Prof. Sir Colin Humphreys is the Director of Research at the University of Cambridge's Department of Materials Science and Metallurgy, and the founder of both the Cambridge Centre for Gallium Nitride and the Rolls-Royce University Technology Centre. In this interview, he talks about the real-world impacts of his wide ranging research, and the importance of electron microscopy in materials science.
WS: Why is materials science such an important area to study?
CH: Materials science is a really important subject, because if you look at objects we use in many areas of life, often the limiting factor is the materials used to make them.
For example, if a blade breaks on your lawnmower, that’s a material that’s failed. If you have a solar panel on your roof, its efficiency is limited by the materials we use. If you look at a jet engine, in a plane flying across the Atlantic – if you can make the materials work at a higher temperature, the engine can be more efficient, using less fuel and reducing carbon emissions. In general, if you can improve the materials in an object, that manufacturer then can sell more of that object.
So materials science just about controls everything we do. It's a really important topic.
WS: Your work is based on using electron microscopy to study advanced materials. Why is electron microscopy such a valuable tool in this area?
CH: Microscopy is essential for studying materials. The materials themselves are often controlled at an atomic level, or a near atomic level, and that's why we need electron microscopes to image the materials. The powerful microscopes we have in our department, like the FEI Titan, can resolve individual atoms in materials and give wonderful results, but it's very difficult to use. You need two years’ training, minimum – and when you want to take an image of a material, if can take perhaps three or four hours to align the microscope first.
So alongside these big microscopes, we sometimes need simpler instruments to look at the materials quickly, at a lower magnification. It’s often good to start imaging at a lower resolution, and work up to the atomic resolution, so you get a complete picture of the sample.
Desktop instruments, like the Phenom, are incredibly useful, because you can train someone to use them in just 15 minutes, and they can be used to image a huge range of samples. Very little preparation is needed – you just put the specimen into the microscope, and 10 minutes later you have your images.
WS: Can you give us an example of a case where the desktop SEM was particularly useful?
CH: The Phenom not only produces images, it also has an x-ray attachment.
By looking at the characteristic x-rays emitted by the material when the electron beam hits the sample, you can tell what that material is.
One interesting example is artificial hips. We study medical materials quite a lot here – artificial hips often fail, and one of the ways that can happen is that metal from the hip shank itself can be deposited on the plastic socket it rotates in.
With the Phenom, we can put the whole hip socket into the microscope – this isn’t possible with the big electron microscopes, where you can only take a small slice of the material. We can see if there is metal deposited on that plastic surface, and we can say without doubt that the metal is titanium from the shank. That’s bad news, because that titanium could get into your bloodstream – so this information could potentially save patient’s lives.
WS: Tell us about your work at the Gallium Nitride Centre.
CH: I founded the Cambridge Centre for Gallium Nitride in the year 2000, when we were fortunate to be donated gallium nitride growth equipment by a local industry, worth £750,000. We’ve been working on gallium nitride ever since.
It’s a very simple compound – just gallium and nitrogen, very strongly bonded together – and it doesn’t exist in nature. It has the most remarkable properties. If you add some indium to it, and pass a tiny electric current through, it emits any colour light you want. 15% indium produces blue light, 25% gives you green light, and so on. If you add some aluminium, you can even get into the ultraviolet.
During the time we’ve been studying gallium nitride, we have had some major developments for LED technology.
Most people are familiar with LED, or light emitting diodes. They are all around us now, in many products, and even in children’s toys. They are increasingly being used to light our homes as well – but one of the main problems is that they are rather expensive. A 60W replacement LED bulb costs £15. Not many people will pay £15, even though it saves you money over its lifetime because the electricity consumption is so low.
We've had a breakthrough here. We've been growing these gallium nitride light-emitting diodes on silicon, instead of on sapphire. All the LEDs you buy in a shop are grown on sapphire, and you can that makes them quite expensive – even artificial sapphire like this is very expensive.
Microscopy is essential for looking at the internal structure of these devices. The light-emitting parts of these devises are called quantum wells because they're so thin - typically 5 or 10 atomic layers. These structures are so small you need to have electron microscopes just to see them.
By growing the material on silicon, the price is going to come down dramatically. That is what we've done – we’ve set up a couple of companies, and these gallium nitride LED chips on silicon are being manufactured right now, in the UK. Plessey made and sold 2 million of these last year - this year they plan to sell 50 million. It's a rapidly expanding business.
So that is our main breakthrough. We are also looking at the limits to efficiency of LEDs, and how we can push the efficiency higher and higher.
Fundamentally this research aims to get LEDs cheap and efficient enough to be in all the homes and offices throughout the world, which would save huge amounts of energy – up to 10% of all the electricity we use, which is remarkable. In the UK alone, that comes to £2 billion.
Throughout the world, if 40% of the lighting was LEDs, we could close over 500 full-size power stations. There’s a huge potential for energy saving, and of course huge reductions in carbon emissions from power stations as well.
WS: LEDs have many applications beyond lighting as well – can you tell us about some of those?
CH: I mentioned that you can make ultraviolet LEDs by adding aluminium to gallium nitride. Ultraviolet light is fascinating – the right UV wavelength kills all bacteria and viruses. It actually damages the nucleic acid, which is part of DNA and RNA.
We want to use this for water purification in the developing world - something like 3 million people a year die of water-related illnesses in the developing world, so a cheap route to sterilized water could save millions of lives. UV sterilization could actually be better than chlorination, so I think we might end up using it in the developed world as well!
There are health benefits to some visible light as well – it turns out that blue light of a certain wavelength kills the bacteria which cause acne. We're working with a local company on this, and they've just produced a device which you hold close to your face, which contains a set of LEDs which are half blue and half red. The blue light kills the bacteria and the acne basically disappears – the red light is there to help the healing process, as it can help skin heal faster.
A few years ago, people realized that gallium nitride was not only a great material to use for LEDs, but also a potential way to save energy in power electronic devices. Any device which changes the voltage or the current in an electricity supply is a power electronic device. They're widely used – in computers, phone chargers, and solar power systems for example – and they are all currently made from silicon.
If we could replace silicon power electronic devices with gallium nitride ones, which are 40% more efficient, we would save another 10% of all electricity. So LED lighting just with current technology could save us 10%; gallium nitride power electronic devices could save another 10%; with further improvements to the efficiency of LEDs we think there is another 5% that could be saved.
That’s 25% of all electricity use, 25% of all carbon emissions from power stations, just from this one material. That’s probably more savings than from wind power, solar, and tidal combined. Gallium nitride really is a wonderful material which is going to help our economy and reduce carbon emissions.
WS: Can you tell us about the Rolls-Royce Technology Centre? How did that get started?
CH: This department has a Rolls-Royce University Technology Centre, which I founded with Rolls-Royce in 1994. We received a small grant from Rolls-Royce at that point, and it's just grown and grown since then.
Rolls-Royce is one of the leading aerospace companies in the world. When I started working with Rolls-Royce, they were the third biggest manufacturer of aerospace engines in the world. General Electric was number one, Pratt & Whitney number two, and then Rolls-Royce. Now it's GE number one, Rolls-Royce number two, Pratt & Whitney number three. That's largely due to improved materials in the engines - materials that we've developed in this department are now flying in Rolls-Royce engines, and helping to make the engines much more efficient.
WS: What parts of the engines have you focused on in particular?
CH: We do a lot of our work on turbine blades and compressor blades. Electron microscopy, again, is essential for looking at the atomic structure of these materials. Sometimes, a blade will fail and it will be sent here.
The first thing we do is look at the surfaces – again SEM is great in this instance, as we can put the whole blade in a Phenom, and see if there are any surface cracks or other features that give us a clue to what has happened.
A large part of our research is really very basic, blue-sky type research. We are studying new materials that people have never looked at before. We’re trying new alloys, with different compositions.
We’re just trying to find materials which are really strong at high temperatures – strong enough to withstand something like a bird strike, whilst operating at the highest temperature possible.
Temperature is very important, because it directly affects the efficiency of the engine. It turns out there is a simple rule, due to the laws of thermodynamics – the higher the temperature the engine operates at, the more efficient the engine. Therefore, the efficiency of the engine is actually limited by the materials used, and the temperatures they can withstand.
So we’re hoping to develop new materials, really strong materials, which can operate at even higher temperatures – so that ultimately planes will be more efficient, need less fuel, and reduce carbon emissions.
We’ve had a lot of success in this area, and we have a strong relationship with Rolls Royce. I’m sure our materials will continue to be used in next-generation Rolls Royce engines.
About Sir Colin Humphreys
Sir Colin Humphreys, CBE, is a Professor and Director of Research in the Department of Materials Science and Metallurgy at the University of Cambridge.
His honours include being elected a Fellow of the Royal Academy of Engineering in 1996, and a Fellow of the Royal Society in 2011. He was awarded a CBE in 2003 for services to science as a researcher and communicator, and was knighted in 2010.
Professor Humphreys' research interests are focused in three main areas: gallium nitride materials and devices; advanced electron microscopy; and high temperature aerospace materials.
In 1994, in an agreement with Rolls-Royce, he founded the Rolls-Royce University Technology Centre at Cambridge, one of over 30 across the world which form a network for collaborative research between industry and academia.
In 2000, Prof. Humphreys founded the Cambridge Centre for Gallium Nitride, now a 20-strong research group working at the cutting edge of research into GaN LEDs, lasers, and other devices.
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