Manipulation of thermal conductivity and thermoelectric power factor via Hf-doping in FeNbSb half-Heusler alloys for enhanced thermoelectric figure of merit

• FeNb 1-x Hf x Sb alloys were synthesized by successive induction melting and mechanical alloying. • Transport and thermoelectric properties were investigated from 300 K to 800 K. • The highest power factor was obtained at 20 µWcm −1 K −2 for the highest Hf-doped alloy. • The maximum figure of merit was observed for the same sample at 0.16, recorded at 700 K. Bulk samples of FeNb 1-x Hf x Sb (x = 0.0, 0.10, 0.20) alloys have been prepared by successive induction melting and mechanical alloying (MA). Electrical and thermoelectrical characteristics have been investigated as functions of temperature in the temperature range 300 to 800 K. Electrical conductivity of the parent FeNbSb alloy and the low doped FeNb 0.9 Hf 0.1 Sb samples showed semiconducting behaviour with smaller electrical conductivity values. On the other hand, the most doped FeNb 0.8 Hf 0.2 Sb alloys showed a metal-like trend. The metallic behaviour indicated that typical acoustic phonon scattering dominates the charge transfer. Thermopower measurements revealed that Seebeck coefficients increase remarkably with the temperature increasing due to Hf-doping at the studied doping range due to the remarkable effect of Hf-doping on the carrier concentration. The relationship between temperature and thermoelectric power factor was evaluated and investigated. High thermoelectric power factor was achieved at 20 µWcm −1 K −2 for the highest Hf-doped alloy. Defects and electron–phonon scatterings reduced the lattice thermal conductivity and consequently improved thermoelectric figure of merit ( zT ). zT has significantly increased with temperature increasing providing evidence that half-Heusler materials are promising thermoelectric options in the mid to high temperature range. In addition, it can be concluded that higher Hf-content as dopant can simultaneously optimizes enhanced power factor besides significantly reduced thermal conductivity leading to better thermoelectric figure of merit. The maximum figure of merit ( zT ) was observed for the same sample with a value of 0.17, recorded at 700 K.