JPH0474755A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To improve critical current density by burning a material containing organometallic compounds of at least one or more metals constituting a metal oxide superconductor as a raw material. CONSTITUTION:Raw material powder comprising Bi2O3, CaCO3 or CuO and at least one organometallic compound, e.g. metal alkoxide, acetylacetonatometal or cyclopentadientylmetal such as cyclopentadienylstrontium is blended so as to give a composition of Bi-based oxide superconductor. Then the blended powder is pelletized, calcined in an Ar gas atmosphere, then powdered and press molded into a given shape. The molded article is burnt in an inert gas atmosphere of N2 gas or a reducing gas atmosphere of H2 gas at a given temperature, then annealed to give an oxide superconductor such as Bi2Sr2Ca1Cu2Oy to improve critical current density under magnetic field.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing an oxide-based superconductor material. More specifically, the present invention provides a method for manufacturing an oxide superconductor that has a higher critical current density than conventional ones under a magnetic field.

[Prior art] Obtaining a superconductor with a high critical temperature has been sought by the scientific and industrial worlds for many years, but in 1987 it was discovered that the critical temperature (Tc) was 90 YBa2Cu30.
An oxide superconducting material called 7-t was discovered, and it became possible to obtain a material that exhibits superconductivity even at liquid nitrogen temperature (77 K). Following this, in 1988, Tc was 110-1.
Bi(Pb)-5r-Ca-Cu even higher than 25
-0 series and T I-Ba-Ca-Cu-○ series oxide superconductors were discovered. On the other hand, in order for oxide superconductors to be put into practical use, it is essential that they have a sufficient critical current density; for example, published IFI Jap, J-App1. Ph
ys, 27 (1988) L622, some small samples of thin films with critical currents sufficient for practical use have been obtained. However, for fishing applications, a sufficient cross-sectional area is required to carry a large transport current. To achieve this, a sintered body (bulk) with a high critical current density is required, but currently, sintered bodies have a much lower critical current density than thin films, and furthermore, this value rapidly decreases under a magnetic field. There is.

As for the manufacturing method of oxide superconductors, in the case of sintered bodies, starting materials such as oxides and carbonates are calcined at about 800°C, subjected to solid phase reaction, and then press-formed to a temperature of 850 to 95°C.
The most common method is to sinter it at 0°C. Regarding manufacturing techniques in this field, for example, the publications Jap, J.
App1. Phys, 27 (1988) L1041.

[Problem to be solved by the invention] From the perspective of practical application of superconductors, it is most important to have a high critical current density when applied to a magnetic field. The critical current density decreases significantly under low temperatures, which poses a major obstacle in practical use. Furthermore, when oxide superconductors are used at liquid nitrogen temperatures, the effects of magnetic flux creep become more pronounced due to increased thermal disturbances than when conventional metallic superconducting materials are used at extremely low temperatures, making it difficult to put into practical use. This has become a big problem.

It is known in principle that the critical current density under a magnetic field can be improved and the effect of magnetic flux creep can be reduced by strengthening the binning force of the material. The exact method was not clear.

This invention was made to solve these problems, and aims to manufacture an oxide-based superconductor that has a small decrease in critical current density even under a magnetic field and has improved magnetic field dependence of critical current density. It is an object.

[Means for Solving the Problems] The method for producing an oxide superconductor of the present invention uses as a starting material an organometallic compound of at least one metal that is a component of the superconductor, and uses an inert gas or a reducing gas. A firing process is performed in a gaseous atmosphere.

[Function] In this invention, an organometallic compound of at least one metal that is a constituent of the oxide superconductor is used as a starting material. When this is fired in an inert gas or a reducing gas, the organometallic compound is dissolved. 8! ! The inventors have discovered that the mass becomes a carbonaceous component, for example, amorphous carbon, and remains dispersed within the sintered body. Through experimental studies, it has been found that carbonaceous matter is dispersed in the vE bonded oxide superconductor, and as a result, the decrease in critical current density under a magnetic field becomes extremely small. Therefore, under a magnetic field, the carbonaceous material in the superconductor acts as a pinning center and prevents the magnetic flux from moving.
It is presumed that this serves to facilitate the flow of current.

[Example] This invention will be described in detail below by giving examples and comparative examples.

Example 1 For a superconductor with the target composition of Bi25r2Ca+Cu2O, as an organometallic compound of Sr as a component
Sr cyclopentadienyl was used and Bi20i, CaC0:+ and CuO were used as starting materials for other components. After mixing in an automatic mortar, the mixture was pelletized and baked at 700°C in argon gas with the plate closed at 10 o'clock. This calcined pellet was again ground in an automatic mortar to obtain a powder. This powder was molded into rod-shaped pellets with a length of 30 mm, a width of 2 mm, and a thickness of 3 mm using a hydraulic press. This was fired in a nitrogen gas stream at 860° C. for 24 hours and then slowly cooled to obtain a sample for measuring characteristics according to the method of the present invention. Before measuring the superconducting properties, the crystal structure was investigated using an X-ray diffraction method and an X-ray microanalyzer, and it was found that Bi25r2Ca+C
Homogeneously dispersed amorphous carbon was identified in the u209 superconductor and sintered body. Next, the critical temperature Tc7 at which the resistance becomes zero, determined from the temperature dependence of resistivity using the four-terminal resistance method.
The critical current density and the dependence of the critical current density on the magnetic field were measured using a zero magnetic field energization method at 7K.

The results are shown in the table and screen. In the figure, the vertical axis is the critical current density (for comparison, the critical current density J when the magnetic field is O
The value normalized by co: J c / J co), the horizontal axis is the external magnetic field B (T), and the characteristic surface "1lA (a) is the normalized critical current density of the sample according to Example 1 of the present invention - This is a measurement curve of an external magnetic field.

Example 2 B1 acetylacetonate and Ca bentoxide were used as organometallic compounds of B1 and Ca as constituent components for a superconductor having a target composition of Bi25raca+cu20y. SrCO3 and CuO were used as other starting materials. After mixing in an automatic mortar, the mixture was pelletized and calcined at 750° C. in a hydrogen stream with a closed door for 3 hours. The calcined pellets were ground in an automatic mortar and then wet-ground in a ball mill for 24 hours to obtain a powder. This powder is processed using a hydraulic press to a length of 30 Irrra, a width of 2 mm, and a thickness of 1 m.
The pellets were molded into rod-shaped pellets with a diameter of m. This is heated to 700℃ in air.
The sample was closed at 3 o'clock and then fired at 860° C. for 24 hours in a nitrogen gas stream, followed by slow cooling to obtain a sample according to the method of the present invention. Before measuring the superconducting properties, we investigated the crystal structure and component elements and found that B i2s r2ca+cu20. superconductor and
Homogeneously dispersed amorphous carbon was also confirmed.

Next, the same measurements as in Example 1 were carried out to evaluate the characteristics. The results are shown in the table and drawings together with the case of Example 1. In the figure, the characteristic curve (b) is a normalized critical current density-external magnetic field measurement curve according to the method of the present invention in Example 2.

Comparative Example B i 2031 according to the most common conventional manufacturing method
SrCO3, CaC(1+, cuo as starting material,
750℃ in the air, 10:1 calcination and 860℃ in the atmosphere
A comparative sample for measuring characteristics was prepared by firing.

In addition, measurements were performed and characteristics were evaluated in exactly the same manner as in the examples. The results are shown in the table and drawings together with those obtained by the method of the invention. In the figure, characteristic curve (c) is a normalized critical current density-external magnetic field measurement curve of the sample of LL Comparative Example.

From the table, it can be seen that the critical temperature Tc at which the resistance becomes zero is the same or slightly higher in the samples of Examples 1 and 2 according to the present invention than in the comparative examples made by the conventional method. It is considered that there is no adverse effect on crystallinity or uniformity. Further, the critical current density at 77 K and zero magnetic field is higher in Examples 1 and 2 according to the present invention than in the comparative example, indicating that current flows easily.

In addition, analysis using an electron microscope W1 and an X-ray microanalyzer revealed that in the oxide superconductor produced by the method of the present invention in Examples, carbonaceous components are contained in CL within the crystal grains.
It was found that the particles were dispersed with a size of l-0.8 μm, and did not affect the connection of superconducting currents.

Next, the reason why the characteristics are normalized by the V-field current density when the magnetic field is zero in the drawings is to compare the differences in magnetic field dependence between samples whose absolute values vary. From this figure, it is clear that in the samples of Examples 1 and 2 according to the present invention, the decrease in critical current due to an external magnetic field is much smaller than that of the comparative example, and the dependence on the magnetic field is improved. The reason for this is presumed to be that carbonaceous components remaining within the crystal grains effectively act as pinning centers.

As the organometallic compound used in this invention, any structure or form can be used, such as metal alkoxide, acetylacetonate (β-diketonate), cyclopentagenyl, etc. as shown in the above examples. .

In addition, organic acid salts such as metal carboxylates can also be used.

In addition, any number of carbon atoms in the organic group in each organometal can be used.

Further, in the present invention, firing in an inert gas or reducing gas atmosphere is performed at least once during the firing process.

As the specific atmosphere, a reducing gas such as hydrogen, an inert gas such as argon, nitrogen gas, etc. can be widely used as in the above embodiments.

The carbonaceous component may exist not only in the form of fine amorphous carbon powder as in the above embodiments, but also in a form containing crystals and organic components, for example.

By the way, in the above embodiment, a Bi-based material was used as an example of an oxide-based superconducting material, but the effects of this invention are not limited to Bi-based materials, but can also be realized in oxide-based superconducting materials such as Y-based, T1-based, Nd-based, etc. It was confirmed through the same study as in the example.

[Effects of the Invention] As explained above, according to the present invention, an inert gas or reducing By performing a firing process in a gas to produce the oxide superconductor, there is an effect of improving the critical current density of the oxide superconductor and its dependence on the magnetic field.

[Brief explanation of the drawing]

The drawing is a characteristic diagram showing the normalized critical current density-external magnetic field characteristic measurement results at 77 for each sample of Example 1.2 of the present invention and Comparative Example.

Claims (1)

[Claims] In the production of a metal oxide superconductor by firing, the starting material containing at least one organometallic compound of one or more metals constituting the superconductor is subjected to a firing process in an inert gas or a reducing gas. A method for producing an oxide superconductor, characterized by: