Homogeneous nucleation energetics

A second problem in these studies concerns cavitation dynamics on the nanometer length scale [86]. If sufficiently energetic, the ultrafast laser excitation of a gold nanoparticle causes strong nonequilibrium heating of the particle lattice and of the water shell close to the particle surface. Above a threshold in the laser power, which defines the onset of homogeneous nucleation, nanoscale water bubbles develop around the particles, expand, and collapse again within the first nanosecond after excitation (Fig. 9). The size of the bubbles may be examined in this way. 

The observation that nanophase anatase sometimes transforms to nanophase brookite and in other cases nanophase brookite transforms to nanophase anatase implies very closely balanced energetics as a function of particle size. Several competing mechanisms (surface nucleation, interface nucleation, homogeneous nucleation) have been proposed for these reactions as a function of temperature and/or particle size (Zhang and Banfield 1999, 2000a,b). 

Nucleation by ideal nucleants should be energetically favorable in comparison with homogeneous nucleation in the polymer matrix and heterogeneous nucleation by other additives or contaminants. This favorable energetic balance could be found in surfaces with weak interactions with the polymer matrix and with radii of curvature larger than the radius of the critical nucleus in CNT. 

The embryos that trigger vapor formation in a superheated liquid are microscopic bubbles small regions where the density is smaller than in the bulk. To calculate the rate of homogeneous nucleation in a superheated liquid according to the classical theory, one must therefore consider the energetics of bubble formation. The contents of vapor embryos can be treated as an ideal gas except near the critical point. Let P be the pressure inside the critical nucleus. Then, P being the bulk pressure in the superheated... 

The energetic cost associated with creating the new interfacial area during nucleation leads to a minimum viable nucleation size (r ) and an activation barrier for nucleation (AG ). For homogeneous nucleation... 

As the factor is always smaller than unity (except for y) = 180°),thenudeationona surface is energetically favorable to homogeneous nucleation, and will thus have drastically higher rates. This is qualitatively shown in Figure 2.21. 

Xu G and Argon A S (2001) Energetics of homogeneous nucleation of dislocation loops under a simple shear stress in perfect crystals. Mater Sci Eng A319-321 144-147. 

Primary Nucleation Homogeneous nucleation requires systems to cross a free energy barrier [34]. For example, crystal nuclei generate sporadically from a supersaturated solution which is energetically metastable and in which density fluctuations occur [35]. A similar situation occurs in the crystallization from the melt. 

For 0 = 180" the energy fluctuation required is the same as for homogeneous nucleation, but for all other cases in which 0 < d < 180° heterogeneous nucleation is a more energetically favourable process. 

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