Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
topography (left) and hardness (right) contrasts between spinel and the glass matrix of the MAS sample in AFM micrographs. (a) Unetched, polished sample and (b) superficially etched sample. Hardness contrast derived from a mapping of the cantilever amplitude damping."/>
Figure 10 Topography (left) and hardness (right) contrasts between spinel and the glass matrix of the MAS sample in AFM micrographs. (a) Unetched, polished sample and (b) superficially etched sample. Hardness contrast derived from a mapping of the cantilever amplitude damping.
Because different phases are not attacked by acid solutions at the same rate, etching of glass‐ceramic surfaces by HF or HF–HNO3 solutions can enhance the microstructure contrast in AFM observations. In the MAS sample, the SiO2‐rich glass is, for instance, more prone to dissolution upon HF etching than spinel and zirconia, so the dendritic‐ or “finger‐like” shape of the spinel growth front is more clearly seen (Figure 10b). In contrast, the large residual glassy areas that are much less etched have a different composition. Indeed, from the EDXS results described in Section 3.2, it is known that these residual glassy areas are strongly enriched in Y.
5 X‐Ray Microscopy
As applied to glasses and glass ceramics, the aforementioned techniques give detailed insights into microstructures and even nanostructures. However, some difficulties remain to be faced. Like electron microscopy or scanning probe microscopy, imaging techniques usually require some sort of (often demanding) sample preparation. If preparation artifacts are avoided, one ends up with a wealth of information about morphology, chemistry, and crystallography for a sample volume of only a few 100 nm3. As stated in the Introduction, it is, therefore, highly desirable either to complement such detailed investigation down to the sub‐nanometer scale with more integrating techniques or to find ways to assess larger volumes, even at the risk of losing spatial resolution.
In this respect, X‐ray microscopy (XRM) has emerged as a new kind of imaging technique that is both nondestructive and capable of offering a three‐dimensional (3‐D) description of the sample bulk. With a maximum 3‐D spatial resolution of 50 nm, it cannot compete with TEM. But sample preparation (for example, with laser micromachining) is rather straightforward, so XRM gives quick answers in the form of a 3‐D density mapping of a reasonably large piece of specimen that may represent the sample bulk.
The technique is based on shadow casting of X‐rays stemming from a point source and going through the sample volume to end up onto a CCD camera. It is similar to X‐ray computed tomography, from which it differs by the fact that the X‐rays are focused within the sample, rather than the sample being illuminated as a whole, and that the signal is further magnified optically on the screen. Because this magnification is made from the central part of the sample, image aberrations are minimized, and enhanced X‐ray optics (derived from synchrotron beamlines) help to achieve an unprecedented resolution. But as with X‐ray computed tomography, the acquisition of series of images for a rotating sample, followed by image processing of the resulting datasets, can yield 3‐D pictures.
Figure 11 Three‐dimensional (3‐D) representation of X‐ray microscopy data from a truncated cone that was laser micromachined from the MAS glass ceramic. (a) Entire dataset with denser volumes in brighter contrast (green in the false color version) and lighter volumes in darker contrast (blue in the false color version). (b) 3‐D visualization of just the high‐density regions of the sample, taken from a clipped internal volume of (a) to make segmentation easier. Volume fractions: dense inclusions: 23%, lower‐density material: 77% (see electronic version for color figures).
For the MAS sample repeatedly considered here, such a 3‐D picture clearly shows how the high‐ and low‐density components are interconnected (Figure 11), the former and latter likely being the Y‐bearing glass and the spinel, respectively. Owing to a lack of elemental and spatial resolution, however, yttria in the residual glass and the zirconia needles cannot be discerned.
6 Perspectives
This review of microstructural techniques is not comprehensive. In addition to the variety of spectroscopy techniques such as IR, Raman, XPS, or micro‐X‐ray fluorescence (Chapters 2.2 and 5.1), which also possess certain spatial‐resolution capabilities, electron backscatter diffraction, transmission Kikuchi diffraction for texture analyses, or time‐of‐flight secondary ion mass spectrometry can also be used to investigate the property–structure relationships in glass‐based materials down to a sub‐nanometer scale. Because an important feature shared by all of these techniques is their ever‐improving resolution and analytical precision, one can expect a better understanding of the way in which valuable mechanical, thermal, optical, or electronic properties are achieved in inhomogeneous materials. Conversely, these observations should help to design and tailor new materials for specific applications. Of particular interest is a better understanding of crystallization processes, whereby the nucleating phases not only differ generally in chemical composition from the starting glass but can be nonstoichiometric and highly disordered. In this respect, more detailed observations thus should serve as a basis for theoretical investigations in the fascinating field of the coordination and valence changes that are taking place during microstructure evolution. And one can even expect that further instrumental advances will make it possible to check material properties online during glass and glass‐ceramic fabrication and figure out at once causes for production failure, such as surface devitrification.
Acknowledgments
We thank Dr. M. Krause and M. Menzel (Fraunhofer IMWS) for performing SEM and AFM experiments, respectively; A. Böbenroth and B. Mühs‐Portius (Fraunhofer IMWS) for AFM (etching) and TEM sample preparation, respectively; and Drs. S. Kelly and B. Hornberger (Carl Zeiss Microscopy) for providing us with 3‐D XRM data. We also thank Dr. A. Gawronski (Friedrich Schiller University Jena) for making of the MAS glass and glass‐ceramic samples. Further, financial support by the German Science Foundation (DFG) by means of various grants is gratefully acknowledged.
References
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4 4 Berndt, S., Gawronski, A., Patzig, C. et al. (2015). Oriented crystallization of a beta‐quartz solid solution from a MgO/Al2O3/SiO2 glass in contact with tetragonal ZrO2 ceramics. RSC Adv. 5: 15164–15171.
5 5 Patzig, C., Höche, T., Wu, Y.F. et al. (2014). Zr coordination change during crystallization of MgO‐Al2O3‐SiO2‐ZrO2 glass ceramic. J. Non‐Cryst. Solids 384: 47–54.
6 6 Patzig, C., Dittmer, M., Gawronski,