JPH06316496A - Production of oxide superconducting material - Google Patents

Abstract

PURPOSE:To obtain a long-sized bulk material by bringing the faces of plural seed crystals of REBa2Cu3Ox (RE is Y, Sm, etc.) having uniform crystal orientation into contact with a mixed compact of the raw-material oxides having a lower peritectic temp. than the seed crystal and growing the crystal. CONSTITUTION:A superconducting material of REBa2Cu3Ox is produced as follows by utilizing melting and crystal growth. The mixed compact 2 of the raw oxide of REBa2Cu3Ox (RE 1) for crystal growth is heated at a temp. higher than the peritectic temp. of RE 1. One face of the plural seed crystals (e.g. 1A and 1B) of REBa2Cu3Ox (RE 2) having a higher peritectic temp. than RE 1 and having uniform crystal orientation is brought into contact with the plural regions on the compact 1 of RE 1 between the peritectic temps. of RE 1 and RE 2 and slowly cooled close to the peritectic temp. of RE 1. Consequently, a crystal is grown from the plural regions, and a bulk material 3 having uniform crystal orientation and free of a large-inclination grain boundary is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a method for producing a large-sized superconducting bulk material which is an oxide superconducting material and has a high critical current density.

[0002]

REBa ₂ Cu ₃ O _x type oxide superconductors have a high critical temperature exceeding the temperature of liquid nitrogen, so that they have great economic merit when put into practical use. However, since the crystal grain boundaries act as weak bonds, a high superconducting current cannot flow across the grain boundaries, and the critical current density decreases especially in a magnetic field, so that practical application in the field of strong electric power has not been realized. It has been known that the critical current density in the crystal grains of REBa ₂ Cu ₃ O _x is relatively high, and if it is possible to manufacture a bulk material without grain boundaries that act as weak bonds, a magnet that generates a strong magnetic field, a strong magnet It can be used as a shield material that shields a magnetic field or a conductive material that allows a large current to flow.

QMG method (Japanese Patent Publication No. 40289/1992)
The melting method typified by is REBa ₂ Cu with large crystal grains.
Research and development of this process is underway in a way that allows the production of ₃ O _x bulk. Figure 2 shows REBa ₂
Cu is a pseudo binary phase diagram of the ₃ O _x. At a high temperature, this substance decomposes into a RE ₂ BaCuO ₅ phase and a liquid phase mainly composed of Ba, Cu, O, and when cooled from this semi-molten state, REBa ₂ Cu ₃ O _x is produced by a peritectic reaction. The melting method is a method for producing large crystal grains by gradually cooling near the peritectic temperature. However, even if annealed, crystal nuclei are generated from all locations, and eventually become polycrystalline.

In order to solve this problem, the inventors of the present invention utilize the fact that the peritectic temperature changes when the element substituting for RE is changed, so that REBa ₂ Cu ₃ O _x having a high peritectic temperature is used as a seed crystal. As a result, a technology for controlling nucleation is being developed (Japanese Patent Application Nos. 3-162360 and 4-55203). By this technique (hereinafter referred to as the seeding method), it is possible to manufacture a large bulk material of several tens cm ³ which has a controlled crystal orientation and does not have a large tilt grain boundary that lowers the critical current density.

However, since the process basically uses crystal growth, strict temperature control is required, and composition shift may occur during crystal growth. Is limited. In particular, when this material is applied to a current lead or a magnetic shield plate, it is necessary to flow a high superconducting current in a large area, and the current crystal grain size is not sufficient.

[0006]

Therefore, it is an object of the present invention to develop a method for producing a larger bulk material in an oxide superconducting material, which does not have a grain boundary that causes a weak bond.

[0007]

In order to solve the above problems, the present invention causes REBa ₂ Cu ₃ O _x to undergo crystal growth.
A mixed / formed body of raw material oxides (hereinafter referred to as RE1) is heated to a temperature higher than the peritectic temperature of RE1, and a plurality of REs having a peritectic temperature higher than RE1 and having a uniform crystal orientation are prepared.
Mixing one crystal face of a Ba ₂ Cu ₃ O _x (hereinafter referred to as RE 2) seed crystal at a temperature between the peritectic temperatures of RE 1 and RE 2 and aligning the crystal orientations of the respective seed crystals with the raw material oxide of RE 1. -Crystals having controlled crystallographic orientations are generated from the plurality of locations by bringing them into contact with a plurality of locations on the molded body and gradually cooling in the vicinity of the peritectic temperature of RE1, and crystallizing by gradually cooling this. It is a means to grow. The above RE is Y, La, Nd, Sm, E.
It means at least one element selected from the group consisting of u, Gd, Dy, Ho, Er, Tm, Yb and Lu.

[0008]

In the present invention, if the crystal orientations of the seed crystals are not aligned, the crystal lattice shifts between RE1s, that is, grain boundaries are generated between RE1s and weak bonds are formed to lower the critical current density. Will end up. Therefore, RE2 must be oriented and free of weak bonds, and their crystal orientations must be aligned. Results of measurement of critical current densities in thin film bicrystals (see, for example, D. Dimos et al. Physica).
l ReviewB Vol. 41 (1990) 4038
It is desirable that the crystal orientation difference in the c-axis and the a-axis / b-axis directions be within 15 degrees in view of the page. Where c
The axis is the longest axis of the unit cell of REBa ₂ Cu ₃ O _x having an orthorhombic structure, and the a axis and the b axis are the other two axes. If these axes are further displaced, a bulk having a high critical current density, which is a feature of the present invention, cannot be obtained.

The REBa ₂ Cu ₃ O _x superconductor is an orthorhombic crystal, but microscopically has a twin structure, and the a-axis and the b-axis are deviated from each other by 90 degrees, and macroscopically observed. No distinction is made between a-axis and b-axis. A twin boundary whose crystal axis is shifted by 90 degrees does not become a weak bond, but when it is deviated from these axes by an intermediate angle (hence 45 degrees at the maximum), this part becomes a weak bond even if the c axes are aligned. turn into. It is desirable that the difference between the crystal orientations in the a-axis and b-axis directions is within 15 degrees, that is, RE2 seed crystals have a twin structure and the a-axis and the b-axis are macroscopically mixed even if they are deviated by 90 degrees. However, it is desirable that the deviation from these axes is within 15 degrees. Hereinafter, the a-axis and the b-axis which are not macroscopically distinguished are referred to as a-b-axis.

The cooling rate near the peritectic temperature depends on the distance between the seed crystals and the temperature gradient in the furnace, but the maximum growth rate of this material is 2 mm / h while maintaining the crystal orientation in the semi-molten state.
However, the cooling rate must be set so that the growth rate is not higher than this.

The raw material used in this manufacturing method is REBa.
_It does not have to be in the form of ₂ Cu ₃ O _x , and the composition does not have to be a stoichiometric composition if crystal growth is possible, and impurities may be contained. Further, the seed crystal has to be oriented, but it does not have to have a stoichiometric composition and may contain impurities.

In FIG. 1, a combination of elements of RE1 and RE2 is selected, and the planes of two seed crystals 1A and 1B (RE2) having the same crystallographic orientation are R having a lower peritectic temperature than pyrolyzed RE2.
Is a schematic diagram when EBa was ₂ Cu ₃ O _x (RE1) in contact with mixing and molding 2 of the raw material oxide by crystal growth. As shown in the figure, RE1 inherits the crystal orientation of RE2 for crystal growth, and the growth surfaces of the two crystals are in contact with each other. Since there is no difference in crystal orientation between the contact surfaces of the two crystals, weak junction does not occur and a large critical current density can be obtained. That is, the two crystals can be regarded as substantially one crystal grain 3, and by making such seeding at two or more places, it is possible to obtain a larger R-angle without a large-angle grain boundary than conventionally obtained.
It enables the production of EBa ₂ Cu ₃ O _x superconducting bulk material.

[0013]

EXAMPLE Using SmBa ₂ Cu ₃ O _x as a seed crystal, YB
The production of a ₂ Cu ₃ O _x was carried out. The SmBa ₂ Cu ₃ O _x crystal used as the seed crystal is Sm ₂ O ₃ powder, Ba
O ₂ powder and CuO powder were weighed so that the element ratio of Sm, Ba, and Cu was 1.2: 1.8: 2.6, 0.5% by weight of Pt powder was added and kneaded, and then oxygen was added. 900 in the airflow
Calcination at 10 ° C for 10 hours, crushing again, molding with a press molding machine to a thickness of 10 mm using a die with a diameter of 28 mm, and further isostatic molding at a pressure of ² t / cm ² were used as raw materials. And This was heated to 1150 ° C., kept at this temperature for 1 hour, cooled to 1070 ° C. in 1 hour, and then gradually cooled to 1030 ° C. at 1 ° C./h to prepare a bulk body.

The crystal grains of this bulk body are large enough to cut out seed crystals without grain boundaries, and the diameter from this is 5 mm.
A plurality of seed crystals having a thickness of 1 mm were cut out by using the cleavage plane (ab plane) so that the thickness direction of the disk was the c-axis direction. When this structure was observed with a polarization microscope, S
It was found that the m ₂ BaCu ₃ O ₅ phase was dispersed, but the matrix was SmBa ₂ Cu ₃ O _x with uniform orientation. The a and b axes were judged from the observed twinning directions.

The raw material of YBa ₂ Cu ₃ O _x is Y ₂ O ₃ powder, BaO ₂ powder and CuO powder such that the element ratio of Y, Ba and Cu is 1.3: 1.7: 2.4. Weigh, add 0.5% Pt powder by weight ratio, knead, calcinate at 800 ° C. for 10 hours in an oxygen stream, pulverize again, and use a press molding machine with a die having a diameter of 80 mm. 15m thick
It was used after being molded to m and further subjected to hydrostatic molding at a pressure of ² t / cm ² . The Pt powder is for promoting the precipitation of the fine dispersed phase which raises the critical current density.

This is heated to 1150 ° C. and at this temperature 1
Held for hours. Then, the mixture was cooled to 1030 ° C. in 30 minutes, and at this temperature, the planes of the two SmBa ₂ Cu ₃ O _x seed crystals were aligned with the crystal orientations and symmetrically from the center of the plane of the raw material, as shown in FIG. They were brought into contact with each other with a distance of about 1 cm. After that, the temperature was cooled to 1005 ° C. in 30 minutes, further gradually cooled to 960 ° C. at an average rate of 0.5 ° C./h, and furnace cooled to room temperature. After that, in order to give superconductivity, annealing was performed at 450 ° C. for 100 hours in an oxygen stream.

As shown in the schematic view of FIG. 3, the sample thus produced has surface-grown on almost the entire surface of the sample (3A, 3B) starting from seed crystals 1A, 1B, and at the center thereof. It was visually observed that the two growth surfaces were in contact with each other. A state of a sample cross section 5 centering on a surface 4 (hereinafter, contact surface) where two crystals grow and contact each other was observed with a polarization microscope. The observation result of this microscope is shown in FIG. Organization is ab
Cracks 11 that are almost parallel to the plane, twin pattern (not shown in FIG. 4 because it is fine), voids 12 and Y ₂ BaCuO ₅ phase dispersed to 2 μm or less (because it is fine, it is shown in FIG. 4). However, the tissue was joined so that the contact surface 4 was indistinguishable by the contrast of the polarization microscope. From the crack and the twinning pattern, the crystals on both sides of the contact surface inherit the orientation of the seed crystal,
It turned out that they were oriented in the same crystal orientation.

In order to investigate the transport characteristics of this contact portion,
As shown by the dotted line in Fig. 3, the length is 15m centering on the contact area
A sample 6 having an area of m and a cross-sectional area of 1 mm ² was cut out, voltage terminals were set on both sides of the contact surface, and the critical current density was measured by a direct current 4-terminal method. As a result, in the magnetic field of 77K, 1T (the magnetic field is c
It was found to have a high critical current density of about 10,000 A / cm ² (parallel to the axis).

[0019]

As described above, it is possible to manufacture a bulk material having a large critical current density even in a magnetic field by aligning the crystal orientation and seeding at a plurality of locations.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a bulk superconducting material produced by seeding seed crystals with uniform crystal orientation at two locations and growing the crystals.

FIG. 2 Phase diagram of pseudo binary system near the REBa ₂ Cu ₃ O _x phase

FIG. 3 is a schematic diagram showing the positions of the bulk material and the seed crystal prepared in the example, the position of the sample from which the critical current density was measured, and the position of the surface observed by a microscope.

FIG. 4 is a schematic diagram showing the results of observation with a polarizing microscope observed in Examples.

[Explanation of symbols]

 1A, 1B seed crystal 2 mixed / formed body of raw material oxide 3, 3A, 3B crystal 4 surface where crystal grows and contacts 5 sample cross section 6 sample 11 crack 12 void

Claims (1)

[Claims] 1. REBa ₂ C utilizing melting and crystal growth
u ₃ O _x system (RE is Y, La, Nd, Sm, Eu, G
d, Dy, Ho, Er, Tm, Yb, and Lu) in a method for producing a superconducting material) of REBa ₂ Cu ₃ O _x (hereinafter, referred to as RE 1) for performing crystal growth. A plurality of REBa ₂ C having uniform crystal orientation and having a peritectic temperature higher than RE1 is prepared by heating the mixed / formed body of the raw material oxides to a temperature higher than the peritectic temperature of RE1.
u ₃ O _x (hereinafter referred to as RE2)
By aligning the crystal orientation at a temperature between the peritectic temperatures of E1 and RE2 and bringing them into contact with a plurality of locations on the mixed / formed body of the raw material oxide of RE1, and gradually cooling near the peritectic temperature of RE1, A method for producing an oxide superconducting material having a large tilt angle grain boundary in which crystal orientations are uniform, which is characterized in that crystals are grown from a plurality of locations.