Solar Thermoelectrics

Mercouri Kanatzidis, Materials Science Division

December 15, 2009

2

Heat to Electrical Energy Directly

Up to 20% conversion efficiency with right materials

http://www.dts-generator.com/

TE devices have no moving parts, no noise, reliable

Thermopower S = ΔV/ΔT

hot cold

3

Figure of Merit

ZT S2

totalT

S2Power factor

Total thermal conductivity

electrical conductivity thermopower

ZT S2

totalT

S2Power factor

Total thermal conductivity

electrical conductivity thermopower

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1 1.5 2 2.5 3

ZTavg

= 0.0 = 0.1

= 1.1

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1 1.5 2 2.5 3

ZTavg

= 0.0 = 0.1

= 1.1

δ=Rc/RFor Th = 800K Tc = 300K

4

2/1

2/3

max r

latt

z

yx

em

mmT

Z

ZT and Electronic Structure

m= effective mass=scattering timer= scattering parameterlatt= lattice thermal conductivityT = temperature

= band degeneracy

Large comes with(a) high symmetry e.g. rhombohedral, cubic(b) off-center band extrema

Isotropic structure

Anisotropic structure

k

E

k

E

k

Ef

For acoustic phonon scatteringr=-1/2

Complex electronic structure

5

Selection criteria for candidate materials

Narrow band-gap semiconductors Heavy elements

High μ, low κ Large unit cell, complex structure

low κ Highly anisotropic or highly symmetric… Complex compositions

low κ, complex electronic structure

6

A2Q + PbQ + M2Q3 –––––> (A2Q)n(PbQ)m(M2Q3)p

Investigating the A/Bi/Q system

A2Q

PbQBi2Q3

A=Ag, K, Rb, CsM=Sb, BiQ=Se, Te

-K2Bi8Se13

Rb0.5Bi1.83Te3

Pb5Bi6Se14

A1+xPb4-2xBi7+xSe15

APb2Bi3Te7

Pb6Bi2Se9 Pb5Bi12Se23

Map generates target compounds

CsBi4Te6

Phases shownare promising new TEmaterials

Cubic materialsAmBnMmQ2m+n

7

AgPbmSbTe2+m (LAST-m) NaPbmSbTe2+m (SALT-m)

Ag

Sb

Te

Pb

6 8 10 12 14 16 18

263

264

265

266

267

268

U

nit

cel

l vo

lum

e (Å

)

Vegard's law

m value

Powder data

Single crystal data

(1) (a) Rodot, H. Compt. Rend. 1959, 249, 1872-4. (2) (a) Rosi, F. D.; Hockings, E. S.; Lindenblad, N. E. Adv. Energy Convers. 1961, 1, 151.

No phase transitions to melting point

8

Synthesis

R. G. Maier Z. Metallkunde 1963, 311

T, K

time, h

1000 ºC

Cool to 50 ºCin 24 h

Gravity induced inhomogeneityLAST-18

time, h

T, K950-1000 ºC

Cool to 50 ºC in 72 h

13 deg/h

Ingot properties very sensitive to cooling profile

Wernick, J. H.. Metallurg. Soc. Conf. Proc. (1960), 5 69-87.

rock

rock

9

7 d4 h

1000 oC700 oC

12 h

50 oC

12 h

LAST-18: Synthesis with Slow Cooling

fast cooled sample

slow cooled sample

Ag Sb Pb Te amount

0.86 1 19 20 105 g

ETN125 (S/cm)

S

(V/K)

PF

(W/cm∙K2)

A 535 -121 7.8

B 959 -128 15.7

C 1026 -158 25.6

D 1341 -180 43.4

~2deg/hr

10

Properties of Ag1-xPb18SbTe20

0

500

1000

1500

2000

-400

-350

-300

-250

-200

-150

-100

300 350 400 450 500 550 600 650 700

(S

/cm

) S (

V/K

)

Temperature, K0

0.5

1

1.5

2

2.5

300 400 500 600 700 800

La

ttic

e T

he

rma

l C

on

du

ctivity (

W/m

K)

Temperature (K)

PbTe

LAST-18

Lattice thermal conductivity

11

LAST-18 ZT~1.6

0.00

0.50

1.00

1.50

2.00

300 350 400 450 500 550 600 650 700

Ag0.86

Pb18

SbTe20

ZT

Temperature, K

PbTe

12

HRTEMof LAST-18

13

What is the dot made of?

Cook, Kramer

14

Nanostructures reduce the lattice thermal conductivity

0.35 W/mK (Harman PbTe/PbSe superlattice)

0

0.5

1

1.5

2

2.5

300 400 500 600 700 800

La

ttic

e T

he

rma

l Con

du

ctiv

ity (

W/m

K)

Temperature (K)

PbTe

LAST-18

Lattice thermal conductivity

Clemens-Drabble theory

15

Why do the LAST materials nanostructure?

Pb Te Pb Te Pb Te Pb

Te Pb Te Pb Te Pb Te

Pb Te Sb Te Pb Te Pb

Te Ag Te Ag Te Pb Te

Pb

Te Pb

Te

Te

Sb

Pb

Te

Te

Pb

Pb

Te

Te

Pb

Ag Te Pb Te Pb Te Pb

Te Pb Te Pb Te Sb Te

Pb Te Pb Te Pb Te Pb

Te Ag Te Pb Te Pb Te

Pb

Te Pb

Te

Te

Pb

Pb

Te

Te

Sb

Pb

Te

Te

Pb

Dissociated state..unstable Associated state..stable

Any +1/+3 pair

Driving force for segregation Ag+/Sb3+ pair: thermodynamics

16

Figure of Merit LASTT (p-type)

J. Androulakis, K. F. Hsu, R. Pcionek, H. Kong, C. Uher, J. J. D'Angelo, A. Downey, T. Hogan, M. G. Kanatzidis, Advanced Materials 2006, 18, 1170

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Ag0.9Pb10.8Sn7.2Sb0.6Te20 Ag0.9Pb7.2Sn10.8Sb0.6Te20 Ag0.9Pb9Sn9Sb0.6Te20 Ag0.6Pb7Sn3Sb0.2Te12 Ag0.5Pb6Sn2Sb0.2Te10Ag0.9Pb5Sn3Sb0.7Te10

ZT

(a)

(b)

ZT

Temperature (K)

Ag0.5Pb6Sn2Sb0.2Te10 Ag0.9Pb5Sn3Sb0.7Te10 Ag0.6Pb7Sn3Sb0.2Te12

TAGS

PbTe

LASTT

17

Na-based materials (SALT-m)

New high ZT p-type material Na1-xPbmSbTe2+m

m~19-21

5 nm

2 nm

18

What is nanostructuring worth?

P. F. P. Poudeu, J. D'Angelo, A. D. Downey, J. L. Short, T. P. Hogan, M. G. Kanatzidis, Angew. Chem. Int. Ed. 2006, 45, 1

Pb Te Pb Te Pb Te Pb

Te Pb Te Pb Te Pb Te

Pb Te Sb Te Pb Te Pb

Te Na Te Na Te Pb Te

Pb

Te Pb

Te

Te

Sb

Pb

Te

Te

Pb

Pb

Te

Te

Pb

19

Matrix Encapsulation as a Route to Nanostructured PbTe

PbTe + X

X = InSb

X = Sb

X =

Bi 20 nm

100 nm

2 nm

2 nm

100 nm

2 nm

20

Nanocrystals of Sb in PbTe

300 400 500 600 7000.5

1.0

1.5

2.0

2.5

PbTe - Sb(16%)PbTe - Sb(4%)

lat,

Wm

-1K

-1

Temperature, K

PbTe - Sb(2%)

PbTe

B

300 400 500 600 7000.5

1.0

1.5

2.0

2.5PbTe

4% InSb

4% Bi

lat,

Wm

-1K

-1

Temperature, K

4% Sb

A

• An optimum concentration of nanoscale second phase is necessary• Mass fluctuations play a role in thermal conductivity reduction• Lattice thermal conductivity reduced, however ZT low due to small Seebeck

20 nm

21

Phonons

22

Electrons

23

24

Completed and Processed Ingot

Composition: Ag0.43Pb18Sb1.2Te20 Weight: 200 grams

0

200

400

600

800

1000

1200

1400

1600

-280

-260

-240

-220

-200

-180

-160

-140

-120

350 400 450 500 550 600 650

1 - Increasing T2 - Decreasing T3 - Increasing T4 - Decreasing T

1 - Increasing T2 - Decreasing T3 - Increasing T4 - Decreasing T

Ele

ctri

cal C

ond

uctiv

ity (

S/c

m)

TE

P (µ

V/K

)

Temperature (K)

Ag0.43

Pb18

Sb1.2

Te20

Temperature cyclability

Schock

25

Module Fabrication

1.78m total 16.0µ·cm2

Hot side diffusion contacts, and cold side solder contacts with <10 µW·cm2 have been achieved.

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Th=495KTh=545KTh=585K

Pow

er G

ener

ated

(W

)

Load resistance (ohms)

26

Best ZT Materials

0

0.5

1

1.5

2

0 200 400 600 800 1000 1200 1400

Temperature (K)

CsBi4Te

6

Bi2Te

3

Tl9BiTe

6

LAST

LASTT

Na0.95

Pb20

SbTe22

TAGS

Ce0.9

Fe3CoSb

12PbTe SiGe -K2Bi

8Se

13

27

Conclusions

LAST, LASTT and SALT: promising thermoelectric materials for next generation power generation modules. (expected device efficiency ~14%)

Nanostructures strongly reduce thermal conductivity. Nanostructures are closely linked to high ZT. Scaleup successful in producing large quantities but material is brittle and contains

microcracks. Hot pressing and powder processing yield 3x improvement in strength. Higher average ZT (>2) needed to reach 20% efficiency.