Initial stages of nucleation

It is established that the first crystals start growing immediately after the appearance of the peak on the switching curves. The repeated switching-on of the current within a short period of time (5-10 s) does not give rise to peak formation. These facts indicate that the crystallization overvoltage is associated with three-dimensional nucleation. Based on the experimental results, we evaluated the crystallization overvoltage due 

A typical switching curve corresponding to carbide electrodeposition onto the native substrate over wide temperature and current-density ranges (973-1173 K, i = 0.02-0.40 A/cm ) indicates the absence of any 

It is characteristic that the height of the overvoltage maximum for the above metals is proportional to the reciprocal time of their formation. This seems to be associated with the penetration of the part of the deposited components into the substrate bulk due to solid-phase diffusion, which allowed us to qualitatively characterize the degree of inertness of the substrate material. Thus, crystallization overvoltage on the 

Ni substrate is observed only at certain current-density values. On a Pt substrate, one observes noticeable depolarization with two pronounced waves that seem to correspond to the formation of alloys with different 

It is shown that the cathodic electrolysis products of melts based on sodium tungstate, in which molybde-num(VI) or tungsten(VI) oxides, lithium or sodium molybdate, tungstate, and carbonate are dissolved, are molybdenum, tungsten, their bronzes and carbides, and carbon. It is found that the phase composition of electrolysis products is determined by the concentration of carbonate in the melt. The conditions are determined for the deposition of galvanic coatings of molybdenum and tungsten carbides on different materials. 

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There are two hmiting cases for Eq. (7.3) for the initial stages of nucleation (low t value). Eirst, for large nucleation constant A, Eq. (7.3) reduces to... 

A number of studies have indicated that a labile cage-like (512) cluster of 20 water molecules around a hydrate guest former may not form preferentially at the initial stages of nucleation. MD simulations of xenon hydrate formation from a xenon-ice system showed that there is no preferential formation of cavities with 20 water molecules, which would be similar to the small hydrate cage. Rather, the statistical mean cage size distribution was found to be between 24 and 27 water molecules. Tse et al. (2002) suggest that this supports the experimental observation that sll SF6 hydrate formation does not require occupation of small cages. 

In the initial stages of nucleation, AGg increases faster with increases in r when compared to AG., and AG remains positive, reaching a maximum at a critical radius r, after which it decreases and eventually becomes negative. This occurs because... 

The formation of SiC on Si substrates in the initial stages of nucleation was also observed by Waite and Shaht 1 in MW PACVD using XPS. A diamond peak appeared after the incubation period with a simultaneous decrease in SiC peak intensity. However, SiC remained as an impurity even after several hours of deposition. 

M. M. Waite, and S. I. Shah, X-ray photoelectron spectroscopy of initial stages of nucleation and growth of diamond thin films during plasma assisted chemical vapor deposition, Appl Phys. Lett., 60(19) 2344-2346 (1992)... 

This allowed L vov et al. [6] to admit that, at sufficiently loose packing of spherical particles, the two neighbouring layers of the powder sample heat up from the container walls in a similar way. Only after complete decomposition of these layers does the decomposition of the next two succeeding layers begin, including initial stages of nucleation and acceleration. However, a discrepancy between the number of layers and that of oscillations may also be due to an error in determination of the mean size of the particles. If the particle radius was 9.5 pm instead of 7.5 pm, the number of layers would decrease from 8 to 4. 

The main concern of the classical homogeneous nucleation theory has been a thermodynamic description of the initial stage of nucleation from embryo to nucleus with a little larger size over the critical one (Seinfeld 1986, Pruppacher and Klett 1997, Seinfeld and Pandis 1998, Kulmala et al. 2000). The change of the free enthalpy of the cluster is at first positive because the decrease of entropy is initially larger (regular structure formation) than the decrease in enthalpy ... 

Zhang et al. used Langmuir monolayers to study the initial stage of nucleation and crystallization of calcium phosphate [160]. The monolayers used in their study consisted of dipalmitoylphosphatidylcholine, arachidic acid, and octadecylamine. The experiments show that the adsorption of calcium ions to the respective mono-layer is a prerequisite for subsequent nucleation. The authors also demonstrate that calcium phosphate forms through a multistage assembly process, in which first an amorphous calcium phosphate dihydrate layer forms, which then recrystallizes to form a crystalline hydroxyapatite layer (Fig. 12). This transformation of an amorphous phase to a crystalline phase provides direct evidence of a multistep crystallization process, which the authors claim is similar to the processes occurring in biomineralization. 

Usually, M02C and WjC coatings are applied onto non-oriented metal substrates by electrolysis of Na2W04-based oxide melts [10, 11]. However, in many instances, it is necessary to obtain continuous cathode deposits with preset properties (structure, orientation, and crystallite size). Therefore, an important role in electrolysis is played by the initial stages of crystal nucleation. In Refs [12-14], the results of such a study for electrodeposited Mo and W are presented. The initial stages of nucleation of Mo and W carbides have not been yet studied. The only known investigation was performed for platinum and glassy carbon electrodes [15]. 

By interrupting a bulk electrodeposition process in the initial stages of nucleation and growth, an electrode surface can be covered by well-separated nanocrystals. This makes electrodeposition a low-cost and easy-to-use approach for the formation of semiconductor nanocrystals in which quantum confinement leads to an atomic-like energy spectrum [199]. Electrodeposition of quantum-confined structures could be of... 

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