Magnetic field resistant Nb3Sn-NbTi superconducting joint and preparation method thereof

Abstract

The invention provides a magnetic field resistant Nb3Sn-NbTi superconducting joint and a preparation method thereof. The magnetic field resistant Nb3Sn-NbTi superconducting joint comprises an Nb3Sn superconducting magnet, a first Nb3Sn lead, a second Nb3Sn lead, a first Nb3Sn joint, a second Nb3Sn joint, a joint manufacturing mold, a first Nb3Sn joint, a second Nb3Sn joint, a first niobium-titanium lead and a second niobium-titanium lead. And the first Nb3Sn joint is formed by sintering the end part of the first Nb3Sn lead and niobium, tin and copper powder. And the first Nb3Sn joint and the second Nb3Sn joint are formed by crimping through a joint manufacturing mold by taking iron materials as cores at the centers. According to the Nb3Sn-NbTi joint, the high magnetic field resistance of the Nb3Sn material is utilized, a magnetic field from a superconducting magnet is shielded, the magnetic field in the center of the joint is low, and the prepared Nb3Sn-NbTi joint has the advantages of being high in magnetic field resistance and low in joint resistance through the low-temperature superconducting solder and niobium-titanium superconducting connection.

Description

Technical Field

[0001] This invention belongs to the field of superconductivity, specifically relating to a magnetic field resistant Nb3Sn-NbTi superconducting joint and its preparation method. Background Technology

[0002] Nb3Sn superconducting materials have become the preferred material for high-field superconducting magnets with a critical temperature (~18K) and upper critical magnetic field (~25T) significantly superior to NbTi alloys, playing an irreplaceable role in high-energy physics, nuclear fusion devices, and high-field nuclear magnetic resonance spectrometers. However, due to limitations in the manufacturing length of a single Nb3Sn wire and the stress grading requirements of magnet design, large Nb3Sn magnet systems must connect multiple wires in series via superconducting connectors and then connect them to NbTi leads with excellent ductility to ultimately form a complete superconducting circuit.

[0003] Existing technologies mainly offer three types of solutions, but none of them can effectively resolve the core challenge of joining Nb3Sn-NbTi dissimilar materials under high-field conditions:

[0004] The first type of solution uses low-temperature superconducting solder (such as Wood's alloy or indium tin alloy) for direct casting connection. The upper critical magnetic field of this type of solder is typically below 1.5T. During fabrication, the joint must be placed in a low-field region (<0.5T) far from the center of the magnet, resulting in a significant increase in lead length and a substantial expansion of the magnet's Dewar volume, severely restricting the miniaturization and compact design of superconducting magnet systems. Furthermore, the solder itself has limited superconducting properties, making it difficult to meet the requirements of high-stability magnets for extremely low joint resistance (<10T). -11 The stringent requirements of Ω).

[0005] The second approach uses a sintered Nb3Sn bulk material directly bonded to NbTi multifilaments. While this method utilizes the Nb3Sn phase as the superconducting medium, its structure is a solid, externally encapsulated type (Nb wire bundle, Nb3Sn bulk material, Wood's alloy layer, Cu layer). The NbTi superconducting wires are only distributed in the surface solder layer of the bulk material, failing to address the magnetic field resistance issue of the solder itself. Under high-field conditions, the surface solder will still experience performance degradation due to magnetic field penetration, resulting in a sharp increase in joint resistance and even loss of superconductivity.

[0006] The third approach addresses NbTi-NbTi homopolymer bonding, using an Nb3Sn bulk as an intermediary and employing a pulse welding process. While this method introduces the concept of "magnetic field resistance," its flat structure is only suitable for homopolymer bonding of NbTi wires and cannot meet the dissimilar bonding requirements of brittle Nb3Sn magnet leads and ductile NbTi leads. Furthermore, the localized high temperatures (>500℃) of pulse welding can cause irreversible damage to the heat-treated brittle Nb3Sn phase, resulting in poor process compatibility.

[0007] In summary, existing technologies have not solved the problem of coordinating the design of high-field magnetic shielding and reliable connection of dissimilar materials, making the joint between the Nb3Sn magnet and the NbTi lead the weakest link in the entire superconducting circuit, which severely limits the engineering application of high-field superconducting magnet systems. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a magnetic field-resistant Nb3Sn-NbTi superconducting joint and its preparation method. The Nb3Sn joint end is fabricated into a block during the heat treatment of an Nb3Sn superconducting magnet. A hollow inner cavity is formed using a mold. Utilizing the high magnetic field resistance of Nb3Sn material, the magnetic field from the superconducting magnet is shielded, resulting in a lower magnetic field at the joint center. The joint is then connected to a niobium-titanium superconducting magnet using low-temperature superconducting solder. The resulting Nb3Sn-NbTi joint exhibits advantages such as strong magnetic field resistance and low joint resistance. The mandrel shape is a small-mouthed, flask-like shape, characterized by a small opening and a large inner cavity, maximizing the shielding of external magnetic fields.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A magnetic field-resistant Nb3Sn-NbTi superconducting connector includes an Nb3Sn superconducting magnet, a first Nb3Sn lead wire and a second Nb3Sn lead wire connected to the Nb3Sn superconducting magnet, a first Nb3Sn connector sintered with the end of the first Nb3Sn lead wire, a second Nb3Sn connector sintered with the end of the second Nb3Sn lead wire, a first niobium-titanium lead wire, a second niobium-titanium lead wire, and a low-temperature superconducting solder. Both the first and second Nb3Sn connectors have hollow cavities, formed by an iron core mold during sintering and then removed by hydrochloric acid etching. The ends of the first and second niobium-titanium lead wires are respectively inserted into the hollow cavities of the first and second Nb3Sn connectors. The low-temperature superconducting solder fills the hollow cavities and welds the niobium-titanium lead wires to the Nb3Sn connectors.

[0011] Preferably, the first Nb3Sn connector and the second Nb3Sn connector are sintered by mixing niobium powder, tin powder and copper powder in a ratio of 3:1:3, and the hollow inner cavity is a flask-like shape with a small mouth and a large inner cavity.

[0012] Preferably, the opening size of the hollow inner cavity is smaller than the maximum cross-sectional size of the inner cavity, and the maximum cross-sectional size of the inner cavity is located in the central region of the joint.

[0013] Preferably, the iron core mold is placed at the center of the joint making mold before sintering, and is removed by hydrochloric acid corrosion after sintering to form a hollow inner cavity.

[0014] Preferably, the ends of the first Nb3Sn lead and the second Nb3Sn lead are cut into a bevel shape, and the beveled ends are inserted into the connector manufacturing mold and sintered together with niobium powder, tin powder and copper powder.

[0015] Preferably, the low-temperature superconducting solder is an indium tin alloy or a Wood alloy, with a melting temperature not exceeding 150 degrees Celsius, and completely fills the hollow inner cavity.

[0016] Preferably, the ends of the first and second niobium-titanium leads are etched to expose the niobium-titanium superconducting wires. The exposed ends are inserted into the hollow cavity and connected to the Nb3Sn connector through low-temperature superconducting solder to form a three-dimensional superconducting connection.

[0017] This invention also provides a method for preparing a magnetic field-resistant Nb3Sn-NbTi superconducting connector, comprising: removing the insulating layer from the ends of the first Nb3Sn lead and the second Nb3Sn lead; cutting the ends into angled bevels; pouring a mixture of niobium powder, tin powder, and copper powder into a connector manufacturing mold; placing an iron core mold at the center of the connector manufacturing mold; inserting the first Nb3Sn lead and the second Nb3Sn lead with angled ends into the connector manufacturing mold and tightening the mold; and then... The superconducting magnet, Nb3Sn leads, and connector fabrication mold are placed together in a heat treatment furnace for heat treatment; the connector fabrication mold is opened and the first Nb3Sn connector and the second Nb3Sn connector are removed; the iron core mold is removed, cleaned and dried; the ends of the first niobium-titanium leads and the second niobium-titanium leads are respectively inserted into the hollow first Nb3Sn connector and the second Nb3Sn connector, and immersed in molten low-temperature superconducting solder to fill the core mold space, so that the niobium-titanium leads are welded to the Nb3Sn connector.

[0018] Preferably, the mixing ratio of niobium powder, tin powder and copper powder is 3:1:3, and the mixing ratio is determined by weight ratio.

[0019] Preferably, the melting temperature of the low-temperature superconducting solder does not exceed 150 degrees Celsius, including indium tin alloy or Wood's alloy, and it completely fills the hollow cavity in the molten state.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. The Nb3Sn block in the connector of this invention adopts a flask-shaped hollow structure design. Utilizing the Nb3Sn material's own upper critical magnetic field (above 20T), it effectively shields against strong external magnetic fields, reducing the internal magnetic field strength to below 0.5T. Low-temperature superconducting solder (such as Wood's alloy) is completely infused into the cavity, maintaining excellent superconducting performance in the shielded magnetic field environment, fundamentally solving the core problem of low-temperature solder's intolerance to high fields. The connector can be deployed close to the magnet's periphery, eliminating the need for long-distance routing to low-field regions, providing a novel solution for the compact design of superconducting magnets.

[0022] 2. This invention employs an integral infusion process to replace traditional spot welding or surface casting. The NbTi leads and the Nb3Sn bulk are connected in a three-dimensional superconducting manner through solder, increasing the contact area by 2-3 orders of magnitude. The measured joint resistance is below 5 × 10⁻⁶. -12 Ω (@4.2K), current decay time constant exceeds 10 5 The system meets the stringent requirements of high-stability magnets such as nuclear magnetic resonance spectrometers for a magnetic field drift rate of <0.01ppm / h, significantly reducing Joule heat loss and quench risk at the connector.

[0023] 3. The hollow block structure of this invention avoids mechanical damage to the Nb3Sn phase in the connection area. The NbTi lead insertion installation and low-temperature solder curing (<150℃) process is entirely stress-free and free from high temperatures, fully protecting the integrity of the heat-treated brittle Nb3Sn phase. The flask-shaped inner cavity's "small opening, large inner cavity" feature further enhances the solder filling density and the joint's resistance to electromagnetic forces, improving structural reliability by an order of magnitude compared to flat plate welding.

[0024] 4. The core mold of this invention is sintered and formed simultaneously with the heat treatment of the magnet, and a precision cavity can be formed by hydrochloric acid etching, without the need for additional precision machining. This process not only reduces manufacturing costs, but also ensures the in-situ integrated preparation of the Nb3Sn block and the magnet leads, avoiding interface contamination and resistance degradation introduced by secondary assembly.

[0025] 5. The connector volume of the present invention is reduced, the amount of lead wire used is reduced, the Dewar space is correspondingly compressed, and the overall volume and weight of the magnet system are reduced, which directly reduces the cost of superconducting wires, cryogenic cooling media and cryogenic containers, and has significant economic value for controlling the engineering cost of large scientific devices (such as accelerators and fusion reactors).

[0026] 6. The connector structure of the present invention maintains stable superconducting performance in the background magnetic field range of 0~15T. It is not only suitable for existing high-field Nb3Sn magnets, but also compatible with the connector requirements of future higher-field magnets (such as 15~20T next-generation nuclear magnetic resonance magnets), providing key support for the development of superconducting magnet technology towards higher field strength. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a magnetic field-resistant Nb3Sn-NbTi superconducting joint according to the present invention;

[0028] Figure 2 This is another schematic diagram of a magnetic field-resistant Nb3Sn-NbTi superconducting connector according to the present invention;

[0029] Figure 3 This is a schematic diagram of the overall Nb3Sn superconducting magnet of the present invention;

[0030] Figure 4 This is a schematic diagram of the core mold of the present invention;

[0031] Figure 5 This is a schematic diagram of the mold for manufacturing the connector of the present invention.

[0032] The attached figures are labeled as follows: 1. Nb3Sn superconducting magnet; 2. First Nb3Sn lead; 3. Second Nb3Sn lead; 4. First Nb3Sn connector; 5. Second Nb3Sn connector; 6. First niobium-titanium lead; 7. Second niobium-titanium lead; 8. Core mold; 9. Connector fabrication mold; 10. Low-temperature superconducting solder. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0034] like Figures 1-3 As shown, the magnetic field-resistant Nb3Sn-NbTi superconducting connector of the present invention, from bottom to top, consists of an Nb3Sn superconducting magnet 1, a first Nb3Sn lead 2 and a second Nb3Sn lead 3, a first Nb3Sn connector 4 and a second Nb3Sn connector 5, a first niobium-titanium lead 6 and a second niobium-titanium lead 7. The first Nb3Sn lead 2 and the second Nb3Sn lead 3 are two externally connected leads of the Nb3Sn superconducting magnet 1; the first Nb3Sn connector 4 is formed by sintering the end of the first Nb3Sn lead 2 with niobium, tin, and copper powder; the second Nb3Sn connector 5 is formed by sintering the end of the second Nb3Sn lead 3 with niobium, tin, and copper powder; the first Nb3Sn connector 4 and the first niobium-titanium lead 6 are connected by low-temperature superconducting solder to form the first connector; the second Nb3Sn connector 5 and the second niobium-titanium lead 7 are connected by low-temperature superconducting solder to form the second connector. The first joint is located at the position of the first Nb3Sn joint 4. The internal core mold 8 is melted away, and the first niobium-titanium lead 6 is inserted before being poured with low-temperature superconducting solder to form the whole. The second joint is located at the position of the second Nb3Sn joint 5. The internal core mold 8 is removed, and the second niobium-titanium lead 7 is inserted before being poured with low-temperature superconducting solder to form the whole. The purpose of the joint fabrication mold is to place the core mold inside, then pour niobium, tin, and copper powder into the mold, insert the first Nb3Sn lead 2 and the second Nb3Sn lead 3 into the powder respectively, and then perform high-temperature heat treatment together to generate the first Nb3Sn joint 4 and the second Nb3Sn joint 5.

[0035] like Figure 2As shown, after etching a niobium-titanium superconducting wire at the end of the first niobium-titanium lead 6, it is inserted into the cavity of the first Nb3Sn connector 4 and immersed in molten low-temperature superconducting solder 10. The low-temperature superconducting solder 10 fills the middle cavity of the first Nb3Sn connector 4, thereby welding the first niobium-titanium lead 6 and the first Nb3Sn connector 4 together, forming a connection structure between the first niobium-titanium lead 6 and the first Nb3Sn lead 2. The high critical magnetic field of the first Nb3Sn connector 4 forms a shield against external magnetic fields for the internally magnetically intolerant low-temperature superconducting solder 10.

[0036] like Figures 1-5 As shown, the present invention also provides a method for preparing a magnetic field-resistant Nb3Sn-NbTi superconducting joint, comprising the following steps:

[0037] Step 1: Remove the insulation layer from the ends of the first Nb3Sn lead 2 and the second Nb3Sn lead 3;

[0038] Step 2: Cut the ends of the first Nb3Sn lead 2 and the second Nb3Sn lead 3 into angled bevels;

[0039] Step 3: Pour the mixed niobium, tin and copper powder into the joint making mold 9, with a ratio of 3:1:3;

[0040] Step 4: Place the iron core mold 8 into the connector manufacturing mold 9, so that the core mold 8 is located in the center of the connector manufacturing mold 9. The core mold 8 has the characteristics of a small opening and a large inner cavity. The core mold 8 is flask-like in shape, with a spherical bottom with a radius of 5-20mm, and the opening gradually narrows towards the top to a small circular opening with a radius of 1-3mm at the very top, through which the first niobium-titanium lead 6 and the second niobium-titanium lead 7 can be inserted.

[0041] Step 5: Insert the first Nb3Sn lead 2 and the second Nb3Sn lead 3, which have beveled ends, into the connector manufacturing mold 9, and tighten the two sides of the connector manufacturing mold 9 with screws.

[0042] Step 6: Place the Nb3Sn superconducting magnet 1, the first Nb3Sn lead 2, the second Nb3Sn lead 3, and the connector mold 9 into a heat treatment furnace for heat treatment. The heat treatment regime is under vacuum conditions, the temperature is 640-680℃, and the treatment time is 50-200h.

[0043] Step 7: Open the connector manufacturing mold 9 and take out the first Nb3Sn connector 4 and the second Nb3Sn connector 5.

[0044] Step 8: Pour hydrochloric acid into the small opening at the top of the core mold 8 to corrode the core mold 8 inside the first Nb3Sn connector 4 and the second Nb3Sn connector 5, and clean the internal impurities with deionized water and alcohol before letting it dry in the air.

[0045] Step 9: Insert the first niobium-titanium lead 6 and the second niobium-titanium lead 7 into the hollow first Nb3Sn connector 4 and the second Nb3Sn connector 5 respectively, and immerse them in molten low-temperature superconducting solder 10. Use the low-temperature superconducting solder 10 to fill the hollow cavity left by the mandrel 8, thereby welding the first niobium-titanium lead 6 and the second niobium-titanium lead 7 to the first Nb3Sn connector 4 and the second Nb3Sn connector 5 respectively.

[0046] Preferably, the low-temperature superconducting solder 10 has a melting temperature not exceeding 150 degrees Celsius, and includes indium tin alloy, Wood's alloy, etc.

[0047] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A magnetic field-resistant Nb3Sn-NbTi superconducting joint, characterized in that, The device includes an Nb3Sn superconducting magnet, a first Nb3Sn lead wire and a second Nb3Sn lead wire connected to the Nb3Sn superconducting magnet, a first Nb3Sn connector sintered with the end of the first Nb3Sn lead wire, a second Nb3Sn connector sintered with the end of the second Nb3Sn lead wire, a first niobium-titanium lead wire, a second niobium-titanium lead wire, and low-temperature superconducting solder. Both the first Nb3Sn connector and the second Nb3Sn connector have hollow cavities inside. The hollow cavities are formed by an iron core mold during the sintering process and then removed by hydrochloric acid etching. The ends of the first niobium-titanium lead wire and the second niobium-titanium lead wire are respectively inserted into the hollow cavities of the first Nb3Sn connector and the second Nb3Sn connector. The low-temperature superconducting solder fills the hollow cavities and welds the niobium-titanium lead wire to the Nb3Sn connector.

2. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 1, characterized in that, The first and second Nb3Sn joints are sintered from a mixture of niobium powder, tin powder and copper powder in a 3:1:3 ratio, and have a hollow inner cavity in the shape of a flask with a small opening and a large inner cavity.

3. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 2, characterized in that, The opening size of the hollow inner cavity is smaller than the maximum cross-sectional size of the inner cavity, and the maximum cross-sectional size of the inner cavity is located in the central region of the joint.

4. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 1, characterized in that, The iron core mold is placed in the center of the joint making mold before sintering. After sintering, it is removed by hydrochloric acid etching to form a hollow inner cavity.

5. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 1, characterized in that, The ends of the first Nb3Sn lead and the second Nb3Sn lead are cut into angled bevels, which are then inserted into a connector manufacturing mold and sintered together with niobium powder, tin powder and copper powder.

6. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 1, characterized in that, The low-temperature superconducting solder is an indium tin alloy or Wood's alloy, with a melting temperature not exceeding 150 degrees Celsius, and is completely filled into the hollow cavity.

7. The magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 1, characterized in that, After the ends of the first and second niobium-titanium leads are etched, the niobium-titanium superconducting wires are exposed. The exposed ends are inserted into the hollow cavity and connected to the Nb3Sn connector through low-temperature superconducting solder to form a three-dimensional superconducting connection.

8. A method for preparing a magnetic field-resistant Nb3Sn-NbTi superconducting joint, characterized in that, Includes the following steps: Remove the insulation layer from the ends of the first and second Nb3Sn leads; cut the ends into angled bevels; pour a mixture of niobium powder, tin powder, and copper powder into the joint making mold; place the iron core mold in the center of the joint making mold; insert the first and second Nb3Sn leads with beveled ends into the joint making mold and tighten the mold; place the Nb3Sn superconducting magnet, Nb3Sn leads, and joint making mold together in a heat treatment furnace for heat treatment; open the joint making mold and remove the first and second Nb3Sn joints; remove the iron core mold by etching with hydrochloric acid, clean and dry; insert the ends of the first and second niobium-titanium leads into the hollow first and second Nb3Sn joints respectively, and immerse them in molten low-temperature superconducting solder to fill the core mold space, so that the niobium-titanium leads are welded to the Nb3Sn joints.

9. The method for preparing the magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 8, characterized in that, The mixing ratio of niobium powder, tin powder and copper powder is 3:1:3, and the mixing ratio is determined by weight ratio.

10. The method for preparing the magnetic field-resistant Nb3Sn-NbTi superconducting joint according to claim 8, characterized in that, The melting temperature of the low-temperature superconducting solder does not exceed 150 degrees Celsius, including indium tin alloy or Wood's alloy, and it completely fills the hollow cavity in the molten state.

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