Frontiers of Oxide Thermoelectric Materials for Power Generation Applications

Thermoelectric (TE) energy conversion generates electricity directly from a heat flux penetrating a solid device consisting of n- and p-type semiconductors. However, limitations in elemental abundance and poor thermal durability of conventional TE materials have prevented a wide commercialization of the TE power generation. Metal oxides are highly durable at high temperature in air, and are generally non-toxic, low cost with minimal environment impact. Boosted by the global environment issues and carbon dioxide mitigation problems, oxide TE materials are becoming more and more of vital importance for recuperation of decentralized waste heat energy for higher total energy efficiency. In this paper, nanostructure engineering in TE oxides will be depicted with focuses on thermal conductivity reduction by introducing complex microstructure, and by carefully choosing the crystal structure. The former approach is exemplified by multinary-doped ZnO with a spontaneously formed microstructure in sintered bodies, while the latter strategy is demonstrated by an ultra_low lattice thermal conductivity found in β-pyrochlore (defect pyrochlore) oxides with an oversized cage-like structure surrounding a small atom inside. Current issues of TE oxides will be discussed with a future prospect of the materials and applications.