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A2: Medium temperature materials

Thermoelectric properties of Tl-filled p-type skutterudites: TlxFe1.5Co2.5Sb12

Seongho Choi1, Ken Kurosaki1, Guanghe Li1, Yuji Ohishi1, Hiroaki Muta1, and Shinsuke Yamanaka1,2
1Graduate School of Engineering, Osaka University, Suita, Japan2Research Institute of Nuclear Engineering, University of Fukui, Tsuruga, Japan

Filled skutterudites are the most promising candidate materials for thermoelectric (TE) applications in the middle temperature range. The voids in the crystal structure of the skutterudits can be fully- or partially-filled with a variety of different atoms, and thus obtained filled skutterudites with quite low lattice thermal conductivity (klat). CoSb3-based skutterudites filled by group 13 elements (Ga, In, and Tl) attracted much attention recently as high-performance TE materials. Since group 13 elements donate their valence electron to the CoSb3 framework, CoSb3-based skutterudites filled by group 13 elements show n-type. Among the group 13 elements, Tl is a good filler element to reduce klat. Although the Tl-filled CoSb3 exhibits excellent TE figure of merit ZT = 0.9 at 600 K1, a similar performance in both n- and p-type TE materials is required for a powerful TE module. Therefore, p-type Tl-filled skutterudites need to be developed. In terms of valence electron counting, the Fe4Sb12 unit is deficient by four electrons compared to the Co4Sb12 unit. Therefore, FeSb3-based skutterudites would be heavily doped p-type material with high hole concentrations. Furthermore, since Fe is considerably less costly than Co, the partial replacement of Fe with Co lowers material costs. For these reasons, our group prepared and characterized two kinds of p-type Tl-filled skutterudetes; one is TlxFe2.5Ni1.5Sb12 (ZT = 0.16 at 422 K)2 as the Co-free system and the other is TlxFe1Co3Sb12 (ZT = 0.36 at 723 K)3 as the Co-reduced system. In the present study, we paid attention to further reduction of Co in the latter system and investigated the TE properties of TlxFe1.5Co2.5Sb12 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.7) in the temperature range of 323 K to 773K. The XRD and SEM/EDX analyses revealed that the Tl-filling fraction limit in this system is around x = 0.6. The samples showed quite complicated phase state, i.e., the samples with x ≤ 0.4 contained three phases: the skutterudite matrix phase, the FeSb2 phase, and the elemental Sb phase; the samples with x = 0.5 and 0.6 contained two phases: the skutterudite matrix phase and a very small amount of the elemental Sb phase; the sample with x = 0.7 contained two phases: the skutterudite matrix phase and the FeSb2 phase. The TE properties of the samples will be discussed.

  

References

 1. A. Harnwunggmoung, K. Kurosaki, H. Muta, and S. Yamanaka, Applied Physics Letters 96, 202107 (2010).

2. S. Choi, K. Kurosaki, Y. Ohishi, H. Muta, and S. Yamanaka, Journal of Applied Physics 115, 023702 (2014).

3. D. Kim, K. Kurosaki, Y. Ohishi, H. Muta, and S. Yamanaka, APL Materials 1, 032115 (2013).