JP2968557B2 - Substrate for oxide superconducting conductor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate for an oxide superconducting conductor whose application is being developed for a superconducting magnet, a superconducting generator, a superconducting energy storage, a superconducting element, and the like. .

"Conventional technology" Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system, Tl-Ba-
2. Description of the Related Art A long superconductor has been developed using an oxide superconductor represented by a Ca—Cu—O system or the like.

Considering the practical use of a long superconducting conductor using this type of oxide superconductor, a substrate made of a material with excellent flexibility and good workability, such as tape-shaped metal, is considered. It is desired to prepare and form an oxide superconducting thin film on this substrate to produce an oxide superconducting conductor.

However, metal substrates for oxide superconducting conductors that have been reported to date are limited to gold, platinum, silver, hastelloy, etc., and an oxide superconducting thin film is formed on a substrate made of these materials. The obtained oxide superconductor is made of strontium titanate (SrTiO ₃ ) or magnesia (Mg
At present, the critical current density is two to three orders of magnitude lower than that of an oxide superconducting conductor obtained by forming an oxide superconducting thin film on a substrate made of a single crystal such as O).

A major cause of the decrease in the critical current density of the oxide superconducting thin film formed on the metal substrate is the problem of the crystal orientation of the oxide superconductor. That is, this kind of oxide superconductor has an anisotropic electrical current that allows current to flow easily in a specific direction of the crystal and makes it difficult to flow current in a specific direction.

Therefore, when manufacturing a superconductor using an oxide superconductor, it is important to control the crystal orientation.

Here, in the crystal structure of the oxide superconductor having a perovskite structure as a basic structure, copper atoms and oxygen atoms are arranged and bonded in the a-axis direction and the b-axis direction of the crystal axis. In the c-axis direction.

Accordingly, the oxide superconductor of this system has a high critical current density in the ab plane of the crystal. Therefore, in order to increase the critical current density, the ab plane of the crystal is arranged in parallel to the base material surface. It is preferable to form the oxide superconductor so that the c-axis of the crystal axis is perpendicular to the substrate surface. Furthermore, from the background that the oxide superconducting thin film needs to be formed on the substrate surface, the size of the crystal lattice on the surface of the substrate to be used is equal to the size of the crystal lattice in the ab-axis direction of the oxide superconductor. Is desirable.

Considering the above-mentioned single crystal substrate from the above viewpoint, in the case of SrTiO ₃ crystal oriented in the (100) plane, a = 3.91 °, and M
In the crystal of gO, while a = 4.21 °, Y ₁ Ba
_In the Y-based oxide superconductor having the composition of ₂ Cu ₃ O _7-δ , a = 3.89 ° and b = 3.82 °, which are very close to the above values.

On the other hand, a substrate made of Ag has a face-centered cubic crystal structure of a = 4.09 °, but a substrate that has been conventionally used for manufacturing an oxide superconductor has a polycrystalline structure. The orientation of the crystals was not uniform. For this reason, when an oxide superconducting thin film is produced on an Ag substrate,
Due to a mismatch between the lattice constant of the ab axis of the oxide superconducting thin film and the surface of the base material, there was a problem that it was difficult to orient the oxide superconducting thin film along the c axis.

In addition, a substrate made of Ag tends to be softened by a heat treatment performed after forming an oxide superconductor, so that there is a problem that the strength becomes insufficient as an application to a device in which an external force caused by an electromagnetic force of a superconducting magnet acts. is there. Further, if the substrate itself has insufficient strength, there is a possibility that the superconducting thin film on the substrate is cracked by the application of stress or the superconducting thin film is separated from the substrate.

The present invention has been made to solve the above problems,
It is an object of the present invention to provide a substrate capable of forming an oxide superconducting thin film in a favorable crystal orientation state, and having a high mechanical strength and capable of obtaining an oxide superconducting conductor strong in applying stress.

Means for Solving the Problems The invention described in claim 1 is a base material for an oxide superconducting conductor formed by forming an oxide superconducting thin film on an outer surface of a base material in order to solve the problem. The base material is a flexible heat-resistant metal base part having a higher strength than the noble metal, and a coating layer made of a crystalline face-centered cubic noble metal material coated on the outer surface of the metal base part. Wherein at least the portion on the outer surface of the coating layer where the oxide superconducting thin film is to be formed is crystallographically oriented in the direction of the (100) plane.

[Function] A substrate provided with a coating layer made of a noble metal having a lattice constant close to the lattice constant of the oxide superconductor is used, and the crystals of the coating layer are oriented in the direction of the (100) plane. If an oxide superconducting thin film is formed on the layer, the oxide superconducting thin film grows while matching the crystal structure on the surface of the coating layer. Therefore, a c-axis oriented oxide superconducting thin film is formed on the surface of the base material. In addition, since the noble metal coating layer is reinforced with a metal base having high strength, the structure is superior in mechanical strength to a substrate made of noble metal alone and has a structure that is strong against stress.

 Hereinafter, the present invention will be described in more detail.

In order to form the base material of the present invention and further produce an oxide superconducting conductor, first, by plastic working such as rolling, thickness,
A long tape-shaped flexible metal base portion 1 shown in FIG. 1 of about 0.1 to 0.5 mm is formed. This metal base 1
It is formed from a heat-resistant Ni-based alloy such as Hastelloy. In addition, a high-strength metal material having higher mechanical strength than a noble metal is used as the heat-resistant metal forming the metal base 1.

Next, as shown in FIG.
A coating layer 2 made of a noble metal such as g, Au, Pt or an alloy thereof is formed, and a base material 3 having a two-layer structure is formed. Here, the thickness of the coating layer 2 is preferably about 10 to 100 μm. The reason for this is that if the coating layer 2 is thinner than 10 μm, the diffusion of the constituent elements of the metal substrate 1 and the continuity of the coating layer 2 are lacking. The method for forming the coating layer 2 may be any of known methods such as a plating method, a cladding method, a pressure bonding method, a CVD method, and a welding method. In short, a coating layer having a necessary thickness is formed. Any means that can be used may be used.

The coating layer 2 made of these noble metals has a crystal structure similar to that of the oxide superconductor and has a similar lattice constant. For example, a crystal of an oxide superconductor having a composition of Y ₁ Ba ₂ Cu ₃ O _7-δ has a crystal structure based on perovskite, and a = 3.89 and b = 3.82. In contrast, Ag, Au, and Pt all have a face-centered cubic structure,
Ag is a = 4.09 °, Au is a = 4.08 °, and Pt is a = 3.92 °.

If the tape-shaped base material 3 having a two-layer structure is formed,
Preferably, under an oxygen partial pressure of 1 atm or less, a temperature exceeding 300 ° C and a temperature lower than 700 ° C, more preferably 500 to
Heat treatment is performed at a temperature of 600 ° C. for 100 hours or less, more preferably for about 1 to 6 hours to orient the crystals on the surface of the coating layer 2.

Here, it is known that when a metal such as silver is subjected to a strong plastic working such as a strong rolling work, it becomes a processed structure, that is, a texture having a preferred orientation, and crystals are aligned in a special orientation. Therefore, by applying a heat treatment to the tape formed by the above-mentioned rolling process to form a tape and develop a texture, the crystal orientation of the crystal grains can be preferentially aligned to a specific orientation. ) Direction, which contributes to the lattice constant and c-axis orientation of the oxide superconductor.

By the heat treatment described above, the crystals on the surface of the coating layer 2 become (10
0) The plane is oriented. In addition, the atmosphere when heat-treating the base material 3 may be at atmospheric pressure. Here, the metal substrate 1
Is formed from a heat-resistant metal such as Hastelloy, so that heat treatment does not cause a decrease in strength or damage.

Next, an oxide superconducting thin film 4 is formed on the substrate 3 as shown in FIG. 3 by using film forming means such as sputtering, molecular beam epitaxy, laser PVD, and CVD. Here, the oxide superconducting thin film 4 is made of a Y-Ba-Cu-O-based, Bi-
It is an oxide represented by Sr-Ca-Cu-O system, Tl-Ba-Ca-Cu-O system and the like. Specifically, for example, Y ₁ Ba ₂ Cu ₃ O _7−δ
, A composition of Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{X, and} a composition of Tl ₂ Ba ₂ Ca ₂ Cu ₃ O _X.

When forming the oxide superconducting thin film 4 on the base material 3,
Since the crystals on the surface of the substrate are oriented in the (100) plane, the crystal orientation of the oxide superconducting thin film 4 formed on the surface of the substrate is also in a well-aligned state. That is, the a-b plane of the crystal of the oxide superconducting thin film 4 is oriented parallel to the upper surface of the coating layer 2 and the c-
The crystal is oriented so that the axis is perpendicular to the surface of the coating layer 2.

Once the oxide superconducting thin film 4 is formed on the substrate 3, the temperature is raised to 500 to 800 ° C. for the purpose of homogenizing the oxide superconducting thin film 4.
A heat treatment of gradually cooling after heating for about a few minutes to several hours may be performed. By this heat treatment, the crystal structure of the oxide superconducting thin film 4 is adjusted, the superconducting properties are improved, and the oxide superconducting conductor 5 is obtained.

If the oxide superconducting thin film 4 is further formed on the coating layer 2 in which the surface crystals are oriented as described above, the film can be formed while the crystal axis of the oxide superconducting thin film 4 matches the crystal axis of the coating layer 2. The oxide superconducting thin film 4 can be formed on the substrate in an oriented state. Therefore, an excellent oxide superconductor 5 having a high critical current density can be obtained.

Further, the obtained oxide superconducting conductor 5 has excellent heat resistance and is reinforced with the high-strength metal base portion 1, so that it is used for a superconducting magnet and is subjected to a stress caused by the electromagnet. Even if it is present, there is little possibility that cracks will occur in the superconducting thin film 4, and the superconducting thin film 4 has a feature that is strong against external force.

"Example" A 0.3 mm thick, 5 mm wide tape-shaped metal base portion made of Hastelloy C-276 was formed by rolling, and a 0.1 mm thick coating layer made of Ag was formed on these surfaces. The whole
By applying a heat treatment at 500 ° C. for 1 hour under an oxygen partial pressure of 0.5 to 1.0 atm, the Ag crystals on the surface of the coating layer are converted to (100)
It was oriented along the plane.

Then, Y ₁ Ba ₂ Cu ₃ O
An oxide superconducting thin film having a composition of _7-δ was formed.

FIG. 5 shows an X-ray diffraction test result of the oxide superconducting thin film manufactured as described above. As is apparent from FIG. 5, a diffraction peak peculiar to the oxide superconductor having the composition of Y ₁ Ba ₂ Cu ₃ O _7-δ is observed, indicating that the oxide superconducting thin film in the oriented state can be formed on the coating layer. It was revealed.

In addition, the tensile strength of the oxide superconducting conductor manufactured as described above was measured and found to be an excellent value of 50 kg / mm ² . On the other hand, the tensile strength of the oxide superconducting conductor obtained by using an Ag tape having the same dimensions as the substrate and heat-treating the same to form a crystal orientation and forming a superconducting thin film thereon is as follows:
It was 14 kg / mm ² .

From the above, it is clear that the oxide superconducting conductor manufactured using the base material of the present invention has an oxide superconducting thin film with good orientation, and also has a higher tensile strength than a tape material of only Ag. Became. Therefore, the oxide superconducting conductor is suitable for a superconducting magnet or the like which is exposed to stress by an electromagnetic force.

[Effects of the Invention] As described above, according to the present invention, a coating layer made of a noble metal having a lattice constant close to the lattice constant of an oxide superconductor and having a similar lattice constant to a crystal is formed on a metal substrate. Since the crystal on the surface of the coating layer of this substrate is oriented in the direction of the (100) plane using the base material thus formed, if an oxide superconducting thin film is formed on the coating layer using this base material, The oxide superconducting thin film grows while matching the crystal structure of the surface of the coating layer. Therefore, an oxide superconducting thin film having c-axis orientation can be formed on the substrate surface, and an oxide superconducting conductor having good crystal orientation and high critical current characteristics can be obtained.

Further, since the use ratio of the noble metal can be reduced as compared with the case where the entire substrate is made of an expensive noble metal, the cost of the oxide superconducting conductor obtained by using the substrate of the present invention can be reduced.

In addition, when a coating layer is formed on a heat-resistant high-strength metal base portion and then an oxide superconducting thin film is formed, the metal base portion exhibits strength when stress is applied, and the superconducting thin film Since the occurrence of cracks is prevented, an oxide superconductor that is strong against external forces can be obtained.

[Brief description of the drawings]

1 to 4 are views for explaining an example of the present invention. FIG. 1 is a sectional view of a metal base portion, and FIG. 2 shows a state in which a coating layer is formed on the metal base portion. FIG. 3 is a sectional view showing a state in which an oxide superconducting thin film is formed on a coating layer, FIG. 4 is a sectional view of an oxide superconducting conductor, and FIG. FIG. 3 is a graph showing an X-ray diffraction peak of a superconducting thin film. 1 ... metal base part, 2 ... coating layer, 3 ... base material, 4 ...
Oxide superconducting thin film, 5 ... Oxide superconducting conductor.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuomi Kakimoto 1-5-1 Kiba, Koto-ku, Tokyo Inside Fujikura Electric Wire Co., Ltd. (72) Inventor Satoshi Kawano 1-1-5-1 Kiba, Koto-ku, Tokyo Fujikura JP-A-3-199107 (JP, A) JP-A-3-75204 (JP, A) JP-A-2-102121 (JP, A) JP-A-1-290524 (JP) JP-A-1-266773 (JP, A) JP-A-1-252534 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C30B 1/00-35/00 C01G 1/00

Claims (1)

(57) [Claims] 1. A substrate used for an oxide superconducting conductor having an oxide superconducting thin film formed on an outer surface of a substrate, wherein the substrate has a higher heat resistance than a noble metal. And a coating layer made of a noble metal material having a crystalline face-centered cubic structure coated on the outer surface of the metal base portion, and at least an oxide superconducting thin film is formed on the outer surface of the coating layer. A substrate for an oxide superconducting conductor, characterized in that the portion to be crystallized is oriented in the direction of the (100) plane.