JPH1027928A - Superconductive coil with permanent current switch and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make feasible of the operation in permanent current mode by a method wherein oxide superconductive wire rods comprising a superconductive coil and a permanent current switch are junctioned each other by mutually diffusion-junctioning with stabilizing materials. SOLUTION: A heater 25 and a thermo-couple 26 are provided near a wire rod junction 22c to be covered with an epoxy resin layer 27. Next, electrodes 28a and 28b for feeding current are formed on a solenoid coil 21. The part of oxide superconductive wire rod heated by a heater 25 fills the role of a permanent current switch for the solenoid coil 21. Next, the oxide superconductive wire rod on the part filling the role of the permanent switch is transferred to constant conductive state so as to feed current to the solenoid 21 from a power supply through the intermediary of the electrodes 28a and 28b. When the solenoid coil 21 is excited, the heating by the heater 25 is stopped to return the oxide superconductor on the switch part to the superconductive state. Through these procedures, the superconductive coil can be stably actuated in the permanent current mode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a superconducting coil with a permanent current switch using an oxide superconducting wire and a method for manufacturing the same.

[0002]

2. Description of the Related Art As a method of operating a superconducting magnet, a method of constantly supplying a current from a power supply to the magnet and a method of connecting a permanent current switch to a coil of the superconducting magnet in parallel with the power supply and exciting the coil by using this switch are provided. There is a method of disconnecting the power supply from the power supply and shifting to the permanent current mode.

FIG. 1 shows a method of operating a permanent current mode using a permanent current switch. As shown in the figure, a short-circuit switch 2 is provided between terminals of the superconducting magnet 1. The short-circuit switch 2 is called a permanent current switch, and usually uses a superconductor in order to reduce the resistance at the time of short-circuit. In a switch using a superconductor, a current is supplied from the power source 3 to the superconducting magnet 1 from the power supply 3 to the rated current value to excite the magnet in a state where resistance is generated by setting the temperature of the superconductor to the critical temperature Tc or more by, for example, heating the heater. . Next, if the switch 2 is shifted to the superconducting state and a short circuit is performed, even if the excitation power supply is removed, the supply of a constant current is continued. In this way, operation in the permanent current mode is possible.

[0004] The permanent current switch is required to have the following characteristics, for example. (1) The resistance at ON is 0 or small.

(2) The resistance at the time of OFF is large. (3) The required current can be supplied stably.

(4) The switching time is short, and the reliability and operability are high. For superconducting magnets using conventional alloy or compound superconducting wires, Nb
A permanent current switch using a wire or an NbTi wire is used. In this case, the temperature for shifting to the permanent current mode is 4.2 K (liquid helium temperature) and cannot be used at a higher temperature.

In recent years, since the discovery of an oxide superconductor that exhibits a superconducting state at a temperature of liquid nitrogen or higher, superconducting magnets using this material have been developed. Therefore, a switch using an Nb line or an NbTi line
It is also conceivable to use at a temperature of 4.2 K in combination with a coil using an oxide superconductor. However, it is relatively difficult to join the Nb wire or NbTi wire to the oxide superconductor.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a permanent current switch which is more suitable for a coil using an oxide superconductor.

It is a further object of the present invention to provide a permanent current switch having a lower ON resistance for a coil using an oxide superconductor.

[0010]

SUMMARY OF THE INVENTION The present invention provides a superconducting coil with a permanent current switch including a superconducting coil and a permanent current switch for short-circuiting the superconducting coil. In the present invention, each of the superconducting coil and the permanent current switch includes an oxide superconducting wire made of a bismuth-based oxide superconductor and a stabilizing material made of silver or a silver alloy covering the bismuth-based oxide superconductor. The oxide superconducting wires constituting the superconducting coil and the permanent current switch are joined by diffusion joining of the stabilizing materials. By this joining, the superconducting coil is short-circuited.

In a preferred embodiment of the present invention, both end portions of the oxide superconducting wire forming the superconducting coil are joined by diffusion bonding of stabilizers, and both end portions are used as permanent current switches. .

In the present invention, the oxide superconducting wire of the permanent current switch can be wound in a coil shape around a core material. In this core material, a portion close to the superconducting coil is preferably made of ceramics having low thermal conductivity, and a portion far from the superconducting coil is preferably made of stainless steel.

Further, in the present invention, the permanent current switch can include a heating element for heating the oxide superconductor, and a heat insulating material covering the heating element.

According to the present invention, there is provided a method for manufacturing a superconducting coil with a permanent current switch. This manufacturing method includes a step of forming a superconducting coil by winding an oxide superconducting wire composed of a sintered body of bismuth-based oxide superconductor and a stabilizing material made of silver or a silver alloy covering the sintered body, leaving both ends of the superconducting wire. When,
The method includes a step of overlapping both end portions with each other and a step of heating the overlapped portion at a temperature of 500 ° C. to 900 ° C. to diffuse and bond the stabilizers at both end portions. The joined two ends are used as a permanent current switch.

The present invention provides another method for manufacturing a superconducting coil with a permanent current switch. This manufacturing method
A bismuth-based oxide superconductor is provided at both ends of a superconducting coil in which a first oxide superconducting wire made of a sintered body of bismuth-based oxide superconductor and a stabilizing material made of silver or silver alloy covering the sintered body is wound. Superposing both ends of a second oxide superconducting wire made of a sintered body of the body and a stabilizing material made of silver or a silver alloy covering the sintered body, and forming the superposed portion at 500 ° C. to 900 ° C. Heating at a temperature of 2. and diffusion bonding the stabilizing materials at both ends. In this method, the second oxide superconducting wire is used as a permanent current switch.

In the manufacturing method of the present invention, both ends of a wire constituting a superconducting coil or a portion of a wire constituting a permanent current switch can be heated while being wound on a core. In this core material, a portion close to the superconducting coil is preferably made of ceramics having low thermal conductivity, and a portion far from the superconducting coil is preferably made of stainless steel.
The portion of the wire to be joined by heating is preferably provided on the portion of the core made of stainless steel.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a bismuth-based oxide superconducting wire is used for a superconducting coil and a permanent current switch. The bismuth-based oxide superconductor, Bi ₂ Sr ₂ Ca
₁ Cu ₂ O _{8- X} , (Bi, Pb) ₂ Sr ₂ Ca ₁ Cu ₂
Bismuth-based oxide superconductor having 2212 phases such as O _8-X (0 ≦ X <1), Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _10-Z ,
(Bi, Pb) ₂ Sr ₂ Ca ₂ Cu ₃ O _10-Z (0 ≦ Z <
Bismuth-based oxide superconductors having 2223 phases, such as 1). The bismuth-based oxide superconductor has a high critical temperature, and a wire having a high critical current density can be relatively easily obtained. Therefore, in particular, if a bismuth-based oxide superconducting wire is used for the permanent current switch, the necessary current can be stably supplied, the switching time is short,
A permanent current switch with high reliability and operability can be provided.

In the present invention, the oxide superconducting wire is filled with raw material powder of the oxide superconductor in a sheath made of a stabilizing material, subjected to plastic working, and then sintered (so-called powder-in-tube). Method) can more preferably be used. In such a method, silver or a silver alloy can be preferably used as a stabilizer, and the formed wire has a structure in which a sintered oxide superconductor is covered with silver or a silver alloy. In this method, examples of plastic working include wire drawing, isostatic pressing, and rolling. Particularly, a tape-shaped superconducting wire can be obtained by a combination of drawing and / or isostatic pressing and rolling, and such a wire can be preferably used. The superconducting wire used may be either a single-core wire or a multi-core wire. The filament of the tape-shaped superconducting wire has a substantially uniform superconducting phase in the longitudinal direction of the tape wire, and the c-axis of the superconducting phase is oriented substantially parallel to the thickness direction of the tape wire. Further, the crystal grains in the filament have a flake shape extending in the longitudinal direction of the tape wire, and the crystal grains are strongly bonded to each other. The flake-like crystal grains are stacked in the thickness direction of the tape wire. A wire using a silver alloy such as an Ag-Au alloy or an Ag-Mn alloy as a stabilizing material has a lower thermal conductivity than a wire using silver as a stabilizing material, and heat enters from the outside and enters the outside. It is more preferable to suppress the release of heat.

In the present invention, the oxide superconducting wire forming the superconducting coil and the oxide superconducting wire forming the permanent current switch are electrically connected by diffusion bonding. A joint is formed by one stabilizer of the wire diffusing into the other stabilizer of the wire. Therefore, no material other than the material contained in the oxide superconducting wire is used for the joint. By such diffusion bonding, the electric resistance generated at the connection between the switch and the coil can be suppressed low. Diffusion joints have significantly lower electrical resistance than joints using eutectic solder, low-temperature solder, or the like. The electrical resistance of silver or silver alloys at low temperatures is significantly lower than that of solder. Therefore, the present invention realizes a superconducting coil with a permanent current switch having a smaller resistance at the time of ON. On the other hand, the electric resistance of the permanent current switch at the time of OFF is sufficiently large due to ceramics.

Further, in the present invention, a structure having both a superconducting coil and a permanent current switch can be obtained by diffusing and joining both ends of one oxide superconducting wire. In this case, both ends of one oxide superconducting wire forming the superconducting coil are diffusion-bonded. These two ends function as permanent current switches. in this case,
Since there is only one joining portion, an increase in electric resistance due to joining can be stopped to a minimum. Permanent current switch is ON
In such a state, the electric resistance of such a structure is remarkably small. According to the present invention, the connection resistance can be kept low, and the current decay can be kept low at the time of ON.

In the present invention, it is preferable that the oxide superconducting wire of the permanent current switch is provided in a state where the distortion is not so much applied. For this reason, it is preferable that the superconducting wire of the permanent current switch is wound in a coil shape. The winding diameter is, for example, 60m so as not to damage the wire.
It is preferable to be not less than mφ. Further, if the oxide superconducting wire is formed into a coil shape in the permanent current switch, a longer wire can be accommodated in a more compact space.

The length of the oxide superconducting wire used for the permanent current switch can be appropriately set according to the number of turns and the size of the superconducting coil.
m, preferably a length in the range of 1 m to 3 m.

In the present invention, the permanent current switch mechanism may be of a thermal type, a magnetic type, or the like. Generally, a thermal switch mechanism is often used. For the heating element used for switching, a material that generates heat by electric resistance such as a manganin wire can be preferably used. When the superconducting switch is used, the heating element is connected to a power supply circuit and a means for controlling heat generation. The heating element may be provided so as to be in contact with the superconducting wire, or may be provided on the superconducting wire via an electrically insulating material.

When the permanent current switch is brought into contact with a refrigerant such as liquid helium or liquid nitrogen, the heating element is covered with a heat insulating material. The thermal insulation material provides an environment for cooling,
An appropriate temperature gradient is formed between the oxide superconducting wire and the heating element, and serves as a thermal buffer so that the heating element and the wire are not rapidly cooled. The heat insulating material protects the heating element so that the superconducting wire can be sufficiently heated when the switch is turned off. As the heat insulating material, for example, a resin material such as an epoxy resin, a resin compound, a silicon compound, or the like can be used. For the heat insulating material, at the temperature used, 10 ⁻² W / cm · K to 10
⁻⁵ W / cm · K, more preferably 10 ⁻³ W / cm · K
A material having a thermal conductivity of 10 ⁻⁵ W / cm · K can be used.

In the manufacturing method according to the present invention, a so-called react-and-wind method is used. In this method, after the sintering step of the oxide superconductor, a processing step for the superconducting coil and the switch is performed. The oxide superconducting wire is wound leaving a predetermined length at both ends to form a superconducting coil. The remaining ends are superimposed on each other or on both ends of the prepared oxide superconducting wire. For example, the overlapped portion can be fixed by winding a glass tape. The superposed part is locally at 500 ° C
To 900 ° C., preferably 700 ° C. to 860 ° C. By heating, the superposed stabilizers are joined together. The heating is preferably performed by winding a superconducting wire around a core connecting stainless steel and ceramics having a lower thermal conductivity than stainless steel such as zirconia and alumina. In this case, it is preferable that the wire portion to be locally heated is wound around a core made of stainless steel, and the wire portion close to the superconducting coil is wound around ceramics. When heating at a temperature of about 800 ° C with the wire wound around a core, if either the wire or the core expands far beyond the thermal expansion coefficient of the other, the wire will be strained and damaged. Is given. Therefore, it is preferable to heat the wire by winding it around a material having a coefficient of thermal expansion close to that of the wire. Further, it is desirable that the material to be wound around the wire has heat resistance and oxidation resistance and does not react with the wire. From such a viewpoint, the core material used for the heating portion is preferably made of stainless steel. On the other hand, care must be taken not to cause thermal damage to the superconducting coil during heating for joining. For this reason, it is preferable that the portion of the core material close to the superconducting coil is made of ceramics having a lower thermal conductivity than stainless steel. Ceramics work to suppress thermal conduction from the joint to the superconducting coil.

In the present invention, the shape of the superconducting coil is not particularly limited. If necessary, a solenoid coil, a pancake coil and the like are used. The superconducting coil and the switch can be directly cooled by a coolant such as liquid helium or liquid nitrogen. Further, cooling may be performed by a refrigerator. When using a refrigerator, it is not necessary to provide a heat insulating material on the heating element of the permanent current switch.

[0027]

【Example】

Example 1 A 3.5 mm wide, 0.24 mm thick, 80 m long oxide superconducting wire in which a bismuth-based oxide superconductor was covered with a silver sheath was wound around a copper bobbin, with an inner diameter of 52 mmφ and an outer diameter of 75 m.
A solenoid coil having mφ and a height of 80 mm was prepared. Insulation between the windings was performed by tape insulation or enamel insulation. The number of turns of the obtained solenoid coil is 800,
The central magnetic field when a current of 20 A was passed was 1000 G.

As shown in FIG. 2, both ends 22a of the wire of the solenoid coil 21 wound on the copper bobbin 20
And 22b were wound around a core material 23 having a diameter of 60 mmφ. The length of both ends of the wound wire was about 1 m. The core material 23 is provided on the copper bobbin 20. In the core material, a portion 23a close to the solenoid coil is made of zirconia ceramics, and another portion 23b is made of stainless steel. The core member 23 has a tube shape and is arranged in the height direction of the solenoid coil 21.

The wire ends 22a and 22
b is wound at an appropriate bending strain rate, and the solenoid coil 21
Superimposed farthest from The overlapped ends are fixed by winding a glass tape from above. The superposed portion 22c was inserted into the heater 24 and heated at a temperature of about 800 ° C. The heater 24 does not cover the entire wire wound around the core material 23 but covers a part thereof. By heating
Diffusion bonding occurred, and both ends of the wire constituting the solenoid coil were bonded.

Next, as shown in FIG.
A heater 25 and a thermocouple 26 are provided near 2c,
They were covered with an epoxy resin layer 27 together with the wire. The thickness of the epoxy resin layer was, for example, 1 to 2 cm. Further, as shown in the figure, electrodes 28a and 28b for supplying a current to the solenoid coil 21 were formed. In such a structure, the portion of the oxide superconducting wire heated by the heater 25 is the solenoid coil 2
1 acts as a permanent current switch for In such a structure, the number of junctions is one, and an increase in resistance value due to the junction is hardly minimized.

The obtained structure is immersed in liquid helium,
A current is supplied to the heater 25 to perform heating, and the portion of the oxide superconducting wire that functions as a permanent current switch is brought into a normal conducting state (1).
(Temperature of 10K or more). In that state, the electrode 28
A current was supplied from the power supply to the solenoid coil 21 through the terminals a and b. When the solenoid coil 21 was excited, the heating of the heater 25 was stopped, and the oxide superconductor at the switch was returned to the superconducting state. As a result of removing the excitation power supply in this state, a state of the permanent current mode was obtained. In the structure shown in FIG. 3, the electrical resistance of the junction is 0.1 n
Ω or less. On the other hand, when bonding was performed using eutectic solder, the measured value of the electrical resistance was about 10 nΩ. As described above, according to the present invention, it was possible to remarkably reduce the electric resistance due to the joining.

Example 2 In Example 1, a superconducting coil with a permanent current switch was manufactured by performing diffusion bonding at one place using one superconducting wire. However, diffusion bonding was performed using two or more superconducting wires. May be performed at two or more locations. For example, as shown in FIG. 4, the bismuth-based oxide superconducting terminals 32 a and 32 b constituting the solenoid coil 31 and a separately prepared bismuth-based oxide superconducting wire 42 are wound around a core 33, respectively, and the terminals are overlapped. Was. In this case, the joining was performed at two places of the joining portions 32c and 42c. Heating furnace 34
As a result, the joints 32c and 42c were covered, and heating was performed in the same manner as in the above-described embodiment, thereby performing diffusion bonding. The core 33 was composed of a cylinder 33a made of stainless steel and a cylinder 33b made of zirconia ceramics, similarly to the first embodiment, and the portion of the wire wound around the stainless steel was heated.

Next, as shown in FIG.
Heaters 35 and thermocouples 36 are located near 2c and 42c.
And these were covered with an epoxy resin 37 together with the wire. Further, electrodes 38a and 38b were formed, and a superconducting coil with a permanent current switch was obtained. The portion of the wire that can be changed to the normal conduction state by heating the heater 35 functions as a permanent current switch. The obtained structure was immersed in liquid helium, and a current was applied to the heater 35 to perform heating, so that the portion of the oxide superconducting wire serving as a permanent current switch was changed to a normal conducting state (temperature of 110 k or more). In this state, a current was supplied from the power supply to the solenoid coil 31 via the electrodes 38a and 38b. When the solenoid coil 31 was excited, the heating of the heater 35 was stopped, and the oxide superconductor in the switch portion was returned to the superconducting state. As a result of removing the excitation power supply in this state, a state of the permanent current mode was obtained. In the structure shown in FIG. 5, the electric resistance of the junction was about 0.15 nΩ or less.

[0034]

As described above, according to the present invention, a superconducting coil with a permanent current switch having the following characteristics can be provided.

(1) The electric resistance is remarkably small when the permanent current switch is ON. (2) The electric resistance is sufficiently large when the permanent current switch is turned off.

(3) A sufficient current can be stably supplied to the permanent current switch. (4) In the permanent current switch, the switching time is short, and the reliability and operability are high.

Further, according to the present invention, the operation of the superconducting coil can be stably performed in the permanent current mode not only at a liquid helium temperature of 4.2 K but also at a temperature equal to or higher than the liquid nitrogen temperature (77.3 K). . The present invention is effective when used for an energy storage magnet, a linear motor, an NMR analysis magnet, and the like.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining an operation method in a permanent current mode.

FIG. 2 is a schematic view showing a process for manufacturing a superconducting coil with a permanent current switch according to the present invention.

FIG. 3 is a schematic view showing an example of a superconducting coil with a permanent current switch according to the present invention.

FIG. 4 is a schematic view showing another process for manufacturing a superconducting coil with a permanent current switch according to the present invention.

FIG. 5 is a schematic view showing another example of a superconducting coil with a permanent current switch according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Superconducting magnet 2 Permanent current switch 3 Power supply 21 Solenoid coil 22a, 22b End of wire 23 Core material 24 Heater 25 Heater 26 Thermocouple 27 Epoxy resin layer 28a, 28b Electrode

Claims (8)

[Claims] 1. A superconducting coil with a permanent current switch, comprising a superconducting coil and a permanent current switch for short-circuiting the superconducting coil, wherein both the superconducting coil and the permanent current switch cover a bismuth-based oxide superconductor. An oxide superconducting wire comprising a stabilizing material made of silver or a silver alloy is provided, and the oxide superconducting wire constituting the superconducting coil and the persistent current switch, respectively,
A superconducting coil with a permanent current switch, wherein the stabilizing members are joined by diffusion joining. 2. The method according to claim 2, wherein both terminal portions of the oxide superconducting wire forming the superconducting coil are joined by diffusion bonding of the stabilizing materials, and the both terminal portions are used as the permanent current switch. The superconducting coil with a permanent current switch according to claim 1, characterized in that: 3. The oxide superconducting wire of the permanent current switch is wound in a coil around a core, and a portion of the core near the superconducting coil is made of ceramics having a low thermal conductivity. 3. The superconducting coil with a permanent current switch according to claim 1, wherein a portion remote from the superconducting coil is made of stainless steel. 4. The permanent current switch further comprises a heating element for heating the oxide superconductor, and a heat insulating material covering the heating element.
4. The superconducting coil with a permanent current switch according to claim 3. 5. A sintered body of a bismuth-based oxide superconductor,
A step of forming a superconducting coil by winding an oxide superconducting wire made of a stabilizing material made of silver or a silver alloy covering the same, leaving both ends thereof, a step of overlapping the both ends with each other, Heating at a temperature of 500 ° C. to 900 ° C. to diffuse and join the stabilizers at both ends, and using the joined both ends as a permanent current switch. Of manufacturing a superconducting coil. 6. Heating is performed with both end portions wound around a core material. In the core material, a portion close to the superconducting coil is made of ceramics having a low thermal conductivity, and a portion far from the superconducting coil is made of stainless steel. The method for manufacturing a superconducting coil with a permanent current switch according to claim 5, wherein the portion to be joined by heating is provided on a portion of the core material made of the stainless steel. 7. A sintered body of a bismuth-based oxide superconductor,
A sintered body of a bismuth-based oxide superconductor and silver or silver covering the superconducting coil are wound at both ends of a superconducting coil wound with a first oxide superconducting wire made of a stabilizing material made of silver or a silver alloy. A step of overlapping both ends of a second oxide superconducting wire made of a stabilizer made of a silver alloy, and a step of heating the overlapped portion at a temperature of 500 ° C. to 900 ° C. A method for producing a superconducting coil with a permanent current switch, comprising: using a second oxide superconducting wire as a permanent current switch. 8. A heating method in which the second oxide superconducting wire is wound around a core material, wherein a portion of the core material close to the superconducting coil is made of ceramics having a low thermal conductivity, and is far from the superconducting coil. The method for manufacturing a superconducting coil with a permanent current switch according to claim 7, wherein the portion is made of stainless steel, and the portion to be joined by heating is provided on a portion of the core material made of the stainless steel. .