Method for manufacturing solder-free welded joint

Abstract

Disclosed in the present invention is a method for manufacturing a solder-free welded joint. The method comprises the following steps: S1, using a flux to pretreat copper surfaces of a plurality of superconducting tapes to be welded, wherein the Auger parameter α' of the surfaces of the pretreated superconducting tapes is 1851±0.5 eV; S2, subjecting the pretreated surfaces of the superconducting tapes to surface-to-surface lamination; and S3, setting the required pressure and temperature to perform hot pressure welding. In the present application, copper oxide and cuprous oxide on the copper surfaces of the superconducting tapes are removed by means of a pretreatment using a specific flux; therefore, during welding, the vast majority of copper (left after the removal of surface residues) on the surfaces of the superconducting tapes is present in the form of elemental copper, and stable copper connection is formed on the surfaces of the lapped superconducting tapes, thereby ultimately achieving a resistance of as low as 1.5 nΩ at a solder-free join length of 10 cm.

Description

A method for manufacturing a solderless welded joint Technical Field

[0001] This invention relates to the field of welding / (superconducting) wire preparation technology, specifically to a method for manufacturing solderless welded joints. Background Technology

[0002] REBa2Cu3O 7-δ REBCO (RE = rare earth element) possesses characteristics such as high critical temperature, high critical current density under magnetic fields, and excellent mechanical properties, and has been widely used in many fields, including high-magnetic-field magnets, superconducting maglev trains, superconducting motors, and superconducting cables. Due to limitations in manufacturing technology, the usable length of REBCO-coated conductors is limited. Currently, the longest single strip length that can be produced is approximately 1 kilometer, with most single strips ranging from 100 to 300 meters. However, the strip lengths required for manufacturing large magnets often reach the kilometer level, and some even exceed 5 kilometers. Therefore, connection technology is needed to connect multiple strips in applications, and a large number of joints require rapid manufacturing methods. In the field of maglev trains, joint technology is essential for the magnet to operate in a closed-loop mode. The lower the joint resistance, the lower the daily attenuation rate of the magnetic field, thereby improving magnet performance. Therefore, rapid manufacturing of low-resistance joint technology plays a crucial role in superconducting applications.

[0003] Currently, REBCO tape joints can be divided into superconducting joints and non-superconducting joints. Superconducting joints are usually formed by atomic diffusion of a semi-molten REBCO layer or by growing a superconducting interlayer on the original REBCO layer, thus directly connecting the superconducting layers of two tape conductors. Park et al. first successfully fabricated a superconducting joint in 2014 through atomic diffusion after laser drilling and annealing in pure oxygen for 350 hours. Jin et al. used the crystalline joint by amelted bulk (CJMB) method to fabricate superconducting joints, shortening the annealing time to 100 hours; however, the critical current (Ic) of the joint decreased by 10%. Ohki et al. fabricated an intermediate-grown superconducting (iGS) joint between REBCO coated conductors through simple heat treatment and a shorter annealing time (less than 24 hours), but the Ic decreased by 50%. Later, Kulikov et al. and other research teams such as Huang explored and optimized the fabrication of superconducting joints. However, superconducting junctions still face challenges such as complex manufacturing processes, long manufacturing times, and Ic decay characteristics.

[0004] Non-superconducting joints mainly include silver diffusion joints and welded joints. When producing welded joints, materials such as InSn and PbSn can be selected as solders. The copper (Cu) stabilizer (i.e., the stabilizing layer) of two strips is joined together by heating and melting the solder layer. The weld joint can be formed by hot pressing, ultrasonic mixing, or manual welding. Welded joints are simple, reliable, and easy to install in the field, making them the most commonly used welding method. A specific joint resistivity (R0) obtained at 77K is shown. sj Greater than 25 nΩ·cm 2 (Rsj is determined by the connector resistance R) j (Multiplied by the area of ​​the joint region). Silver diffusion joints directly connect the silver (Ag) layer through silver atom diffusion, or insert a thin silver layer at the joint interface. This method avoids the bulk resistance between the copper stabilizer (stabilizing layer) and the solder layer, the interface resistance between the copper stabilizer (stabilizing layer) and the solder, and the interface resistance between the silver layer and the copper stabilizer layer. Therefore, the resistance of silver diffusion joints is lower than that of solder joints. First proposed by Kato et al., silver diffusion joints require heating the strip to 500°C in a tube furnace to achieve silver diffusion, followed by oxygen annealing in an oxygen atmosphere (pure oxygen environment) to complete the entire process. Through continuous improvement and optimization of the manufacturing process, the resistance of silver diffusion joints at 77K has been significantly improved. sj Less than 10 nΩ·cm 2 Wu et al. reported that, through encapsulation, the mechanical properties of the silver diffusion joint are similar to those of the original strip, further improving the technical route of the silver diffusion joint.

[0005] In general, superconducting joints have extremely low resistance, but their complex manufacturing process makes them difficult to apply at present. Silver diffusion joints have even lower resistance than welded joints, but require a more complex manufacturing process. Heating in a tube furnace and annealing in an oxygen atmosphere (pure oxygen) typically takes several hours. Furthermore, after manufacturing the silver diffusion junction (i.e., silver diffusion joint), additional processes such as copper plating or encapsulation are usually required before application. Joints used in large magnets are still welded joints, but the individual fabrication time for traditional solder joints is relatively long, and their resistance is generally high. The average resistivity of the welded joints in the magnets used in the EcoSwing project is 36 nΩ·cm. 2 The daily decay rate of the REBCO magnet designed by Mizuno et al. for a life-size magnetic levitation system was 13% because the total solder joint resistance (i.e., joint resistance) of the coils reached 20 μΩ. These cases demonstrate that existing jointing techniques are still insufficient to achieve the desired results. Jointing technology has gradually become a key bottleneck in realizing high-performance superconducting magnets. Therefore, there is an urgent need to design a simple, fast, and efficient joint resistivity R... sj A relatively low-profile connector solution to meet application requirements. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the purpose of the present invention is to provide a method for manufacturing a solderless joint; the technical problem to be solved by the present invention is: to quickly form a joint without using solder; to reduce the joint resistance as much as possible without removing the copper stabilizing layer of the strip; the manufacturing method is simple and reliable, low in cost, fast and efficient; and the mechanical properties of the joint are comparable to those of traditional solder joints.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] <First Aspect>

[0009] This invention provides a method for manufacturing a solderless joint, the method comprising the following steps:

[0010] S1. The copper surface of several sections of superconducting strip to be welded is pretreated with flux; when Al is used as the excitation source, the Auger parameter α′ of the superconducting strip surface after pretreatment is 1851±0.5eV.

[0011] S2. The pretreated surfaces of the superconducting tape are bonded face to face.

[0012] S3. Set the required pressure and temperature for hot-press welding.

[0013] In one embodiment of the present invention, in step S1, the pretreatment involves immersing the copper surface of the superconducting strip in flux; or coating the copper surface of the superconducting strip with flux. Regardless of the pretreatment method used, it is necessary to ensure that the strip connection surface is coated with flux. After pretreatment, the Auger parameter α′ of the superconducting strip surface is 1851±0.5eV; including 1851±0.1eV, 1851±0.2eV, 1851±0.3eV, 1851±0.4eV, etc.

[0014] As one embodiment of the present invention, the immersion time is 3 min to 20 min.

[0015] In one embodiment of the present invention, in step S1, the Auger parameter α′ is calculated using the following formula: α′=(E B ) P +(E K ) A =(E B ) P +[hv-(E B ) A ]

[0016] Where hv is the energy of the photons from the excitation source, which is 1486.6 eV when Al is used as the excitation source. B ) P Represents the photoelectron binding energy, (E K )A Representing the Russian peak energy, (E B ) A It represents the combined energy of the Russian peak.

[0017] In one embodiment of the present invention, the entire manufacturing process is carried out in air. During the manufacturing process, the flux reacts with the original oxide layer on the copper surface of the superconducting strip and also acts as a protective layer, preventing the oxidation of exposed copper on the strip surface during heating. As shown in Example 1; it should be noted that after the two strips are heated and pressurized, they are squeezed together to reduce the gap; during this process, most of the flux residue and / or liquid and / or gas are forced out of the connection area; the connection area tends to be a vacuum. The aforementioned "entire manufacturing process" refers to the external environment / process of the overall operation and does not include vacuum / near-vacuum situations that occur during the operation, such as the aforementioned "connection area tends to be a vacuum".

[0018] In one embodiment of the present invention, the required pressure in step S3 is 208–375 MPa. In some embodiments, the pressure holding time can be 1–20 minutes, including 1–2 minutes, 2–3 minutes, 3–5 minutes, 5–10 minutes, 10–15 minutes, 15–20 minutes, etc. During the pressurization process, some flux residue and / or liquid and / or gas are forced out of the bonding area, and some narrow bonding areas become a vacuum.

[0019] In one embodiment of the present invention, the required temperature in step S3 is 120°C to 210°C.

[0020] As one embodiment of the present invention, in step S3, it is preferable that the pressurization and heating are sequential, that is, the temperature needs to be raised to the set value and stabilized before the pressure is applied.

[0021] In one embodiment of the present invention, the pressurization refers to pressurizing the mold, and the heating refers to heating the mold.

[0022] In one embodiment of the present invention, during the heating period before pressurization, the superconducting tape near the lower mold side is in a state with a gap between it and the surface below, without direct contact. Before heating, the connecting surface of the superconducting tape is coated with flux; without flux as a protective layer, the pure copper on the tape surface would oxidize, forming an oxide film during heating. In this invention, after the lower mold is heated to the set value, the aforementioned gap effectively prevents the superconducting tape from prematurely undergoing excessive oxidation.

[0023] As an example of an implementation of the present invention, the gap is within 2 mm.

[0024] In one embodiment of the present invention, a thermally conductive sheet, which is arched, is placed on the surface of the lower mold; the superconducting tape is placed on top of the thermally conductive sheet. This arrangement of the thermally conductive sheet allows for a gap between the superconducting tape near the lower mold side and the surface of the lower mold during the heating process before pressurization.

[0025] As one embodiment of the present invention, the superconducting tape is a second-generation superconducting tape.

[0026] As one embodiment of the present invention, in step S2, when the strips are bonded, a covering is used to cover the overlapping parts, and the covering area of ​​the covering is not less than the connection area of ​​the joint.

[0027] As one embodiment of the present invention, the covering is an adhesive material, which serves to prevent air from entering the covered area (pressed area).

[0028] As one embodiment of the present invention, in step S1, the abscissa BindingEnergy of the pretreated superconducting tape surface contains two or more peaks in the range of 925eV to 960eV.

[0029] In one embodiment of the present invention, in step S1, the surface energy of the pretreated superconducting tape is greater than 40 mJ / m. 2 Including: 40~80mJ / m2, 40~70mJ / m2, 40~60mJ / m2, 42~70mJ / m2, 42~60mJ / m2, 42~56mJ / m2, etc.

[0030] As one embodiment of the present invention, in step S1, the abscissa BindingEnergy of the Cu2p energy spectrum of the pretreated superconducting tape surface contains two or more peaks in the range of 925eV to 960eV; the surface energy of the pretreated superconducting tape is greater than 40mJ / m2.

[0031] In one embodiment of the present invention, in step S1, the contact angle of water on the surface of the pretreated superconducting tape is less than 77.2°. For example, it can be 50°–75°, 50°–65°, etc.

[0032] And / or, the contact angle of diiodomethane on the surface of the pretreated superconducting tape is less than 44.4°. For example, it can be 30°~44.2°, 35°~44.2°, etc.

[0033] In one embodiment of the present invention, the pretreatment time in step S1 is greater than or equal to 1 second. In some embodiments, the pretreatment time is 1 second to 30 minutes; it can also be 1 second to 10 seconds, 10 seconds to 20 seconds, 20 seconds to 30 seconds, 30 seconds to 1 minute, 1 second to 10 minutes, 10 second to 20 minutes, 20 second to 30 minutes, etc. In some embodiments, after 5 seconds of pretreatment, the BindingEnergy abscissa of the Cu2p energy spectrum shows less than or equal to two peaks in the range of 925 eV to 960 eV. In this case, the pretreatment time is preferably 1 second to 10 seconds, including 1 second, 2 seconds, 3 seconds, 4 seconds, 5 seconds, 6 seconds, 7 seconds, 8 seconds, 9 seconds, and 10 seconds; more preferably, it is 1 second to 5 seconds. In other embodiments, after 180 seconds of pretreatment, the BindingEnergy abscissa of the Cu2p energy spectrum shows less than or equal to two peaks in the range of 925 eV to 960 eV. The preferred preprocessing time is 1s to 180s.

[0034] As one embodiment of the present invention, step S3 includes at least one of the following technical features:

[0035] The required pressure is greater than 63 MPa and less than or equal to 375 MPa;

[0036] The required temperature is greater than 90℃ and less than or equal to 210℃;

[0037] The welding time is greater than or equal to 1 second. The welding time can be from 1 second to 600 seconds; including 1 second to 5 seconds, 5 seconds to 10 seconds, 10 seconds to 20 seconds, 20 seconds to 30 seconds, 30 seconds to 40 seconds, 40 seconds to 50 seconds, 50 seconds to 100 seconds, 100 seconds to 150 seconds, 150 seconds to 200 seconds, 200 seconds to 300 seconds, 300 seconds to 400 seconds, 400 seconds to 500 seconds, and 500 seconds to 600 seconds.

[0038] <Second aspect>

[0039] The solderless joints obtained by the aforementioned method fall within the scope of protection of this invention.

[0040] <Third aspect>

[0041] This invention also protects the application of solderless joints obtained by the aforementioned method in large HTS magnets, superconducting motors, or maglev trains.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1) No solder is required to connect the copper-plated surfaces of two superconducting strips.

[0044] 2) The production method is simple, reliable, and low-cost.

[0045] 3) The joint resistance is significantly reduced compared to traditional solder joints, while the mechanical properties are comparable to those of traditional solder joints.

[0046] 4) This application uses flux to remove copper oxide and cuprous oxide from the copper surface of the superconducting tape, so that the copper element on the surface of the superconducting tape (after removing the surface residue) exists almost in the form of elemental copper during subsequent welding, realizing solderless welding and reducing the resistance to as low as 1.5nΩ under a 10cm solderless overlap length.

[0047] 5) During the heating process of this invention, the flux covering the connecting surface hinders the oxidation of the strip surface; during pressurization, some flux residue and / or liquid and / or gas are forced out of the connecting area, while some strip surfaces covered by flux residue are not completely smooth, and protruding parts first come into contact with each other, leading to squeezing and deformation; when the pressure further increases, the contaminant layer remaining in the connecting area will thin until it breaks, thereby increasing the contact area between the pressurized metals and increasing the amount of deformation. At this time, some narrow bonding areas are very likely to be a vacuum, and little or no new oxides or contaminants will be generated inside; the contaminant film remaining on the surface is crushed under pressure; the intense plastic deformation of the metal produces flow. When the pressure increases and is maintained within a set range, the temperature is maintained within a certain range, and the high temperature makes the copper atoms more and more active, increasing the fluidity of the metal under plastic deformation; the area of ​​exposed pure copper on the strip surface increases, the distance between copper atoms decreases, until the metal connection is formed. Moreover, during the time the pressure is maintained, more metallic copper connections are formed on the bonding surface, thereby strengthening the bonding at the joint and forming an excellent metallurgical connection.

[0048] 6) This invention can also realize the manufacturing of ultrafast copper connectors; the pretreatment time can be controlled within 1s to 10s, and the hot pressing welding time can be controlled within 1s to 30s. As demonstrated by the experimental results in Figures 18 and 19 of Example 2, significantly shortening the connection time to 1 second does not hinder the formation of a good connection interface in the ultrafast copper connector. Attached Figure Description

[0049] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0050] Figure 1 is a side view of the superconducting surfaces of the two superconducting strips after welding.

[0051] Figure 2 is a schematic diagram of the multilayer structure of the superconducting tape;

[0052] Figure 3 is a schematic diagram of a section of base material before welding;

[0053] Figure 4 is a side view of the joint before welding;

[0054] Figure 5 is a schematic diagram of a copper connector; where (a) is a front view of the copper connector; and (b) is a side view of the copper connector.

[0055] Figure 6 is a schematic diagram of the manufacture of copper connectors by thermocompression welding (the asterisk-marked coating in step 3 is formed by a partial reaction product between flux and copper oxide);

[0056] Figure 7 shows the Cu2p photoelectron spectrum on the surface of the original HTS tape (sample 1); where (a) the three single peaks are used to fit the positions of the satellite peaks; and (b) the Cu LMM spectrum on the surface of sample 1.

[0057] Figure 8 shows the surface spectra of the strip (sample 2) after pretreatment and ethanol cleaning; where (a) is the Cu 2p photoelectron spectrum and (b) is the Cu LMM spectrum.

[0058] Figure 9 shows the spectra of the surfaces of samples 3 and 4; where (a) is the Cu 2p photoelectron spectrum and (b) is the Cu LMM spectrum.

[0059] Figure 10 shows the spectrum of the tape surface after heating without using flux and ethanol for cleaning (sample 5); where (a) Cu 2p photoelectron spectrum and (b) Cu LMM spectrum;

[0060] Figure 11 shows an electron microscope image of the copper bonded joint sample;

[0061] Figure 12 shows the temperature dependence of the resistance of the copper connector under 77K and self-field conditions; the temperature range is 120℃ to 210℃, and P is 333 MPa.

[0062] Figure 13 shows the VI curves of copper joints bonded under different bonding conditions (measured at 77K and under self-field conditions, with an overlap length of 10 cm); where (a) are copper joints bonded under different temperatures and 333 MPa pressures; and (b) are copper joints bonded under different P values ​​and 180°C temperatures.

[0063] Figure 14 is a schematic diagram of the manufacturing steps of the ultrafast copper connector;

[0064] Figure 15 shows photographs of the ultrafast copper connector prepared using Example 2; where (a) is a front view of the ultrafast copper connector; and (b) is a top view of the ultrafast copper connector.

[0065] Figure 16 is a schematic diagram of the process flow for manufacturing ultrafast copper I-connectors using the thermo-pressure welding method;

[0066] Figure 17 shows the energy spectrum of the original superconducting band surface and the superconducting band after 5 seconds of Alpha 3355-11 pretreatment: (a) Cu 2p photoelectron spectrum; (b) Cu LMM spectrum;

[0067] Figure 18 shows a scanning electron microscope (SEM) image of the ultrafast copper connector sample;

[0068] Figure 19 shows scanning electron microscope (SEM) images of the ultrafast copper connector; where (a) is a scanning electron microscope (SEM, 20 μm) image; and (b) is a magnified SEM image of the area shown in the (red) dashed box in (a).

[0069] Figure 20 shows the temperature and pressure dependence of the resistance of the ultrafast copper connector at 77K in a self-field; where (a) is the temperature dependence of the resistance of the ultrafast copper connector; (b) is the pressure dependence of the resistance of the ultrafast copper connector; the black squares, red circles, and blue triangles represent the average, maximum, and minimum connector resistance values ​​of the samples prepared under the corresponding conditions, respectively; the area between the two dashed lines represents the standard error of the connector resistance under these conditions; three connectors were prepared and tested for each set of parameters;

[0070] Figure 21 shows the temperature and pressure dependence of the resistance of a conventional copper connector at 77K in a self-field; where (a) is the temperature dependence of the resistance of the conventional copper connector; (b) is the pressure dependence of the resistance of the conventional copper connector; the black squares, red circles, and blue triangles represent the average, maximum, and minimum connector resistance values ​​of the samples prepared under the corresponding conditions, respectively; the area between the two dashed lines represents the standard error of the connector resistance under these conditions; three connectors were prepared and tested for each set of parameters;

[0071] Figure 22 shows the V-I curves of ultrafast copper joints welded with different connection parameters;

[0072] Figure 23 shows the experimental results of the original superconducting tape, welded joint, and copper connector prepared under different temperatures (T), pressures (P), and connection times; among them, (a) the V-I curves of the ultrafast copper connector prepared under different axial tensile forces at T = 110℃ and P = 333MPa; (b) the relationship curve between axial tensile force and normalized critical current.

[0073] Figure 24 is a heat map showing the classification results of ultrafast copper connectors prepared under different temperature (T) and pressure (P) conditions;

[0074] Figure 25 shows the thermal images of copper connectors prepared under different t, T, and P values. During the pretreatment process, the tape was treated with Alpha 3355-11 for 5 seconds. The black crosses represent the copper connectors prepared under the corresponding conditions, and the measured resistance values ​​of the connectors are marked. Among them, (a) t = 30s, (b) t = 1min, (c) t = 3min, (d) t = 5min, and (e) the relationship between Tb and Pb under different t values.

[0075] Figure 26 shows the XPS analysis results of the superconducting band surface under different pretreatment times using Alpha 3355-11 pretreatment / ET-815 pretreatment; where, (a) Cu 2p spectrum after Alpha 3355-11 pretreatment; (b) Cu LMM spectrum after Alpha 3355-11 pretreatment; (c) Cu 2p spectrum after ET-815 pretreatment; (d) Cu LMM spectrum after ET-815 pretreatment.

[0076] Figure 27 is a schematic diagram of the surface energy of the superconducting strip surface after pretreatment; (a) is a schematic diagram of the relationship between contact angle and different surface tension values; (b) is a schematic diagram of solid surface energy testing.

[0077] Figure 28 shows the surface contact angle measurement results of the samples after 5 seconds of Alpha 3355-11 pretreatment and 5 minutes of ET-815 pretreatment; the figure shows the droplet shape captured by the camera and the contact angle value obtained by software fitting;

[0078] Figure 29 summarizes the measurement results of the contact angle and surface energy of the superconducting strip surface after pretreatment with organic and rosin-based fluxes;

[0079] In the attached diagram, 1 is the first copper layer, 2 is the first silver layer, 3 is the base layer, 4 is the buffer layer, 5 is the superconducting layer, 6 is the second silver layer, 7 is the second copper layer, 8 is the superconducting surface, 9 is the flux layer, 10 is the superconducting tape A, and 11 is the superconducting tape B.

[0080] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings. Detailed Implementation

[0081] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0082] This invention proposes a solderless copper bonding joint manufacturing method for copper-plated second-generation high-temperature superconducting (2G HTS) strips. The method requires pretreatment of the strip surface with flux for several minutes to tens of minutes, followed by pressing under appropriate temperature and pressure conditions for several minutes to form a tight bond between the copper-stabilized layers of the two strips. The entire bonding process can be performed in air without solder. This invention clarifies the formation principle of the copper bonding joint and explains the effects of bonding temperature and pressure on the joint resistance, critical current, and mechanical properties under axial tensile force. Furthermore, the joint cross-section was prepared by argon ion polishing, and the microstructure of the joint cross-section was observed. Electron backscatter diffraction (EBSD) analysis was used to observe the copper grains on both sides of the bonding interface. The average resistance value R of a well-bonded copper joint was measured. j The resistance is 3.5 nΩ. This invention tested the original strip, welded joints, and several well-fitted copper connectors under different axial tensile forces. c The attenuation situation is as follows. Copper connectors have a simple manufacturing process, and their resistance is nearly one-third lower than that of welded connectors. Furthermore, their mechanical properties under axial tensile force are similar to those of welded connectors. Copper connectors are ideally suited for applications such as large HTS magnets, superconducting motors, and maglev trains, which require mass production of low-resistance connectors; therefore, the connector manufacturing process must be simple and efficient. See the following examples for details:

[0083] Example 1

[0084] The manufacturing process of copper adhesive joints is as follows:

[0085] First, clean the surface of the strips to be joined with alcohol, preferably ethanol (superconducting strip structure shown in Figure 2), and pre-treat the copper-plated surface by immersing it in flux for several minutes to over ten minutes. After pre-treatment, stack the strips (superconducting strip 1 and superconducting strip 2) together, with the side of the superconducting layer closer to the surface facing each other (as shown in Figures 1, 3, and 4). Secure the stacked area with a covering and / or tape, preferably Kapton tape. The coverage area of ​​the covering and / or tape should generally not be less than the joint area, ensuring that the strip edges are aligned as precisely as possible and forming a near-sealed environment to minimize oxygen entry into the pressing area. Set the bonding temperature and pressure on the control panel. After setting, the hot press equipment starts operating, and the two molds begin heating. Once the heating reaches the set value and stabilizes, the set pressure is applied to the strips in the hot press area. Continuous pressure and a relatively stable temperature are required throughout the entire joining process. After a few minutes, the bonding process ends, and the upper and lower molds separate. The copper joint is then successfully fabricated. The entire process is carried out in air without the need for solder. Figure 3 is a schematic diagram of a section of the base material (superconducting tape) before welding. The superconducting tape includes a first copper layer 1, a first silver layer 2, a base layer 3, a buffer layer 4, a superconducting layer 5, a second silver layer 6, a second copper layer 7, a superconducting surface 8, and a flux layer 9. Figure 4 is a side view of the joint before welding, showing the flux layer 9 between superconducting tape A10 and superconducting tape B11 before welding. Figure 5 shows the copper bonding joint fabricated according to the above method. The connection area of ​​the lap joint is approximately 10 cm long.

[0086] The copper connector achieves direct connection of the copper-stabilized layers of two superconducting strips through a copper metal connection. The formation process of the copper connector is briefly described below. After stacking, the two pre-treated strips are placed in the connection area of ​​a thermoforming device for heating and pressurization. Under plastic deformation, the contamination layer on the strip surface breaks down, exposing deep pure copper. Under higher pressure, the copper exposed on the strip surface adheres to the surface of the other strip. When the copper atoms on the two surfaces are sufficiently close, a copper-copper metal connection is formed, resulting in a relatively good metallurgical bond. Figure 6 is a simplified schematic diagram of this process (note that all figures are enlarged).

[0087] This invention will explain the rationale behind each manufacturing step and the formation mechanism of the copper seam. To verify this process, XPS analysis results were obtained from the surfaces of five tape samples. Table 1 lists the sample numbers and surface treatment methods.

[0088] Table 1. Sample quantity and preparation method used for XPS analysis

[0089] Guo et al. pointed out that even on new REBCO electroplated copper strips, the surface is not pure copper, but rather has a thin layer of cuprous oxide (Cu₂O) with a thickness of 180–360 nanometers. Pure copper exposed to air will rapidly oxidize to Cu₂O, and in a humid environment, it will further oxidize to cuprous oxide (CuO). The oxide layer is very hard and under high pressure, it will severely hinder the exposure and contact of the pure copper layer, thereby hindering the formation of metallic bonds. In addition, grease on the strip surface will also hinder the formation of copper bonds. Therefore, this invention first uses an alcohol solution, preferably ethanol, to clean the strip surface to remove grease. Subsequently, the valence state of copper on the original strip surface is analyzed by XPS, and the copper oxide composition on the strip surface is determined based on the analysis results.

[0090] Figures 7(a) and (b) show the copper 2p and copper LMM Auger spectra on the surface of sample 1. In Figure 7(a), a distinct satellite peak can be observed, indicating the presence of Cu(II) on the tape surface. Since the Cu2p peak position of Cu(I) (932.7 eV) is close to that of metallic copper (932.6 eV), and their peak shapes are also very similar, it is difficult to clearly distinguish them in the Cu2p spectrum. Therefore, this invention combines the Auger spectrum in Figure 7(b) with the improved Auger parameter α′ to determine the valence state of copper. The formula for calculating α′ is as follows: α′=(E B ) P +(E K ) A =(E B ) P +[hv-(E B ) A ]

[0091] Where hv is the energy of the photons from the excitation source, which is 1486.6 eV when Al is used as the excitation source. B ) P 、(E K ) A and (E) B ) A These represent the photoelectron binding energy, Auger kinetic energy, and Auger binding energy, respectively. In Figure 7, (E B ) P It is 932.5 eV, (E B ) A The value is 570.2 eV, and the calculated α′ value is 1848.9 eV. The original strip surface may contain Cu(I). Table 2 lists some α′ values ​​and their corresponding copper valence states. Considering the storage environment of the strip, Cu(I) and Cu(II) should mainly exist in the form of Cu2O and CuO, respectively.

[0092] Table 2. Correspondence between numerical values ​​α′ and copper valence states

[0093] Based on the peak fitting results of the Cu2p spectrum in Figure 7a, the percentages of CuO and Cu2O on the strip surface are approximately 45.8% and 54.2%, respectively. The test results indicate that some strips may have severe surface oxidation.

[0094] The strip was completely immersed in flux, preferably ET-815; the immersion time could be from tens of seconds to several minutes, preferably 5 minutes, to allow it to react with the oxides and complete the pretreatment. To verify the cleaning effect of the flux on the oxides on the strip surface, ethanol was used to clean the flux residue (including residues after the flux reacted with copper oxide and cuprous oxide, and residual flux) on the tape surface. The XPS results of sample 2 are shown in Figure 8. In Figure 8(a), no strong satellite peaks were observed in the Cu2p spectrum, and the Cu2p3 / 2 peak was located at 932.7 eV, indicating that the CuO on the strip surface had been basically removed. To further determine the presence of Cu2O, the peak value of copper on the strip surface (after removing surface residues) was calculated to be 1851.2 eV, which is similar to the peak value of metallic copper. It can be seen that after pretreatment, the oxides on the strip surface have been basically removed, and the metal below the flux residue layer is pure copper.

[0095] As shown in step 3 of Figure 6, samples 3 and 4 were prepared to verify the rationality of the strip surface composition. The XPS results of the two samples are shown in Figure 9. The analytical results of the two samples shown in Figure 9 are listed in Table 3.

[0096] Table 3 Overview of XPS test results for each sample

[0097] These two sets of test results indicate that the surfaces to be joined contained Cu(I)-containing substances before pressure was applied. However, after cleaning with ethanol, these substances were removed, leaving only pure copper. Ethanol cannot remove Cu₂O, unlike the reaction products of rosin and copper oxide in the flux. Therefore, the surfaces to be joined before pressure application were covered with flux residue. Beneath the flux residue was pure copper, which did not oxidize even after a period of heating. This verifies that the flux covering the joining surfaces inhibits the oxidation of the strip surface during heating.

[0098] Two strips are heated and pressurized before being squeezed together to reduce the gap. During this process, most of the flux residue and / or liquid and / or gas is forced out of the joining area. Some surfaces of the strips to be joined, covered by flux residue, are not completely smooth, and protruding parts come into contact with each other first, leading to compression and deformation.

[0099] As pressure increases further, the contaminant layer remaining in the bonding area thins until it fractures, increasing the contact area between the compressed metals and thus increasing deformation. At this point, the narrow bonding area may become a vacuum, with little or no new oxides or contaminants forming inside. The contaminant film remaining on the surface is crushed under pressure. The intense plastic deformation of the metal generates flow, pushing pure copper from deep within the crushed impurity layer to the surface. Contaminants and / or flux liquids and / or gases are then expelled from the bonding area, some diffusing into the deeper metal. The exposed copper metal, in the newly formed, potentially vacuum-like internal region, is not oxidized, or is minimally oxidized, at high temperatures.

[0100] As pressure increases and remains within a set range, temperature is maintained within a certain range. High temperature makes copper atoms increasingly active, increasing the fluidity of the metal under plastic deformation. The area of ​​exposed pure copper on the strip surface increases, and the distance between copper atoms decreases. This continues until a metal connection is formed. During the time pressure is maintained, more copper metal connections form on the joint surface, thus strengthening the bond at the joint. Finally, the upper and lower molds separate. This results in an excellent metallurgical connection and the successful production of a copper connector.

[0101] Before heating, it is essential to ensure that the strip joining surfaces are coated with flux. If the pretreated strip surface is cleaned with ethanol and then joined without flux coverage, a copper joint cannot be formed. Without flux as a protective layer, the pure copper on the strip surface oxidizes, forming an oxide film upon heating. This invention examined the surface composition of pretreated strips to be joined, heated without flux until pressure was applied (i.e., sample 5). Figure 10 and Table 3 list the XPS test results and analysis, respectively. The results show that the pure copper surface oxidizes during heating, forming a Cu2O film. This dense Cu2O oxide film is difficult to break during pressure application, making it difficult to expose the pure copper covered by the oxide film. Therefore, during pressure holding, the pure copper surfaces of the two strips awaiting joining cannot contact each other, leading to the failure of the copper joint manufacturing.

[0102] To observe the microstructure of the copper connector interface, a copper connector sample was fabricated. This sample was prepared by joining the copper connector at 160°C and 333 MPa for 3 minutes. Figure 11 shows a cross-section of the copper connector sample under an electron microscope. Scanning electron microscopy measurements indicate that the total thickness of the copper layer decreased by approximately 12.5% ​​after joining, attributed to significant plastic deformation occurring during the copper bonding process.

[0103] After HTS tape has been subjected to high temperature or sufficient pressure for a long time, I c Attenuation will occur. Experiments have confirmed that to ensure copper connectors manufactured using the strip discussed in this article do not exhibit I... cTo avoid degradation, the connection temperature must be maintained at less than or equal to 210 °C and the connection pressure at less than or equal to 375 MPa.

[0104] Figure 12 shows the temperature dependence of the copper connection joints. The pre-treatment time was 5 minutes and the bonding time was 3 minutes (stable pressure was applied after the upper and lower dies were closed). The overlap length of the copper connection joints was 10 cm. Three to five joints were fabricated and tested for each set of parameters. In Figure 12, the squares, circles, and triangles represent the average, maximum, and minimum joint resistance values of the samples fabricated under the corresponding conditions, respectively. The area between the two dashed lines represents the error of the joint resistance under these conditions. By maintaining the pressure P at 333 MPa and changing the temperature T, the effect of temperature on the electrical properties of the copper connection joints was studied, and the test results are shown in Figure 12.

[0105] As shown in Figure 12, when the P value is 333 MPa, the threshold temperature (i.e., the temperature at which copper connection starts to occur) Tθ is 120 °C, the transition temperature Tt is 135 °C, and the preferable copper connection bonding temperature Tb is 150 °C. When T < Tθ, the copper bonding joints cannot be formed. When Tθ ≤ T < Tt, the copper connection starts to form, and the joint resistance Rj is between 10 and 70 nΩ. Although the resistance decreases significantly with the increase in temperature, it is still very high. When Tt ≤ T < Tb, Rj is less than 10 nΩ, but it is still not practical. When the temperature rises to Tb, the copper connection resistance further decreases and remains stable. Figure 12 shows that when T ≥ Tb, Rj is between 1.5 and 5.8 nΩ, showing randomness. It should be noted that after almost removing CuO and Cu2O on the surface of the superconducting tape, the resistance is as low as 1.5 nΩ for a 10 cm solderless overlap length. With the increase in temperature, there is no obvious change trend. The average value of Rj of the copper connection joints fabricated under the conditions of P = 333 MPa and T ≥ Tb is 3.5 nΩ, indicating that the ET 815 flux can significantly reduce the threshold temperature of the copper connection after successfully removing most of the CuO and Cu2O on the surface of the superconducting tape, making the welding temperature much lower than 210 °C.

[0106] Figure 13 shows the V-I curves of several copper joints fabricated under different bonding conditions for comparison with the V-I curves of the welded joints and the original tape. Comparison was made with the V-I curves of the welded joints and the original tape. Among them, when T ≥ Tb, P is 333 MPa, and when P ≥ Pb (i.e., 250 MPa), T is 180 °C. The average resistance value of the copper connection joints fabricated under the condition of 180 °C (P ≥ Pb) is the lowest; and this value is basically not affected by the changes in T and P.

[0107] Due to testing errors, the binding energy data and Auger parameters, photoelectron peak binding energy, Auger peak kinetic energy, and Auger peak binding energy in the Cu 2p photoelectron spectrum and Cu LMM spectrum are all within ±0.5 of the data in the examples, and are all within the protection range.

[0108] Example 2

[0109] This embodiment relates to the manufacture of an ultrafast copper connector; the manufacturing process of the ultrafast copper connector is shown in Figure 14. The first step is to clean the surface of the superconducting strip with 95% ethanol (hereinafter referred to as ethanol), and then immerse the superconducting strip in the organic flux Alpha 3355-11 for 5 seconds to pre-treat the surface of the 4.8mm wide REBCO superconducting strip. The second step involves aligning the superconducting surfaces of the pretreated superconducting tape and securing it to a silicon steel sheet (the silicon steel sheet is arched with a slight convex curvature at the center) using Kapton tape. A 0.3 mm thick copper sheet is then placed on top of the superconducting tape. The third step involves placing the silicon steel sheet to be bonded onto a lower mold preheated to the bonding temperature, followed immediately by closing the upper and lower molds. The fourth step involves maintaining a constant pressure for 1 second on the surface of the tape to be bonded (thus controlling the bonding time to 1 second), after which the upper and lower molds are separated. Finally, the superconducting tape and silicon steel sheet are removed from the bonding area, and the sample is immersed in a water bath. After 5 seconds, the joint cools, completing the preparation of the ultrafast copper bonding joint.

[0110] Figure 15 shows a photograph of an ultrafast copper connector prepared using the connector preparation method of this embodiment under the following conditions: temperature T = 110°C, pressure P = 333 MPa, and time t = 1 second. The connector interface length is approximately 10 cm.

[0111] The formation mechanism of ultrafast copper joints is also based on thermocompression welding. The joint formation process can be schematically represented by Figure 16 (note that the scale of each figure has been enlarged).

[0112] First, the surface composition of the original superconducting tape was analyzed using XPS, as shown by the black solid line in Figure 17. The Cu 2p spectrum of the original tape showed strong satellite peaks, indicating the presence of copper oxide (CuO) on the surface. Since the surface of the nascent REBCO copper-plated tape is not pure copper but contains cuprous oxide (Cu2O), the superconducting tape used in this study contains both CuO and Cu2O. The copper oxide layer hinders copper bonding, so the tape surface must be pretreated. The pretreatment consisted of two steps: first, the tape surface was cleaned with ethanol to remove the grease film, and then the tape was immersed in the organic flux Alpha 3355-11 for 5 seconds to remove surface oxides.

[0113] In this embodiment, an ultrafast copper connector sample was prepared under connection conditions of T = 110℃, P = 333MPa, and t = 1s. Figure 18 shows an electron microscope image of the connector interface, where some areas show good copper connection, while defects in the form of voids also appear at the connector interface. Figure 19 shows a microscopic image of the well-connected area observed under an electron microscope. The experimental results in Figures 18 and 19 confirm that significantly reducing the connection time to 1 second did not hinder the formation of a good connection interface in the ultrafast copper connector.

[0114] To compare the results of this embodiment with the electrical performance of conventional copper connectors prepared under different parameters, the resistance of the copper connector as a function of temperature was investigated under a constant pressure P of 333 MPa, and the resistance as a function of pressure was investigated under a constant temperature T of 180 °C. For the preparation of the ultrafast copper connector, the pretreatment time was controlled to 5 seconds, and the connection time was 1 second (i.e., the time for applying stable pressure after the upper and lower molds are closed). An ultrafast copper connector with an overlap length of 10 cm was prepared.

[0115] The experimental results in Figure 20(a) can be divided into three parts. The first part occurs at T = 80℃, under which the resistance of the joint prepared ranges from 10 to 70 nΩ. The second part covers the connection temperature range of 80℃ ≤ T < 110℃. As the fabrication temperature increases, the joint connection conditions improve, and the resistance decreases, ranging from 4 to 10 nΩ. The third part occurs at T ≥ 110℃, at which point a good copper connection interface is formed, and the resistance no longer changes significantly with increasing temperature. In Figure 20(a), this invention defines a threshold temperature Tth = 80℃ and a bonding temperature Tb = 110℃. When the temperature is below Tth, copper bonding may not occur (below 70℃). Even if a copper connection interface is formed, the joint resistance is too high for practical application. When the temperature T is between Tth and Tb, the copper connection is formed but insufficient, resulting in a high macroscopic joint resistance. When T ≥ Tb, a good copper connection can be achieved, and within this range, the joint resistance can be as low as 1 nΩ (corresponding to T = 120℃ and T = 180℃). The average Rj of the ultrafast copper bonding joint prepared under pressure of 333 MPa and T≥Tb is approximately 2.5 nΩ, which corresponds to an Rsj value of 12 nΩ·cm. 2 .

[0116] Figure 20(b) shows the pressure dependence curve of the joint resistance at T = 180 °C. The characteristics of this curve are similar to those observed in Figure 20(a). A threshold pressure Pth = 83 MPa and a connection pressure Pb = 167 MPa can be defined. When the pressure P reaches or exceeds Pb, the joint resistance reaches a minimum and is no longer affected by temperature changes. The lowest joint resistance measured in the experiment was 1 nΩ at P = 208 MPa, P = 313 MPa, and P = 375 MPa. For ultrafast copper joints prepared at 180 °C, and with P ≥ Pb, the average value of Rsj is approximately 12 nΩ·cm. 2 .

[0117] By analyzing the results in Figures 20(a) and 20(b), the joints prepared under the conditions of P = 333 MPa and T ≥ Tb, and the joints prepared under the conditions of P ≥ Pb and T = 180 °C, yielded the lowest average resistance values. Among all joint samples prepared within these parameter ranges, The average resistance is 2.5 nΩ.

[0118] Figures 21(a) and 21(b) show the temperature and pressure dependence of conventional copper connectors. Compared to Figures 20(a) and 20(b), although the trends of the curves are similar, the improved copper connector fabrication method significantly reduces the values ​​of Tth, Tb, Pth, and Pb. At a pressure P = 333 MPa, the mold closing temperature decreases from Tb,old = 150 °C to Tb = 110 °C, while at T = 180 °C, the connection pressure decreases from Pb,old = 250 MPa to Pb = 167 MPa. Furthermore, in the region where the copper connection is effective, the average resistance of the ultrafast copper connector fabricated in this study is 2.5 nΩ; this value is nearly 30% lower than the average resistance of 3.5 nΩ for conventional copper connectors.

[0119] Figure 22 shows the V–I curves of several ultrafast copper joints fabricated under different connection conditions, compared with solder joints and the original superconducting tape. The same superconducting tape was used for fabricating the solder joints, with rosin-based flux ET-815 and solder composition Sn63Pb37. Optimized welding parameters were set to T = 190℃ and P = 250MPa, with a welding time of 3 minutes. The length of the welded joint was also approximately 10 cm. The Rj value of this welded joint was approximately 5.4 nΩ.

[0120] Figure 23(a) shows the Ic variation of the ultrafast copper joints prepared at 110°C and 333 MPa under different axial tensile forces. The Ic value decreases when F is 150 N and reaches 95% at F = Fret = 186 N. During this process, the joint resistance does not change significantly; these results indicate that the connection interface of the ultrafast copper welded joint is sufficiently robust and unaffected by the applied tensile force.

[0121] Figure 23(b) shows the variation of the normalized Ic value of copper connectors prepared under different conditions under axial tensile force. This invention tested four ultrafast copper connector samples with good electrical properties prepared under different conditions. The average Fret value, Fret,av, of these connector samples was 191 N. The deviations of Fret,max and Fret,min from Fret,av were all less than 5%. This may represent experimental errors due to differences in the uniformity of the superconducting tape and the experimental test settings. Therefore, for ultrafast copper connectors, if the connection is good, the connection temperature and pressure will not affect the mechanical properties of the connector under axial tensile force. For comparison, Figure 23 also includes the test results of the original tape and the soldered connector. The experimental results show that the mechanical properties of the ultrafast copper connector and the soldered connector are similar under axial tension. However, the Fret value of both connectors is 20% weaker than that of the single-layer superconducting tape, with a Fret value of 235 N.

[0122] Figure 24 shows a heatmap of the joint classification results prepared under different temperature (T) and pressure (P) conditions. To visually represent the differences in joint state under different temperature and pressure conditions when generating the heatmap, the joint state prepared under the (Ti,Pj) condition was considered representative of all joints within the parameter ranges [Ti,Ti+1) and [Pj,Pj+1). The corresponding rectangular areas in the heatmap are filled with specific colors. The red (Bonding failed, located in the lower left corner of the figure), orange (Insufficient bonding), and green (Well-bonded, located in the upper right corner of the figure) areas in the heatmap correspond to connection failure, insufficient bonding, and good bonding states, respectively. This method ensures that within the green (Well-bonded) area shown in the heatmap, any parameter selection can produce a well-bonded copper joint. Therefore, the heatmap is a useful reference for parameter selection during the actual joint preparation process.

[0123] As shown in Figure 24, the boundary line between the red (bonding failed) and orange (insufficient bonding) regions reflects the relationship between Tth and Pth under specified manufacturing conditions, while the boundary line between the orange (insufficient bonding) and green (well-bonded) regions indicates the relationship between Tb and Pb. Based on the experimental results, the following conclusions can be drawn:

[0124] 1. Tb and Pb are not independent variables; as P increases, Tb decreases, and as T increases, Pb decreases. The relationship between Tth and Pth also shows a similar pattern.

[0125] 2. T and P are interchangeable as key connection parameters for copper connectors.

[0126] 3. When manufacturing an ultrafast copper connector with t=1s, the minimum connection temperature Tbmin,1s required to achieve a good copper connection is 100℃, and the pressure P must be at least 354MPa; conversely, the minimum copper connection pressure Pbmin,1s is 125MPa, and the temperature must be at least 200℃.

[0127] Both are external driving factors for the formation of copper-copper connections. When one parameter is insufficient, it can be compensated for by increasing the other. As temperature and pressure gradually increase, the state of the copper connection joint will transition from copper connection failure to insufficient connection, and eventually reach a good connection state.

[0128] To further investigate the effect of crimping time t on the fabrication of copper connectors, thermal images were generated after classifying copper connectors fabricated under different t, T, and P values, as shown in Figures 25(a) to (d). (The corresponding rectangular areas in the thermal images of Figure 25 are marked with specific colors, the division of which is the same as in Figure 24: red (bonding failed, located in the lower left corner), orange (insufficient bonding, located in the middle), and green (well-bonded, located in the upper right corner)). The following conclusions can be drawn:

[0129] 1. There is an interdependence among the parameters t, T, and P used to prepare good copper joints. The range of T and P that can be used to prepare good copper joints expands with increasing connection time. When t = 5 minutes, Tbmin,5min = 90℃ and P = 291 MPa; when T = 200℃, Pbmin,5min = 63 MPa.

[0130] 2. There is a "dead zone" between temperature (T) and pressure (P), within which a good copper connection cannot be formed. When T is less than or equal to 90°C, no matter how much P is increased or how much t is extended, a good copper connection cannot be formed; similarly, when P is less than or equal to 63MPa, no matter how much T is increased or how much t is extended, the formed connection will be insufficiently bonded.

[0131] 3. When T increases to a certain level, its effect on forming a well-bonded copper connection reaches saturation. At this point, further increasing T cannot effectively reduce P required to form a well-bonded connection, and the same phenomenon applies to P.

[0132] 4. Similarly, after extending t to a certain threshold, its effect on forming a well-bonded copper connection reaches a "saturation" effect. When t reaches 3 minutes, further increasing t no longer significantly changes the T and P ranges for forming a well-bonded copper joint.

[0133] Figure 25(e) shows the relationship curves between Tb and Pb at different t values, which are derived from the data summarized in Figures 25(a) to (d). When t is extended to 3 minutes and 5 minutes, the two curves show almost no difference.

[0134] This invention conducted experiments on 12 copper connectors under different connection times at T=100℃, P=313MPa; T=180℃, P=333MPa; and T=200℃, P=83MPa. Figures 25(a) to (d) show the connector resistance values. Despite the increase in connection time, the connector resistance value did not change significantly, with an average value of 2.5nΩ.

[0135] REBCO tapes need to be immersed in flux for a period of time to allow the flux to fully react with the copper oxides on the tape surface. To visually compare the pretreatment effects of Alpha 3355-11 and ET-815 on the tapes, the superconducting tapes were cleaned with ethanol and air-dried, then immersed in the two fluxes for 5 seconds to 3 minutes with Alpha 3355-11 and 30 seconds to 5 minutes with ET-815, respectively. After pretreatment, the superconducting tapes were cleaned with ethanol and dried with a nitrogen gun. Subsequently, XPS was used to analyze the surfaces of the original and pretreated superconducting tapes to obtain their Cu 2p and Cu LMM spectra.

[0136] Figures 26(a) and (b) show the results of pretreatment of the superconducting strip surface with the organic flux Alpha 3355-11 (ORH1) at different pretreatment times. The Cu 2p spectrum of the original sample shows a distinct satellite peak (SAT), indicating the simultaneous presence of CuO and Cu2O on the copper stabilizer surface. After 5 seconds of pretreatment, the satellite peak no longer appears in the Cu 2p spectrum, indicating that CuO has been successfully removed. The photoelectron binding energy (EB)P is 932.6 eV, the Auger binding energy (EB)A is 567.9 eV, and the Auger parameter α' is 1851.3 eV; these values ​​are consistent with the corresponding values ​​for pure copper. These results indicate that Alpha 3355-11 can remove surface copper oxides within 5 seconds. Further extending the pretreatment time to 3 minutes does not change the composition of the superconducting strip surface. However, as shown in the XPS results of Figures 25(c) and (d), rosin-based flux ET-815 (ROM1) requires 5 minutes to complete the pretreatment. The choice of flux directly affects the time required for the pretreatment step. In the production of copper connectors, the use of highly reactive halogen-containing fluxes is crucial for shortening the overall manufacturing time.

[0137] This embodiment continues the analysis of the surface energy of the pretreated superconducting band surface. The surface energy of a solid can be calculated by measuring the contact angle (θ) of the solid surface. θ is determined by the surface tension of the liquid σl, the interfacial tension between the liquid and the solid σsl, and the surface energy of the solid σs. Figure 27(a) shows a schematic diagram of the contact angle θ and the relevant surface tension.

[0138] This invention employs the Owens–Wendt–Rabel–Kaelble (OWRK) two-liquid method to calculate surface energy. Figure 27(b) illustrates the process of testing surface energy by measuring the contact angle. Two standard liquids, water and diiodomethane, are coated onto the surface of a solid sample, and the contact angles θ1 and θ2 are measured; subsequently, the total surface energy of the solid is calculated using the following formula:

[0139] In this equation, the superscripts D and P of σs and σl represent the dispersive and polar components of surface energy and surface tension, respectively. σ(l,1) and σ(l,2) represent the surface tension values ​​of the two standard test liquids, respectively. The relationship between the liquid surface tension σl and the solid surface energy σs and their respective components is as follows:

[0140] Therefore, by measuring the contact angle between the two test liquids on the same solid surface, the system of equations can be solved. This allows for the calculation of the polar and dispersed components of the solid's surface energy. Ultimately, the total surface energy of the solid can be obtained through the equations.

[0141] This invention prepares two types of solid test samples. The first sample involves cleaning the superconducting band with ethanol and then immersing it in Alpha 3355-11 for 5 seconds. The second sample is pretreated by immersion in ET-815 for 5 minutes.

[0142] The contact angle between two test liquids and two test samples was measured using the pendant drop method. The test liquids included a polar liquid (ultrapure water) and a non-polar liquid (diiodomethane).

[0143] Figure 28 shows a set of contact angle results for two types of test samples tested using ultrapure water and diiodomethane. In the image, the black semi-circular objects are real photographs of 5 μL test droplets. The solid lines represent the baseline and the droplet curve fitted by the ellipse, while the angles represent the contact angles obtained through ellipse fitting.

[0144] For superconducting strip surfaces pretreated with two types of flux, ten sets of valid contact angle data were measured for each test liquid (water and diiodomethane). The mean and standard deviation of the contact angle and surface energy components (polar and dispersive components) were calculated. The experimental results are shown in Figure 28.

[0145] Figure 29 summarizes the measurement results of contact angle and surface energy of the superconducting strip surface after pretreatment with organic and rosin-based fluxes. Contact angles are presented as mean ± standard deviation, reflecting the average value and including 1σ measurement uncertainty. Surface energy was calculated using the OWRK two-liquid method and is expressed in mJ / m². 2 The three bar charts on the left, center, and right represent the average values ​​of the dispersed component, polar component, and total surface energy, respectively. The error bars represent the 1σ uncertainty, and the total surface energy and the average and standard deviation of each component are labeled above the bar chart in the format of "mean ± standard deviation".

[0146] The experimental results in Figure 29 show that the surface energy of the superconducting band pretreated with the organic flux Alpha 3355-11 for 5 seconds is 53.3 mJ / m. 2 This value is higher than the 41.3 mJ / m measured on a surface pretreated with rosin-based flux ET-815 for 5 minutes. 2 Higher surface energy indicates greater atomic activity; therefore, surface atoms are more inclined to interact with neighboring atoms to achieve a more stable state. Consequently, solids with higher surface energy are more conducive to stronger interatomic interactions, promoting the formation of chemical bonds between atoms. Higher surface energy corresponds to more active Cu atoms on the surface to be welded; therefore, Cu-Cu metallic bonds are easier to form, and it is easier to prepare well-welded copper joints.

[0147] The higher surface energy of the superconducting tape treated with Alpha 3355-11 makes it easier to fabricate good copper joints, and these joints have lower joint resistance than those fabricated using tape pretreated with ET-815.

[0148] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, any technical solution that can be obtained by those skilled in the art based on the concept of the present invention through simple changes in materials, logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A solderless joint manufacturing method characterized by, The method comprises the following steps: S1, using a flux to pretreat the copper surface of a plurality of sections of superconducting tapes to be welded; the Auger parameter a' of the superconducting tape surface after pretreatment is 1851±0.5eV; S2, the pretreated surfaces of the superconducting tapes are pasted face to face; S3, setting the required pressure and temperature to perform hot pressure welding.

2. The solderless joint manufacturing method according to claim 1, characterized by In step S1, the pretreatment is to immerse the copper surface of the superconducting tape in the flux; or to cover the copper surface of the superconducting tape with the flux.

3. The solderless joint manufacturing method according to claim 2, wherein The immersion time is 3min to 20min.

4. The solderless joint manufacturing method according to claim 1, characterized by In step S1, the Auger parameter a' is calculated by the following equation: a' = (E B ) P +(E K ) A = (E B ) P +[hv-(E B ) A ] where hv is the energy of the exciting light source photon, 1486.6 eV when using Al as the exciting light source, (E B ) P represents the photoelectron peak binding energy, (E K ) A represents the Auger peak kinetic energy, (E B ) A represents the Auger peak binding energy.

5. The solderless joint manufacturing method according to claim 1, characterized by, In step S3, the required pressure is 208MPa to 375MPa; And / or, the required temperature is 120℃ to 210℃.

6. The solderless joint manufacturing method according to claim 1, characterized by In step S3, the pressure and the temperature have a sequence, the temperature needs to be raised first, and then the pressure is applied after the temperature reaches the set value.

7. The solderless joint manufacturing method according to claim 6, wherein The pressure is the pressure of the mold, and the temperature refers to the heating of the mold; during the heating before the pressure, the superconducting tape close to the lower mold side is in a state of having a gap with the surface of the lower mold.

8. The solderless joint manufacturing method according to claim 7, characterized by A heat-conducting sheet is placed on the surface of the lower mold, the sheet is arched, and the superconducting tape is placed above the heat-conducting sheet.

9. The solderless joint manufacturing method according to claim 1, characterized by, In step S3, during the pressure process, part of the flux residue and / or liquid and / or gas is pressed out of the connection area, and part of the narrow bonding area tends to be vacuum.

10. The solderless joint manufacturing method according to claim 1 or 9, characterized by, In step S2, when the tapes are pasted, a cover is used to cover the stacked part, and the coverage area of the cover is not less than the connection area of the joint.

11. The solderless joint manufacturing method according to claim 10, wherein The cover is a material with adhesion, which prevents air from entering the covered area.

12. The solderless joint manufacturing method according to claim 1, characterized by, In step S1, the Cu2p energy spectrum of the superconducting tape surface after pretreatment contains less than or equal to two wave peaks in the range of 925eV to 960eV of the horizontal coordinate BindingEnergy.

13. The solderless joint manufacturing method of claim 1, wherein In step S1, the surface energy of the superconducting tape after the pretreatment is greater than 40 mJ / m 2 .

14. The solderless joint manufacturing method according to claim 1, 12 or 13, characterized by, In step S1, the contact angle of water on the superconducting tape surface after pretreatment is less than 77.2°, and / or the contact angle of diiodomethane on the superconducting tape surface after pretreatment is less than 44.4°.

15. The solderless joint manufacturing method according to claim 1, 12, 13 or 14, characterized by, At least one of the following technical features is included: In step S1, the pretreatment time is greater than or equal to 1s; In step S3, the required pressure is greater than 63MPa and less than or equal to 375MPa; In step S3, the required temperature is greater than 90℃ and less than or equal to 210℃; In step S3, the welding time is greater than or equal to 1s.

Images