Transport and thermoelectric properties of Hf-doped FeVSb half-Heusler alloys

Nearly single-phase FeVSb half-Heusler alloys with fine grains were obtained by induction melting followed by mechanical alloying (MA), spark plasma sintering (SPS) and annealing process. Hf as a heavy element was used as dopant to present point defects aiming at decreasing the material’s thermal conductivity during phonon scattering. Thermoelectric properties of the FeV1−xHfxSb samples (0.0 ≤ x ≤ 0.3) were investigated as a function of temperature in a range from 300 to 600 K. Microstructure investigations showed that grain size decreases with increasing the level of Hf doping (x). Seebeck coefficient of the parent FeVSb compound showed negative sign which refers to n-type conduction. Interestingly, the sign has changed to positive with introducing Hf in the FeVSb lattice at different concentrations (x ≥ 0.1), which is due to one less valence electron in Hf as compared to V. The difference between the host atom (V) and the impurity atom (Hf) in terms of mass and size has resulted in a mass fluctuation and consequently a disorder scattering. The absolute value of the Seebeck coefficient |S| of the FeVSb system was measured at 110 μV/K, while the thermal conductivity value was obtained at 10.46 Wm−1K−1 near room temperature. A maximum power factor of 1.07 mWm−1 K−2 at 420 K was recorded. The thermal conductivity decreased rapidly upon Hf doping due to increased point defect scattering caused by Hf introducing to the FeVSb system. A maximum ZT value of 0.08 at 573 K for FeV0.9Hf0.1Sb was recorded in this study.