JPH02258698A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain the title superconductor with its crystal unidirectionally solidified throughout the width direction of the film by irradiating an oxide superconducting substance with laser beams at a specified tilt angle against said width direction to effect melting said substance followed by cooling and crystallization. CONSTITUTION:A thin film 1 of an oxide superconducting substance or a substance capable of forming the same by melting and sintering is formed on a continuous base 2. The film 1 is then irradiated with laser beams 3 at a tilt angle against the scanning direction; during the process, a scanning is made in the arrow A direction to heat and melt the irradiated zone 1a. As the laser beams 3 move, the melted zone is cooled from behind to effect recrystallization. The irradiation angle theta of the laser beams is regulated so as to accomplish an optimum orientation according to laser output and film thickness. Thereby, the zone 1a begins to cool from behind successively, however, there is no temperature gradient in the width direction at a part where cooling begins and a nearly uniform temperature has been attained, thus crystal will grow parallel to the film surface in the scanning direction.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for manufacturing oxide superconductors such as YBCO-based or Bi-based oxide superconductors, and particularly relates to a method for producing oxide superconductors that enable unidirectional solidification of crystals. It is related to the manufacturing method.

[Prior Art and Problems to be Solved by the Invention] It has been known that oxide superconductors have properties that vary greatly depending on crystal orientation and are anisotropic. In particular, the electrical resistance value in the vertical direction (C axis) of the crystal is much larger than in the horizontal plane (a, b plane), and the characteristics of the oxide superconductor are aSb
It is thought that the structure of the surface is dominant.

By the way, in normal synthesis methods, especially sintered bodies (bulk)
In the case of , the crystal orientation is random, so both the electrical and magnetic properties have not reached a practical level.

As a method of increasing crystal orientation, there is a method of melting an oxide superconductor and recrystallizing it by moving it relatively in an electric furnace having a temperature gradient. It is known that the larger the temperature gradient, the more aligned the crystal orientation is in one direction, but in an electric furnace, the temperature is easily equalized by heat conduction, making it impossible to form a steep temperature gradient. Controlling sexuality is difficult.

In addition, attempts have been made to irradiate oxide superconducting materials with laser light to locally heat and melt it, and then move (scan) it to create a temperature gradient in the scanning direction and sequentially crystallize the irradiated areas. . In particular, when a laser beam having a specific light intensity pattern is used, crystals oriented in the scanning direction can be obtained.

However, as shown in FIG. 3, when the laser beam is irradiated perpendicularly to the sample film 1 formed on the substrate 2, a temperature gradient occurs in the thickness direction of the film.

For this reason, near the film surface, it is cooled from the outside, so crystals grow along the surface.On the other hand, near the substrate 2 inside the film, crystals grow inward because heat is carried away by the substrate. , it is not possible to obtain oriented crystals whose crystal direction is parallel to the film surface and aligned in the scanning direction.

[Objective of the Invention] The present invention has been made in view of the above-mentioned conventional difficulties, and it is an object of the present invention to provide a method for producing an oxide superconductor that enables unidirectional solidification of crystals throughout the thickness of the film. purpose.

[Means for Solving the Problems] In order to achieve such objects, according to the method for producing an oxide superconductor of the present invention, an oxide superconductor or a molten
When irradiating a material that generates an oxide superconducting material by sintering with a laser beam, melting the oxide superconducting material or the substance, and then cooling and crystallizing the material, the laser beam is applied to a predetermined direction in the thickness direction of the film. It irradiates at an angle.

[Embodiments of the Invention] Hereinafter, an embodiment of the method for producing an oxide superconductor according to the present invention will be described in detail with reference to the drawings.

As shown in FIG. 1, the sample 1 is a thin film formed on a long substrate 2, and by scanning in the direction of arrow A while irradiating the laser beam 3, the irradiated portion 1a is heated and melted. As the laser beam 3 moves, it is cooled from the rear and recrystallized or crystallized.

Here, the sample 1 may be either an oxide superconducting material or a material that produces an oxide superconducting material by melting and sintering, and the latter material that produces an oxide superconducting material by melting and sintering is, for example, Y, Ba, etc. , pelletized superconducting materials such as Cu oxides using the solid phase method, superconductor melt coating films using metal alkoxides and other organic/metallic compounds and inorganic compounds, and raw material powders created using the doctor blade method. It is a thick film of a slurry of a solution consisting of an organic binder or the like.

Furthermore, an amorphous thick film created by immersing a substrate in a high-temperature melt of an oxide superconductor raw material and rapidly cooling it can also be used. In this case, the change in density after laser melting is small and cracks are less likely to occur.

Furthermore, as the oxide superconducting material, in addition to those obtained by melting and sintering the above-mentioned materials, those formed on the substrate 2 by a method such as a laser sputtering method using an excimer laser, a CVD method, or a spray pyrolysis method can be used. can.

The substrate 2 may be a plate or a tape, and its material may be a metal substrate such as silver or zirconium, or a buffer layer such as magnesium oxide, yttrium stabilized zirconium (YSZ), or strontium titanate on the metal substrate. Either an insulating substrate made of magnesium oxide, YSZ, etc. can be used. However, when using a metal substrate as the substrate, after laser irradiation, the substrate 2 is forcibly cooled to prevent a reaction between the substrate surface and the sample 1.
Also, when cooling, it is necessary to prevent radial heat diffusion from occurring.

The laser beam 3 may be an ordinary round beam, but in order to obtain crystals oriented in the scanning direction over a wide range, a twin beam 3a1 triple beam 3b or a slit beam 3c having a pattern as shown in FIG. use

When using a laser beam with such a specific pattern, it is possible to form a steep temperature gradient in the same direction as the scanning direction as the heated region moves, and crystallization progresses along this temperature gradient. As a result, highly oriented crystals are formed. The output of the laser beam varies depending on the film thickness of the sample, but typically, for example, in the case of a YAG laser, one with an output of 5 W or more is used. The scanning speed of the laser beam varies depending on the laser beam output and the laser beam diameter, but in the case of a laser with the above laser beam output and a beam radius of 50 μm and a distance between beam focal points of 100 μm, it is 10 cm / s.
Approximately ec.

Such laser light is applied obliquely to the film 1 of the sample from the front of the scan. By irradiating the laser beam obliquely, the boundary between the heated region 1' and the rear unheated region becomes almost perpendicular to the film surface. Here, the irradiation angle θ of the laser beam is appropriately adjusted according to the laser output and the film thickness so as to obtain the optimum orientation.

In this way, by irradiating the laser beam obliquely onto the film and moving it relatively in the direction of the arrow, heating area 1' begins to cool down sequentially from the rear and crystallizes, but the cooling of the film begins here. Since there is no temperature gradient in the thickness direction of the film and the temperature is almost uniform, the crystal grows parallel to the film surface in the scanning direction. Moreover, by irradiating the laser beam diagonally here, the front part in the direction of travel is preheated.
Evaporation of the sample due to rapid heating can be prevented.

Cooling in this case is by air cooling, but in the case of oxides, the thermal conductivity is generally low and it is difficult to create a steep temperature gradient with air cooling alone, so in a preferred embodiment, a laser beam pattern (heating A forced cooling zone shall be provided immediately after the

As means for providing the forced cooling region, for example, a refrigerant such as liquid gallium, liquid nitrogen, or liquid air is used. These coolants are sprayed from one or both sides of the sample 1 and the substrate 2 using a nozzle or the like to perform forced cooling. The refrigerant is recovered by providing a recovery tank opposite the nozzle or surrounding the nozzle.

By forcibly cooling the heated region in this way, the temperature gradient in the laser scanning direction is increased, increasing crystallization and preventing arbitrary nucleation and solidification to achieve the desired orientation. It is possible to obtain crystals with properties.

It should be noted that the oriented crystal obtained by laser melting as described above tends to become a crystal that is highly oxygen-deficient, so oxygen annealing is required as a post-treatment. In this case, oxygen annealing must be performed while maintaining the crystal orientation.

For example, at a relatively low temperature (400'C in the case of YBCO type)
Heat treatment is performed in a high oxygen pressure atmosphere or in an oxygen stream. Preferably, as a heat source for annealing,
A belt-shaped laser parallel to the direction perpendicular to the crystal orientation direction is used to generate a temperature gradient in the crystal orientation direction. This allows heating and oxygen absorption without disturbing the orientation of the crystal.

Example A twin beam with an output of 15W, a beam radius of 50 μm, and a beam spacing of 100 μm was used on a thin film of YBCO superconducting material having a thickness of 10 μm and a width of 200 μm, and the irradiation angle θ=
Recrystallization was performed by irradiation at 80 degrees and a scanning speed of 10 crabsec.

Critical current density Jc of the obtained superconductor (external magnetic field OT,
77K; A/cd) was found to be 20% higher than the critical current density (Jc=10'' to 104) when the sample was irradiated perpendicularly.

[Effects of the Invention] As is clear from the above examples, according to the method for producing an oxide superconductor according to the present invention, when a sample is irradiated with a laser to crystallize it, the crystallization is performed in the thickness direction of the sample. On the other hand, since the irradiation is performed obliquely and scanned, a steep temperature gradient that is uniform in the thickness direction and aligned in the scanning direction can be formed, and a highly oriented oxide superconductor can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a method for manufacturing an oxide superconductor according to the present invention, FIG. 2 is a diagram showing a specific example of a laser beam pattern used in the method for manufacturing an oxide superconductor, and FIG. 3 is a diagram showing a conventional oxide superconductor manufacturing method. It is a figure showing the manufacturing method of a superconductor. 1... Film 2... Substrate 3... Laser light

Claims (1)

[Claims] When irradiating an oxide superconducting material or a material that generates an oxide superconducting material by melting and sintering with a laser beam to melt and crystallize the oxide superconducting material or the material, the laser beam is applied to the film. A method for manufacturing an oxide superconductor, characterized in that irradiation is performed at a predetermined angle with respect to the thickness direction.