Interfacial Segregation in Perovskites and Applications to Boundary Layer Devices

152 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.Interfacial segregation is a term used to describe the compositional heterogeneity that exists between the bulk and interface in materials due to interfacial effects which are distinct from bulk effects. Segregation in solids is of great theoretical importance, and also plays an active role in modifying the properties of many materials. For example, surface segregation in single crystals may change the surface scattering of carriers, and electron emission characteristics. In polycrystalline materials, segregation of defects to grain boundaries may affect corrosion resistance, sintering kinetics, microstructure development and structure-property relations, including dielectric and electric properties. In addition to modifying the properties of materials, controlled segregation may also be made use of for fabricating new boundary layer devices such as, Positive Temperature Coefficient of Resistance (PTCR) Thermistors, internal Boundary Layer Capacitors (IBLC) and Voltage Dependent Resistors (VDR).In this thesis a thermodynamic model was proposed to explain the equilibrium segregation in perovskite materials, and was developed by considering strain energy and electrostatic interactions as the major driving forces for segregation behaviour. The calculated values of thickness for segregation layers were in good agreement with experimental data obtained by Scanning Auger Electron Spectroscopy (SAES), and variations in interfacial compositions were explained in terms of the proposed theory. Direct evidence for impurity segregation was obtained by STEM and SAES analysis; and indirect evidence by lattice parameter and grain size measurements. The charging problem in the Auger analysis of insulators was minimized by a technique based upon current balance. A model based upon heterogeneous defect structure was proposed to explain interrelationships that exist between segregation behaviour-microstructure-electrical properties and processing conditions, and was successfully applied to donor doped BaTiO(,3) and extended to optimize the processing conditions for IBLC and PTCR components.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD