Silicon dioxide reactions

For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Write a balanced equation for the reaction of hydrofluoric acid with Si02. What volume of 2.0 M HF is required to react with one gram of silicon dioxide ... 

Many reactions are being used to deposit silicon dioxide, both experimentally and in production, and many more are being investigated in the laboratory. The selection depends on the application, the temperature limitations, the type of equipment available, and other factors. 

The uses of CVD silicon dioxide films are numerous and include insulation between conductive layers, diffusion masks, and ion-implantation masks for the diffusion of doped oxides, passivation against abrasion, scratches, and the penetration of impurities and moisture. Indeed, Si02 has been called the pivotal material of IC s.1 1 Several CVD reactions are presently used in the production of Si02 films, each having somewhat different characteristics. These reactions are described in Ch. 11. 

Silicon is generally considered to be a congener of carbon and this is also reflected in the evolution of silicon as a reducing agent for metal oxides. Silicon forms a fairly stable solid oxide silica or silicon dioxide (Si02) and also a stable gaseous oxide silicon monoxide (SiO), both of which can be useful in oxide reduction reactions. 

Sodium beryllium fluoride (Na2BeF4) is water-soluble and sodium aluminum fluoride (Na,AlF6) is water-insoluble. A part of the silicon volatilizes off as silicon tetrafluoride (SiF4), while the other part remains in the residue as silicon dioxide (Si02). Fluorination of silicon is unnecessary and it would be economical to recover all of it as silica. This is accomplished by using soda ash, i.e., sodium carbonate (Na2C03) in the reaction mixture ... 

Fig. 4a. TEOS is liquid at room temperature and slowly hydrolyzes into silicon dioxide and ethanol when in contact with ambient moisture. In TEOS, the silicon atom is already oxidized the conversion of TEOS to Si02 is essentially a rearrangement rather than an oxidation reaction. The overall reaction for the Si02 matrix requires the removal of two oxygen atoms from TEOS as shown in Fig. 4b.

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

The reactions of the resulting stabilized ions 153a,b with silicon dioxide produces isolable polyfunctional compounds 155a,b. It should be emphasized that the configuration of the stereocenters in nitronate (153) remains unchanged in the course of the transformation and the reaction is stereoselective with respect to the new stereocenter at the atom bearing the nitro group. 

Preparation ofNb5Si3 Metallic niobium and silicon dioxide do not react if heated (for instance at 1100°C) under vacuum. In the presence of traces of H2 or I2 the formation of transporting compounds (SiO or Nbl4) is observed, followed by their migration and reaction according to the following schemes ... 

SiNW (made of silicon, silicon-covered with silica, or silica) can be either crystalline or amorphous. The interaction of silicon and metal catalysts, the evolution of silicon precipitating out of the catalysts, and the reactions followed the precipitation determine the morphology and crystallinity of SiNW. In many cases, SiNW have been observed with a silicon core covered with an amorphous silicon dioxide sheath. 

Beryllium oxide (BeO) is a beryllium compound produced in significant commercial quantities. The chemical process starts with minerals containing aluminum silicate and silicon dioxide and undergoes a number of chemical reactions, some at high temperatures, to end up with BeO. 

Silicon dioxide plays a critical role in the electronics industry. The silicon used to produce silicon chips is derived from silicon dioxide. Semipure silicon dioxide (to about 99%) is prepared from the reaction of silicon dioxide with coke (a poor grade of graphite) using high temperature and an electronic arc. 

For oxide CMP, the purpose of the solution is two fold. First, water weakens the Si—O bond in a silicon dioxide film and softens the surface as it becomes hydrated with Si—OH bonds [6,7]. Figure 10 shows the reaction mechanism. Second, the solution is to provide a basic environment (pH > 10), which accelerates the hydration rate. An environment with high pH values will allow the polishing-induced reaction to be further accelerated because the surface Si(OH) species will be partially dissolved into water. In the meantime, the zeta potential of silica increases with increasing pH values. At high zeta potentials silica particles will repel each other, whereby a better-suspended slurry is formed. 

The type of attack that occurs in liquid media is highly dependent on the chemical nature of the liquid—that is, molten metal, molten ceramic, or aqueous solution. We will consider two industrially important cases attack by molten metals and attack by aqueous media. The attack of most metal oxide ceramics by molten metals involves a simple exchange of one metal ion for another. For example, silicon dioxide in contact with molten aluminum is susceptible to the following corrosion reaction ... 

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

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