Dielectric Properties of BaTiO₃-Based Ceramics under High Electric Field

The dielectric properties under a high electric field (ac-field) of BaTiO₃-based ceramics with core grains, shell grains and core-shell grains were compared with those of multilayer ceramic capacitors (MLCCs) with these three kinds of grains. The MLCCs with the X7R specification had a core-shell structure, and the relative dielectric permittivity (ε_r) of the dielectric layers in the MLCCs increased with increasing ac-field. Similar behavior was observed in the MLCCs consisting of only cores, indicating that the core predominantly determined the dielectric properties of MLCCs under high ac-fields. The dielectric properties of MLCCs and ceramic plates consisting of only shell grains showed that the shell was the relaxor ferroelectrics. A slight change in the shell composition yielded a large shift of the peak temperature of ε_r. The shell improved the temperature stability of ε_r at low temperatures under low ac-fields. In a ceramic plate with relatively large BaTiO₃ grains (approximately 3 µm), the maximum ε_r was observed at a moderate ac-field, which was explained from the electric displacement vs electric field hysteresis curves of ferroelectric BaTiO₃. The MLCCs and ceramics plates with fine BaTiO₃ grains (0.4 to 0.5 µm) showed similar dielectric behavior to the MLCC with the core-shell structure. The size effect of BaTiO₃ played an important role in determining the temperature stability of ε_r. For future MLCCs with very thin dielectric layers, a microstructure with fine BaTiO₃ grains and grain boundary layers of the shell was proposed.