JPH10310498A - Oxide superconducting bulky material and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a superconductor having a high rate of single crystallization by coating the surface of a compact of starting material for an REBa2 Cu3 Ox superconductor with a material reducing the peritectic temp. of the REBa2 Cu3 Ox , heating the coated compact to the peritectic temp. of the REBa2 Cu3 Ox or above and cooling it. SOLUTION: The material reducing the peritectic temp. of the REBa2 Cu3 Ox (RE is Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb) is a noble metal such as silver or gold, the RE of REBa2 Cu3 Ox having a lower peritectic temp. than the REBa2 Cu3 Ox to be produced or a compd. contg. the RE. The coating is carried out, e.g. by applying paste prepd. by dispersing the material in an org. solvent or adopting a vapor phase method using the material such as sputtering or vapor deposition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a method for manufacturing a large bulk superconductor used for a superconducting bulk magnet, a current lead, a magnetic levitation, a current limiter, and the like.

[0002]

_2. Description of the Related Art Rare earth superconductors represented by YBa ₂ Cu ₃ O _x (represented as REBa ₂ Cu ₃ O _x , where RE has a higher magnetic flux pinning force than other oxide superconductors, In particular, since the critical current density is high even at a high temperature close to the liquid nitrogen temperature (77 K), its use is expected, but the superconductor has a crystal grain boundary that significantly lowers the critical current density. In the present technology, as a method for producing a rare earth-based superconductor having a crystal orientation, a vapor phase method and a melting method in which a film is formed on a substrate having a close lattice constant are cited.

[0003] The QMG method (patent registration number 0186988)
4, and a melting method typified by JP-A-5-193938), once the temperature is raised to a temperature range in which a RE ₂ BaCuO ₅ phase and a liquid phase containing Ba-Cu-O as a main component coexist. REBa ₂ Cu ₃ O _x
In this method, the crystal is grown to a temperature just above the peritectic temperature at which is generated, and the crystal is grown by gradually cooling from this temperature to obtain large crystal grains. In particular, JP-A-5-193 described below
The seeding method of growing a crystal using a seed crystal having a high peritectic temperature disclosed in US Pat.
A bulk superconducting material having the above crystal grains can be manufactured.

In the seeding method, an RE ² Ba ₂ Cu ₃ O _x single crystal sample having a higher melting point (peritectic temperature) than the RE ¹ Ba ₂ Cu ₃ O _x -based superconductor to be produced is used as a seed crystal. . RE ¹ Ba
The feed precursor of ₂ Cu ₃ O _x superconductor is heated to an intermediate temperature of peritectic temperature of RE ¹ Ba ₂ Cu ₃ O _x of peritectic temperature and _{^{RE 2 Ba 2 Cu 3 O x}} ,
RE ¹ Ba ₂ Cu ₃ O _x is decomposed to form a coexistence state of the RE ¹ ₂ BaCuO ₅ phase and the liquid phase containing Ba-Cu-O as a main component, and the precursor thereof is RE ² Ba ₂ Cu ₃ O _x
One side of the crystal is brought into contact. RE ² Ba ₂ Cu ₃ O _x is stable because its peritectic temperature is higher than that temperature. After that, it is cooled to the peritectic temperature of RE ¹ Ba ₂ Cu ₃ O _x to generate RE ¹ Ba ₂ Cu ₃ O _x , but by gradually cooling it near the peritectic temperature, RE ² Ba ₂ Cu ₃ O _x
The crystal is grown in the same orientation as the crystal orientation of the contact surface.

[0005] Table 1 shows the peritectic temperature of the RE ¹ Ba ₂ Cu ₃ O _x by a typical RE ion replacing RE1. Peritectic temperature replace
It varies considerably depending on the RE ion and generally correlates with the ionic radius. As the ionic radius increases, the peritectic temperature increases. Further, RE1 does not need to be one kind of ion, and the peritectic temperature of RE ¹ Ba ₂ Cu ₃ O _x is an average peritectic temperature according to the ratio of the ions constituting RE1. Therefore, the combinations of RE ¹ Ba ₂ Cu ₃ O _x and RE ₂ Ba ₂ Cu ₃ O _x seed crystals to be produced can be infinite.

[0006]

[Table 1]

FIG. 1 shows REBa ₂ Cu ₃ prepared by the seeding method.
Although a schematic diagram of O _x is shown, it is known that the crystal grows toward the outside centering on the seed crystal.
When the crystal orientation is uniform, a crystal habit that is also observed in a silicon single crystal is observed. In the case of such a sound bulk, since there is no crystal grain boundary that reduces the critical current density inside, a superconducting current of 10,000 A / cm ² or more can be applied to the entire bulk. Such bulk superconductors include superconducting bulk magnets, current leads,
Attention has been paid to materials used for magnetic levitation, current limiting devices and the like. In these applications, the larger the bulk, the better the properties and the greater the economic value.

However, as can be inferred from the widely known production of single crystal silicon, the larger the bulk to be produced, the more difficult it is to produce single crystal grains. In the REBa ₂ Cu ₃ O _x bulk manufacturing process,
One of the major problems is nucleation from parts other than the seed crystal. FIG. 2 shows an example in which a nucleus is generated from a portion other than the seed crystal, and is divided into three crystal grains having different crystal orientations. The cause of polycrystalline grain formation is considered to be uneven temperature distribution in the electric furnace and contact / contamination of some inclusions that may cause nucleation, but the starting point of nucleation is often the bulk surface. . The probability of polycrystallization depends on the performance of the electric furnace, but increases as the bulk size increases.

When polycrystallized, a large superconducting current cannot flow at the crystal grain boundaries of each crystal, so that the performance of superconducting bulk magnets, magnetic levitation, current limiters, etc. is significantly reduced. Also, when cut out and used for a current lead or the like, the yield is reduced, so that the economy is low.

[0010]

Accordingly, there is provided a method for producing REBa ₂ Cu ₃ O _x large bulk superconducting material having a high single crystallization rate.

[0011]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems. In order to avoid nucleation from a surface portion other than a seed crystal, the present invention is directed to a peritectic crystal of REBa ₂ Cu ₃ O _x . A means is provided for suppressing nucleation from the surface by lowering the peritectic temperature of the surface by coating a substance that lowers the temperature.

As a substance for lowering the peritectic temperature of REBa ₂ Cu ₃ O _x , there are noble metals such as silver and gold, and RE elements. By applying these to the surface, generation from the surface during cooling is suppressed, and as a result, the production of a large superconducting bulk material having a single crystal grain is facilitated.

[0013] Silver and gold are based on the raw material powder of REBa ₂ Cu ₃ O _x .
When each simple powder is added, 5 per weight%
To lower the peritectic temperature by 10 ° C. Gold and silver are expected to have the effect of improving heat conduction, and can also be expected to have the effect of improving the uniformity of the sample during production. Also, when used directly as a bulk superconducting material, when cooled to liquid nitrogen, etc.,
The whole bulk is uniformly cooled, and the effect of suppressing cracks due to heat shrinkage and the like can be expected. Further, when an electrode is attached to this material and used for current application such as a current lead, the fact that a silver or gold component is contained on the surface is advantageous for reducing the contact resistance.

The silver, gold or RE to be applied does not necessarily have to be a single substance, but may form a composite compound with an oxide or barium or copper. On the other hand, since the substitution of the RE element changes the peritectic temperature as shown in Table 1, RE having a peritectic temperature lower than that of REBa ₂ Cu ₃ O _x to be prepared.
You just have to select

The method of applying silver, gold,
A method of applying paste by dispersing RE element or a compound powder containing these in an organic solvent, and coating with silver, gold, a RE element or a compound containing these elements as a raw material by a gas phase method such as sputtering or vapor deposition. A method can be considered. The coating is desirably applied to the entire surface other than the portion where the seed crystal is brought into contact.

Gold, silver or RE to be produced on the surface
By coating a layer containing an RE element having an ionic radius smaller than that of RE in Ba ₂ Cu ₃ O _x , nucleation from the surface in a semi-molten state during production is suppressed. Since nucleation from parts other than the seed crystal is suppressed, it is easy to manufacture REBa2Cu3Ox large bulk superconductor having single crystal grains.

[0017]

Example 1 Production of a 90 mmF YBa ₂ Cu ₃ O _x large bulk superconductor was attempted. As raw material powder, Y ₂ O ₃ , BaO ₂ , CuO is Y, Ba,
Weigh so that the ratio of Cu is 1.3: 1.7: 2.4, and add 0.5
A powder which was added with wt% platinum, kneaded, calcined at 870 ° C. in an oxygen stream, and pulverized was used. This is finally YBa ₂
The composition is such that 30 mol% of the Y ₂ BaCuO ₅ phase remains in the Cu ₃ O _x bulk. This powder, 30mm in height using a 110mmΦ mold
And then subjected to isostatic pressing at a pressure of ² ton / cm ^{2 to} obtain a raw material molded body. Two molded bodies were prepared, and one of them was coated with a slurry of a commercially available silver paste diluted with acetone. As shown in FIG. 3, the coating was performed on the entire surface except the portion where the seed crystal was placed.

These raw material compacts were subjected to crystal growth heat treatment using the same box-type electric furnace. First 1160
After heating to ℃ for 30 minutes and cooling to 1005 ℃ for 1 hour. 3m square SmBa ₂ C at 1030 ° C
A seeding operation was performed to bring the cleavage plane (ab plane) of u ₃ Ox into contact with the upper surface of the molded body in a semi-molten state. Then 960 ° C
Then, the mixture was gradually cooled at a cooling rate of 0.3 ° C./h, and furnace-cooled from this temperature to room temperature in 8 hours.

Observation of the prepared YBa ₂ Cu ₃ O _x bulk shows that the silver-coated one grows entirely from a seed crystal as shown in FIG. 1 and the c-axis is in the height direction of the cylinder. Was formed. On the other hand, for those not coated with silver, nucleation occurred from the periphery as shown in FIG. The nucleation is presumed to have occurred from the surface based on the form of the crystal, and it is presumed that the nucleation from the surface was suppressed in the silver-coated sample.

[0020]

Example 2 An attempt was made to produce a 45 mmF ErBa ₂ Cu ₃ Ox large bulk superconductor. As raw material powders, Er ₂ O ₃ , BaO ₂ , CuO are Y, B
a, weigh so that the ratio of Cu is 1.2: 1.8: 2.6,
Add 0.5 wt% platinum, knead, 870 ° C in oxygen stream
The powder calcined and pulverized was used. This is finally Er
The composition is such that an Er ₂ BaCuO ₅ phase remains in the Ba ₂ Cu ₃ Ox bulk at 25 mol%. This powder is 20m high using a 60mmφ mold.
m and then subjected to isostatic pressing at a pressure of 2 ton / cm 2 to obtain a raw material molded body. 50 molded bodies were prepared, and half of them were coated with a Yb-Ba-Cu-O layer of about 1 mm. A YbBa ₂ Cu ₃ Ox sintered body was used as a target. The coating was performed on the raw material molded body by masking, and as shown in FIG.

These raw material compacts were subjected to crystal growth heat treatment using the same box-type electric furnace. First 1150
After heating to ℃ for 30 minutes and cooling to 1005 ℃ for 1 hour. 3m square SmBa ₂ C at 1030 ° C
A seeding operation was performed to bring the cleavage plane (ab plane) of u ₃ Ox into contact with the upper surface of the molded body in a semi-molten state. Then 960 ° C
Then, the mixture was gradually cooled at a cooling rate of 0.3 ° C./h, and furnace-cooled from this temperature to room temperature in 8 hours. Comparing the single crystallization ratio of the manufactured ErBa ₂ Cu ₃ Ox bulk, the non-coated Yb-Ba-Cu-O layer was 72%, whereas the coated one was 92%. Significant differences were noted.

[0022]

As described above, the raw material compact of the REBa ₂ Cu ₃ O _x -based superconductor is heated to a temperature equal to or higher than the peritectic temperature of REBa ₂ Cu ₃ O _x , and is cooled to obtain the REBa _2. In a production method for obtaining a Cu ₃ O _x- based bulk,
RE that has a smaller ionic radius than gold, silver, or REBa ₂ Cu ₃ O _x to be produced that lowers the peritectic temperature of 2Cu3Ox
By coating the layer containing the element, nucleation from the surface in a semi-molten state during production is suppressed. Since nucleation from portions other than the seed crystal is suppressed, it is easy to manufacture a REBa ₂ Cu ₃ O _x large bulk superconductor having a single crystal grain.

[Brief description of the drawings]

Fig. 1 External view of a sound YBa ₂ Cu ₃ Ox superconducting bulk composed of single crystal grains

Fig. 2 Polynucleated by nucleation from parts other than the seed crystal
External view of YBa ₂ Cu ₃ Ox superconducting bulk

FIG. 3 is a diagram showing a coating portion on a raw material molded body in Examples 1 and 2.

[Explanation of symbols]

1 YBa ₂ Cu ₃ Ox crystal grown from seed crystal 2 SmBa ₂ Cu ₃ Ox seed crystal 3 Crystal habit 4 YBa ₂ Cu ₃ Ox crystal grown from a portion other than seed crystal 5 Coated part on raw material molded body 6 Uncoated part on raw material compact

Claims (4)

[Claims] 1. A raw material compact of REBa ₂ Cu ₃ O _x -based superconductor is made of RE
In the production method of heating to a temperature equal to or higher than the peritectic temperature of Ba ₂ Cu ₃ O _x and cooling it to obtain a REBa ₂ Cu ₃ O _x -based bulk, the surface of the raw material compact has a REBa ₂ Cu ₃ O _x A method for producing a REBa ₂ Cu ₃ O _x -based superconductor, which comprises performing a heat treatment after coating a substance that lowers the peritectic temperature.
Here, RE is Y, Nd, Sm, Eu, Gd, Dy, H
One or more elements selected from the group consisting of o, Er, Tm, and Yb. 2. The method according to claim 1, wherein the substance to be coated is silver, gold or a compound containing the same. 3. A process according to claim coating substance is characterized in that it is a compound containing RE element or which of trying to REBa ₂ Cu ₃ O _x peritectic temperature lower REBa ₂ Cu ₃ O _x than producing 1 The production method described in 1. 4. A REBa ₂ Cu ₃ O _x -based superconducting bulk material comprising gold or silver in the vicinity of the surface of a bulk body.