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 Differential Thermocouple Material B Material A V T T + T

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

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

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

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

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

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 PGEC concept relies on the ‘rattler’ atom

‘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)

“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)

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 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)

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 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

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

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)

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

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

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)

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

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

YbyCo4Sb12-xGex

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

YbXCo4GeYSb12-Y

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)

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

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