Silicon

Notable characteristics

The outer electron orbitals (half filled subshell holding up to eight electrons) have the same structure as in 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.

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 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.

Occurrence

Measured by mass, silicon makes up 25.7% of the Earth's crust and is the second most abundant element on Earth, after silicate.

Silica occurs in biogenic", silicas.)

Silicon also occurs as 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.

See also Category:Silicate minerals

Isotopes

Main article: isotopes of silicon

Silicon has numerous known S.

Compounds

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For examples of silicon compounds see trichlorosilane (HSiCl₃).

See also Category:Silicon compounds

Applications

As the second most abundant element in the earth's crust, silicon is vital to the construction industry as a principal constituent of natural stone, glass, cement. Silicon's greatest impact on the modern world's economy and lifestyle has resulted from its use as the substrate in the manufacture of discrete electronic devices such as power transistors, and in the development of integrated circuits such as computer chips.

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.)
• alloys.

In electronic applications

• Pure silicon is also used to produce ultra-pure silicon for electronic and photovoltaic applications:
  • semiconductor devices which are used in electronics and other high-tech applications.
  • 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.

Silicones

The second largest application of silicon (about 40% of world consumption) is as a raw material in the production of release agents, mechanical seals, high temperature greases and waxes, caulking compounds and even in applications as diverse as breast implants and explosives and pyrotechnics ^[1] .

• Construction: 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 many different physical properties. Silica is used as a base material to make window glass, containers, insulators, and many other useful objects.
• Silicon carbide is one of the most important abrasives.
• 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.)^{[citation needed]}

See also Category:Silicon compounds

Production

Silicon is commercially prepared by the reaction of high-purity 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).[6]

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 is desired.

Chemical methods

Today, silicon is purified by converting it to a silicon silane are also used. When these gases are blown over silicon at high temperature, they decompose to high-purity silicon.

At one time, 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.

 

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 less than 10⁻⁹.

In 2006 REC announced construction of a plant based on fluidized bed technology using silane ^[2].

3SiCl₄ + Si + 2H₂ → 4HSiCl₃

4HSiCl₃ → 3SiCl₄ + SiH₄

SiH₄ → Si + 2H₂

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 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.

Different forms of silicon

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.

Silicon-based life

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See also: Alternative biochemistry

Since silicon is similar to carbon, particularly in its valency, some people have proposed the possibility of silicon-based life. 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 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 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.

History

Silicon was first identified 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.

References

• Los Alamos National Laboratory: Silicon
• Elastic Waves in Solids II, Eugène Dieulesaint, Daniel Royer (Springer) 2000 (ISBN 3-540-65931-5) (speed of sound)

See also

• List of silicon producers
• silicone
• electronics