1. 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

2. Outline Introduction to Thermoelectric Materials
Previous Skutterudite Research and Promising Results
Description of Measurements
Research and Results
Conclusions

3. 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

4. Thermoelectric Applications
Active Cooling/Warming
Localized cooling
CPUs
Biological specimens
Commercial Coolers/Warmers
Luxury Vehicles – Cool/Warm Seats

5. Thermoelectric (TE) Effects
Seebeck Effect
Differential
Thermocouple
Material B
Material A V T
T + T

6. Heat Absorbed or Expelled
TE Effects
Peltier Effect
Difference in
εF between
Materials A and B
Electric Current
Material A
Heat Absorbed or Expelled
Material B

7. Operating Modes of a Thermoelectric Couple
TE Couple and Module
Operating Modes of a Thermoelectric Couple
Modules
T. M. Tritt, Science 31, 1276 (1996)

8. Thermoelectric Materials
Figure of Merit:
a- Seebeck Coefficient
r- Electrical Resistivity
k- Thermal Conductivity
ke – Electronic ≈ L0T/ρ (W-F relation)
kg – Lattice

9. Current Materials AgPbmSbTe2m Terry M. Tritt & Mas Subramanian
MRS Bulletin TE Theme, March 2006

10. ZT Requirements → Semiconductors and Semi-metals
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 kg = 0,
a Thermopower ≥ 220 μV/K is required
→ Semiconductors and Semi-metals

11. Skutterudite Structure
2M8Pn24
or M4Pn12
Metal Atom (Co, Rh, Ir)
Pnicogen Atom (P, As, Sb)
Void Space/Filler Ion

12. 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
PGEC concept relies on the ‘rattler’ atom

13. ‘Rattling’ ion concept suggested as a means to reduce g
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)

14. “Rattlers” Reduce g 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)
Temperature (K)
G.S. Nolas, et al., J. Appl. Phys. 79, 4002 (1996)

15. 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)

16. 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)

17. Partial Void Filling Partial Filling Yields Largest Reduction
Temperature (K)
La-filled CoSb3
Lattice Thermal Conductivity (mW/cmK)
Partial Filling Yields Largest Reduction
Increased Disorder
Less Impact on Band Structure
G.S. Nolas, J. L. Cohn, and G. A. Slack, Phys. Rev. B 58, 164 (1998)

18. Eu0.42Co4Sb11.37Ge0.50 Reduced Thermal Conductivity
Increased Carrier Mobility
Maintained favorable electronic properties
Lamberton et al., Appl. Phys. Lett. 80, 598 (2002)

19. 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

20. 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)

21. Resistivity and Thermopower
Heater Power, P = I2R, creates ΔT for Thermopower Measurement I+
VR+
VR- T
IHeater
VTEP +
VTEP -
Sample
Cu block
Heater I-
4-probe Resistivity Measurement: Current Reversed to Subtract Thermoelectric Contribution

22. High Temperature Resistivity and Seebeck Measurement
Cu Block
PRT
Operating Range of 100 – 700 K
Standard 4-probe Resistivity Measurement
Voltage vs. ΔT Sweeps at Each Temperature
Sample
Ceramic Posts
Cartridge Heater

23. Differential Thermocouple (1 mil)
Thermal Conductivity
Closed Cycle Helium Cryostat 12 – 300 K
Solid State Heat Flow Method
Strain Gauge
Differential Thermocouple (1 mil)
Sample
(1 mil)
Cu block
A. L. Pope et al., Cryogenics 41, 725 (2001)

24. Thermal Conductivity Analysis
TC Measured from 10 – 300 K
 measured from 10 – 300 K and from 100 – 700 K
e calculated using Wiedemann-Franz relation from 10 – 700 K
Check spelling of Weidemann

25. YbxCo4GeySb12-y

26. 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)

27. 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

28. HF behavior leads to high power factor
High Figure of Merit
Suggests rigid-band 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)

29. 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

30. YbyCo4Sb12-xGex Different Temperature Dependence
Magnitude Scales with Ge Concentration
Decreased Mobility

31. YbyCo4Sb12-xGex

32. YbyCo4Sb12-xGex Reduction over Parent CoSb3 ~ 8 Wm-1K-1 @ 300K
Varies More Than Sn Compensated Samples (Yang et al)
y = 0.066
y > 0.19

33. YbXCo4GeYSb12-Y

34. Figure of Merit – Yb-filled CoSb3
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)

35. ZT vs. Yb Concentration Sensitive to Yb Concentration
Maximum Figure of Merit ~ 0.20 Yb Concentration
Yb Solubility Limit

36. 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

37. 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

38. 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