Nucleation amorphous layer

The H profile in the y direction supports the presence of an outer softer amorphous layer (H = 120 MPa) in the samples with Tmouid below 120 °C (Pig. 7.8). However, on increasing Tmouid above 120 °C, the amorphous layer crystallizes and hardens. It should be noted that for the samples prepared at Tmouid > 120 °C, H shows a distinct maximum value at both surfaces, suggesting an enhancing nucleation effect of the metallic walls of the mould on the crystallizing material. 

Figure 5. Temperature dependence of the nucleation rate of crystals in light (1) and heavy (2) water. Dots on the low-temperature branch of the curve - data on crystallization of amorphous layers on the high temperature-branch - crystallization of droplets.Solid line -...

In conditions of high viscosity of the metastable phase the time of establishment of a stationary concentration of nuclei becomes longer. The process of non-stationary nucleation may be characterized by the stationary nucleation rate J and the lag time. Non-stationary nucleation shows up in the crystallization of amorphous layers of water. The crystallization of such layers proceeds during continuous heating or an isothermal allowance after a stepwise rise in the temperature. 

Fig. 5 gives temperature dependences of the stationary nucleation rate for supercooled and heavy water under crystallization of amorphous layers (dots on the low-temperature branch of the dome of J T)). 

Desre et al. [2.88] have proposed a mechanism for the suppression of nucleation of intermetallics in the case that an amorphous layer has already formed. In this model, nucleation of the intermetallic is impeded by the composition gradient in the growing amorphous interlayer. According to Figs. 2.18, 20, this composition gradient is given by... 

Numerous macromolecular materials are known to be partially crystalline [15]. The crystalline regions are very often lamellar shaped and are formed by backfolding of the chains, as it is schematically drawn in Fig. la. The lamellae build up stacks, in which two adjacent lamellae are separated by an amorphous layer of disordered chains. The stacks might form further superstructures of still larger dimensions. For example, in the case of the crystallization of an unoriented amorphous material, the crystallization process often starts from nucleation centers. Then, from each center a radial symmetric structure, called a spherulite, proceeds to... 

Overall, these experimental results confirm that diamond crystallites are not located directly on the substrate surface but on an intermediate amorphous layer. Nucleation of diamond occurs readily on the... 

The results demonstrate that precursors, which do not easily crystallize, such as ammonium iron EDTA, ammonium iron citrate, and iron gluconate provide the best results. During drying, crystallization does not proceed readily the precursor can fairly completely cover the surface of the carrier with an amorphous layer. During calcination highly dispersed iron(III) oxide results. When the precursor crystallizes to small crystallites, micropores are generated that withdraw the solution from the pores of the support With wide-porous supports nucleation of small crystallites can easily lead to an inhomogeneous distribution of the precursor. 

---