Orientation relationships between phases. Crystal symmetry and crystal space group determinations (Figure 4). The identification of phases and crystal structure types (Figure 3). The electron diffraction patterns can be used for: By varying the condenser apertures of the TEM, different types of diffraction patterns can be produced to provide different information. The patterns produced will vary with different zones and for different crystal structures. The diffraction patterns are typically taken when the sample is tilted so that the electron beam is passing down a particular crystallographic zone (direction in the crystal), with each diffracted spot in the pattern representing a plane which passes through that zone. Each spot represents a plane in the crystal that has diffracted. The pattern is a scaled representation of a section of the reciprocal lattice. Produced from diffraction of the electron beam off of planes in the sample.
Electron diffraction, however, is used to investigate very small regions in the sample and does not provide an overall "view" of the material as in XRD. This is why in XRD methods such as the Lau Method (using white radiation-multiple wavelengths), Rotating Crystal Method (Ewalds sphere remains fixed but reciprocal lattice rotates) and the Powder Method (many randomly oriented crystals) were developed.Įlectron diffraction is very sensitive to changes in the crystal structure, such as small degrees of short range ordering in the material that cannot be detectable through XRD. Whereas in XRD, it can very difficult to produce diffraction from a perfect single crystal. Ewalds Sphere construction for electron and X-ray radiation.īecause the radius of the Ewalds sphere (1/ l ) of the electron beam is large in comparison to the reciprocal lattice, (along with beam convergence and small variations in l ) you are assured to produce diffraction off multiple planes. If the material of interest has a lattice parameter of a o = 3.6Å, a*=0.278Å -1įigure 2. So you are only observing a very small portion of the material. The material must be less than 200 nm thick in order to pass the electron beam through. Limitation of TEM: Sample size and preparation. compositional analysis of individual phases imaging atomic planes and defects in packing associated with dislocations or interfaces determining the growth directions of precipitates or lamella in the material and the type of interfaces between different phases (i.e. determining site occupancy preferences of the atoms in the crystal structures characterizing and identifying defects (antiphase boundaries, dislocations, stacking faults) in the crystal structure and to determine modes of deformation (identifying the slip systems of a material or mode of failure) identifying the phases and crystal structures present in the material composition analysis of individual phases imaging regions or phases of different chemical composition surface topography and analysis of fracture surfaces This type of x-ray production in the sample is the same principle as that of the generation of x-rays used in x-ray diffraction instruments.įigure 1. Detectors in both SEM and TEM instruments collect the characteristic x-rays that are generated from the sample to allow for compositional analysis of the material. In a TEM, the electron beam passes through the sample and produces an image using the transmitted electron beam which contains both diffracted and unscattered electrons. A SEM uses the electrons that are scattered off of the sample surface to produce images of the sample. The electron microscopes also use the many interactions that the electron beam has on the sample (Figure 1) to produce various imaging and diffraction modes that can be used to analyze the material down to the atomic scale. Electron microscopes, such as the scanning electron microscope (SEM) and transmission electron microscope (TEM) are instruments used in the analysis of materials on a scale much smaller than possible by optical microscopy.
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