=superconductor =materials =industrial design =medicine
High-temperature superconductor
(hereafter "HTS") wire is expensive, and that's largely because HTS
materials are all ceramics. They naturally tend to form grains like sand,
but wire requires long thin films.
An alternative way to use HTS is
to make quasi-permanent magnets from them. If a
type 2
superconductor is cooled while in a strong magnetic field, it becomes a
quasi-permanent magnet. Compared to normal permanent magnets, those:
- can have
much stronger fields
- are more expensive
- must be kept cold
-
very slowly decay
In order to use such quasi-permanent magnets in some application, either they need to be magnetized then transported, or they need to be magnetized on-site. Considering the ubiquity of steel, you can probably see why transporting something like an energized MRI magnet would be difficult. As for on-site magnetization, they can only produce as strong a magnetic field as was applied to them, so magnetizing them requires a very strong field.
flux pumping
Imagine a
rectangle of superconductor with a loop of current flowing in it. Now,
suppose it's heated from the bottom, causing a growing region to lose
superconductivity. That pushes the loop of current into a smaller region at
the top. This technique is called
flux pumping.
YBCO was discovered in 1987. Reliable production and discovering field
cooling took a few years. Flux pumping was
patented in 2007
by Tim Coombs and
published in 2008. The patent will expire in 2025.
applications
electric motors
An obvious use for
superconductors is replacing the copper wires in electric motors.
When the current in superconductors changes, they have a little bit of
resistance. At liquid helium temperatures, cooling that in electric motors
is impractical. So, motor designs have either used constant magnetic fields
or HTS coils.
One constant-field design is the superconducting
homopolar motor -
just a
big disk of spinning metal in a magnetic field, with current flowing in
the center and out the rim. Liquid metal was used for the moving electrical
contacts. These work, but their performance wasn't worth dealing with
superconductors and liquid metal.
Large motors using HTS wires
have been made, but the HTS wire is expensive. My understanding is that
cheaper HTS wire production would be needed to make those economically
competitive.
How about replacing permanent
magnets? Current high-performance electric motors use Nd2Fe14B magnets;
superconducting quasi-permanent magnets can be much stronger, and more
stronger is more better.
I'm not aware of any motors made using
pre-magnetized superconductors, due to the issues with transport and
assembly. Also, the rapidly changing fields in motors would probably cause
them to lose their magnetism over time.
Motors using bulk
superconductors magnetized by pulsed fields during cooling have been made.
Here's an
early example from 2005. The field strengths from this have been
unimpressive compared to modern magnets. This is where flux pumping comes
in. That avoids issues with transport, assembly, and demagnetization, and it
can give much stronger fields than neodymium magnets. Could it make
superconductors actually competitive for some large electric motors? My
answer is a strong "maybe".
MRI
An obvious possible way
to use quasi-permanent magnets is to replace superconducting coils used to
make constant fields. The most notable current application for such coils is
MRI machines.
MRI machines need homogeneous fields. Like permanent
magnets, the fields of quasipermanent magnets depend on grain sizes and
orientations, so they're not perfectly smooth. However, people
have gotten <100 ppm field variation with MgB2, low enough for some NMR
and MRI applications. Again, this depends on the grain structure, so there's
room for both improvement and tricky process problems.
With flux
pumping, MRI machines could potentially avoid the need for:
- liquid
helium cooling
- shielding the main magnet from gradient coil fields
-
the expensive magnet "ramping" process
See also my post on using cryogenic aluminum for MRI gradient coils.
