One-process model with slow nucleation and instantaneous growth

We always assume the approximation of a uniform average surface of A. 

One-process model with instantaneous nucleation and slow growth 

In this one-process model, we will have, at the very initial time, the presence of a thin film of the new phase on the whole surface of A, and this layer will be able to grow only normally on the surface. 

We will initially discuss the case of a single grain this case also applies to an A massive solid. 

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Under such conditions, we observe that generally a one-process model with instantaneous nucleation and slow growth is enough to describe the reaction (see Chapter 10). 

The problem of the kinetics of reaction between powder solids is strongly complicated by the design of the mixtures, which can lead to complex laws and even fractional extent that never reach the value of 1 at the end of the reaction. Fortunately, there are several examples of reactions between solids that follow the one-process model with instantaneous nucleation and slow growth, that is, a model of Ml cells and generally with diffusion as the rate determining step. Their application and results are given in the tables of Appendix 3. This undoubtedly explains few studies on the problem of nucleation in this family of reactions. 

We study the reaction of decomposition of a solid A into a solid B and a gas G. Under the experimental conditions discussed below, the speed of the reaction is in good agreement with a one-process model of instantaneous nucleation and slow growth. The reaction proceeds with only a single rate-determining step, and the experimental conditions are very far from that of the equilibrium. Speeds at constant pressure obey Arrhenius law with temperature. 

As we are in pseudo-steady state modes and that extent is enough to define the rate, this means that we can do with a one-process model of instantaneous nucleation and slow growth or slow nucleation and instantaneous growth. The observation of the metal-sulfide interface after partial sufturization shows that we probably have an instantaneous nucleation and slow growth. The increase in mass is thus proportional to the fractional extent and the experiment gives the rate of growth, which is separable, therefore,... 

One-process model with slow nucleation and instantaneous growth... 

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. 

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