solar thermoelectrics
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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.
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