Characteristics

Silicon has many industrial uses.  Elemental Silicon is the principal component of most semiconductor devices.  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. 

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 a combination of Nitric Acid and Hydrofluoric Acid) do not affect it.  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.

Silicon is a crystalline semi-metal or metalloid. One of its forms is shiny, grey and very brittle (it will shatter when struck with a hammer). In another allotropic form silicon is a brown amorphous powder most familiar in "dirty" beach sand. The crystalline form of silicon is the foundation of the semiconductor age.

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 and Silicate.

Silica occurs in minerals consisting of (practically) pure Silicon Dioxide in different crystalline forms (Quartz, Chalcedony, Opal).  Sand, Amethyst, Agate, Quarts, 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).

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.

Applications

Silicon is a very useful element that is vital to many human industries.  Silicon is used frequently in manufacturing computer chips and related hardware.

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 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, 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.
• 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).

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 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)

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 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 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 such as:

2 HSiCl₃ Si + 2 HCl + SiCl₄

Silicon produced from this and similar processes is called polycrystalline silicon.   Polycrystalline silicon typically has impurity levels of less than 10^-9.

At one time, DuPont produced ultra-pure 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, (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.

Compounds

Examples of silicon compounds are Silicate, Silane (SiH₄), Silicic Acid (H₄SiO₄), Silicon Carbide (SiC), Silicon Dioxide (SiO₂), Silicon Tetrachloride (SiCl₄), Silicon Tetrafluoride (SiF₄), and Trichlorosilane (HSiCl₃).

One can notice the color change in Silicon nanopowder.  This is caused by the quantum effects which occur in particles of nanometric dimensions.

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.

Trivia

Since Silicon is similar to Carbon, particularly in its valency, some people have proposed the possibility of Silicon-based life.  This concept is especially popular in science fiction.  One main detraction for silicon-based life is that unlike Carbon, Silicon does not have the tendency to form double and triple bonds.

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 spines 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.

(L. silex, silicis, flint) Davy in 1800 thought silica to be a compound and not an element; later in 1811, Gay Lussac and Thenard probably prepared impure amorphous silicon by heating potassium with silicon tetrafluoride. Berzelius, generally credited with the discovery, in 1824 succeeded in preparing amorphous silicon by the same general method as used earlier, but he purified the product by removing the fluorosilicates by repeated wishing. Deville in 1854 first prepared crystalline silicon, the second allotropic form of the element. Silicon is present in the sun and stars and is a principal component of a class of meteorites known as "aerolites." It is also a component of tektites, a natural glass of uncertain origin. Silicon makes up 25.7% of the earth's crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals. Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. Crystalline silicon has a metallic luster and grayish color. The Czochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. This product can be doped with boron, gallium, phosphorus, or arsenic to produce silicon for use in transistors, solar cells, rectifiers, and other solid-state devices which are used extensively in the electronics and space-age industries. Hydrogenated amorphous silicon has shown promise in producing economical cells for converting solar energy into electricity. Silicon is a relatively inert element, but it is attacked by halogens and dilute alkali. Most acids, except hydrofluoric, do not affect it. Silicones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones, ranging from liquids to hard, glasslike solids with many useful properties. Elemental silicon transmits more than 95% of all wavelengths of infrared, from 1.3 to 6.y micro-m. Silicon is one of man's most useful elements. In the form of sand and clay it is used to make concrete and brick; it is a useful refractory material for high-temperature work, and in the form of silicates it is used in making enamels, pottery, etc. Silica, as sand, is a principal ingredient of glass, one of the most inexpensive of materials with excellent mechanical, optical, thermal, and electrical properties. Glass can be made in a very great variety of shapes, and is used as containers, window glass, insulators, and thousands of other uses. Silicon tetrachloride can be used as iridize glass. Silicon is important to plant and animal life. Diatoms in both fresh and salt water extract Silica from the water to build their cell walls. Silica is present in the ashes of plants and in the human skeleton. Silicon is an important ingredient in steel; silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A. Regular grade silicon (99%) costs about $0.50/g. Silicon 99.9% pure costs about $50/lb; hyperpure silicon may cost as much as $100/oz. Miners, stonecutters, and others engaged in work where siliceous dust is breathed into large quantities often develop a serious lung disease known as silicosis.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond

Silicon was discovered by Jöns Jacob Berzelius, a Swedish chemist, in 1824 by heating chips of Potassium in a silica container and then carefully washing away the residual by-products.  Silicon is the seventh most abundant element in the universe and the second most abundant element in the Earth's crust.  Today, silicon is produced by heating sand (SiO₂) with Carbon to temperatures approaching 2200°C.

Two allotropes of silicon exist at room temperature: amorphous and crystalline. Amorphous appears as a brown powder while crystalline silicon has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown with a process known as the Czochralski process.  These crystals, when doped with elements such as Boron, Gallium, Germanium, Phosphorus or Arsenic, are used in the manufacture of solid-state electronic devices, such as transistors, solar cells, rectifiers and microchips.

Silicon Dioxide (SiO₂), silicon's most common compound, is the most abundant compound in the Earth's crust.   It commonly takes the form of ordinary sand, but also exists as quartz, rock crystal, amethyst, agate, flint, jasper and opal. Silicon dioxide is extensively used in the manufacture of glass and bricks. Silica gel, a colloidal form of silicon dioxide, easily absorbs moisture and is used as a desiccant.

Silicon forms other useful compounds. Silicon carbide (SiC) is nearly as hard as diamond and is used as an abrasive. Sodium Silicate (Na₂SiO₃), also known as water glass, is used in the production of soaps, adhesives and as an egg preservative. Silicon Tetrachloride (SiCl₄) is used to create smoke screens.   Silicon is also an important ingredient in silicone, a class of material that is used for such things as lubricants, polishing agents, electrical insulators and medical implants.