Voids homogeneous nucleation

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). 

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

There exists an extensive literature on nucleation theory. A great diversity of problems have been discussed. They range from homogeneous gas phase nucleation,to condensation of the primodial vapor in the solar system to form meteorites, and to formation of voids in nuclear reactor materials. Several collections of review articles " as well as a book have been published recently devoted solely to nucleation problems. In these works, each author has advocated his own particular approach to nucleation theory or dealt solely with his own pet nucleation problem. 

The cell structure of the polyolefin foam can be further influenced by adding cell nucleation agents into the polymer melt. Highly dispersed fillers and primary gas generators which build small voids (cavities) in the interface, or in the homogeneous melt, act as nucleation agents for further cell formation. 

Figure 2.9. The textural difference between magnetite and the catalyst precursor. The prereduction of wustite opens up voids and provides nucleation centers for the main reduction, which starts at ca 550 K. The promoter oxides prevent the crystallites of iron oxide from growing into large homogeneous and thus unreactive crystals.

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