Mechanism growth

The oxidation reaction of silicon is a process involving breaking Si-Si bonds and formation of Si-0 bonds with conversion of the silicon atoms from a valence of 0 to a valence of 4. In anodization the four valence electrons on a silicon atom are transferred in the overall reaction 

NO3- and HPO4- as N and P are detected in the oxide formed in water-free methanol electrolyte. Once water is present in methanol, the major part of the oxygen required for the oxidation comes from the water instead of the salts. 

Formation of the first layers of oxide (i.e., native oxide) on the surface of silicon, according to Ozanam and Chazalviel, appears to also require the presence of water even in nonaqueous solutions. On immersion into the solution the silicon surface is gradually evolving from a Fi-terminated surface (after FiF cleaning) to a silicon oxide-covered surface due to the residual water present in the nonaqueous electrolyte (10 ppm). Initially the water is molecularly adsorbed at the silicon surface, then slowly oxidizes the surface silicon atoms to form oxide islands. The oxide islands are about 0.6 nm thick and cover about 60% of the surface area after 1 week of immersion in various nonaqueous electrolytes. 

The reaction steps involve Si as an intermediate. It is possible that reactions with higher oxidation states such as Si and Si , which are considered to be present at the silicon/oxide interface, may be involved in addition to reactions (3.6) to (3.8).  

There are many other possible reactions involving increasingly higher energies. 

At normal deposition pressures, the mean free path of the gas molecules is 10 -10 cm and is much smaller than the dimensions of the reactor, so that many intermolecular collisions take place in the process of diffusion to the substrate. An understanding of the growth is made particularly difficult by these secondary reactions. In a typical low power plasma, the fraction of molecular species that is radicals or ions is only about 10 , so that most of the collisions are with silane. An important process is the formation of larger molecules, for example 

The secondary reactions greatly modify the mix of radicals within the plasma (Gallagher 1988). Their concentration is described by the diffusion-reaction equation. 

Within a uniform plasma, the radical concentration is Gx, where the lifetime, t, is kN). Those radicals with a high reaction rate have a low concentration and a short diffusion length and so are less likely to reach the growing surface. The least reactive species survive the collisions longest and have the highest concentrations, irrespective of the initial formation rates. One such radical is SiH which does not react with SiH, unlike radicals with fewer hydro ns, because Sij H structures are possible only with n 6. Thus the plasma contains a combination of long-lived primary radicals, and the secondary products of the more reactive gas species. 

Much of the direct experimental information about the radicals and ions in the silane plasma comes from mass spectrometry. Fig. 2.9 ows the concentrations as a function of argon dilution for a typical low power plasma. Gallagher and Scott (1987) find that SiHj accounts for at least 80 % of the gas radicals in a pure silane plasma. Argon dilution increases the concentration of other radicals and these eventually dominate the plasma. 

Tigerschoid and Ilmoni (T3) attributed the high degree of compaction attained by the granule to a cooperative action of the negative capillary 

In general, the extent of the ultimate consolidation of the granules and the rate of approach to this limiting value are functions of the size and size distribution of the particulate feed, its density and liquid content, and the 

Kapur and Fuerstenau (K5) have presented a unified description of the agglomeration process in which the phenomena of the compaction of the agglomerates and their passage through the various capillary regimes have 

Near the end of the nuclei region the constricted capillaries in the agglomerate begin to fill up with liquid. Eventually, when the interstitial void volume becomes almost equal to the liquid content, the liquid is squeezed onto the surface of the pellet, whose appearance changes from semidry to wet. This movement or demixing of liquid signals the onset of the transition region. From this point onwards, apart from some pockets of trapped air, the pellet is comprised of two phases only, solid and liquid. 

Compaction of nuclei pellets in the course of agglomeration. [From Kapur and Fuerstenau (K6).] 

---

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

An important application of photochemical initiation is in the determination of the rate constants which appear in the overall analysis of the chain-growth mechanism. Although we shall take up the details of this method in Sec. 6.6, it is worthwhile to develop Eq. (6.7) somewhat further at this point. It is not possible to give a detailed treatment of light absorption here. Instead, we summarize some pertinent relationships and refer the reader who desires more information to textbooks of physical or analytical chemistry. The following results will be useful ... 

In the next section we shall examine the distribution of molecular weights for polymerization which follows the chain-growth mechanism. 

In spite of the assortment of things discussed in this chapter, there are also a variety of topics that could be included but which are not owing to space limitations. We do not discuss copolymers formed by the step-growth mechanism, for example, or the use of Ziegler-Natta catalysts to regulate geometrical isomerism in, say, butadiene polymerization. Some other important omissions are noted in passing in the body of the chapter. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

In all of these processes it is possible to increase the yield of whiskers by a dding metallic impurities, and the sublimation process requires such additions. The vapor—Hquid—sohd (VLS) growth mechanism is often thought to be involved. 

There is often complex competition between growth mechanisms. 

In general, analytical solutions are only available for specific initial or inlet size distributions. However, for batch granulation where the only growth mechanism is coalescence, at long times the size distribution may become self-preserving. The size distribution is selfpreserving if the normahzed size distributions

[Pg.1906]

Wagner (1961) examined theoretically the growdr kinetics of a Gaussian particle size distribution, considering two growth mechanisms. When the process is volume diffusion controlled... 

M. Endo. Meeanisme de eroissanee en phase vapeur de fibres de earhone (Tlte growth mechanism of vapor-grown carbon fibers). PhD thesis. University of Orleans, Orleans, France, 1975. (in French). 

While a large body of research has been compiled on VGCF growth mechanisms and the properties of the resulting fiber, very little work has been performed on the properties of composites which are reinforced with VGCF. Essentially, the small quantities of the fiber which has been synthesized, typically in laboratory settings, has not been adequate to support such evaluations. Research efforts at Applied Sciences, Inc. have been motivated by the desire to determine the properties of... 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

Key Words—Carbon nanotubes, vapor-grown carbon fibers, high-resolution transmission electron microscope, graphite structure, nanotube growth mechanism, toroidal network. 

Fig. 10. Growth mechanism proposed for the helical nanotubes (a) and helicity (b), and the model that gives the bridge and laminated tip structure (c).

Although the physical growth mechanism of the cylindrical sheets is not yet fully known, the fact that... 

The growth pathway of various fullerene- and graphene-type nano-objects may be related. They are synthesized in the vapor phase and often appear simultaneously on the same sample. A common growth mechanism with similar nucleation seeds may, therefore, lead to these different structures. 

---