Voids heterogeneous nucleation

It is far more likely that heterogeneous nucleation plays the governing role in nucleation of voids. The effect of a particle or substrate is to lower the free energy barrier AF. Thus, Equations 6.1 and 6.3 remain unchanged, and Equation 6.2 takes on the new form... 

The crystallinity of alumina formed by DMO infiltration into SiC particulate preforms follows the expected behavior of heterogeneous nucleation on SiC particles and large monocrystalline regions with [0001] parallel to the growth direction within the larger voids [83,86]. Manor et al. [82] found that the infiltration rate increased as the SiC particle size decreased. However, no systematic trend was found in another work [83], most probably due to widely dilferent silicon concentrations in the percolation channels resulting from reduction of the silica layer during infiltration. 

A nucleation process is generally described assuming the initial formation of the voids to be in accordance with the classical nucleation theory. It can be assumed that nucleation occurs between resin and fiber or resin and added particle (heterogeneous nucleation) or within the resin itself (homogeneous nucleation). 

The first step in the reduction process, after removal of a layer of chemisorbed water at ca 400 K, is the dehydration and partial reduction of the rust layer. This leads to the formation of a surface containing ferric and ferrous ions, together with structural promoters in the form of pure binary oxides which are dissolved in spinel structures. This step also opens up some of the cracks and pores. Eventually, wustite which initially could not be found on any of the outer surfaces is reduced to iron, and this further increases the voids between the magnetite particles. This step of widening of the cracks and defects, which can be seen only indirectly by photoelectron spectroscopy (e.g., by the induction period found in the UPS experiments), is crucial for the subsequent reduction of the bulk magnetite. This is because it is known that the reduction occurs only along such defects, which provide the sites for heterogeneous nucleation of metallic iron. This led to the core and shell model of activation described in Section 2.5. 

When the dissolved gas reaches a saturation limit in the polymer, it becomes supersaturated and finally diffuses out of the polymer system to form voids or bubbles [17-19]. The formation of bubbles represents a nucleation process in which voids are formed either without nucleating agent (self-nucleation process) or with solid nucleating agents at the liquid-solid interface (heterogeneous process). 

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