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

Cassini - Saturn Mars Science Laboratory

RTG

http://saturn.jpl.nasa.gov/; http://marsprogram.jpl.nasa.gov/msl/

ThermoelectricsCooling

Heat Rejected

h+ h+ e- e-

Seebeck Effect

Power Generation

Peltier Effect

Electronic Cooling/Heating

Heat Source

Heat Sink

h+ h+ e- e-+ -

• 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

Thermoelectric Figure of Merit

S, Seebeck coefficient , electrical conductivity, total thermal conductivityT, temperature

= lattice + electronic

S = V/T

Thermoelectric Materials

S

S2σ

σ

Semiconductors Metals

Arb

itra

ry U

nit

s

1019

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 materials p-type thermoelectric materials

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

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%

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

12

Compaction Hot uniaxial

compression• Need dense pellets for

thermoelectric measurements

• Sintering of nanoparticles ~80-95% of melting point

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

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

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

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

-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

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

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

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

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