Figure 2 – Images of metal particles on a fiber support. Light microscope (top left), SEM (top right), TEM (bottom left, McCrone Associates). High- angle annular dark-field image of metal particle acquired on the TEM operated in scanning transmission mode. Arrows indicate single atoms of metal (bottom right, University of Illinois at Chicago).
The TEM and the polarized light microscope again are more similar be- cause both take advantage of the fact that crystallographic information is present in the back focal plane of the objective lens. In the polarized light microscope, light propagated in different directions through an anisotropic crystal structure forms an interference figure when the crystal is viewed through crossed polarizers. A Bertrand lens inserted into the light path is used to bring the image of the interference figure into focus in the focal plane of the eyepiece lens. The symmetry of the in- terference figure identifies the crystal system and the orientation of the crystal.
In the TEM, no additional detector is required to view diffraction information. The push of a button changes the configuration of the micro- scope lenses to project the diffraction pattern present in the back focal plane of the objec- tive lens onto the viewing screen or camera where the image would be focused in imaging mode. This parallels use of the Bertrand lens in the polarized light microscope to move the interference figure from the back focal plane to a point where it can be observed.
The TEM pattern results from transmission of diffracted electrons through a thin specimen. In the TEM, Kikuchi patterns analogous to SEM EBSD patterns are used primarily for tilting the sample to the proper orientation to acquire high-resolution lattice images and other types of diffraction patterns that provide more detailed information. These include selected
AMERICAN LABORATORY • 25 • SEPTEMBER 2014
Figure 3 – Examples of crystallographic data. Light microscope interfer- ence figure from calcite (top left, McCrone Atlas of Microscopic Particles,
www.mccroneatlas.com). SEM EBSD pattern from nickel (top right, Oxford Instruments). TEM SAED and CBED patterns from silicon (bottom left and right, University of Illinois at Chicago).
area electron diffraction (SAED) patterns, from which atomic plane spacings (d-spacings) and angles can be measured for matching to those of known materials. When combined with EDS analysis in the TEM, SAED patterns can be used for crystalline phase identification. The highest spatial resolution
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