Oxide thermoelectrics: The challenges, progress, and outlook

Most state-of-the-art thermoelectric (TE) materials contain heavy elements Bi, Pb, Sb, or Te and exhibit maximum figure of merit, ZT similar to 1-2. On the other hand, oxides were believed to make poor TEs because of the low carrier mobility and high lattice thermal conductivity. That is why the discoveries of good p-type TE properties in layered cobaltites NaxCoO2, Ca4Co3O9, and Bi2Sr2Co2O9, and promising n-type TE properties in CaMnO3- and SrTiO3-based perovskites and doped ZnO, broke new ground in thermoelectrics study. The past two decades have witnessed more than an order of magnitude enhancement in ZT of oxides. In this article, we briefly review the challenges, progress, and outlook of oxide TE materials in their different forms (bulk, epitaxial film, superlattice, and nanocomposites), with a greater focus on the nanostructuring approach and the late development of the oxide-based TE module.