AU605299B2 - Superconductors and method of making them - Google Patents

Description

COHM4O NWEALTH OF AUFSTRAI .A.

P'ATENT ACT 1952 COMPLETE SPECIFICAT ION 6 (original) 2 9 FOR OFFICE USE Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: :*.Prioritr.

elated Art: Name of Applicant: oa 0 *Actual Inventor(s) Address for Service: METAL MANUFACTURES LIMITED 5 Bridge Street Sydney New South Wales Australia Peter Mather Robinson DAVIES COLLISON, Patent Attorneys, I Little Collins Street, Melbourne, 3000.

J Complete Specification for the invention entitled: I "SUPERCONDUCTORS AND METHOD OF MAKING THEM" The following statement is a full description of this invention, including the best method of performing it known to u.s

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This invention relates to superconductors, especially superconducting power cables, and to a method of making them.

The phenomenon of superconductivity, whereby certain materials carry unidirectional electric current without losses by Joule heating and alternating current with greatly reduced losses has been known since the beginning of the 20th century, and superconducting power cables were demonstrated in the 1960's. However, they were no more than scientific curiosities becase superconductivity is only observed at cryogenic temperatures so that Joule heating losses are exchanged for refrigeration losses, and for the materials then available, which required liquid helium as coulant, it 15 could readily be calculated that the energy throughput at which a nett reduction in losses occured would be so large that it would be more economical to re-site a couple of generating stations.

Currently, materials that are superconductive at "liquid nitrogen" temperatures, say at 77K or above, are becoming available, and this offers the prospect of considerably better economics; but these materials are brittle doped-oxide ceramics and are quite unsuited to making cable conductors of any conventional kind.

In accordance with the present invention, a superconductor4comprises an elongate annular body of

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ii 1 2 superconductive ceramic powder confined and compressed between an inner conductor and a surrounding tube each made of a metallic normal electric conductor of high conductivity, which serves both as a "buffer" to provide a low-resistance path in parallel with any part of the powder that may lose its superconductivity due to coolant failure), so avoiding a potential risk of catastrophic failure, and also as a compres!or to maintain the superconducting particles in adequate e S 0 contact, even at cryogenic temperatures.

Preferred superconducting ceramic powders are *SSo e e. those with a superconducting transition temperature over Coo.

77K, allowing liquid nitrogen, or possibly liquid air, to be used as coolant. More especially we prefer those containing the oxides of barium and yttrium and/or other o rare earth oxides together with copper oxide; for o. these powders optimum particle size is about 1 o* micrometre.

The metallic normally conducting material is 20 preferably copper, silver, aluminium, or a o •o high-conductivity alloy of one of these.

Preferably the inner conductor is also a tube, to maximise efficiency of both buffering and cooling.

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-3- The superconductor structure of the present inventon can be made by drawing-down a preform comprising a central metal tube or rod, a metal outer tube, and an infillitng of loosely compacted superconducting ceramic powder, somewhat on the lines of the process by which a mineral-insulated low-voltage cable is made.

Superconducting cables in accordance with the invention comprise at least one of the superconductor structures already described and means for electrically and thermally insulating it from its surroundings without inhibiting refrigeration.

Where there is an inner tubular metal member to serve as a cooling duct, a solid sleeve of a suitable insulating material can be used; suitable materials include insulating materials, either flame-sprayed or otherwise coated on the outer metal tube of the superconductor structure or else compacted within an outer metallic tube, as well as some polymeric materials suitable for service at the low 15 temperatures involved.

Especially when there is not a central cooling duct, the insulation is preferably discontinuous to allow direct access of coolant to much of the outer metal tube of the superconductor structure; e.g. it may consist of discrete ceramic supports of a helically-applied string of low-temperature polymeric material.

20 A further possibility that may be preferred in many cases is to surround the superconductor structure with a further metallic tube, suitably spaced from the 0e

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5 S 901005,cldspe.009,16693.sp,3 4 .N re. 4 superconductorkto form an annular, or nearly annular, cooling duct, and to surround this further metallic tube with any suitable electrically and thermally insulating material.

The invention will be further described with reference to the single figure of the accompanying drawing which is a diagrammatic cross-section through one form of superconducting cable in accordance with the invention.

10 Preparation of superconducting powder Yttria (Y 2 0 3 barium carbonate (BaCO 3 and cupric oxide (Cu0) each in powder form with an average particle size of 10 micrometres were weighed out in proportions to provide two atoms of barium and three atoms of copper for each atom of yttrium, and the mixture ball-milled for 24 hours under hexane in a nylon container, using zirconia balls. The mixture was calcined at 900 0 C in air for 24 hours, reground by ball-milling to an average particle size of 10 micrometres, calcined at 930 0 C for a 20 further 24 hours and reground again to 10 micrometres.

It was then pressed into cylindrical bodies 12mm long and 24 mm in diameter and sintered for 16 hours at 940 0 C, crushed and ball-milled as before for 10 hours.

After drying for 6 hours at 100 0 C, the powder had a range of particle sizes from 1 to 10 micrometres and a stoichiometry YBa 2 Cu 3 Oz, where z is a little less than 7.

Ii L i 5 A prototype superconducting wire was made using as sole metallic component a silver tube of outside diameter 9.38mm and wall thickness 0.68mm. This was closed at one end by swaging, filled with the superconducting powder prepared as described and vibrated to compact the filling. A plug of a silicone rubber caulk was used to seal the open end against spillages.

The pre-form so made was drawn down (using 10 conventional mineral-insulated cable drawbench technique

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at about 10-15% reduction in area at each die) to a finished diamteter of 3mm, without intermediate annealing. Heat-treatment at finished size was carried out in a tube furnace through which the composite 15 conductor was drawn at 17 mm/hr. This heated the composite to 910 0 C in about 6 hours (100 mm), maintained S that temperature for 6 hours (100 mm) and cooled at about 1 0 C per minute (18 hours, 300 mm) to ambient temperature. The resulting composite conductor was 20 superconducting at 77K with a critical current density of about 2.0 to 2.3 A/m2.

Example The cable shown in the drawing comprises a superconductor 1 itself made up of inner and outer copper tubes 2,3, the space between them being filled with the superconducting ceramic powder 4, and the 6 assembly drawn down to produce the required compaction.

The inner tube 2 forms one roolant duct, and an additional coolant duct is formed by the subsequent addition to the superconductor of an outer copper tube supported by spacers 6 (which may be completely separate from the tube, in which case they are preferably helical so as to avoid the need for any fixings) or may be integrally formed with the outer tube (in which case they preferably extend longitudinally). Outside the 0 outer tube 5 is a layer of any suitable electrical and thermal insulating material 7, which may optionally be enclosed by a further metal tube 8.

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Claims (11)

1. A superconductorkcomprising an elongate annular body of superconductive ceramic powder confined and compressed between an inner conductor and a surrounding tube each made of a metallic normal electrical conductor of high conductivity.

2. A superconductor/as claimed in Claim 1 in which the superconductive powder has a transition temperature 00 0 over 77K.

3. A superconductor as claimed in Claim 1 or Claim 2 10 in which the inner metal conductor is also a tube. .Il.

4. A superconductor as claimed in any one of Claims 1-3 in which the metallic normally conducting material is silver.

A superconducting cable comprising at least one 15 superconductor as claimed in any one of Claims 1-3 and means for electrically and thermally insulating it from its surroundings without inhibiting refrigeration.

6. A superconducting cable as claimed in Claim comprising a further metallic tube surrounding and 9**S 20 spaced from the superconductor to form an annular, or nearly annular, cooling duct and electrically and thermally insulating material surrounding the said further metallic tube.

7. A superconducting cable comprising at least one superconductor as claimed in Claim 4 and means for electrically and thermally insulating it from its 4 4 -8- surroundings without inhibiting refrigeration.

8. A method of making the superconductor structure claimed in any one of claims 1 to 3 comprising the step of drawing down a preform comprising a central metal tube or rod, a metal outer tube and an infilling of loosely compacted superconducting ceramic powder.

9. A superconductor structure substantially as described with reference to the Example.

A method of making a superconductor structure substantially as hereinbefore described with reference to the Example.

11. A superconducting power cable substantially as described with reference to the accompanying drawing. we C. S C C C. S CS C SC s **CC C 0 C C. 00 00 DATED this 5th day of October, 1990. 20 METAL MANUFACTURERS LIMITED By Its Patent Attorneys DAVIES COLLISON /,t 901005,eldspc.009,16693.spe,8