Field scanning, particle size analysis

Appropriate particle size analysis methods with very low shear stress (free settling with particle image velocity, field emission scanning electron microscopy, fluidized bed or vibrating sieve feeding with laser diffraction) have been tested for their reproducibility and minimal dispersion energy. 

Microdiffraction.—Perhaps more important than SAD techniques, particularly in the context of catalyst research, microdiffraction allows the user to benefit from the small probe size generated in STEM in the structural analysis of small particles and localized areas in thin foils. If the small probe is stopped on a particle, then clearly a transmission diffraction pattern will be observable after the beam has traversed the sample, provided we have the means available for its display. In CTEM such a pattern will, of course, be formed by the imaging system in a manner identical to SAD, but in STEM the pattern must be scanned across the detector. This is accomplished by means of a set of post-specimen scan coils which once more scan the diffracted beams across the axial bright-field detector. Such a pattern is shown in Figure 13 where a beam of approximately 10 A FWHM was stopped on a small second-phase particle during the omega-phase transformation in a Zr-Nb alloy. The relatively poor definition of the reflection is a consequence of both the convergent nature in the probe (necessary in order to obtain the smallest probe sizes) and a S/N limited by the available current in the probe. Nevertheless, weak reflections with half-order indices are clearly visible between the main alloy reflections and it is therefore possible to attempt structural... 

With modern scanning electron microscopes many of the restrictions of the transmission electron microscope have been alleviated. Firstly, thin samples are no longer required. With instruments equipped with a field-emission gun, magnifications as low as 50X and as high as lOOOOOX can be achieved routinely. Imaging the back-scattered electrons gives the distribution of the heavy elements at a high resolution, whereas the secondary electrons are indicative of the shape and the size of the solid particles present in the specimen. Analysis of the emitted X-rays can indicate the elemental composition. 

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