Atomic images, grain boundaries

Figure 1.2. FIM image of a tungsten specimen with (110) centre pole A grain boundary runs between B-B, and a single atom is apparent on the plane below the mark X. (Courtesy T.J. Godfrey.)...

These forces are the result of elastic stress fields that. exist near every impurity ion or aggregate and crystal imperfection like a dislocation line or grain boundary. These forces are very strong and are mainly responsible for the creation of second phase impurity aggregates in a host of ionic crystals. If the latent image is considered as a second phase formation of Ag° atoms in the silver halide crystal, then it seems that the elastic forces are those that cause the formation of this Ag aggregate. 

Fig. 10 High-resolution Z-contrast imaging. Z-contrast image of a grain boundary in SrTi03 (perovskite structure) recorded in 0 0 1 direction. One of the unit cells framed in the micrograph is illustrated on the left, the Sr columns (bright) are at the corner of the unit cell, in the center there is a TiO column. The pure oxygen columns, black in the model, are not observable in the Z-contrast image. The atomic number (Z) contrast is apparent with increasing atomic number (Z) of the elements, the intensity increases. (View this art in color at WWW. dekker. com.)...

Fig. 4.23. A plan-view HREM image of 45° ale grain boimdary. The grain boundary is found to have atomically sharp interfaces over 30 pm and the distortion due to mismatch is localized to within about one atomic layer. The inset shows a schematic configuration of the needle-like u-axis grain.

Fig. 10.10. (a) HREM image of one of the dislocation cores in Fig. 10.9. (b) Dislocation core model for [001] tilt grain boundary in YBCO. HREM simulations were performed for two models, (c) Model 1 HREM simulation assumes that the grain boundary core chemical composition is identical to bulk, (d) For the model 2 simulations the Y-Ba atomic columns were replaced by Cu. This model image closely resembles the observed HREM image (after [10.65]). 

The high-Z atomic column positions at a grain boundaries can therefore be determined directly from the image during the experiment. However, on completion of the microscopy, a more accurate statistical method to obtain the column positions is to recover the Z-contrast object function through maximum entropy image analysis [11.12]. Maximum entropy image analysis is based on... 

Crucial in the use of EELS to study carrier concentrations at grain boundaries in high materials is the ability to position the probe with atomic precision a precision only afforded by the Z-contrast image. In order to achieve with EELS the same atomic resolution as the image, the range over... 

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