Where Found (Encyclopedia of Global Resources)
Silicon makes up 25.7 percent of the Earth’s crust and is the second most abundant element after oxygen. It is not found in its elemental form, but rather occurs in compounds such as oxides and various silicate minerals. Silicon is a trace element participating in the metabolism of higher animals, and siliceous structures are found in many biological systems in the form of cell walls, scales, and other skeletal features.
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Primary Uses (Encyclopedia of Global Resources)
Silicon metal and alloys, including ferrosilicon, are used mainly by producers of aluminum, aluminum alloys, and chemicals. Very pure silicon is an essential component of semiconductors and has given its name to the “silicon age,” a term that came into prominence during the 1990’s.
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Technical Definition (Encyclopedia of Global Resources)
Silicon (abbreviated Si) is the fourteenth element of the periodic table, with an atomic number of 28. With carbon, germanium, and tin, it belongs to Group IVA of the periodic table and resembles germanium (Ge) most strongly in its physical, chemical, and electronic properties. Pure silicon is a hard, gray solid with a metallic luster and a cubic crystalline structure similar to that of carbon in diamond form. It has eight isotopes, the most abundant of which are Si28 (92.23 percent), Si29 (4.67 percent), and Si30 (3.10 percent). Its density is 2.329 grams per cubic centimeter, and it has a melting point of 1,410° Celsius and a boiling point of 2,355° Celsius. While the single-crystal form of silicon has been most extensively studied from both basic and practical viewpoints, the polycrystalline and amorphous forms of silicon have also become extremely important: Polycrystalline silicon has been applied in the construction of solar panels and central processing units of computers. Amorphous silicon has been used in thin-film transistors and solar cells.
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Description, Distribution, and Forms (Encyclopedia of Global Resources)
Silicon is widely available in oxides and silicates. The oxide forms include sand, quartz, rock crystal, amethyst, agate, flint, and opal. Granite, feldspar, clay, and mica are some of the common forms of silicates. A basic requirement of silicon in all its preeminent electronic applications is extreme purity—to levels much better than parts per billion (ppb).
The single-crystal form of silicon, while essential for computer chips, has cost and size limitations for a host of other potentially high-volume applications. Hence silicon is also produced in polycrystalline and amorphous forms by techniques such as casting and thin-film deposition. Polycrystalline forms (poly-Si) contain crystalline grains separated by grain boundaries, while amorphous silicon lacks the long-range crystalline order completely. However, both have useful semiconducting properties and have been widely developed for a range of uses.
The interesting and extremely useful electronic and optoelectronic properties of silicon stem from its tetrahedral bonding and diamond cubic structure. Replacing a host silicon atom with a Group V element (such as phosphorus) or Group III element (such as boron) adds a free electron or “hole” (an electron vacancy that behaves like a positively charged free particle. Thus the electrical conductivity of silicon can be changed over several powers of ten simply by controlling the trace quantities...
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History (Encyclopedia of Global Resources)
Although many chemists recognized silicon as an element by the early nineteeth century, its tight bonding with oxygen made it difficult to isolate as a separate element. Jöns Jacob Berzelius achieved the isolation of silicon in 1823 using a method similar to one developed by Sir Humphy Davy, who earlier had tried but failed to isolate silicon. The newly isolated element was named for the Latin word for flint, silex, and subsequently was investigated by German chemist Friedrich Wöhler and others.
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Obtaining Silicon (Encyclopedia of Global Resources)
Semiconductor-grade silicon requires conversion of raw silicon obtained from reducing silica (SiO2) into gaseous compounds such as chlorosilanes. Multiple fractional distillation of the latter leads to high-purity silicon rods. These rods are subsequently melted and grown into dislocation-free single crystals by either the Czochralski (CZ) crystal pulling process or the float zone (FZ) process. Necessary dopants such as boron (for p-type silicon) and phosphorus (for n-type silicon) are added to the melt. CZ silicon ingots are probably the largest single crystals ever produced—more than 3 meters long, with diameters as large as 300 millimeters. Wafers, about a millimeter thick, sliced from the ingots serve as the starting material for the batch fabrication of microelectronic chips, each containing up to a few million transistors.
Silicon by itself is inert, but a number of source gases and reagents used in manufacturing it are highly toxic, so extreme care must be exercised in waste disposal and protection of assembly workers. Silicon has been implicated in silicotic lung diseases and certain cancers.
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Uses of Silicon (Encyclopedia of Global Resources)
The principal applications of high-grade silicon are in microelectronics. The atomic structure of crystalline silicon makes it the most important semiconductor. Silicon in its highly purified form, when “doped” with elements such as boron and phosphorus, becomes the basic element of computer chips, transistors, diodes, and various other electronic switching and control devices. The enormous success of the silicon transistor, the basic electronic amplifying device, was made possible by an extremely pristine interface with silicon dioxide (an insulator readily grown on silicon by heating in oxygen) and by the continual scaling down of transistor feature size, which translates directly to faster computer speed and higher memory capacity.
The field of giant microelectronics, exemplified by portable computer displays and flat-screen television, uses silicon in its polycrystalline or amorphous forms. Another area of great impact for silicon is in terrestrial solar cells, for which extremely large volumes at low cost are necessary. Here computer-grade single crystals are not cost-effective; large-grain polycrystalline silicon holds the key for this crucial renewable energy application.
A late-twentieth century silicon technology extended the same lithographic patterning that helps put millions of transistors on a single computer chip but uses it to make micromachines such as gears, beams, and motors. These...
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Further Reading (Encyclopedia of Global Resources)
Cerofolini, G. F., and L. Meda. Physical Chemistry of, in, and on Silicon. New York: Springer, 1989.
Chatterjee, Kaulir Kisor. “Silicon and Its Minerals.” In Uses of Industrial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009.
Datnoff, L. E., G. H. Snyder, and G. H. Korndörfer. Silicon in Agriculture. New York: Elsevier, 2001.
Grayson, Martin, ed. Encyclopedia of Semiconductor Technology. New York: Wiley, 1984.
Greenwood, N. N., and A. Earnshaw. “Silicon.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Lin, Wen, and Howard Hoff. “Silicon Materials.” In Handbook of Semiconductor Manufacturing Technology, edited by Robert Doering and Yoshio Nishi. 2d ed. Boca Raton, Fla.: CRC Press, 2008.
Massey, A. G. “Group 14: Carbon, Silicon, Germanium, Tin, and Lead.” In Main Group Chemistry. 2d ed. New York: Wiley, 2000.
Siffert, P., and E. F. Krimmel, eds. Silicon: Evolution and Future of a Technology. Berlin: Springer, 2004.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews. http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
U.S. Geological Survey. Silicon: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/silicon
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Silicon (Chemical Elements)
Silicon is a member of Group 14 (IVA) in the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Silicon is also part of the the carbon family. Other carbon family elements include carbon, germanium, tin, and lead. Silicon is a metalloid, one of only a very few elements that have characteristics of both metals and non-metals.
Silicon is the second most abundant element in the Earth's crust, exceeded only by oxygen. Many rocks and minerals contain silicon. Examples include sand, quartz, clays, flint, amethyst, opal, mica, feldspar, garnet, tourmaline, asbestos, talc, zircon, emerald, and aquamarine. Silicon never occurs as a free element. It is always combined with one or more other elements as a compound.
By the early 1800s, silicon was recognized as an element. But chemists had serious problems preparing pure...
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Silicon (World of Earth Science)
Silicon (Si, element 14) is a nonmetallic chemical found in group IV, the carbon family, on the Periodic table. Swedish chemist Jons Jacob Berzelius first isolated and described the element in 1824.
In nature, silicon is always paired with another substance; it combines with oxygen to form quartz and sand (silicon dioxide, SiO2) or with oxygen and a metal to form silicates, which are used to make glass, pottery, china, and other ceramics. The relatively inactive element occurs in nearly all rocks, as well as in soil, sand, and clays. It is the second most abundant element found in the earth's crust, surpassed only by oxygen.
Scientists create pure silicon by heating sand and coke in an electric furnace to remove oxide (oxygen) from the element. Pure silicon is colored dark gray and has a crystalline structure similar to diamond. The crystals are extremely hard and demonstrate remarkable insulating and semiconducting properties, which has made silicon an invaluable resource for the computing and electronics industries. A single purified silicon crystal contains millions of atoms accompanied by loosely attached electrons that break free upon the introduction of energy, such as light or heat. The flowing electrons conduct electricity, hence the term semiconductor. Today, silicon is the backbone of computer chips, transistors and many other electronic components.
Silicones, a chain of alternating silicon and oxygen atoms, are chemically inert and stable in the presence of high heat. The compounds are often used as lubricants, waterproofing materials and varnishes and enamels. Silicone gels have long been used as implants in the human body.
Silicon has an atomic weight of 28.086, a melting point of 2,570°F (1,410°C) and a boiling point of 4,270°F (2,355°C). Only three stable isotopes of silicon are known to exist: silicon-28, silicon-29 and silicon-30.
See also Earth (planet)
Silicon (How Products are Made)
Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates. Silicon's compounds are also found in water, in the atmosphere, in many plants, and even in certain animals.
Silicon is the fourteenth element of the periodic table and is a Group IVA element, along with carbon germanium, tin and lead. Pure silicon is a dark gray solid with the same crystalline structure as diamond. Its chemical and physical properties are similar to this material. Silicon has a melting point of 2570° F (1410° C), a boiling point of 4271° F (2355° C), and a density of 2.33 g/cm3.
When silicon is heated it reacts with the halogens (fluorine, chlorine, bromine, and iodine) to form halides. It reacts with certain metals to form silicides and when heated in an electric furnace with carbon, a wear resistant ceramic called silicon carbide is produced. Hydrofluoric acid is the only acid that affects silicon. At higher temperatures, silicon is attacked by water vapor or by oxygen to form a surface layer of silicon dioxide.
When silicon is purified and doped with such elements as boron, phosphorus and arsenic, it is used as a semiconductor in various applications. For maximum purity, a chemical process is used that reduces silicon tetrachloride or trichlorosilane to silicon. Single crystals are grown by slowly drawing seed crystals from molten silicon.
Silicon of lower purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, alumiinum, and bronze. When small amounts of silicon are added to aluminum, aluminum becomes easier to cast and also has improved strength, hardness, and other properties. In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents.
In 1999, world production was around 640,000 metric tons (excluding China), with Brazil, France, Norway and the United States major producers. This is a continued decline compared to the last several years (653,000 tons in 1998 and 664,000 in 1997). Though data is not available, China is believed to be the largest producer, followed by the United States. One estimate puts China's production capacity as high as 400,000 metric tons per year, with over 400 producers. Exports from this country have increased in recent years.
Consumption of silicon metal in the United States was roughly 262,000 metric tons, at a cost of 57 cents per pound. The annual growth rate during 1980-1995 was about 3.5% for silicon demand by the aluminum industry and about 8% by the chemical industry. Demand by the chemical industry (mainly silicones) was affected by the Asian economic crisis of the late 1990s.
Silicon was first isolated and described as an element in 1824 by a Swedish chemist, Jons Jacob Berzelius. An impure form was obtained in 1811. Crystalline silicon was first produced in 1854 using electrolysis.
The type of furnace now used to make silicon, the electric arc furnace, was first invented in 1899 by French inventor Paul Louis Toussaint Heroult to make steel. The first electric arc furnace in the United States was installed in Syracuse, New York in 1905. In recent years, furnace technology, including the electrodes used for heating elements, has improved.
Silicon metal is made from the reaction of silica (silicon dioxide, SiO2) and carbon materials like coke, coal and wood chips. Silica is typically received in the form of metallurgical grade gravel. This gravel is 99.5% silica, and is 3 x 1 or 6 x 1 in (8 x 3 cm or 15 x 3 cm) in size. The coal is usually of low ash content (1-3% to minimize calcium, aluminum, and iron impurities), contains around 60% carbon, and is sized to match that of the gravel. Wood chips are usually hardwood of 1/2 x 1/8 inch size (1 x. 3 cm size). All materials are received as specified by the manufacturer.
The Manufacturing Process
The basic process heats silica and coke in a submerged electric arc furnace to high temperatures. High temperatures are required to produce a reaction where the oxygen is removed, leaving behind silicon. This is known as a reduction process. In this process, metal carbides usually form first at the lower temperatures. As silicon is formed, it displaces the carbon. Refining processes are used to improve purity.
The Reduction Process
- 1 The raw materials are weighed and then placed into the furnace through the top using the fume hood, buckets, or cars. A typical batch contains 1000 lb (453 kg) each of gravel and chips, and 550 lb (250 kg) of coal. The lid of the furnace, which contains electrodes, is placed into position. Electric current is passed through the electrodes to form an arc. The heat generated by this arc (a temperature of 4000° F or 2350 ° C) melts the material and results in the reaction of sand with carbon to form silicon and carbon monoxide. This process takes about six to eight hours. The furnace is continuously charged with the batches of raw materials.
- 2 While the metal is in the molten state, it is treated with oxygen and air to reduce the amount of calcium and aluminum impurities. Depending on the grade, silicon metal contains 98.5-99.99% silicon with trace amounts of iron, calcium and aluminum.
- 3 Oxidized material, called slag, is poured off into pots and cooled. The silicon metal is cooled in large cast iron trays about 8 ft (2.4 m) across and 8 in (20 cm) deep. After cooling, the metal is dumped from the mold into a truck, weighed and then dumped in the storage pile. Dumping the metal from the mold to the truck breaks it up sufficiently for storage. Before shipping, the metal is sized according to customer specifications, which may require a crushing process using jaw or cone crushers.
- 4 Silicon metal is usually packaged in large sacks or wooden boxes weighing up to 3,000 lb (1,361 kg). In powder form, silicon is packaged in 50-lb (23-kg) plastic pails or paper bags, 500-lb (227-kg) steel drums or 3,000-lb (1,361-kg) large sacks or boxes.
Statistical process control is used to ensure quality. Computer-controlled systems are used to manage the overall process and evaluate statistical data. The two major process parameters that must be controlled are amounts of raw materials used and furnace temperatures. Laboratory testing is used to monitor the chemical composition of the final product and to research methods to improve the composition by adjusting the manufacturing process. Quality audits and regular assessments of suppliers also ensure that quality is maintained from extraction of raw materials through shipping of the final product.
With statistical process control, waste is kept to a minimum. A byproduct of the process, silica fume, is sold to the refractory and cement industries to improve strength of their products. Silica fume also is used for heat insulation, filler for rubber, polymers, grouts and other applications. The cooled slag is broken down into smaller pieces and sold to other companies for further processing. Some companies crush it into sandblasting material. Because electric arc furnaces emit particulate emissions, manufacturers must also comply with the Environmental Protection Agency's (EPA) regulations.
Though industry analysts predicted demand for chemical-grade silicon by Western countries would increase at an annual average rate of about 7% until 2003, this growth may be slower due to recent economic declines in Asia and Japan. If supplies continue to outpace demand, prices may continue to drop. The outlook for the automotive market is positive, as more car makers switch to an aluminum-silicon alloy for various components.
Other methods for making silicon are being investigated, including supercooling liquid to form bulk amorphous silicon and a hydrothermal method for making porous silicon powder for optical applications.
Where to Learn More
Kirk-Othmer. Encyclopedia of Chemical Technology. New York: John Wiley & Sons, Inc. 1985.
Bendix, Jeffrey. "The Heart of Globe is in Cleveland." Cleveland Enterprise (Fall 1991).
Ward, Patti. "Heroult Electric Arc Furnace Stands the Test of Time." Iron and Steelmaker 26, no. 11 (November 1999). .
Annual Minerals Review: Silicon. U.S. Geological Survey, 1998.
Mineral Commodity Summaries: Silicon. U.S. Geological Survey, February 2000.
Mineral Industry Surveys: Silicon in February 2000. U.S. Geological Survey, May 2000.
i>Laurel M. Sheppard