A NbTi multi-core wire and a preparation method thereof

Abstract

The invention discloses a NbTi multi-core wire and a preparation method thereof, belonging to the technical field of superconducting wires. The invention coats a Ta / Cu / Ta sheath on the surface of an oxygen-free copper sheath, uses oxygen-free copper doped with a trace amount of Ta element as the center, and closely arranges NbTi / Cu single core rods and oxygen-free copper rods in the Ta / Cu / Ta sheath as the first central component; takes the first central component as the center, closely arranges NbTi / Cu single core rods and oxygen-free copper rods in the Ta / Cu / Ta sheath as the second central component; takes the second central component as the center, closely arranges NbTi / Cu single core rods and oxygen-free copper rods in the oxygen-free copper sheath, adds copper caps at both ends and seals them with welding, and obtains NbTi / Cu multi-core wires through extrusion and multiple drawing passes. The method of the invention improves the stability of the wire in the stretching stage and the performance of the wound magnet, and significantly reduces the AC loss.

Description

Technical Field

[0001] The invention belongs to the technical field of superconducting wires and relates to a NbTi multi-core wire and a preparation method thereof. Background Art

[0002] NbTi superconducting wire has the advantages of superconducting properties in medium and low magnetic fields, good machinability and relatively low cost. It is widely used in situations requiring high current density and good magnetic field performance, such as superconducting magnets, superconducting cables and magnetic resonance imaging (MRI) systems.

[0003] However, despite the many advantages of NbTi superconducting wires, they still face some challenges in practical applications. Among them, the magnetization of superconductors and magnetic flux creep are two major problems. The magnetization phenomenon will lead to uneven distribution of the magnetic field inside the superconductor, which will affect the spatial field uniformity of the magnet. The magnetic flux creep will cause the performance of the magnet to change over time, reducing the stability of the time field. The combined effect of these two problems not only affects the performance of the superconducting magnet, but may also pose a threat to its operating stability and safety margin.

[0004] In order to reduce the loss caused by magnetization and flux creep, researchers have proposed two solutions. One strategy is to increase the critical current density and reduce hysteresis loss by thinning the core of the superconducting wire, that is, reducing the diameter of the core wire. The other strategy is to improve the uniformity of the core wire of the superconducting wire to ensure that the performance of the wire in the magnetic field is more stable.

[0005] However, in the current manufacturing process of NbTi / Cu composite wire, oxygen-free copper is usually used as the central copper rod, and the internal core wire is processed as a whole. Although this structure improves the electrical conductivity and thermal stability of the wire to a certain extent, it also brings some new problems. Due to the high hardness of the oxygen-free copper rod, the core wire near the center is prone to uneven deformation during the stretching process, which not only increases the difficulty of wire production, but also increases the risk of wire breakage of small-sized wires during the stretching process.

[0006] In addition, due to the difference in thermal expansion coefficients between the oxygen-free copper rod and the core wire, as well as the interaction between the core wires, the hysteresis loss of the NbTi / Cu composite wire is relatively high. This not only reduces the energy efficiency of the wire, but may also have an adverse effect on the long-term stable operation of the superconducting magnet.

[0007] Therefore, how to maintain the excellent performance of NbTi superconducting wire while solving its magnetization and flux creep problems, reducing hysteresis loss, and improving the uniformity and tensile strength of the wire has become a technical problem that needs to be urgently solved in the current superconducting materials field. Summary of the invention

[0008] The present invention aims to solve the problems and defects in the above-mentioned prior art, and provides a NbTi multi-core wire and a preparation method thereof.

[0009] In a first aspect, the present invention provides a method for preparing a NbTi multi-core wire, comprising: using oxygen-free copper doped with Ta or Ag elements as the center, and closely arranging NbTi / Cu single-core rods and oxygen-free copper rods in a Ta / Cu / Ta sheath as a first central component; using the first central component as the center, and closely arranging NbTi / Cu single-core rods and oxygen-free copper rods in a Ta / Cu / Ta sheath as a second central component; using the second central component as the center, and closely arranging NbTi / Cu single-core rods and oxygen-free copper rods in an oxygen-free copper sheath, adding copper covers at both ends and sealing them with welding, and obtaining a NbTi / Cu multi-core wire through extrusion and stretching.

[0010] Furthermore, in the preparation method provided by the present invention, the preparation method of the Ta / Cu / Ta sheath comprises: cleaning and corroding the oxygen-free copper sheath with 30% nitric acid, and then depositing Ta on the surface of the oxygen-free copper sheath to obtain the Ta / Cu / Ta sheath.

[0011] Furthermore, the present invention provides a method for preparing a Ta / Cu / Ta sheath, wherein the cleaning and etching thickness is 3-5 μm;

[0012] The deposition method is any one of chemical vapor deposition, physical vapor deposition, and magnetron sputtering;

[0013] The thickness of the deposited Ta is 10-50 μm.

[0014] Furthermore, in the preparation method provided by the present invention, the diameter of the oxygen-free copper doped with Ta or Ag element is Φ25mm~Φ60mm;

[0015] The doping amount of the Ta element is 0.01%~0.1%;

[0016] The doping amount of the Ag element is 1% to 2%.

[0017] Furthermore, in the preparation method provided by the present invention, the outer diameter of the Ta / Cu / Ta sheath for preparing the first central component is φ55mm~φ120mm, the inner diameter is φ50mm~φ117mm, and the length is 200mm~1000mm.

[0018] Furthermore, in the preparation method provided by the present invention, the outer diameter of the Ta / Cu / Ta sheath for preparing the second central component is φ100mm~φ200mm, the inner diameter is φ95mm~φ197mm, and the length is 200mm~1000mm.

[0019] Furthermore, in the preparation method provided by the present invention, the height of the hexagonal cross-section of the NbTi / Cu single core rod is 3 mm to 30 mm, and the length is 200 mm to 1000 mm; the diameter of the oxygen-free copper rod is 1 mm to 3 mm, and the length is 200 mm to 1000 mm.

[0020] Furthermore, in the preparation method provided by the present invention, the outer diameter of the oxygen-free copper sheath is φ200mm~φ400mm, the inner diameter is φ180mm~φ380mm, and the length is 200mm~1000mm.

[0021] Furthermore, in the preparation method provided by the present invention, the extrusion processing rate is 2% to 3%;

[0022] The stretching speed is 1-100 m / min.

[0023] In a second aspect, the present invention provides a NbTi multi-core wire, which is prepared by the above preparation method.

[0024] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages:

[0025] The present invention obtains a Ta / Cu / Ta sheath by plating a Ta film on the surface of an oxygen-free copper sheath by a deposition method, uses an oxygen-free copper rod doped with trace Ta / Ag elements to replace a central copper rod, and replaces a traditional NbTi / Cu composite wire assembly process with different Ta / Cu / Ta sheaths during the assembly process, so that a Ta layer exists in the structure of the NbTi / Cu composite wire, and the Ta layer divides the NbTi / Cu single core rod inside the NbTi / Cu composite wire into different concentric circle modules, which can effectively reduce the AC loss of the superconducting wire and the operating stability of the superconducting magnet; at the same time, the oxygen-free copper rod doped with trace Ta / Ag elements is used to replace the central copper rod, which ensures the uniformity of the internal core wire deformation, reduces the formation of irregular core wires, and reduces the risk of wire breakage in the subsequent stretching process, thereby ensuring the stability of the wire stretching in the small-size stage, and ensuring the subsequent long-line processing capacity and the stability of the wire performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the cross-sectional structure of Cu / Ta / Cu composite rod for preparing NbTi multi-core wire. 1 is an oxygen-free copper rod doped with Ta or Ag, 2 is a NbTi / Cu single-core rod, the gap is filled with oxygen-free copper rod, 3 is Ta / Cu / Ta sheath, and 4 is oxygen-free copper sheath. DETAILED DESCRIPTION

[0027] The technical scheme of the present invention is described below in conjunction with the embodiments, but the present invention is not limited to the following embodiments. The experimental methods and detection methods described in each embodiment are conventional methods unless otherwise specified; the reagents and materials described are all commercially available unless otherwise specified.

[0028] Example 1

[0029] This embodiment provides a method for preparing a NbTi multi-core wire.

[0030] S1. Select oxygen-free copper sheaths with an outer diameter of φ55mm, an inner diameter of φ50mm, and a length of 200mm and an outer diameter of φ105mm, an inner diameter of φ100mm, and a length of 200mm, use 30% nitric acid to clean and corrode their surfaces with a cleaning and corrosion amount of 10μm, use chemical vapor deposition technology to deposit a Ta film with a thickness of 10μm on the surface of the oxygen-free copper sheath, use 30% nitric acid to clean and corrode it again with a cleaning and corrosion amount of 3μm, and then obtain a Ta / Cu / Ta sheath.

[0031] S2. Select NbTi / Cu single core rods with a hexagonal cross-section height of 3mm and a length of 200mm, oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 200mm for nitric acid (concentration of 30%) corrosion cleaning, and the corrosion amount is 1μm. Oxygen-free copper doped with 0.01% Ta element is used as the central copper rod (diameter of φ25mm and length of 200mm), which is closely packed with the NbTi / Cu single core rod and assembled into a Ta / Cu / Ta sheath with an outer diameter of φ55mm, an inner diameter of φ50mm and a length of 200mm. The gap is filled with oxygen-free copper rods to obtain the first central component.

[0032] S3. The first central component in S2 and the NbTi / Cu single core rod are closely packed into a Ta / Cu / Ta sheath with an outer diameter of φ105mm, an inner diameter of φ100mm, and a length of 200mm. The gap is filled with an oxygen-free copper rod to obtain the second central component.

[0033] S4. The second central component in S3 and the NbTi / Cu single core rod are closely packed and assembled into an oxygen-free copper sheath, and the gaps are filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ200 mm, an inner diameter of φ180 mm, and a length of 200 mm.

[0034] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the NbTi / Cu multi-core wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 20~25mJ / cm 3 .

[0035] Example 2

[0036] This embodiment provides a method for preparing a NbTi multi-core wire.

[0037] S1. Select oxygen-free copper sheaths with an outer diameter of φ60mm, an inner diameter of φ55mm, and a length of 400mm and an outer diameter of φ100mm, an inner diameter of φ95mm, and a length of 400mm, use 30% nitric acid to clean and corrode the surface with a cleaning and corrosion amount of 20μm, use physical vapor deposition technology to deposit a Ta film with a thickness of 30μm on the surface of the oxygen-free copper sheath, then use 30% nitric acid to clean it with a cleaning and corrosion amount of 5μm, and then obtain a Ta / Cu / Ta sheath.

[0038] S2. Select a NbTi / Cu single core rod with a hexagonal cross-section height of 4mm and a length of 400mm and oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 400mm for nitric acid (concentration of 30%) corrosion cleaning, and the corrosion amount is 1μm. Oxygen-free copper doped with 0.05% Ta element is used as the central copper rod (diameter of φ22.5mm and length of 400mm), which is closely packed with the NbTi / Cu single core rod and assembled into a Ta / Cu / Ta sheath with an outer diameter of φ60mm, an inner diameter of φ55mm and a length of 400mm. The gap is filled with oxygen-free copper rods to obtain the first central component.

[0039] S3. The first central component obtained in S2 is closely assembled with the NbTi / Cu single core rod into a Ta / Cu / Ta sheath with an outer diameter of φ100 mm, an inner diameter of φ95 mm, and a length of 400 mm. The gap is filled with an oxygen-free copper rod to obtain the second central component.

[0040] S4. The second central component obtained in S3 is closely assembled with the NbTi / Cu single core rod into an oxygen-free copper sheath, and the gap is filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ220 mm, an inner diameter of φ200 mm, and a length of 400 mm.

[0041] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the superconducting wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 25~30mJ / cm 3 .

[0042] Example 3

[0043] This embodiment provides a method for preparing a NbTi multi-core wire.

[0044] S1. Select oxygen-free copper sheaths with an outer diameter of φ65mm, an inner diameter of φ62mm, and a length of 1000mm and an outer diameter of φ120mm, an inner diameter of φ117mm, and a length of 1000mm, clean their surfaces with 30% nitric acid with a cleaning and etching amount of 30μm, deposit a 50μm thick Ta film on the surface of the oxygen-free copper sheath using magnetron sputtering, clean and corrode its surface with 30% nitric acid with a corrosion amount of 5μm, and then obtain a Ta / Cu / Ta sheath.

[0045] S2. Select a NbTi / Cu single core rod with a hexagonal cross-section height of 6.0mm and a length of 1000mm and oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 1000mm for nitric acid (concentration of 30%) corrosion cleaning, and the corrosion amount is 1μm. Oxygen-free copper doped with 1.0% Ta element is used as the central copper rod (diameter of φ31mm and length of 1000mm), which is closely packed with the NbTi / Cu single core rod and assembled into a Ta / Cu / Ta sheath with an outer diameter of φ65mm, an inner diameter of φ62mm and a length of 1000mm. The gap is filled with oxygen-free copper rods to obtain the first central component.

[0046] S3. The first central component in S2 and the NbTi / Cu single core rod are closely packed into a Ta / Cu / Ta sheath with an outer diameter of φ120 mm, an inner diameter of φ117 mm, and a length of 1000 mm, and the gap is filled with an oxygen-free copper rod to obtain the second central component.

[0047] S4. The second central component in S3 and the NbTi / Cu single core rod are closely packed and assembled into an oxygen-free copper sheath, and the gaps are filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ300mm, an inner diameter of φ280mm, and a length of 1000mm.

[0048] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the superconducting wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 25~30mJ / cm 3 .

[0049] Example 4

[0050] This embodiment provides a method for preparing a NbTi multi-core wire.

[0051] S1. Select oxygen-free copper sheaths with an outer diameter of φ120mm, an inner diameter of φ117mm, and a length of 1000mm and an outer diameter of φ200mm, an inner diameter of φ197mm, and a length of 1000mm, clean their surfaces with 30% nitric acid with a cleaning and etching amount of 150μm, deposit a 50μm thick Ta film on the surface of the oxygen-free copper sheath by magnetron sputtering, and clean and etch its surface again with 30% nitric acid with a etching amount of 5μm, and then obtain a Ta / Cu / Ta sheath.

[0052] S2. Select a NbTi / Cu single core rod with a hexagonal cross-section height of 8.50mm and a length of 1000mm and oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 1000mm for cleaning and etching with nitric acid (concentration of 30%), with an etching amount of 1μm. Oxygen-free copper doped with 1.0% Ag element is used as the central copper rod (diameter of φ50mm and length of 1000mm), which is closely packed with the NbTi / Cu single core rod and assembled into a Ta / Cu / Ta sheath with an outer diameter of φ120mm, an inner diameter of φ117mm and a length of 1000mm. The gap is filled with oxygen-free copper rods to obtain the first central component.

[0053] S3. The first central component in S2 and the NbTi / Cu single core rod are closely packed into a Ta / Cu / Ta sheath with an outer diameter of φ200mm, an inner diameter of φ197mm, and a length of 1000mm, and the gap is filled with an oxygen-free copper rod to obtain the second central component.

[0054] S4. The second central component in S3 and the NbTi / Cu single core rod are closely packed and assembled into an oxygen-free copper sheath, and the gaps are filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ400mm, an inner diameter of φ380mm, and a length of 1000mm.

[0055] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the superconducting wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 25~30mJ / cm 3 .

[0056] Example 5

[0057] This embodiment provides a method for preparing a NbTi multi-core wire.

[0058] S1. Select oxygen-free copper sheaths with an outer diameter of φ120mm, an inner diameter of φ117mm, and a length of 1000mm and an outer diameter of φ200mm, an inner diameter of φ197mm, and a length of 1000mm, clean their surfaces with 30% nitric acid with a cleaning and etching amount of 150μm, deposit a 50μm thick Ta film on the surface of the oxygen-free copper sheath by magnetron sputtering, and clean and etch its surface again with 30% nitric acid with a etching amount of 5μm, and then obtain a Ta / Cu / Ta sheath.

[0059] S2. Select a NbTi / Cu single core rod with a hexagonal cross-section height of 8.50mm and a length of 1000mm and oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 1000mm for cleaning and etching with nitric acid (concentration of 30%), with an etching amount of 1μm. Oxygen-free copper doped with 1.5% Ag element is used as the central copper rod (diameter of φ50mm and length of 1000mm), which is closely packed with the NbTi / Cu single core rod and assembled into a Ta / Cu / Ta sheath with an outer diameter of φ120mm, an inner diameter of φ117mm and a length of 1000mm. The gap is filled with oxygen-free copper rods to obtain the first central component.

[0060] S3. The first central component in S2 and the NbTi / Cu single core rod are closely packed into a Ta / Cu / Ta sheath with an outer diameter of φ200mm, an inner diameter of φ197mm, and a length of 1000mm, and the gap is filled with an oxygen-free copper rod to obtain the second central component.

[0061] S4. The second central component in S3 and the NbTi / Cu single core rod are closely packed and assembled into an oxygen-free copper sheath, and the gaps are filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ400mm, an inner diameter of φ380mm, and a length of 1000mm.

[0062] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the superconducting wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 25~30mJ / cm 3 .

[0063] Example 6

[0064] This embodiment provides a method for preparing a NbTi multi-core wire.

[0065] S1. Select oxygen-free copper sheaths with an outer diameter of φ55mm, an inner diameter of φ50mm, and a length of 200mm and an outer diameter of φ105mm, an inner diameter of φ100mm, and a length of 200mm, use 30% nitric acid to clean and corrode their surfaces with a cleaning and corrosion amount of 10μm, use chemical vapor deposition technology to deposit a Ta film with a thickness of 10μm on the surface of the oxygen-free copper sheath, use 30% nitric acid to clean and corrode it again with a cleaning and corrosion amount of 3μm, and then obtain a Ta / Cu / Ta sheath.

[0066] S2. Select NbTi / Cu single core rods with a hexagonal cross-section height of 3mm and a length of 200mm, and oxygen-free copper rods with diameters of φ1mm, φ2mm, φ3mm and a length of 200mm for nitric acid (concentration of 30%) corrosion cleaning, and the corrosion amount is 1μm. Oxygen-free copper doped with 2.0% Ag element is used as the central copper rod (diameter of φ25mm and length of 200mm), which is closely assembled with the NbTi / Cu single core rod into a Ta / Cu / Ta sheath with an outer diameter of φ55mm, an inner diameter of φ50mm and a length of 200mm, and the gap is filled with oxygen-free copper rods to obtain the first central component.

[0067] S3. The first central component in S2 and the NbTi / Cu single core rod are closely packed into a Ta / Cu / Ta sheath with an outer diameter of φ105mm, an inner diameter of φ100mm, and a length of 200mm. The gap is filled with an oxygen-free copper rod to obtain the second central component.

[0068] S4. The second central component in S3 and the NbTi / Cu single core rod are closely packed and assembled into an oxygen-free copper sheath, and the gaps are filled with oxygen-free copper rods to obtain a NbTi / Cu / Ta composite sheath; the oxygen-free copper sheath has an outer diameter of φ200mm, an inner diameter of / φ180mm, and a length of 200mm.

[0069] S5. Cover both ends of the NbTi / Cu / Ta composite sheath and perform electron beam welding. After extrusion, the processing rate is 2%~30%, and the stretching speed is 1~100m / min. Multiple stretching can obtain φ0.7mm NbTi / Cu multi-core wire. Among them, the NbTi / Cu multi-core wire is tested every 500m after stretching. The test magnetic field is ±3T, and the hysteresis loss is 20~25mJ / cm 3 .

[0070] The embodiments described above are part of the embodiments of the present invention, but not all of them. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention claimed for protection, but only represents selected embodiments of the present invention. All other embodiments obtained without creative work and related deductions and substitutions made by ordinary technicians in the field under the conditions of the concept of the present invention belong to the scope of protection of the present invention.

Claims

1. A method for preparing a NbTi multi-core wire, characterized in that: include: Oxygen-free copper doped with Ta or Ag is used as the center, and NbTi / Cu single core rods and oxygen-free copper rods are densely arranged in a Ta / Cu / Ta sheath as the first center component; the first center component is used as the center, and NbTi / Cu single core rods and oxygen-free copper rods are densely arranged in a Ta / Cu / Ta sheath as the second center component; the second center component is used as the center, and NbTi / Cu single core rods and oxygen-free copper rods are densely arranged in an oxygen-free copper sheath, copper caps are added at both ends and sealed, and NbTi / Cu multi-core wires are obtained through extrusion and stretching; The preparation method of the Ta / Cu / Ta sheath comprises: cleaning and corroding the oxygen-free copper sheath with nitric acid, and then depositing Ta on the surface of the oxygen-free copper sheath to obtain the Ta / Cu / Ta sheath.

2. The preparation method according to claim 1, characterized in that: The concentration of the nitric acid is 30%; The thickness of the cleaning and etching is 3-5 μm; The deposition method is any one of chemical vapor deposition, physical vapor deposition, and magnetron sputtering; The thickness of the deposited Ta is 10-50 μm.

3. The preparation method according to claim 1, characterized in that: The diameter of the oxygen-free copper doped with Ta or Ag element is Φ25mm~Φ60mm; The doping amount of the Ta element is 0.01%~0.1%; The doping amount of the Ag element is 1% to 2%.

4. The preparation method according to claim 1, characterized in that: The Ta / Cu / Ta sheath used to prepare the first central component has an outer diameter of φ55mm~φ120mm, an inner diameter of φ50mm~φ117mm, and a length of 200mm~1000mm.

5. The preparation method according to claim 1, characterized in that: The Ta / Cu / Ta sheath used to prepare the second central component has an outer diameter of φ100mm~φ200mm, an inner diameter of φ95mm~φ197mm, and a length of 200mm~1000mm.

6. The preparation method according to claim 1, characterized in that: The hexagonal cross-section height of the NbTi / Cu single core rod is 3 mm to 30 mm, and the length is 200 mm to 1000 mm; the diameter of the oxygen-free copper rod is 1 mm to 3 mm, and the length is 200 mm to 1000 mm.

7. The preparation method according to claim 1, characterized in that: The oxygen-free copper sheath has an outer diameter of φ200mm~φ400mm, an inner diameter of φ180mm~φ380mm, and a length of 200mm~1000mm.

8. The preparation method according to claim 1, characterized in that: The processing rate of the extrusion is 2% to 3%; The stretching speed is 1-100 m / min.

9. A NbTi multi-core wire, characterized in that: Prepared by the preparation method according to any one of claims 1 to 8.

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