Improving the thermoelectric performance of Ti-doped NbFeSb by substitutional doping of the Sb atoms with the isoelectric and heavy Bi atoms

The inherently low figure of merit (ZT) is the main impediment to the commercialization of half-Heusler thermoelectric materials in energy applications. Herein, we demonstrate the effectiveness of substituting Bi on the Sb site in the promising Nb_0.8Ti_0.2FeSb half-Heusler thermoelectric material to achieve much needed incremental performance improvement. P-Type Nb_0.80Ti_0.20FeSb_1−xBi_x(x= 0-0.07) samples are prepared by arc melting, hot pressing and annealing. After microstructural analysis, a solubility limit of ∼2.4%Bi is determined on the Sb site. It is found that the dissolved Bi atoms not only cause significant phonon scattering, leading to a 12% decrease in thermal conductivity, but also increase the density of states effective mass, leading to an increase in the Seebeck coefficient and thus a 21% increase in the power factor in the samplex= 0.03 compared to the samplex= 0.00 at 973 K. Above the Bi-solubility limit, concomitant Sb vacancies are considered and shown to donate holes that strongly decrease the Seebeck coefficient and disproportionately increase the electronic thermal conductivity, leading to worse thermoelectric performance in samplesx≥ 0.05. Consequently, the effectiveness of this strategy is realized in samplex= 0.03, where the synergistic benefit of suppressed thermal conductivity and increased power factor yields the bestZTof ∼1.2 at 973 K, which is ∼36% higher than that of samplex= 0.00. Moreover, the microstrains introduced by Bi-doping lead to increased microhardness, which is desirable for improving the machinability of parts and increasing wear resistance in application.