JPH02258669A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To form a steep temperature gradient along the scanning direction of a laser beam on a specimen to enable the coagulation of crystals in one direction by disposing a compulsorily cooled portion in proximity to a laser beam-irradiated portion when a long size material comprising an oxide superconductor substance is continuously irradiated with the laser beam to crystallize the material. CONSTITUTION:A laser beam 3 is approximately rectangularly irradiated on a long size material comprising an oxide superconductor substance or a material giving a superconductor by the melting.coagulation thereof, e.g. a thin film 1 formed on a long size substrate 2 and simultaneously scanned in the direction of the arrow to heat and melt an irradiated portion 1a, followed by cooling the melted portion from the backward direction with the movement of the laser beam to recrystallize the melted portion. A compulsorily cooled region is disposed just behind the laser beam- irradiated portion 1a, and liquid nitrogen, etc., is sprayed, immersed, etc., to cool the melted portion, thereby increasing the crystallization of the material in a state wherein the temperature gradient of the thin film 1 is enlarged in the scanning direction of the laser beam. A laser beam having a belt-like pattern is subsequently repeatedly irradiated on the treated portion at a relatively low temperature in a high oxygen pressure atmosphere or in an oxygen stream to anneal the treated portion.

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 oxide superconductors are a,...
It is thought that the structure of the b-plane 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.

One way to increase crystal orientation is to melt an oxide superconductor and recrystallize it by moving it relatively in an electric furnace with 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 a takumi.

In addition, an attempt has been made to irradiate an oxide superconducting material with laser light to locally heat and melt it, and then move it (scanning) to create a temperature gradient in the scanning direction and sequentially crystallize the irradiated areas. However, in this case, the temperature is highest at the center of the irradiated area, so a temperature gradient is created radially from this center, and even though the laser beam is scanned in one direction, the crystal orientation is not the same. Not aligned in the direction. Therefore, there was a problem that the orientation could not be controlled.

Furthermore, YBCO-based oxide superconductors generally have the disadvantage that when melted and recrystallized, they become insulating so-called 211-structure bodies, making it impossible to form 123-structure bodies.

Further, when the crystal is melted and recrystallized by laser light, the solidified crystal is extremely deficient in oxygen, so it is necessary to perform oxygen annealing as a post-treatment. Here, in the conventional annealing method in which the entire sample is heated uniformly, even if crystals are solidified with orientation, the direction of the crystals is disturbed during heating, and as a result, a superconductor with desired characteristics cannot be obtained.

[Object of the Invention] The present invention was 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 oriented solidification of crystals.

[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 recrystallizing or crystallizing a long material made of a substance that generates an oxide superconducting material by solidification with a laser beam, the recrystallized or crystallized superconducting material is immediately forcedly cooled, and the axial It is designed to increase the temperature gradient in the direction.

In this case, by immediately forced cooling of the oxide superconducting material after recrystallization or crystallization, the temperature gradient in the axial direction can be significantly increased compared to the temperature gradient in the direction perpendicular to it, resulting in unidirectional solidification. can be achieved.

In addition, in oxygen annealing the crystal of the oriented oxide superconducting material crystallized by the above manufacturing method,
Oxygen annealing is performed by scanning a strip pattern in the same direction as the crystal orientation in an oxygen atmosphere.

[Embodiments of the Invention] Hereinafter, an example of the method for producing an oxide superconductor according to the present invention will be described.
Examples will be described in detail according 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 sample 1 with a laser beam 3 at approximately right angles, the irradiated area is 1a is heated and melted, and as the laser beam 3 moves, it is cooled from behind and recrystallized.

In actual operation, the laser beam 3 is fixed and the substrate 2 is moved in the opposite direction to the scanning direction.

Here, the sample 1 may be either an oxide superconducting substance or a substance that generates an oxide superconducting substance by melting and solidification, and examples of the latter substance that generates an oxide superconducting substance by melting and solidifying include Y and Ba.

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 and organic materials created using the doctor blade method. It is a thick film of a slurry of a solution consisting of a binder, etc.

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 laser sputtering using excimer laser, CVD, or spray pyrolysis may be used. I can do it.

Further, the substrate 2 may be either a plate-like body or a tape, and may be made of silver,
Metal substrates such as zirconium, magnesium oxide on metal substrates, yttrium stabilized zirconium (YSZ),
Those with a buffer layer such as strontium titanate,
Alternatively, any insulating substrate such as magnesium oxide or YSZ may be used. However, when a metal substrate is used as the substrate, it is necessary to forcibly cool the substrate 2 after laser irradiation to prevent a reaction between the substrate surface and the sample 1.

As the laser beam 3, a normal round beam having the power and beam diameter necessary to melt the entire width of the sample 1 is used.

The above laser beam varies depending on the film thickness of the sample, etc., but normally, for example, in the case of a YAG laser, one with an output of 10 W or more is used. The scanning speed of the laser beam varies depending on the laser beam output and laser beam diameter, but in the case of a laser with the above laser beam output and a beam radius of 50 μm,
~100m/sec.

In this case, since oxides generally have low thermal conductivity and it is difficult to create a steep temperature gradient in the axial direction by air cooling alone, a forced cooling zone 4 is provided immediately after the laser beam irradiation portion 1a.

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 onto the surface of the sample 1 and/or the substrate 2 using a nozzle 5, etc., as shown in FIG. Cooling can also be performed.

The forced cooling region 4 is preferably provided in a spot shape close to the rear of the laser beam pattern 1a, as shown in FIG. The distance from the laser beam pattern 1a is appropriately selected depending on the scanning speed, laser beam output, beam diameter, beam interval, etc.

By providing the forced cooling zone 4 in this manner and forcibly cooling the laser beam pattern 1a from the center, the temperature gradient in the laser scanning direction is increased, crystallization is increased, and nucleation and solidification occur arbitrarily. It is possible to prevent this and achieve crystallization with desired orientation.

Furthermore, by increasing the scanning speed in a narrow heating region, a directional temperature gradient can be maintained;
It is possible to stably crystallize solid phases that cannot be generated through normal melting and recrystallization processes. For example, in the case of a YBCO-based superconductor, only a normal molten 211 phase can be crystallized, but according to the method of the present invention, a 123 phase can be stably crystallized.

Note that the above-described laser melting is preferably performed under oxygen pressure control.

In addition, heat treatment is performed in advance using a preheating source such as a halogen lamp prior to laser irradiation in order to prevent material components from evaporating and foaming due to rapid heating by laser, and to prevent cracks from occurring due to recrystallization. This is desirable.

Next, oxygen annealing of the superconducting material recrystallized as described above will be explained. Since crystals recrystallized by laser melting tend to be extremely oxygen-deficient crystals, oxygen annealing is required as a post-treatment. In this case, oxygen annealing must be performed while maintaining the directionality of the oriented crystal. For example, at a relatively low temperature (for YBCO type, 40
The heat treatment is performed at a temperature of about 0° C. x 10 hrs in a high oxygen pressure atmosphere or an oxygen stream. Preferably, FIG. 4(a),
As shown in (b), a laser with a strip pattern 10 is used as a heat source for annealing. The belt-shaped laser used has an energy distribution that produces a temperature distribution C in the direction B of orientation of 1° of a crystal unidirectionally solidified by laser melting.

For this reason, a slit laser whose width is wider than the width of the crystal is used as the band pattern laser. The laser power used is smaller than that used for laser melting. This allows heating and oxygen absorption without disturbing the orientation of the crystal.

By scanning such a band-shaped laser multiple times in the crystal orientation direction B under a predetermined oxygen partial pressure, the crystal absorbs oxygen and performs oxygen annealing. The scanning speed is the same as that during melting and recrystallization, but under controlled oxygen partial pressure it is necessary to scan multiple times. For example, when the oxygen partial pressure is 0.2 atm, scanning is performed approximately 500 times.

By absorbing oxygen in a belt shape with a temperature distribution in the crystal orientation direction, the crystal orientation is maintained and a highly oriented oxide superconductor can be obtained.

Example 1 A beam with an output of 10W and a half beam was placed on the upper surface of a long thin film of YBCO superconducting material with a thickness of 1 μm and a width of 80 μm. It was irradiated with a beam with a diameter of 50 μm at a scanning speed of 5 cm/sec, recrystallized, and then laser-turned (with a diameter of 150 μm).
Forced cooling areas were established by spraying liquid nitrogen at intervals of .

Controllability of a-b axis orientation and critical current density Jc in this case
The results of the values (OT, 77K) are shown in Table 1.

As a comparative example, Table 1 shows the results of using a single beam (without forced cooling (Comparative Example 1) and when melting and crystallizing in a furnace with a temperature gradient of 30"C/cm (Comparative Example 2). .

As is clear from Table 1, by providing a forced cooling region, both the a-b axis orientation and controllability are excellent, and a high Jc value can be obtained.

Table 1 Example 2 The recrystallized film obtained in Example 1 was subjected to oxygen annealing by scanning a 1 mm x 100 μm strip laser pattern 500 times with an output of 5 W and a scanning speed of 10 cm/see under an oxygen partial pressure of 02° atmospheric pressure. I did it. On the other hand, as Comparative Example 3, uniform annealing was performed at 400° C. under the same oxygen partial pressure, and maintenance of orientation and critical current density J c (A/cry?) at an external magnetic field of 0 T (77 K) were compared.

Table 2 As is clear from Table 2, according to the annealing method of the present invention, oxygen annealing could be performed without impairing crystal orientation, and high Jc values could be obtained.

[Effects of the Invention] As is clear from the above examples, according to the method for manufacturing an oxide superconductor according to the present invention, by providing a forced cooling region close to the laser irradiation part of the elongated member, It is possible to form a steep temperature gradient aligned in the scanning direction on the sample, and the crystal direction can be easily controlled and high unidirectional solidification is possible.

In addition, according to the method for manufacturing an oxide superconductor of the present invention, oxygen annealing is performed by scanning a strip laser when unidirectionally solidified crystals are annealed with oxygen, so that the crystals remain intact even after oxygen annealing. It is possible to maintain the orientation of the oxide superconductor and obtain a highly unidirectionally solidified oxide superconductor.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the state of laser melting in the method for producing an oxide superconductor according to the present invention, FIGS. 2 and 3 are schematic cross-sectional views showing the forced cooling method, and FIG. 4(a)
, (b) are a plan view showing oxygen annealing and a diagram showing temperature distribution in the method for producing an oxide superconductor of the present invention, respectively. 1... Oxide superconducting material (sample) 1a... Irradiated portion 2... Substrate 3... Laser beam 1°... Oriented crystal 4... Forced cooling zone band pattern B...・Orientation direction...Scanning direction

Claims (2)

[Claims] 1. In recrystallizing or crystallizing a long material made of an oxide superconducting substance or a substance that generates an oxide superconducting substance by melting and solidifying the material, the recrystallized or crystallized material is continuously irradiated with laser light. 1. A method for producing an oxide superconductor, characterized in that the superconducting material is immediately forcedly cooled to increase the temperature gradient in the axial direction. 2. Recrystallized or crystallized oxide superconducting materials are
2. The method for manufacturing an oxide superconductor according to claim 1, wherein oxygen annealing is performed in an oxygen atmosphere after cooling by repeated irradiation with a laser beam having a strip pattern parallel to a direction perpendicular to the scanning direction of the laser beam.