New Superconductors Come Through

The materials of two new classes of superconductors--one class containing bismuth and the other thallium--lose their resistance to electric current at higher temperatures than any other known substances. This fact has raised hopes that the goal of practical high-temperature superconductivity will soon be within reach. The latest phase in the search for practical high-temperature superconductors has gone on for nearly a 1 1/2 yr., ever since the discovery by Paul Chu at the University of Houston of an yttrium-barium-copper-oxygen mixture that lost all electrical resistance at 90 deg K. But researchers discovered that the high-temperature superconductors had material problems. The bismuth and thallium compounds become superconducting at higher temperatures than the earlier substances and there are signs that they would be easier to work with.
Sandia National Laboratories in Albuquerque, New Mexico, has made a thin film from a thallium-based superconductor that carries much more electric current than similar films made from earlier superconductors. Thin films, which are used in electronics components, are likely to be the first practical application of high-temperature superconductors. The thallium films are impressive because the strength of the links between the crystalline grains can apparently be made weaker or stronger by changing the processing technique, which could allow for tailor-made materials for a desired critical current density.
A group at Stanford University has succeeded in making an exceptionally high-quality fiber out of a bismuth-based superconductor by using a technique called laser-heated pedestal growth. By changing various conditions of the process--such as the composition of the source material, the speed of withdrawing the seed, or the atmosphere in the growth chamber--the group was able to control the composition of the resulting fiber. One of the impressive things about the process is that the fiber is superconducting as it crystallizes, and needs no further processing. Other techniques for making superconducting wires require an extra step to change the material into a superconducting form.