Hybrid ultra-low resistance connection structure and connection method for high-temperature oxide superconducting wire and metal-based low-temperature superconducting wire

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

[Technical Field]

[0001] The present invention relates to a hybrid ultra-low resistance connection structure for high-temperature oxide superconducting wires and metallic low-temperature superconducting wires, and a connection method therefor. [Background technology]

[0002] In NMR and medical MRI, superconducting magnets made of NbTi (niobium titanium) are used as the superconducting wire material, and the generated magnetic field is approximately 0.5 T to 3 T. However, when generating high magnetic fields, such as 20 T or higher, in NMR and MRI, a combination of metallic low-temperature superconducting wires (NbTi, Nb3Sn) and high-temperature oxide superconducting wires (rare earth superconductors, bismuth superconductors) is used. In particular, one solution for operating superconducting magnets that generate high magnetic fields in persistent current operation mode requires bonding between dissimilar superconducting wires, namely metallic low-temperature superconducting wires and high-temperature oxide superconducting wires.

[0003] When joining metal-based low-temperature superconducting wires together, it is common to use a low-melting-point solder-based superconducting material to achieve a zero-resistance superconducting bond.When joining metal-based low-temperature superconducting wires and high-temperature oxide superconducting wires, even when joining with conventional solder-based low-melting-point superconducting materials, a zero-resistance superconducting bond has not been achieved (see, for example, Non-Patent Document 2). As an alternative technology, in the case of rare earth superconducting wire, Patent Document 1 has developed a method of using normal conducting junctions to increase the junction area (junction length) and thereby reduce the resistance to a level that allows for persistent current operation. However, this method requires a very long junction length and a complex structure, such as winding up the wire, to save space. In the case of bismuth-based superconducting wires, Patent Document 2 and Non-Patent Documents 1 and 3 propose a method of joining by inserting the bismuth-based superconducting wires into tin-containing solder. However, the joint resistance is 9×10 -9 It was not possible to realize a superconducting wire joint that can be used in persistent current modes such as NMR at Ω.

[0004] In Patent Document 3, Bi2Sr2CaCu2O8 (Bi2212) is disclosed as an example, and the junction resistance is 10 -10 The development of superconducting magnets for NMR using Bi2212 wire is being carried out mainly in the United States. On the other hand, in Japan, Bi2Sr2Ca2Cu3O 10 (hereinafter referred to as Bi2223) wire material is used, and if the configuration of Patent Document 3 is applied as is, it is difficult to reduce the resistance to a level that allows persistent current operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2016-535431 [Patent Document 2] Japanese Patent Application Publication No. 2018-129294 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-283660 [Non-patent literature]

[0006] [Non-Patent Document 1] Applied Physics Express 10,093102(2017). doi:10.7567 / APEX.10.093102 [Non-patent document 2] 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 [Non-patent document 3] Kazuaki Inoue et al., "Superconducting Joining of Bi2223 Wire and NbTi Wire Using Bi-Pb-Sn Solder", 98th Spring 2019 Cryogenic Engineering and Superconductivity Society, 2P-p09 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has solved the above-mentioned problems of the prior art, and in the connection between a metallic low-temperature superconducting wire and a high-temperature oxide superconducting wire, the junction resistance is sufficiently low (specifically, 10 -10 The object of the present invention is to provide a joint structure having a resistance of 0.1 Ω or less. [Means for solving the problem]

[0008] [1] The ultra-low resistance connection method of the present invention for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire is as follows: The high-temperature oxide superconducting wire is Bi2Sr2Ca2Cu3O 10 Prepare the wire, As the metallic low-temperature superconducting wire, a NbTi wire is prepared, A lead-bismuth binary alloy is prepared as a joining 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 alloy is, in mass %, 40% to 60% lead, and the remainder is bismuth and inevitable impurities; an end of the superconducting wire made of the high-temperature oxide superconducting wire and an end of the superconducting wire made of the metallic low-temperature superconducting wire are immersed in the molten lead-bismuth binary alloy for a certain period of time, and then cooled and solidified; The high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire are connected via the lead-bismuth binary alloy.

[0009] [2] In the ultra-low resistance connection method [1] of the present invention for connecting a high-temperature oxide superconducting wire and a metallic low-temperature superconducting wire, the temperature of the lead-bismuth binary alloy in a molten state is preferably 210°C or less. [3] In the ultra-low resistance connection method [1] or [2] of the present invention for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire, preferably, the connection length between the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire is 60 cm or less, and the connection resistance between the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire is 10 -10 It is preferable that it is Ω or less. [4] In the ultra-low resistance connection method [1] to [3] of the present invention for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire, preferably, the composition ratio of the lead-bismuth binary alloy is, in mass %, 50% lead, with the remainder being bismuth and unavoidable impurities.

[0010] [5] The superconducting wire joining structure of the present invention is a superconducting wire joining structure in which an end of a superconducting wire made of a high-temperature oxide superconducting wire is joined to an end of a superconducting wire made of a metal-based low-temperature superconducting wire via a joining material, The high-temperature oxide superconducting wire is Bi2Sr2Ca2Cu3O 10 It is a wire rod, The metallic low-temperature superconducting wire is a NbTi wire, The bonding material is a lead-bismuth binary alloy, and the composition ratio of the lead-bismuth binary alloy is, in mass %, 40% to 60% lead, with the remainder being bismuth and unavoidable impurities. [6] The NMR, MRI or superconducting transport device of the present invention is characterized by using the superconducting wire bonding structure [5] of a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire.

[0011] The ultra-low resistance connection method and structure for connecting high-temperature oxide superconducting wires and metallic low-temperature superconducting wires thus constructed operates as follows. [1] The end of the superconducting wire made of NbTi metal-based low-temperature superconducting wire and the joining material, a lead-bismuth alloy superconducting material, form a superconducting junction, and current flows through the joining interface with zero electrical resistance. [2] The end of the superconducting wire made of Bi2223 and the lead-bismuth alloy superconducting material that is the joining material are normally conducting junctions, and ohmic resistance occurs at the joining interface. [3] Therefore, the superconducting wire joint 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 via a solder-based joint material to form a superconducting junction and a normal-conducting junction.

[0012] 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 joint, and we have found a way to achieve this. That is, in the superconducting wire hybrid joint structure of the present invention, we have significantly reduced the joint resistance by optimizing the composition ratio of the lead-bismuth alloy of the joint material, lowering the melting temperature of the lead-bismuth alloy, and increasing the joint length. As a result, we have achieved an ultra-low resistance (10 -10 We have achieved a superconducting wire joint structure with a resistance of 1000 kJ / s (less than 1000 kJ / s). In particular, increasing the length of the joint is an important process, and the joint resistance between the superconducting wire made of Bi2223 and the lead-bismuth alloy superconducting material, which is the joint material, is inversely proportional to the joint length. -10 The resistance is ultra-low below Ω, and a connection is obtained that allows the use of persistent current operation mode in a 30T superconducting magnet for 1GHz super-class NMR. [Effects of the Invention]

[0013] In the hybrid joint structure between the bismuth-based high-temperature oxide superconducting wire and the NbTi-based low-temperature superconducting wire of the present invention, the joint resistance is 10 -10 We succeeded in reducing the resistance to an ultra-low level of less than Ω, resulting in the realization of a superconducting wire joint structure that can be fully used in the persistent current operation mode of a 30T superconducting magnet for NMR at frequencies above 1 GHz. If NMR above 1 GHz becomes operational and widespread, it will greatly contribute to the development of medical, pharmaceutical, and material science fields, such as detailed analysis of proteins. It is also expected to have a variety of applications, such as enabling the use of high-field superconducting magnets in persistent current operation mode for purposes other than NMR and MRI. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram illustrating a configuration of a superconducting wire hybrid joint structure according to an embodiment of the present invention. [Figure 2] 1 is a photograph of a superconducting wire hybrid joint actually fabricated using the method developed in this invention. [Figure 3A] 1 is a diagram for explaining the microstructure of the superconducting wire bonding structure of the present invention, illustrating the bonding interface between the Bi2223 superconducting wire and the bonding material, a lead-bismuth based solder alloy. [Figure 3B] 1 is a diagram for explaining the microstructure of the superconducting wire bonding structure of the present invention, illustrating the bonding interface between the NbTi wire and the bonding material, a lead-bismuth based solder alloy. [Figure 4] FIG. 10 is a diagram showing the inverse proportional relationship between the joint resistance and the joint length of a superconducting wire hybrid joint, illustrating one embodiment of the present invention. [Figure 5A] FIG. 1 is a diagram showing a configuration in which a Bi2223 wire is wound into a ring shape and joined, according to an embodiment of the present invention. [Figure 5B] This is a photograph of an actually fabricated ring-shaped joint sample. [Figure 6] FIG. 10 is a diagram showing the relationship between the bonding heat treatment temperature and the bonding resistance. [Figure 7A] This is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy bonding material, where the bonding heat treatment temperature was 160°C. [Figure 7B] This is an SEM image of the vicinity of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy bonding material, where the bonding heat treatment temperature was 260°C. [Figure 7C] This is an SEM image of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy bonding material, where the bonding heat treatment temperature was 310°C. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of a superconducting wire hybrid joint structure according to an embodiment of the present invention. In the example shown in Fig. 1, a Bi2223 wire was used as the high-temperature oxide superconducting wire, and a NbTi wire was used as the metallic low-temperature superconducting wire using a joining material. A lead-bismuth alloy (Pb-50wt%Bi) was used as the solder-based low-melting-point superconducting material of the joining material.

[0016] The manufacturing process of the superconducting wire hybrid joint structure thus constructed is as follows. [1] When using Bi2223 wire from the DI-BSCCO Type HT series manufactured by Sumitomo Electric Industries, it is recommended to remove the reinforcing material beforehand. [2] It is desirable to remove the copper sheath from the NbTi wire beforehand. [3] The Bi2223 wire and NbTi wire pretreated in steps [1] and [2] are immersed in molten lead-bismuth solder alloy and held for a specified time, after which they are cooled and solidified, resulting in a strong bond between the superconducting wire and the solder alloy.

[0017] Figure 2 shows the superconducting wire joint that was fabricated. FIG. 3 illustrates the microstructure of the superconducting wire joint structure of the present invention. FIG. 3A illustrates the joint interface between the Bi2223 superconducting wire and the lead-bismuth solder alloy joint material. At the joint interface, a silver sheath surrounds the Bi2223 superconducting wire, and current flows through this sheath, resulting in a normal-conducting joint and ohmic resistance at the joint interface. FIG. 3B illustrates the joint interface between the NbTi wire and the lead-bismuth solder alloy joint material. The NbTi wire and the lead-bismuth solder alloy are directly joined to form a superconducting joint. When 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 effectively reduced.

[0018] The composition ratio of lead and bismuth in the lead-bismuth solder alloy, which is the joining material, is preferably 40% to 60% lead and 60% to 40% bismuth by mass. eutectic pointThe preferred composition is around (Pb-50 wt% Bi). The inevitable impurities include the third elements contained in the raw materials of lead and bismuth, and it is practically difficult to prepare 100% pure lead or bismuth, so they are listed here. The junction resistance of the superconducting wire made of Bi2223 and the lead-bismuth alloy superconducting material, which is the joining material, is inversely proportional to the joining length. Several samples with different joining lengths were prepared, and electrodes were attached to the superconducting wire made of Bi2223 and the superconducting wire made of NbTi. The samples were immersed in liquid helium and the resistance was measured using the four-terminal method. Using the measured resistance values ​​of each as a reference, the relationship between the joining resistance value Rj (Ω) and the joining length Lj (cm) was calculated (Rj = 3.6 × 10 -9 / Lj).

[0019] Figure 4 shows the relationship between junction resistance and junction length. j -L j From the relation, the ohmic resistance is 10 when the joint length is 36 cm or more. -10 The resistance is ultra-low below Ω, and a connection is obtained that allows the use of persistent current operation mode in a 30T superconducting magnet for 1GHz super-class NMR. The joining of the superconducting wire made of NbTi and the lead-bismuth alloy superconducting material of the joining material is a superconducting joining, so a joining length of about 1 cm is sufficient.

[0020] Figure 5A shows the structure of joining Bi2223 wires wound into a ring shape. Figure 5B shows a photograph of the actual joint sample. The ring shape allows for a compact joint.

[0021] Figure 6 shows the relationship between the bonding heat treatment temperature and the bonding resistance. A bonded sample with a bonding length of 3 cm was measured. The bonding resistance increases rapidly when the temperature exceeds 210°C. Due to the structural constraints of the superconducting magnet for NMR in this case, the bonding length is preferably approximately 60 cm or less. -10 To achieve a junction resistance of R j (Ω) and joint length L j (cm) relationship (R j =6×10 -9 / Lj ) and calculate backwards, the result is 2.0 × 10 for a joint length of 3 cm. -9 Ω or less. From FIG. 6, it is preferable to set the bonding heat treatment temperature to 210°C or less in order to satisfy this condition.

[0022] Figure 7 shows SEM images of the interface between the Bi2223 superconducting wire and the lead-bismuth alloy bonding material. Figure 7(A) was produced at a bonding heat treatment temperature of 160°C, Figure 7(B) was produced at a bonding heat treatment temperature of 260°C, and Figure 7(C) was produced at a bonding heat treatment temperature of 310°C. Figure 7(A) shows an SEM image of a sample fabricated at a bonding heat treatment temperature of 160°C. The Bi2223 superconducting wire is completely covered with an Ag (silver) sheath and is connected to the lead-bismuth alloy at the outer edge of the Ag. Figures 7(B) and 7(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 gradually removed as the bonding heat treatment temperature is increased. At a bonding heat treatment temperature of 310°C, the Ag still remains in the center, but has almost completely disappeared at the outer edge. From the relationship between bonding heat treatment temperature and bonding resistance in Figure 6 and the SEM images in Figure 7, it is clear that to achieve low bonding resistance, it is desirable to prevent the Ag sheath from being removed, and therefore, a low bonding heat treatment temperature is required.

[0023] Due to the structural constraints of the superconducting magnet for NMR in this case, the junction length is preferably about 60 cm or less. In this embodiment, for example, the junction length is 36 cm, and -10 We have achieved a resistance of less than Ω, overcoming not only the technical challenges of junction resistance but also the technical challenges of the structure of the superconducting magnet for NMR. [Industrial Applicability]

[0024] In the superconducting wire joint structure of the present invention between a bismuth-based high-temperature oxide superconducting wire and a NbTi-based low-temperature superconducting wire, the joint resistance is set to, for example, 10 -10 This allows for an ultra-low resistance of less than Ω, providing a superconducting wire joint structure that can be fully used in the persistent current operation mode of a 30T superconducting magnet for NMR at frequencies above 1 GHz.

Claims

1. A method for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire with ultra-low resistance, comprising: The high-temperature oxide superconducting wire may include Bi. ２ Sr ２ Ca ２ Cu ３ O １０ Prepare the wire, As the metallic low-temperature superconducting wire, a NbTi wire is prepared, a lead-bismuth binary alloy is prepared as a joining material for the high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire, and the lead-bismuth binary alloy has a composition ratio, in mass %, of 40% to 60% lead, and the remainder being bismuth and unavoidable impurities; an end of the superconducting wire made of the high-temperature oxide superconducting wire and an end of the superconducting wire made of the metallic low-temperature superconducting wire are immersed in the molten lead-bismuth binary alloy for a certain period of time, and then cooled and solidified; The high-temperature oxide superconducting wire and the metal-based low-temperature superconducting wire are connected via the lead-bismuth binary alloy, and The joint length of the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire is 36 cm or more and 60 cm or less, and the joint resistance of the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire is 10-10 Ω or less, thereby realizing 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 30 T superconducting magnet for 1 GHz super-class NMR. A method for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire with ultra-low resistance, characterized by the above.

2. 2. The method for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire with ultra-low resistance according to claim 1, wherein the temperature of said lead-bismuth binary alloy in a molten state is 210° C. or lower.

3. 3. The method for connecting a high-temperature oxide superconducting wire and a metal-based low-temperature superconducting wire with ultra-low resistance according to claim 1 or 2, wherein the composition ratio of the lead-bismuth binary alloy is, in mass %, 50% lead, and the remainder being bismuth and unavoidable impurities.

4. A superconducting wire joining structure in which 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 are joined via a joining material, The high-temperature oxide superconducting wire is Bi ２ Sr ２ Ca ２ Cu ３ O １０ It is a wire rod, the metallic low-temperature superconducting wire is a NbTi wire, The bonding material is a lead-bismuth binary alloy, and the composition ratio of the lead-bismuth binary alloy is, in mass %, 40% to 60% lead, and the remainder is bismuth and unavoidable impurities, The joint length of the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire is 36 cm or more and 60 cm or less, and the joint resistance of the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire is 10-10 Ω or less, thereby realizing 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 30 T superconducting magnet for 1 GHz super-class NMR. A superconducting wire joining structure characterized by the above.

5. 5. An NMR having a 30T superconducting magnet for 1 GHz-class NMR using the superconducting wire bonding structure of the high-temperature oxide superconducting wire and the metallic low-temperature superconducting wire according to claim 4.

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