Crystallites nucleation

Figure 5.5. Electron micrographs of different types of diamond film grown on silicon. The white bar shows the scale in micrometres (p.m) (thousandths of a millimetre), (a) The initial stages of diamond growth on a nickel substrate, showing individual diamond crystallites nucleating in scratches and crevices created on the surface by mechanical abrasion, (b) a randomly oriented him,...

Layadi et al. have shown, using in. situ spectroscopic ellipsometry, that both surface and subsurface processes are involved in the formation of /xc-Si [502, 503]. In addition, it was shown that the crystallites nucleate in the highly porous layer below the film surface [502, 504], as a result of energy released by chemical reactions [505, 506] (chemical annealing). In this process four phases can be distinguished incubation, nucleation, growth, and steady state [507]. In the incubation phase, the void fraction increases gradually while the amorphous fraction decreases. Crystallites start to appear when the void fraction reaches a maximum... 

These traps close to the substrate are related to structural defects, which are generated when the 6T crystallites nucleate on the glass surface. 

Glass formation or the absence of crystallite nucleation and growth is also dependent on the confusion principle . This is an empirical observation which suggests that, in a multicomponent melt, several crystalline forms are in competition in the crystallization process the confused situation that results acts as a barrier to microcrystal growth and enhances the formation of the vitreous state. 

The preparation of model catalyst films suitable for investigation by microscopic techniques has been described by Wanke and Bolivar. The most common technique of preparing alumina or silica substrates is oxidation of aluminum or silicon foils. Supp( t films are typically mounted for examination after which a metal film is prepared on the support by vacuum deposition or sputtering to thicknesses ranging from monolayer to 2 nm. Thomal treatment of the sample causes breakup of the metal film into metal crystallites. Table 1 summarizes conditions used by various investigators to convert metal films to crystallites. Apparently, crystallite nucleation is a strong function of metal, atmosphere, temperature, and film thickness and a weak function of support, although these results are only qualitative, since precise conditions for metal film breakup were not available from many of the studies listed. Nevertheless, the more recent studies indicate that breakup of Pt/alumina films to 1.8 nm particles occurs in vacuum at temperatures as low as 473 K. 

Ferris, F.G., Beveridge, T.J. Fyfe, W.S. (1986) Iron-silica crystallite nucleation by bacteria in a geothermal sediment. Nature 320, 609-611. 

The kinetics of many solid state reactions have been reported as being satisfactorily represented by the first-order rate equation [70] (which is also one form of the Avrami-Erofeev equation (n = 1)). Such kinetic behaviour may be expected in decompositions of fine powders if particle nucleation occurs on a random basis and growth does not advance beyond the individual crystallite nucleated. 

The cluster nucleation is the process of random walks in m, while the crystallite nucleation is the diffusion process in the one-dimensional size space. The probabilities of transitions from the vertex to either of the adjacent vertices depend on the free energy of clusters in different structure states. 

The rate of crystallite nucleation /(/ ) is described by a (6.54)-type expression, where N and AG ff are replaced by the critical crystalline nucleus size, N, and the free energy of nucleation, AG. ... 

The overall crystallization rate is determined by the superposition of crystallite nucleation rate and growth rate. 

Barium titanate powders were produced using either an amorphous hydrous Ti gel or anatase precursor in a barium hydroxide (Ba(OH)2) solution via a hydrothermal technique in order to discern the nucleation and formation mechanisms of BaTi03 as a function of Ti precursor characteristics. Isothermal reaction of the amorphous Ti hydrous gel and Ba(OH)2 suspension is believed to be limited by a phase boundary chemicd interaction. In contrast, the proposed BaTi03 formation mechanism from the anatase and Ba(OH>2 mixture entails a dissolution and recrystallization process. BaTi03 crystallite nucleation, studied using high resolution transmission electron microscopy, was observed at relatively low temperatures (38°C) in the amorphous hydrous Ti gel and Ba(OH)2 mixture. Additional solution conditions required to form phase pure crystallites include a C02-free environment, temperature >70°C and solution pH >13.4. Analysis of reaction kinetics at 75°C was performed using Hancock and Sharp s modification of the Johnson-Mehl-Avrami approach to compare observed microstructural evolution by transmission electron microscopy (7). 

Figure 6.12. STM images of Si and Ge grown on crystalline SiNx/Si(lll). (a) 5-nm Si deposited at 750°C (b) 50-nm Ge deposited at 550°C, the top facets on large islands are mostly (111) (c) 80-nm Si deposited at800°C, the top facets of crystallites are mostly (111) aligned with Si substrate and (d) Ge crystallites nucleated on Sb-covered SiNx/Si( 111). Scan area (a), (b) (0.22 om) (c), (d) (0.55 um). ...

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