Magnetic refrigeration material, method for manufacturing the same, AMR bed using the same, and magnetic refrigeration apparatus

Abstract

To provide a magnetic refrigerant material having hydrogen barrier resistance, a method for producing the same, an AMR bed including the same, and a magnetic refrigerator.SOLUTION: A magnetic refrigerant material is a Laves phase compound represented by general formula RT2 (where R is at least one rare earth element and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)), having a barrier layer containing copper (Cu) oxide on its surface.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a magnetic refrigeration material, a method for producing the same, an AMR bed using the same, and a magnetic refrigeration apparatus. [Background technology]

[0002] To realize a future hydrogen society characterized by energy conservation and low carbon emissions, the utilization of liquid hydrogen, which has 1 / 800th the volume of gaseous hydrogen and offers advantages such as mass transport, mass supply, mass storage, space saving, and ultra-high purity, is essential. However, current hydrogen liquefaction technology using compressors has problems such as low liquefaction efficiency during production and losses due to evaporation. Magnetic refrigeration, on the other hand, is a cooling technology that uses magnetic materials exhibiting the magnetocaloric effect as a refrigerant. It induces a ferromagnetic-paramagnetic phase transition through a cycle of increasing and decreasing magnetic fields, and uses the resulting endothermic and exothermic reactions to cool. Compared to refrigerators that use gas compression and expansion, magnetic refrigerators are being developed as refrigerators that can liquefy hydrogen at a lower cost and have higher energy efficiency.

[0003] ErCo2 has been reported as a magnetic refrigeration material (e.g., Non-Patent Document 1). ErCo2 exhibits the largest entropy change among Laves phase compounds and meets the desired Curie temperature T C This demonstrates that it is a promising candidate for a material with a huge magnetocaloric effect for hydrogen liquefaction.

[0004] However, it is known that when such magnetic refrigeration materials are used in a hydrogen atmosphere, hydrogen penetrates the inside of the magnetic refrigeration material, generating hydrides, causing it to pulverize, and significantly altering its magnetic properties. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] H. Wada et al., Cryogenics 39, 1999, 915-919 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Therefore, the object of the present invention is to provide a magnetic refrigeration material having hydrogen barrier properties, a method for producing the same, an AMR bed using the same, and a magnetic refrigeration apparatus. [Means for solving the problem]

[0007] The magnetic refrigeration material of the present invention is a Raves phase compound represented by the general formula RT2 (wherein R is at least one rare earth element, and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)), and has a barrier layer containing copper (Cu) oxide on its surface, thereby solving the above problems. The barrier layer may further contain an oxide of T. The barrier layer may further contain Cu metal. The barrier layer may contain a phase represented by the general formula RT3. The barrier layer may further contain an oxide of R. The barrier layer may have a thickness in the range of 1 μm to 20 μm. The aforementioned barrier layer has the composition formula Cu a R b T c O d (where a, b, c, and d represent the atomic percentage (at%) of each element, and a+b+c+d=100) and parameters a~d are, 45 ≤ a ≤ 90, 0 ≤ b ≤ 8, 0 ≤ c ≤ 8, and, 5 ≤ d ≤ 60 It may satisfy the requirement. The parameters a to d are, respectively, 50 ≤ a ≤ 55, 0.5 ≤ b ≤ 2, 1 ≤ c ≤ 5, and, 38 ≤ d ≤ 50 It may satisfy the requirement. An oxide film containing an oxide of R may be present between the Raves phase compound and the barrier layer. The oxide film may further contain a phase represented by the general formula RT3. The oxide film may be in the range of 1 μm to 30 μm. When using a spherical approximation, the diameter may satisfy the range of 10 μm to 3000 μm. The method for producing the magnetic refrigeration material according to the present invention comprises treating the surface of a Raves phase compound represented by the general formula RT2 (wherein R is at least one rare earth element, and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)), forming a copper (Cu) film on the surface-treated Raves phase compound, and heating and oxidizing the Raves phase compound on which the Cu film has been formed, thereby solving the above problem. The surface treatment may be performed by immersing the Raves phase compound in an acidic aqueous solution. The Cu coating may be formed using an electroless plating method. The Cu coating may have a thickness in the range of 2 μm to 25 μm. The heating and oxidation described above may be performed by heat-treating the Laves phase compound on which the Cu film is formed in an oxygen-containing atmosphere at a temperature range of 200°C to 600°C. Prior to treating the surface, the Raves phase compound may be further homogenized. The AMR bed according to the present invention is equipped with the above-mentioned magnetic refrigeration material, thereby solving the above-mentioned problems. The magnetic refrigeration apparatus according to the present invention comprises an AMR bed equipped with the above-mentioned magnetic refrigeration material, thereby solving the above-mentioned problems. [Effects of the Invention]

[0008] The magnetic refrigeration material according to the present invention is a Laves phase compound represented by the general formula RT2 (where R is at least one rare earth element and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)), and has a barrier layer containing copper (Cu) oxide on its surface. Since the Laves phase compound is the main component, it exhibits the largest entropy change and a desirable Curie temperature T C and has a huge magnetic calorific effect for hydrogen liquefaction. Furthermore, by having a barrier layer containing Cu oxide on the surface, the hydrogen barrier property can be improved. Such a magnetic refrigeration material can be applied to an AMR bed and further provide a magnetic refrigeration device.

[0009] The manufacturing method of the above-mentioned magnetic refrigeration material according to the present invention includes treating the surface of the Laves phase compound, forming a Cu film made of copper (Cu) on the surface-treated Laves phase compound, and heating and oxidizing the Laves phase compound on which the Cu film is formed. By treating the surface, impurities attached to the surface can be removed. After forming the Cu film and oxidizing it, the above-mentioned magnetic refrigeration material can be obtained, so no special technology is required, enabling cost reduction and being advantageous for mass production.

Brief Description of the Drawings

[0010] [Figure 1] Schematic diagram showing the magnetic refrigeration material according to the present invention [Figure 2] Schematic diagram showing the crystal structure of ErCo2 [Figure 3] Schematic diagram showing another magnetic refrigeration material according to the invention [Figure 4] Flow chart showing the process of manufacturing the magnetic refrigeration material according to the present invention [Figure 5] Schematic diagram showing the magnetic refrigeration device according to the present invention [Figure 6] Figure showing the SEM image of ErCo2 particles manufactured by the atomization method [Figure 7] Figure showing the cross-sectional SEM image of ErCo2 particles obtained under the surface treatment conditions of Example 1 [Figure 8] Figure showing an SEM image of a cross-section of a particle with only a Cu coating, as in Example 8. [Figure 9] Figure showing an SEM image of the appearance of the particle in Example 4. [Figure 10] Figure showing an SEM image of the cross-section of the particle in Example 4. [Figure 11] Figure showing an SEM image of the cross-section of the particle in Example 6. [Figure 12] Figure showing an SEM image of the cross-section of the particle in Example 7. [Figure 13] This figure shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 4. [Figure 14] This figure shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 5. [Figure 15] This figure shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 8. [Figure 16] Figures 13 to 15 summarize the results. [Figure 17] Figure showing the magnetic properties of the sample in Example 4 before and after the hydrogen exposure test. [Figure 18] Figure showing the XRD pattern of the sample in Example 4. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the drawings. Similar elements will be given the same numbers, and their descriptions will be omitted. (Embodiment 1) Embodiment 1 describes a magnetic refrigeration material and a method for producing the same according to the present invention.

[0012] Figure 1 is a schematic diagram showing a magnetic refrigeration material according to the present invention. Figure 2 is a schematic diagram showing the crystal structure of ErCo2.

[0013] The magnetic refrigeration material 100 of the present invention mainly comprises a Raves phase compound 110 represented by the general formula RT2 (wherein R is at least one rare earth element, and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)), and has a barrier layer 120 containing copper (Cu) oxide on its surface. Figure 1 shows a cross-section of the magnetic refrigeration material 100 in particle form for clarity.

[0014] Figure 2 shows the crystal structure of ErCo2, one of the Raves phase compounds 110, where R is erbium (Er) and T is cobalt (Co). The structure is the MgCu2 type of the Raves phase, with lattice parameters a=b=c=7.153 Å and α=β=γ=90°. In such a Raves phase, A and B metal elements with an atomic radius ratio of approximately 1.2:1 are bonded together in an AB2 composition ratio to form a compound. The crystal structure, consisting of large A atoms and small B atoms, can be thought of as a packed structure of large and small spheres, which occupy specific lattice positions, namely A sites and B sites. The A site has 4 A atoms and 12 B atoms as neighboring atoms, while the B site is surrounded by 6 A atoms and 6 B atoms. In a hypothetical Raves phase crystal, the AA atoms and BB atoms are in contact with each other, while the atomic packing is such that there is no contact between A and B atoms. In such cases, the relationship RA / RB = √3 / 2 = 1.225 holds for the ratio of the atomic radii of the two atoms. Generally, atoms placed at site A have an arrangement similar to that of a diamond structure, and atoms at site B form a tetrahedron around site A. Since Raves phase compounds are a type of close-packed structure, they have three types of crystal structures: cubic MgCu2 type (C15), hexagonal MgZn2 type (C14), and MgNi2 type (C36), due to differences in the stacking of atoms similar to the difference between face-centered cubic and hexagonal close-packed lattices.

[0015] In RAl2 (R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu), where T is aluminum (Al), ferromagnetic exchange interactions clearly occur between the magnetic moments of the rare earth metals in RAl2. However, as seen in HoAl2, the resulting magnetic moments were found to be lower than the theoretical moment value gI of the trivalent ion of Ho.

[0016] For example, in RT2 (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), magnetic data for RNi2 (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm), RCo2 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu), and RFe2 (R=Ce, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, Y) have already been investigated.

[0017] Thus, it is clear that ErCo2, ErNi2, ErAl2, and ErFe2 are not only MgCu2-type Raves phase intermetallic compounds, but also function as magnetic refrigeration materials. Furthermore, the oxide layers on their surfaces may possess hydrogen barrier properties. The following discussion will focus particularly on ErCo2, but it will be understood by those skilled in the art that similar properties apply to all of these Raves phase intermetallic compounds.

[0018] Thus, since the magnetic refrigeration material 100 of the present invention mainly consists of the Raves phase compound 110, it exhibits the maximum entropy change and a desirable Curie temperature T C This exhibits a large magnetocaloric effect for hydrogen liquefaction. Furthermore, by having a barrier layer 120 containing Cu oxide on the surface, hydride formation is prevented, powdering is suppressed, and hydrogen barrier resistance can be improved. Moreover, a barrier layer 120 containing Cu oxide does not degrade magnetic properties.

[0019] As shown in Figure 1, the magnetic refrigeration material 100 of the present invention preferably has a particle shape. This allows for greater heat exchange with gases and liquids when applied to a magnetic refrigeration device, as described later. Note that although Figure 1 shows the magnetic refrigeration material 100 as a perfect sphere, this is a schematic diagram for clarity, and in reality, it is a spherical shape with various aspect ratios.

[0020] The magnetic refrigeration material 100 of the present invention, when approximated as a sphere, satisfies a particle size (average particle size) in the range of 10 μm to 3000 μm. This makes it applicable to magnetic refrigeration devices. For example, when approximated as a sphere, the particle size may be 50 μm or more, 100 μm or more, or 200 μm or more, or it may be particulate with diameters of 2000 μm or less, 1000 μm or less, or 500 μm or less. In particular, the magnetic refrigeration material 100 of the present invention is preferred when the particle size is in the range of 200 μm to 400 μm, because this maximizes heat exchange.

[0021] The average particle size is determined by measuring the particle size of 100 randomly selected particles in an image observed using a scanning electron microscope (SEM) and employing image analysis software, and then using the average particle size. In this specification, Image J (ver. 1.51n; open-source, public domain image processing software) was used as the image analysis software.

[0022] Naturally, to obtain high hydrogen barrier resistance, it is desirable for the barrier layer 120 to cover the entire surface of the Raves phase compound 110, but it is sufficient for the barrier layer 120 to cover at least 80% of the surface area of ​​the Raves phase compound 110. This can improve hydrogen barrier resistance.

[0023] The barrier layer 120 contains a Cu oxide, which may be copper(II) oxide. The inventors of this application have found that CuO is effective in providing hydrogen barrier resistance. From the viewpoint of suppressing the formation of hydrides, the CuO content in the barrier layer 120 should be 10% by mass or more, preferably 40% by mass or more. The content can be calculated from the intensity ratio of the X-ray diffraction pattern.

[0024] The barrier layer 120 may consist of pure CuO, but may also further contain an oxide of T (where T is at least one element selected from the group consisting of Co, Ni, Al, and Fe) in addition to CuO. Since T is a constituent element of the Raves phase compound 110, the adhesion between the barrier layer 120 and the Raves phase compound 110 can be improved. For example, if T is Co, the oxide of T may be CoO. If T is Ni, Al, or Fe, the oxides of T may be NiO, Al2O3, FeO, Fe2O3, and Fe3O4, respectively. The content of the oxide of T in the barrier layer 120 is preferably 0% by mass or more and 40% by mass or less.

[0025] The barrier layer 120 may further contain unreacted Cu metal remaining during manufacturing. Cu metal readily forms copper oxide and can contribute to hydrogen barrier resistance. The Cu metal content may be between 0% by mass and 10% by mass.

[0026] The barrier layer 120 may further contain a phase represented by the general formula RT3. For example, when R is Er and T is Co, ErCo3 is known as ferrimagnetic, has a trigonal crystal structure, and belongs to the R3-m space group ("-" represents an overbar of 3). ErCo3 may be formed during manufacturing or in the operating environment, but it does not degrade the magnetic properties. Improved adhesion between the barrier layer 120 and the Raves phase compound 110 can be expected. The content of the phase represented by RT3 is preferably 0% by mass or more and 50% by mass or less.

[0027] The barrier layer 120 may further contain an oxide of R (where R is a rare earth element). For example, if R is Er, Er2O3 may be formed during manufacturing or under the operating environment, but this does not degrade the magnetic properties. Improved adhesion between the barrier layer 120 and the Raves phase compound 110 can be expected. The content of the R oxide is preferably 0% by mass or more and 10% by mass or less. The phase content of the barrier layer 120 can be calculated from the peak intensity of the X-ray diffraction pattern.

[0028] The barrier layer 120 has a composition formula Cu a R b T c O d represented by, where a, b, c, and d represent the atomic percentages (at%) of the respective elements, and satisfy a + b + c + d = 100. The parameters a to d are each preferably 45 ≤ a ≤ 90, 0 ≤ b ≤ 8, 0 ≤ c ≤ 8, and 5 ≤ d ≤ 60 are satisfied. Thereby, the hydrogen barrier property can be improved.

[0029] The parameters a to d 45 ≤ a ≤ 90, 0.5 ≤ b ≤ 8, 0.5 ≤ c ≤ 8, and 5 ≤ d ≤ 60 may be satisfied. Thereby, even if the barrier layer 120 is other than a CuO single phase, the hydrogen barrier property can be maintained.

[0030] The parameters a to d are each 50 ≤ a ≤ 55, 0.5 ≤ b ≤ 2, 1 ≤ c ≤ 5, and 38 ≤ d ≤ 50 may be satisfied. Thereby, severe manufacturing conditions are not required, and a magnetic refrigeration material having a barrier layer 120 with hydrogen resistance can be provided.

[0031] The barrier layer 120 preferably has a thickness in the range of 1 μm or more and 20 μm or less. Within this range, the water resistance can be improved. The barrier layer 120 more preferably has a thickness in the range of 3 μm or more and 12 μm or less.

[0032] The barrier layer 120 may preferably consist of an aggregate of fine particles made of the above-described phase. This results in a dense surface, which can improve hydrogen barrier resistance. Such fine particles preferably have a particle size in the range of 100 nm to 300 nm. This results in an even denser surface. Preferably, the fine particles have a particle size in the range of 150 nm to 250 nm.

[0033] Figure 3 is a schematic diagram showing another magnetic refrigeration material according to the invention.

[0034] As shown in Figure 3, the magnetic refrigeration material 100A of the present invention may further include an oxide film 130 between the Raves phase compound 110 and the barrier layer 120. The oxide film 130 can suppress the oxidation of the Raves phase compound 110 and improve the adhesion of the barrier layer 120.

[0035] The oxide film 130 is a native oxide film formed during manufacturing and contains the oxide of R. In addition to the oxide of R, the oxide film 130 may further contain a phase represented by the general formula RT3. The oxide of R and the phase represented by RT3 are as described above and therefore will not be explained further.

[0036] The oxide film 130 preferably has a thickness in the range of 1 μm to 30 μm. Within this range, the magnetic properties do not deteriorate. More preferably, the oxide film 130 has a thickness in the range of 3 μm to 12 μm.

[0037] The ratio of the volume of the barrier layer 120 and oxide film 130 to the total volume of the particles of the magnetic refrigeration material 100 or 100A should be set to 3 / 4 or less, by setting the thickness of the barrier layer 120 and oxide film 130 accordingly. This allows for the suppression of deterioration of magnetic properties while maintaining hydrogen barrier resistance.

[0038] Next, a method for producing magnetic refrigeration materials according to the present invention will be described. Figure 4 is a flowchart showing the process for manufacturing a magnetic refrigeration material according to the present invention.

[0039] The process for manufacturing the magnetic refrigeration material of the present invention comprises the following steps. Step S410: The surface of the Raves phase compound represented by the general formula RT2 (where R is at least one rare earth element and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)) is treated. Step S420: A copper (Cu) film is formed on the surface-treated Laves phase compound. Step S430: The Laves phase compound on which the Cu film has been formed is heated and oxidized.

[0040] The inventors of this invention focused on the Laves phase compound represented by RT2 as a magnetic refrigeration material, and selected Cu, which has excellent conductivity among the many existing metals, as the coating. They were able to coat the Laves phase compound with Cu with good adhesion. Furthermore, they discovered that by heating and oxidizing such a coating, it functions as a barrier film that prevents hydrogen intrusion, and succeeded in manufacturing a magnetic refrigeration material with improved hydrogen barrier resistance. Each process will be described in detail below.

[0041] The Raves phase compound represented by formula RT2 used in step S410 may be prepared by applying gas atomization, mechanical methods, etc., to an ingot obtained by a melting method, etc. Among these, gas atomization is preferred because it can obtain a particulate Raves phase compound having the above-mentioned particle size.

[0042] In step S410, the surface treatment is not particularly limited as long as it removes organic contaminants and native oxide films located on the surface of the Laves phase compound, but examples include etching, degreasing, desmatting, and electropolishing. Such surface treatment can improve the adhesion between the Laves phase compound and the Cu film in step S420 described later, allowing for the formation of a dense Cu film.

[0043] The surface treatment is preferably an etching treatment using an aqueous acid solution, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, or hydrofluoric acid. This allows for the easy removal of organic contaminants and native oxide films from the surface. Specifically, the Raves phase compound is immersed in an aqueous solution with a concentration in the range of 1% to 10% by volume and held for 10 seconds or more. There is no particular upper limit, but the longer the holding time, the more the Raves phase compound itself is removed, so the upper limit is 15 minutes, and more preferably 10 seconds to 30 seconds. After the surface treatment, it is advisable to wash with distilled water or the like.

[0044] In step S420, there are no particular restrictions on the method of forming the Cu film, but the Cu film is preferably formed by electroless plating. This makes it possible to form a uniform Cu film even if the Laves phase compound is in particulate form. Furthermore, even if there are defects such as gaps or irregularities on the surface due to the above surface treatment, electroless plating allows the Cu film to be formed in a way that fills the defects, thus forming a dense Cu film.

[0045] Preferably, the Cu coating should be formed to a thickness of 2 μm to 25 μm. Within this range, the barrier layer 120 (Figures 1 and 2) described above can be formed. Those skilled in the art will understand that a Cu coating of a desired thickness can be formed by controlling conditions such as the plating time of the electroless plating method.

[0046] In step S430, once the Cu film has become an oxide of Cu, there are no particular restrictions on the heating conditions. For example, the Raves phase compound on which the Cu film has been formed may be heat-treated in an oxygen-containing atmosphere at a temperature range of 200°C to 600°C. Within this temperature range, CuO will be formed from the Cu film. The oxygen-containing atmosphere may be air. The heat treatment time depends on the thickness of the Cu film, but for example, it may be in the range of 3 minutes to 60 minutes, more preferably 3 minutes to 35 minutes, and even more preferably 3 minutes to 15 minutes.

[0047] If the heating and oxidation time is prolonged, in addition to oxidation of the Cu film, an oxide film 130 (Figure 3) may be formed between the Cu film and the Laves phase compound.

[0048] Prior to step S410, it is preferable to homogenize the Laves phase compound. This can improve the magnetic properties of the Laves phase compound. For example, the Laves phase compound may be heat-treated in an inert atmosphere such as argon gas, or under vacuum, at a temperature range of 700°C to 900°C for a period of 1 to 10 days.

[0049] (Embodiment 2) Embodiment 2 describes a magnetic refrigeration apparatus using the magnetic refrigeration material described in Embodiment 1. Figure 5 is a schematic diagram showing a magnetic refrigeration apparatus according to the present invention.

[0050] The magnetic refrigeration apparatus 200 further comprises an AMR bed 220 filled with magnetic refrigeration material 210, a magnetic field applying means 230 for applying a magnetic field to the bed, a cooling stage 290 for cooling an object to be cooled by cold temperatures, and a heat exchanger 240 for dissipating the heat generated by the magnetic refrigeration work in the AMR bed 220. Here, the magnetic refrigeration material 210 can be the magnetic refrigeration material 100 or 100A described in Embodiment 1.

[0051] The magnetic field application means 230 can be any means used to apply a magnetic field to the AMR bed 220, and it is practical to use a magnetic field with an intensity of approximately 1 to 10 Tesla, for example. A superconducting magnet, a permanent magnet, etc., can be used as the magnetic field application means 230. Furthermore, the relative position between the magnetic field application means 230 and the AMR bed 220 can be changed by a drive mechanism (not shown), thereby changing the magnitude of the magnetic field applied to the AMR bed 220.

[0052] A pre-cooling stage 260 is provided on the high-temperature side of the AMR bed 220, and an 80K shield 270 is connected to the low-temperature side of the pre-cooling stage 260, while a 300K shield 280 is connected to the high-temperature side of the pre-cooling stage 260. Furthermore, a cooling stage 290 is provided on the low-temperature side of the AMR bed 220, and a liquefaction container 250 is provided, thermally connected to the cooling stage 290. In other words, the gas to be cooled is supplied to the liquefaction container 250 and liquefied. The AMR bed 220 is also provided with an inlet and outlet for the heat-transporting refrigerant, and is structured to allow the heat-transporting refrigerant to flow back and forth inside the AMR bed 220 through the gaps in the magnetic refrigeration material 210.

[0053] The liquefaction container 250 is supplied with a gas 310 to be liquefied (e.g., hydrogen, helium (He), etc.) from a tank not shown. The magnetic refrigeration device 200 may operate as follows: A magnetic field is applied to the AMR bed 220 filled with magnetic refrigeration material 210 by the magnetic field application means 230 to raise the temperature of the magnetic refrigeration material 210. Next, the heat transport refrigerant is flowed in a direction 300A from the low-temperature end to the high-temperature end of the AMR bed 220. The heat transport refrigerant exchanges heat with the magnetic refrigeration material 210 filled inside the AMR bed 220, receiving heat as it flows through the gaps in the magnetic refrigeration material 210 and flows out from the high-temperature end of the AMR bed 220. The heat transport refrigerant that has flowed out from the high-temperature end of the AMR bed 220 flows into a heat exchanger 240 that dissipates heat via a pre-cooling stage 260, and the excess heat is dissipated to the outside. Next, the magnetic field in which the magnetic refrigeration material 210 is filled is removed (reduced), and the temperature of the magnetic refrigeration material 210 is lowered.

[0054] Then, the heat-transporting refrigerant is flowed in a direction 300B from the high-temperature end to the low-temperature end of the AMR bed 220. The heat-transporting refrigerant flows into the high-temperature end of the AMR bed 220 via the pre-cooling stage 260, and while being cooled by heat exchange with the magnetic refrigeration material 210 filled inside, it flows through the gaps in the magnetic refrigeration material 210 and reaches the low-temperature end of the AMR bed 220. The flow of the heat-transporting refrigerant is driven by a refrigerant driving means (not shown). The refrigerant driving means is not particularly limited as long as it can drive an oscillating flow that reciprocates the heat-transporting refrigerant in synchronization with the AMR cycle, and examples include a piston, a blower and valve combination.

[0055] When the temperature at the low-temperature end of the AMR bed 220 drops below the boiling point of liquid hydrogen (20K at atmospheric pressure), the hydrogen gas supplied to the liquefaction container 250 is cooled and concentrated through heat exchange with the cooling stage 290 located on the low-temperature end side of the AMR bed 220. This process is repeated, causing the gas inside the liquefaction container 250 to be periodically liquefied or cooled.

[0056] The present invention will now be described in detail using specific examples, but please note that the present invention is not limited to these examples. [Examples]

[0057] [Synthesis of ErCo2 particles] As raw materials, bulk Er (3N, 99.9% purity) from Fujian Changting Golden Dragon Rare-Earth Co., Ltd. and bulk Co (3N, 99.9% purity) from Sumitomo Metal Mining Co., Ltd. were weighed in a molar ratio to produce ErCO2, and then dissolved in argon using a high-frequency induction furnace to synthesize ErCO2. The acquisition of ErCO2 was identified by powder X-ray diffraction.

[0058] Cast ingot rods were prepared from the synthesized ErCO2. Using a free-fall gas atomization device manufactured by Nisshin Giken Co., Ltd., the tip of these cast ingot rods was high-frequency melted to approximately 1400°C using an electrode. Argon pressurized to 1-5 MPa was then sprayed through a gas jet nozzle onto the resulting falling molten metal flow, stirring and atomizing the molten metal flow. This yielded ErCO2 particles.

[0059] The obtained ErCO2 particles were wrapped in tantalum foil and placed inside a stainless steel tube. Argon gas was then sealed inside this stainless steel tube at approximately 0.5 atmospheres. This sealed tube was placed in a muffle furnace and heated at a rate of approximately 5°C / min from room temperature until it reached 850°C, at which point it was maintained for one week. After that, the furnace power was turned off and it was cooled to room temperature (homogenization process).

[0060] The ErCo2 particles obtained in this manner were observed using a scanning electron microscope (SEM, JEOL Ltd., JSM-7000F). The results are shown in Figure 6.

[0061] Figure 6 shows an SEM image of ErCo2 particles produced by the atomization method.

[0062] According to Figure 6, the ErCo2 particles were spherical with a particle size of 330 μm. The average particle size was calculated to be 300 μm using Image J.

[0063] [Examples 1-3: Preliminary experiments on surface treatment] In Examples 1 to 3, preliminary surface treatment experiments were conducted using homogenized ErCo2 particles (raw material). A 4 vol% hydrochloric acid aqueous solution was used as the surface treatment, and etching (step S410 in Figure 4) was performed under the conditions shown in Table 1 to investigate its effect. Cross-sections of the particles obtained from the etching processes in Examples 1 to 3 were cut and observed using a scanning electron microscope (SEM). The composition of each part of the cross-section was also measured using an energy-dispersive X-ray spectrometer (EDX) attached to the SEM. The results are shown in Table 1 and Figure 7.

[0064] [Table 1]

[0065] Figure 7 shows a cross-sectional SEM image of ErCo2 particles obtained under the surface treatment conditions of Example 1.

[0066] In Figure 7, a brightly colored circular area in grayscale and a darkly colored area outside of it are visible. Outside these areas, a conductive two-part epoxy resin used to fix the particles is shown.

[0067] According to EDX, the composition of the brightly shown region is consistent with ErCo2, with an Er / Co atomic ratio of 1.9. On the other hand, the composition of the darkly shown region is Er 34.86 Co 22.69 O 42.45 The atomic ratio of Er / Co was 1.5. From this, it is thought that more Er than Co was eluted on the surface of the particles, and that the Er and Co in the surface layer were oxidized in the air during subsequent processing such as drying and cutting for SEM observation. The thickness of the darkened region varied depending on the location, but was in the range of 5 μm to 15 μm.

[0068] Furthermore, while the state of ErCo2 particles under the surface treatment conditions in Example 2 did not show significant changes compared to Example 1, the overall particle size of those in Example 3 was smaller. Specifically, the weight reduction rate of ErCo2 particles in Example 1 was 7.4%, while that of Example 3 was 23.5%. It was found that, from the perspective of dissolution loss, an etching time of 10 to 30 seconds is sufficient, as excessive etching occurs with longer etching times.

[0069] Based on these results, subsequent experiments used ErCo2 particles treated under the surface treatment conditions of Example 1.

[0070] [Examples 4-12: Magnetic refrigeration materials with a barrier layer] In Examples 4-7 and 9-12, surface-treated ErCo2 particles from Example 1 were coated with Cu films of various thicknesses, and then heated and oxidized under various conditions to produce magnetic refrigeration materials with various barrier layers. Example 8 is a comparative example of Examples 4-7, and is a magnetic refrigeration material in which a Cu film was formed but heating and oxidation were not performed.

[0071] The electroless Cu plating solution shown in Table 2 was prepared, and ErCo2 particles were immersed in it for 60 or 90 minutes to form a Cu coating (step S420 in Figure 4). Cross-sections of the Cu-coated ErCo2 particles were cut, observed with SEM, and compositional analysis was performed by EDX. The results are shown in Figure 8. Next, the Cu-coated ErCo2 particles were heated in air at a temperature range of 200°C to 500°C for 5 to 30 minutes, if necessary, to oxidize them (step S430 in Figure 4). For clarity, the experimental conditions are summarized in Table 3.

[0072] [Table 2]

[0073] [Table 3]

[0074] The samples obtained under the electroless Cu plating and oxidation conditions of Examples 4 to 12 are simply referred to as the samples of Examples 4 to 12. The appearance of the samples of Examples 4 to 12 was observed using SEM. The results are shown in Figure 9. Cross-sections of the samples of Examples 4 to 12 were cut, observed using SEM, and compositional analysis was performed by EDX. The results are shown in Figures 10 to 12 and Table 4.

[0075] Hydrogen exposure tests were conducted on the samples from Examples 4 to 12. Specifically, 0.5 g of each sample from Examples 4 to 12 was weighed, hydrogen gas was filled into a container at 12 atmospheres, and each container was sealed. The pressure change at room temperature was then investigated. The results are shown in Figures 13 to 16. Magnetic properties and powder X-ray diffraction were also performed on the samples from Examples 4 to 12 before and after the exposure tests. These results are shown in Figures 17 to 18.

[0076] The above results will be summarized and explained below. Figure 8 shows an SEM image of a cross-section of a particle with only a Cu coating, as in Example 8.

[0077] According to Figure 8, a Cu film with a thickness of 6 μm to 10 μm was observed on the outermost surface (Region 1 in Figure 8). Below that, an oxide film was observed (Region 2 in Figure 8). Region 3 in Figure 8 was ErCO2. Region 2 is thought to have been formed during surface treatment with hydrochloric acid, based on the results in Figure 7. The formation of the oxide film can be suppressed by preventing contact with oxygen. Note that the particles with only the Cu film in Example 8 are the same as those in Examples 4 to 7 before oxidation. Although not shown, the SEM images of the cross-sections of the ErCO2 particles after the Cu film in Examples 9 to 12 showed a similar pattern, with the thickness of the Cu film in the range of 4 μm to 7 μm.

[0078] Figure 9 shows an SEM image of the appearance of the particle in Example 4.

[0079] Figure 9 shows the appearance of ErCo2 particles at various magnifications. Comparing Figure 9(A) with Figure 6, the surface of the particle in Example 4 had a rough surface. Figure 9(B) shows that the surface was an aggregate of fine particles. The average particle size of the fine particles was measured using image J and found to be 120 nm. Although not shown, the surfaces of the particles in Examples 5 to 7 and Examples 9 to 12 were similarly aggregates of fine particles with particle sizes in the range of 100 nm to 300 nm.

[0080] Figure 10 shows an SEM image of a cross-section of the particle from Example 4. Figure 11 shows an SEM image of a cross-section of the particle from Example 6. Figure 12 shows an SEM image of a cross-section of the particle from Example 7.

[0081] According to Figure 10, a layer with a thickness of 5 μm to 10 μm, an oxygen concentration of 42.49 at%, an atomic ratio of O to Cu of 0.8:1, and mostly composed of CuO was observed on the outermost surface (Region 1 in Figure 10). Below this, an oxide film containing Er2O3 and ErCo3 was observed (Region 2 in Figure 10). Region 3 in Figure 10 was ErCo2.

[0082] According to Figure 11, the outermost surface layer had a thickness of 4 μm to 7 μm and an oxygen concentration of 9.66 at% (Region 1 in Figure 11). Beneath this, an oxide film containing Er2O3 and ErCo3 was observed (Region 2 in Figure 11). Region 3 in Figure 11 was ErCo2. The oxygen concentration in Region 1 (9.66 at%) increased compared to the oxygen concentration before heating shown in Region 1 of Figure 8 (2.82 at%), suggesting that CuO was generated.

[0083] Similarly, according to Figure 12, the outermost layer had a thickness of 4 μm to 7 μm, an oxygen concentration of 9.76 at%, and contained CuO (region 1 in Figure 12). Below that, an oxide film containing Er2O3 and ErCo3 was observed (region 2 in Figure 12). Region 3 in Figure 12 was ErCo2. These findings indicate that the heating temperature range should be between 200°C and 600°C.

[0084] Figure 13 shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 4. Figure 14 shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 5. Figure 15 shows the change in hydrogen pressure during a hydrogen exposure test using the sample from Example 8. Figure 16 summarizes the results from Figures 13 to 15.

[0085] As shown in Figure 13, when the sample of Example 4, which had a film containing CuO, was used, the hydrogen pressure after sealing (1.272 MPa) was maintained at 1.256 MPa even after one week. The amount of hydrogen reacted in the sample of Example 4 was only 8% of that of the ErCo2 particles used as raw materials, which had only undergone homogenization treatment before Cu coating.

[0086] As shown in Figure 14, when the sample of Example 5, which had a film containing CuO, was used, the hydrogen pressure after sealing (1.270 MPa) decreased to 1.26 MPa within a few hours, and although it decreased slightly over time, it remained at 1.256 MPa even after one week. The amount of hydrogen reacted in the sample of Example 5 was 11% of that of the ErCo2 particles that had only undergone homogenization treatment before Cu coating, which were used as the raw material. Although not shown, the samples of Examples 6-7 and Examples 9-12 showed similar changes in hydrogen pressure.

[0087] As shown in Figure 15, when the sample of Example 8, which had a Cu-containing membrane, was used, the hydrogen pressure after sealing (1.271 MPa) decreased to 1.11 MPa within a few hours and then remained almost constant. The amount of hydrogen reacted in the sample of Example 8 was 81% of that of the ErCo2 particles that had only undergone homogenization treatment before the Cu coating used as the raw material.

[0088] As shown in comparison with Figure 16, the samples of Examples 4 and 5, which were heated after the Cu coating and then fitted with a CuO-containing film, showed dramatically improved hydrogen barrier resistance compared to the sample of Example 8, which had only a Cu coating. From this, it was found that the CuO-containing film functions as a barrier layer for hydrogen barrier resistance.

[0089] Figure 17 shows the magnetic properties of the sample in Example 4 before and after the hydrogen exposure test.

[0090] Figures 17(A) and (B) show the magnetization curves of the sample in Example 4 before and after the hydrogen exposure test, and the temperature dependence of the entropy change ΔS due to the magnetic field, respectively. According to Figure 17, it was found that the magnetic properties of the sample in Example 4 did not change before and after the hydrogen exposure test. Although not shown, the samples in Examples 4 to 7 and Examples 9 to 12 also showed no change in magnetic properties before and after the hydrogen exposure test. Furthermore, these magnetic properties were similar to those of ErCo2 particles that had only undergone homogenization treatment before Cu coating. From this, it was found that the barrier layer containing CuO does not degrade the magnetic properties of ErCo2 particles.

[0091] Figure 18 shows the XRD pattern of the sample from Example 4.

[0092] Figure 18 shows the XRD patterns of homogenized ErCo2 particles (raw material), ErCo2 particles with surface treatment, the sample from Example 8 with a Cu coating, the sample from Example 4 after heating, and the sample from Example 4 after hydrogen exposure testing.

[0093] Focusing on the XRD pattern of the sample from Example 4 before the hydrogen exposure test in Figure 18, in addition to the ErCo2 phase, CuO phase, CoO phase, ErCo3 phase, and Er2O3 phase were observed. This indicates that heating the ErCo2 particles coated with a Cu film on their surface generated the CuO phase, CoO phase, ErCo3 phase, and Er2O3 phase. Based on the relationship of the diffraction peak intensities of the CuO phase, CoO phase, ErCo3 phase, and Er2O3 phase, the CuO phase had a high content, and the CuO phase content of the barrier layer was 43.3 mass%, satisfying the requirement of 40 mass or more. The content of the CoO phase, ErCo3 phase, and Er2O3 phase was 33.9, 27.1, and 3.6 mass%, respectively. From this, it was confirmed that the barrier layer with hydrogen barrier properties is mainly composed of the CuO phase and also contains the CoO phase, ErCo3 phase, and Er2O3 phase. Although not shown in the figures, the samples in Example 5, Example 11, and Example 12 were similarly confirmed to have CuO phase, CoO phase, ErCo3 phase, and Er2O3 phase on their surfaces.

[0094] On the other hand, it was found that the barrier layers of the samples in Examples 6, 7, 9, and 10 contained unreacted Cu phase in addition to the CuO phase, CoO phase, ErCo3 phase, and Er2O3 phase.

[0095] As shown in Table 4, the barrier layer has the composition formula Cu a Er b Co c O d It was found that the following conditions were met: 45≦a≦90, 0.5≦b≦8, 0.5≦c≦8, and 5≦d≦60. In particular, the barrier layers of Examples 4, 5, 11, and 12, which showed high hydrogen barrier resistance, were found to satisfy 50≦a≦55, 0.5≦b≦2, 1≦c≦5, and 38≦d≦50.

[0096] Furthermore, focusing on the XRD pattern of the sample in Example 4 after the hydrogen exposure test, a small amount of hydride (ErCO2H) was observed. 3.7 The formation of ) was confirmed. The amount of this formation was found to be dramatically less than the amount of hydride formed after hydrogen exposure testing of homogenized ErCO2 particles (raw material), and also less than the amount of hydride formed after hydrogen exposure testing of the sample in Example 8 with only Cu coating. Furthermore, the CuO content in the barrier layer after the hydrogen exposure test was 13% by mass, maintaining a level of 10% by mass or higher.

[0097] [Table 4] [Industrial applicability]

[0098] The magnetic refrigeration material of the present invention possesses resistance to hydrogenation, allowing it to withstand long-term use. Such a magnetic refrigeration material can be used in magnetic refrigeration devices and effectively function in processes such as hydrogen liquefaction. This can contribute to the widespread adoption of hydrogen, a promising energy carrier. [Explanation of Symbols]

[0099] 100, 100A, 210 Magnetic Refrigeration Materials 110 Laves phase compound 120 Barrier layer 130 Oxide film 200 Magnetic Refrigeration System 220 AMR bed 230 Magnetic field application means 240 Heat exchanger 250 liquefaction containers 260 Pre-cooling stage 270 80K Shield 280 300K Shield 290 Cooling Stages

Claims

1. General formula RT ２ (wherein R is at least one rare earth element, and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)) is a Raves phase compound, The surface has the composition formula Cu ａ R ｂ T ｃ O ｄ The barrier layer is represented as follows: (a, b, c, and d represent the atomic percentage (at%) of each element, and a + b + c + d = 100) Parameters a to d are, respectively, 45 ≤ a ≤ 90, 0 ≤ b ≤ 8, 0 ≤ c ≤ 8, and, 5 ≤ d ≤ 60 Magnetic refrigeration material that satisfies the requirements.

2. The magnetic refrigeration material according to claim 1, wherein the barrier layer is CuO and further contains an oxide of T.

3. The magnetic refrigeration material according to claim 1 or 2, wherein the barrier layer further contains Cu metal.

4. The aforementioned barrier layer is based on the general formula RT ３ The magnetic refrigeration material according to claim 3, comprising a phase represented by [the specified phase].

5. The magnetic refrigeration material according to claim 4, wherein the barrier layer further contains an oxide of R.

6. The magnetic refrigeration material according to any one of claims 1 to 5, wherein the barrier layer has a thickness in the range of 1 μm to 20 μm.

7. The parameters a to d are, respectively, 50 ≤ a ≤ 55, 0.5 ≤ b ≤ 2, 1 ≤ c ≤ 5, and, 38 ≤ d ≤ 50 A magnetic refrigeration material according to claim 1, satisfying the requirements.

8. The Laves phase compound and the barrier layer have a film containing an oxide consisting only of R and O (oxygen). A magnetic refrigeration material according to any one of claims 1 to 7.

9. The aforementioned film is of the general formula RT ３ The magnetic refrigeration material according to claim 8, further comprising a phase represented by .

10. The magnetic refrigeration material according to claim 8 or 9, wherein the film is in the range of 1 μm to 30 μm.

11. A magnetic refrigeration material according to any one of claims 1 to 10, wherein, when approximated as a sphere, the diameter is in the range of 10 μm to 3000 μm.

12. General formula RT ２ (wherein R is at least one rare earth element, and T is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), aluminum (Al), and iron (Fe)) The surface of the Raves phase compound is treated, Forming a copper (Cu) film on the surface-treated Laves phase compound, The Laves phase compound on which the Cu film is formed is heated and oxidized. A method for producing a magnetic refrigeration material according to any one of claims 1 to 11, comprising the above.

13. The method according to claim 12, wherein the surface treatment is performed by immersing the Raves phase compound in an acidic aqueous solution.

14. The method according to claim 13, wherein the Cu coating is formed by an electroless plating method.

15. The method according to any one of claims 13 to 14, wherein the Cu coating has a thickness in the range of 2 μm to 25 μm.

16. The method according to any one of claims 13 to 15, wherein the heating and oxidation is performed by heat-treating the Laves phase compound on which the Cu film is formed in an oxygen-containing atmosphere at a temperature range of 200°C to 600°C.

17. The method according to any one of claims 13 to 16, further comprising homogenizing the Raves phase compound prior to treating the surface.

18. An AMR bed comprising a magnetic refrigeration material according to any one of claims 1 to 11.

19. A magnetic refrigeration system equipped with an AMR bed, The AMR bed is the AMR bed described in claim 18, in a magnetic refrigeration apparatus.

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