Abstract:
In a method or joint for joining first and second semiconductor wires, each comprising a number of filaments which each comprise a superconductive core within a respective sheath, the filaments being embedded within a matrix and wherein the superconductive cores comprise magnesium diboride and the sheaths comprise niobium, over a certain length a matrix is removed to expose the filaments. The exposed filaments are immersed in molten tin such that the nobium of the sheaths is converted to niobium-tin throughout a thickness of the sheaths. A superconductive path is provided between the superconductive cores of filaments of the first wire through the niobium-tin sheaths of the filaments to the superconductive cores of the second wire.

Description:
BACKGROUND 
       [0001]    The present application relates to methods for joining superconducting wires together, and joints as may be made by such methods. 
         [0002]    When manufacturing equipment such as electromagnets from superconducting wire, it is commonly required to join separate lengths of wire together. In order to maintain the superconductivity of the equipment, the joints must also be superconducting, or at least exhibit very low resistance, if operation in ‘persistent-mode’ is required. Typically, joint resistances of ˜10 −13  ohms are required to enable this mode of operation. Operation in ‘persistent mode’ is highly desirable as this enables the power supply to be dispensed with after initial energization has been achieved. 
         [0003]    Recent developments in superconducting materials have led to the use of magnesium diboride MgB 2  as a superconducting material. Magnesium diboride MgB 2  has the benefit of exhibiting superconductivity at higher temperatures than more conventional materials, avoiding the need to cool the superconductor to very low temperatures. However, the material itself is brittle, and difficult to join to form persistent joints. 
         [0004]      FIG. 1  shows a cut-away view of a typical MgB 2 -core superconducting conductor  10 . Superconducting filaments  4  comprise an MgB 2  core  1  in an essentially granular, powder form, held within sheaths  2  of niobium metal. These MgB 2 -filled niobium sheaths are further encased in a matrix  3  of high strength, conductive metal or alloy, such as the Cu—Ni alloy known as “MONEL”. The matrix  3  and filaments  4  make up superconducting wire  7 . The purpose of the niobium  2  is to prevent unwanted reactions occurring between the MgB 2  and matrix material during wire manufacture. 
         [0005]    In one manufacturing method, known as the ex-situ process, granulated or powdered MgB 2  is placed in a number of niobium lined holes drilled into a billet of matrix material. The complete billet is then drawn to the required final wire diameter. The Niobium-cased superconducting filaments are formed and compacted during the drawing process. 
         [0006]    The matrix  3  provides an electrically conductive shunt and thermal sink. Should any of the superconducting filaments  4  quench, then heat will be carried away from the quenched region by the matrix  3 , and electric current will flow through the lower resistance offered by the matrix. This will allow the quenched part of the filament to cool back to superconducting condition. The matrix also makes the superconducting wire more mechanically robust. 
         [0007]    The conductor  10  typically also comprises a stabilizing channel  5 . This may be of copper or another material, or combination of materials. The channels should be electrically and thermally conductive. In the illustrated example, the wire  7  is soldered at  6  into a cavity of the channel  5 . The channel  5  adds further electrical and thermal stability, and mechanical robustness, to the superconducting wire  7 , in the same manner as explained with reference to matrix  3 . 
         [0008]    In order to make a superconducting joint, two conventional approaches have been adopted: firstly, a joint may be formed directly between the MgB 2  cores  1  of the wires to be joined. Alternatively, another material, which is also superconducting at the temperature of operation of the wire, is used to electrically join the MgB 2  cores  1  of the wires together in a superconducting arrangement. Typically, known joining methods involve exposing the MgB 2  cores of the superconducting wires to be joined, and mechanically pressing the exposed MgB 2  particles of the respective wires together to form the superconducting joint. In some known arrangements, an intermediate layer of a superconducting material, typically a metal such as indium is interposed between the exposed cores of the respective wires, to increase the contact surface area and improve mechanical adhesion between the particles of the respective wires. Such methods require significant mechanical loads to be applied to the MgB 2  particles. The MgB 2  particles are relatively brittle, and applying such significant mechanical loads risks fracturing the MgB 2  superconducting material, leading to failures of the superconducting joint. 
         [0009]    In some known methods, MgB 2  particles are exposed and heated, for example when joined by MgB 2  powder or a reaction between magnesium and boron powders. If the MgB 2  particles are exposed, there is a risk of oxidation. Failures may occur sometime after the jointing process, after the joint is built in to a superconducting device, such as a magnet within a cryogen vessel. Such failures are very expensive and time-consuming to repair, due to the access problems of reaching a joint within a superconducting device built into a cryogen vessel, and/or vacuum vessel, and so on. It is therefore an object to provide methods for joining MgB 2 -cored superconducting wires which reduce the risk of mechanical damage, or oxidation, to the MgB 2  particles. 
         [0010]    However, tests on conventional joints between MgB 2 -based superconducting wires have shown magnetic field tolerance values poorer than expected. This is believed to be due to conduction actually taking place through the niobium of the sheaths  2  rather than through the superconducting joints between MgB 2  particles of the respective wires. Niobium is a “Type II” superconductor, but has a very low upper critical magnetic field strength B c2  when compared to other Type II superconductors such as the alloy niobium titanium. The critical field of niobium is in the range of a few tenths of a tesla with exact value depending on many factors, most notably the current density. Since it is highly desirable that joints for use in superconducting magnets should be able to tolerate quite high magnetic fields, any jointing method that utilizes the niobium sheaths for current transport is likely to be of little use. 
         [0011]    Certain conventional methods for producing superconducting joints are described in WO2007/128635A1, US2008/0236869A1, U.S. Pat. No. 6,921,865B2 and U.S. Pat. No. 7,152,302B2. 
       SUMMARY 
       [0012]    It is an object to produce superconducting joints between niobium-sheathed superconducting wires, such as those with a MgB 2 -core, or those with a NbTi core. 
         [0013]    In a method or joint for joining first and second semiconductor wires, each comprising a number of filaments which each comprise a superconductive core within a respective sheath, the filaments being embedded within a matrix and wherein the superconductive cores comprise magnesium diboride and the sheaths comprise niobium, over a certain length a matrix is removed to expose the filaments. The exposed filaments are immersed in molten tin such that the nobium of the sheaths is converted to niobium-tin throughout a thickness of the sheaths. A superconductive path is provided between the superconductive cores of filaments of the first wire through the niobium-tin sheaths of the filaments to the superconductive cores of the second wire. 
         [0014]    The above and further objects, characteristics and advantages of the present exemplary embodiments will become more apparent from the following description of those certain embodiments of the present invention, in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a cut-away view of a typical niobium-sheathed, MgB 2 -cored superconducting conductor; 
           [0016]      FIG. 2  shows two superconducting wires at an early stage in a joining method of the exemplary embodiment; 
           [0017]      FIG. 3  shows the wires of  FIG. 2  undergoing a later step in the joining method of the exemplary embodiment; 
           [0018]      FIGS. 4A-4C  show cross sections of a niobium-sheathed, MgB 2 -cored superconducting filament at various stages in a joining method of the exemplary embodiment; 
           [0019]      FIG. 5  shows an enlarged partial view of  FIG. 4C ; 
           [0020]      FIG. 6  shows the wires of  FIGS. 2 and 3  at a later stage in the method of the exemplary embodiment; 
           [0021]      FIG. 7  shows a completed joint according to an exemplary embodiment of the present invention, following the step illustrated in  FIG. 6 ; 
           [0022]      FIG. 8  shows a partial cross-section through a joint according to the exemplary embodiment, such as that illustrated in  FIG. 7 ; 
           [0023]      FIG. 9  shows an enlargement of the area identified as IX in  FIG. 8 ; 
           [0024]      FIG. 10  shows a wire prepared for joining according to another exemplary embodiment of the present invention; 
           [0025]      FIG. 11  illustrates various stages in a method of forming a superconducting joint according to an exemplary embodiment of the present invention; and 
           [0026]      FIG. 12  shows a cross-section of a joint according to the exemplary embodiment as may be formed by the method illustrated in  FIG. 11 . 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0027]    For the purpose of promoting an understanding of the principles of the invention, reference will now be made to preferred exemplary embodiments/best mode illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and such alterations and further modifications in the illustrated embodiments and such further applications of the principles of the invention as illustrated as would normally occur to one skilled in the art to which the invention relates are included herein. 
         [0028]    The present exemplary embodiment provides methods for joining niobium-sheathed superconducting wires and joints such as may be prepared by such methods. 
         [0029]    According to the exemplary embodiment, the niobium-sheathed superconducting filaments  4  are immersed in liquid tin (Sn), so that the niobium sheath reacts with the tin to form Nb 3 Sn. Conventionally, Nb 3 Sn superconductor filaments have been prepared by diffusion of tin into filaments of niobium during a long, high temperature, reaction process. A similar process is employed for jointing niobium-sheathed wires according to the exemplary embodiment. 
         [0030]    The Nb 3 Sn is a superconductor having a much higher field tolerance (˜18 T at 4K) than niobium (˜0.5 T at 4K), and a high critical temperature Tc of about 18K. Nb 3 Sn also has a large coherence length, which enables lossless current transfer between the reacted sheath and the MgB 2  superconductor granules or powder. The coherence length indicates the size of gap which may exist between superconductors, yet superconductivity to still exist between them. By having a sheath material such as Nb 3 Sn with a large coherence length, superconductivity may be maintained between the sheath material and enclosed grains of MgB 2 . Thus, if the niobium sheath in superconducting wires can be converted to Nb 3 Sn, the field tolerance of the joint should increase substantially and the transport current between the grains of MgB 2  and the sheath material should be improved. 
         [0031]    In known methods of joining MgB 2  cored wires, it is believed that the majority of electric current passes through the sheath material, rather than directly from the core of one wire to the core of the other. The exemplary embodiment provides a particularly advantageous sheath material to enable such current transfer to operate more effectively. 
         [0032]    The reacted sheaths  2  of filaments  4  are joined by superconducting materials. The exemplary embodiment avoids the need to expose MgB 2  grains and to make mechanical joints between them. It is believed that exposure of MgB 2  to hot tin will result in the formation of undesired compounds as contaminants. This will degrade the attainable quality of the junction. 
         [0033]    Joints according to the exemplary embodiment have a relatively high field tolerance, and a relatively high critical temperature Tc. Joints formed according to the methods of the present exemplary embodiment are believed to provide good electrical and mechanical connectivity between the superconducting filaments of the joined wires, improved magnetic field tolerance of the joints as compared to conventional joints between similar wires, and protection against mechanical damage. 
         [0034]      FIG. 2  illustrates an early step in a joining method of the present exemplary embodiment. Two conductors  12 ,  14  to be joined are stripped of their channel  5  over a certain length to expose the wires  7 . A binding  18 , for example of stainless steel wire, is wrapped around the two conductors in an unstripped region, to hold them mechanically together. The channels  5  of the conductors  12 ,  14  may be soldered together in the unstripped region for further mechanical stability. A certain length  20  of each of the stripped regions of the wires  7  is bent to a radius r. The radius should be selected to be small, yet not so small as to risk degradation of the MgB 2  superconductor. With present MgB 2  wires, a radius of about 80 mm-100 mm may be preferred. The bend may be made to an angle θ preferably in the range 45°-90°. 
         [0035]      FIG. 3  shows a next stage in a method of the present exemplary embodiment, a retaining clip  22  is applied, to hold the bent portion  20  of wires  7  steady in position. The stripped wires  7  in the bent region are immersed in an etchant  24  within a bath  26 . The material and temperature of the etchant is selected with regard to the materials and the topology involved. The etchant is selected to remove the matrix  3  material, and to expose the sheaths  2  of the filaments  4 . 
         [0036]    In a typical example, the matrix  3  is of copper alloy, and the sheaths  2  are of niobium. An etchant  24  of nitric acid may be found suitable, as it etches copper but does not significantly attack niobium. 
         [0037]    In other examples, molten tin (Sn) at a temperature of approximately 300° C. may be found suitable. Cu and copper alloys dissolve readily in hot tin. In this case, the tin will simultaneously etch the copper matrix and form NbSn in a single step. 
         [0038]    Use of hot tin is preferred, and acid etch is preferably used only for sheath materials which are not significantly reactive with hot tin or where the removal of the sheath material with tin would take too long. 
         [0039]    Bath  26  must be chosen to be resistant to the etchant  24 . In the case of a hot tin etchant, the bath may be a crucible. An agitator  28  may be provided to cause circulation of the etchant  24  around and between the wires  7 , and the sheathed  2  filaments  4 . 
         [0040]    Once etching is complete, reaction of the sheath  2  material is carried out. In a crucible, which may be a crucible bath as used in the step of  FIG. 3 , the bent region  20  is immersed in hot tin (Sn) at a temperature of about 600° C. The elemental niobium (Nb) of the sheath  2  reacts with the hot tin (Sn) by diffusion to become Nb 3 Sn, a superconductor. The rate of diffusion of Sn into Nb is highly dependent on the temperature of the molten Sn. Accordingly, the highest practicable temperature for the tin is preferred. An inert gas or vacuum atmosphere may be provided to prevent oxidation of the tin. 
         [0041]      FIGS. 4A-4C  shows three cross-sectional views, each through a single sheathed  2  filament  4  illustrating progression of the reaction. In  FIG. 4A , the MgB 2  core  1  is enclosed within an unreacted Nb sheath  2 , immersed in the Sn bath  24 . In  FIG. 4B , the sheath  2  has begun to react, and an outer part of the sheath has transformed into Nb 3 Sn, while the inner part of the sheath remains as elemental Nb. Reaction continues by diffusion, until the sheath  2  is completely transformed into Nb 3 Sn, as shown in  FIG. 4C . The MgB 2  core  1  remains un-reacted. 
         [0042]    The wires  7  are then removed from the crucible.  FIG. 5  shows a partial cross-section of a single filament  4  following this step. The MgB 2  core  1  is unreacted. The sheath  2  is now entirely of Nb 3 Sn and a thin Sn coating is present on the sheath, from the Sn wetting in the crucible. The grains of MgB 2  within the core are shown. Due to the manufacturing method of the filaments, the MgB 2  grains are in close proximity to the material of the sheath: this distance is typically less than the coherence length of Nb 3 Sn, enabling a persistent superconducting joint to be made between MgB 2  cores, through the Nb 3 Sn material of the sheath  2 . 
         [0043]      FIG. 6  illustrates a further step in a method according to the present exemplary embodiment. The bent portions  20  of wires  7 , now comprising MgB 2  cores in Nb 3 Sn sheaths, are placed within a further crucible or mould  28 . Alternatively, the same crucible may be used, if of suitable construction. A superconducting casting material  30  such as Woods metal or PbBi is added to the crucible or mould  28 , thereby immersing the bent portion  20  of filaments  4 . To assist with mechanical alignment during casting, the retaining clip  22  may be left in place. To provide mechanical strength of the finished joint, adjacent parts of the wires  7  may also be cast into the casting material  30 . The casting material is allowed to cool and harden. The resulting joint  40  is removed from the crucible or mould  28 , as shown in  FIG. 7 . 
         [0044]    It is important that the ends  32  of the filaments are not immersed in the etchant, or in the superconducting casting material to prevent damage to, or contamination of, the MgB 2  core. 
         [0045]      FIG. 8  shows a cross-section through a part of the joint  40  illustrated in  FIG. 7 . Filaments  4  of each wire are shown, still grouped together. The filaments  4  are embedded within the superconducting casting material  30 .  FIG. 9  shows an enlargement of that part of  FIG. 8  indicated at IX. The MgB 2  cores  1  of each filament are mechanically attached, and electrically connected together, through superconducting Nb 3 Sn sheath  2  layers and the superconducting casting material  30 . The Sn coating shown in  FIG. 5  has gone into solution in the superconducting casting material. An electric current i can pass from one core  1  of MgB 2 , through a sheath  2  of Nb 3 Sn, a distance of superconducting casting material  30 , another sheath  2  of Nb 3 Sn, to the MgB 2  core  1  of another filament  4 . In this way, the superconducting joint of the exemplary embodiment may be realized. There is no need to apply a mechanical load to the superconducting wires, reducing the risk of damage to the superconducting filaments as compared to conventional joining methods which involve mechanical compression. 
         [0046]    A variant of this method of forming a superconducting joint according to the exemplary embodiment will be discussed with reference to  FIG. 10 . 
         [0047]    In this embodiment, it is not necessary to bend the wires, allowing a more compact final joint. 
         [0048]      FIG. 10  shows an end of a first wire to be joined. The matrix material such as copper or MONEL  3  has been etched away, for example using acid, over an end portion, leaving Nb filaments in sheath  2  exposed. The ends  42  of the filaments are sealed before immersion in tin in the crucible. This may be achieved by welding or mechanical crimping of the matrix material. 
         [0049]    During a crimping step, adjacent MgB 2  particles are crushed and fall from the filaments, leaving a length of empty sheath which may be sealed by crimping. Alternatively, welding, brazing or similar using a material which is unaffected by tin (Sn) at 600° C. may be used to seal the ends of the filaments. Such sealing has the objective of preventing the MgB 2  particles from coming into contact with the molten tin. 
         [0050]    In the method of  FIGS. 3-7 , the bend in the wires is provided to prevent immersion of the open ends of the sheaths in the etchant, the tin and the superconducting casting material. By using straight wires with sealed ends, as shown in  FIG. 10 , the open ends of the sheaths are also protected from exposure to etchant or casting material. The crucible in which wires such as shown in  FIG. 10  are cast into a joint may be much smaller than that shown in  FIG. 7 : for example, a narrow cylinder. A multi-part mould may be used to form moulding cavities for such joints, as crucibles may be difficult to form and fragile in use if shaped as a narrow cylinder. 
         [0051]    Superconducting joints formed as described above are believed to be suitable for application in the manufacture of dry magnets cooled by a cryogenic refrigerator to a temperature of about 10K. In such an arrangement, it is preferred that the superconducting joints should be positioned close to the refrigerator, to ensure effective cooling of the joints. 
         [0052]    An alternative method for forming superconducting joints will now be discussed, with reference to  FIGS. 11 and 12 . This method shares the feature of causing reaction of the niobium sheaths of the filaments with tin to form Nb 3 Sn superconducting sheaths. However, the resulting joint is crimped together, rather than being cast in a superconducting material. 
         [0053]      FIG. 11(   i ) shows two wires  7  to be joined together according to a method of the present exemplary embodiment. The ends of the wires have been sealed at  44 , for example by crimping, brazing or welding, with a material which is resistant to hot tin. 
         [0054]    As shown in  FIG. 11(   ii ), the matrices  3  are stripped over a certain length at their ends. The filaments  4  are thereby exposed. The material of the seals  44  must be resistant to any etchant used to strip the material of the sheath. The seals  44  prevent exposure of the MgB 2  cores to the etchant. 
         [0055]    As illustrated in  FIG. 11(   iii ), a cylindrical metal crimp  46 , for example of niobium-lined copper tube is placed around the filaments. The niobium lining may be a coating on the inside of the copper crimp, or may be a niobium foil wrapped around the filaments, with a copper crimp then placed over the foil. The crimp should be a snug fit, but not tight, lest damage be caused to the filaments as the crimp is fitted. A mechanical crimping step is then performed, schematically illustrated by arrows  47 . This presses the niobium lining of the crimp into contact with the niobium sheaths of the filaments, and presses the filaments into contact with each other. Although some mechanical compression of filaments  4  is involved, the MgB 2  cores  1  remain encased within the Nb sheaths  2 , which reduces the risk of damage to the cores during mechanical compression, as compared to most conventional methods. 
         [0056]      FIG. 11(   iii )( a ) shows a cross-section through the crimp at this stage. The outer surface  48  of the crimp shows mechanical deformations  50  due to the crimping process. The niobium lining  52  of the crimp  46  is to be seen. Within the crimp, the filaments  4  of the wires  7  are pressed together into mechanical contact. The crimping process must be controlled so as not to damage the MgB 2  cores of the filaments. At this stage, the MgB 2  cores  1  of the filaments  4  are electrically joined through niobium metal sheaths  2 , and the niobium lining of the crimp. 
         [0057]      FIG. 11(   iv ) shows a further stage in this method. The crimped filaments  4 , as illustrated in  FIGS. 11(   iii ) and  11 ( iii )( a ) are immersed in molten tin  54  within a crucible  56 . The molten tin is at a temperature of about 600° C. or more. This step may be performed in a vacuum, or in an inert atmosphere to prevent reaction of atmospheric components with the tin. As discussed with reference to  FIG. 4 , immersion of niobium sheaths  2  in such hot tin causes the niobium to react with the tin by diffusion to form superconducting niobium-tin (Nb 3 Sn). Preferably, this reaction is performed at a suitable temperature, and for a suitable time, for the niobium sheaths  2  to completely transform to Nb 3 Sn, but it is not necessary for the niobium lining  52  to completely transform to Nb 3 Sn. 
         [0058]      FIG. 12  shows a cross-section, similar to the cross-section of  FIG. 11(   iii )( a ), of the resulting crimped joint. The outer surface  48  of the crimp shows mechanical deformations  50  due to the crimping process. The lining  52  of the crimp  46  has been converted to Nb 3 Sn. Within the crimp, the sheaths  2  of filaments  4  of the wires  7  have also been converted to Nb 3 Sn. They are pressed together into mechanical contact. The MgB 2  cores  1  of the filaments  4  are electrically joined through Nb 3 Sn sheaths, and the Nb 3 Sn lining of the crimp. The Nb 3 Sn components are superconducting, as discussed above, and have much better superconducting characteristics than niobium, for example in having a significantly greater field tolerance (about 18 T at a temperature of 4K) and a higher critical temperature (about 18K). Nb 3 Sn also has a relatively large coherence length. The copper crimp  46  is unaffected by immersion in tin, other than in gaining a tin coating. 
         [0059]    While the resulting structure illustrated in  FIG. 12  may be immersed in molten superconducting filler material such as Woods metal or PbBi, which infuses between the filaments  4  and fills the crimp, it is preferred not to include such jointing material. The mechanical and electrical contact between Nb 3 Sn sheaths and Nb 3 Sn crimp lining layer provided by this embodiment of the invention may be sufficient to provide the required superconducting joint. The resultant joint between multiple Nb 3 Sn connections without a filler material is expected to tolerate relatively high strength magnetic fields and remain superconducting at temperatures in excess of 10K. Such joints are expected to be useful in the manufacture of dry magnets cooled by thermal conduction by cryogenic refrigerators operating at about 10K. 
         [0060]    The present exemplary embodiment accordingly provides methods for joining superconducting wires, and joints such as may be produced by such methods. The present exemplary embodiment relates to joints between filaments having a niobium sheath, such as superconducting wires having MgB 2  cores, those having NbTi cores, and joints between a MgB 2  cored wire and a NbTi cored wire. According to the exemplary embodiment, the niobium sheaths are immersed in hot tin (Sn) so as to convert the niobium into Nb 3 Sn, which is a superior superconductor to elemental niobium. The resulting Nb 3 Sn sheaths act as an efficient and effective conductor for introducing transport current into the MgB 2  cored wires. Magnetic field tolerance of the resulting joint is significantly improved as compared to conventional joining methods for such wires, in which it is thought that the niobium sheath carries some or all of the current flowing through the joint. The MgB 2  core is not exposed to the tin (Sn) during joint formation, reducing the risk of contamination or oxidation of the MgB 2  core. 
         [0061]    Some exposure of an MgB 2  core to hot tin may be tolerated, provided that the tin does not penetrate a significant distance into the wire so as to reach the effective part of the joint. 
         [0062]    In some exemplary embodiments of the present invention, multiple joints may be formed in a single tin artifact. Each joint may be of two or more superconducting wires. In a variant of such embodiments, multiple joints may be formed in a single tin artifact, and the tin artifact may then be divided to provide separate joints. 
         [0063]    Although preferred exemplary embodiments are shown and described in detail in the drawings and in the preceding specification, they should be viewed as purely exemplary and not as limiting the invention. It is noted that only preferred exemplary embodiments are shown and described, and all variations and modifications that presently or in the future lie within the protective scope of the invention should be protected.