Silicon oxidation formulation

Oxide Thickness Versus Time. Silicon oxidation has been modeled by using the linear-parabolic macroscopic formulation of Deal and Grove (69). As a starting point for the study of this model, the kinetics of oxidation... 

The many etch rate equations described above are empirical in nature even though mechanistic arguments are made in each specific case. One important omission in these quantitative formulations on the etching kinetics is the lack of consideration of the effect of the structure of silicon oxides. As shown in Fig. 4.40, etch rate can vary over more than three orders of magnitude for different types of oxides. It increases with increasing disorder of the oxide structure with the most ordered oxide, that is, quartz, having the lowest etch rate. The structural disorder of the silicon oxide can be due to impurities, partial oxidation of the silicon atoms, and degree of crystallinity. 

In contrast to household detergents, indnstrial detergents contain much greater amounts of silicate. When dissolved in water, they give solutions of mnch higher pH than domestic formulations [1]. This is the domain of anhydrons sodium metasilicate. It is described by the chemical formula NajSiOj, which corresponds to a one-to-one molar ratio of silicon oxide and sodium oxide (Na20 Si02 = 1 1). It has a crystalline strnctnre and forms monoclinic, colorless crystals or a white powder... 

The continuous demand for new materials have moved to the investigation of polyimides containing metal atoms. Although many polyimide formulations have been reported containing inorganic fillers, of instance silica, it seems that not more than 10% of silica can be incorporated to the composite material at molecular level. Therefore, methods have been outlined to get hybrid polyimides by the sol-gel process. It implies the combination of polyimide with classical precursors of silica employed in the sol-gel methods, particularly tetraethoxysilane and tetramethoxysilane, which lead to silicon oxide through hydrolysis and polycondensation. 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

Stoved phenolics have outstanding acid resistance (up to 200 C in dry conditions and up to 100°C in wet conditions), except to strong oxidizing acids. They are unsuitable for use with alkaline solutions above pH 10, wet chlorine or hypochlorite solutions. Phenolics/silicon formulations can be used for steam up to 180°C without a significant effect on heat transfer rates. 

An industrial blend of ethylene oxide (EO) PEMS marketed as a personal care product was examined by positive ion FIA-APCI-MS and LC-APCI-MS-MS (Fig. 2.8.8) [41]. The FIA-APCI-MS spectrum without LC separation (Fig. 2.8.8(a)) is dominated by ions corresponding to unreacted PEG (m/z 520, 564, 608, 652,...), whilst the ions corresponding to the PEMS (m/z 516, 560, 604, 648,...) could only be clearly observed following LC separation (Fig. 2.8.8(b)). Comparison of the TIC chromatograms of PEMS and PEG (Fig. 2.8.8(c) and (h)) demonstrates the dominance of the PEG by-products in the commercial formulation. It is unclear whether the observed relative intensities are representative of the actual amounts or of the different ionisation efficiencies, due to the confidential nature of the product composition. However, the spectra indicate a trisiloxane surfactant structure of that shown in Fig. 2.8.2 (R = Ac) and FIA-MS analysis of another commercial formulation of this product showed good spectra dominated by the silicone surfactants [48], indicating that the PEG by-product composition can vary significantly in commercially available PEMS formulations. 

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