JPH0818840B2 - Oxide superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to an oxide superconductor, and more particularly to an oxide superconductor having a large critical current density even in a high magnetic field.

[Conventional technology]

In 1988, the critical temperature (T
c) Bi-Sr-Ca-Cu-O having a high critical temperature phase of 105K
The system was discovered. This superconducting material is chemically stable and
-Ba-Cu-O system is an attractive material with little deterioration due to water, which is a problem. However, this material
Although the critical current density in the zero magnetic field is relatively large and close to the level of practical use, the critical current density is
It has been reported to be extremely low. For example, Japane
According to se Journal of Applied Physics 28 (1989), L82-84, powder of Bi _1.6 Pb _0.4 Sr ₂ Ca ₂ Cu ₃ O _x composition is 750-870.
After firing in the air at ℃ for 8 to 200 hours, put it in a silver cheve and subject it to mechanical processing such as rolling and pressing, then at 800 to 870 ℃
It has been baked for ~ 800 hours. Although the critical current density of this silver tape wire at zero magnetic field is relatively large at 6,930 A / cm ² ,
For example, when applying a magnetic field of 1T, the best one is about 500A
/ cm ^{2, which} is a large decrease. Therefore, it has been understood that the reduction of the critical current density due to the application of the magnetic field is a serious problem in the application to high-power wires and the like.

Further, in the conventional Bi-Sr-Ca-Cu-O system, a composition or heat treatment was performed aiming at a homogeneous superconductor so that impurities are not precipitated as much as possible in the crystal grain boundaries or in the grains of superconducting crystals ( JP-A 1-212226 and JP-A 1-212226
-234327).

However, under such process conditions, there is a problem that the critical current density is extremely lowered by applying a magnetic field.

[Problems to be Solved by the Invention]

The above-mentioned conventional oxide superconductor has a problem that the critical current conductivity is extremely lowered by the application of a magnetic field.

An object of the present invention is to provide a superconductor having a large critical current density even under a high magnetic field.

[Means for solving the problem]

In order to achieve the above-mentioned object, in the oxide superconductor of the present invention, a fine oxide phase is formed in the crystal grains of the crystal exhibiting superconductivity.

The oxide superconductor of the present invention is Bi _a -Pb _b -Sr _c -Ca _d -Cu _e -O _x.
System oxide superconductor However, 1.5 ≦ a ≦ 2.5 0 ≦ b ≦ 0.5 1.5 ≦ c ≦ 2.5 0.5 ≦ d ≦ 2.5 1.5 ≦ e ≦ 3.5 7 ≦ x ≦ 17, and Ca ₂ PbO ₄ oxide is the oxide. It is an oxide superconductor dispersed in the crystal grains of a superconductor, and contains 10% Ar and O ₂
A high critical temperature phase exhibiting superconducting properties is synthesized by sintering at 835-855 ℃ in 90% atmosphere.

Further, the high critical temperature phase is heat-treated to generate a Ca ₂ PbO ₄ oxide phase and an oxide superconducting phase or a non-superconducting phase having a lower critical temperature.

[Action]

 The operation of the present invention will be described with reference to the drawings.

FIG. 1 shows Bi _a -Pb _b -Sr _c -Ca _d -Cu _e -O _x -based oxide superconductor, where 1.5 ≦ a ≦ 2.5 0 ≦ b ≦ 0.5 1.5 ≦ c ≦ 2.5 0.5 ≦ d ≦ 2.5 1.5 ≦ e ≦ 3.5 7 ≦ x ≦ 17, Ca ₂ PbO ₄ oxide is dispersed in the crystal grains of the oxide superconductor, and heat treatment is performed for 200 hours by changing the heat treatment temperature and the oxygen partial pressure. It shows the high critical temperature phase separation based on the X-ray analysis result of the sample.

The phase separation of the high critical temperature phase will be examined with reference to FIG. In the figure, the regions A, B, C, and D are the stable region A of the high-critical temperature phase A, the high-critical temperature phase becomes unstable, and the Ca ₂ PbO ₄ oxide phase and the oxide superconducting phase or non-superconducting phase with a low critical temperature A region B in which the phase separation into the body phase, a region C in which the phase separation of the high critical temperature phase does not proceed, and a region D in the liquid phase are shown. As can be seen from FIG. 1, the temperature at which the high critical temperature phase is phase-separated differs depending on the oxygen partial pressure, and the phase-separable B region is preferable. The final material structure of the present invention thus obtained is a fine Ca ₂ PbO ₄ oxide phase inside the crystal exhibiting superconductivity and an oxide superconducting phase or a non-superconducting phase having a lower critical temperature. However, it becomes a finely dispersed structure and acts as effective pinning.

FIG. 2 (a) shows the structure of the material in the conventional method, and there is no different phase in the crystal grains that are the superconductor.
However, there is a case where a different phase exists at the triple point of the grain boundary. On the other hand, in the superconductor material of the present invention, as shown in FIG. 2 (b), a fine Pb and Ca composite oxide phase and an oxide superconductor phase or a non-superconductor phase having a lower critical temperature are used. Has a structure in which is uniformly and finely distributed in the crystal grains of the superconductor.

Since the finely dispersed oxides are finer and more uniform than the oxides deposited in the conventional grain boundary portions, they act as strong pinning. Note that an impurity phase may be generated at the crystal grain boundary. In this way, the pinning center introduced in the crystal grains of the superconductor can strongly trap the magnetic flux lines, so that a material having a large critical current density can be obtained even in a high magnetic field.

Example Next, the oxide superconductor according to the present invention will be described.

(Example 1) Bi _1.8 Pb _0.3 Sr ₂ Ca ₂ Cu ₃ O _x so that Bi ₂ O ₃ , Pb
O, SrCO ₃ , Ca _CO 3, CuO were weighed. The powdered particles were mixed and pulverized, put into an alumina crucible, and held at a temperature of 600 to 900 ° C. for about 10 hours to prepare a precursor. After crushing this precursor, it was pellet-molded at a pressure of 5,000 kg / cm ² using a die. This molded body was placed in an Ar-10% O ₂ atmosphere at 845
After firing at a temperature of ℃ for 200 hours, it was pressed again at a pressure of about 5,000 kg / cm ² . Furthermore, this superconductor is Ar-10% O ₂
Heat treatment was performed under various conditions (720 ° C, 70 to 300 hours) in the atmosphere. Table 1 shows the dispersion state of the Ca ₂ PbO ₄ oxide phase existing in the obtained pellet crystals and the critical current density at 77K. In addition, the dispersed state by a scanning electron microscope,
The critical current conductivity is a value measured by the four-terminal method. First
From the table, when the size of Ca ₂ PbO ₄ oxide phase is 0.01 to 10 μm and the volume ratio of this oxide to the whole oxide superconductor is 0.1 vol% or more and 10 vol% or less, the zero magnetic field is obtained. Good critical current conductivity was obtained in both high and low magnetic fields.

(Example 2) In the same manner as in Example 1, the powder was weighed and mixed so that the composition was Bi _1.8 Pb _0.3 Sr ₂ Ca ₂ Cu ₃ O _x , and a precursor was synthesized.
This precursor is molded again with a mold die in the same manner as in Example 1,
After firing, it was re-pressed and heat treated for 200 hours in an atmosphere with a firing temperature of 850 ° C. and an oxygen partial pressure of 10.0 atm. The result of evaluating the superconducting properties at 77K samples, Atsuta at 51,000A / cm ² in a magnetic field of 52,000A / cm ^2, 10T-zero magnetic field.

(Example _{3) Bi 2 O 3, PbO} , using SrCO _3, CuO raw material _{powder, Bi a -Pb b -Sr c -Ca} d -Cu e -O x based oxide having various compositions shown in Table 2 These raw material powders were weighed so that they would be obtained. This powder was mixed and pulverized, put into an alumina crucible, and kept at a temperature of 600 to 900 ° C. for 10 hours to prepare a precursor. After crushing the precursor in the same manner as in Example 1, pellet molding was performed using a die. This molded body was heated at 845 ° C in an Ar-10% O ₂ atmosphere.
The firing was performed at the temperature of 200 hours. 7 of the obtained pellets
Table 2 shows the results of measuring the critical current density at zero magnetic field at 7K by the four-terminal method. It can be seen that any composition has superconducting properties at 77K.

[Effects of the Invention] According to the present invention, a material having a large critical current density can be synthesized even in a high magnetic field, so that there is a great effect in industrializing Bi-based superconducting materials.

[Brief description of drawings]

FIG. 1 is a diagram showing phase separation of a high critical temperature phase of a Bi-based oxide superconductor, FIG. 2 (a) is a schematic diagram of a material composition obtained by a conventional method, and FIG. 2 (b) is a diagram of the present invention. It is a schematic diagram of the material structure | tissue obtained by a method.

Claims (1)

[Claims] 1. Bi _a -Pb _b -Sr _c -Ca _d -Cu _e -O _x -based oxide superconductor, where 1.5≤a≤2.5 0≤b≤0.5 1.5≤c≤2.5 0.5≤d≤2.5 1.5 ≦ e ≦ 3.5 7 ≦ x ≦ 17, the size of Ca ₂ PbO ₄ oxide is 0.01 to 10 μm, and the volume ratio of the oxide to the entire oxide superconductor is 0.1 vol.
% Or more and 10 vol% or less, an oxide superconductor.