Overview of the Thermoelectric Properties of Yb-filled CoSb 3 Skutterudites

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

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

7. TE Couple and Module Operating Modes of a Thermoelectric Couple Modules www.marlow.com T. M. Tritt, Science31, 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. AgPbmSbTe2m Current Materials Terry M. Tritt & Mas Subramanian MRS Bulletin TE Theme, March 2006

10. 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 ZT Requirements

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

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

13. 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. Lattice Thermal Conductivity (mW/cmK) Temperature (K) “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 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. La-filled CoSb3 Lattice Thermal Conductivity (mW/cmK) Temperature (K) Partial Void Filling • 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. 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 Sample Synthesis

20. Measurement of Electrical Resistivity and Seebeck Coefficient (10 – 300 K) • Helium flow cryostat and closed-cycle refrigerator • High Density of Data2 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. Heater IHeater I+ Cu block VTEP + VR+ Sample T VR- VTEP - I- Cu block Resistivity and Thermopower Heater Power, P = I2R, creates ΔT for Thermopower Measurement 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. 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

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. 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. 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 Concurrent Research

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