CHAPTER 5: Thermoelectric Oxide Materials for Energy Conversion

Thermoelectric modules used to convert thermal energy into electrical energy comprise multiple pairs of n-type and p-type elements connected in parallel thermally and in series electrically. They have been used in niche applications for over 60 years and are starting to gain more widespread acceptance. Traditional metallic thermoelectric materials are confined to modest temperature ranges and limited by cost, and their reliance on rare and sometimes toxic elements. Oxides, offering high temperature stability, along with lower cost and weight, and dependence on more environmentally friendly elements, have attracted increasing attention over the past 30 years. We review the developments of the main p-type and n-type thermoelectric oxides, their current status, the understanding gained from modelling, typical energy harvesting devices and future prospects. The materials are considered in terms of six main families: (i) misfit-layered cobaltites, (ii) ZnO-based materials, (iii) tungsten bronze structured oxides, (iv) CaMnO₃, (v) TiO₂/Magnéli phases and (vi) A-site deficient perovskites, including SrTiO₃. The outlook for oxide thermoelectrics is promising if we can fully exploit available techniques to significantly enhance thermal-electric conversion efficiency and the temperature range of operation.