The ferroelectric properties of barium titanate strongly depend on its microstructure, in particular, grain size and distribution. During sintering, $BaTiO_3$ usually exhibits abnormal grain growth, which deteriorates considerably the ferroelectric properties. The abnormal grain growth of $BaTiO_3$ occurs in different manners according to the sintering temperatures, i.e. above eutectic and below eutectic. Above the eutectic temperature, the abnormal grains are isotropic and grow until they impinge one another. Below the eutectic temperature, however, it was recently reported that the abnormal growth can occur only if the grains have {111} twins. Since twins give easy nucleation sites for growth, grain growth is preferential to the direction of {111} planes. For the different behaviors of abnormal grain growth, the method to suppress the grain growth should differ with respect to the sintering temperature. A typical technique to suppress the abnormal grain growth is the addition of dopants. Dopant addition, however, affects the ferroelectric properties and thus may limit the application of $BaTiO_3$.
In the present investigation, we studied simple and novel techniques to prevent the abnormal grain growth of $BaTiO_3$ and to overcome the limitation of dopant use. The technique that consists of stepwise sintering in a reducing atmosphere and in an oxidizing atmosphere makes possible the suppression of abnormal grain growth above the eutectic temperature. Sintering in a reducing atmosphere, referred to as $H_2$ heat-treatment, gives a specific average grain size with homogeneous microstructures; the suitable hours of heat-treatment could give a required average grain size. Following sintering in an oxidizing atmosphere, the driving force for grain growth is insufficient due to its initial relatively large grain size$(~2.5\mu m)$. Thus in spite of sintering in air, the grain growth is suppressed. In the case of below eutectic temperature sintering, the other technique of sintering in low oxygen partial pressure is available to suppress abnormal grain growth. The formation of {111} twins is strongly connected with the interface structures; {111} twins are formed when the interfaces between $BaTiO_3$ and $Ba_6 Ti_17 O_40$ are faceted. Sintering under low oxygen partial pressure, set by varying the CO/$CO_2$ gas ratio, results in rough $BaTiO_3$/$Ba_6Ti_17O_40$ interfaces. When the hetero-interfaces are changed to rough, no more {111} twins can be formed. Under the oxygen partial pressure presented in this study($70CO-30CO_2$), all the interfaces of $BaTiO_3$/$Ba_6Ti_17O_40$ are rough even though the grain boundaries of BaTiO3 are almost faceted. The {111} twins are not formed under this atmosphere, and so the grain growth of $BaTiO_3$ is perfectly suppressed. The materials prepared by the present techniques exhibit uniform grain size and high dielectric properties. These techniques should provide opportunities for producing $BaTiO_3$-based materials with superior ferroelectric properties.
