HoCu-based cold-storage material, and cold-storage device and refrigerating machine each equipped therewith

Abstract

This invention provides a regenerator material having a high specific heat, particularly in the temperature range of 10 to 25K, and a regenerator and a refrigerator comprising the regenerator material. The present invention specifically provides an HoCu-based regenerator material represented by general formula (1): HoCu.sub.2-xM.sub.x (1), wherein x is 0<x≤1, and M is at least one member selected from the group consisting of Al and transition metal elements (excluding Cu), as well as a regenerator and a refrigerator comprising the regenerator material.

Claims

1. An HoCu-based regenerator material represented by formula (3):
HoCu.sub.2-xAl.sub.x  (3) wherein x is 0<x<1, the HoCu-based regenerator material having a KHg.sub.2-type crystal structure as a main phase and an HoCuAl phase as a second phase, and the HoCu-based regenerator material having a higher specific heat than HoCu.sub.2 in a temperature range of 10K or more.

2. A regenerator filled with the HoCu-based regenerator material according to claim 1, either alone or in combination with one or more other regenerator materials.

3. The regenerator according to claim 2, wherein the HoCu-based regenerator material is in a state of (1) a spherical particle group, or (2) a sintered body of a spherical particle group.

4. A refrigerator comprising the regenerator according to claim 2.

5. An HoCu-based regenerator material represented by formula (4):
(Ho.sub.1-y RE.sub.y)Cu.sub.2-xAl  (4) wherein x is 0<x<1, y is 0<y<1, and RE is one or more rare earth elements with the proviso that RE does not contain Ho, the HoCu-based regenerator material having a KHg.sub.2-type crystal structure as a main phase and an HoCuAl phase as a second phase, and the HoCu-based regenerator material having a higher specific heat than HoCu.sub.2 in a temperature range of 10K or more.

6. A regenerator filled with the HoCu-based regenerator material according to claim 5, either alone or in combination with one or more other regenerator materials.

7. The regenerator according to claim 6, wherein the HoCu-based regenerator material is in a state of (1) a spherical particle group, or (2) a sintered body of a spherical particle group.

8. A refrigerator comprising the regenerator according to claim 6.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the relationship between the temperature and the specific heat of previously known regenerator materials.

(2) FIG. 2 shows X-ray diffraction results of the HoCu-based regenerator materials obtained in Examples 1 and 2, and a regenerator material for comparison (Comparative Example 1).

(3) FIG. 3 shows the relationship between the temperature and the specific heat of the HoCu-based regenerator materials obtained in Examples 1 and 2, and regenerator materials for comparison (Pb, Bi, and Comparative Examples 1 and 3).

(4) FIG. 4 shows X-ray diffraction results of the HoCu-based regenerator material obtained in Example 3, and regenerator materials for comparison (Comparative Examples 1 and 2).

(5) FIG. 5 shows the relationship between the temperature and the specific heat of the HoCu-based regenerator material obtained in Example 3, and regenerator materials for comparison (Bi and Comparative Example 2).

(6) FIG. 6 shows X-ray diffraction results of the HoCu-based regenerator material obtained in Example 4, and regenerator materials for comparison (Comparative Examples 1 and 4).

(7) FIG. 7 shows the relationship between the temperature and the specific heat of the HoCu-based regenerator material obtained in Example 4, and regenerator materials for comparison (Pb, Bi, and Comparative Examples 1 and 5).

DESCRIPTION OF EMBODIMENTS

(8) The HoCu-based regenerator material, and a regenerator and a refrigerator comprising the HoCu-based regenerator material, are described below.

(9) 1. HoCu-Based Regenerator Material

(10) The HoCu-based regenerator material of the present invention has a structure in which Cu contained in HoCu.sub.2 (holmium copper 2) is partially replaced by at least one member selected from the group consisting of Al and transition metal elements (excluding Cu); and is represented by general formula (1) below:
HoCu.sub.2-xM.sub.x  (1)
wherein x is 0<x≤1, and M is at least one member selected from the group consisting of Al and transition metal elements (excluding Cu).

(11) Preferable examples of transition metal elements excluding Cu include, but are not limited to, at least one of Ni (nickel), Co (cobalt), Fe (iron), etc. HoCu.sub.2 has a KHg.sub.2-type (body-centered orthorhombic crystals; Pearson symbol: oI12) crystal structure. It is important that the HoCu-based regenerator material of the present invention has a KHg.sub.2-type structure as a main phase. Such a material is obtained when Cu is partially replaced by M (at least one member selected from the group consisting of Al and transition metal elements excluding Cu). M is more preferably at least one of Ni and Al, among the above.

(12) x, which indicates the amount of M, may be in the range of 0<x≤1, particularly preferably 0<x≤0.8, and more preferably 0<x≤0.5.

(13) In particular, when M is Ni in general formula (1), the HoCu-based regenerator material is represented by general formula (2):
HoCu.sub.2-xNi.sub.x  (2)
wherein x is 0<x≤1.
x may be in the range of 0<x≤1, particularly preferably 0<x≤0.8, and more preferably 0<x≤0.5. The HoCu-based regenerator material represented by general formula (2) is substantially a single-phase alloy (KHg.sub.2-type structure).

(14) In particular, when M is Al in general formula (1), the HoCu-based regenerator material is represented by general formula (3):
HoCu.sub.2-xAl.sub.x  (3)
wherein x is 0<x<1.
x may be in the range of 0<x<1, particularly preferably 0<x≤0.8, and more preferably 0<x≤0.5. The HoCu-based regenerator material represented by general formula (3) has a KHg.sub.2-type structure as a main phase, and further has, for example, an HoCuAl phase as a second phase (minor phase).

(15) The HoCu-based regenerator material of the present invention also includes those with a structure in which Cu contained in HoCu.sub.2 (holmium copper 2) is partially replaced by at least one member selected from the group consisting of Al and transition metal elements (excluding Cu), and Ho is partially replaced by RE (one or more rare earth elements excluding Ho). In this case, it is also important that the HoCu-based regenerator material of the present invention is a material having a KHg.sub.2-type structure as a main phase.

(16) Specifically, the HoCu-based regenerator material is represented by general formula (4):
(Ho.sub.1-yRE.sub.y)Cu.sub.2-xM.sub.x  (4);
wherein x is 0<x≤1, M is at least one member selected from the group consisting of Al and transition metal elements (excluding Cu), y is 0<y<1, and RE is one or more rare earth elements (excluding Ho).

(17) Here, the type of M and x, which indicates the amount of M, are as described above. RE is not limited, as long as it is one or more rare earth elements excluding Ho. Examples include Ce (cerium), Pr (praseodymium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium). A material having a KHg.sub.2-type structure as a main phase is obtained when Ho is partially replaced by one or more rare earth elements described above (RE). As the RE (one or more rare earth elements), Gd, Tb, Dy, Er, Tm, Yb, and Lu, which are heavy rare earth elements, are preferable; and Er is particularly preferable among these. These rare earth elements (RE) may be used singly, or in a combination of two or more.

(18) x, which indicates the amount of M, may be in the range of 0<x≤1, particularly preferably 0<x≤0.8, and more preferably 0<x≤0.5.

(19) y, which indicates the amount of RE, may be in the range of 0<y<1, particularly preferably 0<y≤0.8, and more preferably 0<y≤0.5.

(20) In particular, when M is Ni in general formula (4), the HoCu-based regenerator material is represented by general formula (5):
(Ho.sub.1-yEr.sub.y)Cu.sub.2-xNi.sub.x  (5)
wherein x is 0<x≤1, and y is 0<y<1.
x is preferably 0<x≤0.8, and more preferably 0<x≤0.5. y may be in the range of 0<y<1; and the amount of y may be adjusted so that a predetermined specific heat peak temperature is obtained, taking into consideration the amount of x and the shift of the specific heat peak.

(21) The following is considered to be the reason the regenerator material of the present invention exhibits the predetermined effects of the present invention.

(22) To obtain a magnetic regenerator material with high specific heat characteristics in a broad temperature range, it is also important to utilize the Schottky specific heat. It is possible that a desired material with a large specific heat at 10 to 25K can be obtained by using HoCu.sub.2 as a base; and, for example, 1) applying a replacement element that can increase the magnetic transition temperature, or 2) designing an alloy so that the Schottky specific heat is increased, while maintaining the KHg.sub.2-type structure as a main phase.

(23) An additional explanation is given below for the Schottky specific heat. For example, around room temperature, lattice vibrations mainly contribute to the specific heat of crystals. Thus, the contribution of lattice vibrations to the specific heat decreases in a cryogenic temperature range, and the specific heat of common metals rapidly decreases in a cryogenic temperature range. On the other hand, rare earth ions in crystals have electrons in the 4f orbital, the localization of which is strong; and the 4f orbital electrons are affected by a crystal field, and take on a discrete energy level. That is, the 4f electrons are in a ground state at a temperature near OK, and are excited to a higher level when the temperature increases to reach an energy corresponding to a discrete level. Therefore, a high specific heat is exhibited at a specific temperature. The abnormal specific heat due to this excitation is called “Schottky specific heat.”

(24) Focusing on Cu, which is a constituent element of HoCu.sub.2 alloy, Cu and Ni both have a Cu-type structure (cF4, face-centered cubic crystals) at about room temperature and can form a complete solid solution, with reference to a Cu—Ni binary phase diagram. Specifically, HoCu.sub.2 alloy and HoNi.sub.2 alloy have different crystal structures; however, it is possible that an alloy in which Cu in HoCu.sub.2 alloy is partially replaced by Ni and the KHg.sub.2-type structure is maintained can be obtained as an alloy that cannot be inferred from the heat storage characteristics of the two alloys HoCu.sub.2 and HoNi.sub.2.

(25) Focusing on HoAl.sub.2, HoAl.sub.2 has an MgCu.sub.2-type structure (cF24, face-centered cubic crystals), whereas HoCu.sub.2 has a KHg.sub.2-type structure (oI12, body-centered orthorhombic crystals). HoAl.sub.2 and HoCu.sub.2 have different crystal structures. Al and Cu both have a Cu-type structure (cF4, face-centered cubic crystals) at about room temperature, and Al is solid-soluble in an amount of about 4 to 5 atomic % in Cu, with reference to a Cu—Al binary phase diagram (however, Cu is not solid-soluble in Al).

(26) In Ho—Cu—Al-based alloy, formation of an HoCuAl phase (ZrNiAl structure) as an intermediate phase is indicated. It is possible that an alloy having a KHg.sub.2-type structure, and having characteristics that cannot be inferred from the heat storage characteristics of the two alloys HoCu.sub.2 and HoAl.sub.2, can be obtained by partially replacing Cu in HoCu.sub.2 by Al.

(27) In light of the above, the present invention proposes an HoCu-based regenerator material having specific heat characteristics at cryogenic temperatures that cannot be inferred from the heat storage characteristics of the two alloys HoCu.sub.2 and HoNi.sub.2, by using HoCu.sub.2 alloy as a base; and partially replacing Cu by at least one member selected from the group consisting of Al and transition metal elements (excluding Cu); or in addition to the partial replacement, further partially replacing Ho by RE (a rare earth element excluding Ho).

(28) The HoCu-based regenerator material of the present invention may comprise impurities other than the elements mentioned above in an amount that does not significantly affect the specific heat characteristics of the material. In the present invention, there may be, for example, a case in which a raw material initially contains a trace amount of impurities, or a case in which impurities are introduced when the HoCu-based regenerator material is prepared. The term “impurities” as used herein means components that are not added intentionally in each case.

(29) 2. Regenerator and Refrigerator

(30) A regenerator may be constituted by filling it with the HoCu-based regenerator material of the present invention alone, or in combination with one or more other regenerator materials. The other regenerator materials are not limited, and known regenerator materials may be appropriately combined. Further, a refrigerator (for example, a refrigerator for liquid hydrogen production, or a 10K-specific refrigerator) provided with the regenerator can be constituted. Moreover, in a 4KGM refrigerator, the HoCu-based material of the present invention may be incorporated between a material for the low-temperature end side, and a material for temperatures up to, for example, 80K.

(31) The properties of the HoCu-based regenerator material in the regenerator are not limited, and may be appropriately selected from 1) a state of a spherical particle group; or 2) a state of a sintered body of a spherical particle group, according to the use etc.

(32) When the HoCu-based regenerator material is used in the state of a spherical particle group, for example, a raw material that is mixed so as to have a predetermined composition after dissolution and casting is prepared, and then the raw material is dissolved in a melting furnace such as a vacuum high-frequency melting furnace under an inert gas atmosphere; then, the spherical HoCu-based regenerator material is obtained by an atomizing method such as gas atomization or disk atomization, a rotating electrode method, or the like. In doing so, by preparing the HoCu-based regenerator material under rapid cooling conditions, a single phase is easily obtained in a wide range of composition. The rapid cooling conditions are not limited, and an atomizing method such as gas atomization or water atomization that can be performed at a cooling rate of 10.sup.3/sec or more is preferable. Further, by performing sieving and shape classification as necessary, a desired powder can be obtained. The particle size of the spherical particles is not limited, and is preferably in the range of not less than 100 μm and not more than 750 μm, and more preferably in the range of not less than 100 μm and not more than 300 μm.

(33) The aspect ratio of the spherical HoCu-based regenerator material is preferably 10 or less, more preferably 5 or less, and most preferably 2 or less. By using a spherical HoCu-based regenerator material having a small aspect ratio, it is possible to enhance the filling property into the regenerator, and more easily obtain a sintered body having uniform communication holes when the sintered body of a spherical particle group is obtained. In the measurement of aspect ratio in this specification, the spherical powder of the HoCu-based regenerator material is mixed well, and then a sample obtained by the quartering method is subjected to aspect ratio measurement with an optical microscope by using 100 arbitrary particles. Then, an average value of the measured values is calculated. This method is repeated 3 times, and the average value of 3 measurements is found as the aspect ratio.

(34) When the HoCu-based regenerator material is used in the form of a sintered body of spherical powder, the spherical powder of the HoCu-based regenerator material is introduced into a mold; and then subjected to a heat treatment for not less than an hour and not longer than 40 hours at a temperature of hot less than 700° C. and not more than 1200° C., in an atmosphere furnace in an inert gas atmosphere of Ar, nitrogen, or the like, thereby obtaining a sintered body. By controlling the temperature and the duration of the heat treatment, the filling rate of the HoCu-based regenerator material in the obtained sintered body can be controlled. The heat treatment can also be performed by an electric current sintering method, a hot-pressing method, or the like. The porosity in the sintered body is not limited, and is preferably in the range of 28 to 40%, further preferably in the range of 32 to 37%. When the porosity is within the above range, the HoCu-based regenerator material can be charged into the regenerator at a high filling rate.

(35) The porosity in this specification refers to a value determined according to the following formula.
(1−measured weight/(apparent volume×specific gravity))×100
(provided that the apparent volume means, for example, for a cylindrical sample, a volume obtained from the diameter and the length)

(36) The shape and size of the sintered body are not particularly limited, and may be appropriately selected according to the shape of the regenerator. For example, the shape of the sintered body may be a cylindrical shape, a prismatic shape, or the like. In addition, the sintered body may also have a tapered shape, in view of engagement or the like.

(37) The shape of the sintered body can be adjusted upon sintering of the spherical powder by charging the spherical powder into a container having a desired shape. For example, if the sintered body has a cylindrical shape, a cylindrical container may be filled with the spherical powder, and sintering may be performed.

(38) The sintered body may have a multilayer structure. The multilayer structure herein means, for example, in the case of a cylindrical shape, a structure in which a single or plural outer layers are formed on the outside of the inner layer. Examples of such a multilayer structure include a structure formed from a plurality of layers having different porosities. The multilayer structure may also be a structure in which a plurality of layers are formed from different types of materials. The multilayer structure may further be, for example, a laminate in which a plurality of layers with different specific heat characteristics are laminated in order.

EXAMPLES

(39) The present invention is more specifically explained below in reference to Examples. However, the present invention is not limited to these Examples.

Examples 1 to 4 and Comparative Examples 1 to 5 (Synthesis of Each Regenerator Material Alloy Powder)

(40) First, raw materials, each of which was mixed to have the composition shown in Table 1 after dissolution and casting, were prepared; and dissolved in an argon gas atmosphere in a high-frequency, heat-melting furnace, thereby obtaining molten alloys.

(41) Next, the alloys were sufficiently stirred and rapidly cooled (rapid cooling conditions: 10.sup.3K/sec or more) by an atomizing method, thereby obtaining alloy powders.

(42) Thereafter, in order to increase the homogeneity of the composition of each alloy powder, a homogenization treatment was performed for 0.01 to 40 hours at a temperature equal to 95% of the melting point obtained from the phase diagram; followed by coarse pulverization, if necessary, thereby obtaining alloy powders having an average particle size (D50) of 50 to 300 μm.

(43) Subsequently, X-ray diffraction measurement of each alloy powder was performed. FIGS. 2, 4, and 6 show the results. Further, the specific heat of each alloy powder was determined by a thermal relaxation method using a PPMS (Physical Property Measurement System). FIGS. 3, 5, and 7 show the results. Table 1 shows evaluation (determination) of the composition, the constituent phase, and the specific heat characteristics of each alloy powder. In the determination column of Table 1, when an alloy with a KHg.sub.2-type structure as a main phase had a high specific heat in the temperature range of 10 to 25K, it was evaluated as “A”; and an alloy that did not have such a specific heat was evaluated as “B.”

(44) TABLE-US-00001 TABLE 1 Composition Constituent Phase Determination Example 1 HoCu.sub.1.8Ni.sub.0.2 KHg.sub.2-type phase A Example 2 HoCu.sub.1.5Ni.sub.0.5 KHg.sub.2-type phase A Example 3 (Ho.sub.0.5Er.sub.0.5)CuNi KHg.sub.2-type phase A Example 4 HoCu.sub.1.5Al.sub.0.5 KHg.sub.2-type phase A (main phase) + ZrNiAl phase Comparative HoCu.sub.2 KHg.sub.2-type phase B Example 1 Comparative (Ho.sub.0.5Er.sub.0.5)Cu.sub.2 KHg.sub.2-type phase B Example 2 Comparative HoNi.sub.2 MgCu.sub.2-type phase B Example 3 Comparative HoCuAl ZrNiAl-type phase B Example 4 Comparative HoAl.sub.2 MgCu.sub.2-type phase B Example 5
Specific Heat Characteristics of Alloy Powders Obtained in Examples and Comparative Examples

(45) The results of X-ray diffraction shown in FIG. 2 reveal that each of the alloy powders of Comparative Example 1, and Examples 1 and 2 had a KHg.sub.2-type structure as a main phase.

(46) The results of the specific heat at cryogenic temperatures shown in FIG. 3 reveals that the specific heat peak of each of the alloy powders of Examples 1 and 2 was shifted to the high-temperature side, compared with that of the alloy powder (HoCu.sub.2) of Comparative Example 1. The shift to the high-temperature side is considered to depend to some extent on the replacement amount by Ni. Further, each of the powders of Examples 1 and 2 exhibited improved specific heat characteristics, which are considered to be derived from the Schottky specific heat, in the temperature side higher than the specific heat peak; and showed a higher specific heat than the alloy powder (HoCu.sub.2) of Comparative Example 1 in a temperature range of 10K or more.

(47) With reference to FIG. 3, the alloy powder (HoNi.sub.2) of Comparative Example 3 has the following specific heat characteristics: it exhibited a high specific heat peak at a temperature around 12K, compared with the alloy powder (HoCu.sub.2) of Comparative Example 1; however, it had an extremely low specific heat in the other temperature ranges. In contrast, although the specific heat of the alloy powder of Example 1 decreased in a temperature range higher than the specific heat peak, it increased in a temperature range of, for example, 12K or more due to contribution of the Schottky specific heat, compared with that of the alloy powder of Comparative Example 3. The alloy powder of Example 2 exhibited a specific heat peak at a higher temperature than the alloy powder of Comparative Example 3. The above results show that alloys having high specific heat characteristics at 10 to 25K, which is a target temperature in an Ho(Cu, Ni).sub.2-based alloy having a KHg.sub.2-type structure as a main phase, were obtained.

(48) Many RECu.sub.2 alloys, including HoCu.sub.2 alloy, have a KHg.sub.2-type structure. On the other hand, RENi.sub.2 alloys have an MgCu.sub.2-type structure. Thus, an (RE)(Cu, Ni).sub.2 alloy that maintains a KHg.sub.2-type structure as a main phase as in the alloy powders of Examples 1 and 2 is also expected to exhibit, for example, a specific heat peak at a higher temperature.

(49) Next, the effects of (Ho.sub.0.5Er.sub.0.5)Cu.sub.2 of Comparative Example 2, in which Ho is partially replaced by RE (excluding Ho) and (Ho.sub.0.5Er.sub.0.5)CuNi of Example 3, are described.

(50) As is clear from FIG. 4, the alloy powders of Comparative Example 2 and Example 3 each have a KHg.sub.2-type structure as a main phase.

(51) The results of the specific heat at cryogenic temperatures shown in FIG. 5 reveal that the specific heat peak of the sample of Example 3 was shifted to the high-temperature side, compared with that of Comparative Example 2. Further, the powder of Example 3 exhibited improved specific heat characteristics, which are considered to be derived from the Schottky specific heat, in the temperature side higher than the specific heat peak, specifically in a temperature range of 20K or more. It was confirmed that the alloy that contains as a base another RECu.sub.2-based alloy with a KHg.sub.2-type structure into which the KHg.sub.2-type structure is maintained as a main phase structure, although Ni is introduced, exhibited improved characteristics similar to those of Examples 1 and 2, as compared with Comparative Example 1.

(52) As is clear from FIG. 6, the alloy powder of Example 4 contains a KHg.sub.2-type structure phase as a main phase and an MgCuAl phase (ZrNiAl structure) as a heterogeneous phase, indicating that an alloy having a KHg.sub.2-type structure as a main phase was obtained.

(53) The results of the specific heat at cryogenic temperatures shown in FIG. 7 reveal that, focusing on the specific heat peak of HoCu.sub.1.5Al.sub.0.5 of Example 4, for example, the basic peak temperature is close to those of the HoCu.sub.2 alloy of Comparative Example 1; however, an improvement in specific heat characteristics, which are considered to be derived from the Schottky specific heat, was observed in a temperature range higher than the specific heat peak (around 10 to 23K). In the HoAl.sub.2 alloy of Comparative Example 5 for reference, 1) the Schottky specific heat was low; and 2) compared with the specific heat change between the HoCu.sub.2 alloy and the HoCu.sub.1.5Al.sub.0.5 alloy, the difference in characteristics between the HoCu.sub.2 alloy and the HoAl.sub.2 alloy was large (their characteristics greatly differed from each other). This is considered to be because an improvement in characteristics was achieved by designing the HoCu.sub.1.5Al.sub.0.5 alloy as an alloy that contains a KHg.sub.2-type phase, which is the alloy design principle of the present invention, and is substantially free of the MgCu.sub.2-type phase, in order to obtain an alloy having a desired crystal structure.