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The Element Silicon

Silicon is a 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.

aluminium silicon phosphorus
C
Si
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Image:Si-TableImage.png
General
Name, Symbol, Number silicon, Si, 14
Series metalloid
Group, Period, Block 14 (IVA), 3, p
Density, Hardness 2330 kg/m3, 6.5
Appearance dark grey, bluish tinge
Atomic properties
Atomic weight 28.0855 amu
Atomic radius (calc.) 110 pm (111 pm)
Covalent radius 111 pm
van der Waals radius 210 pm
Electron configuration [Ne]3s2 3p2
e- 's per energy level 2, 8, 4
Oxidation states (Oxide) 4 (amphoteric)
Crystal structure cubic face centered
Physical properties
State of matter solid (nonmagnetic)
Melting point 1687 K (2577 F)
Boiling point 3173 K (5252 F)
Molar volume 12.06 10-6 m3/mol
Heat of vaporization 384.22 kJ/mol
Heat of fusion 50.55 kJ/mol
Vapor pressure 4.77 Pa at 1683 K
Speed of sound __ m/s at __ K
Miscellaneous
Electronegativity 1.90 (Pauling scale)
Specific heat capacity 700 J/(kg*K)
Electrical conductivity 2.52 10-4/m ohm
Thermal conductivity 148 W/(m*K)
1st ionization potential 786.5 kJ/mol
2nd ionization potential 1577.1 kJ/mol
3rd ionization potential 3231.6 kJ/mol
4th ionization potential 4355.5 kJ/mol
5th ionization potential 16091 kJ/mol
6th ionization potential 19805 kJ/mol
7th ionization potential 23780 kJ/mol
8th ionization potential 29287 kJ/mol
9th ionization potential 33878 kJ/mol
10th ionization potential 38726 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP
28Si 92.23% Si is stable with 14 neutrons
29Si 4.67% Si is stable with 15 neutrons
30Si 3.1% Si is stable with 16 neutrons
32Si {syn} 276 y - 0.224 32P
SI units & STP are used except where noted.

In its crystalline form, silicon has a metallic luster and a grayish color. 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.

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.
  • Glass - Silica from sand is a principal component of glass. Glass can be made into a great variety of shapes and is used to make window glass, containers, and insulators, among many other uses.
  • Abrasives - Silicon carbide is one of the most important abrasives.
  • Semiconductor - Ultrapure silicon can be doped with arsenic, boron, gallium, or phosphorus to make silicon more conductive for use in 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 Thenard 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 1900C, 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 99% pure. In 1997, metallurgical grade silicon cost about $ 0.50 per g.

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 were heated to melt at one end. Then, the heater was slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cooled and resolidified behind it. Since most impurities tend to remain in the molten region rather than resolidify, when the process was complete, most of the impurities in the rod had been moved into end that was the last to be melted. This end was 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 1150C. 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 950C, 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 byroduct solidifying and clogging lines) and was evenutally abandoned in favor of the Siemens process.

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 determied 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.

Reference

 

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