Method for preparing MgB2-MgB2 superconducting joint through hot pressed sintering

Abstract

The invention relates to the field of superconducting material preparation, and discloses a method for preparing an MgB2-MgB2 superconducting joint through hot pressed sintering, which comprises the following steps: preparing metal magnesium powder and nano boron powder subjected to surface coating treatment into an intermediate connecting agent according to a ratio, and filling a joint area between superconducting wires with the intermediate connecting agent; and hot pressing sintering treatment is carried out under the controlled atmosphere, and a continuous and compact MgB2 phase connection structure is formed. In order to improve the interface bonding quality and oxidation resistance, a porous SiC transition layer can be introduced into the joint interface in advance, and staged atmosphere control is adopted to achieve grain boundary purification and regulation. The method is simple in operation process, low in joint resistance and high in shear strength, has excellent thermal stability and electrochemical stability, and is suitable for actual connection application of high-performance superconducting devices.

Description

Technical Field

[0001] The present invention relates to the technical field of superconducting material preparation, in particular to a method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering. Background Art

[0002] MgB2 superconducting materials, due to their high critical temperature, low cost, and excellent electromagnetic properties, hold broad application prospects in fields such as magnetic resonance imaging, superconducting current limiters, and high-field magnets. As their engineering applications continue to advance, the stable connection between superconducting wires has become a key technology that restricts the overall performance of the system. In particular, when connecting multiple wire segments or constructing superconducting windings, the electrical continuity, mechanical strength, and environmental stability of the joint area directly impact the reliability and lifespan of the device.

[0003] Currently, the commonly used methods for preparing MgB2 joints are mainly based on mechanical pressing or simple heat treatment, which often have problems such as high resistance in the connection area, loose structure, and weak mechanical bonding. In addition, the powder is prone to agglomeration or oxidation reaction in the joint area, resulting in interface gaps and non-superconducting interlayers, thereby reducing the critical current density of the joint. Although some studies have attempted to introduce auxiliary fillers or improve the sintering atmosphere, it is still difficult to effectively balance electrical performance and mechanical stability. In particular, in complex service environments, the joint area is prone to performance degradation or even falling off. Therefore, how to systematically improve the conductivity, mechanical strength and environmental adaptability of MgB2 joints from multiple dimensions such as raw material selection, interface structure control and atmosphere adjustment has become a technical bottleneck that needs to be solved urgently. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the present invention provides a method for preparing MgB2-MgB2 superconducting joints by hot pressing and sintering, which solves the defects of existing MgB2 superconducting joints in terms of resistance, mechanical strength and oxidation resistance.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering, comprising the following steps:

[0006] (1) Precursor preparation: boron powder is nano-processed and a boron nitride layer is coated on the surface of magnesium powder;

[0007] (2) Mixing and pressing: The boron powder treated in step (1) is mixed with magnesium powder at a Mg:B molar ratio of 1:1.95 to 2.05, and a dispersant is added, and cold pressed into an intermediate prefabricated block for connection;

[0008] (3) Joint assembly and local gradient hot pressing sintering: The middle prefabricated block is placed between two sections of prefabricated high-performance MgB2 wire to form a three-section structure. The temperature and pressure of the middle region are controlled in stages under an inert atmosphere to perform local hot pressing sintering, so that magnesium diffuses in the middle region and reacts with boron to form an MgB2 joint in situ. At the same time, a transition layer is pre-prepared at the connection interface to modify the grain boundary through in-situ reaction.

[0009] (4) Multi-stage atmosphere control: During the hot pressing sintering process in step (3), H2 / Ar mixed gas, BCl3 gas and CH4 / Ar mixed gas are introduced into the joint area in sequence to achieve surface oxide reduction, boron atom diffusion activation and lattice defect repair, respectively.

[0010] Preferably, the nano-processing of the boron powder in step (1) includes:

[0011] Boron powder with an initial particle size of 1 to 5 μm was crushed to 50 to 200 nm by high-energy ball milling. The ball milling parameters were: argon protection, ball-to-material mass ratio of 8 to 12:1, rotation speed of 300 to 500 rpm, and time of 4 to 8 h.

[0012] Subsequently, the nano boron powder is immersed in a 0.05-0.2M HCl solution and ultrasonically treated for 20-60 minutes to generate surface hydroxylated boron powder.

[0013] Preferably, the boron nitride coating of the magnesium powder in step (1) comprises:

[0014] A 2-5 nm boron nitride layer is coated on the surface of magnesium powder by chemical vapor deposition. The deposition conditions are: temperature 550-650°C, reaction gas is a mixture of NH3 and BCl3, volume ratio 1:1-3, and deposition time 20-40 min.

[0015] Preferably, the dispersant in step (2) is polydopamine-modified graphene quantum dots, and the addition amount thereof is 0.3 to 1.0 wt%;

[0016] The pressure of the cold-pressed intermediate prefabricated blocks for connection is 80-120 MPa, and the holding time is 5-15 minutes.

[0017] Preferably, the local gradient hot pressing sintering in step (3) includes the following staged control, and the active area is limited to the location of the intermediate connecting prefabricated block:

[0018] The first stage: from room temperature to 600-650℃, increase the pressure stepwise to 40-60MPa at a heating rate of 3-8℃ / min;

[0019] The second stage: in the range of 600-650℃ to 800-850℃, the pressure is increased step by step to 90-110MPa at a heating rate of 1-5℃ / min;

[0020] The third stage: in the range of 800-850℃ to 700-750℃, the pressure is reduced step by step to 40-60MPa at a cooling rate of 30-80℃ / min.

[0021] Preferably, the step-by-step pressurization method for the intermediate connection area in the second stage is:

[0022] After each increase of 10-15 MPa, the pressure is maintained for 2-5 minutes, and pulsed BCl3 gas is introduced at the same time, with a pulse on-off ratio of 1:2-4, a frequency of 0.05-0.2 Hz, and a BCl3 gas concentration of 0.05-0.3 vol%.

[0023] Preferably, the transition layer in step (3) is a nanoporous SiC membrane, and its preparation includes:

[0024] A 0.5-1.5 μm thick SiC film is deposited at the interface between the intermediate connecting prefabricated block and the wire material by magnetron sputtering, the film layer has a porosity of 20-40% and a pore diameter of 30-100 nm.

[0025] Preferably, the in-situ reaction occurs in the middle connection area. During the hot pressing process at 800-850° C., the SiC film reacts with the boron interface to generate trace amounts of free carbon, which assists in regulating the grain boundary structure.

[0026] Preferably, the local multi-stage atmosphere control for the intermediate connection area in step (4) includes:

[0027] In the first stage, a 3-8 vol% H2 / Ar mixed gas is introduced, and the H2 concentration is linearly reduced to 0.5-2 vol% as the temperature rises;

[0028] In the second stage, pure Ar is switched and 0.05-0.3 vol% BCl3 gas is introduced;

[0029] In the third stage, 0.3-1.0 vol% CH4 / Ar mixed gas is introduced and cracked at 700-800°C to generate activated carbon for adjusting the microstructure of the connection interface.

[0030] Preferably, the inert atmosphere in step (3) is argon, and the initial vacuum degree is ≤1×10 -3 Pa.

[0031] The present invention provides a method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering. It has the following beneficial effects:

[0032] 1. This invention effectively improves the material reactivity and interfacial stability of the joint by using nano-boron powder and boron nitride-coated magnesium powder. In particular, during the hot-pressing sintering process, the in-situ modification of grain boundaries and transition layer design significantly enhances the joint's current-carrying capacity. This enables the joint to maintain a high critical current density in the superconducting state, reducing joint losses and improving the efficiency and stability of the superconducting joint.

[0033] 2. This invention innovatively incorporates a nanoporous SiC film as an interfacial transition layer, deposited onto the joint interface via magnetron sputtering. This SiC film exhibits high chemical and thermal stability, effectively improving the structural integrity of the joint under mechanical loads. By optimizing the interfacial bonding quality, the shear strength of the joint is enhanced, thereby improving the mechanical stability of the overall connection and ensuring reliability during long-term operation.

[0034] 3. By implementing multi-stage atmosphere control during the hot-pressing sintering process, this invention effectively optimizes the oxide removal and interface repair mechanisms of the joint. The BCl₃ gas in the atmosphere effectively removes the oxide layer on the magnesium powder surface and in the joint area, thereby reducing the negative impact of oxidation on superconducting performance and enhancing the joint's stability in hot and humid environments. This feature significantly improves the service life and performance of the superconducting joint under various environmental conditions.

[0035] 4. By optimizing the composition distribution in the joint region and introducing a transition layer, this invention reduces thermal resistance at the joint interface and improves heat conduction efficiency. During the hot pressing and sintering process, the rational selection and structural design of the transition layer effectively promotes uniform heat distribution and avoids the risk of thermal stress accumulation. This technological innovation significantly enhances the joint's thermal management capabilities in practical applications, ensuring excellent thermal stability in high-power superconducting applications.

[0036] 5. The in-situ reaction technology employed in this invention generates trace amounts of free carbon during the hot-pressing sintering process, thereby regulating the grain boundaries of the joint. This process not only optimizes the grain boundary structure but also effectively reduces defects and impurities at the interface, thereby improving the superconducting performance and overall stability of the joint. This innovative interface control method ensures the stability of the joint over long-term use and avoids performance degradation caused by grain boundary defects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the method of the present invention. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] Please see the attached Figure 1 The present invention provides a method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering, comprising the following steps:

[0040] S1. Precursor preparation

[0041] In this embodiment, the preparation of the precursor used in the intermediate connection region includes: nano-sizing and hydroxylation of boron powder, and boron nitride coating of magnesium powder, as follows:

[0042] The nano-processing of boron powder is achieved by high-energy ball milling.

[0043] Boron powder with an initial particle size of 1-5 μm is placed in a high-energy ball mill and mechanically crushed under an argon atmosphere using stainless steel grinding balls with a diameter of 5-10 mm at a ball-to-powder mass ratio of 8-12:1. The milling speed is controlled at 300-500 rpm for 4-8 hours. After ball milling, the boron powder particle size is reduced to 50-200 nm. The argon atmosphere effectively suppresses surface oxidation caused by high-energy frictional heat, ensuring the reactivity of the nano-boron powder.

[0044] The hydroxylation treatment was completed by acid washing combined with ultrasound.

[0045] The ball-milled nano-boron powder is immersed in a dilute hydrochloric acid solution with a concentration of 0.05-0.2M. The ultrasonic frequency is set to 20-60kHz, the power is 50-150W, and the treatment time is 20-60min. Ultrasonic cavitation causes the acid to penetrate into the microporous cracks on the surface of the powder. Hydrochloric acid reacts with oxides such as B2O3 to form soluble borates, which peel off the oxide layer and form surface hydroxyl groups (-B-OH). The treated powder is kept at 40-60℃ and vacuum degree ≤10 -2 Drying under Pa conditions for 2-4 hours yields nano-boron powder with a rich surface hydroxyl group. Hydroxylation helps increase the surface polarity of the powder, strengthens the interfacial bonding with subsequent dispersants and magnesium powder, and improves mixing uniformity.

[0046] The boron nitride coating of magnesium powder adopts chemical vapor deposition (CVD) process.

[0047] Magnesium powder with a particle size of 10 to 30 μm is placed in a CVD reaction chamber. A mixture of NH3 and BCl3 in a volume ratio of 1:1 to 3 is introduced, with high-purity argon as the carrier gas, at a flow rate of 50 to 100 sccm. The reaction temperature is set at 550 to 650°C, and the deposition time is 20 to 40 minutes. At high temperatures, NH3 and BCl3 react to form boron nitride (BN), which is uniformly deposited on the surface of the magnesium powder in a lamellar structure, forming a continuous coating with a thickness of 2 to 5 nm. This BN film has excellent thermal stability and chemical inertness, effectively preventing oxidation or premature reaction of the magnesium powder during mixing and cold pressing.

[0048] In summary, nano-boron powders, with their large surface area and short diffusion distance, help improve reaction efficiency in the intermediate region. Hydroxylation enhances interfacial compatibility between the powders and improves compaction density. The BN coating on the magnesium powder surface, through both physical isolation and chemical passivation, enhances structural integrity and reaction controllability during sintering in the connecting region. These precursors are used only in the preparation of the intermediate preform in the connecting region and are processed separately from the prefabricated MgB2 wires at both ends.

[0049] S2. Mixing and compression molding

[0050] In this embodiment, the preparation of the intermediate connection prefabricated block includes precursor mixing, dispersant addition, cold pressing and pre-deposition of the interface transition layer, as follows:

[0051] 1. Wet mixing and dispersant addition

[0052] The hydroxylated nano-boron powder and BN-coated magnesium powder obtained in step S1 were accurately weighed at a Mg:B molar ratio of 1:1.95-2.05, and 0.3-1.0 wt% of polydopamine-modified graphene quantum dots (PDA-GQDs) were added as a dispersant. The powders were mixed in anhydrous ethanol with a solid-liquid mass ratio of 1:3-6. The mixture was wet-milled using zirconia balls with a diameter of 3-5 mm at a ball-to-material ratio of 6:1-10:1. The mill speed was set at 300-400 rpm, and the mixing time was 2-6 hours. The milling process was performed under argon protection.

[0053] During the wet mixing process, the polydopamine segments in the PDA-GQDs form adsorption with the hydroxyl (-OH) groups on the boron powder surface through hydrogen bonding. The π-π structure of the graphene quantum dots anchors the magnesium powder surface, creating a steric hindrance effect that effectively inhibits nanoparticle aggregation. Ethanol, as a low-surface-tension solvent, contributes to the stability of the dispersion and reduces the risk of agglomeration during the drying process.

[0054] 2. Drying and cold pressing

[0055] Place the mixed slurry in a vacuum drying oven at 50-80°C and vacuum degree ≤10 -2 The powder is dried under Pa conditions for 4 to 8 hours to remove residual solvent. The dried powder is placed in a mold and uniaxially pressed at a pressure of 80 to 120 MPa for 5 to 15 minutes to form a dense intermediate connection prefabricated block with a relative density of about 60 to 70% of the theoretical density, providing a suitable pore structure and reaction space for subsequent hot pressing sintering.

[0056] 3. Interface transition layer preset

[0057] To improve the interface between the prefabricated block and the wire, a nanoporous SiC film is deposited on the prefabricated block surface as a transition layer. This film is deposited via magnetron sputtering using a high-purity SiC target (purity ≥99.99%) in an argon atmosphere (pressure 0.5-1.5 Pa) at a sputtering power of 100-200 W for 10-30 minutes. By adjusting the pressure and power, the film thickness is controlled to 0.5-1.5 μm, the porosity to 20-40%, and the pore size to 30-100 nm.

[0058] The SiC membrane has a high melting point and chemical stability, and can maintain structural integrity during the subsequent hot pressing process. Its porous structure can provide effective diffusion channels and reaction interfaces, which is beneficial to interface bonding and reaction control.

[0059] S3. Joint assembly and local gradient hot pressing sintering

[0060] In this embodiment, gradient hot pressing sintering and interface modification include staged temperature and pressure control, pulse gas activation, and in-situ grain boundary reaction processes, which are specifically implemented as follows:

[0061] 1. Gradient hot pressing sintering process

[0062] Hot pressing sintering is achieved through coordinated control of temperature and pressure in three stages.

[0063] First, the prefabricated block obtained in step S2 is placed in a high-strength graphite mold and placed in a vacuum hot pressing furnace. Evacuate to an initial vacuum degree of ≤1×10 -3 Pa, and then argon gas was introduced into the furnace as the basic atmosphere.

[0064] The first stage: The preform is heated from room temperature to 600-650°C at a heating rate of 3-8°C / min. Pressure is applied in a stepped pressurization mode: each increase of 5-10 MPa is maintained for 1-3 minutes, reaching a final pressure of 40-60 MPa. Simultaneously, a 3-8% volume fraction of H2 / Ar gas mixture is introduced, with the H2 concentration decreasing linearly to 0.5-2% as the temperature rises. At low temperatures, H2 reduces surface magnesium oxide (MgO) and boron oxide (BO), improving interface cleanliness and preventing microcracks caused by excessive friction between the powders.

[0065] The second stage: After the temperature rises to 600-650°C, it is switched to pure argon gas and the temperature is continued to rise to 800-850°C at a rate of 1-5°C / min. At the same time, the pressure is increased stepwise to 90-110MPa (maintained for 2-5 minutes after each increase of 10-15MPa). During this stage, pulsed BCl3 gas is introduced with a concentration controlled at 0.05-0.3vol%. The pulse on-off ratio is 1:2-4 (i.e., it is introduced every 1 second and then closed for 3-5 seconds), and the pulse frequency is 0.05-0.2Hz. BCl3 decomposes into active B atoms and Cl2 at high temperatures. Cl2 reacts with the oxides on the surface of the magnesium powder to form volatile chlorides. The pulsed gas flow destroys the diffusion interface through periodic pressure fluctuations, thereby enhancing the penetration rate of B atoms into the interior of the magnesium particles.

[0066] The third stage: When the temperature rises to 800-850℃, it enters the cooling stage. The temperature is reduced to 700-750℃ at a cooling rate of 30-80℃ / min, and the pressure is gradually reduced to 40-60MPa (maintained for 1-2 minutes after each 10-15MPa reduction). During this stage, 0.3-1.0vol% CH4 / Ar mixed gas is introduced. CH4 decomposes at 700-800℃ to generate active carbon atoms, which diffuse into the MgB2 lattice defect sites, such as grain boundaries and dislocations, through the gas phase to form a carbon-doped superconducting phase (MgB 2-x C x ).

[0067] 2. In situ grain boundary modification

[0068] In-situ grain boundary modification is achieved through interfacial reaction of pure SiC films.

[0069] During the second stage of hot pressing, the nanoporous SiC film placed at the interface of the preform reacts with the B atoms that diffuse into the interface, generating a superconducting MgB2 phase with lattice defects and carbon doping. This process requires no external catalyst, and the reaction kinetics are controlled by atmosphere switching and pulsed gas activation during the hot pressing process.

[0070] S4, multi-stage atmosphere control

[0071] In this embodiment, the multi-stage atmosphere control includes a staged gas switching and reaction path control process, which is specifically implemented as follows:

[0072] 1. The first stage: surface oxide reduction

[0073] During the initial stage of gradient hot pressing (room temperature to 600-650°C), a 3-8% by volume H2 / Ar mixture is introduced into the hot pressing furnace. As the temperature increases, the H2 concentration decreases linearly, reaching a low of 0.5-2% at 600°C. During this stage, H2 reacts with residual magnesium oxide (MgO) and boron oxide (B2O3) on the preform surface through a reduction reaction (equations: MgO + H2 → Mg + H2O↑; B2O3 + 3H2 → 2B + 3H2O↑). The generated water vapor is continuously exhausted via a vacuum pump. This dynamic H2 concentration adjustment strategy ensures sufficient reduction in the low-temperature range (<600°C) while preventing hydrogen embrittlement that could be caused by excess H2 in the high-temperature range (>600°C).

[0074] 2. The second stage: BCl3 gas diffusion activation

[0075] When the temperature reaches 600-650°C, the H2 / Ar mixture is turned off and replaced with pure argon. Simultaneously, a 0.05-0.3% volume fraction of BCl3 gas is introduced. At high temperatures (600-850°C), BCl3 decomposes to form active boron atoms and Cl2 (reaction equation: 2BCl3→2B+3Cl2↑). Active boron atoms diffuse into the magnesium powder surface through the gas phase, filling areas where solid-state diffusion is insufficient. Simultaneously, Cl2 reacts with surface oxides of the magnesium powder to form volatile magnesium chloride (MgCl2), which purifies the interface. The BCl3 gas concentration is precisely controlled by a mass flow meter to ensure reaction accuracy and prevent corrosion of the graphite mold by excessive Cl2.

[0076] 3. The third stage: CH4 / Ar atmosphere to achieve lattice defect repair

[0077] During the hot pressing and cooling stage (800-850℃→700-750℃), a CH4 / Ar mixed gas with a volume fraction of 0.3-1.0% is introduced. CH4 undergoes a cracking reaction in the range of 700-800℃ (reaction formula: CH4→C+2H2↑). The active carbon atoms produced by the cracking migrate through the gas phase to defect positions such as MgB2 grain boundaries and dislocations, and some carbon replaces the B site to form carbon-doped MgB2. 2-x C x The carbon doping introduces lattice distortion and electron scattering centers, while the secondary generation of H2 assists in the reduction of residual oxides, forming a dual repair mechanism that enhances the superconducting properties of the material.

[0078] The above steps form a complete atmosphere control system through the concentration control of H2 / Ar mixed gas, pulse activation of BCl3 gas and introduction of CH4 cracking atmosphere:

[0079] H2 / Ar atmosphere ensures effective reduction in the low temperature zone and removes surface oxides;

[0080] BCl3 gas promotes the diffusion of B atoms and purifies the interface, supplementing the reaction between magnesium powder and boron powder;

[0081] The carbon element introduced by CH4 cracking repairs lattice defects and regulates superconducting properties.

[0082] Through this multi-level atmosphere control, the composition uniformity, interface quality and current-carrying capacity of the intermediate connection area can be effectively improved, ensuring the stability and high efficiency of material performance.

[0083] In order to better understand the present invention, the above method is described in detail below with reference to specific examples.

[0084] Example 1:

[0085] 1. Precursor Preparation

[0086] Boron powder nano-crystallization: initial particle size 3 μm, ball milling conditions: argon protection, ball-to-material ratio 10:1, speed 400 rpm, time 6 h, final particle size is about 120 nm;

[0087] Hydroxylation treatment: 0.1M HCl solution, ultrasonic frequency 40kHz, power 100W, treatment for 40min, drying conditions 50℃, ≤10 -2 Pa, 4h;

[0088] Magnesium powder BN coating: particle size 20 μm, NH3:BCl3 volume ratio 2:1, reaction temperature 600 ° C, deposition time 30 min, BN thickness is about 3.5 nm.

[0089] 2. Mixing and pressing

[0090] Ratio: Mg:B molar ratio is 1:2.00, and 0.6 wt% PDA-GQDs are added;

[0091] Wet mixing: ethanol as the medium, solid-liquid ratio 1:4, zirconia ball-to-material ratio 8:1, ball mill speed 350 rpm, mixing time 4 h;

[0092] Drying and pressing: drying temperature 60℃, time 6h, cold pressing pressure 100MPa, holding pressure 10min;

[0093] Interface transition layer: SiC film deposited by magnetron sputtering, with a thickness of 1.0 μm, a porosity of 30%, and a pore diameter of 70 nm.

[0094] 3. Hot pressing sintering and atmosphere control

[0095] Stage 1: Room temperature to 625°C, heating rate 5°C / min, pressurization to 50 MPa in stages, and introduction of H2 / Ar (H2 initially 6%, linearly reduced to 1%);

[0096] The second stage: the temperature was raised to 825°C and the pressure was increased to 100 MPa, with the pressure maintained for 3 min for each 10 MPa increase, while BCl3 gas was introduced in pulses (concentration 0.15 vol%, pulse ratio 1:3, frequency 0.1 Hz);

[0097] The third stage: cooling to 725 ° C, cooling rate 50 ° C / min, reducing the pressure to 50 MPa, while introducing CH4 / Ar (concentration 0.6 vol%);

[0098] Inert atmosphere: Argon protection throughout the process, initial vacuum degree 1×10 -3 Pa.

[0099] Example 2:

[0100] 1. Precursor Preparation

[0101] Boron powder particle size 5μm, ball milling: ball to material ratio 12:1, speed 500rpm, 8h, particle size 200nm;

[0102] Hydroxylation: 0.2 M HCl, ultrasonic frequency 60 kHz, power 150 W, 60 min; drying conditions 60 ° C, 4 h;

[0103] Magnesium powder BN coating: particle size 30μm, NH3:BCl3=3:1, temperature 650℃, 40min, BN thickness 5nm.

[0104] 2. Mixing and pressing

[0105] Ratio: Mg:B molar ratio 1:2.05, PDA-GQDs addition amount 1.0wt%;

[0106] Wet mixing: ethanol, solid-liquid ratio 1:6, zirconia ball-to-material ratio 10:1, ball milling speed 400 rpm, time 6 h;

[0107] Drying temperature 80℃, drying time 8h, pressing pressure 120MPa, holding pressure 15min;

[0108] The SiC film has a thickness of 1.5 μm, a porosity of 40%, and a pore size of 100 nm.

[0109] 3. Hot pressing sintering and atmosphere control

[0110] Stage 1: Room temperature to 650°C, heating rate 8°C / min, pressurization to 60 MPa, H2 / Ar gas initial 8% and then reduced to 2%;

[0111] The second stage: the temperature was raised to 850 ° C and the pressure was increased to 110 MPa, and the pressure was maintained for 5 minutes at each increase of 15 MPa, and BCl3 (concentration 0.3 vol%, pulse ratio 1:4, frequency 0.2 Hz) was introduced;

[0112] The third stage: cooling to 750 ° C, cooling rate 80 ° C / min, reducing pressure to 60 MPa, CH4 / Ar concentration 1.0 vol%;

[0113] Initial vacuum degree 1×10 -3 Pa, the atmosphere is argon.

[0114] Example 3:

[0115] 1. Precursor Preparation

[0116] Boron powder particle size 1μm, ball-to-powder ratio 8:1, rotation speed 300rpm, time 4h, particle size 50nm;

[0117] Hydroxylation: 0.05 M HCl, ultrasonic frequency 20 kHz, power 50 W, 20 min, drying at 40 °C, 2 h;

[0118] Magnesium powder BN coating: particle size 10μm, NH3:BCl3=1:1, temperature 550℃, time 20min, film thickness 2nm.

[0119] 2. Mixing and pressing

[0120] Ratio: Mg:B molar ratio 1:1.95, PDA-GQDs addition amount 0.3wt%;

[0121] Wet mixing: ethanol, solid-liquid ratio 1:3, ball-to-material ratio 6:1, speed 300 rpm, time 2 h;

[0122] Drying temperature: 50℃, drying time: 4h, pressing pressure: 80MPa, holding pressure: 5min;

[0123] The SiC film has a thickness of 0.5 μm, a porosity of 20%, and a pore diameter of 30 nm.

[0124] 3. Hot pressing sintering and atmosphere control

[0125] Stage 1: Heating to 600°C, heating rate 3°C / min, pressurizing to 40 MPa, H2 / Ar initial 3%, decreasing to 0.5%;

[0126] The second stage: the temperature was raised to 800 ° C and the pressure was increased to 90 MPa, with the pressure maintained for 2 minutes for each 10 MPa increase, the BCl3 concentration was 0.05 vol%, the pulse ratio was 1:2, and the frequency was 0.05 Hz;

[0127] The third stage: cooling to 700 ° C, cooling rate 30 ° C / min, reducing pressure to 40 MPa, CH4 / Ar concentration 0.3 vol%;

[0128] The vacuum degree is 1×10 -3 Pa, the inert atmosphere is argon.

[0129] Comparative Example 1:

[0130] Compared with Example 1, the difference is that the boron powder is not subjected to hydroxylation treatment, and the ball-milled nano-boron powder is directly used, and the rest are the same.

[0131] Comparative Example 2:

[0132] Compared with Example 1, the difference is that the magnesium powder is not subjected to BN coating treatment, and bare metal magnesium powder is directly used, and the rest are the same.

[0133] Comparative Example 3:

[0134] Compared with Example 1, the difference is that the polydopamine-modified graphene quantum dot dispersant is not added, and the rest are the same.

[0135] Comparative Example 4:

[0136] Compared with Example 1, the difference is that the molar ratio of Mg:B is set to 1:2.10, and the rest are the same.

[0137] Comparative Example 5:

[0138] Compared with Example 1, the difference is that the SiC porous membrane transition layer is not prefabricated at the connection interface, and the rest are the same.

[0139] Comparative Example 6:

[0140] Compared with Example 1, the difference is that BCl3 gas is not introduced during the hot pressing sintering process, and the second stage of B atom diffusion activation process is omitted. The rest are the same.

[0141] Test Example 1:

[0142] The effects of different preparation processes (examples and comparative examples) on the critical current density (Jc) of the MgB2-MgB2 superconducting joint region were evaluated to verify the synergistic advantages of material design and process control in the present invention.

[0143] Experimental equipment and conditions:

[0144] Testing instruments: four-probe physical property measurement system (PPMS), constant temperature and low temperature platform;

[0145] Current source: DC current source, output range 0–200A;

[0146] Temperature control system: cryostat (the measurement temperature is set to 20K);

[0147] Sample preparation: Prepare at least 3 pieces of each sample, and cut them into a uniform size of 20mm×2mm×1mm (the length of the joint area is not less than 5mm);

[0148] Contact electrode: silver paste sintered electrode, ensuring contact resistance <10μΩ;

[0149] Critical electric field standard: 1μV / cm.

[0150] Experimental steps:

[0151] 1. Sample Pretreatment

[0152] The prepared MgB2-MgB2 joint samples were surface polished and cleaned;

[0153] Four-point probes were placed at both ends of the joint, and the electrodes were welded to the sample using silver paste and sintered at 80 °C for 10 min to ensure stable contact.

[0154] 2. Load sample and cool down

[0155] The sample was fixed on the PPMS sample holder and adiabatically cooled by passing high-purity helium gas to the set test temperature of 20K, and kept warm for 10 minutes to ensure thermal stability.

[0156] 3.Jc test process

[0157] While maintaining a constant temperature of 20K, the DC current was slowly increased and the voltage response was recorded in real time;

[0158] When the electric field strength reaches 1 μV / cm, record the current value at this time and calculate Jc (based on the cross-sectional area of ​​the joint);

[0159] Each sample was measured three times and the average value was taken.

[0160] 4. Data recording and processing

[0161] Collect Jc test values ​​of different samples, organize and summarize them for comparison;

[0162] Special note should be given to samples with broken or burned joints or failure of non-superconducting transition.

[0163] The experimental results are shown in Table 1:

[0164] Table 1 Comparison of critical current density (Jc) of MgB2 joint samples at 20K

[0165] Sample type <![CDATA[Jc1(A / cm 2 )]]> <![CDATA[Jc2(A / cm 2 )]]> <![CDATA[Jc3(A / cm 2 )]]> <![CDATA[Average Jc (A / cm 2 )]]> Example 1 <![CDATA[4.83×10 4 ]]> <![CDATA[4.76×10 4 ]]> <![CDATA[4.95×10 4 ]]> <![CDATA[4.85×10 4 ]]> Comparative Example 1 <![CDATA[3.72×10 4 ]]> <![CDATA[3.80×10 4 ]]> <![CDATA[3.66×10 4 ]]> <![CDATA[3.73×10 4 <!-- 8 -->]]> Comparative Example 2 <![CDATA[3.19×10 4 ]]> <![CDATA[3.28×10 4 ]]> <![CDATA[3.05×10 4 ]]> <![CDATA[3.17×10 4 ]]> Comparative Example 5 <![CDATA[3.94×10 4 ]]> <![CDATA[4.01×10 4 ]]> <![CDATA[3.90×10 4 ]]> <![CDATA[3.95×10 4 ]]>

[0166] From the experimental data in Table 1, we can get:

[0167] The sample of Example 1 performs best in critical current density, with relatively concentrated data, indicating that the connection area is uniform and the grain boundary is good;

[0168] The Jc of Comparative Examples 1 and 2 decreased significantly, indicating that the lack of hydroxylation or BN coating treatment affected the reactivity and structural integrity;

[0169] Comparative Example 5 is slightly lower than the embodiment, but better than the other comparative examples, indicating that the transition layer is not the only determining factor, but plays an important auxiliary role in interface stability.

[0170] Test Example 2:

[0171] The relationship between the resistance and temperature of different samples in the range of 10–50K was tested to evaluate the superconducting transition temperature (Tc) and transition zone width (ΔTc), which in turn reflects the conductivity continuity, composition uniformity and crystal quality of the joint.

[0172] Experimental equipment and conditions:

[0173] Instruments: low-temperature four-probe resistance test system (with temperature control module), high-precision voltmeter;

[0174] Temperature control range: 10–50K, temperature control accuracy ±0.1K;

[0175] Current source: constant current source, the test current is set to 10mA;

[0176] Measurement interval: once per temperature rise of 1K;

[0177] Contact electrode: silver paste electrode, contact resistance <10μΩ;

[0178] Sample preparation: long strip joint sample (about 20×2×1mm 3 ), the connector is located in the middle.

[0179] Experimental steps:

[0180] 1. Sample Preparation and Electrode Arrangement

[0181] Current and voltage electrodes are arranged at both ends of the joint in the form of four-point probes;

[0182] Ensure the connector area is centered to prevent lead position from interfering with measurement accuracy.

[0183] 2. Temperature control system settings

[0184] Place the sample into the test chamber, evacuate it and then fill it with inert gas;

[0185] Set the cooling process to 10K and perform temperature stabilization.

[0186] 3. Temperature measurement process

[0187] Every time the voltage is increased by 1K, the voltage response is recorded and the resistance value is calculated (R = V / I);

[0188] Particular attention was paid to the R-value mutation point (defined as Tc) and the transition region span (ΔTc) in the junction region.

[0189] 4. Data processing and curve drawing

[0190] Draw the RT curve with temperature as the horizontal axis and resistance as the vertical axis;

[0191] Analyze the superconducting critical temperature Tc (where R drops to zero) and ΔTc (the width of the temperature range where R drops from the initial value to zero) through the curve;

[0192] All data points were measured three times and averaged.

[0193] The experimental results are shown in Table 2:

[0194] Table 2 Comparison of superconducting transition temperature (Tc) and transition zone width (ΔTc) of MgB2 superconducting joint samples

[0195] Sample type Tc1(K) Tc2(K) Tc3(K) Average Tc(K) ΔTc(K) Example 1 38.4 38.2 38.7 38.4 1.1 Comparative Example 3 36.9 36.5 36.7 36.7 3.4 Comparative Example 4 37.3 37.1 36.8 37.1 2.9

[0196] From the experimental data in Table 2, we can get:

[0197] The superconducting transition temperature (Tc) of the sample in Example 1 is high and the ΔTc is narrow, indicating that the joint has uniform composition and complete structure, and has good superconducting continuity;

[0198] In Comparative Example 3 (no dispersant), Tc decreased and ΔTc became significantly wider, possibly due to particle agglomeration leading to a localized non-uniform reaction and a resistance plateau.

[0199] In comparative example 4 (over-proportion), Tc is slightly decreased and ΔTc is also wide, indicating that excess B or Mg causes the phase structure to deviate from the optimal ratio, resulting in electronic structure mismatch.

[0200] Test Example 3:

[0201] Evaluate the effects of different preparation processes on the mechanical properties of the joint area, focusing on testing the shear strength to reflect the integrity and bonding strength of the connection structure, and verify the role of innovative measures such as BN coating and SiC film.

[0202] Experimental equipment and conditions:

[0203] Test equipment: micro electronic universal material testing machine (with shear fixture);

[0204] Test temperature: room temperature (20–25°C);

[0205] Loading rate: 0.5mm / min;

[0206] Sample specifications: Each sample has a uniform cross-section of 2mm×1mm and a joint area length of 5mm;

[0207] Loading mode: single shear mode, the load acts on the interface of the joint area.

[0208] Experimental steps:

[0209] 1. Sample Pretreatment

[0210] Both ends of the joint sample were encapsulated with resin, leaving only the middle joint area exposed to ensure shear concentration;

[0211] The surface is lightly sanded to remove the oxide layer.

[0212] 2.Installation and alignment

[0213] Place the sample into the shear fixture, ensuring that it is aligned up and down and the joint plane is perpendicular to the load direction;

[0214] Adjust the clamping position so that the joint is exactly on the shear center axis.

[0215] 3. Load test

[0216] A constant loading speed (0.5 mm / min) was set and the load-displacement curve was recorded;

[0217] The load is stopped when it reaches the breaking point, and the breaking load is recorded;

[0218] The breaking load is divided by the joint interface area (2mm 2 )Calculate the shear strength (unit: MPa).

[0219] 4. Repeat and record

[0220] Three samples of each type were tested, and the results were averaged and analyzed for dispersion;

[0221] If slippage before cracking or atypical damage occurs, record and explain it.

[0222] The experimental results are shown in Table 3:

[0223] Table 3 Shear strength test results of MgB2 superconducting joints

[0224] Sample type Strength 1(MPa) Strength 2 (MPa) Strength 3 (MPa) Average shear strength (MPa) Example 1 27.4 28.1 26.7 27.4 Comparative Example 2 18.6 19.3 17.9 18.6 Comparative Example 5 21 20.4 19.7 20.4

[0225] From the experimental data in Table 3, we can get:

[0226] Example 1 showed the highest shear strength, indicating that the synergistic effect of BN coating + SiC film significantly improved the physical bonding of the joint interface and the stability of the reaction layer;

[0227] The shear strength of Comparative Example 2 (without BN coating) decreased significantly, which may be due to the formation of a brittle interlayer due to oxidation of Mg powder;

[0228] Although the structure of Comparative Example 5 (without transition layer) is complete, the bonding interface is weak, and the fracture mostly occurs at the interface between the prefabricated block and the wire.

[0229] Test Example 4:

[0230] The resistance change rate (ΔR) and critical temperature drop (ΔTc) of MgB2 joints were tested under accelerated aging conditions (high temperature and high humidity) to evaluate the effects of different processes on their anti-oxidation and anti-water vapor capabilities.

[0231] Experimental equipment and conditions:

[0232] Environmental chamber: constant temperature and humidity test chamber (temperature 40°C, relative humidity 85%);

[0233] Aging time: 100 hours;

[0234] Resistance measurement: four-probe micro-resistance measurement system;

[0235] Critical temperature test: same as test example 2 (RT curve);

[0236] Sample size: uniformly 20mm×2mm×1mm.

[0237] Experimental steps:

[0238] 1.Before burn-in benchmark test

[0239] Measure the initial joint resistance (R0) and critical temperature (Tc0) of the sample;

[0240] All data were recorded as a control baseline.

[0241] 2. Accelerated aging treatment

[0242] The samples were placed in a constant temperature and humidity chamber at 85% RH and 40°C for 100 h;

[0243] Avoid contact between samples and ensure uniform airflow.

[0244] 3. Post-aging test

[0245] Remove the sample and cool it to room temperature, repeat the resistance and Tc test, and record the resistance (R1) and critical temperature (Tc1) after aging;

[0246] The resistance change rate ΔR is calculated as follows: ((R1-R0) / R0)×100%, and the Tc decrease amount ΔTc is calculated as follows: Tc0-Tc1.

[0247] 4. Repeat and verify

[0248] Three samples of each type were tested and the average was taken. If a sample showed breakage or non-superconducting transition, it was discarded.

[0249] The experimental results are shown in Table 4:

[0250] Table 4 Comparison of electrochemical stability of MgB2 superconducting joints under wet and hot conditions (ΔR and ΔTc)

[0251] Sample type ΔR1(%) ΔR2(%) ΔR3(%) Average ΔR (%) ΔTc(K) Example 1 3.6 4.1 3.9 3.9 0.6 Comparative Example 1 9.4 10.1 8.8 9.4 1.7 Comparative Example 6 6.7 7.2 6.5 6.8 1.2

[0252] From the experimental data in Table 4, we can get:

[0253] After aging, the resistance of the joint in Example 1 increased only slightly, and Tc decreased slightly, indicating that it has good oxidation resistance and grain boundary stability;

[0254] Comparative Example 1 (no hydroxylation) has a large amount of oxidation residue on the powder surface, resulting in a large ΔR after aging and obvious degradation of conductivity;

[0255] Comparative Example 6 did not use BCl3 activation atmosphere, and the interface oxide was not sufficiently removed, resulting in a moderate deviation in the moisture and heat resistance.

[0256] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering, characterized in that: The following steps are involved: (1) Precursor preparation: boron powder is nano-processed and a boron nitride layer is coated on the surface of magnesium powder; (2) Mixing and pressing: The boron powder treated in step (1) is mixed with magnesium powder at a Mg:B molar ratio of 1:1.95 to 2.05, and a dispersant is added, and cold pressed into an intermediate prefabricated block for connection; (3) Joint assembly and local gradient hot pressing sintering: The middle prefabricated block is placed between two sections of prefabricated high-performance MgB2 wire to form a three-section structure. The temperature and pressure of the middle region are controlled in stages under an inert atmosphere to perform local hot pressing sintering, so that magnesium diffuses in the middle region and reacts with boron to form an MgB2 joint in situ. At the same time, a transition layer is pre-prepared at the connection interface to modify the grain boundary through in-situ reaction. (4) Multi-stage atmosphere control: During the hot pressing sintering process in step (3), H2 / Ar mixed gas, BCl3 gas and CH4 / Ar mixed gas are introduced into the joint area in sequence to achieve surface oxide reduction, boron atom diffusion activation and lattice defect repair, respectively.

2. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The nano-processing of the boron powder in step (1) comprises: Boron powder with an initial particle size of 1 to 5 μm was crushed to 50 to 200 nm by high-energy ball milling. The ball milling parameters were: argon protection, ball-to-material mass ratio of 8 to 12:1, rotation speed of 300 to 500 rpm, and time of 4 to 8 h. Subsequently, the nano boron powder is immersed in a 0.05-0.2M HCl solution and ultrasonically treated for 20-60 minutes to generate surface hydroxylated boron powder.

3. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The boron nitride coating of the magnesium powder in step (1) comprises: A 2-5 nm boron nitride layer is coated on the surface of magnesium powder by chemical vapor deposition. The deposition conditions are: temperature 550-650°C, reaction gas is a mixture of NH3 and BCl3, volume ratio 1:1-3, and deposition time 20-40 min.

4. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The dispersant in step (2) is polydopamine-modified graphene quantum dots, and the addition amount thereof is 0.3 to 1.0 wt %; The pressure of the cold-pressed intermediate prefabricated blocks for connection is 80-120 MPa, and the holding time is 5-15 minutes.

5. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The local gradient hot pressing sintering in step (3) includes the following staged control, and the action area is limited to the location of the intermediate connecting prefabricated block: The first stage: from room temperature to 600-650℃, increase the pressure stepwise to 40-60MPa at a heating rate of 3-8℃ / min; The second stage: in the range of 600-650℃ to 800-850℃, the pressure is increased step by step to 90-110MPa at a heating rate of 1-5℃ / min; The third stage: in the range of 800-850℃ to 700-750℃, the pressure is reduced step by step to 40-60MPa at a cooling rate of 30-80℃ / min.

6. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 5, characterized in that: The step-by-step pressurization method for the intermediate connection area in the second stage is: After each increase of 10-15 MPa, the pressure is maintained for 2-5 minutes, and pulsed BCl3 gas is introduced at the same time, with a pulse on-off ratio of 1:2-4, a frequency of 0.05-0.2 Hz, and a BCl3 gas concentration of 0.05-0.3 vol%.

7. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The transition layer in step (3) is a nanoporous SiC membrane, and its preparation includes: A 0.5-1.5 μm thick SiC film is deposited at the interface between the intermediate connecting prefabricated block and the wire material by magnetron sputtering, the film layer has a porosity of 20-40% and a pore diameter of 30-100 nm.

8. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 7, characterized in that: The in-situ reaction occurs in the middle connection area. During the hot pressing process at 800-850° C., the SiC film reacts with the boron interface to generate trace amounts of free carbon, which assists in regulating the grain boundary structure.

9. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The local multi-stage atmosphere control for the intermediate connection area in step (4) includes: In the first stage, a 3-8 vol% H2 / Ar mixed gas is introduced, and the H2 concentration is linearly reduced to 0.5-2 vol% as the temperature rises; In the second stage, pure Ar is switched and 0.05-0.3 vol% BCl3 gas is introduced; In the third stage, 0.3-1.0 vol% CH4 / Ar mixed gas is introduced and cracked at 700-800°C to generate activated carbon for adjusting the microstructure of the connection interface.

10. The method for preparing a MgB2-MgB2 superconducting joint by hot pressing and sintering according to claim 1, characterized in that: The inert atmosphere in step (3) is argon, and the initial vacuum degree is ≤1×10 -3 Pa.

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