A magic angle graphene modified niobium tri-tin superconducting wire and a preparation method thereof

Abstract

The application belongs to the technical field of superconducting material processing, and discloses a magic angle graphene modified niobium three tin superconducting wire and a preparation method thereof. The application modifies magic angle graphene on the surface of a core wire of a bronze method niobium three tin superconducting wire, the material can eliminate the work hardening of the bronze matrix, and realize one-time and uninterrupted continuous stretching forming from a composite blank ingot to a final wire. The niobium three tin superconducting wire also has better mechanical properties, superconducting properties and strain resistance, the magic angle graphene layer can effectively inhibit the crack initiation and expansion of the Nb3Sn reaction layer under strain, and improve the mechanical properties of the wire; the material can not only be used as an additional magnetic flux pinning center, but also can induce the formation of finer and stronger grain boundary Nb3Sn grains, and significantly improve the critical current density of the wire; the critical current irreversible strain limit of the magic angle graphene modified niobium three tin superconducting wire is 0.8%, which is significantly higher than that of the traditional bronze method prepared niobium three tin wire.

Description

Technical Field

[0001] This invention belongs to the field of superconducting material processing technology, and relates to a magic-angle graphene-modified niobium-tin superconducting wire and its preparation method. Background Technology

[0002] Niobium-tritin (Nb3Sn) superconductors possess a high critical transition temperature (Tc≈18K), excellent high-field carrying capacity, and good mechanical properties, making them promising candidates for applications in high-field magnets, nuclear magnetic resonance imaging (MRI), and particle accelerators. Currently, the main methods for preparing Nb3Sn superconducting wires include the internal tin method, the bronze method, and the powder-coated tubing method. Among these, the bronze method, with its high process maturity and good product stability, has become the mainstream method for industrial production of Nb3Sn superconducting wires. The process route for preparing Nb3Sn superconducting wires using the bronze method is as follows: First, Nb rods are embedded in a copper-tin bronze alloy matrix, and then subjected to multiple cold working processes such as drawing and twisting to form composite wires. Subsequently, diffusion annealing is performed at a high temperature of 650~750℃ for tens to hundreds of hours. During diffusion annealing, Sn elements diffuse and migrate from the bronze matrix to the Nb rods, and Nb reacts chemically with Sn to form an Nb3Sn superconducting layer, ultimately obtaining a finished wire with superconducting properties.

[0003] Although the bronze method has been industrialized, this technical route still has defects: (1) The production process is not continuous: the copper-tin solid solution as the matrix is ​​prone to work hardening during cold working, which leads to a decrease in the plasticity of the matrix. If the processing continues, it is easy to cause wire breakage. In order to ensure the feasibility of processing, the processing process must be interrupted repeatedly to carry out multiple independent intermediate recrystallization annealing processes to restore the plasticity of the matrix. This "cold working-interruption-annealing" cyclic production mode not only leads to a decrease in production efficiency and an increase in energy consumption, but also makes it difficult to achieve continuous production throughout the entire process. (2) There is a bottleneck in superconducting performance: the grain boundaries of the Nb3Sn layer are the core sites of the magnetic flux pinning center. Their quantity and distribution directly determine the critical current density of the wire. However, the Nb3Sn grain boundaries are inherently brittle and are prone to cracking under strain, which leads to the degradation of the superconducting performance of the wire. On the other hand, in order to achieve a full and uniform diffusion reaction between Nb and Sn and ensure the quality of superconducting phase formation, long-term high-temperature heat treatment is usually required. However, this treatment will inevitably cause grain coarsening, resulting in a reduction in the total area of ​​grain boundaries, weakening the magnetic flux pinning ability, and limiting the further improvement of the critical current density.

[0004] Therefore, it is of great significance to develop a bronze-based method for preparing Nb3Sn wire that can omit or significantly shorten the high-temperature annealing process while simultaneously refining Nb3Sn grains. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a niobium-tritin superconducting wire modified with magic-angle graphene and its preparation method. This invention modifies the surface of the core wire of a bronze-processed niobium-tritin superconducting wire with magic-angle graphene. This material eliminates work hardening of the bronze matrix, enabling one-time, uninterrupted continuous stretching from the composite ingot to the final wire. The niobium-tritin superconducting wire modified with magic-angle graphene of this invention also possesses superior mechanical properties, superconducting properties, and strain tolerance. The magic-angle graphene layer effectively inhibits the initiation and propagation of cracks in the Nb3Sn reaction layer under strain, improving the wire's mechanical properties. Magic-angle graphene not only serves as an additional flux pinning center but also induces the formation of finer, more robust Nb3Sn grains with an average grain size of 70-80 nm, significantly increasing the wire's critical current density. The critical current irreversible strain limit of the niobium-tritin superconducting wire modified with magic-angle graphene of this invention is 0.8%, significantly higher than that of niobium-tritin wire prepared by the traditional bronze method.

[0006] On one hand, the present invention provides a niobium tritin superconducting wire modified with magic angle graphene, wherein the niobium tritin superconducting wire modified with magic angle graphene consists of a stable matrix, a copper-tin alloy matrix and a core rod from the outer layer to the inner layer.

[0007] The core rod is a core filament modified with subcomponents or magic-angle graphene;

[0008] The sub-components, from the outermost layer to the innermost layer, are a magic-angle graphene layer, a copper-tin alloy substrate, a magic-angle graphene layer, and a core filament.

[0009] The number of stacked magic-angle graphene layers in the magic-angle graphene layer is 10;

[0010] The twist angle of the magic-angle graphene layer is 0.8° to 1.3°. Preferably, the twist angle of the magic-angle graphene layer is 1.05° to 1.15°.

[0011] Furthermore, the core wire is a niobium wire or a niobium-titanium alloy wire;

[0012] The titanium content in the niobium-titanium alloy wire is 1~50 wt.%.

[0013] The material of the stabilizing matrix is ​​selected from oxygen-free copper or high-purity aluminum, and the purity of the aluminum is ≥99.999%.

[0014] On the other hand, the present invention provides a method for preparing the above-mentioned magic-angle graphene-modified niobium tin superconducting wire, which specifically includes the following steps:

[0015] S1: The core wire is electrolytically polished to remove the surface oxide layer and obtain an atomically clean surface. It is then ultrasonically cleaned with acetone and ethanol sequentially and dried. A magic-angle graphene layer is grown on the cleaned and dried core wire surface by chemical vapor deposition to obtain a magic-angle graphene-modified core wire.

[0016] Optionally, the magic-angle graphene-modified core wire is assembled with a drilled copper-tin alloy ingot, and then subjected to a first hot isostatic pressing treatment to obtain a single-assembly billet. The single-assembly billet is extruded into shape in one go and then drawn into a rod through a first multi-pass drawing process. Magic-angle graphene is then modified on the surface of the rod by chemical vapor deposition to obtain sub-components.

[0017] Further, the tin content in the copper-tin alloy ingot is 10-16 wt.%. The first hot isostatic pressing treatment is carried out under argon protection at a temperature of 500-650°C, a pressure of 100-200 MPa, and a holding time of 2-3 hours. The extrusion is carried out at a temperature of 300-400°C, and the total reduction of area (RA) of the extrusion is 90-99%. The single-pass drawing rate of the first multi-pass drawing is 20%; in the first multi-pass drawing, a high-frequency alternating current is applied to the wire at the entrance of the drawing die; the frequency of the high-frequency alternating current is 10 kHz-1 MHz, and the current density is 400-450 A / mm. 2 .

[0018] Furthermore, the core wire is a niobium wire or a niobium-titanium alloy wire, wherein the titanium content in the niobium-titanium alloy wire is 1~50 wt.%. The chemical vapor deposition is hot-wire chemical vapor deposition (HFCVD) or plasma-enhanced chemical vapor deposition (PE-CVD).

[0019] Furthermore, the hot-wire chemical vapor deposition is performed under a vacuum of 1×10⁻⁶. -3 The process was carried out under the conditions of Pa, hydrogen flow rate of 40-150 sccm, and gas pressure of 50-1000 Pa. The core filament was placed in the HFCVD reaction chamber, and the core filament substrate temperature was raised to 800-1000℃. A carbon source gas (methane) was introduced at a flow rate of 35-100 sccm for 20-40 min to grow graphene, and then the methane was turned off. The core filament or subcomponent modified with magic-angle graphene was then obtained by a programmed cooling at a cooling rate of 5℃ / s under a hydrogen atmosphere to 800℃, followed by natural cooling to room temperature.

[0020] Furthermore, in the plasma-enhanced chemical vapor deposition, the core wire is first treated at 400~650℃ with argon gas (50~200 sccm) and a small amount of hydrogen gas (5~50 sccm) for 10 min to activate the surface. Then, the radio frequency (13.56MHz) plasma power supply is turned on and the power is set to 50~300W. After the plasma stabilizes, 10~50 sccm of methane is introduced to grow graphene for 5~30 min, thus obtaining the core wire or subunit modified with magic angle graphene.

[0021] S2: Precision machining of arrayed holes is performed on the end face of a cylindrical copper-tin alloy ingot. The magic-angle graphene-modified core wires or sub-components are inserted into the holes, ensuring that all niobium wires are parallel and aligned in their length direction, to obtain a CuNbSn matrix. The CuNbSn matrix is ​​then placed into a stable matrix, followed by a second hot isostatic pressing treatment to obtain a composite ingot.

[0022] Furthermore, the diameter of the arrayed pores is 0.2-2 mm larger than the diameter of the core filament modified with magic-angle graphene. The stabilizing substrate material is selected from oxygen-free copper or high-purity aluminum, and the purity of the high-purity aluminum is ≥99.999%.

[0023] Furthermore, the second hot isostatic pressing treatment is carried out under argon protection at a temperature of 600~650℃, a pressure of 150~200MPa, and a holding time of 2~3h.

[0024] S3: The composite billet is heated to 300~400℃ and extruded into a bar in one pass using an extruder. Then, a second multi-pass drawing is performed at room temperature to obtain the composite wire. A high-frequency alternating current is applied to the wire at the entrance of the drawing die.

[0025] Furthermore, the total reduction rate of the extrusion section is 95%~96%. The single-pass drawing rate of the second multi-pass drawing is 20%.

[0026] Furthermore, the frequency of the high-frequency alternating current is 10kHz~1MHz, and the current density is 400~450A / mm. 2 Applying a high-frequency alternating current to the wire at the entrance of the drawing die can instantly generate a temperature rise of about 350°C on the surface and internal interface of the wire. This temperature rise can effectively reduce the instantaneous rheological stress of the bronze matrix and activate the super-slippery properties between the magic angle graphene layers, thereby achieving high processing rate drawing.

[0027] S4: The composite wire is wound into a small solenoid coil with an inner diameter of 50mm, and the coil is placed in a high vacuum heat treatment furnace for final diffusion annealing.

[0028] Optionally, the coil is placed in a high-vacuum heat treatment furnace for two-stage heat treatment. The first stage is held at 400~600℃ for 50~200h, and then the temperature is raised to 600~800℃ and held for 50~100h for the second stage heat treatment.

[0029] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages:

[0030] (1) This invention provides a niobium tritin superconducting wire modified with magic angle graphene. After adding the magic angle graphene interface layer, the multiple intermediate annealing required due to work hardening is fundamentally eliminated, changing the traditional discontinuous production mode. The niobium tritin superconducting wire modified with magic angle graphene of this invention can be produced continuously from billet to finished product in one go, which greatly improves production efficiency and wire consistency and reduces manufacturing costs.

[0031] (2) In the multi-pass drawing process, the present invention applies a high-frequency alternating current to the wire at the entrance of the drawing die, which can cause the surface and internal interface of the wire to generate a temperature rise of about 350°C instantaneously. This temperature rise can effectively reduce the instantaneous rheological stress of the bronze matrix and activate the super-slippery properties between the magic angle graphene layers, thereby achieving a high processing rate drawing. The cross-sectional reduction of the multi-pass drawing of the present invention can reach more than 99%, and it has good tensile properties.

[0032] (3) The niobium tritin superconducting wire modified with magic angle graphene of the present invention significantly reduces the extrusion temperature. It can be extruded into a rod at a temperature of 300~400℃. This extrusion temperature is much lower than the significant diffusion temperature of Sn, which can realize the densification of the billet and ensure the quality of the billet. It effectively avoids the premature solid-phase reaction between Nb and Sn to form a brittle Nb3Sn phase. The formation of the Nb3Sn phase depends on the directional diffusion of Sn to the Nb core wire during the final vacuum heat treatment, ensuring that the Nb3Sn phase grows uniformly and continuously along the surface of the Nb core wire.

[0033] (4) The present invention provides a two-stage gradient heat treatment process that can achieve grain refinement and high-field pinning optimization. The first stage is at a relatively low temperature, which is conducive to the formation of a large number of fine Nb3Sn crystal nuclei. At the same time, the magic angle graphene layer effectively inhibits the excessively rapid diffusion of tin and avoids premature coarsening of the crystal nuclei. The second stage is at a higher temperature, which can promote the full growth of the Nb3Sn phase and optimize its stoichiometry. At the same time, the fine grain structure that has been formed can be partially retained.

[0034] (5) The niobium-3Sn superconducting wire modified with magic-angle graphene provided by this invention significantly improves the mechanical properties, superconducting properties, and strain tolerance of the wire. As an ultra-strong and ultra-tough nano-interface layer, the magic-angle graphene layer can effectively inhibit the initiation and propagation of cracks in the Nb3Sn reaction layer under strain, greatly improving the mechanical flexibility and tensile / bending fatigue resistance of the wire. The unique electronic state of magic-angle graphene may serve as an additional and excellent flux pinning center; at the same time, the Nb3Sn grains it induces are finer and the grain boundaries are more "tough," resulting in a higher and more stable critical current density under a magnetic field. The graphene layer buffers the thermal expansion mismatch and mechanical stress between the Nb core and the bronze substrate, significantly reducing the sensitivity of the wire's superconducting properties to axial tensile and transverse bending strain. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a cross-sectional view of the core filament modified with magic-angle graphene. The figures are labeled as follows: 1. Magic-angle graphene layer; 2. Core filament.

[0037] Figure 2 This is a cross-sectional view of a niobium-tin superconducting wire modified with magic-angle graphene. The labels in the attached figure are as follows: 3. Copper-tin alloy matrix; 4. Stabilizing matrix.

[0038] Figure 3 This is a cross-sectional view of the subcomponent.

[0039] Figure 4 This is a cross-sectional view of a double-assembled niobium-tritin superconducting wire modified with magic-angle graphene. The figures are labeled as follows: 5. Subunit. Detailed Implementation

[0040] The technical solution of the present invention will be described below with reference to embodiments. However, the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods and detection methods described in each embodiment are conventional methods; unless otherwise specified, the reagents and materials can be purchased commercially. Unless otherwise specified, the percentages in the following embodiments refer to mass percentages.

[0041] The preparation method of magic-angle graphene-modified niobium-tin superconducting wire, the flowchart is as follows: Figure 1 As shown, the specific steps include the following:

[0042] S1: The core wire is electrolytically polished to remove the surface oxide layer and obtain an atomically clean surface. It is then ultrasonically cleaned with acetone and ethanol sequentially and dried. A magic-angle graphene layer is grown on the cleaned and dried core wire surface by chemical vapor deposition to obtain a magic-angle graphene-modified core wire.

[0043] Optionally, the magic-angle graphene-modified core wire is assembled with a drilled copper-tin alloy ingot, and then subjected to a first hot isostatic pressing treatment to obtain a single-assembly billet. The single-assembly billet is extruded into shape in one go and then drawn into a rod through a first multi-pass drawing process. Magic-angle graphene is then modified on the surface of the rod by chemical vapor deposition to obtain sub-components.

[0044] Furthermore, the single-pass drawing rate of the first multi-pass drawing is 20%. At the entrance of the drawing die, a high-frequency alternating current is applied to the wire. The frequency of the high-frequency alternating current is 10kHz~1MHz, and the current density is 400~450A / mm. 2 .

[0045] Further, the tin content in the copper-tin alloy ingot is 10~16 wt.%. The first hot isostatic pressing treatment is carried out under argon protection at a temperature of 500~650℃, a pressure of 100~200MPa, and a holding time of 2~3h. The extrusion is carried out at a temperature of 300~400℃, and the total reduction of area (RA) of the extrusion is 90~99%.

[0046] Furthermore, the core wire is a niobium wire or a niobium-titanium alloy wire. The chemical vapor deposition is hot-wire chemical vapor deposition (HFCVD) or plasma-enhanced chemical vapor deposition (PE-CVD).

[0047] Furthermore, the hot-wire chemical vapor deposition is performed under a vacuum of 1×10⁻⁶. -3 The process was carried out under the conditions of Pa, hydrogen flow rate of 40-150 sccm, and gas pressure of 50-1000 Pa. The core filament was placed in the HFCVD reaction chamber, and the core filament substrate temperature was raised to 800-1000℃. A carbon source gas (methane) was introduced at a flow rate of 35-100 sccm for 20-40 min to grow graphene, and then the methane was turned off. The core filament or subcomponent modified with magic-angle graphene was then obtained by a programmed cooling at a cooling rate of 5℃ / s under a hydrogen atmosphere to 800℃, followed by natural cooling to room temperature.

[0048] Furthermore, in the plasma-enhanced chemical vapor deposition, the core wire is first treated at 400~650℃ with argon gas (50~200 sccm) and a small amount of hydrogen gas (5~50 sccm) for 10 min to activate the surface. Then, the radio frequency (13.56MHz) plasma power supply is turned on and the power is set to 50~300W. After the plasma stabilizes, 10~50 sccm of methane is introduced to grow graphene for 5~30 min, thus obtaining the core wire or subunit modified with magic angle graphene.

[0049] Furthermore, the number of stacked magic-angle graphene layers in the magic-angle graphene layer is 10. The twist angle of the magic-angle graphene layer is 0.8°~1.3°; preferably, the twist angle of the magic-angle graphene layer is 1.05°~1.15°.

[0050] S2: Precision machining of arrayed holes is performed on the end face of a cylindrical copper-tin alloy ingot. The aforementioned magic-angle graphene-modified core wires or sub-components are inserted into the holes, ensuring that all niobium wires are parallel and aligned along their length, thus obtaining a CuNbSn matrix. The CuNbSn matrix is ​​then placed into a stable matrix and subsequently subjected to hot isostatic pressing to obtain a composite ingot.

[0051] Furthermore, the diameter of the arrayed pores is slightly larger than the diameter of the core filaments modified with magic-angle graphene. The stabilizing substrate material is selected from oxygen-free copper or high-purity aluminum, wherein the purity of the high-purity aluminum is ≥99.999%.

[0052] Furthermore, the hot isostatic pressing process is carried out under argon protection at a temperature of 600~650℃, a pressure of 150~200MPa, and a holding time of 2~3h.

[0053] S3: The composite billet is heated to 300~400℃ and extruded into a bar in one pass using an extruder. Then, a second multi-pass drawing is performed at room temperature to obtain the composite wire. A high-frequency alternating current is applied to the wire at the entrance of the drawing die.

[0054] Furthermore, the total reduction rate of the extrusion section is 95%~96%. The single-pass drawing rate of the second multi-pass drawing is 20%.

[0055] Furthermore, the frequency of the high-frequency alternating current is 10kHz~1MHz, and the current density is 400~450A / mm. 2 Applying a high-frequency alternating current to the wire at the entrance of the drawing die can instantly generate a temperature rise of about 350°C on the surface and internal interface of the wire. This temperature rise can effectively reduce the instantaneous rheological stress of the bronze matrix and activate the super-slippery properties between the magic angle graphene layers, thereby achieving high processing rate drawing.

[0056] S4: The composite wire is wound into a small solenoid coil with an inner diameter of 50mm, and the coil is placed in a high vacuum heat treatment furnace for final diffusion annealing.

[0057] Furthermore, the coil is placed in a high-vacuum heat treatment furnace for two-stage heat treatment, with the temperature increasing at 5°C / min. The first stage is held at 400~600°C for 50~200h, and then the temperature is increased to 600~800°C and held for 50~100h for the second stage heat treatment.

[0058] Example 1

[0059] This embodiment provides a method for preparing niobium-tin superconducting wire modified with magic-angle graphene, specifically including the following steps:

[0060] S1: Niobium wires with a diameter of 2 mm and an RRR (Residual Resistivity Ratio) > 250 were electropolished to remove the surface oxide layer and obtain an atomically clean surface. They were then ultrasonically cleaned sequentially with acetone and ethanol and dried. A magic-angle graphene layer was grown on the cleaned and dried niobium wire surface via hot-wire chemical vapor deposition (HFCVD). The niobium wire was placed in an HFCVD reaction chamber, and HFCVD was performed at a vacuum of 1×10⁻⁶. -3 The process was carried out under the conditions of Pa, hydrogen flow rate of 40 sccm, and gas pressure of 50 Pa. The temperature of the niobium wire substrate was raised to 1000℃, and a carbon source gas (methane) with a flow rate of 35 sccm was introduced for 30 min to grow graphene. The methane flow was then turned off. The substrate was then cooled to 800℃ in a hydrogen atmosphere at a cooling rate of 5℃ / s. During the cooling process, thermal stress and airflow disturbance induced the upper graphene to rotate, spontaneously forming a magic-angle stacked structure with an average twist angle of (1.1±0.1)°. The substrate was then allowed to cool naturally to room temperature to obtain niobium wire modified with magic-angle graphene. The thickness of the magic-angle graphene was 0.2 mm. Figure 1 ).

[0061] S2: 481 holes (arranged in an array) with a diameter of 2.3 mm were precisely machined on the end face of a cylindrical copper-tin alloy ingot with a tin content of 14.5 wt.% and a diameter of 300 mm. The niobium wires modified with magic-angle graphene were inserted into these holes, ensuring that all niobium wires were parallel and aligned along their lengths, thus obtaining a CuNbSn matrix. The CuNbSn matrix was then placed into an oxygen-free copper sleeve with an outer diameter of Φ350 mm and an inner diameter of Φ302 mm, and subsequently subjected to hot isostatic pressing at 600 °C and 200 MPa for 2 hours under argon protection to obtain a composite ingot.

[0062] S3: The composite billet is heated to 300°C and extruded into a Φ70mm bar in one pass using an extruder. Subsequently, it undergoes multi-pass drawing at room temperature, with a single-pass drawing yield of 20%, to obtain a composite wire with a target size of Φ1.0mm. At the entrance of the drawing die, a current density of 400A / mm is applied to the wire at a frequency of 300kHz. 2 High-frequency alternating current.

[0063] S4: The composite wire is wound into a small solenoid coil for testing with an inner diameter of 50mm. The coil is then placed in a high-vacuum heat treatment furnace for final diffusion annealing: the temperature is increased at 5℃ / h, and the first stage is held at 400℃ for 200h, followed by raising the temperature to 800℃ and holding for 50h for the second stage of heat treatment. Figure 2 ).

[0064] Performance testing was conducted on the reacted wire (at liquid helium temperature of 4.2K):

[0065] Microstructure: SEM analysis showed that the Nb3Sn reaction layer had a uniform thickness (approximately 2.5 μm) and fine grains with an average size of approximately 80 nm. Energy dispersive spectroscopy (EDS) surface scanning confirmed uniform Sn element distribution, and no significant infiltration into the copper stabilizing layer was observed.

[0066] Superconducting properties: Critical current density (Jc in the non-copper region) reaches 1200 A / mm² under a 12T background magnetic field. 2 .

[0067] Strain performance: Axial tensile tests were performed on the short sample, and the irreversible strain limit of the critical current (Ic) reached 0.7%, and the Ic retention rate was higher than 80% at 0.5% strain.

[0068] Example 2

[0069] This embodiment provides a method for preparing niobium-tin superconducting wire modified with magic-angle graphene, specifically including the following steps:

[0070] S1: A 3mm diameter niobium-titanium alloy (Nb-1wt.%Ti) wire was electropolished to remove the surface oxide layer, resulting in an atomically clean surface. It was then ultrasonically cleaned with acetone and ethanol sequentially and dried. A magic-angle graphene layer was grown on the cleaned and dried niobium wire surface via plasma-enhanced chemical vapor deposition (PECVD). The niobium-titanium alloy wire was placed in a PE-CVD reaction chamber and surface activated at 650℃ with 50 sccm of argon and 10 sccm of hydrogen for 10 minutes. A 13.56MHz RF plasma power supply was turned on and set to 300W. After the plasma stabilized, methane was introduced at a flow rate of 50 sccm. Graphene was grown for 30 minutes under the same temperature and gas conditions. By controlling the plasma power to 300W to induce stress, magic-angle graphene with a twist angle of (1.15±0.05)° was formed, resulting in a 3nm thick niobium-titanium alloy wire modified with magic-angle graphene.

[0071] S2: 57 holes with a diameter of 3.5 mm are precisely machined on the end face of a cylindrical copper-tin alloy ingot with a tin content of 15.8 wt.% and a diameter of 150 mm. The niobium-titanium alloy wire modified with magic angle graphene is inserted into the holes to ensure that all niobium wires are parallel and aligned in the length direction to obtain a single-assembly wire. The single-assembly wire is then subjected to hot isostatic pressing at 500°C and 130 MPa for 2 hours under argon protection to obtain a single-assembly billet.

[0072] S3: The single-assembly billet is heated to 320°C and extruded into a Φ20mm bar using an extruder in one pass. Subsequently, it undergoes multi-pass drawing at room temperature, with a single-pass drawing rate of 20%, to obtain a single-assembly composite wire with a target size of Φ2.0mm. At the entrance of the drawing die, a current density of 350A / mm is applied to the wire at a frequency of 250kHz. 2 High-frequency alternating current.

[0073] S4: After cutting the single-assembly composite wire to length, ultrasonically clean and dry the surface with ethanol. Repeat the plasma-enhanced chemical vapor deposition step of S1 to form a magic-angle graphene layer with a twist angle of (1.15±0.05)° on the surface of the single-assembly composite wire, thus preparing the sub-component ( Figure 3 ).

[0074] S5: 241 holes (arranged in an array) with a diameter of 2.4 mm are precisely machined on the end face of a cylindrical copper-tin alloy ingot with a tin content of 15.8 wt.% and a diameter of Φ150 mm. The sub-component is inserted into the holes, ensuring that all niobium wires are parallel and aligned in their length direction to obtain a CuNbSn matrix. The CuNbSn matrix is ​​then placed into a high-purity aluminum sleeve with an outer diameter of Φ180 mm and an inner diameter of Φ152 mm, and subsequently subjected to hot isostatic pressing at 650 °C and 200 MPa for 3 hours under argon protection to obtain a composite ingot.

[0075] S3: The composite billet is heated to 400°C and extruded into a Φ40mm bar in one pass using an extruder. Subsequently, it undergoes multi-pass drawing at room temperature, with a single-pass drawing yield of 20%, to obtain a composite wire with a target size of Φ1.2mm. At the entrance of the drawing die, a current density of 450A / mm² at a frequency of 350kHz is applied to the wire. 2 High-frequency alternating current.

[0076] S4: The composite wire is wound into a small test solenoid coil with an inner diameter of 50 mm. The coil is placed in a high-vacuum heat treatment furnace for two-stage heat treatment, with the heating rate controlled at 5℃ / h. The first stage is held at 400℃ for 200 h, followed by raising the temperature to 600℃ and holding for another 100 h for the second stage heat treatment. Then, the furnace is cooled to obtain a magic-angle graphene-modified double-assembled niobium-tritin superconducting wire. Figure 4 ).

[0077] Samples of the reacted wire were taken for performance testing (at liquid helium temperature of 4.2K):

[0078] Microstructure: TEM analysis showed that the Nb3Sn reaction layer thickness was 1.8 μm, and thanks to gradient heat treatment, the average grain size was effectively controlled at 70 nm.

[0079] Extreme field current carrying capacity: Under an ultra-high magnetic field of 18T, the critical current density (Jc) in the non-copper region was measured to reach 1500 A / mm². 2 .

[0080] Strain performance: Axial tensile tests were performed on the short sample, and the irreversible strain limit of the critical current (Ic) reached 0.8%, and the Ic retention rate was higher than 90% at 0.5% strain.

[0081] Example 3

[0082] This embodiment provides a method for preparing niobium-tin superconducting wire modified with magic-angle graphene, specifically including the following steps:

[0083] S1: A 1.5 mm diameter niobium wire was electropolished to remove the surface oxide layer, resulting in an atomically clean surface. It was then ultrasonically cleaned sequentially with acetone and ethanol, and dried. A magic-angle graphene layer was grown on the cleaned and dried niobium wire surface via hot-wire chemical vapor deposition (HFCVD). The niobium wire was placed in an HFCVD reaction chamber, and hot-wire HFCVD was performed at a vacuum of 1×10⁻⁶. -3 The process was carried out under the conditions of Pa, hydrogen flow rate of 150 sccm, and gas pressure of 1000 Pa. The temperature of the niobium wire substrate was raised to 800℃, and a carbon source gas (methane) with a flow rate of 100 sccm was introduced for 40 min to grow graphene. The methane was then turned off. The niobium wire was then cooled to 800℃ at a cooling rate of 5℃ / s in a hydrogen atmosphere. During the cooling process, thermal stress and airflow disturbance induced the upper graphene to rotate, spontaneously forming a magic-angle stacked structure with an average twist angle of (0.8±0.1)°. The niobium wire was then naturally cooled to room temperature to obtain a 3 nm thick magic-angle graphene-modified niobium wire.

[0084] S2: 257 holes (arranged in an array) with a diameter of 2.0 mm are precisely machined on the end face of a cylindrical copper-tin alloy ingot with a tin content of 10.5 wt.% and a diameter of 300 mm. The niobium wires modified with magic-angle graphene are inserted into these holes, ensuring that all niobium wires are parallel and aligned along their length, thus obtaining a CuNbSn matrix. The CuNbSn matrix is ​​then placed into an oxygen-free copper sleeve with an outer diameter of Φ340 mm and an inner diameter of Φ302 mm, and subsequently subjected to hot isostatic pressing at 650 °C and 150 MPa for 3 hours under argon protection to obtain a composite ingot.

[0085] S3: The composite billet is heated to 350°C and extruded into a Φ42mm bar in one pass using an extruder. Subsequently, it undergoes multi-pass drawing at room temperature, with a single-pass drawing yield of 20%, to obtain a composite wire with a target size of Φ0.9mm. At the entrance of the drawing die, a current density of 450A / mm² at a frequency of 10kHz is applied to the wire. 2 High-frequency alternating current.

[0086] S4: The composite wire is wound into a small test solenoid coil with an inner diameter of 50 mm. The coil is placed in a high vacuum heat treatment furnace for final diffusion annealing: the temperature is increased at 5℃ / h, the first stage is held at 500℃ for 100h, and then the temperature is increased to 700℃ and held for 75h for the second stage of heat treatment.

[0087] Performance testing was conducted on the reacted wire (at liquid helium temperature of 4.2K):

[0088] Microstructure: SEM analysis showed that the Nb3Sn reaction layer had a uniform thickness (approximately 3 μm) and fine grains with an average size of approximately 85 nm. Energy dispersive spectroscopy (EDS) surface scanning confirmed uniform Sn element distribution, and no significant infiltration into the copper stabilizing layer was observed.

[0089] Superconducting properties: Critical current density (Jc in the non-copper region) reaches 1100 A / mm² under a 12T background magnetic field. 2 .

[0090] Strain performance: Axial tensile tests were performed on the short sample, and the irreversible strain limit of the critical current (Ic) reached 0.6%, and the Ic retention rate was higher than 75% at 0.5% strain.

[0091] Example 4

[0092] This embodiment provides a method for preparing niobium-tin superconducting wire modified with magic-angle graphene, specifically including the following steps:

[0093] S1: A 2.5 mm diameter niobium-titanium alloy (Nb-50 wt.% Ti) wire was electropolished to remove the surface oxide layer, resulting in an atomically clean surface. It was then ultrasonically cleaned with acetone and ethanol sequentially and dried. A magic-angle graphene layer was grown on the cleaned and dried niobium wire surface via plasma-enhanced chemical vapor deposition (PECVD). The niobium-titanium alloy wire was placed in a PE-CVD reaction chamber and surface activated for 10 min at 400 °C, 200 sccm argon, and 50 sccm hydrogen. A 13.56 MHz radio frequency plasma power supply was turned on and set to 50 W. After the plasma stabilized, methane was introduced at a flow rate of 10 sccm, and graphene was grown at the same temperature for 5 min. By controlling the plasma power to 50 W to induce stress, magic-angle graphene with a twist angle of (1.3 ± 0.05) ° was formed, resulting in a 0.3 nm thick niobium-titanium alloy wire modified with magic-angle graphene.

[0094] S2: 360 holes (arranged in an array) with a diameter of 2.7 mm were precisely machined on the end face of a cylindrical copper-tin alloy ingot with a tin content of 13.6 wt.% and a diameter of 400 mm. The niobium wires modified with magic-angle graphene were inserted into these holes, ensuring that all niobium wires were parallel and aligned along their lengths, thus obtaining a CuNbSn matrix. The CuNbSn matrix was then placed into an oxygen-free copper sleeve with an outer diameter of Φ430 mm and an inner diameter of Φ402 mm, and subsequently subjected to hot isostatic pressing at 650 °C and 200 MPa for 3 hours under argon protection to obtain a composite ingot.

[0095] S3: The composite billet is heated to 300°C and extruded into a Φ50mm bar in one pass using an extruder. Subsequently, it undergoes multi-pass drawing at room temperature, with a single-pass drawing yield of 20%, to obtain a composite wire with a target size of Φ1.1mm. At the entrance of the drawing die, a current density of 450A / mm is applied to the wire at a frequency of 1MHz. 2 High-frequency alternating current.

[0096] S4: The composite wire is wound into a small test solenoid coil with an inner diameter of 50mm. The coil is placed in a high vacuum heat treatment furnace for final diffusion annealing: the temperature is increased at 5℃ / h, the first stage is held at 600℃ for 50h, and then the temperature is increased to 800℃ and held for 50h for the second stage of heat treatment.

[0097] Performance testing was conducted on the reacted wire (at liquid helium temperature of 4.2K):

[0098] Microstructure: SEM analysis showed that the Nb3Sn reaction layer had a uniform thickness (approximately 3.2 μm) and fine grains with an average size of approximately 75 nm. Energy dispersive spectroscopy (EDS) surface scanning confirmed the uniform distribution of Sn elements, and no significant infiltration into the copper stabilizing layer was observed.

[0099] Superconducting properties: Critical current density (Jc in the non-copper region) reaches 1150 A / mm² under a 12T background magnetic field. 2 .

[0100] Strain performance: Axial tensile tests were performed on the short sample, and the irreversible strain limit of the critical current (Ic) reached 0.78%, and the Ic retention rate was higher than 85% at 0.5% strain.

[0101] Comparative Example 1

[0102] This comparative example of Nb3Sn is based on the traditional bronze method, using niobium wire and a bronze matrix of the same specifications as in Example 1. During the multi-pass drawing process, after every 3-4 passes (with a reduction in area of ​​approximately 50%), intermediate annealing at 400-700°C for 1-3 hours is required to eliminate work hardening. Finally, after vacuum heat treatment with the same parameters as in Example 1, Nb3Sn superconducting wire is obtained. Performance testing (at 4.2K liquid helium temperature) showed that the average grain size of this comparative example Nb3Sn superconducting wire is 150 nm, and the Jc at 12T is 1000 A / mm². 2 The irreversible strain limit is only 0.5%.

[0103] As described above, the basic principles, main features, and advantages of the present invention have been well described. The above embodiments and specifications are merely descriptions of preferred embodiments of the present invention, and the present invention is not limited to the above embodiments. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit and scope of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A method for preparing a magic-angle graphene-modified niobium-tin superconducting wire, characterized in that, include: S1: The core wire is prepared as a magic-angle graphene-modified core wire by chemical vapor deposition; the magic-angle graphene-modified core wire is assembled with a drilled copper-tin alloy ingot, and then subjected to a first hot isostatic pressing treatment to obtain a single-assembly billet; the single-assembly billet is first extruded and then drawn in a first multi-pass process to obtain a rod; the surface of the rod is modified with magic-angle graphene by chemical vapor deposition to obtain sub-components; S2: Assemble the core wire modified with the subcomponent or magic angle graphene with the drilled copper-tin alloy matrix to obtain CuNbSn matrix. Insert the CuNbSn matrix into a stable matrix and obtain a composite billet after a second hot isostatic pressing treatment. S3: The composite billet is extruded a second time and then drawn a second multi-pass to obtain a wire. S4: The wire is vacuum heat-treated to obtain the niobium tritin superconducting wire modified with magic angle graphene; The single-pass drawing rate of the first multi-pass drawing is 20%; In the first multi-pass drawing process, a high-frequency alternating current is applied to the wire at the entrance of the drawing die; The high-frequency alternating current has a frequency of 10kHz to 1MHz and a current density of 400 to 450A / mm². 2 ; The magic-angle graphene-modified niobium-tin superconducting wire consists of a stable matrix, a copper-tin alloy matrix, and a core rod, from the outermost layer to the innermost layer. The core rod is a core filament modified with subcomponents or magic-angle graphene; The sub-components, from the outermost layer to the innermost layer, are a magic-angle graphene layer, a copper-tin alloy substrate, a magic-angle graphene layer, and a core filament. The number of stacked magic-angle graphene layers in the magic-angle graphene layer is 10; The twist angle of the magic-angle graphene layer is 0.8°~1.3°; The core wire is a niobium wire or a niobium-titanium alloy wire; The titanium content in the niobium-titanium alloy wire is 1~50 wt.%. The material of the stabilizing matrix is ​​selected from oxygen-free copper or high-purity aluminum, and the purity of the aluminum is ≥99.999%.

2. The method for preparing the magic-angle graphene-modified niobium-tin superconducting wire according to claim 1, characterized in that, The chemical vapor deposition is either hot-filament chemical vapor deposition or plasma-enhanced chemical vapor deposition; The hot filament chemical vapor deposition was performed under a vacuum of 1×10⁻⁶. -3 The process is carried out under the conditions of Pa, hydrogen flow rate of 40~150 sccm, and gas pressure of 50~1000 Pa. Then, the substrate temperature of the core wire or rod is raised to 800~1000℃, methane is introduced at a flow rate of 35~100 sccm, and after 20~40 min, the methane is turned off. The temperature is then cooled to 800℃ at a cooling rate of 5℃ / s, and after natural cooling to room temperature, the magic angle graphene modified core wire or sub-component is obtained.

3. The method for preparing the magic-angle graphene-modified niobium-tin superconducting wire according to claim 2, characterized in that, The plasma-enhanced chemical vapor deposition first activates the core wire or rod by treating it at 400-650°C with argon gas at a flow rate of 50-200 sccm and hydrogen gas at a flow rate of 5-50 sccm for 10 min. Then, a 13.56MHz radio frequency plasma power supply is turned on and the power is set to 50-300W. After the plasma stabilizes, 10-50 sccm of methane is introduced to grow graphene for 5-30 min, thus obtaining the magic-angle graphene-modified core wire or subunit.

4. The method for preparing the magic-angle graphene-modified niobium-tritin superconducting wire according to claim 1, characterized in that, The first hot isostatic pressing treatment is carried out under argon protection at a temperature of 500~650℃, a pressure of 100~200MPa, and a holding time of 2~3h. The first extrusion is a one-time extrusion, the extrusion temperature is 300~400℃, and the total cross-sectional reduction rate of the extrusion is 90~99%.

5. The method for preparing the magic-angle graphene-modified niobium-tin superconducting wire according to claim 1, characterized in that, The diameter of the hole in the copper-tin alloy ingot being drilled is 0.2~2mm larger than the diameter of the core wire modified with micro-components or magic angle graphene. The tin content in both the copper-tin alloy ingot and the copper-tin alloy matrix used for drilling is 10-16 wt.

6. The method for preparing the magic-angle graphene-modified niobium-tin superconducting wire according to claim 1, characterized in that, The second hot isostatic pressing treatment is carried out under argon protection at a temperature of 600~650℃, a pressure of 150~200MPa, and a holding time of 2~3h. The second extrusion is a one-time extrusion, with an extrusion temperature of 300~400℃ and a total cross-sectional reduction rate of 95%~96%.

7. The method for preparing the magic-angle graphene-modified niobium-tin superconducting wire according to claim 1, characterized in that, The single-pass drawing rate of the second multi-pass drawing is 20%; In the second multi-pass drawing process, a high-frequency alternating current is applied to the wire at the entrance of the drawing die; The high-frequency alternating current has a frequency of 10kHz to 1MHz and a current density of 400 to 450A / mm². 2 .

8. The method for preparing the magic-angle graphene-modified niobium-tritin superconducting wire according to claim 1, characterized in that, The vacuum heat treatment involves a temperature increase of 5℃ / min and is a two-stage heat treatment. The first stage involves holding the temperature at 400~600℃ for 50~200h, followed by raising the temperature to 600~800℃ and holding it for 50~100h for the second stage heat treatment.

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