Diffusion of Interstitial Particles in a Chemical Concentration Gradient

4 Diffusion of Interstitial Particles in a Chemical Concentration Gradient 

Another system obeying Fick s law is one involving the diffusion of small interstitial solute atoms (component 1) among the interstices of a host crystal in the presence of an interstitial-atom concentration gradient. The large solvent atoms (component 2) essentially remain in their substitutional sites and diffuse much more slowly than do the highly mobile solute atoms, which diffuse by the interstitial diffusion mechanism (described in Section 8.1.4). The solvent atoms may therefore be considered to be immobile. The system is isothermal, the diffusion is not network constrained, and a local C-frame coordinate system can be employed as in Section 3.1.3. Equation 2.21 then reduces to 

Assuming a dilute solution, Henry s law [16] applies, and because c C2 and X — C1/C2, an acceptable approximation, using Eq. 2.2, is 

L ii can be evaluated by introducing the interstitial mobility M, which is the average drift velocity, tq, gained by diffusing interstitials when a unit driving force is applied, that is, 

An interstitial drift velocity causes an interstitial flux, 

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