Elementary Silicon

Metallurgical grade silicon ( 98 % pure) is produced by reducing quartz sand with charcoal or peat coal in an electric shaft furnace, using burned graphite electrodes. 

The volatile silicon monoxide which is first produced from silicon dioxide reacts with carbon in the upper, cooler part of the furnace to form silicon carbide. This reacts with carbon dioxide to form elemental silicon. The furnace is operated continuously and is rotated slowly. It is emptied of metallic silicon, in the form of compact chunks, every 1 to 2 hours. The natural impurities, mainly iron and aluminium, constitute about 2% of the product and are conducive to the synthesis of organic silicon chlorides. 

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The other chemicals mentioned in this book are less dangerous and safety goggles and rubber gloves, which should always be used, are usually sufficient protection. Elementary silicon is inert and shows no toxic effects. In this respect, silicon is different from many other semiconductors, which may contain poisonous compounds. However, sufficient eye protection is required while cleaving wafers, because of the risk of fragmentation. 

Silicon forms two oxides, silica or the dioxide, Si02, and a divalent monoxide, SiO. The latter is obtained by heating silica with elementary silicon at 1,450°C in vacuum. Silicon sublimes as its monoxide, which on rapid cooling forms light brown amorphous SiO ... 

Mass spectrometric studies reveal that silicon vapour produced by heating elementary silicon to 2300 K consists mainly of silicon atoms, but there is also a considerable amount of Si2, Si3 and of higher aggregates present25. ... 

The gaseous SiO monomer is generated by reduction of Si02 with elementary silicon or other reducing compounds at high temperature. On rapid cooling,... 

It can be shown independently that cuprous chloride, the other product of reaction 1, is reduced by elementary silicon at 265° or more ... 

At first it was thought that the copper methyl might react with the silicon halide in the manner of a Grignard reagent, but copper ethyl and copper phenyl prepared in ether suspension did not react with silicon tetrachloride. Neither did free methyl radicals from lead tetramethyl react with elementary, silicon, but they did add on silicon that was being chlorinated. This suggests that the third step in the mechanism is the addition of methyl groups to the chlorinated silicon formed in the cuprous chloride reduction ... 

Table 9.8 Chemical compositions of microsilica from the production of elementary silicon and 75% fcrrosilicon alloy (H53)...

The convenient method of preparation of substituted forms of HAp is described in [39,40], The elementary silicon was added to 3Ca(H2P04)2+Ca0 mixture which was then mechanically activated in planetary mill for 25 min. The sample was smdied by Si MAS NMR spectroscopy. It was shown that three bands of silicon are presented in spectra with chemical shifts of -129.7, -122.7, -112.6 ppm (Fig. 7.19). 

Elementary Silicon and Silicon Alloys. Silicon is a brittle steel-gray metalloid, with m.p. 1420 C, b.p. 2600° C, and density 2.42 g/cm. It can be made by the reduction of silicon tetrachloride by sodium ... 

What is the electronic strui ture of elementary silicon Of silicon carbide ... 

The chemistry of trichlorosilane—tertiary amine combinations Reactions of elementary silicon... 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

We have mentioned in the previous chapter the two crystalline forms of carbon diamond and graphite. The latter structure is unique among the elements, but a number of other elements of Group IV crystallize with the diamond structure ilicon, germanium, and tin (grey modification). In the B subgroup only lead has a typical metallic structure (cubic close-packed), whereas all the elements of the A subgroup crystallize with close-packed structures. In elementary silicon Si-Si = 2-35 A. 

The higher boiling residues obtained in the Direct Synthesis of methylchlorosilanes (from elementary silicon and methyl chloride) include a variety of compounds which contain silicon-silicon bonds. A particularly useful fraction (b.p. 150°-160°C) contains methylchlorodisilanes of the type, (CH3) Cl6-nSi2. The use of this fraction for the preparation of disilanes has been described (2, 62-65) however, few applications leading to the formation of higher polysilanes have been reported. 

A study of the molecular structure of disilane, hexachloiodisilane, and hexamethyldisilane has been carried out (2) with the aid of electron diffraction, and the Si—Si bond lengths obtained (2.32 0.03, 2.32 0.06, and 2.34 0.10 A, respectively) are roughly the same as that in elementary silicon. No information is available on the bond lengths of higher organo-polysilanes. 

Figure 2.5 represents the data as a function CsiP on P. Good linear fit is observed with the plot intersecting the negative part of ordinate axis. Hence it follows that main species in equiUhrium with silicon metal are potassium subions and Si(l). This conclusion consists with the data available on chemical reactions of elementary silicon and nonstatiOTiaiy electroreduction of silicon (IV) in chloride-fluoride melts [14]. 

The species of Si(I) form at the end of the non-stationary cathode reduction. They deposit onto the electrode surface resulting in the formation of the film. Elementary silicon can be obtained only at longer times when the electrode film system with appropriate properties is finally settled down and the solid-phase reduction mechanism enters its play (see Chaps. 1 and 5). If, by any chance, the elementary silicon is available in the system, then the intervalence reaction occurs with ions taking an essential part. 

The prevailing type of charge carriers in the film should affect considerably the apparent character of the inner electrochemical process. Should these carriers be alkali metal cations, their discharge at the inner boundary would be preferable. Thus, the formation of alkali metal is expected to be the side process in such systems resulting in the low current efficiency of the targeted process of the reduction of a polyvalent metal species. Evidently, that accounts for higher current efficiency of elementary silicon electrodepositiOTi from potassium halide baths relatively to the electrolytes based on sodium halides. 

The Direct Reaction of Hydrocarbon Halides with Elementary Silicon ... 

SiF by letting gaseous hydrofluoric acid HF pass over quartz. The tetraflu-oride formed was allowed to pass over hot potassium metal. In a violent reaction a reddish-brown powder was formed. Perhaps an impure elementary silicon. 

Today silicon is classified as a nonmetal but sometimes also as a semi-metal because it displays properties similar to those of both metals and nonmetals. Elementary silicon is produced industrially in large-scale by reduction of quartz with coke. The product is commonly and inappropriately called metallic silicon (see Figure M53). 

The direct process, used for the bulk of methyl silicone manufacture, is based on the reaction of alkyl or aryl halides with elementary silicon in the presence of a catalyst such as copper ... 

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