Crystallization heterogeneous nucleation

In the in situ method the support is put in contact with a synthesis solution or a gel under hydrothermal conditions. At the same time, under appropriate conditions, zeolite nuclei are formed on the support and they will grow by forming a zeolite layer. Zeolite crystals heterogeneously nucleated on the support surface are also present. This technique does not give the possibility of controlling the nucleation of the crystals which is necessary to form a very uniform and compact zeolite layer (Li et al., 2003). 

Fig 7 3 Heterogeneous nucleation takes place on the surface of a solid catalyst. For the catalyst to be effective there must be a strong tendency for it to be "wetted" by the crystal, i.e. 6 must be small. 

Heterogeneous nucleation is most likely to occur when there is a strong tendency for the crystal to stick to the surface of the catalyst. This sticking tendency can be described by the angle of contact, 6, shown in Fig. 7.3 the smaller 6, the better the adhesion. Anyone who has tried to get electronic solder to stick to a strip of copper will understand this well. If the copper is tarnished the solder will just roll around as a molten blob with 6 = 180°, and will not stick to the surface at all. If the tarnished... 

If the nucleus wets the catalyst well, with 6= 10°, say, then eqn. (7.15) tells us that het IS.lrt,. In other words, if we arrange our 10 atoms as a spherical cap on a good catalyst we get a much bigger crystal radius than if we arrange them as a sphere. And, as Fig. 7.4 explains, this means that heterogeneous nucleation always "wins" over... 

Fig. 7.4. Heterogeneous nucleation takes place at higher temperatures because the maximum random fluctuation of 10 atoms gives a bigger crystal radius if the atoms are arranged as a spherical cap.

The nucleation rate is, in fact, critically dependent on temperature, as Fig. 8.3 shows. To see why, let us look at the heterogeneous nucleation of b.c.c. crystals at grain boundaries. We have already looked at grain boundary nucleation in Problems 7.2 and 7.3. Problem 7.2 showed that the critical radius for grain boundary nucleation is given by... 

Heterogeneous nucleation of polymer crystallization resembling a visualized metaphor compare the way meat is prepared in an oriental way shish - kebab. 

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

The melt mixed 80/20 PS/iPP blend displays a set of exotherms, where the amount of the iPP component that was heterogeneously nucleated is substantially reduced as indicated by the decrease of the crystallization enthalpy in the temperature region where the iPP crystallizes in bulk, i.e., at 109-111 °C (exotherm labeled A). This effect is due to the confinement of iPP into a large number of droplets. If the number of droplets of iPP as a dispersed phase is greater than the number of heterogeneities present in the system, fractionated crystallization occurs. The number of droplets for this composition is known (by scanning electron microscopy observations) to be of the order of 1011 particles cm-3 and polarized optical microscopy (POM) experiments have shown that this iPP contains approximately 9 x 106 heterogeneities cm-3. In fact, it can be seen in Fig. 1 that the fractionated crystallization of the iPP compon-... 

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