Investigations of microstructures and thermoelectric properties of TiNiSn half-Heusler compounds with micro- and nano-scale copper additions

The TiNiSn-based half-Heusler compounds have been considered as a promising thermoelectric material system for waste heat recovery applications. There have been numerous studies dedicated to improving its intrinsic low thermoelectric performance, and one strategy is to incorporate certain types of metal dopants into the host materials. Only a few research works have been focused on the effects of the dimensions of metal particles on the thermoelectric properties of materials. In this study, we investigate the roles of micro- (M) and nano-scale (N) Cu powders in affecting the microstructures and thermoelectric properties of TiNiCu_xSn (x = 0−0.1) compounds. Owing to the ease of oxidation of Cu nanoparticles and the tedious preparation procedure, TiNiCu_{x, N}Sn shows lower carrier concentration and electrical conductivity compared to the TiNiCu_{x, M}Sn with the same nominal Cu content. The high-temperature Seebeck coefficient of TiNiCu_{x, N}Sn gradually deteriorates as the temperature rises, partly due to the impurities-induced bandgap narrowing effect. The maximum power factor achieved for TiNiCu_{x, N}Sn is about 20.2% lower than that for TiNiCu_{x, M}Sn. Even though the reduced contribution of electronic component leads to a lower total thermal conductivity at low temperatures, the highest zT obtained in TiNiCu_{x, N}Sn is markedly lower than that in the corresponding TiNiCu_{x, M}Sn. These findings highlight the importance of investigating the dimensions of metal dopants in studies of the thermoelectric properties of materials.