Thermal and electronic charge transport in bulk nanostructured Zr0.25Hf0.75NiSn composites with full-Heusler inclusions

Bulk Zr0.25Hf075NiSn half-Heusler (HH) nanocomposites containing various mole fractions of full-Heusler (FH) inclusions were prepared by solid state reaction of pre-synthesized HH alloy with elemental Ni at 1073 K. The microstructures of spark plasma sintered specimens of the HH/FH nanocomposites were investigated using transmission electron microscopy and their thermoelectric properties were measured from 300 K to 775 K. The formation of coherent FH inclusions into the HH matrix arises from solid-state Ni diffusion into vacant sites of the HH structure. HH(1–y)/FH(y) composites with mole fraction of FH inclusions below the percolation threshold, y∼0.2, show increased electrical conductivity, reduced Seebeck coefficient and increased total thermal conductivity arising from gradual increase in the carrier concentration for composites. A drastic reduction (∼55%) in κl was observed for the composite with y=0.6 and is attributed to enhanced phonon scattering due to mass fluctuations between FH and HH, and high density of HH/FH interfaces.

Graphical abstractLarge reduction in the lattice thermal conductivity of bulk nanostructured half-Heusler/full-Heusler (Zr0.25Hf075NiSn/ Zr0.25Hf075Ni2Sn) composites, obtained by solid-state diffusion at 1073 K of elemental Ni into vacant sites of the half-Heusler structure, arising from the formation of regions of spinodally decomposed HH and FH phases with a spatial composition modulation of ∼2 nm.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Bulk composites from solid state transformation of half-Heusler matrix through Ni diffusion. ► Formation of coherent phase boundaries between half-Heusler matrix and full-Heusler inclusion. ► Alteration of thermal and electronic transports with increasing full-Heusler inclusion. ► Enhanced phonon scattering at half-Heusler/ full-Heusler phase boundaries.