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Industrial Processes

What a superconducting line-up!

   
 
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 80K, 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 orientation.
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 in detail.

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.

 

 

Project Title:  
Composite reaction texturing: a novel fabrication process for high-temperature high current superconductor wires and shaped components

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5399

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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