AU762029B2 - Superconducting leads - Google Patents

Abstract

A superconducting lead comprises a supporting board (1) with at least one superconducting tape (3) adhered to it and extending from one terminal area of the lead to another. Respective metal end-fittings soldered to the superconducting tape (3) at each of its ends, and in each of the terminal areas at least one metal insert is bonded to the board (1) and also soldered to the corresponding one of the said end-fittings, so relieving the tapes (3) of mechanical stress. The board (1), or each of them, is preferably flat and may support two or more superconducting tapes (3) coplanar with one another on one or on each of its major faces. The metal inserts are preferably of copper and may be made by removing most of the copper cladding from a commercially available circuit board (1).

Description

WO 00/1 4827 PCTIUS99/20526 Superconducting Leads This invention relates to superconducting leads, primarily for use as "current leads", meaning (as customary among superconductivity experts) leads for conveying current to a apparatus operating at a cryogenic temperature from a source at a substantially higher temperature. In most applications the cryogenic temperature will be a "liquid helium" temperature in the vicinity of 4K and the higher temperature will be* liquid nitrogen temperature (around 77K) or lower.

Superconducting current leads are preferred, when the level of the higher temperature makes it possible, both because their own losses electrical are smaller than for comparable metal current leads and also because they can hav.:! substantially higher thermal resistance and often appreciably reduce overall refrigeration losses of the apparatus.

Superconducting current leads have been made with "bulk" ceramic superconductor powder, either compressed and sintered into self-supporting blocks or packed into a silver (or silver alloy) tube and suitably compressed. They have also been made with ceramic superconductors in tape form, as they would now normally be used for winding coils or making power cables. However, "bulk" superconductor leads are brittle, mechanically weak, and are liable to be destroyed if fault conditions lead to loss of superconductivity; superconducting tapes are self-protecting from such faults as the silver/silver alloy content is sufficient to carry the current as a normal conductor for short periods and they are flexible and somewhat more resistant to thermal cycling shock, but they still have little inherent strength, and as the thermal stresses are necessarily rather large in a current lead operating with its ends at very different temperatures and subject at least occasionally to cycles to room temperature, they need mechanical support.

One satisfactory way of giving mechanical support is to bond the tapes with a suitable adhesive to support (this may P:\WPDOCS\.d\ pE759220.dc- 17/04/03 -2be of various cross-sections but our own preference is that it should be flat and for convenience we refer to it as a "board") of a resin-bonded fibre glass or other material having thermal expansion characteristics reasonably similar to those of the tape. Even then, substantial stresses arise at the terminations, where the tapes will normally be connected to a solid copper conductors by soldering, and it is the applicants' experience that this may result in partial or complete failure of the soldered connection and that, even under favourable conditions, the contact resistance at the two terminations may account for as much as a fifth of the total losses in the current lead.

The invention seeks to provide a superconducting lead suitable for use as a current lead in which the terminations are secure and the terminal contact resistance stabilised at a low value.

The superconducting lead in accordance with the invention comprises a supporting board, at least one superconducting tape adhered to the supporting board, a terminal member forming a metal end-fitting soldered to the superconducting tape at an end of the supporting board and at least one metal insert bonded to the supporting board and soldered to the metal end-fitting.

In this way, longitudinal mechanical stresses can be carried from the end-fittings directly to the board, bypassing and so relieving of stress the soldered joints to the tape.

:"There may be more than one board, and the board, or each board as the case may be, may support any convenient number of superconducting tapes. Preferably the board, or each of them, is flat and supports two or more tapes coplanar with one another on one or on each of its major faces, so that the whole flat side and both edges of each tape are accessible for soldering without requiring a complex shape for the terminal member.

However, two or three tapes (or more, at an

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WO 00/14827 PCTIUS99/20526 increasing penalty) can be superposed on one another if compactness is of over-riding importance.

The metal inserts, as well as the terminal members, are preferably of copper, especially high-condutivity "oxygen-free" copper(and the terminal members preferably plated with gold or silver). They are conveniently simple rectangular strips along (or parallel and close to) the appropriate edges of the board. Strips only a few millimetres wide are effective and considerered adequate, but if desired they could be wide enough to facilitate reinforcement by drilling to accomodate a transverse pin of metal or other hard material.

Suitable boards with copper very strongly bonded over substantially its whole area is readily available as it is widely used to make circuit boards for electronics applications. One suitable board is a resin-bonded fibreglass board clad with about 35pm of copper and coated with a positive working photoresist to comply with BS 4581, sold under the trademark "Fotoboard" and grade code "FR4" by Mega Electronics. Copper can be removed, except for the required inserts, by imagewise exposure to UV light, development and etching (for which the board is designed) or by machining. It is the applicants' understanding that this board is made by in-situ curing of the fibre-bonding resin on a pre-formed and specially prepared (etched) copper sheet.

Preferably the (or each) superconducting tape is bonded to the board over the whole area of the tape. A suitable adhesive is manufactured by Grace Specialty Polymers and sold in the United Kingdom by Emerson Cumming (UK) Ltd under the trademark "Eccobond 286" and is understood to be an epoxy resin product filled with an oxide of aluminium or of zirconium to improve matching of thermal expansion characteristics.

The lead may incorporate two or more boards with superconducting tapes as described, preferably but not necessarily arranged parallel to one another; they may be WO 00/14827 PCT/US99/20526 alike (typically resulting in a rectangular cross-section) or may differ in width (with or without the number of tapes differing), for example to give an overall round crosssection.

Silver/silver alloy clad ceramic superconductor tapes can be successfully soldered to the terminal members by a suitable solder; we prefer solders comprising (by weight) equal parts of lead and bismuth or 2 parts of lead, 5 parts of bismuth and 3 parts of tin, but ordinary tin/lead eutectic solder can be used.

If it is desired that the lead should be "vapourcooled", the superconducting tape(s) is/are preferably coated to protect from contact with coolant; Nova Tran Ltd offer a custom coating service under the trademark "Parylene" and for this coating a thickness of about 30-40 pm is recommended; but other coatings can be used. The applicants do not know whether the process operated by Nova Tran Ltd (described as a gas phase deposition) resembles the technique using pyrolysed poly-para-xylylene described in US patent 4508760. In most such cases a tubular shroud with appropriate apertures for vapour flow will be added after coating.

If "vapour-cooling" is not desired, then we prefer to protect the lead by adding a closely-fitting enclosure and "potting" by introduction of a suitable hardening resin (such as Eccobond 286 referred to above) into the remaining clearances. Such enclosure is suitable made from a resinbonded fibre glass material similar to the material of the board or boards; a suitable material is sold by Tufnol Ltd under their trademark TUFNOL and the product code 10G/40.

The terminal members may project axially for external connection, but where the installation geometry permits we prefer that they project laterally, so as to accomodate the maximum length of superconductor, and thus minimise thermal conductance of the lead.

The invention will be further described, by way of example, with reference to the accompanying drawing in which: Figure I is a partly diagrammatic elevation of an end part of one form of lead in accordance with~ the invention; Figqure 2 is a cross-section on zhe line Il-11 in Figu~re 1; S and Figure3. is an elevation, corresponding to Figure showing-an alternative form of lead in accordance with the invention afd incorporating a nurnber of independent options; this figure shows the lead in an unfurnished condition, as will be explained below.

The lead of Figures 1 and 2 is based on a narrow strip of double-aided FOTOBOARD circuit board I with the copper layers entirely etched away exccept for narrow Strips 2/at each end:(only one end is shown, the other may be identical).- On each of 4.he so exposed faces of the board superc onducting tapes 3 each consis ting of one or more "filament" of a "high temperature', ceramic superconducting material (preferably of the flISCCO famtily) Are adhered using Eccobond 286 low-exrpansion opoxy adhesive. The end of the board is inserLed in a slot in a gold-plated coppez terminal member 4 and bonded to it by a solder S consisting pf equal parts by weight of lead and bismuth. The solder bonds

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securely both to the tapes 3 (providing a low-reasistance electrical co~nnction) and LC the copper sLrips 2 (providing a strong mechanical connection from the boa.rd I. to the terminal member 4 which is wholly independent of the tapes in the case off a vapour-cooled lead, the exposed surfacLs of the tapes (arid optionally Lhose of the boards as well) are thinly., coated with the PARYLENS polymuer coating idcnLified above and t he board enclosed by a p.istics shroud 6 provided with openings 7 for the flow of coolanrt.

Note thaL in 1.his design, LIM ter-minal metuber 4 CXLends to the side of the load Prop=r it 3s a hflag"i rather than a "spade" type terinal. Mi.; pexmit; the svperconductinc., and celarivel~y thermally insuiatiny, part oL the Icad to be as long as che n4ture of the ins;tallationi pe.Lmics.

The alter-nat:Lvi de.-,ign Lnhown in I--igu.Le 3 .llusrat."s a

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number of variants, any of which may be used separately as well as together.

First, it is a t'spadea type terminal with the axes of the lead and the terminal member coincident, and so it is suitable for direct substitution for a conventional current lead of similar shape.

Second, it comprises two similar boards 1 for increas d supercurrent capacity. Higher numbers can be used, anddi ererit 0 widths of board with the same or different numbers of tapes can be used to mneet design requirements: for example, three paraliel, boards with the central one wider than the others allows an approximation to circular cross-section, when desired.

Third, it is not intended for use with vapour cooling.

Consequientl.y, instead of coating the tapes and adding an opqn tubular housing, the assembly is completed by a jacket made up of two components of resin-bo-nded fibreglass (TTJFNOL lQG/40) maich3.ned to fit around the boards and tormimal members with only a srrzall clearance. Figure 3 shows one of these 3acket components 9* in 0 place and ormuts the other for clarity. Once both components, are in place, the remaining clearances 8 are filled with an epoxy! resin by a vacuum impzreenaLion technique to form a fully potted lead.

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Claims (10)

1. A superconducting lead including a supporting board, at least one superconducting tape adhered to the supporting board, a terminal member forming a metal end-fitting soldered to the superconducting tape at each end of the supporting board and at least one metal insert bonded to the supporting board and soldered to the corresponding one of the said metal end-fittings.

2. A superconducting lead including two terminal members, a supporting board extending between the terminal members, at least one superconducting tape adhered to the supporting board, wherein each terminal member forms a metal end-fitting soldered to the superconducting tape at respective ends of the supporting board and at each end of the supporting board a metal insert bonded to the supporting board and soldered to the corresponding one of said metal end fittings.

3. A superconducting lead as claimed in claim 1 or claim 2 in which the board is flat and supports two or more superconducting tapes coplanar with one another on one or on each major face of the board.

4. A superconducting lead as claimed in any one of claims 1 to 3 in which the metal insert, as well as the terminal members, are of copper. .0•o A superconducting lead as claimed in any one of claims 1 to 3 in which the metal inserts, as well as the terminal members, are of high conductivity oxygen-free copper.

6. A superconducting lead as claimed in any one of claims 1 to 5 in which the metal inserts are rectangular strips along an edge of the board.

7. A superconducting lead as claimed n any one of claims 1 to 6 including two or 30 more said boards which differ in width, with or without the number of tapes differing. P:\WPDOCS\AMUD\speci\7 92230.doc-15 Mah 2001 -8-

8. A superconducting lead as claimed in any one of claims 1 to 7 in which the superconducting tape(s) is/are coated to protect from contact with coolant.

9. A superconducting lead as claimed in claim 8 further including a tubular shroud with apertures for vapour flow. A super conducting lead as claimed in any one of claims 1 to 7 including a closely fitting enclosure and potting resin in remaining clearances.

11. A superconducting lead as claimed in any one of claims 1 to 10 in which the terminal members project laterally.

12. A superconducting lead substantially as described with reference to the S. accompanying drawings. S DATED this 4th day of April, 2001 PIRELLI CABLES (2000) LIMITED By Their Patent Attorneys 20 DAVIES COLLISON CAVE *ee