Electrical Properties of Grain Boundaries in Titanate Ceramics

摘要

Doped titanate ceramics like strontium titanate (SrTiO_3) and barium titanate (BaTiO_3) are being used in a wild field of electronic applications. A decisive feature for their application in electronic devices like varistors, PTC thermistors, or multilayer capacitors are the electrical properties of the grain boundaries. The influence of these grain boundaries on the electrical characteristics of electroceramic devices was intensively studied within the last years. The results of these investigations are reviewed. First of all, a short literature review on the electrical properties of interfaces and grain boundaries in crystalline solids will be given. Then, the bascis of point defect chemistry of alkaline earth titanates are described as far as they are relevant for the electrical properties of grain boundaries. The microstructure of the grain boundary regions leads to special interface states which form electrically active barriers for the charge transport. Using a simulation model, it is possible to calculate the detailed spatial profiles of defect concentrations, space charge density, and partial conductivities along an axis perpendicular to the grain boundary. The ceramic microstructure is modelled by a simplified brick wall model which is required for an equivalent electrical circuit. This circuit is able to explain the observed overall electrical behaviour of electroceramics including the influence of the cores of the grains and the grain boundaries. Furthermore, the charge transport across grain boundaries in the small- and the large-signal-regime will be discussed. For the small-signal-regime, the detailed shape of the grain boundary conductivity profile controls the electrical transport behaviour. For materials with a small mean free path length like SrTiO_3 and BaTiO_3 and for the large-signal-regime, the diffusion theory explains the grain boundary cross transport.