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A3: High temperature materials

Effect of Sn dopants on the thermoelectric properties of p-type Fe-doped Yb-filled skutterudites

S. Hui1, J. R. Salvador2, K. P. Pipe1, and C. Uher1
1Univ. of Michigan, 2GM R&D Center

Band resonant states [1] provide a possible means to improve ZT in p-type CoSb3-based skutterudites. Our previous study [2] indicated that Sn dopants on Sb sites introduce additional states approximately 0.3 eV below the valence band edge. However, even at the solubility limit, the alloy fraction of Sn is too low to allow the Fermi level to reach these states, which are deep within the band. Therefore, in this work we dope Fe on the Co sites to further lower the Fermi level. To make the structure stable, we also add Yb as fillers. YbzFe4-yCoySb12-xSnx samples were synthesized by a melting-quenching-annealing process followed by spark plasma sintering. Powder XRD showed the majority phase is filled skutterudite with only a small amount of Sb impurity. A detailed high-temperature study including Seebeck coefficient, electrical conductivity, Hall coefficient, and thermal conductivity measurements was performed to evaluate the thermoelectric performance of the samples. We find that the addition of Fe and Yb improves the power factor of Sn-doped CoSb3 by a factor of 2.5, but the Seebeck coefficient is nearly constant above a certain carrier concentration, likely indicating Fermi level pinning by the heavy Fe d band, which prevents the Fermi level from reaching the Sn resonant states. However, alloy scattering of phonons causes a significant reduction in thermal conductivity, leading to an increase in ZT from 0.45 (x = 0) to 0.54 (x = 0.1) at 700K. Other p-type dopants such as Mn are currently being investigated as a means to move the Fermi level even closer to the Sn resonant states.

Acknowledgement:

This work is supported by the Department of Energy under CERC-CVC, a joint program between U.S. and China, under the Award No. DE-PI0000012.

References:

[1] J. P. Heremans, V. Jovovic, E. S. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka and G. J. Synder, Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States, Science 321 (2008) 554

[2] S. Hui, M. D. Nielsen, M. R. Homer, D. L. Medlin, J. Tobola, J. R. Salvador, J. P. Heremans, K. P. Pipe, and C. Uher, Influence of substituting Sn for Sb on the thermoelectric transport properties of CoSb3-based skutterudites,  J. Appl. Phys. 115 (2014) 103704