JP3314102B2 - Manufacturing method of oxide superconductor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an oxide superconductor, and more particularly, to a method for manufacturing an oxide superconductor having high density and high orientation.

[0002]

2. Description of the Related Art In recent years, oxide superconductors have been discovered as materials having a higher critical temperature Tc (temperature at which resistance becomes zero) than metal-based superconductors conventionally used as superconductors, and their practical use is expected. Have been.

[0003] At present, oxide superconductors mainly include Y-Ba-Cu-O-based (hereinafter referred to as Y-based), Bi-S
r-Ca-Cu-O system (hereinafter referred to as Bi system) and Tl
-Ba-Ca-Cu-O type (hereinafter referred to as Tl type) 3
Species are known, and these have a critical temperature at which they become superconducting with an inexpensive cooling medium such as liquid nitrogen, so that their practical use is being promoted in various fields. For practical use, these oxide superconductors also need to have a high critical current density (current value at zero resistance). Bi of these
In the system-based oxide superconductor, since the crystal is composed of flake-like particles, it is considered that the critical current density can be increased by orienting the flake-like particles in one direction. It is also said that increasing the relative density and increasing the density of the sintered body is also a major factor in characteristics.

[0004] Therefore, as a method of producing a high-density oxide superconductor, a hot press method of heating while applying a high mechanical pressure is employed.

[0005]

[Problems to be solved by the invention] However, Bi
In the case of producing a system-based oxide superconductor, for example, when calcined powder is hot-pressed, densification itself proceeds and has a certain effect, but the resulting calcined powder is insufficient in terms of particle orientation. The Jc value of the aggregate is at most 1000A / c
m ² or less, the practically level not hardly reached was present.

[0006]

Means for Solving the Problems The present inventors have solved the above problems by first forming and calcining the calcined powder at normal pressure.
By performing a so-called hot forging treatment in which a heat treatment is performed while applying pressure to the sintered body, an excellent oxide superconductor having a high orientation, a high density and a Jc value of about 1500 to 4500 A / cm ² can be obtained. It has been proposed that the density and the degree of orientation of the sintered body can be further increased by repeatedly performing the hot forging treatment. However, when a method for obtaining a higher Jc value was examined, the hot forging was examined. By melting or partially melting during the zing process, the atoms in the particles diffuse in the direction perpendicular to the pressing direction under one-way pressing, increasing the amount of deformation and increasing the aspect ratio of the particles to the pressing surface Therefore, it was found that the degree of orientation was improved, and an oxide superconductor having a uniform structure and a high Jc value was obtained.

That is, in the present invention, a mixture comprising an oxide or an oxide-forming compound of an element constituting the Bi-based oxide superconductor is molded, or the mixture is calcined and then molded. 8 when subsequently fired in a once-oxidizing atmosphere molded article, hot forging process the sintered body
It is characterized by being heated to a temperature of 70 to 1150 ° C. to be melted or partially melted.

Hereinafter, steps (a) to (c) in the method for producing an oxide superconductor of the present invention will be individually described. Preparation process (a) Using an oxide powder of a metal constituting the oxide superconductor or a carbonate or nitrate powder capable of forming an oxide by firing, weighing and mixing them in such a ratio that an oxide superconductor can be formed. I do. Specifically, when preparing the above-mentioned Bi-based oxide superconductor, Bi ₂ O ₃ , SrO, CaCO ₃ , C
Using each powder of uO, these were converted to Sr at an atomic ratio of 2
When Bi is 1.8 to 2.3, Ca is 1.0 to 1.0.
3.5 Weigh so that Cu is in the range of 2.0 to 4.5. Further, for the purpose of generating a high Tc phase, PbO powder, K ₂ CO ₃ , N
a ₂ CO _3, Li ₂ 0.1~0.5 the Pb CO ₂ such as 2 Sr, K, Li, and Na can be mixed at a ratio of 0.05 to 0.6.

The mixed powder obtained as described above is molded by a known molding means. If desired, the above mixed powder is placed in an oxidizing atmosphere at 700 to 850 ° C. for 1 hour.
After calcination for about 30 hours, pulverize and mold similarly. In addition,
Press molding, extrusion molding, doctor blade molding, or the like is employed as a molding method.

Firing step (b) Next, the compact obtained as described above is
It is baked for about 5 to 200 hours in an oxidizing atmosphere at 5 ° C.
If this sintering is performed under no pressure, a low-density sintered body is formed due to the growth of the scale-like crystals, so that hot-press sintering may be performed. At the end of the firing step, the scaly crystals of the sintered body are almost non-oriented.

Hot Forging Step (c) Next, the above sintered body is subjected to hot forging processing. In this hot forging treatment, as shown in FIG. 1, the sintered body 1 is heated by a suitable heating means (not shown) while applying pressure in the direction A by press punches 2 and 3. The hot press method differs from the hot press method in that the horizontal direction of the sintered body 1 is open in FIG. 1, and the degree of freedom of deformation of the sintered body can be increased by opening the horizontal direction in this way.

According to the present invention, it is important to treat the sintered body under such conditions that the sintered body is partially melted or melted. The partial melting or melting may be performed at a temperature of 870 to 1150 ° C. for 5 minutes to 4 hours in the production of a Bi-based oxide superconductor, and then slowly cooled to crystallize.
Hold at 0 ° C. for 0-10 hours. The pressure at this time is 50k
g / cm ² or more, and the atmosphere is preferably an oxidizing atmosphere such as in the air.

In the hot forging treatment, when pressure is applied between the sintered body 1 and the press punches 2 and 3 through a ductile metal plate such as silver, gold or copper in FIG. , The orientation can be further enhanced.

[0014]

In the conventional hot forging treatment without melting, the orientation advances due to the sliding of the particles, but at the same time, the degree of the orientation advances only to some extent because the particles are destroyed. On the other hand, according to the configuration of the present invention, it is most important to include a step of partially melting or melting the sintered body obtained in the firing step (b) during the hot forging treatment. In the process in which the scaly crystal particles generated in the firing step (b) are partially melted or melted under pressure and then recrystallized, the once broken scaly particles are deformed by the diffusion of atoms and become On the other hand, since the particles are converted into particles having a large aspect ratio, the orientation with respect to the pressurized surface increases, and at the same time, the densification proceeds and the contact area between the particles increases, thereby further improving the density and the degree of orientation. As a result, the critical current density of the oxide superconductor can be dramatically increased.

[0015]

【Example】

Example 1 (a) Bi ₂ O ₃ , SrCO ₃ , CaC
The powders of O ₃ and CuO are mixed at a molar ratio of Bi: Sr:
After weighing Ca: Cu = 2.22: 2: 1.11: 2.22, the mixture was calcined at 750 to 810 ° C. for 24 hours and pulverized to obtain a calcined powder having an average particle size of 5 μm. This calcined powder was molded at a molding pressure of 1 ton / cm ² using a mold having a diameter of 12 mm to obtain a disk-shaped compact having a thickness of about 4 mm.

(B) The compact was fired in the air at a temperature of 850 ° C. for 20 hours to obtain a sintered body having a specific gravity of 2.0 (based on the Archimedes method). When the structure was observed, scaly crystals were randomly arranged.

(C) Next, according to FIG.
After raising the temperature to 870 ° C. under a pressure of ton / cm ² to partially melt the sintered body, the temperature was gradually cooled to 845 ° C. over 8 hours, and the temperature was maintained for 1 hour and cooled (Sample No.
1).

As another condition in (c), 1t
After raising the temperature to 870 ° C. under a pressure of on / cm ² to partially melt the sintered body, the temperature was gradually cooled to 845 ° C. over 4 hours, and the temperature was maintained for 1 hour and cooled (Sample No.
2).

Further, as another condition in (c), 1t
After the sintered body was melted by raising the temperature to 900 ° C. under a pressure of on / cm ² , it was gradually cooled to 845 ° C. over 8 hours, kept at this temperature for 1 hour, and cooled (Sample No. 3). .

The specific gravity of each of the obtained sintered bodies was measured by the Archimedes method, X-ray diffraction was measured, and based on the X-ray diffraction chart data,

[0021]

(Equation 1)

Then, the orientation degree f of the (001) plane was determined.

Further, with respect to the above sintered body, the current was gradually increased in liquid nitrogen based on the resistance method, and the current value when a voltage of 1 μV / cm was generated at the high voltage terminal was changed to the critical current density Jc (77K). , 0T) and at the same time the critical temperature T
c was also measured. The results are shown in Table 1.

Comparative Example 1 The sintered body prepared in (a) and (b) described in Example 1 was subjected to hot forging at 840 ° C. for 10 hours under a pressure of 1 ton / cm ² to obtain a sintered body. (Sample No. 4).

Further, (a) and (b) described in Example 1
Under the pressure of 1 ton / cm ² for 85
A hot forging treatment was performed at 0 ° C. for 10 hours to obtain a sintered body (Sample No. 5).

The characteristics of each of the obtained sintered bodies were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0027]

[Table 1]

As is clear from Table 1, the sample No. 1 of the present invention
No. 3 to Comparative Samples No. 4 and No. 5, partial orientation or melting during the hot forging treatment increased the orientation and the critical current density and the critical temperature.

[0029]

As described in detail above, according to the method of the present invention, when producing a Bi-Sr-Ca-Cu-O-based oxide superconductor, the sintered body is partially melted during hot forging. Alternatively, by melting, it is possible to increase the degree of orientation of the crystal grains of the sintered body and to achieve high density, thereby obtaining an oxide superconductor having high orientation and an extremely high critical current density in a stable manner. Can be.

As described above, by obtaining an oxide superconductor having a high critical current density, the practical use of the oxide superconductor can be further promoted.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a hot forging process in a method for manufacturing an oxide superconductor according to the present invention.

[Explanation of symbols]

 1 Sintered body 2, 3 Press punch

Claims (1)

(57) [Claims] 1. A step of molding a mixture comprising an oxide or an oxide-forming compound of an element constituting the Bi-based oxide superconductor, or a step of calcining the mixture, followed by oxidizing the molded article. An oxide superconductor comprising: a step of firing in a neutral atmosphere; and a step of heat-treating the obtained sintered body under a condition of partially melting or melting at 870 to 1150 ° C. during pressurization. Manufacturing method.