Nucleation energy

U.S. Office of NucleaT Energy, Science andTechnology www.nuclear.gov... 

We are assuming an equal nucleation energy for all nucleation sites. In reality, the energy is less where there are breaks in structure such as grain boundaries, dislocations, etc. 

From the linear parts of the Znuc/f curves of Fig. 4.8, the steady state nucleation rate,/, for different overvoltages can be evaluated. Fig. 4.9a represents a typical In /vs. 1/177 P plot for the system of Fig. 4.8. The analysis of the data according to eq. (4.32) shows that, in the overpotential interval of 84 - 106 mV studied, the nucleation energy... 

The equilibrium constant for the reaction is equal to the partial pressure of the hydrazoic acid and was calculated by Feitknecht and Sahli to be 4.5 X lO" mm at 25°C [46]. From the distribution coefficient of hydrazoic acid between air and water [48], the concentration of hydrazoic acid in the water is calculated 5.4 X IO " M at 25°C. It is speculated that the Pb(N3)2-H2O reaction does not proceed beyond the basic lead azide because of the high nucleation energy for the formation of lead hydroxide. 

When 0= 0° this means that the liquid completely wets the solid, [(AGT)max]heterogeneous equals zero, and spontaneous heterogeneous nucleation occurs with no super-saturation at the equilibrium temperature. When 6 = 90° then [(AGT)max]heterogeneous is half the homogeneous value, and when 6= 180° both homo- and heterogeneous nucleation energy barriers are equal. 

The dramatic change in the seed layer morphology at different growth pressures is due to different nucleation energy and lateral growth rate. 

Keywords polymer nanofibers, nanospider, nucleation, energy barrier. 

In the case 6 0 the foreign particle is completely wetted, and the nucleation energy and the supersaturation necessaiy for nucleation tend to zero. The supersaturation which enables the growth of wetted foreign particles to become... 

The diagram of the liquid-vapor phase equilibrium is eharacterized by a decrease in the derivative dp/dT with the polymer concentration (dp/dT —> 0 at k 0). This leads to increase in bofli die nucleation energy and the detachment size of a bubble (equation [7.2.59]) and, consequently, to reduction of the bubbles generation frequency. Note that in reality the critical work, W , for a polymeric liquid may exceed the value predicted by the formula [7.2.59] because of manifestation of the elasticity of macromolecules. 

Fig. 2.8 Theoretical potential-time curves for two-electron process (2.55) and (2.56) with insoluble final product at different values of the parameter b, Eq. (2.69). Sharp maxima can appear at the intersection point because of the nucleation energy required for the formation of new phase...

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. 

Heterogeneous nucleating agent improves cell morphology of the polymer foams made from poly(lactic acid) by microcellular extrusion. Mesoporous sihca was surface modified with heptadecafluoro-l,l,2,2-tetradecyl trimethoxy silane to decease nucleation energy barrier. The modified silica was found to be excellent nucleating agent for PLA foam production. ... 

---