Encyclopedia
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 mass. It does not occur free in nature. It mainly occurs in
minerals consisting of pure silicon dioxide in different crystalline forms and as silicates , 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 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.
Notable characteristics
In its
crystalline form, silicon has a dark gray color and a metallic luster. 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 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.
- The largest application of pure silicon is in aluminum - silicon alloys, often called "light alloys", to produce cast parts, mainly for automotive industry .
- The second largest application of pure silicon is as a raw material in the production of silicones
- 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 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
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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 History
Silicon 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 and silicate.
It is estimated to be the seventh most plentiful element in the universe.
Sand,
amethyst, agate,
quartz, rock crystal, flint,
jasper, and
opal are some of the forms in which silicon dioxide appears .
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 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 SiO
2 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
$ 0.56 per pound ..
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 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 can be used instead.
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
NanoparticleIsotopes
Silicon has numerous known isotopes, with mass numbers ranging from 22 to 44.
28Si ,
29Si , and
30Si 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 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 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 lifeforms; it would be difficult 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 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 as its
p-orbital electrons experience greater shielding and are less able to take on the necessary geometry. Furthermore, although some silicon rings 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 , 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 ,
silicic acid ,
silicon carbide , silicon dioxide ,
silicon tetrachloride ,
silicon tetrafluoride , and trichlorosilane .
See also References
- "Ondes élastiques dans les solides", by E. Dieulesaint and D. Royer
See also
- List of silicon producers.
External links
