JPH04212212A - Oxide superconductive wire, manufacture thereof, and treating method therefor - Google Patents

Abstract

PURPOSE:To obtain a superconductive wire rod including oxide superconductors, in which the critical current density is not lowered so much when bending is applied thereto. CONSTITUTION:A plurality of strands 3, in each of which an oxide superconductor 1 being covered with a first metallic sheath 2, is filled in a second metallic sheath 4. Plastic processing is applied on the second metallic sheath 4 in such a manner as applying compressive load thereto in its cross-sectional direction, whereby the thickness of an oxide superconductor 1 included in each of strands 3 is reduced to 5% or less of the total thickness of the superconducting wire rod 6.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to oxide superconducting wire,
The present invention relates to a method for producing the same and a method for handling the same, and in particular, it relates to an improvement for improving the strain resistance of the critical current density provided by the superconducting wire.

0002

2. Description of the Related Art In recent years, ceramic-based materials, ie, oxide superconducting materials, have attracted attention as superconducting materials exhibiting higher critical temperatures. Among them, yttrium type is 90K, bismuth type is 110K, and thallium type is 120K.
Since it exhibits a relatively high critical temperature, its practical application is expected.

[0003] As a method for obtaining a superconducting wire such as a long superconducting wire or a superconducting pattern wired on a suitable substrate using a superconductor made of such an oxide superconducting material, raw material powder is mixed with metal. There is a known method for manufacturing a superconducting wire in which a superconductor is covered with a metal sheath by turning raw material powder into a desired superconductor by heat-treating the powder coated with a sheath. More specifically, the obtained superconducting wire can be used as a cable,
It can be applied to bus bars, power leads, magnets, coils, etc.

0004

[Problems to be Solved by the Invention] In order to apply the above-mentioned superconducting wire to cables, magnets, etc., it is necessary to have a high critical current density in addition to a high critical temperature. In particular, it is not only necessary to ensure the required critical current density in the magnetic field used, but also to be able to maintain a high critical current density under the used strains.

[0005] However, superconducting wires containing oxide superconductors have extremely poor strain resistance characteristics at critical current density, and have the drawback that, for example, when bent at a certain curvature, the critical current density decreases.

[0006] Therefore, an object of the present invention is to provide a superconducting wire and a method for manufacturing the same, in which the critical current density does not decrease significantly even when strain is applied.

Another object of the present invention is to provide a preferred method for handling superconducting wires as described above.

[0008]

[Means for Solving the Problems] A superconducting wire according to the present invention has a so-called multicore structure, and has a metal sheath,
The metal sheath includes a plurality of superconductors distributed independently of each other in the thickness direction of the metal sheath. The thickness direction dimension of each superconductor is set to 5% or less of the thickness direction external dimension of the metal sheath.

The present invention is particularly advantageously applied when the superconductor is composed of an oxide superconductor.

The present invention also provides a method for manufacturing a superconducting wire containing the above-described oxide superconductor. This manufacturing method includes the steps of: preparing a plurality of wire materials in which an oxide superconductor is covered with a first metal sheath; filling the plurality of wire materials into a second metal sheath; The plurality of wire materials are arranged so that the thickness of the superconductor contained in the wire materials is 5% or less of the external dimension in the thickness direction of the second metal sheath, and the second metal sheath is deformed into a tape shape. and performing plastic working at least once in which a compressive load is applied in the cross-sectional direction to the second metal sheath filled with the second metal sheath. Note that the step of further filling a third metal sheath with a plurality of second metal sheaths each filled with a plurality of wire strands and subjecting the third metal sheath to plastic working may be further repeated at least once.

The present invention also provides a method for handling oxide superconducting wire. The oxide superconducting wire includes a metal sheath having a dimension in the thickness direction, and a plurality of oxide superconductors distributed independently in the thickness direction within the metal sheath, and each of the oxide superconductors The dimension in the thickness direction is 5% or less of the external dimension in the thickness direction of the metal sheath, and such an oxide superconducting wire has a strain (thickness of the metal sheath/bending diameter) of 0.3% or less. It is handled within a controlled manner.

0012

[Operation] Once a crack occurs in a superconductor, it tends to propagate easily. This tendency is remarkable in oxide superconductors which are ceramics. therefore,
If the amount of strain exceeds a certain value, cracks will occur in the superconductor, and once such cracks have occurred, they will propagate to areas with less strain, resulting in a decrease in critical current density. However, according to this invention, since the superconductor is divided so that the thickness of the superconductor is kept below a predetermined value, the propagation of cracks can be prevented without reducing the magnitude of the current that can flow. be able to. This makes it possible to improve the strain resistance of critical current density.

[0013]

Therefore, according to the present invention, a superconducting wire can be obtained in which the critical current density does not decrease significantly even when strain is applied. Therefore, it becomes possible to apply oxide superconducting wires, in particular where strain resistance is a problem, to applications where strain is applied, such as cables or magnets, without any problems.

In the present invention, when an oxide superconductor is used as the superconductor, such oxide superconductor is preferably c-axis oriented in the thickness direction.

The above-mentioned oxide superconductor may be yttrium-based, bismuth-based, or thallium-based. However, bismuth-based oxide superconductors are particularly suitable. Bismuth-based oxide superconductor is B
i-Sr-Ca-Cu-O or (Bi,Pb)-Sr
-Ca-Cu-O, but such components Bi or (Bi, Pb)-Sr-Ca-C
22 exhibiting a critical temperature of 110 K, where u has a composition of 2223
More preferably, the 23 phases have their a-b planes oriented in the direction of current flow. According to this invention, 1
A structure in which the a-b plane of the 2223 phase exhibiting a critical temperature of 10 K is oriented in the direction of current flow can be easily obtained.

Furthermore, compared to yttrium-based or thallium-based oxide superconductors, bismuth-based oxide superconductors satisfy all of the following requirements: high critical temperature and critical current density, low toxicity, and no need for rare earth elements. It can be said that it is particularly preferable also in that it does.

Note that the degree of orientation of yttrium-based and thallium-based materials can be improved to some extent, although not as much as that of bismuth-based materials, so the present invention can also improve the critical current density strain resistance of these materials. It has been found that

[0018] In the present invention, the superconducting wire is in the form of a long wire in order to provide practicality over a wider range.

[0019] When the superconducting wire in the form of a long wire according to the present invention is further coated with an organic coating made of an organic substance, the superconducting properties exhibited by the superconducting wire can be further stabilized against bending of the superconducting wire. .

According to the above-described method for manufacturing an oxide superconducting wire according to the present invention, the thickness of the oxide superconductor covered with the metal sheath is divided into 5% or less of the total thickness of the superconducting wire. is easy.

[0021] Furthermore, drawing the wire material before filling it into the second metal sheath can also reduce the thickness of each superconductor contained in the obtained oxide superconducting wire material. It is valid.

Further, the second metal sheath is preferably deformed into a rectangular tape shape by the plastic working applied in the manufacturing method according to the present invention.

[0023] In order to improve the critical current density, it is preferable to perform heat treatment after plastic working, and it is more preferable to repeat plastic working and heat treatment multiple times.

[0024] Furthermore, by changing the number of wire strands filled in the second metal sheath, the thickness of each superconductor included in the oxide superconducting wire obtained in the plastic working step can be easily adjusted. I can do it. For example, by increasing the number of wires, the thickness of the superconductor after plastic working can be easily reduced.

Furthermore, in the present invention, the strain applied to the superconducting wire containing the oxide superconductor is reduced to 0.3% as described above.
If controlled within the following range, deterioration of superconducting properties due to strain can be substantially prevented. Such distortion control is effective in various processes for manufacturing superconducting wires, including heat treatment, and also in unwinding from a reel after manufacturing, winding onto a reel, and also in cables, busbars, power leads, This is done when it is formed into a coil or the like. If the strain is controlled within the range of 0.3% or less in this manner, there is substantially no decrease in critical current density even if strain is repeatedly applied, and therefore, the superconducting wire can be used for various purposes.

[0026] Furthermore, if the superconducting wire is heat-treated while being wound with a curvature that gives a strain of 0.3% or less, there will be little or no deterioration of the superconducting properties due to strain during the heat treatment and during the unwinding, especially when bending. It is possible to obtain a superconducting wire that exhibits stable superconducting properties against.

[0027]

EXAMPLE In order to obtain a superconducting wire containing an oxide superconductor according to the present invention, for example, the following manufacturing method is applied.

First, a first metal sheath is filled with an oxide superconductor to obtain a wire material. This raw wire material is subjected to a wire drawing process, thereby making it thin. The plurality of wire materials thinned in this way are then filled into the second metal sheath,
In order to further make the wire thinner, plastic working such as wire drawing and rolling is performed. As a result, the thickness of the oxide superconductor contained in each wire material is reduced to 55% of the total thickness.
% or less is obtained. This oxide superconducting wire has, for example, a rectangular tape shape.

In order to increase the critical current density of this oxide superconducting wire, it is then further subjected to wire drawing and rolling, and then heat treated. Furthermore, again,
Rolling and heat treatment may also be performed. At this time,
Instead of rolling, wire drawing may be combined with heat treatment.

In the case of a bismuth-based oxide superconductor, a structure having the desired high critical current density can be obtained by making the heat treatment temperature slightly higher than the temperature at which the 2223 phase is predominantly produced. I can do it.

Furthermore, if the powder initially filled into the first metal sheath is in a submicron state, a superconductor with good uniformity can be obtained.

[0032] The heat treatment temperature cannot be unambiguously determined because the optimum temperature is selected depending on the heat treatment atmosphere. For example, when lowering the oxygen partial pressure in the heat treatment atmosphere, the heat treatment temperature is lowered.

[0033] The metal sheath has the function of stabilizing the superconducting wire. Suitable metals for providing such a metal sheath are those that do not react with the superconductor, have good workability, and have a low specific resistance that functions as a stabilizing material, such as silver, silver alloys, gold, Or a gold alloy is used. In addition, in the manufacturing method according to the present invention, the first and second
In particular, the first metal sheath must not react with the superconductor, but the second metal sheath does not need to satisfy these conditions. However, typically these first and second metal sheaths are preferably constructed of silver, silver alloys, gold or gold alloys. In particular, regarding the first metal sheath, a metal sheath may be used in which only the surface in contact with the superconductor is coated with a layer made of a metal that does not react with the superconductor. In that case, other parts of the metal sheath may be constructed of another metal, such as copper, aluminum, or an alloy thereof.

[0034] Examples of plastic working include wire drawing and rolling. In order to improve the critical current density,
In wire drawing, it is desirable that the degree of work is 80% or more, and in rolling, it is desirable that the degree of work is 80% or more.
% or more is desirable. After such plastic working, heat treatment is preferably performed, and repeating these plastic working and heat treatment multiple times is more effective in improving the critical current density. For example, when rolling is performed multiple times, it is desirable that the degree of work in one pass is 40% or more. When rolling or wire drawing is performed again after heat treatment, the degree of work in such processing is sufficient to be about 10% to 30%. The rolling process is performed using, for example, a roll or a press.

Further, after the heat treatment is completed, the obtained superconducting wire may be further coated with an organic substance. To coat with an organic substance, for example, the superconducting wire is passed through a bath of an organic substance, or the surface of the superconducting wire is coated with an organic substance.

Experimental examples conducted according to the present invention will be explained below.

Experimental Example 1 Bi2 O3, PbO, SrCO3, CaCO3
and CuO, Bi:Pb:Sr:Ca:Cu
=1.85:0.41:2.01:2.19:2.98
These were blended so that the composition ratio was as follows. This blend was heat treated in the air at 750°C for 12 hours, then at 800°C for 8 hours, and then in a reduced pressure atmosphere of 1 Torr at 760°C for 8 hours. In addition, after each heat treatment, pulverization was performed, respectively. The powder obtained through such heat treatment was further pulverized using a ball mill to obtain submicron powder. This powder was subjected to degassing treatment at 800° C. for 10 minutes in a reduced pressure atmosphere.

The obtained powder was filled into a silver pipe with a diameter (outer diameter) of 12 mm, and wire-drawn until the diameter was 1.8 mm. In this way, a wire material was obtained. The wire material obtained in this way is filled as it is or in the desired number into a silver pipe again, then subjected to wire drawing and rolling, then heat treated, and further rolled. By processing and heat treatment,
Sample No. shown in Table 1 below. 1 to 5 were obtained.

[0039]

[Table 1] The thickness of the (individual) superconductor relative to the total thickness of the superconducting wire in each sample is as follows: 32% for 1;
No. 15% for No. 2; 6.2 for 3
%, No. For No. 4, it was 4.3%. For No. 5, it was 2.0%.

[0040] In order to obtain samples in which the thickness of the superconductor was varied in relation to the overall thickness of the wire, the method shown in FIGS. 1 and 2 was used.

1 and 2, a plurality of strands 3 each having an oxide superconductor 1 covered with a first metal sheath 2 are shown. In Fig. 1, seven wire rods 3 are connected to the second
is filled into the metal sheath 4 of. On the other hand, in Figure 2, 19
A real wire material 3 is filled into the second metal sheath 5. In these conditions, plastic working such as rolling is performed. As a result, in FIG. 1, a rectangular tape-shaped superconducting wire 6 is obtained, and in FIG. 2, a rectangular tape-shaped superconducting wire 7 is obtained.

When these superconducting wires 6 and 7 are compared, in the superconducting wire 6 shown in FIG.
%. On the other hand, in the superconducting wire 7 shown in FIG. 2, the thickness of each oxide superconductor 1 is approximately 15% of the thickness of the entire superconducting wire 7.

[0043] In this way, the thickness of each superconductor included in the superconducting wire obtained by plastic working can be adjusted by changing the number of wires filled in the second metal sheath. In each of the samples described above, the thickness ratio of each superconductor was changed by the method shown in FIGS. 1 and 2.

For each of the superconducting wires thus obtained, strain-critical current density (at liquid nitrogen temperature)
The characteristics were compared. The results are shown in Table 1.

[0045] Each number shown in Table 1 is expressed as JC/JCO×1, where JCO is the critical current density when no strain is applied, and JC is the critical current density when a predetermined strain is applied.
It shows the value calculated in 00[%].

From Table 1, sample No. 1 in which the thickness of each superconductor was 5% or less of the total thickness of the wire. 4 and no. 5
It can be seen that the material has excellent strain resistance.

Experimental Example 2 Bi:Pb:Sr:Ca:Cu=1.78:0.44:
Oxides or carbonates containing each element are mixed to have a composition of 1.99:2.23:2.98, and by heat treatment, the ratio of Bi+Pb:Sr:Ca:Cu is approximately 2:
A powder consisting of a 2212 phase and a non-superconducting phase in a ratio of 2:1:2 was prepared.

[0048] This powder was degassed at 720°C for 10 hours in a reduced pressure atmosphere of 8 Torr.

The obtained powder was first covered with a silver pipe having an outer diameter of 12 mm and an inner diameter of 8 mm, and wire-drawn to an outer diameter of 1 mm.Then, this was further put into a silver pipe with a larger diameter. There were 1296 multicore wires. Next, this was wire-drawn to an outer diameter of 1 mm, and then wire-drawn to an outer diameter of 0.17 mm.
It was rolled to a thickness of mm.

Further, this wire rod was heat treated at 840° C. for 50 hours, then rolled at a workability of 11.8%, and then wound around an alumina/silica ceramic cylinder having a diameter of 50 mm. In this winding state, the wire rod is
It had a curvature that gave a strain of 0.3%.

[0051] Under the above conditions, the wire rod was heat treated at 840°C for 50 hours. The critical current density of the wire immediately after this heat treatment at liquid nitrogen temperature was 7000 A/cm2.

Next, the wire was drawn out from the cylinder, bent on the circumferential surface of a cylinder of the same diameter, and returned to a straight shape 40 times.The critical current density was then measured in the same manner, and it was found to be the same as that immediately after heat treatment. The value of was obtained.

Experimental Example 3 Bi:Pb:Sr:Ca:Cu=1.78:0.44:
So that it has a composition of 1.96:2.25:2.99:
Oxides or carbonates containing each element were mixed and heat treated to prepare a powder consisting of a 2212 phase and a non-superconducting phase.

[0054] This powder was degassed at 700°C for 7 hours in a reduced pressure atmosphere of 11 Torr.

[0055] The obtained powder was made into a powder with an outer diameter of 12 mm and an inner diameter of 8 m.
The wire was covered with a silver pipe having a diameter of m and was drawn to an outer diameter of 1 mm, and then further inserted into a larger diameter silver pipe to produce 1260 multifilamentary wires. Next, this multifilamentary wire was drawn to an outer diameter of 1 mm, and then to a diameter of 0.17 mm.
It was rolled to a thickness of mm.

[0056] Two pieces of the obtained wire rods were stacked and brought into close contact with each other, and while they were in close contact, they were heat-treated at 840°C for 50 hours, and then,
It was rolled with a working degree of 15%.

[0057] The two-ply wire rod obtained in this way is
It was wound around a ceramic bobbin with a diameter corresponding to a strain of 0.29%, and heat treated at 840° C. for 50 hours.

[0058] Two wire rods stacked and stuck together in this way are
It was re-wound onto a bobbin of the same diameter, and then spirally wound around a Teflon (trade name) pipe with the same diameter at a pitch of 60 mm.

[0059] This wound product was bent to have a radius of 100 mm and a radius of 200 mm, respectively, and these bending operations were repeated 10 times.

The critical current density of the wire in liquid nitrogen was measured after the heat treatment, after each rewinding, and after each repeated bending. The results are 8000A/
A critical current density of cm2 was obtained.

Experimental Example 4 Bi:Pb:Sr:Ca:Cu=1.76:0.43:
Oxides or carbonates containing each element were mixed so as to have a composition of 1.98:2.20:3.02, and a powder consisting of a 2212 phase and a non-superconducting phase was prepared by heat treatment.

[0062] This powder was degassed at 710°C for 12 hours in a reduced pressure atmosphere of 15 Torr.

The obtained powder was first covered with a silver pipe having an outer diameter of 12 mm and an inner diameter of 8 mm, and wire-drawn to an outer diameter of 1 mm.Then, this was further put into a silver pipe with a larger diameter. There were 1296 multicore wires. Next, this was wire-drawn to an outer diameter of 1 mm, and then wire-drawn to an outer diameter of 0.1 mm.
It was rolled to a thickness of 7 mm.

[0064] Next, this wire rod was heat treated at 840°C for 50 hours, then rolled at a workability of 11.8%, and further wound around an alumina/silica ceramic cylinder having a diameter of 50 mm. In this wound state, the wire rod has a diameter of 0.
It had a curvature that gave a strain of 3%.

[0065] Under the above conditions, the wire rod was heat treated at 840°C for 50 hours.

[0066] Next, the wire is drawn out from the above-mentioned cylinder,
After passing through a formal bath at a linear speed of 20 m/min, 3
Baking at 50℃ was carried out 10 times, and 30 to 5
A formal coating of 0 micron thickness was applied. At this time, strain was controlled to 0.3% or less in all steps.

[0067] This wire was wound around a bobbin having a diameter of 50 mm to produce a coil. This coil exhibited a critical current density of 6000 A/cm2 at liquid nitrogen temperature.

Experimental Example 5 Bi:Pb:Sr:Ca:Cu=1.82:0.42:
Oxides or carbonates containing each element were mixed so as to have a composition of 1.99:2.22:3.01, and a powder consisting of a 2212 phase and a non-superconducting phase was prepared by heat treatment.

This powder was degassed at 700° C. for 15 hours in a reduced pressure atmosphere of 10 Torr.

[0070] The obtained powder was made into a powder with an outer diameter of 12 mm and an inner diameter of 8 m.
The wire was covered with a silver pipe having a diameter of m and was drawn to an outer diameter of 1 mm, and then further inserted into a larger diameter silver pipe to produce 1260 multifilamentary wires. Next, this multifilamentary wire was drawn to an outer diameter of 1 mm, and then 0.17 m
It was rolled until it had a thickness of m.

[0071] Further, this wire rod was heat treated at 840°C for 50 hours, and then rolled at a workability of 15%.

[0072] This wire was wound around a ceramic bobbin having a diameter corresponding to a strain of 0.29%, and heat treated at 840°C for 50 hours.

Next, the wire rod is paid out from the above-mentioned bobbin,
After passing through a formal bath, it is heated in an oven at 400℃ for about 1
Baking was performed for 0 seconds, and this was repeated 10 times. At this time, strain was controlled to be suppressed to 3% or less in all steps. As a result, a wire rod having a formal coating with a thickness of 40 microns was obtained.

[0074] This wire was wound at a pitch of 50 mm around a stainless steel tube with a diameter of 50 mm as a conductor of a cable, and bent with a curvature of 70 cm in radius.

[0075] The critical current density of the wire at liquid nitrogen temperature was measured after heat treatment, after formal coating, after each rewinding, and in the bent state, and a value of 7500 A/cm2 was obtained in each case.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing a plastic working step included in the method for manufacturing an oxide superconducting wire according to the present invention.

FIG. 2 is a diagram corresponding to FIG. 1, showing a state in which the number of strands 3 is increased compared to the case of FIG. 1;

[Explanation of symbols] 1 Oxide superconductor 2 First metal sheath 3. Wire material 4,5 Second metal sheath 6,7 Superconducting wire

Claims (18)

[Claims] [Claim 1] A metal sheath having a thickness direction dimension;
A plurality of superconductors are distributed independently in the thickness direction within the metal sheath, and each of the superconductors has a thickness direction dimension of 5% or less of the thickness direction external dimension of the metal sheath. Superconducting wire. 2. The superconducting wire according to claim 1, wherein the superconductor is an oxide superconductor. 3. The oxide superconductor has c in the thickness direction.
The superconducting wire according to claim 2, which is axially oriented. 4. The oxide superconductor is Bi-Sr-
Ca-Cu-O or (Bi,Pb)-Sr-Ca-C
The superconducting wire according to claim 3, which is a bismuth-based oxide superconductor having a u-O component. 5. The bismuth-based oxide superconductor comprises B
i+Pb:Sr:Ca:Cu=1.5-2.5:1.8
The superconducting wire according to claim 4, having a composition ratio of ~2.2:1.5-2.5:2.5-3.5. 6. The superconducting wire according to claim 1, wherein the metal sheath and the superconductor each have a longitudinal dimension. 7. The superconducting wire according to claim 6, further comprising an organic substance covering the metal sheath. 8. A plurality of strands of oxide superconductor covered with a first metal sheath are prepared, the plurality of strands are filled in a second metal sheath, and each of the oxide superconductors is coated with a first metal sheath. The plurality of wire materials are filled in such a manner that the thickness of the superconductor contained therein is 5% or less of the external dimension in the thickness direction of the second metal sheath, and the second metal sheath is deformed into a tape shape. A method for producing an oxide superconducting wire comprising the steps of performing plastic working at least once in which a compressive load is applied to the metal sheath in the cross-sectional direction. 9. The oxide superconducting wire according to claim 8, further comprising the step of drawing each of the plurality of wire materials before filling the second metal sheath with the plurality of wire materials. manufacturing method. 10. The method for manufacturing an oxide superconducting wire according to claim 8, wherein in the plastic working step, the second metal sheath is deformed into a rectangular tape shape. 11. The method further comprises a step of heat treating the oxide superconductor after the plastic working step.
The method for manufacturing an oxide superconducting wire according to claim 8. 12. The method for producing an oxide superconducting wire according to claim 11, wherein the plastic working step and the heat treatment step are repeated multiple times. 13. The step of further comprising adjusting the thickness of each superconductor included in the oxide superconducting wire obtained in the plastic working step by the number of the wires filled in the second metal sheath. Claim 8 comprising:
A method for producing an oxide superconducting wire according to . 14. The method for manufacturing an oxide superconducting wire according to claim 11, further comprising the step of coating the second metal sheath with an organic substance after the heat treatment step. 15. The method for manufacturing an oxide superconducting wire according to claim 14, wherein the step of coating with an organic substance comprises passing the wire through a bath of an organic substance. 16. The method for manufacturing an oxide superconducting wire according to claim 14, wherein the step of coating with an organic substance comprises a step of applying an organic substance to the surface of the wire. 17. In the heat treatment step, the superconducting wire is heat-treated in a state where it is wound with a curvature that gives a strain of 0.3% or less (strain=thickness of the superconducting wire/bending diameter), and then,
The method for manufacturing an oxide superconducting wire according to claim 11, comprising a step of feeding out the curvature. 18. A metal sheath having a thickness direction dimension, and a plurality of oxide superconductors distributed independently in the thickness direction within the metal sheath, wherein the thickness of each of the oxide superconductors is A method for handling an oxide superconducting wire whose dimension in the direction is 5% or less of the external dimension in the thickness direction of the metal sheath, the method comprising:
A method for handling oxide superconducting wires, which involves handling superconducting wires while controlling the bending diameter) within a range of 0.3% or less.