NIST discovers how strain at grain boundaries suppresses high-temperature superco...( Researchers at the National Institut...)

Researchers at the National Institute of Standards and Technology (NIST) have discovered that a reduction in mechanical strain at the boundaries of crystal grains can significantly improve the performance of high-temperature superconductors (HTS). Their results* could lead to lower cost and significantly improved performance of superconductors in a wide variety of applications, such as power transmission, power grid reliability and advanced physics research.

One of the main challenges in developing long-length, high-quality HTS wires is to mitigate the effect of granularity on wire performance because grain boundaries are prone to block current flow. Dislocationsdefects in the crystalline structurethat grow in number with increasing grain-boundary angle strongly reduce the superconducting crosssection of the grain boundary.

Switching to thin-film designs has led to great improvements in grain alignment and significantly improved performance in, for instance, yttrium-barium-copper-oxide (YBCO) coated conductors. But even in these highly aligned superconductor films grain boundaries still limit their performance. The effect of dislocations can further be mitigated by chemical doping of the grain boundariesfor instance by replacing some of the yttrium atoms with calciumbut it has been difficult to apply this technique to long wire lengths.

Although it is well known that dislocations cause part of the grain boundary crosssection to become non-superconducting, the effect of strainwhich extends from the dislocations into the remaining superconducting bridges over the grain boundarywas previously unknown. NISTs Danko van der Laan and his collaborators have found that this strain plays a key role in reducing current flow over grain boundaries in YBCO. Furthermore, when the strain was removed by applying compression to the grain boundaries, the superconducting properties improved dramatically.

The new understanding of the effects o
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