Mass transport phenomena Migration

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

Concentration profiles are very useful for understanding electrochemical phenomena. Yet It should be kept In mind that they make up only one part of those phenomena, since only diffusion Is visualized. For example, in the case described in appendix A.4.1, the profile of the NO3" anion does not appear flat, although this Ion Is not consumed at the electrodes. Indeed, even if diffusion has a tendency to carry this species from anode to cathode, the migration compensates for this phenomenon in exact terms. Finally, at steady state, there is no overall mass transport for these Ions. Another difficulty arises from the different interpretations of these concentration profiles, in terms of mass consumption or production in transient or steady states. 

At lower stresses and high temperatures, diffusion-controlled mechanisms will be active. Atomic-level mass transport by vacancies and interstitials can cause deformation at the macroscopic level. Vacancies can assist the motion of dislocations by cross-slip processes. As vacancy mobility and/or the density increases, the phenomenon of grain boundary migration will be observed. 

In its simplest form, permeation can be expressed as a product of the solubility and diffusion coefficient of the permeant in the polymer. Permeation of a gas can be calculated from Eq. 12.1. This equation is derived from Pick s first law of mass transfer. Permeation concerns the movement of a species through the molecules of another species, e.g., a gas through a polymer. It does not take into account transport of material through cracks, voids, and in general physical flaws in the structure of the second species such as the polymer. To be sure, both phenomenon result in the migration of chemicals through the structure. This means that after an appropriate plastic material has been selected to meet the permeation requirements of a process, the equipment must be fabricated carefully to avoid flaws in the polymer structure. 

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