Enhancement of Thermoelectric Properties in n-type NbCoSn Half-Heusler Compounds via Ta Alloying

NbCoSn-based half-Heusler alloys are promising n-type high-temperature thermoelectric materials, but their relatively high thermal conductivities prevent the further improvement in thermoelectric properties. In this work, an effective way to reduce the lattice thermal conductivity without degradation of electrical transport properties via the isoelectronic substitution of Ta for Nb in NbCoSn-based alloys has been demonstrated. The synthesis and thermoelectric properties of Nb1-xTaxCoSn0.9Sb0.1 (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25) solid solutions are reported. The large mass fluctuation and chemical bond softening caused by Ta alloying significantly suppress the lattice thermal conductivity. Meanwhile, the Ta content has little effect on the carrier mobility due to the close atomic size and similar chemical nature between Ta and Nb. Benefiting from the significant reduction of lattice thermal conductivity and preservation of electrical transport performance after Ta alloying, a ZT value of similar to 0.7 is achieved in Nb0.75Ta0.25CoSn0.9Sb0.1 at 973 K, which is about 32% higher than that of a NbCoSn0.9Sb0.1 matrix. This work demonstrates a valid approach to enhance the thermoelectric performances of half-Heusler alloys.