  Science-driven design of future thermoelectric materials requires a deep
understanding of the fundamental relationships between microstructure and
transport properties. Grain boundaries in polycrystalline materials influence
the thermoelectric performance through the scattering of phonons or the
trapping of electrons due to space-charge effects. Yet, the current lack of
careful investigations on grain boundary-associated features hinders further
optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler
alloys, which were synthesized by ball milling and spark plasma sintering
(SPS). Post-SPS annealing was performed on one sample, leading to improved
low-temperature electrical conductivity. The microstructure of both samples was
examined by electron microscopy and atom probe tomography. The grain size
increases from ~230 nm to ~2.38 {\mu}m upon annealing. Pt is found within
grains and at grain boundaries, where it locally reduces the resistivity, as
assessed by in situ four-point-probe electrical conductivity measurement. Our
work showcases the correlation between microstructure and electrical
conductivity, providing opportunities for future microstructural optimization
by tuning the chemical composition at grain boundaries.
