Not so cold times for superconductors

Superconductors have at last emerged from being the stuff of science fiction into viable applications.

Superconductors – materials whose electrical resistance is zero – have always fascinated scientists, not to mention science fiction writers.

On the face of it, allowing current to flow through these conductors uninterrupted for ever leads to a whole new realm of possibilities. That magnets will float above them has inspired fanciful discussion of magnetic levitation and frictionless trains, putting an end once and for all to that old ‘leaves on the line’ problem.

But there has always been one major drawback. Because these materials had to be cooled to almost -273ºC, close to absolute zero (meaning expensive liquid helium needs to be used as a refrigerant), the stuff of science fiction has stayed just that. Fiction. Instead, real-life superconductors have remained stuck in specialist applications.

Then, in the mid-1980s, along came ‘high-temperature superconductor’ materials (HTSs), which ignited massive media interest in a hitherto esoteric phenomenon. The term ‘high temperature’ is relative. Some of these new materials will manage to switch from conductor to superconductor properties at -181ºC, or 92ºC above absolute zero.

This is warmer than the -196ºC at which liquid nitrogen boils, which enables the use of liquid nitrogen – according to the sales pitch, cheaper than bottled water – to cool materials sufficiently to take on HTS properties. Suddenly the whole thing looks more feasible.

Expensive search

HTS materials are ceramics, a combination of copper oxide with rare earth elements.

This combination has set researchers off on a long and expensive search for other suitable materials. But there has been a shift in emphasis lately, with universities now trying to perfect the technologies needed to manufacture superconductors on an industrial scale – and to find commercial applications for these new materials.

These studies are starting to pay off. A number of university research groups have developed prototypes of applications such as bearings, electrical hardware, medical diagnostic systems and antennas for mobile communications and digital TV.

The buzzword is interdisciplinary. It takes physicists, chemists and engineers to crack the problem of applications for HTS, and at Cambridge University the job has been handled by its Interdisciplinary Research Centre.

Part of the work has involved creating the HTS materials required: yttrium barium copper oxide (YBCO) and bismuth strontium calcium copper oxide (Bi-2212). These have been made using a patented process that has been taken up for commercial use by a materials company, Advanced Ceramics.

Researchers are also looking at two possible applications: fault current limiters for power transmission networks, and magnetic bearings for flywheel energy storage. An FCL is a high-power device that sits in the electricity transmission and distribution grid and intervenes in the case of power surges.

It is perfect for a superconductor, because under excess power it heats up, loses its conductivity and become resistive. Simply cooling it again can restore the current – so it acts like a fuse that can repair itself. The global market for such devices could be worth billions of dollars.

Energy storage – using flywheels – is another area where superconductors may have a large and expanding market. The key here is low-resistance bearings.

The Cambridge team is working on a bearing that has two permanent magnets (working in attraction) to provide most of the levitation of the 40kg flywheel. Superconductors are then used as part of a ring magnet that stabilises the bearing, allowing it to spin smoothly.

As with the other Cambridge projects, this research has required work on materials processing, and has led to a commercial partnership – in this case with Johnson Matthey – to develop a platinum-doped powder as a precursor for making YBCO. This has led to the world’s first facility for medium-scale production of high-quality single-grain materials.

HTS films

The market for films of HTS materials rather than larger components is one of the most developed commercial applications of superconductors. Here an important use is in base-station antennas for mobile telephones.

The role of HTS in antennas arises from their novel electrical characteristics. Their surface resistance is a factor of 1,000 less than that in copper. But that means starting from scratch in antenna design.

In the US, a growing number of base stations have HTS microwave filters mounted in among the antennas. ‘They allow better reception of signals and reception from longer distances,’ says Professor Mike Lancaster, Head of the Emerging Device Technology Group (EDTG) at the University of Birmingham, which has invented new design techniques for this type of filter. Lancaster has also set up a company, CryoSystems, to commercialise the microwave filters.

At Imperial College, London, a group led by Dr Kwang-Leon Choy, working with Professor Stuart Abell from Birmingham University, has devised an electrostatic spray vapour deposition technique for depositing films of superconducting material. Imperial has set up a company, Innovative Materials Processing Technologies, to commercialise the research, and is also looking for partner companies.

Medical imaging

Professor Neil Alford is another academic who may be travelling down a similar path. He runs a research group at South Bank University, London, and is trying to bring the results of his research, using superconductors in medical imaging, to the market. In particular, he wants to produce an inexpensive, but highly sensitive detector for magnetic resonance imaging (MRI).

The standard means of improving image quality has been to increase the strength of the magnetic fields. But that means the magnets are bigger and more expensive to buy and operate. A side effect is that they are also noisier and more intimidating in operation.

Alford decided that superconducting detectors could improve the signal-to-noise ratio (SNR) at a much lower cost. Adapting the detector works because image quality depends on the temperature and resistance of the scanner’s receive coil and the body being scanned. HTS detectors have to be cooled and they have, by definition, less resistance and can deliver a better SNR. Double the SNR and you reduce the imaging time by a factor of four.

In fact, the South Bank team has achieved a threefold increase in SNR over copper coils cooled to the same liquid nitrogen temperature. Alford is working towards commercialisation of his research, but once again it may well be a case of starting up a company to make this happen.

This is typical of the whole HTS scene in the UK, as it is an area of technology where the commercial sector has effectively thrown in the towel.

Unlike the US, where a number of firms provide materials and commercial HTS systems such as antennas for mobile telecoms, few companies here are in a position to exploit this academic research. Many UK universities have little choice than to link up with an overseas company – or start one up themselves.