Rates of a two-grain level

At a two-grain level, only one nucleus at the contact initiates the reaction whose extent is counted from this time (Fignre 14.10). 

If smface diffusion is very slow, then the growth becomes isotropic and only an interface step can be rate determining. 

We must recognize that very often the reactions between solids have diffusion as the rate determining step. In consequence with different energies of activation, the velocity of surface diffusion is faster at moderate temperature and it is only at veiy high temperatures that the diffusion in the bulk is faster than the diffusion at the surface. It is obviously the fastest of both diffusions (which are two parallel steps) that is essential. 

The growth is counted starting from the moment when the nucleus is formed at the contact point. 

If one of the two grains (C, for example) covers the whole surface of the other, the problem is brought back to that of the Mi model, as seen in Chapter 10. 

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