range. The cross-plane electrical conductivity, Seebeck coefficient and thermal conductivity of 20 microns thick films are extracted using transient Harman technique and thermal imaging of single leg micro refrigerators. Preliminary theoretical calculation and experimental characterization of ZrN/ScN multilayer films are also presented. Potential to reach ZT values exceeding 2-3 are discussed.
12:00 PM - **N4.6
Skutterudite-based Thermoelectrics: Nano-composites and Device Development.
Lidong Chen 1 , Wenqing Zhang 1 , Xiaoya Li 1 , Xiangyang Huang 1 Show Abstract
1 , Institute of Shanghai Ceramics, CAS, Shanghai China
Thermoelectric (TE) conversion receives great attention as a prospective energy conversion technique such as in the harvest of solar energy and recovery of industrial exhausted heat. Recently, significant progress in both material research and device development has been achieved world-wide. This review will provide a summary of some effective techniques for improving TE performance through multi-level microstructure control with focusing on the skutterudite-based materials. A proven approach to elevate figure of merit (ZT) is via formation of nano-composites, in which extrinsic nano-phases are dispersed at grain boundaries and/or within grain. Acting as energy filter and phonon scattering center, nano-phases contribute to both the increase of thermopower and the reduction of thermal conductivity without much degradation of electrical conductivity. The distribution state of the extrinsic phase, including the content, homogeneity and structures, are the key factors for the enhancement of ZT value of the composites. Several novel approaches including in-situ reaction, vapor-transportation and sol-gel process to prepare TE composites with dispersion of extrinsic nano-particles have been developed. The thermal stability of the nano-structure was studied. The design and fabrication of TE device using high performance skutterudite materials have also been carried out using Mo-based alloys as electrode. The interfacial microstructure of the skutterudite/electrode joints were found to give very critical influences on the interfacial electrical resistivity and thermal resistivity and bonding strength, and therefore to give great influence on thermoelectric performance. Some novel processes for fabricating skutterudite/electrode joints have been developed. The reliability and life duration behavior of the TE device have also been studied with focusing on the microstructure evolution during the accelerated test. Some criteria on the development of high performance TE device using skutterudite materials will be also discussed in this report.
12:30 PM - N4.7
Thermal Transport in Rough Silicon Nanowires for Thermoelectric Applications
Sanjiv Sinha 1 , Bair Budaev 2 , Arun Majumdar 2 3 4 Show Abstract
1 Mechanical Science & Engineering, University of Illinois, Urbana, Illinois, United States, 2 Mechanical Engineering, University of California, Berkeley, California, United States, 3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Recently reported data on electrolessly etched rough silicon nanowires  suggest a hundredfold increase in ZT due to reduction in thermal conductivity of the same order. However, no known theoretical model for thermal conductivity of crystalline dielectrics predicts such reduction. Further, the observed linear increase in thermal conductivity with temperature in nanowires with diameters approaching 50 nm defies the widely-accepted Klemens-Callaway  model of thermal conductivity for silicon. Here we discuss a new model that considers the wave-like transmission of acoustic phonons through the wire. We show that such a nanowire behaves in essence, like a waveguide with the resulting wave dispersion becoming size and roughness dependent. Considering a two-dimensional analogue of the wire problem, we solve the Helmholtz wave equation for acoustic waves in a rough film . We show that roughness leads to a coupling of different modes and also the conversion of propagating modes to evanescent modes. This conversion to evanescent localizes phonons in wires with localization lengths approximately greater than a micrometer. We compute the resulting thermal conductivity of rough Si nanowires and show good comparison with experimental data. 1.A.I. Hochbaum et al., Nature 451, 163-167 (January 2008).2.G. A. Slack, Solid State Physics, 34, 1, Academic Press, New York (1969).3.V.D. Freilikher et al., Radiofizika, 13, 73, 1972.
12:45 PM - N4.8
Vacuum Thermionic Energy Conversion Based on Nanostructured Doped Diamond Thin Films.
Franz Koeck 1 , Robert Nemanich 1 , Ken Haenen 2 Show Abstract
1 Department of Physics, Arizona State University, Tempe, Arizona, United States, 2 Institute for Materials Research , Hasselt University, Diepenbeek Belgium
Vacuum thermionic energy conversion is a process where thermally activated electrons from an emitter traverse a vacuum gap and are collected by a counter – electrode. Closing the circuit to the emitter establishes an electrical current where electrons can do work in external devices. Key component of a vacuum energy converter is an efficient electron source which can sustain an electron current at low to moderate temperatures. The electron emission current described by the law of Richardson – Dushman relates emission barrier or work function and emission constant to materials properties. Diamond based emitter configurations can exploit material aspects by means of controlling dopants as well as structural characteristics. By identifying suitable donors a low effective work function material can be synthesized. For nitrogen and phosphorus doped films we have measured work functions of 1.3 eV and < 1 eV, respectively. Here, the effective work function is dependent on surface termination, i.e. negative electron affinity which also alleviates system detrimental space charge effects. In order to obtain significant emission current densities a nitrogen – incorporated ultra – nanocrystalline diamond film (UNCD) was utilized as an interstitial layer between substrate and doped diamond film. This resulted in electron emission commencing at temperatures < 250 °C with a significant value for the Richardson’s constant. High temperature operation in excess of 700 °C was indicative of stable film and surface properties for phosphorus – doped diamond films. In a vacuum thermionic converter configuration nitrogen – incorporated UNCD based emitter and collector structures provided a significant open source voltage of ~ 0.4 V at a moderate temperature of 500 °C.This research is supported by the TEC-MURI project.
Wednesday PM, April 15, 2009
Room 2010 (Moscone West)
2:30 PM - N5.1
First Principles Study of Metal/Bi2Te3 Interfaces: Implications to Improve Contact Resistance.
Ka Xiong 1 , Weichao Wang 1 , Husam Alshareef 1 , Rahul Gupta 1 , Bruce Gnade 1 , Kyeongjae Cho 1 2 Show Abstract
1 Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States, 2 Physics, University of Texas at Dallas, Richardson, Texas, United States
To allow the thermoelectric coolers to reach the next level of performance in terms of efficiency and power density, a low contact resistance value is needed . For current thermoelectric devices, electroless Ni has been used as the contact metal with a contact resistance of ~ 5x10-6 Ωcm2. The contact resistance needs to be at least 10-100 times lower in order to maintain the device scaling . The contact resistance arises from the formation of the Schottky barrier between the metal and the semiconductor, and the interface band alignment has not been systematically optimized yet. Therefore, it is necessary to understand the factors that control the Schottky barrier height between metal and thermoelectric material. Furthermore, our recent experimental data showed that for Ni contacts on Bi2Te3 a NiTe interfacial region is formed, while for Co contacts on Bi2Te3 the CoTe interfacial layer has not been observed. This observation indicates the importance of the chemical stability of the interface and interfacial layer formation. For the purpose of understanding detailed interface electronic structures, we use first principles calculations to investigate the interface chemistry of Ni/Bi2Te3 and Co/Bi2Te3 interfaces. We propose several methods to reduce the Schottky barrier height by forming controlled interfacial dipole layers, which can be achieved by introducing impurities into the interface  or inserting the interfacial layer between the metal and Bi2Te3 . These methods have been previously applied on metal/oxide interfaces to control the band offsets. This study will help us to gain fundamental understanding on the contact resistance, and possible mechanisms to reduce it. .This work is supported by the II-VI Incorporated Foundation. References J. –P. Fleurial, Proc 18th Intl. Conf. on Thermoelectrics, pp294, Baltimore, USA (1999).  V. Semenyuk, Proc 20th Intl. Conf. on Thermoelectrics, pp391, Beijing, CHINA (2001). K. Xiong et al. Appl. Phys. Letts. 92, 113504 (2008). K. Xiong et al, J. Appl. Phys. 104, 074501 (2008).
2:45 PM - N5.2
Studying the Power Factor of a Matrix with High Concentration of Embedded Nanoparticles.
Mona Zebarjadi 1 , Keivan Esfarjani 1 , Ali Shakouri 1 Show Abstract
1 , UC Santa Cruz, Santa Cruz, California, United States
In this presentation we investigate nanoparticle scattering effect on the thermoelectric transport properties using the average T-matrix approach and the coherent potential approximation (CPA). The two methods can have different predictions in the high concentration limit where multiple scattering is important. To include randomness in the nanoparticles sizes, we consider an effective medium for the electron scattering off of nanoparticles of different sizes using the CPA. Enhancement of the thermopower is investigated by looking at the slope of scattering cross section versus energy at the Fermi level. Effect of parameters such as nanoparticle size, height, concentration and the randomness of their distribution will be examined. The study will allow us to engineer nanoparticles with enhanced power factor.
3:00 PM - **N5.3
Enhanced Thermo-electric Transport of Strongly Correlated Electrons.
Veljko Zlatic 1 Show Abstract
1 , Institute of Physics, Zagreb Croatia
We discuss the charge and heat transport of intermetallic compounds with Ce, Eu, and Yb ions using the periodic Anderson model in the limit of infinite correlation between the f
electrons. We show that the normal state properties of the model are governed at low-temperatures by the Fermi liquid (FL) laws, with the characteristic energy scale T0
. At high-temperatures, the model exhibits typical Kondoesque features that can be understood in terms of single impurity Anderson or Kondo models with Kondo scale TK
. Using the slave boson approach, we show that the values of T0
depend on the shape of the conduction electrons density of states (c DOS) in the vicinity of the chemical potential, the degeneracy and the crystal field (CF) splitting of the f states, the number of c and f electrons, and their coupling. The crossover between the high-temperature incoherent regime and the low-temperature coherent one depends on the ratio T0/TK. Unlike in dilute alloys, this ratio is strongly system-dependent, which gives rise to different types of the crossover.
We show that the low-temperature response is enhanced (or reduced) with respect to the predictions based on the single-impurity models that would lead to the same high-temperature behavior as the periodic Anderson model. We also show that the renormalization of transport coefficients in the coherent regime can invalidate the Wiedemann-Franz law and lead to an enhancement of the thermoelectric figure-of-merit. The FL laws explain the correlation between the specific heat coefficient γ=CV/T and the thermopower α(T), or γ and the T2 coefficient of the electrical resistance A=ρ(T)/T2. The FL laws of a N-fold degenerate model explain the deviations of the Kadowaki-Woods ratio RKW=A/γ2 and the q ratio, q=|e|limT→0 α/γT, from the universal values. At high temperature, where α(T) and ρ(T) exhibit large maxima, the perturbation theory shows that the overall temperature dependence of transport coefficients depends on the relative magnitude of TK and the CF splitting Δ. To account for the effects of pressure or doping, we assume that they change the f-c coupling and TK but not Δ.
Using these results, we discuss the thermoelectric response of some typical heavy fermions and valence fluctuators. A sharp peak in the c DOS close to the chemical potential yields T0 << TK, which explains the 'slow crossover' observed in YbAl3. The minimum in the c DOS yields T0 >> TK, which explains the abrupt transition between the high- and low-temperature regimes in YbInCu4. In the case of CeCu2Ge2 and CeCu2Si2, where T0=TK, we show that the pressure-dependence of the A(P) coefficient and of the residual resistance is driven by the change in the degeneracy of the f states. Finally, combining the FL theory and the high-temperature perturbation theory we discuss the drastic modifications of α(T) and ρ(T) induced by pressure, doping or the magnetic field.
3:30 PM - N5.4
Electronic and Vibrational Properties of Filled and Unfilled Ternary Skutterudites.
Dmitri Volja 2 , Fornari Marco 1 , Boris Kozinsky 3 , Nicola Marzari 2 Show Abstract
2 DMSE, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 1 Physics, Central Michigan University, Mt. Pleasant, Michigan, United States, 3 Research and Technology Center, Robert Bosch LLC, Cambridge, Massachusetts, United States
The renewed interest in unfilled and filled ordered ternary skutterudites, XCoA3/2B3/2 (with A and B group IVB and VIB respectively, X a filler), spawns from the possibility to control details of the electronic and vibrational properties of such compounds.Recent experimental work pointed to significant changes in the transport properties with respect CoSb3: unfilled ternary tends to exhibit lower thermal conductivity but larger electrical resistivity [P. Vaqueiro and G. G. Sobany, Mater. Res. Symp. Proc. vol 1044 (2008)].In order to explore optimization strategies for the electronic properties, we performed first principles band structure calculations for CoGe3/2S3/2, CoGe3/2Te3/2, and CoSn3/2Te3/2. In particular we have analyzed the details of the bonding using Wannier function approach as well as studied the electronic transport by a powerful band structure interpolation scheme. Within ab initio density functional pertubation theory we have computed the full phonon dispersions and analyzed the effect of filling with Ba.Our calculations point to a flattening of the bands around the Fermi level that results in larger energy gaps and effective masses especially for the valence band.
3:45 PM - N5.5
The optimized Seebeck coefficient as a determinant of the maximum power factor in Silicon based bulk and nanostructured thermoelectrics
Paothep Pichanusakorn 1 , Prabhakar Bandaru 1 Show Abstract
1 Materials Science program, Mechanical Engineering department, UC, San Diego, La Jolla, California, United States
The efficiency of thermoelectric devices is measured through the Figure of merit,Z=(S2σ)/(ke+kL), and is constituted from both electronic (the Seebeck coefficient: S, the electrical conductivity: σ, and the electronic part of the thermal conductivity: ke) and lattice properties (lattice thermal conductivity: kL) However, the electrical properties can vary by orders of magnitude and depend mainly on the carrier concentration. We show, through extensive analytical work and numerical simulations that there exists an optimum reduced Fermi level (ηopt), where the power factor, S2σ, is maximized, and which is largely independent of material, device geometry, and temperature. Consequently, given the relationship between S and η, we will show that there exists an optimal Seebeck coefficient, Sopt, which can now be set as a practical and direct measure for finding the optimal carrier concentration in any material at any temperature. We also consider the influence of the characteristic scattering exponent (r) in the range of -0.5 to +1.5. For example, given a constant relaxation time (r=0), Sopt in bulk material, quantum well, and quantum wire were calculated to be approximately 130, 167 and 186 µV/K, respectively. However, if acoustic phonon scattering is dominant, then Sopt is always equal to 167 µV/K. Given the optimum Fermi level and Seebeck coefficient, we then determine the minimum quantum well/wire thickness required to achieve an enhancement in S2σ over bulk values. We calculate that the minimum required quantum well and wire thickness for n-Si are both approximately 6.5 nm. We will also discuss the issue of breaking of valley degeneracy in nanostructures, which can reduce the power factor and cause major deviation from the ideal value.
4:30 PM - N5.6
Experimental and Theoretical Studies of the Structure and Properties of Bulk Thermoelectric Bi-Te Compounds
Alfredo Morales 1 , Monica Barney 1 , Douglas Medlin 2 , Peter Sharma 2 , Catalin Spataru 2 , Ana Lima Sharma 3 , Jian He 4 , Fivos Drymiotis 4 , Terry Tritt 4 , James Turner 4 Show Abstract
1 Materials Chemistry, Sandia National Laboratories, Livermore, California, United States, 2 Materials Physics, Sandia National Laboratories, Livermore, California, United States, 3 Department of Physics, Tuskegee University, Tuskegee, Alabama, United States, 4 Department of Physics and Astronomy, Clemson University, Clemson, South Carolina, United States
Bi2 planes can be inserted into the parent Bi2Te3 structure—the most important commercial thermoelectric material—in order to form superlattice structures in a bulk material with the potential to increase thermoelectric efficiency. We have reported the synthesis, property measurements, and theoretical studies of Bi2Te3, Bi6Te7, BiTe, and Bi2Te, and evaluated their potential for thermoelectric applications. Spark plasma sintering was found to be an effective way of eliminating porosity, which negatively affects the interpretation of the intrinsic properties. Superlattice formation has been confirmed using powder x-ray diffraction and TEM studies. Property measurements are consistent with the semimetallic nature of compounds with additional Bi2 planes. A plausible explanation for the observed difference in thermal conductivity between Bi2Te3 and the other compounds could be that interface scattering becomes predominant when the phonon mean free path becomes comparable to the characteristic superlattice spacings. Electronic structure calculations have correctly obtained the structures and electronic nature (semimetallic versus semiconducting) of each compound. Bi2Te3 remains the best thermoelectric material in this series; however, Bi2Te may warrant further investigation for refrigeration near 200 K. This work provides a firm foundation for understanding the role of superlattice formation and its effect on transport properties in a model bulk material that is amenable to a variety of microstructural, transport, and electronic structure studies.
4:45 PM - N5.7
Direct and Indirect Effects of Filling on Band Structure and Phonon Dispersion of Skutterudites.
Daehyun Wee 1 , Boris Kozinsky 2 , Marco Fornari 3 , Nicola Marzari 4 Show Abstract
1 Research and Technology Center, Robert Bosch LLC, Palo Alto, California, United States, 2 Research and Technology Center, Robert Bosch LLC, Cambridge, Massachusetts, United States, 3 Department of Physics, Central Michigan University, Mt. Pleasant, Michigan, United States, 4 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Filled skutterudites are a promising class of medium to high-temperature thermoelectric materials. They exhibit low thermal conductivity, which is typically attributed to the phonon scattering process introduced by the filler. In recent years, however, the exact nature of the physical process involved in the reduction of the thermal conductivity and the associated rattling motion of the filler atom have been the subject of intense investigations and controversy. To provide better insights on the issue, we analyze the effect of filling (Co,Fe)Sb3 with various elements including alkaline earth metals on the band structure and on the full phonon dispersions, using state-of-the-art first principles calculations. In particular, we separate the effects of filler's mass, size, and electronic features on the vibrational spectrum. The effect that pressure has on the vibrational and electronic structure of skutterudite crystals is investigated, and anharmonic vibrational effects are explored by studying thermal expansion as a function of composition in the X(Co,Fe)Sb3 chemical space. We also present electronic transport coefficients estimated within the constant-relaxation-time approximation.
5:00 PM - N5.8
Transport Properties of Thermoelectric Nanocomposites.
Lilia Woods 1 , Adrian Popescu 1 , Joshua Martin 1 , George Nolas 1 Show Abstract
1 Physics, University of South Florida, Tampa, Florida, United States
Transport properties of thermoelectric nanocomposite materials containing granular regions are considered. We present a model describing the carrier conductivity and Seebeck coefficient by taking into account the grain potential boundary scattering mechanism. Other mechanisms, such as carrier-acoustic phonon, carrier-non polar optical phonon, and carrier-impurity scattering are also included in the model. Our calculations reveal that by changing the characteristics of the grains, such as their potential barrier height and size, one can increase the mean energy per carrier and obtain an optimum power factor for improved thermoelectric performance. The model is successfully applied to explain experimental data for PbTe nanocomposites. We also discuss the importance of various material dependent electronic structure parameters for the relevant transport properties in other thermoelectric nanocomposites.
5:15 PM - N5.9
Enhancement of the Thermoelectric Figure of Merit in the Gated Bismuth Telluride Nanowires.
Igor Bejenari 1 , Valeriu Kantser 2 , Alexander Balandin 1 Show Abstract
1 Electrical Engineering and Materials Science and Engineering, University of California - Riverside, Riverside, California, United States, 2 , Institute of Electronic Engineering & Industrial Technologies, ASM, Kishinev Moldova (the Republic of)
Nanostructured materials offer potential for improving the thermoelectric figure of merit and efficiency of the thermal-to-electric energy conversion . In this talk we show that a combination of the quantum confinement effects in bismuth telluride quantum wires with properly applied electric bias can increase the thermoelectric power conversion by an order of magnitude. As an example system we considered square cross-section bismuth telluride quantum wires grown along  direction . The coupled Schrodinger - Poisson equations were solved self-consistently using the trigonometric functions and Gauss – Lobatto - Legendre interpolating polynomials as the basis function elements. At small values of the gate bias the Poisson’s equation was solved analytically by using the Thomas - Fermi approximation. In this case, the solution of the Schrodinger equation was expressed by the Mathieu functions. In the framework of our theoretical model we obtained the dependence of the thermoelectric parameters on the gate voltage as well as on the space variables. It was found that the electric field effect is significant when the quantum wire thickness is less or comparable to the electron Debye screening length. Under applied positive (negative) gate bias the electron (hole) contribution to the thermoelectric power of the intrinsic quantum wires becomes dominant. In nanowires of medium thickness (15 nm) the transverse electric field increases concentration of both types of carriers while in narrow nanowires (7 nm) it increases the concentration of only type of carriers depending on the gate polarity. We have established that both the dimensional confinement and electric field effects lead to the reduction of the nanowire thermal conductivity. The Seebeck coefficient can be increased by a factor of two in nanowires of 7 nm diameter. The overall electric-field effect results in the enhancement of the thermoelectric figure of merit by an order of magnitude. The enhancement is larger for the positive gate bias because the electron mobility exceeds that of the holes. The obtained results may lead to a new method of improving the thermoelectric figure of merit for more efficient thermal-to-electric energy conversion.IB acknowledges the Fulbright Program grant to conduct research at NDL. The work in Balandin group was supported in part through the DARPA UCR – UCLA – UCSB Center for Nanoscience Innovations for Defense (CNID). A.A. Balandin and O.L. Lazarenkova, Mechanism for thermoelectric figure-of-merit enhancement in regimented quantum dot superlattices, Appl. Phys. Lett., 82, 415 (2003). I. Bejenari and V. Kantser, Thermoelectric properties of bismuth telluride nanowires in constant relaxation time approximation, Phys. Rev. B 78, 115322 (2008).
5:30 PM - N5.10
Tailoring Interface Roughness and Superlattice Period Length in Novel Electron Filtering Thermoelectric Materials.
Shidong Wang 1 , Natalio Mingo 2 Show Abstract
1 LETI/DRT/D2NT/LNDE, CEA-Grenoble, Grenoble France, 2 LITEN, CEA-Grenoble, Grenoble France
We quantify the effects of interface roughness and superlattice period on thermoelectric electron filtering [1,2] in superlattices, using the non-equilibrium Green's function method with a realistic description of the interface. In contrast with previous suggestions, we find that rough interfaces do not enhance the power factor more than smooth ones. Electron filtering does not increase the thermoelectric power factor if the well length is longer than the inelastic mean free path. Quantitative results are provided in the case of InGaAs/InAlGaAs superlattices.1. D. Vashaee, & A. Shakouri, J. Appl. Phys. 95, 1233-1245(2004);2. D. Vashaee, & A. Shakouri, Phys. Rev. Lett. 92, 106103(2004);3. S. Wang, & N. Mingo, to be published.
Jihui Yang General Motors R&D Center
George S. Nolas University of South Florida
Kunihito Koumoto Nagoya University
Yuri Grin Max-Planck-Institute for Chemical Physics of Solids
N9: Novel Oxides
Friday AM, April 17, 2009
Room 2010 (Moscone West)
9:30 AM - N9.1
Thermoelectric Zintl Compounds in R-T-Sb (R = Ba, Eu, Yb; T = Zn, Cd) Systems.
X. Wang 1 , H. Zhang 1 , M. Tang 1 , X. Yang 1 , H. Chen 1 , Z. Man 1 , U. Burkhardt 2 , Jing-tai Zhao 1 , Y. Grin 2 Show Abstract
1 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai China, 2 , Max Planck Institute for Chemical Physics of Solids, Dresden Germany
Recently many new promising thermoelectric (TE) materials have been found in a special category of intermetallic compounds, i.e. Zintl phases. The classical Zintl phases are semiconductors with narrow band gap and many have complex crystal structures containing “rattle” heavy atoms to produce low lattice thermal conductivities. In our systematic investigations of potential TE materials in intermetallic compounds with polar character, ternary Zintl phases RxTySz(R=Ba, Yb and Eu; T=Transition elements; S= Semi-metal elements) showed to be promising. Based on first principle calculations on electronic structures and crystal structure analyses, the pnictide candidates such as BaZn2Sb2, YbCd2Sb2 and EuCd2Sb2 were synthesized and their thermal and electrical transport properties were characterized. These compounds show promising TE properties with considerably high figure of merit (e.g. ZT~1at 700K for YbCd2Sb2). The solid solution system YbCdxZn2-xSb2 (x = 0, 0.4, 0.8, 1, 1.2, 1.6 and 2) were synthesized and their TE properties were also characterized. The results revealed that Zn substitution of Cd in YbCd2Sb2 can easily tune carrier concentration and decrease thermal conductivity, which resulted in improvement of ZT value (e.g. ZT ~1.2 at 700K for x = 0.4).
9:45 AM - N9.2
High-temperature Thermoelectric Performance of Sr1-xLaxTiO3-d.
Matthew Scullin 1 2 , J. Ravichandran 2 4 , S. Mukerjee 2 3 , Y. Koh 6 , D. Cahill 6 , J. Moore 2 3 , A. Majumdar 2 5 , R. Ramesh 1 2 3 Show Abstract
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Applied Science and Technology, University of California, Berkeley, Berkeley, California, United States, 3 Physics, University of California, Berkeley, Berkeley, California, United States, 6 Materials Science and Engineering, University of Illinois - Urbana, Urbana, Illinois, United States, 5 Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States
We report the thermoelectric performance of doubly-doped strontium titanate, Sr1-xLaxTiO3-d, in the temperature range 300-1000K. Power factor*T in this n-type oxide semiconductor is large (> 1 W/m-K @300K) due to the enhanced effective mass of electrons from oxygen vacancy doping and high carrier concentrations from lanthanum doping, and increases to 1.6 W/mK @ 500K for some doping levels. Thermal conductivity is also four times lower in these thin-films versus bulk, yielding zT values greater than 0.5 at typical engine exhaust gas temperatures (300-600°C). These attractive high-temperature properties make this material a strong competitor for use in thermoelectric waste heat recovery.
10:00 AM - **N9.3
Why Do Layered Cobaltates Have Good Thermoelectric Properties?
Rongying Jin 1 2 Show Abstract
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee, United States
Layered cobaltates are ideal systems for studying low-dimensionality effects on thermoelectric performance of bulk samples, as they naturally form superlattice structures. By investigating the anisotropy of thermoelectric properties of NaxCoO2 and Ca3Co4O9 single crystals and by comparing these quantities with that obtained from other layered and correlated electron materials, we argue that good thermoelectric properties in these systems are due to their unique crystalline structure and chemical compositions. This is further supported by the results obtained from a series of Bi2Sr2Co2O9 thin films grown in various conditions.
10:45 AM - N9.5
Mechanical and Thermoelectric Properties of Polycrystalline NaxCoO2
Richard Donelson 1 , P. Tsai 2 , Sean Li 2 , Bryce Wood 1 Show Abstract
1 Div of Materials Science and Engineering, CSIRO, Clayton, Victoria, Australia, 2 School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia
Polycrystalline NaxCoO2 (x ≈ 0.7) is being considered for use as the p-type leg in intermediate temperature thermoelectric generators (TEG). This material exhibits a reasonable thermoelectric performance and is relatively easy to process into useful shapes. In addition to the thermal and electrical properties, a TEG designer will require knowledge of the relevant mechanical properties. Here we report on our efforts to obtain a useful property set for NaxCoO2 samples which were produced by solid state reaction.
11:30 AM - **N9.6
Electric Field Induced Giant Thermopower of Two-dimensional Electron Gas at the Gate Insulator/SrTiO3 Heterointeface.
Hiromichi Ohta 1 2 , Akira Yoshikawa 1 , Daisuke Kurita 1 , Kunihito Koumoto 1 , Ryoji Asahi 3 , Yumi Masuoka 3 , Kenji Nomura 4 , Hideo Hosono 4 5 6 Show Abstract
1 Graduate School of Engineering, Nagoya University, Nagoya Japan, 2 PRESTO, Japan Science and Technology Agency, Kawaguchi Japan, 3 , Toyota Central R&D Laboratories, Nagakute Japan, 4 ERATO-SORST, Japan Science and Technology Agency, Yokohama Japan, 5 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 6 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan
Two-dimensionally confined electrons in extremely thin layer (thickness < thermal de Broglie wavelength ~10 nm), the latter being composed of an electron well and a barrier, exhibit unusually large thermoelectric Seebeck coefficient (|S|2D) as compared to the corresponding bulk materials due to the fact that the density of states (DOS) near the bottom of the conduction band increases with decreasing thickness of the electron well. Recently, we briefly reported that a high density two-dimensional electron gas (2DEG), which was confined within a unit cell layer thickness (0.3905 nm) in SrTiO3, exhibited a giant |S|2D, which was about 5 times larger than that of the bulk SrTiO3, whereas the 2DEG system retained a rather high σ2D value. The 2DEG was realized at SrTiO3/SrTi0.8Nb0.2O3 superlattices or TiO2/SrTiO3 heterointerfaces. To realize 2DEG in a SrTiO3 crystal, field effect transistor (FET) would be another good way because applying gate voltage accumulates high density conduction electrons within an extremely thin layer at the gate insulator/SrTiO3 heterointerface. Here we report unusually large |S| observed in a FET fabricated on SrTiO3 single crystal surface. (001)-face of SrTiO3 single crystal plates with stepped & terraced surface were used as active channel. Amorphous LaAlO3 gate insulating film was deposited on the SrTiO3 surface at room temperature through the stainless steel mask. Ti metal films were also deposited as source, drain and gate electrodes. |S| values of the channel were measured by conventional steady state method using two thermocouples (K-type). Temperature difference up to 2 K was introduced between the source and the drain electrodes by using two Peltier devices. The σxx values increased almost proportionally to applied electric field (E) and saturated at 150 μS. Hall effect measurements revealed that the sheet carrier concentration (nxx) and Hall mobility (μHall) values were 4×1014 cm-2 and 2 cm2V-1s-1, respectively. Although |S| values decreased proportionally to log E when σxx < 150 μS, it dramatically increased with E (σxx >150 μS). These results indicate that the electron carriers under a high E become two-dimensional conductance in the channel whose thickness is thinner than the thermal de Broglie wavelength (~6 nm), resulting in a giant |S| due to the quantum size effect.
 L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).
 H. Ohta et al., Nature Mater. 6, 129 (2007).
 K. Ueno et al., Nature Mater. doi:10.1038/nmat2298
12:00 PM - N9.7
New Approach to Improve Figure of Merit in Oxide Thermoelectrics.
Venkat Selvamanickam 1 , Bo Zhang 1 Show Abstract
1 , University of Houston, Houston, Texas, United States
Oxide themoelectrics are well suited for high-temperature operation since their figure of merit increases even at temperatures of about 1000 K. Also, oxides exhibit excellent stability and oxidation resistance at high temperatures. Furthermore, thermoelectric oxides do not include Pb, Te, Se etc and can be processed in air instead of sealed quartz tube conditions. However, oxide thermoelectrics have not been useful so far because of their relatively lower values of figure of merit. The challenge stems from conflicting performance requirements in thermoelectric materials in general, to achieve high electrical conductivity, but low thermal conductivity while maintaining a high Seebeck coefficient. Seebeck coefficient values of 200 μV/k has been demonstrated in both p-type and n-type oxides at 800 K, which compare very favorably with those achieved with other thermoelectric materials. High electrical conductivity has also been achieved in oxides, but mainly along the basal plane of single crystals. However, the thermal conductivity of single crystals has not been reduced due to the lack of phonon scattering defects such as grain boundaries, second-phase precipitates, and dislocations. On the other hand, while polycrystalline granular oxides present opportunities to introduce defect structures, their electrical conductivity has been lower because of random grain orientation. In order to address this problem, we are developing novel approaches to achieve a microstructure with an excellent grain alignment of anisotropic thermoelectric oxides along with fine-scale defects such as precipitates, dislocations, and stacking faults to meet the conflicting performance requirements in thermoelectrics. Techniques that have been well proven in oxide superconductors to achieve excellent large-area grain alignment of the high electrical conductivity basal planes are being adopted to oxide thermoelectrics. These techniques simultaneously enable introduction of fine-scale precipitates, dislocations and stacking faults in the grain-aligned structure that can lower thermal conductivity through phonon scattering. Since the basal plane of the anisotropic oxides is the fast growth plane, large areas of the grain-aligned oxides with fine-scale defects can be synthesized. We will report on the process-microstructure-property relationships of the thermoelectric oxides developed by this new approach.
12:15 PM - N9.8
Transport Properties in Metal Doped In2O3 Thermoelectrics.
Emmanuel Guilmeau 1 , David Berardan 1 , Charles Simon 1 , Antoine Maignan 1 , Bernard Raveau 1 Show Abstract
1 , CRISMAT laboratory, Caen France
Indium oxide is a wide band gap semiconductor which has attracted considerable attention due to its great potential for gas sensing, optical transparency and optoelectronics. Numerous studies have been carried out on thin films of this material as a transparent conductor (TCO). They have shown that the doping with tin increases dramatically the electrical conductivity, as illustrated by the well know Indium Tin Oxide (ITO). More recently, the doping of In2O3 with titanium, zirconium, molybdenium or tungsten has shown the possibility to reach exceptionally high carrier mobility with carrier density higher than 1020 cm-3 on thin films. Very few attempts have been made to study the transport properties of bulk ceramics of In2O3 in view of other applications. Recently, we showed that In2O3 bulk ceramics doped with germanium exhibit a great potential as n-type elements for thermoelectric generators for the direct conversion of waste heat into electricity in air at high temperature (ZT=0.45 at 1273K). One observes that the conductivity of In2O3 is increased by one order of magnitude by Ge doping (0.5 at%) and changes from a semiconducting to a semi-metallic behavior, whereas a high n-type thermopower value is obtained at high temperatures. In order to optimize the thermoelectric properties of such bulk ceramics, it is necessary to study in detail the influence of the nature and concentration of the doping element upon the transport properties of this material. We report in the present study on the substitution of In3+ by selected tetra- and pentavalent dopants. Based on accurate Hall effect measurements to determine the carrier concentration and mobility and XRD analyses of samples, the structural and electrical properties of M4+ and M5+ doped In2O3 bulk specimens are described and the influences of the doping metals on the transport properties (within and beyond their solubility limit in the bixbyite structure) are discussed.