An experimental method for fabricating a zirconium tungstate filled epoxy cured superconducting coil resistant to thermal mismatch

Abstract

The present application belongs to the technical field of superconducting tape testing, and relates to an experimental method for filling epoxy resin cured superconducting coils with zirconium tungstate to resist thermal mismatch. The traditional method has the problem of mismatch between the thermal expansion coefficients of the epoxy resin and the superconducting tape, which leads to the easy delamination damage of the epoxy impregnated superconducting coil and even causes the structural damage of the superconducting coil. The present application aims to prepare a zirconium tungstate / epoxy resin composite impregnated material with a low thermal expansion coefficient, starting from the root cause of the thermal mismatch between the impregnated material and the superconducting material, and proposing a brand new solution to reduce the thermal expansion coefficient of the epoxy resin, thereby reducing the interface cracking failure risk of the impregnated superconducting coil caused by thermal mismatch stress. While reducing the thermal expansion coefficient of the impregnated material, the mechanical properties are also taken into account to ensure that the impregnated coil has high mechanical strength, thereby greatly reducing the failure risk.

Description

Technical Field

[0001] This invention belongs to the field of superconducting tape testing technology, specifically relating to an experimental method for a superconducting coil filled with zirconium tungstate and cured with epoxy resin to resist thermal mismatch. Background Technology

[0002] Due to the significant difference in the coefficients of thermal expansion between traditional epoxy impregnation materials and superconducting tapes, radial tensile stress accumulates during the cooling process of the coil from room temperature to a cryogenic operating environment (such as liquid nitrogen, 77K). Once the radial tensile stress exceeds the interfacial strength of the superconducting tape, delamination failure will occur, leading to cracks in the superconducting layer. This results in a significant and irreversible degradation of the superconducting critical current, and may even cause coil failure.

[0003] To prevent delamination failure of high-temperature superconducting tapes in epoxy-impregnated coils, Yanagisawa et al. developed a polyimide coating manufacturing technique, where delamination failure can be replaced by debonding between polyimide and epoxy resin. Similarly, Mizuno et al. proposed an epoxy-impregnated coil technique where high-temperature superconducting tape is co-wound with polytetrafluoroethylene (PTFE) tape, where the PTFE tape effectively prevents interfacial cracking and mechanical degradation of the coil. Furthermore, Yin et al. proposed a method of coating high-temperature superconducting tapes with a release agent to reduce delamination failure. The basic idea behind these methods is to use other interfacial cracks to replace internal cracks in the superconducting tape. While this reduces the risk of failure due to interfacial cracking, it does not fundamentally reduce thermal mismatch stress. Moreover, cracking at other interfaces reduces interlayer constraint in the coil, decreasing structural stability and causing problems such as interfacial friction and boundary warping, ultimately leading to superconducting coil failure. Summary of the Invention

[0004] To address the problems mentioned in the background art, this invention provides an experimental method for producing a zirconium tungstate-filled epoxy resin-cured superconducting coil resistant to thermal mismatch. The coil is wound using a zirconium tungstate / epoxy resin composite impregnation material with low thermal expansion coefficient and high mechanical strength, thereby achieving an epoxy-impregnated coil that resists thermal mismatch, reduces the risk of structural failure, and is simple to operate and cost-effective.

[0005] Therefore, the present invention adopts the following technical method:

[0006] An experimental method for a zirconium tungstate-filled epoxy resin-cured superconducting coil resistant to thermal mismatch includes the following steps:

[0007] 1) Preparation of superconducting tape: Select the superconducting tape required for the experiment, and weld current leads to both ends of the superconducting tape; arrange voltage solder points at specified intervals along the length of the superconducting tape, and solder silver strips to each voltage solder point.

[0008] 2) Impregnation material preparation: using Stycast@2850FT epoxy resin material, Catalyst 24LV type curing agent, preparing an epoxy resin matrix; doping filler is selected as zirconium tungstate ZrW2O8 powder with a particle size of 1 μm and a purity of more than 99%;

[0009] Mix the epoxy resin and the curing agent in proportion, fully stir them to be uniform, add the zirconium tungstate powder, and fully stir them to be uniform; use a vacuum pump to perform vacuum degassing treatment;

[0010] 3) Coil winding: fix the winding disc of the superconducting tape prepared in step 1) on a wire reel, fix the coil skeleton tooling on a winding machine, and impregnate the coil by using a wet winding method;

[0011] Lead the superconducting tape in the winding disc to the coil skeleton tooling, and glue the superconducting tape; evenly brush the impregnation material prepared in step 2) on the upper and lower surfaces of the superconducting tape, and simultaneously start the winding machine to begin winding the coil, while brushing the impregnation material; pay attention to that the silver tapes should not be stuck together during the winding process, and are spaced apart by the impregnation material.

[0012] After the winding is completed, constant temperature curing is performed, and the required superconducting coil is obtained.

[0013] Further, in step 1), the superconducting tape is provided with voltage welding points at every 10th turn, such as the 1st turn, the 6th turn, the 16th turn, the 26th turn, and so on.

[0014] Further, in step 1), the current lead is a silver tape, and the specification of the silver tape is a thickness of 0.1 mm, a width of 3 mm, and a length of about 10 cm; the specification of the silver tape on each voltage welding point is a thickness of 0.1 mm, a width of 3 mm, and a length of about 3 cm.

[0015] Further, in step 2), the epoxy resin material and the curing agent are mixed in a mass ratio of 100:7-10.

[0016] Further, in step 2, the added mass of the zirconium tungstate powder accounts for 9%-11% of the sum of the mass of the epoxy resin material and the curing agent.

[0017] Further, in step 3), curing is performed in a constant temperature 25℃ drying box for 24 h.

[0018] The present application has the following beneficial effects:

[0019] 1. Starting from the fundamental reason of the thermal mismatch between the impregnation material and the superconducting material, a brand-new scheme of the present application is proposed, the thermal expansion coefficient of the epoxy resin is reduced, and thus the risk of interface cracking failure of the impregnated superconducting coil caused by thermal mismatch stress is reduced;

[0020] 2. Reduce the thermal expansion coefficient of the impregnated material while considering the mechanical properties, ensure that the impregnated coil has high mechanical strength, and greatly reduce the risk of failure;

[0021] 3. The selected doped material, zirconium tungstate, has a wide negative thermal expansion temperature range (2~1443K), covering the application temperature of high-temperature superconducting and low-temperature superconducting;

[0022] 4. By improving the impregnated material, the radial stress of the coil is reduced, more turns of the insulated impregnated coil are wound at a smaller radius, and the application range of the coil is improved;

[0023] 5. Unlike the prior art, which usually introduces a weak interface between the epoxy resin and the superconducting tape, the method directly reduces the thermal expansion coefficient of the impregnated material;

[0024] 6. Unlike the commonly used vacuum pressure impregnation method in the prior art, the improved impregnated material has a larger viscosity, which can be used for wet winding of the coil, and the operation is more simple and convenient, and is more suitable for small coil winding. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is the preparation flowchart of the zirconium tungstate / epoxy resin composite material of the present application;

[0026] Figure 2 is a schematic diagram of the position of the current lead and the voltage lead;

[0027] Figure 3 is a schematic diagram of the coil winding process;

[0028] Figure 4 is a schematic diagram of the impregnated coil structure;

[0029] Figure 5 is the thermal expansion coefficient of four different proportions of zirconium tungstate / epoxy resin composite material;

[0030] Figure 6 is a maximum tensile strength curve diagram of different proportions of zirconium tungstate / epoxy resin composite material at room temperature;

[0031] Figure 7 is a maximum tensile strength curve diagram of different proportions of zirconium tungstate / epoxy resin composite material at 77K temperature;

[0032] Figure 8 is a maximum shear strength curve diagram of different proportions of zirconium tungstate / epoxy resin composite material at room temperature;

[0033] Figure 9 is a maximum shear strength curve diagram of different proportions of zirconium tungstate / epoxy resin composite material at 77K;

[0034] Figure 10 is a structural diagram of a coil former tool. DETAILED DESCRIPTION

[0035] The application will be further described below in conjunction with the accompanying drawings and specific embodiments:

[0036] Superconducting tape preparation:

[0037] (1) Use 95 μm thick, 4 mm wide REBCO tape produced by Shanghai Superconducting Technology Co., Ltd.

[0038] (2) Calculate the amount of wire according to the required number of turns, and wind the tape into a winding reel.

[0039] (3) Use a silver strip with a soldering thickness of 0.1 mm, a width of 3 mm, and a length of about 10 cm as a current lead at the beginning and end of the tape.

[0040] (4) From the innermost turn, place voltage soldering points on the 1st, 6th, 16th, 26th, and so on, every 10 turns, and use a silver strip with a thickness of 0.1 mm, a width of 3 mm, and a length of about 3 cm to lead out, and then solder the two ends of the polished enameled wire to the silver strip.

[0041] Note that the soldering should be as thin as possible to prevent the current and voltage soldering points from protruding and damaging the superconducting tape.

[0042] Note to measure the passage of the two ends of the enameled wire.

[0043] Note to wear gloves during operation to avoid contact between the hands and the tape.

[0044] If the operation environment is humid, the original superconducting tape and the prepared tape winding reel should be stored in a dry cabinet.

[0045] Preparation of impregnated material:

[0046] (1) Use Stycast@2850FT epoxy encapsulating material and Catalyst 24LV type curing agent to prepare the epoxy matrix. The doping filler is zirconium tungstate (ZrW2O8) powder with a particle size of about 1 μm and a purity of more than 99%.

[0047] (2) First, mix Stycast @ 2850FT epoxy resin and Catalyst 24LV curing agent in a mass ratio of 100:8, and stir well.

[0048] (3) Add 10% (±0.5%) by mass of zirconium tungstate powder to the epoxy resin, and stir well.

[0049] (4) Use a vacuum pump to perform vacuum degassing treatment on the well-mixed sample.

[0050] Note: Start winding immediately after the impregnation material is ready.

[0051] Winding procedure:

[0052] (1) The coil pre-stress plan is 10 MPa.

[0053] (2) The coil is impregnated by wet winding method.

[0054] (3) Fix the winding reel with prepared tape to the pay-off reel.

[0055] (4) Fix the coil former tooling to the winding machine.

[0056] (5) Gently pull the prepared tape in the winding reel to the coil former tooling, and fix the first end with Kapton tape.

[0057] (6) Gently straighten the tape in the middle part between the winding reel and the coil former tooling, so that the tension can be applied to the tape when winding starts.

[0058] (7) Use a disposable brush to evenly brush the prepared zirconium tungstate filled epoxy resin impregnation material on the upper and lower surfaces of the tape, and start the winding machine at the same time, with a speed of 4 turns / min.

[0059] (8) After the tape is wound, the end is bonded with Kapton tape, the remaining tape is cut, and a few turns of Kapton tape are gently wound along the outermost turn of the coil without tension.

[0060] Note: The silver tapes should not be stuck together, and can be spaced apart with impregnation material.

[0061] Note: When brushing the impregnation material, it can be slightly more than the expected thickness, and the excess impregnation material will overflow under the action of tension.

[0062] Note: The bottom paper pad prevents epoxy from dripping onto the floor and being difficult to clean.

[0063] (12) After winding is completed, place it in a constant temperature 25°C drying oven for 24h.

[0064] Example 1

[0065] Experimental conditions:

[0066] 4K-285K, Extremely low temperature-electric-magnetic multi-field superconducting material mechanical property testing equipment of Lanzhou University Superconducting Mechanics Laboratory

[0067] Experimental procedure:

[0068] (1) Stycast @2850 FT epoxy resin mixed with Catalyst 24LV curing agent at a mass ratio of 100:8;

[0069] (2) According to the designed formula, zirconium tungstate powder with mass fraction of 0, 10%, 20%, and 30% was added to the epoxy resin, respectively;

[0070] (3) The samples were thoroughly mixed and uniformly stirred;

[0071] (4) The uniformly mixed samples were vacuum degassed using a vacuum pump;

[0072] (5) The samples were injected into the prepared silica gel mold;

[0073] (6) The samples were placed in an incubator and cured at 25°C for 24 hours;

[0074] (7) According to the connection method of Wheatstone bridge, the strain gauges of the test sample and the compensation element were connected in half-bridge mode to the sensor;

[0075] (8) The test process first cooled to 4K, then the refrigerator was turned off, and the temperature was naturally restored to 285K. During the warming process, the strain relative to 4K was recorded every 10 seconds;

[0076] (9) The thermal expansion coefficient results of the composite epoxy material with different zirconium tungstate doping concentrations at 4-285K were obtained by difference-differentiation method.

[0077] Experimental results:

[0078] Figure 5 The experimental measurement results of the thermal expansion coefficient of the composite epoxy material with different mass fraction of zirconium tungstate doping concentration at 4-285K are shown. It can be seen that the addition of zirconium tungstate effectively reduces the thermal strain of the composite material under large temperature change. With the gradual increase of the mass fraction of zirconium tungstate powder in the composite material, the thermal expansion coefficient of the composite material shows a significant downward trend.

[0079] Example 2

[0080] Experimental conditions:

[0081] Room temperature, 77K; Tensile property test according to standard ASTM D638, sample size type IV, test speed 0.2 mm / min. Shear strength test according to GB / T 7124-2008 adhesive test standard, test speed 0.2 mm / min.

[0082] Experimental steps:

[0083] (1) Prepare tensile and shear samples of four different proportions of zirconium tungstate / epoxy resin composite materials in Example 1;

[0084] (2) The sample is clamped in the tensile machine, and the tensile scheme is set;

[0085] (3) The sample is stretched to break, and the stress and strain are recorded;

[0086] (4) The data is processed to obtain the tensile stress-displacement curve and shear stress-displacement curve.

[0087] Experimental results:

[0088] The tensile strength and shear strength of four different composite materials are tested, as shown in Figure 6 and 7 The test results at room temperature and 77K show that the mechanical strength increases first and then decreases with the increase of ZrW2O8 content, and the 10wt.% ZrW2O8 sample exhibits the best mechanical properties: the maximum tensile strength is increased by 11.62% and the shear strength is increased by 7.0% compared with pure epoxy resin at room temperature; at 77K low temperature, the maximum tensile strength is increased by 12.67% and the shear strength is increased by 5.86% compared with pure epoxy resin.

[0089] Example 3

[0090] Experimental conditions:

[0091] The simulation parameters are used Figure 5 The thermal expansion coefficients of the measured pure epoxy resin and 10wt.% zirconium tungstate / epoxy resin composite are measured, the coil turns are 150 turns, the interturn epoxy is 80μm, and the coil inner radius is 8mm and 6mm respectively.

[0092] Experimental steps:

[0093] The radial stress of the simulation coil is reduced from room temperature to liquid nitrogen environment.

[0094] Experimental results:

[0095] The radial stress of the pure epoxy resin impregnated coil is compared with that of the 10wt.% zirconium tungstate / epoxy resin composite impregnated coil, and the radial stress of the improved impregnated coil with inner radius r=8mm is only 4.18% of that of the pure epoxy coil, and the radial stress of the improved impregnated coil with r=6mm is only 4.31% of that of the pure epoxy coil.

[0096] Conclusion: as Figure 8 and 9The results show that the 10wt.% ZrW2O6 / epoxy resin composite material with the lowest thermal expansion coefficient and the best mechanical properties is selected as the improved impregnated material through the thermal expansion coefficient measurement and the mechanical behavior measurement. The simulation results show that the radial stress of the improved impregnated coil is greatly reduced compared with the pure epoxy impregnated coil. The radial stress of the improved impregnated coil with r=8mm is only 4.18% of the pure epoxy coil, and the radial stress of the improved impregnated coil with r=6mm is only 4.31% of the pure epoxy coil.

Claims

1. An experimental method of fabricating a ZrW02-filled epoxy cured superconducting coil resistant to thermal mismatch, characterized in that, The method comprises the following steps: 1) superconducting tape preparation: selecting the superconducting tape required for the experiment, welding current leads at the two ends of the superconducting tape, respectively, spacing voltage welding points on the superconducting tape along the length direction at a specified distance, and welding silver strips on each voltage welding point; the superconducting tape is provided with voltage welding points at the 1st turn, the 6th turn, the 16th turn, the 26th turn, and so on; and a voltage welding point is provided every 10 turns; 2) preparation of impregnated material: using Stycast@2850FT epoxy resin material and Catalyst 24LV curing agent to prepare an epoxy resin matrix; doping fillers selected from zirconium tungstate ZrW2O8 powder with a particle size of 1 μm and a purity of greater than 99%; mixing the epoxy resin and the curing agent in a proportion, fully stirring them to be uniform, adding the zirconium tungstate powder, and fully stirring them to be uniform; reducing the radial stress of the coil to 4.18% or 4.31% of that of pure epoxy resin at 77K, and performing vacuum degassing treatment using a vacuum pump; the added mass of the zirconium tungstate powder accounts for 9% to 11% of the sum of the mass of the epoxy resin material and the curing agent; 3) coil winding: fixing the winding reel of the superconducting tape prepared in step 1) on a wire reel, fixing the coil skeleton tooling on a winding machine, and impregnating the coil using the wet winding method; the inner radius of the coil is 6 mm or 8 mm; introducing the superconducting tape in the winding reel to the coil skeleton tooling, and gluing and fixing the superconducting tape; evenly brushing the impregnated material prepared in step 2) on the upper and lower surfaces of the superconducting tape, simultaneously starting the winding machine, and starting to wind the coil, while brushing the impregnated material; during the winding process, the silver strips should not be stuck together, and should be spaced apart by the impregnated material; after the winding is completed, placing it in a constant-temperature curing oven, and obtaining the required superconducting coil.

2. The experimental method of claim 1, wherein the zirconium tungstate resistive heat mismatched epoxy filled superconducting wire coil is characterized by, In step 1), the current leads are silver strips, and the specifications of the silver strips are a thickness of 0.1 mm, a width of 3 mm, and a length of about 10 cm; and the specifications of the silver strips on each voltage welding point are a thickness of 0.1 mm, a width of 3 mm, and a length of 3 cm.

3. The experimental method of claim 1, wherein the zirconium tungstate resistive heat mismatched epoxy filled superconducting wire coil is characterized by, In step 2), the epoxy resin material and the curing agent are mixed in a mass ratio of 100:7 to 10.

4. The experimental method of claim 1, wherein the zirconium tungstate resistive heat mismatched epoxy filled superconducting wire coil is characterized by, In step 3), curing in a dry box at a constant temperature of 25°C for 24 h.

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