Nanoarchitectured titanium complexes for thermal mitigation in thermoelectric materials

One of the key barriers to commercial development of thermoelectric materials for conversion of heat into electricity is the issue of thermal transport, which diminishes the temperature gradient needed for sustaining the thermoelectric conversion process. The translation of bulk thermoelectric materials into nanostructures such as superlattices, nanowires and quantum dots is useful to mitigate thermal conductivity, through an increase in the mechanisms responsible for phonon scattering. Theoretical principles behind superlattices, phonon glass electron crystals (PGECs) and energy filtering are introduced within the context of thermal conductivity reduction. Case studies using SrTiO3 based oxide materials and titanium-based sulfide materials are used to illustrate structural modifications of the thermoelectric crystal lattice which have been successfully employed to reduce thermal conductivity and hence improve the thermoelectric figure of merit. In particular, the SrTiO3, a perovskite-type oxide class of materials was selected because of the high tunability of its crystal structure through introduction of selective doping, ability to create of an elastic strain field due to radii mismatch of the dopant atom which produces a wider range of phonon scattering and buildable crystal architecture through interleaving with dopant cations to form superlattices. On the other hand, design of TiS2 compounds are a variant on the concept of dichalcogenides, which produce highly anisotropic layered crystal structures, which in turn lend itself to quantum confinement and enhanced phonon scattering effects. Notable nanoarchitectural design of the TiS2 crystal lattice includes nanoblock integration, metal and organic layer intercalation, and misfit layer sulfides which give rise to interesting transport characteristics of the resulting crystal. These strategies all point towards further enhancement of the thermoelectric figure of merit through mitigation of thermal conductivity for ultimate utilization in energy harvesting applications.