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Applied Superconductivity Center
 Magnesium
DiBoride - MgB2 |
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Schematic
of the crystal structure of MgB2
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In
January 2001, a totally new superconductor was discovered, with
a surprising critical temperature of 39 K - MgB2.
This discovery stimulated a global flurry of work seeking higher
Tc and uncovering the basic physics. Although so
far higher Tc has not been found, we now know that
MgB2 is akin to other LTS intermetallics, with high
Tc coming from the exceptionally high vibrational
energies in the graphite-like boron planes. Thus, MgB2
appears to obey conventional models of superconductivity, and
this more simple view (as compared to HTS) opens up a wide range
of practical opportunities. In addition, since magnesium and
boron are both cheap and abundant, practical long-length multifilament
conductors might one day be cheaper than niobium-based LTS and
sterling silver-clad HTS. MgB2 conductors might occupy
a low to mid-field niche, operating in liquid helium or liquid
hydrogen. The density of MgB2 is comparable to aluminum,
perhaps leading to new lightweight applications as well. |
Our goal in the Applied Superconductivity Center is to understand
and explore the potential for MgB2. With help from
collaborators at Princeton and Ames, we found early on that
grain boundaries are not obstacles to current flow, unlike the
situation in HTS. This means that random polycrystalline forms,
such as round wires made by powder-in-tube (PIT) techniques,
can carry substantial critical current densities. This does
not mean that powder-based conductors are without obstacles
to current flow, however. For example, MgO and amorphous regions
are revealed when dense MgB2 samples are probed with
a transmission electron microscope. Avoiding these unwanted
phases remains a central issue for developing wires, tapes,
and other polycrystalline forms. |
The layered crystal structure of MgB2 produces anisotropic
properties when fields and electric currents are applied in
various directions with respect to the boron planes. This anisotropy
is revealed in textured samples, in which all grains share a
common crystallographic orientation, and in single crystals.
Thin films in particular are examples of textured samples, because
grains line up against a flat substrate. By using textured thin
films and, more recently, epitaxial thin films, a phase diagram
of the fields and temperatures that limit superconductivity
and current transport has been mapped out. While upper critical
fields (Hc2, at which superconductivity is destroyed)
determine possibilities for applications, practical limits are
set by irreversibility fields (H*, above which current flow
is no longer lossless), coolant liquids, and refrigeration capacity.
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Critical
field - temperature diagram of MgB2.
Click on the Image to see the Shockwave version of the graph
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| Research
Focus
(1) Understanding basic properties
In collaboration with many groups around the world, we continue
to focus on measuring basic properties, including critical
temperature, critical fields, critical current density, resistivity,
specific heat, and anisotropy. The central question being
addressed is whether small variations in sample chemistry,
preparation, or structure produce discernable differences
in basic properties.
(2) Controlling fabrication
a.
Understanding the phase diagram and potential for alloying
Magnesium diboride can be prepared from a stoichiometric mixture
of pure elements. If this is done carefully, samples with
high Tc, low resistivity and relatively low Hc2
result. A key question is whether Hc2 can be raised
by controllably alloying MgB2 with a third element,
as is the case in many other type-II superconductors, to produce
properties more favorable for high-field applications. Our
early thin film work found evidence for oxygen alloying and
nanoprecipitates of MgO when films are prepared by pulsed
laser deposition from an MgB2 target. Scattering
by these defects increased resistivity and Hc2,
although Tc was also decreased. We are currently
investigating whether nanoparticles added to bulk samples
could produce a similar effect, in collaboration with colleagues
at Imperial College. A related question is how variations
from stoichiometry, such as Mg deficiency, affect the superconducting
properties.
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b. Understanding how to make useful forms of MgB2
We are working extensively on developing wires and thin films
of MgB2. Round wires are best suited for making
stable, tightly packed cables for high field magnets, while
tapes can be used in cables for electric power applications.
Both of these conductors begin as powder-in-tube composites,
and chief concerns are fabricating long, uniform lengths,
chemical reactions with sheath materials, and connectivity
of the powder core. Recently, epitaxial thin films have been
made by a 2-step technique, in which a boron film is deposited
on a sapphire substrate and then converted to MgB2
by a reaction with Mg vapor. Present work addresses making
epitaxial films by a single step process, developing methods
to integrate MgB2 with other materials, and exploring
superconducting electronic devices.
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Cross
Section of MgB2 wire
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(3) Exploring science issues
A number of unique features of MgB2 raise additional
scientific challenges. Recent theoretical and experimental
work suggests that MgB2 may exhibit two-gap superconductivity.
We are therefore curious whether there are novel effects due
to the coupling between the gaps, and whether this produces
new properties when quantized field lines move along grain
boundaries or when MgB2 is exposed to microwave
radiation. The competition between thermal fluctuations and
flux pinning is also being explored.
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