AU619468B2 - Ceramic superconducting devices and fabrication methods - Google Patents

Description

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PCT

WORLD INTELLECTUAL PROPERTY ORGANIZATION Inelrnational Bureau 709r INTERNATIONAL APPLICATION PUBLISHEb UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 In t tion Publ on Number: WO 88/ 08618 HO1L 39/24, 39/14, 39/12 II f H B(4 n ti u io te: 3 November 1988 (03.11.88) (21) International Application Number: PCT/GB88/00330 (81) Designated States: AT, AT (European patent), AU, BB, BE (European patent), BG, BJ (OAPI patent), BR, (22) International Filing Date: 28 April 1988 (28.04.88) CF (OAPI patent), CG (OAPI patent), CH, CH (European patent), CM (OAPI patent), DE, DE (European patent), DK, FI, FR (European patent), GA (31) Priority Application Number: 8710113 (OAPI patent), GB, GB (European patent), HU, IT (European patent), JP, KP, KR, LK, LU, LU (Euro- (32) Priority Date: 29 April 1987 (29.04.87) pean patent), MC, MG, ML (OAPI patent), MR (OA- PI patent), MW, NL, NL (European patent), NO, (33) Priority Country: GB RO, SD, SE, SE (European patent), SN (OAPI patent), SU, TD (OAPI patent), TG (OAPI patent), US.

(71X72) Applicants and Inventors: EVETTS, Jan, Edgar [GB/ GB]: 53 Owlstone Road, Cambridge GLO- Published WACKI, Bartlomiej, Andrzej [PL/GB]; 129 Catharine With international search reporL Street, Cambridge CBI 3AP Before the expiration of the time limit for amending the claims and to be republished in the event of the receipt of (74) Agent: KEITH W. NASH CO; Pearl Assurance amendments.

House, 90-92 Regent Street, Cambridge CB2 IDP (88) Date of publication of the international search report: 1st December 1988 (01.12.88) (54) Title: CERAMIC SUPERCONDUCTING DEVICES AND FABRICATION METHODS (57) Abstract The final step of a method of making a super- conducting composite comprising an outer supporting casing (10, 31) of a non-superconducting inert material having active high temperature superconducting material (14, 27) such as YBa 2 Cu 3 0 7 in the form of a thick walled tube located therewithin, involves passing oxygen through the hollow interior of the tube to produce oxidation of the active material and thereby create a superconducting oxide phase. The internal oxidation process can be performed in more than one step. Dimensional changes which occur during the process can place the superconducting phase under compressive stress. A lining of an oxygen permeable metal (12, such as silver, gold or an alloy, gives support to the superconducting material during fabrication. A barrier layer (16, 29) of a metal such as nickel or 29 tantalum is preferably coated on the internal surface of the outer supporting casing (10, 31). The hollow core may be filled with a removable material for fabrication, and cleared after fabrication. The superconductor can be formed into a coil of other devices. A superconducting composite is further described which includes as an integral part a support sheath for an inner sleeve of brittle and readily damaged superconducting oxide material YBaCu 3 0 7 to protect the latter against degradation and attack from elements such as water, and also provides mechanical structural support. The support sheath will normally conduct electricity to stabilise the superconductor.

PCT/GB88/00330 WO 88/08618

DESCRIPTION

CERAMIC SUPERCONDUCTING DEVICES AND FABRICATION METHODS Field of the invention This invention concerns superconducting materials and their manufacture and is directed to improvements in the design and fabrication of high critical temperature superconducting composite conductors. Typically such conductors depend for th'eir superconducting properties on high field, high critical temperature superconducting oxide or nitride phases, and possess superconducting properties above 77°K.

Background to the invention Although superconducting composite conductors have been, available for some years they have hitherto depended on metallic superconducting materials for their properties.

Examples of existing commercial composites are Nb-Ti alloy composites and composites based on A15 intermetallic compounds such as Nb 3 Sn. A new class of higher critical temperature, high field, superconducting oxide phases presents new opportunities for applications of superconductivity but has created new problems for the design of high performance conductors.

Some features of the new materials make the design of a practical conductor or wire particularly difficult.

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c~I Illlrrpr PCT/G.B88/0 0330 WO 88/08618 2 In the first place the materials are inherently brittle, and this raises acute problems for the support a conductor under the extreme conditions of mechanical stress encountered in many high field applications.

Secondly in their optimum superconducting state these materials are chemically extremely active, in particular exhibiting extreme sensitivity to the presence of oxygen and moisture, and in view'of their extreme chemical activity, the components are also susceptible to corrosion and environmental.degradation.

In certain respects the design of a composite conductor for operation at 77°K and above presents fewer problems than at lower temperatures. Thus for example the specific heat of materials is an order of magnitude, or more, greater, at elevated'temperatures and this makes the design of a thermally stable conductor simpler.

Also, for operation under steady state conditions it is likely that conductors with superconductor cross-sections, larger than commonly used in currently available composites, will be acceptable.

The invention seeks to provide a method of construction and manufacture which enables brittle reactive superconducting oxide or nitride phases to be incorporated into a robust, prestressed, environmentally protected high .critical current composite conductor, and to -provide a form of construction in which the conductor may be reactivated into an optimum state should its performance deteriorate with time.

Summary of the invention 1- -3- According to one aspect of the invention there is provided a method of forming a superconducting composite wire, which comprises the steps of forming an outer supporting casing of a non-superconducting inert material having active high temperature-superconducting material in the form of a thick-walled tube having a hollow interior located therewithin; drawing the composite, to reduce its cross-section and thereby fabricate a wire; and passing oxygen through the hollow interior of the tube to produce oxidation of the active material, to create a superconducting oxide phase after fabrication of the wire.

Fabrication may involve for example laying up, or winding into a coil, IP incorporation into a machine or device., The step of passing oxygen through the hollow tube may be conducted more than once, oxygen being reintroduced into the hollow interior at intervals after previously introduced oxygen has been partially consumed in the oxidation process.

I The oxygen may be introduced under high pressure.

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Where appropriate, oxidation is controlled so as to produce a superconducting phase having the properties for the particular application in mind.

In particular dimensional changes such as contraction on cooling of the superconductor after reaction, and differential thermal contraction as between the conductor and support tube on cooling from the reaction temperature to the working temperature, can be used to place the superconduYting phase under compressive stress. Prestressing in this way avoids brittle in service failure of the superconductor.

Preferably the hollow interior is defined by a tube of oxygen-permeable -4metal to give support to the superconducting material during fabrication and enable the oxidation step to be performed.

The final composite conductor will normally be required to be of relatively small cross-section and relatively much greater length. To this end the composite may be formed by extrusion over a mandrel so as to provide the hollow interior. Thus prior to oxidation the hollow interior may be filled with a suitable material which can be removed readily after fabrication. This material may be left in place during subsequent drawing to further reduce the cross-sectional area of the composite. However at the end, it must be capable of being removed so as to enable oxygen to be pumped along the hollow interior to produce the oxidation of the material coating the inside of the supporting casing to form the superconducting material previously referred to.

This process, often referred to as a substitution process, may for example involve the use of a ductile metal or metal allow, wax or the use of water ':915: or other liquid such as castor oil which may be frozen into the cross-section region of the composite prior to the fabrication stages and then removed by heating or other convenient means typically under pressure so as to enable the cross-section of the final fabrication to be heated and then oxidised in the manner required by the invention.

It is to be understood that the invention is not limited to a composite with only a single superconducting element within the cross-sectional area of the composite, but envisages composites having at least two superconducting elements within the composite, extending the length of the composite.

3 The starting materials from which the superconducting layer is to be formed may for example consistsof mixed powered oxides, carbonates, or other compounds such as co-precipitated nitrates or metals in any combination, and typically the starting materials are pre-treated, reacted, graded and preferably magnetically separated prior to insertion into the composite wire.

Most of the starting materials referred to are in the form of a clinker when in the solid and the first step will typically involve grinding the clinker or other solid form into a powder. If this is then cooled to a temperature at which the material becomes superconducting (typically the temperature of liquid nitrogen) a magnetic field will cause any superconductive powder to move due to the interaction between the magnetic field and the induced currents in the conductive material and this can be used to separate superconductive powder particles from non-superconducting particles. Thus for example the superconductive particles can be caused to levitate be simply to deflected depending on the direction of the magnetic field and the design of the separator. The non- 0 g s** o *ooeo 88/08618 PCT/GB88/00 33 0 WO 88/08618 6 separated material may typically be reprocessed and subjected to a further magnetic separation, to further refine the material.

Carbonates may be used as the raw material if an oxide is not available or is not suitable. Mention has also been made of the use of co-precipitated nitrates as possible starting materials and examples are typically coprecipitates from aqueous solution of yttrium nitrate, barium nitrate and copper nitrate.

Where a substitution process is employed and the hollow core is filled with a material which is to be removed after final fabrication and before the oxidation stage, the composite will normally have to be cooled before it is subjected to extrusion or swaging or wire. drawing techniques since all of which normally increase the temperature of the composite and could cause a failure of the cavity forming characteristic of the substitute material within the core of the composite.

In a preferred embodiment of the invention, the outer supporting casing is formed from copper or stainless steel and the wall of the hollow core is formed from silver -or silver alloy with the high temperature superconductor material sandwiched therebetween. Preferably the inner Ssurface of the outer supporting casing is nickel or silver or tantalum lined.

Typically a composite has an initial diameter of 75mm with an outer support casing thickness of 12.5mm, a powder annulus having a radial thickness of some 15mm, an inner copper liner of Imm thickness and a central core of substitute material of diameter 18mm. After appropriate PCT/GB88/003 3 0 WO 88/08618 processing as by drawing, extrusion etc., the final.

diameter of the conductor would be of the order of 1 to 2mm.

Preferred materials for the substitute core material are low melting point lead alloys, wax, and inorganic salts.

Where materials such as aluminium etc. have been employed as the core substitute, chemical means may be employed for removing the material.

Where the material can be removed by application of gas pressure, the removal of the material and the oxidation of the superconductive material, may be achieved simultaneously by using oxygen under pressure to remove the unwanted substitute material.

Where the final form of the composite requires that different points along the length of the composite must be electrically separated from adjoining sections thereof, as when wound in the form of a coil, the composite preferably includes an outer electrical insulating layer.

Where the environment of the final composite is such that the brittle nature of the superconducting material may cause problems, appropriate non-reactive strengthening rods, fibres or mesh material may be located within the powder sleeve of the initial composite and worked as by extrusion or otherwise with the remaining materials of the composite so as to extend along the length of the final composite within the superconducting region. Since this region is conductive, it is probably irrelevant whether the strengthening material is itself conductive or nonconductive. However where it is conductive, this will/ -8further assist in stabilising the superconductive material which, as is well known, can be damaged if local hot-spots occur.

The nickel, silver or tantalum coating of the interior of the support casing reduces the reaction which may other wise occur between the outer casing and the alkali metals or important rare earth materials which will be present in the superconducting layer. In the alternative, gold may be used in place of nickel, silver or tantalum.

It will be appreciated that gold may in fact be substit ed for the copper or silver previously referred, where circumstances and applications are appropriate.

The invention thus proVides a support sheath for very brittle and readily damaged superconducting materials which not only protects the latter against degradation and attack from elements such as water, but also provides mechanical structural support. Preferably the outer support sheath, if not also any inner support sheath, must be capable of conducting electricity to stabilise the 0:015 superconductor and where reinforcing elements are employed throughout the ~:cross-section of the superconducting material, these are also, advantageously, electrically conductive.

The invention also enables an insulating outer surface to be provided which will enable separation between windings in a coil and the like.

The invention also lies in a method of fabricating a superconducting composite wire involving the steps of forming as an annular sandwich between inner and outer conductive tubular members a Layer of a material which on subsequent processing can be rendered superconductive; drawing the annular p~lfq sandwich, to form a wire of reduced overall cross-section whilst ensuring that the 03 -9inner tubular member does not collapse so that a hollow passage extends throughout the entire length of the wire; 'and a reaction stage comprising passing a reacta;-,t selected from gases, vapours and liquids through the hollow passage of the wire whilst maintaining a temperature at which an appropriate reaction will occur to render superconductive the material between the inner and outer tubular members.

In a development of the method of the invention, the inner tubular member may be filled with a removable substance such as a metal having a melting point lower than that of the other materials which the annular sandwich formed, a metal alloy, cstor oil, wax or even water so as to prevent the collapse of the inner tubular member during the drawing and removing the removable substance before or during the reaction stage.

***Whatever the material thai: is chosen for the inner tubular support, it is of course essential that the material is either removable before the reaction stage or is permeable to said reactant.

According to another aspect of the invention, a layer of nickel or tantalum is applied to the inner surface of the outer conductive tubular member.

According to another development of the method, a layer of gold or silver may advantageously be applied to the inner surface of the nickel or tantalum layer before the material which is to be reacted to from the superconductive material is added.

It is of course not necessary for the inner support tube to be of metal such as silver or copper since other materials may be used such as a plastic pIJ materials, the only requirement being that the material concerned is permeable to '0 0 0 10 the reactive agent which is to be added to the composite at the time when the inaterial saidwiched between the inner and outer supports is to be rendered superconductive.

The invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a cross-section of a composite conductor constructed in accordance with the present invention.

Figure 2 is a cross-section through a silver yttrium barium copper oxide reference wire having an outer diameter of 0 999999 9.9.

.99999 o *go ••go* ooooo W 88/08618 PCT/GB88/00330 11 Figure 3 is a cross-section through a multicore hollow core conductor, Figure 4 is a cross-section through a single hollow core conductor, Figure 5 illustrates the critical current transition for the reference wire at and for silicone at the reference wire with a silica addition Figure 6.is an axial section through a circular section hollow conductor constructed in accordance with the invention, Figure 7 is a cross-section on the line AA through the hollow single core conductor of Figure 6, Figure 8 is a cross-section through a twin conductive tape having a twin core and stainless steel outer, Figure 9 illustrates graphically the resistive transition at 77K in zero magnetic field for external diffusion (ED) conductors sintered for 24 hours at 940 0 C under oxygen, having an outer diameter of yttrium barium copper oxide of 0.7mm, Figure 10 illustrates graphically the critical current value as a function of heat treatment time for ED conductor under oxygen at 940'C (starting material: powder after calcination (SSR)) with the outer diameter of the yttrium barium copper oxide of 0.7mm, Figure 11 illustrates graphically the reduced critical 8808618 PCT/GB88/00 33 0 WO 88/08618 12 current i as a function of preform particle size. The 'applied field H perpendicular to the current direction.

Figure 12 illustrates graphically resistive transitions at internal diffusion twin filament composite and at (b) external diffusion (ED) single filaments, Figure 13 illustrates graphically the reduced critical current i as function of load P for an ED wire under p load, and ED wire as before but after removal of the load and for an ID wire under load.

In the drawing the composite is shown as comprising an outer stainless steel sheath 10 and an inner silver sheath 12 between which is sandwiched an annulus of high temperature superconducting material 14. Between the latter and the outer sheath 10 is a thin film of nickel 16 and within the annulus are located elongated fibres 18 which reinforce the structure. Although shown in the drawing, it is to be understood that these reinforcing fibres are optional.

In accordance with the method of the invention, the hollow interior of the tube 12 is initially filled with a removable material, such as a,low melting point alloy and this serves to ensure that the inner tube 12 does not collapse during the reducing steps.

Also in accordance with the method of the invention, the core of material within the tube 12 is removed before the final stage of processing which involves passing a gas vapour or liquid through the tube which can permeate through the wall of the tube into the material 14 and react therewith to render the latter superconducting.

WO88/08618 PCT/GB88/00330 13 Typically oxygen is passed through the hollow tube 12 after the core material has been removed and the composite is maintained at an appropriate temperature at which oxidation of the material 14 will occur as the oxygen permeates through the wall of the tube 12.

The method of the invention also involves the formation of the composite into a component such as a coil or the like before the core material is removed from the tube 12.

S''e invention also lies in superconducting composite conductors when formed by the method of the invention.

Four major considerations determine the des .gn of a .suitable wire: 1) The most critical problem is to ensure the supply of sufficient oxygen to the final product to obtain an optimum critical temperature. This can be done by exposing the superconductor directly Lo an oxygen source or by diffusing oxygen through a permeable barrier.

Results using an "external diffusion" fabrication route have been reported by S Jin et al. The new "internal diffusion" route of the invention is preferred.

2) Mechanical support is of importance for a brittle material. In commerical composites based on brittle under compression, this can be done by incorporating within the composite a metal with a high thermal expansion.

3) In order to transfer current to the superconductor and achieve adequate electrical stabilisation it is essential to clad the superconductor with a mrtallic coating that WO88/88 PCT/GB88/00 330 WO 88/08618 -14does not develop an insulating oxide barrier at the superconductor interface.

4) FinjLly the composition, state of order, microstructure and defect structure of the superconductor must be optimised to support a high critical current density.

All the composites tested by the inventors used as their starting material finely powdered YBa 2 Cu30 7 with a critical temperature of 91.5K and a transition breadth of 1K. Details of preparation have been reported by R A Camps et al. An initial reference composite was fabricated using the standard method of swaging a simple preform consisting of a silver Ag tube 11 containing superconducting-powder, (see Figure The composite wire was sintered in oxygen at 950*C for 24 hours so that oxygen could permeate through the tube wall 11. It was then annealed at 600°C and 350°C for 8 and 12 hours respectively. The wire exhibited a critical current -2 density of 525 A cm at 77K in zero field. X-ray diffraction analysis of the compound 13 inside the finished wire gave the same result as in the bulk compound.

It is clear that silver is an excellent cladding material both for oxygen transport by diffusion and for making good electrical contact to the superconductor. However the wires were mechanically extremely weak and an attempt was made to reinforce the composite by casing with an outer layer of stainless steel. This composite was not superconducting confirming that diffusion of oxygen through the silver during heat treatment is essential.

i i i PCT/GB88/00330 WO 88/08618 15 In order to facilitate reinforcement without cutting off the oxygen supply a composite was built as shown in Figure S3 or 4 which incorporate a hollow core 15 to allow oxygen penetration by "internal diffusion" in accordance with the invention.

In Figure 3 six cores of superconducting material 17, 19 etc are shown surrounded by silver 21 itself encased in a nitrate sheath 23.

In Figure 4 a single hollow core 15' is surrounded by a silver layer 25 separating 4 from the superconducting material 27 which is itself surrounded by a metal foil 29 separating it from a stainless steel outer 31.

'In the example shown in Figure 3, the critical current of -2 the multicore conductor was about 60 A cm at 77K. At present this is limited by difficulty in making contact to the wire and by contamination of the internal surface of the rather irregular hollow core.

The internal diffusion method of invention enables a good oxygen supply to the superconductor metal especially in the single hollow core arrangement of Figure 4.

Some problems were encountered in making current and voltage contacts to sample wires. Current transfer effects can lead to negative-going voltages when the critical current is large and the superconductive crosssection is non-uniform or cracked. By reversing the S* direction of current flow we have established that these effects are thermoelectric in origin. Current and voltage contacts were made using In, Ag or Au pressed directly onto the surface of the wire. Conventional soldering WO 88/08618 PCT/GB88/003 3 0 -16degraded the, join between the outer cladding and the superconducting core, considerably reducing the value of the measured critical current.

Two different experimental composites were used to investigate the relation of the critical current to microstructure.

First in a tape formed by rolling the reference wire, the critical current had a minimum value for a perpendicular applied field which points to the importance of powder Secondly the importance of trace impurities was investigated by adding 0.lwt% Si02 to the reference wire.

This reduced the critical current by a factor of 2 (see Figure This serious degradation has been correlated with the extensive occurence of silica rich inter-grain precipitates in the work done by R A damps et al.

I Example 1 The Fabrication of a Circular Section Hollow Conductor As shown in Figure 6 a stainless steel tube (20) internal diameter 6mm., external diameter 8mm, was used as the external support cladding for the composite. A copper end stop (22) was screwed into one end of the tube to retain the inner portions of the composite during assembly and fabrication of the composite. The tube was then lined with nickel foil (24) fitted closely to its inner surface.

Then a silver tube filled with low melting points Wood's metal was placed centrally in the stainless steel tube supported on the copper end stop.

WO 88/08618 PCT/GB88/00330 17 The silver tube was prepared by filling a silver tube (26) of wall thickness 0.5mm and internal diameter 5mm with Wood's metal (28) and cold swaging to a final external diameter of 2.5mm prior to the insertion into the stainless steel tube The resulting annular space was filled.up completely with superconducting YBa 2 Cu 3 0 7 powder (30) using a stainless steel rammer.

Finally the upper hole was closed with a second hollow copper end stop (32) screwed into place.

The assembled composite was cold swaged to a wire having 3mm outside diameter.

The copper containing end sections were cut-off and a plastics tube was sealed over one end and connected to a source of compressed air. The composite wire was then Smimmersed in boiling water and the molten Wood's metal extruded under the action of the compressed air. The inside of the silver tube was then cleaned using flowing acid, and the completed composite wire was finally sintered in a furnace at 920°C with a flow of oxygen passing through the hollow core to optimise the oxidation of the superconductor by diffusion of oxygen through the inner silver layer, after which the sample was slowly cooled in flowing oxygen.

A similar method was used to fabricate a hollow core composite containing six cores of silver clad superconductor' as shown in Figure 3.

88/08618 PCT/GB88/00 3 30 WO 88/08618 -18- Example 2 The Fabrication of a twin Conductor Tape In a further composite a twin core stainless steel clad tape was prepared which after fabrication had the crosssection of Figure 8 and contained a single hollow central core (34) and two silver clad superconducting filaments (36 and 38).

A stainless steel tube (40) was prepared by being cold rolled ucitil it had two parallel sides and contained a flattened interior hole. Three silver clad wires of identical diameter surrounded by nickel foil (39) were inserted lengthwisL into the tube The two outer wires contained yytrium barium copper oxide superconducting powder prepared by filling a silver tube having a 0.5mm wall thickness and a-5mm internal diameter with powder, one end sealed with a copper end stop, and swaging the tube down to 2mm diameter. The third central wire.contained Wood's metal.

The whole composite was: further rolled until the wires occupied the whole of the internal space in the tube.

The ends of the wire were then cut-off and the exposed connected to one end of the composite and the other end of the tube was connected to a source of compressed air. The composite was immersed in boiling water and the Wood's alloy was removed by the air pressure.

The internal surface of the tube was cleaned by acid flow after which the now hollow composite was placed in an oven PCT/GB88/003 3 0 19 for sintering. During sintering oxygen was blown through the central hollow core in the wire.

Commercial exploitation of high critical temperature superconductors will be severely limited unless mechanically robust, high critical current, fully stabilised wires and composite conductors can be fabricated with uniform properties over lengths of the order a kilometre. The primary problem is the control of the critical current density, I

C

The production of samples in wire. and tape form enables a range of precisely controlled and reproducible experiments to be performed that would be much harder to achieve using sintered pellet samples. Four major considerations determine the design of a suitable wire, 1) the supply of sufficient oxygen to the final product to give a uniform high critical temperature, 2) mechanical support of the brittle ceramic phase, 3) the achievement of current transfer through normal leads or contacts and 4) the achievement of an optimum composition, state of order and microstructure for the support of a high I

C

Initial work has concentrated on a composite formed by swaging or drawing a preform consisting of a silver tube containing superconducting powder. Preliminary work on steel clad composites has also been reported, although the critical current I c was rather low. The invention provides for.the fabrication of steel clad hollow conductors with I, similar to that of silver clad conductors and with much improved mechanical properties.

Measurements have been made and are reported of the dependence of I c on details of the wire fabrication process.

O 8808 PCT/GB88/00330 WO 88/08618 20 Sample Preparation The preform powders investigated were prepared from BaCO 3 3 and CuO powders (Johnson Matthey, 1 99.999% purity) using two methods. Firstly powders mixed in stoichiometric proportion were prepared by solid state reaction solid state reaction (SSR) powders were investigated both after the initial calcination stage and, when fully superconducting. Secondly the starting powders were dissolved in nitric acid and superconducting powders prepared by a citrate synthesis route (CS).

i) External diffusion (ED) composites Wires for external diffusion were prepared using the. standard method of swaging a simple preform consisting of a silver tube containing, the preform powder being investigated as described below. Different powders could be placed in the same tube separated by solid silver spacer sections.

ii) Internal diffusion (ID) composites Steel clad hollow core composites were constructed in a variety of geometries. Single core, twin core and multicore composites have been tested. In each case the hollow core region was built using a metal core which was removed after fabrication to the final diameter, by chemical means or by melting. Both external and internal composites were reaction sintered after fabrication at about 940*C under oxygen and slowly cooled.

Superconducting Measurements i) External diffusion The critical current for identical wires prepared from different starting materials is shown in Figure 9. The upper three traces used a WO 88/08618 PCT/GB88/00330 21 powder size range 25um to 38um an'd were compacted without evacuation. The lower trace is for a sample with the "asground" particle size spectrum and was evacuated under vacuum prior to compaction. The maximum value of I was obtained for superconducting powder swaged under air.

Problems were experienced with the vacuum compacted wire, after additional heat treatment for 60h under oxygen the value of I only increased from 0.3A to 0.4A. In the case of wire made under air one can see that the value of the critical current increases strongly with the increase of SSR time (see Figure 10). The effect of the preform particle size fraction on the magnetic field dependence of the critical current of standard wires is shown in Figure 11. The value of I c in a 'magnetic field increases with decreasing particle size.

ii) Internal diffusion The values of I obtained in wires based on internal diffusion designs are close to the values obtained for external diffusion designs. Figure 12 shows I c measurements for a twin filament tape form composite, compared with a single filament treated by ED under the same conditions. The individual filaments are made from the same powder and have the same crosssection.

iii) Stress Silver clad wires are very weak and show irreversible degradation under load, in comparison the composites clad in steel are able to withstand high loads without degradation of I c (Figure 13). The improvement in properties arises because the steel does not soften at the sintering temperature. The superconducting ceramic is therefore supported by a strong matrix, in addition, during cooling-the differential thermal expansion places WO 88/08618 PCT/GB88/00 3 -22the superconductor under compression.

Discussion and Conclusions The maximum value of I c observed in YBa2Cu307 x in both ID c in 2 3 7-x -2 and ED conductors is 900Acm in zero field at 77K.

Although I is still very low compared to commercial superconductors the results demonstrate clearly the feasibility of fabricating internal diffusion conductors with steel cladding. The ID designs are mechanically robust because of the rigidity of the cladding and because the brittle ceramic is under compression. Such ID conductors withstand high loads without degradation of I c Vt

Claims (19)

1. A method of forming a superconducting composite wire, which comprises the steps of forming an outer supporting casing of a non-superconducting inert material having active high temperature-superconducting material in the form of a thick-walled tube having a hollow interior located therewithin; drawing the composite, to reduce its cross-section and thereby fabricate a wire; and passing oxygen through the hollow interior of the tube to produce oxidation of the active material, to create a superconducting oxide phase after fabrication of the wire.

2. A method as claimed in claim i, in which the step of passing oxygen through the hollow interior of the tube is e conducted more than once, oxygen being reintroduced into 15 the hollow interior of the tube at intervals after o9 previously introduced oxygen has been partially consumed in the oxidation.

3. A method as claimed in claim 1 or claim 2, in which the oxygen is introduced under pressure. 20

4. A method as claimed in any preceding claim, which :comprises the additional- step of placing, the superconducting phase under compressive stress by means of *999 dimensional changes.

A method as claimed in any preceding claim, in which the hollow interior is defined by a tube of oxygen- permeable metal to give support to the superconducting material during fabrication and enable the oxidation step to be performed.

6. A method as claimed in claim 5, in which the oxygen- permeable metal is silver, gold or an alloy.

7. A method as claimed in any preceding claim, in which the composite is formed by extrusion over a mandrel so as to provide the hollow interior.

8. A method as claimed in any preceding claim, in which the hollow interior is filled with a removable material prior to oxidation, which material is removed after S1 fabrication. fli -24-

9. A method as claimed in claim 8, in which the removable material is left in place during drawing.

A method as claimed in claim 8, in which the removable material is selected from a ductile metal, a ductile metal alloy, and a liquid such as water which is frozen into the cross-sectional region of the composite prior to the fabrication, and then removed by heating, typically under pressure.

11. A method as claimed in any preceding claim, in which there are at least two superconducting elements within the composite and which extend over the length of the composite wire.

12. A method as claimed in any preceding claim, the starting materials from which the superconducting layer is to be tformed is selected from mixed powered ooo oxides, carbonates, co-precipitated nitrates and metals in any combination.

13. A method as claimed in claim 12, in which the starting material includes co-precipitates from aqueous solution of yttrium nitrate, barium nitrate and copper nitrate.

14. A method as claimed in any preceding claim, in which the composite S is cooled before drawing.

15. A method of fabricating a superconducting composite wire involving the steps of forming as an annular sandwich between inner and outer conductive o tubular members a layer of a material which on subsequent processing can be rendered superconductive; drawing the annular sandwich, to form a wire of reduced overall cross-section whilst ensuring that the inner tubular member does not collapse so that a hollow passage extends throughout the entire length of the wire; and a reaction stage comprising passing a reactant selected from gases, vapours and liquids through the hollow passage of the wire whilst maintaining a temperature at which an appropriate reaction will occur to render superconductive the material between the inner and outer tubular members.

16. A method as claimed in claim 15, in which the inner tubular member is filled with a removable substance having a melting point lower than that of the other materials from which the annular sandwich is formed, so as to prevent the collapse of the inner tubular member during the drawing, and removing the r movable substance before or during the reaction stage. Iik i 1III1": I I 25

17. A method as claimed in any one of claims 15 to 16, in which a layer of nickel or tantalum is applied to the inner surface of the outer conductive tubular member.

18. A method as claimed in claim 17, in which a layer of gold or silver is applied to the inner surface of the nickel or tantalum layer.

19. A method as claimed in any preceding claim, which additionally comprises forming the wire into a coil. A method as claimed in any one of claims 1 to 14 and 15 to 19, in which the material of the outer casing or of the outer tubular member, respectively, is stainless steel. a a. 0@ *r S 0 SS esa a S DATED this 11th day of November, 1991 Jan Edgar Evetts and Bartlomiej Andrzej Glowacki By their Patent Attorneys: CALLINAN LAWRIE a 0 a O S