That the boundaries could shift 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 Sibani Lisa Biswalprofessor of chemical and biomolecular engineering at Rice George R. Brown School of Engineeringand graduate student and lead author Dana Lobmeyer, interfacial shear at the crystal vacuum boundary can indeed determine the evolution of microstructures.
The technique reported in Scientists progress could help engineers to design new and improved materials.
To the naked eye, base metals, ceramics and semiconductors appear uniform and solid. But on 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 boundaries moving, controlling their phase transitions has been difficult.
“What sets our study apart is that in the majority of colloidal crystal studies, grain boundaries form and remain stationary,” Lobmeyer said. “They are basically set in stone. But with our rotating magnetic field, the grain 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 to the colloidal particles, Lobmeyer induced the iron oxide-encrusted polystyrene particles to come together and observed that 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.
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