JP3117521B2 - Manufacturing method of oxide superconducting wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to Bi-Si-Ca-Cu.
The present invention relates to a method for producing an oxide superconducting wire using a wire crystallized by a melting method of a -O-based superconducting composition as a starting material, and particularly as a stabilizing material on its outer peripheral surface, a composite plating layer (Cu layer and Ag layer) having a small contact resistance. ) Comprising an oxide superconducting wire.

[0002]

2. Description of the Related Art Conventionally, an oxide superconducting wire has been manufactured by the following method. That is, first, an oxide powder having a superconducting composition is molded under pressure to obtain a molded body. Next, the molded body is filled and sealed in a metal pipe such as silver. Next, this metal pipe is drawn to a desired wire diameter. Next, the powder compact of the core is sintered by performing a heat treatment.

[0003] In the oxide superconducting wire thus manufactured, since the oxide superconductor in the core is coated with a metal, the metal in the coating acts as a stabilizing material for the superconducting wire. In other words, the oxide superconductor has no electric resistance in the superconducting state, but when it transits to the normal conducting state due to some disturbance, its electric resistance is significantly higher than that of a normal metal conductor. In order to prevent the high resistance state at the transition of the normal conduction and the burning of the wire due to this, a stabilizing material for covering the oxide superconductor is required.
Therefore, metals such as Al, Cu, and Ag, which have low electric resistance in a low-temperature region, are usually used as the stabilizing material. Required.

[0004] In the above-described method for producing an oxide superconducting wire, when a silver pipe is used as the metal pipe, it is particularly called a silver sheath method.

[0005] However, the above-described method for producing an oxide superconducting wire has the following disadvantages.

(1) Due to the difference in the packing density of the powder compact, the core diameter tends to vary.

(2) In order to measure the wire diameter of the portion of the wire where the superconducting phenomenon occurs (ie, the core), it is necessary to peel off the silver of the coating.

(3) Since the core portion made of the oxide superconductor is a sintered body, cracks are likely to occur in this portion due to slight bending stress.

(4) Since the core portion made of an oxide superconductor is a sintered body, a large number of voids are present in this portion and the crystal grains are fine, so that the grain boundary area is large and the critical current density is high. Is difficult to obtain.

[0010] In view of the above, a method of forming an oxide superconducting wire having a desired wire diameter by forming a wire having an oxide superconducting composition made of a sintered body, locally melting the wire, and then solidifying the wire is proposed. (RSFeigelson, et.a
l., Science 240, 1642, 1988 and A, Kurosaka, e
t, al., Appl. Phys, Lett. 55 (4), I4, July, 1989, etc.). This melting method has the following advantages.

(1) Since it is possible to obtain a wire having a crystal structure in which voids are small and the ab plane is oriented in the growth direction,
A high critical current density can be easily obtained.

(2) Since the wire can be formed while monitoring the wire diameter during manufacturing, it is easy to make the wire diameter of the wire uniform. Further, the wire diameter of the manufactured wire can be measured nondestructively.

(3) Compared to a wire composed of a sintered body,
Excellent flexibility.

[0014]

However, the above-mentioned method for producing an oxide superconducting wire by the melting method has the following problems. That is, it is necessary to cover the peripheral surface of the oxide superconducting wire produced by the melting method with a stabilizing material in consideration of the case where the oxide superconducting wire transitions to the normal conducting state. Despite this necessity, a technique for coating a peripheral surface of an oxide superconducting wire produced by a melting method with a stabilizing material has not yet been established.

The length of a wire manufactured by the conventional melting method is at most 100 to 200 mm, and it is difficult to manufacture a long wire.

The present invention has been made in view of the above-mentioned problems, and has been developed by using a wire rod of a Bi-based oxide superconducting composition crystallized by a melting method as a starting material, and through a melting and solidifying step, a core material. The present invention provides a method for producing an oxide superconducting wire capable of producing a long oxide superconducting wire in which a stabilizing material is coated on the peripheral surface of the core material with a uniform thickness and with good adhesion. The purpose is to do.

[0017]

According to the present invention, there is provided a method of manufacturing an oxide superconducting wire comprising a Bi-S crystallized by a melting method.
The r-Ca-Cu-O-based superconducting wire is heated to a temperature higher than its melting point in an oxidizing atmosphere to form a melted portion, and the melt is continuously drawn from the melted portion to be solidified. Forming a wire having a thickness of 2 mm or less, covering the outer peripheral surface of the obtained wire with a Cu plating layer having a thickness of 1 to 15 μm by electroplating, and forming a 0.3 A / dm. ^{It is characterized} by comprising a step of forming an Ag plating layer by electroplating Ag at a current density of ² or more, and a step of heating a wire provided with each plating layer at a temperature of 200 ° C. or more.

[0018]

The present inventors have determined that a wire obtained by melting and then solidifying a Bi-based oxide superconducting material is an electric conductor at room temperature, and that a finer crystal than a sintered body is obtained. Paying attention to the structure, various attempts were made to cover the periphery of the wire with metal by electroplating. As the metal for plating, Ag and Cu which can be easily electroplated and can be used on an industrial scale were selected. This has revealed the following facts.

(1) The contact resistance with the Bi-based oxide superconductor is smaller in the plating layer formed by Cu electroplating than in the Ag electroplating.

(2) In a composite plating layer in which a Bi-based oxide superconductor is coated with Cu by electroplating and then an Ag plating layer is formed on the outer peripheral surface, the Cu plating layer and the A
g It is thought that there is contact resistance between the plating layer and
By setting the current density at the time of electroplating g to 0.3 A / dm ² or more, it is possible to obtain the same level of contact resistance as a single-layer plating layer of only Cu.

(3) By setting the wire diameter of the obtained Bi-based oxide superconductor to 2 mm or less, the Cu layer and the Ag
The composite plating layer composed of the layers can be formed with a uniform thickness.

(4) In the composite plating layer composed of the Cu layer and the Ag layer, the Bi-Sr-Ca-Cu-O-based superconductor is in direct contact with the Cu layer in the composite plating layer.
Therefore, by performing an appropriate heat treatment after the formation of each plating layer, Cu in the composite plating layer diffuses into the Bi-based oxide superconductor, thereby further improving the adhesion.
The contact resistance is also reduced. In particular, by performing this heat treatment at a temperature of 200 ° C. or more, the contact resistance between the superconductor and the plating layer can be significantly reduced. Thus,
When the raw material wire is a sintered body (ceramic), it is difficult to obtain a long wire in the melting and solidifying steps. Thus, the present inventors conducted various experimental studies on the elongation of the thin wire of the Bi-based oxide superconductor. As a result, instead of using a sintered body (ceramic) rod as a starting material rod, a crystal was formed by a melting method. It has been found that use of the converted Bi-based oxide superconductor rod as a starting material rod facilitates lengthening. The following phenomena can be considered as the cause. (A) Most disconnections when using a sintered body occur at the melt-solidified interface. (B) A large number of voids are present in the sintered rod, and when these voids form a melt, a gas is generated, and this gas causes disconnection at the melt-solidification interface. (C) By using a crystallized rod, voids are significantly reduced, and hence the amount of gas in the melt is reduced, so that disconnection is reduced. (D) In addition, since the amount of gas contained in the melt decreases, the viscosity of the melt becomes a state suitable for elongation.

As described above, a long superconducting wire can be manufactured by using a wire that has been once melted and crystallized as a raw material wire.

Next, the reason for limiting each numerical value in the present invention will be described.

(1) Reasons for Limiting Wire Diameter When an oxide superconducting wire having a wire diameter exceeding 2 mm is to be manufactured from a raw material wire having a Bi-based oxide superconducting composition through a melting and solidifying process, the ab plane is a wire material. It is extremely difficult to obtain a wire having a crystal structure oriented in the longitudinal direction, and the orientation deteriorates. Therefore, the critical current density of the superconducting wire decreases. Further, it is difficult to form a Bi-based oxide superconducting wire having a wire diameter of more than 2 mm with a uniform thickness and good adhesion in the electroplating step. Therefore, it is necessary that the wire diameter of the wire formed from the raw material wire through the steps of melting and solidification is 2 mm or less.

(2) Reasons for Limiting the Thickness of Cu-Plating Layer If the thickness of the Cu-plating layer is less than 1 μm, the adhesion becomes insufficient and a desired small contact resistance cannot be obtained. On the other hand, when the thickness of the Cu plating layer exceeds 15 μm, the required diffusion distance of Cu during the heat treatment becomes longer, or the amount of oxygen taken in from the superconductor by Cu increases, so that the Cu contained in the superconductor is contained. Superconducting phase (eg, Bi ₂ Sr ₂ Ca ₁
The composition of (Cu ₂ O _x phase) changes, and the superconducting characteristics deteriorate.
Thereby, the critical current density (Jc value) of the wire decreases. For this reason, the thickness of the Cu plating layer is set to 1 to 15 μm.

(3) Reasons for Limiting Current Density During Ag Plating When forming an Ag layer by electroplating, if the current density is less than 0.3 A / dm ² , the thickness is uniform and the Cu plating layer It is difficult to form an Ag plating layer having good adhesion. Therefore, the current density at the time of Ag plating needs to be 0.3 A / dm ² or more.

(4) Reasons for Limiting Heat Treatment Temperature Even at a temperature lower than 200 ° C., that is, for example, at 100 ° C., it is considered that heating for a long time causes diffusion of Cu into the Bi-based oxide superconductor. However, in an actual wire rod manufacturing process, it is desirable that the wire be processed in a short time.

The inventors of the present invention have conducted heat treatments under various conditions. As a result, it has been confirmed that by performing a heat treatment at a temperature of 200 ° C. for 24 hours, the adhesion of the plating layer is improved and a sufficiently low contact resistance can be obtained. did. That is, if heating is performed at a temperature of 200 ° C. or more, a desired effect can be obtained within 24 hours. However, when the heat treatment temperature is lower than 200 ° C., Cu
The time required for sufficient diffusion of chromium is significantly longer than 24 hours. Therefore, when the temperature is lower than 200 ° C., it is difficult to improve the characteristics by heat treatment in practical operation. Therefore, the heat treatment temperature is set to 200 ° C. or higher.

[0030]

Next, an embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a sectional view showing an apparatus for manufacturing a superconducting fine wire used in an embodiment of the present invention.

The raw material wire 1 of the superconducting composition is fixed at its upper end to a wire holder 6a attached to a raw material wire supply drive shaft 8, and is supported with its longitudinal direction vertical.
A pull-down drive shaft 7 is disposed below the supply drive shaft 8, and the pull-down guide wire 2 is fixed to a wire holder 6 b attached to the pull-down drive shaft 7. The supply drive shaft 8 and the lowering drive shaft 7 are
Each can be moved up and down in conjunction with a predetermined relative speed by a driving device (not shown).

A cylindrical heating furnace 9 is disposed in the passage area of the raw material wire 1 so as to surround the raw material wire 1 with its axial direction vertical. The heating furnace 9 has a coil-shaped heating element 10 provided therein. The heating element 10 is supplied with power from an appropriate power source to cause the heating element 10 to generate resistance heat, so that the raw material located inside the heating furnace 9 is heated. The wire 1 and the like are heated to a predetermined temperature.

Inside the heating furnace 9, a resistance heating coil 4 for melting is arranged in a descending region of the raw material wire 1.
The resistance heating coil 4 has, for example, a diameter of 0.2 to 2.
A 0 mm platinum wire is formed into a coil shape. Further, a resistance heating wire 5b is disposed adjacent to and below the coil 4, and the resistance heating wire 5b also has, for example, a diameter of 0.1.
It is formed by winding a platinum wire of about 0.5 mm coaxially with the coil 4 once. The coil 4 and the heating wire 5b are supplied with power from an appropriate power source, and the coil 4 and the heating wire 5b are energized to generate resistance heat, so that the portion surrounded by the coil 4 and the heating wire 5b is heated. Raw material wire 1
Can be heated to a temperature equal to or higher than its melting point to be melted. The melt obtained by this is the coil 4 and the heating wire 5
In the region surrounded by b, the molten material is held by surface tension utilizing the property of wetting of the melt, and the melted portion 3 is formed.

Further, a pipe 5a is disposed directly below the center of the coil 4 coaxially with the coil 4, that is, with its longitudinal direction being vertical. The pipe 5a has, for example, an outer diameter of 0.1 to 2.5 mm and an inner diameter of 0.05 to 2.0 mm.
And is fixed to the resistance heating wire 5b. The pipe 5a is also heated to a temperature equal to or higher than the melting point of the raw material wire 1 by the resistance heat of the heating wire 5b. Since the pipe 5a is disposed in contact with the lower end of the fusion part 3 as described above, it penetrates to the lower end of the pipe 5a by the principle of the capillary of the fusion part 3.

The arrangement positions of the coil 4 and the pipe 5a and their surroundings are maintained in an oxidizing atmosphere. For example, the heating furnace 9 may be entirely in an oxidizing gas atmosphere, or the coil 4 and the pipe 5
An oxidizing gas may be blown around a.

The coil 4, the resistance heating wire 5b and the pipe 5a are not limited to those formed from platinum as described above, but must be usable in this oxidizing atmosphere.

Next, a method of manufacturing an oxide superconducting wire using the above-described manufacturing apparatus will be described.

First, a BSCCO-based oxide powder is filled and sealed in an Ag pipe, and the pipe is reduced in diameter by swaging to form a wire. Thereafter, the Ag sheath on the surface layer is dissolved with methanol nitrate.

Next, the remaining oxide wire rod is heated in an oxidizing atmosphere at a temperature of, for example, 780 ° C. for 10 hours to obtain a raw material rod made of a sintered body of BSCCO-based oxide. Thereafter, the sintered rod is locally melted by the floating zone melting method, and the molten rod is moved in the longitudinal direction of the sintered rod to crystallize the oxide rod. Thus, the raw material wire 1 is obtained.

Next, the upper end of the raw material wire 1 is fixed to the above-mentioned wire holder 6a, and the supply drive shaft 8 is lowered so that the raw material wire 1 is arranged so that the lower end of the raw material wire 1 is fitted into the coil 4. I do. On the other hand, the upper end of the pull-down guide wire 2 smaller than the inner diameter of the pipe 5a is inserted into the pipe 5a, and the lower end is fixed to the wire holder 6b. Then, while supplying an oxidizing gas to the periphery of the coil 4 and the pipe 5a, the preheating element 10 is energized to heat the raw material wire 1 in the heating furnace 9 to a temperature of, for example, 700 ° C. or more. In addition, a current is supplied to the melting resistance heating coil 4 to locally heat and melt the raw material wire 1. Thereby, a fusion part 3 is formed in a region surrounded by the coil 4. The resistance heating wire 5b for supporting the pipe is also energized and heated to a temperature equal to or higher than the melting point of the raw material wire 1.

Next, the supply drive shaft 8 and the pull-down drive shaft 7 are lowered at a predetermined relative speed therebetween. The melt in the melting part 3 penetrates the pipe 5a and descends, goes out of the pipe from its lower end, cools down, and solidifies into a thin wire shape to obtain the oxide superconducting wire 11. This oxide superconducting wire 1
1 is carried downward by the lowering of the drive shaft 7 for lowering. On the other hand, the raw material wire 1 is continuously supplied to the heating furnace 9 from above by lowering the supply drive shaft 8. Thus, the oxide superconducting wire 11 having a wire diameter of 2 mm or less is continuously manufactured.

Next, the obtained oxide superconducting wire 11 is charged into a normal heat treatment furnace and heat-treated at a temperature of 200 ° C. or higher.

Next, the result of actually manufacturing an oxide superconducting wire and testing its characteristics by the above-described method will be described. Example 1 First, a crystallized rod made of a Bi-based oxide superconducting composition and having a wire diameter of 4 mm was prepared by a floating zone melting method, and this rod was used as a starting material and melted by the apparatus shown in FIG. A 1 mm Bi-based oxide superconducting wire was obtained. Next, electroplating of Cu was performed in a cyan bath. In this electroplating, electricity was supplied until a plating layer having a thickness of about 5 μm was theoretically obtained on the outer peripheral surface of the wire. Thereafter, similarly, electroplating of Ag was performed in a cyanide bath, and the outer peripheral surface of the Cu plating layer was theoretically approximately 50
Electric current was supplied until an Ag plating layer having a thickness of μm was obtained. Cu
The electroplating conditions for Ag and Ag are shown in Tables 1 and 2 below, respectively.

[0045]

[Table 1]

[0046]

[Table 2]

After these Cu plating and Ag plating were performed, they were heat-treated at 500 ° C. for 6 hours in the atmosphere.

Example 2 The starting material crystallization rod had a wire diameter of 8 mm and a wire diameter of 2 m.
It was manufactured under the same conditions as in Example 1 except that m Bi-based oxide superconducting wires were obtained.

Example 3 The same procedure as in Example 1 was carried out except that in the electroplating of Cu, electricity was supplied until a plating layer having a theoretical thickness of 1 μm was obtained.

Example 4 The same procedure as in Example 1 was carried out except that in the electroplating of Cu, electricity was supplied until a plating layer having a theoretical thickness of 15 μm was obtained.

Example 5 In the electroplating of Ag, the current density was set to 0.3 A / dm.
Except having changed to ² , it manufactured under the same conditions as Example 1.

Example 6 The same procedure as in Example 1 was carried out except that the heat treatment was performed at 200 ° C. in the air for 24 hours.

Comparative Example 1 An Ag plating layer was formed directly on the outer peripheral surface of a Bi-based oxide superconductor without forming a Cu plating layer. Other conditions are the same as in the first embodiment.

Comparative Example 2 A platinum pipe having an outer diameter of 3.5 mm and an inner diameter of 3.0 mm was used as the pipe 5a of the above-described apparatus, and a B pipe having a wire diameter of 3 mm was used.
It was manufactured under the same conditions as in Example 2 except that an i-based oxide superconducting wire was obtained.

Comparative Example 3 In the electroplating of Cu, the procedure of Example 1 was repeated except that a current was applied until a plating layer having a theoretical thickness of 0.5 μm was obtained.
This is the same condition as.

COMPARATIVE EXAMPLE 4 The same procedure as in Example 1 was carried out except that a current was applied until a plating layer having a theoretical thickness of 20 μm was obtained in Cu electroplating.

Comparative Example 5 In the electroplating of Ag, the current density was set to 0.2 A / dm.
Except having changed to ² , it manufactured under the same conditions as Example 1.

COMPARATIVE EXAMPLE 6 Except that the heat treatment temperature after plating was 100 ° C.
It was manufactured under the same conditions as in Example 1.

Comparative Example 7 A Bi-based oxide superconducting composition rod was used as a starting material in the same manner as in Example 1 except that a sintered body (porosity: 9%) was used.
A Bi-based oxide superconducting wire having a wire diameter of 1 mm was obtained. As a result, in Examples 1 to 6 and Comparative Examples 1 to 6, a wire having a length of 500 mm or more was always stably obtained, whereas in Comparative Example 7, disconnection at the melt-solidification interface was observed. Many, up to about 15
Only the thing of 0 mm was obtained. Therefore, Comparative Example 7
Did not carry out subsequent characteristic evaluations.

The uniformity of the thickness of the plating layer was examined for each of the wire rods of the examples and the comparative examples thus obtained.
The results are shown in Table 3 below. In addition, regarding the uniformity of the thickness of the plating layer, the wire rod (length 50 m
m) A vertical section micrograph was taken, and the thickness of the plating layer was measured using this micrograph.

[0061]

[Table 3]

As is evident from Table 3, the uniformity of the thickness of the plating layer is good except for Comparative Examples 2 and 5. Therefore, the contact resistance between the plating layer and the Bi-based oxide superconductor of the core portion was measured for Examples 1 to 6 and Comparative Examples 1, 3, 4, and 6 in which a uniform plating layer was obtained. A three-terminal method was used as a method for measuring contact resistance.

FIG. 2 shows the measuring method. That is, the plating layers 21 and 22 are formed on the surface of the Bi-based oxide superconductor 20.
23 are formed separately from each other, current leads 24 and 25 are derived from the plating layers 21 and 22, and voltage leads 26 and 27 are derived from the plating layers 22 and 23. Then, the current leads 24 and 25 were connected to a power supply (not shown), and the voltage leads 26 and 27 were connected to a voltmeter (not shown). And
The contact resistance was measured from the voltage value measured by the voltmeter and the current value supplied from the power supply. The measurement results of the contact resistance are shown in FIGS.

As a result, the contact resistances of Examples 1 to 6 and Comparative Example 4 are significantly smaller than those of Comparative Examples 1, 3, and 6. Then, about Examples 1-6 and Comparative Example 4,
The critical current density in liquid nitrogen was measured. The results are shown in Table 4 below.

[0065]

[Table 4]

As shown in Table 4, Comparative Example 4 has a low critical current density, while Examples 1 to 6 have a high critical current density and have sufficient characteristics as a superconducting wire.

[0067]

As described above, according to the method of the present invention, the raw material wire crystallized by the melting method is melt-solidified, and the obtained fine wire is provided with the Cu layer and the A as a stabilizing material.
Since the g layer is formed by electroplating under predetermined conditions, a long superconducting wire can be manufactured, and a stabilizing material composed of these composite layers can be coated on a thin wire with a uniform thickness. The composite layers formed by these electroplating methods have excellent adhesion and extremely low contact resistance. Furthermore, according to the method of the present invention, the stabilizer can be coated without impairing the superconducting characteristics (critical current density) of the Bi-based oxide superconductor.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a method according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a method for measuring contact resistance.

FIG. 3 is a graph showing the contact resistance of Comparative Example 1.

FIG. 4 is a graph showing contact resistances of Comparative Examples 3 and 6.

FIG. 5 is a graph showing contact resistances of Examples 1 to 6 and Comparative Example 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1; Raw material wire 3; Fused part 4; Coil 5a; Pipe 11; Oxide superconducting wire 20; Bi-based oxide superconductor 21, 22, 23; Plating layers 24, 25;

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Haruo Tominaga 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Wire Co., Ltd. (72) Inventor Akira Aji 20 Kitakanyama, Otakacho, Midori-ku, Nagoya-shi, Aichi Prefecture Address No. 1 Chubu Electric Power Co., Inc. Technology Development Headquarters Power Technology Research Laboratories (72) Inventor Toshio Inoue 20-1 Kitakanyama, Odaka-cho, Midori-ku, Nagoya City, Aichi Prefecture (56) References JP-A-3-55718 (JP, A) JP-A-64-54614 (JP, A) JP-A-2-97694 (JP, A) JP-A-3-93685 (JP, A) Kaisho 64-54615 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01B 12/00-13/00 C04B 41/88, 41/90

Claims (1)

(57) [Claims] 1. Bi-Sr-C crystallized by a melting method
A wire having an a-Cu-O-based superconducting composition is heated in an oxidizing atmosphere to a temperature equal to or higher than its melting point to form a molten portion, and the melt is continuously drawn out from the molten portion and solidified to have a wire diameter of 2 m.
m, a step of forming a Cu plating layer having a thickness of 1 to 15 μm on the outer peripheral surface of the obtained wire by electroplating, and a step of forming a 0.3 mm
Ag is electroplated at a current density of A / dm ² or more
A method for manufacturing an oxide superconducting wire, comprising: a step of forming a plating layer; and a step of heating a wire provided with each plating layer at a temperature of 200 ° C. or higher.