From: Mike Lorrey (mlorrey@yahoo.com)
Date: Tue Jul 30 2002 - 09:26:53 MDT
http://www.newswise.com/articles/2002/6/SENSSUP.IEE.html
Magnesium diboride could replace niobium-titanium, a
conventional
low-temperature superconductor, in future MRI magnets
and could also
find use in transformers and fault current limiters.
Until the
magnesium
diboride revelation, engineers trying to apply
superconductivity to the
real world had to grapple with distinctly un-ideal
materials.
Low-temperature (metallic) superconductors, while not
too costly and
with excellent mechanical properties, require cooling
down nearly to
zero--4.2 K--a pricey proposition. High-temperature
(ceramic)
superconductors can be cooled at far less expense to a
less chilly 77 K
but are expensive because the manufacturing process
requires a great
deal of silver.
Magnesium diboride falls between the two types on the
temperature
scale;
it is a conventional (low-temperature metallic)
superconductor. It can
be conveniently cooled with commercial cryocoolers or
liquid hydrogen.
A
powder that can be found in any well-stocked chemistry
laboratory, it
had never been tested for superconductivity until very
recently. The
five Japanese researchers who did so announced their
discovery in
January 2001 at a small conference in the Japanese
city of Sendai.
The materials that go into making magnesium diboride,
magnesium and
boron, are both dirt-cheap. So cheap in fact that
magnesium diboride
cable may eventually be comparable in price with
copper cable. But to
be
useful, scientists must be able to form the material,
a powder in its
original form, into wires.
As early as May of 2001, scientists at Agere Systems
(Allentown, Pa.),
a
spinoff of Lucent Technologies, had made magnesium
diboride tape in
lengths of almost a meter. Using a more
manufacturing-friendly process,
Hypertech Research has already made 100-meter-long
wires. In the United
Kingdom, Diboride Conductors is also working to
commercialize the
technology.
Magnesium diboride needs to be chilled to 20-30 K for
it to be useful.
While this is colder than liquid nitrogen, it is
within the range of a
standard commercial cryocooler, and the cost is not
that high.
Magnesium diboride does have some shortcomings. The
material's
inability
to carry much current was one that surfaced early.
Reports indicated
that the wire could carry only 35 000 A/cm2.
(Real-life superconductor applications require a
larger value, at least
80 000 A/cm2.)
However, that figure has crept upward, and is
currently at around 200
000 A/cm2 for a magnetic field of 1 tesla, typical of
transformers and
motors.
The material also does not stand up very well to
strong magnetic
fields.
Early data showed that its superconductivity vanished
in fields greater
than 2 T, which produces magnetic vortices inside the
alloy.
These vortices move under the Lorenz force created by
the current, and
their motion dissipates energy, which shows up as
electrical
resistance.
The solution turns out to lie in making the material
less structurally
perfect.
Several approaches, which deliberately introduce
structural defects and
impurities, have improved magnesium diboride's
magnetic properties.
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