JPH09118522A - Oxide superconductor having high critical current density - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an oxide superconductor having high critical current density. SOLUTION: Calcined powder prepd. by blending components so as to attain the objective compsn. is compacted and sintered to obtain the objective oxide superconductor. The carbon and water contents of the calcined powder having been regulated to <=1.0wt.%, preferably <=0.1wt.% and <=3.0wt.%, preferably <=0.5wt.%, respectively. The oxide superconductor is preferably a Bi-contg. oxide superconductor and preferably has >=3,000A/cm<2> critical current density.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an oxide superconductor having a high critical current density.

[0002]

2. Description of the Related Art Various materials have been proposed as oxide superconducting materials, but these oxide sintered bodies (bulk bodies) have a uniform critical current density regardless of their composition. Is low. For this reason, there is a common problem that it is difficult to apply to superconducting current leads.

For example, Y, which is a known oxide superconductor
The critical current density (hereinafter, sometimes abbreviated as Jc) of a sintered oxide superconductor such as a system, Bi system, Tl system, Hg system, etc. is generally about 200 to 300 A / cm ² . For example, the Yc sintered body has a Jc of about 100 A / cm ² at most, and therefore, it has been attempted to manufacture it by a melting method, but in this case, the intended shape cannot be obtained. There are difficulties. For Bi system, one report is 1000 A / cm ²
Was reported to have been obtained, and up to 2500
There is also a report that A / cm ² was obtained. However, application to superconducting current leads requires at least 3000 A / cm ² or higher as high as possible Jc, which is not satisfactory. For this reason, developments are being made in various fields to obtain an oxide superconductor capable of flowing a large-capacity superconducting current.

[0004] To increase the Jc in the oxide superconductor material, and measures such as to align the crystal _orientation, it is necessary to a sintered body densified. It is also necessary to reduce impurities as much as possible. Therefore, it is no exaggeration to say that the high Jc of oxide superconductors is determined by the materials used.

In the conventional oxide superconductor, a raw material powder in which each component is mixed so as to have a target composition is molded and sintered to obtain an oxide superconductor material. Generally, the raw material powder is calcined. Powder is used. The calcined powder was obtained by firing a mixture (including coprecipitated powder) of each component once to achieve the target composition, and then repeating the firing and pulverizing process of pulverizing the fired product several times. It is a powder.

[0006]

When the calcined powder is formed and sintered to form an oxide superconductor having a target component composition and crystal structure, the calcined powder itself is a target oxide superconductor. Even if it is precisely controlled to have substantially the same component composition as that of J, and even if the inevitable impurities accompanying the raw materials are reduced as much as possible,
It was found that the effect of improving c is limited. The present invention aims to overcome this limitation.

[0007]

According to the present invention, there is provided an oxide superconductor obtained by molding and sintering a calcined powder in which each component is mixed so as to have a target composition. Carbon content of 1.0% by weight or less, preferably 0.1% by weight or less, and further, the water content of 3.0% by weight or less, preferably 0.5% by weight or less, , 0.0 except impurities (excluding C and water) mixed in during the manufacturing process other than raw materials
Provided is an oxide superconductor having a high critical current density, which is suppressed to 5% by weight or less. The calcined powder referred to here is obtained by, for example, once firing a mixture (including coprecipitated powder) in which each component is mixed so as to have a target composition, and then pulverizing the fired product once or once. The powder obtained by repeating several times.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION To obtain an oxide superconductor material,
First, it is necessary to use high-purity raw material powder. For example in the _{bismuth-based,} specific one-component composition for example Bi
_In order to obtain a sintered body of _1.85 Pb _0.35 Sr _1.90 Ca _2.05 Cu _3.05 O _x , it is necessary to prepare a high-purity calcined powder having a composition ratio as close as possible to the composition ratio thereof. The starting materials for such calcined powder include Bi ₂ O ₃ , PbO, SrCO ₃ , and Ca.
Powders of O and CuO are used, but the raw material powders themselves must have high purity. Also _, when producing such raw materials by the coprecipitation method, it is necessary to avoid mixing impurities.

However, the amount of the components of the calcined powder is precisely controlled so that the composition of the oxide superconductor material is controlled, and high-purity raw materials are used to reduce impurities accompanying the raw materials as much as possible. However, the present inventors have found that there is a limit that cannot be exceeded in improving the Jc of an oxide superconductor, and this is mainly due to water and carbon dioxide in the atmosphere. Carbon (water) is contained in the calcined powder due to water and carbon dioxide in the atmosphere, and this adversely affects Jc of the oxide superconductor. For example, when the carbon content in the calcined powder exceeds 1% by weight, Jc decreases to 500 A / cm ² or less, and even when the water content exceeds 3% by weight, Jc decreases to 500 A / cm ² or less ² .

Further, even if a high-purity raw material is used, impurities (except C and water) inevitably mixed during the production of the calcined powder are found to adversely affect the improvement of Jc and quality characteristics. It was Other impurities that may be mixed in during the calcination powder manufacturing process include Si, Al, Zr, Na, alkaline earth metals and heavy metals, but these may be mixed in by jigs and equipment used during firing and grinding, and by people. all right. If the amount of such impurities exceeds 0.05% by weight, Jc will be 300A.
/ Cm reduced to ² or less.

The route by which carbon is mixed from the atmosphere can be considered as follows. That is, when moisture in the atmosphere comes into contact with a trace amount of a substance that does not form a superconducting crystal, a hydroxide is formed, and this hydroxide reacts with carbon dioxide gas in the atmosphere to form a carbonate, and this carbonate is The content as C is increased, and finally C precipitates at the grain boundaries. The present inventors believe that the precipitation of C at the crystal grain boundaries impedes the superconducting current flowing between the grains.

It has been found that such contamination of water and carbon from the atmosphere cannot be avoided no matter how carefully the starting raw material is selected to be highly pure, and in particular, it occurs in the process of producing the calcined powder. It was Of the firing and crushing processes for producing the calcined powder, there are many opportunities to mix in the crushing process. This is because in the crushed state after firing, the moisture in the atmosphere is easily absorbed due to the rapid increase in the specific surface area and the increase in active points. It has been found that this moisture absorption phenomenon is likely to be absorbed particularly in an atmosphere having a dew point higher than 10 ° C. although it depends on the air temperature. Therefore, if the powder is crushed and held in an atmosphere having a dew point of 10 ° C. or lower, preferably 5 ° C. or lower, the problem of moisture absorption during crushing can be solved. However, the measures to reduce the moisture absorption during crushing are still insufficient, and it is necessary to produce calcined powder with the carbon content as low as possible.

According to the present invention, it has been found that the calcined powder having a low carbon content can be advantageously obtained by performing a heat treatment at a temperature of 600 ° C. or higher and a melting point or lower after the pulverizing step. When the heat treatment temperature is about 500 ° C., carbon cannot be removed, and water having a weak bonding force can be removed, but it is not sufficient. 60
Both carbon and water can be removed by heat treatment at a temperature of 0 ° C. or higher, preferably 700 ° C. or higher. However, it should be excluded because it decomposes into a substance other than the target substance at a temperature above the melting point and becomes a substance other than the target substance. The time to be kept at this heat treatment temperature is determined by the treatment temperature, but may be in the range of 0.1 to 50 hours. More specifically, the yttrium-based calcined powder is preferably heat-treated at 850 to 950 ° C. for 5 to 10 hours, and the bismuth-based calcined powder is preferably heat-treated at 750 to 850 ° C. for 5 to 10 hours.

FIG. 1 shows the temperature and C in the calcined powder when the heat treatment temperature was changed during the production of the calcined powder in Example 1 described later.
It shows the relationship with the content. As is clear from Fig. 1, the C content was 0.1 at heat treatment temperatures of 750 ° C or higher.
If the temperature is higher than that, the C content can be reduced to 0.05% by weight or less.

Similarly, FIG. 2 shows the relationship between the temperature and the C content in the calcined powder when the heat treatment temperature at the time of manufacturing the calcined powder is changed in Example 3 described later. As is clear from 2, the C content becomes 0.1% by weight or less at the heat treatment temperature of 850 ° C. or higher, and the C content can be made 0.05% by weight or less at a higher temperature.

FIG. 3 shows Jc measured values of the bismuth-based superconducting sintered body having the same composition as in Example 1 described later, when the C content in the calcined powder was changed. It is a thing. The C content was adjusted by placing the calcined powder obtained by the same method as in Example 1 in a desiccator containing a small amount of pure water and leaving it for an appropriate time (24 to 72 hours). In this way, calcined powders having various C contents were formed into a disk shape with a diameter of about 20 mm, which was then heated to 850 ° C.
Was primary-sintered, further subjected to CIP (cold isotropic compression) to increase the density, and then secondary-sintered again at 850 ° C. to obtain a sintered body, and Jc of the obtained sintered body was measured. .

The Jc was measured using the obtained sintered body as 1
It was cut out into a strip shape having a square section of mm, Jc measurement electrodes and lead wires were attached, and the measurement was performed. In addition, the C content is measured by sampling a portion of each calcined powder left to stand in a desiccator, heating it to a high temperature of 1000 ° C or higher, and quantifying the carbon dioxide gas emitted in the burned state with an infrared spectroscope. did.

As is clear from FIG. 3, Jc in the sintered body is
It can be seen that (A / cm ² ) sharply decreases as the C content of the calcined powder decreases. In the case of this calcined powder, the C content is 0.0
Jc began to improve sharply from around 8% by weight,
If it is less than 0.04% by weight, it will exceed 3000 A / cm ² .

FIG. 4 shows measured values of water content in the calcined powder and Jc of the superconducting sintered body as in FIG. This adjustment of the amount of water is performed by changing the time of leaving it in the desiccator, just as in the case of FIG. With respect to the same sample as in FIG. 3, the water content generated when heated to 300 was measured using a Karl Fischer water content meter.

As is clear from FIG. 4, Jc in the sintered body is
It can be seen that (A / cm ² ) rapidly improves as the water content in the calcined powder decreases. In the case of this example, when the moisture content of the calcined powder is around 0.2% by weight as the boundary, and when it is less than this, J
The c value rises sharply.

FIG. 5 shows the relationship between the C content and the water content in the calcined powder in the data of FIGS. 3 and 4. As is clear from FIG. 5, there is a clear correlation between the C content and the water content in the calcined powder.

As described above, the C content and the water content in the calcined powder have a great influence on the Jc of the sintered body, and in order to obtain the Jc exceeding the conventional level, the C content and the water content are obtained. It is clear that quantity control is an unavoidable requirement.

However, even if the C content and the water content in the calcined powder can be suppressed as much as possible, if impurities are mixed from the container or the jig during the manufacture of the calcined powder, the effect is also reduced. Will end up. As a means for avoiding the inclusion of impurities from the container during calcination or the crushing jig during crushing, a powder paste having the same composition as the calcined powder to be manufactured is prepared, and this is applied to the surface of the container or jig. Apply to a thickness of about 01 to 1 mm, 70
The coating is preferably baked at 0 to 950 ° C. As a result, a protective film of the same composition having sufficient strength is formed, so that the mixing of the impurities can be effectively prevented.

The calcined powder of the present invention thus produced can have C ≦ 0.05% by weight, water ≦ 0.2% by weight, and other impurities ≦ 0.01% by weight. The critical current density of the sintered superconductor material is stable at 1000 A /
cm ² or more, bismuth-based 3000 A / cm ² or more,
Further, it can be increased to 5000 A / cm ² or more.

[0025]

【Example】

Example 1 _{Bi 2 O 3, PbO, SrCO} 3, CaO and CuO of powdery substance, high-temperature phase of the Bi-based superconducting material (2
223 phase) is mixed at a ratio of 800 ° C. × 10
The process consisting of calcination for 2 hours and pulverization in an atmosphere with a dew point of 10 ° C or lower is repeated twice, and finally heat treatment is performed at 800 ° C for 5 hours in an atmosphere with a dew point of 10 ° C or lower, followed by dew point
It was crushed under an atmosphere of 0 ° C or lower. At that time, the jig used for the crushing and the container used for the heat treatment were coated on the surface with a paste having the same composition in advance, and the coating film was baked. A part of the obtained calcined powder was sampled, and the C concentration, water concentration and impurity concentration were measured by the methods described in the text.

Further, the calcined powder obtained is press-molded,
It was baked at 50 ° C. for 50 hours, further compressed by CIP (cold isotropic compression method), and baked again at 850 ° C. for 50 hours. The obtained sintered body was cut out and Jc was measured by the method described in the text. Table 1 shows the measurement results.

[Example 2] A calcined powder of the same composition was produced under the same conditions as in Example 1 except that a coprecipitated powder in which each component was coprecipitated was used as a raw material powder. The calcined powder was sintered under the same conditions as in Example 1. The Jc of the sintered body, the C concentration of the calcined powder, the water concentration and the amount of impurities were measured in the same manner as in Example 1. The results are shown in Table 1.

Example 3 Y ₂ O ₃ , BaCO ₃ , CuO
Powdery substance of 1: 2: 3 in a molar ratio of 94:
A process consisting of baking at 0 ° C for 30 hours and crushing in an atmosphere with a dew point of 10 ° C or lower is repeated twice, and finally the dew point is 10 ° C.
Heat treatment was performed at 900 ° C. for 10 hours in the following atmosphere, and then pulverization was performed in an atmosphere having a dew point of 10 ° C. or less. The jig and the container used were prepared by applying a paste having the same composition as the composition of this example on the surface thereof and firing the paste.

The calcined powder obtained is press-molded at 950 ° C.
It was baked for 50 hours. A sample for Jc measurement was cut out from the obtained sintered body, and the Jc of the sintered body was measured by the same method as in Example 1. Further, the C concentration, the water concentration and the amount of impurities of the calcined powder were measured by the same method as in Example 1. Table 1 shows the results.
It was shown to.

[Comparative Example 1] In the process of producing the calcined powder,
Except that the final heat treatment was not performed for 5 hours at 800 ° C.
A sintered body was obtained in the same manner as in Example 1. In addition, the jigs and containers used are those without paste coating / sintering treatment.
The crushing was performed in the atmosphere. The Jc of the obtained sintered body, the C concentration of the calcined powder, the water concentration, and the amount of impurities were determined in Example 1.
The measurement was performed in the same manner as in, and the results are shown in Table 1.

[Comparative Example 2] In the process of producing the calcined powder,
A sintered body was obtained in the same manner as in Example 3 except that the final heat treatment was not performed at 900 ° C for 10 hours. The jig and container used were those without paste coating and sintering, and the crushing was performed in the atmosphere. The Jc of the obtained sintered body, the C concentration of the calcined powder, the water concentration and the amount of impurities were measured by the same method as in Example 1, and the results are shown in Table 1.

[0032]

[Table 1]

It can be seen from Table 1 that the amount of C and the amount of water in the Example of the present invention subjected to the high temperature heat treatment are higher than those of the Comparative Example in which the high temperature heat treatment was not applied in the final step of the calcination powder production. It can be seen that the superconducting sintered body using these calcined powders shows a high Jc, which is reduced to an extremely low range. Further, by using a jig or a container whose surface is covered with a fired coating of the same composition, it is possible to avoid mixing of impurities, which also contributes to obtaining a high Jc.

The densities shown in Table 1 were evaluated by measuring the weight per unit volume of each sintered body and evaluating it in the following three stages. ◎: Relative density of 80% or more ○: Relative density of 70 to less than 80% △: Relative density of 60 to less than 70%

The moldability shown in Table 1 is the condition when each calcined powder is subjected to a moldability test by the powder compacting method and the pelletized one is dropped at a predetermined pressure from a height of 100 mm. It is evaluated in the following three stages. ⊚: The pellet shape is maintained. ○ mark: A part collapses. Δ mark: The pellet shape is not maintained. It can be seen from Table 1 that the examples of the present invention have good density and moldability, and can be stably manufactured using the oxide superconducting material as a bulk material.

[0036]

As described above, according to the present invention,
It is possible to provide an oxide superconductor material having a high Jc exceeding the conventional level, and it is possible to achieve a critical current density of about 5000 A / cm ² in a bismuth system. In addition, C ≦ 0.05
% By weight, moisture ≤ 0.2% by weight, other impurities ≤ 0.01
The superconductor of the present invention which uses a high-purity calcined powder such as wt% is also excellent in terms of formability and density. For example, as a result of suppressing blistering during the production of a wire rod, the superconducting wire production technique can be applied. Can also contribute significantly.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the temperature and the C content in a sintered body when the heat treatment temperature in Example 1 is changed.

FIG. 2 is a diagram showing the relationship between the temperature and the C content in the sintered body when the heat treatment temperature in Example 3 is changed.

FIG. 3 is a diagram showing a relationship between C content in a calcined powder and Jc of a sintered body for a Bi-based superconductor.

FIG. 4 is a diagram showing the relationship between the water content in the calcined powder and the Jc of the sintered body for the Bi-based superconductor.

FIG. 5 is a diagram showing a relationship between a carbon content and a water content in a calcined powder for a Bi-based superconductor.

Claims (6)

[Claims] 1. An oxide superconductor obtained by molding and sintering a calcined powder in which each component is blended so as to have a target composition, wherein the calcined powder has a carbon content of 1.0% by weight. % Or less and a water content of 3.0% by weight or less, an oxide superconductor having a high critical current density. 2. The oxide superconductor is a bismuth oxide superconductor, and the calcined powder thereof has a carbon content of 0.1% by weight or less and a water content of 0.5% by weight or less. The oxide superconductor according to claim 1, wherein the critical current density is 3000 A / cm ² or more. 3. The oxide superconductor is a yttrium oxide superconductor, and the calcined powder thereof has a carbon content of 0.1% by weight.
The oxide superconductor according to claim 1, which has a water content of 0.5 wt% or less and has a critical current density of 500 A / cm ² or more. 4. The calcined powder is obtained by calcination of a raw material mixture containing each component after firing, and finally heat-treated at a temperature of 600 ° C. or higher and a melting point or lower. The oxide superconductor described. 5. The oxide superconductor according to claim 4, wherein the calcination powder is pulverized in an atmosphere having a dew point of 10 ° C. or lower. 6. The calcination and crushing of the calcined powder is performed by using a container or a jig having a coating layer having substantially the same composition as the calcined powder. Oxide superconductor.