nature materials | VOL 6 | SEPTEMBER 2007 | www.nature.com/naturematerials 631 REVIEW ARTICLE S. R. FOLTYN 1 , L. CIVALE 1 , J. L. MACMANUS-DRISCOLL 1,2 , Q. X. JIA 1 , B. MAIOROV 1 , H. WANG 3 AND M. MALEY 1 1 Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 Department of Materials Science and Mettallurgy, University of Cambridge, Cambridge CB2 3QZ, UK 3 Department of Electrical and Computer Engineering, Texas A & M University, College Station, Texas 77843-3128, USA *e-mail: sfoltyn@lanl.gov Since the 1986 discovery of high-temperature superconducting (HTS) materials 1 , the promise of zero-resistance devices operating at liquid-nitrogen temperature has fuelled a worldwide research investment that is now around one billion US dollars. Most research has been in the electric power area for applications such as magnets, motors and power-transmission lines; all power applications share a common requirement that the superconducting material be formed into a long, strong and flexible conductor so that it can be used like the copper wire it is intended to replace. And this is where the problems began, because the HTS materials are ceramics that are more like a piece of chalk than the ductile metal copper. e first solution to this problem was to pack Bi–Sr–Ca–Cu–O (BSCCO) superconducting powder into a silver tube 2 . Following a series of rolling and heating steps the end product was a 4-mm-wide tape capable of carrying over 100 A at liquid nitrogen temperature and flexible enough to be used in the above-mentioned applications. is so-called first-generation wire is the workhorse of the present-day HTS industry, with many hundreds of kilometres of tape produced for various demonstration projects, primarily power cables and motors. Unfortunately this type of conductor relies heavily on the use of silver, which makes it too expensive for most commercial applications and severely limits prospects for economy- of-scale cost reductions in the future. Further, BSCCO rapidly loses its ability to carry supercurrent in a magnetic field at liquid nitrogen temperature, negating the advantage of using an inexpensive cryogen in applications such as motors and generators. In the early 1990s an alternative approach was conceived 3–5 , which was quickly shown to offer competitive performance 6,7 . Instead of imparting the requisite strength and crystallinity to the superconductor by the powder-in-tube method, ‘second-generation’ wire uses epitaxial growth of a superconducting coating on a thin metal tape. One advantage is that very little silver is needed, making significant cost reductions possible. Another advantage of this idea is that it allows use of the compound YBa 2 Cu 3 O 7–δ (YBCO), which retains much higher current-carrying ability in a magnetic field. As recently as six years ago 8 , however, there were serious and legitimate questions about whether second-generation wire could be produced in long lengths with the performance required for commercial applications, and whether it could be done at a competitive price. e first of these questions has now been definitively answered by recent announcements from three companies 9–11 about YBCO tapes that are more than 100 metres long, carrying over 200 amperes per centimetre width at 77 K. One of these companies, SuperPower, has delivered 10 km of wire for a power-distribution cable project in Albany, New York 12 . Another, American Superconductor, has delivered a similar amount for various projects, including two Materials science challenges for high-temperature superconducting wire Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and — the downside of high transition temperature — performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials. © 2007 Nature Publishing Group