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silicon,nonmetallic chemical element; symbol Si; at. no. 14; interval in which at. wt. ranges 28.084–28.086; m.p. 1,410°C;; b.p. 2,355°C;; sp. gr. 2.33 at 25°C;; valence usually +4. Silicon is the element directly below carbon and above germanium in Group 14 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . It is more metallic in its properties than carbon; in many ways it resembles germanium. Silicon has two allotropic forms, a brown amorphous form, and a dark crystalline form. The crystalline form has a structure like diamond and the physical properties given above. Silicon forms compounds with metals (silicides) and with nonmetals. With carbon it forms silicon carbidesilicon carbide,
chemical compound, SiC, that forms extremely hard, dark, iridescent crystals that are insoluble in water and other common solvents. Widely used as an abrasive, it is marketed under such familiar trade names as Carborundum and Crystolon.
..... Click the link for more information. ; with oxygen a dioxide, silicasilica
or silicon dioxide,
chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white.
..... Click the link for more information. ; with oxygen and metals, silicatessilicate,
chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids.
..... Click the link for more information. . With hydrogen it forms several hydrides or silanes, the simplest being monosilane, SiH4, a colorless gas. It also forms compounds with the halogens, sulfur, and nitrogen and forms numerous organo-silicon compounds. Silicon is the second most abundant element of the earth's crust; it makes up about 28% of the crust by weight. Oxygen, most abundant, makes up about 47%. Aluminum, third in abundance, makes up about 8%. Silicon is widely distributed, occurring in silica and silicates, but never uncombined. Silicon is obtained commercially by heating sand and coke in an electric furnace. It is used in the steel industry in an alloy known as ferrosilicon, and also to form other alloys, such as those with aluminum, copper, and manganese; in these alloys it contributes hardness and corrosion resistance. A purified silicon is used in the preparation of siliconessilicone,
polymer in which atoms of silicon and oxygen alternate in a chain; various organic radicals, such as the methyl group, CH3, are bound to the silicon atoms.
..... Click the link for more information. . Silicon of very high purity is prepared by thermal decomposition of silanes; it is used in transistors and other semiconductorsemiconductor,
solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction; insulation). At high temperatures its conductivity approaches that of a metal, and at low temperatures it acts as an insulator.
..... Click the link for more information. devices. Silica is widely used in the production of glassglass,
hard substance, usually brittle and transparent, composed chiefly of silicates and an alkali fused at high temperature. Composition and Properties of Glass
..... Click the link for more information. . Silicates in the form of clayclay,
common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts.
..... Click the link for more information. are used in pottery, brick, tile, and other ceramics. Silicon is found in many plants and animals; it is a major component of the test (cell wall) of diatoms. Silicosissilicosis
, occupational disease of the lungs caused by inhalation of free silica (quartz) dust over a prolonged period of time. Free silica is dispersed in the air and inhaled by workers engaged in the mining of lead, hard coal, and gold, in cutting sandstone and granite, in
..... Click the link for more information. is a lung disease caused by inhaling silica dust. Discovery of the element is usually credited to J. J. Berzelius, who in 1824 prepared fairly pure amorphous silicon.
(Si), a chemical element of group IV of Mendeleev’s periodic system. Atomic number, 14; atomic weight, 28.086. Three stable isotepes of silicon exist in nature: 28Si (92.27 percent), 29Si (4.68 percent), and 20Si (3.05 percent).
History. The compounds of silicon are widely distributed on earth and have been known to man since the Stone Age. The use of stone tools for work and hunting continued for several thousand years. The processing of silicon compounds, for example, the production of glass, began about 3000 B.C. (in ancient Egypt). Silicon dioxide, SiO2 (silica), was the first known compound of silicon. In the 18th century silica was considered to be a simple substance and was included among the “earths” (which is reflected in its Russian name kremnezem, “silicon earth”). The compound nature of silica was established by J. J. Berzelius. He was also the first to obtain elemental silicon in 1825 from silicon tetrafluoride by reduction of the latter with metallic potassium. The new element was named silicium (from the Latin silex, “flint”). The Russian name kremnii was introduced in 1834 by G. I. Gess (G. H. Hess).
Occurrence in nature. Silicon is the second most abundant element in the earth’s crust (after oxygen), its content in the lithosphere being 29.5 by weight. Silicon plays as important a role in the earth’s crust as carbon in animals and plants. The exceptionally strong bond between silicon and oxygen is of great importance in the geochemistry of silicon. About 12 percent of the lithosphere consists of silica (SiO2) in the form of the mineral quartz and its varieties. Seventy-five percent of the lithosphere consists of various silicates and aluminosilicates (feldspars, micas, amphiboles, and other minerals). The number of minerals containing silica totals more than 400.
Magmatic processes produce poor differentiation of silica: it accumulates in granitoid rocks (32.3 percent) and in ultrabasic rocks (19 percent). The solubility of SiO2 increases with increasing pressures and temperatures. The migration of silica is also possible with water vapor and, for this reason, significant concentrations of quartz are present in the pegmatites of hydrothermal veins. Quartz is, in this case, frequently accompanied by ore elements (gold-quartz, quartz-cassiterite, and other veins).
Physical and chemical properties. Silicon forms dark gray crystals with a metallic luster. The crystals have a face-centered cubic diamond-type lattice with the constant a = 5.431 Å and density 2.33 g/cm3. A new, apparently hexagonal, modification with the density of 2.55 g/cm3 has been obtained at very high pressures. Silicon melts at 1417°C and boils at 2600°C. The specific heat (at 20°-100°C) is 800 joules (J) per kg.°K (0.191 cal/(g•deg)]. The thermal conductivity is variable even for samples of highest purity and ranges (at 25°C) from 84 to 126 W/m•°K [0.20-0.30 cal/(cm.sec.deg)]. The temperature coefficient of linear expansion is 2.33 × 10−6oK−1; it is negative below 120°K. Silicon is transparent to long-wavelength infrared radiation. The index of refraction (for λ. = 6 μm) is 3.42; the dielectric constant is 11.7. Silicon is diamagnetic and its atomic magnetic susceptibility is 0.13 × 10−6. The hardness of silicon on Mohs’ scale is 7.0; the Brinell hardness is 2.4 giganew-tons per m2 (GN/m2), or 240 kilograms-force per mm2(kgf/mm2). The modulus of elasticity is 109 GN/m2 (10,890 kgf/ mm2); the coefficient of compressibility is 0.325 × 10−6 cm2/kg. Silicon is a brittle matter metal; plastic deformation becomes apparent at temperatures above 800°C.
Silicon is a semiconductor that is finding increasing applications. The electrical properties of silicon depend strongly on impurities. The intrinsic specific electrical volume resistance of silicon at room temperature is assumed to be equal to 2.3 × 103 ohm · m (2.3 × 105 ohm-cm).
Semiconducting silicon with/рype conductivity (B, Al, In, or Ga doping) and n-type conductivity (P, Bi, As, or Sb doping) is less resistant. The width of the forbidden band according to electrical measurements is 1.21 el’ at 0°K and decreases to 1.119 el’ at 300°K.
In accordance with the position of silicon in Mendeleev’s periodic system, the 14 electrons of the silicon atom are distributed among three electron shells. The first shell (nearest to the nucleus) contains two electrons; the second, eight; and the third (valence shell), four. The electron shell configuration is 1s22s22p63s23p2. The successive ionization potentials are (in el’) 8.149, 16.34, 33.46, and 45.13. The atomic radius is 1.33 Å, the covalent radius is 1.17 Å, and the ionic radii are 0.39 Å for Si4+ and 1.98 Å for Si4-.
Silicon is tetravalent (like carbon) in its compounds. However, in contrast to carbon, silicon exhibits the coordination number of 6 in addition to the coordination number of 4, which is explained by its large atomic volume (examples of such compounds are the silicon fluorides, which contain the grouping SiF6]2-).
Chemical bonds between silicon and other atoms are usully formed with the aid of hybrid sp3 orbitals and possibly also with the involvement of two of its five (vacant) 3d orbitals, particularly when silicon is hexacoordinate. Having a low electronegativity of 1.8 (compared to 2.5 for carbon, 3.0 for nitrogen, and so on), silicon is electropositive in its compounds with non-metals; these compouds possess a polar character. The large bond energy with oxygen (Si—O), which is equal to 464 kJ/mole (111 kcal/mole), leads to the stability of its oxygen compounds (SiO2 and silicates). The energy of the Si—Si bond is low, equal to 176 kJ/mole (42 kcal/mole). In contrast to carbon, the formation of long chains and double bonds between Si atoms is not a characteristic of silicon. Silicon is stable in air even at elevated temperatures owing to the formation of a protective oxide film. Silicon is oxidized in oxygen beginning at 400°C to give silicon dioxide (SiO2). The monoxide SiO is also known; it is stable at high temperatures in gaseous form. Rapid cooling gives a solid product, which decomposes readily to give an intimate mixture of Si and SiO2.
Silicon is stable toward acids and dissolves only in a mixture of nitric acid and hydrofluoric acid. It dissolves readily in hot solutions of alkaiis with the evolution of hydrogen. Silicon reacts with fluorine at room temperature and with the remaining halogens on heating to give compounds with the general formula S1X4. Hydrogen does not react with silicon directly; silicon hydrides (silanes) are produced by the decomposition of suicides (see below). Known silicon hydrides include the series from SiH4 to Si8H18, which are analogous in composition to the saturated hydrocarbons. Silicon forms two types of oxygen-containing silanes, namely, the siloxanes and siloxens. Silicon reacts with nitrogen at temperatures above 1000°C. The nitride Si3N4 is of great importance. It resists air oxidation even at 1200°C. It is also stable toward acids (except nitric acid) and alkalis and toward molten metals and slags, which makes it a valuable material for the chemical industry and in the production of heat-resistant materials. Compounds of silicon and carbon (silicon carbide, SiC) and silicon and boron (SiB3, SiB6, SiB12) are distinguished by their hardness and high thermal and chemical stabilities. Silicon reacts on heating (in the presence of catalysts, such as copper) with organic chlorine compounds (for example, CH3CI) to give organohalosilanes [for example Si(CH3)3Cl], which are useful in the synthesis of numerous or-ganosilicon compounds.
Silicon forms compounds with all metals except Bi, Tl, Pb, and Hg to give silicides. More than 250 silicides have been prepared. The compositions of these compounds (MeSi, MeSi2, Me5Si3, Me3Si, Me2Si, and others) are usually inconsistent with the classical valencies. Silicides are distinguished by their refractory properties and hardness. Of most practical importance are ferrosilicon, which is used as a reducing agent in the production of special alloys, and molybdenum silicide (MoSi2), which is used as the construction material for electric furnace heaters, gas turbine blades, and other items.
Preparation and uses. Technical-grade silicon (95–98 percent) is produced in an electric arc by the reduction of silica between graphite electrodes. With the development of semiconductor technology, methods for the production of pure and ultrapure silicon have been developed. This requires the preliminary synthesis of high-purity starting compounds of silicon, from which silicon is produced by reduction or thermal decomposition.
Pure semiconductor silicon is produced in two forms: poly-crystalline (by the reduction of SiCI4 or SiHCI3 with zinc or hydrogen or by the thermal decomposition of SiI4 and SiH4) and single crystals (by floating zone refining and by “withdrawing” a single crystal from molten silicon—the Czohralski method).
Specially alloyed silicon is being widely used as a material for the production of semiconductor devices (transistors, thermistors, power rectifiers, controlled diodes-thyristors, solar cells used in spacecraft). Since silicon is transparent to rays of a wavelength from 1 to 9 μm, it is also used in infrared optics.
Silicon is used in a variety of ever-increasing areas. It is used in metallurgy for the removal of oxygen dissolved in molten metals (deoxidation), and it is a component of a large number of alloys of iron and nonferrous metals. Silicon usually imparts increased corrosion resistance to alloys and improves their casting properties and mechanical strength; however, excessive amounts of silicon may produce brittleness. The most important silicon-containing alloys are those of iron, copper, and aluminum. Increasing quantities of silicon are consumed in the synthesis of organosilicon compounds and silicides. Silica and many silicates (clays, feldspars, micas, talcs) are used as raw materials by the glass, ceramic, and electrotechnical branches of industry.
V. P. BARZAKOVSKI
Silicon in organisms. In organisms silicon is part of various compounds that take part mainly in the formation of hard skeletal members and tissues. Particularly large quantities of silicon may be accumulated by certain marine plants (for example, diatoms) and animals (for example, the Cornacuspongida and Radiolaria), which after death form huge silica deposits on the ocean floor. Cold oceans contain predominantly biogenic silts enriched in silicon, whereas tropical oceans contain predominantly limestone silts of low silicon content. Among terrestrial plants, large quantities of silicon are accumulated by the Gramineae, Cyperaceae, Palmae, and Equisetaceae. Vertebrate animals contain 0.1-0.5 percent silicon dioxide in ash material. The largest quantities of silicon are found in dense connective tissues, in the kidneys, and in the pancreas. The daily human diet contains up to 1 g of silicon. High content of silica dust in air causes silicosis in humans upon inhalation.
V. V. KOVAL’SKII
REFERENCESBerezhnoi, A. S. Kremnii i ego binarnye sistemy. Kiev, 1958.
Krasiuk, B. A., and A. I. Gribov. Poluprovodniki—germanii i kremnii. Moscow, 1961.
Runyan, W. R. Tekhnologiia poluprovodnikovogo kremniia. Moscow, 1969. (Translated from English.)
Salli, I. V., and E. S. Fal’kevich. Proizvodstvo poluprovodnikovogo kremniia. Moscow, 1970.
Kremnii i germanii, fases. 1–2. Collection of articles edited by E. S. Fal’kevich and D. I. Levinzon. Moscow, 1969–70.
Gladyshevskii, E. I. Kristallokhimiia silitsidov i germanidov. Moscow, 1971.
Wolf, H. F. Silicon Semiconductor Data. Oxford-New York, 1965.
silicon(Si) The base material used in chips. Pronounced "sil-i-kin," not "sil-i-cone," the latter used to make sealants (see silicone), silicon is the most abundant element in nature next to oxygen. It is found in a natural state in rocks and sand, and its atomic structure makes it an ideal semiconductor material. For chip making, silicon is mined from white quartz rocks and put through a chemical process at high temperatures to purify it. Pure silicon is not electrically conductive. In order to make it conductive, it is chemically combined with other materials such as boron and phosphorus (see doping). See silicon germanium and black silicon.
|A Silicon Moon|
|This stylized image symbolizes that chips are made from the same material found in sand. The "moon" is a finished wafer containing memory chips. (Image courtesy of Texas Instruments, Inc.)|
|Drawing the Silicon Ingot|
|An ingot is being drawn from a furnace containing molten silicon. High-speed saws slice the ingots into wafers about the thickness of a dime, which are then ground and polished mirror smooth. (Image courtesy of Texas Instruments, Inc.)|