Nuclei, growth

The process as a whole is transient nucleation is predominant initially, and nucleus growth is predominant subsequently. Growth of the nuclei usually continues until they have reached a certain mean size. After some time a quasisteady state is attained, when the number of nuclei that cease to grow in unit time has become equal to the number of nuclei newly formed in unit time. 

Nucleus Growth The basic difference between reactions producing a new crystalline phase and reactions producing a gas or liquid phase is the step of nucleus growth. Difficulties exist in the incorporation of primary reaction products into the lattice. 

The structure of growing crystal faces is inhomogeneous (Fig. 14.11a). In addition to the lattice planes (1), it featnres steps (2) of a growing new two-dimensional metal layer (of atomic thickness), as well as kinks (3) formed by the one-dimensional row of metal atoms growing along the step. Lattice plane holes (4) and edge vacancies (5) can develop when nniform nucleus growth is disrupted. 

The electron affinity (EA) values, relative to the conduction band, indicate that nucleus growth by the Gurney-Mott... 

Surface nucleation diffusion of adsorbed ions partial dehydration formation of two-dimensional nucleus growth to three-dimensional nucleus. 

Three main constraints tend to decrease the rate of product formation through nucleus growth. These are coalesence of nuclei, ingestion of sites potentially capable of generating nuclei, and crystal boundaries, and are explained below. 

The rate of interface advance during nucleus growth is constant for most soild state decompositions for which measurements have been made. The rates of growth for the two nuclei shown here are equal. It is inferred, by extrapolation, that these were formed at times and but, as implied by the dotted lines, growth may have been slower during the very early stages (at or below the limits of microscopic resolution). 

If nucleus growth proceeds in A dimensions at a constant rate of advance, but the effects of overlap are disregarded, and is accompanied by nucleation according to the power law, the resulting rate equation has the form [1,5,8] ... 

This reaction mechanism is supported by the observed quantitative agreement between the rate of the first, diffusion-controlled reaction and the rate of interface advance, together rvith the similarity of values. Diffusive loss of water from within the disorganized solid is therefore identified as the controlling step in both rate processes. The extent of this is limited during the first deceleratory process but subsequently seeded detachment of the recrystallized product enables reaction to be maintained during continued nucleus growth. 

A clear distinction between crystallisation and precipitation is not always possible from a practical point of view [57] hence, it is more convenient to consider precipitation as a very fast crystallisation process. Crystallisation is a result of the combined effects of nucleation, nucleus growth and secondary processes inside the suspension such as agglomeration, ageing and recrystallisation. Depending on the reaction conditions, the above processes can occur together or sequentially during the crystallisation period. 

Formation of networks with secondary nucleation and nucleus growth in the holes and channels of the network. 

Depending on the actual reactant and temperature, the magnitude of the oversaturation Peq/Poo) varies within a very broad range. As we shall see later (Sect. 8.1), the value of Peq/Poo accounts for the dominant contribution of the condensation energy to the enthalpy of nucleus growth. 

Acceleration Nucleus growth into the reactant is conducted by the reaction localized at the product/reactant interface. Unlike the enthalpy of the decomposition reaction occurring at the free surface in the induction period (in the absence of nuclei), that of the reaction at the interface decreases because of a partial contribution of the energy released in the product condensation in this zone. This gives rise to an increase in the rate of reactant decomposition and the onset of the acceleratory period in the kinetics curve. The acceleration depends on the difference between the above enthalpies, i.e., on the condensation energy contributed to the heat of the pure vaporization process. 

Fig. 3.4 Nonisotropic nucleus growth during macrovoid formation in membranes.

An N c (-ln(c)/ > " n-dimertsiorral nucleatiorr/nucleus growth according to Avrami/Erofeev... 

---