15, 2005 --- Researchers at the U.S. Department of
Energy's Brookhaven National Laboratory have discovered
a way to significantly increase the amount of electric
current carried by a high-temperature superconductor,
a material that conducts electricity with no resistance.
This is an important step in the drive to create
superconductor-based electric and power-delivery
devices, such as power transmission lines, motors,
and generators. The results are explained in the
September 12, 2005, online edition of Applied
Physics Letters .
"In theory, superconducting materials can conduct an enormous amount of electric
current. But when incorporated into actual devices, certain factors tend to limit
the current," said Brookhaven materials scientist Qiang Li, a co-author on the
paper. "We studied these factors and found that one, which we call 'substrate
roughness,' can actually significantly increase the current-carrying capacity."
The superconducting material studied here consists of the elements yttrium,
barium, copper, and oxygen. Dubbed YBCO, it is a member of a class of copper-
and oxygen-containing superconductors called "cuprates." Cuprates are "high-temperature" superconductors
because they superconduct at temperatures much "warmer" than conventional superconductors
(although still very cold) - for example, -300°F rather than -440°F.
This difference, while not huge, is enough to make cuprates more viable for
practical applications than materials that must be kept much colder. In many
of these applications, YBCO films are deposited onto a 'normal' metal surface
(the "substrate"), forming components known as coated conductors. One of the
factors widely thought to degrade the performance of coated conductors is the
roughness of the metal surface.
To verify this, Li and his colleagues set out to study and measure how the
roughness of the substrate affects the current-carrying capacity of YBCO. The
researchers deposited a YBCO layer onto a substrate prepared with two distinct
areas: a rough, corrugated region with nanometer (billionth-of-a-meter) sized
ridges and grooves, and a smooth region. This configuration allowed the group
to directly compare the behavior of the YBCO film on both surface types. They
were able to do this using electrical-transport measurement techniques, which
track the amount of supercurrent passing through the material, and "magneto-optical" imaging,
a technique used to study superconductors by following their magnetic behavior.
"What we found is remarkable and surprising," said lead author Zuxin Ye, a graduate
student under Li's supervision. "Rather than limiting the current, the nanoscaled
corrugated surface produces more than a 30 percent increase in the supercurrent
carried by the YBCO films. This suggests that metal substrates with some degree
of roughness at the nanoscale might help improve the performance of high-temperature
superconductors." The work is the result of a collaboration between scientists
in Brookhaven Lab's Materials Science Department, the Condensed Matter Physics
group within the Physics Department, and the Lab's Center for Functional Nanomaterials.
It was supported by the Office of Basic Energy Sciences within the U.S. Department
of Energy's Office of Science.
About Brookhaven National Laboratory:
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.
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