Purdue University

NSF/DOE Thermoelectrics Partnership: Thermoelectrics for Automotive Waste Heat Recovery

Xianfan Xu (ME), Timothy Fisher (ME), Stephen Heister (AAE), Timothy Sands (MSE), Yue Wu (ChemE)

Purdue UniversityIn Partnership with General Motors Global R&D

and Oak Ridge National Laboratory

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NSF/DOE Targeted Areas

Focused areas of this proposal: • TE Materials• Heat sink - high temperature side heat exchanger• Thermal interface materials• Metrology

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TE Materials• Skutterudite (GM) ~ 500oC – advanced characterization on

thermal conductivity reduction (Xu)• Nanowires - room-mid temperature PbTe, Bi2Te3; and high

temperature oxide nanowires (Wu)• Metal-semiconductor superlattice (Sands)

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Advanced Characterization of TE Materials

- Commonly used approach to increase ZT of TE materials is to the reduce the lattice thermal conductivity

- We conduct femtosecond (10-15 s) time-resolved studies of phonon scattering in thermoelectric materials to understand the fundamentals of thermal conductivity reduction

Xianfan Xu

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Phonon Detection – Coherent Phonon (coherent vibration of lattice)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 4 8 12 16

R (a

.u.)

Time delay t (ps)

Random thermal vibration

Optical signal when there is nocoherent lattice oscillation

0

0.2

0.4

0.6

0.8

1

0 4 8 12 16

R (a

.u.)

Time delay t (ps)

With coherent oscillation

Coherent Phonon - LO

Coherent Phonon - TO

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Coherent Phonon Dynamics in Bi

0

1

2

3

4

5

0 4 8 12 16

∆R/R

(%)

time delay t (ps)

0

1

2

3

4

5

-0.8 0 0.8 1.6 2.4 3.2 4 4.8 5.6

ExperimentFitting

0 1 2 3 4 5 6

Am

plitu

de o

f FT

Frequency (THz)

2.88 THz

- Rise of the signal: Photon – electron/exciton (charge) coupling- Decay of the envelope of the signal: electron/exciton (charge) – phonon

coupling - heating- Phonon oscillation and dephasing: Phonon interactions with charges,

other phonon modes, impurities, physical boundaries, etc. – all important for energy transport and conversion processes

Fouriertransform

50 100 150 200

Ram

an S

igna

l (a.

u.)

Raman Shift (cm-1)

98 cm-1 (2.94 THz)

Raman

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Filled Antimony Skutterudites

Oscillation due to filling of guest materials

Co

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Lattice Thermal Conductivity Calculation

Boundary scattering

Defect scattering

Umklappscattering

Resonant oscillation due to filling of heavy elements

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Misch-metal Filled Antimony Skutterudites

Nominal representation

Composition

Co0.9 (unfilled) Co0.9Fe0.1Sb3

Mm0.55 Mm0.55Fe2.44Co1.56Sb11.96

Mm0.65 Mm0.65Fe2.92Co1.08Sb11.98

Mm0.72 Mm0.72Fe3.43Co0.57Sb11.97

Mm0.82 Mm0.82Fe4Sb11.96

Mm: Ce:La:Nd:Pr:Si:Fe:Al:O = 50.75:22.75:16.22:5.72:3.35:0.72:0.50

0

1

2

3

4

1 2 3 4 5∆R

/R (X

10-3

)

Mm0.82

(b)

Mm0.72

Mm0.65

Mm0.55Co

0.9

Delay (ps)

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- Computation of κ based on resonant oscillation model and ultrafast measurement results agree well with k value measured from ~ 0 to 300 K.

- Role of filled material: resonant vibration vs. defect generation

(Phys. Rev. Lett, 2009)

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Phonon Scattering in Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 Superlattice

(in collaboration with RTI)

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Nanowire TE Materials

Yue Wu, School of Chemical Engineering

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Synthesis of Molecular-scale Chalcogenide Nanowires

Scalable and high yield (93%)

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TeNWs

PbTe-Te-PbTeNWs

PbTe-Bi2Te3-PbTe NWs

Se-Te-Se NWs

PbSe-PbTe-PbSeNWs

Bi2Se3-Bi2Te3-Bi2Se3NWsPbTe-Ag2Te-PbTe NWs

PbTe-Cu2Te-PbTe NWs

PbTe-SnTe-PbTeNWs

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Bulk-like Thermionic Energy Conversion Device Fabricated from Laminated Nanostructured Metal/Semiconductor Superlattices

100 nm100-Si

HfN/ScN

Co-PI: Tim SandsResearch Associate: Jeremy SchroederGraduate Student: Bivas Saha

100 µm2 nm

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Benefit of M-S superlattice

Differential Conductivity [(W-cm-J)-1]0 2000 4000

EF

ND= 1.61 x 1023 cm-3

11.2

10.8

10.6

11.0

10.4

0 50000 1e5

ND= 1.61 x 1023 cm-3

metal metal/semiconductorsuperlattice

D. Vashaee and A. Shakouri, Phys. Rev. Lett. 92, 106103 (2004)

hot e-

cold e-

EF

EF

metal metalsemi-cond.

eV

ΦB

hotside

conduction band minima profile

coldside

Thermionic transport

Ener

gy [e

V]

Zebarjadi et al., J Elect. Mat., (2009)

1

1.5

2

2.5

3

3.5

0.75 0.8 0.85 0.9 0.95 1 1.05

601 K700 K794 K900 K1000 K1100 K1200 K1300 K

ZT

dEc (eV)

ZT

knot conserved0

0.5

1

1.5

2

0.7 0.75 0.8 0.85 0.9

601 K700 K794 K900 K1000 K1100 K1200 K1300 K

ZT

dEc (eV)

ZT

k conserved

ZT predictions• Experimental data:

•κ = 1.8 W/m-K•ΦB = 280 meV• S = -820µV/K

280meV

900K

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Laminates

Thermal propertiesκTE = 3 W/m-K κ Au = 300 W/m-KRc,thermal ≈ 7 x 10-8 K-m2/W

Electrical propertiesρc = 1 x 10-8 Ω-cm2

ρ TE = 1 mΩ -cm ρ Au = 25 µΩ -cm

Total thickness

TE layers 50µm

Au bonding layers 20µm

93% of total thermal impedance96% of total electrical impedance

Laminate bilayers

substrate

substrate

5µm polySL

5µm polySL

Bond Etch substrateDeposit metal

1µm bonding metal

70µm

TE layers

Process requires low metal-nitride/metal-bond contact resistance

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Bulk-like laminate

1 mm

sewingpin

• 40 thermoelectric layers (5 µm each)• 80 gold layers (1 µm each)•~300 µm x 300 µm x 280 µm• diced and polished (3 µm diamond lapping disc)

FESEM

100 µm

100 µm

100 µm

100 µm

SEM of four sides of polished cube

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Professor Steve Heister School of Aeronautics & Astronautics

Heat Exchangers for Automotive Exhaust Applications

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Background• Rolls-Royce established a University Technology Center

(UTC) at Purdue in 2003 to study High Mach Propulsion for aerospace applications

• A key technology to enable high Mach flight is a heat exchanger that cools some compressor exhaust gases for use in turbine blade cooling- Air temps 1000-1200F (550-650 C) pressures to 40 bar- Fuel temps to 1000F (550 C) at pressures to 90 bar

• UTC has graduated roughly 10 students working in technologies to enable a fuel/air heat exchanger for operation in difficult environments• fuel coking/thermal stability• tube/shell HEX design• plate/fin HEX design• heat transfer augmentation

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Foil Laminate Construction

21

Chemical etching produces microscale flow passages on individual foils. The foils are then stacked, aligned, and diffusion bonded under heat and pressure.

From “Compact Superalloy Heat Exchangers for Cooled Cooling Air Applications”, G. Campbell, J Fryer, Saddleback Aerspace, Turbine Symposium Presentation ca. 2002

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Heat Exchanger for Waste Heat Recovery

Inflow Exhaust

Fin, or Integrated Fin/TE Assembly (IFTEA)

A A’B

B’

Inflow Exhaust

Fin, or Integrated Fin/TE Assembly (IFTEA)

A A’B

B’

Coolant

TE material

Hot gas

Hot gas

Coolant

TE material

Hot gas

Hot gas

Coolant

TE material

Hot gas

Hot gasInflow

Fins

: high-T TE material: low-T TE material

CoolantThermal interface materials

Hot side heatexchangerInflow

Fins

: high-T TE material: low-T TE material

CoolantThermal interface materials

Hot side heatexchanger

And many other possible topologies (cylindrical etc.)

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CNT Array Thermal Interfaces Materials

Timothy S. Fisher, Mechanical Engineering

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CNT Array Thermal and Electrical Interfaces

Solid 1

Solid 2

Solid 1

Solid 2

One-sided interface Two-sided interface

Solid 1

Solid 2

Cu foil

CNT/foil interface

One-sided (post test)

5 µm

copper

silicon5 µm 100 µm

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1

10

100

1000

0.1 0.15 0.2 0.25 0.3 0.35 0.4

Pressure (MPa)

Inte

rfac

e R

esis

tan

ce (

mm

2 K/W

) Bare Interface (1-D ref. bar)

1-Sided Interface

CNT/foil Interface

2-Sided Interface

Thermal Interface Results: Summary

References:

B.A. Cola, J. Xu, T.S. Fisher,Int J Heat Mass Transfer, 52, 3490-3503 (2009).

B.A. Cola, J. Xu, C. Cheng, H. Hu, X. Xu, and T.S. Fisher, J. Appl. Phys. 101, 054313 (2007).

B.A. Cola, X. Xu, and T.S. Fisher, Appl. Phys. Lett. 90, 093513 (2007).

J. Xu and T.S. Fisher, Int. J. Heat Mass Trans. 49, 1658 (2006).

J. Xu, T.S. Fisher, IEEE Trans. CPT, 29, 261 (2006).

silicon/copper substrates at room temperature

State-of-the-art commercial materials(non-bonded interfaces)

Target performance range

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CNT Interfaces to Thermoelectrics

0

5

10

15

20

25

30

35

0 5 10 15 20 25

Res

ista

nce

(Ω)

Bi2Te3 film thickness (µm)

Ni - Bi2Te3structure

Ni - CNT - Bi2Te3 structure30 µm

TE film

MWCNT array

Ni - CNT - Bi2Te3 structure

Motivation• Parasitic electrical contact resistance can strongly degrade the efficiency of thermoelectric devices• Direct electrodeposited thermoelectric (Bi2Te3) on CNTs shows reduced interface resistance

Mishra et al., Adv Mat 21 4280, 2009

With Sands group

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Improving Thermal Stability

Raidongia et al. (J. Mater. Chem. 2008, 18, 83-90) demonstrated large improvements in thermal stability of nanotubes by conversion to BC4N

Our recent work:

1 μm 1 μm

(a) (b)

MWCNT array B, N modified array

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Thermal Property Characterization: Laser Thermal Reflectance Measurement

• To determine contact resistance and thermal conductivity

• Temperature up to 700oC• Use either a femtosecond

laser pulse or nanosecond laser pulse for different range of contact resistance- femtosecond laser for measuring Rct < 10-6 m2W/K - nanosecond laser for measuring Rct > 10-7 m2W/K

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

4.5 10-6 5 10-6 5.5 10-6 6 10-6 6.5 10-6 7 10-6 7.5 10-6 8 10-6

Exp Data

Model Data

4.5 10-6 5 10-6 5.5 10-6 6 10-6 6.5 10-6 7 10-6 7.5 10-6 8 10-6

Nor

mal

ized

Tem

p

Time (s)

Rct=2 x 10-6 m2W/K

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TEG System-level Design and Testing• System level thermal/TEG design (TE modules, thermal

interface materials, heat sinks, constraints, etc.)• Durability• Cost• Test strategy, i.e., components vs. sub-systems vs. TEG

TE modules

Exhaust gas

Cooled by coolant

TE Generator

(Image courtesy: GM)

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• Project started from Jan. 1st, 2011• Purdue/GM project kick-off meeting on Dec.

13, 2010, at Purdue

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We thank the support from:NSF, PM: Dr. Ted BergmanDOE, PM: Dr. John Fairbanks, Dr. Tom Avedisian

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Coherent Acoustic Phonon

-2

0

2

4

6

0 50 100 150 200 250 300

∆R/R

(10-4

)

Time delay t (ps)

(a)

0

50

100

150

1 2 3 4 5

PeaksPeakFitting

Pea

k (p

s)

Number of peak

0.1076+28.533t

-2

0

2

4

6

10 15 20 25 30 35 40 45 50

APFitting

∆R/R

(10-4

)

Time delay t (ps)

(b)

2

2 2 22

2 2 2

( ') / 4 | '|/( , ) 0

1

31 '

4z z t z

z t

Svt z z

S B T

T T e e dzt

α ξ

η ηρ

β

πα∞ − − −

−∞

∂ ∂ ∂= +

∂ ∂ ∂= − ∆

∆ = ∆ ∫

• Wave propagation equation:

• Driving source:

• Temperature increase:

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Experimental Setup for Pump-and-probe Measurements

Pump-and-probe set-upPulse shaping apparatus

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Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 Superlattice

Bi2Te3 Sb2Te3 Superlattice

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Phonon Scattering in Bi2Te3/Sb2Te3 Superlattice• Scattering of phonon is due to phonon-charge, phonon-phonon, and

phonon-boundary/interface interactions.• Scattering rate of coherent phonon increases linearly with pump

fluence.• Phonon scattering in bulk Bi2Te3, bulk Sb2Te3 and Bi2Te3/Sb2Te3

superlattice is measured and compared.

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Ultrafast Measurement vs. Raman

0

2

4

6

8

60 80 100 120 140 160 180 200

1.6 2.4 3.2 4 4.8 5.6In

tens

ity (A

rb. U

nits

)

Raman Shift ( cm-1)

Mm0.82

Mm0.72

Mm0.65Mm

0.55

Co0.9

Vibration Frequency ( THz)

• Raman measurement detects the Sb4 ring frequency: 4.6 THz• Ultrafast measured frequencies are different from the Raman

frequency, but approach that of Sb4 ring at high filling ratio, indicating strong interactions between the guest atoms and the lattice (Sb4 ring).

Red arrows:frequencies from ultrafast measurements

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ZT ~ 1 in typical bulk thermal electric materials.

A good thermoelectric material means low electric resistivity ρ and low thermal conductivity κ (phonon glass electron crystal – PGEC)

Figure of merit:

• Carnot efficiency:

• ZT = 1 corresponds to η =10% • ZT = 4 corresponds to η = 30% • Home refrigerator: η ~ 30% • Bi2Te3/Sb2Te3 Superlattice: ZT = 2.4

cold

hot cold

TT T

η =−

Rowe, Thermoelectrics Handbook: Macro to Nano, CRC Press (2006).

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- A systematic study of the effect of phonon scattering in TE materials, and the resulting thermal conductivity reduction.

- Effect of filling ratio of misch metal in different family of skutterudites.

- A theoretical understanding on the effect of the rattling behavior of the filled elements.