JP2009074774A - Refrigerant-free refrigerating machine and functional thermal binding body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a refrigerant-free refrigerating machine and a functional thermal binding body capable of reaching a very low temperature in a short time by a simple operation, having high operation efficiency and cooling efficiency at low costs, having a simple structure, and being easily maintained. <P>SOLUTION: In the refrigerant-free refrigerating machine and the functional thermal binding body, a columnar first graphite bar 41 made of graphite having heat conductivity rapidly reduced in proportion to power of a temperature toward absolute zero, is disposed between a 40K plate 13 and a 4K plate 14, heat is conducted between the 40K plate 13 and the 4K plate 14 until the first graphite bar 41 reaches a first temperature equilibrium point when a temperature is lowered to 4K, and the 40K plate 13 and the 4K plate 14 are kept in a state of almost thermally separated from each other at the first temperature equilibrium point or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  INDUSTRIAL APPLICABILITY The present invention can reach a cryogenic temperature of several tens of mK (milli Kelvin) class in a short time, is inexpensive, has a simple structure, and is easy to maintain. It relates to a functional thermal coupling body having a suitable passive thermal switching function.

In fields such as physical physics, material science, material engineering, electricity, and electronics, liquid helium ( ⁴ He) has been used as a cryogen for a long time to cool materials to cryogenic temperatures and investigate their physical properties. Although the boiling point of liquid helium at 1 atm is 4.2K, the temperature can be lowered just by lowering the vapor pressure using a vacuum pump, and the lowest reached temperature of about 1K is realized.

  However, when conducting an experiment to examine physical properties with such a cryostat using liquid helium, it took a long time of about one week including preparation of a cryogen until reaching 1 K (Kelvin). There was an annoyance that a large amount of liquid helium had to be made 1K over such a long time for each experiment. One of the problems with this cryostat using liquid helium is that the running cost is expensive, and the second is that it is difficult to handle unless it is a person who has received specialized education because it is difficult to handle. And when this liquid helium is dissipated in the air, it is also a valuable resource that cannot be recovered.

Therefore, recently, the initial installation cost is high, but the running cost is low, and the overall cost is low. The Gifford-McMahon refrigerator (hereinafter referred to as GM refrigerator), pulse tube refrigerator, Stirling Refrigerant-free (Cryogen) that uses cryocoolers such as refrigerators to achieve extremely low temperatures
Free) cryostat has been developed. Here, the refrigerant-free means that no cryogen such as liquid helium or liquid nitrogen is used. The structure of this GM refrigerator will be described in the detailed description of the present invention.

  Of these, the pulse tube refrigerator is composed of a compressor, a radiator, a regenerator, a heat exchanger, and a pulse tube, and the refrigerant is reciprocated between the pulse tube and the compressor for cooling. Refrigerator to perform. A Stirling refrigerator is composed of a displacer and piston, a regenerator, a cylinder surrounding them, and a heat exchanger for high and low temperatures. The refrigerant is compressed by the piston, and then the displacer and piston move to move the refrigerant. The refrigerator is expanded and cooled, and the displacer returns to its original position and repeats this cycle.

  Now, in order to further lower the minimum temperature with a refrigerant-free cryostat, a cryocooler may be thermally connected in series to be multistage (see Patent Document 1). The GM refrigerator disclosed in Patent Document 1 includes a 40K first stage and a 4K second stage. The GM refrigerator is set to about 4K in two stages, and further uses helium gas to achieve a cryogenic temperature. The first stage refrigerator can move a large amount of heat per unit time and has a high cooling capacity (reach the minimum temperature in a relatively short time), but the reached temperature is not so low (40K). The refrigerator has a small cooling capacity (the amount of heat that can be transferred per unit time is small and reaches the minimum temperature over a long period of time), but the minimum temperature itself has a characteristic that the temperature can be lowered (4K). However, the minimum temperature is limited to 2.7 K, and it takes a long time to reach this temperature.

Thus, a refrigerant-free refrigerator that combines this multistage GM refrigerator and a dilution refrigerator has been proposed (see, for example, Japanese Patent Application No. 2006-188676 by the present inventor). In addition, the conventional normal dilution refrigerator has another internal vacuum container that stores a dilution refrigeration unit in which ³ He circulates in an external vacuum container, in which a diluted ⁴ He gas as a cryogen is put, and this The internal vacuum vessel is cooled to about 4K, and the dilution refrigeration unit is cooled through this diluted ⁴ He gas. However, the inventor's refrigerant-free refrigerator is of a refrigerant-free type that utilizes pre-cooling without using an inner container of ⁴ He gas.

The principle of this dilution refrigerator (hereinafter referred to as a dilution refrigerator unit) will be described. First, ³ He gas is cooled to 40K by the first stage of the GM refrigerator, and then cooled to 4K by the second stage. Thereafter, it is further cooled to about 0.7 K and liquefied by a Joule-Thomson valve.

The ³ He liquefied by this Joule-Thomson valve is lowered in temperature by a tube-in-tube heat exchanger and then further by a step heat exchanger, and sent to a mixer. Here, the liquid ³ He becomes a ³ He concentrated phase liquid located in the upper part of the mixer, absorbs heat because entropy increases when ³ He diffuses and dissolves in the ³ He dilute phase liquid located in the lower part, A cryogenic temperature of 20 mK can be realized.

By the way, in the course of research of a refrigerant-free refrigerator combined with this dilution refrigeration unit or a cryostat using the same, the present inventor said that (1) a multistage refrigerator takes time to reach the minimum temperature. (2) In the dilution refrigeration unit, in addition to the difficult problem that the solidified impurity gas clogs with the Joule-Thompson valve unless the impurity gas mixed in ³ He is removed, the impurity gas is In order to discharge, it was necessary to stop the operation, raise the temperature to room temperature, and then restart the operation, and faced the trouble that it took about 5 days to lower the temperature to 20 mK again. In addition, (3) even though the dilution refrigeration unit uses a step heat exchanger to exchange heat, it also faces the problem that it is difficult to improve its heat exchange efficiency.

  In order to solve the problem (1), pre-cooling may be performed. As a technique for performing this, a conventional gas gap thermal switch has been proposed (for example, see Patent Document 1). The gas gap heat switch is configured such that a plurality of metal pieces are arranged with a narrow gap therebetween, a gas is introduced into the gap and turned on by heat exchange with the gas, and the gas is condensed and solidified to be turned off. The gas to be used is nitrogen gas or neon gas, and if it is higher than the solidification temperature of the gas, the gas carries heat, so that heat conduction is turned on, and if it is lower than the solidification temperature, it is in a vacuum state due to gas condensation. The property which becomes.

  With this gas gap heat switch, when the temperature of the object to be cooled is equal to or higher than the solidification temperature of the gas, the first stage having a large cooling power is brought into thermal contact with the object to be cooled, and the temperature is rapidly lowered to be below the solidification temperature. Sometimes, the first stage is cut off, and the cooling power is lowered by the second stage having a small half-surface temperature and independently lowering the temperature of the object to be cooled.

  However, when the gas of the gas gap heat switch condenses, the solidified gas component remains so as to fill the gaps between the plurality of metal pieces, and heat conduction occurs through the solidified gas component, The thermal connection cannot be broken. In order to avoid this problem, Patent Document 2 provides a separate gas solidification chamber that communicates with the inside of the thermal switch, and controls the solidification and vaporization of the gas with a heater to ensure the thermal connection in the gas gap thermal switch. I try to sever.

  However, in order to produce this gas gap thermal switch, a highly precise and precise work that keeps the gap between the metal pieces to several tens of μm is required, which has become a very expensive member. Further, in addition to the necessity of control for turning on and off the thermal switch, if the control timing is wrong, solidification of gas may occur in the thermal switch, and the metal pieces may be bridged. Furthermore, since there is a stagnation of gas, the mounting direction is limited.

  And the thing which provided the graphite in parallel with the superconducting lead heat switch of the one shot cooling type adiabatic demagnetization refrigerator is also proposed (nonpatent literature 1). Graphite is a support material for preventing the temperature of the sample from rising due to vibration. At the same time, when the adiabatic demagnetization salt and the sample are pre-cooled from room temperature, the thermal conductivity of lead is approximately higher than the liquid nitrogen temperature (77 K). Utilizing the fact that the thermal conductivity of graphite is larger, it is used as a shunt of a superconducting lead thermal switch for precooling faster from room temperature to several tens of K without using a heat exchange gas. Since this adiabatic demagnetization refrigerator operates in a temperature range of 1K or less and is one-shot cooling, the temperature of the sample rises even when a slight heat inflow of the order of 10 μW is made, and the low temperature cannot be maintained. However, since graphite is a perfect thermal insulator with 1K and a thermal conductivity of 1 / 10,000 that of superconducting lead, that is, almost zero, heat flow through it is zero. That is, an on state in a temperature range from room temperature to several tens of K and a completely off state of about 1 K or less are used as passive thermal switches. In the meantime, an intermediate state that continuously changes from on to off in a temperature range of several tens of K to about 1 K is not actively used.

Next, regarding (2) above, it is a well-known technique to remove foreign matter with a filter. Also in the dilution refrigeration unit, mixed impurity gases such as air (nitrogen, oxygen) and moisture are problematic. These become liquefied and solidified as impurities at extremely low temperatures. In the past, ³ He was adsorbed on activated carbon by passing it through a liquid nitrogen trap cooled with liquid nitrogen (77 K). However, preparing such liquid nitrogen separately and maintaining it increases troublesome work. Although the refrigerant-free refrigerator has a great merit that it does not use liquid helium or liquid nitrogen, the use of liquid nitrogen, which is a kind of refrigerant, in the trap greatly impairs this merit.

  The problem (3) is a very important problem in terms of improving the capacity of the dilution refrigeration unit. At present, there is a limit in using only cupronickel (Cu—Ni) having a small thermal conductivity as a partition wall. The conventional step heat exchanger is described in Non-Patent Document 2 and the like.

Japanese Patent No. 2835305 Japanese Patent No. 3881675 F.J. Shore, et al., “Use of Graphite as Low Temperature Support and Shunt for Heat Switch”, THE REVIEW OFSCIENTIFIC INSTRUMENT, Vol. 31, No. 9, November 1960, p. 970 Ukrainian Academy of Sciences, Institute of Low Temperature Physics Engineering, Hideki Yayama, co-translated by I.B. Berktokh, "Ultra Low Temperature Experimental Technology", Kyushu University Press, October 1, 2000, p. 50-p. 53

  As described above, it has been conventionally proposed to use a thermal switch in order to perform precooling in a short time. However, the thermal switch of Patent Document 2 requires a highly accurate work of several tens of μm, and is an expensive device. Moreover, it was inevitable that the solidified gas bridged between the metal pieces. In addition, control for turning on and off the thermal switch is necessary, and there is gas convection inside the thermal switch, so that the mounting direction is limited.

  In addition, the graphite provided in parallel with the superconducting lead thermal switch described in Non-Patent Document 1 has a thermal conductivity of lead at about the liquid nitrogen temperature (77 K) or higher when the adiabatic demagnetizing salt and the sample are precooled from room temperature. By utilizing the fact that the thermal conductivity of graphite is higher, it is for precooling faster from room temperature to several tens of K without using heat exchange gas. On the other hand, in a temperature range of about 1K or less where the adiabatic demagnetizer operates, one-shot cooling is performed and the heat capacity of the substance is extremely small, so the temperature of the sample continues to rise even with a slight heat inflow of the order of 10 μW and keeps the low temperature. Can not. For this reason, graphite has the property of being a perfect thermal insulator with a thermal conductivity of 1K or less and almost zero thermal conductivity (1K is 1 / 10,000 of superconducting lead). As a member to be used, it is used as a thermal switch that is completely off (on from several tens of K to room temperature) below about 1K. In other words, in the temperature range between about OFF and ON (about 1 K to several tens of K), there is no clear temperature that changes continuously from the low heat conduction state to the high heat conduction state as the temperature rises and transitions from off to on. Not actively used. In other words, since there is an intermediate region of about 1K to several tens of K that conducts heat at halfway that is neither on nor off, it cannot be generally used for a refrigerator.

In the dilution refrigeration unit, it is very important to remove impurities mixed in the circulating ³ He. If the impurities are not discharged, they will be clogged in a narrow flow path showing high impedance. Once the operation is stopped in order to remove this, it will take about 5 days to resume the operation and lower the temperature to 20 mK again. Such impurities inevitably lower the cooling rate and cooling capacity.

  Moreover, in a dilution refrigeration unit, unless the heat exchange efficiency of the step heat exchanger located in the lower stage (low temperature side) of the multiple stages is improved, the overall thermal efficiency is deteriorated and the minimum temperature is increased.

  Therefore, the present invention can reach extremely low temperatures with a simple operation in a short time, has a high efficiency of operation, is inexpensive, has a high cooling efficiency, has a simple structure, and is easy to maintain. An object is to provide a thermal bond.

  The refrigerant-free refrigerator of the present invention includes a first cooling stage that can lower the temperature to the first minimum temperature, and a second temperature that can lower the temperature to a second lowest temperature that is lower than the first lowest temperature. And a cooling stage for cooling an object to be cooled in the vacuum vessel using the first and / or second cooling stage, the first cooling stage and the second cooling stage. Passive thermal switch function between refrigerant-free refrigerator stages made of solid material with thermal conductivity that decreases rapidly in proportion to the temperature drop toward absolute zero below a predetermined temperature When the object to be cooled is lowered to a temperature lower than or equal to the first minimum temperature, heat conduction is performed between the first cooling stage and the second cooling stage, and the object to be cooled is disposed. When the temperature of the first cooling stage is high, the thermal coupling degree is high, and as the first temperature equilibrium point is approached, the first cooling stage and the second cooling degree are increased. To mainly characterized in that to abruptly lower state thermal coupling degree between the stages.

  Moreover, the functional thermal coupling body of this invention is arrange | positioned between the non-refrigeration refrigerator in a vacuum vessel, and a to-be-cooled object, and is a continuous operation refrigerator which conducts heat between a non-refrigeration refrigerator and a to-be-cooled object. Functional thermal coupling body for use, having a thermal conductivity that decreases rapidly in proportion to the temperature drop toward absolute zero below a predetermined temperature, and when the temperature is high, the thermal coupling degree is high. It is characterized by comprising a solid material having a passive heat switch function for a refrigerant-free refrigerator whose thermal coupling degree is drastically lowered when the temperature becomes low.

  According to the refrigerant-free refrigerator of the present invention and the functional thermal coupling body, it is possible to reach an extremely low temperature with a simple operation in a short time, high operation efficiency, low cost, high cooling efficiency, and a simple structure. Easy maintenance.

  The first aspect of the present invention includes a first cooling stage capable of lowering the temperature to the first minimum temperature, and a second temperature lowering to a second minimum temperature lower than the first minimum temperature. And a cooling stage for cooling an object to be cooled in the vacuum vessel using the first and / or second cooling stage, the first cooling stage and the second cooling stage. Passive thermal switch function between refrigerant-free refrigerator stages made of solid material with thermal conductivity that decreases rapidly in proportion to the temperature drop toward absolute zero below a predetermined temperature When the object to be cooled is lowered to a temperature lower than or equal to the first minimum temperature, heat conduction is performed between the first cooling stage and the second cooling stage, and the object to be cooled is disposed. When the temperature is high, the thermal coupling degree is high, and as the first temperature equilibrium point is approached, the first cooling stage and the second cooling are performed. A cryogen free cryocooler, characterized by the rapid low state thermal coupling degree between. With this configuration, the cryogenic temperature can be reached in a short time with a simple operation, and the operation efficiency is high, the cost is low, the cooling efficiency is high, and the maintenance is easy with a simple structure.

  The second aspect of the present invention is a form subordinate to the first aspect, and includes a third cooling stage that can lower the temperature to a third minimum temperature lower than the second minimum temperature, A second functional thermal coupling body is disposed between the cooling stage and the third cooling stage, and the second functional thermal coupling body reaches the second temperature equilibrium point with the second cooling thermal stage and the third cooling stage. The third cooling stage is thermally coupled, the degree of thermal coupling is high when the temperature of the third cooling stage is high, and the second cooling stage as the temperature of the third cooling stage approaches the second temperature equilibrium point And a third cooling stage, the degree of coupling is rapidly lowered. With this configuration, the cryogenic temperature can be further reached by the third cooling stage.

  The third aspect of the present invention is a form subordinate to the first or second form, and in order to maintain the object to be cooled in a low temperature state, the first stage or the first stage in which the temperature equilibrium point decreases sequentially between the respective cooling stages. Two or more intermediate stages are provided, and a functional thermal coupling body is provided between the intermediate stages and between the intermediate stage and the cooling stage. With this configuration, the intermediate stage is sequentially thermally separated from the high temperature side as the temperature drops, and the cryogenic temperature can be maintained independently.

A fourth form of the present invention is a form subordinate to the second or third form, wherein the third cooling stage forms a phase interface between a ³ He rich phase and a ³ He dilute phase, and ³ He atoms are A mixer of a dilution refrigeration unit that cools an object to be cooled by a change in entropy when diffusing into a ³ He dilute phase liquid, which is cooled by a first cooling stage, a second cooling stage, and a Joule Thomson heat exchanger ³ He gas is liquefied with a Joule-Thomson valve and heat-exchanged with the still, then introduced into the mixer through a heat exchanger, cooled by entropy change, and ³ He atoms are transferred from the mixer to the still through the heat exchanger The refrigerant-free refrigerator is characterized by being selectively evaporated. With this configuration, an extremely low temperature of 20 mK class can be achieved.

Fifth aspect of the present invention is in a form dependent on the fourth aspect, the step heat exchanging heat between ³ He solution and the mixer to be introduced into the mixer with ³ He liquid moving to still The step exchanger includes a silver plate serving as a partition, a pair of sintered silver sandwiching the silver plate, and a pair of lids covering the sintered silver, and a silver plate and a lid. Is a refrigerant-free refrigerator characterized by being fastened by a fastening member. With this configuration, a silver plate with high thermal conductivity can be used for the partition wall, so that the heat exchange rate of the step heat exchanger can be greatly improved, and even when superfluid ³ He leaks, it can be fastened. It becomes possible to repair by retightening the body.

A sixth aspect of the present invention is a form dependent on any one of the first to fifth aspects, wherein a trap is provided in a flow path for circulating vaporized ³ He, and the trap is provided in the first cooling stage. It is a refrigerant freezer characterized by being provided in. With this configuration, there is no need to separately cool the trap as in the prior art, and the refrigerant freezer is simple and easy to use.

  A seventh aspect of the present invention is a refrigerant-free refrigerator, which is a form subordinate to the sixth aspect, wherein the trap is provided with a heater for vaporizing impurities. With this configuration, the impurities are gasified by locally heating and released using a part of the flow path, so that it is not necessary to stop the operation of the refrigerant-free refrigerator when discharging the impurities.

An eighth aspect of the present invention is a form dependent on any one of the first to seventh aspects, wherein the flow path for circulating the vaporized ³ He is the first flow path, and the first flow path. The helium gas branched into the second flow path for pre-cooling with lower impedance flows through the second flow path only at the time of pre-cooling, and flows into the dilution refrigeration unit through the helium gas cooled in the first and second cooling stages. A refrigerant-free refrigerator that precools a dilution refrigeration unit. With this configuration, it is possible to rapidly cool with a large amount of helium gas using the second flow path, instead of the temperature drop always in the first flow path with high impedance.

  The ninth aspect of the present invention is a function for a continuous operation refrigerator that is disposed between a refrigerant-free refrigerator and a cooled object in a vacuum vessel and conducts heat between the refrigerant-free refrigerator and the cooled object. A thermal conductivity that has a thermal conductivity that decreases rapidly in proportion to the temperature drop toward absolute zero below a predetermined temperature, and when the temperature is high, the degree of thermal coupling is high and the temperature is low. A functional thermal coupling body comprising a solid material having a passive thermal switching function for a refrigerant-free refrigerator in which the thermal coupling degree rapidly decreases. With this configuration, the cryogenic temperature can be reached in a short time with a simple operation, the operation efficiency is high, the cost is low, the cooling efficiency is high, and the maintenance is easy with a simple structure.

  The tenth aspect of the present invention is arranged between the high-temperature side cooling stage and / or intermediate stage and the low-temperature side cooling stage and / or intermediate stage of the refrigerant-free refrigerator in the vacuum vessel, A functional thermal coupling body for a continuous operation refrigerator that conducts heat in a cooling stage and / or an intermediate stage and a low-temperature side cooling stage and / or an intermediate stage, the temperature of the temperature lower than a predetermined temperature toward absolute zero Passive thermal switch for refrigerant-free refrigerator stages that has a thermal conductivity that decreases rapidly in proportion to drought, and that has a high degree of thermal coupling when the temperature is high and suddenly decreases when the temperature is low It is a functional thermal coupling body characterized by comprising a solid substance having a function. With this configuration, the cryogenic temperature can be reached in a short time with a simple operation, the operation efficiency is high, the cost is low, the cooling efficiency is high, and the maintenance is easy with a simple structure. “Cooling stage and / or intermediate stage” means “cooling stage and intermediate stage” or “cooling stage or intermediate stage”.

  An eleventh aspect of the present invention is a form subordinate to the ninth or tenth aspect, wherein the solid material is any one of graphite, alumina, and beryllia. It is a conjugate. With this configuration, it is possible to reliably reach an extremely low temperature in a short time with a simple operation, and the operation efficiency is high, the cost is low, the cooling efficiency is high, and the maintenance is easy with a simple structure.

(Embodiment 1)
Hereinafter, the refrigerant-free refrigerator and the thermal switch according to Embodiment 1 of the present invention will be described. The refrigerant-free refrigerator of the first embodiment is a refrigerant-free refrigerator including a dilution refrigerator unit, and realizes cryogenic temperature by providing a dilution refrigerator unit thermally connected in series with a multistage GM refrigerator. . The dilution refrigeration unit realizes extremely low temperature by using a dilution refrigeration action such as ³ He. And although GM refrigerator is demonstrated below, it will be the same if not only this but the pulse tube refrigerator which does not use a cryogen, a Stirling refrigerator, etc. if it is another stage type.

  1 is a configuration diagram of a refrigerant-free refrigerator in Embodiment 1 of the present invention, FIG. 2 is a configuration diagram of a step heat exchanger of the refrigerant-free refrigerator in Embodiment 1 of the present invention, and FIG. 3 is a graph of graphite and alumina. 4 is a graph showing the thermal conductivity in the low temperature range, FIG. 4 is a graph showing the characteristics of the graphite thermal bond, and FIG. 5 is a graph comparing the cooling time with and without the presence of the graphite thermal bond.

  In FIG. 1, reference numeral 1 denotes a cryostat vacuum vessel provided with a refrigerant-free superconducting magnet. The vacuum vessel 1 is depressurized by a vacuum pump to eliminate heat transfer with the outside through gas. Reference numeral 2 denotes a heat radiation shield, which cuts off heat radiation from the outside and thermally insulates the heat radiation shield 2. In the first embodiment, there is a need for a high degree of heat insulation, and a double heat radiation shield is provided. Reference numeral 2a denotes a second heat radiation shield which is disposed inside the heat radiation shield 2 and doubles heat from outside. Reference numeral 3 denotes a GM refrigerator, which is a multistage refrigerant-free refrigerator, and in the case of the first embodiment, has two stages including a 40K stage and a 4K stage. Furthermore, the number of stages can be increased.

  Here, the basic configuration of the GM refrigerator will be described. Although a detailed configuration is not shown, the cylinder includes a cylinder that can be filled with gas, and a displacer that divides the inside of the cylinder into an upper space and a lower space. It is reciprocated by the drive mechanism. The multi-stage GM refrigerator 3 is provided with such a basic configuration in two stages in series. The first stage cools to 40K, and the second stage cools to 4K. The periphery of each stage is a heat exchanging part, and it is possible to constantly cool the contacted members to 40K and 4K, respectively.

Subsequently, the refrigerant-free refrigerator shown in FIG. 1 will be described. 4 is a ³ He tank filled with refrigerant ³ He gas, 5 is a sample mounting device for mounting a sample in the first embodiment, 5 is a sample mounting device for mounting a sample in Example 1, and 5a is a sample mounting device 5. The supporting upper support cylinder 5b is a lower support cylinder supporting the sample mounting device 5. The sample mounting device 5 will be described later.

6 No refrigerant superconducting magnet for performing measuring physical properties of a sample in a magnetic field consist of superconducting coils, the vacuum pump 7a is to vaporize under reduced pressure the ³ He in liquid, the ³ He gas 7b is vaporized by the vacuum pump 7a pressurizing compressor, 7c impurity gas separated by the activated carbon trap 20 vacuum pump for discharging (nitrogen, oxygen, moisture or the like gas vaporized), 8 a ³ He gas impurities are mixed, details will be described later However, in terms of code, when purifying by circulation in the order of 4 → 17g → 7b → 16 → 17d → 20a → 20 → 17e → 8 → 7a → 17g → 4, the impedance of the flow is used to extremely reduce the circulation speed. is there. Note that the impedance 8 is not necessarily provided if the opening of the valve 17e can be finely adjusted. Reference numeral 9 denotes a low-temperature plate on which a mixer 51 (to be described later) is placed and heat exchange is performed by thermally and physically contacting the tip of the sample mounting device 5.

  Although the refrigerant-free superconducting magnet 6 of the first embodiment is for measuring physical properties, it can be a refrigerant-free superconducting magnet of a magnetic refrigerator for cascade arrangement.

  Now, the detailed structure of the refrigerant-free refrigerator will be described below. Reference numeral 10 denotes a flange that covers the upper portion of the vacuum vessel 1 with a flange, and supports the GM refrigerator 3 and the sample mounting device 5. 11 is a first stage (first cooling stage of the present invention) that is 40K (first minimum temperature of the present invention) of the GM refrigerator 3, and 12 is 4K of the GM refrigerator (second minimum temperature of the present invention). ) Is a second stage (second cooling stage of the present invention). Reference numeral 13 denotes a copper 40K plate which closes the upper part of the heat radiation shield 2 and exchanges heat with the first stage of the GM refrigerator 3 to reach a temperature around 40K, and 14 exchanges heat with the second stage of the GM refrigerator 3. The copper 4K plate 14a having a temperature close to 4K is a flexible heat conductor that absorbs the vibration of the GM refrigerator 3 and transfers the heat of the 40K plate 14 to the second stage.

Next, 15 is an exhaust pipe that guides ³ He gas vaporized from a still 52 constituting a dilution refrigeration unit, which will be described later, and is connected to a vacuum pump 7a. Reference numerals 16, 16a and 16b are flow paths for guiding the ³ He gas pressurized by the compressor 7b through the vacuum pump 7a to the mixer 51 (described later) of the dilution refrigerator, and the flow path 16 is divided into two. The other channel 16a is a channel having a small impedance for flowing helium gas only during the pre-cooling operation, and the other channel 16b divided into two is a channel for flowing ³ He during the normal operation.

  In the conventional precooling method, a small amount of helium gas is introduced into a vacuum vessel immersed in a cryogenic liquid helium, and heat exchange is performed between the outer wall of the dilution refrigeration unit and the inner wall of the 4K vacuum vessel using helium gas. The dilution refrigeration unit is cooled to 4K. In contrast, the refrigerant-free refrigerator of Embodiment 1 precools the cold helium gas cooled by the heat exchangers 18b and 19b by flowing it through the dilution refrigeration unit, bypassing the high impedance portion of the circulation line. .

The reason why the flow is bypassed using the flow path having such a small impedance is as follows. The dilution refrigerator of the first embodiment liquefies ³ He using the Joule-Thomson effect. At this time, the flow rate must be reduced. That original flow path 16b becomes very high impedance of the flow path enough to liquefy the ³ the He. For this reason, when cooling only by the flow path 16b, since the flow rate is very small, it takes a long time to reach a low temperature. The flow path 16b alone is hardly cooled even over several days, and cannot be practically used.

  Therefore, in the first embodiment, the temperature of the dilution refrigeration unit, the low temperature plate 9, the sample holder 30, the sample 31, the heater 32, and the temperature sensor 33 is set to 4K in a short time using the low impedance channel 16a. The flow path is closed and the cryogenic temperature is realized by the flow path 16b. Thereby, circulation type pre-cooling without using heat exchange gas becomes possible. At this time, the flow path 16a is an additional precooling path, and the flow path 16b is an original flow path of the dilution refrigeration unit. This device only constitutes a simple bypass path for pre-cooling, eliminates the need for a large internal vacuum vessel that accommodates heat exchange gas, and is easy to use, efficient in operation, maintenance, and cost. It surpasses conventional technology.

Reference numerals 17a, 17b, 17c, 17d, 17e, 17f, and 17g denote valves. The valves 17a and 17b are valves for switching between the flow paths 16a and 16b, and the valve 17f forms a flow path for discharging impurities (nitrogen, oxygen, moisture) separated by the activated carbon trap 20. The valve 17d is also a valve for setting the circulation amount of ³ He gas. The valve 17e is a valve that is closed except when an impurity discharge circuit is formed, and the valve 17g is a valve for replenishing ³ He and ⁴ He gases.

Reference numerals 18a and 18b denote heat exchangers that are provided in thermal contact with the 40K plate 13 and make ³ He gas pressurized by the compressor 7b substantially the same temperature as the 40K plate 13. 19a and 19b are provided in thermal contact with the 4K plate 14, and ³ He gas sent through the heat exchanger 18a, the heat exchanger 18b, and the activated carbon trap 20, respectively, is almost the same as the 4K plate 14. It is a heat exchanger that makes temperature.

This activated carbon trap 20 (see also the enlarged view of FIG. 1) is a metal container with activated carbon inside, and is used to remove impurities solidified by nitrogen gas, oxygen, water vapor, etc. in the air in ³ He. It is done. 20a is a heat exchanger for pre-cooling provided on the side opposite to the side where the activated carbon trap 20 is in contact with the 40K plate 13, and 20b is made of an alloy such as brass or stainless steel. It is a thermal coupling body having small characteristics. The thermal coupling body 20 b transmits the heat of the activated carbon trap 20 and the heat exchanger 20 a to the 40K plate 13. 20c is a heater (heater of the present invention) used when nitrogen, oxygen, moisture, etc. adsorbed on activated carbon are desorbed and diffused. By heating, the adsorbed gas is vaporized and can be discharged using a vacuum pump. In addition, silica gel, a molecular sieve, zeolite, etc. with a large effective surface area can be used instead of activated carbon.

Here, the details of the activated carbon trap 20 will be described. The dilution refrigeration unit circulates ³ He to condense and vaporize it, and during this time heat is taken away by the dilution refrigeration action to realize a refrigeration cycle. Accordingly, when impurities such as nitrogen, oxygen, and moisture adsorbed on the inside of the pipe are mixed into the ³ He gas during operation, it is cooled and solidified on the low temperature side of the dilution refrigerator, and the JT valve 22 It becomes clogged with the impedance part. For this reason, conventionally, a method of adsorbing on activated carbon cooled with liquid nitrogen at a temperature of 77 K has been taken. However, since liquid nitrogen has to be prepared separately, there is an inconvenience that extra costs are generated and that it is necessary to replenish when liquid nitrogen evaporates and decreases.

Therefore, in the first embodiment, the activated carbon trap 20 is cooled using the 40K plate 13 as a low heat source utilizing the fact that the first stage 11 of the GM refrigerator is 40K. The activated carbon trap 20 is cooled to about 40K through a thermal coupling body 20b such as brass. Further, the circulating ³ He gas is preliminarily lowered in temperature using a heat exchanger 20 a provided on the upper part of the activated carbon trap 20, and is introduced into the activated carbon trap 20. Thereby, impurities can be reliably removed from the ³ He gas.

When the impurities are discharged, the valves 17a, 17b, 17c, 17d, 17e, and 17g are closed to stop the circulation of ³ He gas and the valve 17f is opened. In this state, since the temperature of the adsorption trap 20 is low, the impurity gas remains adsorbed. In discharging, the heater 20c is first energized to raise the temperature of the activated carbon trap 20 locally to room temperature. As a result, the adsorbed or solidified impurities are vaporized, and are discharged to the outside by the vacuum pump 7c through the temporarily formed discharge passage. In addition, since the electric power applied to the heater is small and the thermal conductivity of the thermal coupling body 20b is not so large, the temperature of the 40K plate 13 hardly increases.

Thus, in the refrigerant-free refrigerator of the first embodiment, when discharging impurities, a local discharge channel is formed separately from the circulation channel, and the impurities are released by locally raising the temperature of the activated carbon trap 20. Therefore, it is not necessary to stop the operation of the GM refrigerator 3 during this period, and the circulation of ³ He gas can be started immediately after the impurities are discharged, and the dilution refrigeration operation can be resumed in a short time. When the activated carbon trap is not equipped with a heater as in the prior art, it took a long time to stop and restart the GM refrigerator to perform the discharge process, but it is not necessary to stop the GM refrigerator.

Then, 21 are coiled in the exhaust pipe 15 within, JT (Joule-Thomson) heat exchanger for exchanging heat between ³ He which is cooled by the heat exchanger 19b and the cold ³ He gas increases the exhaust pipe 15 The vessel 22 is a JT (Joule-Thomson) valve.

The JT valve 22 is expanded after the flow rate of the refrigerant is reduced, and the temperature is lowered by the pressure drop at this time so as to be cooled and liquefied. Accordingly, the ³ He gas that has been pressurized by the compressor 7b is cooled by the heat exchangers 18b and 19b in the cooling passage 16b, and then cooled by the JT heat exchanger 21 and then expanded by the JT valve 22 except during the pre-cooling operation. Cooled and liquefied.

Here, the precooling path of the dilution refrigeration unit and the circulation path of the refrigeration cycle will be further described. As described above, the flow path 16 is divided into the flow path 16a and the flow path 16b. One of the flow paths 16a is a precooling path that allows ³ He to flow only during the precooling operation, and the other flow path 16b is ³ He during normal operation. It is used as a flow path for flowing water.

Both the type using the cryogen and the non-refrigerant type using no cryogen once precool the internal dilution refrigeration unit to 4K and then start the dilution refrigerator. For the cryogen type, the vacuum container containing the dilution refrigeration unit is immersed in the cryogen (liquid ⁴ He). First, the vacuum container is filled with a very small amount of ⁴ He gas as a heat exchange gas. Is cooled to 4K. Then, heat is exchanged between the inner surface of the vacuum vessel and the outer wall surface of the dilution refrigeration unit, and the dilution refrigeration unit is cooled to 4K. Thereafter, in order to bring the diluted refrigeration unit into an adiabatic state, the heat exchange gas is exhausted by a vacuum pump over about half a day to one day, and the inside of the internal vacuum vessel is made a vacuum environment. For this reason, it takes extra time to precool the cryogen type.

However, the dilution refrigeration unit of the first embodiment uses a multistage GM refrigerator, is a refrigerant-free type, and cools ³ He to about 4K in the second stage (4K) of the GM refrigerator. Since a refrigerant-free GM refrigerator is used, handling is much easier than when a cryogen is used, and the structure is functionally simple. Further, since the pre-cooling method is a helium gas circulation type, the pre-cooling time is shortened by the amount that the time for exhausting the ⁴ He gas for heat exchange becomes unnecessary.

  In the dilution refrigeration unit, the JT valve 22 is a fluid element having such a high impedance that 1 cc of gas flows over several minutes. Therefore, cooling using the flow path 16b in which the JT valve 22 is disposed requires an enormous amount of time (almost no cooling over several days) until the minimum temperature is reached.

  Therefore, in the dilution refrigeration unit of the first embodiment, the flow path 16 a is provided in parallel to the portion where the JT valve 22 is disposed, and is between the valves 17 a and 17 b and the heat exchanger 53. During the pre-cooling operation, the valve 17a is opened in addition to the valve 17b, and helium gas is allowed to flow using the low-impedance flow path 16a as the pre-cooling path, and the temperature of the portion disposed below the 4K plate is brought to about 4K in a short time. Thereafter, when the valve 17a is closed, only the valve 17b is opened, and the temperature is lowered by the high-impedance channel 16b to realize a cryogenic temperature.

By the way, in the dilution refrigeration unit of Embodiment 1, ³ He and ⁴ He are mixed by the mixer 51, and temperature falls. That is, when the temperature of the mixed solution becomes 0.6K or lower, the mixed solution is separated into two phases, below the ³ He dilute phase solution having a low ³ He concentration and high specific gravity, and above the ³ He concentration. The formation of a ³ He concentrated phase liquid having a high specific gravity and a low specific gravity is utilized. In a dilution refrigerator, ³ He is dissolved (diffused) from a light ³ He concentrated phase liquid into a heavy ³ He dilute phase liquid, and heat absorption (dilution refrigeration action) based on the entropy difference between the two phases occurs, resulting in extremely low temperature ( The third minimum temperature of the present invention is realized. As a result, the temperature drops to around 20 mK.

In FIG. 1, 51 is a mixer (the third cooling stage of the present invention) in which ³ He gas is condensed by the JT valve 22 and mixed with ⁴ He inside when returning as a high concentration condensate of ³ He. It is. Reference numeral 52 denotes a still, and the still 52 is connected to the mixer 51 via a tube-in-tube heat exchanger 55 and a step heat exchanger 56 described later. The exhaust pipe 15 is connected to the still 52, and the vaporized ³ He gas is guided through the exhaust pipe 15 and then to the vacuum pump 7a. Still 52, by vaporizing at 0.7K of ³ He is faster high vaporization saturated vapor pressure compared to ⁴ the He, extracts the ³ He in the dilute phase fluid in the mixer 51, a continuous refrigeration Realize the cycle.

53 is a heat exchanger that absorbs heat of condensation from the ³ He gas cooled to about 1K by the JT heat exchanger 21 and the JT valve 22, and cools ³ He to about 0.7K. It is a plate of a ³ He vaporization stage that transmits the exchanged heat of condensation to the ³ He diluted phase liquid in the still 52. 55 is a tube-in-tube heat exchanger, 56 is a step heat exchanger, and 57 is a plate of about 50 mK that supports the tube-in-tube heat exchanger 55. The plate 54 and the plate 57 are intermediate stages of the first embodiment of the present invention, and two stages are provided between the 4K plate 14 and the low temperature plate 9 in order to maintain the object to be cooled in a low temperature state.

  Here, the above-described tube-in-tube heat exchanger 55 and step heat exchanger 56 of Embodiment 1 will be described.

First, the tube-in-tube heat exchanger 55 is obtained by inserting a coiled small-diameter pipe into a thin-walled large-diameter pipe and further coiling the entire large-diameter pipe, and returns to the mixer 51. ³ He flowed to face the ³ He atoms dense phase fluid and ³ He dilute phase fluid moving in still 52, performs heat exchange between them. Further, the step heat exchanger 56 is for further lowering the minimum temperature of the dilution refrigerator, and the two chambers are in contact with each other with a metal partition sintered with silver powder, and the two chambers are mixed with a concentrated phase liquid and a diluted phase. Liquid flows oppositely. At that time, since the sintered body is porous, a very large heat exchange area (several m ² to several tens m ² ) can be secured.

By the way, in the conventional step heat exchanger, silver sintered bodies (hereinafter, sintered silver) are arranged on both sides of the partition wall, and each sintered silver is covered with a lid and condensed by the JT valve 22. was ³ and the He ³ He atoms dense phase fluid and ³ He dilute phase fluid towards the still 52 to flow in the countercurrent direction was heat exchange. And as a material of a partition, the same cupronickel (Cu-Ni alloy) plate as the two lid | covers with which intensity | strength is requested | required was used. Of course, it is desirable to use a silver plate with high thermal conductivity as the partition wall, but since it is difficult to weld with silver (Ag) and cupronickel (Cu-Ni alloy), argon is used in the surrounding flange portion using the same metal. We were welding. In addition, silver plating was necessary to facilitate the adhesion of silver powder to the partition walls.

In this conventional method, heat is affected when welding, and the sintered silver inside is often damaged by oxidation, the porous voids and voids in the sintered body are crushed, the heat exchange area is reduced, and the step The heat exchange rate of the heat exchanger was lowered. Even when solder is used instead of welding, the same effect due to heat is unavoidable although the damage is low due to the low temperature. In addition, the effective area decreases because the flux enters the pores of the sintered body. There was another problem.

Therefore, the step heat exchanger 56 of the first embodiment employs the following configuration in order to improve the heat exchange rate. In FIG. 2, 61a and 61b are sintered silver having a shape such as a disk, 62 is a partition of a silver plate having high thermal conductivity, and 63a and 63b are made of stainless steel that covers the sintered silver 61a and 61b from above and below, respectively. It is a lid. The diameter of the lids 63a and 63b is about 40 mm, and about 3 g of sintered silver 61a and 61b is used for the step heat exchanger 56. Thus, a heat exchange area of about 6 m ² can be formed.

  Thus, by using the lids 63a and 63b as stainless steel, high strength can be obtained, and a cheaper step heat exchanger can be obtained than when cupronickel is used. Further, since a silver plate is used, the affinity between the sintered silver 61a and 61b and the partition wall 62 is essentially good, and there is no need to apply silver plating. The material of the lids 63a and 63b has a high degree of freedom and does not need to be made of stainless steel, and the materials of the lids 63a and 63b may be different from each other. This is because the step heat exchanger 56 of the first embodiment has a characteristic configuration in which the lids 63a and 63b and the partition wall 62 are joined not by argon welding but by a combination of physical tightening and sealing. It is.

As shown in FIG. 2, 64 is a male screw such as a bolt, and the sintered silver 61a and 61b are covered with lids 63a and 63b across the partition wall 62, and the female screw and the male screw 64 formed on the flange portion are firmly attached. And screwed together. The male screw 64 and the female screw correspond to the fastening member of the present invention. Reference numeral 65 denotes a flow path forming block attached to the lids 63a and 63b. Reference numeral 66 denotes a concentrated liquid flow path formed by perforating the flow path forming block 65 to flow a ³ He concentrated phase liquid. Similarly, 67 is a dilute liquid flow path formed by perforating the flow path forming block 65 in order to flow a ³ He dilute phase liquid. Reference numeral 68 denotes an iridium seal for ensuring the water tightness of the lids 63a and 63b and the partition wall 62. Other seals such as lead, solder, and Kapton (registered trademark) may be used.

With the above configuration, the sintered silver 61a and 61b of the step heat exchanger 56 is affected by argon welding, and the partition wall 62 can also use the silver plate having the highest thermal conductivity. The heat exchange rate can be significantly improved. Even if ³ He leaks from the step heat exchanger 56, in the case of the first embodiment, it is possible to retighten the female screw and the male screw 64 or to disassemble and repair them. On the other hand, a conventional step heat exchanger with argon welding cannot be repaired.

In the dilution refrigeration unit of the first embodiment, the 40K plate 13 is cooled to 40K by the first stage 11 of the GM refrigerator 3, and the 4K plate 14 is cooled to 4K by the second stage 12. Thus, the ³ He gas is cooled to about 40K by the heat exchanger 18b, cooled to about 4K by the heat exchanger 19b, further cooled by the JT heat exchanger 21, and liquefied by the JT valve 22.

Then, ³ He which is cooled back and forth 0.7K in the heat exchanger 53, by the tube-in-tube heat exchanger 55 flows to face the ³ He atoms ³ He dilute phase fluid moving in still 52 The heat is exchanged again, and the temperature is further lowered by the step heat exchanger 56. The liquid ³ He entering the mixer 51 becomes a ³ He concentrated phase liquid located in the upper part of the mixer 51, and is absorbed by the heat of dissolution when ³ He dissolves in the ³ He dilute phase liquid located in the lower part. 02K) can be realized.

At this time, the still 52 is heated to about 0.7K by heat from a heater (not shown), and ³ He having a saturated vapor pressure higher than ⁴ He is vaporized by the negative pressure by the vacuum pump 7a. Thus the concentration gradient is generated between the ³ He in ³ He dilute phase fluid in the ³ He and the mixer 51 in the Still 52, ³ in the mixer 51 to compensate for the shortage of the ³ He in Still 52 ³ He atoms of He dilute phase liquid move. To further supplement the concentration of ³ He in this ³ He dilute phase ^{liquid, 3} He atoms diffuse from the 3 He dense phase fluid. The ³ He gas evaporated from the still 52 rises in temperature through the exhaust pipe 15 and is sucked by the vacuum pump 7a and further pressurized by the compressor 7b. Thereafter, the ³ He gas is sent to the first stage heat exchanger 18b, and the refrigeration cycle described above is repeated.

  Next, the sample mounting device 5 will be briefly described here. In FIG. 1, reference numeral 30 denotes a copper sample holder, and 31 denotes a sample for measuring physical properties (an object to be cooled of the present invention) locked to the sample holder 30. 32 is a heater wound with a manganin wire that raises the temperature of the sample 31, and 33 is a temperature sensor such as a thermocouple or thermistor for detecting the temperature of the sample 31. This temperature sensor 33 is embedded in a recess formed in the sample holder 30. Although not shown, the sample holder 30 is formed with a D-cut recess and a small-diameter portion adjacent to each other. A sample 31 is mounted on the D-cut flat portion while being exposed to a vacuum atmosphere (vacuum space), and a heater 32 is wound around the small-diameter portion. The heater 32 is for locally heating the temperature sensor 32 and the sample 31 through the sample holder 30 by heat conduction. At this time, since the sample holder 30, the temperature sensor 32, and the sample 31 are small, the influence of heat radiation on other components is extremely small.

  When the sample holder 30 is mounted on the low temperature plate 9, the sample 31 is first mounted on the sample holder 30, the sample mounting device 5 is inserted from a position higher than the gate valve of the lower support cylinder 5b, and the gate valve is closed. In this state, the upper support cylinder part 5a and the lower support cylinder part 5b are connected to form a sealed space. In this state, a vacuum pump (not shown) is operated, the air in the upper support cylinder 5a and the like is discharged, and the pressure is reduced to the same degree of vacuum as in the vacuum container 1. When the gate valve is opened in this state, the upper support cylinder 5a, the lower support cylinder 5b, and the vacuum vessel 1 are all in a vacuum state, and the sample holder 30 is inserted therein. Therefore, when the sample mounting device 5 is further inserted, the sample holder 30 at the tip of the sample mounting device 5 is inserted into the sample mounting guide 23 and is guided as it is to the state shown in FIG. The procedure for taking out to the outside may be performed in the reverse order of insertion.

  Now, in the first embodiment of the present invention, a characteristic functional thermal coupling body, that is, a functional thermal coupling body having a passive thermal switching function is used in order to perform precooling in a short time. Here, the passive thermal switch function is a thermal coupling function in which the degree of thermal coupling is high when the temperature is high, and the degree of thermal coupling rapidly decreases when the temperature is low. This is a function that is similar to a thermal switch but has only a negative effect. This is a new heat conduction and heat insulation function that connects the stages of multistage refrigerators that operate continuously. The functional thermal coupling body is a solid substance having a thermal conductivity that decreases rapidly in proportion to the temperature drop below a predetermined temperature toward absolute zero, such as graphite, and has a columnar shape. It is a thermal coupling body that has a passive thermal switching function depending on the physical properties of the thermal coupling body. Because of this simplicity, the functional thermal combination can eliminate the problems of gas gap thermal switches. The “predetermined temperature” is a temperature at which the thermal conductivity starts to show saturation or a peak as the temperature rises because the thermal conductivity mechanism changes as the temperature changes, and the thermal conductivity deviates from the temperature drop. Such a temperature is generally present in an actual material, for example, 110K for graphite and 70K for alumina.

  First, the structure of the graphite thermal coupling body in Embodiment 1 (functional thermal coupling body of the present invention) will be described. In FIG. 1, 41 is a first graphite rod (functional thermal coupling body of the present invention) that connects the 40K plate 13 of the first stage and the 4K plate 14 of the second stage of the GM refrigerator 3, and 42 is 4K. This is a second graphite rod that connects the plate 14 and a plate 54 of about 0.7K. 43 is a third graphite rod for connecting a plate 54 of about 0.7K and a plate 57 of about 50 mK, and 44 is a fourth graphite rod for connecting the plate 57 and a low temperature plate 9 of about 20 mK. Here, when the plates 54 and 57 are not provided between the 4K plate 14 and the low temperature plate 9, the 4K plate 14 and the low temperature plate 9 are thermally replaced with the second to fourth graphite rods 42, 43, and 44. It is only necessary to provide one graphite rod (second functional thermal coupling body of the present invention) connected to the.

Pitch-bonded graphite or the like is preferable as the graphite heat-bonded body of the first embodiment, and the thermal conductivity is proportional to T ^N (where N = 2.6 to 3), where T is the absolute temperature. desirable. At low temperatures, heat conduction depends only on the contact state between the phonons and the particles derived from the graphite structure.

  The functional thermal coupling body composed of the first to fourth graphite rods 41, 42, 43, and 44 is turned on and off (connected and disconnected) between the plates, and physically Is the support for each plate. The first to fourth graphite rods 41, 42, 43, and 44 may be any form of support as long as the composition is graphite and the strength is sufficient. Here, it is referred to as a rod, but it may be a cylinder or a prism as long as it is columnar, and the cross-sectional area may vary depending on the part, not a uniform thickness. Moreover, the apparent on / off temperature changes by changing the shape.

The temperature equilibrium point in the present invention is a temperature in which the heat absorbed by the lower cooling stage to which the functional thermal coupling body is attached and the heat entering from the upper cooling stage through the functional thermal coupling body are balanced. Therefore, the temperature equilibrium point of the first to fourth graphite rods 41, 42, 43, and 44 decreases sequentially as the temperature of each cooling stage and intermediate stage decreases.

  Hereinafter, characteristics of the graphite heat-bonded body having a composition of graphite will be described. A multi-stage refrigerator has a relatively low temperature, but a relatively high-temperature cooling stage with a large cooling capacity, and a plurality of low-temperature cooling stages that can approach a lower temperature but have a low cooling capacity are connected in series. The object to be cooled connected to the lowermost cooling stage is cooled. In general, since the heat capacity of a substance increases as the temperature rises, when it is cooled in a state where it is connected in series, the object to be cooled is gradually cooled from the high temperature state in a cooling stage with a small cooling capacity. It takes a long time for objects to reach the minimum temperature.

  Therefore, in order to rapidly cool the object to be cooled to the minimum temperature, the upper cooling stage having a large cooling capacity is brought into thermal contact with the lower cooling stage while the temperature is high, and the temperature of the lower cooling stage is set to the upper stage. What is necessary is just to cool rapidly to the temperature of a cooling stage, and thermally isolate | separate an upper cooling stage after that, and to cool a to-be-cooled object to the minimum temperature only by a lower cooling stage. At this time, the gas gap heat switch used for separation is extremely difficult to adjust the pressure of gas (helium) for setting the operating temperature below 4K, and a support material is necessary for the structure, and heat conduction through the material is low. It cannot be ignored. It is difficult to use at 4K or less because, for example, heat is generated or absorbed when gas is condensed or vaporized.

  On the other hand, the functional thermal coupling body of Embodiment 1 does not use the properties of gas condensation and vaporization unlike the gas gap thermal switch. Utilize the physical properties of certain solid materials such as graphite. Since these materials exhibit a rapid change in thermal conductivity in proportion to the temperature drop, the ability of the thermal switch-off function increases as the low temperature plate approaches absolute zero. That is, as the temperature decreases, the amount of heat conduction becomes more negligible. Therefore, the conventional gas gap thermal switch can only be thermally turned on and off only once at 30K. However, the functional thermal assembly of the present invention is thermally activated between a plurality of cooling stages approaching absolute zero. It is possible to connect to and disconnect from.

  The operation mechanism of the functional thermal coupling body of the first embodiment is essentially different from the graphite thermal switch used for pre-cooling the one-shot adiabatic demagnetization refrigerator of Non-Patent Document 1 described above. One-shot adiabatic demagnetization refrigerators have a fatal defect that once the temperature is lowered, the temperature gradually rises due to the intrusion heat from the outside. In order to prevent the temperature from rising, the intrusion heat must be cut off, so the refrigeration action is performed in the temperature range (about 1 K or less) where the graphite heat switch is completely turned off. The graphite thermal switch also has the effect of precooling faster than lead. That is, when viewed as a whole, it continuously changes from a low heat conduction state to a high heat conduction state as the temperature rises between a region that turns off at about 1 K or less and a region that turns on at several tens of K to room temperature. An intermediate area exists. In other words, there is no clear temperature transition from off to on, and the temperature range in which the off operation is performed is separated from the temperature range in which the on operation is performed.

  On the other hand, the functional thermal coupling body of the present invention apparently shows heat conduction at high temperatures, but is not completely turned off in a steady operation state, and there is slight heat intrusion through it. However, since it is used for a continuous operation type refrigerator, the lower cooling stage always absorbs heat from the upper stage entering through the functional thermal coupling body, and the temperature is balanced, and apparently off at low temperatures. It should be noted that this is an action similar to the off action of the graphite heat switch of the one-shot adiabatic demagnetization refrigerator, but uses a physically different state. This functional thermal coupling body is a thermal coupling member that functions effectively only when used together with a refrigerator that is continuously operated without a refrigerant, without assuming one-shot.

  The functional thermal coupling body of the present invention has a side that the minimum temperature becomes slightly higher due to intrusion heat, but the time to the minimum temperature can be greatly shortened, control is unnecessary, and it is apparently turned on any number of stages. It can be turned off, has no drawbacks such as a gas gap heat switch, has a function as a support, and has an extremely excellent operational effect that compensates for the side face of adsorbing residual gas in the vacuum vessel. This functional thermal combination is not one-shot, but functions effectively when used with a refrigerator that operates continuously.

  Graphite having this characteristic exhibits a reduction characteristic of thermal conductivity as shown in FIG. The thermal conductivity is large at high temperatures, and the thermal conductivity is extremely small at low temperatures. Therefore, if cooling is performed by connecting the upper cooling stage and the lower cooling stage with a graphite rod, the material to be cooled connected to the lower cooling stage is cooled via the graphite rod because of high thermal conductivity at high temperatures. It is brought into thermal contact with the upper cooling stage having a large capacity and cooled rapidly. However, when the temperature gradually decreases and the temperature of the object to be cooled becomes low, the thermal conductivity of graphite decreases and the thermal conductivity becomes negligible. For this reason, the upper cooling stage and the lower cooling stage are substantially thermally separated from each other, so that the cooling stage is almost cooled by the lower cooling stage. That is, the apparent heat switch is turned off.

  According to FIG. 3, the thermal conductivity of a straight line having a substantially constant gradient below 100K in the log-log graph is shown. This means that the thermal conductivity has a characteristic of decreasing temperature toward 0K with respect to absolute temperature. In addition to graphite, there is alumina as another substance, and it shows a linear thermal conductivity having a substantially constant gradient below 2K in the log-log graph. In the case of alumina, the straight line has a slightly gentle gradient from 2K to 70K. However, the overall thermal conductivity is almost linear. Similarly, although not shown, beryllia has similar properties and can be used as a functional thermal bond.

  A functional thermal coupling body made of graphite, alumina, beryllia or the like has a simple structure and is inexpensive, and functions not only as a thermal switch but also as a support between the plates. A highly precise and precise work such as a gas gap thermal switch is not required, and since it is solid, a bridge made of gas is not formed. Furthermore, it is automatically turned on and off by self-control by temperature change, and it is not necessary to control from the outside. In the case of a gas gap thermal switch, it is necessary to pay attention to the mounting direction because of gas convection, but the functional thermal coupling body does not select the mounting direction. In addition, in the case of a substance having an adsorptive property such as graphite, there is an advantage that the heat insulating property in the container is improved because the functional thermal conjugate adsorbs the residual gas in the vacuum container at a low temperature.

  4 shows (1) one end of one graphite rod having a diameter D = 20 mm and a length L = 100 mm fixed to the second stage of a GM refrigerator equipped with a superconducting magnet of about 10 kg, and the other end of the graphite rod. 2 is a cooling curve in which a copper block of about 200 grams is attached and the temperature of the copper block is measured, and (2) a cooling curve in which the temperature of the second stage is measured. As the temperature of the second stage decreases from room temperature, (1) and (2) decrease almost in parallel, and (1) is a slight heat radiation (about 0.01 W) from the radiation shield of about 40K. Balanced and saturated at about 18K. That is, it is apparently turned on (a state of high thermal coupling and heat conduction) at a high temperature range of 18K or higher, but apparently turned off (a state of low thermal coupling and slightly heat conduction) at about 18K. ing. But not completely off, there is a slight thermal coupling.

  Fig. 5 shows (a) a graphite rod between a first stage (40K) of a GM refrigerator having a cooling capacity of 4W at 4K and a second stage (4K) equipped with a superconducting magnet of about 10kg. When not inserted, (b) When one graphite rod having a diameter D = 20 mm and a length L = 100 mm is inserted and brought into thermal contact, (c) Two graphites having a diameter D = 20 mm and a length L = 50 mm The cooling curve of the 2nd stage in each case when a stick | rod is inserted and it contacts with heat is shown. It can be seen that the cooling in the case of (b) and (c) shortens the time 445 minutes to reach the minimum temperature by 60 to 75 minutes compared to the case of (a). And in the case of one graphite rod, it is cooled more effectively by using two (c) than by (b). Theoretically, the heat transfer should be quadrupled because the graphite cross-sectional area is doubled and halved, but there is a problem of thermal contact at the interface between the graphite and the first and second stages. Since a plurality of factors such as the cooling effect of the second stage that is short-circuited with heat and the like are complicatedly affected, the reduction of the cooling time is not simply four times that of (b).

FIG. 6 is an enlarged graph in the vicinity of the temperature 3K in the graph of FIG. 5 and shows a difference in the reached temperature. The reached temperature (about 2.8 K) of (a) without using graphite corresponds to the second lowest temperature in the present invention. In addition, the reached temperatures (b) and (c) (about 3.0 K and 3.2 K, respectively) using graphite correspond to the temperature equilibrium point in the present invention. Although there is almost no increase in the minimum temperature reached, when viewed microscopically, the temperature equilibrium point in the case of (b) is slightly higher than the minimum temperature of (a), but is slightly higher than the temperature of (c). The equilibrium point is slightly higher than the temperature equilibrium point of (b). However, as shown in FIG. 5, the temperature difference is practically negligible.

Moreover, since the rise in the minimum temperature achieved by adding graphite is so small that it cannot be understood that it does not expand as shown in FIG. 6, there is almost no performance loss as a refrigerator. That is, the passive heat switch using graphite is not completely turned off, but the heat inflow through it is always sufficiently absorbed by the refrigeration capacity of the continuously operating GM refrigerator and ignored near the minimum temperature. It turns out that it is possible. Such a small demerit of a slight increase in the minimum temperature reached is surpassed by a large merit of an improvement in cooling rate.

Utilizing these characteristics, by providing a functional thermal coupling between the plates that support the still, mixer, and heat exchanger of the dilution refrigeration unit, the temperature equilibrium point can be lowered one after another. It is possible to reach the minimum temperature in a short time, and the object to be cooled in thermal contact with the low temperature plate can be stably maintained at a low temperature. Further, the structure of the refrigerant-free refrigerator can be simplified, the cost is reduced, and the structure is strengthened.

  As described above, in the first embodiment, the first to fourth graphite rods 41, 42, 43, and 44 are interposed between the plates as the functional thermal coupling body, and the passive thermal switching function and the support are used. The functions of At the same time, the flow path 16 described above is divided into a flow path 16a and a flow path 16b, and the valves 17a and 17b are switched to make one flow path 16a a precooling path and the other flow path 16b flow during normal operation. The road. If the temperature at which the valves 17a and 17b are switched is close to the temperature equilibrium point of the functional thermal coupling body, it is possible to shift (separate) from the precooling operation to the operation of the dilution refrigeration unit more effectively and smoothly.

  It is to be noted that providing such a functional thermal coupling body and precooling by switching the flow path are compatible, and the flow path switching is performed in parallel with the functional thermal coupling body as in the first embodiment. However, it is also possible to provide only a functional thermal coupling body. In some cases, a precooling path may be formed only by switching the flow path. Importantly, functional thermal assemblies function as thermal switches even in temperatures and environments where gas gap thermal switches cannot be used.

  As described above, according to the first embodiment, it is possible to reach a cryogenic temperature of several tens of mK class in a short time, and it is possible to provide a refrigerant-free refrigerator having a simple structure, light weight, and low cost and a strong structure. Running costs are also significantly lower.

Moreover, according to the step heat exchanger of Embodiment 1, since the sintered silver arrange | positioned inside is familiar with the silver plate currently used for the partition, a silver plate is because of high thermal conductivity. The heat exchange efficiency of the step heat exchanger can be greatly improved. Even when the superfluid ³ He leaks from the step heat exchanger, it is possible to retighten the fastening body or to disassemble and repair both.

Furthermore, in Embodiment 1, since the activated carbon trap is cooled using the first stage of the GM refrigerator, the activated carbon trap can be sufficiently cooled without providing a special configuration, and the heater is heated to locally It is also possible to release impurities by raising the temperature. During this time, it is not necessary to stop the operation of the GM refrigerator 3, and when the circulation of ³ He gas is started after the impurities are discharged, the dilution refrigeration operation can be resumed immediately.

  Further, in the first embodiment, a plurality of graphite rods can be interposed between the plates as a functional thermal coupling body so that a passive thermal switching function and a function as a support can be performed simultaneously. It is not necessary to carry out highly precise precision work such as a gas gap thermal switch, and a gas bridge is not formed. Furthermore, it is not necessary to switch from the outside by self-control by temperature change. And when it becomes low temperature, a residual gas can be adsorb | sucked and heat insulation can be improved.

Also, by dividing the ³ He flow path circulating through the dilution refrigeration unit and switching the valves, one flow path is a precooling path, and the other flow path is a flow path for flowing ³ He during normal operation. can do. Precooling can be performed easily and the minimum temperature can be reached in a short time.

  INDUSTRIAL APPLICABILITY The present invention can be applied to a refrigerant-free refrigerator that can reach an extremely low temperature of several tens of mK class in a short time, is inexpensive, has a simple structure, and is easy to maintain, and a thermal switch.

Configuration diagram of a refrigerant-free refrigerator in Embodiment 1 of the present invention Configuration diagram of step heat exchanger of refrigerant-free refrigerator in embodiment 1 of the present invention Graph showing thermal conductivity of graphite and alumina in low temperature range Graph showing characteristics of graphite thermal bond Graph comparing the cooling time with and without graphite thermal bond A graph showing a part of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum container 2 Thermal radiation shield 2a 2nd thermal radiation shield 3 GM refrigerator 4 ³ He tank 5 Sample mounting apparatus 5a Upper support cylinder part 5b Lower support cylinder part 6 Refrigerant superconducting magnet 7a Vacuum pump 7b Compressor 8 Impedance 9 Low temperature plate 10 Flange 11 First stage 12 Second stage 13 40K plate 14 4K plate 14a Flexible heat conductor 15 Exhaust pipe 16, 16a, 16b Flow path 17a, 17b, 17c, 17d, 17e, 17f, 17g Valve 18a, 18b , 19a, 19b, 20a, 53 Heat exchanger 20 Trap 20b Thermal coupling body 20c Heater 21 JT heat exchanger 22 JT valve 23 Sample mounting guide 30 Sample holder 31 Sample 32 Heater 33 Temperature sensor 41 First graphite rod 42 Second Graphite rod 43 Third graphite rod 44 Fourth graphite rod 51 Mixer 52 Still 54,57 Plate 55 Tube-in-tube heat exchanger 56 Step heat exchanger

Claims (11)

A first cooling stage capable of lowering the temperature to a first minimum temperature and a second cooling stage capable of lowering the temperature to a second minimum temperature lower than the first minimum temperature, and a vacuum vessel A refrigerant-free refrigerator for cooling an object to be cooled using the first and / or second cooling stages in
Between the first cooling stage and the second cooling stage, there is no solid material made of a solid material having a thermal conductivity that rapidly decreases in proportion to a temperature drop toward absolute zero below a predetermined temperature. A functional thermal coupling body having a passive thermal switch function for the refrigerant refrigerator stage is disposed,
When the temperature of the object to be cooled is lowered to the first minimum temperature or less, the functional thermal coupling body thermally couples between the first cooling stage and the second cooling stage, and the object to be cooled When the temperature of the first cooling stage is high, the thermal coupling degree is increased, and the thermal coupling degree between the first cooling stage and the second cooling stage is rapidly increased as the temperature of the object to be cooled approaches the first temperature equilibrium point. A refrigerant-free refrigerator characterized by having a low state. A third cooling stage capable of lowering the temperature to a third minimum temperature lower than the second minimum temperature, wherein a second functionality is provided between the second cooling stage and the third cooling stage; A thermal coupling body is provided,
The second functional thermal coupling body thermally couples between the second cooling stage and the third cooling stage to a second temperature equilibrium point, and heat is generated when the temperature of the third cooling stage is high. The degree of thermal coupling between the second cooling stage and the third cooling stage is rapidly lowered as the degree of coupling is increased and the temperature of the third cooling stage approaches the second temperature equilibrium point. The refrigerant-free refrigerator according to claim 1. In order to maintain the object to be cooled in a low temperature state, one or two or more intermediate stages in which the temperature equilibrium point sequentially decreases are provided between the cooling stages,
The refrigerant-free refrigerator according to claim 1, wherein a functional thermal coupling body is provided between the intermediate stages and between the intermediate stage and the cooling stage. A dilution refrigeration unit for cooling the object to be cooled by an entropy change when the third cooling stage forms a phase interface between a ³ He concentrated phase and a ³ He diluted phase and ³ He atoms diffuse into the ³ He diluted phase liquid The ³ He gas cooled by the first cooling stage, the second cooling stage, and the Joule-Thompson heat exchanger is liquefied by a Joule-Thomson valve and heat-exchanged with a still, and then the mixing is performed. 4. A heat exchanger is introduced into a heat exchanger, cooling is performed by the entropy change, and further, ³ He atoms are transferred from the mixer through the heat exchanger to a still and selectively evaporated. No refrigerant freezer. Comprising the step heat exchanger for heat exchange between the ³ He liquid starts circulating from ³ He solution to the mixer to be introduced into the mixer,
The step exchanger includes a silver plate serving as a partition wall, a pair of sintered silver sandwiching the silver plate, and a pair of lids covering the sintered silver, and fastening the silver plate and the lid The refrigerant-free refrigerator according to claim 4, wherein the refrigerant-free refrigerator is fastened by a member. Trap is provided in a flow path for circulating the vaporized ³ the He, either no refrigerant refrigerator according to claim 1 to 5, wherein said trap, characterized in that provided in the first cooling stage. The refrigerant-free refrigerator according to claim 6, wherein the trap is provided with a heater for vaporizing impurities. A flow path for circulating the vaporized ³ He is branched into a first flow path and a second flow path for precooling having a lower impedance than the first flow path, and the second flow path is only in precooling. The helium gas cooled in the first and second cooling stages is passed through the dilution refrigeration unit to precool the dilution refrigeration unit. Refrigerant refrigerator. A functional thermal coupling body for a continuous operation refrigerator that is disposed between a refrigerant-free refrigerator and a cooled object in a vacuum vessel and conducts heat between the refrigerant-free refrigerator and the cooled object. It has a thermal conductivity that decreases rapidly in proportion to the temperature drop toward absolute zero below a predetermined temperature. When the temperature is high, the thermal coupling degree is high, and when the temperature is low, the thermal coupling degree decreases rapidly. A functional thermal coupling body comprising a solid material having a passive thermal switching function for a refrigerant-free refrigerator. A high temperature side cooling stage and / or intermediate stage and a low temperature side cooling stage and / or intermediate stage of the refrigerant-free refrigerator in the vacuum vessel; A functional thermal coupling body for a continuous operation refrigerator that conducts heat in the cooling stage and / or the intermediate stage on the low temperature side, and rapidly increases in proportion to the temperature drop toward absolute zero below a predetermined temperature. It consists of a solid material with a passive thermal switching function for refrigerant-free refrigerator stages that has a decreasing thermal conductivity, and when the temperature is high, the thermal coupling degree is high and the thermal coupling degree rapidly decreases when the temperature is low A functional thermal bonding body characterized by that. The functional thermal coupling body according to claim 9 or 10, wherein the solid substance is any one of graphite, alumina, and beryllia.

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