Engineers design nanocrystalline dynamics in an easy-to-see system

Grain boundaries are in line with the flow

In a study by Rice University, a polycrystalline material spinning in a magnetic field reshapes the grain boundaries and disappears due to spin at the interface of the voids. The different colors determine the direction of the crystal. Credit: Biswal Research Group / Rice University

Rice University engineers mimic atom-scale processes to make them large enough to see how shear affects grain boundaries in polycrystalline materials.

that border It can change so easily that it wasn’t entirely a surprise to the researchers, who used rotating arrays of magnetic particles to display what they suspected was going on at the interface between deflected crystal domains.

According to Sibanye Lisa Biswal, professor of chemical and biomolecular engineering at the George R. Brown School of Engineering in Rice, and graduate student and lead author Dana Luebmeyer, interfacial shear at the boundary of the crystal space can indeed drive how microstructures evolve.

Technology mentioned in science progress It can help engineers design new and improved materials.

to me With the naked eye, common metals, ceramics, and semiconductors appear homogeneous and solid. But at the molecular level, these materials are polycrystalline, separated by defects known as grain boundaries. The organization of these polycrystalline aggregates controls properties such as conductivity and strength.

Under applied stress, grain boundaries can form, reconfigure, or completely disappear to suit new conditions. despite of colloidal crystals As model systems for seeing and controlling borders moving Phase transitions It was a challenge.

“What is unique about our study is that in the majority of studies of colloidal crystals, grain boundaries are formed and remain constant,” said Luebmeyer. “They were basically set in stone. But with our turn magnetic field, grain boundaries are dynamic and we can watch their movement. “






In the experiments, the researchers induced colloids of quasiparticles to form two-dimensional polycrystalline structures by rotating them with magnetic fields. as such It was recently shown in a previous studythis type of system is well suited for visualizing phase transitions characteristic of atomic systems.

Here, they saw that gaseous and solid phases can coexist, resulting in polycrystalline structures that include particle-free regions. They showed that these voids act as sources and sinks for grain boundary movement.

The new study also shows how it follows their ancient system Reed Shockley’s theory From dense solids that predict misorientation angles and low-angle grain boundary energies, those characterized by slight misalignment between adjacent crystals.

Through the application of a magnetic field On the colloidal particlesLobmeyer push file iron oxide– Polystyrene particles compact to aggregate and watch as crystals form grain boundaries.

“We usually started with many relatively small crystals,” she said. “After some time, the grain boundaries started to disappear, so we thought it might lead to a perfect single crystal.”

Instead, new grain boundaries formed due to shearing at the void interface. Similar to polycrystalline materials, these materials followed the misdirection angle and energy predictions made by Reed and Shockley more than 70 years ago.

“Grain boundaries have a huge impact on material properties, so understanding how to use voids to control crystalline materials provides us with new ways to design them,” Biswal said. “Our next step is to use this tunable colloidal system to study annealing, a process that involves multiple heating and cooling cycles to remove defects within crystalline materials.”

The National Science Foundation (1705703) supported the research. Biswal is the William M. McCardle Professor of Chemical Engineering, and Professor of Chemical and Biomolecular Engineering and Materials Science and Nanoengineering.


Using electron microscopy and automatic atomic tracking to learn more about grain boundaries in metals during deformation


more information:
Dana M. Lobmeyer et al, Grain boundary dynamics driven by magnetically induced spin at the steric interface of 2D colloidal crystals, science progress (2022). DOI: 10.1126 / sciadv.abn5715

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