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
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
Current NASA Missions
• Radioisotope Thermoelectric Generators (RTGs)
powers deep space probes and rovers
RTG
Cassini - Saturn
http://saturn.jpl.nasa.gov/;
Mars Science Laboratory
http://marsprogram.jpl.nasa.gov/msl/
Thermoelectrics
Cooling
Heat Source
+
h+ h+
e- e-
-
h+ h+
e- e-
Heat Sink
Heat Rejected
Seebeck Effect
Peltier Effect
Power Generation
Electronic Cooling/Heating
Terrestrial Applications of
Thermoelectric Devices
• Thermoelectric cooling/heating
Heated and cooled car seats
• Waste heat recovery
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
Thermoelectric Figure of Merit
l = llattice + lelectronic
S = DV/DT
S, Seebeck coefficient
s , electrical conductivity
l, total thermal conductivity
T, temperature
Thermoelectric Materials
σ
Arbitrary Units
S
S2σ
1019
Insulators
Semiconductors
Metals
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
Phonon Mean Free Path and
Thermal Conductivity in Si
1000 K
• 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
300 K
9
Dresselhaus et al
Synthesis
Starting
Materials
High purity
elements
(e.g. Si, Ge)
99.999%
Ball Milling
Nano Bulk
Powder
Unfunctionalized
nanostructured
powders
Hot Uniaxial
Compaction
Nano Bulk
Pellets
Pellets 99% of
theoretical density
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
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
Phase Pure Si,
Crystallite Size
15 nm
a
4000
NSN30_24 15.5 nm crystallites
JCPDS Si 00-027-1402
TEM: Nano Si
Aggregates
b
Intensity
3000
2000
1000
100 nm
0
20
40
60
80
100
Degrees Two Theta Cu K
c
d
Ion milled, 99%
dense pellet with
nanostructured
inclusions
Aggregate made
up of small
nanocrystallites
20 nm
10 nm
Bux, Dresselhaus, Fleurial, Kaner, et al. Adv. Funct. Mater. 2009, 19, 2445
Thermal Conductivity: Bulk
Nanostructured Silicon
910
810
710
l (mW/c mK)
Heavily Doped
n-type Si Single Crystal
610
510
410
310
n-type Nano Si
210
110
10
200
700
T (K)
1200
Up to 90% reduction in the thermal conductivity
Lattice Thermal Conductivity
910
810
710
kL (mW/cm.K)
Heavily Doped
n-type Si Single Crystal
610
510
410
310
n-type Nano Bulk
Si
210
110
10
200
400
600
800
T (K)
1000
1200
1400
Bulk Nanostructured Materials
• Increase phonon
scattering via
interfacial scattering
(reduce thermal
conductivity)
Phonon
Electron
• Minimize electron
scattering (maintain
electrical properties)
Picture courtesy of Gang Chen (MIT)
Nanoparticles
Seebeck
Seebeck Coefficient ( m V/K)
0
Heavily Doped
n-type Si Single Crystal
-50
-100
n-type Nano Bulk Si
-150
-200
-250
200
400
600
800
T (K)
1000
1200
1400
Resistivity of Nano-bulk Silicon
Electrical Resistivity (m  .cm)
1.4
1.2
1.0
0.8
0.6
0.4
Heavily Doped
n-type Si Single
0.2
0.0
200
400
600
800
T (K)
1000
1200
1400
0.8
ZT of Nano-Bulk Si
0.7
0.6
n-type Nano Bulk Si
ZT
0.5
0.4
0.3
0.2
0.1
Heavily Doped
n-type Si Single Crystal
0.0
200
400
600
800
T (K)
1000
1200
1400
Over 250% increase in the ZT over single crystals!
p-type Nanobulk Si
• Substantial
reductions in
thermal conductivity
Lattice Thermal Conductivity (mW/cmK)
• Same process of
high energy ball
milling applied to ptype Si
600
Heavily doped 'single crystal' Si
500
Heavily doped 'nanobulk' Si
400
300
200
100
0
200
400
600
800
T (K)
1000
1200
1400
Bux et al. Mater. Res. Soc. Symp. Proc. (2009), 1166, 1166-N02-04
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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