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In new binary alloy, two layers of boron 'bread'
surround a 'filling' of lithium metal.
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The researchers reported their findings in the May
5 online edition of the journal Physical Review B, Rapid
Communications.
"To the best of our knowledge, this alloy structure
had not been considered before," said Stefano Curtarolo, professor of
mechanical engineering and materials sciences at Duke's Pratt School.
"We have been able to identify synthesis conditions under which the
LiB compound should form. And we believe that if the material can be
synthesized, it should superconduct at a higher temperature, perhaps
more than 10 percent greater, than any other binary alloy
superconductor."
"The significance of the work is not only the
discovery of lithium monoboride itself, but also that this opens the
door to finding derivatives that could aid in the search for
additional novel superconductors," added Aleksey Kolmogorov, lead
author of the study and a postdoctoral fellow at the Pratt School. He
said that once a new superconductive material is identified,
scientists typically can manipulate the substance - twisting it or
doping it with other elements - to create related structures that
might have even more appealing properties.
Superconductors have the potential to produce more
efficient electronics and electric generators, according to the
researchers. The materials also have unique magnetic capabilities that
may enable their use in transportation applications, such as "levitated"
trains that glide over their tracks with virtually no friction.
However, today's superconductors perform only when
cooled to extremely low temperatures near absolute zero, which is
-459.67 degrees Fahrenheit, or 0 degrees Kelvin. This requirement
makes their use prohibitively expensive, the researchers said.
The first superconductive material was identified
in 1911 when a Dutch scientist cooled mercury to 4 degrees Kelvin, the
temperature of liquid helium. Since then, scientists have discovered
superconductivity in various materials, including other pure elements,
complex ceramics, and binary alloys.
Since 1986, ceramics have held the overall record
for highest superconducting temperature - currently 138 degrees
Kelvin. Among pure elements, lithium, when contained under pressure,
holds the record at 20 degrees Kelvin.
Recently, scientists scored an unexpected
breakthrough with the discovery of superconductivity in the simple
binary alloy magnesium diboride (MgB2), Curtarolo said.
This compound holds the current temperature record for its class at 39
degrees Kelvin, and it has attracted much attention because it can be
produced relatively easily from two abundant elements.
"The physics of the superconductivity in MgB2
is now well understood," Kolmogorov said. "However, MgB2
has been shown to be such a unique superconductor - finely tuned by
nature - that attempts to improve it or use it as a model for finding
even better superconducting materials have so far been fruitless."
Curtarolo and Kolmogorov decided it was time to try
something else. Using a theoretical data-mining method developed by
Curtarolo, the pair scoured a database of experimental and
hypothetical compounds, looking for other possible configurations of
binary alloys and tweaking their compositions.
In the process, the team stumbled onto "a path to a
new metal sandwich structure consisting of stacks of metal and boron
layers," Curtarolo said.
Additional calculations identified the binary alloy
lithium monoboride as a promising candidate that might be both
structurally stable and superconductive at temperatures that exceed
those of the current binary alloy record-holder.
"It's a very thin line, because as you try to
increase the temperature at which a material becomes superconducting,
the material tends to lose its stability," Kolmogorov said. "But we
think lithium monoboride should be stable and superconduct at
temperatures greater than 39 degrees Kelvin."
"It was like spotting a $100 bill on the street,"
Curtarolo said of the finding. "It seemed impossible that this could
be real and that no one had seen it before."
The researchers are now conducting more precise
theoretical calculations of LiB's "critical temperature" - that is,
the temperature at which it becomes superconductive - with
computational support from the San Diego Supercomputer Center at the
University of California, San Diego.
The material will have to be synthesized before
experimental tests can confirm any of the theoretical results, the
researchers said. They added that this won't be an easy process, as
manufacturing lithium monoboride will require extremely high
temperatures and pressures. |
