A heat treatment method for Bi-2212 CICC conductors with multi-stage twisting and external sheathing

Abstract

The invention discloses a heat treatment method for Bi-2212CICC conductors for multi-stage twisting and external sheathing, comprising: sealing the ends of Bi-2212 superconducting conductors before heat treatment; introducing high-pressure oxygen-argon mixed gas (>3MPa) into the armor during heat treatment; introducing high-purity argon gas into the armor (the pressure outside the conductor is required to be higher than the pressure inside the conductor); preheating the gas before entering the conductor (the preheating temperature is synchronized with the heat treatment curve); a PLC controller controls the air inlet and outlet flow rates to keep the internal pressure of the conductor stable (fluctuation is less than 1%); oxygen concentration detection at the front and rear ends of the conductor, and real-time calculation of oxygen consumption. The temperature curve is optimized, and a low-rate heat treatment process is used for large conductors to ensure stable temperature rise and fall. The invention provides an efficient and novel method for large Bi2212 conductors and even conductor heat treatment.

Description

Technical Field

[0001] The invention relates to the field of superconducting conductor heat treatment methods, in particular to a heat treatment process for Bi-2212 CICC (armored cable conductor Cable in conduit conductor) conductors with multi-stage twisting and external sheathing. Background Art

[0002] The China Fusion Engineering Test Reactor (CFETR) is a superconducting tokamak device for controlled nuclear fusion research based on China's independent design and development, successful construction and operation of the Advanced Experimental Superconducting Tokamak (EAST) and the construction of the International Thermonuclear Experimental Reactor (ITER). It is of great significance to my country's development of fusion energy and response to energy crises. Superconducting wire is an important component of the tokamak device, and its technology is a key technology in fusion engineering. The salient feature of the next generation of fusion reactors is that the maximum magnetic field of the central solenoid coil and the longitudinal field coil will exceed 12T. Limited by the critical magnetic field on the low-temperature superconducting wire, it is impossible to prepare a conductor with a higher magnetic field using traditional low-temperature superconducting materials. The winding of conductors with higher magnetic fields requires that the material can still maintain a high critical current density in a magnetic field of more than 20T. Therefore, seeking superconducting materials and high-field conductors with better comprehensive performance is a problem that must be solved in the future controlled magnetic confinement fusion demonstration reactor and commercialization process. Ceramic oxide high-temperature superconducting materials are ideal materials for building high-field conductors because they have extremely high irreversible fields and excellent magnetic field current carrying properties at 4.2K. Among high-temperature superconducting materials, Bi2Sr2Ca1Cu2OX (Bi-2212) is the only material that can be prepared into isotropic circular wire. It can still carry engineering current density with practical application significance even when the external field is as high as 45T at 4.2K. Therefore, it is currently the high-temperature superconducting material with the most application prospects under high fields (>20T).

[0003] Heat treatment is an important part of conductor and conductor manufacturing. For CICC conductors made of traditional low-temperature superconducting materials and their wound conductors, such as Nb3Sn, the last step to support CICC conductors and wind them into coils is heat treatment. Usually, a large vacuum furnace is required to heat treat large Nb3Sn coils. However, for high-temperature superconducting materials such as Bi-2212, oxygen is required for the reaction process. Secondly, the temperature of Bi-2212 reaction is much higher than that of Nb3Sn, close to 900°C. Therefore, it is almost impossible to heat treat Bi-2212 conductors like Nb3Sn conductors, because ordinary materials will be severely oxidized at such high temperatures. Even if special materials are used to manufacture heat treatment furnaces, firstly, the technical difficulty is very high, and secondly, the cost is too high and difficult to achieve. In addition, since the Bi-2212 superconducting conductor is inside the armor, the gaps between the superconducting wire and the armor are very small. If you want the superconducting wire in the middle section of the conductor to fully react with oxygen, it will take a long time to heat treat. In a large space, it is almost impossible to keep the temperature stable and uniform for a long time. In summary, the traditional large coil heat treatment method cannot be implemented on the Bi-2212 conductor. Therefore, it is necessary to find a new method to achieve the heat treatment of Bi-2212 conductors. Summary of the invention

[0004] The purpose of the present invention is to provide a heat treatment method for Bi-2212 CICC conductors with multi-stage twisting and external sheathing, so as to solve the problem that the existing heat treatment methods cannot heat treat Bi-2212 superconducting conductors.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] 1. A method for heat treatment of Bi-2212 CICC conductors for multi-stage twisting and sheathing, wherein the Bi-2212CICC conductors are wound from Bi-2212 CICC superconducting cables, and the conductors include less than 1500 superconducting wires, CICC superconducting conductor superconducting wires, and armor coaxially sheathed outside the superconducting wires, characterized in that the method comprises the following steps:

[0007] (1) Heat treatment method: 4.5MPa-5.5MPa high-pressure oxygen-argon mixed gas is passed into the conductor;

[0008] (1) Heat treatment method: 9.5MPa-10.5MPa high-pressure argon gas is introduced into the outside of the conductor;

[0009] (2) The twisted superconducting wires at both ends of the conductor are untied and sealed before heat treatment;

[0010] (3) Use a hollow Bi-2212 conductor armor with an inner diameter 1-4 mm larger than the outer diameter of the cable to install the sealed wire ends and weld the two ends of the armor seamlessly;

[0011] (4) The inlet and outlet ends of the conductor are placed in a low temperature area or wrapped with insulation materials outside the heat treatment equipment;

[0012] (5) Preheating the gas before it enters the heat treatment furnace;

[0013] (6) Keep the temperature at 600℃-890℃ for not less than 24 hours;

[0014] (7) Flow rate control: increase the inlet and outlet gas flow rates before reaching the maximum temperature Tmax = 890°C, and quickly replace the internal gas within 2 hours;

[0015] (8) The heating rate in all stages is less than 50℃ / h;

[0016] (9) During the insulation stage with the maximum temperature Tmax = 890°C, the gas preheating equipment is turned off and normal temperature gas is introduced. Before the temperature drops to 840°C, the cooling rate is controlled to not exceed 20°C / h;

[0017] (10) The melting stage of Bi-2212 superconducting powder does not exceed 8 hours.

[0018] Specifically, a heat treatment method for a Bi-2212 CICC conductor with multi-stage twisting and external sheathing, wherein the Bi-2212 superconducting conductor is composed of a superconducting wire and an armor coaxially sheathed outside the superconducting wire, comprises the following steps:

[0019] (1) Heat treatment method: A high-pressure oxygen-argon mixture is introduced into the conductor, and the same high-pressure argon gas is introduced into the outside for protection;

[0020] (2) End treatment: The twisted superconducting wires at both ends of the conductor are untied and sealed before heat treatment;

[0021] (3) Conductor transfer design: Use a large-caliber hollow Bi-2212 conductor armor to insert the sealed wire ends and weld the two ends of the armor seamlessly;

[0022] (4) The heat treatment process parameters are designed as follows:

[0023] 1. Preheat the gas before it enters the heat treatment furnace; 2. After the conductor starts to heat up, keep it warm at 700℃-850℃ for no less than 4 hours in the first stage; 3. Flow rate control: after the temperature reaches 850℃ and before the maximum temperature Tmax=890℃, increase the inlet and outlet gas flow rate to 50L / min to ensure rapid replacement of the gas inside the conductor within 2 hours; 4. The heating rate in all stages is less than 50℃ / h; 5. During the insulation stage with the maximum temperature Tmax=890℃, turn off the gas preheating equipment, pass in normal temperature gas, and control the cooling rate to no more than 20℃ / h before it drops to 840℃;

[0024] (5) The melting stage (above 870°C) of Bi-2212 superconducting powder shall not exceed 8 hours;

[0025] The present invention solves the problem of heat treatment of Bi-2212 high temperature superconducting conductors. In the superconducting conductors heat treated by this method, the superconducting wires in the conductors can fully react with oxygen, and there is no problem of strand performance degradation, and the method can be applied to the heat treatment of large Bi2-212 superconducting conductors. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the conductor heat treatment system;

[0027] In the figure, 1-gas supply system, 2-binary gas mixer, 3-gas preheating temperature control system, 4-PC control terminal, 5-pressure control module, 6-conductor, 7-high temperature and high pressure furnace, 8-flow controller. DETAILED DESCRIPTION

[0028] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the following embodiments are limited to explaining the present invention, and the protection scope of the present invention should include the entire contents of the claims, and through the description of the following embodiments, those skilled in the art can fully implement the entire contents of the claims of the present invention.

[0029] Figure 1 Figure 1 is a schematic diagram of the conductor heat treatment system. Figure 1 As shown, the conductor heat treatment system includes a gas supply system 1, a binary gas mixer 2, a gas preheating temperature control system 3, a PC control terminal 4, a pressure control module 5, a conductor 6, a high temperature and high pressure furnace 7 and a flow controller 8. The conductor 6 is a Bi-2212 conductor.

[0030] The conductor heat treatment system is used as follows:

[0031] (1) Remove the 15 cm armor at the end of the twisted conductor, untie the 15 cm cable from the wrapping tape and the five-level cable to the superconducting wire in the twisted cable sequence, seal the superconducting wire, wrap it with silver wire, and heat and melt it with a high-temperature flamethrower (greater than or equal to 1000°C) to make it sealed.

[0032] (2) After all superconducting wires are sealed, they are wrapped with tape and inserted into armor with an inner diameter 0.5-1 mm larger than the outer diameter of the cable. The two ends of the armor are welded.

[0033] (3) Use 2mm stainless steel plate welding wire at the end of the armor, and drill a 6-10mm hole in the center of the stainless steel plate.

[0034] (4) A 6-10 mm stainless steel tube is welded to the center hole of the stainless steel plate and connected to an external pressure controller.

[0035] Gas supply system 1 is the oxygen and argon supply source, usually a liquid helium tank or a cylinder gas, and the valve is opened to supply gas.

[0036] Binary gas mixer 2, turn on the binary gas mixer to control the ratio and flow rate of oxygen and argon.

[0037] The gas preheating temperature control system 3 is turned on, and is usually composed of a spiral pipe, a heating belt and a thermocouple. The gas preheating temperature control system 3 includes a gas preheating device.

[0038] PC control terminal 4, opens the computer information control terminal, which is used for setting the parameters of the binary gas mixer, adjusting the gas heating system and collecting information from each system.

[0039] The pressure control module 5 is turned on to balance and maintain the pressure inside the conductor during heat treatment.

[0040] Bi-2212 conductor is a conductor sample, and the gas is passed into the conductor and replaces the internal air for 1 hour.

[0041] The high temperature and high pressure furnace 7 is a heat treatment furnace, which is a tubular furnace. The high temperature and high pressure furnace 7 is an INCONEL617 stainless steel furnace body, which is used for sample heating. Turn on the heating switch to start heat treatment.

[0042] The flow controller 8 is a flow controller for flow rate control.

[0043] First, open the oxygen and argon valves and adjust the pressure to 15kg, then open the binary gas mixer 2, turn on the gas preheating temperature control system 3, and start the control program to control the mixed gas ratio; turn on the pressure control module 5 and the flow controller 8 to replace the gas inside the conductor with the mixed gas. After setting the pressure and flow, turn on the heating switch of the high-temperature and high-pressure furnace 7 to perform heat treatment on the conductor.

[0044] like Figure 1 As shown, a heat treatment process for Bi2212 CICC conductor with multi-stage twisting and external sheathing, the Bi2212 superconducting conductor is composed of a superconducting conductor with a CICC structure, and the CICC superconducting conductor is composed of a superconducting wire and an armor coaxially sheathed outside the superconducting wire.

[0045] A heat treatment process for a Bi2212 CICC conductor with multi-stage twisting and external sheathing, comprising the following steps:

[0046] Before heat treatment, clean all parts of the heat treatment furnace with alcohol to ensure that there is no impurity pollution;

[0047] (1) Leave 20-100cm at both ends of the conductor so that the inlet and outlet ends are placed in a low temperature area or wrapped with insulation materials outside the heat treatment furnace;

[0048] (2) Before heat treatment, the twisted superconducting wires at both ends of the conductor are untied and sealed. The superconducting wires are disassembled one level at a time from the wrapping tape to the five-level cable in the order of twisting, and then wrapped with silver wire and sealed.

[0049] (3) Use a hollow Bi-2212 conductor armor with a diameter 0.5mm-1mm larger than the outer diameter of the sealed cable to install the sealed cable and weld the two ends of the armor together, and weld the air inlet port to the other end of the hollow armor.

[0050] (4) A 5MPa high-pressure oxygen-argon mixture (volume percentage: 2% oxygen, 98% argon) is introduced into the conductor, and a 10MPa high-pressure argon gas is introduced into the outside for protection; the air inside the conductor and the heat treatment furnace is replaced, and the oxygen concentration is observed and recorded with an oxygen analyzer.

[0051] (5) Before heat treatment, the gas preheating equipment is turned on, and the preheating equipment and the tube furnace equipment use the same temperature control program to ensure consistent temperature output;

[0052] (6) Start heating, and the heating rate in all stages is less than 50℃ / h;

[0053] (7) When the temperature is between 700°C and 850°C, keep warm for not less than 4 hours;

[0054] (8) Flow rate control: increase the inlet and outlet gas flow rates before reaching the maximum temperature Tmax = 890°C, and quickly replace the internal gas within 2 hours;

[0055] (9) During the insulation stage with the maximum temperature Tmax = 890°C, the gas preheating equipment is turned off and normal temperature gas is introduced. Before the temperature drops to 840°C, the cooling rate is controlled to not exceed 20°C / h;

[0056] (10) The melting stage of Bi-2212 superconducting powder shall not exceed 8 hours (the melting stage is above 875°C). Wait for the program and workpiece temperature to cool down to below 100°C. The experiment is over and the instrument system is turned off.

[0057] The present invention solves the problem of heat treatment of Bi2212 high temperature superconducting conductors. In the superconducting conductors heat treated by this method, the superconducting wires in the conductors can fully react with oxygen, and there is no problem of strand performance degradation, and the method can be applied to the heat treatment of large Bi2212 superconducting conductors.

[0058] The present invention does not elaborate on some of the common technologies of those skilled in the art. The above-described embodiments are only descriptions of preferred implementations of the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementations described. Without departing from the design spirit of the present invention, various modifications and improvements made by ordinary technicians in this field to the technical solution of the present invention should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A heat treatment method for Bi2212 CICC conductors with multi-stage twisting and external sheathing, wherein the Bi2212 CICC conductors are wound by Bi2212 CICC superconducting cables, and the conductors include less than 1500 superconducting wires, CICC superconducting conductor superconducting wires, and armor coaxially sheathed outside the superconducting wires, characterized in that: The following steps are involved: (1) Heat treatment method: 4.5MPa-5.5MPa high-pressure oxygen-argon mixed gas is passed into the conductor, and 9.5MPa-10.5MPa high-pressure argon gas is passed into the outside of the conductor; (2) The twisted superconducting wires at both ends of the conductor are untied and sealed before heat treatment; (3) Use a hollow Bi-2212 conductor armor with an inner diameter 1-4 mm larger than the outer diameter of the cable to install the sealed wire ends and weld the two ends of the armor seamlessly; (4) The inlet and outlet ends of the conductor are placed in a low-temperature area or wrapped with insulation materials outside the heat treatment equipment; (5) Preheat the gas before it enters the heat treatment furnace; (6) Keep the temperature at 600℃-890℃ for not less than 24 hours; (7) Flow rate control: increase the inlet and outlet gas flow rates before reaching the maximum temperature Tmax = 890 °C, and quickly replace the internal gas within 2 hours; (8) The heating rate in all stages is less than 50℃ / h; (9) During the insulation stage at the maximum temperature Tmax = 890 °C, turn off the gas preheating equipment and introduce normal temperature gas. Before the temperature drops to 840 °C, control the cooling rate to no more than 20 °C / h; (10) The melting stage of Bi-2212 superconducting powder does not exceed 8 hours.

2. The method according to claim 1, characterized in that In step (1), in the mixed gas, the volume percentage of oxygen is 2%, and the volume percentage of argon is 98%.

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