Chemical Fact Sheet
Silicon
| Chemical Abstract Number (CAS #) | 7440-21-3 |
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| Analytical Methods | 200.7 - 6010 - SM4500Si |
| Molecular Formula | Si |
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Synopsis
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Silicon - (L. silex, silicis, flint), Si; at. wt. 28.0855(3); at. no. 14; m.p. 1414 deg C; b.p. 3265 deg C; sp. gr. 2.33 (25 deg C); valence 4. Davy in 1800 thought silica to be a compound and not an element; later in 1811, Gay Lussac and Thenard probably prepared impure amorphous silicon by heating potassium with silicon tetrafluoride. Berzelius, generally credited with the discovery, in 1824 succeeded in preparing amorphous silicon by the same general method as used earlier, but he purified the product by removing the fluosilicates by repeated washings. Deville in 1854 first prepared crystalline silicon, the second allotropic form of the element. Silicon is present in the sun and stars and is a principal component of a class of meteorites known as "aerolites". It is also a component of tektites, a natural glass of uncertain origin. Natural silicon contains three isotopes. Fourteen other radioactive isotopes are recognized. Silicon makes up 25.7% of the earth's crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, claymica, etc. are but a few of the numerous silicate minerals. Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. Crystalline silicon has a metallic luster and grayish color. The Czochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. This product can be doped with boron, gallium, phosphorus, or arsenic to produce silicon for use in transistors, solar cells, rectifiers, and other solid-state devices which are used extensively in the electronics and space-age industries. Hydrogenated amorphous silicon has shown promise in producing economical cells for converting solar energy into electricity. Silicon is a relatively inert element, but it is attacked by halogens and dilute alkali. Most acids except hydrofluoric, do not affect it. Silicones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products or silicones, ranging from liquids to hard, glasslike solids with many useful properties. Elemental silicon transmits more than 95% bf all wavelengths of infrared, from 1.3 to 6.7 um. Silicon is one of man's most useful elements. In the form of sand and clay it is used to make concrete and brick; it is a useful refractory material for high-tempemture work, and in the form of silicates it is used in making enamels, pottery, etc. Silica, as sand, is a principal ingredient of glass, one of the most inexpensive of materials with excellent mechanical, optical, thermal, and electrical properties. Glass can be made in a very great variety of shapes, and is used as containers, window glass, insulators, and thousands of other uses. Silicon tetrachloride can be used to iridize glass. Silicon is important in plant and animal life. Diatoms in both fresh and salt water extract silica from the water to build up their cell walls. Silica is present in ashes of plants and in the human skeleton. Silicon is an important ingredient in steel; silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A. Regular grade silicon (99.5%) costs about $140/kg. Silicon 99.96% pure costs about $250/kg, hyperpure silicon may cost as much as $400/kg. Miners, stonecutters, and other engaged in work where siliceous dust is breathed in large quantities often develop a serious lung disease known as silicosis. |
| Use | FOR MAKING SILANES & SILICONES; MFR OF TRANSISTORS, SILICON DIODES, SEMICONDUCTORS; FOR MAKING ALLOYS SUCH AS FERROSILICON, SILICON BRONZE, SILICON COPPER; REDUCING AGENT IN HIGH TEMP REACTION. SOLID-STATE DEVICES; RECTIFIERS; SOLAR CELLS; MFR STEEL. ALUMINUM ALLOYS; PRODUCTION OF SILICONES; STEEL ALLOYS; PRODN OF ELECTRONIC-GRADE SILICON, EG, FOR SEMICONDUCTORS. Used in the manufacture of silicon tetrachloride. Organosilicon compounds; silicon carbide; cermets and special refractories; halogenated silanes; spring steels. |
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Consumption Patterns |
ALUMINUM ALLOYS, 54%; MFR OF SILICONES, 40%; STEEL ALLOYS, 2%; MISCELLANEOUS USES, 4% (1979) Transportation, 32%; chemicals, 20%; machinery, 17%; construction, 13%; and other, 18% (1986) |
| Apparent Color | BLACK TO GRAY, LUSTROUS, NEEDLE-LIKE CRYSTALS OR OCTAHEDRAL PLATELETS (CUBIC SYSTEM); AMORPHOUS FORM IS DARK BROWN POWDER |
| Boiling Point | 2355 DEG C |
| Melting Point | 1410 DEG C |
| Molecular Weight | 28.0855 |
| Density | 2.33 @ 25 DEG C/4 DEG C |
| Sensitivity Data | Unpleasant deposits of silicon dust in eyes, ears & nasal passages & injury to the skin and mucous membranes may be caused by the dust itself or by cleansing procedures used for its removal. |
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Chemical and Physical Properties |
ATOMIC NUMBER: 14; VALENCE: 4, 2; POOR CONDUCTOR OF ELECTRICITY LATTICE CONSTANT (25 DEG C): 5.41987X10-8 CM; COMPRESSIBILITY (VOL/VOL 0) @ 25X10+3 KG/SQ CM: 0.978; @ 100x10+3 KG/SQ CM: 0.940; DIELECTRIC CONSTANT: 13; COVALENT BOND IONIZATION ENERGY @ 0 DEG K= 1.2 ELECTRON VOLT; BAND GAP: 1.106 ELECTRON VOLT; IMPURITY ATOM IONIZATION ENERGY: APPROX 0.04 ELECTRON VOLT; INTRINSIC RESISTIVITY @ 300 DEG K= 0.23 MEGOHM ELECTRON MOBILITY @ 300 DEG K: 1500 SQ CM/VOLT/SEC; HOLE MOBILITY @ 300 DEG K: 500 SQ CM/VOLT/SEC; INTRINSIC CHARGE DENSITY @ 300 DEG K: 1.5X10+10; ELECTRON DIFFUSION CONSTANT @ 300 DEG K: 38; HOLE DIFFUSION CONSTANT @ 300 DEG K: 13; ATTACKED BY HYDROFLUORIC OR A MIXTURE OF HYDROFLUORIC & NITRIC ACIDS ELEMENTAL SILICON TRANSMITS MORE THAN 95% OF ALL WAVELENGTHS OF IR FROM 1.3 TO 6.7 UM; ATTACKED BY HALOGENS & DILUTE ALKALI Temperature of transition: solid= 1683 degree K; liquid = 2750 degree K; Heat of transition: solid= 11.1 kcal/g mole; liquid= 71 kcal/g mole MOHS HARDNESS: 7 Small single-crystal filaments are very strong and exhibit breaking strengths of up to about 1.4 GPa (200,000 psi). In thin sections, single crystals cleave along planes, but in larger pieces, fracture is conchoidal. At atm pressure, silicon has a diamond cubic structure, ie, two interpenetrating face-centered cubes are displaced 1/4, 1/4, 1/4 from each other. When subjected to about 15 GPa (<150,000 atm), the fcc structure is converted to a body-centered lattice. Vapor deposition below about 500 deg C produces amorphous silicon; upon reheating to a somewhat higher temp, crystallization will occur. Total optical emissivity @ mp= 0.33, at 1500 deg C= 0.33; Reflectivity @ 633 nm = 72% and @ 1500 C= 70% /liquid silicon/ Dielectric constant= 11.8 Oxygen forms strong bonds with silicon. At room temp, silicon is covered with an oxide layer 2.0-3.0 nm thick. This oxide is amorphous. Oxidation consumes silicon at a rate of about 0.4 times the rate of oxidation. The exact values /of oxidation/ depend on the temp, the pressure, the oxidant, the crystal orientation of the surface being oxidized, & the amt & type of impurity in the silicon. Oxygen can also dissolve in the silicon crystal lattice and form SiOx complexes of varying composition that may radically affect the electrical properties of the host. When oxygen-containing silicon is annealed in the 450 deg C range, the -OSi= complexes become donors and contribute to electrical conductivity. Coordination number: 6 |
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Environmental Fate |
Silicon is not found free in nature, but occurs chiefly as the oxide, & as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, & opal are some of the oxide forms. Granite, hornblende, asbestos, feldspar, clay, mica are but a few of the numerous silicate minerals. 3 NATURALLY OCCURRING ISOTOPES: 28 (92.18%); 29 (4.71%); 30 (3.12%) FOUND ASSILICA (SANDSTONE) OR AS SILICATE (ORTHOCLASE, KAOLINITE, ANORTHITE). CONSTITUTES ABOUT 27.6% OF EARTH'S CRUST; SECOND MOST ABUNDANT ELEMENT ON EARTH. |
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