Upton,
NY -- A collaboration led by scientists at the U.S.
Department of Energy's Brookhaven National Laboratory
has revealed a new mechanism that explains why adding
calcium to a high-temperature superconductor increases
its current-carrying capacity. The findings refute
the current explanation and open the door for similar
additives with potentially better current-boosting
abilities. The study is published in the May 26,
2005, edition of Nature.
In theory, high-temperature superconductors conduct
electricity with no resistance. But the most practical,
inexpensive high-temperature superconducting materials
-- those suitable for applications such as electronic
devices and power lines -- are made of many tiny
crystalline grains. The boundaries between grains
act like barriers to electric charge carriers, impeding
the flow of current.
This is the case for the superconducting material
studied here, known as YBCO for its constituent elements:
yttrium, barium, copper, and oxygen. Scientists had
previously discovered that adding calcium to the
boundary between two grains in YBCO improves the
current flow, seemingly because the calcium changed
the electric-charge structure at the boundaries.
Surprisingly, this latest study shows that the position
of the calcium atoms at the boundary is different
than previously assumed -- that is, it is the chemical
structure of YBCO after adding calcium that leads
to improved conductivity.
"At YBCO grain boundaries, calcium atoms replace
some of the barium and copper atoms," said Brookhaven
physicist Robert Klie, the paper's lead author. "Where
the atoms are tightly packed, a calcium atom replaces
a larger barium atom, relieving the strain. Oppositely,
in loosely packed areas, the calcium replaces a smaller
copper atom, which relaxes strained areas that are
nearby."
The
substitutions regulate the atomic structure at
the boundaries, providing additional "pathways" for
electric charge carriers to pass from grain to grain. "This
finding is surprising because we thought only calcium
could improve the grain-boundary conductivity of
YBCO, but our discovery means that similarly sized
elements could be equally or more effective," said
Klie.
Klie and Brookhaven scientist Yimei Zhu, one of
the paper's co-authors, made the discovery using
the scanning transmission electron microscope at
Oak Ridge National Laboratory. The microscope is
a powerful imaging device that uses a beam of high-energy
electrons to examine objects at a very small scale.
The resulting images of YBCO are the first to reveal
the composition of the boundaries at the atomic level,
allowing the researchers to identify, atom-by-atom,
the chemical make-up of the material after the addition
of calcium.
As part of this ongoing collaborative research,
the YBCO sample was fabricated at the University
of Gvttingen in Germany and its electronic properties
were previously measured at Brookhaven by Zhu's group.
The collaboration also includes researchers from
Vanderbilt University, the University of California
at Davis, and The University of Tokyo.
The study was supported by the Office of Basic Energy
Sciences within the U.S. Department of Energy's Office
of Science.
One of the ten national laboratories overseen and
primarily funded by the Office of Science of the
U.S. Department of Energy (DOE), Brookhaven National
Laboratory conducts research in the physical, biomedical,
and environmental sciences, as well as in energy
technologies and national security. Brookhaven Lab
also builds and operates major scientific facilities
available to university, industry and government
researchers. Brookhaven is operated and managed for
DOE's Office of Science by Brookhaven Science Associates,
a limited-liability company founded by Stony Brook
University, the largest academic user of Laboratory
facilities, and Battelle, a nonprofit, applied science
and technology organization. Visit Brookhaven Lab's
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and more: http://www.bnl.gov/newsroom.
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