Enhanced thermoelectric properties of NbCoSn half-Heuslers through in-situ nanocrystallization of amorphous precursors during the consolidation process

Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for half-Heusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity. However, a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases, owing to residual elemental segregation from casting. In this study, we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation, which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization. Moreover, superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process, even at a low sintering temperature (953 K). This resulted in less oxidation at both, the grain boundary and the interior, as the O diffusion pathway was blocked during the sintering process. As a result, the NbCoSn_0.95Sb_0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7, due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.