Dielectric and strain effects of core-shell/barrier layer structures in modified barium titanate ceramics

Current advanced technologies which increasingly require high dielectric constant for miniaturization of electronics and strain properties for actuator applications has motivated many investigations of ferroelectric BaTiO ₃ (BT) ceramics. A new type of BT ceramic with core-shell matrix microstructure and barrier layer structure, therefore, was recently developed with simple one-step processing of BT ceramics, modified by isovalent ZrO₂ and donor Nd₂O₃ dopants. Isovalent ZrO₂ addition alone does not exhibit the barrier layer formation but improves the temperature stability of dielectric constants by forming core-shell grains. The barrier layer (BL) behavior, which consists of insulating oxide layer and a reduced interior, is developed by donor-Nd₂O₃ addition. This novel structure features high field-induced strain and excellent dielectric properties. A critical characteristic is the significantly different oxidation rate and related high internal stress produced, which can be controlled via annealing process.; The objective of this investigation, therefore, was to analyze this internal stress and to understand its influence on the properties.; Firstly, most important discovery made in this study is the macroscopic inhomogeneity in electrical behavior, observed across the cross-sections of donor doped BT ceramics in the form of an oxidized surface layer (insulating) and a reduced interior layer (semiconducting), i.e. a differential in oxidation across the sample cross-section. This BL structure was attributed to high dielectric permittivities.; Consequently the ceramic cross-section was further investigated by using XRD, micro-indentation technique and Raman spectroscopies to study the relationships between its unique structure and the internal stress. The presence of structural inhomogeneities, in terms of c/a ratio, microhardness and internal stress, was found and the configuration of internal stress was evaluated from a macroscopic viewpoint.; A microstructural model, combining core-shell/BL structures, was proposed to demonstrate and feature the uniqueness and the internal stress effect. Good agreement was obtained between the experimental strain data, verifying its validity in the internal stress-enhanced mechanism for the ultra-high strain. The effect of stress on field induced domain reorientation was confirmed by XRD analyses. That is, inhomogeneous distribution of the internal stress is critical to the high strains of BT ceramics.; This study has demonstrated that the BL structure, combined with core-shell microstructure, can be easily fabricated by a simple one-step heat treatment and exhibit high, temperature-stable dielectric permittivities as well as enhanced strains which can also be modulated by means of reoxidation process. (Abstract shortened by UMI.)...