Nanostructured Materials for Thermoelectric Power Generation

1. Nanostructured Materials for Thermoelectric Power Generation Richard B. Kaner1, Sabah K. Bux1,3, and Jean-Pierre Fleurial3 1Department of Chemistry and California NanoSystems Institute, University of California, Los Angeles (UCLA) 2Jet Propulsion Laboratory (JPL), Pasadena, CA Chem 180/280 May 23, 2012

2. 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

3. Cassini - Saturn Mars Science Laboratory Current NASA Missions • Radioisotope Thermoelectric Generators (RTGs) powers deep space probes and rovers RTG http://saturn.jpl.nasa.gov/; http://marsprogram.jpl.nasa.gov/msl/

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

5. Heated and cooled car seats http://www.foursprung.com/2006_10_01_archive.html Terrestrial Applications of Thermoelectric Devices • Thermoelectric cooling/heating • Waste heat recovery Thermoelectric Generator http://www.themotorreport.com.au/23040/bmw-and-nasa-teaming-up-to-devise-regenerative-exhaust-system/

6. Thermoelectric Figure of Merit • = llattice + lelectronic S = DV/DT S, Seebeck coefficient s , electrical conductivity l, total thermal conductivity T, temperature

7. Thermoelectric Materials S σ S2σ Arbitrary Units 1019 Insulators Semiconductors Metals

8. Current State of the Art Bulk Materials n-type thermoelectric materials p-type thermoelectric materials The maximum ZT is about 1.2 over the entire temperature range for bulk materials

9. Phonon Mean Free Path and Thermal Conductivity in Si • Phonon mean free path (MFP) spans multiple orders of magnitude • 80% of the k at 300 K comes from phonons that travel less than 10 mm • 40% of the k at 300 K comes from phonons with MFP<100 nm 1000 K 300 K Dresselhaus et al

10. Synthesis Nano Bulk Pellets Starting Materials Nano Bulk Powder Ball Milling Hot Uniaxial Compaction High purity elements (e.g. Si, Ge) 99.999% Unfunctionalized nanostructured powders Pellets 99% of theoretical density

11. 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

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

13. a b 100 nm c d 10 nm 20 nm Nanostructured Si/SiGe Phase Pure Si, Crystallite Size 15 nm TEM: Nano Si Aggregates Ion milled, 99% dense pellet with nanostructured inclusions Aggregate made up of small nanocrystallites Bux, Dresselhaus, Fleurial, Kaner, et al. Adv. Funct. Mater.2009, 19, 2445

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

15. Lattice Thermal Conductivity

16. Nanoparticles Phonon Electron Bulk Nanostructured Materials • Increase phonon scattering via interfacial scattering (reduce thermal conductivity) • Minimize electron scattering (maintain electrical properties) Picture courtesy of Gang Chen (MIT)

17. Seebeck

18. Resistivity of Nano-bulk Silicon

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

20. Same process of high energy ball milling applied to p-type Si Substantial reductions in thermal conductivity p-type Nanobulk Si 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