Yoichi Nishino

Abstract:

The Heusler compound Fe₂VAl is a potential candidate for thermoelectric applications because of the possession of a deep pseudogap across the Fermi level. Since the Seebeck coefficient varies systematically with the valence electron concentration (VEC), irrespective of doping elements, the net effect of doping is most likely to cause a rigid-band-like shift of the Fermi level from the central region in the pseudogap. Further increase in the Seebeck coefficient can be achieved by the V/Al off-stoichiometric composition change, so that Fe₂V_1+xAl_1-x alloys exhibit a large value of -160 μV/K for the n-type V-rich alloy (x=0.03) and 100 μV/K for the p-type Al-rich alloy (x=-0.03), coupled with a significant decrease in the electrical resistivity. Thus Fe₂V_1.05Al_0.95 achieves a large increase in the power factor up to 6.8x10^-3 W/mK², which is superior to that for half-Heusler compounds, skutterudites and Mg₂Si [1].
We believe that the large Seebeck coefficient for the V-rich alloys could be caused by an electronic structure modification of the pseudogap due to the V/Al off-stoichiometry effect. Improvement of the p-type thermoelectric performance has been investigated for Fe₂V_1.08-yTi_yAl_0.92 alloys, where the Seebeck coefficient changes in its sign from negative to positive at around VEC = 6.0 due to the Ti doping, and the peak value reaches approximately 120 μV/K at 350 K for y=0.22 [2]. As a result of a drastic reduction in the electrical resistivity, the power factor enhances to 3.7×10^-3 W/mK² at 300 K for y=0.30. The thermal conductivity increases with the 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 turns to decrease to approximately 12 W/mK for y=0.34.
One of the issues for thermoelectric Fe₂VAl‐based compounds is to reduce the thermal conductivity as much as possible, while maintaining a high power factor. Heavy‐element Ta doping for the V/Al off‐stoichiometric alloys causes a drastic decrease in the thermal conductivity, leading to a large increase in the figure of merit ZT up to 0.29 at 400 K for Fe₂V_0.98Ta_0.05Al_0.92. High-pressure torsion (HPT) processing further reduces the thermal conductivity because of the production of ultrafine-grained structures with grain sizes less than 100 nm, which can be obtained through the suppression of grain coarsening due to the segregation of Ta during annealing [3]. Thus, a reduced thermal conductivity of 3.5 W/mK for Fe₂V_0.98Ta_0.10Al_0.92, combined with a large power factor, leads to ZT = 0.37 around 400 K, one of the highest values ever achieved for bulk Fe₂VAl-based thermoelectric materials.