Nucleation initial period

It should also be emphasised that an initial period of interaction of elementary substances when there is still no compound layer and consequently there is only one common interface at which substances A and B react directly, is outside the scope of the proposed macroscopic consideration. The stage of nucleation of a chemical compound between initial phases is to be the subject of examination within the framework of a microscopic theory which would have to provide, amongst other parameters of the process, a minimal thickness sufficient to specify the interaction product formed at the A-B interface as a layer of the chemical compound ApBq possessing its typical physical and chemical properties. However, it can already now be said with confidence that this value is small in comparison with really measured thicknesses of compound layers and therefore can hardly have any noticeable effect on the shape of the layer thickness-time kinetic dependences observed in practice. 

For the initial period, i.e., for the period immediately following the nucleation pulse excitation, the probability of observing at least one nucleation event, fm>i. is equal to the complementary probability, Pni=o = (1 - Fm>i), that zero nucleation events are observed. Hence,... 

The initial oversaturation of the melt towards ahte is low and principally the heterogeneous nucleation takes place, primarily as a result of the decrease of Gibbs free energy on phases boundary [42]. In this initial period alite nucleation can occiu... 

On the basis of the above results, it was postulated that particle nucleation took place continuously throughout the polymerization process, in sharp contrast with conventional emulsion polymerization where particle nucleation takes place only in the initial period (interval I)- The results of TEM experiments performed on polyacrylamide samples taken at different extents of conversion [53] also supported the theory of continuous particle nucleation since the number of polymer particles was found to increase proportionally with conversion while the size remained approximately constant. 

Of particular interest is the long-term behavior of voting-rule systems, which turns out to very strongly depend on the initial density of sites with value cr = 1 (= p). While all such systems eventually become either stable or oscillate with period-two, they approach this final state via one of two different mechanisms either through a percolation or nucleation process. Figure 3.60 shows a few snapshots of a Moore-neighborhood voting rule > 4 for p = 0.1, 0.15, 0.25 and 0.3. 

Characteristic features of a—time curves for reactions of solids are discussed with reference to Fig. 1, a generalized reduced-time plot in which time values have been scaled to t0.s = 1.00 when a = 0.5. A is an initial reaction, sometimes associated with the decomposition of impurities or unstable superficial material. B is the induction period, usually regarded as being terminated by the development of stable nuclei (often completed at a low value of a). C is the acceleratory period of growth of such nuclei, perhaps accompanied by further nucleation, and which extends to the... 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

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