Nucleation Silicon

Modifications to Precipitates. Silicon is sometimes added to Al—Cu—Mg alloys to help nucleate S precipitates without the need for cold work prior to the elevated temperature aging treatments. Additions of elements such as tin [7440-31-5] Sn, cadmium [7440-43-9] Cd, and indium [7440-74-6] In, to Al—Cu alloys serve a similar purpose for 9 precipitates. Copper is often added to Al—Mg—Si alloys in the range of about 0.25% to 1.0% Cu to modify the metastable precursor to Mg2Si. The copper additions provide a substantial strength increase. When the copper addition is high, the quaternary Al CuMg Si Q-phase must be considered and dissolved during solution heat treatment. 

Silicone foam thus formed has an open ceU stmcture and is a relatively poor insulating material. Cell size can be controlled by the selection of fillers, which serve as bubble nucleating sites. The addition of quartz as a filler gready improves the flame retardancy of the foam char yields of >65% can be achieved. Because of its excellent dammabiUty characteristics, siUcone foam is used in building and constmction fire-stop systems and as pipe insulation in power plants. Typical physical properties of siUcone foam are Hsted in Table 10. 

Vapor decomposition (14,15) iavolves dryiag, decomposiag, and vaporising a spray of salt precursor solution ia a plasma, and subsequentiy nucleating and growing ceramic particles ia the vapor. Silicon carbide [12504-67-5] SiC, powder is produced by this method. 

Because of the possibility of focusing laser beams, tlrin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible tire production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and tlrus to some unevenness in tire product film. This is because the subsuate is relatively cool, and therefore tire surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the drermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is nanower dran the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geomeU y. 

The nature of the deposit and the rate of nucleation at the very beginning of the deposition are affected, among other factors, by the nature of the substrate. A specific case is that of epitaxy where the structure of the substrate essentially controls the structure of the deposit.Plb lP ] Epitaxy can be defined as the growth of a crystalline film on a crystalline substrate, with the substrate acting as a seed crystal. When both substrate and deposit are of the same material (for instance silicon on silicon) or when their crystalline structures (lattice parameters) are identical or close, the phenomena is known as homoepitaxy. When the lattice parameters are different, it is heteroepitaxy. Epitaxial growth cannot occur if these stmctural differences are too great. 

After the silicon is completely reduced. Reaction (3) stops and Reaction (1) is resumed. The temporary deposition of sacrificial silicon interrupts the tungsten grain growth as new nucleation sites are created. Larger grain and columnar growth are essentially eliminated... 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

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,...

The nature of the nucleation site as well as the kinetics of formation and dissociation of the H-induced and stabilized platelets have yet to be experimentally studied in detail. However, their generation in float-zone (Johnson and Herring, 1988b) as well as Czochralski-grown (Fig. 8) silicon argues against the involvement of oxygen or carbon in platelet nucleation. 

The process described above is expected to produce a random distribution of active and passive spots on the electrode interface. But the electrode surface may also be artificially patterned prior to anodization in order to form nucleation centers for pore growth. This may be a lithographically formed pattern in said passive film or a predetermined pattern of depressions in the electrode material itself, which become pore tips upon subsequent anodization. The latter case applies to silicon electrodes and is discussed in detail in Chapter 9, which is devoted to macropore formation in silicon electrodes. 

In the third study, Miyazaki and Henry (1978) carried out vapor bubble growth experiments with water drops in hot silicone oil under various pressures of argon gas. As conducted, the oil temperature was set so that the interface temperature was below the homogeneous nucleation temperature of water. When bubbles did appear, their growth was followed by... 

Silane and ethylene are present at very high concentrations so that homogeneous nucleation dominates the process. As the gases enter into the hot part of the injector, the silane will decompose and form small Si liquid droplets or solid microcrystals, depending on the temperature. The ethylene will also take part in the reaction, forming microparticles of Si C. It has been noted [39] that even a small addition of hydrocarbons converts the Si droplets to stable particles of Si C (or non-stoichiometric SiC). The stability may, in a hand-waving circumstantial way, be intuitively understood from a solubility point of view. The solubility of carbon in silicon is very low, thus, when carbon is added to the Si droplets, the phase will be solid rather than liquid. 

It should be recalled that the final step in the nodular iron treatment process is termed "post inoculation." The purpose of this procedure is to aid in the elimination of iron carbides and promote enhanced nucleation and proper growth of graphite spheroids. This is accomplished by the introduction of the element silicon (usually a ferrosilicon alloy) along with calcium and maybe some magnesium or rare earth. It has been demonstrated that the benefits of rare earth additions are not affected as a function of the time in the process that they are added (23). For example, the elimination of iron carbides by use of the rare earths is possible if the rare earths are introduced along with the primary nodulizer or with the post inocu-lant. In passing, it should be remarked that both the primary nodulizers and ferrosilicon inoculants contain about 1% calcium. 

Interesting results were obtained by this method as shown in Fig. 13, which depicts the drastic effect of Cu nucleation on the OCP value. These curves first show the initial OCP recorded during 3 min in the dark, in oxygen free pure DHF, on 6 separate n-type silicon samples. This value is quite reproducible for the whole set of samples. 

The substrate for this CVD process is a thin silicon rod, called a slim rod, which serves as a nucleation site for the depositing silicon. After deposition, the EGS, or polysilicon, is processed in the Czochralski (CZ) growth process. 

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