Enhanced thermoelectric performance in the n-type NbFeSb half-Heusler compound with heavy element Ir doping

The p-type NbFeSb half-Heusler compound has been proved to be a promising thermoelectric (TE) material with a figure of merit zT > 1. However, the corresponding n-type (V,Nb)FeSb solid solutions show poor TE performance. Here, we report the enhanced TE performance of n-type NbFe1-xIrxSb (0.01 < x < 0.08) by band modification and heavy element doping compared with that of the state-of-the-art n-type Co-doped (V,Nb)FeSb. The band effective mass m* b is reduced to 0.76 me, and the high-temperature carrier mobility increases obviously. Debye screening length calculation and model analysis show the additional introduced grain boundary scattering, due to small grain sizes, which deteriorates the room temperature carrier mobility. The heavier doping element Ir not only optimizes the carrier concentration but also introduces strong point defect scattering on phonons, resulting in a minimum thermal conductivity of 3.7 Wm−1K−1 at 1000 K for the NbFe0.92Ir0.02Sb sample. The larger bandgap of NbFeSb effectively suppresses minority carrier excitation. A maximum zT value of 0.5 is obtained at 1000 K for the NbFe0.94Ir0.06Sb sample, almost a 50% increase compared with that of the n-type (V0.6Nb0.4)FeSb samples.