Superconducting materials will transform
key magnet, power engineering and medical technologies. The discovery
of high-temperature superconducting ceramics in the 1980s brought
their widespread use closer, but superconductivity is critically dependent
on correct alignment of crystalline grains, and industrial application
has been held back by the difficulty of producing large objects. This
project has developed a new process for texturing bulk ceramics by
seeding them with aligned single crystal fibres of an inert material.
CRT-fabricated products are already in use in research installations
where extremely powerful magnets are required, and are currently being
tested by the national grids of countries around the world.
The Composite Reaction Texturing (CRT) technology
had already been demonstrated in principle, and patented, by Dr
Jan Evetts of the University of Cambridge's Department of Materials
Science and Metallurgy. The three-year project was designed to develop
the process to the stage of full applicability, and has done so
with such success that CRT products are already finding significant
applications. Although only one of a number of competing new superconductor
fabrication technologies, it promises to gain an important position
in the field.
Superconductivity was originally demonstrated at temperatures close
to absolute zero. So-called high-temperature superconductors, on
the other hand, operate at over 80°K, around the temperature of
liquid nitrogen, and are therefore far easier and cheaper to use.
However, prospects for their application are still largely dependent
on the development of new materials processing techniques. Until
recently, there was no known way of producing large, strong, high-current
conductors which could meet the engineering specifications required
for industrial use.
Persuading the brittle to bend
Materials scientists faced two problems in particular. First, ceramics
tend to be brittle. Winding or cutting superconducting ceramic rods
to form magnet coils or other complex artefacts is all but impossible.
Second, their superconductivity is limited by the highly layered
ceramic microstructure. The transport of electrical current is impeded
by grain boundary impurities, and unless the crystals are aligned,
current can only be carried in the plane of the layers.
The CRT process overcomes both these limitations with a single imaginative
leap. The ceramic precursor powders are mixed with plasticisers,
binders and solvents to form a malleable paste, allowing products
to be easily moulded, rolled, bonded and cut before they are fired.
Coils and large cross-section conductors can be shaped with considerable
precision. As there is little expansion, shrinkage or deformation
during melt processing, minimal final machining is needed to achieve
required engineering tolerances.
The easily-handled precursor paste also holds the secret to grain
alignment. Mixed into the paste, the ceramic precursor powders is
around 10% of single crystal fibres of MgO, a material which will
not react with during melting and solidification. The seeds themselves
may be aligned either magnetically or mechanically, by simple extrusion.
The principle of extrusion alignment can be understood by imagining
toothpaste contaminated by tiny needles. As the paste is squeezed
from the tube, the needles all line up, whatever their original
In the CRT process, the molten superconductor crystallises on the
fibres as it cools, and because the fibres have been aligned, so
too are the ceramic grains.
Flexibility for applications engineers
Extrusion is just one of a number of alignment techniques developed
by Dr Evetts and his Greek and Danish partners. By varying the viscosity
of the paste, and the combination of mixing, alignment and forming
processes, a range of different textures can be achieved, offering
applications engineers the opportunity to specify precisely the
electrical and mechanical performance of a finished artefact.
The project was carried out in four stages. First, a number of potential
seed materials were tested, and methods for large-scale production
of the selected material were optimised. Second, a range of mixing,
alignment and forming techniques were developed, with particular
attention to their suitability for subsequent scale-up. Third, composite
reactions during melt processing were investigated in detail, in
order to optimise superconducting properties. Lastly, prototype
artefacts were created, process and quality control procedures were
developed, and the electrical and mechanical properties were characterised
Cutting the cost of NMR imaging
One year from the end of the project, its success is clear from
the speed with which the technology it developed is being taken
up. The first application has been in the field of high-power magnets.
Until very recently, such magnets had to run in a bath of liquid
cryogen, at temperatures close to absolute zero. The technology
was expensive and messy. Newer, cryogen-free high-power magnets
are now being installed, which use small refrigeration units to
cool the magnets to their operating temperature.
However, the cost of this technology is still high. Power consumption
and the use of coolant gases are proportional to the leakage of
heat into the magnet area. The critical component is the current
lead itself. Conventionally made of brass or stainless steel, the
lead not only conducts heat towards the magnet from the external
environment, but may itself generate heat as current flows through
it. High temperature superconducting leads, on the other hand, have
very low thermal conductivity, and because they deliver resistanceless
current, they generate no heat themselves.
Superconducting current leads can cut heat leakage to as little
as one thousandth of that of conventional materials. The project's
partners calculate, for example, that a hospital's Nuclear Magnetic
Resonance (NMR) body scanner, currently needing to be topped up
with coolant once every six months, could extend this to two years
by fitting a superconducting lead.
Protection for the electricity grid
The CRT technology has yet to break into the medical field. So
far, it is only being used in research installations. One of the
largest research projects in the world, the CERN hadron collider
programme, which needs thousands of current leads for very high-power
magnets, is assessing CRT products against other technologies.
In the medium term, the partners expect high-temperature superconducting
components to achieve widespread application in the power generation
and transmission industry, where they will form the basis for a
new generation of fault current limiters. As privatisation and deregulation
link more and more power generators to electricity grids around
the world, the potential for catastrophic failure grows. Fault currents,
whether the result of a lightning strike or of an equipment failure,
must not be allowed to knock out the whole grid.
The new limiters will use a superconducting element with no resistance
to current at normal levels. If the current rises past the fault
threshold, however, this element will lose its superconductivity,
presenting an impedance to the circuit, and limiting the current.
In the UK, research into other possible commercial applications
is continuing, with government support. Rolls-Royce, BOC, Advanced
Ceramics Ltd, Merck, and Brown Boveri are among the companies currently
investing in further development work to build on the technological
platform established by the project.