Correlation between tetragonality (c/a) and direct current (dc) bias characteristics of BaTiO3-based multi-layer ceramic capacitors (MLCC)†
The influence of tetragonality (c/a) of the dielectrics of BaTiO3-based multi-layer ceramic capacitors (MLCC) on the direct current (dc) bias characteristics was investigated. The tetragonality can be reduced by decreasing the grain size, and it can be further decreased for the same grain size condition by increasing the measurement temperature or the A/B ratio in the ABO3 structure. Fine grain samples showed higher dielectric constants than those of coarse grain samples under a high dc-bias field over the entire temperature range below Curie temperature (TC), and they became similar to each other above TC. Whereas the dielectric constants under a high dc-bias field increase slightly with the increase of temperature for the fine grain samples, those of coarse grain samples systematically increase, of which the tetragonality is also supposed to change more distinctly. In fine grain samples, the dielectric constants under a high dc-bias field can be further enhanced by increasing the A/B ratio in ABO3 structures through the excess Ba addition that results in a decrease of tetragonality. All these results demonstrate that the reduction of tetragonality increases the intrinsic dielectric constants along the c-axis direction, and thus is a crucial factor for the enhancement of dielectric constant under a high dc-bias field condition where the domain alignment with the c-axis parallel to the dc-bias field occurs.