GB2278081A

GB2278081A - Manufacture of superconducting tapes

Info

Publication number: GB2278081A
Application number: GB9310058A
Authority: GB
Inventors: Bartlomiej Andrzej Glowacki
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1993-05-15
Filing date: 1993-05-15
Publication date: 1994-11-23
Anticipated expiration: 2013-05-15
Also published as: GB2278081B; GB9310058D0

Abstract

In the manufacture of superconducting tapes by the "powder-in-tube" process, at least one of the reduction steps (preferably many or all, and especially the last) are effected by rolling transversely rather than along the length of the tape precursor.

Description

Manufacture of Superconducting Tapes This invention relates to the manufacture of superconducting tapes by the "powder-in-tube" process in which a tube (usually made of or at least lined with silver) is filled with a superconducting ceramic, or with a material or mixture of materials convertible to superconducting ceramic by subsequent heat-treatment, in particulate form or in the form of blocks pressed from particles and the tube is reduced in cross-section and flattened to densify and orient the ceramic content.

The techniques that have been widely practised in the experimental manufacture of superconducting tapes are extrusion, swaging, drawing and rolling. It is recognised that uniaxial pressing may give superior properties, but this process is difficult to perform on a long tape. More recently we have proposed that rotary forging should be used.

Rolling is an inherently anisotropic process, and we have discovered that when tapes are formed by the known rolling processes, in which, naturally, the tape precursor is passed through a pair of rolls in the direction of its length, the properties of the tape (in particular its critical current density) are superior in a direction across its width compared with the corresponding property in the direction of the length of the tape. This has led us to propose a modified process which we believe, on the basis of preliminary experiments, to offer the possibility of obtaining by a rolling process properties in the direction of the length of the tape that are comparable with those that can be achieved by uniaxial pressing.

In accordance with the invention, at least one of the reduction steps in the manufacture of such superconducting tape is effected by passing the tape precursor between a roll and a support surface (which may be another roll), at least the roll passing transversely across the precursor.

Preferably the direction of movement of the roll is substantially perpendicular to the length of the precursor, but some benefit may be obtained by rolling in any direction that makes a substantial angle with the axis of the tape precursor.

It will be apparent that, except for the manufacture of short lengths of tape, the rolling apparatus will need to be so designed that the tape precursor may pass between the roll and the support surface with its end projecting beyond the ends of the roll(s), and that the precursor needs to be advanced stepwise in the direction of its length between successive rolling passes.

We at present prefer that alternate rolling passes are in opposite directions, to minimise lateral dissymmetry. If preferred, all the rolling passes could be in the same direction to maximise uniformity in the direction of the length of the tape; for example, the roll could be lifted from the support surface to allow the precursor to be returned to starting position with respect to the roll between passes.

When the support surface is formed by a second roll, the rolls may be of the same or different diameters. The results achieved are influenced by the diameter(s) of the roll(s), and in particular by the ratio of the diameter(s) of the rolls to the thickness of the tape (or intermediate) emerging from the apparatus (the roll gap). We have not yet fully explored this effect, but suggest that best results may be obtained with roll diameters several orders of magnitude greater than the roll gap.

As an alternative to a second roll, the support surface may be a flat surface (analogous to a pastryboard). If a flat (or nearly flat) support surface faces upwards, it may provide a continuously-available support for the work (unlike a lower roll from which the precursor will fall as soon as it emerges from the roll gap). Such surface may be stationary, or it may move in the same direction as the roll (to avoid shearing effects between the roll and the support surface) or in the opposite direction (to counter any edgewise bending tendency).

Rolls with plain cylindrical working surfaces aligned parallel to one another or to the support surface may be used, though the ends of such rolls and/ or of the support surface should be rounded or otherwise relieved to avoid marked edge effects. In this case, the precursor may be advanced a distance approaching the roll length between successive passes: overlap areas do not appear to be detrimental.

Alternatively, tapered rolls or rolls with their axes at a small angle to each other or to the support surface may be used, so that the roll gap reduces progressively towards the exit end. Preferably an end portion at the exit end is shaped to give an exit roll gap of uniform thickness, so that the thickness of the emerging tape (or intermediate) is uniform. When this alternative is used, the advance of the tape between passes will normally be a small fraction of the roll length.

While we do not wish to be bound by any theory, microscopic examination of tape structures suggests that the effects observed may be due at least in part to the tendency for the rolling process to create tensile stresses in the rolling direction and consequently microscopic cracks in the ceramic material transverse to that direction while creating compressive forces perpendicular to the rolling direction tending to heal cracks that may extent along the rolling direction.

While we prefer to perform at least most of the reduction steps by transverse rolling as described, swaging and drawing may be preferred in the preliminary stages of reduction (before flattening) and the use of some conventional rolling steps, though disadvantageous, is not wholly excluded.

Example In a preliminary experiment, a silver tube with internal and external diameters of 4 and 6 mm respectively was filled with ceramic superconductor powder of nominal composition Bil gPb0,4Sr2Ca2Cu3010 (bismuth-lead 2-2-2-3) and mean particle size (d50) 5 micrometres. The filled tube was swaged in a series of steps to external diameters of 5, 4, 3.5, 3.2, 3.0, 2.5 and finally 2.0mm without intermediate annealing, then conventionally rolled to 250 micrometre nominal thickness and conventionally heat-treated to form a superconducting precursor tape, which was found to have a critical current in the direction of the length of the tape of 4A but in the direction of its width of 29.5A (measured at 77K and OT).

Short pieces (each a few centimetres) cut from this precursor tape were then rolled in a series of passes in a direction as nearly as possible at right angles to the direction of their lengths between a pair of rolls 200mm in diameter to reduce the thickness by 60%. The pieces were sintered under conventional conditions and found to have a critical current of only 6.9A in the direction of the width of the tape but 19.2A in the direction of its length (at 77K and OT).

Claims

CLP.IMS

1 A method of manufacturing a superconductor tape by the powder-in-tube process in which at least one of the reduction steps is effected by passing the tape precursor between a roll and a support surface, at least the roll passing transversely across the precursor.

2 A method as claimed in claim 1 in which the roll passes perpendicularly across the precursor.

3 A method as claimed in claim 1 or claim 2 in which a plurality of reduction steps including the last are so effected.

4 A method of making a superconductor tape substantially as described with reference to the Example.