For
release: June
25, 2004
Contacts:
Paul Canfield, Condensed Matter Physics, (515) 294-6270, canfield@ameslab.gov
Saren Johnston, Public Affairs, (515) 294-3474, sarenj@ameslab.gov
AMES LAB PHYSICISTS “PERTURB” SUPERCONDUCTOR
TO NEW HEIGHTS
Carbon-doped Magnesium Diboride Superconductors
Withstand Higher Magnetic Fields
AMES, Iowa – At the U. S. Department of Energy’s Ames Laboratory,
a basic research effort to enhance the properties of magnesium diboride,
MgB2, superconductors by doping them with carbon atoms has doubled the
magnetic field the material can withstand. The work may one day ease
the expense associated with current superconducting materials that generate
the intense magnetic fields required for such applications as magnetic
resonance imaging for medical diagnostics, high-field magnets for research,
and superconducting magnets for particle accelerators. The Basic Energy
Sciences Office of the DOE’s Office of Science supports the fundamental
research on MgB2 as part of an experimental effort on superconductivity
and correlated systems at Ames Laboratory.
Unlike ordinary conductors, such as copper, superconductors
conduct electricity perfectly, without energy loss due to
heat. But metallic superconductors (Most
notable among them is triniobium-tin, Nb3Sn.) have always been hampered by the
fact that they must be cooled to an extremely low temperature before they become
superconducting. That critical temperature, or Tc, rests near absolute zero (minus
459 degrees Fahrenheit), thus cooling has always been expensive, requiring large
quantities of liquid helium.
However, things warmed up in 2001 when scientists discovered
the superconducting properties of magnesium diboride. They
were amazed to see that the critical temperature
at which MgB2 becomes superconducting is 39 Kelvin (minus 389 F), far warmer
than the reigning niobium-tin superconductors, which become superconducting at
18 K (minus 427 F). The higher Tc of MgB2 also makes cooling the material more
economical as it allows for the use of less expensive refrigerators in place
of liquid helium.
Magnesium diboride’s unexpected superconducting capabilities brought speculation
about the material’s potential for replacing niobium-tin superconductors
in various applications and thus reducing the expense of those technologies.
But for that speculation to become a reality, more basic research was needed
to increase both the magnetic field MgB2 can withstand and the electric current
it can carry. This research has now been carried out by Ames Laboratory physicists
Paul Canfield, Sergey Bud’ko and Doug Finnemore, and graduate assistant
Derek Wilke.
Canfield, who is also an Iowa State University physics professor, and his group
were the first to describe the mechanism of superconductivity in MgB2. They devised
a method of turning boron of a given form into MgB2 with a similar form by allowing
magnesium vapor to diffuse into the boron matrix. This patented technique has
been used to make pellets, wire segments and thin films. Their familiarity with
MgB2 serves the researchers well in current efforts to enhance its superconducting
properties.
“In this game, once you get an idea of what the pure material
is doing, you want to perturb it,” Canfield said. “You want to mess
with it and see how it responds. The problem is that it’s very hard to
systematically perturb MgB2. It really wants to form in a fundamentally pure
fashion.”
As it turns out, “messing” with the material is one of the things
Canfield and his group do best. They were able to figure out how to get carbon
into MgB2. Experiments done by Wilke showed that a 5 percent substitution of
boron with carbon more than doubles the magnetic field MgB2 can withstand and
still remain superconducting, raising it from 16 Tesla for the pure material
to 36 Tesla with the 5 percent carbon-doping. Even though the carbon-doping of
MgB2 lowers its critical temperature to 35 K (minus 397 F), 4 K less than in
the pure material, the magnetic field as a function of temperature exceeds any
of the NbSn compounds, which “peak out at around 30 Tesla,” according
to Canfield.
“That sounds promising, but there are two things still out
there that need to be resolved,” he cautioned.
“One is determining how much current you can pass through the material
and still have it remain superconducting,” Canfield continued. “That’s
the critical current, and it’s still inferior to triniobium-tin.”
Increasing the critical current that MgB2 can withstand and
still remain superconducting is a challenge because whenever
an electric current passes
through a superconductor,
tiny whirlpools of electrons, called electron vortices, are created. The
motion of electron vortices saps energy and destroys a material’s ability to superconduct.
Finnemore’s specialty is pinpointing the locations of electron vortices,
and knowing their locations makes it possible to “pin” them.
If electrical and magnetic conditions are right, vortices will stick, or
pin, themselves to
nanometer-size precipitates in a superconductor. Once pinned to these impurities,
they no longer move or dissipate energy. The trick is to find just the
right impurity that will trap the vortices yet still allow the electricity
to flow
through the material.
“Titanium diboride is our first try as a precipitate, and it works without
sucking out the carbon we added to increase the magnetic field,” said Finnemore.
He and Wilke added the TiB2 using chemical vapor deposition, which disperses
the element uniformly throughout the material. “We think it’s just
a better way to make samples than mixing powders,” Finnemore added. “Over
the next few years, we hope to try other precipitates using chemical
vapor deposition.”
Although Canfield is pleased with his group’s success
in enhancing the superconducting properties of MgB2, he’s
cautious about predicting too much too soon in terms of the
material replacing Nb3Sn. He reminds us that there’s
still that “second thing” out there that needs
to be resolved. “Even if we can tweak the critical current
to be better or comparable to niobium 3-tin, there’s
still the metallurgy of determining how to get a sheath around
this material and make it a useful wire rather than just a
lab sample,” he said. “That’s just beyond
anything we do as basic physicists; it will have to happen
on some engineering time scale. But as far as temperature and
critical field go, it’s now looking better than
triniobium-tin on both of those parameters. The critical
current needs to
be comparable or better. I think that really is the
trinity of what you care about.”
Ames Laboratory is
operated for the Department
of Energy by Iowa State
University. The Lab conducts research into various areas
of national concern, including energy resources, high-speed
computer design, environmental cleanup and restoration, and
the synthesis and study of new materials.
###
Editors: For an image of a cross section of 5 percent carbon-doped
MgB2 wire, go to: http://www.external.ameslab.gov/news/release/2004rel/mgborideimage.htm
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