JPH0816014B2 - Manufacturing method of oxide superconducting bulk material - 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 producing an oxide superconductor bulk material capable of obtaining a high critical current density in a high magnetic field, and particularly to a step of gradually cooling from a semi-molten state at high temperature, that is, a solid-liquid coexisting region. The present invention relates to a technique for obtaining a superconducting phase by utilizing.

[0002]

2. Description of the Related Art At present, most of the efforts for practical application of bulk materials of oxide superconductors are based on the sintering method (solid-state reaction method) (Reference: Jap. J. Appl. Phys. Vol.26, No.5, 198
7, pp.L624-L626). First, a raw material powder of an oxide superconductor (RE, that is, an oxide or carbonate of RE, a rare earth element containing Y, Ba, or Cu) is RE: Ba: Cu = 1: 2:
REBa ₂ Cu ₃ O mixed with a composition ratio near 3 and calcined
A calcinated powder having a _7-y structure is produced, and this is molded and sintered to obtain a bulk material. As an application example of this sintering method, a study of filling a calcined powder into a metal coating material or the like to form a wire (Jap. J. Appl. Phys.
Vol.26, No.5, 1987, pp. L865-L866). Also,
There is an attempt to form a plate and sinter it into a shield material. However, these attempts have not reached a practical level because the critical current density exhibited by the superconductor produced by the sintering method is low.

A melting method other than the sintering method (solid-state reaction method) as a method of manufacturing an oxide superconductor, that is, a method of heating and melting a raw material at a high temperature and then gradually cooling it, is not suitable for manufacturing a bulk material. Not used, but used for single crystal growth. At this time, Cu or (Cu, Ba) was added to the raw material powder in a considerably excessive amount, and it was generally grown in a platinum or alumina crucible. Target (Jap.
J. Appl. Phys. Vol.26, No.5, 1987, pp. L851-L853,
etc).

[0004]

The oxide superconductor bulk material produced by the sintering method or the like is currently (temperature T = 77).
K, external magnetic field He = 0 Tesla), several thousand A / cm ²
Only a critical current density (Jc) is obtained, and it has not been put to practical use. For practical use, (T = 77K, He =
Under several (several tesla) conditions, it is necessary to improve Jc to about 10 ⁴ A / cm ² . The present invention is different from the conventional production method of the oxide superconductor bulk material by the sintering method (solid phase reaction method).
In the slow cooling process from the higher temperature solid-liquid coexistence region, the superconducting phase (REBa ₂ Cu ₃ O _7-y phase, hereinafter referred to as “123
Abbreviated as "phase". ) Is employed to improve the critical current density (Jc) characteristics in a high magnetic field and to provide a practicable oxide superconductor bulk material.

For the above object, the main problems to be overcome by the present invention are: 1) improvement of critical current density in zero magnetic field and magnetic field, 2) improvement of formability of bulk material in the shape of wire, coil, plate, etc. 3) Improvement of mechanical strength, etc. That is, in the superconductor obtained by the conventional sintering method, since the crystal grain size is several μm to several hundreds μm, a large number of grain boundaries exist therein, and these grain boundaries are weak in terms of superconductivity. In the grain, the current based on a large superconductivity (the critical current density Jc is an index) is limited at the grain boundary and decreases. Especially, in a magnetic field (for example, He = 1 Tesla), it extremely decreases to several tens A / cm ^2. To do. Further, the superconductor obtained by the sintering method is extremely difficult to process after sintering, and it is also very difficult to join the superconductors together. Further, as is well known, the sintered body has a drawback that it is inherently brittle.

[0006]

In order to solve the above problems, the present invention provides a REBa ₂ Cu ₃ O _7-y phase (123 phase).
The RE ₂ BaCuO ₅ phase with a diameter of 20 μm or less (hereinafter,
It is abbreviated as “211 phase” as appropriate. ) Dispersed structure (Fig. 1
The use of (see (a)) as a superconducting material is essential,
Specifically, a high-quality oxide superconductor bulk material based on the above structure (hereinafter, referred to as “high-quality oxide superconductor bulk material”) and an intermediate having a precursor structure suitable for manufacturing the bulk material (hereinafter, Based on the "intermediate"), a method for producing one of the bulk materials (first invention) and another method for producing the bulk material (second invention) are provided. The contents (constitution) of these high-grade oxide superconductor bulk materials, intermediate materials, and the method of the present invention will be briefly described below.

The high-grade oxide superconductor bulk material is R
In the EBa ₂ Cu ₃ O _7-y type oxide superconductor, 12
The oxide superconductor bulk material having a high critical current density in a magnetic field, which has a structure in which 211 phases having a diameter of 20 μm or less are dispersed in three phases. Here, “RE” indicates a rare earth element but also includes Y (yttrium), the “diameter” of the 211 phase means the maximum outer diameter of the 211 phase particles, and the “bulk material” means the thickness or diameter. Refers to a material that is approximately 1 mm or more. As shown in FIG. 1 (a), the structure of this bulk oxide superconductor bulk material is composed of large 123-phase particles having a crystal grain size of several mm or more and containing fine 211-phase particles. As known from FIG. 1 (b), the particles show a single-crystal-like structure with an aligned orientation showing a 123-phase twinning pattern. In this way, the crystal grains are much larger than the conventional material,
The feature is that there are few grain boundaries that lower the (critical current density), and a high Jc is exhibited in the zero magnetic field and the magnetic field. In addition, the particles of the 211 phase that are recognized to exist in the superconducting phase (123 phase) include grain boundaries, cracks, and different phases of the superconducting phase (such as CuO phase existing inside the superconducting phase) other than the 211 phase. It is necessary to some extent to obtain a bulk material having a small amount, and it is desirable that the material be finely distributed.

The intermediate has a chemical composition equivalent to that of a high-quality oxide superconductor, and has a diameter of 50 μm in the BaCuO ₂ phase.
It is an intermediate of an oxide superconductor characterized by having a structure in which the following RE ₂ O ₃ phase is dispersed. Where RE ₂
O ₃ phase of the "diameter" shall refer to the maximum outer diameter of the RE ₂ O ₃ phase particles.

The above-mentioned intermediate is a raw material (RE oxide, Ba oxide, Cu oxide, etc.) compounded with a composition to form a RE-Ba-Cu-O compound consisting of 123 phases containing a fine dispersion of 211 phases. ) Is once melted and then rapidly cooled and solidified, and its structure is as shown in FIG. 1 (c), and RE ₂ O ₃ fine dispersed particles as seen here are the intermediates. Remelting treatment (1000 ° C to 1350
The heating to (° C.) serves as a site for producing 211-phase fine particles that make up the high-quality oxide superconductor bulk material that is the final target.

The first aspect of the present invention is to prepare a starting raw material having a chemical composition for forming the high-quality oxide superconductor (a superconductor consisting of 123 phases containing a fine dispersion of 211 phases), and heating the starting raw material. Then, the melt is rapidly cooled and solidified and molded into a predetermined shape (for example, a plate shape or a linear shape), and the molded body is heated to a temperature belonging to a temperature range of 1000 ° C. to 1350 ° C. to be semi-melted. The production of an oxide superconductor bulk material exhibiting a high critical current density in a magnetic field, characterized in that the semi-molten material is gradually cooled at a cooling rate of 200 ° C./hr or less to precipitate 123 phase by a peritectic reaction. Is the way.

A second invention is a powder of RE ₂ O ₃ and Ba-
Cu oxide powder is mixed with the chemical composition for forming the high-quality oxide superconductor, the mixed powder is molded into a predetermined shape, and the molded body belongs to a temperature range from 1000 ° C to 1350 ° C. A high critical current in a magnetic field, characterized in that the temperature is raised to a temperature to be semi-molten, and the semi-molten material is gradually cooled at a cooling rate of 200 ° C./hr or less to precipitate and form 123 phase by a peritectic reaction. It is a method of manufacturing an oxide superconducting bulk material exhibiting a density.

The present inventors investigated the following three states of the molded body before heat treatment. 1. REBa ₂ Cu ₃ O _7-y phase powder compact 1. _2. Mixed powder of RE ₂ BaCuO ₅ phase and BaCu oxide When using a mixed powder and the mixed powder as a result RE ₂ O ₃ and BaCu oxide of RE ₂ O ₃ and BaCu oxide as shaped bodies, superconducting phase is obtained in which fine 211 phases are uniformly distributed .. In addition, the obtained superconducting phase had a large particle size of several millimeters, had few cracks, and was a superconducting superconductor with less weak-link.

The causes of these are RE ₂ O ₃ and liquid phase (BaC
u oxide) reacts and the 211 phase grows.
It has been found that this is due to the fact that needle-like 211 fibers of the order of microns are formed in the material.

[0014]

The 123 phase is unstable at a high temperature of about 970 ° C. or higher, and decomposes and melts into the 211 phase and the liquid phase (L: BaCu oxide). Further, above about 1250 ° C., the 211 phase is also decomposed and becomes a liquid phase with RE ₂ O ₃ . However, since the fibrous 211 absorbs the liquid phase in the semi-molten state of the molded body when heated at a high temperature, the shape of the molded body is almost maintained. When this semi-molten compact is gradually cooled, the 123 phase is formed by the peritectic reaction between the 211 phase and the L phase. The organization that can be created at this time is small 2
It becomes an aggregate of several millimeters of single crystals including 11 phases. The material produced according to the present invention has very few large-angle tilt boundaries that hinder Jc, and therefore, high Jc can be obtained without a magnetic field, and even in a high magnetic field, Jc is 3 orders of magnitude higher than that of the conventional method. Is obtained. Moreover, in this manufacturing method, once in a semi-molten state at 1000 ° C. to 1350 ° C., it can be easily processed into an arbitrary shape because it has an appropriate viscosity when heated at a high temperature. In addition, the joining of the materials can be easily performed only by bringing them into contact with each other.

The reason for limiting the heating temperature of the molded body is 1000.
When the temperature is lower than 0 ° C, partial melting occurs, but the amount is small and the above effect cannot be obtained, and when the temperature is higher than 1350 ° C, the prototype of the molded body cannot be kept. Further, these temperatures are somewhat changed depending on the type of RE element and the composition of the atmosphere during heating and tend to shift to the higher temperature side as the RE element has a larger ionic radius, the oxygen partial pressure in the atmosphere is larger, and the RE is excessive. .

The reason for limiting the slow cooling rate is that if the temperature is 200 ° C./hr or more, grains of the 123 phase do not grow sufficiently, so that the grain boundaries increase and Jc decreases. By such heat treatment, since the superconducting phase contains the fine 211 phase, the structure is fine and the mechanical strength is improved.

[0017]

EXAMPLES An example of producing an oxide superconductor bulk material by the above-mentioned method of the first invention will be described below. As a molded product, a product having a thickness of 1 mm, a width of 10 mm, and a length of 20 mm, which was obtained by melting YBa ₂ Cu ₃ O _7-y powder and hammer-quenching, was prepared. Figure 1 shows the results of observation of the structure of this material.
It shows in (C). This texture is 50 μm in BaCu oxide
It had a structure in which the following Y ₂ O ₃ was dispersed. This was placed on a platinum net and subjected to the following heat treatment in an oxygen stream. After holding at 1200 ° C for 1 hour, 90 at -30 ° C / hr
The temperature was lowered to 0 ° C. and to room temperature at −100 ° C./hr. When the obtained material was cut out and the superconducting property was measured, the following results were obtained.

At a critical temperature (Tc) of 93 K, a sharp superconducting transition was shown. Critical Current Density (Jc): 77K in Figures 2 and 3, respectively
The transport critical current density by the four-terminal method at 4.2K is shown (however, in the four-terminal method, Jc may be underestimated due to heat generation of the current terminal). Further, FIG. 4 shows that the critical current density obtained from the magnetization measurement of another sample exceeds the value by the four probe method. in this way,
It was found that this manufacturing method can manufacture a superconducting material of extremely high quality as compared with the conventional manufacturing method.

Further, when a molded body was placed on a bent platinum net and the same experiment was conducted, it was found that a superconducting material having a net shape substantially the same as that of the net was formed and that the shape could be easily given. Furthermore, when a part of the two molded bodies were overlapped and the same experiment was conducted, it was found that the superconducting property at the joint hardly changed and the joint was extremely good.

Regarding the mechanical properties, from the result of the structure observation, there are many 211 phases of about 1 micron in the material and the structure is fine, so that the strain due to the phase transition from the tetragonal system to the orthorhombic system does not form twins. It turned out to be relaxed. From this, it is considered that the mechanical toughness is improved.

[0021]

As described above in detail, the present invention enables the production of a high-quality oxide bulk superconducting material which has hitherto been impossible, and can be applied as a molded product in various fields. Yes, it has a great industrial effect. Specific examples are: 1) Superconducting wire rod With this manufacturing method, a wire having a high Jc can be formed from a linear molded body, and since connection is easy, it can be used as a long-distance power transmission line. 2) Superconducting coil A high-quality magnet can be obtained simply by stacking a number of spirally shaped compacts and contacting them at the joint. 3) Superconducting magnetic shield material A high-quality magnetic shield material with less magnetic flux leakage can be obtained because a superconductor of an arbitrary shape can be formed by simply heat-treating a plate-shaped compact on a mold of an arbitrary shape. Etc.

[Brief description of drawings]

FIG. 1 is a micrograph showing a structure of a superconducting material according to the present invention, where (a) is a structure of a superconducting bulk material, (b) is a twin of the superconducting bulk material, and (c) is an intermediate substance of the superconducting bulk material. Each organization is shown.

FIG. 2 shows the magnetic field dependence of the critical current density at a liquid nitrogen temperature of 77K.

FIG. 3 is a diagram showing the magnetic field dependence of Jc at a liquid helium temperature of 4.2K.

FIG. 4 shows the magnetic field dependence of the critical current density obtained from the magnetization characteristics at 77K.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location // H01B 12/00 ZAA (72) Inventor Shoichi Matsuda 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa (1) Reference No. 64-72905 (JP, A) JPN. J. APPL. PHYS. , VOL. 27, NO. 2, PP. L188-L190 (1988.2)

Claims (2)

[Claims] 1. A starting material having a chemical composition for forming a REBa ₂ Cu ₃ O _7-y type oxide superconductor is prepared, the starting material is heated and melted, and the melt is rapidly cooled to a predetermined temperature. Solidified into a shape and made of RE ₂ O ₃ and Ba-Cu oxide
An intermediate body is obtained, this molded body is heated to a temperature within a temperature range of 1000 ° C. to 1350 ° C. to be semi-molten, and this semi-molten body is gradually cooled at a cooling rate of 200 ° C./hr or less to give a peritectic crystal. The reaction produces a 123-phase crystal in which the 211-phase is finely dispersed.
Method of manufacturing an oxide superconductive bulk material exhibiting a high critical current density in a magnetic field, characterized in Rukoto allowed length. RE here
Means Y (yttrium) or a rare earth element. 2. A mixture of RE ₂ O ₃ powder and Ba—Cu oxide is heated to a temperature within a temperature range from 1000 ° C. to 1350 ° C. to be semi-melted, and the semi-melted product is heated to 200 ° C.
211 per hour by peritectic reaction after slow cooling at a cooling rate of less than 1 hour
Phase method of manufacturing an oxide superconductive bulk material exhibiting a high critical current density in a magnetic field, characterized in Rukoto grown crystals finely dispersed 123 phase. Here, RE means Y (yttrium) or a rare earth element.