Oxford Colloid Group

Department of Chemistry University of Oxford

Materials composed of a single crystal are uncommon in nature: many materials are polycrystalline, consisting of individual crystallites packed together. A grain boundary is a planar defect that occurs where two such crystallites meet---the same crystal structure and chemical composition exists on each side but the orientation differs. If a common origin is assumed, the transformation between the two is a pure rotation, which is known as the misorientation. Grain boundaries are central to our understanding of deformation and fracture mechanisms, melting kinetics, and transport properties in a wide range of materials. Some examples include: ductility, brittleness and electrical conductivity. The misorientation affects the properties of the boundary such as its surface tension, mobility and stiffness and is therefore of great importance in the study of grain boundaries. Furthermore, if the orientations of all the crystallites within a polycrystalline material are known, the grain boundary network can be found, along with the misorientation associated with each boundary.

Current Research
Characterization of grain boundary location and misorientation within a colloidal crystal

Colloidal crystals are generated from high volume fraction samples. The colloidal particles are suspended in a refractive index matched solvent and imaged using fluorescent confocal microscopy. Particle identification software measures the 3D coordinates of the particles. From these coordinates, crystalline regions are identified by using the Stainhardt bond-orientational order parameter q_6. The crystalline regions are further probed to identify the orientation of the crystallites, from which a construction of the grain boundary network can be achieved.

Key People

(a) Confocal z-stack image of TPM colloidal crystal. (b) Reconstruction of the particle coordinates, the highlighted grey box appears in (c). To be able to see the crystalline planes in (c) the markers for the particle positions have been made smaller.

Particles in an FCC colloidal crystal coloured by local orientation. The red crosses represent the grain boundary plane.