Electronic imagers detection

EM instmments can be distinguished by the way the infonnation, i.e. the interacting electrons, is detected. Figure Bl.17.2 shows the typical situations for TEM, STEM, and SEM. For TEM the transmitted electron beam of the brightfield illumination is imaged simply as in an light microscope, using the objective and... 

To enable detection of fine mineral particles (<20pm),back-scattered electron imaging was used. Once the minerals were detected, EDS was used for analysis. Selected lignite particles were scanned to determine the distribution of minerals. Mineral types were then differentiated by variation in back scatter intensity and identified using EDS. The relative proportions (major, minor) and size and spatial distributions of the minerals were recorded. The overall surface of the polished section was viewed and massive minerals were analyzed and their distribution and size recorded. 

Fig. 1 Basic spectrometer layout. The source is extended and incoherent. Entrance and exit slits are at equal conjugates. The image is formed electronically after detection at the exit slit.

For studying supported catalysts, TEM is the commonly applied form of electron microscopy. Detection of supported particles is possible provided that there is sufficient contrast between particles and support. This may impede applications of TEM on supported oxides. Determination of particle sizes or of distributions therein rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. Semi in situ studies of catalysts are possible by coupling the instrument to an external reactor. After evacuation of the reactor, the catalyst can be transferred directly into the analysis position, without seeing air [12]. For reviews of TEM on catalysts we refer to refs. [11,12]. 

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

The contrast of a secondary electron image stems from local variations in the number of secondary electrons detected. Secondary electrons are electrons with low energies (< 50 eV). Consequently, their mean free path in the material is low (a few nm). When a sample is tilted at an angle 6 to the plane normal to the beam, the secondary electrons produced at the depth jr cross a smaller distance x cos 6 to the sample surface and are therefore less likely to be absorbed. The amount of secondary electrons emitted is minimal when the electron beam is perpendicular to the surface of the sample, and increases with the sample tilt (Fig. 7.2). The emission of secondary electrons increases very significantly at peaks or points as a greater number of electrons may, in this case, leave the surface all these sharp structures appear bright on a secondary electron image. 

Backscattered electron images arc used to describe the distribution of various phases present in a material and to detect chemical heterogeneities, for example, in order to describe the incorporation of a zeolite into an alumina, or describe the distribution and sizes of particles of a sintered metal (of dispersion less than 30%). 

Backscattered electron images where the contrast is chemical are of far more value to analysis because they can be used to detect rapidly local heterogeneities in a sample. The higher the mean atomic number, the brighter a phase will appear. 

The principle of the scanning transmission electron microscope (STEM) is, at first glance, very different from that of the transmission electron microscope the electrons are focused on a probe scanned on a sample and the transmitted electrons are detected on a scintillator via a collection aperture. There is, however, a so-called reciprocity relationship between transmission electron microscopy and the STEM that can be used to describe image formation using the same formalism and facilitates the understanding of contrast. 

The scanning electron microscope is based on a somewhat different principle than the transmission electron microscope. In the scanning electron microscope, the viewed image is formed by the secondary electrons emitted from the sample surface when an electron beam is scanned across this surface. These secondary electrons are detected by a suitable detector and counted. An image is then formed on a cathode ray tube in which the brightness of the raster spot is proportional to the number of electrons emitted at each point on the sample surface. In order to prevent charging of the surface as the electron beam is scanned across it, the surface is coated with a conductor such as gold. 

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