Thermal storage material, refrigerator, superconducting coil assembly device, and method for manufacturing thermal storage material

Abstract

To provide a thermal storage material, a refrigerator, and a superconducting coil assembly device that have high specific heat properties and mechanical strength in the cryogenic range. [Solution] According to the embodiment, the composition formula Er a Fe b Si 100-a-b A thermal storage material is provided which contains an intermetallic compound represented by (15≦a≦25, 35≦b≦45), wherein the intermetallic compound has a crystalline phase having a ThCr2Si2 type crystal structure as its main phase, and the grain size of the main phase is 0.001 mm or more and 1 mm or less.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a thermal storage material, a refrigerator, and a superconducting coil assembly device. [Background technology]

[0002] Superconducting magnets used in magnetic resonance imaging systems (MRI) and heavy ion accelerators operate in environments below several tens of K. Typically, this environment is achieved by regenerative refrigerators, such as the Gifford-McMahon (GM) refrigerator. In recent years, next-generation devices, including quantum computers, require environments of several mK, and dilution refrigerators are used to achieve this. These dilution refrigerators utilize regenerative refrigerators as components, handling pre-cooling down to below 4K.

[0003] Refrigeration units utilize several types of thermal storage materials with high specific heat values ​​for different operating temperature ranges. In GM refrigerators, which are widely used today, Cu mesh is used in the first stage regenerator, spherical particles of Pb and Bi alloy are used on the high-temperature side of the second stage regenerator, and particles of rare-earth compounds such as Gd2O2S (GOS), HoCu2, and Er3Ni are used on the low-temperature side of the second stage regenerator as thermal storage materials. Among these thermal storage materials, GOS has high specific heat properties in the temperature range around 5K.

[0004] Incidentally, synthesizing oxide refrigerants such as GOS requires a multi-step process, including the synthesis of raw materials, granulation, sintering at high temperatures, and polishing to achieve a perfect sphere.

[0005] Furthermore, in refrigerators such as GM refrigerators, pulse tube refrigerators, and Stirling refrigerators, high-pressure working gas flows back and forth through the gaps in the thermal material packed inside the regenerator. In addition, in GM refrigerators and Stirling refrigerators, the regenerator filled with thermal material vibrates. Therefore, the thermal material is required to have mechanical strength.

[0006] For oxides that require a multi-stage manufacturing process such as synthesis of raw materials, granulation, sintering at high temperatures, and polishing to a perfect sphere, intermetallic compounds that can be manufactured by a simple process of melting and solidifying are preferable from the viewpoint of manufacturing cold storage materials.

[0007] DyCu2Ge2 compounds with a ThCr2Si2 crystal structure are known to exhibit good manufacturability and high specific heat peak values ​​around 6K. However, because DyCu2Ge2 compounds have low specific heat below 5K, refrigeration at temperatures below 5K leads to a decrease in the refrigeration capacity of the refrigerator.

[0008] Furthermore, ErFe2Si2 compounds having a ThCr2Si2-type crystal structure are known to have a high specific heat peak value near 3K. However, because the specific heat peak of ErFe2Si2 compounds has a sharp shape, the integral value of the specific heat peak in the range of 2K to 5K is small, which leads to a decrease in the cooling capacity of refrigerators. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] International Publication No. 2020 / 067356 [Non-patent literature]

[0010] [Non-Patent Document 1] AM Umarji, et. al., J. Magn. Magn. Mat., 36 (1983) 61-65. [Non-Patent Document 2] DS Wang, et. al., Chem. Mater., 36 (2024) 1707-1718. [Overview of the project] [Problems that the invention aims to solve]

[0011] The problem that this invention aims to solve is to provide a thermal storage material, a refrigerator, a refrigerator, and a superconducting coil assembly device that have high specific heat characteristics and mechanical strength in the cryogenic region. [Means for solving the problem]

[0012] According to the embodiment, the composition formula Er a Fe b Si 100-a-b A thermal storage material is provided which contains an intermetallic compound represented by (15≦a≦25, 35≦b≦45), wherein the intermetallic compound has a crystalline phase having a ThCr2Si2 type crystal structure as its main phase, and the grain size of the main phase is 0.001 mm or more and 1 mm or less. [Brief explanation of the drawing]

[0013] [Figure 1] A schematic diagram showing a portion of the crystalline phase of the thermal storage material according to the embodiment. [Figure 2] A schematic diagram showing the shape of the cold storage material 1 according to the embodiment. [Figure 3] A cross-sectional view of a two-stage expansion type GM refrigerator, which is an example of a refrigerator according to this embodiment. [Figure 4] A cross-sectional view of an MRI apparatus, which is an example of a superconducting coil integration device according to this embodiment. [Figure 5] Graphs showing the specific heat characteristics of Example 1 and Comparative Examples 11 to 13 from 2K to 6K. [Figure 6] SEM image of the sample from Example 1 observed at 5000x magnification. [Modes for carrying out the invention]

[0014] The embodiments will be described below with reference to the drawings. In the following description, components that perform the same or similar functions will be given the same reference numerals throughout all drawings, and redundant descriptions will be omitted. Furthermore, each figure is a schematic diagram intended to explain the embodiments and facilitate their understanding, and their shape, dimensions, ratios, etc., may differ from those of the actual device. These can be appropriately modified in consideration of the following description and known technology.

[0015] In this specification, the cryogenic region refers to the temperature range of 2K to 5K. Hereinafter, the temperature range of 2K to 5K may be referred to as the cryogenic region.

[0016] (First Embodiment) In the first embodiment, a cold storage material will be described. The cold storage material according to the embodiment has a composition formula Er a Fe b Si 100-a-b (15 ≤ a ≤ 25, 35 ≤ b ≤ 45) and contains an intermetallic compound represented by, and the intermetallic compound has a crystal phase with a ThCr2Si2-type crystal structure as the main phase, and the crystal grain size of the main phase is 0.001 mm or more and 1 mm or less.

[0017] FIG. 1 is a schematic diagram showing a part of the crystal phase of the cold storage material according to the embodiment. In FIG. 1, there are six main phases 11. The grain boundary phase 12 exists between each main phase 11. Also, the main phase 11 extends in the long axis direction. The crystal phase 10 of the cold storage material 1 according to the embodiment includes the main phase 11 and the grain boundary phase 12. The crystal phase 10 of the cold storage material 1 is, for example, columnar crystals. The crystal phase 10 extends over approximately several hundred μm. The main phase 11 has a larger volume ratio than the grain boundary phase 12 in the cold storage material 1, and the grain boundary phase 12 exists around the main phase 11.

[0018] The main phase 11 is composed of a compound represented by the composition formula Er a Fe b Si 100-a-b . By setting the stoichiometric ratios of Er, Fe, and Si in this composition to 15 ≤ a ≤ 25, 35 ≤ b ≤ 45, and 30 ≤ 100 - a - b ≤ 50, respectively, it is possible to increase the specific heat particularly at 2 to 4K. The most desirable stoichiometric ratio of Er, Fe, and Si is 1:2:2. Therefore, the value of 100 - a - b is preferably 35 or more and 45 or less.

[0019] The main phase 11 is a phase having a ThCr2Si2-type structure and occupies 50% by volume or more in the crystal phase 10 of the cold storage material 1. The main phase 11 is preferably 80% by volume or more. When the main phase 11 is 80% by volume or more, a cold storage material having higher specific heat characteristics in a low temperature region of several tens of K or less can be obtained.

[0020] The grain boundary phase 12 has the same compositional formula as the main phase 11, Er a Fe b Si 100-a-b It is composed of compounds other than those represented by [formula]. The grain boundary phase 12 is, for example, composed of Fe c Si 100-c (45≦c≦55) or / and Er d Fe e Si f O 100-d-e-f The material contains compounds represented by (10≦d≦20, 20≦e≦30, 40≦f≦50). The presence of the above-mentioned compounds, which are not compounds of the main phase 11, in the crystalline phase 10 improves the mechanical strength of the refrigerant particles. The grain boundary phase 12 is preferably 1 volume% or more and 20 volume% or less.

[0021] The cold storage material 1 has the composition formula Er a Fe b Si 100-a-b Whether or not an intermetallic compound represented by (15≦a≦25, 35≦b≦45) is present can be confirmed by analysis using scanning electron microscope energy dispersive X-ray spectrometry (EDX, SEM-EDX), etc.

[0022] Whether or not crystalline phase 10 contains a crystalline phase having a ThCr2Si2 type structure can be measured using powder X-ray diffraction (XRD) or a transmission electron microscope (TEM). The volume percentage of the main phase can be calculated by fitting the XRD pattern of the crystalline phase to a crystal structure model using Rietveld analysis, or from observations using a scanning electron microscope (SEM).

[0023] The grain size of the main phase 11 is between 0.001 mm and 1 mm. Here, the grain size will be explained using Figure 1. First, the surface of the regenerative material 1 is exposed by beam processing or polishing so that the crystalline phase 10 of the regenerative material 1 can be observed. Next, the crystalline phase 10 is observed using an SEM. At this time, the main phase 11 is observed so that there are 3 or more main phases 11 in the image. At this magnification, the main phase 11 and grain boundary phase 12 in the image can be distinguished. The image to be observed is, for example, Figure 1. Next, for all main phases 11 in the image that are sandwiched between or surrounded by grain boundary phases 12, one inscribed circle 13 is drawn for each main phase 11 that is in contact with two grain boundary phases 12 and is the largest in the image, and the diameter of each inscribed circle 13 is measured. Multiple inscribed circles 13 may be drawn within the same main phase 11 if the images show different regions. This makes it possible to obtain a more accurate value of the grain size. In Figure 1, four inscribed circles 13 are drawn for each of the four main phases 11. Then, the SEM image is observed again in the unobserved region so that at least three main phases 11 are included, and inscribed circles 13 are drawn in the same way as before, and the diameter of the inscribed circles 13 is measured. This SEM image observation and measurement of the diameter of the inscribed circles 13 is repeated until 100 inscribed circles 13 are drawn, and the average value of the diameters of all 100 inscribed circles 13 is taken as the grain size.

[0024] By having a grain size in the range of 0.001 mm to 1 mm, good specific heat properties and mechanical strength are achieved in the particles of the thermal storage material. Increasing the grain size increases the integral value of the specific heat peak in the cryogenic region, resulting in a thermal storage material with superior specific heat properties. From the viewpoint of mechanical strength, a grain size of 0.1 mm or less is desirable.

[0025] In the main phase 11 and grain boundary phase 12, some of Er, Fe, and Si can be replaced with other elements. For example, some of Er may be replaced with R (where R is one or more elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Sc, and Y). Some of Fe may also be replaced with T (where T is one or more elements selected from Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Zr, Mo, Ru, Rh, Pd, and Ag). Furthermore, some of Si may be replaced with X (where X is one or more elements selected from B, Al, P, Ga, Ge, As, Sn, Sb, and Te). Elemental substitution may be performed on all of Er, Fe, and Si, or on only one or two elements. In the main phase 11, the substitution ratio of R to Er is preferably 50% or less. In the main phase 11, the substitution ratio of T to Fe is preferably 50% or less. In the main phase 11, the substitution ratio of X to Si is preferably 50% or less.

[0026] Figure 2 is a schematic diagram showing the shape of the thermal storage material 1 according to the embodiment. Here, the thermal storage material 1 is described as being granular. Figure 2 shows a projection image of the granular thermal storage material 1.15 In the particle size φ of the thermal storage material 1, φmax is the length in the longest direction of the granules, and φmin is the length of the longest part perpendicular to the longest direction. It is desirable that both φmax and φmin are within the range of 0.01 mm to 0.1 mm. By having the particle size φ within this range, in the refrigerator described later, the flow of the working gas (He gas, etc.) that reciprocates in the regenerator filled with the thermal storage material is not obstructed, and the pressure loss of the gas is reduced. In addition, the filling rate of the thermal storage material in the regenerator is increased, and good heat exchange between the working gas and the thermal storage material is achieved.

[0027] (Second embodiment) A refrigerator will be described in the second embodiment. The refrigerator according to this embodiment is equipped with the cold storage material according to the first embodiment.

[0028] Figure 3 is a cross-sectional view of a two-stage expansion type GM refrigerator exemplified as a refrigerator 30 according to the embodiment. This refrigerator 30 has a large-diameter first cylinder 31 and a small-diameter second cylinder 32 connected coaxially with the first cylinder 31. A first regenerator 34 is reciprocally mounted in the first cylinder 31, and a second regenerator 35 is reciprocally mounted in the second cylinder 32. Seal rings 36 and 37 are positioned between the first cylinder 31 and the first regenerator 34, and between the second cylinder 32 and the second regenerator 35, respectively.

[0029] A first expansion chamber 41 is provided between the connecting portion of the first regenerator 34 and the second regenerator 35 and the inner wall of the first cylinder 31. A second expansion chamber 42 is provided between the second regenerator 35 and the tip wall of the second cylinder 32. A first cooling stage 43 is formed at the bottom of the first expansion chamber 41, and a second cooling stage 44, which is colder than the first cooling stage 43, is formed at the bottom of the second expansion chamber 42.

[0030] The first regenerator 34 contains a first regenerator material 38, such as a copper alloy mesh, with a passage 33 for the working gas (He gas, etc.). The first regenerator material 38 may be stainless steel mesh, or both. The second regenerator 35 is filled with a second regenerator material 40, with a passage 39 for the working gas. While the diagram shows regenerators 34 and 35 filled separately with the first regenerator material 38 and the second regenerator material 40, they may also be filled into a single regenerator.

[0031] The second cold storage material 40 housed inside the second cold storage container 35 is filled with multiple types of second cold storage materials 40a and 40b separated by a mesh 48. The filling rate of the second cold storage materials 40a and 40b within the space separated by the mesh 48 is preferably 50 to 75%, and more preferably 55 to 65%, taking into consideration the fluidity of the working gas.

[0032] In a two-stage chiller 30, the working gas (He gas, etc.) is compressed by a compressor 45 and supplied to the chiller 30 through a high-pressure line 46. The supplied working gas passes through the gaps in the first thermal storage material 38 housed in the first regenerator 34 and reaches the first expansion chamber 41, where it expands and cools the first cooling stage 43. Next, the working gas passes through the gaps in the second thermal storage material 40 housed in the second regenerator 35 and reaches the second expansion chamber 42, where it expands and cools the second cooling stage 44.

[0033] The working gas, now at low pressure, passes through the second regenerator 35 and the first regenerator 34 in that order (in the opposite direction to when it is at high pressure), and is returned to the compressor 45 via the low-pressure line 47. After that, it is compressed by the compressor 45 and the above cycle is repeated. The expansion of each expansion chamber 41 and 42 is achieved by the reciprocating movement of the regenerators 34 and 35. During this process, each thermal storage material 38 and 40 exchanges thermal energy with the working gas to store and retain cold energy and also regenerate heat.

[0034] Next, the cycle described above will be explained focusing on the flow of heat. The high-pressure working gas supplied from the compressor 45 to the refrigerator 30 is at room temperature (~300K) and is pre-cooled by the first regenerator 38 as it passes through the first regenerator 34 before reaching the first expansion chamber 41. Then, as it expands in the first expansion chamber 41, the temperature of the working gas decreases further, cooling the first cooling stage 43. Subsequently, as the working gas passes through the second regenerator 35, it is pre-cooled by the second regenerator 40 before reaching the second expansion chamber 42. Then, as it expands in the second expansion chamber 42, the temperature of the working gas decreases further, cooling the second cooling stage 44.

[0035] The working gas, now at low pressure, passes through the second regenerator 35 while storing cold energy in the second regenerator 40 (while the working gas itself is being heated). Subsequently, the working gas passes through the first regenerator 34 while storing cold energy in the first regenerator 38 (while the working gas itself is being heated), warming up to near room temperature, and returns to the compressor 45 via the low-pressure line 47.

[0036] During steady-state operation of the refrigeration cycle, a temperature gradient is generated in the cold storage materials 38 and 40 inside the cold storage units 34 and 35. In such a refrigeration cycle, the greater the specific heat of the cold storage material at the operating temperature, the higher the thermal efficiency of the working gas cycle, allowing for even lower temperatures and higher refrigeration performance.

[0037] Incidentally, the specific heat of solids generally changes depending on the temperature. Therefore, in order to enhance the heat recovery effect of the second cold storage material 40, it is effective to selectively arrange the second cold storage material 40, which has good heat recovery characteristics in each temperature range according to the temperature gradient. For this reason, the second cold storage container 35 is filled with multiple second cold storage materials 40 (40a, 40b) with different heat recovery characteristics.

[0038] To obtain a good heat recovery effect, it is important that the thermal storage material has a large heat capacity (specific heat) at the operating temperature of each part in the cycle process, and that the heat exchange between the thermal storage materials 40 and 38 and the working gas is good. In the first thermal storage unit 34, the main operating temperature range is from room temperature to below 100K, so Cu, which has a high specific heat per unit volume in this temperature range, is selected, and since the wire-drawn mesh is easily industrially usable, Cu mesh is widely used as the first thermal storage material 38.

[0039] When the temperature drops below 60K, Pb or Bi, which have a higher specific heat than Cu, are selected as the second cold storage material 40a on the high-temperature side of the second cold storage unit 35. Furthermore, when the temperature drops below 8K, a cold storage material having a ThCr2Si2 type structure according to the first embodiment, which has a higher specific heat than Pb or Bi, is selected as the second cold storage material 40b on the low-temperature side of the second cold storage unit 35. The cold storage materials according to the embodiment exhibit high specific heat characteristics in the cryogenic region, thus contributing to the stable operation of the refrigerator. Thus, for the cold storage materials 38 and 40 of the GM refrigerator, it is preferable to select and arrange materials that have a large volumetric specific heat in the operating temperature range of each part, taking into account the temperature gradient inside the cold storage units 34 and 35. Note that the second cold storage material 40a arranged on the high-temperature side of the second cold storage unit 35 is not limited to Pb or Bi, but may also be HoCu2 or Er3Ni, and the second cold storage material 40 is not limited to the two layers described above, but may be formed in three or more layers.

[0040] Furthermore, the refrigerator equipped with the thermal storage material according to the first embodiment is not limited to the GM refrigerator described above. In refrigerators that generate cryogenic temperatures from room temperature, such as pulse tube refrigerators, Claude refrigerators, and Stirling refrigerators, the thermal storage material according to the embodiment is installed in locations where a large thermal impedance is required, such as the boundary region between the cold / warm section and the hot section generated by the compression / expansion cycle of the working gas. This makes it possible to provide a refrigerator with higher specific heat characteristics in the cryogenic region.

[0041] (Third embodiment) A superconducting coil assembly device will be described in the third embodiment. The superconducting coil assembly device according to this embodiment includes the refrigerator according to the second embodiment.

[0042] Figure 4 is a cross-sectional view of a magnetic resonance imaging (MRI) apparatus 50 showing an example of a superconducting coil integration apparatus according to a third embodiment. Diagnosis using this MRI apparatus 50 involves moving a movable platform (not shown) on which the patient 52 lies down into a tunnel-shaped bore space 51. Then, a static magnetic field is applied by a first electromagnet 53 and a gradient magnetic field is applied by a second electromagnet 54.

[0043] Furthermore, the RF coil 55 transmits radio waves, and a magnetic resonance response signal is received from the subject 52. Due to the presence of a gradient magnetic field, information about the location where the response signal is generated is also received simultaneously. The received response signal is analyzed by a signal processing system (not shown) to reconstruct an image of the inside of the subject 52's body.

[0044] Currently, mainstream MRI devices 50 use superconducting coils that generate high magnetic fields of 1.5T or 3T as the first electromagnet 53. The higher the magnetic field, the better the signal-to-noise ratio (S / N) of the magnetic resonance response signal, allowing for the acquisition of clearer images. The superconducting coils used in the first electromagnet 53 are typically solenoid coils wound with metallic low-temperature superconducting wires such as NbTi or Nb3Sn.

[0045] Since these wires need to be kept below the critical temperature for superconductivity transition, the first electromagnet 53 is placed in a He bath 56 filled with liquid He, which liquefies at temperatures below 4.2K under 1 atmosphere. Because liquid He is scarce and expensive, an insulating vacuum layer 57 is provided on the outside of the He bath 56 to suppress the evaporation of liquid He. Furthermore, to reduce the effects of heat intrusion from the environment in which the MRI device 50 is installed (room temperature: approximately 300K), two radiation shields 58 and 59 are provided inside the insulating vacuum layer 57. The installed refrigerator 30 cools shield 58 to about 4K and shield 59 to about 40K.

[0046] The refrigerator 30 is not particularly limited, and a combination of a GM refrigerator and a JT refrigerator may be used, or a refrigerator such as a GM refrigerator, pulse tube refrigerator, Claude refrigerator, or Stirling refrigerator may be used individually. In particular, the refrigeration performance of GM refrigerators improved dramatically in the 1990s by incorporating magnetic thermal storage materials, making it possible to generate extremely low temperatures below liquid He temperature with a GM refrigerator alone. For this reason, GM refrigerators are widely used in MRI devices 50 that were in use at the time of filing this application.

[0047] As shown in Figure 4, the first cooling stage 43 (Figure 3) of the GM refrigerator 30 is connected to the shield 59, and the second cooling stage 44 (Figure 3) is connected to the shield 58. At the time of filing, GM refrigerators that could stably obtain a cooling capacity of 1W or more at 4K were in widespread use. Therefore, by balancing the heat intrusion into the He bath 56 and the cooling by the GM refrigerator 30, it is possible to maintain extremely low temperatures and almost completely suppress the evaporation of liquid He.

[0048] As a result, in medical institutions such as hospitals, if liquid He is injected during the initial startup of the MRI device 50, there is no need to periodically replenish the expensive and difficult-to-handle liquid He during subsequent operation. This significant improvement in convenience has led to the widespread adoption of MRI devices 50 in small and medium-sized hospitals. Furthermore, MRI devices incorporating direct-cooled superconducting coils, which are conduction-cooled by a refrigerator instead of using liquid He, have also been commercialized. In this case, the He bath 56 can be omitted.

[0049] In recent years, MRI devices using high-temperature superconducting wires such as Y-based, Bi-based, and MgB2 have been developed. Similar to MRI devices using low-temperature superconducting materials, the superconducting coils in these devices must be cooled to below 30K, which is below the critical temperature for superconducting transition and allows for the flow of the current necessary to generate a magnetic field.

[0050] Therefore, in MRI devices using high-temperature superconducting materials, it is necessary to cool the superconducting coils by immersion in liquid He, H2, or Ne, which have a liquefaction temperature of 30K or less at 1 atmosphere, or by conduction cooling of the superconducting coils with a refrigerator. Even in the former method, it is desirable to use a refrigerator to cool the coils in order to prevent evaporation of liquid He, H2, and Ne. To improve the performance of the refrigerator in the 10 to 30K range, it is desirable to install a thermal storage material with a large specific heat in this temperature range into the refrigerator.

[0051] The superconducting coil assembly device according to the third embodiment is equipped with a refrigerator according to the second embodiment that incorporates a thermal storage material according to the first embodiment. This makes it possible to provide a superconducting coil assembly device that has higher specific heat characteristics in the cryogenic region.

[0052] (Manufacturing method) A method for manufacturing a cold storage material will now be described. The method for manufacturing a cold storage material according to the embodiment is, for example, the method for manufacturing a cold storage material according to the first embodiment. The method for manufacturing a cold storage material according to the embodiment includes a melting step of melting elements mixed in a desired stoichiometric ratio to obtain a compound, a cooling step of cooling and solidifying the compound, and a heat treatment step of heat treating the cooled compound at a temperature of 1000°C or higher but below its melting point for 24 hours or more.

[0053] In the melting process, the elements are mixed and melted to achieve the desired stoichiometric ratio. Any method is acceptable as long as the elements become molten metal, such as high-frequency induction heating or arc melting.

[0054] In the cooling process, the molten compound from the melting process is cooled and solidified. Cooling can be done by any method as long as the compound solidifies, but for example, it can be done at a cooling rate of 50°C / s or higher. This allows the compound to be obtained in granular form with excellent specific heat properties. The molten metal is supplied to the running surface of a high-speed rotating body installed in a vacuum or inert gas atmosphere. This molten metal is finely dispersed by the movement of the rotating body and rapidly cooled and solidified at the same time to form spherical granules. Alternatively, the molten metal described above is released into a vacuum or inert gas atmosphere and treated with a non-oxidizing atomizing gas. This causes the molten metal to atomize and disperse while rapidly cooling and solidifying at the same time to form spherical granules. Cooling may also be done by air cooling.

[0055] Such methods include the Rotary Disc Process (RDP), single-roll method, double-roll method, inert gas atomization method, and rotary nozzle method. These methods allow for the production of thermal storage particles very simply and at low cost. It is desirable that the thermal storage particles, after cooling, be spherical.

[0056] In the heat treatment process, the solidified compound is heat-treated. The heat treatment is carried out at a temperature of 1000°C or higher, below the melting point of the compound, for 24 hours or more. The melting point of the compound can be measured in advance using Differential Scanning Calorimetry (DSC). A heat treatment time of 72 hours or more is preferable. This allows the crystalline phase to grow and the grain size to increase, making it possible to increase the integral value of the specific heat peak in the 2K to 5K range. Alternatively, the heat treatment is preferably carried out at a temperature of 1200°C or higher, below the melting point of the compound. This prevents the compound from remelting, making it possible to increase the integral value of the specific heat peak in the 2K to 5K range.

[0057] Furthermore, by heat-treating the manufactured cold storage material at a high temperature below its melting point, a cold storage material with higher specific heat properties in the cryogenic region can be obtained.

[0058] (Example 1) Next, Example 1 will be described. Using each element with a purity of 99.9% or higher as raw materials, the composition is Er 20 Fe 40 Si 40 The elements were weighed to achieve the following ratio. Next, the elements were melted using the arc melting method and then cooled to prepare a bulk sample. The composition of the bulk sample was determined by inductively coupled plasma (ICP) emission spectrometry, which revealed that Er 20 Fe 40 Si 40 This was confirmed. Then, the prepared bulk sample was heat-treated at 1200°C for 72 hours to produce a sample.

[0059] (Examples 2-17, Comparative Examples 1-12) (Example 2) Composition Er 17 Fe 35 Si 48 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0060] (Example 3) Composition Er15 Fe 38 Si 47 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0061] (Example 4) Composition Er 21 Fe 44 Si 35 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0062] (Example 5) Composition Er 25 Fe 41 Si 34 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0063] (Example 6) Composition Er 19 Fe 45 Si 36 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0064] (Example 7) By substituting a portion of Er with Gd and mixing Er and Gd in a 9:1 ratio, the composition is (Er 0.9 Gd 0.1 ) 21 Fe 41 Si 38 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0065] (Example 8) By substituting a portion of Er with Ho and mixing Er and Ho in a 9:1 ratio, the composition is (Er 0.9 Ho 0.1 ) 22 Fe 39 Si 39 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0066] (Example 9) By substituting a portion of Er with Dy and mixing Er and Dy in a 9:1 ratio, the composition is (Er 0.9 Dy 0.1) 19 Fe 41 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0067] (Example 10) By substituting a portion of Er with Y and mixing Er and Y in a 9:1 ratio, the composition is (Er 0.9 Dy 0.1 ) 19 Fe 40 Si 41 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0068] (Example 11) By substituting a portion of Er with Tb and mixing Er and Tb in a 9:1 ratio, the composition is (Er 0.9 Tb 0.1 ) 20 Fe 41 Si 39 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0069] (Example 12) By substituting a portion of Er with Sm and mixing Er and Sm in a 9:1 ratio, the composition is (Er 0.9 Sm 0.1 ) 19 Fe 41 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0070] (Example 13) By substituting some of the Fe with Co and mixing Fe and Ho in a 9:1 ratio, the composition is changed to Er 21 (Fe 0.9 Co 0.1 ) 41 Si 38 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0071] (Example 14) A portion of the Fe is replaced with Mn, and the Fe and Mn are mixed in a 9:1 ratio, and the composition is Er 20 (Fe 0.9Mn 0.1 ) 38 Si 42 A bulk sample was prepared in the same manner as in Example 1, except that it was made into

[0072] (Example 15) Part of Fe was replaced with Ni, and they were mixed so that the composition of Fe and Ni became 9:1. The composition was changed to Er 22 (Fe 0.9 Ni 0.1 ) 37 Si 41 A bulk sample was prepared in the same manner as in Example 1, except that it was made into

[0073] (Example 16) Part of Si was replaced with Ge, and they were mixed so that the composition of Si and Ge became 9:1. The composition was changed to Er2Fe 39 (Si 0.9 Ge 0.1 ) 40 A bulk sample was prepared in the same manner as in Example 1, except that it was made into

[0074] (Example 17) Part of Si was replaced with Ga, and they were mixed so that the composition of Si and Ga became 9:1. The composition was changed to Er 19 Fe 39 (Si 0.9 Ga 0.1 ) 42 A bulk sample was prepared in the same manner as in Example 1, except that it was made into

[0075] (Example 18) The composition was changed to Er 21 Fe 39 Si 40 A bulk sample was prepared by arc melting in the same manner as in Example 1 and heat-treated at 1200 °C for 24 hours.

[0076] (Example 19) The composition was changed to Er 20 Fe 39 Si 41 A bulk sample was prepared by arc melting in the same manner as in Example 1 and heat-treated at 1200 °C for 360 hours.

[0077] (Example 20) Composition Er 20 Fe 39 Si 41 Then, a bulk sample was prepared by arc melting in the same manner as in Example 1, and heat-treated at 1200°C for 480 hours.

[0078] (Example 21) Composition Er 20 Fe 39 Si 41 Then, a bulk sample was prepared by arc melting in the same manner as in Example 1, and heat-treated at 1200°C for 720 hours.

[0079] (Example 22) Composition Er 20 Fe 39 Si 41 Then, a bulk sample was prepared by arc melting, similar to Example 1, and heat-treated at 1200°C for 1440 hours.

[0080] (Comparative Example 1) Composition Er 14 Fe 32 Si 54 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0081] (Comparative Example 2) Composition Er 13 Fe 50 Si 37 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0082] (Comparative Example 3) Composition Er 23 Fe 29 Si 48 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0083] (Comparative Example 4) Composition Er 21 Fe 34 Si 45 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0084] (Comparative Example 5) Composition Er 20 Fe 52 Si 28 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0085] (Comparative Example 6) Composition Er 27 Fe 32 Si 41 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0086] (Comparative Example 7) Composition Er 26 Fe 48 Si 26 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0087] (Comparative Example 8) Composition Er 22 Co 38 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0088] (Comparative Example 9) Composition Er 19 Ni 41 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0089] (Comparative Example 10) Composition Er 20 Cu 38 Si 42 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0090] (Comparative Example 11) Composition Gd 21 Fe 39 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0091] (Comparative Example 12) Composition is Ho18 Fe 42 Si 40 A bulk sample was prepared in the same manner as in Example 1, except for the difference in the preparation method.

[0092] (Comparative Example 13) Composition Er 21 Fe 39 Si 40 In the same manner as in Example 1, a bulk sample was prepared by arc melting, and no heat treatment was performed.

[0093] (Comparative Example 14) Composition Er 19 Fe 40 Si 41 Then, a bulk sample was prepared by arc melting in the same manner as in Example 1, and heat-treated at 800°C for 168 hours.

[0094] Figure 5 is a graph showing the specific heat characteristics of Example 1 and Comparative Examples 11 to 13 from 2K to 6K. The specific heat characteristics were measured using a Physical Property Measurement System (PPMS) manufactured by Quantum Design Co., Ltd. As can be seen from Figure 5, Example 1 has a larger maximum value of specific heat from 2K to 5K compared to Comparative Examples 11 to 13. Therefore, by adopting the cold storage material according to the first embodiment as the cold storage material filled in the chiller of the refrigerator, the cooling capacity of the refrigerator is improved.

[0095] Figure 6 shows an SEM image of the sample from Example 1 observed at 5000x magnification. The SEM image was acquired using a scanning electron microscope (FE-SEM SU8020) manufactured by Hitachi High-Tech Corporation. As shown in Figure 6, it can be seen that there is a grain boundary phase with a different contrast from the main phase. From the compositional analysis results by EDX at each point, in addition to the main phase consisting of Er, Fe, and Si, there is a grain boundary phase with the compositional formula Fe c Si 100-c (45≦c≦55) or Er d Fe e Si f O 100-d-e-fIt was found that there are compounds that contain one or more compounds selected from those represented by (10≦d≦20, 20≦e≦30, 40≦f≦50).

[0096] Tables 1 and 2 show the results for composition, integral values ​​of the specific heat peak from 2K to 5K, and grain size for Examples 1 to 19 and Comparative Examples 1 to 14. Comparing Example 1 with Comparative Examples 7 to 12, it can be seen that the combination of Er, Fe, and Si exhibits a higher specific heat than other elemental combinations below 4K. Furthermore, even when some of Er, Fe, and Si are substituted with other rare earth elements, a high integral value of specific heat is still obtained.

[0097] [Table 1]

[0098] [Table 2]

[0099] Tables 1 and 2 show that the grain size changes when the heat treatment conditions are changed. Heat treatment at temperatures above 1000°C results in a grain size exceeding 0.001 mm, and a specific heat integral value exceeding that of the sample prepared under the same conditions as in Non-Patent Document 2, from 2K to 5K, is obtained.

[0100] In this embodiment, a method for manufacturing a cold storage material having high specific heat properties and mechanical strength in the cryogenic region is provided.

[0101] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0102] The invention of the embodiment is described below.

[0103] <1> Composition formula Er a Fe b Si 100-a-b It includes intermetallic compounds represented by (15≦a≦25, 35≦b≦45), The intermetallic compound is a refrigerant material in which the main phase is a crystalline phase having a ThCr2Si2 type crystal structure, and the grain size of the main phase is 0.001 mm or more and 1 mm or less.

[0104] <2> A portion of Er is substituted with R (where R is one or more elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Sc, and Y). <1> The cold storage material described above.

[0105] <3> Some of the Fe is substituted with T (where T is one or more elements selected from Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Zr, Mo, Ru, Rh, Pd, and Ag). <1> ~ <2> The cold storage material described above.

[0106] <4> A portion of Si is replaced with X (where X is one or more elements selected from B, Al, P, Ga, Ge, As, Sn, Sb, and Te). <1> from <3> A cold storage material as described in any one of the items.

[0107] <5> Composition formula Fe c Si 100-c (45≦c≦55) or Er d Fe e Si f O 100-d-e-f It contains one or more compounds that are represented by (10≦d≦20, 20≦e≦30, 40≦f≦50), <1> from <4> A cold storage material as described in any one of the items.

[0108] <6> The aforementioned compositional formula Fe c Si 100-c (45≦c≦55) or Er d Fee Si f O 100-d-e-f In a compound represented by (10≦d≦20, 20≦e≦30, 40≦f≦50), a portion of Er is substituted with R (where R is one or more elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Sc, and Y), a portion of Fe is substituted with T (where T is one or more elements selected from Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Zr, Mo, Ru, Rh, Pd, and Ag), and a portion of Si is substituted with X (where X is one or more elements selected from B, Al, P, Ga, Ge, As, Sn, Sb, and Te). <1> from <5> A cold storage material as described in any one of the items.

[0109] <7> The particle size is between 0.01 mm and 1 mm. <1> from <6> A cold storage material as described in any one of the items.

[0110] <8> <1> from <7> A refrigerator equipped with a cold storage material as described in any one of the items.

[0111] <9> <8> A superconducting coil assembly device equipped with the refrigerator described above. [Explanation of Symbols]

[0112] 1 Cold storage material 10 Crystalline phase 11 Main phase 12 Grain boundary phase 13 Inscribed Circle 15 Projection image 30 Refrigeration units 31. First cylinder 32 Second cubic centimeter 33 aisles 34 1st regenerator 35 Second regenerator 36, 37 Seal rings 38 1st cold storage material 39 aisle 40 Second cold storage material 41 First Expansion Chamber 42 Second Expansion Chamber 43. First Cooling Stage 44. Second Cooling Stage 45 Compressor 46 High-voltage lines 47 Low-voltage line 48 mesh 50 MRI machine 51 Bore Space 52 subjects 53 First electromagnet 54. Second electromagnet 55 RF coil 56 He bath 57 Insulating vacuum layer 58, 59 Radiation shield

Claims

1. Composition formula Er ａ Fe ｂ Si １００－ａ－ｂ It includes intermetallic compounds represented by (15 ≤ a ≤ 25, 35 ≤ b ≤ 45), The intermetallic compound is ThCr ２ Si ２ A refrigerant material having a crystalline phase with a type crystal structure as its main phase, wherein the crystal grain size of the main phase is 0.001 mm or more and 1 mm or less.

2. The cold storage material according to claim 1, wherein a portion of Er is replaced with R (where R is one or more elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Sc, and Y).

3. The thermal storage material according to claim 1, wherein a portion of Fe is replaced with T (where T is one or more elements selected from Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Zr, Mo, Ru, Rh, Pd, and Ag).

4. The thermal storage material according to claim 1, wherein a portion of Si is replaced with X (where X is one or more elements selected from B, Al, P, Ga, Ge, As, Sn, Sb, and Te).

5. Compositional Fe ｃ Si １００－ｃ (45 ≦ c ≦ 55) or Er ｄ Fe ｅ Si ｆ O １００－ｄ－ｅ－ｆ The cold storage material according to claim 1, comprising one or more compounds represented by (10 ≦ d ≦ 20, 20 ≦ e ≦ 30, 40 ≦ f ≦ 50).

6. The compositional formula Fe ｃ Si １００－ｃ (45 ≤ c ≤ 55) or Er ｄ Fe ｅ Si ｆ O １００－ｄ－ｅ－ｆ The cold storage material according to claim 1, wherein a portion of Er in a compound represented by (10≦d≦20, 20≦e≦30, 40≦f≦50) is replaced with R (R is one or more elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Sc, and Y), a portion of Fe is replaced with T (T is one or more elements selected from Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Zr, Mo, Ru, Rh, Pd, and Ag), and a portion of Si is replaced with X (X is one or more elements selected from B, Al, P, Ga, Ge, As, Sn, Sb, and Te).

7. The cold storage material according to claim 1, wherein the particle size is 0.01 mm or more and 1 mm or less.

8. A refrigerator equipped with a cold storage material according to any one of claims 1 to 7.

9. A superconducting coil assembly device equipped with the refrigerator described in claim 8.

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