Significant enhancement in thermoelectric properties of half-Heusler compound TiNiSn by grain boundary engineering

Half-Heusler compounds MNiSn (M = Ti, Zr, Hf) have attracted increasing attentions in the field of thermoelectrics, since their constituent elements are non-toxic and abundant. Meanwhile, it possesses good thermal stability as well as high mechanical strength. Herein, a substantial enhancement to the thermoelectric performance of intrinsic TiNiSn is achieved by passing the pulverized powders through the sieve prior to sintering. The Seebeck coefficient has increased sharply owing to the energy filtering effect, which is induced by the reduction in the average grain sizes, combined with a slightly decreased electrical conductivity, leading to a significant improvement in power factor. Moreover, the lattice thermal conductivity can be further reduced via the enhancement of the grain boundary scattering. Consequently, a maximum figure-of-merit (zT) value of 0.84 at 820 K is achieved for un-doped TiNiSn. This work indicates that the grain boundary engineering may serve as an effective method for improving the thermoelectric performance of other half-Heusler compounds.