A heat treatment method for MgB2 large superconducting coil pre-winding and post-reaction phase formation

Abstract

The application discloses a heat treatment method for MgB2 large superconducting coil after winding and phase reaction, which comprises the following steps: 1, designing and processing a support framework, and winding MgB2 superconducting cables with an insulation layer on the support framework; 2, carbon removal heat treatment; 3, phase reaction heat treatment; 4, cooling, and completing the heat treatment of the MgB2 large superconducting coil after winding and phase reaction. According to the inherent characteristics of the hard and brittle MgB2 superconducting material, the overall process route of winding the coil first and then performing the phase reaction heat treatment is adopted, the MgB2 superconducting cable with the insulation layer is used as the raw material, is wound on the support framework capable of bearing the high-temperature heat treatment, and then is sintered and heat treated in the MgB2 phase reaction temperature zone, and meanwhile, the carbon removal heat treatment is combined to guarantee the insulation performance between the cables, so that the problem of incomplete or uneven phase reaction in different regions in the phase reaction heat treatment process of the MgB2 large coil can be solved.

Description

Technical Field

[0001] This invention belongs to the field of superconducting coil heat treatment technology, specifically relating to a heat treatment method for a large MgB2 superconducting coil that is first wound and then reacted to form a phase. Background Technology

[0002] Since the discovery of superconductivity in MgB2 materials in 2001, over 20 years of research has led to rapid improvements in the performance of MgB2 superconducting materials. This has resulted in the emergence of MgB2 superconducting wire manufacturers, such as ASG in Italy, HyperTech in the United States, and the Northwest Nonferrous Metals Research Institute in China. Application research based on this material, including superconducting magnetic energy storage devices (SMES), superconducting fault current limiters (SFCL), and superconducting magnetic resonance imaging (MRI), is also gradually unfolding. In these practical applications, MgB2 superconducting wires typically need to be wound into coils. Due to the limitations of the superconducting wire length, and to increase the current transmission of the superconducting coil, multiple strands of superconducting wire are usually twisted into a cable before being wound into a coil.

[0003] However, because MgB2 is an intermetallic compound phase with the hardness and brittleness of ceramic materials, the reacted MgB2 superconducting wires inevitably suffer from additional stresses such as tension, bending, and torsion during stranding or coil winding. This can cause cracks in the superconducting core wires, leading to a significant decrease in superconducting current. A process of winding first and then reacting to form the superconducting phase can effectively avoid this problem. The applicant previously filed a patent application (CN116598063A) for a method of preparing MgB2 superconducting cables, disclosing the structural design, stranding process, and insulation layer wrapping techniques for MgB2 superconducting cables. The resulting problem is that superconducting coils used in practical engineering applications are typically large in size and weight, and significant temperature gradients exist at different locations during heat treatment, making it impossible to use existing heat treatment processes for MgB2 wires or cables.

[0004] To solve this problem, a heat treatment method specifically for MgB2 large superconducting coils, which involves winding first and then reacting to form a phase, is needed. Summary of the Invention

[0005] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a heat treatment method for large MgB2 superconducting coils that involves first winding the coil and then reacting it to form a phase. This method, taking into account the inherent hardness and brittleness of MgB2 superconducting material, adopts a general process route of first winding the coil and then performing phase-forming heat treatment. Using MgB2 superconducting cable with an insulating layer as raw material, it is wound onto a support frame capable of withstanding high-temperature heat treatment. Then, sintering heat treatment is performed in the MgB2 phase-forming reaction temperature range, combined with decarburization heat treatment, to ensure the insulation performance between cables. This method solves the problems of incomplete or uneven phase-forming reaction in different regions and poor breakdown resistance during the phase-forming heat treatment of large MgB2 coils.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a heat treatment method for first winding a large MgB2 superconducting coil and then reacting it to form a phase, characterized in that the method includes the following steps: Step 1: Design and fabricate the support frame of the MgB2 large superconducting coil according to the actual application requirements. Then, wind the MgB2 superconducting cable with insulation layer onto the support frame. During the winding process, monitor the insulation performance between the superconducting cable and the support frame to obtain the wound MgB2 large superconducting coil. Step 2: Perform decarburization heat treatment on the MgB2 large superconducting coil obtained in Step 1 to obtain a decarburized MgB2 large superconducting coil. Step 3: Perform phase-forming heat treatment on the decarbonized MgB2 large superconducting coil obtained in Step 2 to obtain a phase-forming MgB2 large superconducting coil. Step 4: When the phased MgB2 large superconducting coil obtained in Step 3 has cooled to the specified temperature, accelerate the cooling rate. When the temperature is close to room temperature, take out the phased MgB2 large superconducting coil and test the insulation performance between the superconducting cable and the support frame. This completes the heat treatment of the pre-wound and post-reaction MgB2 large superconducting coil.

[0007] This invention first winds an insulating MgB2 superconducting cable to form a large MgB2 superconducting coil, and monitors the insulation performance between the superconducting cable and the supporting frame during the winding process to ensure the performance of the large MgB2 superconducting coil. Then, decarburization heat treatment is used to remove organic matter from the insulation layer to improve the breakdown resistance of the large MgB2 superconducting coil. After that, phase formation heat treatment is performed, and finally, cooling is carried out to realize the pre-winding and reaction phase formation of the large MgB2 superconducting coil.

[0008] This invention designs and processes the support frame of the MgB2 superconducting coil according to actual application requirements. The support frame is designed into a racetrack-shaped coil, a saddle-shaped coil, a solenoid-shaped coil, or a D-shaped coil according to actual requirements.

[0009] This invention designs and processes the support frame for MgB2 superconducting coils based on actual application requirements. Taking into account factors such as material cost, processing cost, and mechanical performance, the support frame is made of 316L stainless steel, which has high strength and can withstand temperatures below 650℃ for up to 10 hours without significant degradation in mechanical properties.

[0010] This invention pre-reserves and secures an inner cable joint outside the support frame before the superconducting cable is wound. After heat treatment, the superconducting cable reacts to form a binary metal compound MgB2 superconducting phase, which has ceramic-like brittleness and cannot withstand large bending deformation. During transportation and transfer, the pre-reserved cable joint in the superconducting cable must be protected and must not be bent, thus ensuring the quality of the MgB2 large superconducting coil.

[0011] This invention employs a winding-then-heat-treatment phase-forming process for MgB2 metal compound phases with ceramic-like brittleness, which can avoid the problem of reduced superconducting transport performance caused by additional stresses such as bending, torsion, and tension on the brittle MgB2 superconducting phase.

[0012] In the aforementioned heat treatment method for the pre-winding and reaction phase formation of a large MgB2 superconducting coil, the insulation performance in step one is monitored using a multimeter set to megohms or directly with a megohmmeter, and the insulation performance meets or exceeds the megohm level. In this invention, the insulation layer of the MgB2 superconducting cable with an insulation layer is obtained by wrapping it with high-temperature resistant glass fiber. While the glass fiber itself possesses certain flexibility and high-temperature resistance, friction and abrasion between cables or between the cable and the coil / lead wheel during the rewinding, transportation, and coil winding processes of the MgB2 superconducting cable can cause damage to the glass fiber in some locations. During the winding process, the insulation performance between the cable and the supporting frame is tested continuously using a megohmmeter or a multimeter set to megohms (MΩ) resistance. Considering factors such as air breakdown, the insulation performance is considered to be up to megohms if the intermediate monitoring process reaches or exceeds the standard. If current conduction occurs, the cable must be unwound to an insulated position, the contact point identified, insulation treatment performed, and winding continued until the entire coil is wound.

[0013] The aforementioned heat treatment method for a large MgB2 superconducting coil, involving winding followed by reaction phase formation, describes a large MgB2 superconducting coil with a weight exceeding 350 kg in step one. The weight of the MgB2 large superconducting coil wound in this invention includes the MgB2 superconducting cable with insulation and the supporting frame, making it suitable for heat treatment of heavier MgB2 superconducting coils and meeting the requirements for heavier superconducting coils in practical applications.

[0014] The aforementioned heat treatment method for the pre-winding and subsequent reaction phase formation of a large MgB2 superconducting coil involves a decarburization heat treatment in step two at a temperature of 300℃~350℃, a holding time of 6h~10h, and an air atmosphere. The decarburization heat treatment process in this invention is primarily due to the fact that the insulation layer is obtained by wrapping with high-temperature resistant glass fiber tape. During the preparation of the glass fiber tape, some organic matter remains on its surface. By sintering at low temperature for a long time in an air atmosphere, the organic matter in the insulation layer is oxidized and released, thus avoiding the influence of carbon elements on the insulation performance. Furthermore, considering the large volume of the superconducting coil and the numerous cable lamination layers in practical applications, a longer holding time is used in the decarburization heat treatment process to ensure that all organic matter in the coil is oxidized and volatilized.

[0015] The above-mentioned heat treatment method for the reaction phase formation of a large MgB2 superconducting coil after winding is as follows: the phase formation heat treatment process in step three is as follows: first, a vacuum is drawn and the gas is purged with inert gas, then the temperature is raised to 450℃~500℃ at a heating rate of 4℃ / min~6℃ / min and held for 3h~5h, then the temperature is raised to 600℃~620℃ at a heating rate of 2℃ / min~3℃ / min and held for 5h~6h. During the heating process, inert gas is continuously replenished to keep the pressure at a slightly positive pressure of no more than 2kPa. This invention utilizes vacuuming and inert gas scrubbing, continuously replenishing inert gas during heating to reduce oxygen content to the PPM level to prevent oxidation. The continuous replenishment of inert gas maintains a slightly positive pressure within the furnace cavity, ensuring an inert atmosphere environment. Unidirectional exhaust prevents air from entering the furnace. This invention is specifically designed for large MgB2 superconducting coils. During heat treatment, the initial reaction temperature for the formation of the MgB2 superconducting phase from Mg and B is around 500℃. A temperature gradient exists within the coil during heat treatment. During heating, heat conduction from the surface of the support frame to the center takes time, with the surface heating up first and the center lagging behind. To ensure temperature uniformity between the coil's central cable and outer cables, a holding period below the initial reaction temperature is designed to maintain temperature balance across different areas of the coil. During cooling, the support frame cools down first. Due to the lag in cooling of the central cable, unlike large pure metal workpieces, the MgB2 large superconducting coil contains multiple different types of materials. The coil frame is made of 316L stainless steel, the MgB2 superconducting cable is a ceramic-like intermetallic compound phase, and the insulation layer is a glass ribbon layer with poor thermal conductivity. This complex multi-component structure further increases the temperature gradient during heat treatment, resulting in incomplete phase formation of the inner MgB2 superconducting cable. To address this issue, based on calculations using a thermal diffusion model and drawing on previously published experience in heat treatment of small solenoid coils, an optimal heat treatment process for MgB2 large superconducting coils for practical engineering applications such as superconducting energy storage coils is proposed. This process involves a relatively fast heating rate, holding at a lower temperature for a certain period, followed by a slower heating rate and holding at a higher temperature for a certain period to complete phase formation.

[0016] In the aforementioned heat treatment method for large MgB2 superconducting coils, which involves winding and then reacting to form a phase, the specified temperature in step four is below 300°C; the temperature close to room temperature is below 50°C. This invention accelerates coil cooling and improves process efficiency by activating the circulating fan within the heat treatment furnace when the temperature drops below 300°C. At this point, the furnace cavity remains inert. Removing the coil only after the temperature drops below 50°C also prevents oxidation of the workpiece.

[0017] In the aforementioned heat treatment method for a large MgB2 superconducting coil, which involves winding the coil before reaction phase formation, the insulation performance test in step four is performed using a multimeter set to the megohm range or directly with a megohmmeter, ensuring the insulation performance meets or exceeds the megohm level. This invention guarantees the performance of the large MgB2 superconducting coil by testing its insulation performance.

[0018] Compared with the prior art, the present invention has the following advantages: 1. This invention addresses the inherent hardness and brittleness of MgB2 superconducting material by employing a general process route of first winding the coil and then performing phase formation heat treatment. MgB2 superconducting cable with an insulating layer is used as raw material and wound onto a support frame capable of withstanding high-temperature heat treatment. Sintering heat treatment is then performed in the MgB2 phase formation reaction temperature range, combined with a decarburization heat treatment process to completely decompose and volatilize the organic matter within the insulating layer, ensuring the insulation performance between cables. This invention primarily solves problems such as incomplete or over-reaction phase formation reaction and poor breakdown resistance in large MgB2 coils.

[0019] 2. This invention mainly targets large-scale MgB2 superconducting coils prepared by a pre-winding and post-reaction process. Due to the large distance between the coil skeleton surface and the inner cable layer, and the poor thermal conductivity of the glass ribbon insulation layer on the outer layer of the cable, a temperature gradient exists inside the coil during the heat treatment process. By controlling the heating and cooling rates, holding time, and heat treatment atmosphere during the decarburization heat treatment and phase formation heat treatment processes, the invention ensures the uniform reaction of precursors in different regions of the MgB2 cable in the coil, completing the reaction and phase formation process. This solves the problem of incomplete phase formation reaction in the inner and outer superconducting cables of this type of MgB2 coil.

[0020] 3. The present invention employs a decarbonization heat treatment, which can completely oxidize and volatilize the organic matter in the glass ribbon insulation layer wrapped around the cable, thus avoiding the problem of reduced breakdown performance caused by carbon accumulation.

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 This is a cross-sectional view of the MgB2 large superconducting coil wound in Embodiment 1 of the present invention.

[0023] Figure 2 This is a physical image of the MgB2 large superconducting coil formed in Embodiment 1 of the present invention.

[0024] Explanation of reference numerals in the attached figures: Detailed Implementation

[0025] Example 1 This embodiment includes the following steps: Step 1: Design and fabricate the support frame for the MgB2 large superconducting coil according to actual application requirements. Then, wind the MgB2 superconducting cable with an insulation layer and a diameter of 10.5mm ± 0.5mm onto the support frame, winding it layer by layer in close succession, for a total of 9 layers, with 10 turns per layer. During the winding process, the insulation performance between the superconducting cable and the support frame is monitored using a multimeter in megohm range. No short circuit was found, resulting in a MgB2 large superconducting coil weighing 392kg. The support frame is D... The coil is shaped like a 316L stainless steel coil. The supporting frame includes an upper flange 1, an inner core cylinder 2, a lower flange 4, and an outer sheath 5 forming a hollow D-shape, as well as a series of screws 3 for fastening. An insulated MgB2 superconducting cable 6 is wound within the supporting frame. The upper flange 1 has vent holes 7 to allow gas to enter and exit during heat treatment, accelerating the temperature equilibrium process inside and outside the supporting frame. Lifting rings 9 are provided for hoisting and easy movement. An inner cable connector 8 is pre-installed on the outer side of the outer sheath 5. (See...) Figure 1 and Figure 2 ; Step 2: The MgB2 large superconducting coil obtained in Step 1 is heated to 350°C in air at a heating rate of 3°C / min and held at that temperature for 6 hours for decarburization heat treatment. It is then cooled in the furnace to obtain a decarburized MgB2 large superconducting coil. The insulation performance of the decarburized MgB2 large superconducting coil meets the megohm level or above. Step 3: Place the decarbonized MgB2 large superconducting coil obtained in Step 2 into the phase formation heat treatment furnace and close the furnace door. Start evacuating the vacuum. When the vacuum level drops below 10 Pa, fill it with argon gas to atmospheric pressure. Repeat the evacuation-argon filling process to clean the gas. Then heat it to 500℃ at a heating rate of 5℃ / min and hold it for 3 hours. Then heat it to 600℃ at a heating rate of 2℃ / min and hold it for 6 hours. During the heating process, argon gas is continuously added to keep the phase formation heat treatment furnace cavity under a slightly positive pressure, with a pressure not exceeding 2 kPa. When the pressure exceeds 2 kPa, the pressure relief valve automatically releases the gas to obtain the phase-formed MgB2 large superconducting coil. Step 4: When the phased MgB2 large superconducting coil obtained in Step 3 cools to 300℃, turn on the circulating fan in the phased heat treatment furnace to accelerate the cooling rate. When the temperature drops to 50℃, take out the phased MgB2 large superconducting coil and use a multimeter in megohm range to test the insulation performance between the superconducting cable and the support frame. No short circuit was found, and the heat treatment of the pre-wound and post-reaction MgB2 large superconducting coil is completed.

[0026] Example 2 This embodiment includes the following steps: Step 1: Design and fabricate the support frame for the MgB2 large superconducting coil according to actual application requirements. Then, wind the MgB2 superconducting cable with an insulation layer and a diameter of 10.5mm±0.5mm onto the support frame, winding it layer by layer in close succession, for a total of 9 layers, with 9 turns in each layer. During the winding process, a megohmmeter is used to monitor the insulation performance between the superconducting cable and the support frame. No short circuits were found, resulting in a MgB2 large superconducting coil weighing 390kg. The support frame is a D-shaped coil made of 316L stainless steel. Step 2: The MgB2 large superconducting coil obtained in Step 1 is heated to 300°C in air at a heating rate of 3°C / min, and held at that temperature for 10 hours for decarburization heat treatment. It is then cooled in the furnace to obtain a decarburized MgB2 large superconducting coil. The insulation performance of the decarburized MgB2 large superconducting coil meets the megohm level or above. Step 3: Place the decarbonized MgB2 large superconducting coil obtained in Step 2 into the phase formation heat treatment furnace and close the furnace door. Start evacuating the vacuum. When the vacuum level drops below 5 Pa, fill it with nitrogen to atmospheric pressure. Repeat the evacuation-nitrogen filling process to clean the gas. Then heat it to 450℃ at a heating rate of 4℃ / min and hold it for 4 hours. Then heat it to 620℃ at a heating rate of 3℃ / min and hold it for 5 hours. During the heating process, continuously replenish nitrogen to keep the phase formation heat treatment furnace cavity under a slight positive pressure, with a pressure not exceeding 2 kPa. When the pressure exceeds 2 kPa, the pressure relief valve will automatically release the gas to obtain the phase-formed MgB2 large superconducting coil. Step 4: When the phased MgB2 large superconducting coil obtained in Step 3 cools to 290°C, turn on the circulating fan in the phased heat treatment furnace to accelerate the cooling rate. When the temperature drops to 45°C, take out the phased MgB2 large superconducting coil and use a megohmmeter to test the insulation performance between the superconducting cable and the support frame. No short circuit was found, and the heat treatment of the pre-wound and post-reaction MgB2 large superconducting coil is completed.

[0027] Example 3 This embodiment includes the following steps: Step 1: Design and fabricate the support frame for the MgB2 large superconducting coil according to actual application requirements. Then, wind the MgB2 superconducting cable with an insulation layer and a diameter of 10.5mm±0.5mm onto the support frame, winding it layer by layer in close succession, for a total of 9 layers, with 10 turns in each layer. During the winding process, the insulation performance between the superconducting cable and the support frame is monitored using a multimeter in megohm range. No short circuit was found, resulting in a MgB2 large superconducting coil weighing 395kg. The support frame is a D-shaped coil made of 316L stainless steel. Step 2: The MgB2 large superconducting coil obtained in Step 1 is heated to 330°C in air at a heating rate of 3°C / min and held at that temperature for 8 hours for decarburization heat treatment. It is then cooled in the furnace to obtain a decarburized MgB2 large superconducting coil. The insulation performance of the decarburized MgB2 large superconducting coil meets the megohm level or above. Step 3: Place the decarbonized MgB2 large superconducting coil obtained in Step 2 into the phase formation heat treatment furnace and close the furnace door. Start evacuation. When the vacuum level drops below 10 Pa, fill it with argon gas to atmospheric pressure. Repeat the evacuation-argon filling process to clean the gas. Then heat it to 480℃ at a heating rate of 6℃ / min and hold it for 5 hours. Then heat it to 610℃ at a heating rate of 2.5℃ / min and hold it for 5.5 hours. During the heating process, argon gas is continuously added to keep the phase formation heat treatment furnace cavity under a slightly positive pressure, with a pressure not exceeding 2 kPa. When the pressure exceeds 2 kPa, the pressure relief valve automatically releases the gas to obtain the phase-formed MgB2 large superconducting coil. Step 4: When the phased MgB2 large superconducting coil obtained in Step 3 cools to 280°C, turn on the circulating fan in the phased heat treatment furnace to accelerate the cooling rate. When the temperature drops to 40°C, take out the phased MgB2 large superconducting coil and use a multimeter in megohm range to test the insulation performance between the superconducting cable and the support frame. No short circuit was found, and the heat treatment of the pre-wound and post-reaction MgB2 large superconducting coil is completed.

[0028] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A heat treatment method for first winding and then reacting a large MgB2 superconducting coil to form a phase, characterized in that, The method includes the following steps: Step 1: Design and fabricate the support frame of the MgB2 large superconducting coil according to the actual application requirements. Then, wind the MgB2 superconducting cable with insulation layer onto the support frame. During the winding process, monitor the insulation performance between the superconducting cable and the support frame to obtain the wound MgB2 large superconducting coil. Step 2: Perform decarburization heat treatment on the MgB2 large superconducting coil obtained in Step 1 to obtain a decarburized MgB2 large superconducting coil. Step 3: Perform phase-forming heat treatment on the decarbonized MgB2 large superconducting coil obtained in Step 2 to obtain a phase-forming MgB2 large superconducting coil. Step 4: When the phased MgB2 large superconducting coil obtained in Step 3 has cooled to the specified temperature, accelerate the cooling rate. When the temperature is close to room temperature, take out the phased MgB2 large superconducting coil and test the insulation performance between the superconducting cable and the support frame. This completes the heat treatment of the pre-wound and post-reaction MgB2 large superconducting coil.

2. The heat treatment method for pre-winding and then reacting a large MgB2 superconducting coil according to claim 1, characterized in that, The insulation performance mentioned in step one is monitored using the megohm range of a multimeter or by directly testing with a megohmmeter, and the insulation performance meets or exceeds the megohm level.

3. The heat treatment method for pre-winding and then reacting phase formation of a large MgB2 superconducting coil according to claim 1, characterized in that, The MgB2 large superconducting coil wound in step one weighs more than 350 kg.

4. The heat treatment method for pre-winding and then reacting phase formation of a large MgB2 superconducting coil according to claim 1, characterized in that, The decarbonization heat treatment in step two is performed at a temperature of 300℃~350℃, with a holding time of 6h~10h, and in an air atmosphere.

5. The heat treatment method for pre-winding and then reacting phase formation of a large MgB2 superconducting coil according to claim 1, characterized in that, The phase-forming heat treatment process described in step three is as follows: First, a vacuum is drawn and the gas is purged with inert gas. Then, the temperature is raised to 450℃~500℃ at a heating rate of 4℃ / min~6℃ / min and held for 3h~5h. Next, the temperature is raised to 600℃~620℃ at a heating rate of 2℃ / min~3℃ / min and held for 5h~6h. During the heating process, inert gas is continuously added to keep the pressure at a slightly positive pressure of no more than 2kPa.

6. The heat treatment method for pre-winding and then reacting phase formation of a large MgB2 superconducting coil according to claim 1, characterized in that, The specified temperature in step four is below 300°C; the temperature close to room temperature is below 50°C.

7. The heat treatment method for pre-winding and then reacting phase formation of a large MgB2 superconducting coil according to claim 1, characterized in that, The insulation performance test described in step four is performed using the megohm range of a multimeter or directly using a megohmmeter, and the insulation performance must meet or exceed the megohm level.

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