Elemental-silicon process

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

Figure 2. Variation of oxidation number of silicon with step and variation of number of oxygens bonded to silicon with step in direct or elemental-silicon process for (EtOJ Si.

The "conventional" methods for the preparation of SiC and Si3N4, the high temperature reaction of fine grade sand and coke (with additions of sawdust and NaCl) in an electric furnace (the Acheson process) for the former and usually the direct nitridation of elemental silicon or the reaction of silicon tetrachloride with ammonia (in the gas phase or in solution) for the latter, do not involve soluble or fusible intermediates. For many applications of these materials this is not necessarily a disadvantage (e.g., for the application of SiC as an abrasive), but for some of the more recent desired applications soluble or fusible (i.e., proces-sable) intermediates are required. 

The synthesis of organosilicones and organosilicone surfactants has been well described elsewhere [36-39] and hence only a brief review is given here. Industrially the manufacture of silicones is performed stepwise via the alkylchlorosilanes, produced through the reaction of elemental silicon with methyl chloride (the Muller—Rochow Process) [40,41]. Inclusion of HC1 and/or H2(g) into the reaction mixture, as in Eq. (1.2), yields CH3HSiCl2, the precursor to the organofunctional silanes, and therefore the silicone surfactants ... 

Sir Humphry Davy attempted to isolate this unidentified element through electrolysis—but failed. It was not until 1824 that Jons Jakob Berzehus (1779—1848), who had earlier discovered cerium, osmium, and iridium, became the first person to separate the element silicon from its compound molecule and then identify it as a new element. Berzehus did this by a two-step process that basically involved heating potassium metal chips with a form of silica (SiF = silicon tetrafluoride) and then separating the resulting mixture of potassium fluoride and silica (SiF + 4K —> 4KF + Si). Today, commercial production of sihcon features a chemical reaction (reduction) between sand (SiO ) and carbon at temperatures over 2,200°C (SiO + 2C + heat— 2CO + Si). 

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. 

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

Silicones are made by hydrolysis of organochlorosilanes RnSiCl4-n, which are produced from elemental silicon (obtainable from the carbon reduction of silica, i.e., sand, Section 17.7) by the Rochow process ... 

Crude elemental silicon can be obtained by reduction of silica sand with coke in the electric furnace (reaction 17.33) and may be adequate for making ferrosilicon alloys (Section 16.7.5) or silicones (Section 3.5). The high purity silicon used for electronic chips can be made from silica via silicon tetrachloride, which, like TiCU, is a volatile liquid (bp 57 °C) susceptible to hydrolysis but readily purifiable by fractional distillation. Indeed, the procedure for silicon resembles the Kroll process for titanium, except that an argon atmosphere is not necessary ... 

Because of the important potential applications of silicon nitride, the use of low-cost starting materials, such as elemental silicon and liquid ammonia or amines, may be more effective than the existing chloride method. In earlier work, litis process was found to form silicon di-imide (Si(NH)2), but required purification steps to remove chloride. 

Until 1982, most alkoxysilanes had been produced from chlorosilanes and alcohols. Hydrochloric acid was therefore still a problem. In 1982, a process was developed in which TMOS could be made directly from elemental silicon and methanol [5]. In the production of silicate coatings, TMOS is first converted to TEOS by an alcoholysis reaction with ethanol. This prevents toxic methanol vapors from escaping from the curing coating. The TEOS is partially hydrolyzed with the rest of the hydrolysis occurring at the time of application. This is therefore a way to produce silicates without chlorine. (If a practical method for converting alkoxysilanes to alkylsilanes could be found, there would also be a nonchlorine method of production of silicones.)... 

This relatively complicated reaction has been replaced by the so-called Direct Process or Rochow Process, 12,16,22 which starts from elemental silicon. It is illustrated by the reaction... 

In this chapter we have summarized selected literature data on the electrodeposition of semiconductors in ionic liquids. It has been demonstrated that elemental silicon, germanium, and selenium can be elecrodeposited in ionic liquids. Furthermore, it is shown that compound semiconductors like InSb, AlSb, CdTe and others can be made, especially at elevated temperatures where kinetic barriers are easier to overcome, even allowing the exclusive electrodeposition of grey selenium. In this context ionic liquids are very promising for semiconductor electrodeposition. Both wide electrochemical and thermal windows allow processes which are impossible in aqueous or organic solvents. 

Although the direct reaction of elemental silicon with methyl chloride shown in Eq. (3) looks simple, it is a complicated reaction and gives many kinds of byproducts.7,8 The yield of methylchlorosilane obtained from the direct reaction varies, and depends upon the reaction conditions such as temperature, pressure, flow rate of reactants, and other processing conditions including particle size and impurities of elemental silicon, catalyst, promoter, reactor type, etc.7... 

Errors in elemental fluxes derived from crustal estimates are larger than, or similar to, the value of the actual flux estimate for many of the major elements silicon, aluminum, iron, manganese, magnesium, and sodium. The fluxes for these elements are thus poorly constrained, but these estimates do serve as conservative bounds on the fluxes. Unfortunately, these bounds overlap the fluxes derived from hydrothermal fluid data and river data. For this reason, current ocean-crust flux estimates do not provide independent evidence for the magnitude of hydrothermal fluxes in the geochemical cycle for these elements. Within the bounds of these uncertainties, the data indicate that ocean floor hydrothermal processes may balance (or compound ) missing global fluxes of these elements. For these reasons, these elements are not discussed here in any detail. 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically usefiil include hydrosilation [7], silane alcdiolysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

The Na AlFg (cryolite) produced as a byproduct in this process is utilized in the aluminum industry and the SiH4, after ultrapurification, is decomposed in a fluidized bed reactor to hydrogen and ultrapure silicon on nuclei of elemental silicon already present there (see Fig. 3.4-3) ... 

The final step in the process is the reduction of purified, liquid silicon tetrachloride with very pure magnesium to give pure elemental silicon. 

At the turn of the century the cost of elemental silicon was 1600/lb. Improvement of reduction conditions allowed this price to fall to 0.10/lb by 1931 and made the use of the element more practical. Elemental silicon is employed as an additive to iron to give silicon-steels, which are more acid resistant, to alloy copper for production of silicon bronzes and as an additive to aluminum or magnesium to improve the strength of these structural materials and provide resistance to corrosion. The use of silicon as a semiconductor had begun by 1940 and after discovery of the direct process reaction became the necessary starting point for the production of silicone polymers103. 

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