Thermal and electronic charge transport in bulk nanostructured Zr{sub 0.25}Hf{sub 0.75}NiSn composites with full-Heusler inclusions
Bulk Zr{sub 0.25}Hf{sub 075}NiSn 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{approx}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 ({approx}55%) in {kappa}{sub l} 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 abstract: Large reduction in the lattice thermal conductivity of bulk nanostructured half-Heusler/full-Heusler (Zr{sub 0.25}Hf{sub 075}NiSn/ Zr{sub 0.25}Hf{sub 075}Ni{sub 2}Sn) 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 {approx}2 nm. Highlights: > 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.