JPH02258697A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain the title superconductor with its crystal unidirectionally orientatively solidified by placing a band cooling member on the central part of a continuous member when said continuous member made of an oxide superconducting substance is to be irradiated with laser beams to effect recrystallization to initiate the solidification from the central part and form a sharp temperature gradient in the scanning direction. CONSTITUTION:A thin film 1 of an oxide superconducting substance or a substance capable of forming the same by melting and sintering on a continuous base 2. The film 1 is then irradiated with laser beams 8 approximately perpendicularly; during the process, a scanning is made in the arrow A direction to heat and melt the irradiated zone 1a, and as the laser beams 3 move, the melted zone is cooled from behind to effect recrystallization. In this case, a band cooling member 4 such as a metal piece is placed on the central part of the laser beam irradiation side of the film 1 to form a negative temperature gradient for the center of the laser beams against both the sides. Thus, crystal with orientation can be grown in the scanning direction with nearly a single phase from the center of the laser beams.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for producing oxide superconductors such as YBCO-based or B1-based oxide superconductors. 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 (aSb plane), and the characteristics of oxide superconductors are a, b
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 form 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. Aligned or not. Therefore, there was a problem that the orientation could not be controlled.

Furthermore, YBCO-based oxide superconductors generally have the disadvantage that when they are melted and then sintered, they become an insulating so-called 211 structure, resulting in no 123 structure.

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 substance by sintering with a laser beam, a belt-shaped cooling member is placed in the center of the long material, and a cooling member is placed in the center of the long material. A temperature gradient is formed that increases from the center toward both ends.

In this case, unidirectional solidification can be more fully achieved by immediately forcibly cooling the oxide superconducting material after recrystallization or crystallization.

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 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 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 material or a material that produces an oxide superconducting material by melting and sintering. .

Superconducting materials such as Cu oxides made into pellets 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 a laser sputtering method using an excimer laser, a CVD method, or a spray pyrolysis method can be used. can.

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, if a metal substrate is used as the substrate, the substrate 2 should be forcibly cooled after laser irradiation to prevent reaction between the substrate surface and the sample 1, and also to prevent radial thermal diffusion from occurring during cooling. There is a need.

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.

A metal piece 4 is arranged at the center of the upper surface of the sample 1, that is, on the laser beam irradiation side. This metal piece can also be embedded integrally with the sample 1 on the surface side and/or inside thereof.

In this case, since metal has a much higher thermal conductivity than oxides, the heat in the center of the irradiated area 1a of the laser beam 3 is released along the metal piece, as shown in FIG. A temperature gradient is formed that increases from the center outward. In other words, the laser beam A steep temperature gradient can be generated in the scanning direction while the center of the beam maintains a negative temperature gradient with respect to both sides. It is possible to grow crystals with This allows arbitrary nucleation from both sides,
Crystal growth in a direction having a component perpendicular to the scanning direction is prevented.

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,
~10 cm/sec.

In the area heated and melted by irradiating the sample 1 with such a laser beam 3, a temperature gradient is generated in the direction of movement due to the relative movement of the laser beam.
Cooling starts from the rear and crystallizes. At this time, since the central part of the beam is cooled more preferentially than both ends thereof, the temperature gradient caused by the cooling is approximately parallel to the direction of movement.

Since crystallization progresses along this temperature gradient, crystals are formed in this region that are substantially single-phase and unidirectionally solidified in the scanning direction.

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 onto the surface of the sample 1 and/or the substrate 2 using a nozzle or the like, or forced cooling is performed by immersing them in a cooling tank containing these liquids.

The forced cooling area 20 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 20 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 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
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 30 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 for melting and sintering, but under controlled oxygen partial pressure, it is necessary to scan multiple times. For example, when the oxygen partial pressure is 0.2 atm, the voltage is changed about 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 metal thin piece with a width of 50 μm was placed on the upper surface of the central part of a long R film of YBCO-based superconducting material with a thickness of 1 μm and a width of 100 μm,
Irradiation was performed using a beam with an output of 10 W and a beam radius of 50 μm at a scanning speed of 5 cm/sec for recrystallization.

Controllability of a-b axis orientation of recrystallization ratio phase and external magnetic field OT,
Critical current density J c (A
/cffl) when using a single beam (one round beam) without using a metal flake (Comparative Example 1) and when melting and crystallizing in a furnace with a temperature gradient of 30 ° C / cm (Comparative Example 2) compared with the results of

The results are shown in Table 1.

Example 2 The same sample and the same laser as in Example 1 were used, and forced cooling zones were provided at intervals of 150 μm with respect to the laser pattern by spraying liquid nitrogen. Table 1 also shows the results of controllability of a-b axis orientation and critical current density Jc value in this case.

As is clear from Table 1, by using metal flakes, both the orientation and controllability of the a-b axes are excellent, and the high Jc
The results were even better with the forced cooling zone.

Below is a blank table 1 Example 3 The recrystallized film obtained in Example 1 was subjected to oxygen annealing by scanning a strip laser pattern of 1 mm x 50 μm with an output of 5 W and a scanning speed of 10 cm/sec 500 times under an oxygen partial pressure of 0.2 atm. I did this. On the other hand, as a comparison N3, uniform annealing was performed at 400° C. under the same oxygen partial pressure, and maintenance of orientation and critical current density Jc (A/CM) 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 producing an oxide superconductor according to the present invention, a strip-shaped cooling member is arranged in the center of the elongated member, and solidification is performed from the center. Since the temperature gradient is started on the sample, a steep temperature gradient aligned in the scanning direction can be formed 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 manufacturing an oxide superconductor according to the present invention, and FIGS. 2(a) and (b)
) is a diagram showing the temperature distribution of the laser beam heating area, Figure 3 is a diagram showing the laser beam pattern and forced cooling area, and Figure 4 (
a) and (b) are a plan view and a diagram showing temperature distribution, respectively, showing oxygen annealing in the method for manufacturing an oxide superconductor of the present invention. 1...Oxide superconducting material (sample) 4...Metal piece 1'...Oriented crystal 20...Forced cooling zone 30...Striped pattern B...Orientation direction A... scanning direction

Claims (1)

[Claims] 1. When recrystallizing or crystallizing a long material made of an oxide superconducting material or a material that produces an oxide superconducting material by melting and sintering with a laser beam, 1. A method for producing an oxide superconductor, comprising arranging a band-shaped cooling member in the center of the material to form a temperature gradient that increases from the center toward both ends. 2. Recrystallized or crystallized oxide superconducting material is
The method for producing an oxide superconductor according to claim 1, wherein the oxide superconductor is immediately forcedly cooled. 3. Recrystallized or crystallized oxide superconducting material is
3. The method for manufacturing an oxide superconductor according to claim 1, wherein after cooling, oxygen annealing is performed in an oxygen atmosphere by repeated irradiation with a laser beam having a strip pattern parallel to a direction perpendicular to the scanning direction.