Diffusion of interstitials

More exotic diffusion processes have been identified, although they may not be fully understood. One example is the substantial enliancement [25] of the diffusivity of interstitial O by H, resulting in the increased fonnation rate of... 

Diffusion of Interstitial Particles in a Chemical Concentration Gradient... 

A gradient in electrostatic potential can produce a driving force for the mass diffusion of a species, as discussed in Section 2.2.2. Two examples of this are the potential-gradient-induced diffusional transport of charged ions in ionic conductors such as those used in solid-electrolyte batteries and the electron-current-induced diffusion of interstitial atoms in metals. 

The diffusion of interstitial atoms in the stress field of a dislocation was considered in Section 3.5.2. Interstitials diffuse about and eventually form an... 

Figure 8.7 Interstitial mechanism for diffusion of interstitial atoms. The smaller...

D. Beshers. Diffusion of interstitial impurities. In Diffusion, pages 209-240, Metals Park, OH, 1973. American Society for Metals. 

Theoretical formulas show that the self-diffusivity of interstitial metal atoms decreases with an increase in the concentration of included hydrogen, which blocks free interstitial sites. A calculation of the coefficient of self-diffusion in the QCA is presented in Fig. 8.14. To calculate concentration dependences of the self-diffusivity, authors used eHh = 0-6 kcal/mol [217]. The curves are normalized to a self-diffusivity of 2.9 x 1CT3 sm/sec at 0H = 0 [218]. As can be seen, the character of curves 1-3 and 5 substantially depends on the value of an = hh/shh, which reflects the difference between... 

Vlasov N.M., Fedik I.I. Diffusion of Interstitial Impurities through Cylindrical Cladding with Residual Stresses. Dokl. Acad. Nauk (Physics) 2002 384 (3) 324-327. 

Figure 2.27. Illustration of the energetics involved for the atomic diffusion of interstitial imprrrities.

Another common theme in FP studies is the richness of possible surface phases -in some cases, dozens of structural isomers are computed to be thermodynamically accessible at room temperature. This has led to speculation that many oxide surfaces are more dynamic than previously thought, but definitive conclusions will only be possible once the processes of surface diffusion are identified and their activation energies are computed. This is perhaps the next frontier in FP oxide simulation. Meanwhile, the flexible surface model for active sites on metals [5] is finding some application in explaining the apparently facile diffusion of interstitial ions in non-stoichiometric oxides, despite the rigidity of the oxide lattice [26]. 

A slightly more complicated situation can exist during sensitization of austenitic stainless steels discussed above. Here, carbon activity is assumed to be at equiKbrium because of fast diffusion of interstitial carbon at all relevant temperatures. Moreover, the bulk carbon content is negKgibly... 

Profiles in samples which had been diffused with Fe at 800 to 1070C were determined by means of deep-level transient spectroscopy. It was found that the diffusivity of interstitial Fe was described by ... 

The mechanisms of nitriding and carburizing involve the transfer of the diffusing species to the surface, the establishment of a diffusing species activity gradient which drives the diffusion process, and the diffusion for itself, may be accompanied by the formation of nitrides or carbides (on the surface or in the core). The diffusion of interstitial species into a metal can only proceed if it exists a chemical potential (or activity) gradient of those species between the surface and the core of the material. 

If an atom on an interstitial site moves to one of the neighbouring interstitial sites, the diffusion occurs by an interstitial mechanism. This is schematically shown in Fig.5.8. Such a movement or jump of the interstitial atom involves a considerable distortion of the lattice, and this mechanism is probable when the interstitial atom is smaller than the atoms on the normal lattice positions. Diffusion of interstitially dissolved hght atoms, e.g. H, C, N, and O in metals provides the best known examples of this mechanism. 

These values for and in the hexagonal close-packed network should be completely applicable to diffusion of interstitial atoms within a metal. Within oxides, however, their use will be limited in the above form because correlation and near-neighbor effects may be important. These will be mentioned below in the discussion of FeS, which has the NiAs structure, the hexagonal equivalent of the NaCl structure and the one to which (41) and (42) apply. 

Diffusion of interstitial cations A from interface b toward interface c. 

Diffusion of interstitial ions B from interface c toward interfaee b. 

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