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.
Electron microscope image of two superconducting thin films that meet at a 6 degree tilt boundary (the dark line running through the image). The numerous smaller lines that intersect the grain boundary at 90 degrees are the individual crystalline layers. The connection between the two films shows distortions in the superconducting layers, which severely limits current flow in these materials. Credit: F.J. Baca, U.S. Air Force Research Lab
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. Dislocations—defects in the crystalline
structure—that 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 boundaries—for instance
by replacing some of the yttrium atoms with calcium—but 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 strain—which
extends from the dislocations into the remaining superconducting bridges over
the grain boundary—was previously unknown. NIST’s 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 of strain on current flow in thin-film
superconductors could significantly advance the development of these materials
for practical applications and could lower their cost. Some of the most promising
uses are in more efficient electrical transmission lines, which already have
been successfully demonstrated by U.S. power companies, and increased electric
power grid reliability. NIST has research programs in both these areas. Improved
HTS thin-film conductors could also enable more powerful high-field particle
accelerators and advanced cancer treatment facilities.
* D.C. van der Laan, T.J. Haugen and P.N. Barnes. Effect of compressive uni-axial
strain on grain boundary critical current density in YBa2Cu3O7-d superconducting
films, Physical Review Letters Accepted papers, June 9, 2009.