Hollow dislocation

A crystal grown in a solvent medium may have hollow dislocations filled with solvent, since the crystal-solvent interfacial energy may be substantially lower than y. For metals, an unusual effect may occur (37). The chemisorption of certain gases onto some metals leads to an appreciable reduction in the value of y. For example (37), y for silver is decreased from 1140 erg cm 2 in vacuo to 450 erg cm 2 ir. Consequently, a dislocation which terminates flush with the surface in vacuo may, at equilibrium in the presence of an adsorbed gas, develop a pit. By the same token, emergent dislocations in tilt and twist boundaries would develop arrays of pits in the surface. 

Figure 5.12. TMAFM image of a thin film (thickness 1 am) of a- 9-NPNN on NaCl(lOO) 250 nm x 250 nm. Reprinted from Surface Science, Vol. 415, J. Fraxedas, J. Caro, A. Figueras, R Gorostiza and F. Sanz, Dislocation hollow cores observed on surfaces of molecular organic thin films p-nitrophenyl nitronyl nitroxide radical, 241-250, Copyright (1998), with permission from Elsevier.

As mentioned before and assuming the vahdity of the continuum elasticity theory at the dislocation core, F. C. Frank derived the expression for the characteristic radius of a hollow core (Frank, 1951) ... 

That a hollow core is formed by the creation of a free surface along a dislocation core implies that the curvature of the spiral step is reversed due to the strain field along the dislocation core. The effect of a strain field upon the advancement of a step was theoretically treated by Cabrera and Levine [14], [15],... 

Only a few crystals exhibit hollow cores at the centers of growth spiral layers. However, on the (0001) faces of SiC, which has a large fx value, hollow cores due to growth have often been observed. According to the summary by Sunagawa and Bennema [16], various degrees of the effect of the strain associated with dislocation cores have been observed depending on the sizes of b and the concentration of dislocations. 

In the case of growth spirals originating from dislocations with large b, hollow cores with diameters of micrometer order are observed at the spiral center however, when a number of dislocations with small b concentrate in a narrow area, a basin-like depression appears at the central area of the composite spirals, since the curvature of advancement of the spiral steps is reversed near the center. A straight step may appear near the spiral center as an intermediate state in the reversal of step curvature. Several examples are shown in Fig. 5.11. 

Hollow cores associated with dislocations may be formed in both growth and etching processes. 

The possibility that the core of a dislocation could be empty was first recognised by Frank [1], If the strain energy density arising from a dislocation is sufficiently large, it may become energetically favourable to remove the material near the core and place it in an unstrained environment far from any dislocations. This process creates additional surface area around the core of the dislocation. The equilibrium radius of the hollow core of a screw dislocation is given by... 

These results are the first dynamic and in situ observations of cores at the center of dislocations characterized by small step spacings. The appearance of hollow cores on the dissolving surface is diagnostic of dissolution occurring with a moderate interfacial undersaturation (57,59,61-63), with the surface kinetics at least partly rate-limiting. Hollow cores were not observed for dissolution under diffusion-controlled conditions (Fig. 37), since the interfacial concentrations of K+ and Br were maintained at a level very close to the saturated value (8). 

Frank predicted the existence of coreless dislocations long before they were unambiguously observed. The idea is that the surface energy associated with a hollow tube may be less than the strain energy associated with a large Burgers vector. Of course, you have to form this large-b dislocation first. Observations of coreless dislocations have been... 

The slip of the majority of dislocations is concentrated in strained metals in slip bands. Figure 17.3 iQustrates the clearly-marked formation of surface steps as a result of a material strain. The hollow initiation on the surface is also seen. Surface microscopic cracks start easily in slip bands. 

The cavities were extensively investigated and no traces of impurities were found to be present. Instead the oxidation hollow has the characteristic corkscrew like shape of a screw dislocation as can be seen from Figure 31. The pits tend to have a vaguely hexagonal shape. 

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