Record-breaker: David Cardwell, University of Cambridge
It barely registered outside the rarefied world of top-level research, but back in the summer academics from the engineering department at the University of Cambridge managed to break a world record that had stood for more than a decade, informing wider work that could herald the arrival of exciting new materials in a broad range of fields.
Specifically, the Cambridge team managed to ‘trap’ a magnetic field with a strength of 17.6 tesla in a high-temperature gadolinium barium copper oxide (GdBCO) superconductor, beating the previous record by 0.4 tesla. That result was the equivalent of harnessing three tonnes of force inside a golfball-sized sample of a material that is normally as brittle as fine china.
The technical nature of the research meant that it failed to garner many headlines. But the academics had effectively demonstrated the extraordinary properties of bulk high-temperature superconductors, and in doing so opened up an enormous range of potential applications across several fields.
Indeed, bulk superconductors are now being touted as potential game-changers for flywheels for energy storage; ‘magnetic separators’ that can be used in mineral refinement and pollution control; and high-speed, levitating, monorail trains.
Superconductors are materials that carry electrical current with little or no resistance when cooled below a certain temperature. While conventional superconductors need to be cooled close to absolute zero (zero degrees on the Kelvin scale or -273°C) before they superconduct, high-temperature superconductors do so above the boiling point of liquid nitrogen (-196°C), which makes them relatively easy to cool and much cheaper to operate.
At present, superconductors are used in scientific and medical applications such as MRI scanners. In the future, they could be used to protect the national grid and increase energy efficiency, because of the amount of electrical current they can carry without losing energy.
The current carried by a superconductor also generates a magnetic field, and the more field strength that can be contained within the superconductor, the more current it can carry. State-of-the-art, practical superconductors can carry currents that are typically 100 times greater than that carried by copper when operated at similar temperatures, which gives them considerable performance advantages over conventional conductors and permanent magnets.
The new record was achieved using two 25mm diameter samples of GdBCO high-temperature superconductor, fabricated in the form of a large, single grain by an established melt processing method and reinforced with a relatively
simple technique.
The previous record of 17.24T, set in 2003 by a team led by Professor Masato Murakami – now at Japan’s Shibaura Institute of Technology – used a specialised superconductor of a similar, but subtly different, composition and structure.
The long-standing nature of the previous record is revealing, says David Cardwell, professor of superconducting engineering at Cambridge, who led the research, in collaboration with Boeing and the National High Field Magnet Laboratory at the Florida State University. “The fact that this record has stood for so long shows just how demanding this field is. There are real potential gains to be had with even small increases in field,” he says.
To contain such a large field, the team used cuprates: thin sheets of copper and oxygen separated by more complex types of atoms. The cuprates were the earliest high-temperature superconductors to be discovered, and have the potential to be used widely in scientific and medical applications.
While they are high-quality superconductors with outstanding potential for practical applications, cuprates can be as brittle as dried pasta when fabricated in the form of sintered ceramics. So trying to contain a strong magnetic field within bulk forms of the cuprates tends to cause them to explode.
To hold in, or trap, the magnetic field, the researchers had to both modify the microstructure of GdBCO to increase its current-carrying and thermal performance, and reinforce it with a stainless steel ring, which was used to ‘shrink-wrap’ the single grain samples. The lines of magnetic flux in a superconductor repel each other strongly, so containing such a large field is difficult. But by engineering the bulk microstructure, the field is retained in the sample by so-called ‘flux pinning centres’ distributed throughout the material. The result is the biggest ever trapped field achieved in a bulk, stand-alone material at any temperature.
The research is significant in terms of applications for the materials, says Cardwell. “This work could herald the arrival of superconductors in real-world applications. To see bulk superconductors applied for everyday use, we need large grains of superconducting material, with the required properties, that can be manufactured by relatively standard processes.”
Circular motion: Rotational movement without contact
The Cambridge team and its collaborators are developing several niche applications, and it is anticipated that widespread commercial applications for superconductors could be seen within the next five years. Cardwell says that the combined abilities of high-strength fields and stable levitation makes superconductors unique, and that industry is becoming increasingly interested in what those capabilities might lead to.
“For a long time now, we have been developing superconducting materials, but from an intrinsic property point of view. It’s only in the past five years that we’ve been looking at medium-scale production techniques using straightforward methods, that give us high reliability and repeatability and large outputs for commercial applications.
“Now we are at a stage where we have developed materials that are very good, and the forces that can be produced are enormous. We’ve cracked a lot of the problems, and we can start thinking about real applications.”
One such application could be a flywheel for load-levelling in the power industry, he says. “You could make a flywheel application where you have a plate of superconductors and you levitate your composite flywheel with magnetic impregnation, spinning it at high speed, getting power out. You could have a number of these flywheels to manage peak and trough power demand through load levelling. It would make energy planning and management much better.”
Another application could be a mineral separator. “You get a very sharp field gradient with these bulk superconductors. That means if you have anything that’s magnetic there will be a big force on it. So you could use these materials for magnetic separators in areas such as mineral processing. You have slurry that contains liquid and magnetic species, you pass it through the gradient, there’s a force on anything that’s magnetic, and it displaces it from the liquid stream,” he says.
Cardwell’s enthusiasm for bulk material superconductors is starting to be shared by some large industrial groups. For instance, Festo, the German control and automation specialist, recently challenged its engineers to imagine how such materials might one day lead to marketable products with real use.
Festo’s fascination with superconductors starts with their unique levitation properties: if you cool them down to their transition temperature, they hover above a permanent magnet as if on an invisible cushion. The principle works in the same way the other way round; the gap between remains stable. This property enables the contact-less positioning and movement of an object – without control technology and without friction losses. That is an exciting prospect for an automation company.
Transport tool: Festo showed how a levitating carriage could be used to move objects in production
Earlier this year, at the Hannover Fair in Germany, Festo demonstrated three technologies that used superconductors to enable contactless transport and the handling of hovering objects. The superconductors exhibited were cooled down using installed electric compressors to a relatively high temperature at a constant
93K (-180°C).
The first exhibit showed linear movement in three spatial planes. The SupraHandling 2.0 system comprised a levitating carriage moving over two magnetic rails,
2.5m in length. This carriage was fitted with three cryostats with superconductors. The entire system could be rotated by up to 180º about its longitudinal axis, enabling the carriage to travel vertically or even upside-down. SupraHandling 2.0 demonstrated stable non-contact positioning, which, for example, could enable the levitated transport of objects in production.
The second technology, SupraShuttle, was designed for the movement of a levitating object in all directions and, for the first time, the handling of the superconductor element itself: the cryostat with the superconducting material was transferred from one electric axis system to another. The exhibit also showed how easily hovering objects could be introduced into a hermetically sealed space and moved within it. This feature would be useful, for example, for applications involving work with gases or in a vacuum.
Finally, Festo also displayed controlled rotation with its SupraCharge technology. In this case, an application for the first time was shown to transmit a rotational movement without contact and in a controlled manner to a magnet, which levitated because of superconductivity. The exhibit showed this effect in three automatically changing, distinct applications: a centrifuge, a blender and a hovering rotary indexing table. A possible future field of application is laboratory automation, in which several processing steps are often performed on an object in sequence.
All superconductors for the Hannover exhibits were cooled by long-life electrical compressors. The cryostats maintained the superconducting material at a constant temperature of around 93K (-180°C), with an energy consumption rate of about 12 watts per cryostat. Festo says each of the technologies could therefore be operated with energy efficiency, and independently of cooling media such as liquid nitrogen.
According to Steve Sands, product management manager at Festo in the UK, the relatively high-temperature nature of the latest bulk materials makes superconductor technology far more feasible than it was previously. “They have real potential now,” he says. “The fact that we are now talking about relatively high-temperature superconductors of -180°C means it’s possible to draw down the temperatures using cryostats. These require very low amounts of energy. Indeed, the ones we used at Hannover were using just 12W each to maintain the temperature. In the past, all the work we were doing in this area was using liquid nitrogen, and that presented handling and safety problems as well as replenishment issues.”
Sands thinks that the future holds several conceivable applications for superconductors in automation technology, in the energy-efficient, stable positioning of objects, without the requirement for measuring or control technology. Superconductors would also enable objects to be moved beyond walls in confined spaces and in all positions. In view of this non-contact handling, superconductors also offer great application potential wherever equipment is required to be cleaned conveniently or during operation, such as in laboratory automation, medical technology or the food industry, he says.
“It opens up interesting areas of automation,” he says. “The ability to hover a slider or a carriage above a bed, and make any distance without any closed-loop control, has distinct advantages. Once you’ve set the distance between the magnet and the superconductor, it is maintained. So you’ve eliminated a whole load of control that would have been required if you were trying to do things electrically.”
Visitors to Festo’s stand at Hannover were initially confused by its interest in superconductors and sceptical of the properties they offered but once the demonstrations were worked through, most were converted, says Sands. “Several companies left Hannover interested in applying these technologies in some exotic ways,” he says.
Looking forward, Festo says that it will carry on assessing the commercial application of bulk superconductors. “We haven’t got a commercial product yet, but we will continue to look at areas where superconductors could be of
interest and use.”
With academics uncovering more about the unusual properties of these materials, and manufacturers showing interest in developing applications, it looks as though superconductors could indeed become game-changers in a variety of fields.