Formation of Core/Shell structure and dielectric properties in rare earth-doped $BaTiO_3$ system

Abstract

For seven decades, barium titanate ($BaTiO_3$) has attracted much attraction from researchers both in academic and industrial circles since it was discovered during World War II in 1941. Because of its simple crystal structure, excellent chemical stability, and dielectric properties, it has been widely used for electronic devices such as PTC-thermistors, piezoelectric generators, and dielectric materials for multilayer ceramic ca-pacitors (MLCCs) so on. In the application of $BaTiO_3$ as dielectric materials of MLCCs, its sensitive temperature dependency of dielectric properties which obeys the Curie-Weiss law, is a critical drawback, hence, it is strongly required to be improved. Since the first report by Kahn, the formation of a core/shell structure which is formed during sintering in doped $BaTiO_3$ ceramics has been a great solution to the problem because of its significant contribution to enhancing dielectric-temperature stability. In the present study, we investigate the microstructural optimization of core/shell structure and its effect on dielectric properties. To be successful in the optimization of core/shell structure, the formation mechanism of core/shell structure has firstly to be understood. In chapter 3, the mechanism of core/shell formation during sintering in $BaTiO_3$-based systems was studied in (Mg, Y)-doped $BaTiO_3$. The effect of ball milling time on core size and shell thickness was first observed. The core size was similar irrespective of ball milling time while the shell thickness increased with increasing ball milling time. The measured powder size after ball milling suggested that the cores were from the larger $BaTiO_3$ particles and the shells formed via dissolution of smaller particles and precipitation of dissolved material, in contrast to the interpretation of the results of a previous investigation. To identify the core/shell formation mechanism, bi-layer samples with different chemical compositions, $94BaTiO_3$-$2Y_2O_3$-$...