JPH07172835A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To provide a method for producing an oxide superconductor by which crystals can be oriented in a certain direction without causing ununiformity in thickness. CONSTITUTION:When a powdery mixture of a precursor of an oxide superconductor convertible into the oxide superconductor by heating with flat platy superconductor crystals is compacted and heated to produce an oxide superconductor, force is applied to the powdery mixture from one direction to orient the flat platy superconductor crystals contained in the mixture before heating or the mixture is wet-mixed, molded into a green sheet and crystallized by heating.

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 having a high current density.

[0002]

2. Description of the Related Art Oxide superconductors (hereinafter referred to as superconductors)
Is generally used by mixing the metal oxides of the respective components and then using heat-treated powder if necessary, or by dissolving the carbonate or alkoxide in the liquid and then coprecipitating the powder to form a plate. Alternatively, it is manufactured by applying it to a metal plate, or putting it in a metal cylinder, stretching it, and then firing it. However, in the superconductor manufactured in this way, the desired temperature and critical current density tend to be low because the desired crystal and crystal arrangement cannot be obtained. However, in the case of raising both the critical temperature and the critical current density exceptionally, a method of firing at the decomposition melting temperature of the superconductor is known.

Further, when the superconductor composition is completely dissolved, the melt is dropped on a silver or copper plate to be rapidly cooled, and then the powder obtained by crushing is fired, both the critical temperature and the critical current density are increased. It is also known that high superconductors can be obtained.

[0004]

However, the method of firing at the decomposition and melting temperature of the superconductor is a manufacturing method in which both the critical temperature and the critical current density increase, but since a part of the superconductor component is melted It is not possible to bake the whole body. Therefore, a superconductor is fired and baked on the surface of a certain kind of base material. However, when the superconductor is manufactured by such a method, there is a drawback in that the flow phenomenon due to melting causes nonuniformity of the superconductor thickness. Also, the crystal arrangement of the superconductor fired at the decomposition and melting temperature changes depending on the thickness of the superconductor layer, and if the thickness is about 50 μm or less, the surface of the base material and the ab plane of the superconductor are likely to be parallel, A high current density can be obtained, but as the thickness of the superconductor increases, the crystal orientation tends to become disordered, so that the critical current density sharply decreases. Since it is not possible to form a thick superconducting layer having a high critical current density so that a large current can flow, a high critical current density and a thin thickness are required so that a necessary critical current can be obtained. At present, many superconductors are stacked and used as conductors.

On the other hand, the method of dropping the melt on a silver or copper plate and quenching it has the drawback that the crystals cannot be oriented in a fixed direction. The present invention provides a method for producing a superconductor which does not have the above-mentioned drawbacks.

[0006]

The present invention provides a superconductor film in which crystals are oriented in a certain direction regardless of the thickness of the superconductor layer, and the critical temperature and the critical current density are both high. It was made as a result of searching for a manufacturing method of. That is, a plate-shaped superconductor crystal is added to and mixed with a superconductor precursor which becomes a superconductor when heated, and after the orientation treatment is performed, the added crystal acts as a crystal nucleus and a crystal growth direction. It was found that a superconducting film in which crystals are oriented in a certain direction can be obtained in order to control the above.

More specifically, a superconducting precursor which becomes a superconductor by heating and a plate-shaped superconductor crystal in which the ab plane is well grown are added and mixed, and then the mixed powder is molded into a predetermined shape to obtain a plate-shaped superconductor. When the crystals are oriented in a certain direction and then heat-treated, crystals of the same kind with the C-axis aligned in a certain direction grow from the superconductor precursor, resulting in highly oriented and heterogeneous crystals, heterogeneous phases such as glass phases, and the like. They have found that a superconductor having a low content can be obtained, and have completed the present invention.

The present invention relates to a method for producing a superconductor by molding a mixed powder of a superconductor precursor and a flat plate-shaped superconductor crystal which become a superconductor by heating and heat-treating the mixture.
A force is applied from one direction of the mixed powder to orient the flat superconducting crystals contained in the mixed powder and then heat treatment is performed, or the mixed powder is wet mixed to form a green sheet, and then heat treated. , A method for producing a superconductor to be crystallized.

In the present invention, the superconducting precursor is homogeneously mixed with each oxide so as to have a superconductor composition, for example, an oxide of a component to be a Bi-based superconductor, and then melted and quenched,
It is desirable to use a crushed powder. The cooling of the melt is not limited to the method of pouring on the metal plate and rapidly cooling. After melting, it may be taken out of the furnace and cooled by air, or may be dropped into water to be cooled. As the superconductor in the present invention,
In addition to Bi-based superconductors, there are Y-based superconductors, Tl-based superconductors, and the like.

When powders obtained by uniformly mixing the respective oxides are used, when powders obtained by heat treating them at a temperature lower than the melting temperature and then pulverizing are used, or for a longer time or by applying a strong shearing force. The effect of the present invention can be obtained by using an amorphous powder.

When a superconductor precursor containing silver is used, the temperature of the heat treatment can be lowered, so that a flat plate-shaped superconductor crystal which acts as a crystal nucleus can stably exist during the heat treatment, so that the orientation effect can be obtained. Is preferred, which is preferable. It is preferable that silver has a fineness of 5 μm or less in the superconductor precursor and 0.5% by weight or more of the portion uniformly distributed in the precursor because the addition effect can be obtained.

The tabular superconductor crystal to be added is a crystal which is obtained by the decomposition and melting method and which grows in both directions ab, which is peculiar to superconductors, and the size thereof is 2 μm in average grain size.
When it is m or more, the effect of controlling the crystal growth orientation is easily obtained, which is preferable. The addition amount is not particularly limited, but it is preferable to add 1% by weight or more to the obtained superconductor.

The mixing method is not particularly limited, but a method in which the added tabular superconductor crystal does not become amorphous or undergo phase transition to another crystal during handling is preferable. The orientation treatment of the flat superconductor crystal is performed by a method such as applying uniaxial pressure to the powder at the time of molding to apply a force from one direction to elongate the molded body, or dry a wet mixed mud commander into a sheet. Further, a method of applying pressure and stretching a dried sheet can be applied if necessary.

The heat treatment for precipitating the superconductor crystal from the superconductor precursor may be carried out at a temperature and an atmospheric condition such that the crystal grows. For example, a temperature of several degrees from the temperature at which the added flat superconductor crystal decomposes and melts. Heat treatment at a low temperature makes it easy to obtain a desired crystal structure. Adjusting the oxygen partial pressure in the treatment atmosphere during or after the heat treatment has the effect of facilitating the precipitation of crystals and improving the superconductor characteristics, and does not impair the effects of the present invention.

The superconductor precursor and the tabular superconductor crystal to be added do not have to have the same composition, as long as they have substantially the same composition as the crystal to be deposited. Note that the superconductor to be deposited is preferably a Bi-based 2212 phase or 2223 phase, and among these, the 2212 phase is preferable because it exhibits a particularly excellent effect in the present invention.

[0016]

EXAMPLES Examples of the present invention will be described below. The present invention is not limited to the scope of the examples below. Example 1 Bismuth oxide having a purity of 99.9% or more (manufactured by Kojundo Chemical Laboratory Co., Ltd.) 913.29 so that the atomic ratio of bismuth, strontium, calcium and copper is 2: 2: 1: 2.
g, strontium carbonate (made by rare metallic) 578.
71 g, calcium carbonate (manufactured by Kojundo Chemical Laboratory) 19
6.17 g and cupric oxide (manufactured by Kojundo Chemical Laboratory) 31
1.82 g was weighed and used as the starting raw material powder.

Next, the above starting raw material powder was filled in a synthetic resin ball mill together with a steel ball covered with a synthetic resin and 2000 g of methanol, and the condition was 50 rpm for 72 minutes.
Wet mixed for hours. After drying, it was placed in a silver container and heated and melted in the atmosphere at 1000 ° C. for 30 minutes in an electric furnace. Then, it was immediately taken out, air-cooled, and dry-ground in a mortar to 200 mesh or less. To 100 parts by weight of the obtained powder, 15 parts by weight of a 1% aqueous solution of polyvinyl alcohol (manufactured by Kuraray) was added to form a paste, which was applied to a silver plate and dried, and then heated at 875 ° C for 10 hours in the atmosphere. Process, Bi
A polycrystalline body of 2212-phase superconductor was obtained. After peeling from the silver plate, lightly crushed in a mortar, the average particle size measured by a laser particle size distribution is 11 μm, and observed by an electron microscope to be 15 μm.
A flat plate-shaped superconductor crystal powder for addition, which contained a large amount of thin flakes, was obtained.

Next, 98% by weight of powder obtained by wet-mixing with the above ball mill and dry pulverization was added to silver powder (average particle size 5 μm).
m, made by Tanaka Massey) and mixed, and the mixture was put in a silver container in the same manner as above and placed in an electric furnace at 100%.
Melted at 0 ° C. for 30 minutes. Immediately after that, take out, air cool, dry pulverize to 200 mesh or less in a mortar, and
A 2212-phase superconductor precursor powder was obtained. Bi obtained
The 2212-phase superconductor precursor powder contained a small amount of 2212 phase and a different phase.

Then, the tabular superconductor crystal powder obtained above was added to the Bi-based 2212 phase superconductor precursor powder obtained above in an amount shown in Table 1 and mixed uniformly. Then, this mixed powder is uniaxially pressed (surface pressure 49 × 10 ⁶ Pa).
Then, a molded body having a thickness of 1.5 mm was obtained. Further, the molded body was placed on a silver plate and heat-treated in the air at 865 ° C. for 10 hours to be crystallized to obtain a sample baked on a silver plate having a thickness of 0.8 mm. Orientation state of each sample, 2212 phase content rate and orientation rate of the sample surface (fired surface), the peeled surface from the silver plate (silver plate surface)
The bent cross section (intermediate part) was examined by a polarization microscope and a powder X-ray diffraction method. The results are shown in Table 1.

The 2212 phase content by X-ray diffraction was 2θ of 3 obtained by X-ray diffracting the surface of the sample or the surface peeled from the silver plate as it was, and X-ray diffracting with Cu as an anticathode.
Of the 2212 phase diffraction peaks between -60 and 00
The sample having the maximum total of the diffraction intensities of the diffraction peaks belonging to L was set as 100, and the relative value of the diffraction intensity of each sample was shown.
Similarly, for the orientation rate, a value obtained by dividing the total value of the diffraction intensities of the diffraction peaks belonging to 00L by the total value of the diffraction intensities of all the diffraction peaks of the 2212 phase was shown in percentage. In addition, the orientation is displayed in a state where the plate-like crystals cover the entire observation surface observed with a polarization microscope, and the crystals grow randomly, almost the entire crystallization orientation, the entire crystallization orientation, and the entire crystallization strong orientation. Shown in stages.

[0021]

[Table 1]

Example 2 Bi system 221 containing 2% by weight of silver powder used in Example 1
17 parts by weight of a 10% aqueous solution of polyvinyl alcohol (manufactured by Kuraray) was added to 100 parts by weight of the two-phase superconductor precursor powder to which the plate-shaped superconductor crystal powder used in Example 1 was added in the amount shown in Table 2. 3 parts by weight of distilled water and 2 parts by weight of dioctyl phthalate (manufactured by Komune Chemical Co., Ltd.) were added to form a paste, which was formed into a thin stainless steel sheet, dried and peeled to obtain a 0.8 mm-thick raw sheet. . Next, the green sheet was pressure-bonded to a 0.1 mm silver plate at a pressure of 10 MPa, and heat-treated in the air at 865 ° C. for 10 hours to be crystallized. For the obtained sample, the orientation state was measured in the same manner as in Example 1,
The 2212 phase content and orientation were investigated. The results are shown in Table 2.
Shown in.

[0023]

[Table 2]

According to Tables 1 and 2, the superconductor obtained by the manufacturing method of the present invention is almost entirely or entirely crystallized.
It is shown that the content of 12 phases is 64% or more and the orientation rate is 71% or more.

[0025]

The superconductor obtained by the manufacturing method of the present invention has a low content of different phases and the crystals are oriented in a certain direction. Further, according to the production method of the present invention, there is no need for a treatment for orienting crystals by pressing after firing and a limitation on the thickness of the superconductor, which are conventionally required. Since the temperature is reached, strict control of the firing temperature is not required, which is an effect.

Front Page Continuation (72) Inventor Shuichiro Shimoda 4-13-1, Higashimachi, Hitachi City, Ibaraki Prefecture Ibaraki Research Laboratory, Hitachi Chemical Co., Ltd.

Claims (2)

[Claims] 1. A method for producing an oxide superconductor by molding a mixed powder of an oxide superconductor precursor and a flat plate superconductor crystal, which are heated to form an oxide superconductor, and heat treating the mixed powder. After applying a force from one direction of the mixed powder to orient the flat superconducting crystals contained in the mixed powder and then performing heat treatment, or wet mixing the mixed powder to form a green sheet,
A method for producing an oxide superconductor, which comprises subjecting to heat treatment to crystallize. 2. The oxide superconductor precursor is rapidly solidified after melting an oxide mixed powder of a component which becomes a Bi-based oxide superconductor,
It is a pulverized powder, and the plate-shaped superconductor crystal is Bi type 22.
The method for producing an oxide superconductor according to claim 1, which is a 12-phase flat plate crystal.