High-temperature thermoelectric properties of Nb-doped MNiSn (M = Ti, Zr) half-Heusler compound

The temperature dependence of electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity of Nb-doped MNiSn (M = Ti, Zr) half-Heusler compounds were investigated at different temperature, ranging from room temperature to 1000 K. The power factor reached 4 × 10−3 Wm−1 K−2 above 600 K for both systems. The power factor for TiNiSn-based samples decreased above 700 K due to the narrower band gap. The Hall mobility was relatively small; however, estimated carrier effective mass was larger by one order of magnitude than that for conventional thermoelectric material. The thermal conductivity increased above 700 K due to the ambipolar diffusion effect. The ambipolar diffusion effect depended on the band gap width and the ratio of electron–hole conductivity. Heavy carrier-doping effectively suppressed the ambipolar diffusion effect, i.e. restrain the increase of thermal conductivity at high temperature. The maximum ZT value of 0.6 at 800 K was obtained for Zr0.98Nb0.02NiSn.