Dislocation contrast densities

Fig. 4.16 HREM image of carbon nanoshells showing that the basal planes roughly follow the curvature of the shell owing to the presence of a high density of dislocations. The image contrast of the basal planes was enhanced by use of fast Fourier transform (FFT) processing. After [60]...

In contrast to the thermodynamics of decomposition, where a few parameters permit the calculation of the equilibrium properties of the system, the determination of decomposition rates is largely an experimental problem, i.e., there are no standard kinetic data from which these rates can be calculated. This is particularly true for the decomposition reactions of solids which are topochem-ical," i.e., where the rate depends on structural factors. One reason for this situation is that it does not yet seem possible to prepare duplicate samples of any solid inorganic salt that are identical in all the properties that may determine the rate of decomposition, e.g., the dislocation density of the crystals. 

FIGURE 9 A crystallite formed on the GaN surface. Note wide band of darker contrast in the subsurface area indicating a high density of small dislocation loops (DL) formed on this side of the crystal. 

More recently Mackwell, Kohlstedt, and Paterson (1985) studied the deformation of single crystals of San Carlos (Arizona) olivine deformed under hydrous conditions at 1,300 C, 300 MPa confining pressure, and 10 s strain-rate and found they were a factor of 1.5-2 weaker than those deformed in an anhydrous environment. TEM observations showed that specimens deformed under dry conditions, in an orientation such that the slip systems (001)[100] and (100)[001] would be activated, were characterized by a microstructure of generally curved dislocations and dislocation loops, but no organization into walls. The dislocation density was 10 -10 cm compared with an initial value of < 10 cm . Most of the dislocations and the loops lie approximately in the (010) plane because they are in contrast for g = 004, they probably have b = [001] dislocations with b = [010] and [100] would be out-of-contrast for this reflection. However, the slip system (010) [001] is not expected to be active. It is not clear, therefore, if these dislocations are actually involved in the deformation. The general geometry of the dislocation microstructure is not inconsistent with some climb mobility in fact, on the basis of the observations of Phakey et al. (1972), climb is certainly expected at 1,300°C. 

This equation shows that in contrast to the steady state current density I ss (c.f. eq. (5.28)) the current density transient depends on the screw dislocation density, Ndis /A. 

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

---