Improving Designs of Halbach Cylinder-Based Magnetic Assembly with High- and Low-Field Regions for a Rotating Magnetic Refrigerator

Magnetic Refrigeration: An Environment-friendly Cooling Technology

Computationally-efficient optimization of the remanence angles of permanent magnet circuits for magnetic refrigeration

This paper presents a novel configuration of the permanent magnet system for rotating magnetic refrigerator, which consists of stator and rotor. The stator of this system includes two permanent magnets and iron yoke. The rotor includes shaft, iron core and magnetic refrigerant material. When the magnetic refrigerant material is located under the pole of the permanent magnet, it is magnetized by a strong magnetic field and has a high temperature. As the rotor rotates, the magnetic refrigerant material is continuously demagnetized and its temperature gets lower and lower. When the rotor has turned 90 °space angle, the magnetic refrigerant material lies in the smallest magnetic field and its temperature becomes minimal. With the rotation of the permanent magnet system rotor, the magnetic refrigerant material is periodically magnetized and demagnetized and produces the magnetic thermal power for magnetic refrigeration. The permanent magnet system can generate the magnetic field of up to 1.2 T. It is much stronger than that generated by the two permanent magnets placed parallel to each other. Compared with the existing ones, this permanent magnet system has the advantages of high magnetic field strength, small size, simple structure, compact, high operation reliability, no one-side magnetic pull on the rotor and small vibration and noise, etc..

Permanent magnet dipole can generate magnetic field higher than the Br of the material with structures such as the Halbach array [1] and Stelter array [2]. These arrays usually generate magnetic field inside a cavity. And the magnetic field is perpendicular to the axis of the cavity. For applications such as Faraday Effect and erasing hard disks perpendicular media, a strong in-plane magnetic field is required. In the case of erasing hard disks, the disk needs to rotate in the air gap with magnetic field perpendicular to the surface of the disks. This requires that the dipole must open along the field direction. And it is often difficult to generate strong magnetic field when magnets can only be placed around the path of the magnetic flux. This paper presents a permanent magnet dipole design for strong in-plane magnetic field. The design starts with a Halbach structure that generates strong magnetic field in the cavity. To create access to the cavity from the in-plane field direction, a pair of magnets with orientation opposite to the magnetic field of the Halbach dipole are substituted along the field direction. The magnetic field from the substitute magnets is analyzed using current sheet equivalent method. This field is then subtracted from the Halbach magnet to generate the magnetic field from the new structure. We then used Bound Analysis software to simulate the structure. For the prototype, we choose an 8-segment Halbach magnet as the basic structure and used triangular magnets so that the magnets fit inside a rectangular steel housing. The magnetic material is NdFeB 48 MGOe with Br of 13600 G. Fig 1 shows the prototype dipole and the magnetic field in the air gap. In conclusion, this paper proposes a permanent magnet dipole with strong in-plane magnetic field. The prototype dipole generates strong in-plane magnetic field that is in line with the predictions from the analytical equation as well as our BEA analysis. This strong dipole can be used in many applications such as Faraday Effect and hard disk erase. The design is granted with US patent 10529362 [3]

As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration.

A general scheme for increasing the difference in magnetic flux density between a high and a low magnetic field region by removing unnecessary magnet material is presented. This is important in, e.g., magnetic refrigeration where magnet arrays has to deliver high field regions in close proximity to low field regions. Also, a general way to replace magnet material with a high permeability soft magnetic material where appropriate is discussed. As an example these schemes are applied to a two dimensional concentric Halbach cylinder design resulting in a reduction of the amount of magnet material used by 42% while increasing the difference in flux density between a high and a low field region by 45%.

This Ph.D. project was mainly devoted to the study of the connection between magnetocaloric properties and first order phase transitions in ferromagnetic materials based on the La(Fe,Si)13 compound. The magneto caloric effect (MCE) and its application in magnetic cooling cycles rely on the reversible magnetization and demagnetization of a magnetic material by an external magnetic field, resulting in a temperature change that is maximal at temperatures close to a magnetic phase transition. The possibility to improve the performance of the active refrigerator materials, depends on many factors: the need of a Curie temperature close to ambient temperature, a low magnetic and thermal hysteresis and a high magnetic entropy variation for magnetic fields below two Tesla. The latter requisite can be found in first order magnetic phase transitions that, unfortunately, are accompanied by intrinsic thermo-magnetic hysteresis. This drawback for magneto cooling cycles, motivates the present study on the phase transitions dynamics. On the other hand, the investigation of magneto-thermal phenomena in magnetic materials is of great importance also for solving fundamental problems of magnetism and solid state physics, for example, it is recognized that the properties of interest of such functional materials are intimately linked to the detailed micro structure, however, the nature of this link itself is not understood very often. In this Ph.D. project, thermo-magnetic phase transitions in La(Fe,Si)13 compounds were investigated through the comparison of various experimental techniques within a collaboration between the applied superconductivity group of Politecnico of Torino and the electromagnetism division of INRiM (National Institute of Metrological Research). To achieve a proper physical understanding of the connection between thermo-magnetic hysteresis at the microscopic level and the microstructure, a magneto optical method was applied to samples of La-F-Si-13 with cobalt substitutions, so to allow the dynamical visualisation of the phase boundaries motion in a first order phase transition. These type of experiments have been compared with low rate calorimetry data and, from the experimental work, it has been found that the presence of avalanches is a characteristic feature of these alloys and it is related to their thermal hysteresis. The difference between first and second order phase transition dynamics were highlighted thanks to the employment of different techniques, which also favoured the separation of the general aspects of hysteresis, common to all irreversible processes, from features more strictly dependent on specific microstructural properties. For the aim of this Ph.D., other techniques were also used to observe temperature induced magnetic phase transitions in functional magnetic materials. Among them an in-temperature ferromagnetic resonance method was implemented for the study of the magnetization dynamics in canted spin structures. The present research activity has been partially related to the European Project DRREAM [1] (a collaborative research project funded by the EC under the Seventh Framework Program 2013-2015), whose goal is to reduce the use of rare earth elements in the life cycle of technologies that use magnetic phase change materials, in particular magnetic refrigerators

Limited resources and the wish for improved prosperity call for efficient use of energy. The UN Advisory Group on Energy and Climate Change recommends a target of 40 % improved efficiency by 2030. Materials research can contribute significantly to reach this target. Magnetic refrigeration offers potential to achieve a 50 % higher energy-efficiency compared to vapor-compression refrigeration. This makes magnetic refrigeration a technology that attracts growing attention. Magnetic refrigeration is based on the magnetocaloric effect, i.e., the temperature change of a magnetic material upon the application or removal of a magnetic field in adiabatic conditions. Magnetocaloric materials play an important role in magnetic refrigeration. Giant magnetocaloric materials which are globally-abundant, non-toxic and can be industrially-mass-produced via a simple fabrication method are particularly attractive for magnetic refrigeration applications. Fe2P-based Mn-Fe-P-Si alloys can meet such requirements. The work presented in this thesis is a study of the magnetocaloric effect and related physical properties in the Mn-Fe-P-Si compounds. Some theoretical aspects of the magnetocaloric effect in general, and the origin of the first-order magneto-elastic transition which enhances the magnetocaloric effect in hexagonal Mn-Fe-P-Si compounds in particular are given in Chapter 2. In Chapter 3, a short review is presented of the experimental techniques and set-ups that have been employed for the sample preparation and the characterization of the physical properties of the Mn-Fe-P-Si compounds. Our efforts to optimize the magnetocaloric effect for refrigeration applications are presented in Chapter 4. We show that a giant magnetocaloric effect and a small thermal hysteresis in Mn-Fe-P-Si compounds of hexagonal Fe2P-type structure can be achieved simultaneously. Furthermore, the working temperature can be controlled over a large interval around room temperature by varying the Mn:Fe and P:Si ratios. In Chapter 5, we report on various types of transition found in (Mn,Fe)1.95P0.50Si0.50 when changing the Mn:Fe ratio. Interestingly, we observe a previously unknown first-order magneto-structural transition and a modified first-order magneto-elastic transition favorable for real refrigeration applications. Using high resolution neutron diffraction, x-ray diffraction and high-temperature magnetic-susceptibility measurements, and based on theoretical calculations, a first-order magneto-elastic transition from high-moment to low-moment in the Mn-Fe-P-Si compounds is presented in Chapter 6. This observation supports our proposal that the competition between moment formation and chemical bonding is at the core of giant magnetocaloric effect displayed in the class of hexagonal Fe2P-based materials with first-order magneto-elastic transition. The effect of the replacement of Fe by Mn on the magnetic moments is also discussed. Chapter 7 is devoted to the effects of P:Si ratio on the magnetic and structural properties of the Mn Fe P Si compounds. In chapter 8, we present magneto-elastic coupling in the Mn-Fe-P-Si compounds. Interestingly, hysteresis and magnetic entropy change are found to be correlated with discontinuous changes of the lattice parameters at the transition temperature. Small thermal hysteresis can be obtained while maintaining the giant magnetocaloric effect. A preliminary comparison of the magneto-elastic coupling and magnetocaloric effect for Mn-Fe-P-As/Ge/Si is also given.

The magnetocaloric effect (MCE) consists, in simple terms, in the heating of a magnetic material by the application of an external magnetic field. This can be easily understood if one but imagines a magnetic material with randomly applied spins; as a magnetic field is applied the spins will tend to alight with this field and as a result the overall Entropy of the system decreases, which consequently results in an exchange in Heat. This idea then becomes analogous to the vapor compression cycle used in our current household refrigerators, making the possibility of assembling a magnetic refrigerator quite tangible. The current thesis is in part an ample exploratory research of several material families with the aim of determining their magnetocaloric potential. In this scope the (Mn,Fe)3(Si,P), (Mn,Co)3(Si,P), (Fe,Co)3(Si,P) and (Mn,Fe)2(P,Ge) systems are studied and characterized. The study in (Mn,Fe)3(Si,P) resulted in the full magnetostructural mapping of this system, revealing many possible applications in a wide variety of physical areas. The (Mn,Co)3(Si,P) and (Fe,Co)3(Si,P), although not wielding positive results for MCE applications, still revealed the existence of other related material systems with extremely relevant physical properties, such as the inverse MCE. Finally the (Mn,Fe)2(P,Ge) revealed a very promising MCE potential, with the added discovery of a definite potential for permanent magnet applications. Finally, the current thesis also covers the assembly of microcalorimetry setup using commercial Xensor chips designed for the measurement of key physical properties for the understanding of the transition phenomenon in materials of MCE interest.

Analytical solution of concentric two-pole Halbach cylinders as a preliminary design tool for magnetic refrigeration systems

The success of a room temperature magnetic refrigerator (RTMR) depends critically on two essential parts: a high magnetic field and a magnetic refrigerant material with large magnetocaloric effect. A carefully designed hollow cylindrical permanent magnet array (HCPMA) can be used to provide strong magnetic field in the cavity, the magnitude of the resulting static field can be even greater than the remanence magnetization of the magnets comprising a HCPMA. A thorough understanding of the magnetic field distribution will provide an invaluable insight into the design and optimization of HCPMA in the reciprocating and rotary RTMR systems. Here, we show a construction of a 16 piece HCPMA with realistic dimensions and we illustrate the mechanism of generating a high magnetic field in such device. We present an effective way to calculate the field distribution of a permanent magnet array with finite size and an unsymmetrical geometry. Furthermore, detailed numerical results of the magnetic field distribution and its dependence on device dimensions are presented.

4.13 Magnetic Energy Conversion

<sec>It has been found that many magnetic materials possess the properties arising from skyrmions at room temperature. In addition to the common interaction energy, chiral interaction is also needed to form the skyrmion in magnetic material. There are four chiral magnetic interactions, namely: 1) Dzyaloshinskii-Moriya (DM) interaction; 2) long-ranged magnetic dipolar interaction; 3) four-spin exchange interaction; 4) frustrated exchanged interaction. Through the competition between exchange interaction and chiral interaction, magnetic skyrmion can be realized in magnetic material subject to a certain magnetic field and temperature. The skyrmion generated by the DM interaction features small size (5–100 nm), which is easy to adjust. The skyrmion can be driven by magnetic field or ultralow current density. The magnetic materials with skyrmion can exhibit the properties related to the skyrmion Hall effect, the topological Hall effect and the emergent electrodynamics, which are closely related to the skyrmion number. The existence of skyrmion in the magnetic material can be indirectly measured by topological Hall effect. The movement of skyrmion can be driven by spin polarized current in the direction either parallel or perpendicular to the current direction. The movement of the skyrmion driven by spin polarized currents will continue when the current is present, and will disappear when the current disappears. </sec><sec>In previous studies, magnetic skyrmions were realized in a variety of materials. However magnetic skyrmions were found only in very limited types of single crystal materials at room temperature or near room temperature. In recent years, scientists have discovered a variety of magnetic skyrmion materials at room temperature, including film materials (such as multilayer materials, artificial skyrmion materials) and crystal materialssuch as <i>β</i>-Mn-type Co<sub>10–<i>x</i>/2</sub>Zn<sub>10–<i>x</i>/2</sub>Mn<i><sub>x</sub></i>, Fe<sub>3</sub>Sn<sub>2</sub>. Among all kinds of room temperature magnetic skyrmion materials, the most valuable one is the multilayer film material. The Skyrmion multilayer film has the advantages of small size, adjustable material type, simple preparation, good temperature stability, good device integration,etc. At the same time, skyrmion multilayer film is very easy to optimize by adjusting and constructing a special structure that has the wanted types of materials each with a certain thickness. Artificial skyrmion material obtains artificial skyrmion by constructing a micro-nano structure, therefore the artificial skyrmion with high-temperature stability can be realized by choosing high Curie temperature materials. There are a variety of materials which can realize the skyrmion above room temperature, such as Co<sub>9</sub>Zn<sub>9</sub>Mn<sub>2</sub> (300–390 K) and Fe<sub>3</sub>Sn<sub>2</sub> (100–400 K). These room temperature materials further widen the temperature application range of skyrmion. The room temperature materials can be prepared or characterized by a variety of techniquesincluding sputtering for fabrication and X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) for characterization. </sec><sec>The discovery of the magnetic skyrmion materials at room temperature not only enriches the research content of materials science, but also makes the skyrmion widely applicable in novel electronic devices (such as racetrack memory, microwave detector, oscillators). Because the skyrmion has the advantages of small size, ultra-low driving current density, and topological stability, it is expected to produce racetrack memory based on the skyrmion with low energy consumption, non-volatile and high density. The MTJ microwave detector based on skyrmion can be achieved with no external magnetic field nor bias current but with low power input (< 1.0 μW); the sensitivity of the microwave detector can reach 2000 V·W<sup>–1</sup>. The frequency of the oscillator based on skyrmion can be tuned by magnetic field or current, and moreover, the oscillato is very easy to integrate with IC. In this paper, first, the basic characteristic of magnetic skyrmion is introduced; and then room temperature magnetic skyrmion is reviewed; finally the advances of the racetrack memory, microwave detectors and oscillators are introduced, highlighting the development trend of room temperature magnetic skyrmion. </sec>

An active magnetic regenerative refrigerator (AMRR) is expected to be useful for hydrogen liquefaction due to its inherent high thermodynamic efficiency. Because the temperature of the cold end of the refrigerator has to be approximately liquid temperature, a large temperature span of the active magnetic regenerator (AMR) is indispensable when the heat sink temperature is liquid nitrogen temperature or higher. Since magnetic refrigerants are only effective in the vicinity of their own transition temperatures, which limit the temperature span of the AMR, an innovative structure is needed to increase the temperature span. The AMR must be a layered structure and the thermophysical matching of magnetic field and flow convection effects is very important. In order to design an AMR for liquefaction of hydrogen, the implementation of multi-layered AMR with different magnetic refrigerants is explored with multi-staging. In this paper, the performance of the multi-layered AMR using four rare-earth compounds (GdNi2, Gd0.1Dy0.9Ni2, Dy0.85Er0.15Al2, Dy0.5Er0.5Al2) is investigated. The experimental apparatus includes two-stage active magnetic regenerator containing two different magnetic refrigerants each. A liquid nitrogen reservoir connected to the warm end of the AMR maintains the temperature of the warm end around 77 K. High-pressure helium gas is employed as a heat transfer fluid in the AMR and the maximum magnetic field of 4 T is supplied by the low temperature superconducting (LTS) magnet. The temperature span with the variation of parameters such as phase difference between magnetic field and mass flow rate of magnetic refrigerants in AMR is investigated. The maximum temperature span in the experiment is recorded as 50 K and several performance issues have been discussed in this paper.

The Physical Principles of Magnetism