Record thermoelectric efficiency in an overlooked compound

The drive toward ever greater energy efficiencies over the past few decades has motivated considerable work on thermoelectric materials, which can convert a temperature gradient ΔT into an electrical voltage ΔV and hence squeeze additional work out of waste heat. The dimensionless figure of merit for these materials is ZT, where T is the absolute temperature and Z is a function of the Seebeck coefficient S (ΔV/ΔT), the electrical conductivity σ, and the heat conductivity Κ. As ZT approaches infinity, the heat-engine efficiency of the material approaches the Carnot limit, but in practice it’s hard to get ZT much above 1. Doing so requires a decoupling of Κ and σ—that is, enhancing the flow of electrons to improve the device power but impeding the flow of phonons to maintain the thermal gradient. The wide variety of ways to do this has led to a large number of thermoelectric candidates, many being quite complex in structure. A figure of merit ZT = 2.2 was reported in 2012 for a doped sample of lead telluride. That record has now been surpassed by Mercouri Kanatzidis and colleagues from Northwestern University and the University of Michigan, who report finding ZT = 2.6 at 923 K—but only along the b axis of the orthorhombic structure. The researchers found that ZT is nearly as high along the c axis (2.4) but only 0.8 along the a axis. The crystal, along with slices along its three axes, is shown in the figure. The work is attracting considerable interest because the material, unlike many other thermoelectrics, is a simple binary compound made of Earth-abundant elements. (L.-D. Zhao et al., Nature 508, 373, 2014 .)