March 25, 2023

The high-speed shutter “camera” reveals a dynamic disturbance of the atoms

At slow shutter speeds, GeTE’s atomic structure appears structured but blurry. Faster exposures reveal a clear complex pattern of dynamic displacements. Credit: Jill Heymann/ORNL, US Department of Energy

Researchers are beginning to understand that materials that do best in sustainable energy applications, such as converting sunlight or waste heat into electricity, often use the collective fluctuations of groups of atoms within a much larger structure. This process is often referred to as “dynamic turbulence”.

Understanding dynamic turbulence in materials can lead to more energy efficient thermoelectric devices, such as solid-state refrigerators and heat pumps, and also better recovery of useful energy from waste heat, such as vehicle exhaust and station exhaust. for electricity. A thermoelectric device was able to extract heat from radioactive plutonium and convert it into electricity to power the Martian rover when there wasn’t enough sunlight.

When working inside an operational device, materials can act as if they are alive and dancing — bits of matter responding and changing in surprising and unexpected ways. This dynamic turbulence is difficult to study because not only are the clusters very small and disorganized, but they also fluctuate over time. In addition, there is a non-floating “disturbing” disorder in the materials that the researchers are not interested in because the disorder does not improve the properties. Until now, it was impossible to see the relevant dynamic chaos against the background of the less relevant static chaos.

new tool

American and French researchers report that they have developed a new type of “camera” that can see through local chaos. Its main advantage is variable shutter speed: because the mixed atomic masses move, when the team used a slow shutter, the dynamic mixing was blurred, but when they used a fast shutter, they could see it. The new method, which they call PDF or vsPDF (for atomic pair distribution function), doesn’t work like a conventional camera — it uses neutrons from a source at the Department of Science’s Oak Ridge National Laboratory (ORNL). The US force measures atomic positions at a shutter speed of about a picosecond, or a million (trillion) times faster than normal camera shutters. The study was published in the journal nature materials.

“Only with this new vsPDF tool can we really see this aspect of materials,” said Simon Pilling, Professor of Materials Science and Applied Physics and Applied Mathematics at Columbia University (USA) and corresponding author of the study. “It gives us a whole new way to unravel the intricacies of what happens in complex materials, and the subtle influences that can cloud their properties. With this technique, we’ll be able to look at a material and see which atoms are in the dance and which are out.”

Detection of atomic structures with a “neutron” camera. Credit: Oak Ridge National Laboratory

new theory

The vsPDF tool allowed the researchers to find atomic symmetries being broken in GeTe, an important thermoelectric material that converts waste heat into electricity (or electricity into cooling). They weren’t able to see the displacements before, or show the dynamic fluctuations and how quickly they fluctuate. As a result of vsPDF’s insights, the team has developed a new theory showing how these local fluctuations in GeTe and related materials might form. This mechanistic understanding of the dance will help researchers search for new materials with these effects and apply external forces to influence the effect, leading to better materials.

Billinge is now working to make his method easier for the research community to use and apply to other dynamically disordered systems. At the moment this technique is not ready for use, but with further development it should become a much more standard metric that can be used in many materials systems where atomic dynamics are important, from watching lithium move around in a battery to studying dynamic processes during water separation with sunlight.

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