Other models for nucleation and growth of compact nuclei

5 Other models for nucleation and growth of compact nuclei 

Topley and Hume [453], in a study of the dehydration of CaC03 6 H20, assumed the rapid initial formation of (on average) a single nucleus on the surface of each particle of reactant, represented as a sphere of radius a. In the absence of preferential surface development, the reaction interface penetrates the reactant at equal rates in all inward directions (kG = dr/df) and the volume of material reacted at time t is that volume of a sphere, having its centre at the site of surface nucleation and of radius kGt, which falls within the reactant. The fractional reaction, the zone of interpenetrating spheres, at time t is 

The sigmoid a—time curve passes through a maximum at a = 16/27 (= 0.593). The assumption that there is a single nucleus per crystallite may be unrealistic for the reaction of CaC03 6 H20 since the expression was applicable only within a restricted range (0.3 a 0.7). 

Nucleation at N0 potential sites is a continuing process expressed by the exponential law 

From the various possible geometric shapes of reactant crystallites, discussion here will be restricted to a consideration of reaction proceeding in rectangular plates knd in spheres [28]. A complication in the quantitative treatment of such rate processes is that reaction in those crystallites which were nucleated first may be completed before other particles have been nucleated. Due allowance for this termination of interface advance, resulting from the finite size of reactant fragments accompanied by slow nucleation, is incorporated into the geometric analysis below. 

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