14
  Si  
28.085500
Silicon

Name: Silicon
Symbol: Si
Atomic Number: 14
Atomic Weight: 28.085500
Family: Metalloids
CAS RN: 7440-21-3
Description: A dark grey solid.
State (25C): Solid
Oxidation states: +2, +4, -4

Molar Volume: 12.05 cm3/mole
Valence Electrons: 3p2

Boiling Point:  2628K, 2355C, 4271F
Melting Point:
1683K, 1410C, 2570F
Electrons Energy Level: 2, 8, 4
Isotopes: 18 + 3 Stable
Heat of Vaporization: 384.22 kJ/mol
Heat of Fusion: 50.55 kJ/mol
Density: 2.33 g/cm3 @ 300K
Specific Heat: 0.71 J/gK
Atomic Radius: 1.46
Ionic Radius: 0.4
Electronegativity: 1.9 (Pauling); 1.74 (Allrod Rochow)
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 Jns Jacob Berzelius, a Swedish chemist, prepared Amorphous Silicon using approximately the same method as Gay-Lussac by heating chips of Potassium in a silica container and then carefully washing away the residual by-products.  The discovery of Silicon has been generally credited to Berzelius. 

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.

The element is second only to oxygen in abundance in the earth's crust.  The most common compound of silicon, SiO2, is THE most abundant chemical compound in the earth's crust.  Silicon is the seventh most abundant element in the universe.

A tetravalent metalloid, Silicon is less reactive than its chemical analog Carbon.   As the seventh or eighth most common element in the universe by mass, Silicon occasionally occurs as the pure free element in nature, but is more widely distributed in dusts, planetoids and planets as various forms of Silicon Dioxide or Silicate.  On Earth, Silicon is the second most abundant element (after Oxygen) in the crust, making up 25.7% of the crust by mass.

1s2 2s2p6 3s2p2

6
C
12.02
14
Si
28.08
32
Ge
72.15
50
Sn
118.7
82
Pb
207.2
114
Uuq
285.0

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.

1s2
2s2 2p6
3s2 3p2

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

Silicon Compounds

Production

Silicon is commercially prepared by the reaction of high-purity silica (sand or SiO2) with wood, charcoal, and coal, in an electric arc furnace using Carbon electrodes.  At temperatures approaching 2200C, the carbon reduces the silica to silicon according to the chemical equation: 

SiO2 + C rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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

Silicon Dioxide (SiO2), silicon's most common compound, is the most abundant compound in the Earth's crust.   It  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.

Examples of silicon compounds are Silicate, Silane (SiH4), Silicic Acid (H4SiO4), Silicon Carbide (SiC), Sodium Silicate (Na2SiO3), Silicon Dioxide (SiO2), Silicon Tetrachloride (SiCl4), Silicon Tetrafluoride (SiF4), Silicon Tetrachloride (SiCl4) and Trichlorosilane (HSiCl3).

Silicon carbide (SiC) is nearly as hard as diamond and is used as an abrasive. Sodium Silicate (Na2SiO3), known as water glass, is used in the production of adhesives, soaps 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: electrical insulators, lubricants, medical implants and  polishing agents. .

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.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Si22 22.035 6 ms
Si23 23.026 >200 ns
Si24 24.0115 102 ms
Si25 25.0041 220 ms
Si26 25.9923 2.234 seconds
Si27 26.9867 4.16 seconds
Si28 27.9769 Stable
Si29 28.9765 Stable
Si30 29.9738 Stable
Si31 30.9754 157.3 minutes
Si32 31.9742 150 years
Si33 32.978 6.18 seconds
Si34 33.9786 2.77 seconds
Si35 34.9846 0.78 seconds
Si36 35.987 0.45 seconds
Si37 36.993 >1 us
Si38 37.996 >1 us
Si39 39.002 >1 us
Si40 40.006 >200 ns
Si41 41.013 >200 ns
Si42 42.016 >200 ns

Precautions

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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.


atom.gif (700 bytes)

Silicon Data

Atomic Radius (): 1.46
Atomic Volume cm3/mol : 12.1cm3/mol
Covalent Radius: 1.11
Crystal Structure: Cubic face centered
Ionic Radius: 0.4

Chemical Properties

Electrochemical Equivalents: 0.26197 g/amp-hr
Electron Work Function: 4.52eV
Electronegativity: 1.9 (Pauling); 1.74 (Allrod Rochow)
Heat of Fusion: 50.55 kJ/mol
Incompatibilities: Chlorine, Fluorine, Oxidizers, Calcium, Cesium Carbide, Alkaline Carbonates
First Ionization Potential: 8.151
Second Ionization Potential: 16.345
Third Ionization Potential: 33.492
Valence Electron Potential: 144
Ionization Energy (eV): 8.152 eV

Physical Properties

Atomic Mass Average: 28.0855
Boiling Point: 2628K, 2355C, 4271F
Melting Point: 1683K, 1410C, 2570F
Heat of Vaporization: 384.22 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 4.2E-6
Electrical Conductivity: 2.52E-12 106/cm
Thermal Conductivity: 1.48 W/cmK
Density: 2.33 g/cm3 @ 300K
Enthalpy of Atomization: 439.3 kJ/mole @ 25C
Enthalpy of Fusion: 46.44 kJ/mole
Enthalpy of Vaporization: 439 kJ/mole
Flammability Class: Combustible Solid in powder form
Molar Volume: 12.05 cm3/mole
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.71 J/gK
Vapor Pressure: 4.77 @ 1410C
Estimated Crustal Abundance: 2.82105 milligrams per kilogram
Estimated Oceanic Abundance: 2.2 milligrams per liter


(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