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A2: Medium temperature materials
To date, only very few complex bulk materials are known to exceed a figure of merit of zT = 1. Most of these materials include elements from the 6th period (such as Ba, La, Yb, Pb, Bi), as heavy elements are known to contribute to low thermal conductivity, an important asset of enhanced thermoelectrics. This appears to be particularly true for materials containing thallium. In particular, Tl9BiTe6 - a ternary variant of Tl5Te3 - exhibits high zT values at intermediate temperatures, namely zT = 0.86 at 590 K after hot-pressing,1 and even up to (extrapolated) zT = 1.2 at 500 K after zone-refining.2 Its lighter homologue, Tl9SbTe6, was investigated as well prior to this work, and reported to achieve zT = 0.41 at 591 K after cold-pressing and sintering.3 The isoelectronic and isostructural materials Tl4SnTe3 and Tl4PbTe3 were found to reach zT = 0.7 at 673 K after hot-pressing.4
We have therefore begun to systematically optimize various variants of Tl5Te3.5,6 In this contribution, we will document that zT values in excess of 1.4 can be achieved with these materials at intermediate temperatures, to a large extent because of their very low lattice thermal conductivity.
[1] Yamanaka, S.; Kosuga, A.; Kurosaki, K. J. Alloys Compd. 2003, 352, 275.
[2] Wölfing, B.; Kloc, C.; Teubner, J.; Bucher, E. Phys. Rev. Lett. 2001, 86, 4350.
[3] Kurosaki, K.; Goto, K.; Kosuga, A.; Muta, H.; Yamanaka, S. Mat. Res. Soc. Symp. Proc. 2006, 886, 281.
[4] Kosuga, A.; Kurosaki, K.; Muta, H.; Yamanaka, S. J. Appl. Phys. 2006, 99, 063705.
[5] S. Bangarigadu-Sanasy, S.; Sankar, C. R.; Assoud, A.; Kleinke, H. Dalton Trans. 2011, 40, 862.
[6] Guo, Q.; Chan, M.; Kuropatwa, B. A.; Kleinke, H. Chem. Mater. 2013, 25, 4097.