Dislocations interaction with vacancies

This section on dislocations will be concluded with a brief discussion of the interaction of point defects and dislocations. Interaction of vacancies and edge dislocations has already been discussed. 

An extension of the kinetic theory on cases when a mechanical pressure interacts with kinetic processes inside solid volume and on interfaces has wide application interests. The elastic deformations in solid are presented from influence of external forces and from presence of internal defects of crystal structure point defects (vacancy, intersite atoms, complexes of atoms, etc.), extended defects (dislocations and inner interfaces in polycrystals), and three-dimensional defects (heterophases crystals, polycrystals). 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

The ASA of carbon materials corresponds to the cumulated surface area of the different types of defects present on the carbon surface (stacking faults, single and multiple vacancies, dislocations) [14, 30] these sites are responsible for the interactions with the adsorbed species. A perfect linear relationship between the irreversible capacity and the value of ASA has been documented for different series of carbon samples [22]. While Cj. can be possibly not correlated with the BET area. Fig. 23.4 shows that it is linearly dependent of the ASA [31]. Moreover, all the samples coated with a thin carbon layer by pyrolytic decomposition of propylene demonstrate the lowest values of irreversible capacity and ASA (Fig. 23.4) [22, 31]. Figure 23.5 illustrates the positive effect of such a coating on the charge-discharge characteristics of carbon fibers from viscose. 

Even the cleanest of all the substrates shows areas where the periodic surface potential is perturbed. These sites can be generally called defects. Defects are generally classified into two main subclasses point defects, like corners, kinks, impurities or missing atoms and extended defects, like dislocations and steps. The type, concentration and characteristics of defects depend on several factors but the nature of the oxide and the history of the sample are no doubt the most important ones. In this section, two of the most commonly found MgO defects21,126 — low coordinated anion sites (steps and corners) and oxygen vacancies — will be considered with special emphasis on their interaction with metal atoms. 

Through the associated strain field, a dislocation is able to interact with other dislocations and also with point defects such as vacancies and impurities. The c.r.s.s. can be affected by the presence of these defects so that the c.r.s.s. is, to some extent. 

Once plastic deformation has started in a specimen an increase in stress is needed to produce further deformation. In microscopic terms this means that as deformation proceeds, the movement of dislocations on their slip planes becomes progressively more difficult. This hardening of the material may arise from elastic interactions between dislocations, through their strain fields, it may arise from dislocation reactions that produce segments that cannot sUp and it may also arise from interactions with other defects such as vacancies, impurities and grain boundaries. 

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