Not to be confused with Silicone.

Silicon (Latin: silicium) is the chemical element in the periodic table that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. It is the second most abundant element in the Earth's crust, making up 25.7% of it by mass. It does not occur free in nature. It mainly occurs in minerals consisting of (practically) pure silicon dioxide in different crystalline forms (quartz, chalcedony, opal) and 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 like granite and sandstone. Silicon is the principal component of most semiconductor devices and, in the form of silica and silicates, in glass, cement, and ceramics. It is also a component of silicones, a name for various plastic substances often confused with silicon itself. 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.

Notable characteristics

In its crystalline form, silicon has a dark gray color and a metallic luster. 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 a combination of nitric acid and hydrofluoric acid) do not affect it. Elemental silicon transmits more than 95% of all wavelengths of infrared light. Pure silicon has a negative temperature co-efficient 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.

Applications

Silicon is a very useful element.

Silicon and alloys

The largest application of pure silicon (metallurgical grade silicon) is in aluminum - 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 ultrapure silicon for electronic and photovoltaic applications:
 

History

Silicon (Latin silex, silicis, meaning flint) was first identified by Antoine Lavoisier in 1787, 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. 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 semiconductor and high-tech devices, the high-tech region of Silicon Valley, California, is named after this element.

Occurrence

Measured by weight, 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 rarely found in nature; natural silicon is usually found in the form of silicon dioxide (also known as silica) and silicate. It is estimated to be the seventh most plentiful element in the universe.
Sand, amethyst, agate, quartz, rock crystal, flint, jasper, and opal are some of the forms in which silicon dioxide appears (they are known as "lithogenic", as opposed to "biogenic", silicas). Granite, asbestos, feldspar, clay, hornblende, and mica are a few of the many silicate minerals. Pure silicon crystals can be found as inclusions with gold and in volcanic exhalations.
Silicon is a principal component of aerolites, which are a class of meteoroids, and also of tektites, which are a natural form of glass.

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

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 99% 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

Purification

The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Historically, a number of methods have been used to produce high-purity silicon.

Chemical methods

Today, silicon is instead purified by converting it to a silicon compound that can be more easily purified than silicon itself, and then converting that silicon element back into pure silicon. Trichlorosilane is the silicon compound most commonly used as the intermediate, although silicon tetrachloride and silane are also used. When these gases are blown over silicon at high temperature, they decompose to high-purity silicon.
In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150 °C. The trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them according to chemical reactions like

2 HSiCl3 → Si + 2 HCl + SiCl4

Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of 1 part per billion or less.

At one time, DuPont produced ultrapure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950 °C, producing silicon according to the chemical equation

SiCl4 + 2 Zn → Si + 2 ZnCl2

However, this technique was plagued with practical problems (such as the zinc chloride byproduct solidifying and clogging lines) and was eventually abandoned in favor of the Siemens process.

Crystallization

The majority of silicon crystals grown for device production are produced by the Czochralski process, since it is the cheapest method available. 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.
 

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 132 years, and it decays by beta emission to 32P (which has a 14.28 day half-life) and then to 32S.

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