Celebrating The Superconductor, As It Turns 100

In 1911, Heike Kamerlingh Onnes discovered that some materials exhibit zero resistance to the flow of electricity at extremely low temperatures—they are superconductors. Physicist David Cardwell discusses their use for applications as diverse as particle physics experiments and magnetic resonance imaging.

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IRA FLATOW, host:

You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

For the rest of the hour, we're celebrating superconductivity, that weird state when a material exhibits no resistance to the flow of electrical current. And though we've only been talking about it, publicly, for a relatively short time - I mean, a few decades -superconductivity was discovered 100 years ago by Dutch experimental physicist Kamerlingh Onnes. And superconductivity isn't something that happens in room temperature world. It requires extremely cold environments. And Onnes happened to run a cryogenics lab, where he became the first man to liquefy helium, something that only happens, oh, a few degrees above absolute zero.

Well, so you're the first guy to liquefy helium. What are you going to do with it, right? You have to have something to do. So, like any curious experimenter, he started throwing stuff into the frigid helium to see what would happen. And joining me to talk more about what he found is my guest, David Cardwell, chair of the Superconductivity Group at the Institute of Physics in the U.K. He's also professor of superconducting engineering at the University of Cambridge. Welcome to SCIENCE FRIDAY, Dr. Cardwell.

Dr. DAVID CARDWELL (Chair, Superconductivity Group, Institute of Physics): Oh, hi, Ira. It's good to be here.

FLATOW: What - so what did he - now he has all this super cold helium -take us back to 1911. What is he doing now with all of that?

Dr. CARDWELL: Well, it was a pretty exciting time for physics and certainly superconductivity. So Kamerlingh Onnes had just achieved these very low temperatures, minus 269 degrees centigrade, and he and a student were doing experiments, measuring resistance of metal. And he found that the resistance of mercury disappeared completely at these very cold temperatures, which was, you know, extraordinary.

Completely unpredicted. No scientist thought it would happen. It took everybody by surprise. And then we spent the next 50 years trying to explain it. So it's so remarkable. I mean, this subject has produced about 13 Nobel Prize winners. So it's - that tells you about the challenge and how it actually captured the imagination of the science world.

FLATOW: Did we ever figure out what is going on inside there?

Dr. CARDWELL: Yeah, kind of. There's a really successful theory called the BCS theory, Bardeen, Cooper and Schrieffer, which was developed in the '60s, and this was very successful, and it predicted the properties of most of the known superconductors. But it had one very important prediction and that was that superconductivity would be limited to low temperatures. And so people stopped looking.

And there's a very important message to all the young scientists listening, and that is that you never actually prove a theory. You only disprove it. And a new class of materials were discovered in 1986 with very high transition temperatures relative to liquid helium. And that's opened up a whole new field of interest in this material.

FLATOW: And where would we find superconducting materials today? It's ubiquitous, right? They're all over the place.

Dr. CARDWELL: They're all over the place. It's just that they're not superconducting because the temperature, usually room temperature, that's far too high for superconductivity. So you only see this property when you cool materials down to an appropriate temperature.

So the new materials got a fairly high transition temperature, and we can actually access those using liquid nitrogen. Whereas Kamerlingh Onnes' materials, you needed liquid helium and that's much more expensive, much more volatile, much more difficult to contain. So the liquid nitrogen - and there's obviously lots of nitrogen around. Seventy percent of the air is nitrogen. Then it's actually easier to see superconductivity. The...

FLATOW: Did we figure out what's going on in the atomic level to make it superconducting?

Dr. CARDWELL: Right, yeah. You certainly know how to ask the questions.

(Soundbite of laughter)

FLATOW: Just - you know, it's an obvious question, like what's going on in there, you know?

(Soundbite of laughter)

Dr. CARDWELL: Yeah, it's a very good question. In a normal metal, you got things called electrons. Electrons come from atoms. And these electrons, as they go through the material, they bump into atoms in the matrix of the material. And every collision results in the dissipation of heat or energy, so the material gets lost. The electrons lose their energy and the current stops slowing. So quite simply, that's how metallic conduction and the generation of heat works.

In a superconductor, what happens is that these charged carriers, these electrons, actually pair up. So they kind of hang around in pairs, not singular. And because they do that, they're able to move through the metal without bumping into these metal atoms. Therefore, there's no loss, nothing to stop them and they just keep moving forever. So if you get a supercurrent going in a loop of superconducting wire, it will flow virtually forever without loss, and that's the difference between superconductors and normal metals. So electrons hunt in pairs in a superconductor.

FLATOW: So we've made all kinds of superconducting magnets now, right, to power things from supercolliders to MRI machines?

Dr. CARDWELL: Absolutely. I mean, the supercollider magnets are hugely impressive. I mean - but they're one-off. I mean, you know, extremely high cost, extremely high field, the kind of thing that the general public never get to see.

MRI magnets, on the other hand, are completely the opposite. I find it extraordinary that people can go for an MRI scan. They can lie in the middle of a superconducting magnet and literally inches away from their body that are cryogenic temperatures, but the insulation is so good that they've got no idea, they have no concept that they are working so closely to this very cold cryogenic environment.

FLATOW: So right next to your ear, your leg or whatever is being scanned at MRI...

DR. CARDWELL: (Unintelligible), yeah.

FLATOW: ...at 300 degrees below centigrade.

DR. CARDWELL: Well, yeah. Well, it's 200 - more than 260 degrees below C (unintelligible) yeah.

FLATOW: Wow.

DR. CARDWELL: You know, you look at an MRI magnet, you see basically a tube, and the tube's got a fairly thick wall. Within that thick wall you've got your superconducting wire carrying this very large current, generating a very big field, and that's what superconductors are really good at. And then you've got all the cryogenics and the insulation. And, as I said, the patient has got no concept of this. It's a wonderful example of very effective, high-tech engineering.

FLATOW: Well, here's a question maybe you can answer for me because it sort of seems counterintuitive. If you're creating a magnetic field with superconducting materials, you put the electricity in there and it goes around and - forever, let's say - and it's created this magnetic field, if I draw energy off of that magnetic field...

Dr. CARDWELL: Yeah.

FLATOW: ...what's going to happen to the magnetic field? I mean, am I getting something for nothing because they've taken away the source of the electricity?

DR. CARDWELL: Yeah. Well, that's a wonderful idea. No, obviously if you take away magnetic energy, then you have to pay for that. But the way a superconductor works is that - there's this thing called Faraday's law. So if you take a superconductor that's absolutely do nothing, just sitting on the bench, basically, and you apply a magnetic field to it, that's a changing field, so that induces a voltage within the superconductor. The voltage will cause charge to flow and that charge will flow indefinitely. Now, that flowing charge, which is a current, will give rise to a magnetic field. So that's a static, stable situation.

As soon as you start to take energy out of a magnetic field, then the penalty you pay is that you reduce the supercurrent. So you're not getting anything for nothing, but you are storing energy in a very effective way.

I once heard this described as you actually - when it comes to looking at generating electricity and essentially electrical systems are inefficient, there are losses, traditionally you can never win. With superconductivity we're getting close to being able to break even.

FLATOW: So the idea that you can regenerate the magnetic field with very little energy putting - being put in there, because it's very efficient.

Dr. CARDWELL: Well, it's more than very efficient. If the magnetic field is there, it's stable, it will persist as long as the supercurrent flows. And until you do something to remove that energy, it will stay there.

FLATOW: Yeah. So if you're using it in an MRI machine, you're taking out some of that energy, right?

Dr. CARDWELL: No, you're not.

FLATOW: You're not?

Dr. CARDWELL: No, you're not, no. You're generating a large magnetic field. And depending on the industry standard, it can be one tesla or it can be three tesla. There are some MRI devices based on five tesla but mainly three tesla and below.

And that, incidentally, is twice the field you can generate using a permanent magnet. You're just generating a constant stable field. And the fact that the patient sits in that stable field means that anything that's got what we call the electron spins will respond to the local field. And it's how those electron spins respond to this very high field and how they behave when we apply an additional field tells us about the structure of the material that the body is made of.

FLATOW: But if so...

Dr. CARDWELL: The background fairly is constant. We don't take energy out. We put additional energy in to get these individual spins in the body excited and spinning around the big field we apply.

FLATOW: So...

Dr. CALDWELL: It's a very clever engineering.

FLATOW: So if you - but if you were to draw some energy off of that superconducting field...

Dr. CALDWELL: Yeah.

FLATOW: ...you would collapse it or would you have to regenerate it?

Dr. CARDWELL: You could take energy out without collapsing the field. But if you're looking at using superconductors as a way of storing energy, the best way to take all the energy out is actually to collapse the field. And the way you can do that is if you have a superconducting loop that continues, you just break the loop.

So it's like you squirt the supercurrent out back into your battery or something as the field collapses, as a changing field on the superconductor, and that produces even more current.

FLATOW: Mm-hmm. When...

Dr. CARDWELL: So all the energy is stored in the current or in the field.

FLATOW: Interesting. 1-800-989-8255. Mark in Oklahoma City, Oklahoma. Hi, Mark.

MARK (Caller): Hi. I just wondered how far we are from having superconducting backbones to our power grids so that we could send, you know, wind power from the Midwest all the way to the East Coast so we'd always have some sort of sustainable energy that was supporting the grid.

FLATOW: Good question, Mark. Let's see if we can get an answer. An interesting question and interesting usage for it, is it not?

Dr. CARDWELL: Yeah, it was. It was a very good question, Mark. And, of course, there's an infinite gain if we can do that. For the moment, a lot of our energy is lost on the transmission. And estimates vary, but typically around about 10 percent of the energy is lost in getting it from A to B.

When you're looking at a continent the size of the U.S., there are enormous gains to be made if we could have a superconducting network of cables to transport the energy around. And I think with sustainable engineering becoming increasingly important, reducing carbon footprints, the diminishing of fossil fuels, that's a very important consideration.

There are one or two groups in the U.S. who are actually working on superconducting cables. So, you know, this may be a reality sooner than you think. At the moment it's on fairly short length, I understand, kilometers. We still have a way to go before we get to hundreds or thousands of kilometers. But you know, I'm pretty sure it's going to come.

FLATOW: Yeah. And we have already seen incredible things on that level, like the Maglev trains in Japan that run on superconductors, right?

Dr. CARDWELL: Absolutely. Yeah. I mean, this - the Maglev train in Japan is absolutely incredible. It's almost impossible to describe. But it's low-altitude flying - it's about two inches off the ground. And this thing travels at hundreds of kilometers, in excess of 500 kilometers per hour. And it's really just levitated.

The superconductor is used to remove all mechanical contact between the train and the track, so there's no friction whatsoever. The only limiting - limit to its velocity is actually air friction and air resistance. So this thing flies along at many hundreds of kilometers an hour. And it's absolutely extraordinary. The downside is that it's very expensive, which is why we don't see widespread application of Maglev. Basically, the cost is in the track. It's very expensive to make track that will levitate to superconducting magnet.

But, you know, it's a spectacular example of what can be done.

FLATOW: This is SCIENCE FRIDAY from NPR. I'm Ira Flatow, talking with David Cardwell, the chair of the superconducting group at the Institute of Physics in the U.K.

How - give us some idea how high you can raise the temperature of a superconductor before it stops doing its thing. I mean, how high have we gotten and is there a theoretical limit?

Dr. CARDWELL: Yeah. Again, another good question. There was a theoretical limit, and this came from the BCS theory. So absolute zero is minus 273 degrees centigrade. You can't get below that. So BCS theory said that superconductivity was limited to about 28 Kelvin, so that's 28 degrees C above this absolute zero.

So when a superconductor was discovered with a Tc close to 28 Kelvin, everybody stopped looking. And again, back to what I said earlier, you can only disprove a theory. A couple of guys, Bednorz and Muller, working in our lab in Zurich, beavered away and reported a material with a Tc of 36 Kelvin, so beyond the BCS limit. And that kind of opened the floodgates. And the record now - remember, BCS had a maximum of 28 degrees centigrade above absolute zero - the world record now is 166 degrees...

FLATOW: Wow.

Dr. CARDWELL: ...above absolute zero. Amazing, I know.

FLATOW: Scorching. Scorchingly hot.

Dr. CARDWELL: Yeah.

FLATOW: And so is there...

Dr. CARDWELL: Well, maybe it's approaching room temperature if you live in Finland or somewhere like that...

(Soundbite of laughter)

Dr. CARDWELL: It's still pretty cold for...

FLATOW: But people are looking for a higher level now, yeah?

Dr. CARDWELL: They are, yeah. I mean, essentially physicists want to look at materials with higher and higher Tcs because that tells you about the mechanism of superconductivity. And I have to say that the mechanism of superconductivity in the new materials - we call them high-temperature superconductors, is still not known. There's you know, some people say there are as many theories as there are theoreticians.

So it's a very complicated phenomenon, and we just don't understand this at the moment. But the hope is that if we can get superconductivity at higher temperatures, we'll understand the mechanism.

FLATOW: So is...

Dr. CARDWELL: The engine yeah, sorry?

FLATOW: So is the idea that you just put different alloys or different metals and throw it back into the nitrogen and see what happens or -like (unintelligible) did in his first experiment?

Dr. CARDWELL: Certainly we've tried that and it doesn't going to get you very far. I mean, materials that superconduct at high temperatures are complex metal oxides. So typically they'll have three or four metallic elements there, and there'll be oxygen, there might be some other non-metal as well.

So it's not a matter of kind of mixing up lots of different materials and seeing. What we need to do is to understand what's going on, look at things like how many electrons are in the material, what's the stretch of the material, how do the electrons interact with those atoms, what type of atoms and kind of constitutions do make good superconductors, and then have a good theoretical basis, and we just don't have that.

So, I mean, the other views that engineers would take, which says that we don't really care about Tc, we don't care if it's room temperature, all we're interested in is how much current it can carry at a temperature that we're going to operate. The more current the better because the more current in the field, the bigger the field.

And currently we operate at about 20 degrees above the boiling point of liquid nitrogen, which is a convenient temperature to operate at. And we get very good properties at liquid nitrogen temperature. So this high-temperature superconductor, this 166 Kelvin superconductor, doesn't have properties that's as good as another material with a Tc of 92 Kelvin.

FLATOW: How...

Dr. CARDWELL: Not Tc, it's Jc, an ability to generate field.

FLATOW: How much total money worldwide is going into this, do you think?

Dr. CARDWELL: Wow...

FLATOW: I mean, is it going to be $100 million, 10 million? Give an order of magnitude...

Dr. CARDWELL: Right. So the U.K., for example, if you look at company investment, research investment, is of the order of millions of pounds a year. How many millions, I can't tell you, but it's less than 10.

FLATOW: So it's very little, actually, compared to other science...

Dr. CARDWELL: It is. In the U.K. it's very little. But to be fair, high-temperature superconductivity was discovered 25 years ago. There's been a lot of investment in the field, particularly in the U.S. as well. I mean, you have some pretty important laboratories there - things of Los Alamos, Oak Ridge, MIT. You know, there's been lots of work done on superconductors.

And you also have good companies - Boeing, for example. I'm working at the moment with Boeing. They're developing a flywheel energy storage system. And I mean, that's got enormous potential to revolutionize the energy storage industry. And again, it relies on superconductors for magnetic levitation to keep the loss of...

FLATOW: We're going to get back - we've run out of time, but I want to pick up on this whole flywheel technology. It is fascinating, very old and very useful, but we'll have to drop it for another day. Thank you very much, Dr. Cardwell, for taking time to be with us.

Dr. CARDWELL: OK. Thanks. Good to speak to you.

FLATOW: Thank you. David Cardwell, chair of the superconductivity group at Institute of Physics in the U.K. and a professor of superconducting engineering at the University of Cambridge.

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