The Fe₂P type Mn–Fe–P–Si alloys exhibit a giant magneto-elastic first-order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe₂P-type (Mn_1.17-xFe_0.73-yV_0.02) (P_0.5Si_0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V-substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn_1.17-xFe_0.73-yV_0.02P_0.5Si_0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet.

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Improving mass transfer in gas diffusion layers is critical to achieving high-performance proton-exchange membrane fuel cells (PEMFCs). Leaks through the interface between the gas and the membrane electrode assembly frame have been widely investigated, and the controllability of the cathode gas diffusion has not been achieved in most studies. In this study, we develop a structural parameter to investigate the controllability of the gas diffusion mechanism in the cathode in order to improve upon the design and performance of PEMFCs. This parameter accounts for the cathode gas diffusion layer porosity and carbon loading inside the catalyst layer. It is comprehensively calculated to relax the two segments’ distribution along three directions of the coordinate axis. The experimental and simulation results show that the obtained values of the parameter vary and change during voltage stabilization. According to the results, regardless of the materials in the cathode gas diffusion layer, the same steady-state voltage is obtained when the parameter is fixed. The cell could be controllably operated for a wide range of diffusion layer thicknesses by selecting the optimal parameter.

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In the field of nanoscale magnetocaloric materials, novel concepts like micro-refrigerators, thermal switches, microfluidic pumps, energy harvesting devices and biomedical applications have been proposed. However, reports on nanoscale (Mn,Fe)₂(P,Si)-based materials, which are one of the most promising bulk materials for solid-state magnetic refrigeration, are rare. In this study we have synthesized (Mn,Fe)₂(P,Si)-based nanoparticles, and systematically investigated the influence of crystallite size and microstructure on the giant magnetocaloric effect. The results show that the decreased saturation magnetization (M_s) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and T_c are reduced. In addition, we determined an optimal temperature window for annealing after synthesis of 300–600 °C and found that gaseous nitriding can enhance M_s from 120 to 148 Am²kg⁻¹ and the magnetic entropy change (ΔS_m) from 0.8 to 1.2 Jkg⁻¹K⁻¹ in a field change of Δμ₀H = 1 T. This improvement can be attributed to the synergetic effect of annealing and nitration, which effectively removes part of the defects inside the particles. The produced superparamagnetic particles have been probed by high-resolution transmission electron microscopy, Mössbauer spectra and magnetic measurements. Our results provide important insight into the performance of giant magnetocaloric materials at the nanoscale.

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Large thermal hysteresis in the (Mn,Fe)2(P,Si) system hinders an efficient heat exchange and thus limits the magnetocaloric applications. Substitution of manganese by vanadium in the Mn1-x1Vx1Fe0.95P0.593Si0.33B0.077 and Mn1-x2Vx2Fe0.95P0.563Si0.36B0.077 compounds enable a significant reduction in the thermal hysteresis without losing the giant magnetocaloric effect. For the composition closest to the critical one, where first-order crossovers to second-order phase transition in the series of x2 = 0.02, Mn0.98V0.02Fe0.95P0.563Si0.36B0.077 exhibits a thermal hysteresis that is reduced from 1.5 to 0.5 K by 67%, yielding an adiabatic temperature change of 2.3 K and magnetic entropy change of 5.6 J/kgK for an applied field of 1 T, which demonstrates its potential for highly efficient magnetic heat pumps utilizing low-cost permanent magnets.@en

The phase diagram of the magnetocaloric Mn_x Fe_2−x P_1−y Si_y quaternary compounds was established by characterising the structure, thermal and magnetic properties in a wide range of compositions (for a Mn fraction of 0.3 ≤ x < 2.0 and a Si fraction of 0.33 ≤ y ≤ 0.60). The highest ferromagnetic transition temperature (Mn_0.3 Fe_1.7 P_0.6 Si_0.4, T_C = 470 K) is found for low Mn and high Si contents, while the lowest is found for low Fe and Si contents (Mn_1.7 Fe_0.3 P_0.6 Si_0.4, T_C = 65 K) in the Mn_x Fe_2−x P_1−y Si_y phase diagram. The largest hysteresis (91 K) was observed for a metal ratio close to Fe:Mn = 1:1 (corresponding to x = 0.9, y = 0.33). Both Mn-rich with high Si and Fe-rich samples with low Si concentration were found to show low hysteresis (≤2 K). These compositions with a low hysteresis form promising candidate materials for thermomagnetic applications.

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The first-order magneto-elastic transition in the Mn–Fe–P–Si alloys can be tailored by vanadium substitution. Alloys with a suitable V substitution provide an excellent magnetocaloric effect with minor hysteresis in low magnetic fields up to 1.2 T. Mössbauer measurements show that the hyperfine field is reduced by V substitution. Neutron diffraction reveals that Fe is substituted by V on the 3f site and the magnetic moment on the 3f site is enhanced by the V substitution. The modified magnetic exchange field around the 3f and 3g positions in the lattice can be utilized to design suitable magnetocaloric materials that operate in low magnetic fields.

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The giant magnetocaloric effect is widely achieved in hexagonal MnMX-based (M = Co or Ni, X = Si or Ge) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. Here, we show an alternative way to realize a reversible low-field magnetocaloric effect in MnMX-based alloys by taking advantage of the second-order phase transition. With introducing Cu into Co in stoichiometric MnCoGe alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with a negligible thermal hysteresis and a large magnetization change can be observed, enabling a reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0-1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To gain a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides an understanding of the MnCoGe alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the MnMX system.

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The magnetocaloric effect (MCE) is a magneto-thermodynamic phenomenon in which a temperature change of a material is caused by exposing the material to a changing magnetic field under adiabatic conditions. There are two main applications based on the MCE. One application is magnetic refrigeration, which can expel heat in a magnetic field cycle. Another application is magnetic energy conversion in thermomagnetic motors/generators, which can transfer waste heat into kinetic/electric energy. Gadolinium metal is the standard reference material for the application of the MCE. However, it has a limited MCE with a second-order magnetic transition. Several intermetallic material systems with first-order magnetic transition resulting in a giant MCE have been discovered, including La(Fe,Si)13 based alloys, MnFeP(As, Ge, Si) alloys and Ni-Mn-based Heusler alloys. To design a magnetocaloric material that is suitable for applications, first of all, requires an estimated recipe, which can be obtained from the phase diagram. Secondly, an appropriate synthesis route should be chosen. Thirdly, the stoichiometry of the material should be optimised to avoid impurity phases. For the energy conversion applications, the desired material should preferentially be in the vicinity of the border between a first-order magnetic phase transition (FOMT) and a second-order magnetic phase transition (SOMT). If it is a FOMT or SOMT, the formula can be adjusted by changing the heat treatment, the element ratios and introducing new elements, until the transition is close to the critical point (CP). Finally, the transition temperature needs to be checked to see it is in the designed working temperature range. If not, the recipe needs to be adjusted until an optimised material is found. Experimental diagrams of the ferromagnetic transition temperature (TC) and the thermal hysteresis as a function of composition were constructed in the (Mn,Fe)2(P,Si) system as a guide to estimate suitable compositions for applications. The structure change across the magnetic phase transition is coupled with the thermal hysteresis of the magnetic transition in the experimental diagram. Both Mn-rich samples and Fe-rich samples with a low Si concentration were found to show a low hysteresis that can form promising candidates for applications in a thermomagnetic motor. The effect of V substitution for Fe is investigating in the Mn0.7Fex-zVzP0.6Si0.4 alloys. The (Mn,Fe)1.91(P,Si) stoichiometry was chosen as a starting point to obtain the smallest impurity content. For an increasing V content the a axis expands and the c axis shrinks (together with the c/a ratio), whereas the unit-cell volume remains about constant. The ferromagnetic transition temperature TC decreases with increasing V content. In the Mn0.7Fe1.18V0.03P0.6Si0.4 compounds, 93% of saturation magnetisation at 5 K was reached in an applied magnetic field of 0.5 T, which makes this compound a promising candidate for low-field applications. The heat treatment clearly affects the amount of the impurity phase, and thereby the composition of the main phase. In this case, oven-cooled samples contain a larger impurity phase fraction than the quenched samples, which results in a lower transition temperature. The currently applied methods to classify FOMT and SOMT materials were applied and compared using a series of samples Mn13Fe0.7P1-ySiy (y = 0.4, 0.5 and 0.6). The FOMT samples are easy to categorise. Every criterion shows that y = 0.4 and 0.5 sample is FOMT materials. However, the SOMT and CP samples are problematic. In this thesis, different criteria were found to result in different conclusions for the y = 0.6 sample. From the latent heat, the y = 0.6 is predicted to undergo a FOMT. From the XRD data and the field dependence of TC, the y = 0.6 sample is right on the CP. However, based on the Arrott plots, the gradual field dependence of the entropy change and the newly proposed field exponent n, the y = 0.6 sample is a SOMT material (but in close proximity to the CP). The structural, magnetic and electronic properties of LaFe11.8-bCobSi1.2 (b = 0.25, 0.69 and 1.13) compounds are studied. With increasing Co content, the material is tuned from a FOMT towards a SOMT, TC increases, and the thermal hysteresis remains neglectable. In the unit cell, the most remarkable change in bond length is between the 8b and 96i sites and for one of the bonds between two neighbouring 96i sites. The negative thermal expansion across the transition correlates with the angle change in the orientation of the cage formed by the atoms on the 96i sites within the cubic unit cell. The experimental electron density maps reveal how the cage rotates within the cubic primitive cell. The samples with a smaller Co content show a larger change in the electron density compared to the sample with the highest Co content when TC is crossed. The choice of synthesis method plays an important role in the physical properties of the prepared materials. For lab-scale samples, the most common way to synthesise (Mn,Fe)2(P,Si) compounds is ball milling. For Ni-Mn based Heusler alloys, the most common synthesis route is arc-melting. In this thesis, ball milling was applied to synthesise Ni-Mn based Heusler alloys. The advantage of ball milling is that the annealing time can be shortened. Based on the optimised sample fabrication, the maximum magnetisation can be tuned by adjusting the Ni/Mn and Mn/Sn ratios. Introducing small amounts of cobalt and aluminium leads to a significant increase in the magnetisation. @en

Ni-Mn-X (X = In, Sn, and Sb) based Heusler alloys show a strong potential for magnetic refrigeration owing to their large magnetocaloric effect (MCE) associated with first-order magnetostructural transition. However, the irreversibility of the MCE under low field change of 0–1 T directly hinders its application as an efficient magnetic coolant. In this work, we systematically investigate thermal and magnetic properties, crystalline structure and magnetocaloric performance in Ni51−xMn33.4In15.6Vx alloys. With the introduction of V, a stable magnetostructural transition near room temperature is observed between martensite and austenite. An extremely small hysteresis of 2.3 K is achieved for the composition x = 0.3. Due to this optimization, the magneticfield induced structural transition is partially reversible under 0–1 T cycles, resulting in a reversible MCE.
Both magnetic and calorimetric measurements consistently show that the largest value for the reversible magnetic entropy change can reach about 5.1 J kg−1 K−1 in a field change of 0–1 T. A considerable and reversible adiabatic temperature change of −1.2 K by the direct measurement is also observed under a field change of 0–1.1 T. Furthermore, the origin of this small hysteresis is discussed. Based on the lattice parameters, the transformation stretch tensor is calculated, which indicates an improved geometric compatibility between the two phases. Our work greatly improves the MCE performance of Ni-Mn-X-based alloys and make them suitable as realistic magnetic refrigeration materials.
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MnMX (M = Co or Ni, X = Si or Ge) alloys with strong magnetostructural coupling exhibit giant magnetic entropy change and are currently extensively studied. However, large thermal hysteresis results in serious irreversibility of the magnetocaloric effect in this well-known system. In this work, we report a low thermal hysteresis and large reversible magnetocaloric effect in a MnNiGe-based system. The introduction of Fe into both Ni and Mn sites can establish stable magnetostructural transitions from paramagnetic hexagonal to ferromagnetic orthorhombic phases. Fascinatingly, a low thermal hysteresis of 5.2 K is achieved in Mn_0.9Fe_0.2Ni_0.9Ge alloy with a large magnetization difference of 62.1 A m²/kg between the two phases. These optimized parameters lead to a partially reversible phase transformation under a magnetic stimulus and bring about a large reversible magnetic entropy change of −18.6 Jkg⁻¹K⁻¹ under the field variation of 0–5 T, which is the largest value reported in MnMX system up to now. Moreover, this low-hysteresis magnetostructural transformation and large reversible magnetocaloric effect can be tuned by doping with Si in a wide temperature range covering room temperature. We also introduce geometrically nonlinear theory to discuss the origin of low hysteresis in MnMX alloys. A strong relation is found between thermal hysteresis and the change of c axis in the orthorhombic structure during the transition. Our work greatly develops the potential of MnMX alloys as magnetocaloric materials and is meaningful to seek or design a MnMX system with low thermal hysteresis.

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