WZR as white lettering backed with rounded shapes in a bright orange. Where does the logo of WZR ceramic solutions come from?

This seemingly arbitrary pattern is a colour-changing representation of an aluminium oxide ceramic in the scanning electron microscope – at a magnification of more than 5,000 times. However, scanning electron microscopy, which was already developed in the first half of the last century, not only provides impressive images (Fig. 1), but is also very important for the characterization of ceramics.

Fig. 1: Gypsum crystals in the JSM-IT200 scanning electron microscope from the InTouchScopeTM scanning electron microscope series from JEOL with built-in EDX, which we have been using since December 2019. The imaging was done by backscattered electrons.
In the following we would like to give you an insight into chemical analysis at WZR and explain the importance of scanning electron microscopy in the field of testing and evaluation.
As with many things, it is the inner values that matter with ceramics. Therefore the investigation of the chemical composition is an important part of the characterization of ceramics. Knowledge of the overall chemistry, the distribution of the elements and the mineral phases to be found is particularly relevant in the development of new materials or in damage analysis.

The most commonly used method for determining the chemical composition of a material is X-ray fluorescence analysis (XRF). A statement about the mineral composition of a ceramic can be made by means of X-ray diffraction analysis (RBA, XRD).

Both methods are typically performed as bulk analyses at WZR: A homogeneous powder is produced from the sample material, which is then analysed. Sometimes it is sufficient to know the overall chemical composition or the mineral content of the ceramic sample to match expectations with reality. But what happens if the ceramic sample does not have the expected resilience?
There are many reasons for this: New mineral formations that bring about volume changes or have other physical properties, a heterogeneous distribution of chemical components, a different or perhaps even altered structure. Sometimes it is also an interaction of several factors that cannot be differentiated with a pure bulk analysis. Spatially resolved investigations are essential at this point.

This is where scanning electron microscopy has been used at WZR for many years. Unlike the well-known light microscope, this special microscope makes it possible to examine the microstructure of a material and its chemical composition by means of energy dispersive X-ray spectroscopy (EDX) at a magnification of up to 300,000 times. The principle of scanning electron microscopy is based on interactions between the sample material and the electron beam with which the sample is scanned.

The most important imaging interaction for us is the deflection of the electrons of the electron beam and the detection of so-called backscattered electrons (BSE). Depending on the mass of the atoms in the sample, the number of backscattered electrons varies and information about the chemical variation in a ceramic is obtained by differences in brightness in the image (Fig. 2). But also the microstructure of a ceramic, which for example indicates the degree of sintering, can be made visible by BSE and provide information about why the ceramic has – or no longer has – certain physical properties.

Another imaging function of the SEM is the detection of secondary electrons (SE), which image the topography of a sample and are therefore particularly important in identifying pore spaces.

Fig. 2: Backscattered electron image of zirconium mullite (exemplary red box), a mixed crystal of ZrSiO4, Al2O3 and SiO2, which is often used in the refractory industry in an alumina ceramic (green arrow)
The following illustrations show you a few highlights of SEM analysis at our company.

Backscatter electron image of a powder
False colour image of the element distribution of chromium (green), copper (red) and magnesium (dark blue) in a ceramic. The imaging is done by BSE and EDX.
Backscattered electron image of a sandstone (left). The false color image of the element distribution (right) allows to assign components of the rock to minerals. It is a predominantly quartz-containing (SiO2) sedimentary rock with feldspars ((K,Na)AlSi3O8) and a partially calcitic (calcareous) matrix (CaCO3)
In addition to the analyses mentioned above, there are other methods for phase identification and imaging that can provide important information about a ceramic.

One of these methods is Raman spectroscopy. This type of spectroscopy is a non-destructive method for the analysis of mineral phases. Of particular interest is the possibility of in-situ measurement of a sample over changing temperatures. This makes it possible to follow the sintering and chemical changes of the ceramic during the process. For this reason, we have recently been in contact with the Institute for Geosciences at the University of Bonn to test the possibilities of this method for the characterization of ceramics.

Chemical analysis and highlights of scanning electron microscopy at WZR