Besides alloying, nanostructuring was implemented to improve the thermoelectric properties in Fe₂VAl. This Heusler alloy indeed displays a thermoelectric figure of merit too small for applications (ZT ~ 0.1 at 300 K) which is caused by a large lattice thermal conductivity (λ_L = 27 W·m⁻¹·K⁻¹ at 300 K). The effect of nanostructuring on the microstructure and on the thermoelectric properties of alloyed Fe₂VAl are therefore reviewed. By mechanical alloying followed by spark plasma sintering, the average grain size (D) was decreased to D ~ 300–400 nm in Fe₂VAl_0.9Si_0.1, Fe₂VAl_0.9Si_0.07Sb_0.03, Fe₂V_1.05Al_0.95, and Fe₂V_0.9W_0.1Al. As expected, phonon scattering at the numerous grain boundaries lead to a strong decrease in the lattice thermal conductivity, which reached values as small as λ_L = 3.3 W·m⁻¹·K⁻¹. However, in all the reviewed examples, the thermoelectric figure of merit (ZT) is only marginally or not even improved when comparing to non-nanostructured samples because the electrical resistivity also increases upon nanostructuring. A significantly improved ZT = 0.3 at 500 K was only recently observed in severely deformed Fe₂VAl_0.95Ta_0.05 by high pressure torsion because the very fine microstructure (D ~ 100 nm) strongly enhanced the thermal conductivity reduction.

A high temperature Seebeck coefficient or electrical resistivity apparatus has been designed and fabricated to measure sample with typical size ∼10 × 1 × 1 mm3. It can measure both transport properties from 300 K to 1000 K in argon atmosphere. The sample lies transversely on top of two metallic half-cylinders, which contain heating cartridges and allow temperature and thermal gradient control and reversal. The temperature gradient is measured by two type N thermocouples pressed against the upper surface of the sample. The key feature of this apparatus is the disk-shaped junction of each type N thermocouple which strongly improves the thermal contact with the sample. The Seebeck coefficient is obtained by averaging over two measured values with opposite thermal gradient directions (∼±2 K). For the resistivity measurements, the temperature is stabilized and the temperature gradient is actively reduced below 0.2 K to make negligible any spurious thermal voltage. Uncertainties of ∼3% for the Seebeck coefficient and 1% for the resistivity were obtained on Ni samples. The Seebeck coefficient and resistivity have also been measured on a skutterudite sample as small as ∼7 × 1.5 × 0.5 mm3 with very good agreement with literature.

In this paper, we report the successful synthesis of a Fe-based highly spin-polarized quaternary Heusler alloy FeMnVGa and its structural, magnetic, and transport properties probed through different experimental methods and theoretical techniques. Density functional theory (DFT) calculations performed on different types of structures reveal that the structure with Ga at $4a$, V at $4b$, Mn at $4c$, and Fe at $4d$ (space group $F\overline{4}3m$) possess minimum energy among all the ordered variants. Ab initio simulations in the most stable ordered structure show that the compound is a ferromagnet having a large spin-polarization (89.9%). Neutron diffraction reveals that the compound crystallizes in disordered type-2 structure (space group $Fm\overline{3}m$) in which Ga occupies at $4a$, V $4b$ and Fe/Mn occupy $4c/4d$ sites with 50:50 proportions. The structural disorder is further confirmed by x-ray diffraction, extended x-ray absorption fine structure, ${}^{57}\mathrm{Fe}$ Mössbauer spectrometry results, and DFT calculations. Magnetization studies suggest that the compound orders ferromagnetically below $T_{\mathrm{C}}\sim 293$ K and the saturation magnetization follows the Slater-Pauling rule. Mössbauer spectrometry, along with neutron diffraction, suggest that Mn is the major contributor to the total magnetism in the compound, consistent with the theoretical calculations, which also indicates that spin polarization remains high (81.3%), even in the presence of such large atomic disorder. The robustness of the half-metallic ferromagnetic (HMF) property in the presence of disorder is a quite unique characteristic over other reported HMF in literature and makes this compound quite promising for spintronics applications.