Wire, superconducting ceramic

The second main part of a PLD system is the vacuum chamber. The author s group developed a large-area PLD process for the double-sided deposition of high-Tc superconducting thin films (see for example [8,10]). As substrate heater an arrangement of KANTHAL wire in ceramic tubes is used... 

At Cambridge s department of physics, Jan Evetts—who has filed at least six patents ranging from the basic physics of ceramic material to designs for superconducting wires— packs ceramic into narrow tubes of silver to create flexible, superconducting wire. Because the silver doesn t oxidize, it allows the oxygen to pass through to the ceramic, which requires it. 

Objects made of high-temperature superconducting ceramics include wires, cylinders, pellets, and tubes. (Photo courtesy of Argonne National Laboratory.)... 

A crucial concern in the application of superconducting ceramics is to devise ways to fabricate the new materials in desired shapes such as wires. This will be quite a challenge because these superconductors are ceramics and have the brittleness and fragility typical of ceramic materials. 

R. W. McCallum, J. D. Verhoeven, M. A. Noack, E. D. Gibson, F. C. Laabs and D. K. Finnemore, Problems in the Production of YBa2Cu3Ox Superconducting Wire, in Ceramic Superconductors, special issue of Advanced Ceramic Materials, ed. D. R. Clarke and D. W. Johnson, to be published July 1987. 

Superconducting ceramics of Y,Ba2Cu30jj g have been used as substrates for the electrochemical deposition of either poly pyrrole or poly thiophene [87]. Ceramics are typically obtained by firing pressed powders of the oxide compounds. Thus, electrical and mechanical contacts made between the ceramics and wires produced mechanically or by vacuum deposition of metal are often found to be inadequate. Superconductors modified by conducting polymers may help improve the contact area. 

Superconducting ceramic materials have some limitations as electrical conductors when compared with an ordinary conductor such as copper wire. What are some of fliese ... 

Note that the issue of the brittleness of ceramics restricting their application is not limited to structural applications. For instance, the brittleness of superconducting ceramics had to be resolved by employing them either as thin films on ductile metal wires or embedded within ductile metal casings. Thus the understanding of how to defeat the brittleness of the ceramics impacts their use in a host of applications. 

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

The current transfer problem that had been identified with low temperature superconducting composites deserves additional mention for the high temperature superconductors, that in the bulk material are frequently not fully dense. Making the electrical connection in such a manner as to obtain uniform current distribution throughout the cross section of the material is difficult. The method described by Jin, et al. (24) with embedded wires or particles may provide for a significant improvement but the present techniques used to determine the critical current by a surface contact on the ceramic sample are subject to this problem. A discussion for the multifilamentary wire of NbsSn is provided by Goodrich and Fickett (30) and this discussion is likely to be similar to the high temperature materials that are not fully dense. 

The copper oxide ceramic superconductors are two-dimensional conductors They can conduct a current parallel to the layers of Cu and O atoms but not between the layers. The fullerides, by contrast, are three-dimensional conductors that conduct equally in all directions. Because of this property, they may prove to be superior materials for making superconducting wires. 

A few months later, at a Boston meeting on superconductivity, a somewhat different response was indicated by Shoji Tanaka, who had led the University of Tokyo team that confirmed IBM s discovery of superconductivity in the lanthanum compound. Asked to comment on recent reports that the Japanese had made great progress in firing a ceramic into a serviceable wire, Tanaka said, My work is not in that direction, adding, after a fairly long pause, but in any event that might be a secret, I think. ... 

Some scientists have foregone the ceramics altogether and attempted to fabricate wire out of metal alloys that can superconduct at temperatures warmer than normal for a metal. At the Massachusetts Institute of Technology, for example, one team has made a superconductor from europium—a soft, silvery metal that was once in short supply but is now more easily acquired—barium, and copper. The alloy apparently works well at 90° K, which means it can run on liquid nitrogen moreover, it is more easily fabri-... 

The ceramics are not, of course, as bad as all that. They most certainly do superconduct. But just barely. Subtle shifts in oxygen balance, too much or too little of one ingredient or another, very high magnetic fields, and high temperatures—all of these reduce their unique capabilities and turn them quickly into insulators or, at best, conductors that offer no real advantage over those spun of more reliable copper wire. 

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