Silicon

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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 chrystalline 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 has a lower reverse leakage current 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. 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 that is vital to many human industries.

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 :
- 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, 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 with a wavelength of 1,698 nm.
- 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 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 ferro-silicon or silico-calcium alloys.

Silicon 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 - It 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.

See also category:Silicon compounds

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.

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.

See also category:Silicate minerals

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

In 2000, metallurgical grade silicon cost about States dollar|$" target="_blank" >^* 0.56 per pound ($1.23/kg).^*.

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.

Physical methods

Early silicon purification techniques were based on the fact that if silicon is melted and re-solidified, the last parts of the mass to solidify contain most of the impurities. The earliest method of silicon purification, first described in 1919 and used on a limited basis to make radar components during World War II, involved crushing metallurgical grade silicon and then partially dissolving the silicon powder in an acid. When crushed, the silicon cracked so that the weaker impurity-rich regions were on the outside of the resulting grains of silicon. As a result, the impurity-rich silicon was the first to be dissolved when treated with acid, leaving behind a more pure product.

In zone melting, also called zone refining, the first silicon purification method to be widely used industrially, rods of metallurgical grade silicon are heated to melt at one end. Then, the heater is slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cools and resolidifies behind it. Since most impurities tend to remain in the molten region rather than resolidify, when the process is complete, most of the impurities in the rod will have been moved into the end that was the last to be melted. This end is then cut off and discarded, and the process repeated if a still higher purity was desired.

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.

Different forms of silicon

Image:Silicon granular 640x480.jpg|Granular silicon Image:Silicon poly 640x480.jpg|Polycrystal silicon Image:Silicon crystal 4 inch interferences 640x480.jpg|Silicon monocrystal Image:Nano Si 640x480.jpg|Silicon nanopowder Image:Monokristalines Silizium für die Waferherstellung.jpg

One can notice the color change in silicon nanopowder. This is caused by the quantum effects which occur in particles of nanometric dimensions. See also Potential well, Quantum dot, and Nanoparticle

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.

Precautions

A serious lung disease known as silicosis often occurred in miners, stonecutters, and others who were engaged in work where siliceous dust was inhaled in great quantities.

Silicon-based life

Since silicon is analogous to carbon, particularly in its valency, some scientists have proposed the possibility of silicon-based life. This concept is especially popular in science fiction.

Although there are no known forms of life that rely entirely on silicon-based chemistry, there are some that rely on silicon minerals for specific functions. Some bacteria and other forms of life, such as the protozoa radiolaria, have silicon dioxide skeletons, and the sea urchin has its sharps made of silicon dioxide. These forms of silicon dioxide are known as biogenic silica. Silicate bacteria use silicates in their metabolism.

Life as we know it could not have developed based on a silicon biochemistry. The main reason for this fact is that life on Earth depends on the carbon cycle: autotrophic entities use carbon dioxide to synthesize organic compounds with carbon, which is then used as food by heterotrophic entities, which produce energy and carbon dioxide from these compounds. If carbon was to be replaced with silicon, there would be a need for a silicon cycle. However, silicon dioxide precipitates in aqueous systems, and cannot be transported among living beings by common biological means.

As such, another solvent would be necessary to sustain silicon-based lifeforms; it would be difficult (if not impossible) to find another common compound with the unusual properties of water which make it an ideal solvent for carbon-based life. Larger silicon compounds analogous to common hydrocarbon chains (silanes) are also generally unstable owing to the larger atomic radius of silicon and the correspondingly weaker silicon-silicon bond; silanes decompose readily and often violently in the presence of oxygen making them unsuitable for an oxidizing atmosphere such as our own. Silicon also does not readily participate in pi-bonding (the second and third bonds in triple bonds and double bonds are pi-bonds) as its p-orbital electrons experience greater shielding and are less able to take on the necessary geometry. Furthermore, although some silicon rings (cyclosilanes) analogous to common the cycloalkanes formed by carbon have been synthesized, these are largely unknown. Their synthesis suffers from the difficulties inherent in producing any silane compound, whereas carbon will readily form five-, six-, and seven-membered rings by a variety of pathways (the Diels-Alder reaction is one naturally-occurring example), even in the presence of oxygen. Silicon's inability to readily form long silane chains, multiple bonds, and rings severely limits the diversity of compounds that can be synthesized from it. Under known conditions, silicon chemistry simply cannot begin to approach the diversity of organic chemistry, a crucial factor in carbon's role in biology.

However, silicon-based life could be construed as being life which exists under a computational substrate. This concept is yet to be explored in mainstream technology but receives ample coverage by sci-fi authors.

Compounds

For examples of silicon compounds see silicate, silane (SiH₄), silicic acid (H₄SiO₄), silicon carbide (SiC), silicon dioxide (SiO₂), silicon tetrachloride (SiCl₄), silicon tetrafluoride (SiF₄), and trichlorosilane (HSiCl₃).