Dislocations kinks

At smooth metal electrodes that have been subjected to annealing, the number of different crystallographic defects (dislocations, kinks, etc.) emerging at the surface is between 10 and lO cm. This number is small relative to the total number of surface atoms (which is on the order of 10 cm ). In the literature, attempts have been described to determine the catalytic activity of electrodes having an artihcially boosted number of surface defects. These experiments gave no unambiguous results in some cases some increase, in other cases some decrease in activity was observed. 

Since indentation hardness is determined by plastic deformation which is determined in turn by dislocation kink mobility, hardness is expected to be proportional to the bond modulus. Figure 5.2 shows that indeed it is for the Group IV elements, and the associated isoelectronic III-V compounds. 

When normal sites in a crystal structure are replaced by impurity atoms, or vacancies, or interstitial atoms, the local electronic structure is disturbed and local electronic states are introduced. Now when a dislocation kink moves into such a site, its energy changes, not by a minute amount but by some significant amount. The resistance to further motion is best described as an increase in the local viscosity coefficient, remembering that plastic deformation is time dependent. A viscosity coefficient, q relates a rate d8/dt with a stress, x ... 

B-B distance is 1.746 A. In pure B,it is 1.75 A. Therefore, covalent B-B bonds may be expected. During the complex deformation in an indentation, these strong bonds must be broken. They are the principal barriers to dislocation kink motion in the diborides. 

Nunes R. W., Bennetto J. and Vanderbilt D., Atomic Structure of Dislocation Kinks in Silicon,... 

All solid surfaces exhibit structural features that can have significant effects on the kinetics of charge transfer reactions and on the stability of the interfacial region. In the case of metals, the most significant structural features for "smooth" surfaces are emergent dislocations, kink sites, steps, and ledges. It has long been known, for example, that the kinetics of some electrodissolution and electrodeposition reactions depend on the density of such sites at the surface, but the exact mechanisms by which the effects occur have not been established. The role of "adion" in these processes is also unclear, as is the sequence of the dehydration-electronation-adsorption-diffusion-incorporation processes, even for the simplest of metals. 

The standard interpretation of the KirkendaU effect micromechanism, for substitutional alloys at least, is this the partial fluxes difference causes a vacancy flux toward the more mobile component of the diffusion couple. Vacancies dismantle extra planes on this component s side, ft means dislocations cHmb and vacancy annihilation at sinks (dislocation kinks). On the slow component s side vacancies are, conversely, generated (to support the flux) bringing to extra planes building up. Thus, on the slow component s side, new lattice planes appear and, on the side of a mobile one, the old lattice planes disappear. New lattice formation is vividly displayed in Mark van Dal s and Frans van Loo s experiments, showing the KirkendaU plane bifurcation into two stable K-planes moving in the opposite directions between these two K-planes new grains appear and none of the markers remain (Chapter 6). 

The Kirkendall effect is commonly accompanied by the Frenkel effect, the void formation in the diffusion zone. In foreign literature, the Frenkel effect is often referred to as Kirkendall voiding, which is rather confusing, as Kirkendall and Frenkel effects are competitive vacancies annihilating at the dislocation kinks and causing the Kirkendall shift, cannot be used for Kirkendall voiding, and vice versa. 

Special activity is often attributed to lattice defects such as dislocations, kinks, vacancies, stacking faults, and intergrain boundaries emerging at the crystal surface. Experiments carried out with catalysts containing different numbers of defects have shown, however, that it will not be justified to identify crystallographic defects emerging at the electrode surface with the active sites responsible for catalytic activity of the electrode as a whole. 

Ftg. lOJ Localized anodic (A) and cathodic (C) zones on the same metal surface covered by an electrolyte. This is a snapshot of a dynamic situation the anodic and cathodic zones may change in shape, size and position. The zones may arise due to differences in, for example, constituent phases, stress levels, thermal history, surface coatings and imperfection levels (such as grain boundaries, dislocations, kink sites, etc). These features tend to become anodic. 

M. W. Barsoum, L. Farber and T. El-Raghy, Dislocations, kink bands, and room-temperature plasticity of Ti3SiC2,Afe/. Mat. Tram. A, 30, 1727-1738(1999). 

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