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

Rice University engineers mimicking processes at the atomic scale to make them large enough to see have modeled how shear affects grain boundaries in polycrystalline materials.

That the boundaries change so easily wasn’t entirely a surprise to the researchers, who used spinning arrays of magnetic particles to see what they suspect happens at the interface between misaligned crystal domains.

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

The technique reported in scientific progress can help engineers design new and improved materials.

To the naked eye, common metals, ceramics and semiconductors appear uniform and solid. But on a molecular scale, these materials are polycrystalline, separated by defects known as: grain boundaries† The organization of these polycrystalline aggregates determines such properties as conductivity and strength.

Under applied stress, grain boundaries can form, reconfigure or even disappear altogether to meet new conditions. Even colloidal crystals have been used as model systems to see boundaries move and their phase transitions been challenging.

“What sets our study apart is that in most colloidal crystal studies, the grain boundaries form and remain stationary,” Lobmeyer said. “They are essentially set in stone. But with our rotating magnetic fieldthe grain boundaries are dynamic and we can watch their movement.”

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

Here they saw that gas and solid phases can coexist, resulting in polycrystalline structures with particle-free regions. They showed that these voids act as sources and sinks for the movement of grain boundaries.

The new study also shows how their system supports the long-standing Read-Shockley Theory of hard condensed matter that predicts the misorientation angles and energies of low-angle grain boundaries, which are characterized by a small deviation between adjacent crystals.

By applying a magnetic field on the colloidal particlesLobmeyer asked the iron oxide– embedded polystyrene particles to assemble and watch as the crystals formed grain boundaries.

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

Instead, new grain boundaries were formed due to shear at the cavity interface. Like polycrystalline materials, these followed the disorientation angle and energy predictions of Read and Shockley more than 70 years ago.

“Grain boundaries have a significant impact on the properties of materials, 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 requires multiple heating and cooling cycles to remove defects in crystalline materials.”

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