JP2023077662A - Hybrid superlow resistance connection structure for high temperature oxide superconducting wire material and metal-based low temperature superconducting wire material, and connection method therefor - Google Patents

Abstract

To reduce a junction length and realize e.g., 1x10-10 Ω or less as junction resistance when connecting a high temperature oxide superconducting wire material and a metal-based low temperature superconducting wire material.SOLUTION: As a high temperature oxide superconducting wire material, a Bi2Sr2Ca2Cu3O10 (described as Bi2223 hereinafter) wire material is prepared and as a metal-based low temperature superconducting wire material, an NbTi wire material is prepared. As a junction material of the high temperature oxide superconducting wire material and the metal-based low temperature superconducting wire material, a lead-bismuth binary alloy is prepared. Regarding a composition ratio of the lead-bismuth binary alloy, lead is 40% to 60% in mass% and the remaining is bismuth and an inevitable impurity. An end of a superconducting wire material consisting of the high temperature oxide superconducting wire material and an end of a superconducting wire material consisting of the metal-based low temperature superconducting wire material are immersed in the lead-bismuth binary alloy in a molten state for a fixed time, cooled and modified, such that the high temperature oxide superconducting wire material and the metal-based low temperature superconducting wire material are connected via the lead-bismuth binary alloy.SELECTED DRAWING: Figure 1

Description

Application for application of Article 30, Paragraph 2 of the Patent Law (1) Released on December 3, 2020 Public Interest Incorporated Association Cryogenics and Superconductivity Society 100th Fall 2020 Cryogenics and Superconductivity Society Lecture Abstracts (https: //www.csj.or.jp/conference/2020a/index.html) (2) Held on December 9, 2020, hosted by the Society of Cryogenics and Superconductivity, the 100th 2020 Autumn Meeting of the Society of Cryogenics and Superconductivity (https://www.csj.or.jp/conference/2020a/index.html) (3) Published on September 16, 2021 Published by the Society of Cryogenics and Superconductivity, the 27th International Conference on Magnet Technology ( (4) November 15-19, 2021 The 27th International Conference on Magnet Technology (MT27) hosted by the Society of Cryogenics and Superconductivity, Fukuoka International Conference Center (Ishiki, Hakata-ku, Fukuoka City, Fukuoka Prefecture) Town 2-1)

The present invention relates to a hybrid type ultra-low resistance connection structure of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire, and a connection method thereof.

In NMR and medical MRI, a superconducting magnet of NbTi (niobium titanium) is used as a superconducting wire material, and the generated magnetic field is about 0.5T to 3T. However, in NMR and MRI, when generating a high magnetic field of, for example, 20 T or more, metal-based low-temperature superconducting wires (NbTi, Nb ₃ Sn) and high-temperature oxide superconducting wires (rare earth-based superconductors, bismuth-based superconductors body) are used in combination. In particular, as one of the solutions for operating a superconducting magnet that generates a high magnetic field in a persistent current operation mode, it is necessary to bond different types of superconducting wires, such as metallic low-temperature superconducting wires and high-temperature oxide superconducting wires. is.

When metal-based low-temperature superconducting wires are joined together, it is common to interpose a low-melting-point solder-based low-melting-point superconducting material to achieve superconducting joining with zero resistance. When joining a metal-based low-temperature superconducting wire and a high-temperature oxide superconducting wire, even if the solder-based low-melting-point superconducting material of the prior art is used for joining, a superconducting joint with zero resistance cannot be realized (for example, non-patent literature 2).
As an alternative technology, in the case of a rare earth-based superconducting wire, Patent Document 1 adopts a normal-conducting joint to increase the joint area (joint length), thereby lowering the resistance value to a value that allows persistent current operation. was developed. However, the joining length becomes very long, and in order to save space, a complicated structure such as winding up the wire is required.
In the case of a bismuth-based superconducting wire, Patent Document 2 and Non-Patent Documents 1 and 3 propose a method of inserting a bismuth-based superconducting wire into tin-containing solder and joining them. However, the joint resistance was 9×10 ⁻⁹ Ω, and a superconducting wire joint that could be used in a persistent current mode such as NMR could not be realized.

Patent Document 3 discloses Bi ₂ Sr ₂ CaCu ₂ O ₈ (Bi2212) as an example, which achieves a junction resistance of 10 ⁻¹⁰ Ω or less with a junction length of 3 cm. The development of superconducting magnets for NMR using Bi2212 wires is mainly carried out in the United States.
On the other hand, in Japan, a wire made of Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ (hereinafter referred to as Bi2223) is used. It was difficult to lower the value.

Japanese Patent Publication No. 2016-535431 JP 2018-129294 A JP-A-2001-283660

Applied Physics Express 10,093102(2017). doi:10.7567/APEX.10.093102 Nobuya Banno et al “A new concept for developing a compact joint structure for reducing joint resistance between high-temperature superconductors (HTS) and low-temperature superconductors (LTS)”, 2020 Supercond. Sci. Technol. 33 115015 Kazuo Inoue et al., "Superconducting Bonding of Bi2223 Wire and NbTi Wire Using Bi-Pb-Sn Solder", 2019 Spring Cryogenics and Superconductivity Society of Japan 2P-p09

The present invention solves the above-mentioned problems of the prior art, and makes it possible to operate a 30T superconducting magnet for over 1 GHz class NMR in a permanent current mode in the connection between a metallic low-temperature superconducting wire and a high-temperature oxide superconducting wire. It is an object of the present invention to provide a junction structure having a sufficiently low junction resistance (specifically, 10 ⁻¹⁰ Ω or less) for

[1] The ultra-low resistance connection method of the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire of the present invention comprises:
preparing a Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ wire as the high-temperature oxide superconducting wire;
preparing an NbTi wire as the metallic low-temperature superconducting wire,
A lead-bismuth binary system alloy is prepared as a bonding material for the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire, and the composition ratio of the lead-bismuth binary system alloy is, in mass%, lead. : 40% to 60%, the balance being bismuth and unavoidable impurities,
An end portion of the superconducting wire made of the high-temperature oxide superconducting wire and an end portion of the superconducting wire made of the metal-based low-temperature superconducting wire are placed in the molten lead-bismuth binary alloy for a certain period of time. , solidified by cooling after immersion,
The high temperature oxide superconducting wire and the metal low temperature superconducting wire are connected via the lead-bismuth binary alloy.

[2] In the method [1] for ultra-low resistance connection of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire according to the present invention, the temperature of the lead-bismuth binary alloy in a molten state is preferably 210°C or less. should be
[3] In the ultra-low resistance connection method [1] or [2] of the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire of the present invention, preferably, the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire are The bonding length of the conductive wire is preferably 60 cm or less, and the bonding resistance between the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire is preferably 10 ⁻¹⁰ Ω or less.
[4] In the method [1] to [3] for ultra-low resistance connection of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire according to the present invention, preferably, the composition ratio of the lead-bismuth binary alloy is In mass %, lead: 50%, the balance being bismuth and unavoidable impurities.

[5] In the superconducting wire bonding structure of the present invention, the end of a superconducting wire made of a high-temperature oxide superconducting wire and the end of a superconducting wire made of a metal low-temperature superconducting wire are joined via a joining material. A superconducting wire joint structure that
_{The high} -temperature oxide _{superconducting} wire is _{a Bi2Sr2Ca2Cu3O10} wire,
The metallic low-temperature superconducting wire is an NbTi wire,
The joint material is a lead-bismuth binary alloy, and the composition ratio of the lead-bismuth binary alloy is, in mass%, lead: 40% to 60%, the balance being bismuth and unavoidable impurities. .
[6] The NMR, MRI, or superconducting transport equipment of the present invention is characterized by using the superconducting wire bonding structure [5] of a high-temperature oxide superconducting wire and a metallic low-temperature superconducting wire.

The ultra-low-resistance connection method and structure of the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire thus constructed operate as follows.
[1] The end of a superconducting wire made of a NbTi metal-based low-temperature superconducting wire and the lead-bismuth alloy superconducting material as a joining material are superconducting joints, and current flows through the joint interface with zero electrical resistance.
[2] The end of the superconducting wire made of Bi2223 and the lead-bismuth-based alloy superconducting material, which is the joining material, are normally conducting joints, and ohmic resistance is generated at the joining interface.
[3] Therefore, the superconducting wire bonding structure of the present invention is characterized by a hybrid (composite) type in which the above two types of superconducting wires are connected in series by superconducting bonding and normal-conducting bonding via a solder-based bonding material. and

In order to realize an ultra-low resistance that can be used in the persistent current mode of a superconducting magnet used in NMR, etc., it is necessary to minimize the ohmic resistance of this normal-conducting junction, and a method has been found. That is, in the superconducting wire hybrid type joint structure of the present invention, the joint resistance is increased by optimizing the composition ratio of the lead-bismuth alloy of the joint material, lowering the melting temperature of the lead-bismuth alloy, and lengthening the joint length. greatly suppressed. As a result, we realized a superconducting wire joint structure with ultra-low resistance (10 ^-10 Ω or less) that can be used in the persistent current operation mode of a 30T superconducting magnet for 1 GHz-class NMR.
In particular, lengthening the joint length is an important process, and the joint resistance between the superconducting wire made of Bi2223 and the lead-bismuth alloy superconducting material as the joint material is inversely proportional to the joint length. At a junction length of 36 cm or more, the ohmic resistance becomes ultra-low resistance of 10 ⁻¹⁰ Ω or less, and a connection is obtained in which a persistent current operation mode can be used with a 30 T superconducting magnet for 1 GHz-class NMR.

In the hybrid joint structure between the bismuth-based high-temperature oxide superconducting wire and the NbTi-based low-temperature superconducting wire according to the present invention, the joint resistance was successfully reduced to an ultra-low resistance of 10 ⁻¹⁰ Ω or less. As a result, we realized a superconducting wire joint structure that can be used sufficiently in the persistent current operation mode of a 30T superconducting magnet for NMR of over 1 GHz.
If NMR at over 1 GHz is operated and spread, it will greatly contribute to the development of the fields of medicine, pharmaceuticals, and material science, such as detailed analysis of proteins. In addition, various applications are expected, such as enabling the use of a high magnetic field superconducting magnet for purposes other than NMR and MRI in a persistent current operation mode.

1 is a configuration diagram of a superconducting wire hybrid joint structure showing an embodiment of the present invention; FIG. 4 is a photograph of a superconducting wire hybrid joint actually produced by the technique developed in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining the microstructure of the superconducting wire bonding structure of the present invention, which illustrates the bonding interface between a Bi2223 superconducting wire and a lead-bismuth-based solder alloy as a bonding material. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining the microstructure of the superconducting wire bonding structure of the present invention, and illustrates the bonding interface between the NbTi wire and the lead-bismuth solder alloy as the bonding material. FIG. 2 is a diagram showing an inversely proportional relationship between the joint resistance and the joint length of a superconducting wire hybrid joint, showing an embodiment of the present invention; FIG. 2 is a configuration diagram showing an embodiment of the present invention when a Bi2223 wire rod is wound into a ring and joined. 4 is a photograph of an actually produced ring-shaped joint sample. It is a figure which shows the relationship between the joining heat treatment temperature and joining resistance. This is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy of the bonding material, at a bonding heat treatment temperature of 160°C. This is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy of the bonding material, at a bonding heat treatment temperature of 260°C. This is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy of the bonding material, at a bonding heat treatment temperature of 310°C.

The present invention will be described below with reference to the drawings.
FIG. 1 is a configuration diagram of a superconducting wire hybrid type joint structure showing an embodiment of the present invention.
In the embodiment shown in FIG. 1, a Bi2223 wire as a high-temperature oxide superconducting wire and an NbTi wire as a metal-based low-temperature superconducting wire were joined using a joining material. A lead-bismuth alloy (Pb-50 wt % Bi) was used as a solder-based low melting point superconducting material for the joining material.

The manufacturing process of the superconducting wire hybrid type joint structure constructed as described above is as follows.
[1] When using Sumitomo Electric's DI-BSCCO Type HT series Bi2223 wire, it is desirable to remove the reinforcing material in advance.
[2] It is desirable to remove the copper sheath in advance from the NbTi wire.
[3] The Bi2223 wire rod and the NbTi wire rod pretreated in steps [1] and [2] are immersed in a molten lead-bismuth solder alloy, held for a predetermined time, and then cooled and solidified. The superconducting wire and the solder alloy are strongly bonded.

FIG. 2 shows the fabricated superconducting wire joint.
FIG. 3 is a diagram illustrating the microstructure of the superconducting wire bonding structure of the present invention, and FIG. 3A illustrates a bonding interface between a Bi2223 superconducting wire and a lead-bismuth solder alloy as a bonding material. At the joint interface, there is a silver sheath around the Bi2223 superconducting wire, through which current flows, so that the joint is a normal-conducting joint and an ohmic resistance is generated at the joint interface. FIG. 3B explains the bonding interface between the NbTi wire and the lead-bismuth solder alloy as the bonding material. The NbTi wire and the lead-bismuth solder alloy are directly joined to form a superconducting joint.
If the temperature of the molten solder is in the range of the melting point of the solder alloy or higher and 210° C. or lower, the joint resistance can be reduced.

The composition ratio of lead and bismuth in the lead-bismuth solder alloy used as the joint material is preferably 40% to 60% lead: 60% to 40% bismuth in mass%, and a lead-bismuth binary confocal composition. Around (Pb-50wt%Bi) is preferred. The unavoidable impurities include third elements contained in raw materials of lead and bismuth, and are indicated because it is practically difficult to prepare lead and bismuth with 100% purity.
The bonding resistance between the superconducting wire made of Bi2223 and the lead-bismuth alloy superconducting material as the bonding material is inversely proportional to the bonding length. A plurality of samples with different joint lengths were prepared, electrodes were attached to a superconducting wire made of Bi2223 and a superconducting wire made of NbTi, and the wires were immersed in liquid helium to measure resistance by the four-probe method. A relational expression (R _j =3.6×10 ⁻⁹ /L _j ) between the junction resistance value R _j (Ω) and the junction length L _j (cm) was derived based on each measured resistance value.

FIG. 4 shows the relationship between junction resistance and junction length. According to the R _j −L _j relational expression, the ohmic resistance becomes ultra-low resistance of 10 ⁻¹⁰ Ω or less at a junction length of 36 cm or more, and a connection that allows the use of a persistent current operation mode with a 30 T superconducting magnet for 1 GHz or higher NMR can be obtained. .
Since the joining of the superconducting wire made of NbTi and the lead-bismuth alloy superconducting material of the joining material is superconducting joining, a joining length of about 1 cm is sufficient.

FIG. 5A is a configuration diagram when a Bi2223 wire is wound into a ring and joined. Moreover, FIG. 5B is a photograph of a bonded sample that was actually produced. By making it a ring shape, we have achieved a more compact joint.

FIG. 6 is a diagram showing the relationship between the bonding heat treatment temperature and the bonding resistance. Bonded samples with a bond length of 3 cm were measured. The junction resistance sharply increases above 210°C. Due to the structural restrictions of the superconducting magnet for NMR, the joint length is preferably about 60 cm or less. In order to achieve 10 ⁻¹⁰ Ω or less with a junction length _{of 60 cm, a relational expression (R j = 6} ×10 ⁻⁹ /L _j ), it corresponds to 2.0×10 ⁻⁹ Ω or less at a junction length of 3 cm. From FIG. 6, in order to satisfy this condition, it is desirable to set the bonding heat treatment temperature to 210° C. or lower.

FIG. 7 is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy of the joining material. 7(A) was manufactured at a bonding heat treatment temperature of 160° C., FIG. 7(B) was manufactured at a bonding heat treatment temperature of 260° C., and FIG. 7(C) was manufactured at a bonding heat treatment temperature of 310° C., respectively.
FIG. 7A shows an SEM image of a sample produced at a bonding heat treatment temperature of 160.degree. The Bi2223 superconducting wire is completely covered with an Ag (silver) sheath and connected with a lead-bismuth alloy at the Ag outer edge. FIGS. 7(B) and (C) show SEM images of samples fabricated at bonding heat treatment temperatures of 260° C. and 310° C., respectively. It can be seen that the Ag sheath is removed as the bonding heat treatment temperature is increased. At a bonding heat treatment temperature of 310° C., Ag still remains in the central portion, but has almost disappeared in the outer edge portion. From the relationship between bonding heat treatment temperature and bonding resistance in FIG. 6 and the SEM photograph in FIG. 7, it is desirable that the Ag sheath is not removed in order to achieve low bonding resistance, and it is necessary to fabricate at a low bonding heat treatment temperature.

Due to the structural restrictions of the superconducting magnet for NMR, the joint length is preferably about 60 cm or less. In the present embodiment, for example, 10 ⁻¹⁰ Ω or less is achieved at a junction length of 36 cm, for example. there is

In the superconducting wire bonding structure between the bismuth-based high-temperature oxide superconducting wire and the NbTi-based low-temperature superconducting wire according to the present invention, the bonding resistance can be reduced to an ultra-low resistance of, for example, 10 ⁻¹⁰ Ω or less. As a result, it is possible to provide a superconducting wire joint structure that can be used sufficiently in the persistent current operation mode of a 30T superconducting magnet for NMR of over 1 GHz, for example.

Claims (6)

A method for ultra-low resistance connection of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire, comprising:
preparing a Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ wire as the high-temperature oxide superconducting wire;
preparing an NbTi wire as the metallic low-temperature superconducting wire,
A lead-bismuth binary system alloy is prepared as a bonding material for the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire, and the composition ratio of the lead-bismuth binary system alloy is, in mass%, lead. : 40% to 60%, the balance being bismuth and unavoidable impurities,
An end portion of the superconducting wire made of the high-temperature oxide superconducting wire and an end portion of the superconducting wire made of the metal-based low-temperature superconducting wire are placed in the molten lead-bismuth binary alloy for a certain period of time. , solidified by cooling after immersion,
and connecting the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire through the lead-bismuth binary alloy;
A method for ultra-low resistance connection of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire, characterized by: 2. A method for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire according to claim 1, wherein said lead-bismuth binary alloy has a temperature of 210.degree. C. or less in a molten state. The bonding length between the high temperature oxide superconducting wire and the metal low temperature superconducting wire is 60 cm or less,
3. The ultra-low junction resistance of the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire according to claim 1 or 2, wherein the joint resistance between the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire is 10 ⁻¹⁰ Ω or less. Resistor connection method. The high-temperature oxide superconducting wire and metal according to any one of claims 1 to 3, wherein the composition ratio of the lead-bismuth binary alloy is, in mass%, lead: 50%, the balance being bismuth and unavoidable impurities. ultra-low-resistance connection method for low-temperature superconducting wires. A superconducting wire joining structure for joining an end of a superconducting wire made of a high-temperature oxide superconducting wire and an end of a superconducting wire made of a metal-based low-temperature superconducting wire through a joining material,
_{The high} -temperature oxide _{superconducting} wire is _{a Bi2Sr2Ca2Cu3O10} wire,
The metallic low-temperature superconducting wire is an NbTi wire,
The joint material is a lead-bismuth binary alloy, and the composition ratio of the lead-bismuth binary alloy is, in mass%, lead: 40% to 60%, the balance being bismuth and unavoidable impurities. ,
Superconducting wire bonding structure. An NMR, MRI, or superconducting transport device using the superconducting wire bonding structure of the high-temperature oxide superconducting wire according to claim 5 and the metal-based low-temperature superconducting wire.

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