Annihilation, point defects

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

Fig. 2.5 Shear operation of (130)i[li0] on ReOj-type structure, (a) Arrangement of the oxygen polyhedra of both slabs after the elimination of one sheet of A plane (oxygen only plane). For visualization the two slabs are separated by vector [110]. The mark denotes the point defects of oxygen on plane A, which are produced by operation (2), and the mark x denotes the point defects produced by the separation of the slabs, (b) New structure formed by shear operation (3). By this operation the point defects are annihilated, which results in the occurrence of edge-sharing octahedra near the shear plane.

If majority point defect concentrations depend on the activities (chemical potentials) of the components, extrinsic disorder prevails. Since the components k are necessarily involved in the defect formation reactions, nonstoichiometry is the result. In crystals with electrically charged regular SE, compensating electronic defects are produced (or annihilated). As an example, consider the equilibrium between oxygen and appropriate SE s of the transition metal oxide CoO. Since all possible kinds of point defects exist in equilibrium, we may choose any convenient reaction between the component oxygen and the appropriate SE s of CoO (e.g., Eqn. (2.64))... 

Fick s second law states the conservation of the diffusing species i no i is produced (or annihilated) in the diffusion zone by chemical reaction. If, however, production (annihilation) occurs, we have to add a (local) reaction term r, to the generalized version of Fick s second law c, = —Vjj + fj. In Section 1.3.1, we introduced the kinetics of point defect production if regular SE s are thermally activated to become irregular SE s (i.e., point defects). These concepts and rate equations can immediately be used to formulate electron-hole formation and annihilation... 

In physical and chemical metallurgy, the Kirkendall effect, which is closely related to point defect relaxation during interdiffusion, has been studied extensively. It can be quantitatively defined as the internal rate of production or annihilation of vacan-... 

We conclude that a crystal which is continuously irradiated with particles of sufficient kinetic energy and in which no further reactions (e.g., phase formations) take place becomes more and more supersaturated with point defects. Recombination starts if the defects can move fast enough by thermal activation. A steady state is reached when the rates of defect production and annihilation (by recombination) are equal. In the homogeneous crystal, the change in local defect concentration (cd) over time is given by (see Section 5.3.3)... 

The influence of plastic deformation on the reaction kinetics is twofold. 1) Plastic deformation occurs mainly through the formation and motion of dislocations. Since dislocations provide one dimensional paths (pipes) of enhanced mobility, they may alter the transport coefficients of the structure elements, with respect to both magnitude and direction. 2) They may thereby decisively affect the nucleation rate of supersaturated components and thus determine the sites of precipitation. However, there is a further influence which plastic deformations have on the kinetics of reactions. If moving dislocations intersect each other, they release point defects into the bulk crystal. The resulting increase in point defect concentration changes the atomic mobility of the components. Let us remember that supersaturated point defects may be annihilated by the climb of edge dislocations (see Section 3.4). By and large, one expects that plasticity will noticeably affect the reactivity of solids. 

These results indicate that the radiation induced defects such as some point defects, dislocations and lattice distortions have no influence on the protonic conduction. However, the electronic conduction is modified by sub-band annihilation in gap between valence and conduction bands after neutron irradiation [2, 6, 7],... 

An isolated CS plane is referred to as a Wadsley defect and a random array of CS planes is considered to constitute planar (extended) defects which are entirely different from point defects. It is obvious that when CS planes occur at regular intervals, the composition of the crystal is stoichiometric, whereas a random array of CS planes results in nonstoichiometric compositions. While we have invoked anion vacancies which are later annihilated in our description of CS plane formation, we must point out that vacancies are not essential precursors for the formation of CS planes. Accommodating anion-deficient nonstoichiometry through CS mechanism is a special feature restricted to d° metal oxides such as W03, Nb205 and TiOz which exhibit soft phonon modes. Soft phonon modes in metal oxides arise from soft metal-orxygen potentials which permit large cation relaxation. The latter... 

Even if the surface is not perfectly smooth, the initial event that must occur in the development of a nucleus is passivity breakdown, in which the protective oxide layer is ruptured to expose the underlying metal to the aqueous environment. The most highly developed theory for this process is the point defect model (PDM) [59-65]. This model postulates that the generation of cation vacancies at the film/solution interface, and their subsequent transport across the barrier layer of the passive film, is the fundamental process fiiat leads to passivity breakdown. Once a vacancy arrives at the metal/film interface, it may be annihilated by reaction (i) in Fig. 31 ... 

At low temperatures, where diffusion in the solid state is unimportant, dislocations move principally by the process of slip (or glide) and may interact with other dislocations which move either in the same or in intersecting planes. Various kinds of lattice imperfections are introduced by such movement, and we shall discuss their identity in this subsection. At higher temperatures dislocation may anneal out by a process of annihilation resulting from slip. Moreover, since diffusion of individual species is now easier, important kinds of interaction between line and point defects are possible. These phenomena are also outlined below. 

Point singularities of equal and opposite strengths attract one another and are annihilated (see fig. 3.5.12). As the total energy of elastic deformation around a point defect increases linearly with the radius of the... 

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