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To illustrate the information depth one can gain with that technique, the aforementioned MAS glass ceramics will be selected. In this sample, precipitation of zirconia and spinel is known from X‐ray diffraction (XRD) experiments, but this technique is inappropriate for investigating the micro‐ and nanostructure because it gives information about the sample as a whole, with a low spatial resolution defined by the X‐ray spot size. Techniques such as SEM or TEM must be used instead, and they can in addition yield the composition of the residual glass and the structure of the crystals – in this case whether zirconia is the tetragonal, cubic, or monoclinic polymorph.

For a piece of this MAS sample, polished and rendered conductive by deposition of a ≈3–5 nm carbon film on its surface, an SEM micrograph (SE) taken with an acceleration voltage of 10 kV clearly shows bright, elongated structures and areas with brighter and darker appearance (Figure 2a). A comparison with an SEM micrograph acquired at a 30 kV acceleration voltage (Figure 2b) illustrates the loss of both resolution and contrast when the energy of the primary electrons increases and results in deeper electron penetration in the bulk. In turn, the effect makes the origin of the image‐forming SE less predictable, so fine details are smeared out at the surface of the sample. At a higher magnification (Figure 2c), the microstructure observed with a 10 kV acceleration voltage from backscattered primary electrons (BSE) shows the dendritic appearance of the brightest, elongated features that indicate their crystalline nature. Resulting from strong scattering of the primary electrons, the marked contrast of these features with their surrounding suggests the presence of heavy elements (and associated high backscattering yield). Given that zirconia is present in the sample and that the other heavy element besides zirconium, i.e. Y, is not expected to enter another crystalline phase in this material, one can conclude that the bright dendritic microstructure is likely made up of zirconia. Around the dendrites, the brighter and darker areas should then be the images of the spinel and the residual glass, respectively.

Figure 2 Scanning electron micrographs of the MAS glass‐ceramic sample: resolution differences between 10 kV (a) and 30 kV (b) acceleration voltages and BSE micrograph taken at 10 kV at higher magnification (c) showing the bright (1) and dark (2) areas for which the EDX spectra of (d) were acquired.

To check this conclusion, EDXS analyses were made in the two areas indicated in Figure 2c. The spectra are presented in Figure 2d. In both, a small carbon peak is an artifact of the sample preparation. Besides the most intense oxygen and silicon peaks, the other peaks are readily assigned to the other elements, but a clear separation of the Y‐L_β and Zr‐L_α peaks is not possible owing to the limited energy resolution of the EDX detector system. The better separated Y‐K (at ≈14.9 keV) and Zr‐K lines (at ≈15.7 keV) would be more suitable, but a higher acceleration voltage would be needed to access that higher‐energy part of the EDX spectrum with its ensuing loss of image resolution.

Nevertheless, these EDX spectra suggest that the brighter areas likely are silica rich, most probably depicting the residual Y‐bearing glass. Conversely, the darker areas should consist of spinel, which is Si‐ and Y‐free, as confirmed by analyses made in the “darker” area of the sample where O, Al, and Mg are found with a higher Al/Si ratio than in the other spectrum. The reason why certain amounts of Si and also of Zr (but not of Y) are nonetheless detected is that a certain interaction volume is also excited below the surface within the sample (cf. Figure 1). As a result, the lateral resolution is worsened, and the probed area does not include only the “dark” region 1 that is visible in Figure 2c, yet also parts of the sample bulk below. In conclusion, like any technique, SEM and EDXS have their own limitations. Other methods having a higher resolving power must be used to overcome them.