George S. Nolas, University of South Florida
Yuri Grin, Max-Planck Institute for Chemical Physics of Solids
Alan Thompson, "Marlow Industries, Inc."
David Johnson, University of Oregon
Symposium Support FCT Systeme GmbH
Fuji Electronic Industrial Co., Ltd.
GE Global Research
General Motors Corp.
Marlow Industries, Inc., Subsidiary of II-VI Incorporated
Sigma-Aldrich Co. LLC
Thermal Technology, LLC
B2: Skutterudites/Device Development
Monday PM, November 26, 2012
Hynes, Level 3, Room 302
2:30 AM - B2.01
Filled n-type IrSb3 Skutterudite
Daniel King 1 2 Thierry Caillat 2 Richard B. Kaner 1 Sabah K. Bux 2 Jean-Pierre Fleurial 2
1UCLA Los Angeles USA2Jet Propulsion Laboratory Pasadena USAShow Abstract
Filling vacancies in the skutterudite structure with rare earth atoms has been used extensively in cobalt and iron based skutterudites. State of the art filled n-type CoSb3 skutterudite has a ZT approximately 30% greater than unfilled, substitutionally doped CoSb3 at equivalent carrier concentration. IrSb3 is a potentially attractive thermoelectric material because it is more refractory than cobalt-based skutterudites and can therefore take advantage of higher operating temperatures, as well as fill some of the gap in performance in segmented device configurations between state of the art filled iron- and cobalt-based skutterudite antimonides and higher temperature thermoelectric materials such as Yb14MnSb11 and La3-xTe4. In the past, it has been found quite challenging to prepare highly doped n-type IrSb3 compositions by doping with impurities such as Pd and Pt. Peak ZT values obtained at elevated temperatures only ranged from 0.1 to 0.15 at best. In contrast, alkaline and rare earth filling of IrSb3 skutterudite has produced greater carrier concentrations and carrier mobilities than ever achieved through substitutional doping alone. As a result, a large increase in ZT values is reported here, with a peak of nearly 0.9 at 1000 K.
2:45 AM - *B2.02
Electron and Phonon Transport in n- and p-type Skutterudites
Jihui Yang 1 Shanyu Wang 1 Jiong Yang 1
1Univ. of Washington Seattle USAShow Abstract
Filled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications in the intermediate temperatures, due to their superior TE and thermomechanical performance as compared to other materials [1-2]. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K . TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. The underlying reasons for this are related to the skutterudites electronic band structures, which results in higher thermal conductivity for the p-type at elevated temperatures due to bipolar lattice thermal conduction; and lower power factor because of the heavy valence bands unlike the conduction bands with beneficial 3-fold degeneracy. In this talk, I will review our recent theoretical and experimental effort on modifying the valence bands of p-type skutterudites and highlight means of improving their TE properties. 1. X. Shi, Jiong Yang, J. R. Salvador, M. Chi, J. Cho, H. Wang, S. Bai, J. Yang, W. Zhang, and L. Chen, “Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports”, J. Am. Chem. Soc. 133, 7837 (2011). 2. J. R. Salvador, J. Yang, A. A. Wereszczak, H. Wang, and J. Y. Chi, “Temperature Dependent Tensile Fracture Stress of n- and p-Type Filled-Skutterudite Materials”, Sci. Adv. Mater. 3, 1 (2011).
3:15 AM - B2.03
Studies on P-type Filled Skutterudites
Qing Jie 1 Xiao Yan 1 Hui Wang 1 Zhifeng Ren 1 Gang Chen 2
1Boston College Chestnut Hill USA2Massachusetts Institute of Technology Cambridge USAShow Abstract
Filled skutterudite materials are very promising for mid-temperature thermoelectric power generation and waste heat recovery, because of their good thermoelectric and mechanical properties. Traditional preparation method needs a very long time annealing (usually 7 to 14 days) to form the right skutterudite phase. The annealing is especially critical for p-type filled skutterudites, since Fe4Sb12 need filler atoms to enter the cage to form a stable phase. In this work, we prepared Ce/Nd double filled p-type skutterudite material by directly ball-milling alloyed and quenched ingot. The results show that, by breaking the ingot into nano-sized particles, ball-milling greatly reduced the distance which filler atoms need to travel and hence accelerated the phase formation. With appropriate ball-milling and handling, pure p-type filled skutterudite phase can be obtained by hot-pressing the ball-milled powder for just 5 minutes, although the powder is still a mixture of FeSb2 and Sb phases. The samples prepared by this way have the same high quality as the samples prepared by the traditional way, and show a peak ZT value above 1 at 750 K. This method greatly saves the processing time and is suitable for large scale industrial production. This research was supported by Bosch and the Solid State Solar-Thermal Energy Conversion Center (S3TEC), and Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001299/DE-FG02-09ER46577 (GC and ZFR).
4:00 AM - B2.04
Concentrating Solar Thermoelectric Generators
Kenneth McEnaney 1 Daniel Kraemer 1 Qing Jie 2 Tulashi Dahal 2 Zhifeng Ren 2 Gang Chen 1
1Massachusetts Institute of Technology Cambridge USA2Boston College Chestnut Hill USAShow Abstract
A model, test setup, and preliminary test results are presented for concentrating solar thermoelectric generators. In these devices, sunlight is concentrated onto an absorbing surface connected to a thermoelectric generator attached to a heat sink. Our previous experiment demonstrated 4.6% solar-to-electricity conversion efficiency without any optical concentration. Optically concentrated solar radiation can create high absorber temperatures with higher conversion efficiency. An efficiency exceeding 10% is possible with current materials. Progress toward achieving this efficiency will be presented.
4:15 AM - *B2.05
Thermoelectric Power Generation over Wide Temperature Range
Ryoji Funahashi 1 2
1National Institute of Advanced Industrial Science amp; Technology Ikeda Japan2Japan Science and Technology Agency Chiyoda-ku JapanShow Abstract
The demand for primary energy in the world was 12,013 million tons of oil per year in 2007. The average total thermal efficiency of the systems utilizing this fuel is as low as 30%, with about 70% of the heat exhausted to the air as waste heat. It is clear that improving the efficiencies of these systems could have a significant impact on energy consumption. Electricity is a convenient form of energy that is easily transported and stored; thus there are a number of advantages to converting the waste heat emitted from our living and industrial activities to electricity. Thermoelectric conversion is attracting attention because it is the strongest candidate to generate electricity from dilute waste heat sources. Oxide and silicide thermoelectric materials are considered to be promising ones because of their durability against high temperature, cost, no content of toxic elements, and so on. Many types of modules using p-type Ca3Co4O9 and n-type CaMnO3 have been produced. They show good generation power density higher than 4.0 kW/m2. An n-type Mn3Si4Al2 discovered recently shows good oxidation resistance at 973 K in air. Thermoelectric modules have been produced with p-type MnSi1.7. The silicide module can generate 9.4 W, which corresponds to 2.3 kW/m2 at 873 K of the hot side temperature.
4:45 AM - B2.06
Practical Electrical Contact Resistance Measurement Method for Bulk Thermoelectric Devices
Rahul Gupta 1 Robin McCarty 1 Jim Bierschenk 1 Jeff Sharp 1 Tao Zheng 2 Bruce Gnade 2
1Marlow Industries Dallas USA2University of Texas at Dallas Richardson USAShow Abstract
As thermoelectric (TE) element length decreases, the impact of contact resistance on TE device performance grows more significant. In fact for a TE device containing 100 µm tall TE elements, the figure of merit ratio (ZTDevice/ZTMaterial) drops from 0.9 to 0.5 as the contact resistivity increases from 5 x 10-07 to 5 x 10-06 Omega;-cm2. To improve and understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance such as Transmission Line Measurements (TLM) and Kelvin Cross Bridge Resistor method (KCBR), but they are only well-suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk thermoelectric material. The authors present a new measurement technique that precisely measures contact resistance (on the order of 5 x 10-07 Omega;-cm2) on bulk thermoelectric materials by processing stacks of bulk, metal-coated TE wafers using TE industry standard processes. One advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure, therefore representing a realistic value for electrical contact resistance in a bulk TE device. Contact resistance values for different manufacturing processes and an estimate of the accuracy of the measurements will be presented.
5:00 AM - B2.07
High Reliability, High Temperature Thermoelectric Power Generation Materials and Technologies
Jean-Pierre Fleurial 1 Jong-Ah Paik 1 Thierry Caillat 1
1Jet Propulsion Laboratory Pasadena USAShow Abstract
Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions (67 missions to date, more than 30 years of life) as well as terrestrial applications where unattended operation in remote locations is required. The successful application of high temperature thermoelectric technology has relied on only a few materials, PbTe, GeTe-AgSbTe2 (TAGS) alloys and Si-Ge alloys, identified in the late 1950s and 1960s. We provide a brief overview of key technologies and system approaches that have enabled such an excellent track record, and reflect on some key lessons learned as we consider infusing new higher performance materials into next generation systems. NASA&’s Radioisotope Power Systems Technology Advancement Program is pursuing the development of more efficient thermoelectric technologies that can increase performance by a factor of 2 to 4X over state-of-practice systems. After several years of maturing new high temperature thermoelectric materials, JPL has started developing segmented couple and module technology using high temperature n-type La3-xTe4 and p-type Yb14MnSb11 Zintl materials combined with lower temperature filled skutterudite compounds. Recent performance tests have demonstrated 11 to 15% conversion efficiencies in the 1273 - 473 K temperature range. The stability of the thermoelectric properties of these new materials have been tested successfully for over 13,000 hours under normal and accelerated operating conditions. Single couple performance tests have been conducted for nearly 9000 hours and technical challenges pertaining to the development and system integration of highly reliable high temperature segmented devices are summarized. The potential for application of this promising new technology to next generation space systems is discussed.
5:15 AM - B2.08
Lossless Coupling of Serial-hybrid-connection between Photovoltaic and Thermoelectric Devices
Kwang-Tae Park 1 Sun-Mi Shin 2 Han-Don Um 1 Jin-Young Jung 2 Sang-Won Jee 2 Min-Wook Oh 3 Bongyoung Yoo 2 Jung-Ho Lee 1 2
1Hanyang University Ansan Republic of Korea2Hanyang University Ansan Republic of Korea3Korea Electrotechnology Research Institute (KERI) Changwon Republic of KoreaShow Abstract
Optimally hybridizing photovoltaic (PV) and thermoelectric (TE) has long been quested as an ideal candidate for more efficiently harnessing solar energy. This focuses mostly on partitioning of the solar spectrum into ultraviolet/visible region for PV and infrared region for TE; especially for PV/TE stack approaches, the TE part aims mainly for converting the transmission and thermalization losses derived from a sun-sided PV device. Cost-ineffectiveness to their conversion performances, however, has been a main obstacle which is still far short of competing with not only fossil fuels but also the sole devices of highly efficient PVs. Here we focus on the Seebeck effect, which is the conversion of temperature differences directly into electricity, depending upon the resistance matching in a serially-connected hybrid circuit of PV and TE devices. We demonstrate that a linear power output of hybrid current-voltage operations can be equal to a nonlinear sum of maximum powers produced separately from each PV and TE circuits when the temperature difference (ΔT) between hot and cold sides of a TE device reaches a threshold value, typically 10~20 °C. This feature is based upon the lossless coupling between open circuit voltages of a serial PV-TE circuit in which the internal TE resistance is low enough to convey the amount of photogenerated current without serious degradation of fill factors in the resistance-matched hybrid operation. At ΔT=16 °C, we could obtain the conversion efficiency of 16.3% in serial operation of the PV/TE stack showing the PV conversion efficiency of 12.5%. This result is, in particular, effective for a position-separated, remote serial operation in which PV and TE parts can share the one set of a control unit, dc-dc converter, charging battery pack, and dc-ac inverter for further utilizing exhaust gas in PV-integrated electrical vehicles. The hybrid generation system consists of crystalline Si PV and bismuth telluride (Bi2Te3) TE, which were placed in tandem and electrically connected in series. To confirm the lossless power coupling in the hybrid circuit, three kinds of TE modules were used; 1) TE87 represents the internal resistance of TE (Ri) of 8.7 Omega;cm2 with the number of thermocouple pairs (N) of 127; 2) TE38 represents the Ri of 3.8 Omega;cm2 with N of 31; TE300 is for Ri=30 Omega;cm2 with N=127. The I-V characteristics were independently recorded for PV, TE, and hybrid circuits to compare the integration results while understanding the operating status of PV and TE parts in the hybrid circuit. To gain further insight into the behavior of the hybrid circuit, output powers were calculated for individual circuit and hybrid circuit. Our theoretical and experimental results suggest that the resistance matching is of significant importance for optimal operation in a hybrid circuit in which the fill factor of a hybrid circuit and a voltage gain are controllable using ΔT and TE variables such as Ri, N, and Seebeck coefficient, S.
5:30 AM - *B2.09
Automotive Thermoelectric Generator and HVAC Development
John Warren Fairbanks 1
1US Dept of Energy Washington USAShow Abstract
The US Department of Energy initiated the application of thermoelectric generators (TEG's) to vehicles in 1994. This TEG was built by Hi-Z Technologies evaluated on a dynamometer test stand. It was then installed on a fully loaded Heavy Duty Diesel truck on the PACCAR test track and run for the equivalence of 550,000 miles. Today every major automobile manufacturer is investigating thermoelectric applications. The US Department of Energy is supporting the development of production prototype TEG's with teams headed by BSST and GM to integrate TEG's to directly convert engine waste heat directly to electricity in the BMW X6, the Ford's Lincoln MKT and the Chevy Suburban. These TEG's will provide a nominal 5 percent improvement in on-highway fuel economy by allowing the alternator to be downsized by at least 1/3. DOE/NETL conducted a completive procurement for automotive thermoelectric air conditioners/heaters (TE HVAC) development and selected teams headed by Ford and GM to develop this technology. Current air conditioners use the R134a refrigerant gas, which has 1300 times the "Greenhouse Gas Effect" as carbon dioxide (CO2)., the primary "Greenhouse Gas". Approximately 41 Million Metric tons of CO2 equivalent (CO2e) are released to the atmosphere in the US annually from air conditioner compressor seal leakage and frontal collisions wherein the the R134a refrigerant gas containment was ruptured. The TE HVAC's are candidates to eliminate refrigerant gases from vehicles. A problem with maintaining occupant comfort in an electrically assisted vehicle was illustrated by Bob Lutz, Vice Chairman, General Motors , who drove a Chevy Volt in January in Detroit and to obtain occupant comfort had to turn on the 5 kW resistive heater which reduced the battery only propulsion mileage from 40 to 28. Preliminary analysis indicates that with TE HVAC a single occupant can be made comfortable using about 630 Watts. Whereas current compressed refrigerant gas air conditioners typically use 3500 to 4,500 Watts. The TE HVAC uses design advantages afforded by Thermoelectrics as a dispersed or zonal system wherein only the occupants are cooled/heated, not the whole cabin. AsTE HVAC is a DC electrical system it only requires a switch to go from the cooling mode to heating. The Zonal System will consist of a thermoelectric seat, thermoelectric units in the overhead and dashboard, A&B pillars focused on each occupant . There will be a cooling loop with either a dedicated radiator or the engine's radiator. TE HVAC is vehicle specific. In this program they are the Cadillac SRX, the Chevy Volt and the Ford Fusion. The latter 2 will also have TEG's. The Department of Energy has initiated a jointly funded program with the National Science Foundation (NSF) to fund university and industrial teams to develop advanced commercially viable Thermoelectrics for 2nd generation automotive thermoelectric applications. Awards were made to 9 of the 48 universities who, with their industrial partners, responded to the DOE/NSF announcement. In 2011 the Department of Energy with it's partner, the Army's TARDEC conducted a competitive procurement to accelerate scale up and manufacture of advanced automotive TEG&’s. The Teams selected and their approaches will be presented.in Boston.
B1: Theoretical Development Materials and Devices/Chalcogenides
Monday AM, November 26, 2012
Hynes, Level 3, Room 302
9:15 AM -
9:30 AM - *B1.01
Transport in Thermoelectric Materials
David Joseph Singh 1 David Parker 1
1Oak Ridge National Laboratory Oak Ridge USAShow Abstract
There is increasing interest in thermoelectric materials motivated in part by recent progress and in part by the potential of these materials in various energy technologies. Thermoelectric performance is a multiply contra-indicated property of matter. For example, it requires (1) high thermopower and high electrical conductivity, (2) high electrical conductivity and low thermal conductivity and (3) low thermal conductivity and high melting point. The keys to progress are finding an optimal balance and finding ways of using complex electronic and phononic structures to avoid the counter-indications mentioned above. In this talk, I discuss some of the issues involved in the context of recent results. These include the surprising doping dependence of the thermopower in PbTe and PbSe, and the interplay between acoustic and optical phonons in PbTe. The potential of some new materials is discussed. This work was supported by the Department of Energy through the Office of Science S3TEC Energy Frontier Research Center and EERE Vehicle Technologies, Propulsion Materials Program.
10:00 AM - B1.02
Thermoelectricity in Bismuth - Magnetic Fields, Nanostructures, Valleytronics, and Carrier Filtering
Lilia M Woods 1 Adrian Popescu 2 3 George S Nolas 1
1University of South Florida Tampa USA2Center for Nanoscale Science and Technology, National Institute of Standards and Technology Gaithersburg USA3Maryland NanoCenter, University of Maryland College Park USAShow Abstract
The thermoelectric transport in bismuth is examined. Although this material has been studied for many years, bismuth is still a great source of new discoveries. The anisotropic transport, Dirac nature of its electronic structure, and the presence of two types of carriers enable us to find some surprising results related to bismuth thermoelectricity. The intricate relationship between the characteristics of the charge and heat transport together with the role of imbedded nanostructures and/or applied an external magnetic field allow us to provide theoretical guidelines for practical advantages and limitations of thermoelectric enhancement in this material. We also demonstrate that an imbalance in the population of the bismuth energy bandstructure can be achieved via a rotating magnetic field in the binary-bisectrix plane. This results in selective carrier filtering and excitations of particular Dirac valleys with specific signatures not only in the charge, but also in the heat transport. These findings suggest novel opportunities for thermoelectricity tuning in a rather wide temperature regime.
10:15 AM - B1.03
Universal Scaling Relations for the Thermoelectric Power Factor of Semiconducting Nanostructures
Oded Rabin 1 Jane E. Cornett 1
1University of Maryland College Park USAShow Abstract
Computational models for the transport properties of nanostructured thermoelectric materials predicted vast improvements in the thermoelectric power factor (PF) values over bulk due to discretization of the electron density-of-states function as the result of confinement. We have developed a model that bridges bulk and nanostructure PF data. The model is analyzed in the framework of the relaxation time approximation, considering different scattering mechanisms. The model shows that the PF of nanowires and thin films in fact falls below the bulk value for most of the experimentally-accessible size range. Under the constant relaxation time approximation, universal scaling relations are obtained for all single-carrier semiconductors. An improvement over bulk is only seen for film thicknesses and nanowire widths associated with quantization energies exceeding ~10 times the thermal energy. The power factor increases with size for most of the size-range investigated. With the consideration of specific scattering mechanisms with energy-dependent scattering times, the size-dependence of the PF of thin films and nanowires becomes material-specific. However, we find that the non-monotonic relationship between the thermoelectric PF and the system size is recurring in all systems studied. The effects of size, dimensionality, temperature, doping, impurity scattering, and phonon scattering in single-carrier semiconductors will be discussed.
10:30 AM - *B1.04
Design of High Performance, Robust and Low Cost Thermoelectric Modules
Ali Shakouri 1 Kazuaki Yazawa 1 Ephraim Suhir 2 Amirkoushyar Ziabari 1
1Purdue University West Lafayette USA2University of California Santa Cruz USAShow Abstract
In this presentation we review recent advances in thermoelectric modules and power generation systems [1, 2]. During the last decade, nanostructured thermoelectric materials have shown improvements in thermoelectric figure-of-merit, ZT. Values from 1.5 to 1.8 have been demonstrated in several systems [1, 2]. Nevertheless, thermoelectric modules with significant improvement in maximum cooling, coefficient of performance, or efficiency, have not been commercialized. In this presentation we describe the efficiency/cost tradeoff in thermoelectric power generation systems . Co-optimization of the thermoelectric module with the heat sink leads to a reduction in the amount of material used in the system and reduces the overall payback time. On the other hand, thermal stresses play a big role in terrestrial thermoelectric systems that are subjected to large temperature gradients and repeated thermal cycles. A new analytic theory is described where the shearing stress can be estimated as a function of the module parameters . We show that there is a significant opportunity to reduce the stress and increase the thermoelectric module lifetime with fractional area coverage of thermoelectric elements in the module.  C.J. Vineis, A. Shakouri, A. Majumdar, M.G. Kanatzidis, "Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features", Advanced Materials, Vol. 22, pp. 3970-3980, 2010.  A. Shakouri, “Recent developments in semiconductor thermoelectric physics and materials”, Annual Review of Materials Research, July 2011.  K. Yazawa and A. Shakouri, "Optimizing Cost-efficiency Trade-offs in the Design of Thermoelectric Power Generators", Environmental Science and Technology, July 2011.  E. Suhir and A. Shakouri, “Assembly bonded at the ends: Could thinner and longer legs result in a lower thermal stress in a thermoelectric module design?”, Journal of Applied Mechanics, to appear in 2012
11:30 AM - B1.05
Thermoelectric Research Activities in Samsung Electronics: Thermoelectric Efficiency Enhancement in BiSbTe Bulk Alloys by Doping and Grain Boundary Nanostructuring by Commercially Viable Technologies
Sang Il Kim 1 Sungwoo Hwang 1 Kyunghan Ahn 1 Jongwook Roh 1 Daejin Yang 1 Kyu Hyoung Lee 1
1Samsung Advanced Institute of Technology, Samsung Electronics Yongin Republic of KoreaShow Abstract
There are numerous potential commercial applications for bismuth antimony telluride BiSbTe thermoelectric bulk alloys for cooling, heating, or power generating application near room temperature, preferably if the dimensionless thermoelectric figure of merit ZT reaches to around 2. In this talk, we summarize our effort in Samsung to reach this goal by commercially-viable and scalable techniques. The ZT value of BiSbTe alloys was enhanced either by enhancing power factor through slight doping/substitution or by reducing thermal conductivity through nanostructuring. The power factor was enhanced by slight doping/substitution by In/Ga. The notable ZT improvement has been achieved by significant reduction of lattice thermal conductivity by forming nanostructured grain boundaries with high-density periodic dislocations. By these approaches combined together, a relatively large amount of BiSbTe alloys was produced with high ZT value higher than 1.50 at around room temperature in lab scale. The main advantage is that these approaches only involves with commercially scalable processes. The results on maximum heat pumping capacity Qc,max measurement of cooling modules made from these materials will be discussed. High-ZT BiSbTe bulk alloy is now commercially-viable technologically, and wider commercialization of thermoelectric BiSbTe materials is closer.
11:45 AM - B1.06
Thermoelectric Properties of Nanostructured p-type PbSe-MSe Systems (M = Mg, Ca, Sr, Ba)
Yeseul Lee 1 John Androulakis 1 Chun-I Wu 2 Duck-Young Chung 3 Tim Hogan 2 Mercouri Kanatzidis 1 3
1Northwestern University Evanston USA2Michigan State University East Lansing USA3Argonne National Laboratory Argonne USAShow Abstract
The binary narrow band gap semiconductor PbSe combines several attractive features for potential thermoelectric applications such as a favorable electronic valence band structure that is comprised of two sub-bands with very different effective masses similarly to that of PbTe. The addition of a few percent of alkaline earth selenide into p-type PbSe generates the second phases that produce a reduction of the lattice thermal conductivity compared to pristine PbSe, but without appreciably affecting the power factors. Here we present a systematic study with respect to characterization and thermoelectric effects of p-type PbSe-MSe systems with PECS samples. The electrical conductivity, thermoelectric power, and thermal conductivity as a function of temperature and carrier density were measured. Our data indicate that the addition of alkaline earth selenides leads to an increase in ZT values (~1.2) compared to those of pristine PbSe (~0.95) around 900 K.
12:00 PM - B1.07
Melt Spun (SnTe)1-x-(SnSe)x and the Observed Enhancement in Power Factor
Li Ping Tan 1 Ady Suwardi 1 Shufen Fan 1 Ting Sun 1 Raju V. Ramanujan 1 Huey Hoon Hng 1 2
1Nanyang Technological University Singapore Singapore2Nanyang Technological University Singapore SingaporeShow Abstract
Energy is an indispensable part of our daily lives, and about 60% of the energy we produce is lost as waste heat. Thus, thermoelectric (TE) materials are of particular interest as they can tap on these waste heat and convert them directly into electricity. Recent developments in TE materials have been focused on multiphase TE materials, due to the ability to concurrently decrease thermal conductivity and tune electrical properties, resulting in improved TE properties. In this work, melt spinning was employed to produce various compositions of (SnTe)1-x-(SnSe)x (10 < x < 50), which to the best of the authors&’ knowledge have not been reported by other groups before. Due to the metastable nature of the samples formed via this processing technique, a second phase was found to precipitate simply by mechanical grinding and then compaction into a pellet, forming a multiphase composite. Experiments suggested that the mass fraction of the second phase can be controlled by the grinding process, and preliminary data showed that a power factor of about 1 mW/m.K^2 can be obtained at 546 K for one of the compositions, which is a 1.2 times improvement in the power factor compared to pure SnTe phase, and about 65 times improvement compared to pure SnSe phase. The large difference in improvement is due to the high Seebeck coefficient but low electrical conductivity of SnSe and high electrical conductivity but low Seebeck coefficient of SnTe, resulting in low and average power factor for SnSe and SnTe respectively. By combining these two phases, a synergistic effect can be obtained, indicating the importance of the contribution of the second phase in enhancing the power factor. Additional characterization, e.g., X-ray diffraction and electron microscopy, were performed to correlate microstructure and phases present with the TE properties obtained. The ease of fabrication of such multiphase materials is attractive for large scale production.
12:15 PM - B1.08
Surface State Effects on Thermoelectric Transport in Bismuth Telluride Nanoplates
Michael Thompson Pettes 1 Li Shi 1 2
1The University of Texas at Austin Austin USA2The University of Texas at Austin Austin USAShow Abstract
Single-crystal nanoplates of bismuth telluride synthesized by the vapor solid method have been assembled directly onto a suspended device consisting of two adjacent low-stress silicon nitride membranes, each with a platinum resistance thermometer and two platinum electrodes and suspended by six silicon nitride beams. The device allows for accurate measurement of themoelectrical (Seebeck), electrical, and thermal transport in the nanostructures and quantification of the electrical and thermal contact resistances in the temperature range of 4-500 K and 60-500 K, respectively. Measurements of the thermoelectric properties, particularly the dimensionless figure of merit (ZT), of four Bi2Te3 nanoplates and one (Bi1-xSbx)2Te3 nanoplate, x~0.1, have been conducted, with thicknesses between 7.6-19.6 nm. All five samples show n-type Seebeck coefficient and ZT < 0.25 due to low Seebeck coefficients in these “as-grown” nanoplates. Moreover, the Seebeck coefficient appears to decrease with decreasing thickness, which can be attributed to two degenerately n-type surfaces that have not yet hybridized to form the band gap suggested in theoretical reports.
12:30 PM - B1.09
Atomic-scale Investigations of Grain Boundary Defect Structure in Bismuth Telluride
Douglas Lloyd Medlin 1 Q. M. Ramasse 2 C. D. Spataru 1 N. Y. Yang 1
1Sandia National Laboratories Livermore USA2STFC Daresbury Daresbury United KingdomShow Abstract
Extended crystallographic defects, such as dislocations, grain boundaries, and stacking faults, can strongly affect the thermal and electronic transport properties of thermoelectric materials. At present, however, our understanding of the structural details of such defects in thermoelectrics is in its infancy, even for such widely used materials as bismuth telluride. In this presentation, we will discuss our experimental analyses of grain boundary defect arrangements in bismuth telluride. We start with a discussion of low-angle grain boundaries in bismuth telluride. In general, such interfaces can be thought of as ordered arrays of individual crystal lattice dislocations. Our HAADF-STEM observations of a <1,0,-1,0> tilt boundary help to clarify the topological details of the relevant defects in bismuth telluride, including their Burgers vectors and relationship to intergranular misorientation. In contrast to low-angle grain boundaries, in many materials, the structure of high angle boundaries (e.g., with misorientations greater than ~15°) is better considered with reference to specific interfaces arising at crystal orientations and interface inclinations with low energy, singular structures. Thus, as a starting point, we consider the (0001) basal twin in bismuth telluride and show how defects in interfaces vicinal to this relatively simple and low energy interface can be interpreted in terms of interfacial disconnections (i.e. step defects possessing dislocation content). For instance, our ab initio, density functional theory calculations indicate a strong energetic preference for terminating (0001) twins at the Te(1)-Te(1) sites of the crystal structure, a result that is consistent with our HAADF-STEM observations. This energetic preference also imposes strong constraints on the possible disconnection arrangements. Our observations of more complex high angle boundaries also identify faceted step formation on tellurium-terminated planes, suggesting such analyses can be extended more generally in this and related layered tetradymite-type materials.
12:45 PM - B1.10
Dopants, Solubility, and Vibrations in PbS-PbTe Alloys
Jeffrey W Doak 1 Vidvuds Ozolins 2 Chris Wolverton 1
1Northwestern University Evanston USA2University of California Los Angeles USAShow Abstract
The creation of nanoscale precipitates via phase separation provides a mechanism for decreasing the lattice thermal conductivity and increasing the figure of merit of some bulk thermoelectric materials, such as PbS-PbTe. It has recently been found that the addition of Na to PbS-PbTe drastically alters the morphology of precipitates in the system. To better understand the phase tranformations giving rise to precipitates in this system, and in particular, the effects of Na doping on these precipitates, we use first-principles density functional theory (DFT) calculations to study the thermodynamics of mixing in PbS-PbTe-Na. We model the PbS-PbTe solid solution with special quasirandom structures (SQS), and treat the Na solubility in PbS and PbTe as dilute. To calculate the dilute-limit solubility of Na in PbS and PbTe, we calculate a large variety of Na, Pb, S, and Te defects in PbS and PbTe, from which we find that Na prefers to substitute for Pb in both PbS and PbTe. Due to the large lattice mismatch between PbS and PbTe, and the anharmonic nature of phonons in PbTe, vibrational entropy is expected to play a large role in the free energy of mixing in PbS-PbTe. As such, we directly obtain the vibrational entropy of mixing for PbS-PbTe through frozen phonon calculations of the SQS&’s. With the free energies of mixing in PbS-PbTe-Na, we calculate the miscibility gap between PbS and PbTe, as well as the solubility limits of Na in PbS and PbTe. We find that vibrational effects play a very important role in the thermodynamics of PbS-PbTe, bringing the calculated miscibility gap into good agreement with experiment. We also find that the solubility of Na in PbTe and PbS depend on the off-stoichiometry of the parent compound and the manner in which Na is added to the system (e.g. doping with Na metal vs. Na2Te).