Materials Case Study 1:
Extreme Ice - What happens in matter at very high temperatures and pressures?

At the Centre for Science in Extreme Conditions scientists look at rocks and minerals under very high pressures and temperatures, also super-conductors – how you can create, prepare and use super-conductors using conditions of high temperature and pressure. This includes study the effects of pressure on things like amino acids, which are the fundamental building blocks of proteins. This has implications for how life evolves at the bottom of the deep oceans and also for food pressurisation techniques which are used to make food safe.

What materials have you looked at?

My own particular area of research is in the study of high pressures and temperatures on the structures of Energetic Materials. Energetic Materials are very interesting materials, represented by explosives, propellants and gas generators – things that give off large amounts of gas, and also pyrotechnics. But most of my research is on explosives and propellants. What I’m interested in is looking at how the structures of these materials change when you subject them to very high temperatures and pressures. The reason that this is important is because that structure affects their properties. In a typical explosion very large pressures, tens of thousands of atmospheres, are generated in the explosive material, and under those sorts of conditions the structure of the material can change and hence its properties. We want to model the performance and characteristics of these compounds at very high temperatures and pressures, we need to know the structures of the materials.

How have you set about looking at these structures at Diamond?

We have used Beamline I15, which is the extreme conditions beamline and we’ve also used the single crystal diffraction Beamline I19. We use a piece of apparatus called a Diamond Anvil Cell which involves squeezing our sample between two gem-quality diamonds. The sample is contained within a small hole no more that 0.3 of a millimetre across and by squeezing these diamonds together we can generate pressures up to 100,000 atmospheres. We then fire an intense beam of x-rays through the sample and from the scattered x-ray beams we are able to monitor the structure as we change the pressure all the way from ambient pressure all the way up to 100,000 atmospheres.

What have you been able to see about the structural changes that happen as the pressure increases?

What we see is fascinating results, which include some quite dramatic structural changes to the crystal structure, the way the molecules are packed in the crystal sometimes change very abruptly. For example, we’ve been looking at a material called RDX which is a very common military explosive. If you squeeze it to above 39,000 atmospheres the molecules in the crystal structure rearrange, until we get a denser structure and a more closely packed set of molecules that has clearly very, very different properties.