A method for sealing the ends of multi-strand twisted Bi-2212 conductors during high pressure heat treatment

Abstract

The invention discloses a method for sealing the end of a multi-strand twisted Bi2212 conductor under high pressure heat treatment, including surface treatment of Bi-2212 round wire and sealing structure design, heating method and control parameter setting. The invention sprays an aerogel insulation coating on the outside of the Bi-2212 superconducting round wire (5 mm from the port), uses a 1.0 mm silver-magnesium alloy wire to wrap around the cable surface, and finally covers the cable end in an elliptical structure, so that when the cable is sealed at high temperature, the external temperature is high and the internal temperature is low. When the silver-magnesium alloy wire melts and falls with gravity, the silver solution covers the superconducting wire end to complete the sealing.

Description

Technical Field

[0001] The invention relates to the field of superconducting strands in CICC armored conductors, in particular to a method for sealing the ends of a multi-strand twisted Bi-2212 conductor subjected to high pressure heat treatment. Background Art

[0002] CICC armored conductor (cable-in-conduit conductor) is internationally recognized as the preferred conductor for manufacturing large superconducting magnet coils, because the coolant in the structured conductor is in direct contact with the cable in the form of a fluid, with a large wet surface area and excellent heat exchange efficiency. In addition, the external armor can provide support for the internal cable and improve the structural strength of the cable. CICC conductors are currently widely used in large scientific facilities such as accelerators and fusion reactors, such as the international cooperation ITER device under construction and the CFETR device to be built in China in the future.

[0003] At present, traditional Nb-based superconducting materials such as Nb3Sn and NbTi are limited by their upper critical field (Hc2) and are gradually unable to meet the high field requirements in the future. Therefore, people have gradually turned their attention to high-temperature superconducting materials with high critical fields, such as Bi-2212, YBCO, etc., which have high upper critical fields and have great development prospects. Bi-2212 can be made into isotropic round wires, which is conducive to the development of CICC conductors. The current common preparation method for Bi-2212 round wires is the powder tube method, that is, the raw materials for forming the superconducting phase are loaded into a silver-based casing and annealed and drawn multiple times to form a wire, and finally subjected to high-temperature heat treatment to form a superconducting phase. The preparation process of Bi-2212 round wire and the ceramic structure of the Bi-2212 phase are destined to have the characteristics of poor mechanical properties of the prepared strands, and its mechanical strength is only about half of that of traditional Nb3Sn materials. In the manufacture and practical application of CICC conductors, superconducting materials will deform due to the forces during the twisting process, the Lorentz force during operation, and the thermal stress during the heating and cooling process, which will cause the critical current of the conductor to decay. In the future, with the development of magnet technology, people will have higher and higher requirements for magnetic field strength.

[0004] Bi-2212 high-temperature superconducting material needs to undergo a special heat treatment process - high temperature and high pressure heat treatment process to achieve high critical current performance that meets engineering applications. High pressure heat treatment (3-10MPa) requires sealing of both ends of the Bi-2212 superconducting wire to ensure high external pressure and low pressure inside the superconducting wire, forming a pressure difference to achieve the purpose of high pressure. The sealing process of Bi-2212 superconducting wire has been studied, and now it is necessary to seal the CICC conductors of thousands of superconducting wires. Summary of the invention

[0005] Bi-2212 conductors are composed of hundreds to tens to thousands of superconducting round wires. Sealing a single superconducting wire alone may take longer, perhaps several months, for large-sized conductors and increase the risk of single-wire sealing failure. In order to quickly seal large-sized cables and conductors, a new and reliable overall sealing method is needed. For this purpose, a high-pressure (3-10MPa) heat-treated end sealing method for multi-strand twisted Bi-2212 conductors has been invented to solve the problem that the Bi-2212 round wires used in CICC conductors in the prior art can only be processed one by one and cannot be sealed quickly and in large quantities.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for sealing the ends of a multi-strand twisted Bi-2212 conductor during high pressure heat treatment, comprising the following steps:

[0008] (1) Before the conductor is sealed, the front 5-10cm armor of the Bi-2212 conductor is removed, the cable is spread out and wrapped with silver-magnesium alloy wire, and the outer surface of the Bi-2212 round wire within 5mm from the end is sprayed with SiO2 aerogel as an insulation layer. Among them, the Bi-2212 conductor is made of twisted Bi-2212 round wire. The SiO2 content of SiO2 aerogel material is greater than 90% by weight, and it cannot contain easily oxidized metal powders such as copper and iron. The thickness of the insulation layer is 5-20μm;

[0009] (2) Then, the front end 5-10 mm of each superconducting wire in the cable is wrapped with a silver-magnesium alloy wire for 3-7 turns, and finally wrapped with a silver sheet for 2-4 turns to tighten, thereby obtaining a cable wrapped with a silver-magnesium alloy;

[0010] (3) Electromagnetic induction heating is used to heat the end of the cable wound with the silver-magnesium alloy; the electromagnetic induction heating device and the sample are installed in a vertical direction, and the sample clamping part is controlled by a stepper motor and a PC end, and moves upward a certain distance (0.5-1 cm) at a constant speed of 5-10 mm / 5 seconds. The sample is the cable wound with the silver-magnesium alloy.

[0011] Furthermore, electromagnetic induction heating is used to heat the cable end wound around the silver-magnesium alloy, the electromagnetic induction coil power is maintained between 1500 Hz and 5000 Hz, and the heating time is 0.5-10 seconds, so that the cable or conductor is sealed.

[0012] Furthermore, after the cable is sealed, no superconducting phase (black phase) or pores should appear around the bottom superconducting wire. The superconducting phase flowing out of the cable head surface needs to be dug out with a knife, and then heated with a torch greater than 1500 degrees Celsius for 5-10 seconds to confirm again whether there is superconducting phase flowing out. If there is a gap in the middle of the bottom superconducting wire, fill it with molten silver solution and quickly cool it with alcohol.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] The present invention solves the problem that the existing superconducting wire sealing intelligent single-wire sealing cannot perform centralized sealing of a large number of superconducting wires without affecting the critical current performance of the superconducting wire. The structure is simple and easy to operate, which is conducive to the realization of large-scale engineering and increases the possibility of the application of Bi-2212 CICC conductors in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic structural diagram of a conductor sealing device of the present invention;

[0016] In the figure, 1-computer, 2-stepping motor, 3-slide rail, 4-clamp, 5-conductor, 6-high frequency induction heating melting machine. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

[0018] Figure 1 Schematic diagram of the conductor sealing device of the present invention. The device comprises a computer 1 (PC), a stepper motor 2, a slide rail 3, a fixture 4, a conductor 5 and a high-frequency induction heating melting machine 6. The conductor 5 is removed and thickened for silver alloy wrapping, and the stepper motor 2 is arranged on the slide rail 3. Then the fixture 4 on the stepper motor 2 is used to fix it in the vertical direction; the program on the computer 1 (PC) is started, and the program controls the high-frequency induction heating melting machine 6 to reach the required heating power within 2 seconds and starts the stepper motor 2 to move the conductor 5 upward, completing the conductor sealing.

[0019] like Figure 1 As shown, a Bi-2212 CICC conductor end sealing process is used to solve the problem that the Bi-2212 round wires used for CICC conductors in the prior art can only be processed one by one and cannot be sealed quickly and in large quantities. The Bi-2212 conductor is made of twisted Bi-2212 round wires.

[0020] First, remove 5-10cm of the outer armor and wrapping tape at the front end of the Bi-2212 conductor cable, spread the cable, clean each superconducting wire with alcohol, and then use SiO2 aerogel spraying as an insulation layer within 5mm of the end of the outer surface of the Bi-2212 round wire. The SiO2 content of the aerogel material is greater than 90wt%, and it cannot contain easily oxidized metal powders such as copper and iron, and the thickness is 5-20μm.

[0021] Each superconducting wire is wound with a silver-magnesium alloy wire. Each superconducting wire in the cable is wound with the silver-magnesium alloy wire for 3-7 turns, and finally wrapped with a 0.1mm thick, 2cm wide silver sheet for 2-4 turns. After winding, it is extruded with two semicircular molds, and the final size is no more than 120% of the original diameter of the cable.

[0022] Then install the heating device and the sample in a vertical direction, and the sample clamping part is controlled by a stepper motor and a PC end, and moves upward a certain distance (0.5-1cm) at a constant speed. The electromagnetic induction heating method is adopted, and the power of the electromagnetic induction coil is maintained between 1500Hz and 15KHz, and the heating time is 0.5-10 seconds.

[0023] After the cable is sealed, no superconducting phase (black phase) or pores should appear around the bottom superconducting wire. The superconducting phase on the upper surface is removed with a knife and heated with a 1500 degree Celsius torch for 5-10 seconds. A gap appears in the middle of the bottom superconducting wire, which is filled with molten silver solution and quickly cooled with alcohol. If a large hole (larger than 1mm) appears at the bottom after melting, cut the cable end and re-seal the conductor (to avoid conductor sealing failure).

[0024] Example 1

[0025] 1. A three-level CICC conductor with a diameter of about 9 mm and composed of 42 superconducting wires was stripped of 10 cm of armor to form a cable;

[0026] 2. Use SiO2 aerogel spraying as the insulation layer at a distance of 5mm from the cable end. The SiO2 content of the SiO2 aerogel material is greater than 90wt%, and the thickness of the insulation layer is 15μm;

[0027] 3. Superconducting wire wrapped with silver-magnesium alloy wire;

[0028] 4. Wrap 3 layers of No. 1 silver-wrapped tape with a thickness of 0.1mm and a width of 10mm on the outer layer;

[0029] 5. Use electromagnetic heating and move upward at a speed of 2mm / s;

[0030] 6. Turn off the heater and spray alcohol to cool the cable from 10 cm above the end to obtain a sealed part;

[0031] 7. After the performance test of the obtained superconducting conductor head, the sealing success rate was statistically greater than 85%.

[0032] The embodiments described in the present invention are merely descriptions of the preferred implementation modes of the present invention, and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should fall within the protection scope of the present invention. The technical contents for which protection is sought in the present invention have all been recorded in the claims.

Claims

1. A method for sealing the ends of a multi-strand twisted Bi-2212 conductor during high pressure heat treatment, characterized in that: The following steps are involved: (1) Before sealing the conductor, remove the 5-10 cm armor at the front end of the Bi-2212 conductor cable, spread the cable and wrap it with silver-magnesium alloy wire. The outer surface of the Bi-2212 round wire within 5 mm from the end is sprayed with SiO2 aerogel as an insulation layer; Among them, the Bi2212 conductor is made of twisted Bi-2212 round wire; (2) Then, the front end 5-10 cm of each superconducting wire in the cable is wrapped with a silver-magnesium alloy wire for 3-7 turns, and finally, it is fixed by wrapping it with a silver sheet for 2-4 turns to obtain a cable wrapped with a silver-magnesium alloy; (3) Electromagnetic induction is used to heat the end of the cable wound with the silver-magnesium alloy; the electromagnetic induction heating device and the sample are installed in a vertical direction, the sample clamping part is controlled by a stepper motor and a PC end, and the cable moves upward at a constant speed of 5-10 mm / 5 seconds; wherein the sample is the cable wound with the silver-magnesium alloy.

2. The method according to claim 1, characterized in that In step (1), the thickness of the insulating layer is 5-20 μm.

3. The method according to claim 1, characterized in that In step (1), the SiO2 content in the SiO2 aerogel is greater than 90wt%, and it cannot contain copper or iron metal powder.

4. The method according to claim 1, characterized in that: In step (2), the silver sheet is 0.1 mm thick and 2 cm wide.

5. The method according to claim 1, characterized in that In step (3), the electromagnetic induction heating parameters are as follows: the electromagnetic induction coil power is maintained between 1500 Hz and 8000 Hz, and the heating time is 0.5-10 seconds.

6. The method according to claim 1, characterized in that In step (3), move upward 0.5-1 cm.

7. The method according to claim 1, characterized in that In step (3), after the cable is sealed, no superconducting phase or pores should appear around the Bi2212 conductor at the bottom.

8. The method according to claim 1, characterized in that In step (3), the superconducting phase on the upper surface is removed with a carving knife and then heated with a flame torch at 1500-3000°C for 5-10 seconds.

9. The method according to claim 1, characterized in that: In step (3), a gap appears in the middle of the bottom Bi2212 conductor, which is filled with molten silver solution and then cooled with alcohol.

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