Thermoelectric and transport properties of half-Heusler FeNb1-xTixSb alloys

In this work, Ti-doped FeNbSb half-Heusler alloys were synthesized by arc and induction melting techniques. The samples were then thermally treated for 48 h at 923 K in evacuated silica tubes. Powders of the prepared samples were alloyed via ball milling for 2 h in an argon atmosphere at milling speed of 450 rpm to obtain nano-composites. Crystal structure, elemental distribution and morphological properties were examined throughout the diffraction patterns of X-ray, scanning by the electronic microscope alongside with energy dispersive x-ray spectroscopy. Transport and thermoelectric properties were investigated over a wide temperature range between 300 and 800 K. The electrical conductivity was significantly improved due to the Ti doping which then resulted in remarkable increase in the thermoelectric power factor. The electrical conductivity showed an increase from 46 O-1cm-1 to 3000 O-1cm-1 with an improvement of 98 %, at room temperature. At high temperature (800 K), The electrical conductivity increased from 150 O-1cm-1 to 1414 O-1cm-1, upon doping. A maximum power factor of 21 mu W cm-1 K-2 for the highest Ti doped sample (FeNb0.8Ti0.2Sb), achieved at 800 K. Alongside improving the electrical conductivity, a remarkable reduction in the lattice and the total thermal conductivity was achieved due to Ti doping, over the whole temperature range. The lattice thermal conductivity exhibited a notable reduction from 12 W/m.K for the pristine FeNbSb alloy to 8.5 W/m.K for the Ti-doped FeNb0.8Ti0.2Sb alloy, at room temperature with a reduction of-30 %. The total thermal conductivity of the parent compound decreased from 12.05 W/m.K to 7.4 W/m.K, at room temperature. Accordingly, a promising figure of merit was recorded at around 0.20 for FeNb0.8Ti0.2Sb sample, at 800 K. A notable improvement in the figure of merit, zT with a percentage of -(84-92) % at 800 K is obtained, in comparison to that of the parent FeNbSb alloy, due to Ti doping. ZT is increased from 0.0011, for the parent FeNbSb sample, to 0.05 for the highest-doped FeNb0.8-Ti0.2Sb sample, at room temperature. Moreover, it increased from 0.015 for the parent sample to 0.2 for the highest-doped sample, at 800 K.