Near Atomic Scale Nanochemistry and Structure: Ceramic Grain Boundaries and Interfaces

Abstract

The local chemistry around microstructural features such as grain boundaries, heterophase interfaces, and inclusions in conventionally processed ceramics is of interest because of its effect on properties, such as the viscosity of thin amorphous phases (Urbain et al., 1981) and its effect on interface bond strength (Evans, 1990). Measurements of local chemistry of interest include qualitative and quantitative composition at these defects and along chemical gradients extending into the adjacent regions, and spectroscopic measurements that provide information about local bonding. Earlier measurements of the structure of these defects by TEM imaging methods by Clarke and Thomas (1977), Clarke (1979) and others showed that they are small, thus chemically sensitive analysis methods with very high spatial resolution are required. In addition these microstructural features are internal in ceramics, and often 2-dimensional (e.g., interfaces), so transmission electron microscopy methods with the feature of interest in edge-on orientation are required. Just as in imaging for defect analysis there is no lower limit on the resolution useful for these chemical analyses. The lower limits attainable are determined by the experimental method and equipment used. At present nanospectroscopy (either electron energy loss or energy dispersive x-ray) or energy selected imaging (using energy loss electrons) are the best choices which produce direct chemical sensitivity. High angle dark field scanning transmission imaging (more often but less descriptively called Z-contrast imaging) is the best indirectly chemically sensitive method for the measurements of interest. To achieve the required spatial resolution field emission sources are required for the small probe methods, which include both nanospectroscopies and Z-contrast imaging.