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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
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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 - Saturn Mars Science Laboratory
RTG
http://saturn.jpl.nasa.gov/; http://marsprogram.jpl.nasa.gov/msl/
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ThermoelectricsCooling
Heat Rejected
h+ h+ e- e-
Seebeck Effect
Power Generation
Peltier Effect
Electronic Cooling/Heating
Heat Source
Heat Sink
h+ h+ e- e-+ -
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• Thermoelectric cooling/heating
• Waste heat recoveryHeated and cooled car seats
Terrestrial Applications of Thermoelectric Devices
http://www.foursprung.com/2006_10_01_archive.htmlhttp://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 conductivityT, temperature
= lattice + electronic
S = V/T
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Thermoelectric Materials
S
S2σ
σ
Semiconductors Metals
Arb
itra
ry U
nit
s
1019
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 materials p-type thermoelectric materials
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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.jsphttp://products.asminternational.org/hbk/index.jsp
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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
4000
3000
2000
1000
0
Inte
nsity
10080604020Degrees Two Theta Cu K
NSN30_24 15.5 nm crystallites JCPDS Si 00-027-1402a 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
10
110
210
310
410
510
610
710
810
910
200 700 1200T (K)
( / )mW cmK
Heavily Doped n-type Si Single Crystal
n-type Nano Si
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Lattice Thermal Conductivity
10
110
210
310
410
510
610
710
810
910
200 400 600 800 1000 1200 1400T (K)
L( / . )mW cm K
n-type Nano Bulk Si
Heavily Dopedn-type Si Single Crystal
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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
-250
-200
-150
-100
-50
0
200 400 600 800 1000 1200 1400T (K)
Seebeck Coefficient (
/ )V K
n-type Nano Bulk Si
Heavily Doped n-type Si Single Crystal
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Resistivity of Nano-bulk Silicon
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
200 400 600 800 1000 1200 1400T (K)
Electrical Resistivity (m
Ω. )cm
Heavily Doped n-type Si Single
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ZT of Nano-Bulk Si
Over 250% increase in the ZT over single crystals!
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
200 400 600 800 1000 1200 1400T (K)
ZT
n-type Nano Bulk Si
Heavily Doped
n-type Si Single Crystal
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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
0
100
200
300
400
500
600
200 400 600 800 1000 1200 1400T (K)
Lat
tice
Th
erm
al C
on
du
ctiv
ity
(mW
/cm
K)
Heavily doped 'single crystal' Si
Heavily doped 'nanobulk' Si
Bux et al. Mater. Res. Soc. Symp. Proc. (2009), 1166, 1166-N02-04
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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