Ultralow Thermal Conductivity Materials for Thermoelectric Device Applications*

High thermoelectric performance, or ZT, necessarily depends on achieving the contradictory combination of low thermal conductivity κ and high electrical conductivity (along with a large Seebeck coefficient). However, for ultralow κ materials (defined here as κ < 1 W/m-K at 300 K) the electrical conductivity requirement is proportionally reduced. There may therefore exist a new opportunity for such materials in the thermoelectric arena. Examples of such materials are Tl₃VSe₄ [1], AgBiSe₂ [2], and SnSe [3].

However, there are several scientific issues that must resolved prior to usage of any of these materials in an actual application. Ultralow κ materials tend to occur near to a ferroelectric transition or other structural instability, due primarily to the large anharmonicity near such an instability. This usually renders materials such as PbTe [4,5,6] unsuitable for device applications due to unfavorable mechanical properties. Is proximity to a structural instability a necessary condition for ultralow κ? Similarly, another ultralow κ route to high ZT is the use of mobile cations, such as Cu in Cu₂Se [7], which presents another hurdle for device applications. Can one create a robust thermoelectric device using a material with a structural instability, or mobile structural disorder, in the operating temperature range? The author will discuss these issues.

References

1. S. Mukhopadhyay, D. S. Parker, B. C. Sales, A. A. Puretzky, M. A. McGuire, and L. Lindsay. Science 360, 1455 (2018).
2. D.S. Parker, A. F. May, and D. J. Singh. Phys. Rev. Appl. 3, 064003 (2015).
3. L.-D. Zhao et al, Nature 508, 373 (2014).
4. D.S. Parker, X. Chen, and D. J. Singh. Phys. Rev. Lett. 110, 146601 (2013).
5. J.P. Heremans et al, Science 321, 554 (2008).
6. O. Delaire et al, Nature Materials 10, 614 (2011).
7. H. Liu et al, Nature Materials 11, 422 (2012).