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

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

Current NASA Missions Radioisotope Thermoelectric Generators (RTGs) powers deep space probes and rovers Cassini - SaturnMars Science Laboratory RTG

Thermoelectrics Cooling Heat Rejected h + e - Seebeck Effect Power Generation Peltier Effect Electronic Cooling/Heating Heat Source Heat Sink h + e - + -

Thermoelectric cooling/heating Waste heat recovery Heated and cooled car seats Terrestrial Applications of Thermoelectric Devices Thermoelectric Generator

Thermoelectric Figure of Merit S, Seebeck coefficient , electrical conductivity, total thermal conductivity T, temperature = lattice + electronic S =  V/  T

Thermoelectric Materials S S2σS2σ σ SemiconductorsMetals Arbitrary Units Insulators

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

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

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) %

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

12 Compaction Hot uniaxial compression Need dense pellets for thermoelectric measurements Sintering of nanoparticles ~80-95% of melting point

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

Thermal Conductivity: Bulk Nanostructured Silicon Up to 90% reduction in the thermal conductivity

Lattice Thermal Conductivity

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

Seebeck

Resistivity of Nano-bulk Silicon

ZT of Nano-Bulk Si Over 250% increase in the ZT over single crystals!

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

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