Women in Science: What’s the crack?
Jassel has studied theoretical physics, music and materials science in Scotland, USAand now London. She is currently pursuing a PhD at Imperial College in hydrogen embrittlement in nuclear reactors – a hot topic in both politics and science. Her ’spare’time is focussed on projects in science communication, which include public science events, and training researchers in communication techniques. Jassel is also a jazz musician. Jassel took part in Soapbox Science 2013, on 5th July where she stood on a soapbox on London’s Southbank and spoke to the public about her work and to help promote the role of women in science. www.soapboxscience.org
The study of materials has long been the most underrated of the sciences. This is because up until the last century, the only materials scientists were blacksmiths. Masters of the manipulation of metals, they were considered to be otherworldly because of their skill, and likened to alchemists.
We humans have been poking and prodding materials for centuries, and materials have shaped every single age of society, from the stone age, to iron, bronze and silicon. Its status as a science was grounded at the beginning of the last century, when advances in transmission electron microscopy – an imaging method that involves firing electrons at things instead of light in order to see something smaller than the wavelength of visible light – enabled scientists to see tiny defects in crystalline structures (an ordered array of atoms, which most metals are made of), and connect these defects to mechanical strength and ductility.
I specialise in materials science and my background is in theoretical physics. This was a particularly interesting transition as I had not expected anything to be quite as cool as superconductors. But every day I am fascinated by the extent to which materials science is shaping our world, from stents in the human body, which are made from shape-memory alloys, to photovoltaic cells which convert sunlight into electricity, and even structural materials that enable us to contain radiation, or shield us from high pressures and temperatures like aircraft or space shuttles. None of these things would be possible without progress in materials science. Because of the nature of materials, they cover many disciplines, and so a whole host of scientists work on materials problems, from physicists, to chemists, mechanical engineers, biologists and also mathematicians.
My research is focussed on hydrogen embrittlement in zirconium alloys, and you’ll see here that size is indeed irrelevant – because the smallest atom in the periodic table can have an impact on even the most cutting edge engineering materials.
Zirconium alloys are used inside nuclear reactors to contain radioactive fuel. They have an amazing property, which is that they remain structurally stable at temperatures up to 500 degrees centigrade. Giant rods made from zirconium alloys are dunked inside a massive water tank, and the heat generated from the nuclear chain reaction heats the water, the water turns to steam, and the steam then turns turbines. Yes, we still haven’t gotten past the classic approach to electricity generation: we have simply found new ways to heat the water.
However zirconium alloys, like many metals in the periodic table, are subject to corrosion – or as it is known for iron and steel, rust. Strangely enough, the corrosion isn’t the annoying bit. When a metal corrodes, such as the pipes used in oil rigs, or other pipelines, what’s happening is that the oxygen from the water grabs on to the metal atoms and leaves hydrogen hanging around. Many metals are like a sponge for hydrogen, soaking up the hydrogen atoms.
Now we have to deal with this hydrogen atom, which is a bit of a menace. Hydrogen likes big spaces. There’s a lot of big space near cracks on the surface of these pipes – even in the tiniest cracks. Hydrogen atoms become attracted to these spaces in the same way that you and I would be attracted to a very heavy person on a trampoline – we’d feel an attractive force – and that’s called an elastic interaction.
The motion of hydrogen is governed by temperature and stress. If it’s hot, the hydrogen atoms vibrate a lot more and have lots of energy, and go wherever they like. When it gets colder, they have less energy and are more likely to fall towards the crack. This is just the same as if I got very tired on the trampoline, I would just fall towards the heavy person, but if I had more energy I could bounce myself away.
When there’s enough hydrogen at the cracks, all the atoms rearrange themselves into a structure that is much easier to break. This means that the crack can grow, and then the whole thing starts again.
This process is known as Delayed Hydride Cracking, and it’s an issue in many structural metals and alloys. What would be amazing is if we could figure out how quickly this process happens, we could then heat up the metal so that the hydrogen has enough energy to move away from the crack. This is what I am trying to find out.
I build computer simulations of hydrogen and zirconium, and bring in all the ingredients from physics to make a realistic model of how hydrogen accumulates at these cracks. One of these ingredients will be the hydrogen atom, and how fast it can move through the metal – this is its diffusion coefficient, which goes up with temperature.
Another ingredient is how the hydrogen atom interacts with the crack – which is similar to how big a dip the heavy person makes on a trampoline. A lot of this can be worked out from experiments and clever computer simulations that solve the Schrodinger equation. Then I simply put the ingredients together and check that I’m getting something similar to what happens in real life. I do this every time I add an ingredient to check that my work is realistic. Eventually I will have a model, based on fundamental physics, that can be used to predict how long I have before Delayed Hydride Cracking sets in.
This kind of problem is not just limited to nuclear reactors, but also oil rigs and pipelines. Steel, iron, titanium and many of the other transition metals that we use every day are subject to embrittlement because of hydrogen. If you can just imagine the scale of it all – hydrogen atoms – 60,000 trillion of these are contained inside a millilitre of water – and yet just a few can impact on the mechanical properties of something the size of a building.
This is the kind of stuff that keeps me fascinated with materials. And I’m just working on a drop in the ocean. There’s so much more to be done in materials science, and it brings together so many disciplines that it’s impossible to be bored.Tagged in: women in science
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