WIRE-IN-CHANNEL SUPERCONDUCTOR

The present invention is directed to a wire-in-channel type superconductive conductor of the type which is used in superconducting magnets, such as are commonly found in magnetic resonance imaging machines, particle accelerators, etc., and methods of producing such wire- in-channel type superconductive conductors.

In wire-in-channel superconductive conductors, a superconducting wire is soldered into a normally-conductive channel of a corresponding dimension. Figure 1 is an illustration of a conventional wire-in-channel superconductor 10. Superconducting wire 12 typically consists of a number of filaments 14 of a superconducting material, such as NbTi, in a normally-conductive matrix material 16, such as copper. Such a superconductive wire 12 is soldered 18 into a corresponding groove in conductive channel 20. The channel 20 is typically of copper, and typically has an approximately rectangular cross-section. The external surface of the channel 20 may be tinned, for example with a suitable solder.

Such wire-in-channel superconductive conductors have been found effective in achieving long continuous lengths, and in providing a superconductive conductor which is suitable for persistent magnets. Wire- in-channel superconductive conductors of this type are typically insulated by a polyester braid 24.

The known copper channel wire-in-channel type superconductive conductor 10 is designed to provide a high copper-to-superconductor ratio, for stability, without the necessity of processing all of the copper with the superconductor. As a result, the cost of production is significantly

reduced as compared to methods of production which require processing all of the copper with the superconductor.

Because of the large quantity of material used in the conductive 5 channel in many applications, it is desirable to reduce the weight of the channel 20 and also to make it more pliable for ease in magnet fabrication. Aluminium is an excellent substitute material for the usual copper, being less costly, more pliable and lighter in weight; however, the use of aluminium as a direct substitute for copper presents several

10 manufacturing difficulties such as quality of solder bond, soldering temperature and compatibility of fluxes as well as the flexibility of the completed wire-in-channel conductor. Accordingly, aluminium has not been used as a direct substitute for the copper material normally used for the channel 20 shown in Fig. 1.

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Japanese patent application JP04071112A describes an arrangement in which an aluminium channel 20 is formed, and at least the interior of the groove 22 is coated in copper by a process such as electroplating, dipping or vapour deposition. The superconducting wire is then soldered

20 18 onto the copper coating within the groove 22, the resulting composite material then being subjected to a drawing process.

The present invention provides alternative and improved methods for producing a copper-clad aluminium conductive channel and a 25 superconducting wire in copper-clad-aluminium channel conductor using this channel.

The present invention also provides a wire-in-channel superconductive conductor wherein the conductive channel is fabricated

from an electrically conductive metal or alloy core with at least one layer of a metallic cladding comprising a layer of a high strength alloy; and an outer layer of copper.

The present invention accordingly provides channel conductors and methods for manufacturing channel conductors as set out in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a cross-section of a wire-in-channel superconductive conductor of the prior art; Fig. 2A shows a cross-section of a copper-clad aluminium wire useful in methods according to the present invention;

Fig. 2B shows a cross-section of a copper-clad aluminium channel conductor, formed from the copper-clad aluminium wire of Fig. 2A, according to an aspect of the present invention; and Fig. 3 shows a cross-section of a wire-in-channel superconductive conductor of the present invention.

According to the present invention, a copper-clad aluminium conductive channel for use in producing a superconducting wire in copper-clad-aluminium channel, is produced from an aluminium wire 30 coated with a metallurgically bonded copper layer 32 such as shown in Fig.

2A.

Rather than forming an aluminium channel section which is then at least partially coated with copper by a process such as electroplating, dipping or vapour deposition, the present invention provides a cost- effective and efficient method for producing copper-clad aluminium channel section suitable for use in producing composite wire-in-channel superconductive conductor, using commercially-available copper clad aluminium wire, such as illustrated in cross-section in Fig. 2A. Copper clad aluminium wire is commercially available having 10% or 15% copper by cross-sectional area, with a circular cross-section.

The cost of commercially available electrical grade copper clad aluminium wire is lower in price per unit length than solid copper. It is also lighter in weight because of the lighter weight of aluminium. A 10% copper clad aluminium wire is 2.8 times as long as a similar wire consisting of an equal weight of copper.

By employing such a composite wire in place of solid copper, such as is used for the channel 20 in conventional wire-in-channel superconductor, manufacturing costs are reduced, the weight of the final conductor is reduced and no change is required in the production process.

By employing such a composite wire, a much simpler manufacturing method may be employed as compared to depositing a copper coating onto at least part of the surface of an aluminium channel, as described by the prior art.

By applying conventional wire drawing and shaping techniques to the copper-clad aluminium wire represented in Fig. 2A, a copper clad

aluminium channel 40 may be formed, as illustrated in cross-section in Fig. 2B.

In an example of a manufacturing process according to an aspect of the present invention, a circular cross-section copper clad aluminium wire, commercially available and fabricated by known industrial manufacturing process, is processed using wire drawing and shaping techniques, conventional in themselves, into a grooved channel, approximately rectangular in profile (Figure 2B). For illustration, the rectangle will be assumed to be 1.27mm by 3.175mm in cross section and containing a

0.813mm wide groove 42. The channel need not be rectangular, or approximately rectangular, but should include groove 42.

The copper clad aluminium wire described is much softer than solid copper wire, which extrapolates to a higher production speed in forming the channel and far less die wear, both of which lower the cost of forming the channel 40 as compared to a corresponding method employing solid copper wire to form the channel 20.

The grooved channel 40 and a 0.813mm diameter superconductive wire 12 are then passed together through a solder bath where the superconductive wire 12 is pressed into the groove 42 in the copper clad aluminium channel 40. The composite superconductive conductor thus formed may then be passed through a die within the solder bath to press the superconductive wire 12 into the groove 42. The die may also serve to draw the conductor to a final cross-section, and in doing so may deform the channel 40 to more securely retain the superconductive wire 12, as illustrated in Fig.3.

The specific heat capacity of the copper clad channel is less than that of the conventional solid copper channel, which results in higher speeds in the soldering process. A controlling parameter in setting the production line speed is the rate that the channel can be heated to the soldering

5 temperature, and since it takes less energy to heat the copper clad aluminium channel to the soldering temperature, as compared to a conventional copper channel, the production line speed can be proportionally increased, as compared to a similar method employing a solid copper channel.

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The resulting structure may be quenched at the exit from the solder bath, or allowed to cool naturally, to solidify the solder and bond the structure together. Figure 3 is a cross section of a finished copper clad aluminium channel with a superconductor wire 12 soldered 18 in place, 15 manufactured according to a method of the present invention. The conductor is then spooled ready for any subsequent processing desired.

The method of forming a composite superconducting conductor having a superconducting wire 12 soldered 18 into a groove 42 in a copper

20 32 clad aluminium 30 channel according to the invention does not require steps such as electroplating, dipping or vapour deposition, as required by the prior art, resulting in a simplified and less costly manufacturing method, in turn resulting in greater reliability and reduced cost of the finished product.

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While copper clad aluminium wire is available commercially, and the formation of the copper-clad aluminium wire does not form part of the present invention, it may be noted that several methods for forming the composite copper clad aluminium wire are available. Some of these are

listed below and the choice again depends on the application. If required, copper-clad aluminium wire may be produced in-situ for use according to the present invention by any of the following processes.

• Hot bonding process — Bonds a relative thick copper layer on the aluminium core.

• Electroplating — Electroplating of copper on aluminium is usually restricted to thin layers of copper up to several micrometers.

• Co-Extrusion

• C o -D rawing

The method employed according to the present invention for converting a copper-clad aluminium wire into a copper-clad channel 40 profile is simpler and more industrially acceptable than the copper coating of an aluminium channel suggested by the prior art.

The aluminium 30 used to form the channel can be replaced by any suitable aluminium alloy, or indeed any other suitable alloy. The choice will depend on the application. In one case, electrical conductivity may dictate the choice, while in other cases the mechanical strength may be the dominant criterion. Although commercially available copper clad aluminium wire has been described, the invention is not limited to this single configuration. By incorporating a multiple layer structure, a reinforced channel conductor can be obtained. For example, a wire comprising aluminium or other electrically conductive alloy core encased within a layer of high strength alloy such as steel, with an outer layer of copper, may be used. Such wire may be commercially produced with a convenient cross-section, typically circular, and formed into the required channel profile by the methods described above. The presence of the high strength alloy will add to the mechanical strength of the final wire-in-

channel conductor, without significantly detriment to its electrical or thermal properties, and without introducing further steps into the manufacturing method for the wire-in-channel conductor. Final assembly of this alternate embodiment would be done on the same production line now used for the conventional copper channel.

Alternatively, or in addition, to the use of a layer of high-strength alloy within the wire used to form the channel, an elongate high-strength member such as a wire or strip may be included with the superconductive wire 12 and the channel 40 when forming the composite wire-in-channel conductor, to mechanically strengthen the composite conductor. The high-strength member can be incorporated in a linear or spiral fashion prior to, during or after the solder operation.

The step of soldering the superconducting wire into the copper clad aluminium profile to form the composite conductor may be performed using any suitable soft solder including but not limited to alloys of Pb, Cu, Sb, Sn and Zn. One particular solder which may be used comprises Sn-5% Sb by weight.

The aluminium used in the copper clad aluminium wire should preferably be high purity aluminium, electrical grade aluminium or high strength aluminium.

The copper used in the copper clad aluminium wire should preferably be high conductivity copper such as HOO(OFHC) grade copper.

The copper used in the copper clad aluminium wire may be an alloyed copper for improved strength.

It may be found advantageous to apply a tinning layer over at least part of the outer copper layer 32 of the channel, particularly the surface in and near the groove 42 which will receive and be soldered to the superconducting wire. Such tinning layers are known to assist soldering. A tinning layer may be of tin, or any suitable solder. Once the channel 40 and the superconductive wire 12 have been soldered 18 together, the whole outer surface of the resultant wire-in-channel superconductor will most likely be tinned.

In further embodiments of the present invention, an elongate member of a material of high specific heat capacity can be incorporated into the composite conductor before, during or after the solder bonding of the superconducting wire to the channel profile, to improve thermal stability. Such material can be treated to accept solder by extrusion, electroplating or co-drawing, etc..