If there are more contributing crystallites, the diffraction image becomes a superposition of individual crystals' diffraction patterns. This pattern can be seen as a two-dimensional projection of reciprocal crystal lattice. The single-crystalline diffractogram depicts a regular pattern of bright spots. This allows to study small objects such as crystallites in polycrystalline material with a broad parallel beam.Ĭharacter of the resulting diffraction image depends on whether the beam is diffracted by one single crystal or by number of differently oriented crystallites for instance in a polycrystalline material. Therefore, the resulting diffraction pattern will only reflect the area selected by the aperture. The user can select the aperture of appropriate size and position it so that it only allows to pass the portion of beam corresponding to the selected area. It is a metallic sheet with several differently sized holes which can be inserted into the beam. For this purpose, there is a selected area aperture located below the sample holder. SAD is called "selected" because it allows the user to select the sample area from which the diffraction pattern will be acquired. Relation between spot and ring diffraction illustrated on 1 to 1000 grains of MgO using simulation engine of CrysTBox. Λ = h c e V ( 2 m 0 c 2 + e V ), is the distance between atoms (if only one row of atoms is assumed as in the illustration aside) or a distance between atomic planes parallel to the beam (in a real 3D atomic structure). Due to the wave–particle duality, the high-energetic electrons behave as waves with wavelength of a few thousandths of a nanometer. At these energies, even metallic samples are transparent for the electrons if the sample is thinned enough (typically less than 100 nm). In transmission electron microscope, a thin crystalline sample is illuminated by parallel beam of electrons accelerated to energy of hundreds of kiloelectron volts. Especially with appropriate analytical software, SAD patterns (SADP) can be used to determine crystal orientation, measure lattice constants or examine its defects. It is a specific case of electron diffraction used primarily in material science and solid state physics as one of the most common experimental techniques. Selected area (electron) diffraction (abbreviated as SAD or SAED), is a crystallographic experimental technique typically performed using a transmission electron microscope (TEM). SADP of a single austenite crystal in a piece of steel
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