Silicon 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 weight. It occurs in
clay,
feldspar,
granite,
quartz and
sand, mainly in the form of
silicon dioxide (also known as silica) and
silicates (compounds containing silicon, oxygen and metals). Silicon is the principal component of
glass,
cement, ceramics, most
semiconductor devices, and
silicones, the latter a plastic substance often confused with silicon. Silicon is widely used in semiconductors because the semiconductor
Germanium has a problem with
reverse leakage current flow, and because its
native oxide forms better semiconductor/dielectric interfaces 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 crystals are rarely found in nature, as natural silicon is usually found as silica (SiO
2). Pure silicon crystals can be found as inclusions in
gold, or in volcanic exhalations. Pure silicon has a negative
temperature co-efficient of
resistance, since the number of free charge carriers increases with temperature.
Applications
Silicon is a very useful element that is vital to many human industries.
Silicon dioxide in the form of
sand and
clay is an important ingredient of
concrete and
brick and is also used to produce
Portland cement. Silicon is a very important element for
plant and
animal life.
Diatoms extract silica from water to build their protective cell walls. Other uses:
- Pottery/Enamel - It is a refractory material used in high-temperature material production and its silicates are used in making enamels and pottery.
- Steel - Silicon is an important constituent of some steels.
- Bronze - Most bronze produced is an alloy of copper and silicon.
- 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, and insulators, and many other useful objects.
- Abrasives - Silicon carbide is one of the most important abrasives.
- 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 and other semiconductor devices which are used in electronics and other high-tech applications.
- Photonics - Silicon can be used in lasers to produce coherent light with a wavelength of 456 nm.
- 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.
- LCDs and solar cells - Hydrogenated amorphous silicon has shown promise 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.
- Construction - Silica is a major ingredient in bricks because of its low chemical activity.
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 of 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
Silicon is a principal component of aerolites which are a class of
meteoroids and also of
tektites which is a natural form of glass.
Measured by weight, silicon makes up 25.7% of the
earth's crust and after
oxygen is also the second most abundant element. Elemental silicon is not found in nature. It occurs most often as
oxides and as silicates.
Sand,
amethyst, agate,
quartz, rock crystal, flint,
jasper, and
opal are some of the forms in which the oxide appears.
Granite,
asbestos,
feldspar, clay,
hornblende, and
mica are a few of the many silicate
minerals.
Production
Silicon is commercially prepared by the heating of high-purity silica 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
:SiO
2 + C → Si + CO
2
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 SiO
2 is kept high, silicon carbide may be eliminated, as explained by this equation:
:2SiC + SiO
2 → 3Si + 2CO
In 2000, metallurgical grade silicon cost about
$ 0.56 per pound ($1.23/kg).
http://minerals.usgs.gov/minerals/pubs/commodity/silicon/760301.pdf.
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, 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 compound 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 HSiCl
3 → Si + 2 HCl + SiCl
4
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
:SiCl
4 + 2 Zn → Si + 2 ZnCl
2
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
Czochralski process is often used to make high-purity single silicon crystals for use in
solid-state/
semiconductor devices.
Isotopes
Silicon has nine
isotopes, with
mass numbers from 25-33. Si-28 (the most abundant isotope, at 92.23%), Si-29 (4.67%), and Si-30 (3.1%) are stable; Si-32 is a radioactive isotope produced by
argon decay. Its
half-life, after much argument, has been determined to be approximately 276 years, and it decays by beta emission to
P-32 (which has a 14.28 year half-life) and then to
S-32.
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 is not silicone
Casual speakers often make the mistake of interchanging the words
silicon and
silicone; they are
not the same. The first, of course, is the element that is the topic of this article. The second is a class of chemical compounds (in particular, inorganic polymers) that contain the element silicon, the most notable members of the class being silicone rubbers and silicone gels.
References
External links
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