The role of Co in the BaTiO₃-Na_0.5Bi_0.5TiO₃ based X9R ceramics

Ceramics in BaTiO₃-Na_0.5Bi_0.5TiO₃-Nb₂O₅-Co₃O₄ (BT-NBT-Nb-Co) system were fabricated to satisfy the EIA-X9R specification (-55 ∼ 200 °C, ΔC/C ≤ ± 15%) of multilayer ceramic capacitors. The role of Co in this system was investigated. The core-shell structure was found to exist in the system, as confirmed by transmission electron microscopy (TEM), accounting for the high dielectric temperature stability. The Co can regulate the reaction between Nb and BT-NBT to adjust the core-shell structure, where the optimum dielectric properties were achieved at Nb/Co = 3, with dielectric constant being 930 at room temperature and capacitance variation being less than 15% in the temperature range from - 55 °C to 200 °C. The impedance spectroscopy was fitted by 3RC equivalent circuit, corresponding to the grain, grain boundary and ceramic/electrode interface region for the Co modified BT-NBT system. The conduction activation energy of grain was found to decrease with increasing Co concentration, where the conduction mechanism of grain and grain boundary are both sensitive to the Co concentration.