Transport properties of niobium doped MNiSn (M = Ti, Zr)

In these years, (Ti, Zr, Hf)NiSn based half-Heusler compounds are expected as new thermoelectric materials at moderate temperatures due to the high power factor, high mechanical strength and non-toxicity [Aliev, FG, et al., 1989, 1990]. In order to improve the figure of merit, the electrical properties have been optimized by carrier doping and the thermal conductivity has been reduced by substitution. In this study, we prepared Ti_1-xNb_xNiSn and Zr_1-yNb_y NiSn (x, y = 0.01, 0.02, 0.05). The Seebeck coefficient, electrical conductivity, thermal conductivity and Hall coefficient were measured from room temperature to 1000 K. The maximum power factor was 4.4 times 10^-3 Wm^-1K^-2 for Zr_0.98Nb_0.02NiSn at 873 K. The carrier mobility was proportional to T^-1.5 from 700 to 900 K, indicating that the acoustic phonon scattering is dominant in the temperature range. The effective mass (m/m₀) was estimated to be 4.5-6.2 at 773 K. The lattice thermal conductivity decreased with increasing niobium content. ZT = 0.68 at 970 K was achieved for Zr_0.98Nb_0.02NiSn