WO2000002208A1

WO2000002208A1 - Superconducting wires and their manufacture

Info

Publication number: WO2000002208A1
Application number: PCT/AU1999/000527
Authority: WO
Inventors: Francis Anthony Darmann
Original assignee: Metal Manufactures Ltd
Current assignee: Metal Manufactures Ltd
Priority date: 1998-06-30
Filing date: 1999-06-30
Publication date: 2000-01-13
Anticipated expiration: 2000-12-30

Abstract

An axially extending superconducting wire includes a substantially axially extending superconducting filament which is flattened in cross section. The superconducting filament is twisted along at least a substantial portion of its length into a substantially helical shape, such that the wire behaves substantially anisotropically in relation to the effect of an applied magnetic field on the critical current Ic at any cross-sectional point along the wire's length, and substantially isotropic in relation to the effect of an applied magnetic field on the self-induced critical current Ic along a substantial length of the wire.

Description

TITLE: SUPERCONDUCTING WIRES AND THEIR MANUFACTURE

FIELD OF THE INVENTION

The present invention relates to ceramic superconductors and the manufacture thereof. In particular, the invention relates to the manufacture of ceramic high temperature superconductors for use in applications requiring

superconducting wire which exhibits isotropic behaviour with respect to magnetic fields.

BACKGROUND OF THE INVENTION

One of the most promising high temperature superconducting (HTS)

materials for power engineering applications at 77K is (Bi_{2 χ}Pb_χ)

Sr₂Ca₂Cu₃O₁₀ (BSCCO-2223) processed in a silver sheath via the powder-

in-tube (PIT) technique. As it is competing for the most part with copper at

approximately 293K, the economic viability of powder-in-tube HTS conductors

in electrical engineering depends on the ability to manufacture long lengths of

relatively cheap tape or wire, with a 77K, self-field critical current density, J_c, >

2 x 10⁸ Am^*2 and a 77K, self-field critical current, l_c, greater than 30A.

At present, short (»50mm) intermediately-pressed, PIT tapes have

shown 77K, self-fields of J_c = 7x10⁸ Arrr² on selected areas, whilst longer

length (10-1000m) powder-in-tube tapes manufactured using conventional or

shear/high pressure rolling have shown 77K, self-field J_c values of 3 + 1 x 10⁸

Am^-2. These J_c values show that powder-in-tube tapes are just entering the

zone where they may be considered as economic competitors to conventional

copper-based high power devices. > The PIT technique is essentially a deformation process whereby a

tube, typically silver or a silver alloy, is packed with the powdered ceramic

superconductor or a precursor thereof, and repeatedly drawn and/or rolled

whereby the ceramic powder is compacted in a sheath. The compacted

powder is subsequently sintered. This process may typically be used to

produce a metal/superconductor composite wire or tape which consists of one

or more continuous superconducting filaments embedded in a metal matrix. It

is desirable that the metal provide mechanical support to the ceramic

superconductor material, has good thermal conductivity to provide

cryostability (maintenance of an even temperature throughout the ceramic

superconductor material) and is capable of serving as an electrical conductor

in the event that the ceramic superconductor material reverts to the state in

which it has normal conductivity, i.e. is no longer superconducting.

In one example of the PIT technique, a multifilament tape may be

manufactured by packing a precursor of partially reacted oxides into a sealed

silver tube or billet which is then reduced to around 1 mm diameter by drawing

through dies. The wire is then be passed through a rolling mill to produce a

tape. The drawing and rolling processes is typically repeated several times,

with intermediate annealing of the silver sheath and heat treatment/sintering

of the ceramic, resulting in high temperature superconductor tape typically 3

to 4mm wide and 0.2 to 0.5mm thick. A multifilament tape is produced by

stacking a bundle of drawn tapes or wires into another tube, drawing it down

to 1~2mm diameter and rolling and heat treating as before. It is preferred to use BSCCO-2223 high temperature superconductors since high critical

current density, J_c, is typically obtained with a microstructure of well-aligned

superconducting grains with clean grain boundaries and strong flux pinning.

Achievement of acceptable critical current and other parameters has

been found to require superconductors in flat tape form, most preferably

having a cross-sectional height to width aspect ratio of approximately 1 :10.

These tapes are grossly anisotropic and react to magnetic fields in a very

anisotropic way: a field applied parallel to the tape may degrade J_c by 10%

but an equal field applied perpendicular to it may degrade it by 50%. In some

applications, an anisotropic superconductor can become twisted along its

length in an unpredictable manner, resulting in a J_c that varies along the

length of the superconductor. In many situations, this is undesirable.

However, in some cases it is desirable that a superconducting wire behaves

substantially isotropically along its length. A conventional approach of

achieving this is to make a roundish assembly with anisotropic tapes disposed

in different orientations to achieve overall symmetry.

A physically or geometrically isotropic or symmetrical wire is a wire of

at least substantially round cross-section in which all points within the wire

and along the wire axis may be mapped onto each other according to the rule

(R, ψ) < > (R, -ψ) for any arbitrary value of φ, where R is the radial distance

from the wire.

Magnetic isotropy at any cross-section along a wire may be satisfied

if: l_c (R. φ) = l_c (R, φ - α). where _α is an arbitrary angular offset. (Eqn 1 )

Rotational symmetry exists in a wire which has both physical and magnetic isotropy.

European Patent Application EP 0798749, describes the manufacture

of a wire with a circular or polygonal cross-section having a high degree of

rotational symmetry. The manufacture of such wires involves sophisticated

and complex manufacturing processes to achieve the high degree of

rotational symmetry, rendering them too expensive to compete with copper

conductors.

Unless the context clearly requires otherwise, throughout the description and

the claims, the words 'comprise', 'comprising', and the like are to be construed

in an inclusive sense as opposed to an exclusive or exhaustive sense; that is

to say, in the sense of "including, but not limited to".

It is an object of the present invention to overcome or ameliorate at

least one of the disadvantages of the prior art, or to provide a useful

alternative.

DISCLOSURE OF THE INVENTION

According to a first aspect of the present invention there is provided an

axially extending wire including a substantially axially extending

superconducting filament which is flattened in cross section, the

superconducting filament being twisted along at least a substantial portion of

its length into a substantially helical shape, such that the wire is: substantially anisotropic in relation to the effect of an applied magnetic

field on the critical current, l_c, at any cross-sectional point along the wire's length; and

substantially isotropic in relation to the effect of an applied magnetic

field on the critical current, l_c, along a substantial length of the wire.

Preferably, the filament is rotationally asymmetrical about the axis at

each point along the length of the wire.

Preferably, the anisotropic and isotropic conditions are satisfied in

relation to a magnetic field applied in a plane substantially normal to the axis.

Preferably, the wire includes a plurality of the filaments arranged, in

cross section, in at least two parallel stacks.

Preferably, each stack contains a plurality of the filaments.

Preferably also, the filaments are embedded in a silver or silver alloy

matrix.

Preferably, a pitch of the helical shape is selected on the basis of an

application to which the wire is to be applied.

Preferably, the pitch of the helical shape is selected on the basis of an

expected minimum radius of curvature which the application to which the wire

is to be applied.

Preferably, the filaments include Pb stabilised BSCCO-2223 ceramic.

According to a second aspect of the invention, there is provided a

method of fabricating a superconducting wire, the method including the steps

of: > providing a primary metallic tube;

packing precursors of ceramic high temperature superconducting

material into the primary metallic tube;

applying heat to degas the contents of the primary metallic tube;

closing the primary metallic tube;

swaging and drawing the primary metallic tube to reduce its diameter;

rolling the primary metallic tube to form a tape;

twisting the tape into a generally helical shape; and

applying sufficient heat to the primary metallic tube to sinter the

precursors, thereby to form a ceramic-composite superconducting tape with

an inner superconducting ceramic filament and an outer metallic sheath.

Preferably, the method further includes the step, prior to the sintering

step, of positioning one or more of the tapes within a secondary metallic tube,

and drawing the secondary tube to reduce its diameter.

Preferably also, the tapes are positioned within the secondary tube

prior to twisting the tape.

Preferably, the tapes are positioned within the secondary tube in one or

more parallel stacks.

More preferably, the twisting step includes the sub-step of twisting the

secondary tube after it has been drawn, thereby to impart the requisite twist to

the tapes prior to sintering. Preferably, the twisting step is performed in a number of twisting sub-

steps, at least some of the twisting sub-steps being separated by annealing steps.

Preferably, the drawing step is performed in a number of drawing sub-

steps, at least some of the drawing sub-steps being separated by annealing

steps.

Preferably, the degassing step includes the sub-step of placing the

primary tube into a non-oxidising atmosphere. Preferably, the non-oxidising

atmosphere is substantially nitrogen.

Preferably, the primary and/or secondary tubes are formed from silver

or a silver alloy.

Preferably, the secondary tube includes a rectangular bore for

receiving the tapes.

Preferably, the precursors packed into the primary tube includes Pb

stabilised BSCCO-2223 ceramic material.

Substantial isotropy in the wire may be ascertained by placing a pair

of voltage taps at a distance of D apart on the surface of the wire where D is

greater than the pitch length of the twist about the longitudinal axis of the wire.

Preferably, D is a whole number of the half twist pitches. In other

embodiments, D is a whole number of twist pitches. A DC magnetic field of

non zero magnitude is incident on the wire at an angle of 90 degrees to the

longitudinal axis, and at an initial nominal angle of _α = 0 degrees in the R-ψ > plane. The DC l_c is measured according to accepted practice. Magnetic isotropy is satisfied if equation 1 is met.

In a preferred embodiment, the anisotropic wires may be

manufactured by stacking tapes, formed, for example, as described above, in

parallel together, and placing the stack or stacks in a tube. The tube may

then be drawn down. The anisotropic wire is then twisted such that overall

the wire exhibits isotropic behaviour with respect to the effect of an applied

magnetic field on l_c. The twisting may be performed in stages,

advantageously with an annealing step between each stage. The tube may

then be sintered and further drawn down prior to a subsequent heat

treatment.

The incorporation of tapes of ceramic superconductors into a

preferably round tube having a round bore may result in approximately 28% of

the available cross-section of the tube bore being unfilled. In a preferred

embodiment, the tube comprises a rectangular cross-section bore adapted to

receive a multiplicity of layers of superconducting ceramic tape.

Suitable pitches required to achieve the desired degree of isotropy in

the wire may be determined experimentally depending upon the application

for which the wire is intended. For example, a transformer winding would

desirably incorporate as short a twist as possible. In preferred embodiments,

pitches of about 10mm may be obtained without resulting in a lowering of l_c.

In applications where the wires are formed into a cable, pitches of about

14mm may be suitable. > -

Ceramic superconductor materials for use in the present invention include, but are not limited to, ceramic high temperature superconductors

such as Yttrium Barium Copper Oxide (Y₁Ba₂Cu₃0₇ ) (YBCO-123); Bismuth Strontium Calcium Copper Oxide (Bi Sr₂ Ca₂ Cu₁ 0₂)_χ (BSCCO-2212);

Thallium Barium Calcium Copper Oxide (Ti^a^a^U_gO ) (TBCCO-1223);

Thallium Barium Calcium Copper Oxide (TI₂Ba₂Ca₁Cu₂O_y) (TBCCO-2212);

Thallium Barium Calcium Copper Oxide (TI₂Ba₂Ca₂Cu₃O_y) (TBCCO-2223); Mercury Barium Calcium Copper Oxide (Hg₁Ba₂Ca₂Cu₃O_y) (HBCCO-1223):

and -1212 type superconductors such as [(Pb.Cd). Sr₂(Y,Ca) Cu₂0₇]; but

BSCCO-2223, Lead stabilised Bismuth Strontium Calcium Copper Oxide, is

preferred.

Precursor materials of the ceramic superconductor materials include

oxides, carbonates, nitrates and other compounds which contain the metal

elements in the desired proportions. Where the precursor materials are a

mixture of separate components, the components are generally combined in

amounts which promote the formation of the ceramic superconductor material.

For example, in the formation of BSCCO-2223, the desired precursor

materials are powders prepared by co-decomposition of metal nitrate

solutions having the cation ratio of Bi:Pb:Sr:Ca:Cu = 1.84:0.35:1.91:2.05:3.06.

The powders may be calcined and may contain the -2212 as the major phase.

The precalcined powders may be transformed to the -2223 phase by sintering

at about 1103K. The ceramic superconductor material or precursor material thereof

may be formed into a structure suitable for deformation by any convenient

means, including by moulding, pressing, slip casting and extrusion.

The at least one ceramic superconducting tape may be formed by any

convenient means. Such means include by the PIT technique, a Doctor/Blade

process, a dip coating process and the like. Processes such as the PIT

technique result in the ceramic superconductor material, or precursor material

thereof being encased in a metal conductor. Processes such as the

doctor/blade and the dip coating process result in the ceramic superconductor

material or precursor material thereof forming a thick film on a metal

substrate. It will be understood that the process by which the tape is formed

is not narrowly critical. However, in preferred embodiments of the invention

the formation of the elongate metal composite with a ceramic superconductor

material or precursor material thereof by the PIT technique is particularly

advantageous.

The metal selected to support the ceramic superconducting material is

typically selected according to the desired properties of the tape. For

example, in the production of high temperature superconducting wires or

tapes, it is desirable to use silver or silver alloys to support the high

temperature superconducting material in particular, while other metals and

metal alloys may be identified, silver, dispersion strengthened silver, and

dilute silver alloys such as with up to 4% gold for example 2% gold, have

been found to be very effective. Other effective materials include silver- > magnesium alloys, whereby the magnesium content is preferably less than

1 %.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way

of example only, with reference to the accompanying drawings in which:

Figure 1 is a cross-sectional micrograph of a 1X11 z-stack

multifilamentary tape-in-tube according to the invention;

Figure 2 is a cross-sectional micrograph of a 2X11 multifilamentary

tape-in-tube example according to the invention;

Figure 3 is a graph of l_c as a function of applied magnetic field for the

example of Figure 2, and included for comparison, l_c behaviour with applied

magnetic field directed along the a-b and c tape axes respectively of a

conventional 27 multifilamentary flat tape;

Figure 4 is a graph of l_c as a function of applied magnetic field for the

example of Figure 2 at 77K;

Figure 5 is a graph of l_c as a function of applied magnetic field for the

example of Figure 2 at 4.2K;

Figure 6 is a micrograph of a restacked 2X7 multifilamentary tape-in-

tube example according to the invention;

Figure 7 illustrates the experimental arrangement employed in

characterising the example of Figure 6; and Figure 8 graphically illustrates l_c as a function of the resulting

magnetic field performance of the twisted sample (curve 1) of Figure 6 and an

untwisted sample (curve 2).

PREFERRED EMBODIMENTS OF THE INVENTION

EXAMPLE 1

A charge of partially-reacted commercial (Bi, Pb)-2223 powder of

nominal stoichiometry Bi_{1 84}Pb₀₃₄Sr_{1 91}Ca₂₀₃Cu_{3 06}O₁₀₈ was packed into a

silver (99.99%) tube (1mm wall) which had been crimp-sealed at one end.

After the powder was fully loaded, the packing end was also closed.

After a de-gassing procedure, the packed tube was swaged and then

subjected to a series of drawing passes, reducing its diameter, to form a

ceramic-metal composite wire with an inner ceramic core and outer metallic

sheath. All mechanical deformation in the experiment, including the drawing

schedule consisted of reduction of approximately 15% per die pass. That is,

the packed tube was drawn through a die a plurality of times, reducing its

diameter approximately 15% per draw. After the final drawing reduction, the

round composite wire was rolled into a flat tape 0.3mm thick.

Multifilamentary samples of tape-in-tube wires were fabricated by

packing a plurality of the flat tapes into another round Ag tube (99.99%, wall

thickness = 1 mm), in the following manner, with control of the tape orientation

inside the round tube ensuring that the tapes remained substantially parallel.

A particular re-stack tube was fabricated in one example by placing a

stack of 11 tapes on top of each other and inserting the stack into the Ag > tube, the tube having an inner diameter of 8mm and an outer diameter of

10mm. Another example was fabricated in by placing 2 parallel stacks of 11

tapes side by side into an identical tube. After stacking the 11 or 2 x 11 flat

tapes into the second tube, the restacked tube was drawn to wire and heat

treated. There was no rolling operation.

The twisting procedure for both examples was as follows. After

drawing to a diameter of 1.54mm, the wire was twisted to a pitch of 10mm in

the following steps: Pitch = 66,33,15mm, with intermediate annealing at 873K

for 10 minutes in between each step. Twisting of short lengths was achieved

by clamping the ends of the wire in vice grips, and counter rotating the ends a

half turn to achieve one twist pitch in the wire. About 6 twist pitches were

imparted to the wire between each annealing step.

Longer lengths of wire may be twisted by varying degrees at each

later stage of reduction, with intermediate annealing between each draw pass.

This may be achieved with a rotating die assembly.

After a first sinter, the wires of the examples were reduced further in

diameter to 1.42mm without further twisting. This elongated the twist pitch by

about 10%.

Finally the wires were sintered at 1043K<T<1113K for times up to 100

hours, in flowing synthetic air.

The current-voltage (l-V) characteristics of the wires at 77K, in self-

field, were determined using a four-point technique with voltage contact

spaced 70mm apart, using a voltage criterion of O.lμVmrrr¹. After each > measurement, the current was reversed to prevent spurious results arising

from localised heating. Isothermal transport critical current measurements were performed in applied DC fields up to 7 x 10⁵ Am-¹ at 77K, with the

sample able to be rotated ψ=360° about the longitudinal axis of the wire, while

suspended between the poles of an electromagnet. In addition, l_c measurements at temperatures T=4.2 K and 70K in applied fields up to 8 x

10⁶Aιτr¹ orientated along the wire in a first axial alignment and in a second

perpendicular axial alignment were also carried out on the wires and

compared with corresponding measurements on tape.

The critical current density was calculated from l_c/s, where s is the

cross-sectional area in the ceramic core. A Kontron KS 300 image analysis

software package connected to a Carl Zeiss optical microscope was used to

obtain the area, s. The wire samples were analysed by employing back-

scattered electron microscopy (SEM-BS) using a Jeol JSM 5400 LV electron

microscope. SEM samples (8 mm in length) were prepared by mounting in

epoxy resin and mechanical polishing without any chemical etching.

Figure 1 shows a cross-sectional micrograph of the 1 x 11 z-stack

multifilamentary tape-in-tube twisted wire of the example. This sample had a

fill-factor of 18% and filament thickness of 60 + 20 _μm. At 77K, in self-field,

the sample shown in Figure 1 had an l_c=22 + 1 A, corresponding to a J_c = 7.7

x 10⁷ Am-² (J_E=1.4 x 10⁷ Arrr²). Geometrical calculations show that the multitape structure of Figure 1 has a non-ideal initial packing fraction = 0.56. The

packing fraction can be advantageously increased to 0.76 by using different cross sectional area tapes to re-stack. Such a wire is illustrated in Figure 2

which is a cross-sectional micrograph of the 22(2 x 11 ) multifilamentary tape-

in-tube wire of the example. The sample in Figure 2 had a fill factor of 20%,

and at 77K, in self-field, also had an l_c= 22+1 A, corresponding to a J = 7 x

10⁷ Am^-2 (similarly with J_E= 1.4 x 10⁷ Arrr²). The packing fraction of

approximately 0.80 for this tape compares favourably to that obtained from a

conventional round wire re-stacking procedure which yields an initial packing

fraction of 0.74.

Figure 3 shows a plot of the observed I versus applied field at 77K of

the twisted round wire of the 2X11 multifilamentary tape-in-tube example,

having a 14mm twist pitch. For comparison, the observed I behaviour with

applied field along the a-b and c tape axis respectively is also shown for a

conventional 27 multifilamentary flat-tape. The observed l_c overall

dependence on an applied field for the round z-stack wire was in-between that

of the a-b:c axial applied field anisotropy for the conventional multifilamentary

powder-in-tube tape, showing that the 14mm twist pitch provides a degree of

homogeneity of the field direction - dependence of these tapes. The results

were also supported by magnetisation measurements.

Plotted data comparing l_c versus applied magnetic field (l_c-B),

measured by the procedure described above, for the tape and round wire

compared in Figure 3 shown in Figure 4 (77K) and Figure 5 (4.2K). The figure

legends are as follows:

H//c l-B for tape with field applied perpendicular to the tape surface; > -

H//ab l-B for tape with field applied parallel to tape surface; wire l-B for isotropic wire; and

wire2 l-B for isotropic wire with field applied 90° from previous set.

It is to be noted that, at each temperature, the two data sets for the wire are

virtually co-incident, indicating an overall isotropic l-B property for the wire.

EXAMPLE 2

A partially reacted commercial (Bi, Pb)-2223 powder of nominal stoichiometry Bi_{1 84}Pb₀₃₄Sr_{1 91}Ca₂₀₃Cu₃₀₆O₁₀₈, was packed into a 99.99%

silver tube having a 1 mm wall thickness and crimp sealed at one end. After

the powder was fully loaded, the packing end was closed. In other

embodiments, however, the silver packing tube may include an inner cross-

section being square or polygonal.

The loaded tube was degassed by heating to 923K for approximately 3

hours. The packed tube was then swaged and subjected to a series of die

drawing passes, reducing its diameter, to form a ceramic-metal composite

wire with an inner ceramic core and outer metallic sheath. All mechanical

deformation in the process, including drawing, provided a reduction in

diameter of approximately 15% per pass. After the final drawing reduction,

the composite was drawn down to a diameter of 2.13mm.

The wire was rolled to a flat tape of thickness 0.80mm. This rolling was

conducted in steps such that the completion of each step corresponded to a - ^• flat tape thickness of 2.13, 1.80, 1.50, 1.20, 1.00 and 0.80mm. No annealing of the wire occurred to this point.

A plurality of flat tapes were stacked in an array inside a round 99.99%

silver tube having a wall thickness of 0.75mm. The flat rolled tape was

divided into sections of approximately 100mm length and arranged in two parallel stacks of 7 tapes each. This arrangement was found to be most

suitable to fit into a silver tube having an internal diameter of 8.50mm.

However, in other embodiments of the invention, the number of parallel stacks

having a plurality of tapes in each can be selected to suit a silver tube having

an arbitrary internal diameter.

This composite structure, or "restack", was degassed by heating to

923K for approximately 3 hours. The degassing was executed in a vacuum

whose pressure did not exceed 1.33 Nnrr².

The restacked composite structure was then drawn down to a diameter

of 1.54mm. This result is shown in Figure 6.

The annealing of the composite restacked structure during drawing

was conducted in an inert atmosphere by flushing with nitrogen gas. The

nitrogen gas advantageously prevented the magnesium alloy present in the

packing tube from oxidising, and hence hardening, during the mechanical

deformation stages.

The resulting drawn restacked composite structure, or wire, had three

150mm lengths divided from it. Each wire was subject to twisting about its

longitudinal axis wherein each of the three wires respectively experienced 15, 8 and 4 longitudinal turns over their lengths. The respective pitches of the twisted wires was 5, 10 and 20mm.

During the twisting procedure, annealing was carried out periodically to relieve the work hardening and deterioration of the filament structure. The

annealing during this process was carried out in an oven having an inert

atmosphere of nitrogen at 773K for three minutes. The annealing was

effected after every two 360° twists of each end of the respective wires. In

other embodiments, annealing is effected after each 360° twist of the wire.

That is, the annealing was effected for ends of the wire being rotated 360°

with respect to each other.

In addition to the three 150mm sample lengths used in the twisting

process, an untwisted control sample of 200mm length was also selected.

The control sample was drawn down to a diameter of 1.54mm, the same as

the three 150mm lengths. Both the three 150mm sample lengths and the

control sample were sintered in identical fashions during the drawing process.

Upon completion of the first sinter for each of the wires, their diameters by

drawing were further reduced to 1.45mm. This represented a reduction in cross-sectional area of 14% and a reduction in diameter of 6%. Each of the

samples were then subjected to a second sinter as previously detailed.

Figure 6 presents a representative example of the cross-section of one

of the three 150mm samples. The diameter of the wire illustrated in figure 6 is

1.45mm. An arbitrary one of these twisted 150mm wires and the control wire

were characterised by employing the following technique. A sample of twisted

wire was cooled to 77K in a magnetic field of fixed magnitude of 4.05mT and directed perpendicular to the longitudinal axis of the wire. Voltage contacts in the wire structure, used to measure the critical current of the sample, were

placed an integral number of twist pitches apart. In this embodiment, the

sample having a pitch of 5mm corresponding to 15 turns along its 150mm

length, was chosen. Distance between these voltage contacts was arbitrarily

chosen to be 10mm. Repeated measurements suggested voltage contacts

were spaced a distance of 11 + 0.5mm as a result of practicalities in soldering

the contacts.

Having an initially randomly oriented magnetic field perpendicular to

the longitudinal axis of strength 4.05mT, the critical current was found to be

5.5A. Figure 7 schematically depicts the arrangement of the magnetic field.

Figure 8 graphically illustrates the resulting magnetic field performance

of the twisted sample (curve 1 ) and the untwisted sample, or control wire

(curve 2), when the magnetic field direction was swept in an arc of 180° of 5°

steps at a constant magnitude. The magnetic field was constantly directed

normal to the longitudinal axis of the wire, and the critical current after each 5°

was measured. As is apparent from Figure 8, the critical current density for

the twisted wire varied by less than 5% of the initial critical current for each 5°

rotation of the magnetic field. Curve 2 of the figure denotes the results for the 200mm control sample

of untwisted wire subjected to the same test. Results of the normalised

critical current shows a variation of more than 30% from the initial critical

current for the initial critical current for the untwisted wire. It is apparent,

therefore, that a clear advantageous result is achieved by twisting the wire

about its longitudinal axis.

These results suggest that the tape-in-tube technique may provide a

suitable route to manufacture practical circular wire superconductors. From

an engineering point-of-view, optimum tape-in-tube round wires should have

as homogenous a transverse J_c dependence as possible, and be

manufactured to tight confirmation tolerances. This is because, unlike the

high a-b:c aspect ratio tapes, round wires will have no physically discernible

external features to characterise their anisotropy. However, the

superconducting filamentary structure within the wires will still be anisotropic

and so the best I c will still be obtained if the filaments contain well-textured,

aligned grains.

Those skilled in the art will appreciate that the invention described

herein is susceptible to variations and modifications other than those

specifically described. It is to be understood that the invention includes all

such variations and modifications which fall within its spirit and scope. The

invention also includes all of the steps, features, compositions and

compounds referred to or indicated in this specification, individually or

collectively, and any and all combinations of any two or more of said steps or features. Although the invention has been described with reference to

particular examples, it will be appreciated by those skilled in the art that it may be embodied in many other forms.

Claims

CLAIMS:-

1. An axially extending wire including a substantially axially extending

superconducting filament which is flattened in cross section, the

superconducting filament being twisted along at least a substantial portion of

its length into a substantially helical shape, such that the wire is:

substantially anisotropic in relation to the effect of an applied magnetic

field on the critical current l_c at any cross-sectional point along the wire's

length; and

substantially isotropic in relation to the effect of an applied magnetic

field on the critical current l_c along a substantial length of the wire.

2. An axially extending wire according to claim 1 , wherein the filament is

rotationally asymmetrical about the axis at each point along the length of the

wire.

3. An axially extending wire according to claim 1 or 2, wherein the

anisotropic and isotropic conditions are satisfied in relation to a magnetic field

applied in a plane substantially normal to the axis.

4. An axially extending wire according to any one of the preceding claims,

including a plurality of the filaments arranged, in cross section, in at least two

parallel stacks.

5. An axially extending wire according to claim 4, wherein each stack

contains a plurality of the filaments.

6. An axially extending wire according to any one of the preceding claims,

wherein the filaments are embedded in a silver or silver alloy matrix.

7. An axially extending wire according to any one of the preceding claims,

wherein a pitch of the helical shape is selected on the basis of an application

to which the wire is to be applied.

8. An axially extending wire according to claim 7, wherein the pitch of the

helical shape is selected on the basis of an expected minimum radius of

curvature which the application to which the wire is to be applied.

9. An axially extending wire according to any one of the preceding claims,

wherein the filaments include Pb stabilised BSCCO-2223 ceramic.

10. A method of fabricating a superconducting wire, the method including

the steps of:

providing a primary metallic tube;

packing precursors of ceramic high temperature superconducting

material into the primary metallic tube;

applying heat to degas the contents of the primary metallic tube;

closing the primary metallic tube;

swaging and drawing the primary metallic tube to reduce its diameter;

rolling the primary metallic tube to form a tape;

twisting the tape into a generally helical shape; and

applying sufficient heat to the primary metallic tube to sinter the

precursors, thereby to form a ceramic-composite superconducting tape with

an inner superconducting ceramic filament and an outer metallic sheath.

11. A method of fabricating a superconducting wire according to claim 10,

further including the step, prior to the sintering step, of positioning one or more of the tapes within a secondary metallic tube, and drawing the

secondary tube to reduce its diameter.

12. A method of fabricating a superconducting wire according to claim 11 ,

wherein the tapes are positioned within the secondary tube prior to twisting

the tape.

13. A method of fabricating a superconducting wire according to claim 11 ,

wherein the tapes are positioned within the secondary tube in one or more

parallel stacks.

14. A method according to claim 12 or 13, wherein the twisting step

includes the sub-step of twisting the secondary tube after it has been drawn,

thereby to impart the requisite twist to the tapes prior to sintering.

15. A method according to any one of claims 10 to 14, wherein the twisting

step is performed in a number of twisting sub-steps, at least some of the

twisting sub-steps being separated by annealing steps.

16. A method according to any one of claims 10 to 15, wherein the drawing

step is performed in a number of drawing sub-steps, at least some of the

drawing sub-steps being separated by annealing steps.

17. A method according to any one of claims 10 to 16, wherein the

degassing step includes the sub-step of placing the primary tube into a non-

oxidising atmosphere.

18. A method according to any one of claims 10 to 17, wherein the non-

oxidising atmosphere is substantially nitrogen.

19. A method according to any one of claims 10 to 18, wherein the primary and/or secondary tubes are formed from silver or a silver alloy.

20. A method according to any one of claims 10 to 19, wherein the secondary tube includes a rectangular bore for receiving the tapes.

21. A method according to any one of the preceding claims, wherein the

precursors packed into the primary tube includes Pb stabilised BSCCO-2223

ceramic material.

22. A superconducting wire manufactured in accordance with the method

of any one of claims 10 to 22.