A superconducting enameled wire and its preparation method

Abstract

This invention belongs to the field of superconducting wire technology and relates to an enameled superconducting round wire and its preparation method. The invention provides a method for preparing an enameled superconducting round wire, comprising: coating a bare NbTi superconducting wire with 20-25 passes of modified acetal varnish, followed by 5-10 passes of acetal varnish coating; wherein the modified acetal varnish is a mixture of acetal varnish, citrate, and β-cyclodextrin in a mass ratio of 100:0.2-1.3:1-3. The modified acetal varnish provided by this invention, by simultaneously adding citrate and β-cyclodextrin to the acetal varnish, can effectively improve the thermal conductivity and low-temperature resistance of the enameled superconducting round wire.

Description

Technical Field

[0001] This invention belongs to the field of superconducting wire technology and relates to an enameled superconducting round wire and its preparation method. Background Technology

[0002] Superconducting enameled wire is a core material in fields such as superconducting magnets, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), and high-energy physics detectors. Its performance directly affects the stability, efficiency, and lifespan of the entire superconducting device. Especially with the rapid development of low-liquid-helium or liquid-helium-free superconducting magnet systems, more stringent requirements have been placed on the mechanical properties, insulation reliability, thermal conductivity, and thermal cycling resistance of enameled superconducting round wires at low temperatures. Currently, most common enameled superconducting round wires use acetal resins (such as polyvinyl formal and polyvinyl acetal) as the insulating coating. While this type of coating possesses good electrical insulation properties and a certain degree of adhesion, it still faces the following prominent problems in practical applications: First, conventional acetal coatings have poor thermal conductivity, affecting the thermal stability of superconducting wires at low temperatures, especially prone to superconductor failure under transient thermal shock or localized overheating. Second, the coating is prone to embrittlement at low temperatures, and after repeated thermal cycles or mechanical stress, it is susceptible to microcracks, wrinkles, or even powdering, leading to insulation failure. Third, to improve the mechanical properties or functionality of the coating, nanofillers are often added to the paint solution, but these fillers tend to agglomerate and disperse unevenly, resulting in increased internal defects, higher pinhole rate, and decreased insulation strength. Fourth, current processes often employ post-stretching to improve yield strength, but the coating is easily damaged during stretching, requiring secondary coating for repair, which is complex and inefficient. To address these issues, some research has attempted improvements through paint modification or process optimization. For example, some technologies use alternating coatings of different paint types to enhance coating adhesion, or increase the solid content of the paint solution to enhance film density. However, these methods often sacrifice other properties while improving one. For example, increasing thermal conductivity may reduce the flexibility of the coating film, while increasing low-temperature resistance may affect the insulation strength. A good balance in overall performance has not yet been achieved. Therefore, how to simultaneously improve the thermal conductivity, low-temperature toughness, and interfacial adhesion of the coating film while ensuring its insulation performance has become a critical technical bottleneck that urgently needs to be overcome in the preparation of high-reliability enameled superconducting round wires. Summary of the Invention

[0003] The purpose of this invention is to provide an enameled superconducting round wire, which is made by coating with modified acetal varnish, and can effectively improve the thermal conductivity and low-temperature resistance of the enameled superconducting round wire.

[0004] To address this need, the present invention provides an enameled superconducting round wire and its preparation method.

[0005] On one hand, the present invention relates to a method for preparing an enameled superconducting round wire, which includes: coating a bare NbTi superconducting wire with 20 to 25 layers of modified acetal varnish and then coating it with 5 to 10 layers of acetal varnish.

[0006] The modified acetal paint is a mixture of acetal paint, citrate and β-cyclodextrin in a mass ratio of 100:0.2~1.3:1~3.

[0007] Furthermore, in the preparation method of enameled superconducting round wire provided by the present invention, the thickness of each pass of the modified acetal varnish is 1~5μm, and the thickness of each pass of the acetal varnish is 10~15μm.

[0008] Furthermore, in the method for preparing enameled superconducting round wire provided by the present invention, the acetal varnish is baked and sized after coating.

[0009] Furthermore, in the method for preparing enameled superconducting round wires provided by the present invention, the baking temperature is 260~300℃ and the baking time is 90~120s.

[0010] Furthermore, in the preparation method of the enameled superconducting round wire provided by the present invention, after the modified acetal varnish is applied, it is first baked at 260~300℃ for 10~30s, then naturally cooled to 80~100℃ and held for 10~30s, and then the acetal varnish is applied again, and finally baked at 260~300℃ for 60~90s.

[0011] Furthermore, in the method for preparing enameled superconducting round wire provided by the present invention, the sizing is performed using a high-precision mold, wherein the average particle size of the high-precision mold is not higher than 5 μm and the roughness is 0.1 μm to 0.15 μm.

[0012] Furthermore, in the preparation method of the enameled superconducting round wire provided by the present invention, the solid content of any of the acetal varnishes is 20~23wt%.

[0013] Furthermore, in the method for preparing enameled superconducting round wires provided by the present invention, the citrate is at least one of potassium citrate, sodium citrate, or ammonium citrate.

[0014] Furthermore, in the method for preparing enameled superconducting round wire provided by the present invention, the diameter of the NbTi superconducting bare wire is 0.10~2.00 mm.

[0015] On the other hand, the present invention relates to an enameled superconducting round wire, which is prepared by the aforementioned method for preparing enameled superconducting round wire.

[0016] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages:

[0017] In existing technologies, improving the thermal conductivity of paint films often involves adding high thermal conductivity fillers, but this easily leads to agglomeration, increases internal stress in the paint film, and impairs its flexibility and adhesion. Using organic toughening agents alone, on the other hand, reduces thermal conductivity and insulation performance. This invention employs a strategy of composite modification with citrate and β-cyclodextrin. The cavity structure of β-cyclodextrin effectively encapsulates citrate ions, allowing them to disperse uniformly in the paint solution and be gradually released during curing. The interaction between the cationic ions in citrate and the β-cyclodextrin and acetal resin chains constructs a more stable cross-linked network within the paint film, enhancing the film's cohesion and interfacial bonding with the superconducting bare wire. This synergistic effect allows the paint film to achieve excellent thermal conductivity while maintaining flexibility and mechanical strength at high and low temperatures, resolving the performance trade-offs inherent in traditional solutions. Conventional acetal paint films, under liquid nitrogen temperatures and repeated thermal cycling, are prone to microcracks that propagate and lead to powdering failure due to resin chain freezing and uneven volume shrinkage. In this invention, the rigid hydrophobic cavity and hydrophilic outer wall structure of β-cyclodextrin effectively absorbs and disperses localized stress caused by rapid temperature changes, inhibiting the initiation and propagation of microcracks. Simultaneously, the uniformly dispersed citrate helps improve the crystallization behavior of the coating film and reduces its glass transition region's temperature sensitivity. High filler content or uneven dispersion often introduces pinhole defects, impairing insulation. This invention, through the pre-assembly and controlled release of the modified components by β-cyclodextrin, and an optimized layered coating process, ensures extremely high density and uniformity of the coating film. The modified coating layer first forms a defect-free, dense underlayer, filling the microscopic unevenness of the bare wire surface; subsequent conventional coating layers are further thickened and leveled. This structure minimizes inherent defects such as bubbles and pinholes. Detailed Implementation

[0018] 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 and detection methods described in each embodiment are conventional methods; the reagents and materials described are commercially available unless otherwise specified. Unless otherwise specified, all percentages in the following embodiments refer to mass percentages. Unless otherwise specified, all proportions in the following embodiments refer to mass ratios.

[0019] Example 1

[0020] This embodiment provides a process for preparing enameled superconducting round wire.

[0021] NbTi superconducting bare wire with a diameter of Φ0.60mm was selected, cleaned by a combination of ultrasonic cleaning and brush cleaning, and then dried at 120℃ for later use. Modified acetal paint was prepared by mixing polyethylene acetal paint with a solid content of 20wt%, sodium citrate, and β-cyclodextrin at a mass ratio of 100:0.5:1.5, stirring evenly, and then ultrasonically degassing for 30min. The acetal paint used was polyethylene acetal paint with a solid content of 20wt%.

[0022] The treated NbTi superconducting bare wire was coated with 20 passes of modified acetal varnish using a high-pressure annular nozzle. Each pass was varnished to a thickness of 3 μm, and each pass was allowed to level naturally for 5 seconds. After all the modified varnish was applied, the wire was baked at 270℃ for 20 seconds, then allowed to cool naturally to 90℃ and held at that temperature for 20 seconds. After cooling, 10 passes of acetal varnish were applied, each pass 12 μm thick, with a spacing of 1.5 cm between passes. After the acetal varnish was applied, the wire was baked at 270℃ for 80 seconds. Sizing was performed using a high-precision artificial diamond die with an average particle size of 3 μm and a surface roughness of 0.12 μm. The dimensional accuracy of the sized wire was controlled within ±0.005 mm. The wire tested showed no alarm under a 1000V pinhole detector, had a thermal conductivity of 0.85 W / (m·K), and showed no wrinkling or powdering after being placed in liquid nitrogen for 7 days and then returned to room temperature.

[0023] Example 2

[0024] This embodiment provides a process for preparing enameled superconducting round wire.

[0025] NbTi superconducting bare wire with a diameter of Φ1.20mm was selected, and after surface polishing and ultrasonic cleaning, it was annealed (380℃, 30min). Modified acetal paint was prepared by mixing polyethylene acetal paint with a solid content of 20wt%, potassium citrate, and β-cyclodextrin at a mass ratio of 100:0.9:2.2, stirring, and filtering to remove impurities with a particle size >1μm. Polyethylene acetal paint with a solid content of 20wt% was selected as the acetal paint.

[0026] The annealed bare wire was coated with 22 coats of modified acetal varnish, each coat being 4 μm thick, using a high-pressure annular nozzle (outlet temperature 42℃). After the modified varnish was applied, it was baked at 285℃ for 30 seconds and then naturally cooled to 80℃ for 15 seconds. Subsequent 8 coats of acetal varnish were applied, each coat being 14 μm thick, with the uniformity of the varnish film monitored in real time during the coating process.

[0027] After the acetal paint was applied, it was baked at 285℃ for 70 seconds. A high-precision diamond die with an average particle size of 4μm and a roughness of 0.10μm was used for sizing. The wire size after sizing was Φ1.32mm±0.005mm. Tests showed no alarm under 1000V pinhole detection, and the thermal conductivity was 0.82W / (m·k). After 100 batches of samples were placed in liquid nitrogen for 7 days and then returned to room temperature, no wrinkles or powdering were observed.

[0028] Example 3

[0029] This embodiment provides a process for preparing enameled superconducting round wire.

[0030] NbTi superconducting bare wire with a diameter of Φ1.80mm was selected, ultrasonically cleaned, and then dried with a high-speed air knife for later use. Modified acetal paint was prepared by mixing polyimide acetal paint, ammonium citrate, and β-cyclodextrin with a solid content of 20wt% in a mass ratio of 100:1.3:3.0, stirring, and then letting it stand for 2 hours to remove air bubbles. The acetal paint used was polyimide acetal paint with a solid content of 20wt%.

[0031] The bare wire was coated with 25 coats of modified acetal varnish, with a single coat thickness of 5 μm. The outlet temperature of the high-pressure annular nozzle was controlled at 45℃. After the modified varnish was applied, it was baked at 295℃ for 10 seconds and then naturally cooled to 100℃ and held for 30 seconds. Subsequently, 5 coats of acetal varnish were applied, with a single coat thickness of 15 μm, and the coating was allowed to level naturally for 8 seconds.

[0032] After the acetal paint was applied, the wire was baked at 295℃ for 90 seconds. The sizing die had an average particle size of 5μm and a surface roughness of 0.15μm, resulting in a dimensional accuracy of ±0.005mm for the sized wire. The wire passed a 1000V pinhole test without alarming, had a thermal conductivity of 0.88W / (m·k), and after 100 batches of samples were placed in liquid nitrogen for 7 days and then returned to room temperature, no wrinkles or powdering were observed.

[0033] Example 4

[0034] This embodiment provides a process for preparing enameled superconducting round wire.

[0035] This embodiment is the same as Embodiment 1, except that after all the modified paints were applied, the sample was baked at 260°C for 10 seconds, then naturally cooled to 80°C and held for 10 seconds. After the acetal paint was applied, it was baked at 260°C for 80 seconds. Tests showed no alarm under a 1500V pinhole detector, and the thermal conductivity was 0.95 W / (m·K). After 100 batches of samples were placed in liquid nitrogen for 14 days and then returned to room temperature, no wrinkles or powdering occurred.

[0036] Example 5

[0037] This embodiment provides a process for preparing enameled superconducting round wire.

[0038] This embodiment is the same as Embodiment 2, except that after all the modified paints were applied, they were baked at 280℃ for 20 seconds and then naturally cooled to 90℃ and held for 20 seconds. After the acetal paint was applied, it was baked at 280℃ for 70 seconds. Tests showed no alarm under a 1500V pinhole detector, and the thermal conductivity was 0.91 W / (m·K). After 100 batches of samples were placed in liquid nitrogen for 14 days and then returned to room temperature, no wrinkles or powdering occurred.

[0039] Example 6

[0040] This embodiment provides a process for preparing enameled superconducting round wire.

[0041] This embodiment is the same as Embodiment 3, except that after all the modified paints were applied, they were baked at 300℃ for 30 seconds and then naturally cooled to 100℃ and held for 30 seconds. After the acetal paint was applied, it was baked at 300℃ for 90 seconds. Tests showed no alarm under a 1500V pinhole detector, and the thermal conductivity was 0.94 W / (m·K). After 100 batches of samples were placed in liquid nitrogen for 14 days and then returned to room temperature, no wrinkles or powdering occurred.

[0042] Comparative Example 1

[0043] This comparative example is the same as Example 1, except that citrate was not added to the modified acetal paint. Tests showed no alarm under 500V pinhole detection, and the thermal conductivity was 0.48 W / (m·K). Of 100 batches of samples placed in liquid nitrogen for 7 days and then returned to room temperature, 23 batches showed wrinkles or powdering.

[0044] Comparative Example 2

[0045] This comparative example is the same as Example 1, except that β-cyclodextrin was not added to the modified acetal paint. Tests showed no alarm under 500V pinhole detection, and the thermal conductivity was 0.55 W / (m·K). Of 100 batches of samples placed in liquid nitrogen for 7 days and then returned to room temperature, 12 batches showed wrinkles or powdering.

[0046] The embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate preferred embodiments. All other embodiments obtained by those skilled in the art through related deductions and substitutions based on the inventive concept, without inventive effort, are within the scope of protection of the present invention.

Claims

1. A method for preparing enameled superconducting round wire, characterized in that, include: The NbTi superconducting bare wire is coated with 20 to 25 layers of modified acetal varnish, and then coated with 5 to 10 layers of acetal varnish. After the acetal varnish coating is completed, it is baked and sized. The modified acetal paint is a mixture of acetal paint, citrate and β-cyclodextrin in a mass ratio of 100:0.2~1.3:1~3; After the modified acetal paint is applied, it is first baked at 260~300℃ for 10~30s, then naturally cooled to 80~100℃ and held for 10~30s before applying the acetal paint again, and finally baked at 260~300℃ for 60~90s. The solid content of any of the acetal paints is 20-23 wt%; The citrate is at least one of potassium citrate, sodium citrate, or ammonium citrate.

2. The method for preparing enameled superconducting round wire according to claim 1, characterized in that, The thickness of each coat of the modified acetal paint is 1~5μm, and the thickness of each coat of the acetal paint is 10~15μm.

3. The method for preparing enameled superconducting round wire according to claim 1, characterized in that, The sizing process uses a high-precision mold, the average particle size of which is no higher than 5μm and the roughness is 0.1μm~0.15μm.

4. The method for preparing enameled superconducting round wire according to claim 1, characterized in that, The diameter of the NbTi superconducting bare wire is 0.10~2.00 mm.

5. An enameled superconducting round wire, characterized in that, It is prepared by the method for preparing enameled superconducting round wire according to any one of claims 1 to 4.