JP2013108574A - Reflective heat insulation material, heat insulation container, and very low temperature device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure excellent durability capable of maintaining a high reflectance of a reflective film on the surface of a reflective heat insulation material.SOLUTION: By forming a reflective film 3 of the surface of a resin film 2 as a base material from an Ag-based alloy containing Ag as its main component and 0.005 to 3.2 atom% of one or more kinds of alloy elements selected from Bi, Sb, and Nd, chemical stability is improved higher than pure Ag and thereby excellent durability capable of maintaining a high reflectance of the reflective film 3 is secured.

Description

  The present invention relates to a reflective heat insulating material, a heat insulating container using the reflective heat insulating material, and a cryogenic apparatus including the heat insulating container.

  In order to insulate the inside and outside by suppressing heat transfer due to radiation, a sheet-like reflective heat insulating material provided with a highly reflective reflective film on one or both surfaces is used for containers and building walls. Sometimes. Conventionally, aluminum is used as a material for forming a reflective film (see, for example, Patent Documents 1 and 2).

  In what was described in Patent Document 1, an aluminum film was attached to the surface of the foamed polyethylene sheet as the reflective heat insulating material, and the reflective heat insulating material was applied to the outer wall surface of the building. I try to insulate.

  Moreover, in what is described in Patent Document 2, the reflective heat insulating material is an aluminum sheet or a sheet having an aluminized surface, and the reflective heat insulating material stacked in a plurality of layers at intervals is used as an MRI (magnetic resonance) which is a cryogenic device. A superconducting magnet assembly for a tomography apparatus) is disposed in a vacuum container that accommodates a helium container or the like so as to insulate between the outer vacuum container and the inner helium container.

Special table 2011-521187 gazette JP 2000-152922 A

  As described in Patent Documents 1 and 2, the reflective film of the conventional reflective heat insulating material is formed of aluminum. Aluminum is a representative metal having high reflectivity together with gold and silver, and shows high reflectivity with respect to long-wave electromagnetic waves such as infrared rays having large radiant heat energy, but the reflectivity is lower than that of gold or silver. For this reason, for example, when a heat insulating container of a cryogenic device such as a superconducting magnet or a refrigerator that requires high heat insulation is to be insulated, it is necessary to increase the number of layers on which the reflective heat insulating material is stacked. There is a problem that the required heat insulating layer is thickened and the bulk of the heat insulating container is increased. Aluminum is inferior in chemical stability and thermal stability to gold and silver, and inferior in durability to maintain reflectivity.

  In order to cope with such a problem, it is conceivable to form the reflective film with gold or silver. However, gold is a very expensive metal and is not practical. On the other hand, silver is cheaper than gold and has higher durability than aluminum to maintain high reflectivity. However, chemical stability is inferior to gold, and sufficient durability cannot be ensured.

  Then, the subject of this invention is ensuring the outstanding durability which can maintain the reflective film of the surface of a reflective heat insulating material with a high reflectance.

  In order to solve the above problems, the present invention provides a sheet-like reflective heat insulating material provided with a reflective film on one or both surfaces, wherein the reflective film is mainly composed of Ag and selected from Bi, Sb, and Nd. The composition formed with an Ag-based alloy containing 0.005 to 3.2 atomic% of one or more alloy elements was adopted.

  The inventor conducted a high-temperature and high-humidity test and a salt water immersion test on an Ag-based alloy to which various alloy elements were added, and investigated the reflectance change before and after the high-temperature and high humidity test and the appearance change after the salt water immersion test. As a result, as shown in Table 1 below, the reflective film formed of an Ag-based alloy containing 0.005 to 3.2 atomic% of one or more alloy elements selected from Bi, Sb, and Nd It was confirmed that the reflectivity was close to the reflectivity of pure Ag, and the decrease in reflectivity after the test was greatly reduced as compared with pure Ag.

  Based on such a result, by adopting the above configuration, the chemical stability is improved more than that of pure Ag, and excellent durability capable of maintaining the reflective film at a high reflectance can be secured. In addition, the addition amount of the alloy element is set to 0.005 to 3.2 atomic%. If the amount is less than 0.005 atomic%, sufficient durability cannot be ensured, and if it exceeds 3.2 atomic%. This is because the amount of decrease in the initial reflectance may be increased.

  By forming the reflective film on a sheet-like base material by vapor deposition, a reflective heat insulating material having a large area can be easily obtained. As a vapor deposition method, a vacuum evaporation method, a sputtering method, an ion plating method, or the like can be employed. In particular, the reflective film formed by the sputtering method is excellent in the alloy element distribution and film thickness uniformity of the Ag-based alloy, and the reflectance can be maintained at a higher level.

  By forming the sheet-like base material with a material having an average volume thermal conductivity at 300 K of 0.2 W / m · K or less, heat transfer generated by heat conduction due to contact with the support member of the reflective heat insulating material is generated. Can be suppressed. Examples of the material having an average volume thermal conductivity of 0.2 W / m · K or less include polyester resin, polyimide resin, polypropylene resin, polystyrene resin, polyphenylene sulfide resin, polyvinyl chloride resin, and methacrylic resin.

  The average thickness of the reflective film is preferably in the range of 10 to 1000 nm. When the average film thickness is less than 10 nm, electromagnetic waves may be transmitted. When the average film thickness exceeds 1000 nm, the reflective heat insulating material becomes thicker than necessary, and the surface roughness becomes rough due to strain due to stress generated during the film formation process, etc. It is because it becomes the cause which lowers.

  The surface roughness Ra of the reflective film is preferably 100 nm or less. This is because the reflectance decreases as the surface roughness Ra increases, and the amount of decrease increases when the surface roughness Ra exceeds 100 nm.

  The present invention also provides any one of the above-described reflective heat insulations in a heat insulating container provided with at least a double inner case and an outer case, and a heat insulating space that separates the inner case and the outer case in the inner and outer directions. A configuration in which the materials are arranged in a plurality of layers in the inner and outer directions of the heat insulating space was also adopted.

  The radiant heat flow velocity between two parallel flat plates with a temperature difference is reduced to 1 / (N + 1) when an insulating material having the same emissivity (1-reflectance << 1) as that of the flat plate is arranged between these flat plates in the N layer. It is known that Therefore, a heat insulating container having a very high heat insulating property can be obtained by arranging the above-described reflective heat insulating material having a high reflectance (low emissivity) in a plurality of layers in the inner and outer directions of the heat insulating space.

  By making the said heat insulation space into a vacuum, the heat transfer by a convection can be eliminated and higher heat insulation can be ensured.

  By fixing the reflective heat insulating material arranged in the plurality of layers to the inner housing side, the reflective heat insulating material can be easily fixed. In addition, the inner housing can be insulated with a reflective heat insulating material having a smaller area.

  Furthermore, this invention also employ | adopted the structure of the cryogenic apparatus provided with one of the heat insulation containers mentioned above. For example, in a cryogenic device, a heat insulating container provided with a heat insulating space between an inner casing called a heat shield tank surrounding a magnet tank in which a superconducting magnet is immersed in liquid helium and an outer casing exposed to the outside air. If the superconducting magnet assembly for MRI that cools the inner casing in the one-stage cooling section of the cryogenic refrigerator that cools the magnet tank is provided, the above-mentioned high space is provided in the heat insulating space separating the inner casing and the outer casing. By arranging the reflective heat insulating material of reflectivity in multiple layers in the inner and outer directions, the heat radiation that occupies most of the heat intrusion from the outer housing to the inner housing is effectively blocked, and the inner housing is cooled The load on the refrigerator can be reduced. Therefore, the capacity of the refrigerator can be reduced, which can greatly contribute to energy saving.

  In the reflective heat insulating material according to the present invention, the reflective film is formed of an Ag-based alloy containing 0.005 to 3.2 atomic% of one or more alloy elements selected from Bi, Sb, and Nd as a main component. Therefore, it is possible to improve the chemical stability more than pure Ag and to ensure excellent durability that can maintain the reflective film at a high reflectance.

Longitudinal sectional view showing an embodiment of the reflective heat insulating material The longitudinal cross-sectional view which shows the cryogenic apparatus provided with the heat insulation container using the reflective heat insulating material of FIG. Sectional view along line III-III in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, this reflective heat insulating material 1 has 0.005-3 one or more alloy elements selected from Bi, Sb and Nd on the surface of one side of a resin film 2 as a sheet-like base material. A reflective film 3 is formed by vapor-depositing an Ag-based alloy containing 2 atomic% by a sputtering method.

  The reflective film 3 has an average film thickness in the range of 10 to 1000 nm and a surface roughness Ra of 100 nm or less. The resin film 2 is a polyester resin having an average volume thermal conductivity at 300 K of 0.2 W / m · K or less. The resin for the resin film 2 may be a polyimide resin, a polypropylene resin, a polystyrene resin, a polyphenylene sulfide resin, a polyvinyl chloride resin, a methacrylic resin, or the like.

  FIG. 2 shows a cryogenic apparatus provided with a heat insulating container using the reflective heat insulating material 1. This cryogenic apparatus is a superconducting magnet assembly for MRI, and includes an inner housing 15 called a heat shield tank that forms a vacuum chamber 14 surrounding a magnet tank 13 in which a superconducting magnet 11 is immersed in liquid helium 12, and an outside air. A heat insulating container provided with a vacuum heat insulating space 17 between the exposed outer casing 16 and an inner casing 15 made of aluminum alloy at a one-stage cooling unit of a cryogenic refrigerator 18 for cooling the magnet tank 13. The reflective heat insulating material 1 is arranged in multiple layers in the inner and outer directions so as to cover the outer diameter side and the axially outer side of the inner housing 15 in the vacuum heat insulating space 17. The magnet tank 13, the inner casing 15 and the outer casing 16 are formed in a donut shape so that the subject and the test object pass through the center hole 16 a of the outer casing 16.

  A cylindrical refrigerator sleeve 19 is attached to the upper part of the magnet tank 13, and the refrigerator 18 is refrigerated through the cylindrical openings 15 a and 16 a provided on the inner casing 15 and the outer casing 16. The machine sleeve 19 is detachably inserted. The openings 15 a and 16 a are sealed with a lid member 20. The lid member 20 is integrally attached to the refrigerator 18 and seals the openings 15a and 16a when the refrigerator 18 is inserted.

  The refrigerator 18 is a two-stage refrigerator using a Gifford McMahon system, and includes a drive unit 21, a first cylinder 22 and a second cylinder 23 sequentially connected below the drive unit 21, and the first cylinder A first cooling stage 22 a is provided at the lower end of 22, and a second cooling stage 23 a is provided at the lower end of the second cylinder 23. A helium gas recondenser 24 is attached to the lower side of the second cooling stage 23a.

  Helium gas is supplied to the drive unit 21, and the supplied helium gas is ejected from the lower ends of the first cylinder 22 and the second cylinder 23. The first cylinder 22 is located in the cylindrical opening 15a of the inner casing 15, and the inner casing 15 is cooled to about 40K by the first cooling stage 22a. The second cylinder 23 is located in the refrigerator sleeve 19 and cools the inside of the magnet tank 13 to about 4K by the second cooling stage 23a.

  A safety discharge pipe 25 penetrating the inner casing 15 and the outer casing 16 is also provided at the top of the magnet tank 13. This safety discharge pipe 25 discharges the helium gas in the magnet tank 13 to the outside when the amount of heat penetration into the magnet tank 13 increases for some reason and the pressure in the magnet tank 13 increases excessively. It is a safety measure. A burst valve 26 that breaks when the pressure increases excessively is provided at the distal end of the safety discharge pipe 25, and a plurality of baffles that suppress intrusion of radiant heat from the burst valve 26 side are provided in the safety discharge pipe 25. A plate 27 is provided.

  As shown in FIG. 3, the reflective heat insulating materials 1 arranged in a plurality of layers in the vacuum heat insulating space 17 are connected to each other by a plurality of stakes 4 with a slight gap between them, and the outer periphery of the doughnut-shaped inner casing 15. It is wound around the surface. Each reflective heat insulating material 1 is fixed with the reflective film 3 facing outward. Therefore, electromagnetic waves such as infrared rays radiated from the outer casing 16 on the outer peripheral side are reflected by the reflective film 3 having a high reflectivity of each reflective heat insulating material 1, and heat intrusion due to radiation to the inner casing 15 is effectively blocked. Is done.

  As an example, as shown in FIG. 1, the reflective film 3 is formed by sputtering with an Ag-based alloy containing 0.005 to 3.2 at (atomic%) of one or more alloy elements selected from Bi, Sb, and Nd. The reflective heat insulating material (Examples 1-18) formed in 1 was prepared. As a comparative example, a reflective heat insulating material (Comparative Example 1) having a reflective film formed of pure Ag was also prepared.

About each reflective heat insulating material of the said Examples 1-18 and the comparative example 1, the initial stage reflectance of the reflecting film was measured. In addition, a high-temperature and high-humidity test and a salt water immersion test were performed as accelerated durability tests on each of these reflective heat insulating materials. In the high-temperature and high-humidity test, the reflectance and surface roughness Ra of the reflective film before and after the test were measured, and changes in the reflectance and surface roughness Ra were investigated. In the salt water immersion test, the appearance after the test was visually observed, and the presence or absence of appearance changes such as discoloration and peeling was investigated. Here, it was difficult to accurately measure the reflectance of long-wavelength electromagnetic waves in the infrared region, so in general, the longer the wavelength, the higher the reflectance. The reflectance of red light having a side wavelength of 650 nm was measured. The test conditions for each test are as follows.
(High temperature and humidity test)
・ Temperature: 80 ℃
・ Humidity: 90% RH
・ Retention time: 48 hours (salt water immersion test)
Salt concentration: 0.5 mol / liter (NaCl)
・ Salt water temperature: 20 ℃
・ Immersion time: 5 minutes

  Table 1 shows the measurement results of the initial reflectivity of the reflective heat insulating materials of the examples and comparative examples. From this measurement result, the initial reflectivity of each example is within a decrease of several percent with respect to the initial reflectivity of Comparative Example 1 in which the reflective film is formed of pure Ag. However, it can be estimated that a high reflectance can be secured.

  The test results of the high temperature and high humidity test are also shown in Table 1. The change in reflectance before and after the test is greatly reduced to about -3% in Comparative Example 1 in which the reflective film is formed of pure Ag, whereas in each Example, the reflectance change is less than -1% It can be seen that the durability capable of maintaining a high reflectance is very excellent. Further, the change in the surface roughness Ra before and after the test was also about 3 nm for the comparative example 1, whereas it was 0.3 nm or less for each of the examples. Except for Examples 1 and 5 in which the amount was 0.01 at% or more, the change in roughness Ra was hardly changed to 0.05 nm or less. This confirms that high reflectivity can be maintained.

  The test results of the salt water immersion test are also shown in Table 1. In Comparative Example 1 in which the reflective film was formed of pure Ag, a remarkable change in appearance was observed, whereas in each Example, no change in appearance was observed. This also confirms that each of the examples can maintain a high reflectance. Table 1 also shows a comprehensive evaluation of durability based on the test results of the high temperature and high humidity test and the salt water immersion test.

  In the embodiment described above, the reflective heat insulating material is provided with the reflective film only on the surface on one side, but the reflective heat insulating material may be provided with the reflective film on the surface on both sides.

  In the above-described embodiment, the cryogenic apparatus is an MRI superconducting magnet assembly including a heat insulating container having a vacuum heat insulating space, and a plurality of layers of reflective heat insulating materials are disposed in the vacuum heat insulating space. Can also be applied to other insulated containers such as refrigerators and freezers. These heat insulating containers may have a heat insulating space in which gas exists, or may be one in which a reflective heat insulating material is arranged in a single layer in the heat insulating space. Further, the cryogenic apparatus according to the present invention is not limited to the superconducting magnet assembly for MRI, but can be applied to other superconducting magnet assemblies or other cryogenic apparatuses using liquid nitrogen or the like.

  Further, in the above-described embodiment, the reflective heat insulating material is disposed so as to cover the inner housing in the vacuum heat insulating space of the heat insulating container, but also covers internal parts of the heat insulating container such as bolts and nuts for fixing the inner housing and the like. Thus, you may arrange | position a reflective heat insulating material in one layer or multiple layers.

DESCRIPTION OF SYMBOLS 1 Reflection heat insulating material 2 Resin film 3 Reflection film 4 Stake 11 Superconducting magnet 12 Liquid helium 13 Magnet tank 14 Vacuum chamber 15 Inner housing | casing 16 Outer housing | casing 15a, 16a Opening part 17 Vacuum insulation space 18 Refrigerator 19 Refrigerator sleeve 20 Lid member 21 Drive unit 22 First cylinder 22a First stage cooling stage 23 Second cylinder 23a Second stage cooling stage 24 Recondenser 25 Safety discharge interval 26 Rupture valve 27 Baffle plate

Claims (9)

  In a sheet-like reflective heat insulating material provided with a reflective film on one or both surfaces, the reflective film is made of Ag as a main component, and one or more alloy elements selected from Bi, Sb, and Nd are added in an amount of 0.005-3. A reflective heat insulating material formed of an Ag-based alloy containing 2 atomic%.   The reflective heat insulating material according to claim 1, wherein the reflective film is formed by vapor deposition on a sheet-like base material.   The reflective heat insulating material according to claim 2, wherein the sheet-like base material is formed of a material having an average volume thermal conductivity at 300 K of 0.2 W / m · K or less.   The reflective heat insulating material in any one of Claims 1 thru | or 3 which made the average film thickness of the said reflective film the range of 10-1000 nm.   The reflective heat insulating material according to any one of claims 1 to 4, wherein a surface roughness Ra of the reflective film is 100 nm or less.   The heat insulation container provided with the heat insulation space which separated the inner housing | casing and the outer housing | casing made into at least 2 layers, and these inner housing | casing and the outer housing | casing in the inside and outside direction, The reflective heat insulation in any one of Claim 1 thru | or 5 The heat insulation container characterized by arrange | positioning material in multiple layers in the inside and outside direction of the said heat insulation space.   The heat insulation container according to claim 6 which made the heat insulation space a vacuum.   The heat insulation container according to claim 6 or 7, wherein the reflective heat insulating material arranged in the plurality of layers is fixed to the inner housing side.   A cryogenic apparatus provided with the heat insulating container according to claim 6.

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