JPH04245112A - Manufacture of oxide superconductive material - Google Patents

Abstract

PURPOSE:To easily realize a long size and enhance a critical current density by forming an oxide superconductive film on a substrate made of metal or an alloy manufactured by one-way coagulation by a chemical deposition method or a solution application method. CONSTITUTION:In order to coagulate silver as a substrate material in one direction, molten silver 2 is filled into a heated pipe-lie graphite crucible 1, and the lower end of the crucible 1 is brought into contact with water 3, for coagulation. An oxide superconductive film is formed on the resultant silver substrate by a chemical deposition method or solution application method. Since a one-way coagulant has a high dislocation density due to its high growing speed, a dislocation density of the oxide superconductive film becomes high. Consequently, it is possible to efficiently manufacture a long size oxide superconductive material having a high critical current density. Furthermore, it is possible to easily provide satisfactory flexibility because the substrate is made of metal or an alloy.

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 superconducting material, and more particularly to a method for producing an oxide superconducting material provided as a film formed on a substrate.

0002

BACKGROUND OF THE INVENTION In recent years, oxide superconducting materials with high critical temperatures have been discovered one after another, and active research is being conducted to put them into practical use, including the production of wires. In particular, for making wire rods,
Many methods are being considered.

[0003] A typical method for forming wire rods is the silver sheath method, in which a raw material powder is filled into a silver pipe, and wire drawing, rolling, and heat treatment are appropriately performed to obtain a tape-shaped wire rod. This method is effective as a means for making wire rods because it is easy to lengthen the wire and a relatively high critical current density can be obtained due to mechanical orientation during rolling.

However, the critical current density of the wire obtained by the above-mentioned silver sheath method is a little less than 50,000 A/cm2 under the condition of zero external magnetic field, and up to 10,000 A/cm2 under the magnetic field of 1 T (Tesla). This is about A/cm2, which is insufficient for widespread use in cables and magnets.

[0005] Although a solution coating method is being considered to produce a wire, it is significantly inferior to the silver sheath method in terms of critical current density.

On the other hand, in research on thinning the film, 4 million A/cm2
Thin films having critical current densities above the above have been obtained. However, such a thin film with high properties can only be formed on a single crystal of an oxide such as strontium titanate. In addition, the production of large oxide single crystals is technically difficult, and such materials lack flexibility and
cannot be bent. Therefore, as long as an oxide single crystal is used as a substrate, a long material that can be used as a wire or a large material such as an acceleration cavity cannot be manufactured.

[0007]

[Problems to be Solved by the Invention] Therefore, an object of the present invention is to provide a method for manufacturing an oxide superconducting material that can be easily made into a long length and has a critical current density that can be used in a wide range of applications. That's true.

[0008]

[Means for Solving the Problems] The first feature of the method for producing an oxide superconducting material according to the present invention is that a substrate made of a metal or alloy produced by unidirectional solidification is used. A second feature of the present invention is that a chemical vapor deposition method or a solution coating method is used to form an oxide superconducting film on the substrate as described above.

Note that "unidirectional solidification" as used in this specification means that a large temperature gradient is applied to the solid-liquid interface in one direction during the solidification process of the molten metal or alloy, so that any crystal nuclei in the liquid phase are This refers to a method that prevents crystal growth from occurring and causes crystal growth only at the solid-liquid interface, giving the structure a unidirectional orientation.

[0010] "Chemical vapor deposition" is a method in which a vapor of a compound of an element constituting a film is introduced onto the surface of a substrate, and this compound is decomposed by heat, light, or plasma to form a film on the substrate. shall mean.

[0011] Also, in the "solution coating method", a solution of a compound dissolved in water or an organic solvent is applied onto a substrate, and this compound is decomposed by heat, light, plasma, etc. to form a film on the substrate. shall mean the method. Specific examples of the solution coating method include a dipping method, a spray method, a spin coating method, a spray pyrolysis method, and the like.

0012

[Function] From experiments on organic acid salt solution coating methods for Y-Ba-Cu-O based oxide superconducting materials, the present inventors discovered that oxides formed by solution coating methods on commercially available silver single crystal substrates. It has been discovered that a superconducting film achieves a critical current density of about 50,000 A/cm2. However, since ordinary silver single crystals are produced by the Bridgman method or the like, there is a limit to their length or size.

[0013] On the other hand, although the unidirectionally solidified material has inferior crystallinity compared to the single crystal produced by the Bridgman method,
There are no or very few distinct grain boundaries. Taking note of this, the present inventor formed an oxide superconducting film by chemical vapor deposition or solution coating on a substrate made of a unidirectionally solidified silver material, and surprisingly found that We have found that an oxide superconducting film on a solidified material exhibits a higher critical current density than a commercially available oxide superconducting film on a silver single crystal. Since the unidirectionally solidified material used in this invention has a high growth rate, it has a high dislocation density, and as a result, the dislocation density of the oxide superconducting film is higher than that of the film on the commercially available silver single crystal substrate mentioned above. It is presumed that this leads to an increase in pinning points and increases the critical current density.

[0014] In this specification, unless otherwise specified, when the numerical value of "critical current density" is cited, it refers to the normal four-terminal transport method at zero external magnetic field and a temperature of 77 K (temperature cooled by liquid nitrogen). Shows current density.

[0015]

As described above, according to the present invention, a unidirectionally solidified material that can be easily made into a long length is used as a substrate, and furthermore,
In order to form an oxide superconducting film on such a substrate,
A chemical vapor deposition method or a solution coating method is used, which makes it easy to increase the length and size. Therefore, a long oxide superconducting material having a high critical current density can be efficiently manufactured. In particular, regarding critical current density, it can exhibit higher characteristics than commercially available oxide superconducting films on silver single crystals. Furthermore, since the substrate produced by unidirectional solidification is made of metal or alloy, it is easy to provide appropriate flexibility.

Therefore, the present invention can be advantageously applied to the production of superconducting wires used in cables and magnets. The invention can also be advantageously applied to the manufacture of large materials such as acceleration cavities.

In addition, oxide superconducting materials are attracting attention as superconducting materials that do not require the use of rare liquid helium, but their coherence length is short and it is difficult to improve critical current density. According to the present invention, this problem is also solved and it is extremely useful for putting oxide superconducting materials into practical use as industrial materials.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a substrate made of metal or alloy produced by unidirectional solidification is prepared. For example, silver is used as a material for such a substrate. In order to solidify silver in one direction, for example, as shown in FIG. 1, molten silver 2 is poured into a heated pipe-shaped graphite crucible 1, and water 3 is brought into contact with the lower end of the crucible 1 to solidify the silver. As a unidirectional silver solidification method, methods other than this method can also be used. For example, molten silver is flowed out from a nozzle, water is poured on the flow part to solidify it, and the solidified material, for example, a plate, is continuously pulled out. method can also be applied. This latter method is particularly suitable for obtaining long substrates. According to the results of research conducted by the present inventors, the larger the temperature gradient at the solidification interface, the higher the critical current density of the oxide superconducting film formed thereon tends to be.

Furthermore, the critical current density of the oxide superconducting film is:
The highest value is obtained when a substrate that has been unidirectionally solidified is used, but even when a substrate obtained by rolling or wire drawing is used on a plate material obtained by unidirectional solidification, , have also been found to be effective in providing high critical current densities. For example, in order to increase the overall critical current density including the substrate, it is preferable that the cross-sectional area of the substrate be small. In this respect, it is also meaningful to use a substrate whose cross-sectional area is reduced by subjecting the plate material obtained by unidirectional solidification to further processing such as rolling or wire drawing.

Further, as oxide superconducting materials to which this invention is applied, Y-Ba-Cu-O system, Bi-Sr-C
There are a-Cu-O type, Tl-Ba-Ca-Cu-O type, etc., and the effects of the present invention can be expected in either case.

[0021] In the Tl-Ba-Cu-O system, Tl
Safety measures are necessary due to the toxicity of Regarding the critical current density at 77K, the Y-Ba-Cu-O system is
It seems to be superior to the i-Sr-Ca-Cu-O system. Therefore, it can be said that Y--Ba--Cu--O system is most suitable as the oxide superconducting material to which this invention is applied.

Further, when an oxide superconducting film is formed on a substrate according to the present invention, the thickness of this oxide superconducting film is as follows:
It is preferable that the thickness is 10 μm or less per time. This is because, in either chemical vapor deposition or solution coating, the critical current density decreases with increasing film thickness, and
This is because if it exceeds m, the critical current density may fall below the critical current density of oxide superconducting materials obtained by other methods. However, in the solution coating method, by repeating solution coating and baking, it is possible to increase the thickness while maintaining a high critical current density.

Experimental examples carried out according to the present invention will be explained below. Experimental Example 1 From experiments on the production of silver substrates, it was found that with silver, a large single-crystalline material can be easily obtained by simply cooling from one direction. Although the silver material produced in this way has a dislocation density that is more than an order of magnitude higher than that of commercially available silver single crystals,
No clear grain boundaries were observed.

Therefore, a silver substrate produced by the floating zone melting method shown in FIG. 2 was used. In other words, a silver tape 4 having a thickness of 1.5 mm and a width of 10 mm is placed at a speed of 5.
While pulling the silver tape 4 at a rate of 0 mm/min, a 100 W infrared lamp 5 was irradiated on both sides of the silver tape 4 to a length of 1 mm.
The melted part 6 was formed by focusing the light in the range of . silver tape 4
was held by the surface tension of the melted part 6, and a 60 cm tape-shaped silver substrate 7 could be obtained continuously in this state.

The crystal orientation of this 60 cm long silver substrate along its length was investigated using an X-ray diffraction method. As a result, it was found that the surface was a (100) plane over almost the entire length, and the [011] direction was aligned with the longitudinal direction of the tape-shaped silver substrate.

Next, a Y--Ba--Cu--O based superconducting film was formed on this silver substrate by an organic acid salt coating method under the following conditions. ＊＊＊＊＊＊＊ Application solution: Yttrium laurate
2.80wt%
barium laurate
1.08wt%
copper laurate
5.11w
t% Propionic acid:Pyridine (2:1) Balance Coating method: Dipping-pyrolysis (600°C in the air, 1 minute) repeated 3 times
Firing: 950°C in oxygen for 10 minutes＊＊＊＊＊＊＊ The thickness of the obtained oxide superconducting film was 1.6 μm,
The critical current density was 75,000 A/cm2.

Experimental Example 2 The number of repetitions of immersion and thermal decomposition in Experimental Example 1 was varied to investigate the influence of film thickness on critical current density. As shown in Figure 3, as the film thickness increases, the critical current density decreases, and when it exceeds 10 μm, the critical current density decreases to 6,000 A/cm2.
As a result, it has fallen to a level where it cannot be put into practical use.

Experimental Example 3 Using the same substrate, solution and final firing conditions as in Experimental Example 1, Y-Ba-
A Cu-O based oxide superconducting film was produced. The coating method, coating conditions and critical current density are shown in Table 1 below.

[0029]

[Table 1]

Experimental Example 4 Y--Ba--Cu--O based oxide superconducting films were fabricated by chemical vapor deposition using various unidirectionally solidified silver materials as substrates.

The chemical vapor deposition method was carried out under the following conditions. ＊＊＊＊＊＊＊ Raw material and evaporation temperature Y (thd) 13
0℃ B
a 350℃
Cu
120℃ (thd:2,2,6,6-tet
ramethyl-3,5-heptanediona
to) Carrier gas and flow rate Ar
7.5×10-6m3/s Oxygen gas flow rate
4.
2×10-6m3/s pressure
1to
rr temperature
850℃ Film formation rate

200 angstrom/min film formation time
1 hour ***** The solidification method for producing the substrate, its production conditions, and critical current density are as shown in Table 2 below.

[0032]

[Table 2]

Experimental Example 5 The long silver substrate obtained by the drawing method of Experimental Example 4 was rolled to produce a silver tape having a thickness of 0.1 mm and a width of 15 mm. Next, Bi was applied onto this silver tape by the sol-gel method.
- A superconducting wire was produced by forming a Pb-Sr-Ca-Cu-O based oxide superconducting film.

In the sol-gel method, acetylacetonate of Bi, Pb, and Cu, dimethoxystrontium, and dimethoxycalcium are used in a molar ratio of Bi, Pb, Sr, Ca, and Cu, respectively, of 1.9:0.4:2:2: 3 was dissolved in xylene and heated at 100° C. for 5 hours to form a sol. Next, the above-mentioned silver tape was passed through this sol solution to apply the sol solution and thoroughly dried. Next, the sol was made into a ceramic by firing at 950° C. for 30 minutes in oxygen.

In the superconducting wire thus obtained, the thickness of the oxide superconducting film varied from 5 to 20 μm, but the critical current was 100 A and the average critical current density was 68,000 A.
/cm2, overall critical current density 8,800 A
/cm2 could be obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing steps for producing a substrate by unidirectional solidification included in one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing another example of steps for producing a substrate by unidirectional solidification included in one embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the film thickness and critical current density of an oxide superconducting film obtained in the step of forming an oxide superconducting film by a solution coating method included in an embodiment of the present invention.

[Explanation of symbols]

1 Graphite crucible 2 Molten silver 3 Water 4 Silver tape 5 Infrared lamp 6 Melting part 7 Silver substrate

Claims (1)

[Claims] 1. A substrate made of a metal or alloy produced by unidirectional solidification is prepared, and an oxide superconducting film is formed on the substrate by a chemical vapor deposition method or a solution coating method.
A method for producing an oxide superconducting material, comprising steps.