It may be possible to use stronger materials for nuclear fusion thanks to images of small aluminum crystals

Abstract technology graphene material concept

Laser compression of the aluminum crystal provides a clearer view of the material’s plastic deformation, which could lead to the design of stronger nuclear fusion materials and armor for spacecraft.

Imagine dropping a tennis ball on the bedroom mattress. The tennis ball will bend the mattress slightly, but not permanently – pick the ball back up, and the mattress will return to its original position and strength. Scientists call this elastic state.

On the other hand, if you drop something heavy – like a refrigerator – the force pushes the arranged into what scientists call the plastic state. In this sense, the plastic case is not the same as the plastic milk jug in your refrigerator, but rather a permanent rearrangement of the material’s atomic structure. When you remove the refrigerator, the mattress will compress and be uncomfortable to say the least.

But changing the material’s elastic plastic is about more than the comfort of the mattress. Understanding what happens to matter at the atomic level when it transitions from flexible to plastic under high pressures could allow scientists to design stronger materials for spacecraft and nuclear fusion experiments.

So far, scientists have failed to capture clear images of a material’s transition to elasticity in the past, leaving them in the dark about what microscopic atoms are doing when they decide to leave their comfortable flexible state and journey into the realm of plastic.

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory have captured high-resolution images of a small sample of a single crystal of aluminum as it transitions from an elastic to a plastic state for the first time. The images will allow scientists to predict the behavior of the material as it undergoes a plastic transformation within five trillionths of a second of the phenomena that occur. The results were recently published in the journal Nature Communications.

Final moments of crystallization

The scientists needed to apply force to the aluminum crystal sample in order to take the pictures, and the refrigerator was clearly very large. Instead, they used a high-powered laser to hammer the crystal hard enough to change its state from malleable to plastic.

The scientists used SLAC’s fast “electron camera,” or Megaelectronvolt Ultrafast Electron Diffraction (MeV-UED) instrument to send a high-energy electron beam through the crystal as the laser produces shock waves that compress it. The scattering of this electron beam from the aluminum nuclei and the electrons in the crystal allowed the scientists to accurately determine the atomic structure. As the laser proceeded to compress the sample, the scientists took several shots, resulting in a kind of flip-book film — a stop-motion film of the crystal’s dance in plasticity.

More specifically, the high-resolution footage showed scientists when and how streak defects appeared in the sample — the first sign that a material was hit more hard than it recovered.

Line defects are like broken strings on a tennis racket. For example, if you use a tennis racket to gently hit a tennis ball, the strings of your racket will vibrate slightly, but return to their original position. However, if you hit a bowling ball with your racket, the strings will shift out of place, and you won’t be able to bounce. Similarly, when a high-powered laser hit the aluminum crystal sample, some of the rows of atoms in the crystal shifted out of place. Tracking these transitions – line defects – using the MeV-UED electron camera showed the crystal’s flexible journey into plastic.

SLAC scientist Mianzhen Mu said scientists now have high-resolution images of these linear defects, revealing how quickly the defects grow and how they move once they appear.

“Understanding the dynamics of plastic deformation will allow scientists to add artificial defects to the material’s lattice structure,” Mo said. “These synthetic defects can provide a protective barrier to prevent materials from deforming at high pressures in harsh environments.”

UED’s moment to shine

Key to the experimenters’ fast, clear images were the high-energy electrons of the MeV-UED, which allowed the team to sample images every half-second.

“Most people use relatively small electronic energies in the UED experiments, but we use 100 times more energetic electrons in our experiment,” said Xijie Wang, distinguished scientist at SLAC. “When you use higher power, you get more particles in a shorter pulse, which provides excellent quality 3D images and a more complete picture of the process.”

The researchers hope to apply their new concept of plasticity to a variety of scientific applications, such as strengthening materials used in high-temperature nuclear fusion experiments. A better understanding of materials’ responses in extreme environments is urgently needed to predict their performance in a fusion reactor in the future, said Siegfried Glenzer, director of high energy density sciences.

“We hope that the success of this study will stimulate the application of higher laser powers to test a larger range of important materials,” Glenzer said.

The team is interested in testing materials for experiments that will be conducted at ITER Tokamak, a facility they hope will be the first to produce sustainable fusion energy.

MeV-UED is a tool of the Linac Coherent Light Source (LCLS) User Facility, operated by SLAC on behalf of the Department of Energy’s Office of Science. Part of the research was conducted at the Center for Integrated Nanotechnology at Los Alamos National Laboratory, a user facility of the Department of Energy’s Office of Science. Support was provided by the Department of Energy’s Office of Science, in part through the Laboratory-Driven Research and Development Program at SLAC.

Reference: “Ultra-fast visualization of elementary plasticity in dynamically compressed materials” by Mianzhen Mu, Minxu Tang, Zhejiang Chen, C. Adrien Descamps, Benjamin K. Ofori-Okai, Renkai Li, Sheng-Nian Luo, Xijie Wang and Siegfried Glenzer, February 25, 2022 Available here Nature Communications.
DOI: 10.1038 / s41467-022-28684-z