High-Zt Half-Heusler Thermoelectrics, Ti0.5Zr0.5NiSn and Ti0.5Zr0.5NiSn0.98sSb0.02: Physical Properties at Low Temperatures

Abstract

With a small gap in the density of states and a substantially semiconducting behavior half Heusler alloys have drawn attention as thermoelectric materials. For this study we have selected Hf-free compounds, Ti0.5Zr0.5NiSn, Ti0.5Zr0.5NiSn (with a densification aid (DA)) and Ti0.5Zr0.5NiSn0.98Sb0.02 as well their parent alloys TiNiSn and ZrNiSn as cheap thermoelectrics with a figure of merit reaching ZT=1.2 at 875 K. Electrical resistivity, thermal conductivity and specific heat were evaluated below room temperature (4.2 - 300 K) in order to get insight into the mechanism of transport properties. KSEM and TEM investigations as well as DFT (density functional theory) calculations accompany this research. The fine-grained epitaxial microstructure with a large number of dislocations warrants a low thermal conductivity at ultralow values (~30 mW/cmK at 300 K) at a narrow band gap with a sufficiently high density of states at the Femi level. High order of components mixing strongly affects the stability of the solid solutions by the configuration entropy term, which causes a shrinkage of the miscibility gap. For the electronic density of states (DOS) the split Zr band and impurity Ni band induce a significant reduction of the effective energy gap and thus explain n-type of conductivity of the compounds and solid solutions studied.

Keywords: Half Heusler Alloys, DFT, Nanostructured Materials, Specific Heat, Thermal Effect