Magnetic refrigeration material and its manufacturing method

Abstract

To provide means for preventing mechanical damage due to the manufacturing environment and usage environment of a magnetic refrigeration material.SOLUTION: Since it has been found that a magnetic refrigeration material may be mechanically damaged by hydrogenation, a magnetic refrigeration material with excellent hydrogen resistance is provided. In a Laves phase compound represented by RT2, R is at least one rare earth element, and T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, and the thickness of the surface layer containing oxide is 10 nm or more, and the ratio of the volume of the formed oxide layer to the total volume is 3 / 4 or less.SELECTED DRAWING: Figure 14

Description

[Technical Field]

[0001] The present invention relates to a magnetic refrigeration material and a method for producing the same. [Background technology]

[0002] To realize a future hydrogen-based society with energy conservation and a low-carbon society, it is essential to utilize liquid hydrogen, which has characteristics of mass transportation, mass supply, mass storage, space-saving, and ultra-high purity, with a volume 1 / 800 that of gaseous hydrogen. However, current hydrogen liquefaction technology using compressors has problems such as low liquefaction efficiency during production and evaporation loss. Magnetic refrigeration is a cooling technology that uses a magnetic material exhibiting the magnetocaloric effect as a refrigerant. A ferromagnetic-paramagnetic phase transition occurs through cycles of increasing and decreasing magnetic fields, resulting in endothermic and exothermic reactions. Magnetic refrigerators are being developed as refrigerators that are more energy-efficient and capable of liquefying hydrogen at low cost compared to refrigerators that use gas compression and expansion (see, for example, https: / / www.jst.go.jp / mirai / jp / program / large-scale-type / theme06.html).

[0003] Various magnetic refrigeration materials have been proposed (for example, Patent Documents 1 to 3). In addition, in the development of magnetic refrigeration materials, it is important to bring about a large magnetic entropy change when the magnetic field of the magnetic material is turned on and off at a target temperature (20 Kelvin for hydrogen liquefaction). Up until now, room-temperature magnetic refrigeration materials (for example, La(FeSi) 13 ) is a representative example of the development of materials. It is also known that materials such as Laves phases (e.g., ErCo2) have large magnetic entropy changes even near the hydrogen liquefaction temperature.

[0004] However, magnetic refrigeration materials can form hydrides depending on the environment in which they are used. Excessive hydriding usually embrittles the base metal, resulting in deterioration of mechanical properties.

[0005] On the other hand, materials that have a slower hydrogen diffusion rate than metals, such as metal oxides such as Al2O3 and Cr2O3, metal nitrides such as TiN and SiN, and metal carbides such as SiC, are known (for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-214733 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-262457 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-265631 [Non-patent literature]

[0007] [Non-Patent Document 1] Vincenc Nemanic, “Hydrogen permeation barriers: Basic requirements, materials selection, deposition methods, and quality evaluation”, Nuclear Materials and Energy 19(2019)451-457 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, various magnetic refrigeration materials have been proposed, but the behavior of these materials in their manufacturing and usage environments has not been thoroughly investigated. In particular, the chemical reactions of magnetic refrigeration materials with hydrogen, which are thought to come into frequent contact with hydrogen, deserve attention, but it is difficult to say that sufficient knowledge has been gained about this issue. [Means for solving the problem]

[0009] As a result of intensive research conducted by the inventors against this background, they discovered that when hydrogenation progresses in magnetic refrigeration materials made of metals, many of them may shatter due to the volume expansion after hydride formation. They also discovered that hydrogenation significantly changes the magnetic properties, resulting in a decline in the functionality of the magnetic refrigeration material.

[0010] Hydrogenation of metallic magnetic refrigeration materials leads to the formation of hydrides, which affect their mechanical and magnetic properties. To prevent this, a hydrogen barrier layer can be used. Possible hydrogen barrier materials include metals such as Mo and W, oxides such as Al2O3 and Cr2O3, nitrides such as TiN and SiN, and carbides such as SiC.

[0011] To prevent this, we discovered that forming a hydrogen barrier layer on the surface of the magnetic refrigeration material can effectively prevent hydrogenation. For example, by performing heat treatment in an oxygen atmosphere to form an oxide layer on the surface, we can realize a hydrogen barrier coating that has good adhesion to the underlying magnetic refrigeration material and is inexpensive and easy to process. This makes it easier to use Laves phase compounds, which have a large magnetocaloric effect but are easily hydrogenated, as magnetic refrigeration materials in a hydrogen atmosphere.

[0012] When used in magnetic refrigerators, magnetic refrigeration materials are often formed into particles (spheres) with a diameter of approximately 300 microns to maximize heat exchange with gases or liquids. Magnetic refrigeration materials with a large magnetocaloric effect, such as ErCo2, a Laves phase compound, are often subjected to a homogenization heat treatment for approximately one week at approximately 800 °C in an inert atmosphere or vacuum to improve their magnetic properties. If a sample after homogenization heat treatment is exposed to a hydrogen atmosphere, it may shatter rapidly due to hydrogenation, significantly altering its magnetic properties and potentially rendering it nonfunctional as a magnetic refrigeration material. Furthermore, uniformly depositing conventional hydrogen barrier materials onto 300-micron-diameter particles (spheres) with gaps and irregularities on the surface is not only technically challenging but also expensive, even if feasible.

[0013] Therefore, the present inventors have succeeded in forming a hydrogen barrier layer by treating the surface of a magnetic refrigeration material, rather than attaching a hydrogen barrier to the surface.

[0014] Such a treatment can be carried out by heating in an oxidizing atmosphere. The oxidizing atmosphere may be, for example, an atmosphere containing oxygen or an atmosphere containing a compound that releases oxygen. Heat treatment in such an oxidizing atmosphere is preferably carried out after a homogenizing heat treatment. If the homogenizing heat treatment is carried out after a heat treatment in an oxidizing atmosphere containing oxygen, oxygen may diffuse further inward, potentially impairing the hydrogen barrier effect. It is preferable that the formed oxide layer be at least 10 nm thicker than the native oxide film. If the oxide layer is too thin, a sufficient hydrogen barrier effect may not be obtained. On the other hand, if the oxide layer is too thick, the magnetic properties as a magnetic refrigeration material may be degraded. The volume ratio of the oxide layer to the entire material is preferably 3 / 4 or less.

[0015] Furthermore, after the oxide layer is formed, it is preferable to perform a heat treatment at a temperature below the homogenization heat treatment temperature in an inert atmosphere or under vacuum. This may further enhance the hydrogen barrier effect. The magnetic refrigeration material may be in the form of a bulk, thin film, granules, fine particles, ultrafine particles, etc. For example, in the case of a magnetic refrigeration material consisting of grains or spheres with a size of 30 nm to 3000 μm, heat treatment in an oxygen atmosphere to form an oxide layer can form a uniform oxide layer all the way to the surface of the cavities inside the grains or spheres connected by gaps. During the formation of grains or spheres, gaps or irregularities can form on the surface, or cavities can form inside. Therefore, it is not always easy to form a uniform oxide layer using dry or wet film formation methods such as sputtering or plating, and these methods are relatively expensive.

[0016] Specifically, this may include the following: It is a Raves phase compound represented by RT2, where R is at least one rare earth element, T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, the thickness of the surface layer containing an oxide is 10 nm or more, and the ratio of the volume of the formed oxide layer to the total volume is 3 / 4 or less. A magnetic refrigeration material. In any of the above magnetic refrigeration materials, R is a magnetic refrigeration material that is at least one element selected from the group consisting of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. In any of the above magnetic refrigeration materials, R is an erbium magnetic refrigeration material. In any of the above magnetic refrigeration materials, T is a cobalt magnetic refrigeration material. The oxide layer may be thicker than a so-called natural oxide film. The oxide layer is Er x Co y O z A magnetic refrigeration material containing a compound represented by (x + y + z = 1), where 0 ≦ x ≦ 0.6, 0 < y ≦ 0.6, 0 < z ≦ 0.6. Here, 0 ≦ x ≦ 0.7 may be satisfied, or 0 ≦ x ≦ 0.65 may be satisfied. 0 < y ≦ 0.7 may be satisfied, or 0 < y ≦ 0.65 may be satisfied. 0 < z ≦ 0.7 may be satisfied, or 0 < z ≦ 0.65 may be satisfied. In any of the above magnetic refrigeration materials, the shape of the magnetic refrigeration material is a shape that can be approximated by a sphere, an ellipsoid, or a flattened ellipsoid. In any of the above magnetic refrigeration materials, when the shape is approximated by a sphere, the diameter is 60 nm or more and 6000 μm or less. In any of the above magnetic refrigeration materials, the average particle size of each constituent particle, when approximated as a sphere as described above, is 60 nm or more and 6000 μm or less. The average particle size may be 100 nm or more, 200 nm or more, or 300 nm or more. The average particle size may be 6000 μm or less, 1000 μm or less, or 300 μm or less. An AMR bed comprising any of the magnetic refrigeration materials described above. A magnetic refrigerator equipped with any of the above-described AMR beds. Any of the above-described magnetic refrigeration devices further comprising a magnetic field application means for applying a magnetic field to the AMR bed, a refrigeration stage for cooling an object to be cooled by cold, and a heat exchanger for dissipating heat generated by the AMR bed. A method for improving the hydrogen resistance of a magnetic refrigeration material containing a Laves phase compound represented by RT2, wherein R is at least one rare earth element and T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, the method comprising the steps of: molding the magnetic refrigeration material; and forming a hydrogen barrier layer on a surface of the magnetic refrigeration material. A method for improving the hydrogen resistance of a magnetic refrigeration material containing a Laves phase compound represented by RT2, where R is at least one rare earth element and T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, comprising the steps of: molding the magnetic refrigeration material; homogenizing the molded magnetic refrigeration material; and forming a hydrogen barrier layer on the surface of the magnetic refrigeration material. The step of forming the hydrogen barrier layer may include heat-treating the magnetic refrigeration material in a predetermined atmosphere. The predetermined atmosphere may include an oxidizing atmosphere. The oxidizing atmosphere may include an atmosphere containing oxygen or a compound capable of releasing active oxygen. In the oxidizing atmosphere containing oxygen, the partial pressure of oxygen is preferably 100 Pa or more, more preferably 1000 Pa or more, and even more preferably 10,000 Pa or more. There is no particular upper limit to the partial pressure of oxygen, but a pressure that allows the formation of a desirable hydrogen barrier layer (which may include an oxide layer) is preferred, and may be approximately the partial pressure of oxygen at atmospheric pressure, for example. Any of the methods described above, further comprising the step of heat-treating the magnetic refrigeration material in an inert atmosphere after the step of forming the hydrogen barrier layer, wherein the step of forming the hydrogen barrier layer may include the step of forming an oxide layer. The step of forming the hydrogen barrier layer may include a step of maintaining the molded magnetic refrigeration material in an oxidizing atmosphere within a predetermined temperature range. The predetermined temperature range may be 250°C or higher and 850°C or lower. The predetermined temperature range may be 150°C or higher, 200°C or higher, or 250°C or higher. The predetermined temperature range may be 1000°C or lower, 900°C or lower, or 850°C or lower. A method for producing a magnetic refrigeration material, the method including a method for improving the hydrogen resistance of any of the magnetic refrigeration materials described above. [Effects of the Invention]

[0017] As described above, in the examples of the present invention, hydrogenation of the magnetic refrigeration material can be reduced, so that the shape of the molded body of the magnetic refrigeration material can be maintained and the function as a magnetic refrigeration material can be maintained. [Brief explanation of the drawings]

[0018] [Figure 1] 1 illustrates the crystal structure of ErCo2, an example of a Laves phase compound that may comprise a magnetic refrigeration material usable in embodiments of the present invention. [Figure 2] FIG. 1 is a diagram showing changes in hydrogen pressure when Sample 1 is sealed in hydrogen gas. [Figure 3] FIG. 2 shows the results of observation of grains of Sample 1 with an optical microscope before and after exposure to hydrogen. [Figure 4] FIG. 10 is a diagram showing the temperature change of magnetization of Sample 1 before and after exposure to hydrogen. [Figure 5] FIG. 1 is a diagram showing the change in magnetization of Sample 1 with respect to the magnetic field before and after exposure to hydrogen. [Figure 6] FIG. 10 is a diagram showing changes in hydrogen pressure when Sample 2 is sealed in hydrogen gas. [Figure 7] FIG. 10 is a diagram showing the results of observation of grains of Sample 2 with an optical microscope before and after exposure to hydrogen. [Figure 8] FIG. 10 is a diagram showing changes in hydrogen pressure when Sample 3 is sealed in hydrogen gas. [Figure 9] FIG. 10 is a diagram showing the results of observation of grains of Sample 3 with an optical microscope before and after exposure to hydrogen. [Figure 10] FIG. 1 shows EDS spectra of samples 1 to 3. [Figure 11] FIG. 2 is a diagram showing the change in magnetization of (1) to (4) with respect to a magnetic field in an example of the present invention. [Figure 12] FIG. 2 is a diagram showing the temperature change of magnetization in (1) to (4) in an example of the present invention. [Figure 13] FIG. 1 is a diagram showing the temperature change of normalized magnetization of (1) to (4) in an example of the present invention. [Figure 14]1 shows a diagram of an analysis of a cross section of a particle of the magnetic refrigeration material of Sample 1. [Figure 15] 1 shows a diagram of an analysis of a cross section of a particle of the magnetic refrigeration material of Sample 2. [Figure 16] 1 shows a diagram of an analysis of a cross section of a particle of the magnetic refrigeration material of Sample 3. [Figure 17] FIG. 1 is a diagram schematically illustrating a magnetic refrigeration device. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, in which like elements are designated by like reference numerals and their description will be omitted.

[0020] Figure 1 illustrates the crystal structure of ErCo2 in an embodiment of the present invention. The structure is a Laves phase MgCu2 type with lattice parameters a = b = c = 0.71536 nm and α = β = γ = 90°. In such a Laves phase, A and B metal elements with an atomic radius ratio of approximately 1.2:1 are bonded in a composition ratio of AB2 to form a compound. The crystal structure, consisting of large A atoms and small B atoms, can be thought of as a packing structure of large and small spheres, occupying specific lattice positions, the A site and the B site. The A site has four A atoms and 12 B atoms as neighboring atoms, while the B site is surrounded by six A atoms and six B atoms. In a realistic Laves phase crystal, the atomic packing is such that the AA atoms and BB atoms are in contact with each other, and the AB atoms are not in contact with each other. In such a case, the atomic radius ratio of the two atoms satisfies the relationship RA / RB = √3 / 2 = 1.225. Generally, atoms arranged in the A site are arranged in a similar manner to the diamond structure, and atoms in the B site form a tetrahedron around the A site. Because Laves phase compounds are a type of close-packed structure, there are three types of crystal structures due to differences in atomic stacking similar to the difference between a face-centered cubic lattice and a hexagonal close-packed lattice: cubic MgCu2 type (C15), hexagonal MgZn2 type (C14), and MgNi2 type (C36).

[0021] In RAl2 (R = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu), there is clearly a ferromagnetic exchange interaction between the magnetic moments of the rare-earth metals in RAl2, but as seen in HoAl2, the obtained magnetic moment is lower than the theoretical moment value gI of the trivalent Ho ion.

[0022] For example, in RT2 (R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), magnetic data of 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.

[0023] Thus, it is clear that ErCo2, ErNi2, ErAl2, and ErFe2 are not only MgCu2-type Laves phase intermetallic compounds, but also function as magnetic refrigeration materials. Furthermore, the oxide layers on their surfaces can function as hydrogen barrier layers. While the following experimental examples will focus on ErCo2, those skilled in the art will understand that the same applies to any of these Laves phase intermetallic compounds.

[0024] [Experimental Example] (Preparation of ErCo2) The raw materials, lump erbium (3N, 99.9% purity) from Fujian Changting Golden Dragon Rare-Earth Co., Ltd. and lump cobalt (3N, 99.9% purity) from Sumitomo Metal Mining Co., Ltd., were weighed to a molar ratio of ErCo2 and melted in an argon-filled high-frequency furnace to synthesize ErCo2. Cast ingot rods were fabricated from the synthesized ErCo2. Using a free-fall gas atomization device manufactured by Nisshin Giken Co., Ltd., the tip of this cast ingot rod was high-frequency melted to approximately 1400°C using the electrode. Argon pressurized to 1-5 MPa was sprayed through a gas jet nozzle onto the resulting falling molten metal stream, stirring and atomizing it. This resulted in the formation of grains approximately 300 microns in diameter. The resulting particles, approximately 300 microns in diameter, were wrapped in tantalum foil and placed in a stainless steel tube. Argon gas was then sealed in the tube at approximately 0.5 atmospheres. The sealed tube was placed in a muffle furnace and heated from room temperature at a rate of approximately 5°C / min. After reaching 850°C, the temperature was maintained for one week. The furnace was then turned off and cooled to room temperature. A portion of the sample was subjected to homogenization heat treatment at 850°C in an Ar atmosphere for one week, and designated Sample 1. The average diameter of the particles, approximated as a sphere, was approximately 300 μm. The average particle size can be determined using the method described below. The particle sizes ranged from approximately 212 μm to approximately 355 μm.

[0025] (oxidation heat treatment) The granular sample after homogenization was placed in an alumina crucible, the lid was closed with a small gap, and the sample was heated in a muffle furnace from room temperature to 500°C at a heating rate of approximately 25°C / min in an air atmosphere, and then held at 500°C for 30 minutes to perform an oxidation treatment. The furnace was then turned off, the sample was cooled to 350°C at a rate of approximately 5°C / min, the muffle furnace door was opened, the crucible lid was closed, and the sample was cooled to below 200°C at a rate of approximately 10°C / min. The covered crucible was then removed from the furnace and left in air at room temperature for 10 minutes before sample 2 was removed. The resulting granular samples adhered to each other at their contact points, but they disintegrated and returned to their granular form when lightly shaken.

[0026] (vacuum heat treatment) A portion of the granular sample (sample 2) after oxidation heat treatment was wrapped in tantalum foil and placed in a one-sided sealed quartz tube. A vacuum was drawn, and a 3 × 10 -4 The sample was heated from room temperature to 530°C in an electric furnace at a rate of 6.7°C / min under a vacuum of about Pa, and then maintained at this temperature for 5 hours (heat treatment at 500°C under vacuum for 5 hours). After that, the power was turned off and the sample was cooled to room temperature, the evacuation was stopped, air was introduced into the quartz tube, and the sample was removed. In this way, sample 3 was obtained.

[0027] (Hydrogen exposure test) Samples 1 to 3 were weighed in equal amounts and placed in separate containers of equal volume. They were filled with hydrogen gas at 12 atmospheres and sealed. The hydrogenation process was investigated by measuring the subsequent pressure change at room temperature. The results are shown in Figures 2, 6, and 8. In these figures, the upper plot represents hydrogen pressure, and the lower plot represents temperature. After sealing, observations were continued for up to 7 days. For Sample 1, the hydrogen pressure dropped significantly from 1.28 MPa to 1.08 MPa in approximately 1 hour and then remained almost constant (Figure 2). This indicates that hydrogenation was complete. In other words, Sample 1, which was only homogenized, did not exhibit the so-called hydrogen barrier effect. The temperature at this time was between 24°C and 23°C. For Sample 2, there was no sudden drop in hydrogen pressure within 1 hour of sealing. Instead, the hydrogen pressure gradually decreased over time, from an initial 1.28 MPa to 1.25 MPa (a 2.2% decrease in hydrogen pressure) after 7 days (Figure 6). As a result, hydrogenation of sample 2 progressed gradually in proportion to time, and hydrogenation of approximately 14% of the amount of reacted hydrogen of sample 1 was observed over seven days. The temperature at this time was 23.5°C to 22.5°C. In sample 3, no decrease in hydrogen pressure was observed for two days after injection, and then it gradually decreased, from 1.27 MPa to 1.25 MPa after seven days (a 2.0% decrease in hydrogen pressure) (Figure 8). Furthermore, the rate of decrease in hydrogen pressure became particularly slow from around the sixth day. Thus, although hydrogenation of sample 3 progressed gradually over time, the reaction tended to saturate, and hydrogenation of approximately 13% of the amount of reacted hydrogen of sample 1 was observed over seven days.

[0028] (Optical microscope observation) The ErCo2 grains of Samples 1 to 3 were observed under an optical microscope before and after hydrogen exposure (Figures 3, 7, and 9). As can be seen in Figure 3, (A) grains were clearly visible before the test, but (B) after the test, almost all of the grains were broken into pieces. This is thought to be due to the grains being broken into pieces due to volume expansion caused by hydrogenation after hydrogen exposure. Even without measuring the magnetic properties, it is clear that such broken grains are unsuitable for magnetic refrigeration materials. As can be seen from Figures 7 and 8, in Samples 2 and 3, (A) most of the grains remained intact before the test, but (B) only some of the grains were broken into pieces after the test. This indicates that volume expansion due to hydrogenation was suppressed after hydrogen exposure. This indicates that Samples 2 and 3 have a clear hydrogen barrier effect, and that the hydrogen barrier effect is greater in Sample 3 than in Sample 2.

[0029] (Evaluation of magnetic properties 1) The magnetic properties of Sample 1 described above were evaluated before and after hydrogen exposure. The results are shown in Figures 4 and 5. In these figures, Sample 1 before hydrogen exposure is labeled "Ar-850°C-1w" and Sample 1 after hydrogen exposure is labeled "H2-exposed." Figure 4 shows that Sample 1 before hydrogen exposure exhibits a significant change in magnetization at approximately 30 K, indicating excellent magnetic properties. However, after hydrogen exposure, such properties are no longer observed, indicating that Sample 1 is not suitable as a magnetic refrigeration material. Figure 5 also shows that Sample 1 before hydrogen exposure exhibits higher magnetization and superior magnetic properties across a wide range of magnetic fields, compared to Sample 1 after hydrogen exposure. Furthermore, the magnetic properties and their temperature dependence of Sample 1 change significantly before and after hydrogen exposure, indicating that hydrogenation makes it difficult for Sample 1 to function as a magnetic refrigeration material. Samples 2 and 3 did not show significant changes in their magnetic properties before and after hydrogen exposure, indicating that they function as magnetic refrigeration materials even after hydrogen exposure. The magnitude relationship of ΔS (magnetic entropy change) per unit volume, which indicates the performance of a magnetic refrigeration material, is estimated to be Sample 1 > Sample 2 > Sample 3, based on the temperature dependence of the magnetization characteristics. This is presumably a reflection of the decrease in ErCo2 volume due to oxide formation, etc. Magnetic refrigeration materials are typically used in temperatures below liquid nitrogen. In magnetic refrigeration systems that undergo maintenance at room temperature for approximately one day once a year, exposure to hydrogen at room temperature during this period is thought to be the most severe environment for hydrogen barrier coating materials. However, magnetic refrigeration materials such as those used in this example are estimated to be able to withstand several years of maintenance.

[0030] (Evaluation of magnetic properties 2) Table 1 summarizes the details of the samples whose magnetic properties were evaluated before and after hydrogen exposure. [Table 1] (1) was held at 850°C in an Ar atmosphere for one week (homogenization heat treatment), followed by 500°C in an air atmosphere for 30 minutes (oxidation heat treatment). (2) was held at 850°C in an Ar atmosphere for one week (homogenization heat treatment), followed by 500°C in an air atmosphere for 30 minutes (oxidation heat treatment), followed by hydrogen exposure. (3) was held at 850°C in an Ar atmosphere for one week (homogenization heat treatment), followed by 500°C in an air atmosphere for 30 minutes (oxidation heat treatment), followed by 530°C in a vacuum for five hours (vacuum heat treatment). (4) was held at 850°C in an Ar atmosphere for one week (homogenization heat treatment), followed by 500°C in an air atmosphere for 30 minutes (oxidation heat treatment), followed by 530°C in a vacuum for five hours (vacuum heat treatment), followed by hydrogen exposure.

[0031] Figures 11 to 13 show the results of evaluating the magnetic properties for the above-mentioned (1) to (4). Figure 11 plots magnetization versus magnetic field, and it can be seen that there is no significant difference among (1) to (4). Figure 12 plots magnetization versus temperature, and Figure 13 normalizes the magnetization value at 60 K. Figure 12 shows that (1) has the best magnetic properties. Furthermore, comparing (1) and (2) and (3) and (4) before and after hydrogen exposure, it appears that the magnetic properties deteriorated due to hydrogen exposure. Next, in Figure 13, which is normalized, a similar comparison shows almost no difference between before and after hydrogen exposure. Furthermore, the magnitude relationship of ΔS predicted from the jump in magnetization at the Curie temperature (up to 32 K) is thought to be as follows: ΔS(Ar-850℃-1w)>ΔS((1)&(2))>ΔS((3)&(4)) This suggests that the decrease in volume of ErCo2 due to oxidation and the formation of ErCo3 may have had an effect.

[0032] (Grain surface composition) Figure 10 shows the EDS spectra of samples 1 to 3. Sample 1 underwent homogenization heat treatment but not oxidation heat treatment, resulting in a small oxygen (O) signal (see Figure 10A). On the other hand, sample 2 underwent oxidation heat treatment, resulting in a significant increase in the oxygen signal, likely reflecting the oxidation of the magnetic refrigeration material. At the same time, the erbium (Er) signal significantly decreased. This suggests a change in the composition of the surface layer of the magnetic refrigeration material. Considering the blackening of the sample after oxidation, it is highly likely that Co oxides formed on the surface layer of the magnetic refrigeration material. Cobalt oxide likely functions as a hydrogen barrier (see Figure 10B). Furthermore, for sample 3, which underwent vacuum heat treatment, the oxygen (O) signal intensity decreased, but the erbium (Er) signal intensity did not recover (see Figure 10C). The grayish-white color of the sample surfaces suggests that sample 2 is likely Co3O4 or Co2O3, and sample 3 is likely CoO.

[0033] (Cross-sectional structure of grains) Figures 14 to 16 show diagrams analyzing the cross sections of each particle of the magnetic refrigeration material of Samples 1 to 3. In Figure 14, no oxidizing heat treatment was performed, so it is thought that there is a so-called natural oxide film on the surface. In fact, when the signal intensity due to Er, Co, and O elements is measured from the surface layer, as shown in the graph on the bottom right, a small peak for O is seen on the outermost surface, and the intensity of Co is relatively low, so it is presumed that erbium oxide has formed. Figure 15 shows a cross section of Sample 2, which has been subjected to oxidizing heat treatment. Elemental analysis revealed an oxide layer of less than a few microns on the outermost surface, mainly composed of Co oxide. Beneath this, there is a layer of Er x Co y O z Figure 16 shows a cross-sectional view of Sample 3, which was subjected to a vacuum heat treatment after the oxidizing heat treatment. Elemental analysis revealed an oxide layer of less than several microns on the outermost surface, mainly composed of Co oxide. Beneath this was a layer of Er x Co y O z A 20-30 micron thick semi-oxidized layer consisting mainly of SiO2 was observed.

[0034] (oxide layer) In the examples of the present invention, as described with reference to Figures 15 and 16, the surface and surface oxide layer can be easily observed by cross-sectional observation. For example, as described with reference to Figure 14, by scanning the cross-section with an electron gun or the like and continuously measuring the intensity of an element, the change in concentration of that element can be determined. In this case, if the scanning speed is constant, the time is proportional to the scanning distance, and the change in detected concentration over time can be determined as a change in distance. For example, elements such as Er, Co, and O can be investigated. For example, the detected concentration of oxygen (O) may be low and almost constant inside the grain, but may suddenly increase toward the surface, reach a certain constant value, and then suddenly decrease at the surface. The midpoint between the point where the detected concentration suddenly increases and reaches a certain constant value can be determined as the boundary of the oxide layer. The distance from the surface to this boundary can then be used to determine the thickness of the oxide layer. Generally, multiple observation fields are set up, and the oxide layer thickness is measured at at least 10 locations in each field. The arithmetic average can then be used to determine the thickness of the oxide layer on the surface of the sample grain.

[0035] Here, if the radius of the particle when approximated as a sphere is R and the thickness of the oxide layer is L, the volume of the particle is 4 / 3πR 3 and the volume of the oxide layer is 4 / 3π{R 3 -(RL) 3}, so if the volume ratio is 3 / 4 or less, L≦(1-0.25 (1 / 3) )R. To satisfy L≧10nm, R≧27nm may be required. The maximum value of L increases with increasing R, so the practical maximum value of L is (1-0.25 (1 / 3))R. In general, the amount of penetration is thought to be inversely proportional to the penetration distance, which corresponds to the thickness of the so-called barrier layer. Therefore, if the volume of the oxide layer is ¾ or less of the volume of the entire grain, its thickness may be set to 10 nm or more, 100 nm or more, or 1000 nm or more as R increases. On the other hand, considering the deterioration of magnetic properties, it is preferable that the thickness of the oxide layer be as thin as possible within the range in which the desired hydrogen barrier effect is obtained. However, if the oxide layer is too thin, the hydrogen barrier function will decrease, and if it is 10 nm or less, the effect will be low.

[0036] FIG. 17 is a schematic diagram showing the main components of a magnetic refrigeration device. Magnetic refrigeration materials, including those described in the above-described embodiments, can be used. One form of this magnetic refrigeration material is particles with a particle diameter ranging from 50 μm to 1000 μm. For example, the particles may be approximately spherical, with diameters ranging from 50 μm to 100 μm, or from 200 μm to 2000 μm, 1000 μm to 500 μm. These upper and lower limits may also be appropriately combined to create a specific range. The particle form can increase the filling rate of the AMR bed, and the particle size can change the heat exchange cross-sectional area and pressure loss with the heat transport refrigerant. Smaller particle sizes increase the heat exchange cross-sectional area, which is effective in improving refrigeration performance. However, smaller particle sizes also increase pressure loss, resulting in reduced refrigeration performance. The actual pressure loss depends not only on the particle size but also on the type of heat transport refrigerant and operating conditions. Here, the particle size is the volume-based median diameter (d50), and the volume-based average particle size can be measured, for example, using a microtrack or laser scattering method. More specifically, static image analysis and dynamic image analysis can be used. The former involves taking multiple particle images (e.g., SEM images) and using image analysis software to calculate the particle diameter converted into a circular shape from the area of ​​each particle. A magnetic refrigeration device 200 equipped with such a magnetic refrigeration material can be used to generate ultra-low temperatures, such as hydrogen liquefaction. The magnetic refrigeration device 200 further includes an AMR bed 220 filled with a magnetic refrigeration material 210, a magnetic field application means 230 that applies a magnetic field to the bed, a cooling stage 290 that cools the object to be cooled by cold, and a heat exchanger 240 that dissipates heat generated by the magnetic refrigeration work in the AMR bed 220.

[0037] The magnetic field application means 230 can be any means for applying a magnetic field to the AMR bed 220, and it is practical to use a magnetic field with a strength of about 1 to 10 T (tesla), for example. A superconducting magnet, permanent magnet, or the like can be used as the magnetic field application means 230. Furthermore, the relative positions of the magnetic field application means 230 and the AMR bed 220 can be changed by a drive mechanism (not shown), thereby changing the strength of the magnetic field applied to the AMR bed 220.

[0038] A pre-cooling stage 260 is provided on the high-temperature side of the AMR bed 220, with an 80K shield 270 connected to the low-temperature side of the pre-cooling stage 260 and a 300K shield 280 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, with a liquefaction vessel 250 thermally connected to the cooling stage 290. That is, gas to be cooled is supplied to the liquefaction vessel 250 and liquefied. The AMR bed 220 is also provided with inlet and outlet ports for the heat transport refrigerant, allowing the heat transport refrigerant to flow back and forth within the AMR bed 220 through the gaps in the magnetic refrigeration material 210.

[0039] The liquefaction vessel 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 the 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 caused to flow 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, while flowing through the gaps in the magnetic refrigeration material 210 and flowing out from the high-temperature end of the AMR bed 220. The heat transport refrigerant flowing out from the high-temperature end of the AMR bed 220 flows through the pre-cooling stage 260 into the heat exchanger 240, which releases the heat, and excess heat is released to the outside. Next, the magnetic field in which the magnetic refrigeration material 210 is filled is removed (reduced), causing the temperature of the magnetic refrigeration material 210 to drop.

[0040] The heat transport refrigerant is then caused to flow in direction 300B from the high-temperature end to the low-temperature end of the AMR bed 220. The heat transport refrigerant flows into the high-temperature end of the AMR bed 220 via the pre-cooling stage 260, where it is cooled by heat exchange with the magnetic refrigeration material 210 filled inside, 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 transport 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 causes the heat transport refrigerant to flow back and forth in synchronization with the AMR cycle, and examples include a system that combines a piston, a blower, and a valve.

[0041] When the temperature of the low-temperature end of the AMR bed 220 drops below the boiling point of liquid hydrogen (20 K at atmospheric pressure), the hydrogen gas supplied to the liquefaction vessel 250 is cooled and concentrated by heat exchange with the cooling stage 290 installed on the low-temperature end side of the AMR bed 220. This process is repeated, and the gas inside the liquefaction vessel 250 is periodically liquefied or cooled. [Industrial Applicability]

[0042] In the embodiments of the present invention, the magnetic refrigeration material is resistant to hydrogenation and can withstand long-term use. Such a magnetic refrigeration material is used in magnetic refrigeration devices and effectively functions for liquefying hydrogen, etc. This can contribute to the widespread use of hydrogen, which is a promising energy carrier. [Explanation of symbols]

[0043] 200 Magnetic Refrigeration Device 220 AMR bed 230 Magnetic field application means 240 Heat exchanger 250 Liquefaction vessel 260 Pre-cooling stage 270 80K Shield 280 300K Shield 290 Cooling Stage 300A Heat transport refrigerant flow direction 300B Heat transport refrigerant movement direction

Claims

1. RT ２ is a Laves phase compound represented by R is at least one rare earth element, T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, The thickness of the surface layer containing oxide is 10 nm or more, A magnetic refrigeration material characterized in that the ratio of the volume of the formed oxide layer to the total volume is 3 / 4 or less.

2. 2. The magnetic refrigeration material according to claim 1, wherein R is at least one element selected from the group consisting of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

3. 3. The magnetic refrigeration material according to claim 1, wherein the diameter of the particles is 30 nm or more and 3000 μm or less when approximated as a sphere.

4. An AMR bed comprising the magnetic refrigeration material according to any one of claims 1 to 3.

5. A magnetic refrigeration apparatus comprising an AMR bed, wherein the AMR bed is the AMR bed according to claim 4.

6. 6. The magnetic refrigeration apparatus according to claim 5, further comprising: a magnetic field applying means for applying a magnetic field to the AMR bed; a freezing stage for cooling an object to be cooled by cold; and a heat exchanger for discharging heat generated by the AMR bed.

7. RT ２ A method for improving the hydrogen resistance of a magnetic refrigeration material containing a Laves phase compound represented by the formula: R is at least one rare earth element, T is at least one element selected from the group consisting of Co, Ni, Al, and Fe, forming a magnetic refrigeration material; a step of homogenizing the formed magnetic refrigeration material; and a step of subjecting the magnetic refrigeration material after the homogenization treatment to an oxidation treatment.

8. 8. The method according to claim 7, further comprising the step of heat treating the magnetic refrigeration material in an inert atmosphere after the step of oxidizing the magnetic refrigeration material.

9. A method for producing a magnetic refrigeration material, comprising the method for improving the hydrogen resistance of the magnetic refrigeration material according to claim 7 or 8.

Images