Overview of the Thermoelectric
Properties of Yb-filled
CoSb3 Skutterudites
Gary A. Lamberton, Jr.1
Terry M. Tritt2
R. W. Ertenberg3
M. Beekman3
George S. Nolas3
1 National
Center for Physical Acoustics, University of Mississippi
2 Department of Physics and Astronomy, Clemson University
3 Department of Physics, University of South Florida
Outline
• Introduction to Thermoelectric Materials
• Previous Skutterudite Research and
Promising Results
• Description of Measurements
• Research and Results
• Conclusions
Thermoelectric Applications
• Power Generation
• Radioisotope Thermal Generators (RTGs)
• Cassini, Voyager missions
• Lifespan of more than 14 years
• Waste Heat Recovery
• Large scale – Power Plants
•Small scale - Automobiles
Thermoelectric Applications
•Active Cooling/Warming
• Localized cooling
• CPUs
• Biological specimens
• Commercial Coolers/Warmers
• Luxury Vehicles – Cool/Warm Seats
Thermoelectric (TE) Effects
Seebeck Effect
Material A
Differential
Thermocouple
T + T
T
Vab
α=
ΔT
Material B
Material B
V
TE Effects
Electric Current
Peltier Effect
Material
Difference in
A
εF between
Materials A and B
Material
B
Heat
Absorbed
or
Expelled
TE Couple and Module
Heat Source
P
N
Heat Sink
I
Power Generation Mode
Active Cooling
P
N
Heat Rejection
I
Cooling Mode
Operating Modes of a
Thermoelectric Couple
T. M. Tritt, Science 31, 1276 (1996)
Modules
www.marlow.com
Thermoelectric Materials
Figure of Merit:
 2T
ZT 
 ( e   g )
- Seebeck Coefficient
- Electrical Resistivity
- Thermal Conductivity
e – Electronic ≈ L0T/ρ (W-F relation)
g – Lattice
Current Materials
AgPbmSbTe2m
Terry M. Tritt & Mas Subramanian
MRS Bulletin TE Theme, March 2006
ZT Requirements
For a ZT = 1, e.g. Optimized Bi2Te3 (300 K)
– Resistivity ~ 1.25 mΩ-cm
– Thermopower ~ 220 μV/K
– Thermal Conductivity ~ 1.25 Wm-1K-1
For a ZT > 2,
Assuming a hypothetical g = 0,
a Thermopower ≥ 220 μV/K is required
→ Semiconductors and Semi-metals
Skutterudite Structure
2M8Pn24
or M4Pn12
Metal Atom (Co, Rh, Ir)
Pnicogen Atom (P, As, Sb)
Void Space/Filler Ion
History
• Discovered in Skutterud, Norway
• Studied in the 1950s-60s for potential
thermoelectric applications (binary)
• Sparse research until the early 1990s
– Slack’s Phonon Glass – Electron Crystal
Concept
 T
ZT 
  e   g 
2
IrSb3 and Ir0.5Rh0.5Sb3
• Mass
fluctuation
scattering reduces
the lattice thermal
conductivity to 58%
of the original value
• ‘Rattling’ ion
concept suggested
as a means to
reduce g
Slack et al., J. Appl. Phys 76, 1665 (1994)
• Order of Magnitude
Reduction
• Ge substitution could
only reduce to 30%
• CeFe4Sb12 has g 10%
of that of FeSb3
• “Rattling” of Void
Filling Ion is the source
of the reduced g
Lattice Thermal Conductivity
(mW/cmK)
“Rattlers” Reduce g
Temperature (K)
G.S. Nolas, et al., J. Appl. Phys. 79, 4002 (1996)
Previous Work
CeFe4-xCoxSb12: ZT ~ 1.4 (900 K) JPL
Fleurial, et al., Proc. 16th International Conference on Thermoelectrics,
IEEE Catalog Number 97TH8291, Piscataway, NJ, p. 1 (1997)
Ce “Rattlers” in CoPn3
• Fe4Sb12 has
largest cage size
• More efficient
scattering with
heavier atoms in
the lattice
Watcharapasorn et al., J. Appl. Phys 91, 1344 (2002)
• Partial Filling
Yields Largest
Reduction
• Increased
Disorder
• Less Impact on
Band Structure
Lattice Thermal Conductivity (mW/cmK)
Partial Void Filling
La-filled
CoSb3
Temperature (K)
G.S. Nolas, J. L. Cohn, and G. A. Slack, Phys. Rev. B 58, 164 (1998)
Eu0.42Co4Sb11.37Ge0.50
• Reduced Thermal
Conductivity
• Increased Carrier
Mobility
• Maintained
favorable electronic
properties
Lamberton et al., Appl. Phys. Lett. 80, 598 (2002)
Sample Synthesis
Dr. George S. Nolas (USF/Marlow)
• Stoichiometric amounts of high purity
elements mixed and reacted at 800 ˚C
under Ar atmosphere for 2 days, ground,
and reacted at 800 ˚C for 2 additional days
• Resulting polycrystalline powders were
densified using a HIP at 600 ˚C for 2 hours
• Compositional analysis by Electron
Microprobe
Measurement of Electrical Resistivity
and Seebeck Coefficient (10 – 300 K)
• Helium flow cryostat
and closed-cycle
refrigerator
• High Density of Data
2 samples simultaneously
– 24 hours per experiment
•Typical sample size:
2-4 mm x 2-4 mm
x 6-10 mm
•Mounted on chip that
plugs into system
Pope et al, Rev. Sci. Instrum. 72, 3129 (2001)
Resistivity and Thermopower
Heater
I+
VTEP +
IHeater
Cu block
VR+
Sample
VRVTEP I-
Heater Power,
P = I2R, creates ΔT
for Thermopower
Measurement
T
VR 
4-probe Resistivity
Measurement:
Current Reversed to
Subtract Thermoelectric
Contribution
IR  VTEP -  - I  R - VTEP
Cu block
2
High Temperature Resistivity and
Seebeck Measurement
Cu Block
PRT
Sample
Ceramic Posts
Cartridge Heater
• Operating Range of
100 – 700 K
• Standard 4-probe
Resistivity
Measurement
• Voltage vs. ΔT
Sweeps at Each
Temperature
Thermal Conductivity
Closed Cycle
Helium Cryostat
12 – 300 K
Strain
Gauge
Sample
Cu block
Differential
Thermocouple
mil)
(1(1mil)
Solid State Heat
Flow Method
P  KT
L
κ K
A
A. L. Pope et al., Cryogenics 41, 725 (2001)
Thermal Conductivity
Analysis
-1
-1
Thermal Conductivity (Wm K )
Thermal Conductivity Extrapolation
3.5
• TC Measured
from 10 – 300 K
3.0
2.5
•  measured from
10 – 300 K and
from 100 – 700 K
2.0
1.5
Measured 
Curve Fit 
Total
Wiedemann-Franz 
1.0
Lattice = 
0.5
-
Total
g
e
g
Wiedemann-Franz 
Curve Fit  + WF 
g
e
e
0.0
0
100 200 300 400 500 600 700 800
Temperature (K)
• e calculated
using WiedemannFranz relation from
10 – 700 K
YbxCo4GeySb12-y
Dilley et al.
• Intermediate valence detected in
YbFe4Sb12 between 2+ and 3+
• Heavy Fermion behavior at low
temperature
• Low carrier density leads to relatively high
resistivity, ~ 3 m-cm at 300 K
• ZT < 0.02 at 300 K
Dilley et al., Phys. Rev. B 58, 6287 (1998)
Dilley et al., Phys. Rev. B 61, 4608 (2000)
Motivation for YbxCo4Sb12
• RE-filled Skutterudites have shown relatively large
Figures of Merit
– Reduced Lattice Thermal Conductivity
• Yb – Large Mass, Small Atomic Radius
– Electronic Properties Sensitive to Doping Level
• Reported Mixed Valence in YbFe4Sb12
– Heavy Fermion Behavior
• Increased Seebeck Coefficient
Reduce g + Increase 
High ZT
High Figure of Merit
• Suggests rigidband behavior
(maintain electronic
properties) with
varying Yb
concentration
(x = 0.066, 0.19)
• HF behavior leads
to high power factor
Nolas et al., Appl. Phys. Lett. 77, 1855 (2000)
Concurrent Research
• Anno et al – ZT ~ 1 (700 K) in Yb0.25Co4Sb12 and
Yb0.25Co3.88Pt0.12Sb12
• UCSD and GM: YbyCo4Sb12-xSnx
– x > 0.8 reduces Seebeck Coefficient
• p-type if x > 0.83
– Lattice Thermal Conductivity reduced
• Dependent upon Yb concentration
• Unaffected by Sn compensation
Resistivity (m-cm)
YbyCo4Sb12-xGex
Yb
0.066
4
12
Yb
Co Sb
Yb
Co Ge
Sb
Yb
Co Ge
Sb
Yb
Co Ge
Sb
Yb
Co Ge
Sb
0.19
20
Co Sb
0.49
0.65
0.46
0.46
10
4
4
4
4
4
12
1.00
0.96
0.79
0.86
10.92
11.21
11.21
11.13
0
0
200
400
600
Temperature (K)
800
• Different
Temperature
Dependence
• Magnitude
Scales with Ge
Concentration
• Decreased
Mobility
0
-100
-200
-300
Seebeck Coefficient (V/K)
YbyCo4Sb12-xGex
0
Total
0
 n n   p p

n  p
Yb0.066Co4Sb12
Yb
0.066
Co Sb
4
12
Yb Yb CoCoSb
Sb
0.190.19 4 4 1212
-100
Yb CoCoSb
Sb (Sales)
(Sales)
Yb0.20
0.20 4 4 1212
Ge Sb
Sb
Yb Yb0.49
CoCoGe
4
1.00
10.92
0.49
-200
Yb
Yb
4
Co Ge
4
Co Ge
0.65
0.65
1.00
4
10.92
0.96
Sb
Sb 11.21
0.96
11.21
Yb
Co Ge
Sb
Yb
Co Ge
Sb
4
0.79
11.21
Yb0.460.46
Co4Ge
Sb
0.79
11.21
-300
Yb
0
200
200
400
600
Temperature
400
600 (K) 800
800
0.46
0.46
4
Co Ge
4
0.86
0.86
Sb
11.13
11.13
y = 0.066
5
• Reduction over
Parent CoSb3
~ 8 Wm-1K-1
@ 300K
SK181
Sk193
SK281
Sk282
SK283
SK284
-1
-1
(Wm K )
Lattice Thermal Conductivity
YbyCo4Sb12-xGex
3
y > 0.19
1
0
100
200
Temperature (K)
300
• Varies More
Than Sn
Compensated
Samples
(Yang et al)
YbXCo4GeYSb12-Y
Figure of Merit – Yb-filled CoSb3
1.2
Yb
Co Sb
Yb
Co Sb
Yb
0.066
0.19
12
Co Sb
12
0.066
4
12
4
Co Sb
Yb Co SbYb0.19
(Sales)
4
12
0.8
ZT
0.2
Yb
Co Sb
Yb
0.49
Yb
0.65
200
400
600
Temperature (K)
800
3.88
0.12
Yb
4
12
Co
Pt
3.88
(Anno)
0.25
12
Yb
Co Sb
Co Ge 0.25Sb4
4
12
0.49
4
Sb
1.00
Yb0.96 Co11.21
Ge
0.65
4
Sb
12
(Anno
(Anno)
Yb1.00 Co10.92
Ge
Co Ge
4
0.12
0.96
Sb
Sb
10.92
11.21
Yb
Co Ge
Yb
Yb
Co Ge
Yb Sb
Co Ge Sb
0.46
4
0.86
11.13
0.46
0
12
Sb (Anno)
(Sales)
Co Yb
Pt0.2CoSb
4
12
0.25
0.4
4
Yb
0.25
0.0
4
0.46
4
4
Sb
Co11.21
Ge Sb
0.79
0.46
4
0.79
11.21
0.86
11.13
Sales, B., March APS (2002)
H. Anno et al., Mat. Res. Soc. Symp. Proc. Vol. 691, 49 (2002)
G. S. Nolas, M. Kaeser, R. T. Littleton IV, and T. M. Tritt, Appl. Phys. Lett. 77, 1855 (2000)
ZT vs. Yb Concentration
1.2
•Sensitive to Yb
Concentration
0.8
Yb Solubility Limit
ZT (600 K)
1.0
0.6
0.4
0.2
0.0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Yb Concentration
•Maximum
Figure of Merit
~ 0.20 Yb
Concentration
Conclusions
• Yb-doped skutterudites show significant
promise for thermoelectric applications
• Figure of Merit - Sensitive to Yb
concentration
• Ge Charge Compensation
– Reduces Seebeck Coefficient at Elevated
Temperatures
– Reduces Carrier Mobility Leading to
Increased Resistivity
Future Direction
• Yb ~ 0.20 concentrations
• High temp YbxCo4Sb12-ySny data
y  0.80
• Focus on keeping large magnitude
thermopower while incorporating ‘rattling’
atoms
– Beware of charge compensating
– Perhaps Co site
Acknowledgements
• Dr. Terry M. Tritt (Dissertation Advisor)
• Dr. George S. Nolas – Synthesis
• NASA South Carolina Space Grant
• Project Supported through:
– Clemson - DOE EPSCoR Partnership
Grant No. DOE-DE-FG02_00ER45850