Rate of a granular cell

For a two-grain scale, only one nucleus at each contact initiates the reactioa But in a cell, if no surface diffusion intervenes, the various contacts nucleate generally at different time points and all the isotropic types of growth could be considered a priori. 

However, the one-process model with slow nucleation and instantaneous growth is forever highlighted in the reactions between sohds. This is dne imdoubtedly to the fact that the network of grains A and C is almost always cormected (except, perhaps, for the systems with very diluted C), whereas for the solid-gas systems, for example, the grains are independent of each other and react individnally with the gas. 

In addition, it is known that the isotropic growth with a bnlk diffusion as the rate determining step is not conceivable. 

If the tangential diffusion of one reactant (C, for example) on the surface of product AC is easy, the tangential growth of AC can be carried ont very quickly as soon as a nnclens of AC is formed at one of contacts of the powder A/C the cormectivity of the network of the inter-grains bonds distributes quasi-instantaneously the matter of AC on the surface of all the grains of A, leading to shell growth. 

In the case of two-process models in which the growth starts from a point, only those with an interface reaction as the rate determining step will be examined at the cell level. On the other hand, the one-process models with slow growth will be studied in detail. 

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