Effect of Off-Stoichiometry on the Thermoelectric Properties of Heusler-Type Fe2VAl Sintered Alloys

Abstract

Heusler-type Fe2V1−xAl1+x sintered alloys with micrometer-sized grains were fabricated by the powder metallurgical process using mechanical alloying and pulse-current sintering. Both positive (∼90 μV/K) and negative (∼−140 μV/K) Seebeck coefficients were obtained for the composition ranges of x > 0 and x < 0, respectively, resulting from a Fermi level shift caused by the change in the valence electron concentration. The electrical resistivity was reduced by the carrier doping effect, especially at lower temperatures, resulting in an increased thermoelectric power factor of 2.8 mW/m-K2 for the p-type alloy with x = 0.06 and 5.0 mW/m-K2 for the n-type alloy with x = −0.06. In addition, the lattice thermal conductivity decreased with |x| because of phonon scattering at crystal lattice defects induced by the off-stoichiometry. Consequently, the thermoelectric figure of merit, ZT, was enhanced and reached 0.07 for p-type alloys with 0.06 < x < 0.15 and 0.18 for n-type alloys with −0.15 < x < −0.10 around 500 K. The ZT value was especially enhanced at higher temperatures by the off-stoichiometric composition control, which could extend the range of heat source temperatures for thermoelectric power generation applications using this alloy.