Superconducting material joint based on nanocrystalline copper bonding and preparation method thereof

Abstract

The invention relates to the field of superconducting material preparation, in particular to a superconducting material joint based on nanocrystalline copper bonding and a preparation method thereof.The preparation method comprises the following steps that the to-be-bonded surface of a strip is mechanically polished; electroplating a nanocrystalline copper layer on the surface to be bonded; electrochemically polishing the nanocrystalline copper layer; and the nanocrystalline copper layers of the two strips are in butt joint and bonded in a pressurized and heated mode, and then the connector is obtained. After the nanocrystalline copper layer is introduced on the surface, the bonding pressure and the bonding temperature can be reduced to be 50 MPa and 140 DEG C, interface grains are small, and even if defects exist on the bonding contact surface, the defects are gradually healed under the driving of the grain boundary, and the interface resistance is reduced.

Description

Technical Field

[0001] This invention relates to the field of superconducting material preparation, and more specifically, to a superconducting material joint based on nanocrystalline copper bonding and its preparation method. Background Technology

[0002] Since Professor Onnes of Leiden University in the Netherlands first observed the superconducting phenomenon of mercury's electrical resistance suddenly dropping to zero at liquid helium temperature in 1911, superconductivity research has become an extremely active frontier field in condensed matter physics and materials science. Hundreds of superconducting materials have been discovered, but only superconducting materials such as NbTi, Nb3Sn, MgB2, Bi-based, and yttrium-based materials have practical application value in engineering applications.

[0003] Second-generation high-temperature superconducting rare-earth barium copper oxide (REBCO) tapes possess superior properties compared to other practical superconducting materials, including high critical temperature, high critical current density, and strong current-carrying capacity under high fields. They are widely used in advanced scientific equipment and large-scale scientific facilities such as superconducting current limiters, high-field superconducting magnets, superconducting energy storage devices, high-energy particle accelerators, and thermonuclear fusion reactors. However, the continuous fabrication length of existing uniform REBCO superconducting materials is less than 1 kilometer, while the requirements for large-scale superconducting devices are tens or even hundreds of kilometers. Therefore, numerous joints are necessary to connect REBCO tapes to meet the demands of large-scale superconducting equipment. Simultaneously, during the current transfer from the current source to the superconducting material, joints also exist at the connection points between the conventional conductor and the superconducting material. During current-carrying operation, Joule heating (the only heat source) caused by resistance at these joints not only significantly increases the cooling load of the cryogenic system but may also trigger local overheating, leading to superconducting instability or even loss of superconductivity, seriously threatening the reliability and operational efficiency of the device. Therefore, achieving extremely low joint resistance is crucial for controlling the heat load and improving system stability, especially under high current-carrying conditions.

[0004] Based on welding processes, existing joint types mainly include superconducting joints, silver diffusion joints, and welded joints. Superconducting joints are formed by directly sintering superconducting materials together, then adding a metal protective layer on the outside. However, this method leads to degradation exceeding 10% of the critical current, and the connection time exceeds 10 hours. Silver diffusion joints are formed by welding superconducting materials containing only a silver layer, typically achieving a characteristic resistivity (R0). sj (resistance × contact area) in 10 nΩ·cm 2 The above methods result in numerous defects and voids at the connection points. While joints made using these two methods have lower resistance, their strength is also lower, making them unsuitable for industrial applications. Soldered joints, on the other hand, involve soldering the outer copper layer of the REBCO strip with low-resistance solder. Soldered joints typically use low-melting-point, low-resistance solder as the solder, followed by pressure heat treatment to achieve a lower joint resistance (R0). sj>26 nΩ·cm²). This method has been widely adopted in industry due to its simplicity, convenience, and high joint strength. However, introducing a solder layer not only introduces additional solder layer resistance and interface resistance between the solder and copper layers, but also leads to electromigration between the solder and copper layers under prolonged high current conditions, increasing the interface resistance and raising the risk of quenching in superconducting equipment. Therefore, to ensure long-term stable operation of the equipment, it is necessary to remove the solder layer and achieve direct copper-to-copper bonding. However, conventional copper bonding methods have drawbacks such as requiring high pressure, high temperature, and long time, which are not conducive to direct engineering applications.

[0005] Therefore, it is necessary to explore simpler manufacturing methods and joint preparation processes with low resistance, high mechanical strength, and good stability to promote the engineering application of superconducting materials represented by REBCO superconducting tapes. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a superconducting material joint based on nanocrystalline copper bonding. By introducing a nanocrystalline copper layer on the surface, the bonding pressure and temperature can be reduced to (50 MPa, 140℃). Moreover, the interface grains are smaller, and even if there are defects at the bonding contact surface, the defects will gradually heal and grow under the drive of the grain boundaries, thereby reducing the interface resistance.

[0007] Another object of the present invention is to provide a superconducting material joint based on nanocrystalline copper bonding, which has low interfacial resistance.

[0008] The technical problem solved by this invention is achieved by the following technical solution.

[0009] On one hand, embodiments of the present invention provide a method for preparing a superconducting material joint based on nanocrystalline copper bonding, comprising the following steps:

[0010] Mechanically polished strip surface to be bonded;

[0011] Electroplating a nanocrystalline copper layer onto the surfaces to be bonded;

[0012] Electrochemical polishing of nanocrystalline copper layers;

[0013] By butt-bonding the nanocrystalline copper layers of the two strips together, and then applying pressure and heating, a joint is obtained.

[0014] In some embodiments of the present invention, the thickness of the nanocrystalline copper layer is 0.1-3 μm.

[0015] In some embodiments of the present invention, after mechanical polishing of the bonding surface of the strip, the surface roughness of the bonding surface is less than 100 nm.

[0016] In some embodiments of the present invention, after electrochemical polishing of the nanocrystalline copper layer, the surface roughness of the nanocrystalline copper layer is less than 10 nm.

[0017] In some embodiments of the present invention, during pressurized and heated bonding, the pressure is 10-70 MPa, the temperature is 80-180°C, and the bonding time is 0.1-5 min.

[0018] In some embodiments of the present invention, the tape is a copper-clad flat superconducting tape, comprising a copper layer, a Hastelloy layer, a buffer layer, a rare earth barium copper oxide layer, a silver layer, and a copper layer connected in sequence.

[0019] In some embodiments of the present invention, the bonding surface is a copper layer near the silver layer.

[0020] This invention provides a superconducting material connector based on nanocrystalline copper bonding, which is prepared by the above method.

[0021] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects:

[0022] The bonding method provided by this invention, after introducing a nanocrystalline layer on the surface of the copper layer, can reduce the bonding pressure and temperature to 50 MPa and 140°C, respectively. The bonding conditions are more moderate, and the interface grains are smaller. Defects existing on the bonding contact surface will gradually heal under the drive of the grain boundaries, thereby reducing the interface resistance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a diagram of the strip structure according to an embodiment of the present invention;

[0025] Figure 2 This is a diagram of the connector structure according to an embodiment of the present invention;

[0026] Figure 3 for Figure 2 Enlarged view of section A;

[0027] Figure 4 Current-voltage curves of joints bonded at 50 MPa and different temperatures;

[0028] Figure 5 The current-voltage curves of the joints bonded at 180℃ and under different pressures are shown.

[0029] Figure 6 This is a microscopic view of the connector according to an embodiment of the present invention;

[0030] Icons: 1-Copper layer, 2-Silver layer, 3-Rare earth barium copper oxide layer, 4-Buffer layer, 5-Hastelloy layer, 6-Nanocrystalline copper layer, 7-Nanocrystalline copper bonding interface. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to specific embodiments.

[0033] This invention provides a method for preparing a superconducting material joint based on nanocrystalline copper bonding, comprising the following steps:

[0034] Mechanically polished strip surface to be bonded;

[0035] Electroplating a nanocrystalline copper layer onto the surfaces to be bonded;

[0036] Electrochemical polishing of nanocrystalline copper layers;

[0037] By butt-bonding the nanocrystalline copper layers of the two strips together, and then applying pressure and heating, a joint is obtained.

[0038] The thickness of the nanocrystalline copper layer is 0.1-3 μm. After mechanical polishing of the bonding surface of the strip, the surface roughness of the bonding surface is less than 100 nm. After polishing the nanocrystalline copper layer, the surface roughness of the nanocrystalline copper layer is less than 10 nm.

[0039] During pressurized and heated bonding, the pressure is 10-70 MPa, the temperature is 80-180℃, and the bonding time is 0.1-5 min.

[0040] The tape is a flat, copper-clad superconducting tape, comprising a copper layer, a Hastelloy layer, a buffer layer, a rare-earth barium copper oxide layer, a silver layer, and another copper layer connected in sequence. The bonding surface is the copper layer closest to the silver layer.

[0041] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0042] Example

[0043] Perform the bonding connection as follows:

[0044] Select a flat, copper-clad superconducting tape with a width of 4 mm, as shown in the attached image. Figure 1 As shown, it includes a copper layer 1, a Hastelloy layer 5, a buffer layer 4, a rare earth barium copper oxide layer 3, a silver layer 2, and a copper layer 1 connected in sequence; wherein, the material of the buffer layer 4 is CeO2.

[0045] Using the copper layer 1 near the silver layer 2 as the bonding surface, the bonding surface of the strip is polished, and the surface roughness after mechanical polishing is less than 100 nm.

[0046] A nanocrystalline copper layer 6 was electroplated on the surface to be bonded (current density 0.04 A / cm²). 2 ,68 s / μm), the thickness of the nanocrystalline copper layer 6 is 2μm;

[0047] Electrochemical polishing of nanocrystalline copper layer 6 resulted in a surface roughness of less than 10 nm after polishing;

[0048] The nanocrystalline copper layers 6 of the two strips are butt-bonded together, with a joint length of 4 cm. Pressure is applied to 10-70 MPa, and the temperature is raised to 80-180℃. Bonding is performed under pressure for 0.1-5 minutes to form the nanocrystalline copper bonding interface 7, thus obtaining the joint. The structure of the joint is shown in the attached figure. Figure 2 and attached Figure 3 As shown, the microstructure of the joint cross-section is as follows Figure 6 As shown. Figure 6 In the diagram, 6 represents the nanocrystalline copper layer, and 7 represents the nanocrystalline copper bonding interface. It can be observed that the bonding interface almost disappears, proving that the bonding effect is excellent.

[0049] Specifically, under bonding conditions of 50 MPa pressure, 5 min bonding (pressurization) time, and temperatures of 80℃, 100℃, 120℃, and 140℃, different joints were obtained, and the current and voltage of each joint were tested to obtain current-voltage curves, as shown in the attached figure. Figure 4 As shown.

[0050] Different joints were obtained under pressures of 10 MPa, 30 MPa, 50 MPa, and 70 MPa, a temperature of 180℃, and a bonding time of 2 min. The current and voltage of each joint were tested, and the current-voltage curves were obtained, as shown in the attached figure. Figure 5 As shown.

[0051] From the appendix Figure 4 and 5 It can be concluded that the slope gradually decreases with increasing temperature and pressure, meaning the joint resistance gradually decreases. Calculations show that the lowest characteristic resistivity reaches 5.87 nΩ·cm. 2 (50% lower than the current international minimum).

[0052] In summary, the bonding method provided by the embodiments of the present invention, after introducing nanocrystals on the surface of the copper layer, can reduce the bonding pressure and temperature to 50 MPa and 140°C, respectively. The bonding conditions are more moderate, and the interface grains are smaller. Defects existing on the bonding contact surface will gradually heal and grow under the drive of the grain boundaries, thereby reducing the interface resistance.

[0053] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a superconducting material joint based on nanocrystalline copper bonding, characterized in that, Includes the following steps: Mechanically polished strip surface to be bonded; Electroplating a nanocrystalline copper layer onto the surfaces to be bonded; Electrochemical polishing of nanocrystalline copper layers; By butt-bonding the nanocrystalline copper layers of the two strips together, and then applying pressure and heating, a joint is obtained.

2. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 1, characterized in that, The thickness of the nanocrystalline copper layer is 0.1-3 μm.

3. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 1, characterized in that, After mechanical polishing of the bonding surface of the strip, the surface roughness of the bonding surface is less than 100 nm.

4. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 1, characterized in that, After electrochemical polishing, the surface roughness of the nanocrystalline copper layer is less than 10 nm.

5. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 1, characterized in that, During pressurized and heated bonding, the pressure is 10-70 MPa, the temperature is 80-180℃, and the bonding time is 0.1-5 min.

6. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 1, characterized in that, The tape is a flat superconducting tape clad in copper, comprising a copper layer, a Hastelloy layer, a buffer layer, a rare earth barium copper oxide layer, a silver layer, and another copper layer connected in sequence.

7. The method for preparing a superconducting material joint based on nanocrystalline copper bonding according to claim 6, characterized in that, The surface to be bonded is a copper layer on the side closest to the silver layer.

8. A superconducting material joint based on nanocrystalline copper bonding, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.

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