JPH05319829A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To easily enhance the critical current characteristics of an oxide superconductor contg. Bi as a constituent element, the applicability in a high magnetic field, the uniform temp. property, yield, etc., by heat-treating the oxide superconductor in a Bi-contg. atmosphere. CONSTITUTION:Calcined powder of an oxide superconductor having a compsn. represented by Bi2Sr2Ca1Cu2Ox (where (x) is the amt. of oxygen) is mixed with a binder, etc., and molded to obtain thick filmlike molded body such as a thick film having 2212 phase as the crystal phase. This molded body is superposed on silver foil as a substrate and the resulting laminated thick film is put on an alumina holder fixed on an alumina substrate. A powdery Al2O3-Bi2O3 mixture is laid on the alumina substrate so that it does not come in contact with the laminated thick film and they are covered with an alumina crucible and put in an electric furnace, where the laminated thick film is heated to 885-920 deg.C at about 300 deg.C/hr heating rate, slowly cooled to about 870 deg.C at about 10 deg.C/hr cooling rate and further cooled to room temp. at about 600 deg.C/hr cooling rate to produce an oxide superconductor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an oxide superconductor, and particularly to an oxide superconducting material having a high critical current density using a thick film material obtained by a doctor blade method or the like. The present invention relates to a method for producing a stable product.

The present invention is applicable to the production of superconducting materials such as superconducting coils typified by coils for hybrid magnets that generate a strong magnetic field, superconducting wiring for electronic parts, magnetic shield materials and the like, which are made using thick films or tapes. Can be used.

[0003]

2. Description of the Related Art Conventionally, Nb-Ti, Nb3Sn, V3Ga and the like are known as superconducting materials that have been put into practical use. Among these, (Nb, Ti) 3Sn and V3Ga with Ti added to Nb3Sn are known to have a high critical current density even in a high magnetic field, and at a liquid helium temperature of 4.2K, even in a 20T magnetic field,
The entire superconducting wire has a critical current density of about 10,000 A / cm2. However, in a magnetic field higher than that, the critical current density dropped sharply, and there was no superconducting wire that has been put to practical use above 20T.

In recent years, oxide superconductors exhibiting a superconducting transition above the temperature of liquid nitrogen have been discovered one after another, and a method of processing into wire rods and tapes has been devised for its application. An example thereof is a tape material prepared by the doctor blade method. In this method, an organic solvent having the functions of a dispersant, a binder, and a plasticizer is added to fine powder of an oxide superconductor to prepare a slurry-like raw material, which has good releasability from a gap of about 1 to 300 μm. A green sheet is formed by continuously pouring out on a film, processed into a desired shape, and then heat treatment is performed to evaporate and remove the organic solvent and sinter oxide superconductor powder to obtain a tape material. Is.

Recently, a dip coating method has also been developed in which a metal substrate is immersed in a slurry-like raw material to continuously form an oxide superconductor layer on the surface of the metal substrate. Compared with the doctor blade method, the dip coating method can manufacture a long tape material with a simpler apparatus.

For Bi-based oxide superconductors, the composition ratio is
Crystal phase composed of Bi2Sr2Ca1Cu2Ox (x is oxygen content)
In 12 phases), a superconducting material having a high critical current density can be manufactured by combining the doctor blade method or the dip coating method with the melt solidification method. At this time, from a temperature slightly higher than 880 ° C at which the 2212 phase decomposes and melts to a temperature lower than the temperature at which it solidifies, for example, 885 ° C.
In the case of gradually cooling from 1 to 870 ° C., an oriented structure in which a superconducting current easily flows can be obtained, and a superconducting material having a high critical current density can be obtained. At this time, it has been clarified that silver is preferable as the substrate for supporting the superconductor.

The Bi-based oxide superconductor manufactured in this manner becomes a material having a high critical current density, and it is 2K at 77K, 0T.
Values over 000A / cm2 and over 4.2K, 12T are over 200,000A / cm2. Especially when cooled to 4.2K, the critical current density does not decrease even in a high magnetic field of 20T or more, and a value of 100,000A / cm2 or more is obtained, which is expected as a material for a high magnetic field generating magnet. There is.

[0008]

However, Bi, which is one of the constituent elements of this material, has a drawback that it is highly volatile and is scattered during the melt heat treatment. For this reason, the optimum melting temperature range was extremely narrow at 881 to 886 ° C, and an electric furnace with high temperature uniformity was required for the production of superconductors.
That is, when the maximum melting temperature is higher than 890 ° C, Bi scatters and the composition becomes non-uniform,
There was a drawback that the second phase rich in Sr-Ca-Cu-O was precipitated in the 2212 phase, which is a superconductor, or the low melting point liquid phase was generated and separated and flowed out of the tape. ..

For this reason, the critical current characteristics of the manufactured superconductor also greatly vary, and the reproducibility deteriorates. Particularly in a long tape or a large sample such as a coil shape, the influence of Bi scattering caused by the non-uniformity of the temperature distribution is large, which is a factor that deteriorates the superconducting property of the entire tape.

[0010]

Means for Solving the Problems The present invention uses Bi as a constituent element.
The present invention provides a method for producing an oxide superconductor, which comprises subjecting an oxide superconductor containing Si to a heat treatment in a Bi atmosphere.

In the present invention, the Bi-containing oxide superconductor is not particularly limited, but 2212 phase is preferably adopted.
Hereinafter, the 2212 phase will be described as an example.

A typical method for producing an oxide superconductor composed of a Bi-based 2212 phase using the present invention is shown below. First, using a doctor blade method or a dip coating method, a thick film composed of a calcined powder of Bi type 2212 phase is formed on the surface of a silver substrate. The molded body is heated to a temperature of 880 ° C. or higher at which the 2212 phase is decomposed and melted, and is melted and then solidified. 20 ° C for coagulation
It is preferable to cool at a rate slower than / h.

This melting and solidifying process is performed in a Bi atmosphere. In the present invention, “in a Bi atmosphere” refers to a gas containing a Bi-containing molecule that is a component volatilized from a Bi-based superconductor. An atmosphere having a partial pressure higher than the Bi vapor pressure of the Bi-based superconductor at that temperature is preferable for the molecule. To achieve such an atmosphere, it is preferable to place a substance having a vapor pressure of Bi-containing molecules higher than that of the Bi-based superconductor in a closed container.

However, when a substance such as Bi2O3, which is liquid at the melting and solidifying treatment temperature of the oxide superconductor and has a high Bi vapor pressure, is used, the Bi vapor pressure is too high.
In the process of melting and solidifying the superconducting system superconductor, the viscosity of the liquid phase generated from the superconductor becomes low and the superconductor easily flows, which is not preferable because the oxide superconductor may not be kept in a predetermined shape. Therefore, it is preferable to control the Bi vapor pressure by mixing the Bi2O3 powder with another oxide powder.

When the Al2O3 powder is mixed with the Bi2O3 powder, the molar ratio is preferably Bi2O3 / Al2O3≤0.5. This mixture forms the compound Bi2Al4O9 upon heat treatment. This compound has a melting point of 1070 ° C., is a solid phase at the melting and solidifying temperature of the Bi-based oxide superconductor, and stably supplies Bi vapor. From the beginning, it is also possible to use powders such as Bi2Al4O9.

As the constitution of the closed container, various shapes such as a crucible with a lid, a flat substrate covered with a boat or a crucible upside down, a pipe and the like can be used according to the shape of a desired material. You can At this time, with Bi oxide in the container
The 2212 phases are arranged so that they do not touch each other. After solidification, it is preferable to cool rapidly to room temperature so that the newly formed 2212 phase does not decompose.

Bi, which is one of the constituent elements of Bi-based superconductors
Has a high volatility and is scattered when the melt heat treatment is performed in air. In the manufacturing method of the present invention, since the melting / solidifying heat treatment is performed in the Bi atmosphere, it is possible to suppress the scattering of Bi.

Therefore, the composition does not change even after melting and solidification, and the precipitation of the second phase and the separation of the liquid phase can be suppressed. Since the superconducting material after heat treatment is not affected by the composition shift due to the scattering of Bi, it becomes possible to manufacture a tape or coil having a high critical current density with good reproducibility.

As a result of suppressing the scattering of Bi, the optimum melting temperature range is greatly expanded, and in the case of the Bi type 2212 phase, melting can be performed in the temperature range from 880 ° C to around 930 ° C. When the temperature is 880 ° C or lower, melting of 2212 phase does not occur, the critical current characteristics do not improve, and when the temperature rises to 930 ° C or higher, when silver is used as the metal substrate, it is
This is not preferable because there is a possibility that the Bi-based superconductor reacts and the silver substrate is melted, and the morphology as a material is lost.

When the melting and solidification are performed in the Bi atmosphere as in the present invention, the width of the optimum melting temperature is widened, so that the allowable range is widened even in the temperature equalizing region in the electric furnace. Particularly in the case of a large sample such as a long tape or coil, it is possible to remarkably reduce the deterioration of the characteristics caused by the non-uniformity of the temperature distribution.

The manufacturing method of the present invention is particularly effective for manufacturing the 2212 phase among Bi-based oxide superconductors. Further, it is preferable to use silver or an alloy containing silver as the metal substrate.

The method for forming the oxide superconductor thick film on the metal substrate is not particularly limited, and various thick film forming methods can be adopted. Specifically, the doctor blade method, the screen printing method or the dip coating method is preferably used because a film having a uniform thickness can be easily formed.

When the dip coating method is adopted, since the oxide thick film can be molded to the final shape without any mechanical deformation, a molded body having a complicated shape can be easily obtained.
In addition, the dip coating method has an advantage that, when a metal foil is used for the base material, a thick film of the oxide superconductor is formed on both surfaces of the base material, so that there is little deformation when heat treatment is performed. ..

[0024]

[Function] With respect to the Bi-based oxide superconductor, a superconducting material having a high critical current density can be manufactured by applying the melting / solidifying method. However, since the volatility of Bi is high, it is difficult to control the melting conditions, and there is a problem in increasing the size and reproducibility of superconducting properties. The heat treatment in the Bi atmosphere suppresses the volatilization of Bi, widens the allowable temperature range, and facilitates temperature control. Also, the reproducibility of superconducting properties is greatly improved.

[0025]

【Example】

(Example) An oxide superconductor calcination powder having a composition of Bi2Sr2Ca1Cu2Ox (x is oxygen content) was mixed with a polyvinyl binder, a plasticizer, and a dispersant, and the mixture was molded by a doctor blade method to form a thick film with a thickness of 50 μm. A shaped body was obtained. This thick film is cut into 25 mm × 3 mm pieces, and 50 μm thick silver foil is cut into 2 pieces.
It was placed on top of a substrate cut into 7 mm x 4 mm. This laminated thick film is placed on a support table placed on an alumina substrate, and Bi2O3 is placed on the substrate so that it does not come into contact with the laminated thick film.
And Al2O3 mixed powder (molar ratio Bi2O3 / Al2O3 = 0.1)
Spread, cover the alumina crucible upside down from the top,
An enclosed space was formed. This state is shown in FIG.

The laminated thick film, together with the substrate and the crucible, was placed in an electric furnace, and the maximum melting temperature was changed from 885 to 920 ° C. to carry out melting / slow cooling heat treatment. That is, the temperature was raised to a predetermined maximum temperature at 300 ° C / h, gradually cooled to 870 ° C at 10 ° C / h, and further cooled to room temperature at 600 ° C / h.

After the heat treatment, the surface of the thick oxide superconductor film appeared as if the melt had solidified, but there was no evidence that the liquid phase had flowed, and the edges remained clear as before melting. It was Observation of the cross section with a scanning electron microscope showed that the film thickness of the oxide superconductor was reduced to 15 μm and that the crystal grains of the oxide superconductor were oriented along the silver foil of the base material. The tape material thus obtained was cooled to 4.2 K with liquid helium, and the critical current density was measured in a magnetic field of 12 T by the DC four-terminal method.

(Comparative Example) Bi2Sr2Ca1Cu2Ox (x is the oxygen amount)
The oxide superconductor calcined powder having the composition was mixed with a polyvinyl binder, a plasticizer, and a dispersant, and molded by a doctor blade method to obtain a thick film-shaped molded body having a thickness of 50 μm. This thick film is cut into a size of 25 mm × 3 mm and a thickness of 50 μ
m foil was placed on the substrate cut to 27 mm x 4 mm. This laminated thick film was placed in an electric furnace as it was, and the maximum melting temperature was changed to 885 to 920 ° C. in the same manner as in Example, and the melting / slow cooling heat treatment was performed. That is, the temperature is raised up to a predetermined maximum temperature at 300 ° C / h, and then 10 ° C / h at 870 ° C.
The mixture was gradually cooled to ℃ and further cooled to room temperature at 600 ℃ / h.

After the heat treatment, the surface of the oxide superconductor thick film appeared as if the melt had solidified, but many needle-like impurity crystals were observed when treated at a high temperature such as 920 ° C. .. Observation of the cross section with a scanning electron microscope showed that the film thickness of the oxide superconductor was reduced to 15 μm, and the crystal grains of the oxide superconductor were oriented along the silver foil of the base material, but the orientation texture was needle-like. Many areas were also blocked by crystals.
The tape material thus obtained was treated with liquid helium.
Table 1 shows the results of measuring the critical current density in a 12T magnetic field by the DC four-terminal method after cooling to 4.2K.

[0030]

[Table 1]

[0031]

By the manufacturing method of the present invention, it becomes possible to easily manufacture an oxide superconductor, which has been difficult to control the temperature in the past. Furthermore, the oxide superconductor produced by the method of the present invention exhibits good critical current characteristics with good reproducibility. In particular, when manufacturing a superconductor having a large volume and a large surface area having a shape such as a coil, the allowable range of temperature uniformity of an electric furnace or the like becomes large, so that the effect of the present invention becomes remarkable. Recently, application of Bi-based oxide superconductors, which is cooled to 4.2K and generates a high magnetic field of 20T or more, has been studied, and the present invention is expected to realize the application in such a high magnetic field.

The conventional practical superconducting material could not be used because the critical current density sharply decreased in a high magnetic field of 20 T or more. The tape material and coil manufactured according to the present invention can be used while maintaining the superconducting state even at 20 T or more, and the energy loss in high magnetic field application can be minimized. Also in the manufacturing process, the allowable range for composition deviation and temperature uniformity is expanded by simple atmosphere control, so that the manufacturing apparatus can be simplified and the yield can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a heat treatment apparatus in an embodiment.

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[Procedure amendment]

[Submission date] September 9, 1992

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[0003]

2. Description of the Related Art Conventionally, Nb-Ti, Nb ₃ Sn, V ₃ Ga and the like are known as superconducting materials that have been put to practical use. Of these, (Nb, Ti) ₃ Sn and V ₃ Ga, which are Ti-added Nb ₃ Sn, are known to have high critical current densities even in high magnetic fields, and a magnetic field of 20T at a liquid helium temperature of 4.2K. Above all,
The entire superconducting wire has a critical current density of about 10,000 A / cm ² . However, in a magnetic field higher than that, the critical current density dropped sharply, and there was no superconducting wire that has been put to practical use above 20T.

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For Bi-based oxide superconductors, the composition ratio is
A crystal phase composed of Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x (x is the amount of oxygen) (hereinafter 22)
In 12 phases), a superconducting material having a high critical current density can be manufactured by combining the doctor blade method or the dip coating method with the melt solidification method. At this time, from a temperature slightly higher than 880 ° C at which the 2212 phase decomposes and melts to a temperature lower than the temperature at which it solidifies, for example, 885 ° C.
In the case of gradually cooling from 1 to 870 ° C., an oriented structure in which a superconducting current easily flows can be obtained, and a superconducting material having a high critical current density can be obtained. At this time, it has been clarified that silver is preferable as the substrate for supporting the superconductor.

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The Bi-based oxide superconductor manufactured in this manner becomes a material having a high critical current density, and it is 2K at 77K, 0T.
Values over 000A / cm ^{2 and} over 4.2K, 12T are over 200,000A / cm ² . Especially when cooled to 4.2K, the critical current density does not decrease even in a high magnetic field of 20T or more, and a value of 100,000A / cm ² or more is obtained, which is expected as a material for a high magnetic field generating magnet. ing.

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However, when a substance such as Bi ₂ O ₃ that is liquid at the melting and solidifying treatment temperature of the oxide superconductor and has a high Bi vapor pressure is used, conversely, the Bi vapor pressure is too high.
In the process of melting and solidifying the superconducting system superconductor, the viscosity of the liquid phase generated from the superconductor becomes low and the superconductor easily flows, which is not preferable because the oxide superconductor may not be kept in a predetermined shape. Therefore, it is preferable to control the Bi vapor pressure by mixing the Bi ₂ O ₃ powder with another oxide powder.

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When the Al ₂ O ₃ powder is mixed with the Bi ₂ O ₃ powder, the molar ratio is preferably Bi ₂ O ₃ / Al ₂ O ₃ ≤0.5. This mixture forms the compound Bi ₂ Al ₄ O ₉ upon heat treatment. This compound has a melting point of 1070 ° C., is a solid phase at the melting and solidifying temperature of the Bi-based oxide superconductor, and stably supplies Bi vapor. From the beginning, it is also possible to use powders such as Bi ₂ Al ₄ O ₉ .

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[0025]

Example An oxide superconductor calcined powder having a composition of Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x (x is an oxygen amount) was mixed with a polyvinyl binder, a plasticizer, and a dispersant, and a doctor was used. Molding was carried out by the blade method to obtain a thick film molded body having a thickness of 50 μm. This thick film is cut into 25 mm × 3 mm pieces, and 50 μm thick silver foil is cut into 2 pieces.
It was placed on top of a substrate cut into 7 mm x 4 mm. Place this laminated thick film on a support stand placed on an alumina substrate, and place Bi ₂ O ₃ on the substrate so as not to contact the laminated thick film.
And Al ₂ O ₃ mixed powder (molar ratio Bi ₂ O ₃ / Al ₂ O ₃ = 0.1)
Spread, cover the alumina crucible upside down from the top,
An enclosed space was formed. This state is shown in FIG.

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(Comparative Example) Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x (x is the amount of oxygen)
The oxide superconductor calcined powder having the composition was mixed with a polyvinyl binder, a plasticizer, and a dispersant, and molded by a doctor blade method to obtain a thick film-shaped molded body having a thickness of 50 μm. This thick film is cut into a size of 25 mm × 3 mm and a thickness of 50 μ
m foil was placed on the substrate cut to 27 mm x 4 mm. This laminated thick film was placed in an electric furnace as it was, and the maximum melting temperature was changed to 885 to 920 ° C. in the same manner as in Example, and the melting / slow cooling heat treatment was performed. That is, the temperature is raised up to a predetermined maximum temperature at 300 ° C / h, and then 10 ° C / h at 870 ° C.
The mixture was gradually cooled to ℃ and further cooled to room temperature at 600 ℃ / h.

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[0030]

[Table 1]

Front page continued (72) Inventor Toshiya Matsubara 1150 Hazawa-machi, Kanagawa-ku, Yokohama, Kanagawa Prefecture Asahi Glass Co., Ltd. Central Research Laboratory (72) Inventor Tsuyoshi Morimoto 1150, Hazawa-machi, Kanagawa-ku Yokohama City, Kanagawa Prefecture Asahi Glass Co., Ltd. Central Research Center (72) Inventor Kazumasa Toba 1-2-1 Sengen, Tsukuba City, Ibaraki Prefectural Government, Science and Technology Agency, Research Institute for Metal Materials (72) Inventor Hiroaki Kumakura 1-2-1, Sengen, Tsukuba City, Ibaraki Prefecture In-house (72) Inventor Hitoshi Kitaguchi 1-2-1 Sengen, Tsukuba-shi, Ibaraki Prefectural Institute of Metals Technology (72) Inventor Hiroshi Maeda 1-2-1 Sengen, Tsukuba-shi, Ibaraki Prefectural Government In-house

Claims (6)

[Claims] 1. A method for producing an oxide superconductor, which comprises subjecting an oxide superconductor containing Bi as a constituent element to a heat treatment in a Bi atmosphere. 2. The method for producing an oxide superconductor according to claim 1, wherein the heat treatment of the oxide superconductor is a melt solidification method. 3. The oxide superconductor according to claim 1 or 2, wherein the oxide superconductor containing Bi as a constituent element is mainly formed of a crystalline compound having a composition ratio of Bi2Sr2Ca1Cu2Ox (x is the amount of oxygen). Body manufacturing method. 4. The Bi atmosphere is a gas containing a Bi-containing molecule that is a component volatilized from the Bi-based superconductor.
3. A method for manufacturing an oxide superconductor according to any one of 3 above. 5. The oxide superconductor is higher than the Bi-based superconductor.
The method for producing an oxide superconductor according to any one of claims 1 to 4, wherein the heat treatment is performed in a closed container on which a substance having a Bi-containing molecular vapor pressure is placed. 6. A compound having a Bi-containing molecular vapor pressure higher than that of a Bi-based superconductor, a compound containing Bi2O3 and Al2O3, or B.
The method for producing an oxide superconductor according to claim 5, which is a mixture of i2O3 and Al2O3.