WO1996037002A1

WO1996037002A1 - Method and apparatus for making superconductor wires

Info

Publication number: WO1996037002A1
Application number: PCT/US1996/006553
Authority: WO
Inventors: James Charles Mckinnell
Original assignee: Teledyne Industries, Inc.
Priority date: 1995-05-16
Filing date: 1996-05-15
Publication date: 1996-11-21
Also published as: AU5919396A

Abstract

A method of making superconductor-containing wires, where the wires are fabricated of compositions that are or can become superconducting comprising components where the components are labelled A, B, C, et seq., by providing a core of either a non-superconducting metal or a super-conducting metal then wrapping strips either separately or together containing at least two components A and B, in a spiral arrangement on the surface of the core, to form a wrapped core, so that at least the strips substantially overlap, forming interfaces therebetween; then twisting the wrapped core; then drawing the wrapped core lengthwise to reduce its diameter to form a wire; and then optionally treating the composition during the processing to produce superconducting wire where the compositions of the strips require such treatment to exhibit the desired superconducting properties in the wire.

Description

METHOD AND APPARATUS FOR MAKING SUPERCONDUCTOR WIRES BACKGROUND OF THE INVENTION This invention relates generally to superconductors and more particularly to a method of making multifila ent wires for electromagnets and the like.

Superconductors are usually formed together with another, non-superconducting material such as copper, aluminum, or silver to shunt currents. The non- superconducting component may be a core wire and/or a sheath surrounding the superconductor.

For making multifilament superconducting wires, there are a number of techniques including the "jelly roll" method. For a thorough description of that method, one may refer to U.S. Patent Nos. 4,262,412 and 4,414,428 to McDonald and No. 4,973,527 to Smathers, the entire disclosures of which are incorporated herein by reference.

In the jelly roll method, thin sheets of at least two components labelled A, B, (C) , etc. are wrapped in a spiral around a cylindrical core to form a billet. The composition of the core will vary depending on the superconducting system. For example, for the Nb-Ti system the core can be Cu or Nb-Ti; for the Nb₃Sn system the core can be bronze, Sn or Cu; for the Nb₃Al system the core can be Cu; and for the Nb-Ti artificial pinning center (APC) systems the core can be Nb-Ti or Cu.

The billet is inserted into a sleeve of copper or the like. The billet, which can be from a few inches long to a few feet long, is drawn to many times its original length, forming a thin wire. The wire may be cut into lengths which are stacked in an array to form a second billet, which is again drawn lengthwise.

The subsequent treatments which may need to be given will depend on the various superconducting systems.

For ductile systems such as the Nb-Ti system a heat treatment during the processing causes alpha-Ti to precipitate out and improve the superconducting properties. For a system where Nb₃Sn is being formed it is possible to have Sn being supplied from the core and the Nb being supplied by layer "A". In this case, layer "B" can be Cu. Here, layers A and B do not react, rather Sn reacts with Nb. Heat treatment will permit the Sn to flow through the Cu layers and reach the Nb to form the desired Nb₃Sn.

For systems such as Nb₃Al where the A and B components react, then a heat treatment at the final form will permit the components to interdiffuse and form the superconductor material.

For systems were APC is being utilized, they may not require any heat treatment. Instead the material is drawn to a fine enough size wire that the artificial pinning centers are formed. For systems were precursor materials are being used, they may require a heat treating under oxidative conditions to oxidize the components to form a superconducting wire.

The jelly roll technique is essentially a batch process. It makes rolls of 1 to 6 inches in diameter which must be drawn down considerably. It would be desirable to be able to produce superconducting multifilament wire of indefinite length in a continuous process. It would also be desirable to have a roll made with a smaller diameter so that less working would be required to make the final small diameter wire. The present invention provides a simple, but effective solution to these goals.

OBJECTS OF THE INVENTION An object of the invention is to simplify the manufacture of superconducting multifilament wire. Another object of the invention is to make it possible to make superconducting wire of indefinite length in a continuous process.

It is a further object of this invention to provide an apparatus to produce superconducting wire of indefinite length in a continuous process. These and further objects will become apparent as the description of the invention proceeds.

SUMMARY OF THE INVENTION This invention relates to a continuous method for forming a wire by wrapping at least two strips made of a first component A and a second component B which together with a preferable core component will produce a super¬ conducting wire.

In one embodiment, a core of a non-superconducting metal is provided and the strips of components A and B are wrapped in a helical arrangement on the surface of the core, so that the strips A and B substantially overlap, forming A-B interfaces therebetween. When a multi- filamentary wire is desired, the wrapped core can then be combined with other like wrapped cores to form an array which is drawn lengthwise to greatly reduce its diameter, and then heat treated to produce the intermetallic reaction compound of A and B at the interfaces of the strips. Alternatively, in the second embodiment the core may be omitted, in which case the A and B strips are simply twisted together.

An apparatus for carrying out the method includes a means for supporting a cylindrical core of indefinite length when the core is used, a supply of a strip of component A of indefinite length, a supply of a strip of component B of indefinite length, means for drawing the core in a downstream direction away from the strip supplies, means for wrapping the strips widthwise on the core in a spiral fashion as the strips are drawn from their respective supplies, so there are interfaces between components A and B at which superconducting reaction compounds can be produced by heat treatment, and means for twisting the wrapped core as it is drawn away from the strip supplies. It is contemplated the invention may be used with a variety of materials which form superconductors including ductile metal superconductors or the ductile metal precursors of brittle superconductors and especially the A15 compounds.

The methods described herein are useful for making composite metal wire which is near the size needed to make sub-elements for restacking into a billet which is subsequently processed into a wire which is made from these ductile metal superconductors or the ductile metal precursors of brittle superconductors. The restacked composite may be processed into wire by extrusion, wire drawing, rolling, swaging, or combinations thereof. The final cross-sectional shape of the composite may be round or polygonal.

The methods have application to a wide range of ductile input materials, including titanium, zirconium, hafnium, vanadium, niobium, tantalum, copper or alloys thereof, and niobium-tin alloys; A15 compounds formed from any of niobium, vanadium, aluminum, gallium, tin and germanium; and High-T_c compounds formed from ductile precursors containing La, Sr, Cs, Y, Ba, Cu, Bi, Be, Tl.

The methods reduce the degree of cold working required to manufacture large quantities of thin multilayer wires, and readily permit the use of very thin input foils which facilitate the formation of thin multilayers (following further processing into wire) in materials which work hard substantially.

The methods further facilitate the formation of the fine multilayers needed to form artificial pinning centers which are used to pin fluxons formed in type II superconducting materials. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings.

Fig. 1 is a perspective simplified view of an apparatus suitable for carrying out the invention, showing superconducting component strips being wrapped around a non-superconducting core;

Fig. 2 is a cross-section of the core, taken on the plane 2-2 in Figure l;

Fig. 3 is a cross-section of the core after wrapping, taken on the plane 3-3 in Figure 1; and

Fig. 4 is a similar view, showing three strips being twisted, but without a core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An implementation of the invention is shown in Figure 1. A cylindrical core 10 of indefinite length is supported between a source reel 12 and a take-up reel 14 while it is wrapped with strips of components A and B supported on respective reels 16, 18. The core wire preferably has a diameter in the range of .02 mm to 13 mm. Components A and B might be niobium and aluminum, for example, and the core can be a low cost metal such as copper. The additional strip C shown, leading from reel 20, will form a sheath around the twisted A and B components. This strip is preferably copper, for the example given. Each strip is pulled from its reel lengthwise along an axis which is substantially parallel to the cylinder axis. The combined metal strips are maintained, by braking of the supply reels, under a tension sufficient to produce smooth wrapping, yet insufficient to break the strips. Just downstream of the reels, the strips are passed between a pair of rolls 22, 24 which serve mainly as guides which keep the strips flat and parallel.

Preferably, the strips next pass through a funnel 26, having the shape of a segment of a hollow frustum of a cone, which guides the strips onto the surface of the core. Only the upper inner surface 28 of the cone contacts the strips; the gap 30 at the bottom permits the wire to be inserted laterally into the cone. The function of the longitudinal groove 32 (Figure 2) in the core surface is to receive one edge of the strips, so that a smooth spiral wrap results.

A cross-section of the wrapped core is shown in Figure 3. The groove has a depth of less than about 3/4 of the core wire diameter, and a width of less than about half of the core wire diameter.

Further downstream, the composite wire is passed through a rotating compaction die 34 which twists it, en route to the take up spool 14. The take-up spool is supported by a fixture 38 parallel to the length of the wire coming from the compaction die. The fixture is rotated by means not shown in the same direction as the compaction die, so as not to untwist the wire. As the fixture rotates, the reel is turned about its own axis, to pull the composite wire through the die. The rotational speed of the compaction die is chosen, with respect to the winding speed, to produce a twist pitch of less than one meter. Other apparatus can be used in place of this unit 34, 14 and 38 such as a cabling machine or a twister.

While use of a guide cone is presently preferred, to produce smooth wrapping, it may prove unnecessary in some applications. Alternatively, other types of guide devices may be used.

The embodiment of Fig. 4 is like that of Fig. 1, except both the core wire and the guide cone are omitted. The apparatus and method of using it are otherwise the same.

Regardless of whether the A and B strips are wrapped onto a core, or simply twisted together, the resulting composite wire can be drawn to produce a wire of greatly reduced diameter. Depending on which superconducting system is employed, various treatments can be used with or without heat treatment and with further drawing which will insure that a superconducting wire will be formed. Alternatively, the resulting composite wire from the wrapping operation can be combined with a number of other like composite wires, and the resultant array is then drawn to produce a multifilament wire of greatly reduced diameter. Again, depending on which superconducting system is employed, various treatments can be used with or without heat treatment and with further drawing which will insure that a superconducting wire will be formed.

The width of the strips is preferably between 3 mm and 1 and the thickness is less than 3 mm. The diameter of the core wire 10 is preferably between 20 microns and about 13 mm. The twist pitch length of the composite is preferably less than one meter and the compaction die diameter ranges from about 0.1 mm to 60 mm.

The wires made by the present method form super- conductors. The superconductors can conduct electricity with virtually no electrical resistance and are useful for a wide variety of devices including high field electro¬ magnets.

There are many types of materials which form superconductors. The so-called A15 superconducting compounds are cubic crystal intermetallic compounds of the generic type A₃B, where "A" and "B" represent different atomic elements. The A atoms are from the groups IVA, VA and VIA transition metals, and the B atoms are from groups IIIB, IVB and VB and some transition metals including osmium, iridium, platinum, gold and technetium. In an A15 compound, the B atoms are arranged in a body centered cubic array, and two A atoms are centered on each face of the cube. The A atoms of successive cubes are aligned, so that there are orthogonal (mutually perpendicular) chains of A atoms running through the crystal. Within these chains, the A atoms are very close together, closer than in pure A metal, which contributes to superconductive behavior.

Of the seventy-six known A15 compounds, 46 are known to be superconducting. These 46 compounds are called low- temperature superconductors because they superconduct only when they are at a temperature below 25"Kelvin. The critical temperature at which they cease to superconduct is called the critical temperature: this temperature varies according to the components A and B, and to some degree on impurities. Of the A15 superconductors, Nb₃Ge has the highest critical temperature, T_c, of about 23°K. Some of the 46 compounds are not particularly useful because they are unstable at low temperatures. Examples of the preferred compositions are Nb₃Sn, Nb₃Al, Nb₃Ga, Nb₃Ge, Nb₃Si, V₃Sn, V₃A1, V₃Ga, V₃Ge, and V₃Si, as well as these compositions which are doped with an alloying ingredient which can include elements such as Ti, Ta, or Ge.

The fact that intermetallic compounds in A15 superconductors are brittle (they fail at strains of about 0.5%) presents special problems when forming electro¬ magnets. These compounds are formed during a heat treating step, so where the conductors must ultimately be formed into windings, either (a) the winding is done first, and then the heat treatment applied, or (b) the winding is done after heat treatment, but carefully, at large radii of curvature. Where the compound is expected to undergo bending stresses, either during manufacture or in use, it is obviously important to keep the brittle layers very thin, to minimize strain induced by bending. Multifilament wires may be used to minimize this problem.

Another series of superconducting materials are the copper-oxide-based planar structures. There are three classes of these superconductors, all of which share the common feature that they contain one or more conducting planes of copper and oxygen atoms. The first class is designated by the chemical formula La₂__χA_χCu0₄ where the A atom can be barium, strontium or calcium. The second class of copper-oxide superconductors is designated by the chemical formula Y-Ba-.Cu-.O-,.-, where d is <0.4. The third class is the most complicated. These compounds contain either single thallium-oxygen layers represented by the chemical formula Tl₁Ca_n.1Ba₂Cu_r)0_2n+3 where n refers to the number of copper-oxygen planes, or double thallium-oxygen layers represented by the chemical formula Tl₂Ca_n-Ba₂Cu_n0_2n+4. The number of copper-oxygen planes may be varied and as many as three planes have been included in the structure. Other peroskite structures can be used such as those containing Bi,Sr,Ca,Cu, and 0, thus generating a second family of superconductors. In all of these compositions, they may also be doped with a small quantity of other alloying ingredients.

Another series of superconducting materials are the niobium and titanium alloys. These include Nb47wt%Ti; compositions formed from a Nb-Ti alloy and another ductile metal such as Nb, Ti or Cu; NbTi alloys which are doped with a small quantity of other alloying ingredients; and Nb-Ti-Ta alloys.

The best presently contemplated modes of practicing the invention are described above. It should be understood that some of the elements described may be unnecessary, particularly the guide cone and/or the pressure rolls. Additionally, while some preferred A and B materials are mentioned above, it is expected that the invention will be useful with a variety of other known A15 components and other superconductors.

Since the invention is subject to modifications and variations, it is intended that the foregoing description and the accompanying drawings shall be interpreted as illustrative of only one form of the invention, whose scope is to be measured by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method of making superconductor-containing wires, said wires being fabricated of compositions that are or can become superconducting comprising components where the components are labelled A, B, C, et seq. , said method comprising the steps of: providing a core of either a non-superconducting metal or a superconducting metal; wrapping strips either separately or together containing at least two components A and B, in a spiral arrangement on the surface of the core, to form a wrapped core, so that at least the strips substantially overlap, forming interfaces therebetween; twisting the wrapped core; drawing the wrapped core lengthwise to reduce its diameter to form a wire; and optionally treating the composition during the processing to produce superconducting wire where the compositions of the strips require such treatment to exhibit the desired superconducting properties in the wire.

2. The method of Claim 1, wherein the step of treating the wire while being produced comprises either: a) heat treating at final form to interdiffuse the components and form a superconducting material; b) heat treating during intermediate processing to form precipitation; c) drawing the wire to a fine enough size such that artificial pinning centers are formed; or d) heat treating under oxidative conditions to oxidize the components to form a superconducting wire.

3. The method of Claim 1, wherein the strips are fed from stationary supplies, and the core is twisted and moved lengthwise away from the strip supplies as wrapping progresses.

4. The method of Claim 3, wherein the strips are fed lengthwise towards the core along axes substantially parallel to the core.

5. The method of Claim 4, further comprising a step of guiding each strip onto the core by drawing it through a conical spiral forming mechanism.

6. The method of Claim 5, wherein the conical spiral forming mechanism is a funnel.

7. The method of Claim 1, wherein the strips have a width of from about 3 mm to about 1 meter.

8. The method of Claim 1, wherein the components are of the type A₃B and wherein A and B represent different metallic elements.

9. The method of Claim 1, wherein the components are niobium and titanium.

10. The method of Claim 1, wherein the components form a High-T_c compound which is known as an oxide superconductor.

11. A method of making multifilamentary wires containing a superconducting composition, said wires being fabricated of two or more components where the components are labelled A, B, C, et seq. , said method comprising the steps of: providing a core of either a non-superconducting metal or a superconducting metal; wrapping strips of at least two components A and B in a spiral arrangement on the surface of the core, to form a wrapped core, so that at least the strips A and B substantially overlap, forming A-B interfaces therebetween; twisting the wrapped core; combining the wrapped core with other like wrapped cores to form an array thereof; drawing the array of wrapped cores lengthwise to reduce its diameter to form a wire array; and treating the wire while being produced in an appropriate atmosphere to produce superconducting wire.

12. The method of Claim 11, wherein the step of treating the wire while being produced comprises either: a) heat treating at final form to interdiffuse the components and form a superconducting material; b) heat treating during intermediate processing to form precipitation; c) drawing the wire to a fine enough size such that artificial pinning centers are formed; or d) heat treating under oxidative conditions to oxidize the components to form a superconducting wire.

13. The method of Claim 12, wherein the strips are fed from stationary supplies, and the core is twisted and moved lengthwise away from the strip supplies as wrapping progresses.

14. The method of Claim 13, wherein the strips are fed lengthwise towards the core along axes substantially parallel to the core.

15. The method of Claim 12, further comprising a step of guiding each strip onto the core by drawing it through a conical spiral forming mechanism.

16. The method of Claim 15, wherein the conical spiral forming mechanism is a funnel.

17. A continuous method of making multifilamentary wires according to Claim 12, wherein the components are of the type A₃B and wherein A and B represent different metallic elements.

18. The method of Claim 12, wherein the strips have a width of from about 3 mm to about 1 meter.

19. A method of making multifilamentary wires containing a superconducting composition, said wires being fabricated of two or more components where the components are labelled A, B, C, et seq., said method comprising steps of: contacting strips of at least elements A and B to form a composite strip having A-B interfaces; twisting the strip to form a composite wire; combining the composite wire with other like composite wires to form an array thereof; drawing the array of composite wires lengthwise to reduce its diameter; and treating the wire while being produced in an appropriate atmosphere to produce superconducting wire.

20. The method of Claim 19, further comprising a step of providing a metallic core, and wherein the contacting step includes wrapping said strips in a spiral fashion on said core.

21. The method of Claim 20, further comprising a step of forming a continuous lengthwise groove in said core, and inserting an edge of each of said strips into the groove prior to the laminating step.

22. An apparatus for wrapping a cylindrical core with strips of at least components A and B, preparatory to producing a multifilament superconductor wire comprising a superconductive material made from components A and B, said apparatus comprising: a supply of a strip of component A of indefinite length; a supply of a strip of component B of indefinite length; means for drawing the strips in a downstream direction away from said strip supplies; means for wrapping said strips widthwise about each other in a spiral fashion to form a wrapped core as the strips are drawn from their respective supplies, so there are interfaces between elements A and B ; and means for twisting the wrapped core as it is drawn away from the strip supplies.

23. The apparatus of Claim 22, wherein each of said supplies is a reel upon which a respective one of said strips is initially wound.

24. The apparatus of Claim 22, wherein the twisting means comprises a rotary die through which the wrapped core is drawn.

25. An apparatus for wrapping a cylindrical core with strips of components A and B, preparatory to producing a multifilament superconductor wire, said apparatus comprising: means for supporting the cylindrical core of indefinite length; a supply of a strip of component A of indefinite length; a supply of a strip of component B of indefinite length; means for drawing the core in a downstream direction away from said strip supplies; means for wrapping said strips widthwise on the core in a spiral fashion as the strips are drawn from their respective supplies, so there are interfaces between components A and B; and means for twisting the wrapped core as it is drawn away from the strip supplies.

26. The apparatus of Claim 25, further comprising a conical spiral forming mechanism for guiding said strips onto the surface of the core.

27. The apparatus of Claim 26, wherein the conical spiral forming mechanism is a funnel.

28. The apparatus of Claim 25, wherein each of said supplies is a reel upon which a respective one of said strips is initially wound.

29. The apparatus of Claim 28, wherein the reels are disposed on diametrically opposite sides of said core.

30. The apparatus of Claim 25, wherein the twisting means comprises a rotary die through which the wrapped core is drawn.