Mastering hysteresis in magnetocaloric materials Gutfleisch, O.; Gottschall, T.; Fries, M. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
08/2016, Letnik:
374, Številka:
2074
Journal Article
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Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic ...refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2P-type compounds.
This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.
We report on (1) direct measurements of ΔTad for binary Fe49Rh51 during field cycling and (2) maximum possible ΔTad measured under discontinuous protocol. Our results show that the ΔTad is 9.2 K on ...the first application of magnetic field of Δμ0H = 1.9 T and it remains as high as 6.2 K during the cycling in alternated field of the same magnitude. In addition, the adiabatic temperature change and magnetic entropy change under the first application of magnetic field and under cyclic conditions were determined indirectly using three different approaches: (1) from magnetic measurements (M(T)H dependences, Maxwell relations), (2) from calorimetry (C(T)p,H, S-T diagram) and (3) from H-T diagram. While the indirectly measured maximum possible ΔTad lies in the range of 10.5–12 K, the reduced value of ΔTad measured directly under cycling (6.2 K) is still extraordinarily high and is 15% higher than in Gd in similar magnetic field. This demonstrates the potential of materials with a first order metamagnetic transition for magnetocaloric applications despite the presence of hysteresis.
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It is commonly understood that among the intermetallic phases used for permanent magnets, practically none can fully realize its potential based on the intrinsic magnetic properties. We discuss ...different reasons leading to this limitation, known as the Brown paradox, and propose some possible ways of overcoming it. We compare the intrinsic magnetic properties of (Nd1−xCex)2(Fe1−yCoy)14B single crystals with the extrinsic characteristics of sintered and hot compacted magnets made from the very same alloys. In addition, looking at RE-free materials, our results obtained on Mn- and Co-based RE-free single crystals are compared with the hard magnetic properties of Mn-based permanent magnets.
The magnetocaloric effect of gadolinium has been measured directly in pulsed magnetic fields up to 62T. The maximum observed adiabatic temperature change is ΔTad=60.5K, the initial temperature T0 ...being just above 300K. The field dependence of ΔTad is found to follow the usual H2/3 law, with a small correction in H4/3. However, as H is increased, a radical change is observed in the dependence of ΔTad on T0, at H=const. The familiar caret-shaped peak situated at T0=TC becomes distinctly asymmetric, its high-temperature slope becoming more gentle and evolving into a broad plateau. For yet higher magnetic fields, μ0H≳140T, calculations predict a complete disappearance of the maximum near TC and an emergence of a new very broad maximum far above TC.
Achieving a very strong magnetic anisotropy in a 3d material is a difficult, but not an impossible task. It is difficult because there is no general recipe (necessary condition) for a strong ...anisotropy in a band magnet. Several strategies can be pursued in this situation. One of them is to re-examine the less studied 3d compounds, somewhat neglected since the discovery of the Nd-Fe-B magnets 30 years ago. As an example, a single crystal of (Fe0.7Co0.3)2B has been investigated in this work.
The response of a magnetocaloric material to periodic variations of magnetic field and temperature corresponding to those occurring during a magnetic refrigeration process is studied. A series of ...simple measurement protocols are suggested which are used to obtain a value for the cyclic response of the magnetic entropy change associated with the magnetic transition. The entropy values are compared to direct measurements of the temperature change under adiabatic conditions. The procedure is illustrated on the first order magnetocaloric material La0.6Pr0.4Fe11.6Si1.4 and provides a basis for comparison of the suitability of different hysteretic magnetocaloric materials for application in a magnetic refrigerator. For the alloy studied here the peak magnetic entropy change of −28±1Jkg−1K−1 in a field change of 2T is not affected by cycling, but the full width at half maximum of the peak decreases from 8.7K to 3.8K.
•Measurement protocols are presented to assess materials for magnetic refrigeration.•Entropy changes are compared to the temperature change under adiabatic conditions.•The procedure overcomes limitations of the Maxwell relation for first order materials.•The first order magnetocaloric material La0.6Pr0.4(Fe,Si)13 is investigated.
•A comprehensive study of ThMn12-type Ce(Fe,Ti)12 material system.•Twinned grains of CeFe11Ti with misorientation angle of 58∘± 2∘.•Invar-type anomaly at the Curie temperature of CeFe11Ti.•Reduction ...of anisotropy field of CeFe11Ti under hydrostatic pressure in contrast to an increase for chemical expansion of CeFe11TiHx.
The criticality of rare-earth elements and its impact on the permanent magnet market have initiated a search for alternative material systems, either to fill the magnetic performance gap between ferrites and Nd-Fe-B magnets or to even replace the existing benchmark systems. Ce-based systems are attractive due to the abundance of cerium, however the formation of secondary phases leads to challenges for Ce-based compounds. We investigated the formation and magnetic properties of ThMn12-type Ce1+xFe11Ti samples for x = 0.15 and 0.20 compositions. Microstructural investigations reveal the formation of twin boundaries in the ThMn12 grains with a misorientation angle of 58∘± 2∘. By varying the chemical composition and lowering the annealing temperatures, a grain refinement and reduction in twin boundary density was achieved. In addition, the magnetostrictive characteristic of CeFe11Ti-phase was investigated. Around the Curie temperature, an M2 behaviour is noted, in contrast to an M3 behaviour at lower temperatures. To probe the effect of unit cell compression and expansion on the magnetic properties, magnetic characterizations were carried out under hydrostatic pressure and after hydrogenation. A slight reduction is observed for the measured anisotropy field under 0.45 GPa hydrostatic pressure in the temperature interval of 10–300 K, which is consistent with the small increase detected for the hydrogenated compound.
In the present work, we explore the influence of a surface-bulk coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. In our ...systematic and comprehensive study we diffused four different rare earth elements (Dy, Tb, Ce and Gd) in two different kinds of commercial Nd-Fe-B magnets, one very Dy-lean and one Dy-rich. By means of cutting the magnets into thin slices we obtain lateral coercivity profiles, from which diffusion constants are extracted. We find that in both magnets Tb diffuses significantly faster than Dy. The diffusion is generally slower in the Dy-lean magnet, which is attributed to the different chemistry and a smaller grain size. Ce diffuses slightly slower than Dy and the overall coercivity decrease is similar for Ce and Gd. With scanning electron microscopy it is revealed that, contrary to the magnets diffused with the heavy rare earths, the microstructure in the magnets treated with Ce show no (Nd,Ce)-Fe-B shells in the surface regions. While not of practical importance this allows some interesting insights into the metallurgy of (Nd,Ce)-Fe-B system. High-resolution scanning transmission electron microscopy coupled with electron probe microanalysis show the nano-scale distribution of Tb around the grain boundaries located in the bulk of the magnet. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into a FEM software. Our calculated demagnetization curves correspond very well for the Dy and Tb samples, but deviate significantly for Ce and Gd.
Coercivity changes in Nd-Fe-B permanent magnets after diffusion of several rare earth elements and resulting microstructures. Display omitted
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