GB2498961A

GB2498961A - Methods of joining superconducting wires

Info

Publication number: GB2498961A
Application number: GB1201679.6A
Authority: GB
Inventors: Mark David Blumenthal; M Hamed Lakrimi
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2012-02-01
Filing date: 2012-02-01
Publication date: 2013-08-07
Also published as: GB201201679D0

Abstract

A method of joining superconducting wires 50, each having at least one superconducting filament 52 (e.g. niobium-titanium, NbTi) enclosed within an outer 54 (e.g. copper, Cu), comprises: stripping a length of the outer 54 from the filament(s) 52 of each wire 50, thereby exposing the filaments 52 over the length; placing the exposed length of the filaments 52 into an electrically conducting collar 56; and applying pressure and mechanical oscillation to the collar 56 in an ultrasonic welding process. The superconducting filaments 52 are thereby pressed into mechanical and electrical contact and contaminants are removed from the surface of the filaments. The electrically conducting material of the collar 56 has a melting point that is less than that of the superconducting filaments 52 and an electrical conductivity greater than that of the superconducting filaments 52 at the temperature of operation of the ultrasonic welding process. The application of pressure and mechanical oscillation may be performed by placing the collar 56 containing the filaments 52 onto a surface of an anvil structure 60 of an ultrasonic welding machine, whereby an actuator (e.g. sonotrode 62) bears upon the collar and oscillates in a plane 64 parallel or perpendicular to a surface of the anvil structure 60.

Description

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METHODS FOR FORMING SUPERCONDUCTING JOINTS

The present invention relates to methods for joining together filaments of niobium-titanium conductor in a superconductive joint. Niobium-titanium (NbTi) filaments are 5 commonly used in superconductive wire, as used for example in producing magnets for MRI imaging systems. The method of the present invention may also be applied to the joining together of filaments of other superconducting materials.

Conventional methods of forming superconductive joints between NbTi filaments 10 involve stripping the filaments with hydrofluoric acid, and embedding them in a jointing material such as Wood's metal.

Wood's metal contains lead and cadmium. Known health risks result from exposure to hydrofluoric acid. The use of lead containing materials is sought to be reduced, 15 and eliminated where possible. For example, a recent EU directive has an objective of reducing the use or dependence on lead compounds.

Some conductive materials may be joined by simple methods, such as ultrasonic welding. Ultrasonic welding is commonly used to provide low resistance joints 20 between two copper conductors. This has, however, been found difficult to apply to joining NbTi. Without wishing to be bound by any particular theory, the inventors believe that this is due to the much higher melting point of the NbTi material, almost double that of pure copper. Typical currently available ultrasonic welding machines are incapable of providing enough amplitude to directly ultrasonically weld NbTi parts 25 together. Similar difficulties may be encountered with other materials, including superconducting ones.

Conventional joints between NbTi filaments of superconducting wire may be formed by crimping. A deformable collar, typically of copper, is placed around the filaments 30 of the wires to be joined. Mechanical deformation is applied to the collar to force the filaments into closer contact with one another. A cross-section through a typical joint of this type is illustrated in Fig. 1. As can be seen, the filaments are not pressed particularly closely together, and oxide is understood to form on the surfaces of the NbTi filaments, providing electrical resistance between the joined superconducting 35 wires.

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It is important to note that the crimping procedure does not alter the metallurgical composition - the atomic structure - of the superconducting filaments. Crimping must not cause mechanical damage to the filaments. For these reasons, only a limited pressure and I or heating may be used in the crimping of superconducting 5 joints. This limited pressure and / or heating may be found insufficient to provide good electrical contact between filaments, and may not prevent the formation of oxides or other surface contaminants on the surfaces of the filaments, which introduces electrical resistance.

10 The present invention provides methods which enable ultrasonic welding methods to be used to join superconducting wires, including NbTi-based superconducting wires.

The present invention accordingly provides methods as defined in the appended claims.

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The above, and further, objects, advantages and characteristics of the present invention will become more apparent from the following discussion of certain embodiments thereof, with reference to the accompanying drawings, wherein:

20 Fig. 1 shows a photograph of a cross-section through a conventional crimped joint used to join NbTi filaments together;

Fig. 2 shows a photograph of a cross-section through a structure at an early stage in a joining method of the present invention;

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Fig. 3 shows a photograph of a cross-section through a structure at a later stage in a joining method of the present invention;

Fig. 4 shows a photograph of a cross-section through a finished joint structure 30 formed in a joining method of the present invention; and

Figs. 5A-5G show steps in a process according to the present invention.

While conventional methods of joining NbTi superconducting filaments by ultrasonic 35 welding have generally been unsuccessful, the present invention provides a method

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for joining NbTi filaments using an ultrasonic welding technique, which is relatively simple to perform and which produces effective joints.

Both the melting temperature and the electrical resistivity of NbTi at room 5 temperature and above are much higher than that of copper, and while relatively small ultrasonic welding machines can be used successfully to ultrasonically weld copper to copper, attempts to ultrasonically weld NbTi to NbTi have in general been unsuccessful. The present inventors believe that this is due to the amplitude of the signal required to soften the NbTi requiring much more power to complete an 10 ultrasonic weld than typically available in ultrasonic welding machines..

According to an example embodiment of the present invention, NbTi filaments to be joined are placed within a collar which comprises copper at least on its outer surface, and an ultrasonic welding technique is applied to the copper collar as in a 15 conventional method of ultrasonically welding copper.

Preferably, the copper collar has a layer of superconducting material on its inner surface. That superconducting layer may serve to shield the NbTi joint produced according to the present invention from exposure to the background field it is located 20 in. A joint shield is a superconductor that shields completely a background magnetic field. When a joint is surrounded by a joint shield, it sees truly zero magnetic field intensity; this effect is called the Meissner effect. The layer of superconducting material may be a layer of NbTi or other superconductor.

25 An example method according to an embodiment of the present invention will now be described with reference to Figs. 5A-5E.

In this example, two wires will be joined. Each wire comprises a number of NbTi filaments within a copper outer. As is well known in the art, the NbTi filaments 30 provide the superconducting path, while the copper outer provides mechanical resilience, improved thermal behaviour and a path for electrical conduction in case of a quench in one or more of the filaments.

In preparation for jointing, the copper outer also known as a matrix material must be 35 removed from the NbTi filaments in each wire. This may be achieved by dipping a desired length at the end of each wire into nitric acid for long enough that the

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immersed portion of the copper outer dissolves in the nitric acid. Alternatively, an electrolysis process may be used, for example in sulphuric acid. It is also possible to remove the copper matrix by dipping the wire in molten tin. Each wire 50 will then appear as shown in Fig. 5A, with a number of NbTi filaments 52 extending beyond 5 the end of copper outer 54.

Fig. 5B shows an axial cross-section through a collar 56 suitable for use in the method of the present invention. The collar may be a plain copper cylinder, or may have a lining 58 of NbTi on the inner surface or other superconductor material. The 10 collar may have a circular cross-section, or may be of any other suitable cross-section, for example oval or hexagonal.

For mechanical considerations, it is preferable that the collar 56 has a large enough bore b that the copper outer 54 of the wires to be joined may be accommodated 15 within the collar. In an example, a circular cylindrical copper collar is used, with an outer diameter of less than 5mm.

As shown in Fig. 5C, the joint may then be assembled. A wire 50 is introduced into each end of the collar 56, such that a part of the copper outer 54 of each wire passes 20 into the collar 56, and the NbTi filaments 52 of the wires overlap.

This structure is then placed into an ultrasonic welding machine. If desired, the collar 56 may be crimped first, to retain the wires in position as they are transferred to the ultrasonic welding machine. Such crimping may be done onto the copper outers 54, 25 to protect the NbTi filaments from damage.

Fig. 5D schematically illustrates the joint of Fig. 5C placed into an ultrasonic welding machine 59 ready for treatment. The collar 56 carrying wires 50 is placed on an anvil structure 60, while a sonotrode 62 is brought to bear upon the collar with a pressure 30 of several bar (hundreds of kilopascals). The ultrasonic welding equipment causes the sonotrode 62 to oscillate, either parallel 64 or perpendicular to a surface of the anvil structure 60, depending on the design of the equipment. The mechanical oscillation of the sonotrode causes the collar 56 and the filaments 52 to be rubbed together, resulting in localised friction between the copper and the NbTi filaments, 35 and between the NbTi filaments themselves. The combination of rubbing together of filaments, and filaments with the collar, and pressure applied by the sonotrode cleans

any surface contamination from the filaments and presses them into intimate contact, forming molecular bonds between the filaments, and between the filaments and the collar.

5 In a conventional copper-to-copper weld, this oscillation causes the two copper wires to form a molecular bond in a cold weld. In the joint of the present invention, the NbTi filaments are rubbed together and the process removes any oxides and other contaminants from their surfaces. The mechanical pressure applied by the ultrasonic welding equipment increases the area of contact between the filaments. The 10 filaments deform into closer packing with greater contact surface area between the filaments. The copper of the collar 56 may soften and permeate between filaments 52 and provide mechanical strength and protection of the NbTi filaments, but the electrical joint is made between NbTi filaments in a mechanical and molecular bond contact with one another. The NbTi filaments are pressed into tighter contact by the 15 operation of the ultrasonic welding equipment, and some deformation of the NbTi filaments may result. The required settings for the ultrasonic welding equipment will depend on many factors, including features of the wire and of the equipment. However, the correct settings for any particular combinations of wires, collar, collar lining and equipment may be rapidly determined by simple trial-and-error by 20 someone familiar with the ultrasonic welding apparatus and process.

The copper collar lowers the electrical resistance between the anvil and the sonotrode of the ultrasonic welding machine. The copper also enables mechanical motion to a greater extent than would be permitted by the higher-melting point NbTi 25 alone. The copper collar retains the NbTi filaments in a confined space, ensuring that friction between the filaments can remove surface contamination from the filaments.

Fig. 5E schematically illustrates a completed joint. The collar 56 has been 30 compressed onto the filaments 52. It may also, to some extent, have been compressed onto the copper outers 54. Retaining the copper outers in this way will improve the mechanical strength of the joint. The NbTi filaments 52 are held in close mechanical end electrical contact with one another, and are molecularly joined by the process of the present invention.

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Fig. 5F schematically illustrates a radial cross-section through the joint, along the plane VF shown in Fig. 5E. The collar 56 is crushed onto the filaments 52 (only a few shown in the drawing) which are pressed into close contact with one another. The filaments themselves may be deformed due to the pressure and action of the 5 ultrasonic welding process. In some embodiments, the copper collar 56 is provided with a lining of a superconducting material, for example NbTi. This lining may be helpful in the finished joint, by improving the mechanical retention of the filaments 52, by shielding the joint from a magnetic field or by providing superconducting material electrically connecting certain filaments 52 together. In the finished structure, it is 10 believed that the filaments 52 are cold welded together by molecular bonds, and are protected from oxidisation by the material of the collar 56.

In alternative arrangements, two or more wires may be joined, and two or more wires may enter the copper collar 56 at the same end. Preferably, the bore b of the collar 15 36 is sufficiently large to accommodate the copper outers 54 of the wires 50. Fig. 5G shows a schematic cross-section through such a joint, by way of example.

It is also possible to do the jointing in two stages, using a relatively small bore collar for ultrasonically welding the filaments together, followed by a collar of larger bore for 20 retaining the copper outers 54.

An example of such a procedure is illustrated in Figs. 6A and 6B. First, a collar 70 of relatively small bore diameter is used to ultrasonically bond the filaments 52 of at least two wires 50 together. Then a second collar 72, with a larger bore diameter is 25 provided over the outers 54. The second collar may then be either crimped or also ultrasonically welded to hold the outers 54 together. As shown in Fig. 6A, a gap 74 may be left between the collars, or the smaller bore collar 70 may abut the end of outers 54, as shown in Fig. 6B. Alternatively, the larger-bore collar may be crimped or ultrasonically welded into place before the small bore collar is welded into place. 30 An oversleeve material may be provided, extending over part of the small-bore collar and part of the larger-bore collar for added mechanical strength. This oversleeve may be a further collar of copper or similar, or a thermoplastic, such as a heat-shrink material.

35 Fig. 7 shows a variant of such an arrangement. Here, filaments 52 are ultrasonically welded together with small-bore collar 70 as in Figs. 6A, 6B. Next, a larger bore

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collar 72 is slid over the outers 54 of wires 50 and also over at least part of the length of small bore collar 70. Ultrasonic welding or crimping may then be applied to the larger-bore collar. In the resultant structure, the larger bore collar 72 retains the joint firmly in place and protects the joint from damage which might otherwise be caused 5 by relative movement between wires 50 and small bore collar 70. The larger bore collar 72 may be welded to the small-bore collar, or may be left unattached.

Figs. 8A and 8B show a variant of the joint of Fig. 7. Here, filaments 52 from two wires 54 enter a small bore collar 70 from opposite ends, and are ultrasonically 10 welded within that small bore collar. Larger-bore collar 72 passes over the smallbore collar 70 and is ultrasonically welded or crimped onto the outers 54 of the wires 50 and the small bore collar 70. In the resultant structure, the larger bore collar 72 retains the joint firmly in place and protects the joint from damage which might otherwise be caused by relative movement between wires 50 and small bore collar 15 70.

Figs. 9A and 9B illustrate another arrangement, combining features of the arrangement of Fig. 7 and the arrangement of Figs. 8A, 8B. The small-bore collar 70 is placed over the filaments 52, and ultrasonic welding applied. A larger-bore collar 20 72 is then placed over outers 54 of wires 50. Crimping or ultrasonic welding is then applied to larger-bore collar 72, as in the arrangement of Figs. 6A and 6B. Finally, an oversleeve 76 is slid over both the small-bore collar 70 and the larger-bore collar. This oversleeve may be of copper, or another metal, or may be of a suitable polymer such as a heat-shrink material. The oversleeve may be ultrasonically welded, 25 crimped or shrunk into place, and may serve to seal the joint against contamination.

In all cases, the joint may be immersed in a sealing material such as a solder or a thermosetting resin to provide mechanical protection and resistance to contamination.

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In another embodiment a wire could enter the copper tube from one side and another wire enter from the other side. Similarly one can have two or more wires from one end of the collar 56 and one or more wire(s) entering the collar from the other end.

35 The photograph of Fig. 1 shows a conventional crimped joint, in which a copper collar is mechanically crimped onto NbTi filaments. The NbTi filaments are not very

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densely packed, and surface contamination such as oxidation may occur on and between the filaments, providing resistance to electrical current.

Fig. 2 shows a photograph of a cross-section through a partially formed joint of the 5 present invention, at the stage shown in Fig. 5D. Mechanical crimping has been applied to retain the wires in position. The NbTi filaments are packed with a density similar to that of the conventional joint of Fig. 1.

Fig. 3 shows a photograph of a cross-section through a joint at a later stage in the 10 method of the present invention. Ultrasonic welding techniques have been applied, which have resulted in the NbTi filaments being pressed into much closer contact. The mechanical oscillations of the ultrasonic welding apparatus will have rubbed the filaments against one another, thus breaking and removing any surface contamination such as oxides and enabling effective electrical contact between the 15 filaments.

Fig. 4 shows a photograph of a cross-section through a completed joint formed according to the method of the present invention. The NbTi filaments are pressed into very close proximity, to become molecularly bonded together. In places, material 20 of the copper collar has permeated between the NbTi filaments, protecting them from further contamination and reducing the likelihood of deterioration of the quality of the ultrasonically welded joint. In arrangements where the copper collar is lined with a superconductor such as NbTi, the superconducting material may be caused to permeate between the NbTi filaments, providing a superconducting current path of 25 increased cross-section. The layer of superconductor on the inside of the copper collar shields the joint from the external background magnetic field.

According to the method of the present invention, a method is provided for joining NbTi filaments using ultrasonic welding techniques. The NbTi filaments are 30 themselves ultrasonically welded together, and the ultrasonic welding process is assisted by having the NbTi filaments placed within a copper collar. That copper collar is subjected to an ultrasonic welding technique. The technique causes mechanical oscillation and compression of the collar and the NbTi filaments, rubbing them together to drive off contaminants or oxide on the surface of the filaments and 35 force the filaments into close electrical and mechanical contact to provide molecular bonding by ultrasonic welding caused by friction and pressure between the filaments.

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The oxide removal method conventionally performed uses hydrofluoric acid, HF, and it is desired to reduce the use of this hazardous chemical. The present invention relies on a mechanical process to remove the oxides, so no HF step is required. The NbTi filaments may themselves be deformed by the compression and oscillation. 5 Material of the copper collar may permeate between the NbTi filaments, providing improved mechanical strength and protection of the NbTi filaments from further contamination. The copper collar also provides an electrical conduction path for carrying current in case of quench of the superconducting joint itself.

10 In some embodiments, a sealing material may be applied around the joint to further reduce the exposure of the NbTi filaments to contaminants. For example, epoxy resin or a solder may be applied by conventional techniques to enclose a joint made according to the method of the present invention.

While the present invention has made specific mention of copper collars, this material has been provided as an example. Copper has the desirable properties of malleability and electrical conductivity greater than that of the superconducting filaments to be joined, at the temperature of operation of the ultrasonic welding equipment. Other suitable materials include aluminium, MONEL® (an alloy of nickel (65-70%) and copper (20-29%) with iron and manganese), or GLIDCOP® (a copper matrix containing finely dispersed sub-microscopic particles of Al203). The chosen material should provide good electrical conductivity during quench, to provide an alternative current path around the ultrasonically welded joints between NbTi filaments. Use of a superconductive material on the inside such as NbTi, MgB2, Nb3Sn, PbSn may provide additional or further magnetic shielding. The collars referred to throughout the description may be tubes, sheets or tapes of copper or other suitable material, cut to an appropriate size and folded over, or wrapped around, the filaments to assist with forming the joints. Tubular collars need not necessarily have circular cross-section, but other cross sections may be used as appropriate, for example oval or hexagonal cross-sections.

It is believed that the importance of electrical conductivity of the collar may be explained as follows. To a first approximation, a good electrical conductor is also a good conductor of heat. This means that the heat deposited on the surface of the 35 collar will find its way to the inside of the collar and thus onto the filaments. In the

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case of a typical high electrical resistance material, the heat is not sufficiently conducted away from the surface to reach the filaments.

Although the present invention has been particularly described with reference to 5 wires having multiple cores of NbTi, the method of the present invention may be applied to wires having a single core of NbTi, or tapes of NbTi having a single flat core. The method of the present invention may also be applied to joining dissimilar wires selected from among these types.

10 Similarly, while the present invention has been described with reference to NbTi filaments, the method may be applied to joining magnesium diboride MgB2 or niobium tin Nb3Sn filaments. Those skilled in the art will appreciate that this technique can be extended to high temperature cuprate superconductors such as BiSCC (bismuth strontium calcium copper) and YBCO (yttrium barium copper oxide) 15 which are in the shape of wires or tapes.

Conventionally, joints between superconductive wires are placed in a joint cup and embedded in a superconducting alloy such as Wood's metal. Using the method of the present invention, embedding in such cups of alloy is unnecessary, and avoids 20 the need to use lead containing alloys. However, such cups provide a useful place to retain any excess lengths of superconducting wire leading to the joint, by coiling it into the cup. In devices containing joints formed according to the method of the present invention, similar cups may be provided, for the purpose of keeping the excess lengths of superconducting wire leading to the joint and sealing the joint 25 against contaminants. As such cups perform no electrical function, and do not need to tolerate temperatures of molten alloys, they may be made more inexpensively than the conventional metallic cups. For example, moulded cups of nylon or other plastic may be used, and the joint may be embedded in a relatively inexpensive sealing material such as wax, grease, or epoxy resin. As is well known, superconducting 30 electromagnets for MRI imaging systems employ many superconducting joints, which may be formed according to the method of the present invention. In a typical superconducting electromagnet, spaces are provided between coaxial annular superconducting coils. Joints made according to the present invention may be placed in a cup and filled with wax, grease or epoxy resin.

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Claims

CLAIMS:

1. A method for joining superconducting wires (50) each comprising at least one superconducting filament (52) enclosed within an outer (54), the method comprising

5 the steps of:

- stripping the outer from the filament(s) of each wire for a certain length at an end of each wire to be joined, to expose the filaments over a certain length;

- placing the exposed length of the filaments into a collar (56) of an electrically conducting material;

10 - applying pressure and mechanical oscillation to the collar in an ultrasonic welding process, thereby pressing the filaments into mechanical and electrical contact and removing surface contaminants from the superconducting filaments,

wherein the electrically conducting material has a melting point less than the melting point of the superconducting filaments and has an electrical conductivity greater than 15 an electrical conductivity of the superconducting filaments at the temperature of operation of the ultrasonic welding process.

2. A method according to claim 1, wherein the collar is formed from a sheet or tape of electrically conducting material cut to an appropriate size and folded over, or

20 wrapped around, the filaments.

3. A method according to claim 1 or claim 2 wherein the electrically conducting material comprises copper.

25 4. A method according to any preceding claim wherein the collar of electrically conducting material is lined with a superconductive material.

5. A method according to any preceding claim wherein the step of applying pressure and mechanical oscillation to the collar in an ultrasonic welding process is 30 performed by placing the collar containing the filaments onto a surface of an anvil structure (60) of an ultrasonic welding machine (59) comprising the anvil structure (60) and an actuator (62), which bears upon the collar and oscillates in a plane (64) parallel or perpendicular to a surface of the anvil structure (60).

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6. A method according to any preceding claim wherein the collar (56) nas a large enough bore that the copper outer (54) of the wires to be joined may be accommodated within the collar.

5 7. A method according to any preceding claim wherein the collar is lined with a non-superconducting material for mechanically retaining the filaments together.

8. A method according to any preceding claim wherein the collar is crimped prior to the step of applying pressure and mechanical oscillation, to retain the wires in

10 position.

9. A method according to claim 8 wherein the crimping is done onto the outers.

10. A method according to any preceding claim wherein a sealing material is 15 applied around the joint.

11. A method according to claim 10 in which the joint is embedded in a sealing material.

20 12. A method according to claim 1 wherein a relatively small bore collar (70) is used for ultrasonically welding the filaments (52) together, and a collar (72) of larger bore is used for retaining the copper outers (54).

13. A method according to claim 12 wherein an oversleeve material is provided, 25 extending over part of the small bore collar (70) and part of the larger bore collar (72)

for added mechanical strength.

14. A method according to claim 12 wherein a relatively small bore collar (70) is used for ultrasonically welding the filaments (52) together, then a larger-bore collar

30 (72) is slid over the outers (54) of wires (50) and also over at least part of the length of small-bore collar (70), then ultrasonic welding or crimping is applied to the larger-bore collar.

15. A method according to claim 14 wherein the larger-bore collar (72) passes 35 over the small-bore collar (70) and adjacent parts of outers (54) of wires (50), and is

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ultrasonically welded or crimped onto the outers (54) of the wires (50) and the sman bore collar (70).

16. A method according to claim 12 wherein an oversleeve (76) is slid over both 5 the small-bore collar (70) and the larger-bore collar (72), and is ultrasonically welded,

crimped or shrunk into place.

17. A method according to any preceding claim wherein the joint is immersed in a sealing material to provide mechanical protection and resistance to contamination.

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18. A method substantially as described and as illustrated in Figs. 2-9B of the accompanying drawings.