Microstructures and Thermoelectric Properties of an Annealed Ti0.5(Hf0.5Zr0.5)0.5NiSn0.998Sb0.002 Ribbon

Abstract

The thermoelectric half-Heusler compound Ti 0.5 CHf 0.5 Zr 0.5 ) 0.5 NiSn 0.998 Sb 0.002 was fabricated by spin-casting and subsequent annealing. ZT at room temperature increased with annealing time through an increase in absolute Seebeck coefficients despite a decrease in electrical conductivity. ZT reached 0.10 after annealing at 1050 K for 48 h. In powder x-ray diffraction analysis, each half-Heusler peak was accompanied by a bump at the high-angle side, corresponding to a minor Ti-rich half-Heusler phase. The quantity and Ti composition of the minor phase increased with annealing time, although those of the major half-Heusler phase were almost constant. In transmission electron microscopic analysis, granular domains, several nanometers in size, with atomic ordering or disordering were observed. Thermoelectric properties were improved by annealing through the growth of heterogeneous microstructures of the Ti-rich and Ti-poor half-Heusler grains and of the granular domains.