Inorganic Clathrates for Thermoelectric Applications

The unique crystal structures clathrates possess, in which atoms reside inside polyhedral cavities typically formed by framework group-14 elements, directly results in unconventional physical properties, including low thermal conductivity for these crystalline solids and their potential for thermoelectric heat-to-electrical energy conversion. A unique "tunablility" exists in these materials by altering the type of guest and host atoms, as well as the guest content. Poor thermal conduction, akin to amorphous solids, and electronic properties typical for crystalline materials define the so called phonon glass-electron crystal (PGEC) concept. This concept, first proposed by Glen Slack, identified clathrates as promising thermoelectric materials. In order to manufacture efficient thermoelectric devices the required thermoelectric performance must be realized in mechanically robust materials comprised of earth-abundant non-toxic elements, and low-cost manufacturing processes. Considering the recent advances in thermoelectric energy conversion technologies and the current interest for research in materials with excellent thermoelectric properties, it is likely that clathrates will play a role in thermoelectric power generation applications. In this chapter we provide an overview of the interesting transport properties these materials possess, then present some of the recent developments in the field of thermoelectric clathrates while highlighting potential strategies for further improvement of their thermoelectric properties.