Valence band structure of PbTe1 − xSex.
Researchers have long tried to enhance the thermoelectric effect (turning heat into electricity) to make the devices practical. In doing so, the goal is typically to increase a property in the materials known as ZT, which depends on a set of factors that include a material’s ability to conduct heat and its electrical conductivity. An alloy of lead telluride (PbTe), for example, which has long been used to generate electricity aboard satellites, has a ZT of around 0.8. Researchers have now made one of the most common thermoelectric materials more efficient with a ZT of 1.8. A ZT of 1.8 at 850K has a energy conversion efficiency of about 20 to 22% instead of 13 % at ZT of 0.8.
To increase ZT, researchers typically try to increase a material’s electrical conductivity as much as possible while holding down its thermal conductivity. In 2008, researchers led by Jeffrey Snyder, a materials scientist at the California Institute of Technology in Pasadena, spiked PbTe with thallium, which boosted the ZT to 1.5. The group later determined that the thallium altered the electronic structure of the crystal, improving its electrical conductivity.
But thallium is toxic, so Snyder and his colleagues wanted to determine if they could match the improvement with other additives. Earlier this year, Snyder and his team at Caltech reported in Energy & Environmental Science that substituting sodium for thallium produced a ZT of 1.4. Now, Snyder’s team, in combination with researchers from the Chinese Academy of Sciences’ Shanghai Institute of Ceramics, report online today in Nature that adding selenium and sodium gives them a maximum ZT of 1.8. The selenium not only further improves the electrical conductivity, it also reduces the thermal conductivity, Snyder explains.
Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity which has led to the investigation of a variety of complex material systems. In this vein, it is well known that a high valley degeneracy (typically less than or equal to 6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe1 − xSex alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity.