Epitaxial heterogeneous nucleation

Manecki et al. (2000b) used the same three apatites to study both homogeneous and epitaxial heterogeneous nucleation. For this, AFM, SEM, optical microscopy, EDS, Fourier-transformed infrared (FTIR) spectroscopy, and XRD were used to characterize the apatites and the reaction... 

Valsami-Jones et al. (1998) conducted similar studies with a synthetic, microcrystalline hydroxyapatite in either a Pb or Cd buffered solution. Using AFM, hydroxypyromorphite was observed to grow epitaxially on the hydroxyapatite surface in a clear example of heterogeneous nucleation. Cd removal differed, with the likely formation of a Ca-Cd phosphate solid solution. 

If a c = a sA then = 0 and the new phase C wets the interface. If nucleation barrier for the new phase C will vanish. On the other hand, if homogeneous nucleation. This would be the case if the original interface were nearly coherent, i.e., A/B is an epitaxial structure. Heterogeneous nucleation would be effectively suppressed in this case. In general, the nucleation barrier for C, AG UC depends on

0. These general features are applicable irrespective of whether C is a metastable... 

In the case of the crystaUization of the matrix in the presence of already solidified or crystallizins particles, migration of nuclei still can play an important role. However, several other phenomena have to be taken into account. First of all, the solidified domains can act as efficient nucleators. Furthermore, retarded crystallization of finely dispersed droplets can nucleate the matrix and leads to coincident crystallization of both phases. Finally, it has been reported that epitaxial crystallization at the interfaces sporadically occurs. All these phenomena lead to an increased heterogeneous nucleation of the matrix phase. 

A typical shish-kebab crystalline structure has been foimd by Maiti and Okamoto (2003) and Kim et al. (2001) in polyamide/organoclay nanocomposite and by Choi and Kim (2004) in PP/EPR/talc nanocomposite where a preferential orientation of polymer lamellae perpendicular to the surface of organoclay layers was inspected by TEM measurements. The unique observation of lamellar orientation (Ml the clay layers was ascribed to nucleation and epitaxial crystallization at the interface between layered silicate and polymer matrix especially the surfaces of clay platelets acted as heterogeneous nucleation sites. Orientation of iPP crystals was also enhanced in rPP/PP-MA/o-MMT injection-moulded parts, especially manufactured by dynamic packing injection moulding (Wang et al. 2005). MMT... 

Perovic, D. D., Weatherly, G. G., Baribeau, J. M. and Houghton, D. C. (1989), Heterogeneous nucleation sources in molecular beam epitaxy grown Gea,Sii a /Si strained layer superlattices, Thin Solid Films 183, 141-156. 

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