Growth, restricted, nuclei

What defines a spherulite is its shape and structural symmetry. All radii of an ideal spherulite are equivalent, at least at lengths greater than the 0.1-pm resolution of optical microscopy. The spherical (or circular) envelope of growth fronts is established by advance at rate G of separate fibrillar crystals into the melt from a common nucleus. This ideal shape is altered by growth restrictions presented by other spherulites (impingement/ truncation) or by melt interfaces (two-dimensional spherulites). 

Once a nucleus has been initiated, it draws solvent into it, in order to achieve the thermodynamic equilibrium of the phase. If affinity between solvent and nonsolvent is low, the driving force for this diffusion process will also be low, and pores growth would be restricted. On the other hand, if affinity is high, pores will grow fast. 

The kinetics of the thermal decomposition of solids are reviewed, with emphasis on topological considerations. The general model of nucleation in the bulk of the reactant is explored in detail and the kinetic equations appropriate to this model are derived. It is pointed out that a multistage nucleation process leads to a power law whenever the characteristic time for nucleus formation is long compared with the observation time, and that the assumption of equal rate constants for successive steps is unnecessarily restrictive. The problem of the induction period is examined and two possible reasons for the critical time to, namely the use of an incorrect model, and time-dependent growth rates (including, as a special case, aggregation without chemical decomposition) are advanced. Finally, the consequences of nucleation only on the surface of the reactant are mentioned briefly. 

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