Bathing in oxygen
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. |
