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.
Magnetic cooling could be a radically different energy solution substituting conventional vapour compression refrigeration in the future. For the largest cooling effects of most potential ...refrigerants we need to fully exploit the different degrees of freedom such as magnetism and crystal structure. We report now for Heusler-type Ni–Mn–In–(Co) magnetic shape-memory alloys, the adiabatic temperature change ΔT(ad) = −3.6 to −6.2 K under a moderate field of 2 T. Here it is the structural transition that plays the dominant role towards the net cooling effect. A phenomenological model is established that reveals the parameters essential for such a large ΔT(ad). We also demonstrate that obstacles to the application of Heusler alloys, namely the usually large hysteresis and limited operating temperature window, can be overcome by using the multi-response to different external stimuli and/or fine-tuning the lattice parameters, and by stacking a series of alloys with tailored magnetostructural transitions.
The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for ...both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of n > 2 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean-Rodbell model.
NdFeB permanent magnets have different life cycles, depending on the applications: from as short as 2–3 years in consumer electronics to 20–30 years in wind turbines. The size of the magnets ranges ...from less than 1 g in small consumer electronics to about 1 kg in electric vehicles (EVs) and hybrid and electric vehicles (HEVs), and can be as large as 1000–2000 kg in the generators of modern wind turbines. NdFeB permanent magnets contain about 31–32 wt% of rare-earth elements (REEs). Recycling of REEs contained in this type of magnets from the End-of-Life (EOL) products will play an important and complementary role in the total supply of REEs in the future. However, collection and recovery of the magnets from small consumer electronics imposes great social and technological challenges. This paper gives an overview of the sources of NdFeB permanent magnets related to their applications, followed by a summary of the various available technologies to recover the REEs from these magnets, including physical processing and separation, direct alloy production, and metallurgical extraction and recovery. At present, no commercial operation has been identified for recycling the EOL NdFeB permanent magnets and the recovery of the associated REE content. Most of the processing methods are still at various research and development stages. It is estimated that in the coming 10–15 years, the recycled REEs from EOL permanent magnets will play a significant role in the total REE supply in the magnet sector, provided that efficient technologies will be developed and implemented in practice.
A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency in the total energy lifecycle, has accelerated research into ...energy‐related technologies. Due to their ubiquity, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conditioning, conversion, transportation, and other energy‐use sectors of the economy. This review focuses on the state‐of‐the‐art hard and soft magnets and magnetocaloric materials, with an emphasis on their optimization for energy applications. Specifically, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure–property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy efficiency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare‐earth intermetallics for hard magnets will be discussed.
A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency, has accelerated research in energy‐related technologies. Magnetic materials play an important role in improving the efficiency and performance of many devices. The impacts of hard magnets on electric motor and transportation technologies, of soft magnets on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration are discussed.
Abstract
Autonomous materials discovery with desired properties is one of the ultimate goals for materials science, and the current studies have been focusing mostly on high-throughput screening ...based on density functional theory calculations and forward modeling of physical properties using machine learning. Applying the deep learning techniques, we have developed a generative model, which can predict distinct stable crystal structures by optimizing the formation energy in the latent space. It is demonstrated that the optimization of physical properties can be integrated into the generative model as on-top screening or backward propagator, both with their own advantages. Applying the generative models on the binary Bi-Se system reveals that distinct crystal structures can be obtained covering the whole composition range, and the phases on the convex hull can be reproduced after the generated structures are fully relaxed to the equilibrium. The method can be extended to multicomponent systems for multi-objective optimization, which paves the way to achieve the inverse design of materials with optimal properties.
Tetragonal (Formula: see text) FeNi is a promising material for high-performance rare-earth-free permanent magnets. Pure tetragonal FeNi is very difficult to synthesize due to its low chemical ...order-disorder transition temperature (Formula: see text K), and thus one must consider alternative non-equilibrium processing routes and alloy design strategies that make the formation of tetragonal FeNi feasible. In this paper, we investigate by density functional theory as implemented in the exact muffin-tin orbitals method whether alloying FeNi with a suitable element can have a positive impact on the phase formation and ordering properties while largely maintaining its attractive intrinsic magnetic properties. We find that small amount of non-magnetic (Al and Ti) or magnetic (Cr and Co) elements increase the order-disorder transition temperature. Adding Mo to the Co-doped system further enhances the ordering temperature while the Curie temperature is decreased only by a few degrees. Our results show that alloying is a viable route to stabilizing the ordered tetragonal phase of FeNi.
► Optimized annealing temperature and time in La-Fe-Si. ► Lamellar structure as an intermediate phase ► Large adiabatic temperature change of about 7
K for La-Fe-Si.
A systematic study of the ...microstructure and magnetocaloric effect in LaFe
11.8Si
1.2 and LaFe
11.6Si
1.4 alloys over a large range of annealing temperatures and times has been carried out. With the aim of obtaining the pure 1:13 phase and maximum magnetic entropy change the annealing temperature was optimized at 1373
K for LaFe
11.8Si
1.2 and 1323
K for LaFe
11.6Si
1.4. We found a unique morphology of eutectoid-type lamellae, which is suggested to be an intermediate phase upon formation of the 1:13 phase. Adiabatic temperature change Δ
T
ad measurements were employed to directly assess the magnetocaloric effect. By application of a magnetic field of 1.9 T large Δ
T
ad values of 7.3
K and 7.0
K in the vicinity of the transition temperatures were found for LaFe
11.8Si
1.2 and LaFe
11.6Si
1.4, respectively, after optimized annealing. By considering the partial irreversibility of magnetostructural transition the influence of thermal and magnetic hysteresis on magnetic entropy change and Δ
T
ad is also discussed.
•Magnetically anisotropic flake-shaped Mn–Al–C powders were prepared by ball milling.•Magnetization decreased due to the defects introduced during the milling process.•Annealing the as-milled powders ...could recover the crystal structure of τ-phase MnAl.•The easy axis lies in the planar direction of the flakes.
Magnetically anisotropic Mn–Al–C powders have been prepared by ball milling the τ-phase Mn53.3Al45C1.7 alloy. Flake-shaped powders were obtained after milling for 2h and the thickness decreased continuously with extended milling time. No sign of decomposition was observed in the as-milled state, however, magnetization decreased and coercivity increased continuously with milling time because of the defects introduced in the milling process. μ0iHc of 0.52T was obtained in powders milled for 12h. By annealing the as-milled powders at 773K for 30min, the magnetization increased significantly because of the recovered crystal structure of τ-phase. A degree of texture ranging from 17.8% to 37.1% has been obtained and the magnetically easy axis lies within the plane of the flakes. Optimized magnetic properties, with Mr of 54.8Am2kg−1 and μ0iHc of 0.28T, was obtained in Mn53.3Al45C1.7 powders milled for 5h followed by heat treatment at 773K.
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