The demand for refrigeration has substantially increased since 1990s. The refrigerants employed in conventional cooling systems are known to cause ozone depletion and subsequent environmental impacts. Thermoelectric (TE) cooling and magnetocaloric (MC) cooling technologies face some challenges in replacing conventional vapor compression systems due to their relatively low coefficients of performance (COP) and heat transfer efficiency. A combination of TE and MC cooling could be a promising way to improve the heat-exchange efficiency. Recent studies have focused extensively on thermo-electro-magnetic materials that simultaneously exhibit TE and MC properties. These materials include both MC/TE composites and intrinsic thermo-electro-magnetic materials. However, inherent heterogeneous interfacial reactions in MC/TE composites simultaneously degrade both MC and TE performance. Intrinsic thermo-electromagnetic materials as single-phase compounds can effectively avoid such a degradation. Fe₂VAl-based Heusler alloys exhibit promising thermoelectric properties due to their unique electronic structure. The incorporation of magnetic Co atoms is expected to simultaneously enhance the Curie temperature (T_C), and synergistically regulate both MC and TE properties in Fe₂VAl-based Heusler compounds. This study prepared a series of Fe_2-xCo_xVAl alloys via arc-melting and characterized their phase composition, MC properties, and TE performance. This study demonstrated that controlled Co doping could be critical for optimizing the thermo-electro-magnetic properties of intrinsic Fe_2-xCo_xVAl alloys, thus providing an effective material design strategy for solid-state hybrid MC/TE cooling technologies.

A series of Fe_2-xCo_xVAl (x = 0, 0.5, 1.0, 1.5, 2.0) ingots were prepared via arc melting in argon (Ar) atmosphere with high purity Fe (99.99%, blocks), Co (99.9%, pieces), V (99.95%, pieces), and Al (99.99%, granules). To ensure a homogeneity, the ingots were subjected to five remelting cycles with intermediate flipping. The ingots were cut into desired shapes using wire electrical discharge machining for the measurement of thermal and electrical transport properties. The phase structure of the materials was determined by powder X-ray diffraction (XRD, SmartLab, Rigaku Co., Japan). The microstructures were determined by scanning electron microscopy (JSM-IT800, Japan) equipped with a back-scattered electron detector and a Bruker energy-dispersive spectroscopy detector. The lattice constants were refined according to the Rietveld method using the Fullprof program. The Seebeck coefficient (α) and electrical conductivity (σ) were measured simultaneously by a standard four-probe method (CTA-3, Beijing Cryoall Science and Technology Co., Ltd., China) in a helium atmosphere at 300-750 K. The thermal diffusivity (λ) was measured by a laser flash method (Netzsch LFA-467, Netzsch Co., Germany). The thermal conductivity was calculated according to κ = λρC_p, where C_p was the heat capacity according to the Dulong-Petit formula, and the density ρ was measured based on the Archimedes prinicple. The magnetization versus magnetic field (M-H) curves and thermomagnetic (M-T) curves were measured with a multi-VersaLab vibrating sample magnetometer (VSM, VersaLab, Quantum Design INC., USA). The magnetic entropy change curves (ΔS-T) were calculated using the Maxwell relations.

The results indicate that the incorporation of the magnetic element Co into Fe₂VAl Heusler alloy can synergistically regulate its MC and TE properties, showing a potential to become a high-performance intrinsic thermo-electro-magnetic material. The XRD patterns show that the Bragg diffraction peaks of all the samples match the standard cards without impurity peaks detected. The lattice constant increases with the increase of Co doping, indicating the continuous substitution of atoms Fe by atoms Co. The SEM images reveal that Fe, V, and Al are homogeneously distributed in the materials without any impurity phases. The incorporation of Co significantly elevates the T_C of Fe₂VAl-based Heusler alloys to industrially viable ranges, which is consistent with the Slater-Pauling rule predictions and the Bethe-Slater curve validation. At a magnetic field of 2.5 T, the maximum magnetic entropy change (ΔS_max) increases to 1.04 J·kg^-1·K^-1 for the sample with x of 1.5 J·kg^-1·K^-1 and 0.81 J·kg^-1·K^-1 for the sample with x of 2.0. The sample with x of 2.0 achieves an enhanced relative cooling power (RCP) of 110.7 J·kg^-1, benefiting from its significantly broadened full-width at half-maximum (δ_FWHM). The introduction of Co significantly increases the electrical conductivity (σ), shifting a transport behavior from semiconducting to metallic. The σ of the sample with x of 2.0 is the maximum vaule of 4.2×10⁵ S·m^-1 at 300 K. The substitution of Co introduces extra electrons, switching the Seebeck coefficient (α) from positive to negative values, indicating that the majority carriers shift from holes to electrons. The α of the sample with x of 0.5 is the maximum vaule of -68 µV/K at 550 K. The sample with x of 0.5 sample exhibits a peak power factor (α²σ) of 1.14 mW·K^-2·m^-1 at 550 K. The decrease in α²σ at high temperatures is due to the combined effects of reduced electrical conductivity and the decrease in the Seebeck coefficient caused by intrinsic excitation. At 300 K, The lattice thermal conductivity of the sample with x of 2.0 is the minimum value of 4.1 W·m^-1·K^-1. The maximum zT value reaches 0.073 K at 600 K for the sample with x of 0.5, which is 260 times higher than that of the sample with x of 0.

The introduction of the magnetic element Co could synergistically regulate the magnetocaloric and thermoelectric properties of Fe₂VAl Heusler alloy. The sharp increase in T_C could be attributed to the intense Fe-Co exchange interaction. The sample with x of 1.5 had the ΔS_max of 1.04 J·kg^-1·K^-1. The sample with x of 2.0 had the maximum RCP of 110.7 J·kg^-1. The sample with x of 0.5 had the maximum zT value of 0.073 at 600 K. The existing research showed that the T_C could be further increased and a better balance between MC and TE properties could be achieved via optimizing the Co doping content. To further enhance the TE performance without affecting T_C, doping heavy elements at the V site to increase phonon scattering or introducing V/Al antisite defects to improve electrical transport properties could be considered. This work could provide an effective material design strategy for developing intrinsic thermo-electro-magnetic materials based on Full Heusler alloys.