Thermoelectric Materials

Thermoelectric Materials 

Dan Lorenc 

Katrina Ellison 

March 12, 2009 

1

Moral of the Story 

• Biggest challenges for 

us: 

• Packaging 

• Integration 

• Bulk manufacturing 

http://www.thermoelectric.com/2005/images/te-1.jpg 

© Dan Lorenc and Katrina Ellison 2009 

3 

Slide

Physical Overview 

TH 

Current 

flow 

V+ 

Thermoelectric 

Material 

V- 

Tc 

Power output 


Slide 4

History 

• 1821: Seebeck effect 

• 1834: Peltier effect 

• 1854: Thomson effect 

• 1910: Figure of merit 

• 1950: First space 

application 

First Thermopile 

circa 1840 

http://www.dself.dsl.pipex.com/MUSEUM/POWER/thermoelectric/ 

thermoelectric.htm 


5 

Slide

Applications 

• Cooling 

www.micropelt.com 

• Power generation 

• Space 

• Waste heat 

Radioisotope Thermoelectric Generator (RTG) 

Used on Voyager 1 & 2 

http://thermoelectrics.caltech.edu/history_page.htm 

www.nextbigfuture.com 

© Dan Lorenc and Katrina Ellison 2009 Slide 6

Strengths and Weaknesses 

Strengths 

High reliability 

Lightweight 

Quiet 

High scalability 

Weaknesses 

Low efficiency 

Expensive 

Exotic materials 

Difficult to manufacture 


7 

Slide

Typical Specs 

Custom 

Thermoelectric 

Nextreme 

Lincoln Lab 

Thot 260° C ~ 210° C 327° C 

Tcold 50° C ~ 90° C 107° C 

ΔT 210° C 120° C 220° C 

Imatched load 1.43 A 0.875 A - 

Vmatched load 4.33 V 0.28 V - 

Powermatched load 6.19 W 0.240 W - 

Size 1600 mm 2 5.12* mm 2 120 mm 2 

Power Density 0.39 W/cm 2 4.69 W/cm 2 18 W/cm 2 

Weight 25 g - 19 g (w/ combustor) 

Output Resistance 3.0 Ω 0.336 Ω - 

Cost $52.88 $75 - 

Watts per Dollar 0.12 0.0032 - 

* TE material size. Actual module size: 12.75 mm 


8 

Slide

Practical Considerations: Thermal Contact 

http://www.customthermoelectric.com/app_notes/app_note_TEG%20Install.pdf 

• Module surface must be as smooth as possible 

for good thermal contact 

• Thermal Interface Material (TIM) needed 

between module and heat sink 


9 

Slide

Practical Considerations: Clamping Force 

http://www.customthermoelectric.com/app_notes/app_note_TEG%20Install.pdf 

• Clamping force must be applied evenly 

• Typical clamping pressure ~ 1275 kPa 


10 

Slide

Form, F’ysics and Flows 

http://www.wikipedia.com 


11 

Slide

Physics 

V = (S B 

− S A 

)(T 2 

− T 1 

) 

• Seebeck Effect 

• Peltier Effect 

• Thomson Effect 

dQ 

dt 

= Π AB 

I = (Π B 

− Π A 

)I 

q = ρJ 2 − µJ dT 

dx 


12 

Slide

Physics 

• Thomson relations show 

these are all one effect 

Peltier Coefficient 

Thomson Coefficient 

∏ = S ⋅ T 

µ = T dS 

dT 

Seebeck Coefficient 

William Thomson (Lord Kelvin) 

www.abdn.ac.uk 


13 

Slide

ZT Number 

Electrical Conductivity 

ZT = σS2 

λ 

Thermal Conductivity 

T 

• Dimensionless figure 

of merit 

• Linked to 

thermodynamic 

efficiency 

• Z is often used as 

well - simply drop T 

from each side 


14 

Slide

Z vs. Efficiency 

Thermoelectrics Handbook: Macro to Nano p. 1-5 


15 

Slide

Z of Practical Materials 

Z (K^-1*10^3) 

4 

3 

2 

Sb-BiTe-Se 

Bi2Te3-75Sb2 Te3 

(n-type) 

4N-PbTe 

Bi2Te3-75Sb2 Te3 

(p-type) 

3N-PbTe 

ZT=1 

• Z has no fundamental 

limit 

• Theoretical max with 

current materials ~ 4 

• Best commercially 

available ~ 1 

1 

SiGe n-type 

SiGe p-type 

• Best synthesized in lab 

~ 2 

0 

-200 0 200 400 600 800 

Temperature (K) 

Thermoelectrics Handbook: Macro to Nano p. 1-9 

• Lincoln Lab material is 

PbTe 


16 

Slide

Current Research: Nanostructuring 

Growth 

Direction 

Material 1 

Material 2 

Material 1 

Material 2 

Material 1 

Substrate 

• Allows creation of new 

lattice structures 

(“superlattices”) which 

reduce thermal 

conductivity 

ZT = σS2 

λ 

T 

• Utilizes thin-film 

manufacturing methods 

• Technique Lincoln Lab is 

using 


17 

Slide

Current Research: Low-Dimensional Materials 

• At least one dimension 

no wider than lattice 

constant 

• Most promising for 

large increases in ZT (up 

to ZT=4) 

http://en.wikipedia.org/wiki/File:UnitCell.png 


18 

Slide

Current Decreasing Research: Thermal PGEC Conductivity Materials 

PGEC = “Phonon-Glass, Electron- 

Crystal” 

• High electrical conductivity 

• Low thermal conductivity 

Created by adding a loose “rattler” 

atom within the crystal structure 

http://www.nature.com/nmat/journal/v7/n10/thumbs/nmat2271-f1.jpg 


19 

Slide

Manufacturing: Czochralski Method 

Molten material 

Formed Crystal 


20 

Slide

Manufacturing: Zone Melting 

Impurities 

Molten Region 

Heating Elements 

Purified Crystal 


21 

Slide

Manufacturing: Pressing 

Plunger 

Heaters 

Die 

Thermoelectric material 


22 

Slide

Advanced Manufacturing Techniques 

Thermoelectrics Handbook 

• Contamination a problem 

• Electron-beam evaporation 

• Layered cutting into modules 

• Micromilling? 

www.micropelt.com 


23 

Slide

Manufacturing: Sputter Deposition 

• Widely used for 

semiconductor 

applications, tool bit 

coating, etc. 

http://upload.wikimedia.org/wikipedia/en/7/72/Sputtering.gif 

• Utilized by companies 

marketing thin-film 

thermoelectrics 


24 

Slide

Manufacturing: Epitaxy 

• Growth of 

monocrystalline thin-film 

material on substrate 

• Highest quality thin-film 

technique 

http://www.bel-india.com/BELWebsite/images/epitaxy1.jpg 


25 

Slide

Manufacturing: Comparison 

Method Pros Cons 

Czochralski 

Zone Melting 

Pressing 

-Highest Quality 

standard technique 

-Well understood 

-Cheaper 

-Impurities OK 

-Cheapest 

-Exact Dimensions 

-Resistant to Shocks 

-Expensive 

-Post machining 

-Brittle 

-Post machining 

-Brittle 

-Lowest Quality 

Sputter 

Deposition 

Epitaxy 

- Versatile 

- Well-characterized 

- Precise 

- Highest Quality Thin 

Film Technique 

- 

- Expensive 

- Expensive 


26 

Slide

References 

Thermoelectrics Handbook: Macro to Nano. CRC Press, 2006. 

Rowe, D.M. “General Principles and Basic Considerations”. pp. 1-2 : 1-14. 

Goldsmid, H.J. “A New Upper Limit to the Thermoelectric Figure-of-Merit”. pp. 10-1 : 10-10. 

Nurnus, J., H. Böttner, and A. Lambrecht. “Nanoscale Thermoelectrics”. pp. 48-1 : 48-23. 

Min, Gao. “Thermoelectric Module Design Theories”. pp-11-1:11-14. 

Cook, B.A. Harringa J.L. “Solid State Synthesis of Thermoelectric Materials”. pp. 19-1 : 19-14. 


27 

Slide

Reading List 

Chapter 1: Thermoelectrics Handbook 

Rowe, D.M. “General Principles and Basic Considerations”. pp. 1-2 : 1-14. 

Will be posted/emailed shortly. 


28 

Slide

Solid-State Approaches 

• Thermoelectric (TE) and thermal photovoltaic (TPV) 

– Spacecraft power 

– Navy development 

– TE used for larger generators 

Heatsink 

– Limited micropower efforts Coolant air 

Fan 

in 

Teledyne TE Generator 

CERDEC Sponsor 

HC fuel in 

Voltage: 

Power: 

24V to 36V 

adjustable 

120 W 

Efficiency: ~3% 

Weight: 16.4 kg 

w/o fuel 

Fuel: Diesel, JP8 

Era: 1988 

Exhaust out 

Combustor 

Converter 

modules 

Electric 

power out 

• Share common conversion architecture 

– Air-breathing combustor at Th w/ optional recuperator 

– Converter 

– Heatsink to maintain Tc 

slide- 

GSH 3/15/2007 

MIT Lincoln Laboratory

TPV and TE Thermal Conversion 

• Thermal photovoltaic is a straightforward extension of traditional solar 

PV using photoelectric effect 

– Thermal source couples photons to semiconductor diode junction which 

has a quantum energy level structure 

– Photon energy greater than semiconductor bandgap is absorbed to create 

an electron-hole-pair 

– For TPV, the thermal source is local, typically heated by combustion 

processes 

– Characteristic temperatures are lower than solar, so lower bandgap 

devices are needed 

• Thermoelectric effect also related to energy absorption of charge 

carriers in solids 

– Thermal and electrical conduction in non-insulating solids are 

manifestations of the same transport phenomena 

– Charge carriers act as working fluid with “specific heat” 

– Charge carrier thermal energy diffusion confined by voltage just as gas 

thermal energy diffusion confined by pressure 

– Carrier sign provides opportunity for negative voltage 

– Solid lattice introduces challenging but potentially beneficial added 

complexity 

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In Practice 

TPV Application 

+ 

Potential 

Heat In 

(HC combustion) 

P-type 

N-type 

Heat Out 

Load 

hν > E bg 

Emitter 

T ~ 1000K 

- 

V(I,Th,Tl) 

Photodiode 

Qh(I,Th,Tl) 

Thomson Relations 

TE Application 

+ 

Temperature 

Potential 

Heat In 


P-type 

N-type 

Heat Out 

Load 

T ~ 700K 

- 

V(I,Th,Tl) 

TE couple 

Qh(I,Th,Tl) 

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GSH 3/15/2007 


In Practice 

volumetric heat 

generation 

TPV Application 

Material Heat Inelectrical 


conductivity 

Thomson 

heat 

current 

density 

+ 

1d spatial 

temperature 

gradient 

P-type 

N-type 

Potential 

Heat Out 

Load 

hν > E bg 

Emitter 

material Seebeck coefficient (ΔV/ 

ΔT) 

T ~ 1000K 

- 

Photodiode 

V(I,Th,Tl) 

Qh(I,Th,Tl) 

Thomson Relations 

Relative Peltier heat 

(energy / charge) 

TE Application 

+ 

Potential Color 

Temperature 

Heat In 


N-type P-type 

P-type N-type 

Heat Out 

Load 

T ~ 700K 

- 

TE couple 

V(I,Th,Tl) 

Qh(I,Th,Tl) 

slide- 

GSH 3/15/2007 


Campus - Lincoln TPV ACC Effort 

• Joint research program to 

demonstrate micro size TPV 

– Blackbody radiation matched PV 

cells on both sides 

– Antireflective coatings and 

spectral filters 

– Matched PV cell and combustor 

dimensions 

– Thermal management for high 

temperature operation 

– Modular fabrication 

• Substantial progress 

demonstrated 

Top PV cell 

and filter 

Spacer 

Burner 

Spacer 

Bottom filter 

and PV Cell 

slide- 

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Packaged 20W Portable Generator 

Featuring Lincoln NDSL TE Material 

3 in 

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GSH 3/15/2007 


Overview of Thermoelectrics 

• Thermoelectric materials provide solid-state, direct conversion 

between thermal and electrical energy 

– Obtain electrical power from heat (NASA RTGs) 

– Solid-state cooling (TE coolers for lasers, car seats, mini fridges) 

• Advantages: small size, no moving parts, quiet, no pollutants 

The key figure of merit is ZT (unitless) 

Seebeck 

Coefficient 

(=ΔV/ΔT) 

Electrical conductivity 

Temperature 

Device Conversion Efficiency 

T c =300 K 

ZT = 5 

ZT = 3 

ZT = 2 

ZT = 1 

ZT = 0.5 

Lattice and electronic 

thermal conductivity 

slide- 

GSH 3/15/2007 


Optimization of ZT 

Parameter values (arb. units) 

S 

ZT 

σ 

• Best TE materials today have ZT~1 at 

300 K, rising to ~1.5-2 at 600-1000 K 

• Difficulty of increasing ZT due to 

interplay between S, σ, and κ e 

– As carrier concentration increases, S 

decreases while σ and κ e increase 

– Maximum ZT typically at ~1x10 18 to 1x10 20 

cm -3 depending on material 

• Two approaches to increase ZT 

– Decrease κ L (phonon conductivity) through 

nanostructuring 1-4 

– Increase power factor (=S 2 σ) through 

electron filtering 5 or quantum confinement 6 

κ e 

1 

T.C. Harman et al., Science 297, 2229 (2002). 

2 

R. Venkatasubramanian et al., Nature 413, 597 (2001). 

3 

W. Kim et al., Phys. Rev. Lett. 96, 045901 (2006). 

4 

K.F. Hsu et al., Science 303, 818 (2004). 

5 

D. Vashaee and A. Shakouri, J. Appl. Phys. 95, 1233 (2004). 

6 

L.D. Hicks and M.S. Dresselhaus, Phys. Rev. B 47, 12727 (1993). 

slide- 

GSH 3/15/2007 


Intrinsic ZT for Various Materials 

• Lincoln performed 

pioneering work to 

demonstrate ZT 

enhancement 

– MBE grown 

spontaneous 

nanostructure 

– nano dot superlattice 

(NDSL) 

• Work continues in a 

number of material 

systems 

– Lincoln NDSL among 

the best 

– Inconsistent 

measurement results 

[1] T.M. Tritt et al., MRS Bulletin 31, 188 (2006). [2] B. Poudel et al., Science 320, 634 (2008). [3] H. Bottner et al., MRS Bulletin 31, 211 (2006). 

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Power Generation: 

100-µm-Thick n-NDSL 

10 W/cm 2 

= -235 µV/K 

T c = 25 °C 

= -187 µV/K 

• Electrical power generated from thermal gradient across sample 

• n-PbTe/PbSe NDSL: 1 mm 2 , 100 µm length, Sn/Ni/Au top and Sn bottom contact 

• Power density is 5-6x higher than previous best 1 100-µm-thick NDSL (G-207) 

• Power density is ~10x higher than TPV, ~100x higher than 1-sun solar cell 

1 

T.C. Harman et al., Appl. Phys. Lett. 88, 243504 (2006). 

slide- 

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Part II 

Lincoln Laboratory Project Overview 

and 

Link to Campus Project 

slide- 

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20 Watt Generator 

TE vs. TPV 

TPV, 25% combustor efficiency 



LL NDSL TE 

slide- 

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20W TPV and TE System Comparison 

TPV 

(high eff.) 

TPV 

(high power) 

TE 

(optimum) 

Hot side temperature 1200K 1200K 600K 

Cold side temperature 320K 350K 380K 

Combustor Efficiency 70% 70% 80% 

Converter Efficiency 20.3% 18.2% 7.6% 

Device area 47 cm² 52 cm² 1.2 cm² 

Junction power density 0.46 W/cm² 0.40 W/cm² 18 W/cm² 

Combustor heat flux 2.3 W/cm² 2.2 W/cm² 25 W/cm² 

Overall efficiency 12.3% 13.3% 5.7% 

Converter / Combustor mass 146 gm 164 gm 19 gm 

Heatsink mass 126 gm 35 gm 116 gm 

System mass 364 gm 289 gm 300 gm 

Packaging issues 

Vacuum, high temperature braze, parasitic 

heat transfer effects 

Junction thermal stress, 

high temperature 

stability 

slide- 

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Comparative System Performance 

500 MWe Steam Power 

6 month fuel 

• 60% mass reduction 

for 10 hour mission 

• Rechargeable 

• Logistics fuel 

5 MWe Gas Turbine 

6 hour fuel 

240 kW APU 

1 hour fuel 

Solid Oxide 

FC, 9 days 

Direct Meth 

FC, 3.5 days 

Palm Power 

35W JP8 Stirling 

3 days 

Air-cooled 

TPV, 1 day 

Air-cooled 

TE, 11 hrs 

Water-cooled 

TPV, 13 hr 

Water-cooled 

TE, 90 min 

DARPA WASP 

Zinc-Air 

Primary 

Reformed 

Meth FC, 9 hr 

Zinc-Air 

10hr 

BA-5590 

Li Primary 

BB-2590 

Li-ion secondary 

cell 

Zinc-Air 

30min 

Nanoparticle 

Li-ion cell 

Top Fuel Dragster 

w/ generator 

~5 sec 

NRL ARSENIC 

UGS 

ARMY Future 

Force Warrior 

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System Block Diagram 

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Individual Junction Testing 

First thermo-element tested. 

In-situ Sn face down 

TE Device Junction Test Apparatus 

MITLL Engineering Division 

TE Measurement Apparatus 

MITLL Electrooptical Materials Group 

slide- 

GSH 3/15/2007 

• Extend previous material measurement work 

– T.C. Harman, C.J. Vineis 

– Thermal conductivity uncertainty 

• Develop device oriented measurement capability 

– Vacuum operation with heat flux measurement 

– Measure V(I,Th,Tl) and Q(I,Th,Tl) 


• Working module design 

completed 

– Thermal stress, passivation, and 

operational lifetime identified as 

critical risks 

– Still need insulating coating for hotside 

interconnects 

• Successfully soldered junctions 

to copper substrates 

– Soldered junctions under test: 

T < 200ºC so far 

• Moving forward with small 

module fabrication and testing 

– Substrate ordered 

– Fixturing design and fab proceeding 

Module Devleopment 

slide- 

GSH 3/15/2007

Thermoelectric Materials

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