The potential benefits of compounds that behave as superconductors at slightly higher temperatures, has spurred a lot of research activity in the field
ALEX Muller and Georg Bednorz of the International Business Machines had reported observing superconductivity at 35 Kelvin in 1986. This report caused a stir in the scientific community, because the phenomenon of super- conductivity had hitherto, only been observed at much lower temperatures. Since then, over 100 high temperature superconductivity (HTS) compounds have been discovered, and the highest critical temperature (Tc, the temperature below which the compound is superconducting) has now reached 134 Kelvin. And all this in a short span of only a decade.
By early 1987, C W Chu of the University of Houston had discovered a compound -yttrium barium copper oxide (YBCO) -which had a critical temperature of 93 Kelvin. This was a major breakthrough because this temperature can be achieved by using liquid nitrogen which is cheap and easily available rather than liquid helium which is very expensive and cumbersome to handle. Almost immediately, there was talk of the high Tc revolution: high speed trains levitating on superconducting magnets, trans- mission wires with no loss in power and so on.
Unfortunately, most of the initial enthusiasm for commercial applications of high Tc seems to have been misplaced and the commercial development of applications has not been upto expectations. Nevertheless, there has been real progress. One of the first practical applications to be tried out was superconducting wires. Initially, researchers tried to use YBCO, but it was soon realised that the nature of the material was unsuit- able for wire making. The next material of choice was bismuth strontium calcium copper oxygen (BSSCO ). The grains of BSSCO are flat and thus, can be easily aligned. Researchers have been able to make BSSCO wires which can carry 100 times more current than the conventional copper wires. And now there are several wiremaking companies in the us and Japan which are claiming that they can manufacture about a kilometre of such wire in a week. In fact, recently, a consortium of European and American power companies wrapped about six kilometres of BSSCO ribbon into a 50 metre underground transmission cable. The cable is sheathed around a core of a hollow tube carrying liquid nitrogen. This is considered a breakthrough because the wire has been made in a factory as opposed to a laboratory.
Another area of interest and growth has been the fabrication of high Tc films. Grains of high Tc material are grown on top of a crystalline template, and a thin film of such a material is obtained. These films have already found applications in a wide range of areas including filters for cellular phone base stations.
These filters screen out the ambient noise accompanying the signal from the cellular handset, and were conventionally made of copper. Now with high Tc thin film filters, the efficiency has considerably increased and it is possible to detect the signals from further out.
Though a satisfactory theory of why some materials are superconducting at high temperatures is still eluding the scientists, the applications, both actual and potential, will keep the scientists going on to discover newer materials with unique properties (Science, Vol 271, No 5258).
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