Silicon vapor phase hydrolysis

Transparent vitreous sihca is made by electric melting of natural quartz minerals such as sand in vacuum. It also may be made by fusing quartz in flame or by vapor phase hydrolysis or oxidation of pure silicon compounds by heating electrically or using a flame or plasma. Translucent form is made by fusion of high purity quartz sand crystals. 

In the typical flame or vapor-phase hydrolysis process, which uses SiCl4 as the precursor, silicon tetrachloride vapor is fed into a flame using an oxygen carrier gas. Fine (<50 nm), amorphous silica particles form in the flame (57). 

Silicon Dioxide occurs as an amorphous substance that shows a noncrystalline pattern when examined by X-ray diffraction. It is produced synthetically, either by a vapor-phase hydrolysis process, yielding fumed silica, or by a wet process, yielding precipitated silica, silica gel, colloidal silica, or hydrous silica. Fumed silica is produced in an essentially anhydrous state, whereas the wet-process products are obtained as hydrates or contain surface-adsorbed water. 

Fumed silica is traditionally produced by vapor phase hydrolysis of silicon tetrachloride in a hydrogen-oxygen flame [83]. Figure 7.16 shows the reaction sequence for this process. 

Fumed silica, or fumed silicon dioxide, is produced by the vapor phase hydrolysis of silicon tetrachloride in an H2/O2 flame. The reactions are shovm in Chapter 19. Hydrophilic fumed silica bearing hydroxyl groups on its surface is produced by this process. Hydrophobic fumed silica is made by processing fumed hydrophilic silica through in-line hydrophobic treatments such as with silanes, siloxanes, silazanes, and so on [1]. Examples of different types of hydrophobic fumed silica coatings include DMDS (dimethyldichlorosilane), TMOS (trimethoxyoctylsilane), HMDS (hexamethyldisilazane). 

Aerosils or pyrogenic silica. Powders made by condensing silica from the vapor phase at elevated temperature. The silica vapor is produced by a) direct volatilization of SiOj, (6) reduction of SiOj to volatile SiO which is reoxidized, (c) oxidation of volatile silicon compounds such as chloride or esters, or (d) vapor phase hydrolysis of SiF. ... 

Fumed silica is produced from the vapor-phase hydrolysis of silicon tetrachloride as shown in Figure 8.7. 

Silicon Dioxide occurs in nature as agate, amethyst, chalcedony, flint, quartz, sand, and tridymite. Silicon dioxide is obtained by acid precipitation from a sodium silicate solution to yield veiy fine particles. If silicon dioxide is obtained by the addition of sodium silicate to a mineral acid, the product is termed a silica gel. The pharmaceutical industry uses colloidal silicon dioxide, which is a submicroscopic fumed silica prepared by the vapor phase hydrolysis of a silica compound, such as silicon tetrachloride. Other names for colloidal silicon dioxide include light anhydrous silicic acid, silicic anhydride, and silicon dioxide fumed. Colloidal Silicon Dioxide NF must meet the following standards ... 

Thin films of mixed metal oxides are usually obtained from a mixture of two different kinds of alkoxide precursors. However, this method suffers from problems with stoichiometry control since extensive efforts must be made to control the vapor phase concentration of two precursors with often dissimilar vapor pressures. Also of importance here is the near impossible task of matching rates of hydrolysis/oxidation to give pure , non-phase segregated films, i.e., those having a homogeneous composition and structure. In an effort to solve these problems, research effort has been aimed at single-source precursors, i.e., those containing both aluminum and silicon. 

CAB-O-SIL Also written Cab-o-Sil. A vapor-phase process for making an AEROSIL silica powder by the flame hydrolysis of silicon tetrachloride. Also a trade name for the product. Developed by Cabot Corp. See also AEROSIL. 

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