Fresnel contrast analysis of composition changes and space charge at grain boundaries in mullite

Grain boundaries in two mullite specimens have been characterised using Fresnel contrast analysis in the transmission electron microscope. A ratio of the scattering potential differences ΔV between the grains and the grain boundaries in the two specimens was obtained without needing to know the absolute specimen thicknesses of the regions examined. The characterisation of a glassy intergranular film in one specimen using energy dispersive X-ray spectroscopy then allowed the magnitude of ΔV at a crystalline boundary in the second specimen to be determined. For the first time, this scattering potential difference was interpreted in terms of contributions to the contrast from space-charge layers as well as changes in electron scattering factor and density.