| References | Silicon is the chemical element that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. As the eighth most common element in the universe by mass, silicon occasionally occurs as the pure free element in nature, but is more widely distributed in dusts, planetoids and planets as various forms of silicon dioxide or silicate. On Earth, silicon is the second most abundant element (after oxygen) in the crust, making up 25.7% of the crust by mass.
Silicon has many industrial uses. Elemental silicon is the principal component of most semiconductor devices, most importantly integrated circuits or microchips. Silicon is widely used in semiconductors because it remains a semiconductor at higher temperatures than the semiconductor germanium and because its native oxide is easily grown in a furnace and forms a better semiconductor/dielectric interface than almost all other material combinations. In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics. It is also a component of silicones, a class-name for various synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often confused with silicon itself. Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals. It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatoms. |
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| Notable characteristics | Having the same structure to the outer electron orbitals (half filled subshell holding up to eight electrons) as carbon, the two elements are very similar chemically and both are semiconductors readily either donating or sharing their four outer electrons allowing many different forms of chemical bonding. Pure silicon has a negative temperature coefficient of resistance, since the number of free charge carriers increases with temperature. The electrical resistance of single crystal silicon significantly changes under the application of mechanical stress due to the piezoresistive effect.
In its elemental crystalline form, silicon has a gray color and a metallic luster which increases with the size of the crystal. It is similar to glass in that it is rather strong, very brittle, and prone to chipping. Even though it is a relatively inert element, silicon still reacts with halogens and dilute alkalis, but most acids (except for some hyper-reactive combinations of nitric acid and hydrofluoric acid) do not affect it. Having four bonding electrons however gives it, like carbon, many opportunities to combine with other elements or compounds under the right circumstances. |
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| Applications | As the second most common element on earth, silicon is a very useful element that is vital to many human industries, and impacts much of modern life as a principal component in glass, concrete and cements of many kinds. Outside of the many modern world features its construction uses enable, perhaps silicon's most lifestyle affecting application is its use as the fundamental substrate in manufacturing electronics integrated circuits such as computer chips, and discrete active devices such as power transistors. Further, the element and its compounds find widespread use in explosives and pyrotechnics and further uses in mechanical seals, high temperature silicon based greases, caulking compounds and so forth.
Alloys The largest application of pure silicon (metallurgical grade silicon) is in aluminium-silicon alloys, often called "light alloys", to produce cast parts, mainly for automotive industry. (This represents about 55% of the world consumption of pure silicon.) The second largest application of pure silicon is as a raw material in the production of silicones (about 40% of the world consumption of silicon) Pure silicon is also used to produce ultra-pure silicon for electronic and photovoltaic applications: Semiconductor: Ultrapure silicon can be doped with other elements to adjust its electrical response by controlling the number and charge (positive or negative) of current carriers. Such control is necessary for transistors, solar cells, microprocessors, semiconductor detectors and other semiconductor devices which are used in electronics and other high-tech applications. Photonics: Silicon can be used as a continuous wave Raman laser to produce coherent light. (Though it is ineffective as a light source.) LCDs and solar cells: Hydrogenated amorphous silicon is widely used in the production of low-cost, large-area electronics in applications such as LCDs. It has also shown promise for large-area, low-cost thin-film solar cells. Steel and cast iron: Silicon is an important constituent of some steels, and it is used in the production process of cast iron. It is introduced as ferrosilicon or silicocalcium alloys. Compounds Construction: Silicon dioxide or silica in the form of sand and clay is an important ingredient of concrete and brick and is also used to produce Portland cement. Pottery/Enamel is a refractory material used in high-temperature material production and its silicates are used in making enamels and pottery. Glass: Silica from sand is a principal component of glass. Glass can be made into a great variety of shapes and with a many different physical properties. Silica is used as a base material to make window glass, containers, insulators, and many other useful objects. Abrasives: Silicon carbide is one of the most important abrasives. Medical materials: Silicones are flexible compounds containing silicon-oxygen and silicon-carbon bonds; they are widely used in applications such as artificial breast implants and contact lenses. Silicones are also used in many other applications. Silly Putty was originally made by adding boric acid to silicone oil. Now name-brand Silly Putty also contains significant amounts of elemental silicon. (Silicon binds to the silicone and allows the material to bounce 20% higher.) |
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| History | Silicon was first identified by Antoine Lavoisier in 1787 (as a component of the Latin silex, or silicis (meaning what were more generally termed "the flints" or "Hard Rocks" during the Early Modern era where nowadays as we would say "silica" or "silicates"), and was later mistaken by Humphry Davy in 1800 for a compound. In 1811 Gay-Lussac and Thénard probably prepared impure amorphous silicon through the heating of potassium with silicon tetrafluoride. It was first discovered as an element by Berzelius in 1823. In 1824, Berzelius prepared amorphous silicon using approximately the same method as Lussac. Berzelius also purified the product by repeatedly washing it.
Because silicon is an important element in semiconductors and high-tech devices, the high-tech region of Silicon Valley, California, is named after this element. |
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| Occurrence | Measured by mass, silicon makes up 25.7% of the Earth's crust and is the second most abundant element on Earth, after oxygen. Pure silicon crystals are only occasionally found in nature; they can be found as inclusions with gold and in volcanic exhalations. Silicon is usually found in the form of silicon dioxide (also known as silica), and silicate.
Silica occurs in minerals consisting of (practically) pure silicon dioxide in different crystalline forms. Sand, amethyst, agate, quartz, rock crystal, chalcedony, flint, jasper, and opal are some of the forms in which silicon dioxide appears. (They are known as "lithogenic", as opposed to "biogenic", silicas.) Silicon also occurs as silicates (various minerals containing silicon, oxygen and one or another metal), for example feldspar. These minerals occur in clay, sand and various types of rock such as granite and sandstone. Asbestos, feldspar, clay, hornblende, and mica are a few of the many silicate minerals. Silicon is a principal component of aerolites, which are a class of meteoroids, and also is a component of tektites, which are a natural form of glass. |
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| Production | Silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal, in an electric arc furnace using carbon electrodes. At temperatures over 1900 °C, the carbon reduces the silica to silicon according to the chemical equation
SiO2 + C > Si + CO2. SiO2 + 2C > Si + 2CO. Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 98% pure. Using this method, silicon carbide, SiC, can form. However, provided the amount of SiO2 is kept high, silicon carbide may be eliminated, as explained by this equation: 2 SiC + SiO2 > 3 Si + 2 CO. In 2005, metallurgical grade silicon cost about $ 0.77 per pound ($1.70/kg). |
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| Crystallization | The majority of silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) since it is the cheapest method available and it is capable of producing large size crystals. However, silicon single-crystals grown by the Czochralski method contain impurities since the crucible which contains the melt dissolves. For certain electronic devices, particularly those required for high power applications, silicon grown by the Czochralski method is not pure enough. For these applications, float-zone silicon (FZ-Si) can be used instead. It is worth mentioning though, in contrast with CZ-Si method in which the seed is dipped into the silicon melt and the growing crystal is pulled upward, the thin seed crystal in the FZ-Si method sustains the growing crystal as well as the polysilicon rod from the bottom. As a result, it is difficult to grow large size crystals using the float-zone method. Today, all the dislocation-free silicon crystals used in semiconductor industry with diameter 300mm or larger are grown by the Czochralski method with purity level significantly improved. | ||||||
| Isotopes | Silicon has numerous known isotopes, with mass numbers ranging from 22 to 44. 28Si (the most abundant isotope, at 92.23%), 29Si (4.67%), and 30Si (3.1%) are stable; 32Si is a radioactive isotope produced by argon decay. Its half-life has been determined to be approximately 170 years (0.21 MeV), and it decays by beta - emission to 32P (which has a 14.28 day half-life [3]) and then to 32S. | ||||||