Atom transport diffusion

The Ru surface is one of the simplest known, but, like virtually all surfaces, it includes defects, evident as a step in figure C2.7.6. The observations show that the sites where the NO dissociates (active sites) are such steps. The evidence for this conclusion is the locations of the N and O atoms there are gradients in the surface concentrations of these elements, indicating that the transport (diffusion) of the O atoms is more rapid than that of the N atoms thus, the slow-moving N atoms are markers for the sites where the dissociation reaction must have occurred, where their surface concentrations are highest. 

If samples of two metals widr polished faces are placed in contact then it is clear that atomic transport must occur in both directions until finally an alloy can be formed which has a composition showing die relative numbers of gram-atoms in each section. It is vety unlikely that the diffusion coefficients, of A in B and of B in A, will be equal. Therefore there will be formation of an increasingly substantial vacancy concentration in the metal in which diffusion occurs more rapidly. In fact, if chemically inert marker wires were placed at the original interface, they would be found to move progressively in the direction of slowest diffusion widr a parabolic relationship between the displacement distance and time. 

In all of the structures based upon hexagonal close-packed anions illustrated, continuous diffusion paths through empty sites can be traced, and a population of point defects is not mandatory to facilitate atom transport. 

Suppose that vacancy diffusion is the principal mechanism involved in atom transport. An expression for the fraction of vacancies in a pure crystal is [Eq. (2.6)]... 

J. Philibert, Atom Movements Diffusion and Mass Transport, translated by S. J. Rothman, Les Editions de Physique, F-91944 Les Ulis, 1991. 

The interpretation of this data on metals in terms of microscopic mechanisms of surface atom transport is not totally understood. The original papers[ 11] proposed that during surface transport the controlling process was adatom terrace diffusion between steps with the adatom concentration being that in local equilibrium with the atomic steps. This may indeed be the case, but in light of other experiments on adatom diffusion[13] and exchange processes at steps[14] the possibility of step attachment/detachment limited kinetics caimot be raled out. 

One of the most important aspects of point defects is that they make it possible for atoms or ions to move through the structure. If a crystal structure were perfect, it would be difficult to envisage how the movement of atoms, either diffusion through the lattice or ionic conductivity (ion transport under the influence of an external electric field) could take place. Setting up equations to describe either diffusion or conductivity in solids is a very similar process, and so we have chosen to concentrate here on conductivity, because many of the examples later in the chapter are of solid electrolytes. 

Molecular Dynamics and Monte Carlo simulations have been used to predict the adsorption isotherms and transport diffusivities of Xe adsorbed in A1P04-31. The results of these calculations can be used to predict the properties of Xe diffusing through membranes made from A1P04-31 crystals directly from atomic-scale principles. 

We see from Figure 3-1 that edge dislocations possess a compressed region above, and a dilated region below the glide plane. Therefore, in the dilated area around the dislocation line, the transport coefficients will be larger than in the bulk crystal. Thus, dislocations can serve as fast diffusion pipes for atomic transport. 

Decomposition reactions are solid-gas reactions which do not involve diffusional transport through the solid. Their reaction rates are determined by surface kinetics and possibly pore diffusion. The assumption of local equilibrium is not valid. The course of an isothermal decomposition is schematically illustrated in Figure 15-15. There is often an induction period followed by a rapid increase in relative yield until, after the inflection point, the reaction eventually ceases (the yield will not always be 100%). Since atomic transport in crystals is normally not involved in these decomposition reactions, we shall restrict ourselves to a few comments only. 

TWo remarks, however, seem appropriate. 1) If the distance, a, between individual dislocations is very small on an atomic scale, diffusion coefficients obtained from macroscopic experiments can not be used in Eqn. (14.29) (as explained in Sections.1.3). 2) Since diffusional transport takes place in the stress field of dislocations, in principle, fluxes in the form of Eqn. (14.18) should be used. This, however, would complicate the formal treatment appreciably. In the zeroth order approach, one therefore neglects the influence of the stress gradient, which can partly be justified by the symmetry of the transport problem. 

An analysis of the rate of elongation of a wire possessing a bamboo-type grain structure is given in Section 16.1.3. An essential aspect of the analysis is the assumption that the stress-induced atomic transport producing the elongation is diffusion-limited. Now, construct the main framework of a model for the same system in which the atomic transport is source-limited, as indicated below, and explain how the model works. 

An alternative method for distinguishing simple diffusion from carrier-mediated transport can be used if the solute contains a chiral atom. Simple diffusion will occur for both enantiomers and at the same rate, but carrier-mediated transport is invariably stereospecific i.e., the carrier will recognize only one of the enantiomers. 

The motion of atoms both in and on the surface can also occur through surface diffusion. The concept of atom transport along the surface plane is important in equihbrium surface structure, nucleation and growth of thin films, and surface reactivity. 

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