Enhanced thermoelectric figure of merit in Ta-doped TiCoSb half-Heusler bulks through simultaneously optimising electrical transport property and reducing thermal conductivity

Ta-doped TiCoSb-based half-Heusler (HH) bulks were fabricated via microwave synthesis combined with spark plasma sintering. Ta doping can provide additional electrons to the matrix while increasing the density of states effective mass, optimising the carrier concentration and carrier mobility, and thus simultaneously increasing the electrical conductivity and Seebeck coefficient. The power factor is increased to 2467.31 μWm⁻¹ K⁻², which is 11.89 times higher than that of the pure sample. On the other hand, there are large atomic mass and size differences between Ta and Ti atoms, and Ta doping introduces mass fluctuation scattering and stress field fluctuation scattering. The point defects, lattice distortion and phonon scattering brought by doping further reduce the lattice thermal conductivity, resulting in a significant reduction of the thermal conductivity from 5.26 to 2.41 Wm⁻¹ K⁻¹. And Ti_0.875Ta_0.125CoSb achieves a maximum ZT of 0.69 at 873 K.