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Nanostructured Materials for Thermoelectric Power Generation Richard B. Kaner 1, Sabah K. Bux 1,3, and Jean-Pierre Fleurial 3 1 Department of Chemistry and California NanoSystems Institute, University of California, Los Angeles (UCLA) 2 Jet Propulsion Laboratory (JPL), Pasadena, CA Chem 180/280 May 23, 2012
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Why Thermoelectrics? NASA’s deep space missions –Not enough solar flux beyond Mars Compact, solid-state devices –Survives the vibrations from launch Long lifetimes –Voyager ~30 years Space and terrestrial applications http://www.its.caltech.edu/~jsnyder/thermoelectrics
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Current NASA Missions Radioisotope Thermoelectric Generators (RTGs) powers deep space probes and rovers Cassini - SaturnMars Science Laboratory RTG http://saturn.jpl.nasa.gov/; http://marsprogram.jpl.nasa.gov/msl/http://saturn.jpl.nasa.gov/http://marsprogram.jpl.nasa.gov/msl/
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Thermoelectrics Cooling Heat Rejected h + e - Seebeck Effect Power Generation Peltier Effect Electronic Cooling/Heating Heat Source Heat Sink h + e - + -
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Thermoelectric cooling/heating Waste heat recovery Heated and cooled car seats Terrestrial Applications of Thermoelectric Devices http://www.foursprung.com/2006_10_01_archive.html http://www.themotorreport.com.au/23040/bmw-and-nasa-teaming-up-to-devise-regenerative-exhaust-system/ Thermoelectric Generator
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Thermoelectric Figure of Merit S, Seebeck coefficient , electrical conductivity, total thermal conductivity T, temperature = lattice + electronic S = V/ T
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Thermoelectric Materials S S2σS2σ σ SemiconductorsMetals Arbitrary Units 10 19 Insulators
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Current State of the Art Bulk Materials The maximum ZT is about 1.2 over the entire temperature range for bulk materials n-type thermoelectric materialsp-type thermoelectric materials
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9 1000 K 300 K Phonon Mean Free Path and Thermal Conductivity in Si Dresselhaus et al Phonon mean free path (MFP) spans multiple orders of magnitude 80% of the at 300 K comes from phonons that travel less than 10 m 40% of the at 300 K comes from phonons with MFP<100 nm
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Synthesis Starting Materials Ball Milling Nano Bulk Powder Hot Uniaxial Compaction Nano Bulk Pellets Pellets 99% of theoretical density Unfunctionalized nanostructured powders High purity elements (e.g. Si, Ge) 99.999%
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Mechanical Alloying/High Energy Ball Milling Nanostructured materials are formed from constant welding and fracturing Scalable technique –Processing conditions must be adapted for each materials Mechanochemical process http://products.asminternational.org/hbk/index.jsp
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12 Compaction Hot uniaxial compression Need dense pellets for thermoelectric measurements Sintering of nanoparticles ~80-95% of melting point
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Nanostructured Si/SiGe a b c d 20 nm 10 nm 100 nm Bux, Dresselhaus, Fleurial, Kaner, et al. Adv. Funct. Mater. 2009, 19, 2445 Phase Pure Si, Crystallite Size 15 nm TEM: Nano Si Aggregates Aggregate made up of small nanocrystallites Ion milled, 99% dense pellet with nanostructured inclusions
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Thermal Conductivity: Bulk Nanostructured Silicon Up to 90% reduction in the thermal conductivity
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Lattice Thermal Conductivity
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Bulk Nanostructured Materials Increase phonon scattering via interfacial scattering (reduce thermal conductivity) Minimize electron scattering (maintain electrical properties) Phonon Electron Picture courtesy of Gang Chen (MIT) Nanoparticles
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Seebeck
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Resistivity of Nano-bulk Silicon
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ZT of Nano-Bulk Si Over 250% increase in the ZT over single crystals!
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p-type Nanobulk Si Same process of high energy ball milling applied to p- type Si Substantial reductions in thermal conductivity Bux et al. Mater. Res. Soc. Symp. Proc. (2009), 1166, 1166-N02-04
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21 Conclusions Ball milling can be used to decrease the particle size of Si ZT increases by a factor of ~250% due to the decrease in thermal conductivity This method can be applied to SiGe alloys such as those used in RTG generators for space applications
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