Elemental silicon preparation

After oxygen, silicon is the most abundant element in the earth s crust, It occurs extensively as the oxide, silica, in various forms, for example, flint, quartz, sand, and as silicates in rocks and clays, but not as the free element, silicon. Silicon is prepared by reduction of silica, Si02- Powdered amorphous silicon can be obtained by heating dry powdered silica with either powdered magnesium or a... 

C04-0145. Silicon tetrachloride is used in the electronics industry to make elemental silicon for computer chips. Silicon tetrachloride is prepared from silicon dioxide, carbon graphite, and chlorine gas. 

Pure elemental silicon is a hard, dark gray solid with a metallic luster and with a crystalline structure the same as that of the diamond form of carbon. For this reason, silicon shows many chemical and physical similarities. There is also a brown, powdery form of silicon having a microcrystalline form. The element is prepared commercially by reducing the oxide by reacting it with carbon (as coke) in electric furnaces. On a small scale, silicon has been obtained from the oxide by reduction with aluminum meted. 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

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. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

The ability to insert in many element-element bonds is an important property of 1 the r -p1 rearrangement of the pentamethylcyclopentadienyl ligands during the reaction is a prerequisite to show a silylene-type reactivity. From a preparative point of view it is worth mentioning that element-silicon bonds which otherwise are difficult to form are easily accessible with the help of 1. In addition, the leaving group character of the pentamethylcyclopentadienyl substituents allows further chemical transformations (vide infra). 

Gay Lussac and Thenard in 1809 obtained very impure amorphous silicon by passing silicon tetrafluoride over heated potassium. Berzelius in 1823 prepared elemental silicon in high purity by the same method. He also obtained silicon by heating potassium fluosilicate with potassium metal. Deville produced crystalline silicon in 1854 by electrolysis of a molten mixture of impure sodium aluminum chloride containing 10% silicon and a small quantity of aluminum. 

Silicon [7440-21-3], Si, from the Latin silex, silicis for flint, is the fourteenth element of the Periodic Table, has atomic wt 28.083, and a room temperature density of 2.3 gm/cm3. Silicon is brittle, has a gray, metallic luster, and melts at 1412°C. In 1787 Lavoisier suggested that silica (qv), of which flint is one form, was the oxide of an unknown element. Gay-Lussac and Thenard apparently produced elemental silicon in 1811 by reducing silicon tetrafluoride with potassium but did not recognize it as an element. In 1817 Berzelius reported evidence of silicon occurring as a precipitate in cast iron. Elemental silicon does not occur in nature. As a constituent of various minerals, eg, silica and silicates such as the feldspars and kaolins, however, silicon comprises about 28% of the earth s crust. There are three stable isotopes that occur naturally and several that can be prepared artificially and are radioactive (Table 1) (1). 

Wide variations in Cs and Na leaching observed in some of the first samples produced were found to result from the formation of the corresponding molybdate compounds which are highly water soluble. This problem was eliminated by the addition of 1-2% by weight of elemental silicon which served as a reducing agent and prevented molybdate formation. The silicon was added during the preparation of the sodium titanate material. 

Preparation and properties of silicon. Elemental silicon of about 98% purity may be produced by the reduction of silicon dioxide by aluminum. 

A substrate 10 of silicon comprising an array of switching elements is prepared. Contact pads such as 16 and 18 are connected to inputs of the switching elements. A protective nitride layer 20 is formed, with openings for the contact pads, on top of the substrate. A layer of photoresist 25 is patterned on top of the nitride layer. A layer of indium alloy 26, fabricated from a combination of indium, bismuth, lead, cadmium and tin, is deposited on the substate so as to contact the contact pads. This layer is built up such that its upper surface is higher than the structures that constitute the topography of the substrate. Portions of the conductive layer overlying the photo-resist are removed by a lift off process... 

Combination of the 13 most important main group elements (H, B, C, Si, N, P, O, S, F, Cl, Br, I, Xe) allows the construction of 1638 triatomic molecules, of which 1183 are linear and 455 are cyclic. With rather few exceptions such as Sil2, most of them are already known61. The novel triatomic molecule can be prepared in analogy to other dihalogensilylenes10,16 by the reaction of especially purified, white crystalline SU4 with elemental silicon (Figure 20). 

The elements can be obtained by reduction of oxides or halides. Highly divide carbon black is used as a catalyst and black pigment, and impure carbon (coke) for reducing some metal oxides (e.g., in the manufacture of iron). Pure silicon prepared by reduction of SiCl4 with Mg is used in electronics (silicon chips) although much larger quantities of impure Si are used in steels. 

The products were purified by preparative GLC and subsequent spectral analysis led to the assignments of the structures for the products. The results obtained from the direct reaction of elemental silicon with mixtures of various (dichloromethyl)silanes 6a-d and hydrogen chloride are summarized in Table I. 

The alkali metal salts of the six-coordinate anion (61) may readily be prepared (equation 61) by using Si02 (or Si(OMe)4) as starting material, thus forming monomeric silicon compoimds from the polymeric Si02 without the need for the isolation of elemental silicon. ... 

A nonmetallic element, silicon, was prepared sonodiemically by reducing tetraethyl orthosilicate (TEOS) with a colloidal solution of sodium. The product was obtained as 2-5 nm sized, highly aggregated partides. The silicon exhibited a luminescence similar to that of porous silicon. This procedure is suggested as a general sonochemical reduction leading to the formation of metallic nanopartides [26]. 

Elemental silicon is usually prepared by the high-temperature reduction of silica (sand) with coke. Excess Si02 prevents the formation of silicon carbide. 

Silicon dibromide and diiodide, SiBr2 and Sil267, can both be prepared by passing the tetrahalides SiX4 over elemental silicon (cf. Figure 5). A schematic radical cation state correlation of all silicon dihalides (scheme 13) based on the SiCl 20 sequence [scheme... 

Elemental silicon is frequently used in steel and aluminum industries for the preparation of aluminum alloys. In the production of semiconductors, amplifiers, photovoltaic and solar cells, ultra-pure silicon is needed. The world s computers and all computer-controlled equipment have, at their heart, silicon chips - crystals of silica... 

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