Extensions to the coalescence of solids with several components

Extensions to the coalescence of solids with several components 

In these solids, the production and consumption of the defects is often done by quasi-chemical reactions that use the gas phase above the powder. In a certain number of cases and in the presence of a gas, we can find structure elements that are 

The space functions, already calcnlated in section 12.4, remain valid whatever the nature of the solid. Only the reactivities will be expressed from surface rates or flux of diffusion by means of either eqnation [12.12] or [12.14]. 

To illustrate what preceded, we will treat in detail the case of the coalescence of the titanium dioxide (anatase) in the presence of water vapor and of oxygen. We will mention the results obtained for the anatase in the presence of oxygen and hydrogen chloride, with or without water vapor, and we will approach, in connection with ceria, the influence of foreign elements. These examples will enable us to see how the influences of the intensive constraints are introduced into rates of coalescence via their influences on the reactivity. 

This study [HEB 90], to our knowledge is the first one, which introduces a mechanism with elementary steps and makes it possible to include the effects of the partial pressures of water and oxygen on the speed of coalescence. The authors indeed showed that at 823 K, the speed of variation of specific area (of initial value of approximately 100 m /g), at constant specific area, for anatase powder varied with these two pressures according to a law proportional to the product PnJ) withp = 2 and q ranging between 6 and 14. 

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