Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance

TL;DR

This study investigates how dopant segregation at grain boundaries in NbCo(Pt)Sn half-Heusler alloys affects electrical conductivity, revealing microstructure-property relationships crucial for optimizing thermoelectric performance.

Contribution

It provides detailed microstructural analysis of dopant segregation effects on electrical conductivity in NbCo(Pt)Sn alloys, advancing understanding of grain boundary engineering for thermoelectrics.

Findings

Pt segregation reduces resistivity at grain boundaries.

Annealing increases grain size and improves conductivity.

Microstructure influences thermoelectric properties.

Abstract

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.