Modeling nanoscale crystal dynamics in an easy-to-view system

Engineers from rice university who mimic atomic-scale processes to make them large enough to see have modeled how shear affects grain boundaries in polycrystalline materials.

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. Image Credit: Biswal Research Group/Rice University

The fact that borders are able to change so easily was not a complete surprise to scientists. They used rotating arrays of magnetic particles to observe what they thought was happening at the interface between the misaligned crystal domains.

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 believe that interfacial shear at the crystal-vacuum boundary could definitely determine the progression of microstructures. .

The method published in the journal Scientists progress could help engineers to design new and improved materials.

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

Under applied stress, grain boundaries have the potential to grow, reconfigure, or even completely disappear to harbor new conditions. Although colloidal crystals have been used as model systems to visualize boundary movement, regulating their phase transitions has always been difficult.

What sets our study apart is that in the majority of colloidal crystal studies, grain boundaries form and remain stationary. They are basically carved in stone. But with our rotating magnetic field, the grain boundaries are dynamic and we can observe their movement.

Dana Lobmeye, lead study author, Rice University

In experiments that were conducted, scientists induced colloids of paramagnetic particles to grow 2D polycrystalline structures by rotating them with magnetic fields. As recently shown in a previous study, this type of system is perfectly suited to consider the phase transition characteristics of atomic systems.

In this context, the researchers were able to observe that the gaseous and solid phases had the potential to coexist. This leads to a polycrystalline structure consisting of particle-free regions. They illustrated that these voids serve as sinks and sources for grain boundary movement.

In addition, the new study illustrates how their system follows the long-term Read-Shockley theory of hard condensed matter that anticipates misorientation angles and low-angle grain boundary energies. These were characterized by a small misalignment between adjacent crystals.

Using a magnetic field on the colloidal particles, Lobmeyer induced the iron oxide-encrusted polystyrene particles to come together and observed that the crystals developed grain boundaries.

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

Dana Lobmeye, lead study author, Rice University

Instead, shear at the vacuum interface caused new grain boundaries to form. These, like polycrystalline materials, followed the angle and disorientation energy predictions of 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.. 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..

Sibani Lisa Biswal, Professor, Chemical and Biomolecular Engineering, George R. Brown School of Engineering, Rice University

The study was financially supported by the National Science Foundation (1705703). Biswal is the William M. McCardell Professor of Chemical Engineering, a professor of chemical and biomolecular engineering, as well as materials science and nanoengineering.

Journal reference:

Lobmeyer, DM & Biswal, SL (2022) Grain boundary dynamics driven by magnetically induced circulation at the vacuum interface of 2D colloidal crystals. Scientists progress.


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