Effects of off-stoichiometry and Ti doping on thermoelectric performance of Fe2VAl Heusler compound

Improvement of p-type thermoelectric performance for Heusler-type Fe2V1.08−yTiyAl0.92 alloys with y = 0–0.34 has been investigated, focusing on the valence electron concentration (VEC) effect for tuning the Fermi level across the center of the pseudogap via Ti doping, combined with a constructive modification of the electronic structure around the pseudogap through the off-stoichiometric V/Al composition change. The Seebeck coefficient changes its sign from negative to positive at around VEC = 6.0 due to Ti doping, and the peak value reaches approximately 120 μV/K at 350 K for y = 0.22. As a result of a drastic reduction in the electrical resistivity at 300–600 K, the power factor enhances up to 3.7 × 10−3 W/mK2 at 300 K for y = 0.30. The thermal conductivity increases with Ti doping to 15.5 W/mK at 350 K for y = 0.16, because of an increased stability of the L21 structure for VEC closer to 6.0, but then decreases to approximately 12 W/mK for y = 0.34. While the dimensionless figure of merit ZT = 0.13 is achieved at 500 K for y = 0.34, the codoping of Ta (Fe2V0.69Ti0.34Ta0.05Al0.92) leads to a further enhancement of up to ZT = 0.18 at 500 K.