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ECCI

ECCI: Electron channeling contrast imaging
Definition:An imaging technique in scanning electron microscopy (→SEM) based on electron channelling applying a backscatter electron detector. It is used for direct observation of lattice defects
Explanation:Understanding ECCI is done best by looking into the physics of interaction of an electron beam with a crystal; when high energy primary beam electrons of a scanning electron microscope enter into a crystalline sample, they form a standing electron-density wave inside the lattice which is coherent with the crystal lattice, the so-called primary wave field. Depending on the direction of the primary electron beam with respect to the lattice, the maxima of the electron density waves may change from a position at the atomic nuclei to one in between them. In the former case, strong backscattering of electrons out of the primary wave field occurs, while in the latter case, only few electrons are backscattered. As a consequence, the backscatter signal carries information about the crystal lattice and its orientation relative to the primary beam. Minimum backscattering occurs when the primary beam almost exactly fullfills the Bragg angle with one of the lattice planes. The electrons then travel deep into the crystal without intense interaction with it; this case is called as electron channelling.

If a defect, e.g. a dislocation or a stacking fault (→SF) is present in the crystal then the coherency of the channelling primary electron wave field with the lattice is disturbed and strong backscattering occurs at the position of the defect. As a result, the defect is visible as a bright feature on a dark background when the sample is observed with a backscatter electron detector.

ECCI shows very comparable contrast features to dark-field transmission electron microscopy (TEM) with the advantage that the images are obtained on bulk samples rather than on thin foils. On the other hand, the resolution and contrast are not as good as in TEM.
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Diagram:

Individual dislocations and dense dislocation walls in a Fe 3% Si alloy, fatigued by 180 cycles of cyclic loading of 0.5 % of strain.
SFB-Link:ECCI is a very powerful characterization method for crystal defects on bulk materials. It complements electron backscattered diffraction (→EBSD, which does not allow direct defect observation) and replaces classical TEM to a certain extend.
References:S. Zaefferer, N.-N. Elhami: Theory and application of electron channelling contrast imaging, Acta Materialia 75 (2014) 20–50