Grain boundaries in dielectric and mixed-conducting ceramics

Presented are the physics and chemistry of an important class of grain boundary types in oxide ceramics, which is characterized by a depletion of the major charge carrier in an area around the grain boundary core. Experimental results obtained in the time domain and in the frequency domain are shown for acceptor-doped titanates as the model material. The space charge layer width as well as the electronic and ionic charge transport across the grain boundary barrier, as a function of all relevant parameters, are taken from the results of the impedance analysis. A comprehensive and quantitative model is established which explains the experimental findings on the basis of the point defect structure and the Poisson equation. Depending on the height of the potential barrier at the grain boundary, inversion layers close to the grain boundary occur. Under long-term d.c. voltage stress, a macroscopic de-mixing of ionic defects takes place in these materials which lead to resistance degradation effects. The impact of grain boundaries on these phenomena and on the chemical diffusion processes is described.