JPH02217352A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain an oxide superconductor having critical current density characteristics sufficiently large even in a magnetic field, a size and shape suitable for practical use and excellent mechanical strength by solidifying a raw material composition for oxide superconductor containing a molten state to give polycrystal and pressurizing the polycrystal at a specific temperature. CONSTITUTION:A raw material composition for oxide superconductor is at least partially made into a molten or semi-molten state and solidified to give polycrystal. Then the polycrystal is pressurized at a temperature to partially soften the polycrystal and < the melting point of the oxide superconductor. The shape of the raw material composition for oxide superconductor is not particularly limited, the above-mentioned pressurizing treatment can be applied to a molded article of desired shape such as tape, linear material, laminar material or tubular material. Pressurizing in the uniaxial direction like hot press or hot hydrostatic press may be used as the pressurizing method. The pressure depends on temperature and is usually about >=10kg/mm<2>.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing an oxide superconducting conductor used for cables, magnets, magnetic shielding elements, etc. The present invention relates to a method for manufacturing a superconducting conductor having (Jc).

(Prior art) In recent years, many high-temperature superconductors such as He, which do not require extremely low temperatures, have been found in oxides, especially oxides with a layered perovskite structure containing Cu. For example, LnBatCax.
Ot-x (Ln:Y%Hu,!'11 etc. rare earth elements)% B i rich srzcacumo*-x, B it
s rgcaxcuso+*-x, TlxBazCa
CuOm-x, Tj! 1BazCazCusO+e-x
, Tl1B; 1zCazCuzOx-x, etc. These oxide superconductors become superconducting at temperatures above liquid N2 temperature, so they are much more economical than conventional metal superconductors that exhibit superconductivity at liquid He temperatures, and their use in various fields is being considered. .

By the way, the above-mentioned oxide superconductor is brittle, so it cannot be plastically worked into the metallic birch material.In order to process these into wires, strips, etc., a pre-filled powder of oxide raw material powder must be pre-filled using a powder metallurgy method. After forming into a shape, it is sintered or a composite oxide that can be used as a conductor is formed into a thin film on a substrate such as SUS or Hastelloy alloy using a vapor phase growth method such as PVD or CVD. Methods such as directly depositing oxides on oxides and heat-treating them to form oxide superconductors have been adopted.

(Problem to be solved by the invention) However, among these methods, the method of sintering a molded body of oxide powder yields only a superconductor with a small critical current density (JC), and J due to magnetic field
There was a problem that c-characteristics deteriorated. On the other hand, thin films obtained by PVD methods such as sputtering and vapor deposition, CVD methods, etc. have relatively large J values, and also have Jc characteristics that do not deteriorate much even in a magnetic field. However, the thickness is lI! limited to around m or less,
In addition, because of low productivity, this method was thought to be useful only for some applications such as electronics.

Therefore, various studies have been made to improve the Jc characteristics of oxide superconductors using powder methods. For example, methods have been attempted in which oxides are melted and then solidified to form high-density oxide crystals. There is.

The crystals obtained by this method have individual crystal grains with large Jc values comparable to those of the thin film. However, when the entire crystal was measured, the properties were significantly reduced, and the properties were reduced to almost the same level as in the case of oxide sintered bodies produced by conventional methods. By making some adjustments, the length can be reduced to 20.1~
Several mm, thickness, width: 0-01~0. Although it is possible to grow up to several mm, there are natural restrictions on the size, and the size is insufficient for practical applications such as cables, magnets, and magnetic shielding elements.

[Means to solve problems]

The present invention has been made as a result of intensive studies in view of the above points, and its purpose is to provide a sufficiently large critical current density (JC) characteristic even in a magnetic field, and a large enough current to be suitable for practical use. It is an object of the present invention to provide a method for efficiently molding an oxide superconducting conductor having a good shape and excellent mechanical strength.

The present inventors conducted intensive studies to solve the above problems, and found that a polycrystalline body obtained by solidifying a raw material composition for an oxide superconductor including a melt state is We discovered that by applying pressure at a temperature that softens the low-melting-point impurities contained between crystal grains, the bonds between each crystal grain are strengthened and a product with a large critical current density (Jc) can be obtained. Thus, the present invention has been completed.

That is, the present invention heats a raw material composition for an oxide superconductor to make at least a part of it into a molten or semi-molten state, and then solidifies it to obtain a polycrystalline body. This is a method for producing an oxide superconductor, characterized in that pressure treatment is carried out at a temperature at which a portion of the oxide superconductor softens and is lower than the melting point of the oxide superconductor.

The configuration of the present invention will be specifically explained below.

The raw material composition for an oxide superconductor in the present invention is a raw material composition containing a stoichiometric ratio or a specific component in excess or in a small amount, such as an oxide, a nitrate, a carbonate, or a mixture thereof. Of course, it is desirable to pre-sinter the material in advance to convert most of it into an oxide superconductor.

The oxide superconductor raw material composition is completely melted as a whole or partially kept in a semi-molten state, and then cooled and solidified. At this time, it is more preferable to cool and solidify in a furnace with a temperature gradient in one direction because the crystal grains will elongate in the cooling direction and orientation will be obtained.The shape of the oxide superconductor raw material composition in this step is The heat treatment is not particularly limited, and the above-described heat treatment can be applied to molded bodies of desired shapes such as tapes, linear bodies, plate-shaped bodies, and tubular bodies. Alternatively, the polycrystalline material obtained by such heat treatment may be used as a raw material, and the polycrystalline material may be molded into a desired shape and then subjected to the next pressure treatment.

Next, the polycrystalline body thus obtained is heated and subjected to pressure treatment, but if the temperature is below the softening temperature of the low melting point impurities contained between each crystal grain of the polycrystalline body, Since the bond between each crystal grain is not strengthened and therefore a material with a large critical current density (JC) value cannot be obtained, it is necessary to perform the pressure treatment at a temperature higher than the softening temperature of the low melting point impurity.

Furthermore, if the temperature is higher than the melting point of the oxide superconductor, impurities with a high melting point that are finely dispersed within the grains of the polycrystalline material and have an effective effect on superconducting properties such as improving the pinning force will also melt. Therefore, it is necessary to perform pressure treatment at a temperature below the melting point of the oxide superconductor.

The pressurization method may be uniaxial pressure such as hot press, or hydrostatic pressure such as HIP (hot isostatic press), and the effective pressure for pressure treatment depends on temperature. Although it depends, it is usually 10 kg/mm” or more, more preferably 50 kg/mm” or more.
Kg 7mmt or more.

When the pressure treatment is performed at a low partial pressure or in a vacuum or inert gas atmosphere, the melting of low melting point impurities existing between crystal grains is promoted, and at the same time, the bonding reaction between crystal grains is accelerated. The low O3 partial pressure, which often yields good results, is desirably 0.1 atm or less, more preferably O,OS atm or less.

The polycrystalline material that has undergone the above pressure treatment may have a 0. It is heat-treated in an atmosphere to cause reactions such as oxidation, recrystallization, and phase transition to become an oxide superconductor.

[Effect]

In the method of the present invention, since the oxide superconductor raw material composition is once melted and then cooled to solidify, a high-density polycrystalline oxide can be obtained. In particular, when solidifying in one direction during cooling, the a-axis or b-axis direction, where current flows easily in the longitudinal direction, advances crystal growth while discharging low melting point impurities to the oriented grain boundaries, so the electrical bond at the grain boundaries is weak. It is something.

In other words, many of these low melting point impurities are Cu oxides, composite oxides of Cu and alkaline earth metals, oxides of Bi, T1, etc., and composites of these with oxides of other components, etc. Or a semiconductor, which prevents current from flowing between crystal grains.

The present invention was made by elucidating the above facts and discovering that bonding between crystal grains of an oxide superconductor can be quickly obtained by applying pressure treatment at a temperature above the softening point of these low-melting point impurities. be. Although the details of the reaction that causes the bonding between crystal grains are not clear, it is thought that the pressure treatment brings the crystal grains closer together, which favorably affects the liquid phase or solid phase reaction between the crystal grains.

It is obvious that the above-mentioned pressure treatment is also effective in improving mechanical strength.

(Example〕 Next, the present invention will be explained in more detail with reference to Examples.

Example 1 Powders of Y2O2, B a COs and CuO were converted into Y:Ba
:Cu was blended in an atomic ratio of 1:2.05:12, and then this mixed powder was heated at 920'CX for 6 hours in the air.
It was pre-fired. After crushing and classifying this calcined body, 5X10X
Molded into a 40mm rod and heated at 900°C for 12hrs.
The sintered main island was placed in a Pt box, placed in an electric furnace, and heated for 10 minutes in a 0° air flow (latm).
After heating the rod to 90'C to a semi-molten state, it was then moved at a moving speed of 0°8 mm/hr in a part with a temperature gradient of about 50°C/cm in one direction in the furnace. The rod was cooled and solidified.

Next, this rod-shaped body was subjected to HIP treatment for 12 hours in a Pot-0.
After heating for 5 minutes, the mixture was slowly cooled to room temperature at a cooling rate of 1.5° C./min to obtain an oxide superconductor.

Example 2 An oxide superconducting conductor was obtained in the same manner as in Example 1 except that the pressure in the HIP treatment was 30 atm.

Comparative Example I An oxide superconducting conductor was obtained in the same manner as in Example 1 except that the HIP treatment was not performed.

Comparative Example 2 An oxide superconducting conductor was obtained in the same manner as in Example 1 except that the pressure in the HIP treatment was 5 atm.

Regarding the superconducting conductors obtained in Examples 1 to 2 and Comparative Examples 1 to 2 above, current terminals were placed at both ends of the rod-shaped body, voltage terminals were placed in the center at intervals of 20 mm, and a 10 ms e c
The critical current density (J) was measured by applying a pulse current of . The 2+1 constant is based on IpV/cm, and a magnetic field of 0°5 T perpendicular to the rod-shaped sample surface is applied to the sample in liquid nitrogen (7
7K).

The results obtained are shown in Table 1.

Table 71 As is clear from Table 1, the oxide superconducting conductors (Examples 1 and 2 products) produced by the method of the present invention are the same as those of Comparative Examples 1 and 2.
The critical current density (JC) value is significantly improved compared to the product. Regarding the polycrystalline material of Comparative Example 1, Au manufactured by Semiconductor Industry
Using the wire pole bonding method, a terminal was attached inside the crystal grain, and J was measured under the same conditions as in Example 1 except for the terminal position.
” was obtained, which had a high value on the same order as the products of Examples 1 and 2. However, when measured through one or more grain boundaries, the value of J was 1150 or less (in the case of multiple 100~l
/1000). From this experimental fact, it was found that the method of the present invention strengthened the bonds at the grain boundaries and improved the J of the polycrystalline material to a level close to the Jc within the crystal grains.

Example 3 Instead of HIP treatment, commercially available N, in a gas stream, temperature: 9
101 of the rod-shaped body under the conditions of 50℃ and 250 atm pressure.
An oxide superconducting conductor was obtained under the same conditions as in Example 1 except that the 0x40 plane was pressed from above and below by hot pressing for 4 hours.

Comparative Example 3 An oxide superconductor was obtained under the same conditions as in Example 3, except that the temperature of the pressure treatment was 800°C.

The critical current density (JC) of the superconducting conductors obtained in Example 3 and Comparative Example 3 was measured in the same manner as in Example 1, and the results are shown in Table 2.

Table 2 As is clear from Table 2, the oxide superconducting conductors manufactured by the method of the present invention (3 products near X route) had a lower temperature of J than the 3 products of comparative examples in which the temperature of the pressure treatment was too low.

The value has improved significantly.

Example 4 Bit's, S r COs, CaC0, and CuO powders were prepared in an atomic ratio of Bi:Sr:Ca:Cu of λ2:2.
The powder mixture was blended in a ratio of 0:1.1:2.1, and then this mixed powder was calcined at 820° C. for 20 hours in the atmosphere.

The calcined powder obtained by crushing and classifying this calcined body is
．． After filling a 5 mmφ Ag tube, the thickness was 0.
．． Six products rolled to 5 mm were rolled to 905' in air.
After heating CX for 2 hr, Po, -Q.

latm Ar + O in atmosphere, temperature: 840
HIP treatment was performed for 8 hours at 350 atm at a temperature of 0.degree.

Finally, this was heat-treated at 810° C. for 6 hours in the atmosphere to obtain an oxide superconducting conductor.

Comparative Example 4 An oxide superconducting conductor was obtained in the same manner as in Example 4, except that the HIP treatment was performed in the atmosphere for 8 hours.

Comparative Example 5 An oxide superconducting conductor was obtained in the same manner as in Example 4 except that the HIP treatment was performed in the atmosphere for 100 hours.

Comparative Example 6 An oxide superconductor was obtained in the same manner as in Example 4 except that the HIP treatment was performed at 750°C.

Regarding the superconducting conductors obtained in Example 4 and Comparative Examples 4 to 6 above, current terminals were placed at both ends of a 40 mm long tape, voltage terminals were placed at intervals of 10 mm in the center, and the critical current was Density (Jc) was measured. Measurements are taken perpendicular to the tape surface at 0. The test was carried out in liquid nitrogen (77K) by applying J4i volume of IT.

The results obtained are shown in Table 3.

Table 3 As is clear from Table 3, the product of Example 4 manufactured by the method of the present invention has excellent superconducting properties. On the other hand, Comparative Example 4 products whose 08 partial pressure during pressure treatment was too high had a J value, which was very small, and Comparative Example Product 5 had a J value that was second to the invention example even though the 01 partial pressure was too high. Jc value has been obtained, but 'JF?' due to pressure treatment? 5 takes time,
From this point as well, it is clear that the method of the present invention is excellent in productivity. Moreover, in the 6 comparative examples that were subjected to pressure treatment under conditions where no liquid phase was formed, only very small Jc values were obtained.

〔Effect of the invention〕

As is clear from the above, according to the method of the present invention, it is possible to obtain a material in which the poor superconducting properties, particularly the critical current density (Jt) properties, which were a fatal flaw in the practical application of oxide superconducting conductors, have been significantly improved. I can do it.

The pressure treatment in the present invention can be performed at any step leading to the final molded product, and can also be performed using hydrostatic pressure using, for example, HIP, so even if the molded product has a complex shape, Pressure treatment can be applied homogeneously.

It has been exemplified above that an oxide superconductor with significantly improved properties can be obtained by the method of the present invention. It is obvious that it can be improved.

The above two methods of the present invention enable the wide-ranging practical application of oxide superconducting conductors, and their industrial value is extremely large.

Claims (1)

[Claims] After heating the raw material composition for an oxide superconductor to make at least a part of it into a molten or semi-molten state, it is solidified to obtain a polycrystalline body, in which a part of the polycrystalline body is softened. 1. A method for producing an oxide superconducting conductor, comprising pressure treatment at a temperature lower than the melting point of the oxide superconductor.