RESUMO
Formation of composition conserving defects is an inherent feature that appears in compounds for thermoelectric applications during the processes of their fabrication. Different types of such defects including exchange antisite defects, Schottky defects, and triple-, quatro- and penta-defects in TiNiSn are considered. Density functional theory calculations of the energy of formation of these defects are carried out. It is demonstrated that their formation may lead to a significant decrease of the band gap (Eg), simultaneously causing a transformation to p-type or semi-metal conductivity in this material. The role of nanopores is discussed. It is shown that preparing nanoporous compounds may be an efficient way to create p-type TiNiSn, simultaneously decreasing the thermal conductivity and improving its thermoelectric parameters.
RESUMO
Novel approaches for the development of highly efficient thermoelectric materials capable of a direct conversion of heat into electricity, are being constantly investigated. TiNiSn based half-Heusler alloys exhibit a high thermoelectric potential for practical, renewable power generation applications. The main challenge of further enhancement of the thermoelectric efficiency of these alloys lies in the reduction of the associated high lattice thermal conductivity values without adversely affecting the electronic transport properties. The current manuscript theoretically investigates two possible routes for overcoming this limitation in TiNiSn alloys. On the one hand, the influence of nano-grained structure of TiNiSn on the electronic structure of the material is theoretically demonstrated. On the other hand, the potential for thermal conductivity reduction upon increasing the Ni fraction in the intermetallic TiNiSn compound via the formation of metallic TiNi2Sn nanoparticles is also shown. Using the applied approach, a useful route for optimizing both the electronic and thermal properties of half-Heusler TiNiSn, for practical thermoelectric applications, is demonstrated.
