A magnesium diboride superconducting flat wire and its preparation method

Abstract

The present invention belongs to the technical field of superconducting wire materials, and discloses a magnesium diboride superconducting flat wire and a preparation method thereof. It includes: S1. Select a copper groove wire and a MgB2 round wire workpiece; S2. Tin-plate the copper groove wire, and then pass it through a U-shaped groove die to obtain a soft-state thin-walled copper groove wire; S3. Pass the soft-state thin-walled copper groove wire and the MgB2 round wire through a rectangular die at the same time for cold processing combination to obtain a nested structure MgB2 tin-plated flat wire; S4. Perform surface polishing and tin removal procedures on the nested structure MgB2 tin-plated flat wire, and only retain the tin-plated layer in the groove to obtain a MgB2 tin-removed flat wire; S5. Wind the MgB2 tin-removed flat wire on a steel wheel and place it in a heat treatment furnace for heat treatment to obtain a MgB2 superconducting flat wire. The present invention effectively solves the problems of poor bending resistance and poor mechanical properties of the MgB2 superconducting flat wire, and realizes that good current-carrying performance can still be maintained after repeated winding.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting wire materials, and relates to a magnesium diboride superconducting flat wire and a preparation method thereof. Background Art

[0002] Magnesium diboride (MgB2) is a high-temperature superconductor discovered in 2001, which has the advantages of a relatively high critical temperature (39K), simple chemical composition and structure, no weak connection at grain boundaries, low material cost, and excellent processing performance. Considering the refrigeration cost and raw material cost of MgB2 superconducting wire materials, it has an obvious price advantage in the temperature range of refrigerators and liquid hydrogen (10 - 20K) and low magnetic fields (1 - 5T), and is mainly applied to fields such as nuclear magnetic resonance imaging systems (MRI), special cables, wind turbines, superconducting energy storage, etc.

[0003] Since MgB2 superconducting wire materials are a kind of ceramic structure materials, it faces great challenges to manufacture bendable and durable MgB2 superconducting wire materials. At the same time, the mechanical properties of MgB2 superconducting wire materials are poor, and they are easily affected by external forces in practical applications, resulting in unstable mechanical properties.

[0004] To address the problems of poor bendability and mechanical properties of MgB2 superconducting wire materials, high-strength and high-toughness CuNi alloys are usually used as the matrix of MgB2 superconducting wire materials to reduce the risk of fracture during the stretching and bending processes. On this basis, if referring to the nested structure of the Wire in channel NbTi superconducting wire materials (abbreviated as WIC wire materials) for MRI, after adding a copper protective layer outside the MgB2 superconducting wire materials and performing reaction heat treatment, the obtained MgB2 superconducting wire materials are flat wires. Without turning over and passing through small-diameter guide wheels, the brittle MgB2 superconducting phase will not break during the winding process. Furthermore, it is possible to perform eddy current flaw detection, insulation braiding, etc. after the reaction heat treatment of the MgB2 superconducting flat wire to ensure the reliable quality of the wire materials for winding magnets.

[0005] However, in the above process, the MgB2 superconducting flat wire has a CuNi alloy matrix. When the Ni content is relatively high, the chemical affinity with tin is poor. If the traditional dip soldering process is used, there are many soldering defects and poor solder filling, and it cannot be produced by the dip soldering method. Therefore, it is necessary to improve the technical defects existing in this process to prepare a bend-resistant and excellent mechanical property MgB2 superconducting flat wire. Summary of the Invention

[0006] In order to produce an MgB2 superconducting flat wire with excellent bending resistance and mechanical properties, the present invention combines cold working of a tinned copper groove wire and an MgB2 round wire, and then removes the surface tin plating layer by detinning, only retaining the tin plating layer in the groove. During the reaction heat treatment process, the soft thin-walled copper groove wire and the CuNi matrix in the MgB2 round wire diffuse into each other, using the surface tin layer of the soft thin-walled copper groove wire as a diffusing agent to achieve stable bonding between the two and obtain an MgB2 superconducting flat wire. Based on this technical route, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing an MgB2 superconducting flat wire, the method comprising:

[0008] S1. Select a copper groove wire and an MgB2 round wire workpiece;

[0009] S2. Tin plate the copper groove wire, and then pass it through a U-shaped groove die to obtain a soft thin-walled copper groove wire;

[0010] S3. Pass the soft thin-walled copper groove wire and the MgB2 round wire through a rectangular die simultaneously for cold working combination to obtain an MgB2 tinned flat wire with a nested structure;

[0011] S4. Perform surface polishing and detinning on the MgB2 tinned flat wire with a nested structure, only retaining the tin plating layer in the groove to obtain an MgB2 detinned flat wire;

[0012] S5. Wind the MgB2 detinned flat wire on a steel wheel and place it in a heat treatment furnace for heat treatment to obtain an MgB2 superconducting flat wire.

[0013] Further, in S2 of the above method, the tin plating temperature is 400 - 500 °C and the speed is 70 - 120 m / min.

[0014] Further, in S2 of the above method, the wall thickness of the soft thin-walled copper groove wire is ≥ 0.25 mm.

[0015] Further, in S3 of the above method, the cold working combination processing rate is 10 - 15%.

[0016] Further, in S4 of the above method, the polishing conditions are: polishing with a copper fiber polishing wheel with a diameter of 50 - 80 mm, and the polishing wheel rotation speed is 15000 - 20000 r / min.

[0017] Further, in S4 of the above method, the detinning conditions are: a nitric acid-based detinning solution, the detinning solution temperature is 40 - 50 °C, the immersion length of the wire in the detinning solution is 30 m, and the wire running speed is 60 - 80 m / min.

[0018] Further, in step S5 of the above method, when the MgB2 tin-removed flat wire is wound around the steel wheel, it shall not turn over, and the diameter of the steel wheel is greater than 200 mm.

[0019] Further, in step S5 of the above method, the MgB2 tin-removed flat wire is heat-treated in a protective atmosphere, and the protective atmosphere is argon.

[0020] Further, in step S5 of the above method, the heat treatment conditions are: heat treatment at 500 - 700 °C for 5 - 15 h.

[0021] The present invention also claims the MgB2 superconducting flat wire prepared by the above preparation method.

[0022] Compared with the prior art, the "magnesium diboride superconducting flat wire and its preparation method" of the present invention has at least the following beneficial effects:

[0023] In the preparation method of the MgB2 superconducting flat wire provided by the present invention, the soft thin-walled copper groove wire described in S2 can ensure its full combination with the MgB2 round wire in S3. The reason is that: the matrix of the MgB2 round wire is a CuNi alloy, and the overall hardness is relatively large. The soft thin-walled copper groove wire prepared in S2 reduces the difficulty of cold processing combination between the two.

[0024] In the preparation method of the MgB2 superconducting flat wire provided by the present invention, the copper groove wire is tinned to obtain a thin tin coating on its surface. The tin coating serves as a diffusing agent to accelerate the diffusion reaction between the copper groove wire and the CuNi matrix of the MgB2 round wire, facilitating the stable combination of the copper groove wire and the MgB2 round wire within a relatively short reaction heat treatment time (500 - 700 °C, 5 - 15 h).

[0025] In the preparation method of the MgB2 superconducting flat wire provided by the present invention, the tin-removed treatment of the nested structure MgB2 tinned flat wire in S4 avoids the problem of mutual adhesion between the wound wires due to tin diffusion during the reaction heat treatment process in S5.

[0026] In the preparation method of the MgB2 superconducting flat wire provided by the present invention, during the winding process, it is required that the front and back orientations of the wire are consistent with the orientation when finally winding the magnet, without turning over, avoiding the rupture of the superconducting phase and the reduction of performance caused by the need to flip the front and back of the wire when rewinding after the formation of the brittle MgB2 superconducting phase.

[0027] The present invention effectively solves the problems of poor bending resistance and poor mechanical properties of the MgB2 superconducting flat wire, realizes that good current-carrying performance can still be maintained after repeated winding, and there is no gap between the soft thin-walled copper groove wire and the MgB2 round wire, and the combination is stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a cross-sectional view of the copper groove wire.

[0029] Figure 2 Cross-sectional view of a nested structure MgB2 tinned flat wire

[0030] Figure 3 MgB2 de-tinned flat wire diagram

[0031] Figure 4 Cross-sectional view of MgB2 superconducting flat wire Specific implementation mode

[0032] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0033] The embodiment of the present invention provides a method for preparing MgB2 superconducting flat wire, and the method includes:

[0034] S1. Select a copper groove wire and a MgB2 round wire workpiece; the cross-section of the copper groove wire is as Figure 1 shown.

[0035] S2. Tin the copper groove wire, and then pass it through a U-shaped groove die to obtain a soft thin-walled copper groove wire.

[0036] S3. Pass the soft thin-walled copper groove wire and the MgB2 round wire through a rectangular die at the same time for cold working combination to obtain a nested structure MgB2 tinned flat wire; the cross-section of the nested structure MgB2 tinned flat wire is as Figure 2 shown.

[0037] S4. Perform surface polishing and de-tinning on the nested structure MgB2 tinned flat wire, and only retain the tinned layer in the groove to obtain a MgB2 de-tinned flat wire; the MgB2 de-tinned flat wire is as Figure 3 shown.

[0038] S5. Wind the MgB2 de-tinned flat wire on a steel wheel and place it in a heat treatment furnace for heat treatment to obtain a MgB2 superconducting flat wire; the cross-section of the MgB2 superconducting flat wire is as Figure 4 shown.

[0039] Exemplarily, in S2 of the above method, the tinning temperature is 400-500°C and the speed is 70-120 m / min.

[0040] Exemplarily, in S2 of the above method, the wall thickness of the soft thin-walled copper groove wire is ≥0.25 mm.

[0041] Exemplarily, in S3 of the above method, the cold working combination processing rate is 10-15%.

[0042] Exemplarily, in S4 of the above method, the polishing conditions are as follows: polishing is carried out using a copper fiber polishing wheel with a diameter of 50 - 80 mm, and the rotational speed of the polishing wheel is 15000 - 20000 r / min.

[0043] Exemplarily, in S4 of the above method, the tin stripping conditions are as follows: a nitric acid-based tin stripping solution, the temperature of the tin stripping solution is 40 - 50 °C, the immersion length of the wire in the tin stripping solution is 30 m, and the running speed of the wire is 60 - 80 m / min.

[0044] Exemplarily, in S5 of the above method, when the MgB₂ tin-stripped flat wire is wound around the steel wheel, it shall not be turned over, and the diameter of the steel wheel is greater than 200 mm.

[0045] Exemplarily, in S5 of the above method, heat treatment is carried out under a protective atmosphere, and the protective atmosphere is argon.

[0046] Exemplarily, in S5 of the above method, the heat treatment conditions are as follows: heat treatment is carried out at 500 - 700 °C for 5 - 15 h.

[0047] Example 1

[0048] This example provides a method for preparing MgB₂ superconducting flat wire.

[0049] S1. Select a copper groove wire and a MgB₂ round wire workpiece; in this example, the width of the copper groove wire is 2.6 mm, the height is 2.0 mm, and the groove width is 1.4 mm; the diameter of the MgB₂ round wire is 1.45 mm.

[0050] S2. The copper groove wire selected in S1 is first passed through a soldering flux and then immersed in a tin bath at 500 °C for tin plating at a tin plating speed of 120 m / min, and then passed through a U-shaped groove die with a width of 2.4 mm, a height of 1.85 mm, and a groove width of 1.5 mm to obtain a soft thin-walled copper groove wire, and the wall thickness of the soft thin-walled copper groove wire is 0.35 mm.

[0051] S3. The soft thin-walled copper groove wire and the MgB₂ round wire are simultaneously passed through a rectangular die with a width of 2.24 mm and a height of 1.73 mm for cold working combination, and the processing rate of the cold working combination is 14.5% to obtain a nested structure MgB₂ tin-plated flat wire.

[0052] S4. The nested structure MgB₂ tin-plated flat wire is respectively subjected to surface polishing and tin stripping procedures to completely strip the tin on the wire surface and only retain the tin plating layer in the groove to obtain a MgB₂ tin-stripped flat wire. In this step, the polishing conditions are as follows: polishing is carried out using a copper fiber polishing wheel with a diameter of 80 mm, and the rotational speed of the polishing wheel is 20000 r / min. The tin stripping conditions are as follows: a nitric acid-based tin stripping solution, the temperature of the tin stripping solution is 50 °C, the immersion length of the wire in the tin stripping solution is 30 m, and the running speed of the wire is 80 m / min.

[0053] S5. Wind the MgB₂ tin - removed flat wire around a steel wheel with a diameter of 350 mm, and ensure that the MgB₂ tin - removed flat wire does not turn over during winding; then place it in a heat treatment furnace and conduct heat treatment under an argon - protected atmosphere. The treatment conditions are: heat treatment at 700 °C for 15 h to obtain MgB₂ superconducting flat wire.

[0054] Example 2

[0055] This example provides a method for preparing MgB₂ superconducting flat wire.

[0056] S1. Select a copper groove wire and a MgB₂ round wire workpiece; in this example, the width of the copper groove wire is 1.65 mm, the height is 1.25 mm, and the groove width is 0.82 mm; the diameter of the MgB₂ round wire is 0.88 mm.

[0057] S2. First pass the copper groove wire selected in S1 through a flux, then immerse it in a 400 °C tin bath for tin plating at a tin - plating speed of 70 m / min, and then pass it through a U - shaped groove die with a width of 1.55 mm, a height of 1.15 mm, and a groove width of 0.9 mm to obtain a soft - state thin - walled copper groove wire, and the wall thickness of the soft - state thin - walled copper groove wire is 0.25 mm.

[0058] S3. Pass the soft - state thin - walled copper groove wire and the MgB₂ round wire simultaneously through a rectangular die with a width of 1.45 mm and a height of 1.08 mm for cold - working combination, and the cold - working combination processing rate is 13.8% to obtain a nested - structure MgB₂ tin - plated flat wire.

[0059] S4. Conduct surface polishing and tin - removal procedures on the nested - structure MgB₂ tin - plated flat wire respectively to completely remove the tin on the wire surface, and only retain the tin - plating layer in the groove to obtain MgB₂ tin - removed flat wire. In this step, the polishing conditions are: polish with a copper fiber polishing wheel with a diameter of 50 mm, and the rotation speed of the polishing wheel is 15000 r / min. The tin - removal conditions are: nitric acid - based tin - removal solution, the temperature of the tin - removal solution is 40 °C, the immersion length of the wire in the tin - removal solution is 30 m, and the running speed of the wire is 60 m / min.

[0060] S5. Wind the MgB₂ tin - removed flat wire around a steel wheel with a diameter of 200 mm, and ensure that the MgB₂ tin - removed flat wire does not turn over during winding; then place it in a heat treatment furnace and conduct heat treatment under an argon - protected atmosphere. The treatment conditions are: heat treatment at 550 °C for 5 h to obtain MgB₂ superconducting flat wire.

[0061] Example 3

[0062] This example provides a method for preparing MgB₂ superconducting flat wire.

[0063] S1. Select a copper groove wire and a MgB2 round wire workpiece; in this embodiment, the width of the copper groove wire is 2.05 mm, the height is 1.52 mm, and the groove width is 1.03 mm; the diameter of the MgB2 round wire is 1.08 mm.

[0064] S2. First pass the copper groove wire selected in S1 through a soldering flux, then immerse it in a tin bath at 450 °C for tin plating at a tin plating speed of 100 m / min, and then pass it through a U-shaped groove die with a width of 1.9 mm, a height of 1.4 mm, and a groove width of 1.12 mm to obtain a soft thin-walled copper groove wire, and the wall thickness of the soft thin-walled copper groove wire is 0.28 mm.

[0065] S3. Pass the soft thin-walled copper groove wire and the MgB2 round wire through a rectangular die with a width of 1.78 mm and a height of 1.32 mm at the same time for cold processing combination, and the processing rate of the cold processing combination is 13.2% to obtain a nested structure MgB2 tin-plated flat wire.

[0066] S4. Perform surface polishing and tin removal procedures on the nested structure MgB2 tin-plated flat wire respectively to completely remove the tin on the wire surface and only retain the tin plating layer in the groove to obtain a MgB2 tin-removed flat wire. In this step, the polishing conditions are as follows: polishing is performed using a copper fiber polishing wheel with a diameter of 65 mm, and the polishing wheel speed is 18000 r / min. The tin removal conditions are as follows: a nitric acid-based tin removal solution, the temperature of the tin removal solution is 50 °C, the immersion length of the wire in the tin removal solution is 30 m, and the running speed of the wire is 75 m / min.

[0067] S5. Wind the MgB2 tin-removed flat wire around a steel wheel with a diameter of 300 mm, and the MgB2 tin-removed flat wire does not turn over during the winding process; then place it in a heat treatment furnace and perform heat treatment under an argon protection atmosphere. The treatment conditions are as follows: heat treatment at 630 °C for 12 h to obtain a MgB2 superconducting flat wire.

[0068] Example 4

[0069] This example describes the performance testing of the MgB2 superconducting flat wires prepared in Examples 1 to 3, including the critical current (Ic value) and cross-sectional observation.

[0070] 1. Ic value

[0071] Perform Ic value testing on the MgB2 superconducting flat wires prepared in Examples 1 to 3 as follows:

[0072] 1) Take the MgB2 superconducting flat wire obtained in S5 and name it "superconducting wire - 1";

[0073] 2) Take the MgB2 superconducting flat wire obtained in S5, repeat winding it 2 times on the steel wheel, and do not turn it over during the winding process. The diameter of the winding steel wheel is the same as that in S5, and name it "superconducting wire - 2";

[0074] 3) After repeating the winding of the "superconducting wire - 2" sample 2 more times and taking samples without turning it over during the winding process, the diameter of the winding steel wheel is the same as that of S5, and it is named "superconducting wire - 3".

[0075] 4) Measure the Ic values (4.2K, 3T) of the samples of "superconducting wire - 1", "superconducting wire - 2" and "superconducting wire - 3" respectively. The results are shown in Table 1.

[0076] Table 1. Ic values (4.2K, 3T) of the MgB2 superconducting flat wires prepared in Examples 1 - 3

[0077]

[0078] As can be seen from Table 1, in Examples 1 - 3, the Ic values of the samples of "superconducting wire - 1", "superconducting wire - 2" and "superconducting wire - 3" are basically the same, indicating that the current - carrying capacity of the MgB2 superconducting flat wire is stable, and it can still maintain good wire performance after repeated winding, with good bending resistance and mechanical properties.

[0079] 2. Cross - section observation

[0080] In addition, intercept the cross - sections of the samples of each example respectively to observe whether there are gaps between the soft - state thin - walled copper groove wire and the MgB2 round wire. After observing the cross - sections of the MgB2 superconducting flat wires prepared in Examples 1 - 3, it is confirmed that there are no gaps between the soft - state thin - walled copper groove wire and the MgB2 round wire, and the combination is stable.

[0081] In summary, for the preparation method of the MgB2 superconducting flat wire provided by the present invention, after the copper groove wire is tinned and cold - worked and combined with the MgB2 round wire, the surface tinning layer is removed by desnolding, and only the tinning layer in the groove is retained. In the subsequent reaction heat treatment process, the soft - state thin - walled copper groove wire and the CuNi matrix in the MgB2 round wire diffuse with each other. Since the Ni content in the CuNi matrix is relatively high and the reaction time is short (500 - 700 °C, 5 - 15 h), using the surface tin layer of the soft - state thin - walled copper groove wire as the diffusing agent, stable combination of the two can be achieved, and the MgB2 superconducting flat wire with stable current - carrying performance can be obtained. It has been verified that the present invention effectively solves the problems of poor bending resistance and mechanical properties of the MgB2 superconducting flat wire, realizes that it can still maintain good current - carrying performance after repeated winding, and there are no gaps between the soft - state thin - walled copper groove wire and the MgB2 round wire, and the combination is stable.

[0082] The above - mentioned embodiments are only a part of the embodiments of the present invention, rather than all embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the present invention claimed, but only represents the selected embodiments of the present invention. All other embodiments obtained by relevant deductions and substitutions made by those of ordinary skill in the art under the premise of not making creative labor according to the concept of the present invention belong to the scope of protection of the present invention.

Claims

1. A method for preparing a MgB2 superconducting flat wire, characterized in that: The method comprises: S1. Select copper slot wire and MgB2 round wire processing parts; S2. The copper slot wire is firstly passed through a flux, then immersed in a 400~500°C tin bath for tinning, and then passed through a U-shaped slot mold to obtain a soft thin-walled copper slot wire; S3. The soft thin-walled copper slot wire and the MgB2 round wire are simultaneously passed through a rectangular die and cold worked to obtain a nested structure MgB2 tinned flat wire; S4. The nested structure MgB2 tinned flat wire is subjected to surface polishing and tin stripping steps, respectively, leaving only the tinned layer in the groove to obtain MgB2 tin stripping flat wire; S5. The MgB2 detinning flat wire is wound on a steel wheel and placed in a heat treatment furnace for heat treatment to obtain a MgB2 superconducting flat wire; the MgB2 detinning flat wire is heat treated under an argon protective atmosphere; heat treatment conditions: 500~700℃ heat treatment for 5~15h.

2. The preparation method according to claim 1, characterized in that: In S2, the tinning temperature is 400~500℃ and the speed is 70~120m / min.

3. The preparation method according to claim 1, characterized in that: In S2, the wall thickness of the soft thin-walled copper slot line is ≥0.25 mm.

4. The preparation method according to claim 1, characterized in that: In S3, the processing rate of cold working is 10~15%.

5. The preparation method according to claim 1, characterized in that: In S4, the polishing conditions are: polishing with a copper fiber polishing wheel with a diameter of 50~80mm and a polishing wheel speed of 15000~20000r / min.

6. The preparation method according to claim 1, characterized in that: In S4, the tin stripping conditions are: nitric acid-based tin stripping solution, the tin stripping solution temperature is 40~50℃, the wire immersion length in the tin stripping solution is 30m, and the wire running speed is 60~80m / min.

7. The preparation method according to claim 1, characterized in that: In S5, the MgB2 detinning flat wire must not turn over when it is wound on the steel wheel, and the diameter of the steel wheel is greater than 200 mm.

8. MgB2 superconducting flat wire prepared by the preparation method according to claim 1.

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