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 any other material.
In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics. It is also a constituent 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.
Both silicon and carbon 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 crystalline form, pure silicon has a gray color and a metallic luster. It is similar to glass in that it is rather strong, very brittle, and prone to chipping.
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.
See also Silicate minerals
Silicon forms binary compounds called silicides with many metallic elements whose properties range from reactive compounds e.g. magnesium silicide , Mg2Si through to high melting refractory compounds such as molybdenum disilicide, MoSi2.
Silicon carbide, SiC, (carborundum) is a hard, high melting solid and is the well known abrasive.
Silane, SiH4, is a pyrophoric gas with a similar tetrahedral structure to methane, CH4. Additionally there is a range of catenated silicon hydrides that form an homologous series of compounds, SinH2n+2 where n = 2-8 (analogous to the alkanes). These are all readily hydrolysed and are thermally unstable, particularly the heavier members.
Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are generally highly reactive requiring large substituent groups to stabilise them.
Tetrahalides, SiX4, are formed with all of the halogens. Silicon tetrachloride for example readily reacts with water unlike its carbon analogue, carbon tetrachloride. Silicon dihalides are formed by the high temperature reaction of tetrahalides and silicon, with a structure analogous to a carbene they are reactive compounds. Silicon difluoride condenses to form a polymeric compound, (SiF2)n.
Silicon dioxide, is a high melting solid with a number of different crystal forms the most familiar of which is the mineral quartz. In quartz each silicon atom is surrounded by four oxygen atoms that bridge to other silicon atoms to form a three dimensional lattice. Silica is soluble in water at high temperatures forming monosilicic acid (Si(OH)4) and this property is used in the manufacture of quartz crystals used in electronics. ,
Under the right conditions monosilicic acid readily polymerises to form more complex silicic acids, ranging from the simplest condensate, disilicic acid (H6Si2O7)to linear, ribbon, layer and lattice structures which form the basis of the many different silicate minerals. Silicates are also important constituents of concretes.
With oxides of other elements the high temperature reaction of silicon dioxide can give a wide range of glasses with various properties. Examples include soda lime glass, borosilicate glass and lead crystal glass.
Silicon sulfide, SiS2 is a polymeric solid (unlike its carbon analogue the gas CS2.
Silicon forms a nitride, Si3N4 which is a ceramic. Silatranes a group of tricyclic compounds containing five coordinate silicon may have physiological properties.
Many transition metal complexes containing a metal-silicon bond are now known, which include complexes containing, SiHnX3-n ligands, SiX3 ligands, Si(OR)3 ligands.
Silicones are large group of polymeric compounds with an (Si-O-Si) backbone an example is the silicone oil, PDMS, polydimethylsiloxane). These polymers can be crosslinked to produce resins and elestomers.
Many organosilicon compounds are known which contain a silicon-carbon single bond. Many of these are based on a central tetrahedral silicon atom and some are optically active when central chirality exists. In 1981 a silene with a silicon-carbon double bond and a disilene with a silicon-silicon double bond were isolated. A compound with silicon-silicon triple bond was first isolated in 2004, although as the compound is non-linear, the bonding is dissimilar to that in alkynes.
Long chain polymers containing a silicon backbone are known for example polydimethysilylene (SiMe2)n. Polycarbosilane, [(SiMe2)2CH2]n with a backbone containing a repeating -Si-Si-C unit is a precursor in the production of silicon carbide fibres.
See also Silicon compounds
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:
In 2005, metallurgical grade silicon cost about $ 0.77 per pound ($1.70/kg).
It has been reported in recent years that, by molten salt electrolysis, pure silicon can be directly extracted from solid silica and this new electrolysis method, known as the FFC Cambridge Process, has the potential to produce directly the solar grade silicon without any CO2 emission at much lower energy consumption.
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 re-solidifies behind it. Since most impurities tend to remain in the molten region rather than re-solidify, 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 is desired.
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.
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
Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of less than 10−9.
In 2006 REC announced construction of a plant based on fluidized bed technology using silane.
Silicon, like carbon and other group IV elements form face-centered diamond cubic crystal structure. Silicon, in particular, forms a face-centered cubic structure with a lattice spacing of 5.430710 Å (0.5430710 nm).
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.
Although there are no known forms of life that rely entirely on silicon-based chemistry, some use silica for specific functions. The polycystine radiolaria and diatoms have skeletons of opaline silicon dioxide, and the Hexactinellid sponges secrete spicules 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 life forms; 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.
A. G. Cairns-Smith has proposed that the first living organisms to exist were forms of clay minerals—which were probably based around the silicon atom.
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. Other geographic locations with connections to the industry have since characterized themselves as siliconia as well.
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