The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more ...important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future.
Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials is a promising alternative to conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant magnetocaloric material families is presented and compared in terms of their adiabatic temperature and isothermal entropy change under cycling, criticality aspects, and the amount of heat that they can transfer per cycle.
•Structural and magnetic properties of (Nd1−xREx)13.6FebalCo6.6Ga0.6B5.6 (RE = Ce, La) melt-spun alloys.•Ce induced grain coarsening whereas La had a grain refinement effect.•Moderate Ce substitution ...mildly deteriorated magnetic properties.•Dilute La addition enhanced remanence and saturation polarization and maintained a high Curie temperature.•Ce substitution improved hot-workability.
Ce and La as very cheap rare-earth elements were used to substitute Nd in nanocrystalline melt-spun ribbons of nominal compositions (Nd1−xREx)13.6FebalCo6.6Ga0.6B5.6 (x = 0, 0.1, 0.2, … 1 for RE = Ce) and (x = 0, 0.1, 0.2, … 0.5 for RE = La). Ce substitution gradually decreased the Nd2Fe14B lattice constants and produced CeFe2 segregation from x = 0.7. La substitution led to lattice expansion along the c-axis and induced segregation of α-Fe and Nd2Fe17 at x = 0.5. Grain coarsening was observed in the Ce-substituted samples while La was found to suppress grain growth. Cerium worsened the magnetic properties of as-spun powders after an initial improvement in (Nd0.9Ce0.1)13.6FebalCo6.6Ga0.6B5.6 alloy which showed a coercivity (µ0Hc) of 1.54 T and a remanence (Br) of 0.81 T. Coercivity dropped with increasing La concentration but remanence increased from 0.73 T in the base composition to 0.88 T at x = 0.3. The Curie temperatures (TC) showed a slight decrease in both cases until x = 0.4. It then dropped abruptly for increasing Ce fractions and increased at x = 0.5 La. For x = 0.2 and 0.3 Ce and x = 0.2 La fractions, the melt-spun samples were further processed by hot-pressing and hot-deformation. The hot-pressed (Nd0.8La0.2)13.6FebalCo6.6Ga0.6B5.6 alloy measured lower coercivity but increased remanence comparing to the Ce-substituted alloys. However, this composition responded poorly to hot-deformation, severe cracking being induced in the process. Due to enhanced hot-workability, best magnetic properties were obtained after deformation for the (Nd0.7Ce0.3)13.6FebalCo6.6Ga0.6B5.6 alloy (µ0Hc = 1.09 T, Br = 0.97 T and energy product (BH)max = 170 kJ/m3).
Ni-Mn-based metamagnetic shape-memory alloys exhibit a giant thermal response to magnetic fields and uniaxial stress which can be utilized in single caloric or multicaloric cooling concepts for ...energy-efficient and sustainable refrigeration. However, during cyclic operation these alloys suffer from structural and functional fatigue as a result of their high intrinsic brittleness. Here, we present based on Fe-doping of Ni-Mn-In a microstructure design strategy which simultaneously improves cyclic stability and maintains the excellent magnetocaloric and elastocaloric properties. Our results reveal that precipitation of a strongly Fe-enriched and In-depleted coherent secondary γ-phase at grain boundaries can ensure excellent mechanical stability by hindering intergranular fracture during cyclic loading. In this way, a large elastocaloric effect of -4.5 K was achieved for more than 16000 cycles without structural or functional degradation, which corresponds to an increase of the cyclic stability by more than three orders of magnitude as compared to single-phase Ni-Mn-In-(Fe). In addition, we demonstrate that the large magnetocaloric effect of single-phase Ni-Mn-In-(Fe) can be preserved in the dual-phase material when the secondary γ-phase is exclusively formed at grain boundaries as the martensitic transformation within the Heusler matrix is barely affected. This way, an adiabatic temperature change of -3 K and an isothermal entropy change of 15 Jkg−1K−1 was obtained in 2 T for dual-phase Ni-Mn-In-Fe. We expect that this concept can be applied to other single caloric and mutlicaloric materials, therewith paving the way for solid-state caloric cooling applications.
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•In situ alignment of platelet-shaped filler during LPBF process.•Processability of composites in LPBF decreased in comparison to pure polymers.•Ratio dependent trade-off relation between mechanical ...and magnetic performance.•Platelet shaped filler improves demagnetization behavior.
Bonded permanent magnets are key components in many energy conversion, sensor and actuator devices. These applications require high magnetic performance and freedom of shape. With additive manufacturing processes, for example laser powder bed fusion (LPBF), it is possible to produce bonded magnets with customized stray field distribution. Up to now, most studies use spherical powders as magnetic fillers due to their good flowability. Here, the behavior of large SmFeN platelets with a high aspect ratio as filler material and its influence on the arrangement and the resulting magnetic properties are examined in comparison to a spherical magnetic filler. The 3D distribution and orientation of the magnetic filler was studied by computed tomography and digital image analysis. The platelet-shaped particles align themselves perpendicular to the buildup direction during the process, which offers a new and cost-effective way of producing composites by LPBF with anisotropic structural and functional properties. The influence of LPBF parameters on the properties of the composites is investigated. Highest filling fractions are required for high magnetic remanence, however the powder itself limits this maximum due to particle shape and required minimal polymer fraction to form mechanically stable magnets. The coercivity decreases for higher filling fractions, which is attributed to increased rotation of insufficiently embedded magnetic particles in the matrix. It is discussed how filler morphology influences the observed change in coercivity since the rotation of spherical particles in comparison to platelet-shaped particles requires less energy. Our work shows the challenges and opportunities of large platelet shaped fillers used in LPBF for the production of anisotropic functional and structural composites.
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The direct use of an advanced binder‐free additive manufacturing technique, namely laser powder bed fusion (L‐PBF), does not easily allow obtaining variously shaped, fully dense Nd–Fe–B magnets with ...high coercivity. The process inherently leads to the re‐melting of the powder and appearance/disappearance of undesired/desired microstructural features responsible for low and large coercivity. In this work, the development of a useful microstructure responsible for high coercivity in Pr21Fe73.5Cu2B3.5 and Nd21Fe73.5Cu2B3.5 alloys and a possible way to produce fully dense permanent magnets via additive manufacturing processes is demonstrated using: (i) suction casting technique, which provides a high cooling rate and thus similar microstructures as in L‐PBF but requires only very small amounts of powder; (ii) conventional L‐PBF processing using kg of powder, and (iii) a subsequent annealing treatment that is similar to a conventional sintering treatment. The subsequent heat treatment is necessary to develop high coercivity by forming a novel microstructure: hard magnetic (Nd,Pr)2Fe14B grains embedded in a matrix of intermetallic (Nd,Pr)6Fe13Cu phase. Furthermore, it is demonstrated that Pr21Fe73.5Cu2B3.5 exhibits a higher coercivity than Nd21Fe73.5Cu2B3.5 because of a finer and more homogeneous grain size distribution of the Pr2Fe14B phase. The final L‐PBF printed Pr21Fe73.5Cu2B3.5 samples provide a coercivity of 0.75 T.
A new alloy and grain boundary design strategy to produce fully dense permanent magnets by additive manufacturing processes is proposed. After solidification of a Pr–Fe–Cu–B‐based alloy by laser powder bed fusion, the initial low coercivity due to an inappropriate microstructure is restored by temperature‐controlled phase transformation for efficient and stable magnetic hardening in 3D‐printed magnets.
We present a comprehensive study on three selected Heusler alloy systems. Ni‐Mn‐X(‐Co) systems with X = Al, In, Sn are compared with respect to the relevant magnetocaloric properties of their ...magnetostructural phase transition, namely martensitic transition temperature as well as its field dependence, magnetization change, and width of the thermal hysteresis. The latter one is strongly determining the reversibility of the magnetocaloric effect. Therefore the understanding of how to tailor it by extrinsic and intrinsic factors is of great importance. Our study of the magnetocaloric properties leads to the conclusion that the width of thermal hysteresis can be correlated to the magnetization change of the phase transition. Consequently, the adiabatic temperature change under cycling can largely vary despite similar values of isothermal entropy change for Ni‐Mn‐In‐Co and Ni‐Mn‐Sn‐Co. This result therefore shows the importance of tailoring sharpness, thermal hysteresis, and field dependence of the phase transition to achieve high values for the isothermal entropy change as well as a large magnetocaloric cooling effect in the different Heusler alloys.
This study presents a comprehensive overview on the crucial properties for a systematic improvement of the magnetocaloric performance of three selected Ni‐Mn‐X(‐Co) Heusler systems. After optimization of heat treatment, the magnetic field dependence and thermal hysteresis are evaluated. The different influences of these properties on isothermal entropy change and adiabatic temperature change are finally compared for the three systems.
Ni-Mn-based Heusler alloys, in particular all-d-metal Ni(-Co)-Mn-Ti, are highly promising materials for energy-efficient solid-state refrigeration as large multicaloric effects can be achieved across ...their magnetostructural martensitic transformation. However, no comprehensive study on the crucially important transition entropy change Δst exists so far for Ni(-Co)-Mn-Ti. Here, we present a systematic study analyzing the composition and temperature dependence of Δst. Our results reveal a substantial structural entropy change contribution of approximately 65 J(kgK)-1, which is compensated at lower temperatures by an increasingly negative entropy change associated with the magnetic subsystem. This leads to compensation temperatures Tcomp of 75 K and 300 K in Ni35Co15Mn50-yTiy and Ni33Co17Mn50-yTiy, respectively, below which the martensitic transformations are arrested. In addition, we simultaneously measured the responses of the magnetic, structural and electronic subsystems to the temperature- and field-induced martensitic transformation near Tcomp, showing an abnormal increase of hysteresis and consequently dissipation energy at cryogenic temperatures. Simultaneous measurements of magnetization and adiabatic temperature change ΔTad in pulsed magnetic fields reveal a change in sign of ΔTad and a substantial positive and irreversible ΔTad up to 15 K at 15 K as a consequence of increased dissipation losses and decreased heat capacity. Most importantly, this phenomenon is universal, it applies to any first-order material with non-negligible hysteresis and any stimulus, effectively limiting the utilization of their caloric effects for gas liquefaction at cryogenic temperatures.
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Microstructure design allows to prevent intergranular cracking and premature failure in Co–Ni–Ga shape memory alloys. Favorable grain boundary configurations are established using additive ...manufacturing techniques, namely, direct energy deposition (DED) and laser powder bed fusion (L‐PBF). L‐PBF allows to establish a columnar grain structure. In the Co–Ni–Ga alloy processed by DED, a microstructure with strong ⟨001⟩ texture is obtained. In line with optimized microstructures, the general transformation behavior is essential for performance. Transition parameters such as transition temperature and thermal hysteresis depend on chemical composition, homogeneity, and presence of precipitates. However, these parameters are highly dependent on the processing method used. Herein, the first‐order magnetostructural transformation and magnetization properties of Co–Ni–Ga processed by DED and L‐PBF are compared with single‐crystalline and as‐cast material. In the alloy processed by L‐PBF, Ga evaporation and extensive formation of the ferromagnetic Co‐rich γ'‐phase are observed, promoting a very wide transformation range and large thermal hysteresis. In comparison, following DED, the material is characterized by minor chemical inhomogeneity and transition and magnetization behavior being similar to that of a single crystal. This clearly renders DED‐processed Co–Ni–Ga to become a promising candidate material for future shape memory applications.
Microstructure design of Co–Ni–Ga shape memory alloys by additive manufacturing can be used to prevent intergranular cracking and premature failure. The comparison of microstructure, composition, and magnetic properties of Co–Ni–Ga Heusler alloy processed by direct energy deposition and laser powder bed fusion shows large differences in terms of precipitate formation and martensitic transformation behavior.
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