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When calcium atoms (yellow spheres) in these iron-based crystals (left) are replaced on some occasions with praseodymium (blue sphere in right image), the crystals are able to superconduct at up to 47 K—but the crystals can also collapse, shrinking by about 10% in size. Adding a sufficient amount of praseodymium is necessary to avoid the collapse, which compromises the materials usability in electronics applications. Image: NIST |
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A team from NIST and the University of Maryland
has found an iron-based superconductor that operates at the highest known
temperature for a material in its class. The discovery inches iron-based superconductors—valued
for their ease of manufacturability and other properties—closer to being useful
in many practical applications.
Iron-based superconductors, which were discovered only about
four years ago, are a hot research topic, in part because they are more
amenable to commercial applications than copper-based superconductors, which
are more difficult to make and are frequently brittle. Of the four broad
classes of iron-based superconductors, the 1:2:2 class—so named because their
crystals are built around a hub of one atom of calcium, two of iron and two of
arsenic—is particularly promising because these superconductors’ properties can
be custom-tailored by substituting other atoms for these basic elements.
Magnets made with low-temperature superconductors have
already found use in hospital MRI machines, but less expensive MRI machines and
other applications, such as superconducting cables for resistance-free power
transmission over long distances, become closer to reality the more choices
manufacturers have among superconductors.
Working at the NIST
Center for Neutron Research (NCNR) and
the University of
Maryland, the team found
that a particular type of 1:2:2 superconductor possesses some unexpected
properties. Of perhaps greatest value to manufacturers is that its threshold
temperature of superconductivity is 47 K, the highest yet for the 1:2:2 class,
whose previous record was 38 K.
But the crystal also has a highly curious property: It can
superconduct at this record temperature when a smaller atom is substituted for
the crystal’s original calcium in some of its hubs, and when this substitution
is performed, the overall crystal actually shrinks by about 10%, a dramatic
size change. "It's almost like what would happen if you cut off a few inches
from the bottom of your chair's legs," says the NCNR’s Jeff Lynn. "The crystal
just collapses. The change is quite visible in neutron scans."
This effect is likely one that manufacturers will want to
avoid. But Lynn
says the group's research has determined how to make the substitution while
eluding the collapsed state altogether, so that as it is cooled, the potential
mechanical instabilities associated with the collapse are sidestepped. "This
understanding should enable manufacturers to use the superconductor in
electronic devices," he says.
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