Defects path diffusion

These are depicted schematically in Figure 18.4 in the case of metal A deposited on metal B. Bulk diffusion, as noted above, is the transfer of B into A or A into B through the crystal lattice. This is characterized by the coefficient D in the figure. Defect path diffusion is the migration along lattice defects such as grain boundaries, characterized by the coefficient D in the figure. Ordered A B, possible phases are indicated between the metals. Finally, Kirkendall void porosity is indicated and will be expected to be present if the interdiffusion rates from one metal to the other are not equal in both directions. 

Diffusion along defect paths (grain boundaries, dislocations)... 

Galy, J. and Carpy, A. (1974) Defects and diffusion paths in perovskite-like structures ABO3+. Philos. Mag., 29 (5), 1207-1211. 

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. 

The nature of bulk defects is straightforward. Pinholes, cracks, or other surface imperfections result in rapid diffusion of saline into the implant resulting in corrosion of electrical conductors. This corrosion lowers the resistance of the original leakage path until normal operation of the electronic circuit is impared and the Implant falls. 

With the help of structural defects in aluminosilicate MCM-41, diffusion of reactant and product could proceed across instead of just along the channels as in the case of defect-free MCM-41. In this way, the activity will improve since the diffusion path to and from the active site is shorter than that in defect-free MCM-41. It is also possible that the large internal channel at the center of the tubule is freely accessible to reactant and product. In fact, there exist a two-way diffusion system in aluminosilicate Def-MCM41 which minimized traffic congestion. 

Experiments demonstrate that along crystal imperfections such as dislocations, internal interfaces, and free surfaces, diffusion rates can be orders of magnitude faster than in crystals containing only point defects. These line and planar defects provide short-circuit diffusion paths, analogous to high-conductivity paths in electrical systems. Short-circuit diffusion paths can provide the dominant contribution to diffusion in a crystalline material under conditions described in this chapter. 

Fig. 5. A donor impurity is diffused into the silicon from a gaseous phase, (a) A shallow n region lias been created, (b) Continued diffusion, longer times, or higher temperatures increase the extent of the n region, (c) Surface diffusion has caused spreading of the n region along the SiCVsilicon interface, (d) A crystal defect, such as a dislocation, has provided a path for anomalously high diffusion and led to penetration of the junction to unanticipated distance from the surface. (See Fig. I for legend)...

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. 

Self-diffusion in materials occurs by repeated occupation of defects. Depending on the defects involved one can distinguish between (1) vacancy, (2) interstial, and (3) interstitialcy mechanisms [107], As an example, different diffusion paths for oxygen interstitials are illustrated in Fig. 1.16 [129]. For a detailed description of diffusion paths for oxygen vacancies, zinc vacancies and zinc interstitials the reader is also referred to literature [129,130]. 

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