JPS63274016A - Oxide superconductor and its manufacture - Google Patents

Abstract

PURPOSE:To improve the critical temperature and the critical current density by using an oxide superconductor as a core, and covering its periphery with silver or a silver alloy. CONSTITUTION:As the material of a sheath 2 for an oxide superconductor 1, silver or a silver alloy is used. Since oxygen is fed suffeciently to the oxide superconductor for a core when it is in a sintering heat-treatment, the deterioration of the property after the sintering heat-treatment is not generated even though it is used as covered with the sheath. Since the electric conductibility and thermal conductibility of silver itself is good, an oxide superconductor with a stabilized silver or silver alloy can be provided, and the critical temperature and the critical current density are improved extensively. The melting point of silver or the silver alloy is below 960 deg.C, and the sintering temperature is made also below 920 to 930 deg.C. In this case, as the oxide superconductor, an oxide superconductor including a layer-form perovskite structure or a perovskite structure ceramics of such as Y-Ba-Cu-O, Er-Ba-Cu-O, or Yb-Ba-Cu-O is used. In such a composition, it can be used as a superconductor even under the liquid nitrogen condition.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to superconducting conductors, particularly high temperature oxide superconducting conductors.

[Prior art and its problems] La-Ba, -Cu-0 system, La-8n-Cu-0 system,
In layered perovskite type or perovskite type oxide ceramics such as Y-Ba-Cu-0 series, high-temperature superconductors with critical temperatures Tc of 40 to 125 have been discovered, but their development as conductors is still insufficient. Therefore, the critical current density Jc is naturally at a low level.

The first example of making wire rods is at Yozai Research Institute.
Put La-8n-Cu-0 powder into a Cu-Ni tube,
It is drawn and made into wire, and the critical temperature is 35, the critical current is 4.2, and 100 OA/cj is produced in IOT. However, this property was measured after removing the Cu-Ni sheath on the outer periphery after forming the wire into a wire, and then heat-treating the wire, and not the characteristics as a clad wire.

As a second example of wire rod production, there is an example by Tohoku University's Institute of Materials Research, in which only the metal components of the La-5r-Cu-0 system excluding oxygen are made into an amorphous alloy tape by a molten metal rapid cooling method and then placed in an oxygen atmosphere. There is a report that the material is heat-treated to form a high-temperature superconducting ceramic tape, and that it has a critical temperature of 40°C, but the critical current value is not specified.

Even in the Y-Ba-Cu-0 system, which has a high critical temperature, it is made into a conductor, with a critical temperature of 87 and a critical current value of 6 A/cd.
(at 77.3 K), but its conductor structure has not been clarified.

Incidentally, an example of powder sintering of the Y-Ba-Cu-0 system without forming it into a wire is the case of ATT, which has a high criticality of 1100 A/c- at the boiling point temperature of liquid nitrogen (77.3 K). There are reports that the current density was shown.

In the case of the oxide-based superconducting material shown above, it is possible to easily make it into a wire by enclosing the powder in a metal tube and reducing the area by drawing, swaging, etc., but it is not in a superconducting state as it is. It is almost impossible to flow current at this temperature, and high critical temperatures and high critical current densities cannot be expected.

The reason why properties are not obtained when the wire is formed by plastic working is that the properties deteriorate due to the deformation of the superconducting fine powder, and the compacting effect of the processing process makes it difficult to connect the highly anisotropic superconducting fine powder with mere physical contact. I think it's because it's impossible. In the case of oxide-based superconducting materials, it is said that in order to expect a tunnel effect due to close proximity, it is necessary to get close to the order of 10 to 20, and this is thought to be the reason why the characteristics are not obtained with compacted powder alone.

In order to interpret this, it is necessary to perform sintering heat treatment after making wire rods, but in the case of oxide-based superconducting materials, it is necessary to heat treat them at high temperatures in an oxygen atmosphere, at least in the air, and Cu
- When placed in a Ni or Cu metal tube, the superconducting powder does not contain oxygen. Furthermore, the superconducting powder and the metal sheath undergo a diffusion reaction and the composition changes, so superconducting properties are still not achieved.

[Object of the Invention] An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a novel oxide superconducting wire having a high critical temperature and high critical current density.

[Summary of the Invention] The gist of the present invention is that silver or a silver alloy material is used as the sheath material of the oxide superconducting wire.
The main feature is that sufficient oxygen is supplied to the oxide superconducting portion of the core during the sintering heat treatment, so that the characteristics after the sintering heat treatment do not deteriorate even if the sheath remains covered.

Silver-white bodies have good electrical conductivity and thermal conductivity, and as a result,
We can provide silver or silver alloy stabilized oxide superconducting wire,
We were able to significantly improve the critical temperature and critical current density.

The melting point of silver or a silver alloy is 960°C or lower, which means that the sintering temperature is also limited to 920-930°C or lower.

Taking the Y-Ba-Cu-0 system as an example, methods for producing superconducting powder include a powder mixing method, a coprecipitation kneading method, etc.
The optimum sintering temperature varies depending on the particle size. In the case of the powder mixing method, the optimum temperature is 950-1000°C, while in the case of the coprecipitation kneading method, the optimum temperature is about 50-60°C.
The temperature can be lowered by 0°C to 900-950°C. Due to the relationship with the melting point of the silver sheath material, the coprecipitation kneading method is a more suitable powder manufacturing method.

The oxide superconducting materials used here are Y-Ba-Cu-O, E
r-6Ba-Cu-0, Yb-Ba-Cu-0, La-
It means an oxide superconducting material containing layered perovskite type ceramics such as Ba-Cu-0, La-5n-Cu-0, or perovskite type ceramics.

[Example] An example of a manufacturing process in which the present invention is implemented is shown in FIG.

The high-temperature superconducting oxide fine powder used was (YBaO,330,87)8Cu307-δ. Two types were used: a coprecipitation kneading method and a powder mixing method.
℃, and the powder mixing method is to pulverize and finely powder the calcined material at 950 to 1000℃.

The superconducting fine powder is compacted, assembled into a silver sheath tube, subjected to surface reduction plastic processing together with the sheath tube, and then drawn out.
After rolling into a linear or tape-like cross-sectional shape, 8
The sintering heat treatment was carried out in the temperature range of 50 to 950°C in 02 atmosphere or air.

Thereafter, if necessary, an annealing heat treatment was performed at 500 to 800°C to improve the characteristics and complete the required superconducting wire. FIG. 2 shows its cross-sectional shape.

Here, we will discuss the advantages of using silver or silver alloy tubes as the sheath material. The first condition required for the sheath material is
It is important that the sheath tube material has ductility in order to protect and reduce the area of the superconducting powder in the core, and silver or silver alloy materials are suitable for this purpose. Second, during sintering heat treatment in an oxygen atmosphere, the properties of the superconducting material and sheath material should not be deteriorated, and good superconducting properties should be exhibited. There are three factors regarding this second condition, one of which is a diffusion reaction between the sheath material and the superconducting powder, resulting in property deterioration that changes the composition.
Since silver is a precious metal, there is little need to worry about this. Furthermore, L
In the case of the a-Ba-Cu-0 system, it is known that the addition of Ag element has the effect of increasing the critical temperature;
It can be expected that it will not become worse in the case of Ba-Cu-0 system. The second factor is that the sheath material oxidizes in an oxygen atmosphere, making it unusable.The third factor that poses a problem with silver precious metals is that the sheath material oxidizes in an oxygen atmosphere, making it unusable. The problem is that oxygen vacancies, which play an important role in the superconducting mechanism of oxide superconductors, are blocked by blocking oxygen in and out of the material, and the properties deteriorate. The material allows oxygen to diffuse more easily than other metals, and there are no problems with oxygen vacancies. Table 1 shows 90 as a value near the sintering temperature.
It shows the value of the diffusion coefficient of oxygen into metal at 0°C,
Silver has an order of magnitude higher diffusion coefficient than other metals. Steel is also relatively large, but steel cannot be used because it becomes crumbly when heated in oxygen, which changes the copper content in the superconducting material.

The third condition required of the sheath material is its effectiveness as a superconductor stabilizing material, and it has good conductivity and thermal conductivity, so there is no problem.

Table 2 shows the low-temperature electrical properties of the wire produced as shown in FIG. 1, with a silver diameter of 1.6 and a core superconducting portion diameter of 1.0, together with comparative examples. The measurement uses a four-terminal method, the outer sheath material is removed with nitric acid, and the resistance change is determined from only the superconducting portion of the core. The critical current density value Jc is 77.
3 is the constant value of n1 in OT.

In Examples 1 and 2 in which silver was used as the sheath material, a superconducting state was achieved, and in comparative examples using other metals, a superconducting state was not achieved. In the case of Example 1, superconducting fine powder produced by the coprecipitation kneading method is used, and the sintering temperature can be lowered, so the critical temperature (fln
al) Tc-89 has the boiling point temperature of liquid nitrogen (77,
3K), the critical current density is Jc=565A/c-.

In the case of Example 2, superconducting fine powder was used by the powder mixing method, and the critical temperature (1'1n
al) to Tc-82, 77. Although the critical current density at No. 3 is low at Jc-2A/c-, a superconducting state is achieved.

Comparative Example 1 is an example in which superconducting fine powder produced by the coprecipitation kneading method was used, and Cu-30%Ni was used as the sheath material, and only surface reduction plastic processing was performed without heat treatment. When measured, it did not show any superconducting properties. The characteristics of this sample were measured separately from changes in magnetic susceptibility using an inductance method, and it was found that the critical temperature (onset
) It was found that it exhibits superconducting properties at 92 K. In other words, this means that the oxide raw material powder, which already exhibits superconducting properties, has not deteriorated even after surface-reducing plastic processing, and some of the powders have shown superconductivity, which is a phenomenon recognized by the inductance method. However, it can be interpreted that the oxide powders were not connected in a superconducting state and did not exhibit superconducting properties in the four-terminal method.

In Comparative Example 2, the wire of Comparative Example 1 was sintered and heat-treated at 900° C., but the oxide powder of the core and the sheath material reacted, and the wire did not exhibit superconducting properties.

Comparative Example 3 uses copper as the sheath material, and the oxide powder of the core reacts with the sheath material, so it does not exhibit superconducting properties. Furthermore, in this case, the sheath material becomes crumbly and oxidized and does not function as a sheath material.

Comparative Example 4 used platinum as the sheath material, and although the appearance was good in this case, superconducting properties were not exhibited, probably because oxygen vacancies were not adequate.

Table 1 In Figure 1.2, single-core structure Ag/Y-Ba-C
Although the example of the u-0 wire rod is shown, it is possible to create a multicore structure as shown in FIG. 3 when the wire rod is assembled into the sheath, or by re-bundling the wire rods whose area has been reduced and incorporating them into the sheath. . Specifically, when a set of 7 wires with a diameter of 41.6 mm was manufactured and subjected to sintering heat treatment, it was confirmed that the wire rod had a value of 77.3 or higher and exhibited superconducting properties. By using multiple wires, it is expected that the stability characteristics of the wire will increase and the critical current value will increase.

[Effects of the invention] According to the present invention, it is possible to make an oxide superconductor into a wire,
In addition, the problem of stability, which was a drawback of oxide-based superconductors, can be solved, and the superconductor exhibits a high critical current value.

This makes it possible to manufacture wires that can be used as superconducting conductors even under liquid nitrogen.

[Brief explanation of drawings]

FIG. 1 shows an example of the manufacturing process of the oxide superconducting material of the present invention. FIG. 2 shows a cross-sectional configuration diagram of the single-core wire rod and ribbon-shaped superconductor of the present invention. FIG. 3 shows a cross-sectional configuration diagram of a multi-wire, multi-ribbon superconductor of the present invention. 1: Oxide superconducting material, 2: Sheath material (silver).

Claims (4)

[Claims] (1) A single-core oxide superconductor having a structure in which an oxide superconducting material is used as a core and silver or a silver alloy is coated around the core. (2) A multi-oxide superconducting conductor in which a number of cores of oxide superconducting materials are dispersed in a matrix consisting of a stabilizing material of silver or silver alloy. (3) As a method for manufacturing the oxide superconducting material in (1) and (2) above, oxide superconducting fine powder that already exhibits superconducting properties is sealed in a tube material made of silver or a silver alloy in a single core or in a multi-core form, After that, the area is reduced by plastic working, and then sintering heat treatment is applied. (4) A method in which a powder produced by a coprecipitation kneading method is used as the oxide superconducting fine powder in the production method of 3 above.