SUPERCONDUCTING JOINT

The present invention addresses the issue of joining superconducting wires in such a manner to produce a superconducting joint. The present invention aims to provide such a method which is effective, produces mechanically robust joints quickly, and avoids or reduces the use of dangerous or harmful chemicals.

An application for superconducting wire is in producing superconducting electromagnets for generating strong magnetic field for magnetic resonance imaging (MRI) . Several coils are required, each made up of several kilometres of wire. It is necessary to join these coils together in an electrical circuit, and it is commonly required to make joints within the length of wire used for any particular coil. The joints usually take the form of joint cups filled with a superconducting alloy in which exposed superconducting filaments are immersed. The resultant joints are mechanically attached to the structure of the magnet as required. The present invention relates to the structure of such joints. An example of a conventional superconducting wire comprises a number of NbTi filaments within a copper matrix material. A conventional method of joining such wires proceeds as follows. The ends of the wires are etched in nitric acid to remove the copper matrix material, and then further etched in hydrofluoric acid to remove any oxide from the surfaces of the NbTi filaments. It is also well known to those skilled in the art that it is possible to remove the copper matrix and oxide using an electrolytic process. The filaments of the wires to be joined are twisted or plaited together to provide an area of contact between the filaments. The plaited or twisted filaments are immersed in a small cup of molten superconducting alloy such as PbBiSn or Wood's metal. While this method has been successfully employed for some time, safety and regulatory concerns mean that it is desired to phase out the use of lead, as used in PbBiSn and Wood's metal, and hydrofluoric acid.

The present invention accordingly aims to provide a method for electrically jointing superconducting wires to produce superconducting joints in a manner which does not require the use of lead or lead alloys, or hydrofluoric acid. Such method should be rapid and produce electrically and mechanically reliable joints. It is believed to be impossible to use conventional welding to join superconducting NbTi filaments together into a superconducting joint, as the required rise in temperature of the filaments would alter the molecular structure of the NbTi and ruin its superconducting properties. The NbTi filaments themselves may be found too resistive to hold significant transport currents, and so the process will be slightly inefficient or one would have to rely on very high powers from the capacitor banks which drive the magnetic coils.

Magnetic welding is a known method of welding electrically conductive components together without heat. Similar or dissimilar materials may be welded using this technique. Essentially, the technique relies on an oscillating magnetic field generating eddy currents which interact to cause the extreme acceleration of one piece towards another. On impact, usually at closing speeds of hundreds of metres per second, any surface oxide is broken away, and airflow caused by the relative motion of the pieces blows oxide away from the contact area. The resulting high-energy impact between clean metallic surfaces results in a cold weld - or molecular bond - between the materials of the pieces.

The prior art document available at the filing date of the present invention at

http : / /www . englis . pstproduct s . com/ index_htm_ iles/empt %2 Oforming%20welding%2 Qc imping%20and%2 Ocutt ing . df

provides a background explanation of mechanical welding and mechanical crimping. A copy of that document is filed with the priority application GB1207624.6. A known method of forming superconducting joints by pressure welding is described in US2011/0028327, but that method does not benefit from some of the advantages of the present invention: for example, the present invention provides for removal of oxides from surfaces to be welded, which is not the case for this prior art method. The present invention also provides a method in which the opportunity for movement of the filaments is reduced as compared to that in US2011/0028327, encouraging more filaments to be welded in the method of the present invention.

Other known methods, such as described in US5082164 and US5111574 involve the use of heat, which may be detrimental to the superconducting properties of the filaments .

The present invention accordingly provides methods of joining superconducting filaments together to form superconducting wires without the use of lead, or hydrofluoric acid, and without the application of heat.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments, given by way of non-limiting example only, in conjunction with the accompanying drawings, wherein: Figs. 1A-1C illustrate steps in a method of the present invention, and a superconducting joint according to the present invention;

Figs. 2A-2C illustrate steps in a method of the present invention, and a superconducting joint according to the present invention;

Figs. 3A-3C illustrate steps in a method of the present invention, and a superconducting joint according to the present invention;

Figs. 4A-4C illustrate steps in a method of the present invention, and a superconducting joint according to the present invention;

Fig. 5 illustrates a joint of the present invention protectively embedded in a filler material within a joint cup;

Fig. 6A illustrates a cross-section through an assembly at a certain stage in a method of forming a joint according to an embodiment of the invention; Fig. 6B illustrates a cross-section through a joint according to an embodiment of the invention, as may be produced using the assembly shown in Fig. 6A;

Figs. 7A and 7B schematically illustrate perspective and cross-sectional views of an assembly of parts useful in manufacture of a joint according to an embodiment of the invention;

Figs. 7C-7E schematically illustrate perspective views of certain stages in a method of producing a superconductive joint according to the present invention;

Fig. 8 illustrates a cross-section through an assembly at a certain stage in a method of forming a joint according to an embodiment of the invention; and

Figs. 9A - 9D schematically illustrate cross-sections through certain stages in a method of producing a superconductive joint according to the present invention, resembling the assembly shown in Fig. 8.

The present invention employs the technique of magnetic welding for the production of superconducting joints between superconducting filaments.

Although NbTi filaments themselves are not ideally suitable as a participant in magnetic welding because the eddy current is dependent on the resistivity of the material and NbTi has a relatively high resistivity, the present invention provides methods of producing electrically and mechanically effective superconducting joints using magnetic welding.

Figs. 1A-1C illustrate a cross-section of components employed in steps of a method of the present invention. As shown in Fig. 1A, an electrically conductive tube or cup 10 is provided. It may be open-ended, or closed at one end 12. It may be preferred to use a tube 10 which is open at both ends to avoid entrapment of air within the joint, which might later contaminate the interior of a vacuum vessel or cryogen vessel.

An electrically conductive insert 14 is also provided. This insert preferably has a tapered end 15, which is preferably closed. The insert preferably has an outer cross-section at plane A-A which is the same shape as, but slightly smaller than, an inner cross-section of the tube or cup 10 at plane B-B. The cross-sections are preferably circular.

In preparation for jointing, the matrix material, typically copper, of the wires to be joined, is stripped from ends of the wires to be joined, leaving exposed superconducting filaments for joining.

As shown in Fig. IB, in this embodiment of the invention, filaments 16 of at least two superconducting wires to be joined are twisted or plaited together and wrapped around the insert 14. Plaiting of the filaments is preferred, as that ensures a greater surface area of the filaments being in contact with each other, and provides more reliable pattern of contact points between filaments. The outer dimension of the filaments 16 may be slightly larger than the inner dimension of the tube or cup 10 at plane B-B. A retaining feature may be incorporated into the shape of insert 14. The tapered end 15 of insert 14 is inserted into an open end 17 of tube or cup 10, to a sufficient distance that filaments 16 are trapped between insert 14 and tube or cup 10. A magnetic welding step is then applied, as is conventional in itself, other than for the presence of filaments 16. Tube or cup 10 is violently compressed onto insert 14, and moulds itself to the contours of the insert 14. The impact caused by the magnetic welding step cleans oxides from the surfaces of the filaments 16, the insert 14 and tube or cup 10. The pressure exerted due to the magnetic welding causes a solid-state cold weld between the tube or cup 10 and the insert 14. Similarly, the impact also cleans oxides from the NbTi filaments 16, and the pressure drives the filaments together to form a solid state cold weld between filaments, and also between the filaments and the insert 14 and the tube or cup 10.

Fig. IC schematically illustrates the resultant structure, wherein tube or cup 10 is magnetically welded by compression onto insert 14 and filaments 16. A bulge 18 may be visible on the outer surface of the tube or cup 10, where the tube or cup 10 has been forced onto the insert 14 and filaments 16.

A similar but alternative method is shown in Figs. 2A-2C. In this method, the step of Fig. 2A corresponds in all respects with the step of Fig. 1A. However, as shown in Fig. 2B, instead of wrapping filaments 16 around the insert, the filaments are formed into a loop which is placed within the tube or cup 10. The insert 14 is then inserted into tube or cup 10 such that filaments 16 are trapped between tube or cup 10 and insert 14. In the magnetic welding step, the tube or cup 10 is violently compressed onto insert 14 and the loop of filaments 16. As described above, the impact of the tube or cup 10 meeting the insert 14 and filaments 16 at a speed of several hundred metres per second dislodges all surface oxides from the surfaces of the insert 14, tube or cup 10 and filaments 16. This, in conjunction with the pressure exerted on the surfaces of the insert 14, the tube or cup 10 and the filaments 16, causes a cold, solid state weld between the filaments 16, and between the filaments and the cup or tube 10, and the insert 14. A bulge 18 may again be visible on the outer surface of the tube or cup 10 as the tube or cup 10 is forced onto the insert 14 and the loop of filaments 16.

Figs. 3A-3B show an alternative method and resulting structure. In this method, the step of Fig. 3A corresponds in all respects with the step of Fig. 1A. However, ends of filaments 16 are simply placed within the tube or cup 10 rather than being formed into a loop, as shown in Fig. 3B. Fig. 3C illustrates the magnetic welding step, in which the tube or cup 10 is violently compressed onto insert 14, and traps the filaments 16 between an outer surface of the insert 14 and an inner surface of the tube or cup 10. As described above, the impact of the tube or cup 10 meeting the insert 14 at a speed of several hundred metres per second dislodges all surface oxides from contacting surfaces of the filaments 16, the insert 14 and the tube or cup 10. This, in conjunction with the pressure exerted on the surfaces of the insert 14, the tube or cup 10 and the filaments 16, causes a solid state cold weld between the filaments 16, and between the filaments 16 and the cup or tube 10, and the insert 14. A bulge 18 may again be visible on the outer surface of the tube or cup 10, where the tube or cup 10 has been forced into impact with the insert 14 and filaments 16.

Figs. 4A-4C show another alternative method and resulting structure. In this method, the step of Fig. 4A corresponds in all respects with the step of Fig. 1A. However, in this embodiment, the exposed ends of filaments 16 are wrapped over the tapered end 15 of the insert 14 prior to the magnetic welding step. Fig. 4C illustrates the result of the magnetic welding step, in which the tube or cup 10 is violently compressed into contact with insert 14, and traps the filaments 16 between an outer surface of the insert and an inner surface of the tube or cup 10. As described above, the impact of the tube or cup 10 meeting the insert 14 and the filaments 16 at a speed of several hundred metres per second dislodges all surface oxides from contacting surfaces of the filaments 16, the insert 14 and the tube or cup 10. This, in conjunction with the pressure exerted on the surfaces of the insert 14, the tube or cup 10 and the filaments 16, causes a solid state cold weld between the filaments 16, and between the filaments 16 and the cup or tube 10, and the insert 14. A bulge 18 may again be visible on the outer surface of the tube or cup 10, where the tube or cup 10 has been forced into contact with the insert 14 and filaments 16.

While the above examples are illustrative of the methods and joints of the present invention, numerous variants will be apparent to those skilled in the art. For example, while the illustrated tube or cup and insert is believed to be effective in ensuring an appropriate pressure between the components, any typical magnetic welding arrangement may be employed. For example, the filaments 16 may be twisted, plaited or simply placed together between two flat surfaces of respective pieces of conductive material such as copper or aluminium, and those pieces of conductive material magnetically welded together, trapping the filaments between them. In a particular variant, the filaments are simply placed side by side, not twisted or plaited. Such an arrangement is best used where at least the tube or cup, or the insert, is made of, or coated with, a superconducting material.

Fig. 6A shows a cross-section through an assembly at a certain stage in such a method, and Fig. 6B shows a cross-section through a finished joint 100 produced by such a method.

As in the methods described above, two pieces of conductive material 102, 104 are driven together at high speed and high energy in a magnetic welding step. On impact, oxides are driven from the surfaces of the pieces of conductive material 102, 104, and a cold weld 106, being a molecular bond, is formed between the material of the two pieces 102, 104. Exposed filaments 16 of superconducting wires 108 to be joined are located between respective surfaces 112, 114 of the pieces of conductive material 102, 104 before the magnetic welding step. The magnetic welding step involves driving the surfaces 112, 114 of pieces of conductive material together, in a direction at least approximately perpendicular to the surfaces. On impact between the pieces of conductive material 102, 104, oxides are driven from the surfaces of the superconducting filaments and the surfaces 112, 114 of the pieces of conductive material 102, 104. A molecular bond 106 is formed between the filaments 16 of the superconducting wires, and between the filaments 16 and the pieces of conductive material 102, 104.

Figs. 7A-7E shows various sub-assemblies and stages in the manufacture of joint 100 according to an embodiment of the invention. A conductive enclosure 120, here an open-ended hollow cylinder of a material such as copper or aluminium, is provided. The enclosure 120 can alternatively be closed at one end. As shown in cross-section in Fig. 7B and in perspective view in Fig. 7A, pieces 102, 104 of conductive material are placed within the enclosure 120 with their surfaces 112, 114 facing each other.

A method for manufacture of a joint according to the present invention may proceed as follows.

As shown in Fig. 7C, a first piece 102 is placed within the enclosure 120. Exposed filaments 16 of the superconducting wires to be joined are placed within the enclosure, adjacent the relevant surface 112 of piece 102. The filaments 16 should be in close proximity to one another. They may be straight; they may be twisted together or, preferably, they may be plaited together. The wires 108 may be bound together near to the joint, to prevent respective movement between the wires, which could place excessive strain on the filaments 16 or the wires 108 once the join is formed. As shown in Fig. 7D, the second piece 104 is placed into the enclosure with its surface 114 facing the surface 112 of piece 102. This assembly is then subjected to magnetic welding. The two pieces 102, 104 are driven towards one another, and the enclosure 120 is radially compressed. The compression of enclosure 120 may serve to accelerate the pieces towards one another, or may simply act to help to bind the finished joint together. The energy of the motion of pieces 102, 104 towards one another causes a cold weld 106, a molecular bond, between the pieces 102, 104, and between filaments 16, and between filaments 16 and pieces 102, 104, as described above. Any oxides which may have been present on the surfaces 112, 114 or the surfaces of filaments 16 will be driven off or embedded in the cold weld, ensuring an effective cold weld, and an effective superconductive joint between filaments 16. Twisting the filaments together ensures that they cross one another several times, each providing a potential point for molecular bonding. A larger number of such potential molecular bonding locations are believed to be provided by plaiting the filaments together. The filaments may simply be placed side-by-side, but this arrangement is preferably used where one or more of the pieces 102, 104 are of, or are coated with, a superconducting material.

Numerous variants of this arrangement will be apparent to those skilled in the art, within the scope of the present invention. For example, the enclosure 120 may be of square, oval, rectangular or any other cross-section, with pieces 102, 104 suitably shaped to fit within the enclosure. The enclosure may be of a highly conductive material suitable for magnetic welding, such as copper or aluminium. Alternatively, the material of the enclosure may be magnetically transparent and unsuitable for magnetic welding, and may serve only to mechanically retain pieces 102, 104 and filaments 16 in their required relative positions prior to the magnetic welding step. The pieces 102, 104 need to be of a material suitable for cold welding and of suitable hardness to cause the required cold welding, molecular bonding, of filaments 16. Such materials include copper, NbTi. In preferred embodiments, the pieces 102, 104 may be of the same material as the matrix of the wires 108; or a different material having a hardness at least equal to that of the material of the matrix material. The materials of the pieces 102, 104 preferably have a hardness higher than that of the material of the filaments 16, which may be NbTi, to ensure that filaments 16 are cold welded, molecular bonded, together, rather than simply becoming embedded in one or other of the pieces 102, 104. Figs. 8-9D schematically illustrate a further variant of the present invention. In this variant, a first of the conductive pieces includes a surface feature in the surface to be magnetically welded to a second conductive piece, while the corresponding surface of the second conductive piece carries a complementary feature.

Fig. 8 schematically represents conductive pieces 102, 104 within enclosure 120, similar to the illustration of Fig. 7A. However, in this embodiment, one conductive piece 102 includes a trough 122 extending axially of the enclosure 120, while another conductive piece 104 includes a complementary ridge or protrusion 124. In use, the filaments 16, which may be twisted or plaited together, or just lain side-by-side, may be placed in trough 122, making alignment easier. When the second conductive piece 104 is added, the ridge 124 may assist in retaining the filaments in position. In other respects, the method and joint according to this variant is as explained with reference to Figs. 7A-7E.

Figs. 9A-9D schematically illustrate steps in a method according to another variant of the present invention. The assembly may closely resemble the assembly of Fig. 7A or Fig. 9A. In this variant, each of the conductive pieces 102, 104, has at least one corrugation 126, 128, running perpendicular to the direction of placement of the filaments 16. The corrugations 126, 128 themselves should not be sharply angled, to reduce the chance of damage to the filaments 16.

In the illustrated process, a first conductive piece 102 is provided, with one or more corrugations 126 running approximately perpendicular to the intended direction of the filaments.

As shown in Fig. 9B, filaments 16 of superconducting wires 108 are placed on the first conductive piece 102, in a direction approximately perpendicular to the corrugations 126. A second conductive piece 104, having corrugations 128 complimentary to the corrugations 126 of first piece, is placed over the filaments. The assembly of conductive pieces 102, 104 and filaments 16 is then placed within enclosure 120, as discussed above in relation to the embodiment of Figs. 7A-7E. A magnetic welding step is then applied, as in the other described embodiments. The presence of the corrugations provides particular pressure points, encouraging molecular bonding between filaments at the pressure points.

In any of the described methods, magnetic welding is believed to work best with highly-conductive pieces 10, 14, 102, 104 to be joined. For that reason, copper or aluminium pieces are preferred. However, other materials such as stainless steel may be used, and the magnetically-welded parts may be of dissimilar materials, such as a copper piece and an aluminium piece. It is also apparent for those skilled in the art that one can have a material whereby a thin layer of a highly conductive, or indeed superconductive, material is deposited by a conventional method onto a less-conductive material, e.g. Copper or aluminium deposited on stainless steel.

In its general scope, the present invention provides a method for joining superconducting wires which involves stripping matrix material from superconducting filaments, placing these filaments between electrically conductive pieces 10, 14, 102, 104 and applying magnetic welding to drive those pieces together, so that at least some of the filaments 16 become cold welded with molecular bonds between one another and between the filaments and the two electrically conductive pieces. Particular examples of electrically conductive pieces are described, and preferred, but are not considered essential.

The joint produced by the methods described thus far is as shown in Figs. 1C, 2C, 3C, 4C, 7E, 9D. Two electrically conductive pieces - here, the insert 14 and the tube or cup 10 or pieces 102, 104 - are magnetically welded together with superconducting filaments 16 trapped between them. The superconducting filaments 16 are themselves cold welded to each other and to the electrically conductive pieces. As the superconducting filaments are themselves cold welded together, the joint is superconducting as electrical current can pass directly from one superconducting filament to another without passing through any other material.

Further steps may be applied as required by the application. Fig. 5 shows a joint 50 according to the invention, as illustrated in Fig. 3C, placed in a joint cup 52 and embedded in a filler material 54. Such joint cups are conventionally made of brass or copper, but in joints of the present invention, they may also be of plastic, in circumstances when they do not need to have any particular thermal or electrical properties. The filler material serves to mechanically protect the joint, and need not have any particular thermal or mechanical properties. Grease, wax or thermosetting plastic, or a resin may be used. Wires 56 may be bound 58, for example with cable ties, lacing or wire, to hold them together to avoid placing strain in the joint itself. Joint cup 52 may then be mounted within equipment as any conventional superconducting joint using a joint cup. Similar to known joint cups, any excess length of wires 56 may be coiled into the joint cup and held in place with the filler material 54. In use, the joint will be cooled to below the transition temperature of the superconducting wires .

In alternative arrangements, any excess wire may be radially placed on the outer surface of coils, or in a groove cut into a former between coils, while the joint of the present invention is placed in a cup or trough filled with a wax, or a grease or a resin.

While the invention has been described with reference to NbTi filaments in the superconducting wires, it may also be applied to superconducting wires having NbsSn filaments and MgB ₂ or any other low- and high- temperature superconducting material for those skilled in the art. These superconductors could be in the form of wires or tapes without any restriction on cross-section.

While the particularly-described embodiments involve superconducting wires with multiple superconducting filaments, it may also be applied to superconducting wires which have a single superconducting filament, or "tapes" which are flat wires.

In the above-mentioned embodiments, the insert 14 or cup 10, or one of the conductive pieces 102, 104, may be made of, or coated with, a superconducting material, such as NbTi. This will increase the superconducting cross- section of the resulting joint, as any welds between filaments and the insert will from part of the superconducting joint, rather than only the welds between filaments.