Dislocation and Grain Boundary Diffusion

The diffusion of oxide ions may also dominate an oxidation reaction. In a similar fashion to that described for cations, the anions can be accompanied by a flow of holes or a counterflow of electrons (Fig. 5.22c and 5.22d). The arriving O2-anions extend the oxide film at the metal-metal oxide boundary. Once again, which of these mechanisms is theoretically possible will depend upon whether the oxide is naturally rc-type or p-typc. 

Many metal oxides are insulators. In these cases oxidation can only occur if charge neutrality is maintained by way of a significant counterdiffusion of cations and anions (Fig. 5.22e). Once again, mobilities will be equalized by the Nemst field set up when one species moves faster than the other. When counterdiffusion of ions is involved, the oxide film grows at both the inner and outer surfaces. 

The likelihood that Fick s laws will be obeyed in a crystal containing dislocations is dependent upon the spacing between the defects. Provided that this spacing is much greater than the diffusion length (Dt)1/2, where D is the bulk diffusion coefficient, Fick s laws are obeyed, with an effective (measured) diffusion coefficient, Deff, given by 

Dislocations can attract a population of impurities, vacancies, or self-interstitials that are bound to the dislocation core by a binding energy Agb. These will be liberated and become free to contribute to the overall diffusion at higher temperatures, so that it is possible to write 

The situation with respect to grain boundary diffusion is similar. The contribution of the grain boundaries to the effective diffusion coefficient can be written as follows  

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