JPH07192837A - Connecting method for oxide superconducting wire material - Google Patents

Abstract

PURPOSE:To provide a connecting method by which oxide superconducting wire materials can be connected to each other in low connecting resistance without reducing a critical current. CONSTITUTION:In a connecting method for oxide superconducting wire materials 3 to be connected to each other in the superposed part by superposing a part of the oxide superconducting wire materials 3 formed by covering an outer peripheral surface of an oxide superconductor 1 with conductive metal 2 on each other, a thickness of at least either one conductive metal 2 in an opposing part is thinned and removed by 95% at its maximum by superposition of the oxide superconducting wire materials 3. Next, a paste layer which contains metal to form the conductive metal 2 and has a thickness not more than 200mum is laid and formed on a layer surface of the thinned and removed conductive metal 2. The oxide superconducting wire materials 3 are superposed on each other through the laid and formed paste layer, and heat treatment is applied after drying.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for connecting oxide superconducting wires, which is capable of connecting oxide superconducting wires with a low connection resistance without lowering the critical current of the wires.

[0002]

2. Description of the Related Art As is well known, since the advent of La-based oxide superconductors, the critical temperature of superconducting materials has exceeded the liquid nitrogen temperature of 77.3K, and these oxide superconductors have
Since the required superconducting properties are expected in the state of being cooled by liquid nitrogen, the range of its application is significantly expanded compared to metal-based superconductors that require advanced cryogenic technology for cooling liquid helium. Is being noticed.

By the way, the field of application of the oxide superconductor is, for example, electric power / industrial electronic equipment, and specifically, for example, a power transmission cable and a superconducting coil for generating a high magnetic field. But,
In either case, the oxide superconductor is generally a composite with a metal in consideration of mechanical strength and stability of characteristics.
Since it is used in the form of a wire rod by lengthening it, it can be said that the focus is on making the oxide superconductor wire rod.

Various methods have been proposed for manufacturing oxide superconducting wire, and at present, as shown in FIG.
The powder method (powder in tube method) whose manufacturing process is shown in a flow chart is generally used. Then, this powder method is applied to a calcined oxide superconductor such as Bi _1.72 Pb _0.34 S.
r _1.83 Ca _1.97 Cu _3.13 O _x powder is filled into a metal tube that serves as a sheath, for example, a silver tube, and then drawn by cold surface-reduction processing such as swaging and drawing, and further subjected to final heat treatment. This is a method of forming the tape-shaped oxide superconducting wire 3 by coating the outer peripheral surface of the oxide superconductor 1 with the conductive metal layer 2.

The connection of the superconducting wire is an important issue in practical use regardless of whether it is a metal-based superconductor or an oxide-based superconductor. In the case of a coil, it is necessary to make the resistance of the connection portion when forming a closed loop circuit as close to zero as possible in order to prevent current decay. Regarding this connection resistance, in the case of an MRI magnet configured by using a niobium-titanium wire, a connection is formed by solid-phase diffusion of filaments, and a low connection resistance of 10 ^-8 Ω or less is obtained. on the other hand,
To make an MRI magnet with oxide superconducting wire,
The same connection resistance must be realized, and in the configuration of power transmission cables and laminated superconducting coils, the connection resistance value is not as strict as MRI, but the length of the wire can be increased by the connection or the pancake coil When connecting between, so-called wire connecting technology is required.

In response to such demands, various attempts have been made to connect oxide superconducting wire materials. Broadly speaking, (A) a conductive metal layer (sheath material) covering the oxide superconductor 1 is used. 2) a method of peeling and connecting 2), and (B) a method of directly connecting the conductive metal layer 2 without peeling. In the former, as shown in a perspective view in FIG. 11, the silver sheath 2 on one surface of the tape-shaped oxide superconducting wire 3 is peeled and removed, and the exposed oxide superconductors 1 are brought into contact with each other,
Further, there is a method of connecting by repeating pressing and heat treatment (connection method (a)). The latter is a method of connecting the tape-shaped oxide superconducting wires 3 by soldering (connection method (b-1)), overlapping the tape-shaped oxide superconducting wires 3 with each other, and performing solid phase diffusion by heat treatment. In the connection method (connection method (b-2)) and the connection method (b-2), the tape-shaped oxide superconducting wires 3 are pressure-bonded and heat-treated by a press (connection method (b-3)). ).

Here, in considering the superiority or inferiority of the connection method, (1) the connection resistance is small, (2) the critical current (Ic) flowing through the wire is not deteriorated near the connection point, etc. . In particular, regarding the point (2), mechanical deterioration such as chemical deterioration such as melting or decomposition of oxide superconductor and cracking due to mechanical processing is not caused with connection work. Requires that.

[0008]

However, in any of the above connection methods (a), (b-1), (b-2), and (b-3), the above (1) and (2) It is not possible to satisfy both conditions at the same time. For example, in the case of the connection method (a), although the connection resistance can be almost completely reduced to zero, as shown in the perspective view developed in FIG. Ic deteriorates. So in this connection method (a),
Since the thickness of the oxide superconducting layer is discontinuous in the portions A and A ', when the tape-shaped oxide superconducting wire rods 3 are stacked and pressed, excessive shear stress is concentrated and cracks occur. It is thought to occur. Also, in the case of the connection method (b-3), although the degree of deterioration of the critical current Ic is smaller than that of the connection method (a), stress concentration is likely to occur similarly.

On the other hand, in the case of the connection methods (b-1) and (b-2), although the deterioration of the critical current Ic is hardly seen, the connection resistance value is relatively large. For example, in the case of soldering with connection method (b-1), the connection resistance is the resistivity (Pb-6
20 ° C. in 3% Sn system, in 14.8Myuomega _cm, are governed by about 10-fold) of silver, in the connection area of for example 2.8 × 5 mm, as shown in FIG. 12, the order to 10 ^-6 Omega connection resistance I can only get it. In the case of the connection method (b-2), a sheath material layer is formed as shown in FIG. 13 (a) schematically and in FIG. 14 (a) with a micrograph showing the cross-sectional structure of the connection portion. A void 4 remains between the upper and lower silver layers 2 and 2, as shown in FIG.
At present, only connection resistance of about 10 ^-6 Ω can be obtained.

The present invention has been made in consideration of the above circumstances, and provides an oxide superconducting wire rod which can be easily achieved such as lengthening of the oxide superconducting wire rod with a low connection resistance without lowering the critical current. The purpose is to provide a connection method.

[0011]

A method of connecting oxide superconducting wires according to the present invention is a method in which oxide superconducting wires formed by coating an outer peripheral surface of an oxide superconductor with a conductive metal layer are partially overlapped with each other. , In the connecting method of the oxide superconducting wire to be connected at the overlapping portion, at least one of the portions of the conductive metal layer facing each other due to the superposition of the oxide superconducting wire is reduced in thickness by up to 95%. The step of removing, the step of depositing a paste layer having a thickness of 200 μm or less containing the metal forming the conductive metal layer on the surface of the thinned / removed conductive metal layer, and the paste layer formed by the deposition. It is characterized by comprising a step of superposing the oxide superconducting wires through the layer, drying and then heat treating.

The method for connecting oxide superconducting wires according to the present invention is a method for forming a pseudo superconducting connection by arranging a thin normal conductive metal layer between oxide superconductor layers that are in contact with each other and connected. By making the resistance of the normal-conducting layer as small as possible, it is possible to obtain connection characteristics that can be regarded as superconducting connection in practical use. Here, the purpose of thinning / removing the conductive metal layer in the overlapping portion (region) for connection is to reduce the thickness of the intermediate normal conductor layer,
Examples of the method of thinning / removing include a mechanical method using sandpaper, a grinder or the like, or a chemical method of dissolving and electrolytically etching with an acid (sulfuric acid, nitric acid, etc.). Then, the thickness d ₁ of the portion of wall thinning and removal is, d ₁ / d ₀ ≦ 0 the original sheath thickness d _0.
95, preferably 0.1 ≦ d ₁ / d ₀ ≦ 0.95, more preferably 0.5 ≦ d ₁ / d ₀ ≦ 0.8. Experimentally, d ₁ is 0
Even if it is close to the above, it is effective in reducing the connection resistance only by removing oils and fats and salts attached to the surface of the conductive metal. Furthermore, if the thickness of the intermediate conductive metal layer is reduced, it is possible to obtain a lower connection resistance. on the other hand,
If d ₁ / d ₀ exceeds 0.95, the oxide superconductor layer may be partially exposed to the acid and deteriorate due to uneven etching.

Next, the purpose of interposing a paste containing a metal of the same kind as the conductive metal functioning as a coating material between the surfaces on which the oxide superconducting wire is superposed is to contact the thinned conductive metal layers. To make the surface contact as close as possible. For example, as shown in perspective in FIG.
When there is nothing intervening between the conductive metal layers 2 and 2 when superposing the tape-shaped oxide superconducting wires, in other words, when they correspond to the connection method (b-3), the contact between them is point contact. As a result, as shown schematically in FIG. 13 (b) and in the micrograph in FIG. 14 (b), which show the cross-sectional structure of the connection portion, sufficient integration does not occur in the subsequent heat treatment step. In other words, the voltage-current characteristic when connecting with this connection method is
As shown in FIG. 15, the situation is such that the characteristics cannot be satisfied. However, applying (adhering) the paste,
By interposing, the fine conductive metal particles (usually a few μm or less) contained in the paste material can fill the irregularities on the surface of the conductive metal sheath and bring them into close contact with each other. Becomes In other words, the paste layer thickness is 2
After application to a thickness of 00 μm or less, the conductive metal layers are pressure-bonded to each other at the stage of exhibiting fluidity and dried as it is, whereby a dense connection structure can be formed. Here, the thickness of the applied paste is preferably as thin as possible so long as there is no coating unevenness, and if it exceeds 200 μm, the connection resistance becomes larger than that in the case of solder connection.

Furthermore, the heat treatment conditions (temperature, time, etc.) for integrating the conductive metal sheaths forming the connected portions by solid-phase reaction via the paste layer formed by the deposition form the sheath material. It is appropriately selected and set according to the type of conductive metal and the thickness after thinning / removing. For example, when the sheath material is silver and the thickness after thinning / removing is about 6 to 15 μm, 820 to 850 ℃, 10 to 50 hours is enough.

The oxide superconductors to which the present invention is applicable include rare earth element-containing oxide superconductors represented by Y system, La-Sr-Cu-O system, and Bi-Sr-Ca- system.
Cu-O system, Tl-Ba-Ca-Cu-O system, Nd-C
Various oxide superconductors, such as an e-Cu-O-based oxide superconductor, may be mentioned. The rare earth element-containing oxide superconductor may be any one as long as it can realize a superconducting state. For example, ReM ₂ Cu ₃ O _7-δ (Re is Y, La, S
c, Nd, Sm, Eu, Gd, Dy, Ho, Er, T
At least one element selected from rare earth elements such as m, Yb, and Lu, M is at least one element selected from Bi, Sr, and Ca, δ is an oxygen defect, usually a number of 1 or less, Cu Some are Ti, V, Cr, Mn, Fe, Co,
Examples thereof include oxides represented by (which can be replaced by Ni, Zn, etc.).

The Bi-based oxide superconductor has the chemical formula: Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x ,: Bi ₂ (Sr,
Ca) ₃ Cu ₂ O _{x and the} like, and the Tl-based oxide superconductor has a chemical formula: Tl ₂ Ba _{2
Ca 2 Cu 3 O y,:} Tl 2 (Sr, Ca) 3 Cu 2 O
It is substantially represented by _{y and the} like, and silver oxide superconductors to which silver is added may be used.

On the other hand, the conductive metal sheath material of the oxide superconducting wire is a material which is not oxidized by the internal (embedding) oxide superconductor at room temperature or high temperature (heat treatment temperature of 800 to 1000 ° C.). And must be electrically good conductors so that they can bypass the current flowing when the superconductivity is broken. Specifically, silver, gold, platinum, palladium, copper and alloys thereof are preferable, and silver is more preferable. When silver or silver alloy is used as the sheath material, silver paste is selected as the paste. Further, for the purpose of improving the mechanical strength of the conductive metal sheath material, a composite material to which a trace amount of metal oxide (tin oxide, zinc oxide, aluminum oxide, etc.) is added can be used. Further, before or after the heat treatment, the connecting portion can be reinforced with a metal splint 5 of the same kind as the sheath material, as shown in a perspective view in FIG.

[0018]

According to the present invention, the conductive metal sheath surfaces of the connecting portions to be superposed on each other are substantially cleaned by thinning / removing, while the paste layer is interposed between the facing and facing surfaces. Integral by solid-state reaction in the form The conductive metal sheath surface of the connected portion facing and confronting with each other by the paste layer containing the fine metal particles exhibits uniform surface contact and a solid-phase reaction is performed, so that a void or the like remains. A precise integration is achieved. For example, as shown schematically in Fig. 3 (a) and in a micrograph in Fig. 3 (b), the cross-sectional structure of the connecting portion is shown respectively, and the upper and lower silver layers 2 and 2 forming the sheath material layer are uniform. It is possible to easily and surely form a precise and low resistance connecting portion. That is, according to the connection method of the present invention, the connection resistance is smaller than that of a general solder connection method by one digit or more, the connection location has sufficient strength, and the critical current (Ic) deteriorates in the vicinity of the connection section. It is possible to provide a wire rod that does not wake up (provide a long wire rod).

[0019]

EXAMPLES Examples of the present invention will be described below.

Example 1 Prepared by the oxalate coprecipitation method, the result of IPC analysis was as follows:
Bi: Pb: Sr: Ca: Cu = 1.72: 0.34: 1.83: 1.9
A powder having a composition of 7: 3.13 was prepared. This powder 100
The g was stored in a MgO sample dish, calcined in the air at 800 ° C. for 20 hours, and then pulverized to a diameter of 1 to 10 μm to obtain a calcined powder. When the phase of this calcined powder was examined by X-ray diffraction,
(Bi, Pb) ₂ Sr ₂ CaCu ₂ O _x (low Tc phase)
And a mixed phase of Ca ₂ PbO ₄ and CuO.

The calcined powder was filled in a silver tube having an outer diameter of 6 mm and an inner diameter of 5 mm, and a tape material having a width of 2.5 mm and a thickness of 0.15 mm was obtained by drawing (drawing) and rolling (rolling). After that, oxygen was heat-treated at 835 ° C for 50 hours in an atmosphere of 7.7% -argon (balance) system, and again,
Secondary rolling was applied to obtain a tape-shaped oxide superconducting wire with a width of 2.8 mm and a thickness of 0.13 mm. That is, B added with Pb
i-based (Bi, Pb) ₂ Sr ₂ Ca ₂ Cu ₃ O _x (high T
The c-phase) silver sheath tape wire was manufactured by the so-called powder method (powder in tube method).

The tape wire was cut to a length of 20 mm, and the portion corresponding to the connecting portion was stored in a beaker and had a concentration of 20% nitric acid (3 times that of commercially available concentrated nitric acid), as schematically shown in FIG. (Diluted product) 6 and then the platinum electrode 7 is used as a cathode (-) and the tape wire piece 8 is used as an anode (+) at 100 mA, t
A direct current was applied for 2 seconds to perform etching treatment. The energization time t seconds was changed to 0, 10, 30, 60, 120, 180, and etching treatment was performed, followed by washing with water for several seconds to remove nitric acid and then drying. As a result of the etching, the oxide film and the dirt on the surface of the silver sheath were completely removed, and the thickness of the silver sheath was reduced from the initial 30 μm at the rate of the thickness shown in Table 1. FIG. 5 is a cross-sectional view showing a state where the silver sheath 2 is thinned / removed by the etching, and in Table 1, d ₀ is a value before etching and d ₁ is a value after etching. Next, as shown in a perspective view in FIG. 6, two pieces of the above-mentioned etched tape wire rods are faced to each other, and silver paste is applied in a thickness of 10 μm on the overlapping portions (areas), and the overlapping margin is set to 5 mm. After drying, they were placed in a furnace and heat-treated in the air at 840 ° C for 50 hours to connect and integrate them. The sample integrated by the heat treatment is taken out of the furnace, the voltage terminals are arranged in the portion including the connection region and the portion not including the connection region, and the critical current in liquid nitrogen and zero magnetic field is measured by the 4-terminal resistance method. (Ic) When measured, the connection resistance values at that time are shown in Table 1.
It was as shown together with. Note that FIG. 7 shows an example of voltage-current characteristics in the connection portion connected by the connection method.

For comparison, when a tape piece cut into a length of 20 mm is heat treated in the same manner as described above, the overlap margin is set to 5 mm in any case, and nitric acid etching treatment is performed before solder connection. Table 2 shows the results of measuring the connection resistance and the like under the same conditions as above (Comparative Example 1) and the case of connecting by soldering without etching (Comparative Example 2). As can be seen from Table 1 and Table 2, in the case of the connection method according to the present invention, the connection resistance is lower than that of the comparative example, and in particular, the connection resistance is two or more orders of magnitude lower than that of the comparative example 2. Can be realized. Further, it can be seen that the connection resistance can be reduced as the etching amount increases.

Example 2 Length 20 mm cut from the tape wire produced in Example 1
The tape wire piece of No. 1 was subjected to a nitric acid etching treatment for 120 seconds, and the coating layer thickness d _p of the silver paste was 5, 10, 20, 50, 100, 200 μm.
Table 3 shows the results of examining the connection resistance by connecting the tape wire rod pieces in the same manner as in Example 1 by changing the above. In addition,
The connection resistance when the thickness of the paste coating layer is 200 μm is
It is almost the same as the case of connection by soldering. Example 3 Length 20 mm cut out from the tape wire produced in Example 1
The piece of tape wire of No. 1 was subjected to nitric acid etching for 120 seconds, and the thickness d _p of the coating layer of silver paste was set to 10 μm.
Table 4 shows the results of examining the connection resistance by changing the l (mm) to 5, 10, 15 mm, connecting the tape wire pieces in the same manner as in Example 1. It can be seen that the connection resistance decreases in inverse proportion to the connection area.

Example 4 Using the tape wire material produced in Example 1, a one-turn coil having a diameter of 8 cm was produced as shown in a perspective view in FIG.
That is, the tape wire is cut in advance to a required length, and in the silver sheath region forming the joint, paraffin is applied to the opposite surface so that only the surfaces facing each other are etched (masking), After immersion in nitric acid for 60 seconds for etching, after removing paraffin with acetone, the overlap margin is set to 20 mm, the tape wire piece 3'is connected in the same manner as in Example 1, and one turn coil is produced. did. In this case, since the strength is maintained by the one-side sheath material that remains without being etched, a low resistance connection can be formed without significantly reducing the strength.

In order to evaluate the connection resistance of the one-turn coil manufactured as described above, a loop current was caused to flow by electromagnetic induction while the coil was immersed in liquid nitrogen, and its decay was examined. As a result, a decay time constant of 126 sec was obtained. . Since the inductance of the coil is about 10 µH, the resistance of the connection is estimated to be about 10 ^-7 Ω. When a similar coil was produced by solder connection (Comparative Example 3), the decay time constant was 26 microseconds, and the connection resistance was only 3.5 × 10 ⁻⁴ Ω.

Example 5 Using the tape wire produced in Example 1, a double pancake coil having an outer diameter of 35 mm, an inner diameter of 20 mm and a height of 15 mm, as shown in a perspective view in FIG. 9, was produced. That is, the tape wire is cut into a length of 4 m, wound on a cylindrical winding frame 9, and then joined at one outermost turn at the final stage of winding. First, in the silver sheath region forming the joined portion. Apply paraffin to the opposite surface so that only the surfaces facing each other are etched, dip it in nitric acid and perform an etching treatment for 120 seconds, remove the paraffin with acetone, and then stack it. To 4 cm, apply a silver paste to the overlap area to a thickness of 10 μm, and then temporarily superimpose the surfaces to be joined via a silver paste layer and then in the air at 840 ° C.
It was heat-treated for 50 hours to prepare a double pancake coil.

In the double pancake coil produced above,
After the resin impregnation treatment and the insulation treatment, a loop current was caused to flow by electromagnetic induction in the state of being immersed in liquid nitrogen as in the case of Example 4, and the decay was examined. The decay time constant was 226 seconds. From this value, the connection resistance is 6.0 × 10
^-8 Ω was required.

[0029]

As can be seen from the above description, the method of connecting oxide superconducting wires according to the present invention enables connection with a connection resistance that is 1 to 2 orders of magnitude lower than that in the case of soldering connection. It is possible to realize an excellent connection without the critical current and strength of the wire material being deteriorated around the connection point. In other words, it also opens the way to applications requiring permanent current mode energization such as MRI. On the other hand, by this connection method, it is possible to reduce power transmission loss in the power transmission cable configured by connecting and lengthening the oxide superconducting wire.
In addition, the superconducting coil configured can reduce heat generation during energization. In any case, it can be said that the connection method according to the present invention greatly contributes to the practical application of the oxide superconducting wire, which has received much attention from the viewpoint of excellent characteristics.

[0030]

[Brief description of drawings]

FIG. 1 is a perspective view used for explaining an example of an embodiment of a connection method according to the present invention.

FIG. 2 is a schematic perspective view showing an embodiment example of a connection method according to the present invention.

3A and 3B show an example of a cross-sectional structure of a connecting portion by the connecting method according to the present invention, in which FIG. 3A is a schematic diagram and FIG.

FIG. 4 is a schematic diagram showing an example of an embodiment for processing a connected portion in the connection method according to the present invention.

FIG. 5 is a cross-sectional view schematically showing a state after processing a connected portion in the connecting method according to the present invention.

FIG. 6 is a schematic perspective view showing another embodiment example of the connection method according to the present invention.

FIG. 7 is a diagram showing an example of voltage-current characteristics of a wire connected by the connecting method according to the present invention.

FIG. 8 is a perspective view of a one-turn coil showing an application example of the connection method according to the present invention.

FIG. 9 is a perspective view of a double pancake coil showing another application example of the connection method according to the present invention.

FIG. 10 is a flowchart showing a manufacturing process of an oxide superconducting tape wire by a powder method.

FIG. 11 is a schematic perspective view showing an example of an embodiment of a conventional connecting method.

FIG. 12 is a voltage-current characteristic diagram of wires connected by the first conventional connecting method.

13 (a) and 13 (b) are schematic views showing examples of cross-sectional structures of connecting portions by different conventional connecting methods.

14 (a) and 14 (b) are photomicrographs showing examples of the cross-sectional structure of a connecting portion by different conventional connecting methods.

FIG. 15 is a voltage-current characteristic diagram of wires connected by a second conventional connecting method.

FIG. 16 is a voltage-current characteristic diagram of wires connected by a third conventional connecting method.

[Explanation of symbols]

1 ... Oxide superconductor 2 ... Conductive metal layer (sheath material)
3 ... Oxide superconducting wire 3 '... Oxide superconducting wire piece 4 ... Void part 5 ... Metal splint 6 ... Nitric acid (etching liquid) 7 ... Platinum electrode 8 ... Tape-shaped wire piece 9 ... Winding frame

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[Procedure amendment]

[Submission date] November 18, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be amended] Detailed explanation of the invention

[Correction method] Change

[Correction content]

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for connecting oxide superconducting wires, which is capable of connecting oxide superconducting wires with a low connection resistance without lowering the critical current of the wires.

[0002]

2. Description of the Related Art As is well known, since the advent of La-based oxide superconductors, the critical temperature of superconducting materials has exceeded the liquid nitrogen temperature of 77.3 K, and these oxide superconductors have liquid nitrogen. In the cooled state, the expected superconducting properties are expected, so it should be noted that the range of its application will be much wider than that of metal-based superconductors that require advanced cryogenic technology for cooling liquid helium. Has been done.

By the way, the field of application of the oxide superconductor is, for example, electric power / industrial electronic equipment, and specifically, for example, a power transmission cable and a superconducting coil for generating a high magnetic field. But,
In either case, the oxide superconductor is generally a composite with a metal in consideration of mechanical strength and stability of characteristics.
Since it is used in the form of a wire rod by lengthening it, it can be said that the focus is on making the oxide superconductor wire rod.

Various methods have been proposed for manufacturing oxide superconducting wire, and at present, as shown in FIG.
The powder method (powder in tube method) whose manufacturing process is shown in FIG. And this powder method is based on a calcined oxide superconductor such as Bi _1.72 Pb.
A powder of _0.34 Sr _1.83 Ca _1.97 Cu _3.13 O _x is filled in a metal tube that serves as a sheath, for example, a silver tube, and then expanded by cold surface-reduction processing such as swaging and drawing. Line, and then perform the final heat treatment,
This is a method of forming a tape-shaped oxide superconducting wire 3 in which the outer peripheral surface of the oxide superconductor 1 is covered with a conductive metal layer 2.

The connection of the superconducting wire is an important issue in practical use regardless of whether it is a metal-based superconductor or an oxide-based superconductor. In the case of a coil, it is necessary to make the resistance of the connection portion when forming a closed loop circuit as close to zero as possible in order to prevent current decay. Regarding this connection resistance, in the case of an MRI magnet constructed by using a niobium-titanium wire, a connection is formed by solid-phase diffusion of filaments, and a low connection resistance of 10 ⁻⁸ Ω or less is obtained. On the other hand, in order to manufacture an MRI magnet with an oxide superconducting wire, the same connection resistance must be realized, and in the structure of a power transmission cable or a laminated superconducting coil, the connection resistance value is not as strict as MRI. Though
A so-called wire connecting technique is required when a wire is lengthened by connection or when pancake coils are connected.

In response to such demands, various methods for connecting oxide superconducting wires have been attempted. Broadly speaking, (A) a conductive metal layer (sheath material) covering the oxide superconductor 1 is used. 2) a method of peeling and connecting 2), and (B) a method of directly connecting the conductive metal layer 2 without peeling. In the former case, as shown in a perspective view in FIG. 11, the silver sheath 2 on one surface of the tape-shaped oxide superconducting wire 3 is peeled off and removed, and the exposed oxide superconductors 1 are brought into contact with each other, and further pressed. And a method of connecting by repeating heat treatment (connection method (a)). Further, on the latter, a method of connecting the tape-shaped oxide superconducting wires 3 with each other by soldering (connection method (b-1)), overlapping the tape-shaped oxide superconducting wires 3 with each other, and performing solid phase diffusion by heat treatment. Connection method (connection method (b-2)), and the connection method (b-
2)), a method in which the tape-shaped oxide superconducting wire rods 3 are pressure-bonded and heat-treated by a press (connection method (b-
There is 3)).

Here, in considering the superiority or inferiority of the connection method, (1) the connection resistance is small, (2) the critical current (Ic) flowing through the wire is not deteriorated in the vicinity of the connection point, and so on. . In particular, regarding the point (2), mechanical deterioration such as chemical deterioration such as melting or decomposition of the oxide superconductor and generation of cracks due to mechanical processing are not caused with connection work. Requires that.

[0008]

However, in any of the connection methods (a), (b-1), (b-2), and (b-3) described above, the above (1) and (2) are used. It is not possible to satisfy both conditions at the same time. For example, in the case of the connection method (a), although the connection resistance can be almost completely reduced to zero, as shown in the developed and perspective view of FIG. Ic deteriorates. That is, in this connection method (a), since the thickness of the oxide superconducting layer is discontinuous in the portions A and A ′, when the tape-shaped oxide superconducting wires 3 are stacked and pressed, It is considered that excessive shear stress concentration causes cracks. In the case of the connection method (b-3), the critical current I
Although the degree of deterioration of c is smaller than that of the connection method (a), stress concentration is likely to occur similarly.

On the other hand, the connection methods (b-1) and (b-2)
In the case of 1, the deterioration of the critical current Ic is hardly seen, but the connection resistance value is relatively large. For example, in the case of soldering in the connection method (b-1), the connection resistance is a resistivity peculiar to the solder (Pb-63% Sn system, 20 ° C., 14.8 μ
Ω _cm, which is about 10 times that of silver). For example, a connection area of 2.8 × 5 mm gives a connection resistance of about 10 ⁻⁶ Ω as shown in FIG. In the case of the connection method (b-2), a sheath material layer is formed as shown in FIG. 13 (a) schematically and in FIG. 14 (a) with a photomicrograph, showing the cross-sectional structure of each connection portion. At present, the void 4 remains between the upper and lower silver layers 2 and 2, and as shown in FIG. 15, only a connection resistance of about 10 ⁻⁶ Ω can be obtained.

The present invention has been made in consideration of the above circumstances, and provides an oxide superconducting wire rod which can be easily achieved such as lengthening of the oxide superconducting wire rod with a low connection resistance without lowering the critical current. The purpose is to provide a connection method.

[0011]

A method of connecting oxide superconducting wires according to the present invention is a method in which oxide superconducting wires formed by coating an outer peripheral surface of an oxide superconductor with a conductive metal layer are partially overlapped with each other. In the method of connecting oxide superconducting wires connected at the overlapping portion, the thickness of at least one of the conductive metal layers in the contacting portion due to the overlapping of the oxide superconducting wires is reduced by up to 95%. Removal process,
A step of depositing a paste layer having a thickness of 200 μm or less containing a metal forming the conductive metal layer on the surface of the thinned / removed conductive metal layer, and an oxide through the paste layer formed by the deposition. After superposing the superconducting wires and drying,
It is characterized by including a step of performing heat treatment.

The method for connecting oxide superconducting wires according to the present invention is a method for forming a pseudo superconducting connection by arranging a thin normal conductive metal layer between oxide superconductor layers that are in contact with each other and connected. By making the resistance of the normal-conducting layer as small as possible, it is possible to obtain connection characteristics that can be regarded as superconducting connection in practical use. Here, the purpose of thinning / removing the conductive metal layer in the overlapping portion (region) for connection is to reduce the thickness of the intermediate normal conductor layer,
Examples of the method of thinning / removing include a mechanical method using sandpaper, a grinder or the like, or a chemical method of dissolving and electrolytically etching with an acid (sulfuric acid, nitric acid, etc.). The thickness d ₁ of the portion to be thinned / removed is d ₁ / d ₀ ≦ the original sheath thickness d ₀ .
0.95, preferably 0.1 ≦ d ₁ / d ₀ ≦ 0.95,
More preferably, 0.5 ≦ d ₁ / d ₀ ≦ 0.8. Experimentally, even if d ₁ is close to 0, it is effective in reducing the connection resistance only by removing oils and fats and salt adhering to the surface of the conductive metal. Furthermore, if the thickness of the intermediate conductive metal layer is reduced, it is possible to obtain a lower connection resistance. On the other hand, if d ₁ / d ₀ exceeds 0.95, the oxide superconductor layer may be partially exposed to the acid and deteriorate due to uneven etching.

Next, the purpose of interposing a paste containing a metal of the same kind as the conductive metal functioning as a coating material between the surfaces on which the oxide superconducting wire is superposed is to contact the thinned conductive metal layers. To make the surface contact as close as possible. For example, as shown in perspective in FIG.
When the tape-shaped oxide superconducting wires are superposed, if there is nothing interposed between the conductive metal layers 2 and 2, in other words, if they correspond to the connection method (b-3), the contact between them is point contact. As a result, as shown schematically in FIG. 13 (b) and in the micrograph in FIG. 14 (b), which show the cross-sectional structure of the connecting portion, sufficient integration does not occur in the subsequent heat treatment step. That is, the voltage-current characteristics when the connection is made by this connection method are as shown in FIG. 15, and the characteristics cannot be satisfied. However, when the paste is applied (deposited) and interposed, fine conductive metal particles (usually several μm or less) contained in the paste material are
It becomes possible to fill the irregularities on the surface of the conductive metal sheath and bring them into contact with each other in a state close to surface contact. That is, after the paste layer is applied to a thickness of 200 μm or less, the conductive metal layers are pressure-bonded to each other at the stage of exhibiting fluidity and dried as it is, whereby a dense connection structure can be formed.
Here, the thickness of the paste to be applied is preferably as thin as possible so long as there is no coating unevenness, and if it exceeds 200 μm, the connection resistance becomes larger than in the case of solder connection.

Furthermore, the heat treatment conditions (temperature, time, etc.) for integrating the conductive metal sheaths forming the connected portions by solid-phase reaction via the paste layer formed by the deposition form the sheath material. It is appropriately selected and set according to the type of the conductive metal, the thickness after thinning / removing, and the like. For example, when the sheath material is silver and the thickness after thinning / removing is about 6 to 15 μm, 820 to 850 ° C. It may be about 10 to 50 hours.

The oxide superconductors to which the present invention is applicable include rare earth element-containing oxide superconductors represented by Y system, La-Sr-Cu-O system, and Bi-Sr-Ca- system.
Cu-O system, Tl-Ba-Ca-Cu-O system, Nd-C
Various oxide superconductors, such as an e-Cu-O-based oxide superconductor, may be mentioned. The rare earth element-containing oxide superconductor may be any one that can realize a superconducting state, and for example, ReM ₂ Cu ₃ O _7-δ (Re is Y, La, S.
c, Nd, Sm, Eu, Gd, Dy, Ho, Er, T
At least one element selected from rare earth elements such as m, Yb, and Lu, M is at least one element selected from Bi, Sr, and Ca, δ is an oxygen defect, usually a number of 1 or less, Cu Some are Ti, V, Cr, Mn, Fe, Co,
Examples thereof include oxides represented by (which can be replaced by Ni, Zn, etc.).

The Bi-based oxide superconductor has the chemical formula: Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x ,: Bi ₂ (Sr,
Ca) ₃ Cu ₂ O _x or the like, and the Tl-based oxide superconductor has a chemical formula: Tl ₂ Ba _{2
Ca 2 Cu 3 O y,:} Tl 2 (Sr, Ca) 3 Cu 2 O
It is substantially represented by _y or the like, and silver oxide superconductors to which silver is added may be used.

On the other hand, the conductive metal sheath material of the oxide superconducting wire is a material which is not oxidized by the internal (embedding) oxide superconductor at room temperature or high temperature (heat treatment temperature of 800 to 1000 ° C.). And must be electrically good conductors so that they can bypass the current flowing when the superconductivity is broken. Specifically, silver, gold, platinum, palladium, copper and alloys thereof are preferable, and silver is more preferable. And
When silver or silver alloy is used as the sheath material, silver paste is selected as the paste. Further, for the purpose of improving the mechanical strength of the conductive metal sheath material, a composite material to which a trace amount of metal oxide (tin oxide, zinc oxide, aluminum oxide, etc.) is added can be used. Further, before or after the heat treatment, the connecting portion can be reinforced with a metal splint 5 of the same kind as the sheath material, as shown in a perspective view in FIG.

[0018]

According to the present invention, the conductive metal sheath surfaces of the connecting portions to be superposed on each other are substantially cleaned by thinning / removing, while the paste layer is interposed between the facing and facing surfaces. Integral by solid-state reaction in the form The conductive metal sheath surface of the connected portion facing and confronting with each other by the paste layer containing the fine metal particles exhibits uniform surface contact and a solid-phase reaction is performed, so that a void or the like remains. A precise integration is achieved. For example, as shown schematically in FIG. 3 (a) and in a micrograph in FIG. 3 (b), the upper and lower silver layers 2 and 2 forming the sheath material layer are uniform as shown in the cross-sectional structure of the connecting portion. It is possible to easily and surely form a precise and low resistance connecting portion.
That is, according to the connection method of the present invention, the connection resistance is smaller than that of a general solder connection method by one digit or more, the connection portion has sufficient strength, and the critical current (Ic) deteriorates in the vicinity of the connection portion. Providing wire rods that do not wake up (providing long wire rods)
Is possible.

[0019]

EXAMPLES Examples of the present invention will be described below.

Example 1 Prepared by the oxalate coprecipitation method, the result of IPC analysis was as follows:
Bi: Pb: Sr: Ca: Cu = 1.72: 0.34:
A powder having a composition of 1.83: 1.97: 3.13 was prepared. 100 g of this powder was housed in a MgO sample dish and calcined in the atmosphere at 800 ° C. for 20 hours, and then the diameter of 1 to 1
It was pulverized to 10 μm to obtain a calcined powder. The phase of this calcined powder is X
When examined by line diffraction, (Bi, Pb) ₂ Sr ₂
CaCu ₂ O _x (low Tc phase), Ca ₂ PbO ₄ , Cu
It was a mixed phase with O.

The calcined powder was filled in a silver tube having an outer diameter of 6 mm and an inner diameter of 5 mm, and the width was 2.5 mm and the thickness was 0.1 mm by drawing (drawing) and rolling (rolling).
A 15 mm tape material was used. After that, 7.7% oxygen
In an atmosphere of argon (balance) system, 835 ° C, 50
2.8m in width after heat treatment for 2 hours and secondary rolling again
A tape-shaped oxide superconducting wire having a thickness of 0.13 mm and a thickness of 0.13 mm was obtained. That is, the Bi-based (Bi, Pb) added with Pb
_{A 2} Sr ₂ Ca ₂ Cu ₃ O _x (high Tc phase) silver sheath tape wire is so-called powder method (powder in tube method).
Manufactured in.

The above tape wire was cut into a length of 20 mm, and the portion corresponding to the connection portion was nitric acid (concentration of 3 times that of commercially available concentrated nitric acid) contained in a beaker, as shown schematically in FIG. Diluted) 6 and then platinum electrode 7
10 as the cathode (-) and the tape wire piece 8 as the anode (+).
An etching treatment was carried out by applying a direct current of 0 mA for t seconds. This energization time t seconds is 0, 10, 30, 60, 1,
After changing the etching process to 20, 180, it was washed with water for several seconds to remove nitric acid and then dried. As a result of the etching, the oxide film and stains on the surface of the silver sheath were completely removed, and the thickness of the silver sheath was reduced from the initial 30 μm at the rate of the thickness shown in Table 1. FIG. 5 is a sectional view showing a state in which the silver sheath 2 is thinned / removed by the etching, and in Table 1, d ₀ is a value before etching and d ₁ is a value after etching. Next, as shown in a perspective view in FIG. 6, two pieces of the tape wire rods subjected to the etching treatment are opposed to each other, and a silver paste is applied to the overlapping portion (area) to a thickness of 10 μm, and the overlapping margin is set to 5 mm. After drying, they were placed in a furnace and heat-treated in the atmosphere at 840 ° C. for 50 hours to connect and integrate them. The sample integrated by the heat treatment is taken out from the furnace, and a portion including a connection region,
When a voltage terminal was arranged in a portion not including a connection region and a critical current (Ic) was measured in liquid nitrogen and zero magnetic field by a four-terminal resistance method, the connection resistance values at that time are shown in Table 1. It was as shown together with. Note that FIG.
Shows an example of voltage-current characteristics in the connection portion connected by the connection method.

For comparison, the same as the above, 20 mm
When the tape pieces cut into long lengths are first heat-treated, then the overlap margin is 5 mm in any case, and nitric acid etching treatment is performed before solder connection (Comparative Example 1), soldering is performed without etching. Table 2 shows the results of measuring the connection resistance and the like under the same conditions as described above for the case of connection in Comparative Example 2 (Comparative Example 2). As can be seen from Table 1 and Table 2, in the case of the connection method according to the present invention, the connection resistance is lower than that of the comparative example, and in particular, the connection resistance is two or more orders of magnitude lower than that of the comparative example 2. Can be realized. Further, it can be seen that the connection resistance can be reduced as the etching amount increases.

Example 2 A length 20 cut from the tape wire produced in Example 1
mm piece of tape wire rod is subjected to nitric acid etching treatment for 120 seconds, and the coating layer thickness d _p of the silver paste is 5, 10, 20,
Table 3 shows the results of examining the connection resistance by connecting the tape wire pieces in the same manner as in Example 1 while changing the thickness to 50, 100, and 200 μm. The thickness of the paste coating layer is 200
The connection resistance when μm is almost the same as that in the case of connection by soldering. Example 3 Length 20 cut out from the tape wire produced in Example 1
mm piece of tape wire material was subjected to nitric acid etching treatment for 120 seconds, and the coating layer thickness d _p of the silver paste was set to 10 μm,
Table 4 shows the results of examining the connection resistance by connecting the tape wire rod pieces in the same manner as in Example 1 while changing the overlap margin 1 (mm) to 5, 10, and 15 mm. It can be seen that the connection resistance decreases in inverse proportion to the connection area.

Example 4 Using the tape wire material produced in Example 1, a one-turn coil having a diameter of 8 cm as shown in a perspective view in FIG. 8 was produced. That is, the tape wire is cut in advance to a required length, and in the silver sheath region forming the joint, paraffin is applied to the opposite surface so that only the surfaces facing each other are etched (masking), Immerse in nitric acid and perform etching for 60 seconds, remove paraffin with acetone, and then set the overlap margin to 20 mm.
Similar to the case of the first embodiment, the tape wire rod pieces 3 ′ are connected and
A turn coil was produced. In this case, since the strength is maintained by the one-side sheath material that remains without being etched, a low resistance connection can be formed without significantly reducing the strength.

In order to evaluate the connection resistance of the one-turn coil manufactured as described above, a loop current was caused to flow by electromagnetic induction while the coil was immersed in liquid nitrogen, and its decay was examined. As a result, a decay time constant of 126 sec was obtained. . Since the inductance of the coil is about 10 μH, the resistance of the connection part is estimated to be about 10 ⁻⁷ Ω. When a similar coil was produced by soldering (Comparative Example 3),
The decay time constant is 26 microseconds and the connection resistance is 3.5 x
Only a value of 10 ⁻⁴ Ω was obtained.

Example 5 Using the tape wire produced in Example 1, as shown in a perspective view in FIG. 9, the outer diameter is 35 mm, the inner diameter is 20 mm, and the height is 15.
mm double pancake coil was made. That is, the tape wire is cut into a length of 4 m, wound on the winding frame 9 on the cylinder, and then joined at the outermost one turn at the final stage of winding. First, in the silver sheath region forming the joined portion. Is
Apply paraffin to the opposite surface so that only the surfaces facing each other are etched, and then dip it in nitric acid.
After performing an etching treatment for 0 second and then removing paraffin with acetone, the overlapping margin is set to 4 cm, and a silver paste having a thickness of 10 μm is applied to the overlapping margin, and then the surface to be bonded is inserted through the silver paste layer. After lap-temporary joining, heat treatment was performed at 840 ° C. for 50 hours in the atmosphere to produce a double pancake coil.

In the double pancake coil produced above,
After the resin impregnation treatment and the insulation treatment, a loop current was caused to flow by electromagnetic induction in a state of being immersed in liquid nitrogen in the same manner as in the case of Example 4, and its decay was examined. The decay time constant was 226 seconds. From this value, the connection resistance 6.0
× 10 ⁻⁸ Ω was obtained.

[0029]

As can be seen from the above description, the method of connecting oxide superconducting wires according to the present invention enables connection with a connection resistance that is 1 to 2 orders of magnitude lower than that in the case of soldering connection. It is possible to realize an excellent connection without the critical current and strength of the wire material being deteriorated around the connection point. In other words, it also opens the way to applications requiring permanent current mode energization such as MRI. On the other hand, by this connection method, it is possible to reduce power transmission loss in the power transmission cable configured by connecting and lengthening the oxide superconducting wire.
In addition, the superconducting coil configured can reduce heat generation during energization. In any case, it can be said that the connection method according to the present invention greatly contributes to the practical application of the oxide superconducting wire, which has received much attention from the viewpoint of excellent characteristics.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] Figure 3

[Correction method] Change

[Correction content]

3A and 3B show an example of a cross-sectional structure of a connection portion by a connection method according to the present invention, FIG. 3A is a schematic diagram, and FIG. 3B is a micrograph of a material composition of a ceramic connection portion.

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] Fig. 14

[Correction method] Change

[Correction content]

14 (a) and 14 (b) are micrographs showing the material composition of the connection part of the ceramic connection part composition by different conventional connection methods.

Claims (1)

[Claims] 1. A method for connecting oxide superconducting wires, wherein oxide superconducting wires formed by coating an outer peripheral surface of an oxide superconductor with a conductive metal layer are partly overlapped with each other, and the superposed parts are connected with each other. A portion of the oxide superconducting wire that is in contact with each other by superposition, at least one of the steps of thinning / removing the thickness of at least one conductive metal layer, the thinning / removing conductive metal layer surface, A step of depositing a paste layer having a thickness of 200 μm or less containing a metal forming a conductive metal layer, and a step of stacking oxide superconducting wires through the paste layer deposited, drying and then heat treating A method for connecting an oxide superconducting wire, comprising: