Enhanced thermoelectric performance of NbCoSb half-Heusler alloys by using an amorphous precursor

Amorphous flakes with a stoichiometric composition of NbCoSb were successfully synthesized by introducing a rapid solidification technique. The amorphous phase transformed into a nanostructured NbCoSb half-Heusler phase with a valence electron count (VEC) of 19 during spark plasma sintering. Due to the chemically homogeneous nature of amorphous phase, uniform nanostructure of NbCoSb half-Heusler phase was obtained within the disk samples without impurity phase formation as well as long term annealing treatment required in the conventional manufacturing process. Nanostructuring of NbCoSb half-Heusler phase significantly increased grain boundary area in which electron and phonon scattering occurred and was effective to enhance thermoelectric performance. Furthermore, the carrier concentration was reduced by doping with Sn causing an increased dimensionless figure-of-merit ZT value. A maximum ZT value of -0.78, corresponding to a two-fold improvement compared to the ZT value of the as-cast sample, was achieved at 1173 K for the sintered NbCoSb0.95Sn0.05 sample.