Magnetic refrigeration system using an all-solid thermal switch and method for manufacturing a material for the thermal switch

Abstract

To provide a magnetic refrigeration system that is preferably used when being located in an atmosphere of around hydrogen liquefaction temperature (20.3K) or cooling a wide temperature range between ten K to several tens K or so.SOLUTION: A magnetic refrigeration system includes: a low-temperature-side heat bath 37; a high-temperature-side heat bath 38 having temperature higher than that of the low-temperature-side heat bath; a magnetic body 30 for pumping up heat on a side of the high-temperature-side heat bath 38 from the low-temperature-side heat bath 37; a first thermal switch 34 for turning on and off thermal connection between the low-temperature-side heat bath 37 and the magnetic body 30; a second thermal switch 30 for turning on and off thermal connection between the high-temperature-side heat bath 38 and the magnetic body 30; and a magnetic field application part 42 for applying a magnetic field to the magnetic body 30, and the first and second thermal switches (34, 30) at the same time to magnetize them. The magnetic refrigeration system controls such that, when the magnetic body 30 generates heat due to magnetization, the first thermal switch 34 turns off and the second thermal switch 30 turns on and when the magnetic body 30 absorbs heat due to demagnetization, the first thermal switch 34 turns on and the second thermal switch 30 turns off.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a magnetic refrigeration system using an all-solid-state thermal switch and a method for manufacturing a material for the thermal switch.

Background Art

[0002] A magnetic refrigeration system that utilizes the phenomenon of heat generation or heat absorption in a substance by applying or removing a magnetic field to a substance such as a magnetic material has attracted attention as an alternative to a conventional thermal cycle system that utilizes the heat generation or heat absorption phenomenon due to compression and expansion of a gas. A method of cooling an object to be cooled using the heat quantity change of a magnetic material that generates heat or absorbs heat when a magnetic field is applied or removed has been performed by flowing a gas or liquid refrigerant in one direction. Specifically, when the magnetic material generates heat, the refrigerant is caused to flow in one direction, and when the magnetic material absorbs heat, the refrigerant is caused to flow in the opposite direction, thereby transporting the heat quantity in one direction.

[0003] On the other hand, as a method of cooling liquid hydrogen, which is considered as one of the candidates for storing renewable energy, expectations for magnetic refrigeration have increased, and research and development of magnetic refrigeration technology have been actively carried out for extremely low temperatures. Also, even in a temperature range lower than the hydrogen liquefaction temperature, with the recent practical application of quantum devices, the demand for an efficient cooling method at extremely low temperatures has been increasing. Magnetic refrigeration technology is based on the magnetocaloric effect. The magnetocaloric effect is a phenomenon in which the temperature of a magnetic material changes when an external applied magnetic field is changed for the magnetic material in an adiabatic state.

[0004] As a magnetic refrigeration system, an AMR (Active Magnetic Regenerative Refrigeration) method has been proposed (see Patent Document 1). This AMR method is a method of transporting heat in one direction by flowing a liquid or gas refrigerant through a magnetic material and exchanging heat with the magnetic material using the heat generated by the magnetocaloric effect in the magnetic material. AMR has been demonstrated so far in a temperature range from room temperature to near the hydrogen liquefaction temperature. Furthermore, regarding contact thermal switches, they are used for the purpose of cooling a non-refrigerant superconducting magnet (see Patent Document 2). Patent Document 3 proposes using single-crystalline beryllium as a thermal switch.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] Heat flow control at extremely low temperatures plays an important role in hydrogen liquefaction technology and quantum device implementation. Currently, heat flow control in the extremely low temperature region mainly uses gas-gap thermal switches, superconducting thermal switches, and contact thermal switches. Gas-gap thermal switches control heat flow by the entry and exit of gas, making it difficult for the device to perform high-speed on / off. Superconducting thermal switches are limited to extremely low temperatures below 1K for operation. In addition, contact thermal switches have problems such as the need for mechanical repetitive operation at extremely low temperatures and the enlargement of the device.

[0008] In addition, in Non-Patent Documents 1 to 3 and Patent Document 3, a high-purity single-crystal thermal switch has been proposed to obtain a large magnetic-field change in thermal conductivity. On the other hand, when the thermal switch is incorporated into a magnetic refrigeration system or other cooling devices, it is required to process the thermal switch into various shapes in order to control the heat flow in the thermal switch. However, single-crystal materials have poor workability and have practical problems when used as materials for thermal switches. There have also been reports on polycrystalline pure metals, but there is a problem that the thermal conductivity ratio Δκ indicating the performance of the thermal switch is as small as about 4 and is not practical. Also, in the temperature range below 1K, polycrystalline pure metals are used as superconducting thermal switches, but there is a problem that superconducting thermal switches cannot be used when targeting quantum device cooling (tens of mK to tens of K) which requires cooling in the vicinity of the hydrogen liquefaction temperature (20.3K) or over a wide temperature range.

Means for Solving the Problems

[0009] The present invention solves the above-described problems, and is suitable for use when cooling in the vicinity of the hydrogen liquefaction temperature (20.3K) or over a wide temperature range of about ten K to several tens of K. Without using a gas as a refrigerant, it has a wide operating temperature range, is compact, and can control the heat flow at extremely low temperatures by a magnetic field, and an object thereof is to provide a magnetic refrigeration system using a magnetic-field-driven all-solid-state thermal switch.

[0010] 〔1〕The magnetic refrigeration system of the present invention, for example, as shown in FIG. 1, includes a low-temperature-side heat bath 37, a high-temperature-side heat bath 38 having a temperature higher than that of the low-temperature-side heat bath 37, a magnetic body 30 that pumps heat from the low-temperature-side heat bath 37 to the high-temperature-side heat bath 38 side, a first thermal switch 34 that turns on and off the thermal connection between the low-temperature-side heat bath 37 and the magnetic body 30, a second thermal switch 30 that turns on and off the thermal connection between the high-temperature-side heat bath 38 and the magnetic body 30, and a magnetic-field application unit 42 that simultaneously applies a magnetic field to the magnetic body 30 and the first and second thermal switches (34, 30) to perform excitation. According to the magnetic refrigeration system of the present invention configured as described above, by cooperatively controlling the on / off states of the first and second thermal switches (34, 30), heat can be continuously pumped from the low-temperature side heat bath 37 to the high-temperature side heat bath 38, and the cooled part, which is the low-temperature side heat bath 37, can be continuously cooled. 〔2〕In the magnetic refrigeration system 〔1〕 or 〔2〕 of the present invention, preferably, when the magnetic body 30 generates heat due to excitation, the first thermal switch 34 connected to the low-temperature side heat bath 37 is turned off, and the second thermal switch 30 connected to the high-temperature side heat bath 38 is turned on. When the magnetic body 30 absorbs heat due to demagnetization, it is preferable to provide a thermal switch control unit 44 that controls the first thermal switch 34 connected to the low-temperature side heat bath 37 to be turned on and the second thermal switch 30 connected to the high-temperature side heat bath 38 to be turned off.

[0011] 〔3〕In the magnetic refrigeration system 〔1〕 or 〔2〕 of the present invention, preferably, at least one of the first thermal switch 34 or the second thermal switch 30 uses a polycrystalline metal material composed of any one of tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn), and inevitable impurities. 〔4〕In the magnetic refrigeration system 〔3〕 of the present invention, preferably, the inevitable impurities are preferably 1000 ppm or less by weight ratio. If the content of inevitable impurities is 1000 ppm or less, it means using polycrystalline metals of tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn) with a purity of 99.9% or more of polycrystalline metals. Since the thermal conductivity changes significantly when a magnetic field is applied, it is suitable for use in at least one of the first thermal switch 34 or the second thermal switch 30. More preferably, a polycrystalline metal of tungsten (W), copper (Cu), aluminum (Al), or zinc with an unavoidable impurity content of 100 ppm or less and a polycrystalline metal purity of 99.99% or more may be used. Even more preferably, a polycrystalline metal of tungsten (W), copper (Cu), aluminum (Al), or zinc with an unavoidable impurity content of 10 ppm or less and a polycrystalline metal purity of 99.999% or more may be used. Most preferably, a polycrystalline metal of tungsten (W), copper (Cu), aluminum (Al), or zinc with an unavoidable impurity content of 1 ppm or less and a polycrystalline metal purity of 99.9999% or more may be used. 〔5〕In the magnetic refrigeration system 〔3〕or 〔4〕of the present invention, preferably, the polycrystalline metal of tungsten (W), copper (Cu), aluminum (Al), or zinc has a thermal conductivity ratio {κ(H = 0) / κ(H = 8.5T)} of the thermal conductivity {κ(H = 0)} in a zero magnetic field at 15K and the thermal conductivity {κ(H = 8.5T)} in a magnetic field of 8.5T at 15K of 4 or more. 〔6〕In the magnetic refrigeration system 〔3〕or 〔4〕of the present invention, preferably, the polycrystalline metal of tungsten (W), copper (Cu), or zinc has a thermal conductivity ratio {κ(H = 0) / κ(H = 8.5T)} of the thermal conductivity {κ(H = 0)} in a zero magnetic field at 15K and the thermal conductivity {κ(H = 8.5T)} in a magnetic field of 8.5T at 15K of 6 or more. 〔7〕In the magnetic refrigeration system 〔3〕to 〔6〕of the present invention, preferably, at least one of the first or second thermal switches (34, 30) is a polycrystalline metal material composed of tungsten (W) and unavoidable impurities, and the ratio {R(300K) / R(4K)} of the residual resistance value {R(300K)} at 300K and the residual resistance value at 4K {R(4K)} is 100 or more. 〔8〕In the magnetic refrigeration system 〔1〕to 〔7〕of the present invention, preferably, further, the set temperature of the low-temperature side heat bath is in the range of the hydrogen liquefaction temperature (20.3K) ± 2K.

[0012] [9] The manufacturing method of the material for the thermal switch of the present invention is a manufacturing method of the material for the thermal switch used for at least one of the first or second thermal switches (34, 30) used in the magnetic refrigeration system [3] to [7], wherein the polycrystalline metal tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn) is heat-treated in a vacuum or a reducing atmosphere.

[10] In the manufacturing method [9] of the material for the thermal switch of the present invention, preferably, the first thermal switch 34 is a polycrystalline metal material composed of tungsten (W) and inevitable impurities, and it is preferably heat-treated at 800 °C or higher and 2300 °C or lower. This is because pure tungsten undergoes primary recrystallization at 1300 - 1500 K and subsequently secondary recrystallization at 1500 K - 1800 K.

[11] In the manufacturing method

[10] of the material for the thermal switch of the present invention, preferably, the polycrystalline metal tungsten (W) is heat-treated at 900 °C or higher and 1600 °C or lower.

[12] In the manufacturing method [9] of the material for the thermal switch of the present invention, preferably, the first thermal switch is a polycrystalline metal material composed of copper (Cu) and inevitable impurities, and it is preferably heat-treated at 250 °C or higher and 800 °C or lower.

[13] In the magnetic refrigeration system

[12] of the present invention, preferably, the polycrystalline copper (Cu) is heat-treated at 300 °C or higher and 600 °C or lower.

[14] In the magnetic refrigeration system [9] to

[13] of the present invention, preferably, the time of the heat treatment is 10 seconds or more and 48 hours or less. More preferably, it is 10 minutes or more and 36 hours or less, and the optimal range is 6 hours or more and 30 hours or less. If the heat treatment time is less than 10 seconds, recrystallization does not occur sufficiently. If the heat treatment time exceeds 48 hours, from the perspective of the operation of the heat treatment furnace, the heat treatment cost increases, which is not industrially preferable. Regarding the cooling rate in the heat treatment, preferably, it is 0.2 °C / min or more and 2000 °C / sec or less. More preferably, it is 0.4 °C / min or more and 500 °C / min or less, and the optimal range is 0.8 °C / min or more and 30 °C / min or less. Regarding the heating rate in heat treatment, preferably, it is 0.2 °C / min or more and 1000 °C / min or less, more preferably 0.4 °C / min or more and 500 °C / min or less, and the optimal range is 0.8 °C / min or more and 30 °C / min or less.

Advantages of the Invention

[0013] According to the magnetic refrigeration system of the present invention, it is possible to construct a cooling system using an all-solid cooling element made of all solid materials without using a liquid or gas refrigerant. By using all-solid elements, the magnetic refrigeration system of the present invention eliminates the need to use complex devices such as compressors for gas flow that were incorporated in conventional AMR systems, and also eliminates the need to make the magnetic material spherical in order to improve the heat exchange efficiency between the gas and the magnetic material. Therefore, the system is extremely simplified. Furthermore, in conventional AMR, a certain amount of time was required for heat exchange to flow a liquid or gas, but in the present invention, since solid heat conduction is used, it is possible to realize an extremely high-speed heat cycle.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2A

Figure 2B

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Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0015] FIG. 1 is a configuration diagram for explaining the outline of a magnetic refrigeration system showing an embodiment of the present invention. In the figure, the magnetic refrigeration system of the present invention includes adiabatic vacuum vessels 10, 12, 14, 16, a magnetic refrigeration system housing vessel 18, a refrigerator 20, a refrigerator cold head 22, a heat transfer component 24, an electromagnet 40, a hydrogen gas introduction pipe 50, a heat exchanger 52, and a liquid hydrogen storage vessel 54. The magnetic refrigerator housed in the magnetic refrigeration system housing vessel 18 includes second thermal switches 30a, 30b, 30c using a three-stage all-solid-state cooling element, magnetic bodies (magnetic refrigeration materials) 32a, 32b, 32c, first thermal switches 34a, 34b, 34c, and heat baths 36a, 36b, 36c.

[0016] The heat-insulating vacuum container is a container insulated by a vacuum double structure using, for example, stainless steel or the like. It has high heat retention by vacuum insulation and can minimize the influence on the surroundings whether filled with a refrigerant such as liquid nitrogen or hot water. The heat-insulating vacuum container is composed of a cylindrical peripheral wall portion 10, a connecting bottom plate 12, a magnetic refrigerator housing peripheral wall portion 14, and a bottom plate portion 16. The cylindrical peripheral wall portion 10 houses a refrigerator cold head 22, a heat transfer component 24, and a heat exchanger 52. The connecting bottom plate 12 has a structure that can absorb the difference in inner diameters between the cylindrical peripheral wall portion 10 and the magnetic refrigerator housing peripheral wall portion 14 and maintain a vacuum state. Inside the magnetic refrigerator housing peripheral wall portion 14, a magnetic refrigerator and a liquid hydrogen storage container 54 are housed. The bottom plate portion 16 corresponds to the bottom plate of the magnetic refrigerator housing peripheral wall portion 14. The refrigerator 20 is thermally connected to a second heat switch 30a of a magnetic refrigerator connected via a refrigerator cold head 22 and a heat transfer component 24. The refrigerator 20 can generate liquid hydrogen from the lower end of the magnetic refrigerator by cooling the upper end of the magnetic refrigerator, and for example, a GM (Gifford-McMahon) refrigerator is used.

[0017] Here, the magnetic refrigerator has a three-stage series configuration of a second heat switch 30 using all-solid-state cooling elements, a magnetic body (magnetic refrigeration material) 32, and a first heat switch 34. The all-solid-state cooling elements of each stage are represented by subscripts a, b, and c. The heat bath 36 is located between the all-solid-state cooling elements of each stage and corresponds to the low-temperature side heat bath and the high-temperature side heat bath described in FIG. 4 according to the temperatures at both ends of the all-solid-state cooling elements. The electromagnet 40 is arranged on the circumferential surface of the circumferential wall portion 14 of the magnetic refrigerator housing, and applies a magnetic field to all the solid-state cooling elements constituting the magnetic refrigerator. The application of the magnetic field from the electromagnet 40 to the solid-state cooling elements can be controlled to be turned on and off. By this on-off control of the magnetic field, each stage of the solid-state cooling elements alternately repeats endothermic and exothermic processes. Therefore, the exothermic process of one magnetic body and the endothermic process of the magnetic body operating in a magnetic refrigeration cycle at a higher temperature than that can be combined. That is, when a magnetic field generated by the electromagnet 40 acts on the magnetic body, the magnetic body generates heat. In this process, the thermal switch on the high-temperature side of the magnetic body is outside the magnetic field, and heat conduction is in an on state and becomes good, so the generated heat is transferred to the magnetic body on the high-temperature side, and the magnetic body on the high-temperature side exhausts this heat in the next cycle. The hydrogen gas introduction pipe 50, the heat exchanger 52, and the liquid hydrogen storage container 54 are paths for sending hydrogen gas to the magnetic refrigerator and storing the generated liquid hydrogen.

[0018] In the apparatus configured as described above, it operates as follows. The hydrogen gas sent from the hydrogen gas introduction pipe 50 is precooled by the heat exchanger 52 from room temperature to about 50 K (77 K in the case of liquid nitrogen) by the refrigerator 20 or liquid nitrogen. The precooled hydrogen gas is introduced into the magnetic refrigerator, and all solid-state cooling elements composed of units of heat switch 2 - magnetic body - heat switch 1 are stacked in multiple stages to cool the hydrogen gas step by step. When cooling is performed so that the heat bath 36c at the lowest stage becomes 20 K or lower, the hydrogen gas is liquefied and accumulates in the liquid hydrogen storage container 54.

[0019] Figure 2 is a configuration diagram for explaining the outline of a magnetic refrigeration system showing an embodiment of the present invention. (A) is an explanatory diagram when the magnetic field is turned on by turning on the drive current to the electromagnet, and (B) is an explanatory diagram when the magnetic field is turned off by turning off the drive current to the electromagnet. The drive circuit 44 to the electromagnet as the heat switch control unit 44 switches between the on state 44a and the off state 44b.

[0020] FIG. 3 is a configuration diagram for explaining the outline of a magnetic refrigeration system showing another embodiment of the present invention, and is an explanatory diagram in the case where a magnetic field is turned on and off by relatively moving an electromagnet and an all-solid-state cooling element. For a magnetic refrigerator having a three-stage series configuration of a second thermal switch 30 using an all-solid-state cooling element, a magnetic material (magnetic refrigeration material) 32, and a first thermal switch 34, the position of the electromagnet 40 is moved up and down to turn on and off the applied magnetic field. In the embodiments of FIGS. 1 to 3, a magnetic refrigerator having a three-stage series configuration of all-solid-state cooling elements is shown, but the present invention is not limited thereto, and a magnetic refrigerator having a two-stage series configuration or a magnetic refrigerator having four or more stages may be used. By combining the heat absorption process and the heat rejection process of the magnetic refrigeration cycles of a plurality of magnetic materials in this way, heat can be sequentially conducted, and it is possible to use a heat rejection switch immersed in the refrigerator 20 for the heat rejection of the magnetic material on the hottest side. Therefore, it is suitable for use in the liquefaction of hydrogen gas.

[0021] FIG. 4 is a configuration diagram for explaining the outline of a magnetic refrigeration system using an all-solid-state cooling method showing an embodiment of the present invention. In the figure, a single-stage all-solid-state cooling element is composed of a magnetic refrigeration material 32, a first thermal switch 34, and a second thermal switch 30. When pumping heat from the low-temperature side heat bath 37 to the high-temperature side heat bath 38 in a wider temperature range such as from the hydrogen liquefaction temperature to the LNG liquefaction temperature or room temperature, it is preferable to configure a single-stage all-solid-state cooling element composed of the magnetic refrigeration material 32, the first thermal switch 34, and the second thermal switch 30 in multiple stages. The low-temperature side heat bath 37 and the high-temperature side heat bath 38 are provided as heat baths 36a, 36b, and 36c between, for example, all-solid-state cooling elements configured in three stages as shown in FIG. 1 above. Regarding the heat baths 36a, 36b, and 36c of each stage, whether it becomes the low-temperature side heat bath 37 or the high-temperature side heat bath 38 in that stage is determined by the temperature relationship of the heat bath at the end of the all-solid-state cooling element of each stage. The magnetic field application unit 42 and the thermal switch control unit 44 are commonly provided for each stage of the all-solid-state cooling element.

[0022] For the low-temperature side heat bath 37 and the high-temperature side heat bath 38, materials such as lead and tantalum that exhibit a large specific heat at several tens of K are used. Here, a heat bath refers to an ideal heat source with a large specific heat, and its temperature T does not change even when heat is added or removed. The magnetic refrigeration material 32 uses substances such as HoB 2 and Ho that exhibit a large magnetocaloric effect in the temperature range of several tens of K. For the first heat switch 34, a high-purity polycrystalline metal material (W, Cu, Al, Zn) as described later, whose thermal conductivity changes greatly with a magnetic field, is used. For the second heat switch 30, a material (such as a high-purity alumina material) that does not change its thermal conductivity much with a magnetic field and exhibits a large change in thermal conductivity when the temperature is changed is used.

[0023] The magnetic field application unit 42 simultaneously applies a magnetic field to the magnetic body 30 and the first and second heat switches (34, 30) for excitation, and for example, an electromagnet is used. When a permanent magnet is used for the magnetic field application unit 42, the relative positional relationship between the permanent magnet, the magnetic body 30, and the first and second heat switches (34, 30) may be changed to change the on / off state of the magnetic field application. When the magnetic body 30 generates heat due to excitation, the heat switch control unit 44 turns off the first heat switch 34 connected to the low-temperature side heat bath 37 and turns on the second heat switch 30 connected to the high-temperature side heat bath 38. When the magnetic body 30 absorbs heat due to demagnetization, the heat switch control unit 44 turns on the first heat switch 34 connected to the low-temperature side heat bath 37 and turns off the second heat switch 30 connected to the high-temperature side heat bath 38 for control.

[0024] <Example 1> The temperature change and magnetic field change of the thermal conductivity of a polycrystalline tungsten wire with a purity of 99.999% heat-treated in a vacuum were measured at different heat treatment temperatures. In Examples 1 to 5, the heat treatment time is set to 24 hours. In the cooling after heat treatment, basically no quenching is performed, and it is cooled slowly. The fastest cooling rate is about 20 °C / min, and usually it is cooled at about 1.0 °C / min. Figure 5 shows the dependence of the thermal conductivity temperature change of tungsten polycrystalline wire on the heat treatment temperature, where the magnetic fields are zero magnetic field and 8.5 T. The magnetic field change of the thermal conductivity increases significantly when the heat treatment temperature is 800 °C or higher. Figure 6 shows the dependence of the magnetic field change of the thermal conductivity (temperature 15 K) of tungsten polycrystalline wire on the heat treatment temperature. The magnetic field change of the thermal conductivity increases significantly when the heat treatment temperature is 800 °C or higher. As shown in Figure 5, the thermal conductivity in the zero magnetic field decreases significantly due to the application of the magnetic field.

[0025] Table 1 shows the zero magnetic field thermal conductivity and magnetic field change of pure metal tungsten polycrystalline wire at 15 K. As shown in Table 1, for tungsten wire, the ratio of the thermal conductivity in the zero magnetic field to the thermal conductivity in the magnetic field (Δκ(H)=κ(H = 0) / κ(H)), which characterizes the performance of the thermal switch by performing heat treatment at a temperature exceeding 800 °C, increases rapidly, and the performance of the thermal switch improves dramatically by heat treatment. For example, when the heat treatment temperature in vacuum is less than 600 °C, the thermal conductivity ratio Δκ(H = 8.5 T) is about 3 at 15 K, but when it exceeds 800 °C, the thermal conductivity ratio Δκ increases rapidly, and at 1000 °C, the thermal conductivity ratio Δκ = 20 exceeds that of some single crystal thermal switch materials. Also, by performing heat treatment at a temperature of 800 °C or higher, a ratio of the residual resistance value {R(300 K)} at 300 K to the residual resistance value at 4 K {R(4 K)}, i.e., {R(300 K) / R(4 K)}, of 100 or more can be obtained.

Table 1

[0026] <Example 2> The temperature change and magnetic field change of the thermal conductivity of a polycrystalline copper wire with a purity of 99.9999% heat-treated in vacuum were measured at different heat treatment temperatures. Figure 7 is a diagram showing the heat treatment temperature dependence of the temperature change of the thermal conductivity of a copper polycrystalline plate. The magnetic fields are zero magnetic field and 8.5 T. The magnetic field change of the thermal conductivity increases significantly at a heat treatment temperature of 400 °C. Figure 8 is a diagram showing the heat treatment temperature dependence of the magnetic field change of the thermal conductivity (temperature 15 K) of a copper polycrystalline plate. The magnetic field change of the thermal conductivity increases significantly at a heat treatment temperature of 400 °C. As shown in Figure 7, the thermal conductivity in the zero magnetic field decreases significantly due to the application of the magnetic field.

[0027] Table 2 shows the zero magnetic field thermal conductivity and magnetic field change at 15 K of a pure metal copper polycrystalline wire. As shown in Table 2, for the copper wire, Δκ(H), which characterizes the performance of the thermal switch, increases rapidly by performing heat treatment at a temperature exceeding 300 °C. For example, when the heat treatment temperature in vacuum is less than 200 °C, the thermal conductivity ratio Δκ(H = 8.5 T) is about 3 at 15 K, but when it is 300 °C or higher, the thermal conductivity ratio Δκ becomes 7 or more, and at 400 °C, the thermal conductivity ratio Δκ increases to 14.0, exceeding the performance of a single crystal copper thermal switch material.

Table 2

[0028] <Example 3> The temperature change and magnetic field change of the thermal conductivity of a polycrystalline copper plate with a purity of 99.9999% heat-treated in vacuum were measured at different heat treatment temperatures. Figure 9 is a diagram showing the heat treatment temperature dependence of the temperature change of the thermal conductivity of a copper polycrystalline plate. The magnetic fields are zero magnetic field and 8.5 T. The magnetic field change of the thermal conductivity increases significantly from 300 °C to 600 °C. Figure 10 is a diagram showing the heat treatment temperature dependence of the magnetic field change of the thermal conductivity (temperature 15 K) of a copper polycrystalline plate. The magnetic field change of the thermal conductivity increases significantly from 300 °C to 600 °C. As shown in Figure 9, the thermal conductivity in the zero magnetic field decreases significantly due to the application of the magnetic field.

[0029] Table 3 shows the zero-field thermal conductivity and magnetic field change of a pure metal copper plate at 15K. As shown in Table 3, for the copper plate, the thermal conductivity ratio Δκ(H) that characterizes the performance of the thermal switch increases rapidly by performing heat treatment at a temperature exceeding 300°C, and when heat treatment is performed at a temperature exceeding 700°C, Δκ(H) decreases. For example, when the heat treatment temperature in vacuum is less than 200°C, Δκ(H = 8.5T) is about 3 at 15K, but between 300°C and 700°C, Δκ increases to 9 or more, exceeding the performance of single-crystal copper thermal switch materials.

Table 3

[0030] <Example 4> The temperature change and magnetic field change of the thermal conductivity of a polycrystalline aluminum plate with a purity of 99.999% heat-treated in vacuum were measured at different heat treatment temperatures. The thermal conductivity in zero magnetic field decreases significantly when a magnetic field is applied. Table 4 shows the zero-field thermal conductivity and magnetic field change of a pure metal aluminum plate at 15K. As shown in Table 4, for the aluminum plate, no significant change in the thermal conductivity ratio Δκ was observed due to heat treatment. It is comparable to the performance of single-crystal zinc thermal switch materials.

Table 4

[0031] <Example 5> The temperature change and magnetic field change of the thermal conductivity of a polycrystalline zinc plate with a purity of 99.99% heat-treated in vacuum were measured at different heat treatment temperatures. The thermal conductivity in zero magnetic field decreases significantly when a magnetic field is applied. Table 5 shows the zero-field thermal conductivity and magnetic field change of a pure metal zinc plate at 15K. As shown in Table 5, for the zinc plate, no significant change in the thermal conductivity ratio Δκ was observed due to heat treatment. It is comparable to the performance of single-crystal zinc thermal switch materials.

Table 5

[0032] <Comparative Example> In the prior art of magnetic field-driven thermal switches, attempts have been made to manufacture thermal switches using single crystals of pure tungsten metal (see, for example, Non-Patent Document 1). FIG. 11 shows the magnetic field dependence of the thermal conductivity when a magnetic field is applied to a tungsten single crystal. At a temperature of 4.2 K in zero magnetic field, the thermal conductivity was about 4 W / cmK (= 400 W / mK), and it decreased to about 0.2 W / cmK under a magnetic field of 3 T.

[0033] On the other hand, in addition to tungsten, a decrease in thermal conductivity due to magnetic field application has been reported in pure metal single crystals or polycrystals of copper. Table 6 shows the amount of change in thermal conductivity in pure metal single crystals, and the source of citation is also indicated. [Table 6]

[0034] Also, although pure metals are used as superconducting thermal switches in the temperature range below 1 K (see Non-Patent Documents 2 and 3), superconducting thermal switches cannot be used when targeting hydrogen liquefaction temperature (20.3 K) vicinity or quantum device cooling (tens of mK to tens of K) that requires cooling over a wide temperature range.

[0035] There have also been reports on pure metal polycrystals, but the thermal conductivity ratio Δκ indicating the performance of the thermal switch is as small as about 4 and not practical. (FIG. 12)

[0036] The thermal conductivity and electrical resistance of Examples 1-5 and the comparative example were measured as follows. FIG. 13 is an electrical circuit diagram for explaining the measurement method of thermal conductivity and electrical resistance. Regarding the thermal conductivity measurement, four lead wires (copper) 2 were adhered to the measurement sample 4 with silver paste 3, and 10 -4Place it under adiabatic conditions below torr and supply heat quantity by the heater source 1. At this time, measure the temperature difference (Th - Tl) with the first and second temperature sensors 6 and 7. The fourth lead wire 2 is grounded to the low-temperature heat bath 5. Calculate the thermal conductivity using the input heat quantity, the measured temperature difference, the distance between electrodes and the thickness measured in advance.

[0037] Regarding the electrical resistance measurement, with an arrangement similar to the thermal conductivity measurement, pass an electric current from the heater source 1 to the low-temperature heat bath 5, and measure the voltage generated in the first and second temperature sensors 6 and 7 at this time to measure the electrical resistance. As common matters for the thermal conductivity measurement and the electrical resistance measurement, insert the sample 4 into a cryomagnet that enables temperature change between 5K and 300K and magnetic field generation from 0T to 8.5T, and measure the temperature change. The direction of the magnetic field is perpendicular to the direction of the heat flow flowing through the sample 4. The residual resistance ratio is the value obtained by dividing the electrical resistance value at 4K by the electrical resistance value at 300K.

Industrial Applicability

[0038] The magnetic refrigeration system of the present invention is suitable for use in hydrogen liquefaction technology and quantum device implementation that require heat flow control at extremely low temperatures. The manufacturing method of the material for the heat switch of the present invention is suitable for use in the material for the heat switch of the magnetic refrigeration system suitable for use in hydrogen liquefaction technology and quantum device implementation.

Explanation of Signs

[0039] 10, 12, 14, 16 Heat-insulating vacuum container 18 Magnetic refrigeration medium storage container 20 Refrigerator 30, 30a, 30b, 30c Second heat switch 32, 32a, 32b, 32c Magnetic substance (magnetic refrigeration material) 34, 34a, 34b, 34c First heat switch 36, 36a, 36b, 36c Heat bath 37 Low-temperature side heat bath (cooled part) 38 High-temperature side heat bath 40 Magnet 42 Magnetic Field Application Unit 44 Thermal Switch Control Unit 50 Hydrogen Gas Introduction Pipe 52 Heat Exchanger 54 Liquid Hydrogen Storage Container

Claims

1. A low-temperature side heat bath, A high-temperature side heat bath having a temperature higher than that of the low-temperature side heat bath, A magnetic body that pumps heat from the low-temperature side heat bath side to the high-temperature side heat bath side, A first heat switch that turns on and off the thermal connection between the low-temperature side heat bath and the magnetic body, A second heat switch that turns on and off the thermal connection between the high-temperature side heat bath and the magnetic body, A magnetic field application unit that simultaneously applies a magnetic field to the magnetic body and the first and second heat switches to perform excitation, Comprising, by coordinately controlling the on / off of the first and second heat switches, continuously pumping heat from the low-temperature side heat bath to the high-temperature side heat bath, the cooled part which is the low-temperature side heat bath is continuously cooled, and One of the first or second heat switches uses a polycrystalline metal material composed of any one of tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn), and inevitable impurities, A magnetic refrigeration system characterized by that.

2. Furthermore, when the magnetic body generates heat due to excitation, the first heat switch connected to the low-temperature side heat bath is turned off, the second heat switch connected to the high-temperature side heat bath is turned on, and when the magnetic body absorbs heat due to demagnetization, a heat switch control unit is provided to control so that the first heat switch connected to the low-temperature side heat bath is turned on and the second heat switch connected to the high-temperature side heat bath is turned off, The magnetic refrigeration system according to claim 1, wherein the second heat switch is a switch that turns on due to heat generation and turns off due to heat absorption.

3. The magnetic refrigeration system according to claim 1 or 2, wherein the inevitable impurities are 1000 ppm or less by weight ratio.

4. The magnetic refrigeration system according to any one of claims 1 to 3, wherein the polycrystalline metal tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn) has a thermal conductivity ratio {κ(H = 0) / κ(H = 8.5T)} of the thermal conductivity {κ(H = 0)} in zero magnetic field at 15K and the thermal conductivity {κ(H = 8.5T)} in a magnetic field of 8.5T at 15K is 4 or more.

5. The magnetic refrigeration system according to any one of claims 1 to 3, wherein the polycrystalline metal tungsten (W), copper (Cu), or zinc (Zn) has a thermal conductivity ratio {κ(H = 0) / κ(H = 8.5T)} of the thermal conductivity {κ(H = 0)} in zero magnetic field at 15K and the thermal conductivity {κ(H = 8.5T)} in a magnetic field of 8.5T at 15K is 6 or more.

6. ​ One of the first or second thermal switches is a polycrystalline metal material composed of tungsten (W) and inevitable impurities, and the ratio {R(300K) / R(4K)} of the residual resistance value {R(300K)} at 300 K to the residual resistance value at 4 K {R(4K)} is 100 or more. The magnetic refrigeration system according to claims 1 to 5.

7. The set temperature of the low-temperature side heat bath is in the range of hydrogen liquefaction temperature (20.3 K) ± 2 K. The magnetic refrigeration system according to claims 1 to 6.

8. A method for manufacturing a thermal switch material used for one of the first or second thermal switches used in the magnetic refrigeration system according to claims 1 to 7, The polycrystalline tungsten (W), copper (Cu), aluminum (Al), or zinc of the metal is heat-treated in a vacuum or a reducing atmosphere. A method for manufacturing a thermal switch material.

9. The polycrystalline tungsten (W) is heat-treated at 800 °C or higher and 2300 °C or lower. The method for manufacturing a thermal switch material according to claim 8.

10. The polycrystalline tungsten (W) is heat-treated at 900 °C or higher and 1600 °C or lower. The method for manufacturing a thermal switch material according to claim 9.

11. The polycrystalline copper (Cu) is heat-treated at 250 °C or higher and 800 °C or lower. The method for manufacturing a thermal switch material according to claim 8.

12. The polycrystalline copper (Cu) is heat-treated at 300 °C or higher and 600 °C or lower. The method for manufacturing a thermal switch material according to claim 11.

13. The heat treatment time is 10 seconds or more and 48 hours or less. The method for manufacturing a thermal switch material according to claims 8 to 12.

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