JP7690200B2 - Magnetic refrigeration system using all-solid-state heat switch and method for manufacturing heat switch material - Google Patents

Description

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

Magnetic refrigeration systems, which utilize the phenomenon in which materials such as magnetic materials generate or absorb heat by applying or removing a magnetic field to them, are attracting attention as an alternative to conventional heat cycle systems that utilize the phenomenon of heat generation or absorption caused by the compression and expansion of gases. Until now, cooling of objects to be cooled using the change in heat generated or absorbed by magnetic materials when a magnetic field is applied or removed has been achieved by flowing a gas or liquid refrigerant in one direction. Specifically, when the magnetic material generates heat, the refrigerant is made to flow in one direction, and when the magnetic material absorbs heat, the refrigerant is made to flow in the opposite direction, transporting heat in one direction.

On the other hand, expectations are rising for magnetic refrigeration as a cooling method for liquid hydrogen, which is considered to be one of the candidates for storing renewable energy, and research and development of magnetic refrigeration technology targeting extremely low temperatures is becoming more active. Furthermore, with the recent practical application of quantum devices, there is an increasing demand for efficient cooling methods at extremely low temperatures, even in the range lower than the hydrogen liquefaction temperature. The basic principle of magnetic refrigeration technology is the magnetocaloric effect. The magnetocaloric effect is a phenomenon in which the temperature of a magnetic material changes when an external magnetic field is applied to the magnetic material in an adiabatic state.

As a magnetic refrigeration system, the AMR (Active Magnetic Regenerative Refrigeration) system has been proposed (see Patent Document 1). The AMR system is a method of transporting heat in one direction by flowing a liquid or gas refrigerant and exchanging heat with the magnetic material, which generates heat due to the magnetocaloric effect in the magnetic material. AMR has been demonstrated in the temperature range from room temperature to near the hydrogen liquefaction temperature.
Furthermore, a contact type thermal switch is used for the purpose of cooling a cryogen-free superconducting magnet (see Patent Document 2). Patent Document 3 proposes using single crystal beryllium as a thermal switch.

US20020040583A1 JP 2016-211803 A Japanese Patent Application Publication No. 8-278066

Cryocoolers 13, p567-573 (2004) C. V. Heer and J. G. Daunt, Phys. Rev. 76, 854 (1949) Soviet Physics JETP 21 258 (1965)

Heat flow control at extremely low temperatures plays an important role in hydrogen liquefaction technology and quantum device implementation. Currently, gas gap heat switches, superconducting heat switches, and contact-type heat switches are mainly used to control heat flow in the extremely low temperature range. Gas gap heat switches control heat flow by allowing gas to flow in and out, making it difficult to quickly turn the device on and off. Superconducting heat switches are limited to extremely low temperatures of 1 K or less in operating temperature. Furthermore, contact-type heat switches have drawbacks such as the need for repeated mechanical operations at extremely low temperatures and the large size of the device.

In addition, in Non-Patent Documents 1 to 3 and Patent Document 3, high-purity single crystal thermal switches have been proposed to obtain a magnetic field change with a large thermal conductivity. On the other hand, when the thermal switch is incorporated into a magnetic refrigeration system or other cooling device, it is required to process it into various shapes in order to control the heat flow in the thermal switch. However, single crystal materials have poor processability, and there are practical problems in using them as materials for thermal switches.
There have been reports on pure metal polycrystals, but the thermal conductivity ratio Δκ, which indicates the performance of the thermal switch, is small and impractical, at about 4. In addition, pure metal polycrystals are used as superconducting thermal switches in the temperature range below 1K, but there is an issue that superconducting thermal switches cannot be used near the hydrogen liquefaction temperature (20.3K) or when targeting quantum device cooling (tens of mK to tens of K), which requires cooling over a wide temperature range.

The present invention has been made to solve the above-mentioned problems, and aims to provide a magnetic refrigeration system that uses a magnetic field-driven all-solid-state heat switch, does not use a gas as a refrigerant, is compact, has a wide operating temperature range, and can control heat flow at extremely low temperatures using a magnetic field, and is suitable for cooling a wide temperature range, such as near the hydrogen liquefaction temperature (20.3 K) or a wide range of temperatures from tens to several tens of K.

[1] As shown in FIG. 1, for example, the magnetic refrigeration system of the present invention comprises a low-temperature side heat bath 37, a high-temperature side heat bath 38 which is at a higher temperature than the low-temperature side heat bath 37, a magnetic body 30 which pumps up heat from the low-temperature side heat bath 37 to the high-temperature side heat bath 38, a first thermal switch 34 which 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 which 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 which simultaneously applies a magnetic field to the magnetic body 30 and the first and second thermal switches (34, 30) to excite them.
According to the magnetic refrigeration system of the present invention configured in this manner, by controlling the on/off of the first and second thermal switches (34, 30) in a coordinated manner, heat can be continuously pumped up 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, it is preferable to further provide a heat switch control unit 44 which controls the first heat switch 34 connected to the low-temperature side heat bath 37 to be turned off and the second heat switch 30 connected to the high-temperature side heat bath 38 to be turned on when the magnetic material 30 generates heat due to excitation, and controls the first heat switch 34 connected to the low-temperature side heat bath 37 to be turned on and the second heat switch 30 connected to the high-temperature side heat bath 38 to be turned off when the magnetic material 30 absorbs heat due to demagnetization.

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

[9] The present invention relates to a method for producing a thermal switch material for at least one of the first and second thermal switches (34, 30) used in the magnetic refrigeration systems [3] to [7], wherein the metallic polycrystalline tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn) is heat-treated in a vacuum or in a reducing atmosphere.
[10] In the manufacturing method [9] for a thermal switch material of the present invention, the first thermal switch 34 is preferably a metallic polycrystalline material consisting of tungsten (W) and unavoidable impurities, and is heat-treated at 800°C or more and 2300°C or less.
This is because pure tungsten undergoes primary recrystallization at 1300 to 1500K, followed by secondary recrystallization at 1500 to 1800K.
[11] In the method for producing a thermal switch material of the present invention [10], the metallic polycrystalline tungsten (W) is preferably heat-treated at 900°C or higher and 1600°C or lower.
[12] In the manufacturing method [9] for a thermal switch material of the present invention, preferably, the first thermal switch is a metal polycrystalline material consisting of copper (Cu) and unavoidable impurities, and is heat-treated at 250°C or more and 800°C or less.
[13] In the magnetic refrigeration system [12] of the present invention, preferably, the metallic polycrystalline copper (Cu) is heat-treated at 300° C. or more and 600° C. or less.
[14] In the magnetic refrigeration systems [9] to [13] of the present invention, the heat treatment time is preferably 10 seconds or more and 48 hours or less. More preferably, it is 10 minutes or more and 36 hours or less, and the optimum 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, the heat treatment cost increases from the viewpoint of the operation of the heat treatment furnace, and it is not industrially preferable.
The cooling rate in the heat treatment is preferably 0.2° C./min to 2000° C./sec, more preferably 0.4° C./min to 500° C./min, and optimally 0.8° C./min to 30° C./min.
The heating rate in the heat treatment is preferably 0.2° C./min to 1000° C./min, more preferably 0.4° C./min to 500° C./min, and optimally 0.8° C./min to 30° C./min.

According to the magnetic refrigeration system of the present invention, a system can be constructed that performs cooling using an all-solid cooling element made of all solid materials, without using a liquid or gas refrigerant.
The magnetic refrigeration system of the present invention uses all-solid-state elements, which eliminates the need to use complex equipment such as a compressor for gas flow that was incorporated in conventional AMR systems, and also eliminates the need to make the magnetic material spherical to improve the heat exchange efficiency between the gas and the magnetic material, resulting in an extremely simplified system. Furthermore, while conventional AMR systems require a certain amount of time for heat exchange to move liquid or gas, the present invention uses the thermal conduction of solids, making it possible to achieve extremely high-speed heat cycles.

1 is a configuration diagram illustrating an outline of a magnetic refrigeration system according to an embodiment of the present invention; FIG. 1 is a configuration diagram illustrating an outline of a magnetic refrigeration system according to an embodiment of the present invention, and is an explanatory diagram illustrating a case where an electromagnet is included and a magnetic field is turned on by turning on a drive current. FIG. 1 is a configuration diagram illustrating an outline of a magnetic refrigeration system according to an embodiment of the present invention, and is an explanatory diagram illustrating a case where an electromagnet is included and the magnetic field is turned off by turning off the drive current. FIG. 11 is a block diagram illustrating an outline of a magnetic refrigeration system according to another embodiment of the present invention, and is an explanatory diagram illustrating a case in which a magnetic field is turned on and off by relatively moving an electromagnet and a refrigerator. 1 is a configuration diagram illustrating an outline of a magnetic refrigeration system using an all-solid-state cooling method according to an embodiment of the present invention. FIG. 1 is a graph showing the heat treatment temperature dependence of the thermal conductivity of a tungsten polycrystalline wire with respect to temperature. FIG. 13 is a graph showing the heat treatment temperature dependence of the thermal conductivity magnetic field change (temperature 15 K) of a tungsten polycrystalline wire. FIG. 1 is a diagram showing the heat treatment temperature dependence of the thermal conductivity change of a polycrystalline copper plate material with respect to temperature. FIG. 1 is a graph showing the heat treatment temperature dependence of the thermal conductivity of a polycrystalline copper plate as a function of magnetic field (at 15 K). 1 is a diagram showing the heat treatment temperature dependence of the thermal conductivity temperature change of a copper polycrystalline plate material, with the magnetic field being zero and 8.5 T. The magnetic field change of the thermal conductivity increases significantly when the heat treatment temperature is from 300° C. to 600° C. 1 is a diagram showing the heat treatment temperature dependence of the thermal conductivity magnetic field change (temperature 15 K) of a copper polycrystalline plate, in which the thermal conductivity magnetic field change increases significantly when the heat treatment temperature is from 300° C. to 600° C. FIG. 1 is a diagram showing the magnetic field dependence of the thermal conductivity of a tungsten single crystal. FIG. 1 is a diagram showing the magnetic field dependence of thermal conductivity of polycrystalline copper. FIG. 2 is an electrical circuit diagram illustrating a method for measuring thermal conductivity and electrical resistance.

FIG. 1 is a configuration diagram illustrating an outline of a magnetic refrigeration system according to an embodiment of the present invention.
In the figure, the magnetic refrigeration system of the present invention includes heat-insulating vacuum vessels 10, 12, 14, and 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 inlet pipe 50, a heat exchanger 52, and a liquid hydrogen housing vessel 54. The magnetic refrigerator housed in the magnetic refrigeration system housing vessel 18 includes second thermal switches 30a, 30b, and 30c using a three-stage all-solid-state cooling element, magnetic bodies (magnetic refrigeration materials) 32a, 32b, and 32c, first thermal switches 34a, 34b, and 34c, and heat baths 36a, 36b, and 36c.

The insulated vacuum vessel is a vessel insulated by a vacuum double structure using, for example, stainless steel, and has high heat retention due to vacuum insulation, and can minimize the impact on the surroundings even if a refrigerant such as liquid nitrogen or hot water is poured into it. The insulated vacuum vessel is composed of a cylindrical peripheral wall portion 10, a connecting bottom plate 12, a magnetic refrigerator-accommodating peripheral wall portion 14, and a bottom plate portion 16. The cylindrical peripheral wall portion 10 accommodates a refrigerator cold head 22, a heat transfer part 24, and a heat exchanger 52. The connecting bottom plate 12 is structured to absorb the difference in inner diameter between the cylindrical peripheral wall portion 10 and the magnetic refrigerator-accommodating peripheral wall portion 14 and to maintain a vacuum state. The magnetic refrigerator and a liquid hydrogen storage vessel 54 are accommodated inside the magnetic refrigerator-accommodating peripheral wall portion 14. The bottom plate portion 16 corresponds to the bottom plate of the magnetic refrigerator-accommodating peripheral wall portion 14.
The refrigerator 20 is thermally connected to a second thermal switch 30a of the magnetic refrigerator connected via a refrigerator cold head 22 and a heat transfer part 24. The refrigerator 20 is capable of producing liquid hydrogen from the bottom end of the magnetic refrigerator by cooling the top end of the magnetic refrigerator, and a GM (Gifford McMahon) refrigerator, for example, is used.

The magnetic refrigerator here has a three-stage configuration in which a second thermal switch 30 using an all-solid-state cooling element, a magnetic body (magnetic refrigeration material) 32, and a first thermal switch 34 are connected in series, and the all-solid-state cooling elements in each stage are represented by the subscripts a, b, and c. A heat bath 36 is located between the all-solid-state cooling elements in 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 element.
The electromagnet 40 is disposed on the peripheral surface of the magnetic refrigerator housing peripheral wall 14, and applies a magnetic field to the all-solid-state cooling element that constitutes the magnetic refrigerator. The application of the magnetic field to the all-solid-state cooling element by the electromagnet 40 can be controlled on and off, and by controlling the on and off of the magnetic field, the all-solid-state cooling elements of each stage alternately absorb and release heat, so that it is possible to combine the heat release process of one magnetic body with the heat absorption process of a magnetic body operating in a magnetic refrigeration cycle at a higher temperature. In other words, when the magnetic field generated by the electromagnet 40 is applied to a magnetic body, the magnetic body generates heat, and in this process, the heat switch on the high-temperature side of the magnetic body is outside the magnetic field, and heat conduction is turned on and good, so the heat is transmitted to the magnetic body on the high-temperature side, and the magnetic body on the high-temperature side releases this heat in the next cycle.
The hydrogen gas introduction pipe 50, the heat exchanger 52, and the liquid hydrogen storage vessel 54 form a path for sending hydrogen gas to the magnetic refrigerator and storing the generated liquid hydrogen.

The device configured in this manner operates as follows.
The hydrogen gas sent from the hydrogen gas inlet pipe 50 is pre-cooled in the heat exchanger 52 from room temperature to about 50 K (77 K in the case of liquid nitrogen) using the refrigerator 20 or liquid nitrogen or the like.
Pre-cooled hydrogen gas is introduced into a magnetic refrigerator, and the hydrogen gas is cooled in stages by stacking all-solid-state cooling elements consisting of a thermal switch 2-magnetic material-thermal switch 1 unit in multiple stages.
When the heat bath 36 c in the lowest stage is cooled to 20 K or less, the hydrogen gas is liquefied and collected in the liquid hydrogen storage vessel 54 .

FIG. 2 is a block diagram illustrating an outline of a magnetic refrigeration system according to one embodiment of the present invention, in which (A) is an explanatory diagram for the case where the magnetic field is turned on by turning on the drive current to the electromagnet, and (B) is an explanatory diagram for the case where the magnetic field is turned off by turning off the drive current to the electromagnet.
A drive circuit 44 for the electromagnet, which serves as a thermal switch control section 44, switches between an on state 44a and an off state 44b.

FIG. 3 is a block diagram illustrating an outline of a magnetic refrigeration system according to another embodiment of the present invention, and is an explanatory diagram of a case in which a magnetic field is turned on and off by moving an electromagnet and an all-solid-state cooling element relatively.
The magnetic field applied to the magnetic refrigerator having a three-stage series configuration of a second thermal switch 30 using an all-solid-state cooling element, a magnetic body (magnetic refrigeration material) 32, and a first thermal switch 34 is turned on and off by vertically moving the position of the electromagnet 40. Note that, although the embodiment in Figures 1 to 3 shows a magnetic refrigerator having three stages of all-solid-state cooling elements in series, the present invention is not limited to this, and may be a magnetic refrigerator having two stages in series, or may be a magnetic refrigerator having four or more stages.
In this way, by combining the heat absorption process and heat exhaust process of the magnetic refrigeration cycle of multiple magnetic bodies, heat can be conducted sequentially, and a heat exhaust switch immersed in the refrigerator 20 can be used to exhaust the heat of the magnetic body with the highest temperature, making it suitable for use in liquefying hydrogen gas.

FIG. 4 is a block diagram illustrating an outline of a magnetic refrigeration system using an all-solid-state cooling method according to 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 heat switch 34, and a second heat switch 30. When pumping heat from a low-temperature side heat bath 37 to a 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 heat switch 34, and the second heat 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 the all-solid-state cooling elements configured in three stages, as shown in Fig. 1. The heat baths 36a, 36b, and 36c in each stage are determined to be the low-temperature side heat bath 37 or the high-temperature side heat bath 38 in that stage depending on the temperature relationship of the heat baths at the ends of the all-solid-state cooling elements in each stage. The magnetic field application unit 42 and the thermal switch control unit 44 are provided in common to each stage of the all-solid-state cooling elements.

The low-temperature heat bath 37 and the high-temperature heat bath 38 are made of materials such as lead and tantalum that have a large specific heat at several tens of K. Here, the heat bath refers to an ideal heat source that has a large specific heat and whose temperature T does not change even when heat is input or output.
The magnetic refrigeration material 32 is made of a substance such as _HoB2 or Ho that exhibits a large magnetocaloric effect in the temperature range of several tens of K.
The first thermal switch 34 is made of a high-purity metal polycrystalline material (W, Cu, Al, Zn) as described below, whose thermal conductivity changes significantly depending on the magnetic field.
The second thermal switch 30 is made of a material (such as a high-purity alumina material) whose thermal conductivity does not change significantly with a magnetic field and whose thermal conductivity changes significantly when the temperature is changed.

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

Example 1
The thermal conductivity of a 99.999% pure polycrystalline tungsten wire heat-treated in a vacuum was measured for temperature and magnetic field changes at different heat treatment temperatures. In Examples 1 to 5, the heat treatment time was 24 hours. The wire was cooled slowly after heat treatment without quenching. The fastest cooling rate was about 20°C/min, and the normal rate was about 1.0°C/min.
5 is a graph showing the heat treatment temperature dependence of the thermal conductivity temperature change of a tungsten polycrystalline wire, where the magnetic field is zero and 8.5 T. When the heat treatment temperature is 800° C. or higher, the magnetic field change of the thermal conductivity increases significantly.
Fig. 6 is a diagram showing the heat treatment temperature dependence of the thermal conductivity of a tungsten polycrystalline wire on the magnetic field (temperature 15 K). The magnetic field dependence of the thermal conductivity increases significantly when the heat treatment temperature is 800°C or higher. As shown in Fig. 5, the thermal conductivity in zero magnetic field is greatly reduced by the application of a magnetic field.

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

Example 2
The temperature change and magnetic field change of the thermal conductivity of a 99.9999% pure polycrystalline copper wire heat-treated in a vacuum were measured at different heat treatment temperatures. Figure 7 is a diagram showing the heat treatment temperature dependence of the thermal conductivity temperature change of a copper polycrystalline plate. The magnetic field is 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 thermal conductivity magnetic field change (temperature 15K) 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 zero magnetic field is greatly reduced by the application of a magnetic field.

Table 2 shows the zero-field thermal conductivity and magnetic field change of pure metal copper polycrystalline wire at 15K. As shown in Table 2, in copper wire, Δκ (H), which characterizes the performance of a thermal switch, increases rapidly by performing heat treatment at a temperature exceeding 300°C. For example, when the heat treatment temperature in a vacuum is less than 200°C, the thermal conductivity ratio Δκ (H = 8.5T) is about 3 at 15K, but at 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.

Example 3
The temperature change and magnetic field change of the thermal conductivity of a polycrystalline copper plate with a purity of 99.9999% that was heat-treated in a vacuum were measured at different heat treatment temperatures. Figure 9 is a diagram showing the heat treatment temperature dependence of the thermal conductivity temperature change of the copper polycrystalline plate material. The magnetic field is zero magnetic field and 8.5 T. The magnetic field change of the thermal conductivity increases significantly when the heat treatment temperature is from 300 ° C to 600 ° C. Figure 10 is a diagram showing the heat treatment temperature dependence of the thermal conductivity magnetic field change (temperature 15 K) of the copper polycrystalline plate. The magnetic field change of the thermal conductivity increases significantly when the heat treatment temperature is from 300 ° C to 600 ° C. As shown in Figure 9, the thermal conductivity in zero magnetic field is greatly reduced by the application of a magnetic field.

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, the thermal conductivity ratio Δκ(H), which characterizes the performance of a thermal switch, increases rapidly when the copper plate is heat-treated at a temperature exceeding 300°C, and Δκ(H) decreases when the heat treatment exceeds 700°C. For example, when the heat treatment temperature in a 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 a single crystal copper thermal switch material.

Example 4
The thermal conductivity of a 99.999% pure polycrystalline aluminum plate was measured at different heat treatment temperatures in a vacuum. The thermal conductivity at zero magnetic field was significantly reduced by the application of a magnetic field.
Table 4 shows the zero magnetic field thermal conductivity and magnetic field change of the pure metal aluminum plate at 15 K. As shown in Table 4, the aluminum plate did not show a large change in the thermal conductivity ratio Δκ due to heat treatment. The performance is comparable to that of single crystal zinc thermal switch material.

Example 5
The thermal conductivity of a 99.99% pure polycrystalline zinc plate was measured at different heat treatment temperatures in a vacuum. The thermal conductivity at zero magnetic field was significantly reduced by the application of a magnetic field.
Table 5 shows the zero magnetic field thermal conductivity and magnetic field change of the pure metal zinc plate at 15 K. As shown in Table 5, the zinc plate did not show a large change in the thermal conductivity ratio Δκ due to heat treatment. The performance is comparable to that of single crystal zinc thermal switch material.

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

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

In addition, pure metals are used as superconducting heat switches in the temperature range below 1 K (see non-patent documents 2 and 3), but superconducting heat switches cannot be used near the hydrogen liquefaction temperature (20.3 K) or when targeting quantum device cooling (tens of mK to tens of K), which requires cooling a wide temperature range.

There have been reports on pure metal polycrystals, but the thermal conductivity ratio Δκ, which indicates the performance of the thermal switch, is small at around 4, making it impractical. (Figure 12)

The thermal conductivity and electrical resistance of Examples 1 to 5 and the comparative example were measured as follows: Fig. 13 is an electrical circuit diagram for explaining the method of measuring the thermal conductivity and electrical resistance.
For the thermal conductivity measurement, four lead wires (copper) 2 are attached to the measurement sample 4 with silver paste 3, and placed under adiabatic conditions of 10 ^-4 torr or less, with heat supplied by a heater source 1. At this time, the temperature difference (Th-Tl) is measured by first and second temperature sensors 6, 7. The fourth lead wire 2 is grounded to a low-temperature heat bath 5. The thermal conductivity is calculated using the input heat amount, the measured temperature difference, and the distance and thickness between the electrodes that have been measured in advance.

For the electrical resistance measurement, a current is passed from the heater source 1 to the low-temperature heat bath 5 in the same arrangement as in the thermal conductivity measurement, and the voltage generated in the first and second temperature sensors 6, 7 at that time is measured to measure the electrical resistance.
As a common feature between the thermal conductivity measurement and the electrical resistance measurement, the sample 4 is inserted into a cryomagnet capable of generating a temperature change between 5 K and 300 K and a magnetic field between 0 T and 8.5 T, and the temperature change is measured. The direction of the magnetic field is perpendicular to the direction of the heat flow through the sample 4. The residual resistance ratio is the value obtained by dividing the electrical resistance value at 4 K by the electrical resistance value at 300 K.

The magnetic refrigeration system of the present invention is suitable for use in hydrogen liquefaction technology and quantum device implementation, which require heat flow control at extremely low temperatures.
The method for producing a thermal switch material of the present invention is suitable for use in a thermal switch material for a magnetic refrigeration system suitable for use in hydrogen liquefaction technology and quantum device implementation.

10, 12, 14, 16: Insulated vacuum container 18: Magnetic refrigeration medium container 20: Refrigerator 30, 30a, 30b, 30c: Second heat switch 32, 32a, 32b, 32c: Magnetic body (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 vessel

Claims (13)

A low-temperature side heat bath;
A high-temperature side heat bath having a higher temperature than the low-temperature side heat bath;
A magnetic body that pumps heat from the low-temperature side heat bath to the high-temperature side heat bath;
a first thermal switch that turns on and off a thermal connection between the low-temperature side heat bath and the magnetic body;
a second thermal switch that turns on and off the thermal connection between the high-temperature side heat bath and the magnetic body;
a magnetic field applying unit that simultaneously applies a magnetic field to the magnetic body and the first and second thermal switches to excite them;
The first and second thermal switches are controlled to be turned on and off in a coordinated manner to continuously pump up heat from the low-temperature side heat bath to the high-temperature side heat bath, thereby continuously cooling the cooled part which is the low-temperature side heat bath.
A magnetic refrigeration system, wherein one of the first and second thermal switches uses a metal polycrystalline material made of any one of tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn), as well as unavoidable impurities . a heat switch control unit that controls the first heat switch connected to the low-temperature side heat bath to be turned off and the second heat switch connected to the high-temperature side heat bath to be turned on when the magnetic body generates heat due to excitation, and controls the first heat switch connected to the low-temperature side heat bath to be turned on and the second heat switch connected to the high-temperature side heat bath to be turned off when the magnetic body absorbs heat due to demagnetization ;
The second thermal switch is a switch that is turned on by heat generation and turned off by heat absorption.
The magnetic refrigeration system according to claim 1 . 3. The magnetic refrigeration system according to claim 1, wherein the unavoidable impurities are contained in an amount of 1000 ppm or less by weight. 4. The magnetic refrigeration system according to claim 1, 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 in zero magnetic field at 15K {κ(H=0)} to the thermal conductivity in a magnetic field of 8.5T at 15K {κ(H=8.5T)}, which is 4 or more. 4. The magnetic refrigeration system according to claim 1, 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 in zero magnetic field at 15K {κ(H=0)} to the thermal conductivity in a magnetic field of 8.5T at 15K {κ(H=8.5T)} is 6 or more. 6. The magnetic refrigeration system according to claim 1, wherein one of the first and second thermal switches is a metal polycrystalline material consisting of tungsten (W) and unavoidable impurities, and the ratio of the residual resistance value at 300 K {R(300K)} to the residual resistance value at 4 K {R(4K)} {R(300K)/R(4K)} is 100 or more. 7. The magnetic refrigeration system according to claim 1 , wherein the set temperature of the low-temperature side heat bath is within a range of hydrogen liquefaction temperature (20.3 K) ±2 K. A method for manufacturing a thermal switch material used in one of the first and second thermal switches used in the magnetic refrigeration system according to any one of claims 1 to 7 , comprising the steps of:
The method for producing a thermal switch material, wherein the metallic polycrystalline tungsten (W), copper (Cu), aluminum (Al), or zinc (Zn) is heat-treated in a vacuum or in a reducing atmosphere. The method for producing a thermal switch material according to claim 8 , wherein the metal polycrystalline tungsten (W) is heat-treated at 800°C or more and 2300°C or less. The method for producing a thermal switch material according to claim 9 , wherein the metal polycrystalline tungsten (W) is heat-treated at 900°C or more and 1600°C or less. The method for producing a thermal switch material according to claim 8 , wherein the metal polycrystalline copper (Cu) is heat-treated at 250°C or more and 800°C or less. The method for producing a thermal switch material according to claim 11 , wherein the metal polycrystalline copper (Cu) is heat-treated at 300°C or more and 600°C or less. The method for producing a thermal switching material according to any one of claims 8 to 12 , wherein the heat treatment time is from 10 seconds to 48 hours.