Nucleation free surface

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

The validity of mean field theory for N —y oo has striking consequences for the initial stages of phase separation. " In a metastable state slightly inside the coexistence curve, the nucleation free energy barrier is due to spherical droplets with a radius R The free energy excess of a droplet is written in terms of bulk and surface terms " "... 

Fig. 10). With the completion of the structure transition, the current should drop to zero, which is indeed the case except for peak B, where a slight leak current is seen (ascribed to the side reaction Cu++ I c > Cu+). According to the theory by Bewick, Fleischmann and Thirsk (BFT) the transients can be used to distinguish between instantaneous and progressive nucleation [45], A corresponding analysis revealed that the falling part of the transients agrees well with the model for instantaneous nucleation, while the rising part shows a systematic deviation. This was explained by the existence of surface defects on a real electrode in contrast to the ideal case of a defect-free surface assumed in the theoretical model. By including an adsorption term in the BFT theory to account for Cu deposition at defects, the experimentally obtained transients could indeed be reproduced very well [44], We shall return to the important role of surface defects in metal deposition later (sec. 3.2).

The "classical" theory of nucleation concentrates primarily on calculating the nucleation free energy barrier, AG. Chemical interactions are included under the form of thermodynamic quantities, such as the surface tension. A link with chemistry is made by relating the surface tension to the solubility which provides a kinetic explanation of the Ostwald Step Rule and the often observed disequilibrium conditions in natural systems. Can the chemical model be complemented and expanded by considering specific chemical interactions (surface complex formation) of the components of the cluster with the surface ... 

ITie Henry and Fauske model employs curves similar to Fig. 16. Immediately upon initial contact, they assume that there is rapid pressurization at the interface. Nucleation in this vicinity is then prevented [Po in Eq. (7) is large and so is Dq] until the pressure is acoustically relieved by the wave moving to a free surface and returning. During this period, the thermal boundary layer in the cold liquid continues to develop. At relief, there still may be no intersection of the t-Do curve (in Fig. 16), so until such a time... 

If a glass is held for a long period at an elevated temperature it may start to crystallize or devitrijy. Devitrification of fused quartz (silica glass) to cristabolite is slow. Nucleation is usually at a free surface and is often stimulated by contamination from alkali ions such as sodium. The rate of growth of cristabolite is increased by oxygen and water vapor. With surface contamination, devitrification of fused quartz may occur at temperatures as low as 1000 °C. However, if the surface is clean it rarely occurs below 1150 °C. 

Here Ii(t - u) is the growth current of a single cluster born at time t = u, [1 - 0(u)] is the actual free surface fraction available for the nucleus formation, and J u) is the nucleation rate at time t = u. Similarly to the case of - instantaneous nucleation an expression for the current density jN(t)canbe derived accounting either for direct clusters coalescence [iii-vi] or for overlapping of planar diffusion zones within which nucleation is fully arrested [vii-xi], In the latter case,... 

The local plasticization mechanism is described pictorially in the sketch shown in Fig. 31. The advancing craze, nucleated from free surfaces or other occasional imperfections, cuts into the dispersed population PB-2.76K pools (which at this low molecular weight acts like a relatively low viscosity liquid) and drains the contents of these pools onto the surfaces of the craze. Although the solubility v of the PB-2.76K into PS under standard conditions of room temperature and atmospheric pressure is negligibly small (of the order of 4x 10 ), this should increase greatly in the presence of a negative pressure, according to the expression... 

In region B-C, the heater temperature is further increased, and bubbles form at such great rates at such a large number of nucleation sites that they form numerous cotitinuotis columns of vapor in the liquid. These hubbies move all the way up to the free surface, where they break up and release their vapor content. The large heat fluxes obtainable in this region are caused by the combined effect of liquid entrainment and evaporation. 

D nucleation and growth can be excluded. This is a necessary but not sufficient criterion for a nucleation-free Meads overlayer formation process on quasi-homogeneous substrate surfaces. 

Polyurethane RIM (PU-RIM) was selected for the Initial RIM application of automotive fascia not only because 1t provides the required damage resistance and other performance requirements, but also because the reactivity of the urethane raw materials, Isocyanate and polyol, provide acceptable productivity. Because the urethane mnomers can be nucleated, high quality sink free surfaces that are suitable for automotive exteriors are produced at the time of demold. Little, 1f any, post mold surface repair 1s needed 1n preparation for painting the RIM part. 

The solution constructed above provides the necessary tools from elasticity theory in order to consider nucleation of a dislocation at a crack tip. The philosophical perspective adopted here is that due to Rice (1992) in which a comparison is made between the work needed to create new free surface and that needed to create a slip distribution corresponding ultimately to a dislocation. We present this model not so much with the hope that it will deliver quantitative insights but on the grounds that it is highly instructive concerning the atomic-level processes near a crack tip. In particular, the model is appealing because the treatment of dissipative processes such as dislocation nucleation is endowed with atomic-level realism, while still maintaining an overall continuum description. 

It should also be noted that the crystallization behavior of melt-spun ribbons may be different on both ribbon sides [4.18]. Nucleation for primary crystallization of the transition metals is observed on both sides of the glassy ribbons, while other crystallization reactions have been observed to prefer usually either the free surface or the contact side of the ribbon [4.77]. This phenomenon may lead to different structural and chemical properties of the two ribbon sides, and consequently also to large anisotropy in the catalysts prepared from such ribbons [4.18, 19]. 

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