Wednesday, 3 December 2014

Materials research - Small Balls and Pretty Patterns

By Alex Cackett and Jim Hickey

As we all know, magnetic confinement fusion is around the next corner and we’re just dotting the i’s and crossing the t’s. Such a statement would be true if we wanted to build a very expensive power source that would disintegrate in a few years. This highlights a vital area of fusion research, developing materials that can withstand the harsh environment plasmas impose on their containers. Culham Centre for Fusion Energy (CCFE) is currently a large component of a resurgence of nuclear materials research in the UK, known as the National Nuclear Users Facility (NNUF) which aims to, in a nutshell, figure out the following:
     1. How long can we keep the current fleet of nuclear fission plants running?
     2. How will the recently developed materials behave in the new reactors currently under construction? (e.g. at Hinkley Point)
     3. Can we solve some of the materials conundrums for future fusion power plants so that they are economical and last longer?

The Materials Research Laboratory (MRL) at CCFE is a baby lab with the primary aim of looking at materials on the small-scale. It contains some very cool pieces of kit with even cooler names: a nanoindenter, a Focused Ion Beam (FIB) and a Scanning Electron Microscope (SEM). More on these bad boys later but in short, the MRL is a precursor to the Materials Research Facility (MRF), which is currently being built here at Culham. The major difference between the two is that the MRF will be able to do everything we’re currently doing in the MRL, and more, on mildly radioactive materials (think lamb bhuna spicy).

Focused ion beam beast in the MRL
Both nuclear fission and fusion reactions we currently use, or would like to use for power generation in the future, produce neutrons. These neutrons can be thought of as sub-atomic snooker balls (great band name) that simply smash into the atoms that form the components of your reactor. This can cause all sorts of phenomena including knocking atoms out of their usual locations, degrading the mechanical properties of the material and causing further nuclear reactions that can cause radioactivity.
We use all sorts of different materials in a nuclear reactor to satisfy different roles. For obvious reasons, certain metals and alloys (e.g. steels, zirconium, CuCrZr) and some of his/her mates are used for lots of structural applications. They are usually stiff, strong and retain these properties reasonably well as they are bombarded with neutrons. We want to understand exactly what happens to these mechanical properties as their exciting existence proceeds in a reactor. This has historically been done by either pulling apart quite large lumps of irradiated material to investigate the change in their properties or by simply replacing parts once they have exceeded very conservative lifetime expectancies.
Cherenkov radiation in a small fission test reactor in the USA. Although it looks very lovely, it is the result of high energy particles being emitted from nuclear reactions within the core.
 Small balls - why size matters

Our graduate project is based on developing a technique, based on a process called nanoindentation, to extract the mechanical properties of very small (micron-scale) volumes of materials for nuclear applications. Nanoindentation is basically a glorified word for prodding a little diamond tip into the surface of a material and measuring the response. Tips come in different geometries and we are using tips of a spherical shape. Spheres are great as they elastically deform a material before permanently deforming it compared to tips with a sharp end (think knifey-spooney). This is very important as we can therefore plot stress-strain curves, a sort of fingerprint of the mechanical properties of a sample. Nanoindentation is also semi non-destructive in that you leave a tiny little pimple in your sample but you don’t completely destroy it. This has obvious benefits if you want to figure out how your expensive reactor component is doing throughout its service life.
Size really does matter here and researchers in the field have always been trying to reduce the volume of test specimens. The smaller a radioactive test sample, the easier it will be to handle. Also, most techniques currently used to irradiate/mimic irradiation in materials only manage to do this on a very small volume of material anyway, usually by firing ions at the material. As mentioned above, small-scale testing will also not destroy your very expensive reactor component and you don’t have to shut down your reactor for a long time to get at it, keeping the money gods happy. The issue is though, the smaller you go, the less representative the properties are compared to bulk components. This is called the size effect and is one of the banes of a materials scientist life or a happy challenge, depending on what you’re researching/ approach to life is.  Our challenge is to characterise the size effect for different materials by using different size spherical tips, ranging from 1 to 150 micrometre in radius. This way the strength of the material can be plotted against tip size to characterise the size effect.
TKD - what, why and how?
Transmission Kikuchi Diffraction (TKD) is a recently developed technique that’s currently being explored in the MRL. It can be used to gather crystallographic information (atomic configuration) using only tiny volumes of material to do so, but that’s not the only reason it’s attractive to material scientists; it can also be performed inside the SEM and is capable of achieving spatial resolutions as small as 2 nanometres (nm), which is an order of magnitude improvement on the next best thing - electron backscatter diffraction (EBSD). Traditionally, high resolution imaging could only be obtained using transmission electron microscopes (TEMs) which are tricky to use and bloomin’ expensive. Now with TKD, all you need is a SEM with an EBSD detector (essentially a CCD camera and phosphor screen) and you’re good to go!
A lot of materials have some form of crystal structure, which means their constituent atoms are regularly arranged in a repeating 3D pattern that forms a lattice. When electrons are fired at a material some of them are diffracted by the planes of atoms and can be collected by a detector forming patterns, like the one seen in the figure below, called Kikuchi patterns. Each Kikuchi pattern is characteristic of the sample material and crystal orientation, so by scanning the electron beam over the whole material surface it’s possible to form a map of crystallographic information. This is essentially how EBSD works, but TKD is a slight variation on this method. Rather than collecting the backscattered electrons bouncing off the sample surface, instead it is the electrons that have been transmitted through the material that are used.

Kikuchi pattern of single crystal Si with orientation <001> normal to the sample surface

As you can probably imagine, for this to be possible the samples have to be extremely thin, ~200 nm (1/50th of the longest dimension of a human blood cell!). This is where the focused ion beam comes in handy. Gallium ions are used to bombard the bulk specimen (think sand-blaster but on a much, much smaller scale) and mill away material to form a standing wafer in the surface. The wafer can then be stuck on to the end of a precisely controlled needle using platinum ‘glue’ and moved to a small holder where it’s mounted and ready for thinning. Thinning is also performed using gallium ions, but at a much lower velocity, so the ions are not implanted into the wafer (this can damage the material and ruin the Kikuchi patterns). With some precision milling, and more than a little patience, the wafer can eventually be thinned down to a skinny 200 nm. It’s then ready to be placed in the SEM for TKD analysis. The results of all this hard work can be seen here - a rainbow-coloured, first of its kind, strain map around a spherical indentation, tah-dah! With the MRF opening next year there is the possibility of using this technique on irradiated materials, so stay tuned for a follow-up blog post on that.

Strain mapping around a spherical indentation placed at the edge of a grain boundary

More information on the NNUF and the MRL can be found at the following websites:

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