Influence of the Milling Process on Microstructure and Electrical Properties for BaTiO₃-Based Ni-MLCC

Abstract

The influence of the process parameter in the milling process on core-shell microstructure and electrical properties of X7R type multi-layer ceramic capacitors with an Ni internal electrode (Ni-MLCC) was investigated in a BaTiO₃ (BT)-Ho-Mg system. Elaborate transmission electron microscopy (TEM) observation and statistical analysis for 100 grains for each MLCC sample fired at 1320°C revealed that microstructure was dependent on the degree of damage of BT particles caused by the milling process. The rate of frequency of grains without 90° domain pattern increased, and the volumetric ratio of the shell region increased as the damage increased. It was found that the concentration gradients of Ho were observed in the shell phase for all samples, and the concentration of Ho near the grain boundaries (GBs) became poorer as the damage increased, judging from chemical composition analysis by TEM with an energy dispersive X-ray spectroscope (EDS). Furthermore, the microstructural evolution had a fatal influence on the electrical properties of MLCC. The relative permittivity decreased and the broad peak around 40°C shifted to a higher temperature as the damage increased. The insulation resistance (IR) increased and the lifetime during the highly accelerated lifetime testing (HALT) was improved as the damage increased. Measurement of the impedance response at elevated temperatures revealed that the εr of the core phase decreased, however, the relative resistivity of core phase increased with increasing damage.