Reviewed by Alex SmithJun 6 2022
Engineers from Rice University who mimic atom-scale processes to make them large enough to see have modeled how shear impacts grain boundaries in polycrystalline materials.
In a Rice University study, a polycrystalline material spinning in a magnetic field reconfigures as grain boundaries appear and disappear due to circulation at the interface of the voids. The various colors identify the crystal orientation. Image Credit: Biswal Research Group/Rice University
The fact that the boundaries were capable of changing so readily was not a complete surprise to the scientists. They utilized spinning arrays of magnetic particles to observe what they believed to happen at the interface present between the misaligned crystal domains.
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 feels that interfacial shear at the crystal–void boundary could certainly drive how microstructures advance.
The method published in the journal Science Advances could help engineers design new and enhanced materials.
Ceramics, common metals, and semiconductors appear even and solid to the naked eye. However, at the molecular scale, these materials are known to be polycrystalline, segregated by defects called grain boundaries. The organization of such polycrystalline aggregates governs these properties like strength and conductivity.
Under applied stress, grain boundaries have the potential to develop, reconfigure, or even disappear completely to house new conditions. Although colloidal crystals have been utilized as model systems to visualize the motion of boundaries, regulating their phase transitions has always been a hard one.
What sets our study apart is that in the majority of colloidal crystal studies, the grain boundaries form and remain stationary. They’re essentially set in stone. But with our rotating magnetic field, the grain boundaries are dynamic and we can watch their motion.
Dana Lobmeye, Study Lead Author, Rice University
In experiments that were conducted, the scientists induced colloids of paramagnetic particles to develop 2D polycrystalline structures by spinning them along with the magnetic fields. As recently displayed in an earlier study, this kind of system is ideally suited for envisioning the phase transition characteristics of atomic systems.
In this context, the researchers were able to notice that gas and solid phases have the potential to coexist. This leads to a polycrystalline structure that consists of particle-free regions. They illustrated that these voids serve as sinks and sources for the movement of grain boundaries.
Also, the new study illustrates how their system tracks the long-term Read-Shockley theory of hard condensed matter that anticipates the misorientation angles and energies of low-angle grain boundaries. These have been characterized by a small misalignment between adjacent crystals.
Employing a magnetic field on the colloidal particles, Lobmeyer induced the iron oxide-embedded polystyrene particles to gather and watched as the crystals developed grain boundaries.
We typically started out with many relatively small crystals. After some time, the grain boundaries began to disappear, so we thought it might lead to a single, perfect crystal.
Dana Lobmeye, Study Lead Author, Rice University
Instead, shear at the void interface made new grain boundaries form. These, like polycrystalline materials, followed Read and Shockley’s misorientation angle and energy predictions from 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 offers us new ways to design them. 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.
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 in Chemical Engineering, a professor of chemical and biomolecular engineering, and of materials science and nanoengineering.
Journal Reference:
Lobmeyer, D M & Biswal, S L (2022) Grain boundary dynamics driven by magnetically induced circulation at the void interface of 2D colloidal crystals. Science Advances. doi.org/10.1126/sciadv.abn5715