Beamtime Galore

by Mark Wolfman on 2015-04-26

My PhD research project involves using X-rays to probe the chemistry of lithium-ion battery electrodes. In order to see something with a high level of detail, you need to look at it in bright light. Analogously, I need access to very high quality X-rays. The last two weekends I traveled to two different synchrotron; particle accelerators whose sole purpose is to generate high intensity X-rays. Each one has 10-30 X-ray beams coming off of it, set up to run a variety of experiments. At Argonne National Lab's Advanced Photon Source (APS), we used diffraction to measure the strain withing a micrometer sized particle. At the Stanford Synchrotron Radiation Lightsource (SSRL) we collected the data to make three-dimensional reconstructions of the micro-structure of some samples using X-ray Transmission Tomography.

Brian in the hutch at Argonne

Fellow grad-student Brian May at beamline 34-ID-E at Argonne National Lab.

The Physics

Viewed from above, the synchrotron is a big ring. In the case of Argonne, it's 1.1km (3,622ft) around. Stanford's is a little smaller. Electrons travel around the ring at over 99.99% of the speed of light. Anytime a charged particle accelerates, it gives off electromagnetic light waves. [NB: "Light" doesn't necessarily mean visible light, it can also refer to microwaves, UV light, X-rays, etc.] From a physicists perspective, moving in a circle is a form of acceleration, so as these electrons travel around the ring they give off electromagnetic radiation. Because of how fast they're going, the light given off is in the X-ray region of the electromagnetic spectrum. The electrons also go through special magnets, either "undulators" and "wigglers", that give off even better light. These X-rays are very bright, quite straight and highly coherent; perfect for my research.

Argonne Advanced Photon Source

The Advance Photon Source at Argonne National Lab, Chicago IL.

The Chemistry

X-ray micrograph of secondary particles

X-ray micrograph of secondary cathode particles. Each particle is ~2-5µm across.

In my research group, we look at cathode materials for lithium-ion batteries. These materials are usually crystalline, meaning the atoms form a regular, repeating pattern. X-rays are really good at probing crystal structures and so with the right experiment, we can get a lot of information about our sample. The experiment at SSRL made use of an X-ray microscope. The concept is analogous to a conventional microscope except that X-rays have a much smaller wavelength so we can, in theory, resolve much smaller distances. Also, different elements absorb X-rays at different wavelengths, so by tuning the beam we can actually collect images of just certain elements, something conventional microscopy cannot do. We looked at the secondary particles of a battery matherial. Then we rotated the sample and by taking multiple images were able to reconstruct a 3-dimensional model of where specific elements are located.

The Experience

The first reaction to being at a synchrotron was awe. These machines are enormous and incredibly complex. At first, the equipment can be overwhelming. Fairly quickly, though, I came to realize that each piece is straightforward. I certainly don't claim to understand what all (or even most) of the pieces do, but there's nothing mysterious about their operation. There's also a fair amount of downtime, since it takes time to collect all the data and much of the process is automated. This provides a good opportunity to wander around experiment hall and read the various research posters that the different beamlines have created. Each beamline is set up for a different technique, so seeing what each is capable of was a fascinating tour.

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