Anderson transition in stoichiometric Fe2VAl: high thermoelectric performance from impurity bands

Abstract

Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for ’the best thermoelectric’—a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.

Improved Power Factor in Self-Substituted Fe2VAl Thermoelectric Thin Films Prepared by Co-sputtering

“…For the n-type film showing the highest P F , n is close to −10 22 cm –3 . This high value remains within the range of carrier concentrations obtained in the literature on Fe 2+x V 1+y Al 1+z , showing the power factor ranging from 2 to 7.6 mW/K 2 m. , …”

“…It corresponds to twice the maximum power factor obtained at 350 K by Mikami et al in Ti-substituted Fe 2 VAl thin films and at 300 K by Kudo et al in Fe 2–x V 1+x Al films grown by the MBE technique . It should be mentioned that our results are close to the highest values obtained in Fe 2 VAl-based alloys in the bulk state with P F = 7.6 mW/K 2 m recently obtained at 300 K in a 1653 K-quenched stoichiometric Fe 2 VAl sample or P F = 6.7 mW/K 2 m reported in bulk n-type Fe 1.98 V 1.02 Al 0.9 Si 0.1 at 300 K and Fe 2 V 1.03 Al 0.97 at 250 K.…”

“…Fe 2 VAl alloy shows high S when the valence electron concentration (VEC) per atom is close to 6. , Miyazaki et al and Diack-Rasselio et al also observed that the thermoelectric power factor ( P F = S 2 /ρ) is enhanced in n-type off-stoichiometric Fe 2– x V 1+ x Al bulk alloys ( x ∼ 0.03–0.05). The highest power factors P F = 7.6 and 6.7 mW/K 2 m at room temperature are larger for such alloys than that for Bi 2 Te 3 materials. However, the figure of merit ( ZT ∼ 0.1 at 300 K) is lower than that of Bi 2 Te 3 compounds because of its high lattice thermal conductivity ( λ L ) of about 20 W/(m·K).…”

“…Film annealing was carried out in a tube furnace under secondary vacuum close to 10 −6 mbar for a film deposited at 733 K. Four successive temperatures were chosen: T annealing = 873, 923, 973, and 1023 K for a constant duration of 15 h. The atomic film composition after deposition was Fe 48.4 V 30.1 Al 21. 5 The Seebeck Table 1. VEC and Electronic Transport Properties of Thin Films Deposited at 733 K (in Red) and 873 K (in Blue) a sample VEC T deposition (K) S (μV/K) +/−20% ρ (mΩ•cm) +/−10% n/p (10 21 cm −3 ) +/−10% μ (cm 2 /Vs) +/−20% P F (mW/K 2 m) +/−50% The black line corresponds to the transition from p-type to n-type films.…”

Roadmap on energy harvesting materials

“…Only recently, it has been demonstrated that the electronic structure itself can be severely modified, exhibiting remarkable tune-ability. By either opening the band gap [291] or incorporating narrow resonant or semi-localized impurity states at E F [292,293], the Seebeck coefficient was significantly enhanced, compared to the conventional S(n) behaviour (see coloured symbols, deviating from the black solid line in figure 48). Consequently, thermoelectric power factors 2-3 times larger than those of optimized Bi 2 Te 3 systems in the 0 • C-100 • C range have been achieved [291].…”