The Science of Superconductivity

Chris - We've talked up superconductivity quite a lot and people have been hearing us doing some experiments there, but maybe we should back pedal a bit. What actually is a superconductor?

Tim - Some people call them the super heroes of materials science. They're very surprising materials because you cool them down through a special temperature called the transition temperature, and at that point they lose all of their electrical resistance. Electrical engineers can employ this rather surprising property in a variety of different situations.

Chris - Why do they lose all this resistance? And what is resistance for a start?

Tim - Well we have to go back a bit and think about electric current going through the copper wires in your house. The electrons are more or less free to do that as they pushed by the voltage through the wires. But they do encounter some resistance and that resistance is actually that they collide with the atoms in the material, they lose a bit of energy when they do that, and that energy comes out as heat. That's why a light bulb works; it's really the resistance of the filament that makes the bulb hot and it glows white. Now in a superconductor at room temperature, the electrons are moving around and doing their own thing without taking much notice of each other. You now cool it down, and you really have to cool it to about minus 180 degrees centigrade, and as you get close to that, the electrons start to take more notice of each other. You can think of them as starting to flirt with each other a little bit. You go through that transition temperature and they start to hold hands. Once electrons are holding hands like that, they can't be knocked off course, so they move through without any problems.

Chris - And I suppose the goal of this is that 180 degrees below zero is not a practical temperature to be working at. If we could make this happen at something closer to room temperature, it would be ideal.

Tim - Well you'd be surprised to be honest. Minus 180 degrees centigrade is the temperature of liquid nitrogen, and nitrogen makes up 80% of the air that we breathe. As far as technology goes, it's not so hard to cool down to those sorts of temperatures. Twenty years ago, there was a class of superconductors discovered that we call high-temperature superconductors. High and low are all relative in this context. If you can cool things down and make them superconduct at liquid nitrogen temperatures, there's an easy technology there. Between 1911 and 1987, most superconductors that were known had to be cooled down to within a few degrees of absolute zero, which is minus 273 degrees centigrade. That was a barrier to most applications.

Chris - So how do you make something superconduct? How do you decide that this is the recipe needed to make a chemical combination that will have these properties?

Tim - That's a very good question. There was a theory in the 1950s about how these very low temperature superconductors worked and most people thought that that meant you couldn't make a superconductor that worked above about 25 degrees above absolute zero. But people following the guidelines of those theories were looking at other materials and in particular, oxide materials. These are materials that are a little bit like ceramics; there are three metallic elements and oxygen in a compound. What they found twenty years ago was that these would superconduct at much higher temperatures. It is now still a big challenge to theoretical physicists to work out exactly how that takes place.

Chris - Now what sorts of applications might there be if we are able to crack this nut and make these materials in the way that we want? What will we be able to do with them, and how are they being employed at the moment?

Tim - One of the applications that our research team are working on in Birmingham is making filters for radioastronomers at Jodrell Bank radio telescopes.

Chris - And when you say filters, what does that do and how does that work?

Tim - A filter is a kind of electrical equivalent of a bouncer at a nightclub. A bouncer at a nightclub with a selective door policy has been told that certain people who come up to the door are to be allowed through with no interference. If they're the wrong people wearing the wrong type of clothes, just send them back and don't let them through. Now a filter is an electrical circuit that does just that. What the circuit does is allow through particular electrical frequencies and electrical signals that you want. The problem for radio astronomers is that there's a lot of electromagnetic pollution. So for example, you're looking for a distant object in the universe and you want to study its properties. But all the satellite communications we have, all our mobile phones, and all our televisions are polluting the frequency space. That means that the astronomers have to spend a great deal of time on their telescopes averaging out the data. If you can find a way to help them use their telescope more efficiently, they can do more experiments. Our superconducting filters do just that because they cut out that noise.

Chris - Why do you need a superconductor to make that kind of filter though? Why won't standard electronics do it?

Tim - The reason you can't do it with standard electronics is that there are always some losses. As we talked about earlier, in a normal metal there is always some resistance. When it comes to making electronic filters, that sort of loss means that you need large devices and also they will never pass through the signals you want without some attenuation. If you want the signals to come through clean and undiminished, then you need a superconductor.

Chris - Now hasn't Birmingham got another claim to fame in terms of superconductivity in terms of the Maglev. Is that at Birmingham, where they actually had a train that drifted along on magnets so that it wasn't touching the ground.

Tim - That's right. There used to be a train, and it hasn't been running now for 10 or 15 years now, which went from the passenger terminal at Birmingham international airport to the railway station. That was a magnetic levitating train but it wasn't a superconducting train and there were some problems with it. People thought it was a bit wobbly and bobbly and that might have been because the technology was much earlier.

Chris - So is that the kind of thing we can look forward to if we are able to get a handle on getting superconductivity working as well as we'd like to?

Tim - Better transport is one of the large scale applications that's getting people excited at the moment. There's a test track in Japan at Yamanashi, which contains wires made of superconducting materials in the floor of the train. They're cooled down, and you drive a current through those coils and that repels against some other coils in the track and lifts the train up. The advantage of doing this is that there's no rolling resistance, and this train has now been tested up to speeds of 360 miles per hour. It's exciting because this gives you high speed transport without the need for aeroplanes, which have very high environmental costs at take off and landing.