Structure of Σ7 grain boundary in Al2O3

Abstract

In the past, extensive high resolution electron microscopy has been applied to the atomic structure of grain boundaries of cubic symmetry. In order to have a better understanding of generalization of the grain boundary theory, it could be fruiful to study grain boundary structure of non-cubic and low symmetry crystals in which case the exact CSL’s may not exist. Al2O3 has a hexagonal crystal structure ( non-cubic). In the case of hexagonal crystals, three dimensional coincidence site lattices (CSL’s) are only possible for rational values of (c/a), except for rotations about the [0001] axis. The (c/a) of α-Al2O3 is very close to a rational number (15/2) such that constrained coincidence-site lattice (CCSL) misorientations can be found. In this research, we study the atomic structure of Σ7 grain boundary. The misonentation of Σ7 is [011]/180°. The bicrystals of Σ7 were made by diffusion bonding in high temperature and high vacuum.Figs. 1 (a) and (b). show typical HRTEM images of Σ7 Al2O3 boundary recorded at the underfocus values -48 nm and -96 nm, respectively. The beam direction is parallel to a common axis [20].