High temperature thermoelectric properties of MNiSn (M=Zr, Hf)

The high temperature transport properties in a series of intermetallic half-Heusler alloys of the form MNiSn, where M=Zr, Hf, have been examined. The semiconducting nature of these materials due to the formation of a pseudo-gap in the density of states make them promising candidates for intermediate temperature thermoelectric applications. Samples of pure and Sb-doped ZrNiSn, HfNiSn, and (Zr-Hf)NiSn were prepared by arc melting and homogenized by heat treatment. Phase purity was determined by X-ray diffraction and the microstructures were examined by scanning electron microscopy. The temperature dependence of the electrical resistivity and Seebeck coefficient of these samples was characterized between 300 K and 1050 K. At room temperature, the data match closely with the results recently reported by us. The thermopower initially increases with temperature, exhibits a broad maximum between 400 K and 600 K, and decreases to a common value, characteristic of the magnitude of the forbidden gap. The electrical resistivity decreases with temperature following a T/sup -1/ dependence. A correlation between the magnitude of the thermopower and the Hf/Zr ratio was observed. An estimate of the magnitude of the gap was made from a plot of 1n(/spl sigma/) versus reciprocal temperature, giving a value of 0.21 eV which is in good agreement with previous estimates. The effects of antimony and bismuth doping on the electrical properties are discussed.