TEM analysis of grain boundaries in SrTiO ₃-based semiconductor ceramics

Structures of grain boundaries of SrTiO ₃ semiconductive capacitor have not been fully elucidated. Several studies were focused on the SrTiO ₃ systems which include additives, such as, Al ₂O ₃ or SiO ₂. In this study, we first investigated the grain boundaries of Sr _0.975Ca _0.025TiNb _0.004O ₃ which did not contain any additives. Then Bi ₂O ₃/CuO, which is a grain boundary modifier, was also added in order to investigate the process that changes grain boundaries into insulators. Samples heat-treated with Bi ₂O ₃/CuO were quenched in water and TEM (transmission electron microscopy) was carried out on these microstructures. Grain boundaries of samples sintered in a reduced atmosphere showed segregation of TiO ₂ oxide phase, which included no traces of Sr. On the other hand, in grain boundaries of samples heat-treated with the Bi ₂O ₃/CuO agent, at least two phases existed. One phase included large amounts of Bi and Cu, while the other phase showed only traces of these elements. Near the grain boundaries Bi and Cu diffused into the matrix grains showing inhomogeneous diffusion of which diffusion lengths were different at each side of the grain boundary and seemingly lattice coincident with both grain boundary layers and matrix grains. In addition, steps of about 2 nm in height were observed at grain boundaries.