A Heat Treatment Method and System for Bi-2212 Superconducting Material to Improve Heat Transfer Efficiency

Abstract

The present invention discloses a heat treatment method and system for Bi-2212 superconducting materials to improve heat transfer efficiency, which includes the following steps: placing the Bi-2212 superconducting magnet in a heat treatment furnace, where the heat treatment furnace chamber has a protective atmosphere of helium, and introducing a certain proportion of oxygen-helium mixed gas into the magnet; under the action of a blower, the furnace chamber is in a strong convection hot air circulation environment; starting to heat up for high-temperature and high-pressure heat treatment of Bi-2212; stopping the gas supply to the furnace chamber and closing the strong convection circulation system, and the Bi-2212 superconducting magnet can be taken out after exhausting and relieving pressure. The present invention can effectively solve the problems of uneven heat transfer and temperature lag existing in the traditional heat treatment process through two major ways: the oxygen-helium mixed gas atmosphere and the strong convection heat transfer of hot air. The heat treatment method disclosed by the present invention improves the heat treatment efficiency of Bi-2212 superconducting materials by more than 8 times.

Description

Technical Field

[0001] The present invention belongs to the field of heat treatment methods for superconducting materials, and particularly relates to a heat treatment method and system for Bi-2212 superconducting materials to improve heat transfer efficiency. Background Art

[0002] Controlled nuclear fusion is an important way to completely solve the problem of future energy shortage in human society, and superconducting tokamak devices are one of the most promising devices to achieve controllable utilization of nuclear fusion. The superconducting magnet system is one of the essential core systems in a tokamak device. Currently, the superconducting materials used to manufacture superconducting magnets are mainly low-temperature superconducting materials NbTi and Nb3Sn. With the continuous breakthroughs in the cause of controlled nuclear fusion, the superconducting magnets prepared from low-temperature superconducting materials have gradually approached their performance limits and cannot meet the significant characteristics of large current and strong magnetic field in future fusion reactors. Among practical high-temperature superconducting materials, the upper critical magnetic field of Bi-2212 can reach 100 T at a temperature of 4.2 K. In addition, Bi-2212 not only has excellent high-field current-carrying characteristics but is also the only known high-temperature superconducting material that can be fabricated into isotropic round wires, making it suitable for the development of superconducting magnets for large CICC (cable in conduit conductor) fusion reactors.

[0003] The interior of a Bi-2212 superconducting strand is superconducting powder, and the outer layer is a silver-magnesium alloy sleeve. For Bi-2212 high-temperature superconducting materials, the dense and uniform ceramic-type crystals formed after high-temperature and high-pressure heat treatment have a crucial impact on the current-carrying characteristics of superconducting wires. The heat treatment of Bi-2212 superconducting materials has the characteristics of a fast heating rate (160 °C / h), a short holding time (30 min), a stringent requirement for temperature uniformity (±3 °C), high pressure, and rich oxygen. Therefore, the heat treatment technology for Bi-2212 conductors and magnets is extremely difficult and risky. The most prominent problem is mainly reflected in the fact that the existing technologies are difficult to meet the heat treatment requirements of future conductors, especially magnets, because the heat transfer efficiency during heat treatment is extremely poor, making it difficult to ensure temperature uniformity. Summary of the Invention

[0004] To solve the problems of poor heat treatment temperature uniformity and temperature hysteresis of large-sized Bi-2212 superconducting magnets under the existing heat treatment system, the present invention provides a heat treatment method and system for Bi-2212 superconducting materials that improve the heat transfer efficiency, enhance the internal heat conduction efficiency of the furnace, fill helium gas in the furnace, and the heat transfer efficiency of helium gas is 8 times that of other gases. The ratio of helium gas to oxygen gas is used to not only meet the oxygen demand of the Bi-2212 superconducting material formation but also effectively improve the temperature uniformity in the furnace. In addition, under the stirring action of the fan, the heat transfer efficiency in the furnace will be further improved and the temperature uniformity will be better. In view of this, the present invention has increased the heat transfer efficiency in the furnace by more than 8 times, and through analysis, it can meet the future heat treatment requirements of Bi-2212 superconducting magnets.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A heat treatment method for Bi-2212 superconducting materials that improves the heat transfer efficiency, comprising the following steps:

[0007] Step (1) Gas configuration: Mix oxygen and helium gases in a ratio of 98% helium and 2% oxygen to form an oxygen-helium mixed gas.

[0008] Step (2) Pressurization and pre-vacuum pumping: Evacuate the gas inside the superconducting magnet to below 1×10 -2 atm using a vacuum pump unit, and pressurize the mixed gas through a booster pump with a pressure increase value of more than 5.5 MPa.

[0009] Step (3) Calibration: Calibrate the oxygen partial pressure in the booster pump to 2 ± 0.1%, and after reaching the standard, introduce it into the evacuated superconducting magnet.

[0010] Step (4) Gas stirring: Start the fan of the heat treatment furnace to perform strong convection circulation on the protective atmosphere in the furnace chamber of the heat treatment furnace.

[0011] Step (5) Heat treatment of the superconducting magnet; the heat treatment process is as follows: 1. Heat from room temperature to T1, 800°C < T1 < 850°C, with a heating rate of less than 50°C / h; 2. Keep at 840°C for a holding time greater than 30 min and less than 2 h; 3. Heat from 840°C to the maximum temperature Tmax, with a heating rate of less than 20°C / h, where 880°C < Tmax < 900°C; 4. Hold at the maximum temperature Tmax for a holding time greater than 30 min and less than 1 h; 5. Cool from the maximum temperature Tmax to T2, where 870°C < T2 < 880°C, with a cooling rate of less than 30°C / h; 6. Cool from T2 to T1, where 800°C < T1 < 850°C, with a cooling rate of less than 5°C / h; 7. Cool from T1 to room temperature, cooling with the furnace.

[0012] Further, for the gas configuration requirements in step (1), the purity of single-component helium and oxygen is >99.99%. The set helium gas flow rate is not less than 10 L / min, and the oxygen gas flow rate is not less than 1 L / min. They are mixed and configured according to a helium gas flow rate to oxygen gas flow rate of 9760 ± 100 sccm: 240 ± 30 sccm.

[0013] Further, in step (4), in order to increase the heat transfer efficiency, the inside of the furnace chamber of the heat treatment furnace is filled with helium as the protective gas, and the gas circulation flow rate inside the furnace of the heat treatment furnace exceeds 1 m / s.

[0014] Further, in step (5), during the entire heat treatment period, the flow rate of the helium-oxygen mixed gas discharged from the outlet of the superconducting magnet is not greater than 1 L / min.

[0015] The present invention also provides a heat treatment system for Bi-2212 superconducting materials to improve heat transfer efficiency, including a helium pressure-resistant tank, an oxygen pressure-resistant tank, a binary gas mixer, a storage tank, a booster pump, a vacuum pump unit, an oxygen analyzer, a flow controller, a heat treatment furnace fan, a superconducting magnet, a fairing, a heat treatment furnace, and a pressure controller;

[0016] The helium pressure-resistant tank and the oxygen pressure-resistant tank are used to store high-pressure gases or liquids, providing sufficient gas sources for the mixing system; the binary gas mixer mixes helium and oxygen into the gas ratio required for heat treatment; the storage tank stores the helium and oxygen mixed and configured in proportion; the booster pump boosts the mixed gas to the required pressure; the vacuum pump unit evacuates the air inside the magnet to ensure the purity of the mixed gas; the oxygen analyzer detects the oxygen partial pressure of the mixed gas and the outlet gas; the flow controller controls and records the gas flow rate; the heat treatment furnace fan stirs the gas inside the furnace to make it undergo strong convective heat transfer and improve the temperature uniformity; the fairing enables the gas to perform strong convective circulation along the specified channel; the rotation of the heat treatment furnace fan drives the gas to circulate inside the furnace, making the overall temperature inside the furnace more uniform; the pressure controller controls the internal pressure of the superconducting magnet to be stable.

[0017] Beneficial effects:

[0018] The present invention uses a helium-oxygen mixed gas as the heating medium, which is several times more efficient than other gas heating methods, greatly reducing the temperature hysteresis of the workpiece during heating - shortening the heating time and improving the temperature uniformity of the workpiece. Description of the drawings

[0019] Figure 1 It is a schematic diagram of the heat treatment system used for the heat treatment method of Bi-2212 superconducting materials to provide heat transfer efficiency according to the present invention. Specific embodiments

[0020] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] As Figure 1 shown, the heat treatment system adopted by the present invention includes a helium pressure-resistant tank 1, an oxygen pressure-resistant tank 2, a binary gas mixer 3, a storage tank 4, a booster pump 5, a vacuum pump unit 6, an oxygen analyzer 7, a flow controller 8, a heat treatment furnace fan 9, a superconducting magnet 10, a fairing 11, a heat treatment furnace 12 and a pressure controller 13.

[0022] The helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2 are connected in parallel and then connected to the binary gas mixer 3. The binary gas mixer 3 is connected to the storage tank 4. The storage tank 4 is connected to a valve. One end of the valve is connected to the vacuum pump unit 6, one end is connected to the oxygen analyzer 7, and one end is connected to the flow controller 8. The flow controller 8 is connected to the bottom of the superconducting magnet 10. The superconducting magnet 10 is arranged in the fairing 11, and the fairing 11 is arranged in the heat treatment furnace 12. The heat treatment furnace fan 9 is arranged on the top of the heat treatment furnace 12. The top of the other end of the superconducting magnet 10 is connected to the pressure controller 13, the oxygen analyzer 7 and the flow controller 8.

[0023] Before the mixed gas is input, the vacuum pump unit 6 first evacuates the superconducting magnet 10 and the heat treatment furnace 12.

[0024] The helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2 transport helium and oxygen to the binary gas mixer 3. The binary gas mixer 3 fully mixes the gases and then transports them to the storage tank 4. The storage tank 4 stably supplies the gases to the booster pump 5. The booster pump 5 transports the gases to the oxygen analyzer 7 and the flow controller 8. The flow controller 8 supplies the gases to the superconducting magnet 10 and the heat treatment furnace 12 according to the set flow rate. After the pressure of the heat treatment furnace 12 is stable, the flow valve is adjusted, and at this time, the gases are transported to the superconducting magnet 10. The gases inside the superconducting magnet 10 are introduced into the pressure controller 13, and the gases pass through the pressure controller 13 and then enter the flow controller 8.

[0025] The helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2 are used to store high-pressure gases or liquids, providing sufficient gas sources for the mixing system; the binary gas mixer 3 mixes helium and oxygen into the gas ratio required for heat treatment; the storage tank 4 stores the helium and oxygen mixed in proportion; the booster pump 5 boosts the mixed gas to the required pressure; the vacuum pump unit 6 evacuates the air inside the magnet to ensure the purity of the mixed gas; the oxygen analyzer detects the oxygen partial pressure of the mixed gas and the outlet gas; the flow controller controls and records the gas flow; the heat treatment furnace fan 9 stirs the gas in the furnace body to perform strong convective heat transfer and improve the temperature uniformity; the fairing 11 enables the gas to perform strong convective circulation along the specified channel; the heat treatment furnace 12 provides a heating environment and temperature control for the superconducting magnet; the pressure controller 13 controls the internal pressure of the superconducting magnet 10 to be stable.

[0026] In order to achieve the purpose of efficient heat treatment, a heat treatment method for Bi-2212 superconducting materials that provides heat transfer efficiency according to the present invention includes the following steps:

[0027] Step (1) Gas configuration: Mix oxygen and helium gases into an oxygen-helium mixed gas in a ratio of 98% helium and 2% oxygen. It is required that the purity of helium and oxygen is >99.99%. Set the maximum flow rate of helium to be not less than 10 L / min, and the flow rate of oxygen to be not less than 1 L / min. Mix according to a helium flow rate and an oxygen flow rate of 9760±100 sccm:240±3 sccm.

[0028] Step (2) Boosting and pre-evacuating: Evacuate the gas inside the superconducting magnet 10 to below 1×10 - 2 atm by the vacuum pump unit 6, and boost the mixed gas to above 5.5 MPa by the booster pump 5;

[0029] Step (3) Calibration: Calibrate the oxygen partial pressure in the booster pump 5 to 2±0.1%. After passing the standard, introduce it into the evacuated superconducting magnet 10;

[0030] Step (4) Gas stirring: Start the heat treatment furnace fan 9 to perform strong convective circulation on the helium protective gas in the furnace chamber. The circulating flow rate of the atmosphere in the furnace needs to exceed 1 m / s;

[0031] Step (5), turn on the heating program; the heat treatment process system is as follows: 1. Heat up from room temperature to T1, where 800°C < T1 < 850°C, and the heating rate is less than 50°C / h; 2. Keep the temperature at 840°C for insulation, and the insulation time is greater than 30 min and less than 2 h; 3. Heat up from 840°C to the maximum temperature Tmax, where the heating rate is less than 20°C / h, and 880°C < Tmax < 900°C; 4. Keep the temperature at the maximum temperature Tmax for insulation for a time greater than 30 min and less than 1 h; 5. Cool down from the maximum temperature Tmax to T2, where 870°C < T2 < 880°C, and the cooling rate is less than 30°C / h; 6. Cool down from T2 to T1, where 800°C < T1 < 850°C, and the cooling rate is less than 5°C / h; 7. Cool down from T1 to room temperature, cooling with the furnace.

[0032] Specifically, according to Figure 1 the heat treatment system shown, the present invention preferably provides a heat treatment method for Bi-2212 superconducting materials to improve heat transfer efficiency, which specifically includes the following steps:

[0033] Step (1) Close both ends of the superconducting magnet 10 shown in Figure 1 with flanges and evacuate the inside with a vacuum pump unit 6, and the vacuum degree is less than 1×10 -2 atm;

[0034] Step (2) Open the valves of the helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2, and introduce helium and oxygen into the binary gas mixer 3;

[0035] Step (3) After 30 min or more, close the gas input of the helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2 and evacuate the gas in the binary gas mixer 3 to the air;

[0036] Step (4) After the oxygen content meets the standard, introduce the gas into the booster pump 5 and make the pump pressure greater than 5.5 MPa;

[0037] Step (5) Re-open the gas valves of the helium pressure-resistant tank 1 and the oxygen pressure-resistant tank 2, mix helium and oxygen, and turn on the oxygen analyzer 7 to detect whether the gas composition meets the requirements, that is, detect whether the oxygen content is 1.95% - 2.05%;

[0038] Step (6) After the oxygen content meets the requirements, introduce the gas in the booster pump 5 into the inside of the furnace chamber of the heat treatment furnace 12 and the inside of the superconducting magnet 10. The inside of the furnace chamber of the heat treatment furnace 12 is pure helium, and the inside of the superconducting magnet 10 is a helium-oxygen mixture. The air pressure inside the superconducting magnet 10 is stabilized at 5 ± 0.1 MPa through the flow controller 8 and the pressure controller 13, and the pressure inside the furnace chamber of the heat treatment furnace 12 is less than 1 MPa;

[0039] After the pressure stabilizes in step (7), the flow controller 8 controls the gas input and output flows inside the superconducting magnet 10, specifically reflected in micro-charging and micro-discharging of the flow, with the outlet flow not exceeding 1 L / min, and the gas pressure inside the heat treatment furnace 12 remaining unchanged without inflating or deflating.

[0040] In step (8), turn on the heat treatment furnace blower 9 to enable strong convective circulation of the gas inside the furnace body of the heat treatment furnace 12, and the circulating flow rate of the gas inside the heat treatment furnace 12 needs to exceed 1 m / s.

[0041] After the gas circulation starts in step (9), turn on the heating system of the heat treatment furnace 12 and execute the heat treatment process according to the heat treatment process settings: 1. Heat up from room temperature to T1, where 800 °C < T1 < 850 °C, and the heating rate is less than 50 °C / h; 2. Hold at 840 °C, and the holding time is greater than 30 min and less than 2 h; 3. Heat up from 840 °C to the maximum temperature Tmax, where 880 °C < Tmax < 900 °C, and the heating rate is less than 20 °C / h; 4. Hold at the maximum temperature Tmax for a time greater than 30 min and less than 1 h; 5. Cool down from Tmax to T2, where 870 °C < T2 < 880 °C, and the cooling rate is less than 30 °C / h; 6. Cool down from T2 to T1, where 800 °C < T1 < 850 °C, and the cooling rate is less than 5 °C / h; 7. Cool down from T1 to room temperature, cooling with the furnace.

[0042] After the heat treatment in step (10), close the pressure tank, mixing device, booster pump, etc., open the outlet valve to exhaust the internal gas and then take it out.

[0043] The embodiments described in the present invention are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various variations and improvements made by those skilled in the art to the technical solutions of the present invention should fall within the protection scope of the present invention. The technical content claimed by the present invention has been fully recorded in the claims.

Claims

1. A heat treatment method for Bi-2212 superconducting material to improve heat transfer efficiency, characterized in that, It includes the following steps: Step (1) Gas configuration: Mix oxygen and helium gases in a ratio of 98% helium and 2% oxygen to form an oxygen-helium mixed gas; Step (2) Pressurization and preliminary vacuum pumping: evacuate the gas inside the superconducting magnet to below 1×10 -2 atm with a vacuum pump unit, and pressurize the premixed gas with a booster pump to a pressure increase value of more than 5.5 MPa; Step (3) Calibration: Calibrate the oxygen partial pressure in the booster pump to 2 ± 0.1%, and after passing the standard, introduce it into the superconductor magnet that has been evacuated; Step (4) Gas stirring: Start the heat treatment furnace fan to perform strong convection circulation on the protective atmosphere in the furnace chamber of the heat treatment furnace; Step (5) Heat treatment of the superconducting magnet; The heat treatment process is as follows:

1. Heat up from room temperature to T1, 800 °C < T1 < 850 °C, and the heating rate is less than 50 °C / h; 2. Keep the temperature at 840 °C, and the holding time is greater than 30 min and less than 2 h; 3. Heat up from 840 °C to the maximum temperature Tmax, and the heating rate is less than 20 °C / h, where 880 °C < Tmax < 900 °C; 4. Keep the temperature at the maximum temperature Tmax for a holding time greater than 30 min and less than 1 h; 5. Cool down from the maximum temperature Tmax to T2, where 870 °C < T2 < 880 °C, and the cooling rate is less than 30 °C / h; 6. Cool down from T2 to T1, where 800 °C < T1 < 850 °C, and the cooling rate is less than 5 °C / h; 7. Cool down from T1 to room temperature, cooling with the furnace.

2. The heat treatment method of Bi-2212 superconducting material for improving heat transfer efficiency according to claim 1, characterized in that: The gas configuration in step (1) requires that the purity of single-component helium and oxygen is > 99.99%, the set helium gas flow rate is not less than 10 L / min, the oxygen gas flow rate is not less than 1 L / min, and they are mixed in accordance with a helium gas flow rate and an oxygen gas flow rate of 9760 ± 100 sccm: 240 ± 30 sccm.

3. A heat treatment method for Bi-2212 superconducting material to improve heat transfer efficiency according to claim 1, characterized in that: In step (4), in order to increase the heat transfer efficiency, the inside of the furnace chamber of the heat treatment furnace is filled with helium as the protective gas, and the gas circulation flow rate inside the furnace of the heat treatment furnace exceeds 1 m / s.

4. A heat treatment method for Bi-2212 superconducting material to improve heat transfer efficiency according to claim 1, characterized in that: In step (5), during the entire heat treatment period, the flow rate of the helium-oxygen mixed gas discharged from the outlet of the superconducting magnet is not greater than 1 L / min.

5. A system for implementing a heat treatment method of a Bi-2212 superconducting material for improving heat transfer efficiency according to any one of claims 1-4, characterized in that: It includes a helium pressure-resistant tank, an oxygen pressure-resistant tank, a binary gas mixer, a storage tank, a booster pump, a vacuum pump unit, an oxygen analyzer, a flow controller, a heat treatment furnace fan, a superconducting magnet, a fairing, a heat treatment furnace, and a pressure controller; The helium pressure-resistant tank and the oxygen pressure-resistant tank are used to store high-pressure gases or liquids and provide sufficient gas sources for the mixing system; the binary gas mixer mixes helium and oxygen to form the gas ratio required for heat treatment; the storage tank stores the helium and oxygen mixed in proportion; the booster pump boosts the mixed gas to the required pressure; the vacuum pump unit evacuates the air inside the magnet to ensure the purity of the mixed gas; the oxygen analyzer detects the oxygen partial pressure of the mixed gas and the outlet gas; the flow controller controls and records the gas flow rate; the heat treatment furnace fan stirs the gas inside the furnace to perform strong convection heat transfer and improve the temperature uniformity; the fairing makes the gas perform strong convection circulation along the specified channel; the heat treatment furnace provides a heating environment and temperature control for the superconducting magnet; the pressure controller controls the internal pressure of the superconducting magnet to be stable.

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