The new process is cheap and simple to fit into existing manufacturing techniques. It takes away impediments which have hindered making competitive superconducting cables until now. Since the discovery of high-temperature superconductivity in 1986 one of the most important aims has always been a nitrogen-cooled superconducting power cable: a cable to conduct current without resistance, for example for high-power applications in the energy branch, and in powerful magnets for particle accelerators and advanced hospital machinery. The advantage of 'high-temperature' superconductors is that they are already superconducting at the temperature of liquid nitrogen: this is still -196 %C, but cooling them is a lot simpler than the existing superconducting cables that work near the absolute zero and need to be cooled with liquid helium: complicated, especially over longer distances.
In practice the manufacture of cables turned out to be extremely complicated. Cables consist of microscopically small grains, and the supercurrent has to flow from one grain to the next, over boundaries between the grains. These grain boundaries are the limiting factor. If the grains differ a lot in direction of orientation, the link is much less strong because of that. A great deal of effort has therefore been expended on precisely aligning the grains. This is, however, time-consuming and costly.
The researchers of the research group Low Temperature Physics and the MESA+ institute of the UT and the Physics Institute of the University of Augsburg opted for another approach. To this end, UT-researcher dr. ir. Hans Hilgenkamp, as specialist in the area of thin superconducting layers, worked in Augsburg for a time. In Nature of 14 September the researchers describe a way to improve the current on grain boundaries. For this they used the most popular high-temperature superconductor: yttrium barium copper oxide, Yba2Cu3O7. On the interface of the grains they add a material like Calcium. In this way they 'dope' the superconductor, which seems to affect the conductivity favourably. Especially stacking alternating layers of undoped and calcium-doped superconducting turns out to improve performance. The critical current - the maximally achievable current before resistance starts working - can be raised for grainboundaries with a factor of six [h2], experiments show.
The new discovery is cheap and simple to fit into existing manufacturing techniques. The researchers expect therefore that industrial-scale applications are coming close.
