INFLUENCE OF GRAIN-BOUNDARY, DEFECT AND INTERNAL-STRESS STATES ON PROPERTIES OF FERROELECTRIC MATERIALS

Performance characteristics in ferroelectric based components, sensors and thin films for memory and storage applications are significantly influenced by aging instability in these perovskite materials. The aging changes can be attributed both to dc electrochemical effects, where migration of ionic charges lead to degradation in permittivity, loss and resistance, and to changes in domain structure and mobility. This latter reflects the presence within the microstructure of defect states, diffuse grain boundary structures and internal stresses. Aging in these materials is also dependent on microstructure, impurities, dopants and stoichiometry, as well as on ambient temperature and applied dc field stresses. Significant improvements in aging and performance characteristics have been achieved with selected dopant additions to BaTiO3, such as ZrO2, which control grain morphology, resulting in structural modifications and decreased domain mobility. Permittivity has been found to be highly dependent on internal stress and this dependence has been mathematically modeled and related to the aging instability. Similarly, a model has been developed which explains the changes in grain morphology, resistivity, aging and PTC effects in terms of site occupancy of the dopants in the BaTiO3 lattice. Properties of ferroelectric films are also significantly influenced by structure, dopant incorporation and grain size, but understanding of these interrelationships is less advanced.