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"A new materials technology - such as a technology
based on high-temperature superconductivity - is required to make the
huge leap from 21 Tesla to 30 Tesla," said William P. Halperin, John
Evans Professor of Physics and Astronomy in the Weinberg College of
Arts and Sciences at Northwestern, who led the team. "We have shown
that Bi-2212 could be operated at the same temperature as is presently
the case for magnets made with niobium - 4 degrees Kelvin - and also
achieve the stable state necessary for a 30 Tesla magnet."
The findings was published online Feb. 11 by the
journal Nature Physics.
"We are exploring nature's limitations, and our
discovery has basic implications for the study of superconductors and
for applications to magnetic resonance imaging," said Halperin. "The
dream would be to have powerful magnets that don't require helium for
cooling. Some day new materials might be discovered where this
restriction is lifted, but it isn't possible at the present time."
A superconductor, when cooled to its appropriate
temperature, conducts electricity without any resistance.
Superconductivity first appears in Bi-2212 at a high temperature of 90
degrees Kelvin, but Halperin and his colleagues found that the stable
state required in high-magnetic fields can be established only when
the temperature falls below 12 degrees Kelvin. The team is the first
to establish this limit for Bi-2212.
"Sometimes what seems to be bad can be good," said
Bo Chen, lead author of the paper and a graduate student of Halperin's.
"Our findings set a speed limit. If you go beyond this speed you may
have trouble. Knowing the upper temperature limit is a kind of
security."
"To create a 30 Tesla magnet, we need a
superconducting material that can carry the required amount of
electricity without blowing up," said Halperin. "We have found that
the operating temperature for Bi-2212 must be below 12 degrees Kelvin.
The good news is that this temperature can be reached by cooling the
magnet with liquid helium. If we had found the upper limit to be 2
degrees Kelvin then the cryogenic requirements would be intractable."
MR imaging is widely used by hospitals for medical
diagnosis, and scientists at universities, national laboratories and
pharmaceutical companies use even more powerful MR technology to study
DNA, proteins and other complex molecules. About a dozen labs around
the country take advantage of the highest magnetic field now in use -
21.1 Tesla, which produces a magnetic field 10 times larger than your
average hospital machine. Increasing the field of the magnet even a
small amount, from 21.1 to 22.2 Tesla, would increase the cost of the
machine by two million dollars.
"A holy grail of the scientific community, as set
out recently by the National Research Council, is to build a
superconducting magnet of 30 Tesla," said Halperin. "In MR imaging,
the higher the magnetic field, the higher the resolution, which
provides scientists with more detail for analysis. A 30 Tesla magnet
could drive significant advances in chemistry, biology and medicine."
Using MR techniques at the National High Magnetic
Field Laboratory in Tallahassee, Fla., Halperin and his team studied
Bi-2212, one of the "darlings" of superconductivity. To measure its
properties, they put the rare isotope oxygen-17 into a crystal of
Bi-2212, with the isotope acting as a probe, much like a fluorescent
dye. They then determined the phase diagram of the material where
superconductivity is stable, which showed high temperature and high
magnetic field could not be achieved together.
"Now that we have this information about Bi-2212,
the next question is, 'Can such a magnet actually be made?'" said
Halperin. "I really don't know - it depends on engineering and
processing the materials to make them into wires. My fellow scientists
and engineers will have to solve the materials problems, and they
don't like to accept no as an answer." |
