Symposium Organizers
Kornelius Nielsch University of Hamburg
SaskiaF. Fischer Ruhr-Univ. of Bochum
BethanieJ. H. Stadler University of Minnesota
Ted Kamins Stanford University
I1: Silicon-based TE Nanostructures
Session Chairs
Tuesday PM, April 26, 2011
Room 2007 (Moscone West)
9:30 AM - I1.1
Ordered Si-Ge Nanostructures by Glancing Angle Deposition via Ion Beam Sputtering.
Jens Bauer 1 , Michael Weise 1 , Chinmay Khare 1 , Bernd Rauschenbach 1
1 , Leibniz Institute of Surface Science, Leipzig Germany
Show AbstractThe preparation of efficient thermoelectric nanomaterials is a big challenge in modern material science. A special focus is set on the defined adjustment of structure geometry, size and orientation. For this purpose we present PVD investigations on self-organized a-Si and a-Ge nanostructures by a recent nanofabrication technique, the glancing angle deposition (GLAD). In particular, a Si or Ge particle beam provided by ion-beam sputtering is directed under an extremely flat angle towards the substrate surface (glancing angle < 5°). At low substrate temperatures and thus reduced surface diffusion a ballistic deposition process occurs. As a result of shadowing separated nanoneedles evolve competitively with a defined inclination towards the particle incidence direction. The additional use of substrate rotation allows the shaping of this nanostructure, i.e. with continuous substrate rotation vertical posts, screws or even spirals can be realized with respect to the rotation speed. With a direction-modulated particle beam the morphology can be further customized. Axial Si/Ge multi-heterojunctions can be realized by sequential deposition.To improve arrangement and uniformity of GLAD nanostructures patterned substrates with symmetrically ordered surface mounds were applied. The artificial definition of pre-patterns was performed by electron beam lithography with square and hcp order as well as nanosphere lithography with honeycomb or hcp order. Very uniform nanocolumns of different shape are obtained.In the temporal evolution of periodic GLAD nanostructures three morphological stages need to be distinguished: During the initial stage of deposition the shape and size of the pre-pattern mounds determine the GLAD structures’ shape. The structure thickness shows a successive broadening. Between the ordered nanostructures additionally disordered arrays of intermediate GLAD structures evolve. With decreasing glancing angle and decreasing pre-pattern periodicity this intermediate deposition decreases and the saturation shape is reached earlier. In this 2nd stage the intermediate deposition has terminated and the ordered nanostructures exhibit a columnar shape with parallel border areas reflecting the pattern symmetry: square cross-section for square order, three-fold for honeycomb, and hexagonal to round for hcp. The nanostructures’ tops show characteristic caps. Both the size of the border areas and the cap size are strongly influenced by the glancing angle and the pre-pattern periodicity. The mutual shadowing of the ordered columns is accompanied by increasing competition with time. As a result the formation of individual GLAD columns stops and the pre-pattern arrangement gets lost in the 3rd stage.As a general observation with patterned and unpatterned substrates we found that the nanostructure area density is pattern independent and depends on the glancing angle only.
9:45 AM - I1.2
Thermoelectric Properties of Si–Ge Superlattices.
Katrin Bertram 1 , Markus Trutschel 1 , Stefan Kretschmer 1 , Johannes de Boor 2 , Bodo Fuhrmann 1 , Alexander Tonkikh 2 , Peter Werner 2 , Hartmut Leipner 1
1 Center of Materials Science, Martin Luther University, Halle Germany, 2 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractThermoelectric bulk materials are widely used to change waste heat into electrical energy. The efficiency of such materials is determined by the figure of merit ZT. In addition to conventional materials like Bi2Te3 and Sb2Te3, which are suitable for room temperature applications, SiGe mixed crystals show a great potential for high temperature applications. Regarding theoretical predictions, nanostructured materials such as superlattices or nanowires, are able to show a higher figure of merit than bulk materials. An enhancement in the figure of merit of Si–Ge films by nanostructuring offers a new temperature range for a Si-based thin film technology. In this study, we have investigated thermoelectric and structural properties of Si–Ge superlattices. Sim–Gen superlattices with stacks of m Si and n Ge layers of various thicknesses were grown by molecular beam epitaxy on (001) or (111) Si substrates. The structure was characterized by cross-section transmission electron microscopy. The lithographic preparation schemes for the measurement of the cross plane transport properties in dependence of temperature, such as the Seebeck coefficient, the electrical and the thermal conductivity, are outlined and results are presented.
10:00 AM - I1.3
Nanocrystalline Silicon for Thermoelectricity.
Gabi Schierning 1 , Dominik Schwesig 1 , Ralf Theissmann 1 , Niklas Stein 1 , Nils Petermann 1 , Hartmut Wiggers 1 , Roland Schmechel 1 , Dietrich Wolf 1
1 Center for NanoIntegration (CeNIDE), University of Duisburg-Essen, Duisburg Germany
Show AbstractA main focus in thermoelectricity is on the development and utilization of materials with high availability and sustainability. From this point of view, nanocrystalline silicon is a potential candidate for thermoelectric application, motivating the effort of optimizing its thermoelectric properties. Our bottom up approach starts with tailored nanoparticles from a gas phase process. Compaction of the nanopowder is done by a dc-current sintering. While usually the final stage sintering is accompanied by rapid grain growth which spoils nanocristallinity, field activated sintering techniques overcome this problem. For electrically conducting nanoparticles, small grain sizes have been maintained in spite of consolidation.In a combined experimental and theoretical work we show how the developing microstructure during sintering correlates to the percolation paths of the current through the powder. To achieve nanocrystalline bulk silicon and a homogeneous microstructure, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. Observed density fluctuations on the micrometer scale are attributed to the specific heat profile of the simulated powder networks.Using silicon nanoparticles from our bottom-up approach via gas phase synthesis implies two further possible difficulties. These are the contamination of a material with a high surface–to–volume ratio, especially with oxygen, and the incorporation and electrical activation of the dopant atoms. Considering the whole process chain from precursor decomposition to sintered samples, it is technologically highly demanding to exclude any contamination of the nanopowder with oxygen and moisture. As a result, a Si/SiO2 core/shell structure is always observed for nanocrystalline silicon powders having been in contact with air. In the sintered body, the initial core shell structure is dissolved. Instead, oxidic precipitates have formed during sintering. The role of these oxidic precipitates with respect to the sintering mechanism and the thermoelectric transport properties will be discussed.
10:15 AM - I1.4
Thermoelectric Properties of Spark-plasma Sintered Nanocrystalline Silicon/Germanium Composites.
Niklas Stein 1 , Gabi Schierning 1 , Nils Petermann 1 , Hartmut Wiggers 1 , Ralf Theissmann 1 , Benedikt Stoib 2 , Roland Schmechel 1
1 Center for NanoIntegration (CeNIDE), University of Duisburg-Essen, Duisburg Germany, 2 Walter Schottky Institut, Technische Universitaet Muenchen, Munich Germany
Show AbstractSilicon/germanium alloys are known for their good thermoelectric performance at elevated temperatures. The alloy structure scatters the short wavelength phonons in the material which leads to an appreciable figure of merit. By introducing a nanostructure to the material, it is expected to generate a barrier for the mid to long wavelength phonons as well and therewith cause an increase of thermoelectric efficiency.Therefore, silicon/germanium alloy nanoparticles are synthesized in a gas phase process based on a microwave plasma reactor. By varying the microwave power, chamber pressure and concentration of the precursor gases silane (SiH4) and germane (GeH4) as well as the plasma gases Ar and H2, crystalline particles with a mean diameter of 10 to 100 nanometer can be obtained. N-type or p-type doping can be realized by adding phosphine (PH3) or diborane (B2H6) respectively to the precursor gases.In a second process step the nanoparticles are compacted in a spark-plasma sintering process. Several grams of the nanoparticles are pre-compacted and subsequently exposed to high current (~600A at 5V) and a pressure of 35MPa obtaining dense nanocrystalline bulk pellets. Thereby the sintering temperature is around 1000°C and the hold time at maximum temperature is 3 minutes.Structural information of raw powder and sintered pellets is gained by transmission electron microscopy and x-ray diffraction combined with Rietveld-analysis. The Seebeck-coefficient α and the electrical conductivity σ are measured using the direct measurement setup ZEM3 from ‘ULVAC technologies, Inc.’ while the thermal conductivity κ is obtained by a laser flash method from ‘NETZSCH’.With a thermal conductivity of 1.5 W/m/K at 500°C a promising ZT of 0.51 was found for the nanocrystalline Si/Ge composites.
10:30 AM - I1.5
Formation of Ge Quantum Dots in Silicon Oxide and Silicon Nitride Matrices and Associated Optical and Thermal Properties.
Yu-Jui Chang 1 , Jung-En Chang 1 , Chung-Yen Chien 1 , Jung-Chao Hsu 2 , Ming-Tsung Hung 2 , Sheng-Wei Lee 3 , Pei-Wen Li 1
1 Department of Electrical Engineering, National Central University, Jhongli Taiwan, 2 Mechanical Engineering, National Central University, Jongli Taiwan, 3 Material Science and Engineering, National Central University, Jongli Taiwan
Show Abstract Taking full advantage of quantum confinement effects and state-of-the-art Si complementary metal-oxide-semiconductor (CMOS) process infrastructure, silicon-based quantum-dot (QD) nanostructures have demonstrated their unprecedented versatility and never before seen potentials for advanced lighting, sensing, computing, memory, and information technologies, Consequently, it is a nature extension to employ Si-based QDs into photovoltaic (PV) and thermoelectric (TE) devices for boosting the efficiency of energy saving and even energy harvest or conversion technologies. Experimental works have demonstrated the effectiveness and the feasibility of QD TE devices. However, many of them are not practical for large-scaled commercial use because they are fabricated by either top-down patterning or bottom-up epitaxy techniques at the expense of high cost and low throughput. In this paper, we report a manageable growth method for placing dense 3D Ge QD arrays in a uniform or a grading size distribution, based on thermally oxidizing stacked poly-SiGe in a layer-cake technique. The QD size and spatial density in each stack are able to be modulated by conditions of the Ge content, oxidation, and the underlay buffer layer. In addition, we present results of a comprehensive study on the influence of oxidation time and host ambiance on growth kinetics of Ge QDs. The structural properties of Ge QDs in oxide and nitride matrices, such as size, shape, density, and chemical composition were examined using high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy. The optical and thermal conductivity properties of Ge QDs embedded in both oxide and nitride were evaluated in terms of photoluminescence (PL) spectroscopy and thermal resistance measurement using a 4-probes metrology. Power-dependent PL analysis indicates that a sharp, strong light emission at 3.3 eV relates to the free exciton transition of QDs, and a broad, weak luminescence centered at 2.7 eV is inferred from free-to-bound transitions between QDs and host ambiances. In-plane thermal conductivity (k) of Ge QD/SiO2 system measured at 300-360 K is ~0.9-1.1 W/mK, which is a 60-fold and a 1.5-fold reduction in magnitude as compared with bulk Ge (60 W/mK) and SiO2 (1.4-1.6 W/mK), respectively. Remarkably the thermal conductivity of Ge QDs/Si3N4 system could be further reduced to 0.3-0.5 W/mK. This indicates that a high density of interfaces states between Ge QDs and oxynitride effectively drag phonons transportation. We believe that there is ample room for implementing Ge QDs/oxynitride system for advanced TE applications.
10:45 AM - I1.6
Tunability of the Thermal Conductance of Silicon Nanowires through Porosity Control.
Abhijeet Paul 1 , Mathieu Luisier 1 , Gerhard Klimeck 1
1 , Purdue University, West Lafayette, Indiana, United States
Show AbstractReducing device structures to the nanometer scale provides the ability to control their material parameters such as the electrical and thermal conductance, thereby converting the relatively lousy bulk Si into a potential thermoelectric material. The present study is motivated by the recent experimental demonstration of a silicon-based phonon nanomesh (Yu et. al, Nature Nano., July, 2010) allowing for a substantial reduction of its thermal conductivity as compared to bulk Si. Here, the effect of porosity on the thermal conductance of silicon nanowires (SiNWs) with different channel orientations is theoretically investigated using the complete phonon spectra of these structures calculated using a modified Valence Force Field (MVFF) model. The thermal conductance is calculated in the ballistic limit using Landauer's approach and assuming a low temperature difference between the SiNWs ends. We show that 3nm x 3nm square <100> and <111> SiNWs exhibit a reduction of roughly 30% of their thermal conductance when ~18% (a hole of ~0.7nm radius) of the atoms are removed from their center, while <110> SiNWs experience a ~20% reduction of their thermal conductance for a similar atom reduction. As the size of the wire cross-section increases the hole diameter must increase to obtain a similar reduction of the thermal conductance. For example, in a 4nm x 4nm <100> SiNW, nearly 25% of the center atoms must be removed (a hole of ~0.8nm radius) to obtain a 30% reduction of the thermal conductance. The reduction in the thermal conductance can be attributed to the strong phonon confinement and the localization of the vibrational energy density at the surface atoms. Hence, porous SiNWs provide an alternative route for tuning the thermal conductance of nanostructures and further improve their thermoelectric efficiency (ZT).
11:30 AM - I1.7
Thermoelectric Power Microgenerator Based on Silicon Nanowires Dense Arrangements as Thermoelectric Material.
Diana Davila 1 , Albert Tarancon 1 2 , Marta Fernandez-Regulez 1 , Neus Sabate 1 , Marc Salleras 1 , Carlos Calaza 1 , Alvaro San Paulo 1 , Luis Fonseca 1
1 Dept. Micro & Nanosystems, Instituto de Microelectronica de Barcelona – Centro Nacional de Microelectronica (IMB-CNM, CSIC), Bellaterra, Barcelona, Spain, 2 Department of Advanced Materials for Energy Applications, Catalonia Institute for Energy Research (IREC), Barcelona, Barcelona, Spain
Show AbstractThe increasing demand of miniaturized systems with long-lasting operation caused by the progress made in fields such as embedded systems or wireless sensor networks is driving the development of new technologies to achieve efficient energy generation. Due to the large amount of residual heat produced by the current energy generation technology based on fossil fuels, thermoelectric energy harvesters have received special attention in recent years. Low-dimensional structures have been recently discovered to be a promising approach for enhancing the thermoelectric properties of semiconductors providing fresh perspectives for the integration of thermoelectric generators in silicon microtechnology.With this aim, well-oriented and size-controlled Silicon nanowires (Si NWs) arrays have been implemented in microfabricated structures to develop planar unileg thermoelectric microgenerators (μTEGs) monolithically integrated in silicon. The compatibility of typical microfabrication technologies (including bulk micromachining, and patterning of appropriate metal contacts and heaters) and the vapor-liquid-solid (VLS) mechanism used for growing silicon nanowires has been successfully evaluated. Different low-thermal mass suspended structures have been designed and microfabricated on Silicon On Insulator substrates to passively generate thermal gradients and operate as microgenerators using Si NWs arrays as thermoelectric material. Such arrays were achieved by horizontally synthesizing silicon nanowires between the opposing (111)-oriented sidewalls of microfabricated thermally-isolated silicon platforms. This design allows passive generation of thermal gradients between those low thermal structures and the surrounding silicon bulk, which is in contact with a heat source. The temperature difference attainable in such devices is limited by the short length of VLS Si NWs (typically 10µm). In order to obtain suspended Si NWs arrays of longitudes bigger than the ones normally achieved by the VLS technique, structures composed by multiple ordered arrays consecutively linked by transversal microspacers were fabricated. This increases the thermal gradient generated across the device. In addition, interdigitated silicon suspended structures were also designed to maximize the contact area of the thermoelectric material and improve the power output obtainable for a given temperature difference.Electrical and thermal characterization in order to evaluate the performance of the devices when subjected to a thermal gradient as well as the electrical conductivity of the silicon nanowires will be shown.
11:45 AM - I1.8
Phonon Transport in Metal Assisted Chemically Etched Silicon Nanowires.
Myunghoon Seong 1 , Krishna Valavala 1 , Bruno Azeredo 1 , Jun Ma 1 , Marc Ghossoub 1 , Jyothi Sadhu 1 , Placid Ferreira 1 , Sanjiv Sinha 1
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractWhile bulk silicon (Si) is a poor thermoelectric material due to its relatively large thermal conductivity, nanostructured Si may show an improved figure of merit by as much as two orders of magnitude. The improvement stems from the reduction of thermal conductivity in Si nanowires and nanomesh structures [1-5]. In this talk, we report measurements of thermoelectric properties of Si nanowires fabricated using a top-down method. The method involves a combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical etching (MacEtch). Roughening control was partially achieved by using MacEtch solutions with different H2O2-HF ratios, and by varying the etching time. In order to measure the thermoelectric properties of the roughened Si nanowires, we have fabricated a heater-sensor platform similar to that described in the literature [6]. We discuss measured conductivities for Si nanowires with various diameter, length, and roughness.References[1] A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008)[2] A. I. Boukai et al, Nature Lett. 451, 168-171 (2008)[3] S. Sinha, B. Budhaev, and A. Majumdar, Mater. Res. Soc. Symp. Proc. 1166 (2009)[4] J. Yu et al, Nature Nano. 5, 718-721 (2010)[5] J. Tang et al, Nano Lett. 10, 4279-4283 (2010)[6] L. Shi et al., J. Heat Trans. 125, 881-888 (2003)
12:00 PM - I1.9
Effect of Nanocavities on the Thermoelectric Properties of Polycrystalline Silicon.
Ekaterina Selezneva 1 , Andrea Arcari 1 , Elisabetta Romano 1 , Gianfranco Cerofolini 1 , Rita Tonini 2 , Giampiero Ottaviani 2 , Dario Narducci 1
1 Department of Materials Science, University of Milano Bicocca, Milano Italy, 2 Department of Physics, University of Modena and Reggio Emilia, Modena Italy
Show AbstractNanostructuring has opened new ways to increase the thermoelectric figure of merit (ZT) of a host of materials, mainly by decreasing their thermal conductivity κ due to phonon surface scattering. Experiments on Si nanowires [1,2] actually demonstrated a remarkable decrease of κ. However, killing thermal conductivity is of little use when nanostructuring does not preserve electronic conductivity. In porous Si, for instance, thermal conductivity is found to be up to three orders of magnitude smaller than in single-crystal Si. However, a severe degradation of the electronic conductivity was also observed [3]. This has motivated the quest for phonon scatterers able to preserve the charge carrier mean free path. Among them, nanocavities (NCs) may be considered of special interest because of the possibility of continually modulating their size and spacing by annealing; and of decorating their internal surfaces with oxygen, hydrogen, and hydroxyl groups, thereby enabling the fine tuning of the inner surface transparency for phonons and electrons. Polycrystalline 450-nm thick Si films were deposited on top of SiO_2 insulating layer and were then heavily implanted with boron. NCs were then generated by using a well established process [4,5] based upon high-fluence He implantation followed by an annealing at temperatures above 500 °C. Additional isochronal thermal treatments at temperatures ranging between 500 and 1000 °C were finally used to modify the shape of the NCs. The same isochronal annealing cycle was also carried out on non He-implanted samples to compare the evolution of the electronic and thermal properties of the material.We found that, for the annealing temperatures above 800 °C the Seebeck coefficient quite unexpectedly increases with increasing electrical conductivity while the hole mobility retains values close to those of the single-crystal [6]. The NC injection is found not to change this picture once the implantation damage is recovered. The effect of NCs on κ will also be presented, and the competing role played by grain borders and precipitates will be discussed.[1] A. I. Hochbaum, R. Chen, R. D. Delgado, et al., Nature Lett., 451, 163 (2008)[2] A. I. Boukai, Y. Bunimovich, J. Tahir – Kheli, et al., Nature Lett., 451, 168 (2008)[3] A. Yamamoto, H. Takazawa, T. Ohta, Proc. Int. Conf. Thermoelectrics, 428 (1999)[4] V. Raineri, M. Saggio, E. Rimini, J. Mater. Res. 15, 1449 (2000)[5] S. Frabboni, F. Corni, C. Nobili, R. Tonini, and G. Ottaviani, Phys. Rev. B 69, 165209 (2004)[6] D. Narducci, E. Selezneva, G.F. Cerofolini, et al., Proc. 8th European Conf. Thermoelectrics, (2010)
12:15 PM - I1.10
Design and Thermoreflectance Imaging of High-Speed SiGe Superlattice Microrefrigerators.
Bjorn Vermeersch 1 , Je-Hyeong Bahk 1 , James Christofferson 1 , Ali Shakouri 1
1 Baskin School of Engineering, UC Santa Cruz, Santa Cruz, California, United States
Show AbstractOver the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterization techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium.In this paper, we present the design and thermal characterization of integrated high-speed microcoolers based on SiGe superlattices. The electrode metalisation is layed out as a coplanar waveguide, enabling to supply steep and short electrical pulses to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ~100ns temporal and submicron spatial resolution without the need to scan the sample.Net cooling in the 2 to 3 degree range is observed, with response times in the order of 1us. This is more than one order of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. We will discuss limits to directly measuring Peltier cooling in the nanosecond time regime and below, and the potential to study electron-phonon interaction during Peltier effects.
12:30 PM - I1.11
Silicide-based Composites for Waste Heat Recovery Applications.
Sabah Bux 1 , Jean-Pierre Fleurial 1
1 Thermal Energy Conversion Technologies, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California, United States
Show AbstractThermoelectric technology is a possible candidate for producing electrical power by recovering high grade waste heat produced by energy intensive industrial processes and transportation vehicles. Compared to other high temperature thermoelectric compounds, silicon-based materials are attractive due to their low- toxicity, low cost of production, thermal stability and low mass density. Significant advancements in the thermoelectric performance of silicon have been achieved recently through large reductions in the lattice thermal conductivity due to increased phonon scattering resulting from the introduction of nanoscale features. To date, across the full 1275 to 500 K temperature range of interest, average ZT values for combined p-type and n-type Si1-xGex alloys have increased from about 0.53 for state-of-practice space power systems to about 0.75 for the best nanostructured alloys. However, larger performance increases are still needed (factor of 2) for viable application of these materials to large scale waste heat recovery. One method of enhancing ZT is through forming nanostructured composites with nanoscale inclusions. The inclusions are predicted to not only reduce the lattice thermal conductivity but also enhance the Seebeck coefficient via carrier injection. Recent results on silicon-based composites formed by ball milling and high pressure sintering will be presented and discussed.
Symposium Organizers
Kornelius Nielsch University of Hamburg
SaskiaF. Fischer Ruhr-Univ. of Bochum
BethanieJ. H. Stadler University of Minnesota
Ted Kamins Stanford University
I4: Bi2Te3, PbTe and Related Materials
Session Chairs
Joseph Heremans
Kornelius Nielsch
Wednesday AM, April 27, 2011
Room 2007 (Moscone West)
9:15 AM - I4.1
Lattice Thermal Conductivity Reduction and Thermoelectric Figure of Merit Enhancement in Nanoporous Bismuth Tellurium Structures from Doped Nanoplate Crystals.
Yanliang Zhang 1 , Rutvik Mehta 2 , Matthew Belley 1 , Ganpati Ramanath 2 , Theodorian Borca-Tasciuc 1
1 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThis work reports thermal and thermoelectric transport properties in nanoporous Bi2Te3 pellets of 70%-97% densities synthesized from a bottom up approach. The nanoporous pellets are fabricated by compacting and sintering 5-25nm thick Bi2Te3 nanoplates prepared by surfactant-mediated microwave-stimulated synthesis. We demonstrate ultra low lattice thermal conductivity κL of 0.3 W/mK in the 70% dense Bi2Te3 nanostructures, ~75% lower than the value of the single-crystal bulk Bi2Te3 and approaching the theoretically predicted lower limit. The responsible mechanisms for κL reduction were quantitatively studied by employing a modified Debye-Callaway model and a modified effective medium formulation. The modeling results indicate that the nanograins and nanopores are the major factors for the κL reduction. The carrier concentration and electron mobility are determined via Hall effect measurement. We found that the electron mobility of the chemically-synthesized nanoporous Bi2Te3 structures is enhanced compared with the single-crystal bulk counterpart of the same carrier concentration. The room temperature thermoelectric figure of merit of the nanoporous Bi2Te3 of 30% porosity is 0.86, 70% increase compared with the bulk counterpart with the same carrier concentration. Further figure of merit enhancement is highly anticipated by the nanoporous approach with the optimized carrier concentration. Our bottom-up approach yields electron-transmitting phonon-blocking structures, opening up new possibilities in obtaining high performance thermoelectrics from nanoporous structures.
9:30 AM - I4.2
Effect of Sn Substitution on the Thermoelectric Properties of Nanostructured Bulk Bi2-xSbxTe3 Alloy.
Sumithra Santhanam 1 , Nathan Takas 1 , Westly Nolting 1 3 , Pierre P.Poudeu 1 2 , Kevin Stokes 1 3
1 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States, 3 Department of Physics, University of New Orleans, New Orleans, Louisiana, United States, 2 Department of Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractBulk nanostructured Bi2-xSbxTe3 alloy has shown enhanced thermoelectric properties arising due to reduction in lattice thermal conductivity without deterioration of the electrical transport parameters [1]. In this study, we present the effect of Sn substitution at Bi site, on the thermoelectric properties of Bi2-xSbxTe3 alloy. The Bi2-xSbxTe3 alloy with small percentages of Sn at Bi site was synthesized by planetary ball-milling. The resultant alloy powder was consolidated by uniaxial hot pressing at 100MPa and 5000C. The Seebeck coefficient, electrical and thermal conductivity were measured from RT-1500C. The carrier concentration of the alloy at room temperature was deduced from Hall effect measurements. An enhancement in the thermoelectric figure of merit observed due to Sn substitution in Bi2-xSbxTe3 alloy will be presented. The lattice thermal conductivity of these alloys simulated from Callaway model, considering the alloying (of Sn at Bi site) effect and comparison with the experimental lattice thermal conductivity will be presented. [1] B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M.S. Dresselhaus, G. Chen and Z. Ren, Science, 320, 634, (2008).
9:45 AM - **I4.3
Bi2Te3 Related Nanostructures.
Jan Koenig 1 , Harald Böttner 1
1 Thermoelectric Systems, Fraunhofer IPM, FReiburg Germany
Show AbstractBi2Te3 related compounds are since more than 30 years the material of choice for thermoelectric room temperature applications. Starting with the work of Rama Venkatasubramanian [1] regarding Bi2Te3 – Sb2Te§ superlattices the nanostructuring of these materials is the main focus to increase the thermoelectric quality. Here we will present a survey of themoelectric bulk and thin film Bi2Te3 related nanostructures and their structural and thermoelectric properties. Bi2Te3 thin films can be synthezised by molecular beam epitaxy, sputtering, nanoalloying and electroplating. The progress regarding these techniques will be discussed and compared. Especially the fabrication of superlattices will be addressed.Bulk Bi2Te3 nanostructures materials could be obtained in different ways. One possible approach is the formation of nanoinclusions due to phase separation. In 2008 a big success was published for nanostructured V2VI3 nanocomposites, which means that nanopowders compacted while preserving the nanostructure. The nano-powders are typically fabricated by ball milling. The compaction is done e.g. by spark-plasma sintering. Due to the fast heating, which is characteristic for these sinterprocess, the crystal growth is small and the nanostructure could be remained. The nanostructure results in lower thermal conductivities - good for thermoelectrics -, as well as higher mechanical strength – good for thermoelectric module fabrication. A survey on the most recent of the latest thermoelectric characterization and the module fabrication accordingly will be presented.[1] Venkatasubramanian, R. et al., Thin-film thermoelectric devices with high room-temperature figures of merit, Nature, Vol. 413 (2001), pp. 597.[2] Poudel, B. et al., High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Science, 1156446, 2008, p. 2
10:15 AM - I4.4
Sb2Te3 and Bi2Te3 Thin Films Grown by Molecular Beam Epitaxy at Room Temperature.
Zainul Aabdin 1 , Markus Winkler 3 , Dimitrios Bessas 2 , Jan Koenig 3 , Nicola Peranio 1 , Oliver Eibl 1 , Raphael Hermann 2 , Harald Boettner 3
1 Institut fuer Angewandte Physik, Universitaet Tuebingen, Tuebingen Germany, 3 , Fraunhofer Institut Physikalische Messtechnik, Freiburg Germany, 2 Institut fuer Festkoerperforschung, Forschungszentrum Juelich GmbH, Jülich Germany
Show AbstractNanoalloyed p-type Sb_2Te_3 and n-type Bi_2Te_3 thin-films were grown on SiO_2/Si and BaF_2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of five-layer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature. (ii) Phase formation of Sb_2Te_3 or Bi_2Te_3 during annealing at 250°C for two hours. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi2Te3 thin-films, which were epitaxially grown on BaF2 substrates at 290°C [1]. X-ray diffraction revealed single phase, poly-crystallinity, and texture. The films showed grain sizes of 400 nm for Sb_2Te_3 and 250 nm for Bi_2Te_3, analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S, electrical conductivity \sigma, charge carrier density n, charge carrier mobility \mu, power factor S^2\sigma) were measured at room temperature. The nanoalloyed Sb_2Te_3 thin-film revealed a remarkably high power factor of 29 \mu W cm^{-1} K^{-2} similar to epitaxially grown Bi_2Te_3 thin-films [1] and Sb_2Te_3 single crystalline bulk materials [2]. This large power factor can be attributed to a high charge carrier mobility of 402 cm^2 V^{-1} s^{-1} similar to high-ZT Bi_2Te_3/Sb_2Te_3 superlattices [3]. However, for the nanoalloyed Bi_2Te_3 thin-film a low power factor of 8 \mu W cm^{-1} K^{-2} and a low charge carrier mobility of 80 cm^2 V^{-1} s^{-1} were observed. Detailed microstructure and phase analyses were carried out by TEM on cross-sections. Energy-filtered TEM and diffraction contrast were used to analyze extended crystal defects. Plasmon spectroscopy by electron energy-loss spectroscopy (EELS) and quantitative chemical analysis by energy-dispersive X ray spectroscopy (EDS) were also applied. In Bi_2Te_3 thin-films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.5 at.% within the grains were observed. The structural modulation [natural nanostructure (nns)] with a wave length of 10 nm was found, which is known to be of general character for Bi2Te3 materials [1]. References: [1] N. Peranio et al., J. Appl. Phys. 100, 114306 (2006); J. Appl. Phys. 103, 024314 (2008)[2] D.M. Rowe, CRC Handbook of Thermoelectrics, (CRC Press, Boca Raton, FL, 1995), page 222 [3] R. Venkatasubramanian et al., Nature 413, 597 (2001); Appl. Phys. Lett. 75, 1106 (1999); J Cryst. Growth 170, 817 (1997)
10:30 AM - I4.5
Synthesis, Fabrication, and Characterization of Multidimensional Nanoparticle Based Thermoelectric Materials Composed of Bismuth, Antimony, and Tellurium.
Derrick Mott 1 , Nguyen Mai 1 , Nguyen Thuy 1 , Teruyoshi Sakata 1 , Mikio Koyano 1 , Shinya Maenosono 1
1 Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
Show AbstractWith the event of nanotechnology, the field of thermoelectric (TE) materials has been re-invigorated with many recent advances towards materials with high thermoelectric efficiency (dimensionless figure of merit, ZT). The realization of such materials opens up new avenues to the creation of devices that can be used in freon-less refrigeration, micro-electronic cooling, or for harnessing lost heat energy from sources such as car engines. In our own research work, we have successfully devised a synthetic technique towards nanoparticles composed of bismuth, antimony, and tellurium that has proven highly versatile in tuning both the composition and shape/structure of the resulting nanoparticles. The ability to control the nanoparticle composition and shape/structure are highly important as these are critical parameters that dictate the resulting devices TE activity. In a modified polyol synthetic technique, it was found that many complex composition, shape, and structure combinations could be obtained for the nanoparticles including Te/Bi2Te3 nanowires, Bi2Te3/Sb2Te3 nanodiscs, BiSbTe3 platelets or a heterostructure composed of Bi2Te3 nanodiscs deposited on Te nanowires, just to name a few. By simply changing the capping ligands used in the synthesis, the nanoparticles resulting composition, morphology and structure could be changed, leading to a straightforward route towards TE nanoparticles with interesting properties. The unique morphology of these particles made them interesting candidates for processing into functional devices to study the resulting TE properties. This presentation focuses on our recent study of the synthesis and processing of bismuth, antimony, and tellurium composite nanoparticles into functional thermoelectric devices with enhanced activity in terms of understanding the underlying mechanisms of the synthetic technique, and characterization of the resulting nanomaterial properties (composition, morphology, TE properties, etc.). The resulting materials and devices are characterized using techniques such as HR-TEM, EDS, XPS, XRD, and SEM, as well as many others.
11:15 AM - **I4.6
Formation and Control of Thermoelectric Composites at the Nanoscale.
Jeff Snyder 1
1 Materials Science, Caltech, Pasadena, California, United States
Show AbstractMany new high efficiency thermoelectric materials are multi-phase composites at the nanometer scale. These include materials such as PbTe alloys, and recently Zn4Sb3, which contain small ~10nm nanoparticles that can be difficult to control. Typically the goal is to reduce the lattice thermal conductivity of the matrix by scattering phonons at the interfaces of the particles. Thus it is not the small size of the particles but the spacing between particles that matter. We have found that in PbTe with large (~50nm – 200nm) nanoparticles of Ag2Te high zT ~ 1.5 can be obtained through the independent control of thermal conductivity reducing Ag2Te large nanoparticles and electronic doping with La.
11:45 AM - I4.7
Formation of Widmansaetten Precipitates and Thermal Conductivities in Bulk PbTe-base Compounds.
Teruyuki Ikeda 1 2 , Neal Bansal 2 , Eric Toberer 2 , Kristin Bergum 2 , Marcus Toussaint 2 , Nathan Marolf 2 , G. Jeffrey Snyder 2
1 PRESTO, Japan Science and Technology Agency, Kawaguchi Japan, 2 , California Institute of Technology, Pasadena, California, United States
Show AbstractNanostructures could significantly reduce the lattice thermal conductivities of thermoelectric materials due to phonon scattering at boundaries. In some PbTe based compounds, plate-like precipitates (Widmanstätten plates) are formed due to the decrease in the solubility in PbTe with decreasing temperature. The Widmanstätten precipitates introduce phase boundaries with high density. In this paper, we demonstrate the routes to control the phase boundary area. We also show the lattice thermal conductivity as a function of phase boundary area and discuss the effects of the nanoscale microstructure on the lattice thermal conductivity.
12:00 PM - **I4.8
Hierarchical Length-scale Architecture for High Performance Thermoelectrics.
Vinayak Dravid 1 , Mercouri Kanatzidis 1 2
1 Mat Sci & Engg, Northwestern University, Evanston, Illinois, United States, 2 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractThe core intellectual challenge in the next generation bulk thermoelectric materials revolves around synthesis and fabrication of hierarchically organized microstructure that does not appreciably compromise the innate high power factor of the chosen thermoelectric matrix systems but significantly reduces lattice thermal conductivity to enhance the overall figure of merit, ZT. An effective emerging strategy involves introducing nanostructures in the bulk thermoelectric materials; wherein nanoscale precipitates do not appreciably compromise the charge mobility (thus the power factor) but significantly reduce the lattice thermal conductivity via multiple phonon scattering pathways. As a result, there are exciting opportunities for understanding and tailoring microstructural elements with due attention to the hierarchical length-scale influence across atomic-, nano- and micro-meter dimensions. We will present this intricate but tractable relationship between various microstructural attributes (point, line and interfacial defects as well as associated elastic and plastic strain) and lattice thermal conductivity in representative nanostructured thermoelectric systems such as: PbTe-X (X = Sb, Bi, Pb or a combination) and PbTe/PbS. Advanced S/TEM imaging, spectroscopy and spectroscopic imaging approaches are utilized to obtain quantitative information about the “nanostructured microstructure”, including spatially distributed plastic and elastic strain. We will show that despite its limitations, the modified Callaway model provides a rationale basis based on microstructure for optimizing microstructure for effective phonon scattering. The presentation will highlight the need for overarching approach for understanding hierarchical length-scale influence on phonon scattering in bulk thermoelectric materials for enhancing the figure of merit - thus, paving the way to specific design strategies for the next generation thermoelectric materials.
12:30 PM - **I4.9
New Compounds Consisting of Turbostratic Intergrowths:Ultra Low Thermal Conductivities and Tunable Electric Properties.
Ngoc Nguyen 1 , Krista Hill 1 , Matthew Beekman 1 , David Johnson 1
1 Department of Chemistry, University of Oregon, Eugene, Oregon, United States
Show AbstractMisfit layered compounds are naturally occurring nanostructured solids consisting of intergrowths of two constituents, [(MX)1+δ]n[TX2]m where M is Pb, Sn, Bi, Sb, RE; X is S or Se, T is a group IV or V transition metal, and δ is the difference in the areal densities of the two constituents. MX has a distorted rock salt structure and the trilayers of TX2 contain hexagonal layers of chalcogen X on either side of a hexagonal layer of T. These compounds are potentially interesting thermoelectric materials with low thermal conductivities (~0.8 Wm-1K-1) and non optimized figures of merit ZT as large as 0.4 reported for [(PbS)1+δ]1[TiS2]2. We have reported that introducing turbostratic disorder into the structurally related compounds [(PbSe)1.00]m[MoSe2]n and [(PbSe)0.99]m[WSe2]n reduces the in-plane thermal to 0.4-0.5 Wm-1K-1, results in ultra-low cross plane thermal conductivity of 0.06 Wm-1K-1 and that the turbostratic disorder remains after annealing for 24 hours at 500°C in a controlled partial pressure of selenium, reducing carrier concentrations in nominally undoped materials to ~ 1017cm-3. We have recently found that varying the composition profile with the repeating unit of the precursor while maintaining the precursor period results in the ability to make structural isomers for each value of m and n. We have also found that this synthesis route can also prepare intergrowths of other structures. The synthesis, structure and properties of these new compounds will be discussed.
Symposium Organizers
Kornelius Nielsch University of Hamburg
SaskiaF. Fischer Ruhr-Univ. of Bochum
BethanieJ. H. Stadler University of Minnesota
Ted Kamins Stanford University
I10: Poster Session: Thermoeletric Nanostructures and Materials
Session Chairs
Saskia Fischer
Kornelius Nielsch
Thursday PM, April 28, 2011
Salons 7-9 (Marriott)
I8: Spin Caloritronics II
Session Chairs
Jean-Philippe Ansermet
Gerrit Bauer
Thursday PM, April 28, 2011
Room 2007 (Moscone West)
2:30 PM - **I8.1
The Spin-Seebeck Effect Seen from an Experimentalist's Point of View.
Joseph Heremans 1 , Christopher Jaworski 1 , Jing Yang 1 , Shawn Mack 2 , David Awschalom 2 , Roberto Myers 1
1 , Ohio State University, Columbus, Ohio, United States, 2 , UCSB, Santa Barbara, California, United States
Show AbstractThe spin-Seebeck effect is a spin-polarization induced by a temperature difference. This effect was observed experimentally in a ferromagnetic metal [1], ferromagnetic semiconductor [2] and ferrimagnetic insulator [3]. In all cases, the authors use the inverse spin-Hall effect in Pt strips as a spin-polarization detector, and report a signal proportional to the temperature differences along the sample. The signal switches sign in the presence of a magnetic field parallel to the temperature gradient. Our experimental observations in the ferromagnetic semiconductor GaMnAs allow us to follow the signal as a function of temperature through the Curie temperature, and to study its dependence on the angle between easy axis and directions of heat flow and applied magnetic field. We also prove, by simply scratching the ferromagnetic film off its substrate, that there is no macroscopic spin flow parallel to the temperature gradient. New data on MnAs/GaMnAs bilayers further suggest that the polarity of the effect is material-dependent. No theory suggested so far [4] can explain all observations, but the spatial and temperature-dependence of the effect suggests that might be related to a coupling between magnons in the films and phonons in the substrate. Acknowledgements: work at OSU supported by NSF (CBET- 0754023, DMR-0820414), and OSU-IMR, at UCSB by NSF.[1] K. Ushida & al., Nature 455 778 (2008)[2] C.M. Jaworski & al., Nature Materials 9 898 (2010)[3] K. Ushida & al., Nature Materials 9 894 (2010)[4] H. Adachi & al., arXiv:1010.4600v1 (2010), Xiao et al., PRB 81 214418 & erratum PRB 82 099904 (2010)
3:00 PM - **I8.2
Thermoelectricity in Magnetic Nanostructures.
Yaroslav Tserkovnyak 1 , Alexey Kovalev 1
1 Physics and Astronomy, UCLA, Los Angeles, California, United States
Show AbstractWe introduce and study a magnetocaloritronic circuit element based on a domain wall that can move under applied voltage, magnetic field and temperature gradient. Analogies are drawn between Carnot machines and possible devices employing such a circuit element. We propose a realization of magnetocaloritronic cooling and point out the parallels between the operational principles of magnetocaloritronic and thermoelectric cooling and power generation. Following this analogy, we introduce a magnetocaloritronic figure of merit that encodes information about the maximum efficiency of such devices. While the magnetocaloritronic figure of merit turns out to be very small for transition-metal based magnets, larger numbers are expected in ferromagnetic insulators.
3:30 PM - **I8.3
Topological Thermoelectrics.
Jairo Sinova 1
1 Department of Physics, Texas A&M University, College Station, Texas, United States
Show AbstractWe have introduced a novel approach to obtain a high ZT in topological insulators at room temperature. The increase of ZT is obtained by engineering lattice dislocations that would reduce thermal conductivity while increasing electrical conductivity. The idea is based on the fact that the screw dislocations in certain 3D topological insulators have topologically protected 1D conducting channels. Our theoretical prediction is that at high dislocation densities ZT can be dominated by these 1D states, which reduce the thermal conductivity while simultaneously increasing the conductivity and thermopower. We have shown that in principle a very high ZT of order 10 can be achieved at room temperature. Thus this system, if successful, would be an ideal candidate for possible applications in heat management of nanodevices at room temperature.
I9: Thermoelectric Oxides and Heusler Alloys
Session Chairs
Ted Kamins
Gaby Schierning
Thursday PM, April 28, 2011
Room 2007 (Moscone West)
4:30 PM - **I9.1
Heusler Compounds for Thermoelectric and Spincaloric Applications.
Claudia Felser 1
1 Graduate School of Excellence Materials Science, Mainz Inst. for Inorganic and Analytical Chemistry, Mainz Germany
Show AbstractSilicon is still the most important semiconductor for various applications such as electronic devices and solar cells. Binary semiconductors YZ such as GaN and GaAs are alternative materials because it is possible to tune their properties by choosing different element combinations. Recently, ternary and quaternary semiconducting materials have been designed for more complex applications. Ternary semiconductors derived from the ZnS structure exhibit the C1b structure type, the so called half-Heusler structure (in case that the compound contains a transition metal). The C1b structure can be understood as ``filled” ZnS structure type. Instead of a few possible binary combinations crystallizing in the ZnS structure type more than 250 mainly semiconducting compounds are known to crystallize in the C1b structure type. Half-Heusler compounds XYZ have attracted attention as potential candidates for thermoelectric applications and as topological insulators. Favorable for thermoelectric applications are semiconductors with a small band gap and high densities of states. Similar to the binary semiconductors, the band gap depends on the electronegativity difference of the elements, which build the diamond-like substructure (van Vlechten). Complex C1b compounds such as TiNiSn phases are promising n-type thermoelectrical materials illustrated by exceptionally large figure of merit, ZT ~1.5 at high temperatures. The p-type C1b Heusler compounds show lower ZT values due to their high thermal conductivities. Strategies to improve the thermoelectric properties will be discussed. Magnetic Co2TiZ Heusler compounds are half metallic compounds and fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. The Z=Si, Ge and Sn compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. The Seebeck coefficients are relative high and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for spincaloric investigations.
5:00 PM - I9.2
Thermoelectric Properties of LaCoO Thin Films.
Hanns-Ulrich Habermeier 1 , Ahmed Sleem 1
1 , MPI-FKF, Stuttgart Germany
Show AbstractPerovskite oxides containing transition elements, in particular Co, exhibit a high Seebeck coefficient, and also a low κ. In these oxides there is strong electron-electron interaction. The 3d electrons are degenerate due to spin and orbital degrees of freedom. The competition between crystal field and Hund’s rule coupling is responsible for the degeneracy of the electronic states of Co3+ and Co4+ ion causing low-spin, intermediate spin and high spin states. Particularly interesting are LaCoO3 based perovskites with the possibilities of substitution on the La-site. Whereas LaCoO3 and Sr-substituted counterparts have been investigated already, little is published about LaCoO3 thin films. We have prepared La1-xSrxCoO3 thin films by PLD techniques on LaAlO3 substrates and investigated their structural, transport, and magnetic properties as well as their Seebeck coefficient as a function of Sr substitution in the range from room temperature to 4000C. The room temperature values for the Seebeck coefficient decrease monotonically from 275 μV/K to 175 μV/K for the parent compound and a 4% Sr substituted LaCoO3. Furthermore, we observed for LaCoO3 thin films an unexpected ferromagnetic transition at 85K. The results will be discussed within the frame of strain-induced modifications of the electronic structure of the films.
5:15 PM - I9.3
Doped Zinc Oxide Nanocrystal Assemblies for Thermoelectric Waste-heat Scavenging.
Priyanka Jood 1 3 , Rutvik Mehta 1 , Yanliang Zhang 2 , C. Karthik 1 , Richard Siegel 1 , Theo Borca-Tasciuc 2 , G. Peleckis 3 , X. Wang 3 , Shi Dou 3 , Ganpati Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales, Australia, 2 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThermoelectric materials are a means to realize solid-state refrigeration, and electrical power generation from waste heat, e.g., in cars and power plants. The latter application requires materials with high figures of merit ZT at high temperatures. Doped n-type ZnO is a promising candidate for such applications due to its high melting point, high chemical stability, high electrical conductivity (σ) and a high Seebeck coefficient (α), but is limited by the high lattice thermal conductivity κL (~5 W/mK 1273K). Nanostructuring and doping are promising strategies to decrease κL while retaining high power factor α2σ to obtain higher ZT values necessary for applications. Here, we report a novel bottom-up approach to obtain bulk nanostructured n- and p-type ZnO through assembly of ZnO nanocrystals doped with Al, In and Bi. ZnO nanocrystals were synthesized by a a rapid (~60-120s) scalable (>5 g/minute) microwave-activated thermal decomposition of zinc salts in the presence of oleic acid and oleylamine as surfactants together with dopant-containing metal salts. The nanocrystals obtained have a narrow size range between 5 to 10 nm while the doping is controllable between 0.5 to 2 at.%. We show that controlled doping alters the band gap by as much as 0.26 eV through creation of states near the band edges and correlates with the change in the lattice parameters besides impacting the defect structure, morphology and grain growth of the sintered nanocrystal assemblies. The bulk nanostructured pellets fabricated from the ZnO nanocrystals exhibit a 10-fold diminution compared to the bulk values, resulting in ultra-low κL in the 1.6~2 W/mK range. The κL decrease arises from enhanced phonon scattering in the 10-35 nm grains in the bulk nanostructured samples, in good agreement with κL values calculated using a modified Debye-Callaway model. ZnO doped with individual dopants lead to large room temperature Seebeck coefficients in the -500 μV/K< α <-200 μV/K range together with high σ~1-1000 Ω-1m-1. Based upon these results combined with carrier concentrations and mobilities determined by Hall measurements, we identify the possible pathways to enhance ZT through the use of multiple-dopants, e.g., Al and In. We will also present temperature-dependent thermoelectric properties of doped- nanostructured ZnO to obtain insights into the effects of nanostructuring and doping on ZT at temperatures of interest for power harvesting applications.
5:30 PM - I9.4
Nanocrystalline Thermoelectric Oxide for High Efficiency Energy Harvesting.
Feiyue Ma 1 , Jiangyu Li 1
1 , University of Washington, Seattle, Washington, United States
Show AbstractLayered cobalt oxides such as NaCo2O4 and Ca3Co4O9 have great potential in thermoelectric energy harvesting due to their excellent thermal stability at high temperature. Their thermal conductivity is dominated by phonons, making it possible to substantially reduce their thermal conductivity by phonon scattering at nanoscrystalline boundaries. While our previous studies demonstrated that grain size as small as 5~10nm can be realized in isolated NaCo2O4 nanofibers processed by sol-gel based electrospinning, substantial grain growth was observed when large amount of nanofibers were annealed together. In this paper we focus on controlling the grain size in NaCo2O4 nanofibers during large scale fabrication. By carefully controlling the nucleation and grain growth during annealing process, aggregates of large amount of NaCo2O4 nanofibers with nanocrystalline grain sizes and uniform grain size distribution have been achieved. Those nanofibers have been consolidated into bulk ceramics by spark plasma sintering without substantially grain growth, as verified by SEM, XRD and TEM, resulting greatly improved thermal resistivity without compromising their electrical conductivity.
5:45 PM - I9.5
Impacts of Phase Transition on Thermoelectric Figure of Merit in [0001]ZnO Nanowires.
Shi Lihong 1 , Zhang Gang 2 , Li Baowen 1 3
1 Department of Physics and Centre for Computational Science and Engineering, National University of Singapore, Singapore Singapore, 2 Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing China, 3 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore Singapore
Show AbstractRecently, Znic Oxide (ZnO) nanowires (NWs) have shown promise for nanodevice applications. The high surface-to-volume ratio of NWs is important for many applications, such as gas sensors. Through the coupling of piezoelectric and semiconducting properties of ZnO, nanoscale mechanical energy can be converted into electrical energy by using ZnO NW arrays. Moreover, compared to traditionally TiO2 films, ZnO NW arrays are considered promising anode material for photovoltaic device because of the highly controllable single-crystalline morphology which provides direct electron transport pathway. In addition to piezoelectric and optoelectronic applications, thermoelectric effect is also an important part of the solution to the energy crisis by converting waste heat into electricity. The maximum energy conversion efficiency of a uniform thermoelectric material is limited by the dimensionless figure of merit ZT, which is a function of Seebeck coefficient, electrical conductivity and thermal conductivity. To our best knowledges, rare researches are concerning about the thermoelectric properties in ZnO NWs. In this work, we use 1D transport theory and first-principle method to investigate the impacts of phase transition on the thermoelectric properties in [0001] ZnO NWs. Phase transition has played an important role in electronic conduction[1] and thermal conduction in ZnO NWs [2], but this effect on thermoelectric is still unclear. Our results show that the electronic band gap of ZnO NWs for Wurtzite (W) phase is larger than that of Hexagonal (H) phase. For a certain carrier concentration, the Seebeck coefficient S for W-phase is larger than that for H-phase, while electrical conductivity with H-Phase is much higher than that of W-Phase because of the higher electron mobility in H-Phase. There is an optimal carrier concentration to achieve the maximum value of power factor P for both W and H phases. The maximum value of P (Pmax) for H phase (Pmax=1638uW/m-K2) is larger than that of W phase (Pmax=1213uW/m-K2) due to its high electrical conductivity. In Ref [2], it is referred that the thermal conductivity for H phase is about 20% larger than that for W phase. Combined the calculations of power factor and the thermal conductivity, the maximum achievable value of figure of merit ZT for H phase is larger than that for W phase (1.1 times). Our work provides design rules for possible ZnO NW arrays based piezoelectric, optoelectronic and thermoelectric hybrid energy generator.Reference:[1]Y. Zhang et al. Appl. Phys. Lett., 94, 113114 (2009). [2] A.J. Kulkarni et al. Nanotechnology, 18, 435706(2007).
I10: Poster Session: Thermoeletric Nanostructures and Materials
Session Chairs
Saskia Fischer
Kornelius Nielsch
Friday AM, April 29, 2011
Salons 7-9 (Marriott)
9:00 PM - I10.1
Thermoelectric Properties of Coupled Double Quantum Dots in a Nanowire.
Ming Ting Kuo 1 , Yia chung Chang 2
1 , National central university, Jungli Taiwan, 2 , Academic sinica, Taipei Taiwan
Show AbstractWe theoretically investigate the thermoelectric properties of coupled double quantum dots in series connected with metallic electrodes. A coupled two-level Anderson model including electron hopping between dots and electron Coulomb interactions is employed to simulate the electronic transport of the system. The charge and heat currents in the Coulomb blockade regime are calculated in the framework of Keldysh Green's function technique. Based on the equation of motion, we obtain a closed form solution for the spectral function, which can simultaneously reveal the cases of strong and weak coupling between two dots (tc). The electrical conductance, Seebeck coefficient, thermal conductance, and figure of merit of the nanostructure junction are calculated. The figure of merit is seriously suppressed when tc decreases. We attribute this to the fact that the electrical conductance reduction is faster than that of thermal conductance. On the other hand, the Seebeck coefficient is found to be insensitive to the variation of tc.
9:00 PM - I10.10
First-principles Investigation of Point Defects in the Thermoelectric Bi2Te3 Material.
Adham Hashibon 1 , Nicola Peranio 2 , Christian Elsaesser 1
1 , Fraunhofer-Institut fuer Werkstoffmechanik IWM, Freiburg Germany, 2 , Institut fuer Angewandte Physik, Universitaet Tuebingen, Tuebingen Germany
Show AbstractThe best thermoelectric materials for applications at room temperature are based on Bi2Te3 and its alloys, such as Bi2-xSbxTe3. Much effort has been made to enhance the thermoelectric properties of these materials in order to increase the figure of merit ZT beyond 1 at room temperature. Bi2Te3 is a narrow-gap semi-conductor having a tetradymite layered structure with the fundamental building block of five atomic layers of the sequence: Te1-Bi-Te2-Bi-Te1. Small deviations from stoichiometry or the presence of impurities are known to affect charge carrier concentrations [1] and hence also the electronic properties. Yet little is known about the formation energies of native defects or impurities, their concentrations, and their effect on the thermoelectric properties [1,2]. We have investigated by means of first-principles density functional theory (DFT) calculations the formation energies and the atomistic and electronic structures of various point defects in Bi2Te3, namely atomic vacancies, antisite atoms, and substitutional impurities (Sb, Se). It was found that despite the believe that the Te1-Te1 interaction is of the weak van der Waals type, the structure and bonding are apparently well described by the local-density approximation (LDA) to the DFT. While formation energies of vacancies were found to be relatively large, the lowest formation energy turned out to be that of the antisite defect BiTe1, where a Bi atom replaces a Te1 atom in the unit cell of Bi2Te3 . This may be a result of the weak Te1-Te1 bond and the relatively large distance between them. The effect of these defects on the thermoelectric properties will be further discussed in terms of variations in the electronic densities of states (DOS), which directly affects the thermopower and the charge carrier mobility. Calculated electronic DOS were compared to experimental electron energy-loss spectra (EELS) measured in the transmission electron microscope (TEM). This is done by relating the measured energy-loss intensity to the density of unoccupied states according to Fermi’s golden rule [3,4]. [1] J. Horák, J. Navrátil, and Z. Starý, J. Phys. Chem. Solids 53 (1992) 1067. [2] T. Thonhauser et al., Phys, Rev. B 68 (2003) 205207. [3] S. Köstlmeier and C. Elsässer, Phys. Rev. B 60 (1999) 14025.[4] S. Köstlmeier and C. Elsässer, Ultramicroscopy 80 (1999) 145.
9:00 PM - I10.11
Electrochemical Formations of Bismuth Telluride-based Thin Films on Stainless Steel.
Xuan Cheng 1 2 , Linqin Qiu 1 , Jian Zhou 1 , Qinhan Lin 1
1 Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian, China, 2 Fujian Key Laboratory of Advanced Materials, Xiamen University, Xiamen, Fujian, China
Show AbstractBismuth telluride (Bi2Te3) and its derivative compounds have proven to be one of the best thermoelectric materials with the highest figure of merit for thermoelectric applications at room temperature such as thermoelectric generators and coolers. It has been shown that Bi2Te3-based thin films can be electrochemically prepared on relatively expensive substrates using gold or gold sputtered-aluminum, platinum, and titanium. In this work, the thin films with a stoichiometry of Bi2Te3, Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3 on stainless steel substrates were obtained by electrochemical deposition at selected potentials from aqueous electrolytes consisting mainly of nitric acid, Bi3+, HTeO2+, SbO+ and H2SeO3. The microstructure, composition and conductivity of the thin films were investigated using cyclic voltammetry, electrochemical impedance spectroscopy, X-ray diffraction spectroscopy, electron microprobe analysis technique, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results revealed that the thin films were single phase with the rhombohedral Bi2Te3 structure. The morphology and growth orientation of the films were dependent on the deposition potentials. Possible formation mechanisms of the thin films on stainless steel are suggested based on experimental data.
9:00 PM - I10.12
Fabrication and Characterization of Flexible and Array-type Surface Temperature Sensor by Screen Printing.
Han Wook Song 1 , In Mook Choi 1 , Kwangsoo No 2
1 , KRISS, Daejeon Korea (the Republic of), 2 , KAIST, Daejeon Korea (the Republic of)
Show AbstractA flexible and array-type surface temperature sensor (FASTS) was developed for measuring the temperature distribution on human body or the surface of an object. The FASTS consist of conductive paste(Ag) used for upper and bottom electrode(5 μm), polymer-based paste(consisted of S, Cl, C, O, P, etc) between electrodes on polyethylene substrate. The FASTS was fabricated by screen printing and cured at 90, 110, 130 and 150 °C for 5, 10, 15, 30 and 60 minutes. From the measurement of resistance, the optimum curing conditions were 150 °C for 5 minutes. The thickness of polymer layer were varied from 20 μm to 50 μm. Films with the thickness about 20 μm showed the contact regions between the top and the bottom electrodes, while films with the thickness about 50 μm showed serious exothermic behavior due to the high resistance of the polymer layer. The optimum thickness was 40 μm. The temperature coefficient of resistance was characterized in oil bath. This FASTS showed the characteristics of negative temperature coefficients (NTC) thermistor. Compared to the commercial NTC thermistors, FASTS showed superior sensitivity at the temperature range from 5 °C to 100 °C(-4.5 %/°C at 25 °C and -2 %/°C at 100 °C), which meant that our FASTS was suitable to measure the temperature distribution on human body.
9:00 PM - I10.13
Simple and Large-scale Synthesis of Uniform-sized Bi2-xSbxTe3 Nanocrystals and Characterization of Their Composition-dependent Thermoelectric Properties.
Mi-kyung Han 1 , Sol Kim 1 , Sung-Jin Kim 1
1 Chemistryand and Nano Science, Ewha womans University, Seoul Korea (the Republic of)
Show AbstractThe ability of thermoelectric device is measured by figure of merit (ZT). ZT is given by ZT=S2σT /κ, where σ is the electrical conductivity, S the thermo power, and κ is the thermal conductivity. For several decades, the ZT of the thermoelectric materials have been less than 1. Current research results often are reported ZT > 2 due to quantum confinement effect. We prepared high-quality, nearly uniform-sized Bi2Te3, Sb2Te3, and BixSb2-xTe3 nanocrystallites by colloidal method. The composition, size and morphology of the nanoparticles were controlled by varying the starting materials and stabilizing agents. The organic ligands on the as-prepared nanoparticles are exchanged with inorganic ligand of [SnS4]2-, and then the nanoparticles are pressed to a pellet. We also characterized composition-dependent thermoelectric properties of BixSb2-xTe3 nanocrystals. The concentration dependence of thermoelectric properties of such pressed BixSb2-xTe3 shows a highly reduced thermal conductivity.
9:00 PM - I10.14
Thermoelectric Characterization of Sb2Te3/Bi2Te3 Superlattice Grown by Cyclic PECVD.
Young Kuk Lee 1 , Chang wan Lee 1 , Seong Gu Kang 1 , Ki-SeoK An 1 , Sun Suk Lee 1 , Taek-Mo Chung 1 , Chang-Gyoun Kim 1
1 Advanced Materials Division, Korea Research Institute of Chemical Technology, Taejon Korea (the Republic of)
Show AbstractSb2Te3/Bi2Te3 superlattices were deposited on TiO2/SiO2/Si substrate by cyclic plasma-enhanced metallorganic chemical vapor deposition at temperature ranging from 75 to 200 °C using triisopropyl antimony, using triisopropyl bismuth and diisopropyl tellurium, as Sb, Bi and Te precursors, respectively. The conditions for deposition have been investigated as a function substrate temperature, plasma power, working pressure and optimized to achieve a high thermoelectric power factor. The quality of the deposited films, e.g. structure, composition, thickness and morphology, has been examined by X-ray diffraction (XRD), X-ray fluorescence analyzer (XRF), field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The chemical composition of the films was controlled by the cycling ratio of Sb, Bi and Te precursors pulse. The thermoelectric properties of the thin films have been studied by room-temperature measurement of the Seebeck coefficient, Hall coefficient and electrical resistivity.
9:00 PM - I10.15
Bi2Te3-Sb2Te3 Superlattices Grown by Nanoalloying.
Jan Koenig 1 , Markus Winkler 1 , Harald Boettner 1 , Saskia Buller 2 , Ullrich Schuermann 3 , Lorenz Kienle 3 , Wolfgang Bensch 2
1 Thermoelectric Systems, Fraunhofer IPM, Freiburg Germany, 2 Institute of Inorganic Chemistry, Christian-Albrechts-Universitaet zu Kiel, Kiel Germany, 3 Faculty of Engineering, Christian-Albrechts-Universitaet zu Kiel, Kiel Germany
Show AbstractThe legendary breakthrough in ZT from R. Venkatasubramanian is an outstanding result in thermoelectricity. ZT values as high as ~2.4 and ~1.7 were measured for p/n-V2VI3-superlattices [1]. However, several important questions still remain open since the works of Venkatasubramanian lack a systematic examination of the thermoelectric as well as the structural properties. In this work, Bi2Te3, Sb2Te3 and Bi2Te3-Sb2Te3 multilayers similar to those of R. Venkatasubramanian were synthesized and examined. A novel approach called “nanoalloying” was used which has the potential to create V2-VI3 nanostructured layers with good thermoelectric properties [2]. Element nanolayers of Bi, Sb and Te were deposited near the binary stoichiometry on a cold substrate (BaF2 and SiO2) and subjected to an annealing process, in which a solid state reaction yielding Bi2Te3, Sb2Te3 takes place. The phase formation of these compounds is investigated using in-situ temperature dependent XRD measurements. The synthesis of the binary compounds and the multilayers is investigated regarding conduction type, high Seebeck coefficients and good electrical properties. Besides the thermoelectrical properties the thermal stability of the multilayers is of major interest for a future application. Therefore the temperature stability of the multilayer structures is investigated by annealing experiments using TEM and SIMS analysis.The above mentioned investigations and the obtained results of the nanoalloyed Bi2Te3, Sb2Te3 and Bi2Te3-Sb2Te3 multilayers will be presented. This work was funded by the German Research Society (DFG) via the priority program SPP1386 “Nanothermoelectrics”. 1.R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature 413 (2001), p. 5972.Koenig, J.D.; Boettner, H.; Tomforde, J., Bensch W., Proceedings ICT 07: 26th International Conference on Thermoelectrics, 2007, p.395
9:00 PM - I10.16
Fabrication of a Subwavelength Grating Broadband, Wide-angle Solar Thermal Absorber for Thermophotovoltaics Applications.
Mark Losego 1 , Andrew Gardner 2 , Eden Rephaeli 3 , Shanhui Fan 3 , William King 2 , Paul Braun 1
1 Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States, 2 Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois, United States, 3 Department of Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractThermophotovoltaics hold promise for improving solar cell efficiencies by reshaping the solar spectrum to better match the absorption bandgap of semiconductor solar cells. A key component to this technology is an efficient solar absorber capable of broadband absorption across the solar spectrum and over a wide angular range. Herein, we describe our work to fabricate a pyramidal nanostructured tungsten surface that has been predicted by finite-difference time domain (FDTD) simulations to fully absorb over the solar spectrum and over a >120° angular range. To achieve such performance, the tungsten features must have high aspect ratios (greater than 2:1) and a subwavelength periodicity (<300 nm). A periodic pyramidal mold is formed in a polymer layer using nano-indentation. These features are then replicated into a tungsten surface by using a combination of physical vapor deposition and electrodeposition. Reflectance measurements reveal that the nanostructured tungsten surface has a modified absorbance. Comparisons to FDTD simulations will be discussed.
9:00 PM - I10.18
Thermoelectric Properties of (Bi2Te3)1-X(In2Se3)X Composite Thin Films Prepared by Off-axis Rf Sputter–Continuous Composition Spread System.
Kwang-Chon Kim 1 2 , Won Chel Choi 1 , Hyun Jae Kim 2 , Ji won Choi 1 , Jin-Sang Kim 1
1 Electronic Materials Center, Korea Institute of Science and Technology, Seoul, Cheongryang, Korea (the Republic of), 2 School of Electrical and Electronic Engineering, Yonsei University, Seoul, Seodaemun-Gu, Korea (the Republic of)
Show Abstract(Bi2Te3)1-X(In2Se3)X (x=0~1) compound thin films were prepared by off-axis Rf sputter - continuous composition spread (CCS) system. The off-axis RF sputter–CCS technique is widely recognized in combinatorial material science as a powerful technique for investigating the properties of new functional materials at various composite ratios. (Bi2Te3)1-X(In2Se3)X composite thin films were possible to obtain of high performance thermoelectric material at various temperatures by controlling compositions. In previous report, we achieved the 0.1 of ZT (figure of merit, ZT=S2σ/κ, where σ is electric conductivity, S is Seebeck coefficient, and κ is thermal conductivity) at room temperature in x=0.1. But the thermoelectric properties of this compound material are not clear yet. Hence, we would like to investigate value of ZT at various temperatures to find a high performance of thermoelectric properties.In this study, ZT values of (Bi2Te3)1-X(In2Se3)X system were investigated in the measurement temperature range of 300~500K according to x value. Structural characterizations were carried out using transmission electron microscope (TEM). The formation of compound was confirmed by high angle annular dark field-scanning transmission electron microscope (HAADF-STEM). And, the compositional ratio could be analyzed by Rutherford backscattering spectrometry (RBS).
9:00 PM - I10.19
The Effective Thermoelectric Figure of Merit of Composite Materials.
Yang Yang 1 , Feiyue Ma 1 , Jiangyu Li 1
1 Mechanical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractIt is well known that many of the effective properties of a composite material can exceed its constituents, when the microstructure of the composite is appropriately designed and engineered. For thermoelectric composites, however, it was concluded that the effective thermoelectric figure of merit cannot exceed the highest thermoelectric figure of merit of their constituent when the size and interface effects are excluded, regardless of the microstructural details. This conclusion, however, was drawn on questionable assumptions, and in this work, we reexamine the effective thermoelectric properties of composite materials using more physically sound derivation, and conclude that the effective thermoelectric figure of merit of composites can actually exceed their constituents in the absence of size and interface effects, offering a new strategy in search for thermoelectric materials with high figure of merit. Many interesting phenomena related to thermal and electric transport in thermoelectric composites are also discovered and discussed.
9:00 PM - I10.2
Dimensional Crossover of the Lattice Thermal Conductivity of Layered Materials.
Bo Qiu 1 , Xiulin Ruan 1
1 School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractIn this work, we use molecular dynamics simulations and spectral analysis methods to study the dimensional crossover of the lattice thermal conductivity of layered materials, namely, Bi2Te3 thin films and few-layer graphene. From molecular dynamics simulations we find that the dependence of lattice thermal conductivity on number of stacked layers are quite different for Bi2Te3 thin films and few-layer graphene. With the help of spectral analysis, we find that such difference is closely related to the different cross-plane coupling strength and the interplay between Umklapp process and boundary scattering.
9:00 PM - I10.20
Power Dissipation in a Quasi-1D Ballistic Electron System.
Rasmus Ballmer 1 , Elmar Sternemann 1 , Sven Buchholz 1 , Andreas Wieck 2 , Dirk Reuter 2 , Saskia Fischer 3 1
1 Werkstoffe und Nanoelektronik, Ruhr-Universitaet Bochum, Bochum Germany, 2 Angewandte Festkoerperphysik, Ruhr-Universitaet Bochum, Bochum Germany, 3 Neue Materialien, Humbold-Universitaet zu Berlin, Berlin Germany
Show AbstractThe fundamental mechanisms of heat transport in low dimensional electron systems are of great interest in order to build novel thermoelectric devices [1][2]. Regarding thermopower one has to distinguish between two contributions: charge carrier diffusion and the phonon-drag. In the Bloch-Grüneisen (BG) regime these two contributions have a different temperature dependency. A theoretical study of these two contributions exists [3], however, the experimental verification remains to be done. Here we investigate the power dissipation and hence the electron-phonon scattering rate in quasi-1D nanowires and in quantum point contacts (QPC’s) by evaluating the rise of the electron temperature versus applied heating power. We use a high-mobility GaAs/AlGaAs multilayer system in which the BG regime is extended to higher temperatures.In the BG regime the charge carriers are almost completely decoupled from phonons and an electron temperature higher than the lattice temperature can be established by the current heating technique. The correct determination of the electron temperature is of utmost importance, hence two different methods to determine the electron temperature are applied and compared with each other, i.e. thermometry via QPC’s and thermal noise measurements. The first method utilizes the thermovoltage that is build up when one side of a QPC is connected to a heating channel while the other side is connected to an electron reservoir kept at lattice temperature. The temperature difference can be determined by evaluating the peaks of the oscillating thermal voltage [4]. The second method applies the Nyquist formula to thermal white noise. A cross-correlated measurement setup is used to reduce noise contributions from amplifiers and leads [5]. The amplitude of the white noise is extracted from the frequency resolved spectral density of the thermal voltage fluctuations.The dissipated power per electron is determined. From the temperature dependence of the dissipated power we can identify the mechanism of heat loss in our system. At low heating currents we find that the heat loss arises from electron diffusion.[1] O’ Dwyer M F,Humphrey T E and Linke H, Nanotechnology 17, 338 (2006)[2] Scheibner R et al., New Journal of Physics 10, 083016 (2008)[3] Tsaousidou M and Butcher P M, Phys. Rev. B 56, 10044 (1997)[4] Appleyard et al., Phys. Rev. Lett. 81 No.16, 3491 (1998)[5] Sampietro M et al., Rev. Sci. Instrum. 70 No.5, 2520 (1999)
9:00 PM - I10.21
Low Loss EELS and EFTEM Study of Bi2Te3 Based Bulk and Nanomaterials.
Nicola Peranio 1 , Zainul Aabdin 1 , William Toellner 2 , Markus Winkler 3 , Jan Koenig 3 , Oliver Eibl 1 , Kornelius Nielsch 2 , Harald Boettner 3
1 Institut fuer Angewandte Physik, Universitaet Tuebingen, Tuebingen Germany, 2 Institut fuer Angewandte Physik, Universitaet Hamburg, Hamburg Germany, 3 , Fraunhofer Institut Physikalische Messtechnik, Freiburg Germany
Show AbstractEnergy-filtered transmission electron microscopy (EFTEM) yields new possibilities for the understanding of Bi_2Te_3 based nanomaterials. Combined low-loss electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray microanalysis (EDX) [1] and energy-filtered TEM were applied on a Zeiss 912Omega TEM to investigate nanowires, thin-films, and bulk materials. Multilayered Bi-Sb-Te nanowires with a diameter of 65 nm and a period of 200 nm were grown by electrodeposition under variation of the deposition potential. Tellurium elemental maps of the layered structure were obtained by two-window edge-jump images (EJI) acquired at the Te N_{4,5} ionization edge with energy losses of 36 eV and 42 eV. Chemical analysis by EDX showed that small Te fluctuations of 3 at.% yielded significant contrast in EJI in contrast to conventional imaging by diffraction contrast, as used in Bi2Te3 superlattice structures [1]. Energy-filtered TEM applied on nanoalloyed Bi_2Te_3 thin-films grown by molecular beam epitaxy (MBE) revealed few nanometer thick Bi-rich blocking layers at grain boundaries. Plasmon spectroscopy by EELS in the TEM was applied on bulk and nanoalloyed thin-films with a lateral resolution of 10 nm nanometer. Plasmon energies of 16.3 eV, 16.6 eV, and 16.4 eV, were found in Bi_2(Te_{0.91},Se_{0.09})_3 (54 at.% Te) bulk, Bi_2Te_3 (59 at.% Te) and Sb_2Te_3 (56 at.% Te) nanoalloyed thin-films, respectively. Finally, plasmon dispersion was measured for Bi_2(Te_{0.91},Se_{0.09})_3 bulk by angle-resolved EELS with a lateral resolution of 800 nm, which yields a fingerprint of the anisotropy and the dimensionality of the electronic structure of the materials [2]. For this, EEL spectra were acquired for momentum transfer in directions perpendicular and parallel to the c-axis with scattering angles up to 11 mrad and with a resolution of 1.1 mrad. For both directions a quadratic dependence of the plasmon energy on the momentum transfer was obtained. Plasmon dispersion turned out to be anisotropic with a ratio of 1.5 for the dispersion coefficients for momentum transfer perpendicular and parallel to the c-axis, respectively.References: [1] N. Peranio et al.; J. Appl. Phys. 100, 114306 (2006); Phys. Status Solidi A 204, 3243 (2007)[2] J. Fink et al., Phys. Rev B 40, 5799 (1989); Phys. Rev B 44, 7155 (1991); Phys. Rev. Letters 100, 196803 (2008)
9:00 PM - I10.22
Nanostructured Bi2Te3 Bulk Obtained by High-Energy Ball Milling and Spark-plasma Sintering.
Nicola Peranio 1 , Vicente Pacheco 2 , Juergen Schmidt 2 , Oliver Eibl 1
1 Institut fuer Angewandte Physik, Universitaet Tuebingen, Tuebingen Germany, 2 , Fraunhofer Institut fuer Fertigungstechnik und Angewandte Materialforschung (IFAM-DD), Dresden Germany
Show AbstractElectron-conducting Bi_2(Te,Se)_3 (4.5 at.% Se) and hole-conducting (Bi,Sb)_2Te_3 (29.8 at.% Sb) bulk samples (Peltron) were processed into nanostructured bulk by high-energy ball milling and subsequent spark plasma sintering (SPS). Powder X-ray diffraction investigations (XRD) were performed on the as cast, ball milled, and SPS-sintered samples. Depending on the milling conditions, crystallite sizes in the range of 15 nm to 23 nm were determined from Profile Analysis of selected XRD reflections (Full Width at Half of Maximum, FWHM) and Williamson-Hall analysis of the FWHM. The FWHM of the (00l) Bragg reflection is larger than other (hkl) reflections, and it deviates from the linear regression in the Williamson-Hall plot. This is an indication of a platelet like morphology of the crystallites with the thinnest side running parallel to the c-axis of the crystallites. The ball milled Powders were also analyzed by transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) for a quantitative chemical analysis [1]. Three types of morphologies were found by TEM in the n- and p-Bi_2Te_3 ball-milled powders: nano-crystallites of 5 to 30 nm, platelets of 50 to 200 nm, and agglomerates of crystallites and platelets of 200 nm to 3 \mu m. The chemical composition of the agglomerates agreed with the as-cast material whereas the chemical composition of the crystallites and platelets deviated from the starting material. Peak broadening in X-ray diffraction confirmed the nanostructure of thematerials after SPS yielding densities exceeding 90% of the theoreticaldensity. Thermal diffusivity was significantly lower for nanostructuredmaterials up to 30% as compared to otherwise identical conventionally preparedmaterials with grain size in the micrometer range. The influence of the crystallite size on the thermoelectric properties of the bulk sintered nanomaterial will be discussed.References: [1] N. Peranio et al.; Phys. Status Solidi A 204, 3243 (2007)
9:00 PM - I10.23
Thermal Conductivity of Oxidized Silicon Membranes with a Semi-analytical Elastic Continuum Approach.
Emigdio Chavez 1 2 , Pierre Oliver Chapuis 1 , John Cuffe 1 , Francesc Alzina 1 , Clivia Sotomayor Torres 1 2 3
1 , Catalan Institute of Nanotechnology,(ICN-CIN2), Barcelona Spain, 2 Dept. of Physics, Universitat Autònoma de Barcelona, Barcelona Spain, 3 , Catalan Institute for Research and Advances Studies ICREA, Barcelona Spain
Show AbstractWe report on thermal conductivity calculations of silicon free-standing membranes and clamped layers. Free-standing membranes are used in many applications, for example in nanosensors, nanomechanical systems and optomechanical systems. The reduction of the structure dimensions leads to the confinement of acoustics modes and a discretization of the acoustic spectrum, which results in a change in electron and phonon propagation. We apply the elastic continuum model to analyse the acoustic phonon modes. Single and three-layer membranes are investigated subjected to two different boundary conditions: free standing and clamped to a rigid surface. Of particular interest are the effects of the (native) silicon oxide on the membrane surfaces. The dispersion relations are computed using a numerical approach and are compared successfully to experimental and theoretical results. The phonon relaxation rates are also computed taking special care of the three-phonon Umklapp process. It is demonstrated that the confinement of acoustics phonons significantly affects this process increase the phonons relaxations rates.We finally calculate the thermal conductivities and analyse the origins of the reduction.
9:00 PM - I10.24
Wet Chemical Synthesis and Characterization of Nanoscale Transition Metal Antimonides.
Gregor Kieslich 1 , Christina Birkel 1 , Wolfgang Tremel 1 , Tanja Claudio 2 , Raphael Hermann 2
1 , Johannes Gutenberg-Universität, Institut für Anorganische Chemie und Analytische Chemie, Mainz Germany, 2 , Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich Germany
Show AbstractDuring the last decades nanosciences and nanotechnology have been among the fastest growing fields in material science. Many research groups focus on synthesizing tailor-made nanomaterials with distinct physical and chemical properties[1]. Nanosized materials are well-known to exhibit properties, which differ from the corresponding bulk phases. One example is the reduced thermal conductivity due to enhanced phonon scattering at the grain boundaries, whereas the electrical conductivity is not affected significantly[2,3]. Therefore, much interest has been devoted to down-scaling well known high ZT bulk phases to further improve the figure of merit, e.g. thin film layer structures, nanocompounds and nanosegregrations.Motivated by the reported colossal Seebeck coefficient of bulk-FeSb2 below 20K[4] we have developed a wet chemistry route towards nanoscaled FeSb2 with a phase purity of >95%, confirmed by 57Fe-Möβbauer spectroscopy and X-ray powder diffraction (XRD). As expected, nuclear inelastic scattering (NIS) on the nano-FeSb2 powder sample revealed a reduced thermal conductivity. In order to perform transport measurements the powder was pressed into a pellet with spark plasma sintering (SPS). Optimizing sintering conditions to obtain a dense pellet without destroying the nano-FeSb2 phase is a major goal in the future.Furthermore, we could show that our optimized wet chemistry route can be used for the synthesis of several other binary transition metal antimonides, e.g. FeSb, CoSb, NiSb and Cu2Sb. All compounds were synthesized with “activated“ Sb nanoparticles5 and metal nanoparticles as precursor materials. XRD and transmission electron microscopy (TEM) were used to characterize the nanoparticles. In all cases, phase pure metal antimonides were obtained. First studies of the particle growth based on refined XRD data provide hints for a two step diffusion and crystallization mechanism[6].The nature and the role of the particles’ surface during sintering are subject of ongoing research.References:(1) Y. Li and G. A. Somorjai; Nano Letters, 2010, 10(7), 2289-2295.(2) G. J. Snyder* and E. S. Toberer; Nature Materials, 2007, 7, 105 – 114.(3) R. Venkatasubramanian, E. Siivola, T. Colpitts; B. O'Quinn; Nature Materials, 2001 413, 597-602.(4) P. Sun, N. Oeschler, S. Johnsen, Bo B. Iversen and F. Steglich; Dalton Trans., 2010, 39, 1012–1019.(5) S. Schlecht, C. Erk, M. Yosef; Inorg. Chem., 2006, 45 , pp 1693–1697.(6) M. D. Hornbostel, E.J. Hyer, J.Thiel, D. C. Johnson; J. Am. Chem. Soc., 1997, 119 (11), pp 2665–2668
9:00 PM - I10.26
Thermal Conductivity of Silicon Nanowire Arrays.
Trong Tong 1 , Jun Ma 2 , Bruno Azeredo 2 , Keng Hsu 2 , Sanjiv Sinha 2 , David Cahill 1
1 Materials science and Engineering, University of Illinois Urbana Champaign, Urbana, Illinois, United States, 2 Mechanical Science and Engineering, University of Illinois Urbana Champaign, Urbana, Illinois, United States
Show AbstractThe ability to measure and understand heat flow in nanowire composites is crucial for applications ranging from high-speed electronics to thermoelectrics. Here we demonstrate the measurement of the thermal conductance of Si nanowire arrays embedded in spin-on-glass (SOG) using time-domain thermoreflectance (TDTR). The nanowire arrays, approximately 1000 nm length, 120 nm diameter and 23% areal packing density, are fabricated by using Ag-Au-lift-off patterning on Si substrates, followed by metal assisted chemical etch (MacEtch). Roughening is done by a solution with different H2O2/HF ratios and can be controlled by etching time. SOG is spun onto the sample to fill the gap between nanowires. A 80 nm Al thin film is deposited on top of the nanowire array and SOG for TDTR measurement. Our data show that the thermal conductivity of the nanowire arrays is independent of the penetration depth of the thermal wave, d, if d is sufficiently larger than the thermal healing length. A effective medium model for heat flow in the nanowire array is used to extract the thermal conductivity of the Si nanowires; we find W m-1 K-1. This result about half of the value typically measured for smooth Si nanowires fabricated by vapor-solid-liquid methods.
9:00 PM - I10.27
Synthesis of Fulleride Thin Films with Ultra-low Thermal Conductivity.
Michael Check 1 2 , Douglas Dudis 2 , John Ferguson 2 , Jamie Gengler 3 2 , Jianjun Hu 4 2 , Zachary Votaw 5 2 , Andrey Voevodin 2
1 , UTC, Dayton, Ohio, United States, 2 , Thermal Sciences and Materials Branch / AFRL, Wright- Patterson AFB, Ohio, United States, 3 , Spectral Energiers, LLC, Dayton, Ohio, United States, 4 , University of Dayton Research Institute, Dayton, Ohio, United States, 5 , SOCHE, Dayton, Ohio, United States
Show AbstractIn the past two decades, there has been growing interests in the design and improvement of thermoelectric (TE) materials and devices largely due to their potential use in technologies such as: 1) the conversion of waste heat to electricity, 2) solid-state refrigeration and heating, 3) biomedical batteries, and 4) power sources for both ground and space-based electronics.1 Recent research has suggested that by using nanotechnology (i.e. nanostructuring / nanoengineering) large advances can be gained in controlling interfaces to hinder thermal transport while allowing electrical movement. Thin film structuring of thermoelectric materials potentially offers several advantages over bulk thermoelectric materials. Furthermore, others have advocated that by making thermoelectric materials very small, one can achieve an enhanced ZT (the thermoelectric figure of merit) due to quantum confinement effects.2-5 The structure and physical properties of doped fullerene materials were investigated for use as electrically conducting phonon blocking layers. The synthesis and thermal properties of ZnxC60 and SbxC60 thin films are reported. Preliminary results have shown the formation of amorphous fullerides structures with thermal conductivities as low as 0.13 Wm-1K-1. Further physical and structural measurements (i.e Electron Microscopy, Electron Diffraction, Raman Spectroscopy, and 4-point probe) will be reported detailing the unique structure-property relationships in these materials. 1. Tritt, T. M.; Subramanian, M. A. Thermoelectric materials, phenomena, and applications: a bird's eye view. MRS Bulletin 2006, 31 (3), 188.2. Balandin, A.; Wang, K. L. Effect of phonon confinement on the thermoelectric figure of merit of quantum wells. Journal of Applied Physics 1998, 84 (11), 6149-6153.3. Bulusu, A.; Walker, D. G. Effect of quantum confinement on the thermoelectric properties of semiconductor 2D thin films and 1D wires. In Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference; Institute of Electrical and Electronics Engineers Inc.: San Diego, CA, United states, 2006; pp 1299-1305.4. Hicks, L. D.; Harman, T. C.; Sun, X.; Dresselhaus, M. S. Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit. Physical Review B (Condensed Matter) 1996, 53 (16), 10493-10496.5. Huber, T. E.; Nikolaeva, A.; Gitsu, D.; Konopko, L.; Graf, M. J. Quantum confinement and surface-state effects in bismuth nanowires. Physica E 2007, 37 (1-2), 194-199.
9:00 PM - I10.28
Impact of Impurities on the Electronic Structure of Thermoelectric Material Mg2Si.
Ka Xiong 1 , Sabina Sobhani 1 , Rahul Gupta 1 , Bruce Gnade 1 , Kyeongjae Cho 1 2
1 Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States, 2 Department of Physics, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractThermoelectric (TE) materials have attracted a lot of attention because of its capability of converting heat into electricity or vice versa. However, the growth of the TE industry has been hindered by the low energy conversion efficiency (ZT) of the TE materials. Recent experimental work showed that doping Tl into bulk PbTe could double ZT as compared to that of the undoped PbTe [1]. This increase is because the distortion of the electronic density of states would enhance the Seebeck coefficient of PbTe and corresponding ZT value. This finding motivates us to explore effects of dopants in other TE materials. In this work, we focus on Mg2Si, which is a promising TE material candidate because it is abundant, light, and environmental-friendly in comparison to heavy-metal based materials such as Bi2Te3 and PbTe [2]. We use first principles calculations to investigate the electronic structures and stability of dopants (e.g. Al, In, P, As, Sb, Bi, Ag, Cu, Zn, Cd etc.) in Mg2Si with various charge states. These methods have been previously applied to investigate the impact of defects in PbTe [3]. This study will help us to gain insights on the behavior of these dopants in Mg2Si and the mechanisms which cause the modulation of the TE efficiency. Our preliminary results show that Al, P, and Tl act as donors and make Mg2Si n-type, while Ag and Cu act as acceptors and make Mg2Si p-type. Sn, which is isoelectronic to Si, acts as neither a donor nor an acceptor. Among all these impurities, Al and Tl introduce a defect state in the band gap of Mg2Si. These electronic structure data will help to develop a doping strategy to optimize the TE efficiency. This work is supported by the II-VI Foundation, a private foundation. References[1] J. P. Heremans et al, Science 321, 554 (1999).[2] D. M. Rowe, Thermoelectrics Handbook (CRC, Boca Raton, 2006).[3] K. Xiong et al, J. Phys. D: Appl. Phys. 43, 405403 (2010).
9:00 PM - I10.29
Photoheating with Coated Gold Nanoparticles.
Pete Tandler 1
1 , Walsh University, North Canton, Ohio, United States
Show AbstractThere is much interest in the physical and chemical properties of metal nanostructures. The optical properties and the effect of coating the surface of nanoparticles are two areas of intense interest. Recently there has been an increased interest in using nanoparticles to generate heat by transforming light energy to thermal energy. Using nanoparticles for localized heating has found application in biological imaging, detection of biological molecules, drug delivery, and treating cancer. In this presentation, a simple method for measuring macroscopic heating of liquid droplets by nanoparticles will be applied to coated gold nanoparticles. Previously, we have shown that moderately high concentrations of aqueous colloidal gold are extremely efficient at light-to-heat conversion. However, these initial studies are limited in scope because the gold nanoparticles were not functionalized and the experiments were conducted in a simple matrix. This presentation extends the previous study to functionalized gold nanoparticles of different morphologies. Spherical and cylindrical nanopaparticles have been created with coating varying in length, rotatable bonds, branching, and polarity of end groups. The efficiency of coated nanoparticles for converting light-to-heat was determined and compared to that of naked nanoparticles whose efficiency is nearly 1. These studies are of importance because most current applications of nanoheaters use coated nanoparticles.
9:00 PM - I10.30
Assessment and Prediction of Thermal Transport at Solid-SAM Junctions.
John Duda 1 , Christopher Saltonstall 1 , Pamela Norris 1 , Patrick Hopkins 2 1
1 , University of Virginia, Charlottesville, Virginia, United States, 2 , Sandia National Laboratories, Albuquerque, Virginia, United States
Show AbstractSelf-assembled monolayers (SAMs) have recently garnered much interest due to their unique electrical, chemical, and thermal properties. Several studies have focused on thermal transport across solid-SAM junctions, demonstrating that interface conductance is largely insensitive to changes in SAM length. In the present study, we have investigated the vibrational spectra of alkanedithiol-based SAMs as a function of the number of methylene groups forming the molecular backbone via Hartree-Fock methods. In the case of Au-alkanedithiol junctions, it is found that despite the addition of nine new vibrational modes per added methylene group, only one of these modes falls below the maximum phonon frequency of Au. In addition, the alkanedithiol one-dimensional density of states is nearly constant regardless of chain length, explaining the observed insensitivity. Furthermore, we developed a diffusive transport model intended to predict interface conductance at solid-SAM junctions. It is shown that this predictive model is in excellent agreement with prior experimental and computational data available in the literature.
9:00 PM - I10.31
Thermoelectric Properties of SiO2/SiO2+Au Superlattices Effected by MeV Si Ions.
Satilmis Budak 1 , J. Chacha 1 , M. Pugh 1 , D. McElhaney 1 , T. Wright 1 , C. Smith 2 3 , K. Heidary 1 , R. Johnson 3 , C. Muntele 2 , D. Ila 2 3
1 Electrical Engineering, Alabama A.&M. University, NORMAL, Alabama, United States, 2 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States, 3 Physics, Alabama A&M University, Normal, Alabama, United States
Show AbstractThe performance of thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and k is the thermal conductivity. ZT can be increased by increasing S, increasing σ, or decreasing k. In this study we have prepared a thermoelectric generator from 100 alternating layers of SiO2/SiO2+Au superlattice films using DC/RF sputtering. In order to determine the stoichiometry of SiO2 and Au in the grown multilayer films and the thickness of the grown multi-layer films, Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used. The 5 MeV Si ion bombardments were performed using the CIM Pelletron ion beam accelerator to make nanoclusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity.Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, Rutherford backscattering, Figure of merit.*Corresponding author: S. Budak; Tel.: 256-372-5894; Fax: 256-372-5855; Email:
[email protected] Acknowledgement Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425.
9:00 PM - I10.32
Ag-Based Nanofluids for High-thermal Conductivity Applications.
Glorimar Garcia 1 , Oscar Perales Perez 1
1 , University of Puerto Rico, Mayaguez, Puerto Rico, United States
Show AbstractMetal-bearing nanofluids are receiving increased attention due to their expected enhanced thermal conductivity when compared with bare cooling fluids. Nanosize metals, e. g. gold, silver, copper or conductive carbon nanotubes are considered promising candidate materials to suspend them in typical cooling fluids. Our work is focused on the evaluation of the effect of silver crystal size and shape on the thermal conductivity of ethylene glycol, a typical cooling fluid. As the first step of our research, we have synthesized silver in ethylene glycol. Control of crystal size and shape at the nanoscale are attempted by suitable selection of the synthesis conditions and the presence of shape-control agents like chloride salts. Synthesized silver nanoparticles were characterized by X-ray diffraction, UV-vis spectroscopy, SEM, TEM and thermal conductivity.
9:00 PM - I10.33
Bismuth Telluride Thin Film Prepared by Cyclic Voltammetry Method and Its Ooptical Properties.
Yan Yang 1 , Zhigang Zeng 1 , Zhiyu Hu 1
1 , Institute of NanoMicroEnergy, Shanghai University, Shanghai China
Show Abstract As we all know, Bi-Te alloys are most mature and widely used thermoelectric materials, which have superior thermoelectric properties than other materials at room temperature, and thus one of the most extensively investigated thermoelectric materials. In addition to thermoelectric performance, few systematic studies of optical properties were done. In this paper, alloy films of Bi-Te were successfully deposited onto Ti film substrate by cyclic voltammetry (CV) methods, and their optical reflectivity in the visible and near infrared broad band was measured. The UV-VIS-NIR spectra showed that the reflectivity of the films was below 5% for wavelengths from 300 to 1700 nm. CV deposition was a process of repeated erosion deposition, and the film fabricated by CV method received a very rough morphology, which similar to the “black silicon” structure. Test results showed that the reflectivity of incident light had a relationship with the surface roughness. The comparison of the morphology and optical properties of Bi-Te films prepared in different technologies was conducted, including galvanostatic and magnetron sputtering. The results showed that films with compact structure and smooth surface possessed higher reflectivity. We can obtain the surface structure information from the reflectance spectra. That was because, for microscopically rough surfaces, the light interacts with the whole surface rather than with each rough spot. In this work, Bi-Te alloy films with low reflectivity in the visible and near infrared broad band of wavelength (300-1700nm) were fabricated by cyclic voltammetry (CV) method. Coupled with good thermoelectric properties of Bi-Te alloys, films obtained by this method can absorb the thermal energy and then convert into electricity more effectively. And this material will have important value in the comprehensive use of thermal photovoltaic and thermoelectric power generation.
9:00 PM - I10.37
Characterization of Electrodeposited Nano-crystalline Sb2Te3 Films.
Jessica Lensch-Falk 1 , Dhego Banga 1 , Vitalie Stavila 2 , Peter Sharma 1 , Douglas Medlin 1
1 Materials Physics Department, Sandia National Laboratories, Livermore, California, United States, 2 Combustion Research Facility, Sandia National Laboratories, Livermore, California, United States
Show AbstractNanostructured thermoelectric materials have been shown to improve thermoelectric properties by reducing thermal conductivity and improving the power factor as compared to bulk materials. Electrodeposition is a promising low-cost method to fabricate thermoelectric thin films such as Sb2Te3 and its alloys. However, electrodeposition of crystalline Sb2Te3 without the need for additional processing and with good compositional control has presented a challenge. In this presentation we describe the electrodeposition of nano-crystalline Sb2Te3 thin films at room temperature from a tartaric-nitric electrolyte using a pulsed potentiostatic process. We correlate the processing conditions with the resulting microstructure and compositional homogeneity using x-ray diffraction, electron diffraction, high resolution electron microscopy, and analytical electron microscopy. We observe a unique microstructure with lamellar grains that measure 10-40 nm thick by up to 750 μm long present in films deposited at short pulse times (< 50 ms). Within these grains, the Sb2Te3 (000l) planes are parallel to the long axis giving the overall film a (000l) texture which is not observed at longer pulse times. The composition of these films is found to be homogeneous across the thickness of the thin film. Variation of electrodeposition parameters such as pulse time, duty cycle, and potential allow for investigation of the nucleation and growth mechanism.
9:00 PM - I10.38
Thermoelectric and Magnetic Properties of Yb14-xCexMnSb11.
Jason Grebenkemper 1 , Catherine Uvarov 1 , John Roudebush 1 , Tanghong Yi 1 , Susan Kauzlarich 1
1 Chemistry, University of California, Davis, California, United States
Show AbstractYb14MnSb11 is currently one of the highest efficiency thermoelectric materials for high temperature use. This high figure of merit could be related to an enhanced Seebeck coefficient due to its magnetic properties, which result from a high spin tetrahedral Mn2+ with an anti-aligned Sb 5p hole compensating for one of the spins. Yb14MnSb11 is ferromagnetic below 53 K, and has been described as a heavy fermion compound with a Kondo effect. Previous studies have replaced some Yb2+ with La3+, leading to a reduced carrier concentration and increased thermoelectric efficiency. This study will examine the effects of doping Ce3+ to replace Yb2+. While this will lower the carrier concentration, the Ce3+ also has a single unpaired electron, leading to additional effects on magnetic properties. Single crystals of Yb14-xCexMnSb11 were grown in Sn flux and characterized. Microprobe showed diminishing amounts of Ce were able to be added to the system, with x = 0.59(2) achieved for a synthetic x = 2. Magnetic susceptibility was measured from 5 to 300 K with fields varying from 0 to 3 T. The samples showed a Tc ~ 40 K, which is lower than undoped Yb14MnSb11, but close to that of the La doped material. The thermoelectric properties of the Ce doped material will also be examined and compared with previously investigated materials.
9:00 PM - I10.39
Band Structure and Thermoelectric Properties of Y-doped Mg2Si Prepared by Field-activated Reactive Sintering.
Shaoping Chen 1 2 , Wenhao Fan 3 , Qingsen Meng 1 , Tanghong Yi 4 , Susan Kauzlarich 4 , Zuhair Munir 2
1 Materials Science and Engineering, Taiyuan University of Technology, Taiyuan China, 2 Chemistry Engineering and Materials Science, University of California, Davis, California, United States, 3 Physics, Taiyuan University of Technology, Taiyuan China, 4 Chemistry, University of California, Davis, California, United States
Show AbstractFirst principle band structure calculations on Y-doped Mg2Si suggest that it should be n-type with Y replacing Mg in the structure. Y-doped Mg2Si with different content of Y dopant were synthesized from elemental powders of magnesium and silicon by field-activated pressure-¬assisted synthesis (FAPAS). The thermoelectric properties were evaluated from 300 to 850 K and the sample doped with 2 wt.% Y provided the highest Seebeck coefficient, electric conductivity, and power factor. Y-doped Mg2Si was also prepared by spark plasma sintering with nano silicon. X-ray powder diffraction results show that the product is nano-scaled with full density. The thermoelectric properties of Y-doped Mg2Si prepared by these two methods will be presented and the effect of nano-Si powder on the properties will be discussed.
9:00 PM - I10.6
Thermoelectric Power of Nanotube/Polymer Heterogeneous Multilayer Thin Film Fabrics.
Corey Hewitt 1 , David Carroll 1
1 Physics, Wake Forest University, Winston Salem , North Carolina, United States
Show AbstractThermoelectrics are materials that are capable of the solid state conversion between thermal and electrical energy. In this study, the Seebeck coefficient α and σ were determined for SWNT/PVDF heterogeneous thin films. NT/polymer composites were used because of the NT’s favorable electrical conductivity, and the polymer’s low thermal conductivity, which can both be used to increase the figure of merit ZT. Films of varying SWNT weight percent values from 5% to 100% with T from 20K to 290K were tested to determine the effects of NT concentration and T on ZT. Values of α and σ ranged from 16 µV/K and 10000 S/m for 100% to 32 µV/K and 100 S/m for 5%. This test resulted in a maximum ZT of 0.005 for the 100% film at 290 K. To further enhance the ZT of the films, the SWNT/PVDF films were stacked with N-doped NT/PVDF films (α of -15 µV/K) in alternating layers with successive junctions at opposing ends to form multiple layer thermoelectric “fabrics”. This results in an increased α, while maintaining a constant σ. Using this technique for an 11 layer fabric resulted in a α of 225 µV/K and a ZT of about 0.3, greatly enhancing the figure of merit over that for the single layer. This technique could theoretically be used for any number of layers provided the heat source is capable of supplying an adequate temperature gradient.
9:00 PM - I10.7
Effect of Interface Roughness on the Thermal Conductivity of GaAs/AlAs Superlattices.
Abdelhak Saci 1 , Duquesne Jean-Yves 1
1 , Institut des NanoSciences de Paris, Paris France
Show AbstractWe present thermal conductivity measurements in short period GaAs/AlAs superlattices. The aim of this study is to get a better understanding of the various processes which restrict the phonon transport in semiconductor superlattices. In particular, the role of interfacescattering is still under debate and very few experiments have been devoted to address this issue. Using the so-called 3w method, we measured the cross-plane thermal conductivity of (GaAs)n(AlAs)n where n=3, 8 and 20. The interface roughness is controlled thanks to the growth temperature of the samples. We also present and discussresults as a function of temperature, between 20 and 300 K. For a given period, no significant change as a function of interface roughness is observed, in contrast to theoretical predictions. However, recent molecular dynamic simulations, using realistic interface models, could give clues to this low sensitivity versus interface quality.
9:00 PM - I10.8
Thermoelectric Properties of Na(Co1-xMx)2O4 (M: Ag, Cu).
Kyeongsoon Park 1 , C. Choi 1
1 , Sejong University, Seoul Korea (the Republic of)
Show Abstract A series of Na(Co1-xAgx)2O4 (x = 0, 0.1, 0.15, 0.2, and 0.25) and Na(Co1-xCux)2O4 (x = 0, 0.04, 0.08, 0.12, 0.16, and 0.2) powders was synthesized by solution combustion process. The partial incorporation of Ag or Cu for Co in NaCo2O4 led to increases in density and hole concentration of the system, thereby increasing electrical conductivity. The electrical conductivities of Na(Co1-xAgx)2O4 and Na(Co1-xCux)2O4 samples were in the range of 105.0 to 226.3 and 125.3 to 245.2 Ω-1cm-1 , respectively, at 800 °C. The Seebeck coefficients of the Na(Co1-xMx)2O4 (M: Ag and Cu) increased with increases in the Ag and Cu content up to x=0.1 and 0.04, respectively, and then decreased with further increased Ag and Cu content. The values of the power factors for the Ag- and Cu-added Na(Co1-xMx)2O4 ranged from 0.25×10-3 to 1.76×10-3 and 0.43×10-3 to 4.28×10-3 Wm-1K-2, respectively, at 800 °C. The maximal power factors, 1.76×10-3 and 4.27×10-3 Wm-1K-2, were obtained for Na(Co0.8Ag0.2)2O4 and Na(Co0.84Cu0.16)2O4, respectively, at 800 °C.
9:00 PM - I10.9
Transport Properties of p-type Bi0.5Sb1.5Te3 Thermoelectric Materials.
Kyeongsoon Park 1 , S. Nam 1
1 , Sejong University, Seoul Korea (the Republic of)
Show AbstractIn this study, we fabricated high-efficiency Bi0.5Sb1.5Te3 thermoelectric materials with a high figure-of-merit by controlling both initial powder size and sintering temperature. The Bi0.5Sb1.5Te3 sintered at 450 or 500°C with various powder sizes (<45, 45-105, 106-150μm) was a solid solution of the constituent elements, which had a rhombohedral structure, regardless of initial powder size and sintering temperature. The electrical and thermal conductivities of the Bi0.5Sb1.5Te3 sintered at 500°C were higher than those of the Bi0.5Sb1.5Te3 sintered at 450°C mainly because of a higher hole concentration. The Bi0.5Sb1.5Te3 fabricated with intermediate powders (45-105μm) in size had the highest hole concentration and the lowest porosity, resulting in the highest electrical conductivity. The Bi0.5Sb1.5Te3 sintered at 450°C with the smallest powders (<45μm) in size showed the highest value of the Seebeck coefficient (240μV/K) mainly because of the lowest hole concentration (0.439×1025m-3). We obtained the highest figure-of-merit (2.82×10-3/K) for the Bi0.5Sb1.5Te3 sintered at 450°C with intermediate powders.
Symposium Organizers
Kornelius Nielsch University of Hamburg
SaskiaF. Fischer Ruhr-Univ. of Bochum
BethanieJ. H. Stadler University of Minnesota
Ted Kamins Stanford University
I11: Thermophotovoltaics and Emerging Thermal Devices
Session Chairs
Saskia Fischer
Anke Weidekaff
Friday AM, April 29, 2011
Room 2007 (Moscone West)
9:30 AM - I11.1
Seebeck Coefficient Effects of Nanoscale Conductors in a Gaseous Flow Environment.
Patrick Garrity 1 , Kevin Stokes 2
1 Physics, Loyola University New Orleans, New Orleans, Louisiana, United States, 2 Physics, Advanced Materials Research Institute, New Orleans, Louisiana, United States
Show AbstractThe surrounding ambient introduces a gaseous boundary to many potential nanotechnology applications such as nanoscale thermoelectric devices and low dimensional thermal control devices. Despite the large surface area to volume ratio of nanostructures, a formal study of the surface scattering effects induced by a gaseous boundary has received little attention. In this work, we consider the perturbing effects to the electron cloud or jellium of conducting nanostructures when submitted to a gaseous interface of varying interaction energies. Specifically, we incorporate the novel experimental method of Dynamic Electron Scattering (DES) to measure the Seebeck coefficient of 30 nm thick Au and Cu metal films in He and Ar atmospheres. The gas particle impact energy is varied by changing the flow speed from stationary (non-moving gas field) to high speed flow over the metal films. The scattering effects of each gas are clearly observable through a Seebeck coefficient increase as the gas impact energy increases. We find the high collision density of He to induce a greater increase in thermopower than the much heavier Ar with lower collision density. The perturbed transport properties of the Au and Cu thin films are explained by kinetic surface scattering mechanisms that dominate the scattering landscape of high surface area to volume ratio materials as suggested by comparative measurements on bulk Cu.
9:45 AM - I11.2
Electrothermally Driven Current Vortices in Inhomogeneous Bipolar Semiconductors.
Deyi Fu 1 2 3 , Alejandro Levander 1 4 , Rong Zhang 2 3 , Joel Ager 4 , Junqiao Wu 1 4
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing, Jiangsu, China, 3 Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing, Jiangsu, China, 4 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractFree carriers motion in a semiconductor is driven by gradients in electric potential, carrier density (described by the chemical potential), and/or temperature. In a homogeneous system in electrical open circuit, the balance among the electric field driven (drift current), density gradient driven (diffusion current), and thermal gradient driven currents leads to a static electrothermal state: the electrochemical potential (Fermi level) varies along the temperature gradient, but the net current is zero. The situation is more complicated in inhomogeneous systems containing regions with different doping or composition. Such systems are frequently encountered in the search for high thermopower and simultaneous high electrical conductivity in modern thermoelectric devices. Unlike in homogeneous semiconductors, electrothermal transport in inhomogeneous structures is subject to microscopic interactions between the temperature field and the built-in electric field. To our knowledge, the effect of these interactions on the electrothermal transport has not been elucidated yet. Here we analyze a prototype bipolar structure in which the gradient of doping inhomogeneity is orthogonal to that of temperature. By solving the coupled Poisson and Boltzmann transport equations we have discovered a dynamic electrothermal effect. Steady current vortices develop in the system even in the absence of external carrier injection. When the system size is comparable to carrier depletion length, the effective thermopower is significantly different from what is expected in a static bipolar model. Also, Joule heating arising from the current vortices effectively reduces the thermal conductivity by an amount that may be comparable to the electronic thermal conductivity of the system.
10:00 AM - I11.3
Microstructural Characterization of Nanocrystalline Silicon and Silicon-germanium Composites for Thermoelectricity.
Ralf Theissmann 1 , Niklas Stein 1 , Nils Petermann 1 , André Becker 1 , Hartmut Wiggers 1 , Gabi Schierning 1 , Roland Schmechel 1
1 Center for NanoIntegration (CeNIDE), University of Duisburg Essen, Duisburg Germany
Show AbstractAn optimized thermoelectric material combines a high Seebeck-coefficient and electrical conductivity with a low thermal conductivity. The latter is achieved by the incorporation of scattering centres for phonons. In a given material system, it can be realized by alloying, by the use of nanometer-scaled material or structural modulations at the nanometer scale, by the incorporation of precipitates, or by the use of other obstacles for phonons like structural domains. By transmission electron microscopy (TEM), nanostructures in thermoelectric silicon and silicon-germanium composites are investigated and analysed with respect to their ability to improve the thermoelectric performance.Our bottom up approach starts with tailored silicon, germanium, and silicon-germanium alloy nanoparticles from a gas phase process. The compaction is done by a dc-current sintering. Samples are further prepared for TEM by sawing, grinding, polishing, and final thinning with Ar-ions. Different imaging techniques are employed to investigate specific feature of the nanostructure: By direct imaging, the overall nanocrystalline structure of the composites is demonstrated. High resolution imaging techniques are used to show the quality of the crystalline interfaces. Element specific analysis was used to investigate the chemical distribution of the alloy. Energy filtered imaging technique was employed to increase the contrast between silicon and silicon dioxide. Thus, the distribution of the native oxide within the samples could be studied. A special preparation with focussed ion beam allowed for the production of TEM-samples with a well defined thickness. These samples were used for a 3-dimensional tomography. Reconstruction of the data confirmed the formation of oxidic precipitates within a well connected silicon matrix.Different techniques of nanocomposite formation for the silicon-germanium system were studied. While the simple mixing of silicon and germanium nanoparticles by a milling process resulted in an inhomogeneously sintered nanocomposite, the direct alloying of the nanoparticles in the gas phase gave rise to a well sintered, homogeneous alloy nanocomposite with a thermal conductivity around 1.5 W/m/K and a figure of merit of about 0.5 around 500 °C.
10:15 AM - I11.4
Realization of a Low Thermal Resistance Hybrid Material Using Carbon Graphite for Chip Cooling Applications.
Yoichi Taira 1 , Keiji Matsumoto 1 , Kuniaki Sueoka 1
1 , IBM Research - Tokyo, Yamato Japan
Show AbstractSoft and compliant high heat transport material is important in the computer system especially in the chip cooling system. Carbon based materials such as carbon nanotubes, graphite rods, and graphite sheet have very high thermal conductivity when fabricated at higher temperature. Therefore this type of material systems have more use when it is made soft compliant without losing the thermal performance. We already showed that a hybrid system consisting of stacked 2D graphite sheets has much higher thermal conductivity than any metal. Vertically aligned graphite also shows similar high conductivity while vertically aligned carbon nanotube system has a limited value because of its low volume filling factor. In order to utilize their high thermal conductivity for chop cooling applications, the system has to be thermally connected with chips and heat removal element. There are several ways to connect the system thermally with the facing elements we can use, 1) liquid metal, 2) inorganic oil, 3) polymer gel, or 4) polymer network, where these agents fill the gap between the system and the neighboring element. We found that the overall thermal resistance differs heavily depending on the connecting agent regardless of the carbon system. For the realistic application of chip cooling, material system 3) and 4) are more realistic choice because 1) and 2) are not favored because the liquid part has to be contained so that it won’t go out into the computer system. Although the microscopic heat transport at the interface between the carbon system and the interface agent has to be take into account in these cases, the heat transport can be handled well with a simply bulk model especially when the density of the carbon system is 10% or more in the volume fraction. A simple series resistance model shows that the interfacial layer material determines the most of the thermal performance. To realize a low thermal resistance, the end graphene in the carbon system needs to couple with the interfacial material thermally well and the heat transport within the interfacial material. Since the heat path is all connected in the fully cured polymer network, the thermal resistance is much lower than the polymer gel were the heat path is not continuous. We could actually show that the thermal resistance of a polymer embedded graphite system in a sheet form is much low comparable to the flexible metal sheet and the actual heat resistance is less than 10 C mm2/W even at the thickness of 0.5 mm, where the equivalent thermal conductivity is 50 W/m-K. The fully cured polymer network also has good features including compliance and also some spring effect which the indium metal sheet does not have. The system demonstrates give a good thermal performance together with compliance and spring effect, which are the key requirement for the chip cooling. Efficient heat removal also gives us other benefits for reducing power consumption of the computer system by effective recycling.
10:30 AM - **I11.5
Hotwater Cooling For Energy-Hungry Datacenters.
Gerhard Meijer 1 , Juergen Marschall 2 , Stephan Paredes 1 , Manfred Ries 2 , Thomas Brunschwiler 1 , Gottfried Goldrian 2 , Bruno Michel 1
1 , IBM Research - Zurich, Rueschlikon Switzerland, 2 , IBM Germany, Boeblingen Germany
Show AbstractSoon after the Internet took off in the mid-1990s, enterprise computing infrastructures with warehouses full of servers, known as data centers, became commonplace. The energy consumption challenges posed by such data centers are considerable. The power dissipation of servers has to be managed skillfully. Perhaps surprisingly, the power consumption of the cooling infrastructure that is required to keep the microelectronic components from overheating is on a par with that of the servers themselves. ——— The real key to ratcheting down the energy consumption of a computing facility is liquid cooling. The reason is that thermodynamically liquid cooling is much more efficient than air cooling because the heat capacity of liquids is orders of magnitude larger than that of air. Once the heat has been transferred to the liquid, it can be handled very efficiently. ——— Recently, chilled-liquid cooling was reintroduced in high-end mainframes and densely packed servers to cope with the high heat fluxes. Yet, liquid cooling can be taken further if we consider a microfluidic heat sink. Microchannel heat sinks can be designed such that the thermal resistance between the transistor and the fluid is reduced to the extent that even cooling water temperatures of 60° to 70°C ensure no overheating of the microprocessors. This hotwater cooling has compelling advantages. First, chillers are no longer required yearround and thus the data-center energy consumption plummets by up to 50%. Second, and perhaps most important, direct utilization of the collected thermal energy becomes feasible, either using synergies with district heating or specific industrial applications. With such an appealing waste-heat recovery system, the green diligence of data centers would be upped substantially. ——— In this presentation I will discuss this hotwater cooling concept in detail and will present a first-of-a-kind water-cooled supercomputer - dubbed Aquasar - which was built for the Swiss Federal Institute of Technology Zurich that marks a new era in energy-aware computing.
11:30 AM - I11.6
Interfacial Thermal Conductance of Metal Films Transfer-printed using Microtip Stamps.
Seok Kim 1 , Dongwook Oh 1 , Sanjiv Sinha 2 , David Cahill 1 3 , John Rogers 1 3 4
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractResearch on thermoelectric generators is often directed at finding new materials, the scaling of components, and the development of new fabrication methods needed to achieve higher performance. Assembly using transfer-printing is emerging as one of the most promising approaches for the fabrication of flexible modules. Transfer-printing enables heterogeneous integration of multi-scale materials in a deterministic way and has been widely exploited in the fabrication of high-performance flexible electronics. Transfer-printing can, in principle, be used to assemble flexible thermoelectric generators but the thermal properties of the interfaces between transfer-printed components are essentially unknown. Here, we present results for the thermal conductance of interfaces between various common substrates (Si, SiO2, and Al2O3) and 100 nm thick metal films (Au and Au(Pd) alloy) which are formed by transfer-printing. We use 100 μm by 100 μm square metal films since this dimension is similar to the size of the potentially scaled-down thermoelectric generators. We use recently developed microtip elastomeric stamps to deterministically transfer-print the films without using any adhesives. The interfacial thermal conductance G is measured using time domain thermo-reflectance (TDTR) as a function of temperature 300 < T < 600 K and the results are compared with that of deposited films. The thermal conductance of transfer-printed Au films is surprisingly high, G>30 MW/m2-K, and comparable to the thermal conductance of interfaces formed by sputter deposition of Au.
11:45 AM - I11.7
Plasmon-assisted Heating in Concentric Nanotubes.
Marc Ghossoub 1 , Sanjiv Sinha 1
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractPlasmonic nanostructures absorb infrared and visible radiation to become nanometer scale heat sources that can be exploited for cancer therapy [1], drug delivery, surface chemistry [2], memory storage, and microfluidics [3]. Numerical simulations on metallic nanoparticles and nano-antennas using either the Greens’ dyadics [4] or the FDTD method show that the optical and thermal hot spots do not coincide in such structures. The local field enhancement is proportional to the accumulation of static charges while the heat generation is a direct consequence of the induced current densities in the metal. This dependence has been recently confirmed experimentally using a combination of two photon luminescence and fluorescence polarization anisotropy measurements on gold nanowires and nanorods [5]. Here, we use the Green’s dyadic method to simulate the local field enhancement and heat generation in concentric nanotubes formed from a Si-Gold bilayer. We show the dependence of local field and current distributions and the location of the thermal hot spot on gold thickness and the wavelength, polarization and incidence angle of the excitation. Such cylindrical patterns can be readily fabricated using a superionic electrochemical nanoimprinting technique with metal assisted chemical etching [6]. This work shows how such structures can be exploited in applications related to precisely controlled temperature rise at sub-100 nm scales.1.A. M. Gobin et al., Nano Lett. 7, 1929 (2007).2.L. Cao, D. Barsic, A. Guichard, and M. Brongersma, Nano Lett. 7, 3523 (2007).3.N. J. Florous, K. Saitoh, and M. Koshiba, IEEE Trans. Nanotechnol. 6, 549 (2007).4.O. J. F. Martin, C. Girard, and A. Dereux, Phys. Rev. Lett. 74, 526 (1995).5.G. Baffou, C. Girard, and R. Quidant, Phys. Rev. Lett. 104, 136805 (2010).6.K. H. Hsu, P. L. Schultz, P. M. Ferreira, and N. X. Fang, Nano Letters, 7:2, 446-451, (2007).
12:00 PM - I11.8
Thermally Stable Tungsten Inverse Opals for Thermophotovoltaic Applications.
Kevin Arpin 1 , Paul Braun 1
1 Materials Science and Engineering , University of Illinois at Urbana Champaign, Urbana , Illinois, United States
Show AbstractMetallic photonic crystals have been proposed as structures, which may exhibit a highly modified thermal emission relative to conventional emitters. When properly designed, they may present a narrowed thermal emission, which can be used to minimize losses in solar cells and achieve ultra high efficiencies (thermophotovoltaics). Previous reports of tungsten woodpile structures demonstrate modified thermal emission, however, these structures were made via a complicated fabrication process and because of their relatively large characteristic dimensions, exhibit photonic properties in the infrared regime. It is desirable to fabricate a tungsten photonic crystal by simple self-assembly that exhibits useful optical properties close to the visible regime. However, the fabrication of a thermally stable tungsten inverse opal for use as a narrow band emitter remains an unsolved challenge. Herein, we demonstrate a tungsten inverse opal, fabricated via electrodeposition inside a self-assembled 3D template. Thermal stability (up to at least 1000°C) of such a structure is achieved, for the first time, using an alumina passivation layer that confines and controls sintering and grain growth that nominally would destroy the 3D periodicity. The optical response of these structures is reported and related to the thermal emission.
12:15 PM - I11.9
Solid-state Device Architecture for Photon-enhanced Thermionic Emission.
Samuel Rosenthal 1 2 , Jared Schwede 1 3 , Daniel Riley 1 3 , Kunal Sahasrabuddhe 1 3 , Vijay Narasimhan 1 2 , Igor Bargatin 4 , Roger Howe 4 , Zhi-Xun Shen 1 3 , Nicholas Melosh 1 2
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States, 2 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 3 Department of Physics and Applied Physics, Stanford University, Stanford, California, United States, 4 Department of Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractPhoton-enhanced thermionic emission (PETE) is a new method of solar energy conversion that operates by thermionic emission of photoexcited electrons from a high-temperature semiconductor cathode to a lower- temperature anode. Previous work has focused on a vacuum gaparchitecture, which ensures thermal separation of the electrodes butrequires challenging surface engineering to minimize surfacerecombination and reduce electrode work functions.Replacing the vacuum gap with a wide-bandgap semiconductor wouldeffectively passivate the emissive surface while offering greatertunability of energy level offsets, but at the cost of thermallyshorting the electrodes. Here we propose an all solid-statenanostructured architecture for PETE. This structure can maintain alarge temperature difference between the device electrodes whileallowing efficient diffusion of excited carriers across a sparse fieldof wide-bandgap semiconductor nanowires. By combining the thermalisolation of the vacuum gap architecture with the electronicadvantages of solid-state interfaces, this work establishes a pathtoward stable, high-efficiency devices.
12:30 PM - I11.10
High Efficiency pBn Thermophotovoltaic Cells.
Dante DeMeo 1 , Thomas Vandervelde 1
1 Electrical Engineering, Tufts University, Medford, Massachusetts, United States
Show AbstractThe focus of this paper is the characterization of novel thermophotovoltaic (TPV) cell designs which employ a barrier layer in the pn junction. The use of a barrier layer enables these cells to operate at longer wavelengths, higher efficiencies, and higher operating temperatures. Designs were examined incorporating bulk materials as well as type-II strained layer superlattice (SLS) structures, where the effective bandgap is adjustable and a function of the thickness of the individual layers. Additionally, the type-II nature of the SLS causes the charge carriers to be spatially separated helping to minimize Auger recombination. Simulations performed using Sentaurus were used to determine barrier materials, while the testing was done on a cryogenic thermal simulator and mid-range blackbody tester. The cells employing SLS structures have the ability to extract energy from long wavelength infrared photons, which will enable devices to harvest energy from lower temperature sources than has been previously possible, thus opening new applications in thermal energy harvesting.
12:45 PM - I11.11
Optimization and Simplification of Spectrally Coherent Thermal Sources.
Jeremie Drevillon 1 , Elyes Nefzaoui 1 , Karl Joulain 1 , Philippe Ben-Abdallah 2
1 Department FTC, Institut PPRIM, Poitiers France, 2 Laboratoire Charles Fabry, Insitut d'optique, Palaiseau France
Show AbstractIn the last decade, there has been an increased interest to the conception of micro/nanostructures with unusual radiative properties, far away from those of blackbody, especially thermal sources with temporal coherent emission [1-4]. Such structures are especially extremely interesting for energy conversion systems as thermal photovoltaics (TPV). Indeed, in TPV systems, one important point is the PV cell conversion gap since the PV cell converts photons into electricity only near this gap. If the thermal source emits also only near this gap, the conversion efficiency will increase compare to a black body source. Up to now, temporal coherent thermal sources had rather complex structure and despite the development of nanofabrication techniques including techniques for thin film deposition it was therefore difficult to integrate them into a TPV system. To design coherent thermal sources with simpler structure, we developed numerical optimization methods such as genetic algorithms [4] and particle swarms. We present several coherent thermal sources with a structure a simple as possible, each of these sources emitting photons in a specific wavelength range. We show that a bilayer structure composed with one coat of germanium and one of silicon carbide is able to emit in narrow spectral peak in mid-infrared [5]. We design also very simple multilayer metallic structures emitting in near infrared region. The integration of these structures in TPV systems should allow increasing significantly the conversion efficiency. [1] J. J. Greffet, R. Carminati, K. Joulain, J. P. Mulet, S. Mainguy, and Y. Chen, Nature,416, 61 (2002)[2] P. Ben-Abdallah and B. Ni, J. Appl. Phys., 97, 104910 (2005)[3] B. J. Lee, C. J. Fu, and Z. M. Zhang, Appl. Phys. Lett., 87, 071904 (2005)[4] J. Drevillon and P. Ben-Abdallah, J. Appl. Phys., 102, 114305 (2007)[5] J. Drevillon, P. Ben-Abdallah and K. Joulain, JQSRT, (2010)
I12: Novel TE Materials
Session Chairs
Jan Koenig
Hartmut Leipner
Friday PM, April 29, 2011
Room 2007 (Moscone West)
2:30 PM - I12.1
Approaching Glass-like Thermal Conductivity in Nanostructured Zn4Sb3.
Eric Toberer 1 2 , Greg Pomrehn 2 , Protima Rauwel 3 , Johan Tafto 3 , Axel van de Walle 2 , Jeff Snyder 2
1 Physics Dept., Colorado School of Mines, Golden, Colorado, United States, 2 Materials Science, California Institute of Technology, Pasadena, California, United States, 3 Physics Dept., University of Oslo, Oslo Norway
Show AbstractZn4Sb3 is an attractive thermoelectric material (zT > 1) with a high mobility and extremely low lattice thermal conductivity. The low lattice thermal conductivity has been attributed to point defect scattering and a soft mode associated with dumbbell 'rattling' of the Sb-Sb dimer. Here, transmission electron microscopy reveals a high density of nanoscale features (precipitates and pores) which additionally contribute to the observed low thermal conductivity. Embedded nanoparticles of ZnSb and Zn are understood to arise from cooling through the relevant phase boundaries. Voids are believed to form due to retrograde solubility of Zn-rich Zn4Sb3, where Zn nanoparticles precipitate and then are re-adsorbed on further cooling. Ab initio calculations of the Zn-Sb phases yields a phase diagram which is consistent with these experimental results. Such an understanding of the phase-formation behavior, resulting nanostructures, and impact on lattice thermal conductivity is critical for commercial applications of Zn4Sb3.
3:00 PM - I12.3
Effect of Nanostructuring on Thermoelectric Properties and Microstructure of Cobalt Antimony Skutterudites with Addition of Indium and Cerium.
Andreas Sesselmann 1 , Christian Stiewe 1 , Klemens Kelm 1 , Eckhard Mueller 1
1 Inst. of Materials Research, German Aerospace Center (DLR), Cologne Germany
Show AbstractRecently nanostructured cerium and indium added cobalt antimony based skutterudites have shown high thermoelectric figure of merit (ZT) at temperatures between 300-700 K. The ZT enhancement of the material is mostly attributed to high filling fractions and a non-equilibrium synthesis method, which comprises rapid soldification and short term sintering of the material. The objective of this paper is to investigate the effect of different synthesis parameters and planetary ball milling of indium and cerium filled skutterudites with high and low filling fractions on high temperature thermoelectric properties and microstructure.In a previous study we have shown that good thermoelectric efficiency of indium added cobalt antimony skutterudites can be achieved at lower filling than the reported maximum filling fraction. Additionally, with an equilibirium synthesis route, showing InSb precipitations of several μm in size in indium filled CoSb3, we are able to exclude the effect of nanosized InSb precipitates on the thermoelectric properties and thereby have an experimental opportunity of comparison.Phase homogeneity of the bulk material has been evaluated by X-ray powder diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). Electrical conductivity, Seebeck coefficient and thermal conductivity have been analyzed in the temperature range from 300 K to 700 K. The functional homogeneity of the as cast material and the compacted samples were determined by the Potential & Seebeck Microprobe (PSM) and the microstructure was analyzed by scanning electron microscopy (SEM). Based on these results the effect of the synthesis parameters, planetary ball-milling and high and low filling fractions of indium and cerium on the efficiency of the thermoelectric skutterudite material will be discussed.