(Dr. C. Bocker)
For the characterization of the micro structure of glasses and glass-ceramics, electron microscope images are very valuable. This method is based on the interaction of the electron beam with matter and enables advances analyses. In OSI, a transmission electron microscope (TEM) for replica and bulk samples (image and diffraction mode), a scanning electron microscope (SEM) as well as wavelength dispersive (WDS) and energy dispersive X-ray spectroscopy (EDS) are used. In dependence of the used method, spatial resolutions from several µm up to a few Å are possible.
Dr. Christian Bocker, scanning electron microscopy laboratory (SEM).
The limiting factor in the maximum resolution for each technique is the material of the sample under investigation itself and the preparation method. Chemical analysis by EDS and WDS are possible in the range of several µm in the SEM and a few nm in the TEM. The group of electron microscopy is integrated in many research activities of the Otto-Schott-Institut and hence the preparation technique has to be adapted or developed for each specific demand. The traditional imaging of fracture surfaces by replica technique in the mid range of resolution (≈10 nm) by TEM is used especially for the investigation of phase separation droplets in glass samples. In the case of phase separated glasses, the thinning of the bulk sample usually shows a very low or even no contrast. However, chemical statements are not possible using this indirect method. Furthermore the replica technique is going to be replaced more and more by modern SEM because also here the imaging of phase separations <100 nm sometimes is possible. The main application field of TEM in combination with EDS is the imaging and the characterization of micro and nano crystals in glass-ceramics by using micro analysis or nano diffraction mode.
a) 
b)
TEM replica image of phase separation droplets in a sodium silicate glass a) and a diffraction pattern of Si single crystal b) (images: C. Bocker).
A main topic in recent research is the characterization of crystal orientation in various glass-ceramics, e.g. fresnoite and cordierite crystal phases. This can be carried out by using the diffraction of the back scattered electrons (EBSD = electron backscatter diffraction). This method is well known in metal science but scarcely introduced into the glass and ceramics community.
Furthermore it is possible to study the crystallization in-situ up to 1200 °C via a heating stage using SEM. Also samples that are non conductive and cannot or should not be coated, can be investigated by SEM in a low vacuum chamber (10 to 270 Pa).
Orientation map of fresnoite glass-ceramics which were oriented crystallized (image: W. Wisniewski, M. Nagel).
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