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Washington University in St.
Louis Advance hastens practicality of superconductors
Nobody completely understands
superconductors. So fathom how James S.Schilling, Ph.D., led a team that makes
the phenomenon work better. Schilling, a professor of physics in Arts & Sciences
at Washington University in St.
Louis, collaborated with recent doctoral graduate Takahiro Tomita and
scientists at Argonne (Ill.) National Laboratory to determine whether one region
in superconductors, called grain boundaries (GB), are oxygen deficient. Such
oxygen deficiency impairs superconductor performance.
Their paper, titled
“Enhancement of the Critical Current Density of YBa2Cu3Ox Superconductors under
Hydrostatic Pressure,” is published in the Feb.24 issue of the highly regarded
journal Physical Review Letters.
A superconductor is a solid
material that conducts electricity without resistance when it is cooled to
certain subzero temperatures. Because there is no resistance, current uniquely
travels through superconductors without losing energy.
Their study involves the newer,
so-called “high-temperature” ceramic superconductors. They superconduct at less
frigid temperatures than other superconductors, although still in the subzero
realm.
The superconducting material
used in this study was a ceramic compound consisting of millions of microscopic
crystals (grains). The WUSTL/Argonne team specifically developed a technique to
determine whether a desired maximum number of possible sites are filled with
oxygen in the GB, which surrounds every crystalline grain. The GB is a region of
misfit between the grains and usually is only a few atoms wide.
The study used the most widely
employed ceramic superconductor, known as YBCO. YBCO (or YBa2Cu3Ox) simply
represents its “yttrium-barium-copper-oxide” content.
Fully oxygenated
Full oxygenation is essential
for the manufacture of reliable ceramic superconductors. Maximizing oxygen in
the GB helps maximize critical current density (Jc), or the maximum current that
a superconductor can carry. In the subatomic world of superconductors,
unrestricted current flow must be the outcome.
“Even in the best
superconductors,” Schilling noted, “GBs limit their ability to carry the high
electric currents required for applications in electric power grids or to
generate enormous magnetic fields. To enhance the current carrying capacity, it
is essential to bathe the grain boundaries in as much oxygen as possible.
Unfortunately, it is very difficult to determine how much oxygen is really
present in the GB.
“We have developed a method
which allows one to estimate this, called pressure-induced oxygen relaxation.”
Boyd W.Veal, Ph.D., an Argonne
physicist and a co-author of their paper, said the technique “could tremendously
ease the superconductor manufacturing problem. There is hope that these
discoveries can make (superconductor) materials more accessible for practical
applications."
Until now, science had
determined how to check ceramic superconductors’ crystalline structures – but
not their GBs – to ensure all potential oxygen sites were filled. It also was
known that full oxygenation is essential. The investigators note in the paper,
“Even when the bulk material is fully oxygenated, the GBs are likely oxygen
deficient.”
“This is the most applied thing
we’ve ever done,” Schilling said of his WUSTL research. “But we’ve done a huge
amount of work in the past on oxygen ordering; that was in the (superconductor
crystalline structure) bulk itself – not in the grain boundary.”
Current flowing without
resistance
Electrical systems would run
more efficiently if current flowed without resistance. Electrical voltage simply
is current multiplied by resistance. At room temperature, all known materials
resist electric current in varying amounts, including today’s electrical wiring
– which, therefore, loses energy.
“There’s no way to explain
superconductivity in simple terms. It’s against intuition,” Schilling said,
finding no commonplace analogy for superconductors, which only can be explained
using quantum mechanics. “It’s like nothing you’ve ever experienced.”
The phenomenon has been tweaked
by scientists, including a few Nobel Prize winners, in an effort to achieve
maximum current flow (Jc) at higher temperatures (as close as possible to room
temperature) using various compounds. Generally, the lower the temperature and
the higher the pressure, the better the current capacity (Jc). Magnetic field is
another complicated variable in the mix. The goal of finding a superconductor
that will function at room temperature is desired for many widespread practical
applications.
For its superconductor, the
WUSTL/Argonne study used a recently developed YBCO bicrystalline melt-textured
ceramic ring – a small, brittle object that is about the size of a tiny washer.
Chemical pressure up to 6,000 atmospheres (0.6 GPa) – or 6,000 times the air
pressure of the earth’s atmosphere – was applied by transmitting high-pressure
helium gas into a compression chamber holding the ring. Then a magnetic field,
which generates an electrical current in the ring, was applied.
In this study, the new
“pressure-induced Jc relaxation” technique revealed whether there were vacant
oxygen sites in the GB.
When there was a markedly and
measurably strong change in the Jc with changes in pressure, it indicated that
oxygen ordering (realignment) was occurring in the GB. Conversely, if all the GB
oxygen sites already were filled when pressure was applied, there were only
small changes in the superconductor’s current – because the oxygen did not move.
When the oxygen moved into vacant sites, “we knew because it affected the
current capacity (Jc) in the grain boundary and the Jc went up,” Schilling
explained.
To preserve a superconductor
with a fully oxygenated GB for manufacture, pressure would have to be released
at “temperatures sufficiently low (less than 200 K or less than –73 C for YBCO)
to prevent the oxygen (atoms) from diffusing back, thus effectively freezing in
the higher degree of order,” the investigators say in the paper.
Schilling said researching the
oxygenation of GBs under pressure was built on Veal’s earlier work. “This turned
out to be a very challenging thing – not an easy solution,” said Veal, who is
one of the world’s most cited physicists in the physical sciences. “Solving this
GB problem could have huge commercial impact.”
Room temperature is the ideal
Like analyzing plant life for
pharmaceutical answers to disease, one broader quest for physicists is to
discover the most practical combination of elements that will superconduct
current – ideally closer to or at room temperature. Since the phenomenon first
was encountered in 1911 by a physicist applying an electric current to mercury
at nearly absolute zero (4.2 K or –269 degrees C), the basic process has
undergone innumerable substitutions. As in perpetual motion, current will flow
forever in a closed loop of superconducting material.
In one atmosphere of pressure,
the YBCO superconducts at 93 K (or –180 C) — which is well above the temperature
required of earlier superconductors. Sometimes, this critical transition
temperature (Tc), or the temperature below which a material begins to
superconduct, can be pushed higher with the application of higher pressure.
YBCOs can superconduct at temperatures as high as 110 K (–163 C) at highest
pressure (about 100,000 atmospheres). But, to date, no superconductor Tc has
remotely neared room temperature.
Schilling, who joined the WUSTL
faculty as a professor in 1990, earlier conducted research for 21 years in
Germany. He was a professor of applied physics at the University of Munich and
primarily worked in high-pressure physics research. The Little Rock native is a
Fellow of the American Physical Society and is the only faculty physicist at
WUSTL studying superconductors.
“In Munich, we discovered the
effect that oxygen rearranges under pressure in the superconductor bulk and
causes a big change in the Tc. Then, we were studying the crystal (structure
itself) instead of the GB,” Schilling said.
For more than a decade, his
research at WUSTL has been supported by the National Science Foundation, which
also helped fund this study. Argonne is supported by the U.S. Department of
Energy. Argonne’s other authors on the paper are Helmut Claus, Ph.D., and
doctoral student Lihua Chen.
--By Leslie R.
Myers
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