A superconducting magnet and refrigeration method for magnetically controlled CZ single crystal

Abstract

The invention relates to the technical field of superconducting magnets, in particular to a superconducting magnet and a refrigeration method for magnetically controlled CZ single crystals, comprising a coil group, a coil box, a fixing frame, a hollow cold shield, an external vacuum dewar, a magnetic shield, a cooling pipe, a pull rod support and a refrigerator, wherein a pair of oppositely arranged annular coils constitute a coil group, the annular coil wound by a superconducting wire with an insulating layer comprises a winding skeleton, a winding and a coil box, the winding skeleton is used for forming the winding and supporting the winding, the coil box is used for supporting the winding and containing cooling media such as liquid helium, the fixing frame of the annular structure is used for accurately positioning three groups of coils, the hollow cold shield is insulated and separated by high thermal conductivity metal and composite materials to reduce eddy current and Joule heat, the pull rod support is used for positioning the coil fixing frame, and the magnetic shield is used for reducing leakage magnetic flux generated by the winding and improving the magnetic field strength and uniformity in the central area of ​​the crucible.

Description

Technical Field

[0001] The invention relates to the technical field of superconducting magnets, in particular to a superconducting magnet and a refrigeration method for magnetically controlled CZ-pulling single crystals. Background Art

[0002] In recent years, with the rise of downstream industries such as semiconductors and photovoltaics, the market size of monocrystalline silicon has continued to expand, the market share has continued to increase, and the rapid development of the electronic communication semiconductor industry has driven the growth of the market size of electronic-grade monocrystalline silicon. With the rapid development of the integrated circuit industry, China's integrated circuit output has also continued to grow, providing a strong impetus for the development of the electronic-grade monocrystalline silicon industry. Monocrystalline silicon is divided into magnetron CZ method (MCZ), zone melting method (FZ) and epitaxy method according to the crystal growth method. Among them, magnetron CZ method and zone melting method are used to grow single crystal silicon rods, and epitaxy method is used to grow single crystal silicon thin film. Monocrystalline silicon grown by magnetron CZ method is mainly used in semiconductor integrated circuits, diodes, epitaxial wafer substrates, and solar cells; monocrystalline silicon grown by zone melting method is mainly used in the field of high-voltage and high-power controlled rectifier devices, including high-power transmission and transformation, electric locomotives, rectification, frequency conversion, mechatronics, energy-saving lamps, televisions and other products; monocrystalline silicon grown by epitaxy method is mainly used in the field of integrated circuits.

[0003] The magnet system is one of the key components of the single crystal silicon pulling furnace. It is used to provide a constant magnetic field with high intensity and a certain uniformity. It has a significant impact on the melt flow pattern during the growth of single crystal silicon and can improve the composition and impurity distribution of the crystal. In the process of growing single crystal silicon rods, the use of magnetic fields to suppress the convection of silicon melt can grow high-quality electronic-grade semiconductor single crystals. In order to achieve a smaller volume, lower power consumption, higher magnetic field intensity, higher magnetic field uniformity and higher magnetic field stability, the existing magnetic control CZ single crystal pulling furnaces used to pull large-sized single crystal silicon rods basically use superconducting magnets.

[0004] In terms of the superconducting magnet structure of the magnetron controlled single crystal pulling furnace:

[0005] In China, Liancheng Kex Technology Co., Ltd. applied for an invention patent: a magnetically controlled single crystal superconducting magnet coil, with the application publication number CN115762954A. The invention includes two groups of superconducting magnet coils with arc-shaped ring structures, and the two groups of superconducting magnet coils are respectively composed of six annular sub-coils. The superconducting magnet coil with a six-coil structure not only reduces the difficulty of winding the coil, but also improves production efficiency. At the same time, the six-coil structure has a more reasonable force, which can reduce the risk of the coil losing superconductivity during operation, thereby reducing the maintenance cost of the product.

[0006] The utility model patent applied for by Xi'an Juneng Superconducting Magnet Technology Co., Ltd. is a superconducting magnet coil and superconducting magnet device for magnetostrictive single crystal pulling. The application publication number is CN216450468U. The superconducting magnet coil in this patent is composed of two circular coils set at an angle. The circular coil used in this patent has a lower winding difficulty than the saddle coil, which is conducive to improving production efficiency. At the same time, the circular coil can provide a more uniform magnetic field under the same size, improving the efficiency of magnetostrictive single crystal production. The coils proposed in the above two patents do not involve cooling methods, cooling structures and positioning structures. Due to the differences in the thermal expansion coefficients of the coil conductor, coil insulation and coil support structure materials, the coils wound at room temperature will produce higher thermal stress when running at low temperatures. In addition, if the positioning structure of the coil is unreasonable, it may also produce higher mechanical stress, and the combined effect of electromagnetic force can easily cause the coil to lose supersonic force or even be damaged.

[0007] In China, Xi'an Juneng Superconducting Magnet Technology Co., Ltd. has applied for a utility model patent: a quadrangular coil distribution superconducting magnet for magnetron CZ single crystal, with the authorization announcement number CN210837338U. The coils in the patent are symmetrically distributed in a quadrangular shape with the center of the crucible, and the cold mass parts inside the superconducting magnet cryostat are conduction cooled by a GM refrigerator. Although the use of GM refrigerators to cool the coils can get rid of the plight of liquid helium resource shortage, the coils require a long cooling time and a long superconducting recovery time after quenching, which poses a great safety hazard for pulling electronic-grade single crystal silicon rods with a diameter of more than 300 mm.

[0008] In terms of magnetron CZ single crystal superconducting magnet structure and magnetic field shielding:

[0009] In China, Hangzhou Huixiang Electro-hydraulic Technology Development Co., Ltd. applied for an invention patent: a magnet structure for magnetically controlled vertical pulling of single crystals and a method for magnetically controlled vertical pulling of single crystals, application publication number: CN110129883A. The superconducting magnet of the present invention includes 4 groups of coils circumferentially arranged on the periphery of the crucible, each group of coils includes a main coil and a secondary coil, which are respectively used to generate the magnetic field required for single crystal pulling and reduce the leakage magnetic field. The 4 groups of coils are arranged in pairs around the center of the single crystal furnace crucible, and the magnetic field generated by the secondary coil offsets the magnetic field generated by the main coil outside to achieve the effect of magnetic field shielding. However, the patent adopts a toroidal coil and the outer diameter of the secondary coil is larger than the outer diameter of the main coil, resulting in a significant increase in the outer diameter of the coil cryostat. In addition, the design of the superconducting magnet structure does not take into account the complexity of the electrical connection of the coil and the complexity of the support structure design. Although the superconducting magnet with a main coil + secondary coil structure is theoretically feasible, it is difficult to implement in engineering.

[0010] In China, Xi'an Juneng Superconducting Magnet Technology Co., Ltd. applied for an invention patent: a magnetically controlled single crystal superconducting magnet device, application announcement number: CN114582583A. In addition to arranging a magnetic shielding iron yoke upper plate, a magnetic shielding iron yoke lower plate, and an iron yoke embedded in a magnetic shielding iron yoke tube outside the saddle-shaped coil, the patent also embeds an iron yoke in the coil, thereby reducing the leakage flux and the Lorentz force generated by the coil; Patent: A shielding structure for magnetically controlled single crystal superconducting magnets, application announcement number: CN215680369U. This patent has carried out multi-layer optimization on the single-layer magnetic shielding structure, further reducing the magnetic leakage capacity of the magnet. Compared with the single-layer magnetic shielding structure, the multi-layer magnetic shielding structure can effectively prevent the magnet stability problem caused by the magnetic force between the magnetic shielding and the coil.

[0011] In terms of magnetron CZ single crystal superconducting magnet structure and cooling:

[0012] In China, Xi'an Juneng Superconducting Magnet Technology Co., Ltd. applied for an invention patent: a method for conduction cooling of magnetron single crystal superconducting magnets, application publication number: CN215933291U, the coil skeleton of the superconducting magnet is connected to the conduction cooling device, the first cold head of the GM refrigerator is connected to the second cold head of the GM refrigerator by a heat pipe, and the refrigerant gas is filled into the heat pipe during the refrigeration step, and then the refrigerator is turned on for cooling. This patent mainly claims protection for the cooling method proposed for the magnetron single crystal superconducting magnet of the existing structure, and the proposed conduction cooling of the superconducting magnet using a combination of a GM refrigerator and a heat pipe has the problem of long pre-cooling time and long time required to restore the superconducting state after the superconducting winding is quenched; the invention patent applied by China Aerospace Science and Industry Flight Technology Research Institute (China Aerospace Sea Eagle Electromechanical Technology Research Institute): superconducting coil support device and superconducting magnet, application publication number: CN115132443A, the superconducting magnet of the present invention is composed of an inner dewar, a clamp and a spacer, and the coil support device is realized by using a headband with a through hole. The patent increases the contact area between the coil and the liquid helium through the through-hole headband, thereby improving the heat exchange efficiency between the coil and the liquid helium. However, the patent does not fully consider the displacement problem of the coil at low temperatures, and only using the headband to fix the coil cannot compensate for the cold contraction of the fixing frame and the coil in a low-temperature environment. Summary of the invention

[0013] In view of the deficiencies in the prior art, the present invention provides a superconducting magnet and a refrigeration method for magnetronically controlled CZ single crystal pulling, which solve the problems of long expected cooling time, temperature difference of coils, poor ability of coils to resist quench, and poor ability of coils to recover to a superconducting state after quenching in the current magnetronically controlled single crystal pulling furnace using a refrigerator as a cold source.

[0014] The technical solution adopted by the present invention to solve the above technical problems is: a superconducting magnet for magnetically controlled CZ single crystal pulling, comprising a coil assembly, a coil, a pull rod support, a refrigerator, a cold shield, an external vacuum dewar and a magnetic shield;

[0015] The coil group is composed of a plurality of pairs of annular coils which are arranged opposite to each other and whose central axes coincide with each other and intersect perpendicularly with the central axis of the single crystal furnace crucible;

[0016] The coil assembly comprises a coil box, a winding, a fixing frame, a Z-shaped fixing plate and a cooling tube. The winding is made of a circular cross-section superconducting wire with an outer insulating layer tightly wound on a circular ring frame made of an epoxy resin-based composite material. The winding is placed inside the coil box. The blank area between the winding and the metal coil box is a cooling medium channel. The coil box is fixed on a spoke-type bracket outside the fixing frame. A parallel structure is used between the cooling tube and the coil box and they are respectively sealed and welded at the bottom and top of the coil box. The coil box and the spoke frame of the fixing frame are connected by a Z-shaped fixing plate and bolts.

[0017] The pull rod support is made of carbon fiber or glass fiber reinforced epoxy resin composite material, is used for spatial positioning of the coil fixing frame and the cold shield position, and has a regulating function of compensating for cold shrinkage of internal components of the external vacuum dewar due to thermal stress.

[0018] The coil boxes at the same end of the paired coils are connected in parallel through an S-shaped or U-shaped bending cooling pipe with flexible compensation capability. The cooling medium is injected from the bottom of the coil box and flows through the winding from bottom to top. After the cooling medium fills the coil box, it flows out from the S-shaped or U-shaped bending cooling pipe welded to the upper part of the coil box to complete the cooling of the winding structure by the cooling medium. This method can greatly reduce the amount of cooling medium used for pre-cooling the coil, and the vaporized cooling medium is discharged from the top of the coil box after the coil quenches, thereby improving the ability of the coil to resist quenching and the ability to recover to a superconducting state after quenching.

[0019] A number of semicircular groove flow channels are provided on the inner surface of the coil box, and the direction of the flow channel is parallel to the direction of the cooling medium flow, ensuring that the cooling medium can freely pass through the inner flow channel of the coil box from the bottom to the top. The gap area formed by the different radii of the chamfer inside the coil box and the chamfer outside the winding further provides a channel for the flow of the cooling medium.

[0020] The cold screen is located inside the outer vacuum dewar and contains the fixing frame and the coil; the outer vacuum dewar is made of non-magnetic, low outgassing rate metal material, is located outside the cold screen and closes and contains the cold screen and its internal cold mass components; the magnetic shielding body is made of high magnetic permeability materials such as pure iron, and consists of an outer annular ring and upper and lower annular flanges, which are used to reduce the magnetic field leakage generated by the winding and improve the magnetic field strength in the central area of ​​the crucible of the magnetically controlled CZ single crystal pulling furnace.

[0021] As a further solution of the present invention: the skeleton and winding are both circular ring structures, the skeleton is made of non-magnetic, extremely low temperature resistant, insulating, and low temperature fatigue resistant epoxy resin-based composite materials, the winding is tightly wound around the circular ring skeleton by a superconducting wire with pre-tightening force, the superconducting wire adopts a circular cross-section wire with an outer insulating layer, the coil box is made of non-magnetic, extremely low temperature resistant, and low temperature fatigue resistant metal material, used to support the winding structure and hold the cooling medium used to cool the winding structure; the fixing frame is used for the precise positioning of the coil box together with the coil sealed and welded in the coil box in space, the outer side of the fixing frame has spokes, and the top of the outer end of each spoke has a curved baffle for installing the coil box together with the coil sealed and welded in the coil box, and the coil box and the cross are self-pretightened at low temperature and are detachably connected.

[0022] As a further solution of the present invention: the fixing frame is made of non-magnetic, extremely low temperature resistant, and low temperature fatigue resistant metal materials to form an annular ring structure. The coil box together with the coil welded in the coil box is installed on the outside of the coil fixing frame with multiple spokes. The arc surface of the outer edge of the spokes of the fixing frame is consistent with the curvature of the inner ring surface of the coil box, so the coil box can be firmly installed on the fixing frame. The outside of the annular ring structure of the fixing frame has a rectangular plate and arc-shaped fixing blocks at the four corners of the rectangular plate. Part of the rectangular plate is irregularly hollowed out to improve the strength and stability of the coil fixing frame, significantly reduce the weight, eliminate eddy current generation and improve the stability of the magnetic field. In addition to being used to position the coil box together with the coil welded in the coil box and the GM refrigerator, the fixing frame also acts as a heat sink for the coil box together with the coil welded in the coil box. The fixing frame is cooled by conduction cooling of the GM refrigerator to a certain temperature to achieve zero evaporation of the coil cooling medium.

[0023] As a further scheme of the present invention: the cold screen is made of a plurality of thin-walled high thermal conductivity tile-like metal plates and tile-like insulating partitions. The surfaces of the thin-walled high thermal conductivity tile-like metal plates of the cold screen are polished, gold-plated or silver-plated to reduce the surface emissivity. The cold screen is irregularly obliquely hollowed out, and the edges of two adjacent thin-walled high thermal conductivity tile-like metal plates are inserted into the grooves at the ends of the tile-like insulating partitions and then fastened together by screws or rivets. The insulating partition is made of epoxy resin-based fiber-reinforced composite materials and has the characteristics of being non-magnetic, insulating, low-temperature resistant and high-strength. The entire cold screen is composed of a plurality of thin-walled high thermal conductivity tile-like metal plates with holes and seams + a tile-like insulating partition structure, which can eliminate the eddy currents induced by the cold screen and the Joule heat generated.

[0024] As a further solution of the present invention: the pull rod support is divided into two parts, upper and lower, and is composed of a main pull rod, a fixed flange, a cold screen support, an insulation section, a connecting pin, screws, a sealing flange, a nut, a round cap and a disc spring. The pull rod made of composite materials such as carbon fiber or fiberglass can achieve precise positioning of the fixed frame, the coil box and the coil. The insulation section can effectively reduce the heat conduction from the pull rod. The disc spring connection that vertically crosses the insulation section and the lower pull rod can adjust the influence of thermal stress and Lorentz magnetic force on the fixed bracket.

[0025] A cooling method for a superconducting magnet used for magnetically controlled CZ single crystals comprises the following steps:

[0026] Step 1: Input hot nitrogen gas with gradually increasing temperature into the outer vacuum dewar until the nitrogen temperature reaches 105°C and keep it warm for a certain period of time to complete the dehumidification inside the outer vacuum dewar;

[0027] Step 2: After the internal dehumidification of the outer vacuum dewar, turn on the vacuum pump to evacuate the inner part of the outer vacuum dewar. When the pressure inside the outer vacuum dewar is lower than 10 -3 Turn off the vacuum pump at Pa;

[0028] Step 3: The hot nitrogen gas with gradually rising temperature is transported to the coil box through the cooling pipe. According to the humidity of the coil box, the hot nitrogen gas transport time is controlled to dehumidify the coil box;

[0029] Step 4: After the dehumidification of the coil box is completed, the nitrogen gas with gradually lowered temperature is transported to the coil box through the cooling pipe. After the temperature of the coil box is lowered to near room temperature, the refrigerator is started to cool the coil;

[0030] Step 5: After the coil temperature drops to the liquid nitrogen temperature under the combined action of nitrogen and the refrigerator, liquid helium begins to flow into the cooling pipe. When the liquid helium covers the entire winding and reaches the specified rated volume, check that the overpressure relief valve of the coil box is in normal working condition, then close the liquid helium filling valve to stop the input of liquid helium, check that the temperature of the winding is stable at 4.2K, and then energize the excitation.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The present invention comprises a plurality of groups of circular coils which are arranged opposite to each other and whose central axes coincide with each other, and forms a coil group, and improves the strength and uniformity of the magnetic field at the center of the single crystal furnace by superposition of magnetic fields; the present invention adopts a coil box made of non-magnetic, extremely low temperature resistant and low temperature fatigue resistant metal materials, which is used to support the winding and contain liquid helium and other cooling media for cooling the winding, thereby reducing the use of liquid helium and ensuring that the cooling efficiency does not decrease, thereby greatly reducing the operating cost of the superconducting magnet.

[0033] (2) The inner surface of the coil box of the present invention is provided with a plurality of semicircular parallel groove flow channels. Under this condition, the volume of the coil box will be smaller, the amount of cooling medium used will be lower, and the chamfer radius inside the coil box is different from the chamfer radius outside the coil, which further provides more channels for the flow of cooling medium; the cooling tube with flexible compensation capability connected to the coil box of the present invention can effectively prevent thermal expansion and contraction from causing damage to the welding joint between the cooling tube and the coil box, and can accelerate the cooling of the coil compared with the structure of using a set of low-temperature containers to contain multiple coils.

[0034] (3) The annular ring fixing frame with a hollow rectangular plate and a divergent spoke reinforcement structure of the present invention can not only improve the strength and stability of the fixing frame and reduce the weight, but also accurately position the coil box together with the coil sealed and welded in the coil box and eliminate possible eddy currents; the cold shield of the present invention with a hollow thin-walled high thermal conductivity metal plate with a polished surface and gold or silver plating to reduce the surface emissivity, the thin-walled high thermal conductivity tile-shaped metal plate and the insulating partition structure can greatly reduce the eddy current induced by the cold shield and the Joule heat generated, thereby improving the superconducting stability of the coil.

[0035] (4) The left and right end surfaces of the insulating partition of the present invention are T-slot structures, and the cold shield with tile-like structural units is inserted into the T-slot from the top to the bottom edges on the left and right sides of the ring and then connected by rivets or bolts. While reducing the eddy current induced by the cold shield and the Joule heat generated, the regularity of the appearance is also ensured, which is conducive to realizing the compact structure of the superconducting magnet. The cold shield and the fixing frame of the present invention are respectively positioned on the flange of the outer vacuum dewar through a plurality of tie rods made of composite materials such as carbon fiber or fiberglass. The hollow thin-walled cylindrical tube support of the cold shield is sleeved on the main tie rod and does not contact the dewar to fix the cold shield. The hollow thin-walled cylindrical tube support can not only play a good supporting role, but also block the low-temperature heat load generated by heat conduction on the upper and lower cold mass components of the outer vacuum dewar.

[0036] (5) The present invention uses a magnetic shielding body made of high magnetic permeability materials such as pure iron to reduce the magnetic field leakage generated by the winding and improve the magnetic field strength in the central area of ​​the crucible of the magnetron CZ single crystal pulling furnace; in addition, the magnetic shielding body is attached to the upper and lower flanges and the outer ring of the outer vacuum dewar and the two are fastened by screws, which plays a role in improving the strength of the outer vacuum dewar; the lifting ears welded on the outer surface of the magnetic shielding body can realize the lifting of the entire superconducting magnet. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0038] Figure 1 It is a schematic diagram of the overall structure of a superconducting magnet used for magnetically controlled CZ-pull single crystals according to the present invention;

[0039] Figure 2It is a schematic diagram of a partial disassembly of a superconducting magnet used for magnetically controlled CZ-pull single crystal of the present invention;

[0040] Figure 3 It is a cross-sectional schematic diagram of a superconducting magnet coil used for magnetically controlled CZ-pull single crystal of the present invention;

[0041] Figure 4 It is a schematic diagram of the disassembly of a pull rod support member of a superconducting magnet used for magnetically controlled CZ single crystal pulling of the present invention;

[0042] Figure 5 It is a schematic diagram of the disassembly of a superconducting magnet pull rod support member for magnetically controlled CZ single crystal pulling according to the present invention;

[0043] Figure 6 A schematic diagram of a vacuum dewar and a magnetic shielding body outside a superconducting magnet for magnetically controlled CZ-pulling single crystals according to the present invention;

[0044] Figure 7 Schematic diagram of the magnetic field of the Helmholtz coil of the present invention.

[0045] In the figure:

[0046] 1. Coil assembly; 11. Coil; 111. Coil box; 112. Winding; 113. Blank area; 12. Fixing frame; 13. Z-shaped fixing plate; 14. Cooling pipe;

[0047] 2. Upper tie rod support; 21. Fixed flange 1; 22. Main tie rod 1; 23. Cold screen support 1; 24. Heat insulation section 1; 25. Connecting pin 1; 26. Bolt; 27. Sealing flange; 28. Nut 1; 29. ​​Round cap;

[0048] 3. Lower tie rod support; 31. Main tie rod 2; 32. Fixed flange 2; 33. Cold screen support 2; 34. Connecting pin 2; 35. Heat insulation section 2; 36. Disc spring; 37. Nut 2; 38. X-shaped fixing piece;

[0049] 4. Refrigerator; 41. Primary cold head; 42. Secondary cold head;

[0050] 5. Cold shield; 51. Inner wall of cold shield; 52. Outer wall of cold shield; 53. Upper flange of cold shield; 54. Lower flange of cold shield; 55. Insulating partition;

[0051] 6. External vacuum dewar; 61. External wall of vacuum dewar; 62. Internal wall of vacuum dewar; 63. Upper flange of vacuum dewar; 64. Lower flange of vacuum dewar;

[0052] 7. Magnetic shielding body; 71. Outer annular ring of the magnetic shielding body; 72. Upper annular flange at the end of the magnetic shielding body; 73. Lower annular flange at the end of the magnetic shielding body. DETAILED DESCRIPTION

[0053] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0054] Embodiment 1:

[0055] See also Figure 1-7 , a superconducting magnet for magnetically controlled CZ single crystal, comprising a coil assembly 1, an upper pull rod support 2, a lower pull rod support 3, a refrigerator 4, a cold shield 5, an outer vacuum dewar 6 and a magnetic shield 7;

[0056] The coil group 1 is composed of four pairs of circular coils 11 which are arranged opposite to each other and whose central axes coincide with each other. The coil group 1 includes a coil box 111, a winding 112, a fixing frame 12, a Z-shaped fixing plate 13 and a cooling tube 14. The winding 112 is a circular cross-section superconducting wire with an outer insulating layer tightly wound on a circular ring frame made of epoxy resin-based composite material. The winding 112 is placed inside the coil box 111. The blank area 113 between the winding 112 and the metal coil box 111 is a cooling medium channel. The coil box 111 is fixed on a spoke-type bracket outside the fixing frame 12. The coil box 111 is fixed by a total of eight fixing frames 12, thereby indirectly ensuring that the position of the coil in space meets the requirements for single crystal rod drawing; a parallel structure is used between the cooling tube 14 and the coil box 111 and they are sealed and welded at the bottom and top of the coil box 111 respectively.

[0057] A plurality of tile-shaped cold shield 5 structural units are inserted into the T-slot of the insulating partition 55 and connected by bolts or rivets to form a complete cold shield 5; the fixing frame 12, the coil box 111, the coil sealed and welded in the coil box 111, and the cooling pipe 14 connected to the coil box 111 are contained by the cold shield 5; the middle thickness layer of the insulating partition 55 and the middle thickness layer of the tile-shaped hollow thin-walled metal plate have the same central axis and diameter; the cold shield 5 includes a cold shield inner wall 51, The outer wall 52 of the cold screen, the upper flange 53 of the cold screen, the lower flange 54 of the cold screen, the inner wall 51 of the cold screen are fitted with the outer wall 61 of the vacuum dewar of the external vacuum dewar 6, the outer wall 52 of the cold screen is fitted with the inner wall 62 of the vacuum dewar, the inner wall 51 of the cold screen and the outer wall 52 of the cold screen are arranged in parallel, and one end of the inner wall 51 of the cold screen and the outer wall 52 of the cold screen are connected to the upper flange 53 of the cold screen, and the other end of the inner wall 51 of the cold screen and the outer wall 52 of the cold screen are connected to the lower flange 54 of the cold screen.

[0058] The external vacuum dewar 6 made of non-magnetic metal material is located outside the cold shield 5 and encloses the cold shield 5 and its internal cold mass components; the cold shield 5 and the fixing frame 12 are respectively fixed to the annular flange 72 on the end of the magnetic shield and the lower flange 64 of the vacuum dewar through the super-strong upper tie rod support 2 and the lower tie rod support 3 made of carbon fiber or fiberglass composite materials located at the upper and lower parts of each.

[0059] Specifically, the upper pull rod support 2 includes a fixed flange 21 connected to the flange on the fixing frame 12, and is fixed to the annular flange 72 on the end of the magnetic shielding through a main pull rod 22, an insulation section 24, a connecting pin 25, a bolt 26, and a nut 28. The cold screen support 23 is hung in the middle part of the main pull rod 22, and the sealing flange 27 is wrapped by a round cap 29 on the upper sealing flange 27, bolts 26 and nuts 28 of the annular flange 72 on the end of the magnetic shielding. One end of the main pull rod 22 is rotatably connected to the fixing frame 12, and the other end of the main pull rod 22 is connected to the bolt 26 through a connecting pin 25. A cold screen support 23 is installed on the main pull rod 22, and one end of the bolt 26 is fixed with a sealing flange 27, and the end of the bolt 26 is installed with a nut 28; the lower pull rod support 3 includes a main pull rod 21, a fixed flange 22, a cold screen support 23, and a sealing flange 27. Support 23, connecting pin 234, insulation section 235, disc spring 36, nut 237 and I-shaped fixing piece 38, one end of main pull rod 231 is rotatably connected to fixing flange 232, the other end of main pull rod 231 is rotatably connected to insulation section 235, and insulation section 235 passes through I-shaped fixing piece 38 and is connected to nut 237, disc spring 36 is installed between nut 237 and I-shaped fixing piece 38, the I-shaped fixing piece 38 at the lower part of lower pull rod support 3 is fixed on the lower flange 64 of vacuum dewar, disc spring 36 is added to nut 237 and hung on the middle cold screen support 23 of main pull rod 12, which can not only play a good supporting role, but also block the low-temperature heat load generated by heat conduction of the upper and lower cold mass components of the outer vacuum dewar 6, and fine-tune the influence of thermal stress and Lorentz magnetic force on the fixing frame 12 through the disc spring 36 connection of the lower pull rod support 3.

[0060] The magnetic shielding body 7 is composed of an outer circular ring 71 of the magnetic shielding body, an upper circular flange 72 of the magnetic shielding end, and a lower circular flange 73 of the magnetic shielding end. One end of the outer circular ring 71 of the magnetic shielding body is provided with an upper circular flange 72 of the magnetic shielding end, and the other end of the outer circular ring 71 of the magnetic shielding body is provided with a lower circular flange 73 of the magnetic shielding end. The inner surface of the outer circular ring 71 of the magnetic shielding body is in contact with the outer surface of the inner wall 62 of the vacuum dewar of the outer vacuum dewar 6, and the countersunk magnetic conductive bolt passes through the through hole of the outer circular ring 71 of the magnetic shielding body and is threaded with the blind hole on the outer side of the inner wall 62 of the vacuum dewar Connection; the lower surface of the upper annular flange 72 at the end of the magnetic shielding is fitted with the upper bottom surface of the upper flange 63 of the vacuum Dewar of the external vacuum Dewar 6, and the countersunk magnetic screws pass through the through holes of the upper annular flange 72 at the end of the magnetic shielding and are threadedly connected with the blind holes on the upper bottom surface of the upper flange 63 of the vacuum Dewar of the external vacuum Dewar 6; the upper surface of the lower annular flange 73 at the end of the magnetic shielding is fitted with the lower bottom surface of the lower flange 64 of the vacuum Dewar of the external vacuum Dewar 6, and the countersunk magnetic screws pass through the through holes of the lower annular flange 73 at the end of the magnetic shielding and are threadedly connected with the blind holes on the lower bottom surface of the lower flange 64 of the vacuum Dewar.

[0061] The first-stage cold head 41 and the second-stage cold head 42 of the refrigerator 4 pass through the magnetic shielding body 7 and the external vacuum dewar 6 respectively. The first-stage cold head 41 is connected to the cold shield 5 through a flexible high thermal conductivity structural part, and the second-stage cold head 42 is connected to the fixing frame 12 through a flexible high thermal conductivity structural part; the flexible high thermal conductivity structural part is a copper braided belt.

[0062] Embodiment 2:

[0063] A cooling method for a superconducting magnet used for magnetically controlled CZ single crystals comprises the following steps:

[0064] Step 1: Input hot nitrogen gas with gradually increasing temperature into the outer vacuum Dewar 6 until the nitrogen temperature reaches 105°C and keep it warm for a certain period of time to complete the dehumidification inside the outer vacuum Dewar 6;

[0065] Step 2: Turn on the vacuum pump to evacuate the interior of the external vacuum dewar 6. When the pressure inside the external vacuum dewar 6 is lower than 10 - 3 Turn off the vacuum pump at Pa;

[0066] Step 3: The hot nitrogen gas with gradually rising temperature is input into the coil box 111 through the lower cooling pipe 14 and output from the upper cooling pipe 14 until the input hot nitrogen gas temperature reaches 105° C. and is kept warm for a certain period of time to complete the dehumidification inside the coil box 111;

[0067] Step 4: After the dehumidification of the coil box 111 is completed, the nitrogen gas with gradually decreasing temperature is input into the coil box 111 through the lower cooling pipe 14 and output from the upper cooling pipe 14 until the temperature of the nitrogen gas input into the coil box 111 is reduced to near room temperature, and then the refrigerator 4 is started to cool the coil box 111 and the coil 11;

[0068] Step 5: After the temperature of the coil 11 is reduced to the liquid nitrogen temperature under the joint action of the refrigerator 4, liquid helium is input into the coil box 111 through the lower cooling pipe 14. When the liquid helium soaks the entire winding 112 and reaches the specified rated volume, check that the overpressure relief valve of the coil box 111 is in normal working state, then close the liquid helium filling valve to stop the input of liquid helium, check that the temperature of the winding 112 is stable at 4.2K, and then energize the excitation.

[0069] The basic principle of the coil assembly 1 of the present invention conforms to the Helmholtz coil theory. The Helmholtz coil is composed of a pair of circular coils with a radius R, coaxially placed, and a spacing a equal to the radius R. Because it has a simple structure and can generate a magnetic field with good uniformity, for the central axis magnetic field of the two coils, Figure 7 As shown, from the Biot-Savart law, we can get the unit measurement of the magnetic field: the magnetic field strength at the center of a single-turn coil ,make (where a is the distance between the two coils), then for the tile-shaped unit in the cold shield 5, the eddy current satisfies the continuity condition. Since the wall thickness is much smaller than the length and width of the cold shield, the eddy current can be described by surface current, so the surface current density has only the component parallel to the surface of the cold shield 5.

[0070] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. A superconducting magnet for magnetically controlled CZ single crystal pulling, characterized in that: The invention comprises a coil assembly (1), a coil (11), a pull rod support, a refrigerator (4), a cold shield (5), an external vacuum dewar (6) and a magnetic shield (7); the coil assembly (1) comprises a plurality of pairs of annular coils (11) which are arranged opposite to each other and whose central axes coincide with each other and which intersect perpendicularly with the central axis of the single crystal furnace crucible; the coil assembly (1) comprises a coil box (111), a winding (112), a fixing frame (12), a Z-shaped fixing plate (13) and a cooling tube (14); the winding (112) is made of a circular cross-section superconducting wire with an outer insulating layer tightly wound on a circular ring frame made of an epoxy resin-based composite material; the winding (112) is placed inside the coil box (111); the winding (112) and the The blank area (113) between the metal coil boxes (111) is a cooling medium channel. The coil box (111) is fixed to a spoke-type bracket on the periphery of the fixing frame (12). A parallel structure is used between the cooling tube (14) and the coil box (111) and the cooling tube (14) is respectively sealed and welded to the bottom and top of the coil box (111). The coil box (111) and the spoke frame of the fixing frame (12) are connected by a Z-shaped fixing plate (13) and bolts. The pull rod support is made of a carbon fiber or glass fiber reinforced epoxy resin composite material, is used for spatial positioning of the coil fixing frame (12) and the cold shield (5), and has a regulating function of compensating for cold shrinkage of internal components of the external vacuum dewar (6) due to thermal stress. The cold shield (5) is located inside the outer vacuum dewar (6) and contains a fixing frame (12), a coil box (111), a coil (11) sealed and welded inside the coil box (111), and an S-shaped or U-shaped bending cooling pipe (14) connected to the coil box (111). The cold shield (5) structural unit composed of a plurality of tile-shaped hollow thin-walled high thermal conductivity metal plates in the annular direction is inserted into the T-slot of the insulating partition (55) and connected by bolts or rivets to form a complete cold shield (5); The middle thickness layer of the insulating partition (55) and the middle thickness layer of the tile-shaped hollow thin-walled metal plate have the same central axis and diameter; The magnetic shielding body (7) is composed of an outer magnetic shielding ring (71), an upper magnetic shielding end ring flange (72), and a lower magnetic shielding end ring flange (73), and the magnetic shielding body (7) is made of a high magnetic permeability material; the magnetic shielding body (7) contains an external vacuum dewar (6); The coil boxes (111) at the same end of the paired coils (11) are connected in parallel via an S-shaped or U-shaped bent cooling pipe (14) having a flexible compensation capability. A cooling medium is initially injected from the bottom of a coil box (111) at the same end of the paired coils (11). The cooling medium flows through the winding (112) from bottom to top. After the cooling medium fills the coil box (111), it flows out from the S-shaped or U-shaped bent cooling pipe (14) welded to the upper part of the coil box (111), completing the immersion cooling of the winding structure by the cooling medium. The internal chamfer radius of the coil box (111) is different from the external chamfer radius of the coil (11); The magnetic shielding body (7) is composed of an outer magnetic shielding ring (71), an upper magnetic shielding end ring-shaped flange (72) and a lower magnetic shielding end ring-shaped flange (73). The inner surface of the outer magnetic shielding ring (71) is in contact with the outer surface of the vacuum dewar inner wall (62) of the outer vacuum dewar (6). The countersunk magnetic conductive bolt passes through the through hole of the outer magnetic shielding ring (71) and is threadedly connected with the blind hole on the outer side of the vacuum dewar inner wall (62). The lower surface of the upper magnetic shielding end ring-shaped flange (72) is in contact with the vacuum dewar inner wall (62) of the outer vacuum dewar (6). The upper bottom surface of the upper flange (63) of the hollow dewar is fitted, and the countersunk magnetic conductive screw passes through the through hole of the upper annular flange (72) of the magnetic shielding end and is threadedly matched with the blind hole on the upper bottom surface of the upper flange (63) of the vacuum dewar of the external vacuum dewar (6); the upper surface of the lower annular flange (73) of the magnetic shielding end is fitted with the lower bottom surface of the lower flange (64) of the vacuum dewar of the external vacuum dewar (6), and the countersunk magnetic conductive screw passes through the through hole of the lower annular flange (73) of the magnetic shielding end and is threadedly matched with the blind hole on the lower bottom surface of the lower flange (64) of the vacuum dewar; The tie rod support comprises an upper tie rod support (2) and a lower tie rod support (3); an external vacuum dewar (6) made of non-magnetic metal material is located outside the cold screen (5) and encloses and contains the cold screen (5) and its internal cold mass components; the cold screen (5) and the fixing frame (12) are fixed to the annular flange (72) on the end of the magnetic shield and the lower flange (64) of the vacuum dewar through the super-strong upper tie rod support (2) and the lower tie rod support (3) made of carbon fiber or glass fiber reinforced plastic composite materials located at the upper and lower parts of the respective parts; The upper tie rod support (2) includes a fixing flange (21) connected to the upper flange of the fixing frame (12), and is fixed to the annular flange (72) on the end of the magnetic shield through a main tie rod (22), a heat insulation section (24), a connecting pin (25), a bolt (26), and a nut (28). A cold shield support (23) is hung on the middle part of the main tie rod (22). The sealing flange (27) is wrapped by a round cap (29) on the upper sealing flange (27), the bolt (26) and the nut (28) of the annular flange (72) on the end of the magnetic shield. The lower part of the lower tie rod support (3) is fixed to the lower flange (64) of the vacuum dewar, and the nut (37) is provided with a disc spring (36) and is hung on the cold shield support (23) in the middle of the main tie rod (22).

2. A superconducting magnet for magnetically controlled CZ single crystal pulling according to claim 1, characterized in that: The refrigerator (4) comprises a primary cold head (41) and a secondary cold head (42), wherein the primary cold head (41) and the secondary cold head (42) respectively pass through a magnetic shielding body (7) and an external vacuum dewar (6), the primary cold head (41) is connected to a cold shield (5) via a flexible high thermal conductivity structural member, and the secondary cold head (42) is connected to a fixing frame (12) via a flexible high thermal conductivity structural member.

3. A superconducting magnet for magnetically controlled CZ single crystal pulling according to claim 2, characterized in that: The flexible, highly thermally conductive structural member is a copper braid.

4. A method for cooling a superconducting magnet for magnetically controlled CZ single crystal according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: step 1: inputting hot nitrogen gas with gradually increasing temperature into the external vacuum dewar (6) until the temperature of the nitrogen gas reaches 105°C and keeping the temperature for a certain period of time to complete the dehumidification inside the external vacuum dewar (6); step 2: turning on the vacuum pump to evacuate the inside of the external vacuum dewar (6), and turning off the vacuum pump when the pressure inside the external vacuum dewar (6) is lower than 10-3Pa; step 3: inputting the hot nitrogen gas with gradually increasing temperature into the coil box (111) through the lower cooling pipe (14) and outputting it from the upper cooling pipe (14) until the temperature of the input hot nitrogen gas reaches 105°C and keeping the temperature for a certain period of time to complete the dehumidification inside the coil box (111); step 4: after the dehumidification of the coil box (111) is completed, the nitrogen gas with gradually decreasing temperature is outputted from the lower cooling pipe (14). The cooling tube (14) inputs the nitrogen into the coil box (111) and outputs it from the upper cooling tube (14), until the temperature of the nitrogen input into the coil box (111) drops to near room temperature, and then the refrigerator (4) is started to cool the coil box (111) and the coil (11); step five: after the temperature of the coil (11) drops to the temperature of liquid nitrogen under the joint action of the refrigerator (4), liquid helium is input into the coil box (111) from the lower cooling tube (14), and when the liquid helium soaks the entire winding (112) and reaches a specified rated volume, after checking that the overpressure relief valve of the coil box (111) is in a normal working state, the liquid helium filling valve is closed to stop the input of liquid helium, and after checking that the temperature of the winding (112) is stable at 4.2K, the magnetization is energized.

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