Abstract
Half-Heusler (HH) compounds are promising thermoelectric materials for mid-to-high temperature waste heat recovery, yet their performance is limited by relatively high lattice thermal conductivity. Nanostructuring has emerged as a key strategy to reduce thermal conductivity by enhancing phonon scattering at grain boundaries and interfaces. This study systematically investigates the effect of nanostructuring on the thermoelectric properties of p-type NbCoSn and n-type TiNiSn-based HH compounds. Samples with varying grain sizes (50–500 nm) were synthesized using ball milling and spark plasma sintering. A combination of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy confirmed the formation of nanograined microstructures. Transport properties were measured from 300 K to 900 K. The results show that reducing grain size to ~100 nm leads to a significant reduction in lattice thermal conductivity by up to 40% compared to microcrystalline samples, while maintaining a high power factor. The maximum figure of merit ZT reached 1.2 at 873 K for NbCoSn with 100 nm grains, a 50% improvement over coarse-grained counterparts. This work demonstrates that controlled nanostructuring is an effective route to enhance thermoelectric performance in HH compounds, and provides guidelines for optimizing processing parameters.