Effect of grain boundary structure on grain boundary migration and densification behavior in $BaTiO_3$

Abstract

Barium titanate is a typical ferroelectric material with a high dielectric constant, widely utilized to manufacture electronic components including multi-layer ceramic capacitors (MLCC). The ferroelectric properties of $BaTiO_3$ strongly depend on its microstructure, in particular, grain size and distribution. Understanding of grain growth and densification behavior should, therefore, be a prerequisite for the control of microstructure and properties of $BaTiO_3$. Recently, novel metal (Pt, Pd, etc.) electrodes of MLCC (multi-layer ceramic capacitor) has been replaced by base metal (Ni, Cu, etc.) electrodes and sintering under a reducing atmosphere becomes necessary to prevent oxidation of electrode materials. Understanding the effect of sintering atmosphere on grain growth and densification behavior should be of use in processing of $BaTiO_3$ based materials. It has been reported that the grain boundary structure of $BaTiO_3$ is faceted (atomically ordered) in oxidizing atmospheres, and is rounded (atomically rough) in reducing atmospheres. With changing atmosphere, the grain growth behavior also varies, abnormal grain growth (AGG) with and without {111} double twins, stagnant grain growth (SGG), and normal grain growth (NGG) depending on the migration behavior with respect to the boundary structure, similar to that of solid/liquid interface or solid/vapor interface. In the present thesis, several aspects on the effect of faceted boundaries have been studied in a $BaTiO_3$ model system. In chapter III, we studied the effect of oxygen partial pressure on the morphology of triple junctions between a large abnormal grain with a (111) plane and two adjacent grains. When $P_{O2}$ is high, (111) plane was retained and the two adjacent grains formed a T-shaped junction without a grain boundary glassy phase. A torque, $d\gamma/d\Phi$ on (111) plane is needed to retain (111) plane against the grain boundary tension between two adjacent small grains and the torque b...