| Pseudogap Engineering of Fe2VAl Thermoelectric Heusler Compounds Yoichi Nishino1; 1NAGOYA INSTITUTE OF TECHNOLOGY, Nagoya, Japan; PAPER: 379/SISAM/Invited (Oral) SCHEDULED: 15:55/Mon. 28 Nov. 2022/Ballroom A ABSTRACT: The Heusler compound Fe<sub>2</sub>VAl 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 Fe<sub>2</sub>V<sub>1+x</sub>Al<sub>1-x</sub> 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 Fe<sub>2</sub>V<sub>1.05</sub>Al<sub>0.95</sub> achieves a large increase in the power factor up to 6.8x10<sup>-3</sup> W/mK<sup>2</sup>, which is superior to that for half-Heusler compounds, skutterudites and Mg<sub>2</sub>Si [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 Fe<sub>2</sub>V<sub>1.08-y</sub>Ti<sub>y</sub>Al<sub>0.92</sub> 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<sup>-3</sup> W/mK<sup>2</sup> 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 L2<sub>1</sub> 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 Fe<sub>2</sub>VAl‐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 <i>ZT</i> up to 0.29 at 400 K for Fe<sub>2</sub>V<sub>0.98</sub>Ta<sub>0.05</sub>Al<sub>0.92</sub>. 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 Fe<sub>2</sub>V<sub>0.98</sub>Ta<sub>0.10</sub>Al<sub>0.92</sub>, combined with a large power factor, leads to <i>ZT</i> = 0.37 around 400 K, one of the highest values ever achieved for bulk Fe<sub>2</sub>VAl-based thermoelectric materials. References: [1] Y. Nishino, Thermoelectric Energy Conversion: Theories and Mechanisms, Materials, Devices, and Applications, ed. by R. Funahashi (Woodhead Publishing, 2021) pp. 143-156. [2] Y. Nishino, S. Kamizono, H. Miyazaki, K. Kimura, AIP Advances <b>9</b> (2019) 125003. [3] K. Fukuta, K. Tsuchiya, H. Miyazaki, Y. Nishino, Appl. Phys. A <b>128</b> (2022) 184. |
