JP5162088B2 - Cooling method using nitrogen-oxygen mixed refrigerant - Google Patents

Description

  The present invention relates to a cooling method using a nitrogen-oxygen mixed refrigerant to a temperature range of 63K or lower for a cooled object such as a superconductor.

  Superconducting materials exhibit superconducting properties below the critical temperature Tc, but oxide high-temperature superconductors are expected to be used at liquid nitrogen temperatures of 77K due to their high critical temperature Tc. There are two main means for cooling the superconductor. One is conduction cooling by a refrigerator or the like, and the other is a method using liquid helium or liquid nitrogen as a liquid refrigerant. Cooling using a refrigerant is desirable for cooling the coil or bulk body from the viewpoint of heat transfer, heat conduction efficiency, and temperature uniformity. Moreover, when it has a heat generating part, coolant cooling is especially desirable.

  In order to exhibit the superiority of the high critical temperature Tc of the oxide superconductor over the metallic superconductor such as NbTi, the temperature is higher than the liquid helium temperature (4.2K) and lower than the boiling point of liquid nitrogen (77K). Cheaper, simpler and safer cooling methods to the area have been studied. A substance having a boiling point in the temperature range of 77 K to 4.2 K is shown in Non-Patent Document 1, and includes hydrogen and neon. Liquid hydrogen (20K) liquefies oxygen when exposed to air. When liquid oxygen and liquid hydrogen are mixed, there is an extremely high risk of explosion. Neon is a rare gas and an extremely expensive substance.

  In order to lower the cooling temperature, a method of cooling to the melting point or triple point instead of the boiling point of the refrigerant has been studied. Patent Document 1 describes cooling at a triple point (63.1K) and a melting point (63.9K) of nitrogen. Patent Document 2 also describes cooling at a triple point (54.36 K) and a melting point (54.4 K) of oxygen.

Japanese Patent Laid-Open No. 4-822077 JP-A-9-36444 JP-A-6-72712 Cryogenic engineering / superconductivity handbook Low Temperature Engineering Vol.37, No.11, p.589 (2002) Unexplored Science and Technology Association, New Superconducting Materials Study Group, New Superconducting Materials Forum News No.10 p.15

  In the cooling using liquid helium (2.19K, 4.2K), the helium itself is expensive and inconvenient to handle. Therefore, although the critical current density is improved as compared with 77K, the high-temperature oxide superconductor The advantage of high critical temperature cannot be utilized.

On the other hand, when used at a liquid nitrogen temperature (77 K), the QMG material (Non-patent Document 3) currently produced by the melting method is about 30,000 A / cm ^{2 in a} 1 T magnetic field, and the Bi-based silver sheath wire is 4000 A. The critical current value Jc of / cm ² is recorded, approaching a full-scale practical level.

  However, in order to promote the practical application of these oxide superconductors, a low-temperature refrigerant that is easy to handle is used, and in order to bring out higher superconducting characteristics, it is easy and stable to a temperature range of about 63K or less, or about 50K. A cooling method is desired.

  However, when liquid oxygen is used as a refrigerant, the triple point is 54.36 K and 152 Pa, and the vapor pressure is low. Therefore, a pump with a large displacement is required to obtain about 54 K by decompression. In addition, since high concentration oxygen has a high combustion promoting property, care should be taken from the viewpoint of safety compared to liquid nitrogen.

  Accordingly, an object of the present invention is to provide a simple and safe cooling method to a temperature range below the melting point or triple point (about 63 K) at which liquid nitrogen solidifies.

The gist of the present invention is as follows.
(1) Nitrogen solidification of 50K or more and less than 63K can be achieved by depressurizing a refrigerant mixed with liquid nitrogen and liquid oxygen having a mixed molar ratio of nitrogen and oxygen in the range of 99: 1 to 30:70 to less than 12600 Pa. nitrogen is happening is characterized in that to cool the cooled object in a state with the liquid phase - cooling method using oxygen mixed refrigerant.
( 2 ) The cooling method using a nitrogen-oxygen mixed refrigerant according to ( 1 ), wherein the object to be cooled includes an oxide superconductor.
( 3 ) The oxide superconductor is REBa ₂ Cu ₃ O _7-x (RE is one of Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, or those The cooling method using a nitrogen-oxygen mixed refrigerant according to ( 2 ), wherein 0.0 ≦ x ≦ 0.3).
( 4 ) The oxide superconductor is a single crystal REBa ₂ Cu ₃ O _7-x (RE is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu. One or a combination thereof (0.0 ≦ x ≦ 0.3) a superconducting material in which a 0.1 to ₅ μm RE ₂ BaCuO ₅ phase is dispersed in a phase, and the nitrogen according to ( 2 ) -Cooling method with oxygen mixed refrigerant.

  ADVANTAGE OF THE INVENTION According to this invention, the liquid oxygen is mixed with liquid nitrogen and the method which can cool a to-be-cooled body simply, cheaply and safely in the temperature range of about 63K-50K vicinity is provided. This cooling method is useful for many general materials, but is particularly useful for cooling an oxide superconductor having a critical temperature of 50K or higher or equipment using the same. Therefore, such a cooling method can be applied in various fields and a great industrial effect can be expected.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.
In view of the above-mentioned problems, the present invention can easily cool an oxide superconducting bulk body or a cooled object such as a magnet in a temperature region of 63.1 K or less by mixing an appropriate amount of oxygen with nitrogen that is inexpensive and relatively easy to handle. A method for cooling is provided.

The present invention, by reducing the pressure mixed refrigerant liquid nitrogen and liquid oxygen cooling, to a means for conveniently cooled from 63K to the triple point or melting temperature (about 54K) Temperature of the region near oxygen. Although it is not a method to which the present invention is applied, a method for safely cooling a mixed refrigerant of liquid nitrogen and liquid oxygen at about 54 K using latent heat between the liquid phase and the solid phase at atmospheric pressure can be considered. The Further , although not a method to which the present invention is applied, a method of cooling to a temperature range of about 40K by further reducing the pressure in a state where the mixed refrigerant of liquid nitrogen and liquid oxygen is solidified is conceivable .

  When cooling with high-purity nitrogen using reduced pressure, it is only liquid at the triple point and the vapor pressure is about 12600 Pa, and cooling can be easily performed by an exhaust device such as a rotary pump having a relatively low exhaust speed.

  However, solidification of nitrogen proceeds below the triple point. At this time, since the gaseous nitrogen is involved and crystallizes as a collection of fine crystals such as snow, the volume substantially expands rapidly. When the object to be cooled is cooled by nitrogen solidified like snow, latent heat is taken away from the surface of the solidified nitrogen by sublimation, so that a gap is formed between the solidified nitrogen and the object to be cooled. If there is any heat generation or heat transfer from another heat source in the body to be cooled, attempting to lower the temperature of the solidified nitrogen by further reducing the pressure will increase the temperature of the body to be cooled.

  On the other hand, when cooling by depressurization using high-purity oxygen, the triple point is 152 Pa, and since the vapor pressure is low, it is necessary to use a pump such as a rotary pump with high exhaust speed or a mechanical booster. Further, since the vapor pressure at the triple point is about two orders of magnitude lower than that of nitrogen, rapid volume expansion occurs when liquid oxygen is vaporized, which is accompanied by severe bumping. The liquid oxygen scattered by this bumping strikes the upper part (high temperature part) in the cryostat container and evaporates, and the pressure in the cryostat is rapidly increased. Due to such bumping, in the case of high-purity oxygen, cooling by decompression becomes inefficient.

  Therefore, when the pressure is reduced in a state where oxygen is mixed with nitrogen, the boiling point and triple point of the mixed liquid at atmospheric pressure change as shown in FIG. By adding only about 10% oxygen, the freezing point is lowered to about 60K, and when added to 20%, the freezing point is lowered to about 56K, and near the triple point temperature of oxygen. Furthermore, since the ratio of nitrogen is large, the vapor pressure is in the order of 1000 Pa, and can be reached by pressure reduction using a rotary pump or the like having a relatively low exhaust speed. For the same reason, the cooling efficiency is high without bumping.

  Also, even in a temperature range lower than the triple point (freezing point), unlike the case of high-purity nitrogen, since it does not solidify rapidly, it does not become a snowy solid, it becomes a relatively dense solid, In close contact with the medium to be cooled. Therefore, even in the sublimation state at a low temperature of about 40K, it is possible to maintain the thermal contact with the object to be cooled and cool the object to be cooled by heat conduction in the solid.

  The above effect appears even when the ratio of oxygen mixed with nitrogen is several percent, for example, about 1%. Also, as can be seen from FIG. 1, when the oxygen pressure is 70% or more, the vapor pressure is on the order of several hundred Pa, and bumping is likely to occur. From the standpoint of safety, remarkable utility can be obtained compared to pure oxygen. Disappear.

Although it is not a method to which the present invention is applied, when cooling is performed by heat conduction at normal pressure using a refrigerator or the like, it is desirable that the refrigerant maintain a liquid phase to a colder temperature, and from a safety standpoint, Desirably not oxygen concentration. Therefore, even if cooling is performed by heat conduction at normal pressure using a refrigerator, etc., it is easy, cheap and safe to cool by using a refrigerant with a low freezing point and a high nitrogen ratio by mixing oxygen with nitrogen It is desirable to do.

  As the object to be cooled, a substance exhibiting superconductivity at about 40K or more and an apparatus including these materials can be most effectively used by this cooling method. As the superconducting material, a Bi-based superconductor called 2212 or 2223, a rare earth element (RE: Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu or one kind thereof 90K class superconducting material (REBaCuO system).

  Moreover, the silver sheath Bi type | system | group wire material containing these substances, the wire material and device etc. which have a REBaCuO type | system | group thin film are mentioned.

Furthermore, in the bulk material (QMG) which is a conductor having the largest current carrying capacity at present, a critical current characteristic higher than that of 77K or 63K can be obtained, and the application is expanded. Here, QMG is a bulk material in which a 0.1 to ₅ μm RE ₂ BaCuO ₅ phase is finely dispersed in a single crystal REBa ₂ Cu ₃ O _7-x phase (0.0 ≦ x ≦ 0.3). Means. In QMG, since the RE ₂ BaCuO ₅ phase is finely dispersed, a high critical current value Jc can be obtained by functioning as a magnetic flux pinning site. In particular, when the size of the RE ₂ BaCuO ₅ phase is 0.1 to 2 μm, an interface between the superconducting phase that functions as a pinning site and a non-superconducting phase is sufficiently obtained, which is more preferable. Moreover, oxygen deficiency x of REBa ₂ Cu ₃ O _7-x phase, if the range of _{0.0 ≦ x ≦ 0.3, REBa 2} Cu 3 O 7-x phase phase transition in orthorhombic Therefore, it is preferable.

Example 1
A silver-added Gd-based cylindrical column having a diameter of about 60 mm and a thickness of 20 mm in which a Gd ₂ BaCuO ₅ phase of about 1 μm is finely dispersed in a single-crystal GdBa ₂ Cu ₃ O _7-x phase and 10% by mass of silver is added. The sample was produced based on the manufacturing methods described in Patent Document 3 and Non-Patent Document 2. This was sliced and cut to a thickness of 1.0 mm, and then processed into a spiral shape having an outer diameter of 55 mm and 7 turns (line width: about 2.3 mm, line interval: 0.5 mm). After sputtering about 2 μm of Ag on both sides, oxygen annealing was performed. The obtained sample is shown in FIG. Eight coils were stacked in the state where the spiral directions were alternately reversed, and the ends were connected in series. In addition, a voltage terminal was attached so that voltage generation in the superconductor and in the connection portion could be measured. The coils laminated as shown in FIG. 3 were placed in a reinforcing NiCr ring, connected to a copper electrode, and then molded using stycast and GFRP to produce a magnet. Furthermore, in order to measure the generated magnetic field, a Hall element was arranged at the center of the magnet.

  After cooling to 77K in liquid nitrogen, the liquid nitrogen was depressurized to 12600 Pa in a sealed container, and measurement was performed at the triple point (63K). Next, it mixed so that the molar ratio (mol) of liquid nitrogen and liquid oxygen might be set to 8: 2, and was used as a refrigerant | coolant. The mixed refrigerant was put into a sealed container, and the pressure was reduced to 1500 Pa with a rotary pump, reaching 56K. Furthermore, by reducing the pressure to 1000 Pa, the mixed refrigerant was cooled to 48K where it was solidified. For decompression, an oil rotary pump having an exhaust speed of 240 L / min was used.

  At each cooling temperature (77K, 63K, 56K, 47K), electricity was supplied using a constant current power source, and the generated magnetic field was measured simultaneously with recording the voltage of each part. The critical current value and the magnetic field generation value at that time were measured using the voltage generation value of 20 μV in the superconductivity as a threshold value. The results are shown in Table 1 below.

  For comparison, pure nitrogen was decompressed and solidified below the triple point, and cooling to 56K and 48K was attempted. However, cooling to 60K or less was not possible, and conversely the temperature rose to about 70K due to decompression. .

  Furthermore, as a comparison, pure oxygen was depressurized using an oil rotary pump having a pumping speed of 240 L / min, and cooling was attempted. However, it was saturated at about 65 K and 1450 Pa, and could not be cooled to a lower temperature. It was.

  From the results of Table 1 and the comparative experiment, it was shown that a 1.53 T magnetic field at 63K provides 1.30 and 1.42 times improvement in characteristics at 56K and 48K, respectively, by the nitrogen-oxygen mixed refrigerant.

(Example 2 )
A silver sheath Bi2223 series wire and a Y series thin film wire were attached to a holder via a copper bus bar, and the critical current was measured by energization. The holder was placed in a refrigerant mixed so that the molar ratio of liquid nitrogen to liquid oxygen was 6: 4, and the refrigerant was cooled to about 55 K by reducing the pressure to about 1200 Pa with a rotary pump. The critical current in the self magnetic field was 205A for the Bi-based wire and 230A for the Y-based wire at 55K.

  Next, as a comparison, only liquid nitrogen was decompressed without mixing oxygen, and cooling to 55K was attempted in a sublimation state, but although it could be cooled to 59K once, the wire rod and bus bar The temperature has risen to about 71K. Therefore, measurement was performed at a triple point 61K having a liquid phase under pressure. As a result, the critical currents in the self magnetic field were 150 A and 170 A, respectively, at 61K.

  As described above, it has been clarified that superior characteristics of each wire can be obtained by mixing oxygen with nitrogen as a refrigerant.

It is an assumed nitrogen-oxygen binary phase diagram. 2 is a photograph showing a single-layer QMG coil used in Example 1. FIG. 6 is a photograph showing an internal structure of a QMG magnet laminated in 8 layers used in Example 1. FIG.

Claims (4)

Nitrogen solidification of 50 K or more and less than 63 K occurs by reducing the pressure of the mixed liquid nitrogen and liquid oxygen mixture in the range of 99: 1 to 30:70 to less than 12600 Pa . cooling method according to oxygen mixed refrigerant - nitrogen, characterized in that but for cooling the object to be cooled in a state with the liquid phase. The cooling method using a nitrogen-oxygen mixed refrigerant according to claim 1 , wherein the object to be cooled includes an oxide superconductor. The oxide superconductor is REBa ₂ Cu ₃ O _7-x (RE is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, or a combination thereof. The cooling method using a nitrogen-oxygen mixed refrigerant according to claim 2 , wherein 0.0 ≦ x ≦ 0.3). The oxide superconductor is a single crystal REBa ₂ Cu ₃ O _7-x (RE is one of Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, or The nitrogen-oxygen mixture according to claim 2 , which is a superconducting material in which a 0.1 to ₅ μm RE ₂ BaCuO ₅ phase is dispersed in a 0.0 ≦ x ≦ 0.3) phase. Cooling method with refrigerant.

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