Bismuth nanoprecipitation at grain boundaries during microstructural evolution in (Sr, Ba)TiO3 ceramics

Metallic Bi nanoprecipitates (NPs) were observed at grain boundaries (GBs) in a series of Bi-doped Sr1−xBaxTiO3 ceramics sintered at low temperature, using analytical electron microscopy. Most GBs fell into two groups, according to their local structure and chemistry, one with the NPs and the other with titania-based amorphous film. The solution of Ba into the lattice was only about one-third of the nominal value, and the remaining Ba dopants were driven into triple pockets or wetted the GBs during the sintering. Lower Ba doping initiated less liquid, resulting in a core–shell structure. Bi dopants were co-soluted with Ba, thereby suppressing the Bi segregation at GBs. The extra Bi could not diffuse out through the dewetted GB as fast as via the titania-based film, compelling them to precipitate locally. These metallic NPs may initiate several modes of polarization that increase the dielectric constants detected at 1 kHz.