Electron microscopy analysis of the boundary layer structure of SrTiO3 semiconducting ceramic

In a boundary layer (BL) semiconducting SrTiO3-based ceramic condenser, the BL structure has been investigated using high-resolution field-emission scanning electron microscopy (FE-SEM) and field-emission (scanning) transmission electron microscopy (FE-(S)TEM). In an initial TEM observation, a double layered structure was observed at the grain boundary region. It consisted of a grain boundary (second phase) and a pair of the metal diffusion layers of up to several nanometres in width across the grain boundary where the change of the crystal lattice distance was undetected by the high-resolution TEM image. A facet structure was often observed on the grain boundaries. It was particularly formed on (020) plane of the grain crystal. High resolution SEM showed a jagged striped structure on the surface of the bulk material and on the inside grain as revealed by fracture. Using the similarity in shape and size, it can be identified to correspond to the facet boundary structure. Its formation mechanism can be explained as that during the reoxidization process when the oxide flux of the mixture of Bi2O3, PbO and CuO, painted on the bulk material, surfaces migrates into the ceramic along the grain boundary. The oxide corrodes the grain surfaces including the bulk surfaces. This corrosion particularly occurs on (020) plane of the grain so that the facet structure is produced. In this paper, by using the atomic scale high angle annular dark field STEM, it has been determined that Bi atoms preferentially replace Sr atoms on (020) in the diffusion layers. The atom position displacement was also detected at the grain surfaces and this altered atomic assignment can be determined as an origin of production of Sr2Bi4Ti5O18 at the grain boundary. Also, it was observed that the layer width of the metal diffusion layers was often different between the both grains and changed locally so that the ribbon of the diffusion layers meandered around the straight grain boundary. Its possible mechanism is also proposed.