1911 Encyclopædia Britannica/Silicon
SILICON [symbol Si, atomic weight 28.3 (O=16)], a non-metallic chemical element. It is not found in the uncombined condition, but in combination with other elements it is, with perhaps the exception of oxygen, the most widely distributed and abundant of all the elements. It is found in the form of oxide (silica), either anhydrous or hydrated as quartz, flint, sand, chalcedony, tridymite, opal, &c, but occurs chiefly in the form of silicates of aluminium, magnesium, iron, and the alkali and alkaline earth metals, forming the chief constituent of various clays, soils and rocks. It has also been found as a constituent of various parts of plants and has been recognized in the stars. The element exists in two forms, one amorphous, the other crystalline. The older methods used for the preparation of the amorphous form, namely the decomposition of silicon halides or silicofluorides by the alkali metals, or of silica by magnesium, do not give good results, since the silicon obtained is always contaminated with various impurities, but a pure variety may be prepared according to E. Vigouroux (Ann. chim. phys., 1897, (7) 12, p. 153) by heating silica with magnesium in the presence of magnesia, or by heating silica with aluminium. The crystalline form may be prepared by heating potassium silicofluoride with sodium or aluminium (F. Wohler, Ann., 1856, 97, p. 266; 1857, 102, p. 382); by heating silica with magnesium in the presence of zinc (L. Gattermann, Ber., 1889, 22, p. 186); and by the reduction of silica in the presence of carbon and iron (H. N. Warren, Chem. News, 1888, 57, p. 54; 1893, 67, p. 136). Another crystalline form, differing from the former by its solubility in hydrofluoric acid, was prepared by H. Moissan and F. Siemens (Comptes rendus, 1904, 138, p. 1299). A somewhat impure silicon (containing 90–98% of the element) is made by the Carborundum Company of Niagara Falls (United States Patents 745122 and 842273, 1908) by heating coke and sand in an electric furnace. The product is a crystalline solid of specific gravity 2.34, and melts at about 1430 C. See also German Patent 108817 for the production of crystallized silicon from silica and carborundum.
Amorphous silicon is a brown coloured powder, the crystalline variety being grey, but it presents somewhat different appearances according to the method used for its preparation. The specific gravity of the amorphous form is 2.35 (Vigouroux), that of the crystalline variety varying, according to the method of preparation, from 2.004 to 2.493. The specific heat varies with the temperature, from 0.136 at -39 C. to 0.2029 at 232° C. Silicon distils readily at the temperature of the electric furnace. It is attacked rapidly by fluorine at ordinary temperature, and by chlorine when heated in a current of the gas. It undergoes a slight superficial oxidation when heated in oxygen. It combines directly with many metals on heating, whilst others merely dissolve it. When heated with sodium and potassium, apparently no action takes place, but if heated with lithium it forms a lithium silicide, Li6Si2 (H. Moissan, Comptes rendus, 1902, 134, p. 1083). It decomposes ammonia at a red heat, liberating hydrogen and yielding a compound containing silicon and nitrogen. It reduces many non-metallic oxides. It is only soluble in a mixture of hydrofluoric and nitric acid, or in solutions of the caustic alkalis, in the latter case yielding hydrogen and a silicate: Si+2KHO+H2 = K2SiO3+2H2. On fusion with alkaline carbonates and hydroxides it undergoes oxidation to silica which dissolves on the excess of alkali yielding an alkaline silicate.
Silicon hydride, SiH4, is obtained in an impure condition, as a spontaneously inflammable gas, by decomposing magnesium silicide with hydrochloric acid, or by the direct union of silicon and hydrogen in the electric arc. In the pure state it may be prepared by decomposing ethyl silicoformate in the presence of sodium (C. Friedel and A. Ladenburg, Comptes rendus, 1867, 64, pp. 359, 1267); 4SiH(OC2H5)3 = SiH4+3Si(OC2H5)4. When pure, it is a colourless gas which is not spontaneously inflammable at ordinary temperature and pressure, but a slight increase of temperature or decrease of pressure sets up decomposition. It is almost insoluble in water. It burns when brought into contact with chlorine, forming silicon chloride and hydrochloric acid. It decomposes solutions of silver nitrate and copper sulphate. A second hydride of silicon, of composition Si2H6, was prepared by H. Moissan and S. Smiles (Comptes rendus, 1902, pp. 569, 1549) from the products obtained in the action of hydrochloric acid on magnesium silicide. These are passed through a vessel surrounded by a freezing mixture and on fractionating the product the hydride distils over as a colourless liquid which boils at 52° C. It is also obtained by the decomposition of lithium silicide with concentrated hydrochloric acid. Its vapour is spontaneously inflammable when exposed to air. It behaves as a reducing agent. For a possible hydride (Si2H3)n see J. Ogier, Ann. chim. phys., 1880, (5), 20, p. 5.
Only one oxide of silicon, namely the dioxide or silica, is known (see Silica).
Silicon fluoride, SiF4, is formed when silicon is brought into contact with fluorine (Moissan); or by decomposing a mixture of acid potassium fluoride and silica, or of calcium fluoride and silica with concentrated sulphuric acid. It is a colourless, strongly fuming gas which has a suffocating smell. It is decomposed with great violence when heated in contact with either sodium or potassium. It combines directly with ammonia to form the compound SiF4·2NH3, and is absorbed by dry boric acid and by many metallic oxides. Water decomposes it into silicofluoric acid and silicic acid: 3SiF4+3H2O = 2H2SiF6+H2SiO3. With potassium hydroxide it yields potassium silicofluoride, whilst with sodium hydroxide, sodium fluoride is produced: 3SiF4+4KHO = SiO2+2K2SiF6+2H2O; SiF4+4NaOH = SiO2+4NaF+2H2O. It combines directly with acetone and with various amines. Silicon fluoroform, SiHF3, was obtained by O. Ruff and Curt Albert (Ber., 1905, 38, p. 53) by decomposing titanium fluoride with silicon chloroform in sealed vessels at 100–120° C. It is a colourless gas which may be condensed to a liquid boiling at -80.2° C. On solidification it melts at about –110 C. The gas is very unstable, decomposing slowly, even at ordinary temperatures, into hydrogen, silicon fluoride and silicon: 4SiHF3 = 2H2+3SiF4+Si. It burns with a pale-blue flame forming silicon fluoride, silicofluoric acid and silicic acid. It is decomposed readily by water, sodium hydroxide, alcohol and ether.
2SiHF3+4H2O = H4SiO4+H2SiF6+2H2;
SiHF3+3NaOH+H2 = H4SiO4+3NaF+H2;
2SiHF3+4C2H6OH = Si(OC2H6)4+H2SiF6+2H2;
SiHF3+3(C2H6)2 = SiH(OC2H6)3+3C2H5F.
Silicofluoryc acid, H2SiF6, is obtained as shown above, and also by the action of sulphuric acid on barium silicofluoride, or by absorbing silicon fluoride in aqueous hydrofluoric acid. The solution on evaporation deposits a hydrated form, H2SiF6·2H2O, which decomposes when heated. The anhydrous acid is not known, since on evaporating the aqueous solution it gradually decomposes into silicon fluoride and hydrofluoric acid.
Silicon chloride, SiCl4, was prepared by J. J. Berzelius (Jahresb., 1825, 4, p. 91) by the action of chlorine on silicon, and is also obtained when an intimate mixture of silica and carbon is heated in a stream of chlorine and the products of reaction fractionated. It is a very stable colourless liquid which boils at 58° C. Oxygen only attacks it at very high temperatures. When heated with the alkali and alkaline earth metals it yields silicon and the corresponding metallic chlorides. Water decomposes it into hydrochloric and silicic acids. It combines directly with ammonia gas to form SiCl4·6NH3, and it also serves as the starting point for the preparation of numerous organic derivatives of silicon. The hexachloride, Si2Cl6, is formed when silicon chloride vapour is passed over strongly heated silicon; by the action of chlorine on the corresponding iodo-compound, or by heating the iodo-compound with mercuric chloride (C. Friedel, Comptes rendus, 1871, 73, p. 497). It is a colourless fuming liquid which boils at 146–148° C. It is decomposed by water, and also when heated between 350° and 1000° C., but it is stable both below and above these temperatures. The octochloride, Si3Cl8, is formed to the extent of about ½ to 1% in the action of chlorine on silicon (L. Gattermann, Ber., 1899, 32, p. 1114). It is a colourless liquid which boils at 210° C. Water decomposes it with the formation of silico-mesoxalic acid, HOOSi·Si(OH)2·SiOOH. Silicon chloroform, SiHCl3, first prepared by H. Buff and F. Wöhler (Ann., 1857, 104, p. 94), is formed by heating crystallized silicon in hydrochloric acid gas at a temperature below red heat, or by the action of hydro-chloric acid gas on copper silicide, the products being condensed by liquid air and afterwards fractionated (O. Ruff and Curt Albert, Ber., 1905, 38, p. 2222). It is a colourless liquid which boils at 33° C. It fumes in air and burns with a green flame. It is decomposed by cold water with the formation of silicoformic anhydride, H2Si2O3. It unites directly with ammonia gas yielding a compound of variable composition. It is decomposed by chlorine.
Similar bromo-compounds of composition SiBr4, Si2Br6 and SiHBr3 are known. Silicon tetraiodide, SiI4, is formed by passing iodine vapour mixed with carbon dioxide over strongly-heated silicon (C. Friedel, Comptes rendus, 1868, 67, p. 98) ; the iodo-compound condenses in the colder portion of the apparatus and is purified by shaking with carbon bisulphide and with mercury. It crystallizes in octahedra which melt at 120.5° C. and boil at 290° C. Its vapour burns with a red flame. It is decomposed by alcohol and also by ether when heated to 100° C.: SiI4+2C2H6OH = SiO2+2C2H5I+2HI; SiI4+4(C2H5)2O = Si(OC2H5)4+4C2H5I. The hexaiodide, Si2I6, is obtained by heating the tetraiodide with finely divided silver to 300° C. It crystallizes in hexagonal prisms which exhibit double refraction. It is soluble in carbon bisulphide, and is decomposed by water and also by heat, in the latter case yielding the tetraiodide and the di-iodide, Si2I4, an orange-coloured solid which is not soluble in carbon bisulphide. Silicon iodoform, SiHI3, is formed by the action of hydriodic acid on silicon, the product, which contains silicon tetraiodide, being separated by fractionation. It is also obtained by the action of hydriodic acid on silicon nitrogen hydride suspended in carbon bisulphide, or by the action of a benzene solution of hydriodic acid on trianilino-silicon hydride (O. Ruff, Ber., 1907, 41, p. 3738). It is a colourless, strongly refracting liquid, which boils at about 220° C, slight decomposition setting in above 150° C. Water decomposes it with production of leucone. Numerous chloro-iodides and bromoiodides of silicon have been described.
Silicon nitrogen hydride, SiNH, is a white powder formed with silicon amide when ammonia gas (diluted with hydrogen) is brought into contact with the vapour of silicon chloroform at –10° C. Trianilino silicon hydride, SiH(NHC6H6)3, is obtained by the action of aniline on a benzene solution of silicon chloroform. It crystallizes in needles which decompose at 114° C. Silicon amide, Si(NH2)4, is obtained as a white amorphous unstable solid by the action of dry ammonia on silicon chloride at –50° C. (E. Vigouroux and C. Hugot, Comptes rendus, 1903, 136, p. 1670). It is readily decomposed by water: Si(NH2)4+2H2O = 4NH3+SiO2. Above 0° C. it decomposes thus: Si(NH2)4 = 2HN3+Si(NH)2.
Silicon sulphide, SiS2, is formed by the direct union of silicon with sulphur; by the action of sulphuretted hydrogen on crystallized silicon at red heat (P. Sabatier, Comptes rendus, 1880, 90, p. 819); or by passing the vapour of carbon bisulphide over a heated mixture of silica and carbon. It crystallizes in needles which rapidly decompose when exposed to moist air. By heating crystallized silicon with boron in the electric furnace H. Moissan and A. Stock (Comptes rendus, 1900, 131, p. 139) obtained two borides, SiB3 and SiB6. They are both very stable crystalline solids. The former is completely decomposed when fused with caustic potash and the latter by a prolonged boiling with nitric acid. For silicon carbide see carborundum. Numerous methods have been given for the preparation of magnesium silicide, Mg2Si, in a more or less pure state, but the pure substance appears to have been obtained by P. Lebeau (Comptes rendus, 1908, 146, p. 282) in the following manner. Alloys of magnesium and silicon are prepared by heating fragments of magnesium with magnesium filings and potassium silico-fluoride. From the alloy containing 25% of silicon, the excess of magnesium is removed by a mixture of ethyl iodide and ether and a residue consisting of slate-blue octahedral crystals of magnesium silicide is left. It decomposes water at ordinary temperature with evolution of hydrogen but without production of silicon hydride, whilst cold hydrochloric acid attacks it vigorously with evolution of hydrogen and spontaneously inflammable silicon hydride.
Organic Derivatives of Silicon.
The organic derivatives of silicon resemble the corresponding carbon compounds except in so far that the silicon atom is not capable of combining with itself to form a complex chain in the same manner as the carbon atom, the limit at present being a chain of three silicon atoms. Many of the earlier-known silicon alkyl compounds were isolated by Friedel and Crafts and by Ladenburg, the method adopted consisting in the interaction of the zinc alkyl compounds with silicon halides or esters of silicic acids. SiCl4+2Zn(C2H6)2 = 2ZnCl2+Si(C2H6)4. This method has been modified by F. S. Kipping (Jour. Chem. Soc, 1901, 79, p. 449) and F. Taurke (Ber., 1905, 38, p. 1663) by condensing silicon halides with alkyl chlorides in the presence of sodium: SiCl4+4R·Cl+8Na = SiR4+8NaCl; SiHCl3+3R·Cl+6Na=SiHR3+6NaCl; whilst Kipping (Proc. Chem. Soc., 1904, 20, p. 15) has used silicon halides with the Grignard reagent: C2H5MgBr(+SiCl4) → C2H5SiCl3(+MgBrPh) → Ph·C2H5·SiCl2(+MgBrC3H 7) → Ph·C2H5·C3H7·SiCl.
Silicon Tetramethyl, Si(CH3)4 (tetramethyl silicane), and silicon tetraethyl, Si(C2H5)4, are both liquids. The latter reacts with chlorine to give silicon nonyl-chloride Si(C2H5)3·C2H4Cl, which condenses with potassium acetate to give the acetic ester of silicon nonyl alcohol from which the alcohol (a camphor-smelling liquid) may be obtained by hydrolysis. Triethyl silicon, (C2H6)3Si·OH, is a true alcohol, obtained by condensing zinc ethyl with silicic ester, the resulting substance of composition, (C2H6)3·SiOC2H5, with acetyl chloride yielding a chloro-compound (C2H5)3SiCl, which with aqueous ammonia yields the alcohol. Silicon tetraphenyl, Si(C6H5)4, a solid melting at 231° C, is obtained by the action of chlorobenzene on silicon tetrachloride in the presence of sodium. Silico-oxalic acid, (SiO·OH)2, obtained by decomposing silicon hexachloride with ice-cold water, is an unstable solid which is readily decomposed by the inorganic bases, with evolution of hydrogen and production of a silicate. Silicomesoxalic acid, HO·OSiSi(OH)2·SiO·OH, formed by the action of moist air on silicon octochloride at 0° C, is very unstable, and hot water decomposes it with evolution of hydrogen and formation of silicic acid (L. Gattermann, Ber., 1899, 32, p. 1114). Silico-benzoic acid, C6H6·SiO·OH, results from the action of dilute aqueous ammonia on phenyl silicon chloride (obtained from mercury diphenyl and silicon tetrachloride). It is a colourless solid which melts at 92° C. For silicon derivatives of the amines see Michaelis, Ber., 1896, 29, p. 710; on asymmetric silicon and the resolution of dl-benzyl-ethyl-propyl-silicol see F. S. Kipping, Jour. Chem. Soc., 1907, 91, pp. 209 et seq.
The atomic weight of silicon has been determined usually by analysis of the halide compounds or by conversion of the halides into silica. The determination of W. Becker and G. Meyer (Zeit. anorg. Chem., 1905, 43, p. 251) gives the value 28.21, and the International Commission in 1910 has adopted the value 28.3.