Nucleation, Nuclei

Embiyo formation Nucleation Nuclei Growth Impingement... 

The reactant species impinging from the vapor phase is attracted to the surface of the substrate mainly due to the instantaneous dipole moment of the substrate surface atoms. Adsorption of the condensed species is accompanied by lateral diffusion, i.e., the species moves along the surface of the substrate for a particular distance before it resides on the surface. An aggregate of condensed reactant species that reside on the surface of the substrate is termed a "nucleus". This could vary from a single atom or molecule to a cluster of species, and the process is referred to as nucleation. Nuclei are usually three dimensional island type structures formed on the surface of the substrate and nucleation is usually the initial stage of the film formation. It is worthwhile to note that, while simple condensation is usually exothermic, CVD adsorption and subsequent film formation is usually an endothermic reaction with few exceptions. 

Crystallites grow from nuclei. In homogeneous nucleation, nuclei are created by random statistical thermodynamic fluctuations of the local structure of the polymer. In heterogeneous nucleation, nuclei are created on surfaces of randomly distributed microscopic insoluble particles present in the melt. Since thermodynamic fluctuations are very small in magnitude while impurities are almost always present in significant amounts in a polymer, heterogeneous nucleation normally dominates. However, homogeneous nucleation can also become important at low values of Tx [186,187]. 

According to the classical theory of nucleation nuclei are bom by the successive addition of units following the formation scheme ... 

The parameter A, in turn, is inversely proportional to the rate constants for nucleation, nuclei and chains growth (Kgm, Kgs, Kgi) respectively. Thus, the analysis of non-isothermal reduction of nickel from NiO allows determination of the possible mechanism for this process, which is interpreted as quasi-chain reaction accompanied by the nuclei growth with equal probability of the size distribution. The proposed kinetic analysis can be used for other thermally activated processes from unstable precursors such as oxalates, metal carbonyls etc. 

As the structural match varies from a perfect to a poor match, m decreases from 1 to 0, - 1. The extreme case will be m 1, corresponding to the situation where there is no correlation between the nucleating phase and the substrate. This is the case where the substrates exert almost no influence on nucleation, which is equivalent to homogeneous nucleation. Nuclei emerging in this case are completely disordered, bearing no correlation to the substrate. One has then/(m) = 1. 

The kinetics of metal deposition on crystal faces free from screw dislocations is determined by the rate of nucleation /, nuclei cm sec of new lattice nets and their rate of propagation v, cm sec over the face. Almost simultaneously Nielsen/ Chernov/ and Hillig " showed that, depending on the values of these two parameters, two different mechanisms can be distinguished, whereby the extension of the surface plays a significant role (i) a layer by layer growth and (ii) a multinuclear multilayer growth. 

The aim of this study was to develop a kinetic model for the reduction of a supported chromia catalyst. The reduction rate data was examined by various methods and typical kinetic models were tested. Based on the information obtained, the nucleation/nuclei growth models were investigated more closely and derived in a revised form. 

It has been noted previously [6, 14] (and also in this chapter) that nucleation is always a progressive process except that, for a fast nucleation (the so-called instantaneous nucleation), nuclei are forming so fast that it is 60-fold more likely for a given site to be covered by the act of nucleation than by the act of growth. It is, therefore, simply incorrect to relate fast nucleation to a scenario in which the number of nuclei on the... 

For homogeneous nucleation, nuclei of the new phase form uniformly throughout the parent phase. 

For heterogeneous nucleation, nuclei form preferentially at the surfaces of structural inhomogeneities (e.g., container surfaces, insoluble impurities). 

Table 18.23. Specific frequencies of nucleation (nuclei m s ) at 353 K for various water pressures...

The AG, Npir and /dt values can be used to describe how easy or difficult it is for a solute to nucleate. Nuclei form easily when a system has small values of and since the clusters need to overcome only a small energy barrier and incorporate few atoms to become stable. In contrast, if r- is large and is very positive, the formation of stable nuclei is difficult. Only a small portion of clusters can then grow into stable nuclei, and a slow nucleation rate is expected. 

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