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image European Research News Centre > Pure Science > How to turn powder into 'wire'
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image image image Date published : 24/02/03
  image How to turn powder into 'wire'
RTD info 36
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  Changing powder into ‘wire’ is the simple yet infuriating puzzle that hundreds of high-tech laboratories worldwide have been grappling with for more than a decade. The non-metallic physical consistency of the new HTS (High Temperature Superconductor) compounds is altogether different to traditionally extruded copper wire. How, then, is it possible to convert them into 'resistance-free' conductors of electric current?
   
     
   

The feverish activities of chemists and physicists following the discovery of HTS produced a growing list of compounds with relatively high critical temperatures (the record set recently is 138°K). However, they show very different performances in terms of magnetic field and the critical current they can carry.

The BSCCO family

Although the list may well grow further – and some new surprises may be in store (see box: Waiting in the wings) – the other major problem concerns the technologies that will enable these compounds to be used. The ceramic nature of copper oxide-based HTS compounds means that they have to be deposited along a long support – in fact on tapes which are similar to those developed decades ago for audio or video cassettes. Over the past decade, the search for a solution to this problem of materials engineering has been a major scientific, multidisciplinary and industrial challenge, in which European research is actively involved.

An initial avenue of research was opened up with the use of a family of materials jointly known as BSCCO (pronounced 'bisco'), which are copper oxides containing the elements bismuth, strontium and calcium. Their principal quality lies in their grain structure, which quite systematically shows a favourable alignment, a necessary pre-condition for allowing the passage of an electric current in the superconductive state. In the widely used ‘powder in tube’ technique, BSCCO precursors were successfully incorporated into silver tubes, from where it was possible to draw or extrude multifilament-long strands. Despite the limited performance of this solution (cost of silver support, limits on intensity of current density carried through and relatively low magnetic fields tolerated), it has the merit of having given rise to instructive experiments that are useful in the electrotechnical sector.

The promising but problematic case of YBCO

Today, there is general agreement on the potential of technological developments based on the HTS group known as YBCO (yttrium barium copper oxide). YBCO is resistant to high critical magnetic fields and, at temperatures above those at which nitrogen liquefies, it can conduct electric currents estimated at one million amperes per cm2 !

This is certainly a tempting prospect, but the headache comes when working with the compounds to prepare industrially useable quantities of the superconducting tapes capable of carrying the current. It is a genuine technological feat to deposit the compounds on the necessary supports. The process involves superposing extremely complex and perfectly dosed chemical elements – on the scale of the nanometre and with utmost precision – to obtain impeccable critical performances. At the same time, the processes must be workable on an industrial scale with competitive manufacturing costs.

Eamonn Maher, on behalf of the company Oxford Instruments (UK) and coordinator of the European Ready project for the secondary coiling of a prototype 45 KVA superconducting transformer, takes the view that: 'The options are so many that the final choices must be made by industrialists, even if they do need the expertise of university research laboratories to assist them. It is they who must ultimately evaluate the technological opportunities on the basis of their know-how and market conditions. In the Ready project, coated conductor tapes produced by thermal evaporation and chemical deposition techniques will be used in the secondary windings of a transformer integrated with a pulse tube refrigerator.'

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Design for a prototype of an integrated device, combining an HTS superconducting transformer, a cryostat and a complete refrigeration system, as developed by the industrial partners working on the European Ready project (Oxford Instruments, Air Liquide, Schneider), with the participation of the French CNRS (Centre National de la Recherche Scientifique).

Design for a prototype of an integrated device, combining an HTS superconducting transformer, a cryostat and a complete refrigeration system, as developed by the industrial partners working on the European Ready project (Oxford Instruments, Air Liquide, Schneider), with the participation of the French CNRS (Centre National de la Recherche Scientifique).
EFMaher@aol.com

The image shows a classic demonstration of the 'diamagnetic levitation' of a superconducting disc made of an HTS compound when it approaches a magnet.

The image shows a classic demonstration of the 'diamagnetic levitation' of a superconducting disc made of an HTS compound when it approaches a magnet.
(Photo from the Solid State Physics Group, KTH, Sweden)

Cross-section of the superposed layers required to manufacture an HTS superconducting tape.

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Cross-section of the superposed layers required to manufacture an HTS superconducting tape.

 


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