11 - Interface mass transport in oxide materials

Abstract

Publisher Summary The life of a functional material (fabrication, service, and degradation) involves several diffusion-related processes, in which interfaces play a key role, as they spawn additional equilibria and diffusion paths. The knowledge of the defect structures and diffusion effects of grain boundaries are necessary to optimize the properties of ceramics and, furthermore, of the nanocrystalline oxides. This chapter describes a novel approach to characterize defects induced by impurities by calling for Positron Annihilation Lifetime Spectroscopy (PALS). It highlights the present state-of-the-art concerning grain boundary diffusion in oxides, refers to the available literature, as a comprehensive basis, and focuses on some relevant aspects encountered during the research in this field that shed light on the complexity of oxides compared to metals. PALS is sensitive to vacancy-like defects in material. In oxide ceramics, the dissolution of aliovalent cations requires that charged point defects be created in order to maintain electric neutrality. In alumina, these extrinsic defects dominate, by far, the intrinsic ones at any temperature. Positively charged defects repel positrons whereas the neutral or negatively charged ones can act as positron traps. In practice, the materials contain various impurities greater or lesser in valence than the host cation. Hence, interactions between extrinsic defects are expected during the elaboration process as well as during the material use.