Fundamental studies of the inter-relationship between grain boundary properties and the macroscopic properties of polycrystalline materials. Final report, October 1991--December 1996

Abstract

The research performed under this grant has been principally devoted to understanding and quantifying the relationship between the macroscopic electrical transport properties of ZnO based materials and the properties of their grain boundaries. Two forms of polycrystalline ZnO have been extensively investigated, polycrystalline thin films, such as are used as optically transmitting, conducting electrodes and as piezoelectric films, and polycrystalline bulk forms, such as are widely used as surge arrestors. The former are essentially two-dimensional and the latter three-dimensional. The research has included both simulation and experimental studies. The simulation studies have been primarily addressing how the macroscopic properties of bulk ZnO ceramics are determined by the electrical and crystallographic properties of their grain boundaries. The behavior of varistors has been the focus since the highly nonlinear electrical characteristics provide an opportunity to test the models in much greater detail than is possible if the characteristics were simply ohmic. Furthermore, there is a continuing desire to improve varistor characteristics, such as the sharpness of the switching voltage and the degree of nonlinearity, so the effect of grain boundary variations on these parameters have been specifically addressed and found to quantitatively depend on the variation in both grain size and grain boundary barrier height. New methods of quantifying the effect of microstructural variations on the I-V characteristics have been introduced. The simulations have included both electrical network methods and effective medium methods. During the course of the research, the studies were extended to describe electrical breakdown, specifically on how microstructural variations lead to current localization which in turn leads to a form of electrical discharge failure, a common form of failure of varistors under electrical loading.