Preparation method of superconducting joint for MgB2 multi-core wire

Abstract

The invention discloses a preparation method of a superconducting joint for an MgB2 multi-core wire, and the method comprises the steps: 1, corroding a middle section of the MgB2 multi-core wire through concentrated nitric acid, and enabling an internal core wire to be exposed; 2, immersing the core wire into the molten Ti metal liquid to form a Ti protective layer, and cutting off the core wire from the middle part to obtain two wires to be connected; 3, inserting the Ti-plated ends of two wires to be connected into the Mg / B welding agent through the hole, and compacting to form a joint structure; and 4, phase-forming heat treatment. The Ti protection layer is introduced to the surface of the Mg / B / Nb core wire, contact reaction of the metal sheath and an Mg / B welding agent is effectively isolated, the MgB2 superconducting effective area of a joint area is increased, distortion or breakage of the Mg / B / Nb core wire is prevented, the thermal stability of a joint structure is enhanced, the electric-thermal-mechanical comprehensive stability of the superconducting joint for the MgB2 multi-core wire is cooperatively improved, and the superconducting joint has the advantages of being high in reliability, high in reliability and the like. And the MgB2 magnet is assisted to realize continuous current operation.

Description

Technical Field

[0001] This invention belongs to the field of superconducting wire preparation technology, specifically relating to a method for preparing a superconducting connector for MgB2 multi-core wire. Background Technology

[0002] Magnetic resonance imaging (MRI) is a core piece of equipment in modern precision medicine, crucial for safeguarding public health and improving diagnostic and treatment levels. The key material for achieving the high-field magnets in traditional MRI is a superconducting wire capable of transmitting large currents without loss. Current MRI systems generally use niobium-titanium cryogenic superconducting wires, but these rely on expensive liquid helium cooling systems, resulting in high equipment purchase costs, complex maintenance systems, and stringent site requirements, limiting the widespread adoption and application of MRI technology. Therefore, developing low-cost, miniaturized, and lightweight MRI technology is crucial for promoting the universal adoption of precision medicine and achieving the strategic goal of "building a healthy China by 2035." Against this backdrop, magnesium diboride (MgB2), as a practical superconducting material, offers a new path to overcome these bottlenecks. Its core advantages are mainly reflected in the following three aspects: First, MgB2 has the lowest theoretical density currently available (approximately 2.6 g / cm³). 3 The practical superconducting material ( ) will help in the development of lighter MRI magnets; secondly, its superconducting transition temperature ( ) T c With a K of 39K, it can eliminate the dependence on liquid helium, significantly simplify the cooling system and reduce the overall weight; finally, its main raw materials, magnesium (Mg) and boron (B), are abundant and inexpensive, giving it a clear cost advantage over high-temperature superconducting materials such as yttrium barium copper oxide.

[0003] In MRI equipment, to generate a highly stable magnetic field for imaging (the magnetic field attenuation rate needs to be less than 0.1 ppm·h), -1When using MgB2 multi-core wire to wind superconducting magnets, superconducting connectors are needed to connect the individual magnets to form a continuous current mode operation. Therefore, the preparation of high-performance superconducting connectors suitable for MgB2 multi-core wires is a key step in achieving breakthroughs and practical applications in MRI magnet technology. Currently, MgB2 magnets for MRI are generally prepared using a "winding-reaction" process. In this process, the structure of the MgB2 wire mainly consists of Mg rods / powders and B powders (referred to as Mg / B core wires) covered with a metal sheath. The superconducting connectors for this wire are typically prepared using Mg powder and B powder as a bonding agent, followed by heat treatment to form the MgB2 superconducting connector. However, the external metal sheath of the wire (such as Monel alloy or copper, Cu) easily reacts with the filler material, and the resulting reaction layer reduces the formation area of ​​the MgB2 superconducting phase, leading to a decrease in the current-carrying capacity of the connector. To circumvent this problem, the conventional method is to remove the Monel and Cu sheaths through strong acid etching, retaining only the Mg / B core wire (referred to as Mg / B / Nb core wire, with a diameter of approximately hundreds of micrometers) wrapped in niobium (Nb). However, this process introduces new challenges: ① The Mg / B / Nb core wire has low strength and poor plasticity, making it prone to structural distortion or even breakage during fabrication, resulting in a reduction in the effective current-carrying area of ​​the joint; ② The mechanical properties of the etched Mg / B / Nb core wire at the connection with the wire are fragile, and even minor disturbances can cause breakage at the core wire connection; ③ After a large amount of the metal sheath is removed, the heat conduction capacity of the joint area decreases, and the thermal stability deteriorates. These problems are interconnected and collectively restrict the fabrication of high-performance MgB2 superconducting joints, becoming a key bottleneck for the practical application of this technology. Summary of the Invention

[0004] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a method for preparing a superconducting connector for MgB2 multi-core wires. This method introduces a Ti protective layer onto the surface of the Mg / B / Nb core wire, effectively isolating the metal sheath from contact reaction with the Mg / B flux, increasing the effective superconducting area of ​​MgB2 in the connector region, and also serving as a high-strength protective layer to prevent distortion or breakage of the Mg / B / Nb core wire. Simultaneously, it improves the thermal conductivity of the connector region, enhances the thermal stability of the connector structure, and synergistically improves the overall electro-thermal-mechanical stability of the MgB2 multi-core wire superconducting connector, enabling continuous current operation of the MgB2 magnet. This solves the problem that conventional strong acid corrosion of the sheath leads to a reduction in the effective area of ​​the connector, weakened mechanical properties, and poor thermal stability, making it impossible to obtain a high-performance MgB2 superconducting connector.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for preparing a superconducting connector for MgB2 multi-core wire, characterized in that the method includes the following steps: Step 1: Use concentrated nitric acid to corrode the middle section of the MgB2 multi-core wire to remove the outer Monel sheath and Cu sheath, exposing the internal Mg / B / Nb core wires to form the core wire section to be connected. Step 2: Immerse the exposed core wire segments to be connected obtained in Step 1 into molten Ti metal. Then, remove excess molten Ti metal by passing the polycrystalline drawing die with an aperture matching the diameter of the MgB2 multi-core wire in Step 1. After cooling, an attached Ti protective layer is formed on the surface of the exposed core wire segments to be connected, resulting in MgB2 multi-core wires with Ti-plated sections. Then, cut the wires from the middle of the Ti-plated sections to expose the internal Mg / B core wires, thereby obtaining two wires with Ti-plated ends to be connected. Step 3: Fill the stainless steel cylindrical container with Mg / B welding flux, and drill two independent holes on the lower side of the stainless steel cylindrical container with a diameter equivalent to that of the MgB2 multi-core wire in Step 1. Then, insert the Ti-plated ends of the two wires to be connected in Step 2 into the stainless steel cylindrical container through the two independent holes, ensuring that the exposed Mg / B core wire end face is in full contact with the Mg / B welding flux. Press the Cu plug at the top of the stainless steel cylindrical container to compact it and form a dense joint structure. Step 4: Perform phase-forming heat treatment on the joint structure formed in Step 3 and the connected MgB2 multi-core wire to promote the reaction of Mg / B welding agent and form a connected MgB2 superconducting phase, thereby obtaining a superconducting joint for MgB2 multi-core wire.

[0006] The above-mentioned method for preparing a superconducting connector for MgB2 multi-core wire is characterized in that the MgB2 multi-core wire in step one is made of Monel outer sheath, Cu inner sheath, Nb barrier layer and black precursor including Mg and B powder, wherein the Nb barrier layer wraps the black precursor to form Mg / B / Nb core wire.

[0007] The above-mentioned method for preparing a superconducting connector for MgB2 multi-core wire is characterized in that the molten Ti metal liquid in step two has a mass purity of not less than 98%, and the tensile strength of the Ti protective layer formed after cooling is not less than 300 MPa, and the thermal conductivity is not less than 15 W·K. -1 ·m -1 .

[0008] Compared with the prior art, the present invention has the following advantages: 1. This invention obtains a superconducting connector for MgB2 multi-core wires by attaching molten Ti metal to the surface of Mg / B / Nb core wires after removing the metal sheath and then cooling and solidifying it to form a Ti protective layer. Combined with the connector preparation process and phase formation heat treatment, a superconducting connector for MgB2 multi-core wires is obtained. By using the Ti protective layer to cover the Mg / B / Nb core wires, not only is the contact reaction between the metal sheath and the Mg / B welding agent avoided, increasing the effective superconducting area of ​​MgB2 in the connector region, but it also plays the role of a high-strength protective layer, preventing the Mg / B / Nb core wires from being distorted or broken during processing, thereby ensuring the smooth preparation of the superconducting connector.

[0009] 2. This invention improves the thermal conductivity of the joint area and enhances the thermal stability of the joint structure by introducing a Ti protective layer. It also utilizes the excellent mechanical properties (yield strength greater than 300 MPa) and high thermal conductivity (not less than 15 W·K) of the Ti protective layer. -1 ·m -1 This collaboratively enhances the overall electro-thermal-mechanical stability of superconducting connectors for MgB2 multi-core wires, helping MgB2 magnets achieve continuous current operation.

[0010] 3. The critical current density of the superconducting connector for MgB2 multi-core wire prepared in this invention ( Jc The maximum content can reach 85% of the parent material (MgB2 multi-core wire), providing technical support for MRI magnets to achieve continuous current mode operation and promoting the development of low-cost, miniaturized and lightweight MRI.

[0011] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0012] Figure 1 This is a schematic cross-sectional view of the 30-core MgB2 wire in step one of Embodiment 1 of the present invention.

[0013] Figure 2 This is a schematic diagram of the core wire segment to be connected prepared in step one of Embodiment 1 of the present invention.

[0014] Figure 3 This is a schematic diagram of the structure of the MgB2 multi-core wire with a Ti-plated section prepared in step two of Embodiment 1 of the present invention.

[0015] Figure 4 This is a schematic diagram of the dense joint structure prepared in step three of Embodiment 1 of the present invention.

[0016] Figure 5 This is a schematic diagram of the dense joint structure prepared in step three of embodiment 2 of the present invention. Detailed Implementation

[0017] Example 1 This embodiment includes the following steps: Step 1: Take the in-situ PIT ( in-situ A 30-core MgB2 wire with a diameter × length of Φ1.0mm × 100mm, prepared by the PIT method, has the following cross-section: Figure 1 As shown, the 30-core MgB2 wire is made of a Monel outer sheath, a Cu inner sheath, an Nb barrier layer, and a black precursor including Mg and B powder. The Nb barrier layer encapsulates the black precursor to form a Mg / B / Nb core wire. The outer Monel and Cu sheaths are etched with concentrated nitric acid to a 20mm section in the middle of the MgB2 wire, exposing the inner Mg / B / Nb core wire and forming the core wire segment to be connected. Figure 2 As shown; Step 2: Immerse the exposed core wire segment to be connected obtained in Step 1 into molten Ti metal. The molten Ti metal has a purity of not less than 98%, and the tensile strength of the Ti protective layer formed after cooling is not less than 300 MPa, and the thermal conductivity is not less than 15 W·K. -1 ·m -1 Then, using a polycrystalline drawing die with a Φ1.1mm aperture, excess molten Ti metal is removed. After cooling, an adhering Ti protective layer is formed on the surface of the exposed core wire segments to be connected, resulting in MgB2 multi-core wires with Ti-plated sections, such as... Figure 3 As shown, the section with the Ti-plated layer is then cut in the middle to expose the Mg / B core wire inside, thereby obtaining two wires with Ti-plated ends to be connected. Step 3: Weigh 24.5g of Mg powder and 21.8g of B powder according to a stoichiometric ratio of 1:2. Grind them for 30 minutes and then place them into a stainless steel cylindrical container with a diameter × wall thickness × length of Φ20mm × 2mm × 30mm as the Mg / B welding agent. Drill two independent holes with a diameter × depth of Φ1.1mm × 9mm on the lower side of the stainless steel cylindrical container. Then, insert the Ti-plated ends of the two wires to be connected in Step 2 into the stainless steel cylindrical container through the two independent holes, ensuring that the exposed Mg / B core wire end face is in full contact with the Mg / B welding agent. Press the Cu plug at the top of the stainless steel cylindrical container to compact it, forming a dense joint structure. Figure 4 As shown; Step 4: The joint structure and the connected MgB2 wire formed in Step 3 are heated to 600℃ under vacuum and held for 2 hours for phase formation heat treatment, which promotes the reaction of Mg / B welding agent and forms the connected MgB2 superconducting phase to obtain a superconducting joint for 30-core MgB2 wire.

[0018] Testing showed that the 30-core MgB2 wire superconducting connector prepared in this embodiment performed well at 4.2K and 3T. Jc =8×10 4 A / cm2 It can reach 65% of 30-core MgB2 wire, which is about 30% higher than the joint without Ti protective layer.

[0019] Example 2 This embodiment includes the following steps: Step 1: Take a 30-core MgB2 wire with a diameter × length of Φ1.0mm × 100mm prepared by the central Mg diffusion method (IMD), and use concentrated nitric acid to etch the outer Monel cladding and Cu cladding of the middle 20mm section of the MgB2 wire to expose the inner Mg / B / Nb core wire, forming the core wire segment to be connected. Step 2: Immerse the exposed core wire segments to be connected obtained in Step 1 into molten Ti metal, and then remove excess molten Ti metal by passing it through a polycrystalline drawing die with a diameter of Φ1.1mm. After cooling, an attached Ti protective layer is formed on the surface of the exposed core wire segments to be connected, resulting in MgB2 multi-core wires with Ti-plated sections. Then, cut the wires from the middle of the Ti-plated sections to expose the internal Mg / B core wires, thereby obtaining two wires with Ti-plated ends to be connected. Step 3: Weigh 36.7g of Mg powder and 32.7g of B powder according to a stoichiometric ratio of 1:2. Press them into Mg blocks with a diameter × height of Φ18mm × 10mm and B blocks with a diameter × height of Φ18mm × 12mm, respectively. Place them into a stainless steel cylindrical container with a diameter × wall thickness × length of Φ20mm × 2mm × 30mm as the Mg / B welding agent, with the B block at the bottom and the Mg block at the top. Drill two independent holes with a diameter × depth of Φ0.9mm × 9mm on the lower side of the stainless steel cylindrical container. Then, insert the Ti-plated ends of the two wires to be connected in Step 2 into the stainless steel cylindrical container through the two independent holes, ensuring that the exposed Mg / B core wire ends are in full contact with the Mg / B welding agent. Press the Cu plug at the top of the stainless steel cylindrical container to compact it, forming a dense joint structure. Figure 5 As shown; Step 4: The joint structure and the connected MgB2 wire formed in Step 3 are heated to 650℃ under vacuum and held for 4 hours for phase formation heat treatment, which promotes the reaction of Mg / B welding agent and forms the connected MgB2 superconducting phase to obtain a superconducting joint for 30-core MgB2 wire.

[0020] Testing showed that the 30-core MgB2 wire superconducting connector prepared in this embodiment performed well at 4.2K and 3T. Jc =2.4×10 4 A / cm 2 It can reach 65% of 30-core MgB2 wire, which is about 25% higher than the joint without Ti protective layer.

[0021] Example 3 This embodiment includes the following steps: Step 1: Take the in-situ PIT ( in-situ A 30-core C-doped MgB2 wire with a diameter × length of Φ1.0mm × 100mm was prepared by PIT method. The outer Monel cladding and Cu cladding of the middle 20mm section of the C-doped MgB2 wire were etched with concentrated nitric acid to expose the inner Mg / B / Nb core wire, forming the core wire segment to be connected. Step 2: Immerse the exposed core wire segments to be connected obtained in Step 1 into molten Ti metal, and then remove excess molten Ti metal by passing it through a polycrystalline drawing die with a diameter of Φ1.1mm. After cooling, an attached Ti protective layer is formed on the surface of the exposed core wire segments to be connected, resulting in MgB2 multi-core wires with Ti-plated sections. Then, cut the wires from the middle of the Ti-plated sections to expose the internal Mg / B core wires, thereby obtaining two wires with Ti-plated ends to be connected. Step 3: Weigh 24.5g of Mg powder and 21.8g of C-coated B powder according to a stoichiometric ratio of 1:2. Grind them for 30 minutes and then put them into a stainless steel cylindrical container with a diameter × wall thickness × length of Φ20mm × 2mm × 30mm as Mg / B welding agent. Drill two independent holes with a diameter × depth of Φ1.3mm × 9mm on the lower side of the stainless steel cylindrical container. Then, insert the Ti-plated ends of the two wires to be connected in Step 2 into the stainless steel cylindrical container through the two independent holes to ensure that the exposed Mg / B core wire end face is in full contact with the Mg / B welding agent. Press the Cu plug at the top of the stainless steel cylindrical container to compact it and form a dense joint structure. Step 4: The joint structure formed in Step 3 and the connected C-doped MgB2 wire are heated to 630℃ under vacuum and held for 2 hours for phase formation heat treatment, which promotes the reaction of Mg / B welding agent and forms the connected MgB2 superconducting phase, to obtain a superconducting joint for 30-core C-doped MgB2 wire.

[0022] Testing showed that the 30-core C-doped MgB2 wire superconducting connector prepared in this embodiment performed well at 4.2K and 3T. Jc =2.1×10 5 A / cm 2 It can reach 70% of the 30-core C-doped MgB2 wire, which is about 20% higher than the joint without the attached Ti protective layer.

[0023] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a superconducting connector for MgB2 multi-core wire, characterized in that, The method includes the following steps: Step 1: Use concentrated nitric acid to corrode the middle section of the MgB2 multi-core wire to remove the outer Monel sheath and Cu sheath, exposing the internal Mg / B / Nb core wires to form the core wire section to be connected. Step 2: Immerse the exposed core wire segments to be connected obtained in Step 1 into molten Ti metal. Then, remove excess molten Ti metal by passing the polycrystalline drawing die with an aperture matching the diameter of the MgB2 multi-core wire in Step 1. After cooling, an attached Ti protective layer is formed on the surface of the exposed core wire segments to be connected, resulting in MgB2 multi-core wires with Ti-plated sections. Then, cut the wires from the middle of the Ti-plated sections to expose the internal Mg / B core wires, thereby obtaining two wires with Ti-plated ends to be connected. Step 3: Fill the stainless steel cylindrical container with Mg / B welding flux, and drill two independent holes on the lower side of the stainless steel cylindrical container with a diameter equivalent to that of the MgB2 multi-core wire in Step 1. Then, insert the Ti-plated ends of the two wires to be connected in Step 2 into the stainless steel cylindrical container through the two independent holes, ensuring that the exposed Mg / B core wire end face is in full contact with the Mg / B welding flux. Press the Cu plug at the top of the stainless steel cylindrical container to compact it and form a dense joint structure. Step 4: Perform phase-forming heat treatment on the joint structure formed in Step 3 and the connected MgB2 multi-core wire to promote the reaction of Mg / B welding agent and form a connected MgB2 superconducting phase, thereby obtaining a superconducting joint for MgB2 multi-core wire.

2. The method for preparing a superconducting connector for MgB2 multi-core wire according to claim 1, characterized in that, The MgB2 multi-core wire described in step one is made of a Monel outer sheath, a Cu inner sheath, an Nb barrier layer, and a black precursor including Mg and B powders. The Nb barrier layer encapsulates the black precursor to form a Mg / B / Nb core wire.

3. The method for preparing a superconducting connector for MgB2 multi-core wire according to claim 1, characterized in that, The molten Ti metal liquid mentioned in step two has a purity of not less than 98%, and the tensile strength of the Ti protective layer formed after cooling is not less than 300 MPa, and the thermal conductivity is not less than 15 W·K. -1 ·m -1 .

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