JPH08241635A - Oxide superconductive wire material and manufacture thereof - Google Patents

Abstract

PURPOSE: To obtain the oxide superconducting wire material, which can improve the strength of sheath material and to which drawing can be performed, by forming the wire material of the blunt silver, which is arranged adjacent to the oxide superconductor so as to surround the superconductor, and the silver, which includes the predetermined quantity of at least one kind of MgO, NiO for surrounding the blunt silver. CONSTITUTION: An AgNi0.3 Mg0.3 alloy pipe as a sheath 1 is arranged most outside, and hexagonal pipes 3 filled with the raw material powder 2 of oxide superconductor 4 are inserted into the alloy pipe for fixation. After drawing is performed to the hexagonal blunt silver pipes 3, the heating that the temperature is raised from the room temperature to 835 deg.C at 400 deg.C/h, and maintained at 835 deg.C for eight hours, and furthermore, raised from 835 deg.C to 890 deg.C in four hours, and maintained at 890 deg.C for 10 minutes, and thereafter, gradually cooled from 890 deg.C to 835 deg.C at 5 deg.C/h, and cooled from 835 deg.C to the room temperature 10 about six hours is performed so as to manufacture the oxide superconducting wire material. As the raw material of the superconductor 4, Bi, Sr, Ca, Cu are mixed at Bi:Sr:Ca:Cu=2:2:1:2 of the atomic molar ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconducting wire used for producing a magnet for generating a strong magnetic field such as high resolution NMR.

[0002]

2. Description of the Related Art The superconducting phenomenon is being widely used in various fields such as high-current power transmission and strong magnetic field generation equipment by taking advantage of the fact that a large current can flow with zero resistance. Presently used for such practical use are materials commonly called metallic materials such as NbTi and Nb3Sn. In recent years, oxide superconductors whose critical temperature exceeds liquid nitrogen temperature have been discovered, and it is expected that this material will further expand superconducting applications.
In addition to having a high critical temperature, this oxide superconducting material also has the feature that the critical magnetic field (the maximum magnetic field that can maintain superconductivity) is dramatically larger than that of metallic materials. Utilizing this feature, it is expected to be applied as the innermost layer coil of a super strong magnetic field magnet.

[0003]

A large electric current is applied to the superconducting wire of the innermost coil of the strong magnetic field magnet under a strong magnetic field, so that a strong electromagnetic force is exerted. For example, in the case of a typical inner layer coil generating 3T in 20T, the tensile stress acting on the wire reaches several hundred MPa. Therefore, since the oxide superconducting material is ceramics, it is deformed, and the critical current density (Jc) is immediately lowered by this deformation. The amount of deformation that an oxide superconducting material can withstand is about 0.2%.

The oxide superconducting wire that has been conventionally manufactured is a so-called silver sheath wire in which the oxide superconducting material is wrapped with pure silver. Pure silver has weak strength (low Young's modulus), and distortion occurs when it is used in a high magnetic field as described above.
It is about 0.5% or more, and it cannot be used as it is. Therefore, some attempts have been made to increase the strength of the silver sheath wire. Nomura et al. Have reported that the addition of MgO or NiO to silver improves the strength of the silver sheath (Autumn 1993 Autumn Low Temperature Engineering and Superconductivity Society Proceedings p.32).

However, using this method, silver is replaced by Mg.
When a pipe with O 2 and NiO added was made and the powder of oxide superconducting material was packed into this pipe for wire drawing and processing, the pipe became hard and the pipe was damaged during the drawing and processing. It was common for cracks to occur and damage. That is, it has been strongly desired to develop an oxide superconducting wire having a high tensile strength and capable of being drawn, and a manufacturing method thereof.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an oxide superconducting wire capable of increasing the strength of a sheath material and performing wire drawing, and a method for producing the same. .

[0007]

Means for Solving the Problems The gist of the present invention is to provide pure silver surrounding the oxide superconductor adjacent to the oxide superconductor and at least one of MgO and NiO surrounding the pure silver.
An oxide superconducting wire composed of 0.01 to 0.5 mass% of silver converted to g and Ni.

Further, a pure silver pipe filled with oxide superconductor raw material powder is wrapped with a silver alloy containing 0.01 to 0.5 mass% of at least one of Mg and Ni to form a composite, and this composite is wire-drawn. Heat treated in an atmosphere containing oxygen to MgO, NiO
Is a method for producing an oxide superconducting wire, in which at least one of the above is deposited and dispersed in the silver alloy.

Further, in the above method for producing an oxide superconducting wire, the heat treatment after wire drawing is carried out at a temperature of 800 to 900 ° C. for 5 to 50 hours in an atmosphere containing oxygen.

[0010]

The present inventors conducted the following experiment in order to increase the strength of the silver sheath. That is, a method of increasing the strength of the silver sheath by heat treating a silver alloy containing Mg and Ni to internally oxidize it and depositing MgO 2 and NiO 2 was examined. The silver alloys having the compositions shown in Table 1 were heat-treated in the atmosphere at a temperature of 850 ° C., and the oxidation penetration depth with respect to time was examined. The result is shown in FIG.

As shown in FIG. 1, it has been found that oxidation permeates with time and the internal Mg and Ni change to MgO and NiO, respectively. In the typical case of containing Mg and Ni at 0.3% respectively, heat treatment at 850 ° C for 5 hours in air will give about 70
The oxidation penetrated to a depth of μm. The workability of silver alloys in which Mg and Ni are not oxidized is not so different from that of pure silver, and wire drawing is very easy. The present invention has been made based on this finding. That is, the present invention uses a silver alloy containing at least one of Mg and Ni as a silver sheath, and heat-treats after wire drawing to oxidize and deposit an appropriate amount of Mg and Ni in the silver sheath to enhance the strength of the silver sheath. It is a thing.

[0012]

[Table 1]

If the amount of Mg or Ni contained in the silver alloy is too large, the amount of Mg and Ni becomes hard and the workability before heat treatment deteriorates.
It was 0.5% by mass. On the other hand, if the amount is too small, M after heat treatment
Since gO 2 and NiO 2 cannot be expected to have high strength, the lower limit was made 0.01% by mass.

The heat treatment performed after wire drawing is carried out at a temperature of 800 to 900 ° C. for 5 to 50 hours in an atmosphere containing oxygen. The reason for this is to secure a sufficient time for oxidative permeation and to promote sintering and high orientation of the oxide superconductor to secure a sufficiently high Jc. Regarding the processing temperature, when the oxide superconductor does not sinter when heat-treated at less than 800 ° C,
It needs to be more than this. Also, when heated above 900 ° C, the Bi-based oxide powder decomposes and melts. At this time, CaO
Remains as a solid phase, and as it cools, CaO remains as a solid non-superconducting impurity. This lowers the crystallinity and orientation of the oxide superconductor and hinders the improvement of Jc. Therefore, the upper limit of the heat treatment temperature must be 900 ° C.

[0015]

EXAMPLES Examples of the present invention will be described below.
The manufacturing procedure of the oxide superconducting wire of the example is shown in FIG. In the examples, a multi-core structured wire is illustrated, but it goes without saying that a single-core structured wire is also included in the scope of the present invention. The manufacturing procedure of the oxide superconducting wire is as follows.

The example of the present invention is AgNi _0.3 Mg _0.3 which is the sheath 1.
An alloy pipe was arranged on the outermost periphery, and a hexagonal pipe 3 in which the raw material powder 2 of the oxide superconductor 4 was enclosed was charged and fixed therein. In the example of the present invention, the hexagonal pipe 3 is a pure silver pipe. After wire drawing, raise the temperature from room temperature to 835 ° C at 400 ° C / h in air, hold at 835 ° C for 8 hours,
It is said that the temperature is raised from 835 ° C to 890 ° C in 4 hours, kept at 890 ° C for 10 minutes, gradually cooled from 890 ° C to 835 ° C at 5 ° C / h, and the furnace is cooled from 835 ° C to room temperature in about 6 hours. An oxide superconducting wire was manufactured by heat treatment. The powder used as the raw material of the oxide superconductor 4 contains Bi, Sr, Ca and Cu in an atomic molar ratio of Bi: Sr: C.
It is a mixture such that a: Cu = 2: 2: 1: 2.

In Comparative Example 1, a pure silver pipe was used in place of the outermost silver alloy pipe of the present invention example, and an oxide superconducting wire was manufactured in exactly the same steps as the present invention example. This corresponds to the conventional method for producing a silver sheath oxide superconducting wire.

In Comparative Example 2, instead of the outermost silver alloy pipe of the present invention, MgO and NiO were converted into Mg and Ni, respectively.
An oxide superconducting wire was manufactured in exactly the same process as in the example of the present invention using a silver pipe added with 0.3% by mass. However,
In this case, the workability was poor as described above, and the outermost pipe was broken before the final heat treatment, and the critical current density could not be measured.

In Comparative Example 3, the outermost pipe and the hexagonal pipe for enclosing the raw material powder of the oxide superconductor are made of Ag.
Using an Ni _0.3 Mg _0.3 alloy pipe, an oxide superconducting wire was manufactured in exactly the same process as the example of the present invention.

The critical current density Jc and Young's modulus of the oxide superconducting wire manufactured by the above manufacturing procedure were measured at a temperature of 4.2K and an external magnetic field of 20T. The results are shown in Table 2.

[0021]

[Table 2]

As shown in Table 2, in the case of the pure silver sheath of Comparative Example 1, Jc was sufficiently large, but the Young's modulus was
Only 30 GPa and insufficient strength were obtained. In Comparative Example 2, although the Young's modulus was 75 GPa, which was a sufficient strength, the outermost peripheral pipe broke during processing, and wire drawing could not be performed to the final shape.

In Comparative Example 3, the strength was finally improved almost completely, but Jc was only 1/8 of that in Comparative Example 1, and it was not at a practical level. The change in magnetic susceptibility of this wire was measured using SQUID. The result is shown in FIG. As shown in Fig. 3, the onset temperature of superconducting transition is Bi-
Although the transition temperature of system 2212 phase is near 80K, the transition region is very wide. This is considered to be because the oxide superconductor was in direct contact with the silver alloy containing Mg and Ni, so that Mg and Ni diffused to the side of the superconductor during the heat treatment and deteriorated the characteristics of the oxide superconductor.

On the other hand, in the examples of the present invention, Jc has a sufficient size and sufficient strength is secured, and a practical wire rod is obtained. The change in magnetic susceptibility of this wire was measured using SQUID. FIG. 4 shows the results. As shown in Figure 4, the onset temperature of superconducting transition is around 80K,
The transition area is also very narrow. Considering this in comparison with Comparative Example 3, it was determined that the oxide superconductor was prevented from being contaminated by the diffusion of Mg and Ni during the heat treatment by arranging pure silver so as to directly contact and wrap the oxide superconductor. Conceivable.

[0025]

As is apparent from the above description,
According to the present invention, a practically large critical current density Jc
An oxide superconducting wire having high strength can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the depth of oxidation penetration with respect to time when a silver alloy having the composition shown in Table 1 was heat-treated in the atmosphere at a temperature of 850 ° C.

FIG. 2 is a diagram showing a procedure for manufacturing an oxide superconducting wire according to an example.

FIG. 3 is a diagram showing changes in magnetic susceptibility with temperature in Comparative Example 3.

FIG. 4 is a diagram showing changes in magnetic susceptibility with temperature in an example of the present invention.

[Explanation of symbols]

1 ... sheath, 2 ... raw material powder, 3 ... hexagonal pipe, 4
… Superconductors.

Claims (3)

[Claims] 1. Pure silver surrounding the oxide superconductor adjacent to the oxide superconductor, and silver containing at least one of MgO and NiO in 0.01 to 0.5 mass% in terms of Mg and Ni. An oxide superconducting wire, which is characterized by being composed of 2. A pure silver pipe filled with oxide superconductor raw material powder is wrapped with a silver alloy containing 0.01 to 0.5 mass% of at least one of Mg and Ni to form a composite, and the composite is wire-drawn. A method for producing an oxide superconducting wire, comprising heat treating in an atmosphere containing oxygen to precipitate and disperse at least one of MgO and NiO in the silver alloy. 3. The method for producing an oxide superconducting wire according to claim 2, wherein the heat treatment after wire drawing is performed in an atmosphere containing oxygen at a temperature of 800 to 900 ° C. for 5 to 50 hours.