JPH02258694A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain the title superconductor with its crystal unidirectionally orientative solidified by melting and recrystallizing an oxide superconducting substance through irradiation with laser beams having a band pattern in the direction rectangular to 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 is formed on a continuous base 2. The thin film 1 is then irradiated with laser beams 3 approximately rectangularly; during the process, a scanning is made in the arrow A direction to heat and melt an irradiated zone, and as the laser beams move, the melted zone is cooled and recrystallized. Said laser beams 3 have a band pattern, and the cooling is made in such a manner that a forcedly cooling zone due to e.g. liquid nitrogen is provided immediately after the laser beam pattern. Thence, the band laser is plurally scanned in the direction of crystal orientation under a specified oxygen partial pressure to carry out oxygen annealing.

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 higher than in the horizontal plane (a, b plane), and the characteristics of oxide superconductors are
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. However, in this case, the center of the irradiated area is at its highest temperature, so a temperature gradient forms radially from the center, and even though the laser beam is scanned in one direction,
Crystal orientation is not aligned in one direction. 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 material that generates an oxide superconducting material through sintering by irradiating it with laser light, a laser light having a strip pattern parallel to a direction perpendicular to the scanning direction is used as the laser light. .

In this case, solidification in the -direction 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 six directions of arrows while irradiating the sample 1 with a laser beam 3 approximately at 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, and the latter material that produces an oxide superconducting material by melting and sintering is, for example, YSBa.

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 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.

The laser beam 3 used is a beam having a strip pattern formed by an optical device using a laser source 10, a single lens 11, a cylindrical lens 12, and a mirror 13 as shown in FIG. This beam 3 is irradiated parallel to a direction xSx' perpendicular to the scanning direction A, as shown in FIG. As such a laser beam,
Although it varies depending on the film thickness of the sample, normally, for example, in the case of a YAG laser, one with an output of 40 to 50 W or more is used.

The scanning speed of the laser beam is 1 mm x 50 mm.
In the case of μm, it is approximately 5 cm71 sec.

By irradiating the sample 1 with the laser beam 3 having such a shape, the heated and melted region will generate a temperature gradient in the direction of movement due to the relative movement of the laser beam, and will begin to cool down sequentially from the rear. crystallize. Since crystallization progresses along this temperature gradient, crystals are formed that are substantially single-phase and are 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 region 20 is preferably provided 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 shape diameter, etc., but it is approximately 1
A value of about mm is appropriate.

In this way, the forced cooling zone 20 is provided, and by forcibly cooling the temperature gradient in the laser scanning direction is increased, crystallization is increased, and arbitrary nucleation and solidification are prevented and the desired orientation is achieved. Crystallization can be achieved.

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 211 phases can be crystallized using ordinary melting and solidification methods, but 123 phases can be stably crystallized using the method of the present invention.

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. 5(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 orientation direction B of the crystal 1, which is 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 orientation direction B of the crystal under a predetermined oxygen partial pressure, oxygen is absorbed into the crystal and oxygen annealing is performed. 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 with a band-shaped beam having a temperature distribution in the crystal orientation direction, the crystal orientation is maintained and a highly oriented oxide superconductor can be obtained.

Example 1 Long YB formed into a substrate shape with a thickness of 1 μm and a width of 400 μm
Output 50 W 1 beam shape 1 r on a thin film of CO-based superconducting material
Scanning speed 5 cm/ using a beam of nm x 50 μm
sec to recrystallize. At this time, a forced cooling area was provided parallel to the beam at an interval of 1 mm.

Controllability of a-b axis orientation and critical current density J for recrystallization
c (A/crtr) value (external magnetic field 0T177K (
The same applies hereinafter)) was compared with the case (Comparative Example 1) using a single beam (one round beam).

The output power for this single beam is 5W, and the beam radius is 5
Scanning was performed multiple times at 0 μm.

Table 1 Example 2 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 am/sec 500 times under an oxygen partial pressure of 0.2 atm. Ta. On the other hand, as Comparative Example 2, uniform annealing was performed at 400° C. under the same oxygen partial pressure, and maintenance of orientation and critical current density Jc (A/C Bo) 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 laser having a specific beam shape is used as a heat source for melting the sample, and the laser is applied to the sample. On the other hand, since scanning is performed at high speed, 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 producing an oxide superconductor according to the present invention, FIG. 2 is a schematic diagram of a laser beam generator having a specific beam shape, and FIG. Figure 4 shows the heating area, Figure 4 shows the laser beam pattern and forced cooling area, Figure 5 (a), (b)
These are a plan view and a diagram showing temperature distribution, respectively, showing oxygen annealing in the method for producing an oxide superconductor of the present invention. 1... Oxide superconducting material (sample) 3... Laser light 1°... Oriented crystal 30... Band-shaped pattern B... Orientation direction A... Scanning direction

Claims (1)

[Claims] 1. When recrystallizing or crystallizing an oxide superconducting material or a material that generates an oxide superconducting material by melting and sintering a laser beam, the laser beam is perpendicular to the scanning direction. 1. A method for producing an oxide superconductor, characterized in that a laser beam having a strip pattern parallel to the direction is used. 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.