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EMU Notes in Mineralogy - volume 16

Mineral reaction kinetics: Microstructures, textures, chemical and isotopic signatures
(W. Heinrich and R. Abart, editors)

Chapter 3. Scanning electron microscopy and electron backscatter diffraction
S. Zaefferer and G. Habler

Scanning electron microscopy (SEM) is one of the most frequently used techniques for near- surface characterization of most solid (and some liquid or liquid-containing) materials with a lateral resolution ranging between 1 nm (for surface-morphology observations) and 1 mm (for certain elemental-composition measurements). It works by scanning a finely focused electron beam over a surface and recording a variety of signals obtained from the electron-beam illuminated volume. Besides observation of the morphology of the surface of materials by recording of secondary electrons, SEM allows the measurement of elemental composition (e.g. by X-ray spectroscopy) and crystallographic nature (by backscattered electrons) of microscopic volumes beneath the surface. Even information on the chemical bonding (by Auger electrons) and on the electronic state (e.g. by cathodoluminescence) of microscopically small-volume elements can be obtained. Furthermore, direct observation of crystal lattice defects is possible.Many of the observations and measurements can be performed simultaneously, thus referring to the same position on a sample surface. Furthermore, materials may be modified during the measurements, e.g. by heating or mechanical loading, and changes can be observed either in situ or by means of interrupted tests.

Particularly powerful techniques for characterization of crystalline materials in SEM are electron backscatter diffraction (EBSD) and EBSD-based orientation microscopy. These techniques allow quantitative characterization of microstructures (i.e. defect arrangements) of bulk crystalline materials down to a lateral resolution of ~50 to 200 nm (depending on material and microscope conditions). The largely automated analysis of electron backscatter diffraction patterns (EBSPs) yields the crystallographic phase, orientation, defect density, and, potentially, elastic stress state of the illuminated crystal volume. Crystal orientation mapping (COM) is performed by scanning the electron beam over the sample and recording and analyzing a diffraction pattern from every point of the scan grid. The data obtained can then be plotted, for example, in the form of orientation, misorientation or phase maps of the scanned area. These maps reveal all kinds of morphological data such as grain size, grain shape, spatial distribution of phases and defects and much more. Besides this, the orientation data represent the texture of the investigated area. Together with EBSD scanning, further signals can be recorded, e.g. the elemental composition via energy-dispersive X-ray spectroscopy (EDX) or optoelectronic properties via cathodoluminescence (CL). This enhances the strength of the technique even further.

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