Engineers model nanoscale crystal dynamics in an easy-to-view system

In a Rice University study, a polycrystalline material rotating in a magnetic field reconfigures itself as grain boundaries appear and disappear due to interface circulation of voids. The different colors identify the orientation of the crystal. Credit: Biswal Research Group/Rice University

Rice University engineers who mimic atomic-scale processes to make them large enough to see have modeled how shear influences grain boundaries in polycrystalline materials.

That the borders can change so easily wasn’t entirely a surprise to the researchers, who used rotating arrays of magnetic particles to see what they suspected was happening at the interface between misaligned crystal domains.

According to Sibani Lisa Biswal, professor of chemical and biomolecular engineering at Rice’s George R. Brown School of Engineering, and graduate student and lead author Dana Lobmeyer, interfacial shear at the crystal vacuum boundary may indeed determine the evolution of microstructures. .

The technique reported in Scientists progress could help engineers to design new and improved materials.

At naked eye, base metals, ceramics and semiconductors appear uniform and solid. But at the molecular scale, these materials are polycrystalline, separated by defects called grain boundaries. The organization of these polycrystalline aggregates governs properties such as conductivity and resistance.

Under applied stress, grain boundaries can form, reconfigure, or even disappear entirely to adapt to new conditions. Even though colloidal crystals have been used as model systems to see borders move, to control their phase transitions was difficult.

“What sets our study apart is that in the majority of colloidal crystal studies, grain boundaries form and remain stationary,” Lobmeyer said. “They’re basically set in stone. But with our rotation magnetic fieldgrain boundaries are dynamic and we can observe their movement.”






In experiments, researchers induced colloids of paramagnetic particles to form 2D polycrystalline structures by rotating them with magnetic fields. As recently shown in a previous studythis type of system is well suited to visualize the phase transitions characteristic of atomic systems.

Here they saw that gas 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 demonstrates how their system follows the long tradition Read–Shockley theory of hard condensed matter that predicts misorientation angles and low-angle grain boundary energies, those characterized by small misalignment between adjacent crystals.

By applying a magnetic field on the colloidal particlesLobmeyer prompted the iron oxide-embedded polystyrene particles to assemble and watched as the crystals formed grain boundaries.

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

Instead, new grain boundaries formed due to shear at the vacuum interface. Similar to polycrystalline materials, these followed the misorientation angle and energy predictions made by Read and Shockley over 70 years ago.

“Grain boundaries have a significant impact on material properties, so understanding how voids can be used to control crystalline materials gives us 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 cycles of heating and cooling to remove defects in crystalline materials.”

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


Using electron microscopy and automatic atom 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 circulation at the vacuum interface of 2D colloidal crystals, Scientists progress (2022). DOI: 10.1126/sciadv.abn5715

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