Fast diffusion paths

Fast diffusion paths grain boundary and dislocation core diffusion... 

We have pointed out before that during creep, demixing of solid solutions is to be expected. Creep in compounds, however, occurs in such a way that the rate is determined by the slowest constituent since complete lattice molecules have to be displaced and the various constituent fluxes are therefore coupled. If extra fast diffusion paths operate for one (or several) of the components in the compound crystal, the coupling is cancelled. Therefore, if creep takes place in an oxide semiconductor surrounded by oxygen gas, it is not necessarily the slow oxygen diffusion that determines the creep rate. Rather, the much faster cations may determine it if oxygen can be supplied to or taken away from the external surfaces via dislocation pipes. 

C. Piehl, Z. Tokei, H.J. Grabke. The role of fast diffusion paths on the selective oxidation of chromium steels // Defect Diffusion Forum.- 2001.- V.194-199.- P.1689-1694. 

Figure 19.8 Schematic visualising the possible influence of the film stmcture on the metal diffusion. Grain boundaries might either act as fast diffusion paths or act as trapping and nucleation sites for clusters.

Ag concentration with respect to the surface concentration is one order of magni-mde lower as compared with the film with the higher grain boundary density (DIP02). This indicates that grain boundaries are acting as fast diffusion paths. 

It was found that grain boundaries act as fast diffusion paths. While DIP and TMC-PC have the same cohesive energy they differ in their microstructure. DIP forms polycrystalline, well ordered films and TMC-PC films are amorphous. The existence of grain boundaries in DIP and their absence in TMC-PC can explain the difference in the ratios and the varying effective-... 

A second-phase that forms a liquid at the firing temperature can provide a fast diffusion path for densification but grain growth by the Ostwald ripening process may also be enhanced. In this case, high density is normally accompanied by appreciable grain growth. This commonly used fabrication approach is the subject... 

Depending on which of the above factors dominates during deformation, the accommodation mechanism may be regarded as viscous flow, solution-precipitation, or cavitation creep. In general, cavitation creep can be discarded as an accommodation mechanism for superplasticity, as the strain-to-failure afforded by this mechanism is rather small. Therefore, only viscous flow and solution-precipitation mechanisms are important. Obviously, too, whether these mechanisms apply depends on the presence or absence of a liquid phase. Solution-precipitation requires a liquid phase to envelop the grains, while viscous flow is facilitated by the fast diffusion path of the liquid, although it may also occur in a dry polycrystal via diffusional creep. 

Dislocations are one-dimensional defects. They are largely responsible for the plastic behaviour of solids. Two of their properties are particularly important in connection with solid state reactions 1. They can act as sites of repeatable growth within a crystal. 2. They can serve as fast diffusion paths. They also act as preferential nucleation sites for the formation... 

Oxygen diffuses to the reinforcing steel surface through the porous concrete, with cracks acting as fast diffusion paths, especially if they are not filled with water. The Fe + ions produced at the anodes combine with the OH ions from the cathodic reaction to ultimately produce a stable passive film. This electrochemical process is illustrated schematically in Fig. 2.29. 

However, the hydrogen bond network in the liquid ensures fast diffusion paths for the proton and the HO ion, so tliat the reaction may as well proceed though a direct attack of the aquo ligand by the hydroxyl ... 

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