MANUFACTURE OF ELONGATE SUPERCONDUCTORS

Abstract: An elongate superconductor is made by enclosing anisotropic particles that are superconducting or are capable of being converted to a superconducting form by subsequent treatment in a tube of a ductile metal, compacting it and reducing it in one or more reduction steps to an elongate tape. At least one of these reduction steps is effected by a rotary forging process, preferably one in which one of the platens is stationary at least in a rotational sense while the movable platen spins about a first axis that is inclined at a small angle alpha to the direction of the urging force while the first axis rotates about a second axis that is substantially coincident with the direction of the urging force to produce "wobble". The rotary forging operation may be incremental or quasi-continuous. Substantially better alignment of plate-like superconductive particles is achieved compared with conventional rolling processes, and higher critical-current density follows.

Description: Manufacture of Elongate Superconductors This invention relates to the manufacture of elongate superconductors from particles (especially ceramic particles) that are superconducting or can be made superconducting by an appropriate subsequent treatment. More particularly it relates to the manufacture of superconductors from particles which are plate-like or otherwise anisotropic and which need to be aligned to achieve improved critical-current density. An important application is to the "BiSCCO" range of high- temperature superconducting ceramics of nominal composition M 2 Sr 2 CaCu 2 0g and M 2 Sr 2 Ca 2 Cu 3 0, Q (where M is either Bi or Bi partly substituted by Pb) . These form as plate-like crystallites and it is known that spectacular increases in critical-current density can be achieved if the particles can be aligned to maximise face-to-face contacts between the particles. Another application is to the related Tl series superconductors of nominal compositions Tl«Ba CaCu«O β/ TlBa 2 Ca 2 Cu 3 O g and Tl 2 Ba 2 Ca 2 Cu 3 0., Q . The simplest and potentially most satisfactory way of producing an elongate superconductor from superconducting particles is to fill a metal tube (in the case of ceramic particles usually of silver, because many other metals have a deleterious effect on superconducting properties of the particles and few other metals are sufficiently permeable to oxygen to allow the heat-treatment steps that are normally required after reduction) with particulate material and reduce the cross-section of the filled tube to compact the particles and lengthen the assembly. However, there is no viable way to introduce the particles into the tube in any but random alignment, and drawing and swaging processes of the kind used to reduce the cross-section of metal rods or the filled-tube preforms for mineral-insulated electric cables produce only a small degree of alignment. Substantially increased alignment can be achieved by first reducing by drawing or swaging sufficiently to form a compact body without substantial voids and then further reducing by a series of rolling steps (with intermediate annealing as required) to produce a superconductor in tape form. A tape thickness below 50 micrometres is currently 5 recommended and is capable of achieving critical current densities (J 's) with good-quality BiSCCO materials in the region of lOG c Am.-2, but it has been established (by experiments using reducing techniques that are applicable only to short specimens) that better alignment could improve - j this figure at least tenfold. We have therefore recognised a need for a technique capable of processing long lengths on a continuous or nearly continuous basis; - capable of maintaining continuity of a thin 15 metal covering? - suited to the production of tapes with thickness in the range from about 50 down to about 20 micrometers (below which mechanical strength is unlikely to be adequate) ; and 0 - which produces a substantially higher degree of particle alignment than rolling. In accordance with the present invention, a process for the manufacture of an elongate superconductor comprises enclosing anisotropic particles that are superconducting or 5 are capable of being converted to a superconducting form by subsequent treatment in a tube of a ductile metal, compacting it and reducing it in one or more reduction steps to an elongate tape and is characterized by the fact that at least one of the said reduction steps is effected by a rotary 0 forging operation. Rotary forging is a specialised but well-established metalworking technique using two platens which are urged together to engage a workpiece while rotating relative to one another simultaneously about two axes that make a small angle to each other. The angular velocities of rotation about the two axes, the force urging them together and the shapes of*. the platens are selected to produce a rotating localized zone of plastic deformation where neither platen slips on the surface of the work. A number of ways of achieving the relative motions required for rotary forging are known; at present we believe that the one most appropriate for use in the present invention is the one in which one of the platens (usually the lower one and hereinafter referred to as the anvil) is stationary at least in a rotational sense while the movable platen spins about a first axis that is inclined at a small angle ( alpha ) to the direction of the urging force while the first axis rotates about a second axis that is substantially coincident with the direction of the urging force to produce "wobble". Alpha will normally be less than 15° and a convenient geometry uses a flat anvil and a moving platen that is conical with a semi-angle close to the value 90° minus alpha and a cone axis coinciding with the first rotation axis. Preferably rotary forging is used for all the reduction steps except possibly for initial compacting. Heat-treatment steps may be used between and/or after reduction steps as required to obtain required properties in the metal and/or the superconducting material. Ideally the, or each, rotary forging operation would be continuous with the workpiece continuously advancing through the working zone between the platens. This is unlikely to be feasible, in view of the large forces acting on the workpiece during reduction, and an incremental operation may be adopted in which the platens disengage from the workpiece for part of their rotational cycle or the force urging the platens together is repeatedly withdrawn or reduced to allow the workpiece to be advanced a distance short compared with the length of the working zone; it may be possible (in either case) to achieve quasi-continuous operation by continuously applying a biassing force to the workpiece by pulling from downstream, pushing from upstream, or both. Preferably the anvil has a guide groove in its working surface extending inwardly of the latter from an edge of such working surface which is disposed at the inlet side 5 of the machine, which guide groove has a depth over a portion of its length adjacent the inlet side of the machine which is greater than the corresponding dimension of the workpiece. The guide groove may extend diametrically relative to the cone axis, or there maybe one or more guide grooves which are -JO spaced from the axis. The invention includes an elongate BiSCCO superconductor with a critical current density in the range -2 from about 60 to about 90GAm The invention will now be further described, by way 1 of example only, with reference to the accompanying drawings, in which :- Fig. 1 is an axial section through a rotary forging machine suitable for use in the present invention. Fig. 2 is cross-section showing a detail of the o machine of Fig. 1, Fig. 3(i) is a schematic view illustrating the mutual arrangement of first and second die members of the machine in a workpiece feeding condition. Fig. 3(ii) is a schematic illustration showing the 5 mutual arrangement of the die members of the machine in a rotary forging position. Fig. 4(i) is a plan view of one of the die members of the machine of Fig. 1, Fig. 4(ii) is a plan view similar to Fig. 4(i) of a 0 modified machine, and Fig. 5 is a schematic view of a roll drive and synchronizing arrangement. This example used BiSCCO superconductive particles having the composition Bi lβ84 Pb 0β 3 4 Sr 1#gι Ca 2 _ 03 Cu 3#06 O y (where y was not determined but can be assumed to be fractionally over 10) and a nominal particle size (d j - n ) estimated by a light-scattering technique as 5 micrometres. These particles were packed into a pure silver tube 150 mm long and with internal and external diameters of 4.0 and 6.0 mm respectively. The open end of the tube was sealed with a force-fitted silver plug, the seal swaged and the assembly drawn through a conventional series of tube-forming dies to a diameter of 0.6 mm. The compacted assembly was then reduced in a series of rotary forging steps with heat-treatment after each. Referring now to Fig. 1 of the drawings, the rotary forging machine has a machine frame 10 comprising a bed 12, four vertical columns 14 (only two are visible) fixed to the bed 12 and extending vertically upwardly therefrom, and an upper crosshead 16 in which the vertical columns 14 are secured. The columns 14 are equi-spaced around the central vertical axis of the machine. A slide 18 has four sleeves 20 which are slidably mounted on the respective columns 14. The slide 18 is therefore fixed against rotation relative to the machine frame 10 but is displaceable vertically relative thereto. The machine further comprises a horizontally disposed rotary screw 22 carrying a pair of oppositely threaded captive nuts 24. Each nut 24 is pivotally connected to the lower end of the respective link 26 whose upper end is pivotally connected to the slide 18 through the intermediary of a belleville washer 27 (a spring plate capable of withstanding high loads). Thus, it will be appreciated that rotation of the screw 22 results in displacement of the slide 18 vertically upwardly or downwardly depending upon the direction of rotation of the screw 22. The slide 18 carries a first platen (the anvil) 28 having a planar upper working surface 30. Thus, the anvil 28 is displaceable upon rotation of the screw 22 along a vertical displacement axis indicated by dotted line 32 in Fig. 1, but is fixed against rotation relative to the machine frame 10. This axis 32 coincides with the central axis of the machine. Referring now to Fig. 4(i), the upper working surface 30 of the anvil 28 has a guide groove 34 therein which extends diametrically across the anvil 28 so as to intersect the displacement axis 32. The guide groove 34 extends perpendicularly to the plane of the axial section of Fig. 1 from an inlet side 36 of the machine to an outlet side 38 thereof. The depth of the guide groove 34 decreases progressively from the inlet side 36 to a location which is about 7/8ths across the diameter of the anvil 28 from the inlet side 36. The inclination of the guide groove 34 and the depth thereof are chosen such that its depth at the centre of the anvil 28 (i.e, at the displacement axis 32) is marginally greater than the diameter of the superconductor precursor workpiece W to be forged. Therefore, it will be appreciated that, when the workpiece W is fed into the rotary forging machine along the base of the guide groove 34, it will only project above the planar upper working surface 30 of the anvil 28 over the region thereof which is disposed on the opposite side of the displacement axis 32 to the inlet side 36. Thus, the effective working region of the machine is constituted by the region towards the outlet side 38 of the machine. As can be seen from Fig. 4(i), the width of the guide groove 34 is substantially constant over the whole of the length thereof which is disposed between the inlet side 36 and the displacement axis 32, such width being marginally greater than the diameter of the workpiece W. Between the displacement axis 32 and the outlet side 38 of the machine, the width of the groove 34 progressively increases up to the location which is 7/8ths across the diameter of the die member 28 and is then constant up to the outlet side 38. Over the last l/7th of the diameter, the depth of the groove 34 is very small. Returning to Fig. 1, the rotary forging machine further comprises a second platen 40 mounted on a sleeve 42 rotatably mounted through upper and lower bearings 44 and 46 on a shaft 48 arranged on axis 50 which is disposed at an angle alpha relative to the displacement axis 32 and which is pivotabley about a horizontal axis at a point 51 at which the axes 32 and 50 intersect. The second platen 40 is freely rotatable about axis 50. The shaft 48 is arranged to be driven by a precession drive which is indicated generally by arrow 52 and which includes a vertical drive shaft 54 mounted in upper and lower bearings 56 and 58 carried by the crosshead 16. The vertical drive shaft 54 is rotatable about a vertical axis which is coincident with the displacement axis 32. The lower end of the vertical drive shaft 54 is of rectangular shape (as will be apparent from Fig. 2), is concavely curved over the arc centred on intersection point 51, and has a slot 54b therein extending diametrically relative to the axis of rotation of the shaft 54. The slot 54b has a concavely curved based 54c which is parallel to the lower end face 54a. Upper end region 48a of the shaft 48 is a close sliding fit within the slot 54b. The end region 48a has a convexly curved end face 48b which mates with the base 54c of the slot 54b. The shaft 48 also includes a pair of convexly curved shoulders 48c which mate with respective portions of the lower end face 54a on opposite slides of the slot 54b. The end region 48a has an internally screw-threaded hole 48d extending therethrough diametrically with respect to the axis 50 and longitudinally with respect to the slot 54b. A screw-threaded rod 62 is engaged in the hole 48d and passes with clearance through respective holes in a pair of U-shaped locating members 64 which embrace the shaft 54 at opposite ends of the slot 54. Inner and outer tapered shims 66a and 66b and a nut 68 are engaged with each end region of the rod 62 outwardly of each locating member 64 serve to lock the rod 62 at the desired angle relative to the longitudinal axis of drive shaft 54. The shims 66a and 66b of each pair are relatively rotatable and can be appropriately set to take up the misalignment between the nut 68 and the associated collar 64 caused by the angular offset of the axis of the rod 62. As a result of the above described arrangement, rotation of the shaft 54 causes precession of shaft 48 and thereby of the rotary axis 50 of the second platen 40 at angle alpha about the displacement axis 32. The axes 32 and 50 mutually intersect at point 51 which is at the centre of working surface 76 of the platen 40. The working surface 76 is of downwardly tapering conical shape with a cone angle equal to 180° minus alpha . The horizontal pivot axis 51 intersects the apex of the conical working surface 76. In this embodiment, alpha is 5°, but can be adjusted before a rotary forging operation by replacing the die member 40 with one having the desired cone angle, and re-setting by loosening at least one of the nuts 68, rotating rod 62 within the hole 48d to adjust the angle alpha to the appropriate value and then tightening the nuts 68. Referring now to Fig. 5, the rotary forging machine further comprises workpiece-moving means in the form of a pair of haul-off rolls 80 which pull on the rotary forged wire emerging from the outlet side 38 of the machine. The rolls 80 are driven by a stepping motor 82 under the control of a control unit 84 to which an angular position sensor 8 is connected. The angular position sensor 86 is disposed adjacent the drive shaft 54 which itself is driven by motor 90. In operation, the rolls 80 are controlled so that they operate to advance the workpiece W in stepwise fashion between the working surfaces 30 and 76 of the die members 28 and 40 in synchronism with precession of the die member 40 about displacement axis 32. In the condition illustrated in Fig. 3(i), it will be appreciated that the die member 40 is in a position in which it is not in contact with the workpiece W since that region of the working surface 76 which is in contact with the working surface 30 overlies the relatively deep region of the groove 34 at the inlet side 36 of the machine. It is at this stage that the stepping motor 82 advances the workpiece W a short distance. In this embodiment, such distance corresponds to approximately 25% of the radius of the die member 28. The drive to the rolls 80 is then stopped and a rotary forging operation is performed on the freshly exposed portion of the workpiece W as the die member 40 continues its precession to the position illustrated in Fig. 3(ii). At the same time, a further rotary forging operation is performed upon those regions of the workpiece W which have already been rotary forged at least once in previous passes but which have not yet left the outlet side 38 of the machine. In this way, the workpiece W which is originally of circular cross section (in the first pass) is rotary forged into a thin ribbon with the advantageous particle orientation described. During movement of the platen 40 relative to the anvil 28, the latter is held in position without rotation of the screw 22. The belleville washer 27 imparts a small degree of resilience in the direction of axis 32 but is sufficiently stiff to permit useful work to be performed on the workpiece W during each forging pass. The compression of the washer 27 can be altered by appropriate adjustment of the screw 22. Thus, the forging pressure may remain substantially constant up to a pre-set value. Alternatively, the screw 22 may be adjusted automatically during a rotary forging operation so as to vary the forging pressure for the purpose of optimising the properties of the product being forged. In the modification of Fig. 4(ii), the upper working surface 30 of the anvil 28 has a pair of guide grooves 34 therein. The guide grooves 34, in this embodiment, are mutually parallel and extend across the anvil 28 from the inlet side 36 to the outlet side 38 with at positions spaced from the axis 32. With such an arrangement, it will be appreciated that, as the platen 40 precesses about the axis 32, the workpieces in the grooves 34 are contacted in turn during each forging pass. Furthermore, since the direction of the length of each workpieces is at an angle to the line of rolling contact between the die members 28 and 40, the whole of the exposed length of wire is progressively forged (with the working zone progressing along it) during each pass rather than being substantially simultaneously forged (with the working zone progressing across it) as in the case of the embodiment of Fig. 4(i). After the first such rotary forging step, the product of the examples was a flat superconductor with a thickness (including the silver cladding) of 0.1 mm and a width of about 4 mm, and after heat-treatment for 50 hours at 848°C 2 had already a critical current density of about 5GA/m ; it is confidently predicted that further reduction to a thickness below 50 micrometres by rotary forging steps (with similar intermediate heat-treatments) will raise this value 2 to at least 60GA/m .

Claims: CLAIMS 1 A process for the manufacture of an elongate superconductor comprising enclosing anisotropic particles that are superconducting or are capable of being converted to a superconducting form by subsequent treatment in a tube of a ductile metal, compacting it and reducing it in one or more reduction steps to an elongate tape and is characterized by the fact that at least one of the said reduction steps is effected by a rotary forging operation. 2 A process as claimed in claim 1 in which the said rotary forging operation is one in which one of the platens (the anvil ) is stationary at least in a rotational sense while the movable platen spins about a first axis that is inclined at a small angle alpha to the direction of the urging force while the first axis rotates about a second axis that is substantially coincident with the direction of the urging force to produce "wobble". 3 A process as claimed in claim 2 in which the said rotary forging operation uses a flat anvil and a moving platen that is conical with a semi-angle close to 90° minus alpha and a cone axis coinciding with the first said rotation axis. 4 A process as claimed in any one of the preceding claims in which rotary forging operations are used for all the reduction steps. 5 A process as claimed in any one of claims 1-3 in which a rotary forging operation is used for all the reduction steps except for initial compacting. 6 A process as claimed in any one of the preceding claims in which at least one heat-treatment step is used between and/or after reduction steps. 7 A process as claimed in any one of the preceding claims in which in the, or each, rotary forging operation the platens disengage from the workpiece for part of their rotational cycle to allow the workpiece to be advanced a distance short compared with the length of the working zone. 8 A process as claimed in any one of claims 1-6 in which the force urging the platens together is repeatedly withdrawn or reduced to allow the workpiece to be advanced a distance short compared with the length of the working zone. 9 A process as claimed in claim 7 or claim 8 in which a quasi-continuous rotary forging operation is achieved by continuously applying a biassing force to the workpiece by pulling from downstream, pushing from upstream, or both. 10 A elongate superconductor made by the method claimed an any one of claims 1 - 9. 11 An elongate ceramic superconductor made by the method claimed in any one of claims 1-9. 12 An elongate BiSCCO superconductor with a critical _2 current density m the range from about 60 to about 90GAm .

Inventors: ASHRAF MUFTI MOHMED, BEALES TIMOTHY PAUL, MARSHALL STEVEN, TEDSTONE SEAN FRANCIS, ASHRAF, MUFTI, MOHMED, BEALES, TIMOTHY, PAUL, MARSHALL, STEVEN, TEDSTONE, SEAN, FRANCIS
Applicants: BICC PLC, PENNY & GILES BLACKWOOD LTD, BICC PUBLIC LIMITED COMPANY, PENNY & GILES BLACKWOOD LIMITED
Application year: 1993
Main classification: H01L39
IPC codes: B21J9, C04B35, H01B12, H01B13, H01L39