Researchers now say that the best-performing materials in sustainable energy applications — e.g., converting sunlight or waste heat to electricity — often use collective fluctuations of clusters of atoms within a much larger structure; this process is often referred to as “dynamic disorder.”
Understanding dynamic disorder in materials could lead to more energy-efficient thermoelectric devices and to better recovery of useful energy from waste heat by converting it directly to electricity.
When materials function inside an operating device, they can behave as if they are dancing. This dynamic disorder is difficult to study because the clusters are not only so small and disordered, but they also fluctuate in time. In addition, there is “boring” non-fluctuating disorder in materials that researchers aren’t interested in because the disorder doesn’t improve properties. Until now, it has been impossible to see the relevant dynamic disorder from the background of less relevant static disorder.
Researchers at Columbia Engineering and Université de Bourgogne report that they have developed a new kind of “camera” that can see the local disorder. Its key feature is a variable shutter speed: because the disordered atomic clusters are moving, when the team used a slow shutter, the dynamic disorder blurred out, but when they used a fast shutter, they could see it.
The new method, variable shutter PDF (vsPDF), for atomic pair distribution function, doesn’t work like a conventional camera. It uses neutrons from a source at the U.S. Department of Energy’s Oak Ridge National Laboratory to measure atomic positions with a shutter speed of around one picosecond, or a trillion times faster than normal camera shutters. The study was published in Nature Materials.
“It’s only with this new vsPDF tool that we can really see this side of materials,” said Professor Simon Billinge. “It gives us a whole new way to untangle the complexities of what is going on in complex materials, hidden effects that can supercharge their properties. With this technique, we’ll be able to watch a material and see which atoms are in the dance and which are sitting it out.”
The vsPDF technology enabled the researchers to find atomic symmetries being broken in GeTe, an important material for thermoelectricity that converts waste heat to electricity, or electricity into cooling. They hadn’t previously been able to see the displacements, or to show the dynamic fluctuations and how quickly they fluctuated. Via vsPDF, the team developed a theory that shows just how such local fluctuations can form in GeTe and related materials. Such an understanding will help researchers to look for new materials with these effects and to apply external forces to influence the effect, leading to even better materials.
Billinge is now working on making the technique easier to use for the research community and applying it to other systems with dynamic disorder. The technique is not currently turn-key, but with further development it should become a much more standard measurement.
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