INTERFACE DEFECT CHEMISTRY OF OXIDE CERAMIC MATERIALS - UNRESOLVED PROBLEMS

Segregation causes changes of chemical composition of interfaces such as external surfaces and grain boundaries. The depth of the interface region, involving the segregation-induced chemical potential gradients, may reach hundreds of lattice layers. The interface region has a strong impact on properties of polycrystalline materials especially of fine grain ceramics. When the concentration of segregated species exceeds a certain critical value then a change in crystalline ordering occurs within the interface layer leading to the formation of low-dimensional interface structures. These structures may exhibit extraordinary properties not displayed in the bulk phase. The paper considers unresolved problems concerning the effect of segregation on processing of ceramic materials and their final properties. Segregation of both intrinsic and extrinsic defects is discussed for several binary metal oxides such as NiO, CoO, ZrO2, SnO2, TiO2 and Al2O3. Also ternary oxides, represented by BaTiO3, and quaternary oxides, represented by YBa2Cu3O7-x, are briefly considered.