Growth on Foreign Nuclei

Growth on Foreign Nuclei As noted above, foreign nuclei are often present in abundance and permit fog formation at much lower supersaturation. For example,... 

After formation of a primary deposit layer on foreign substrates, further layer growth will follow the laws of metal deposition on the metal itself. But when the current is interrapted even briefly, the surface of the metal already deposited will become passivated, and when the current is turned back on, difficulties will again arise in the formation of first nuclei, exactly as at the start of deposition on a foreign substrate (see Section 14.5.3). This passivation is caused by the adsorption of organic additives or contaminants from the solution. Careful prepurification of the solution can prolong the delay with which this passivation will develop. 

High density of nuclei on foreign surface causes disturbance in spherulites growth, which results in the only possible growth - perpendicularly to the surface. As a result, transcrystalline front will emerge parallel to the surface. Transcrystallization is possible when nucleation energy conditions are more favorable on the surface than in the bulk of the melt. The microscopic photograph of TCL is shown in Fig. 10.6. 

The situation is more complex if crystalline nuclei are formed not on structureless but on foreign crystalline substrates. In that case it is necessary to take into consideration both the orientation effect of the substrate and the crystallographic lattice mismatch at the nucleus-substrate interface boundary. Such phenomena are known as epitaxial crystal growth and for more information on this important subject the readers are referred to [1.72, 1.82-1.86]. Here we shall present oidy the widely used classification of the different modes of epitaxial crystal growth proposed by Bauer [1.77] in 1958 (Figure 1.18). 

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

Homogeneous nucleation takes place only when the system does not contain any surfaces on which the formation and growth of nuclei can occur at a sufficient rate. If such surfaces are present (e.g. vessel walls, or especially the surfaces of foreign inclusions), heterogeneous nucleation may become much more probable, depending on the nature of the surfaces. If seeds of the substance forming a new phase or those of a substance with similar structure and properties are introduced into the system, the new phase starts to form on the surface of such seeds. 

These include the qualitative or quantitative interpretation of phenomena, effects, and regular trends observed in the course of thermal decomposition. The most significant achievements are the explanation of the mechanism of the nucleation and growth of nuclei and the interpretation of such effects as localization of the decomposition process, the T-S effect, a decrease in the decomposition rate upon melting of the reactant, differences in the structure (amorphous or crystalline) of the solid product, and the effect of gaseous products and foreign gases on the decomposition rate. 

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