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= Silicon =
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Introduction
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Silicon is a chemical element; it has symbol Si and atomic number 14.
It is a hard, brittle crystalline solid with a blue-grey metallic
lustre, and is a tetravalent metalloid (sometimes considered as a
non-metal) and semiconductor. It is a member of group 14 in the
periodic table: carbon is above it; and germanium, tin, lead, and
flerovium are below it. It is relatively unreactive. Silicon is a
significant element that is essential for several physiological and
metabolic processes in plants. Silicon is widely regarded as the
predominant semiconductor material due to its versatile applications
in various electrical devices such as transistors, solar cells,
integrated circuits, and others. These may be due to its significant
band gap, expansive optical transmission range, extensive absorption
spectrum, surface roughening, and effective anti-reflection coating.
Because of its high chemical affinity for oxygen, it was not until
1823 that Jöns Jakob Berzelius was first able to prepare it and
characterize it in pure form. Its oxides form a family of anions known
as silicates. Its melting and boiling points of 1414 °C and 3265 °C,
respectively, are the second highest among all the metalloids and
nonmetals, being surpassed only by boron.
Silicon is the eighth most common element in the universe by mass, but
very rarely occurs in its pure form in the Earth's crust. It is widely
distributed throughout space in cosmic dusts, planetoids, and planets
as various forms of silicon dioxide (silica) or silicates. More than
90% of the Earth's crust is composed of silicate minerals, making
silicon the second most abundant element in the Earth's crust (about
28% by mass), after oxygen.
Most silicon is used commercially without being separated, often with
very little processing of the natural minerals. Such use includes
industrial construction with clays, silica sand, and stone. Silicates
are used in Portland cement for mortar and stucco, and mixed with
silica sand and gravel to make concrete for walkways, foundations, and
roads. They are also used in whiteware ceramics such as porcelain, and
in traditional silicate-based soda-lime glass and many other specialty
glasses. Silicon compounds such as silicon carbide are used as
abrasives and components of high-strength ceramics. Silicon is the
basis of the widely used synthetic polymers called silicones.
The late 20th century to early 21st century has been described as the
Silicon Age (also known as the Digital Age or Information Age) because
of the large impact that elemental silicon has on the modern world
economy. The small portion of very highly purified elemental silicon
used in semiconductor electronics (<15%) is essential to the
transistors and integrated circuit chips used in most modern
technology such as smartphones and other computers. In 2019, 32.4% of
the semiconductor market segment was for networks and communications
devices, and the semiconductors industry is projected to reach $726.73
billion by 2027.
Silicon is an essential element in biology. Only traces are required
by most animals, but some sea sponges and microorganisms, such as
diatoms and radiolaria, secrete skeletal structures made of silica.
Silica is deposited in many plant tissues.
History
======================================================================
Owing to the abundance of silicon in the Earth's crust, natural
silicon-based materials have been used for thousands of years. Silicon
rock crystals were familiar to various ancient civilizations, such as
the predynastic Egyptians who used it for beads and small vases, as
well as the ancient Chinese. Glass containing silica was manufactured
by the Egyptians since at least 1500 BC, as well as by the ancient
Phoenicians. Natural silicate compounds were also used in various
types of mortar for construction of early human dwellings.
Discovery
===========
In 1787, Antoine Lavoisier suspected that silica might be an oxide of
a fundamental chemical element, but the chemical affinity of silicon
for oxygen is high enough that he had no means to reduce the oxide and
isolate the element. After an attempt to isolate silicon in 1808, Sir
Humphry Davy proposed the name "silicium" for silicon, from the Latin
, 'silicis' for flint, and adding the "-ium" ending because he
believed it to be a metal. Most other languages use transliterated
forms of Davy's name, sometimes adapted to local phonology (e.g.
German , Turkish ', Catalan ', Armenian or 'Silitzioum'). A few
others use instead a calque of the Latin root (e.g. Russian , from
"flint"; Greek ' from "fire"; Finnish from "flint", Czech from
"quartz", "flint").
Gay-Lussac and Thénard are thought to have prepared impure amorphous
silicon in 1811, through the heating of recently isolated potassium
metal with silicon tetrafluoride, but they did not purify and
characterize the product, nor identify it as a new element. Silicon
was given its present name in 1817 by Scottish chemist Thomas Thomson.
He retained part of Davy's name but added "-on" because he believed
that silicon was a nonmetal similar to boron and carbon. In 1824, Jöns
Jacob Berzelius prepared amorphous silicon using approximately the
same method as Gay-Lussac (reducing potassium fluorosilicate with
molten potassium metal), but purifying the product to a brown powder
by repeatedly washing it.See
* Berzelius announced his discovery of silicon ("silicium") in:
Berzelius, J. (presented: 1823; published: 1824)
[
https://books.google.com/books?id=pJlPAAAAYAAJ&pg=PA46
"Undersökning af flusspatssyran och dess märkvärdigaste föreningar"]
(Investigation of hydrofluoric acid and of its most noteworthy
compounds), 'Kongliga Vetenskaps-Academiens Handlingar' [Proceedings
of the Royal Science Academy], 12 : 46-98. The isolation of silicon
and its characterization are detailed in the section titled
"Flussspatssyrad kisseljords sönderdelning med kalium," pp. 46-68.
* The above article was printed in German in: J.J. Berzelius (1824)
[
http://gallica.bnf.fr/ark:/12148/bpt6k15086x/f185.image.langEN "'II.
Untersuchungen über Flussspathsäure und deren merkwürdigsten
Verbindungen'"] (II. Investigations of hydrofluoric acid and its most
noteworthy compounds), 'Annalen der Physik', 77: 169-230. The
isolation of silicon is detailed in the section titled:
[
http://gallica.bnf.fr/ark:/12148/bpt6k15086x/f220.image.langEN
"Zersetzung der flussspaths. Kieselerde durch Kalium"] (Decomposition
of silicate fluoride by potassium), pp. 204-210.
* The above article was reprinted in French in: Berzelius (1824) [
"Décomposition du fluate de silice par le potassium"] (Decomposition
of silica fluoride by potassium), 'Annales de Chimie et de Physique',
27: 337-359.
* Reprinted in English in: As a result, he is usually given credit
for the element's discovery. The same year, Berzelius became the first
to prepare silicon tetrachloride; silicon tetrafluoride had already
been prepared long before in 1771 by Carl Wilhelm Scheele by
dissolving silica in hydrofluoric acid. In 1823 for the first time
Jacob Berzelius discovered silicon tetrachloride (SiCl4). In 1846 Von
Ebelman's synthesized tetraethyl orthosilicate (Si(OC2H5)4).
Silicon in its more common crystalline form was not prepared until 31
years later, by Deville.Subsequently, Deville obtained crystalline
silicon by heating the chloride or fluoride of silicon with sodium
metal, isolating the amorphous silicon, then melting the amorphous
form with salt and heating the mixture until most of the salt
evaporated. See: By electrolyzing a mixture of sodium chloride and
aluminium chloride containing approximately 10% silicon, he was able
to obtain a slightly impure allotrope of silicon in 1854. Later, more
cost-effective methods have been developed to isolate several
allotrope forms, the most recent being silicene in 2010. Meanwhile,
research on the chemistry of silicon continued; Friedrich Wöhler
discovered the first volatile hydrides of silicon, synthesising
trichlorosilane in 1857 and silane itself in 1858, but a detailed
investigation of the silanes was only carried out in the early 20th
century by Alfred Stock, despite early speculation on the matter
dating as far back as the beginnings of synthetic organic chemistry in
the 1830s. Similarly, the first organosilicon compound,
tetraethylsilane, was synthesised by Charles Friedel and James Crafts
in 1863, but detailed characterisation of organosilicon chemistry was
only done in the early 20th century by Frederic Kipping.
Starting in the 1920s, the work of William Lawrence Bragg on X-ray
crystallography elucidated the compositions of the silicates, which
had previously been known from analytical chemistry but had not yet
been understood, together with Linus Pauling's development of crystal
chemistry and Victor Goldschmidt's development of geochemistry. The
middle of the 20th century saw the development of the chemistry and
industrial use of siloxanes and the growing use of silicone polymers,
elastomers, and resins. In the late 20th century, the complexity of
the crystal chemistry of silicides was mapped, along with the
solid-state physics of doped semiconductors.
Silicon semiconductors
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The first semiconductor devices did not use silicon, but used galena,
including German physicist Ferdinand Braun's crystal detector in 1874
and Indian physicist Jagadish Chandra Bose's radio crystal detector in
1901. The first silicon semiconductor device was a silicon radio
crystal detector, developed by American engineer Greenleaf Whittier
Pickard in 1906.
In 1940, Russell Ohl discovered the p-n junction and photovoltaic
effects in silicon. In 1941, techniques for producing high-purity
germanium and silicon crystals were developed for radar microwave
detector crystals during World War II. In 1947, physicist William
Shockley theorized a field-effect amplifier made from germanium and
silicon, but he failed to build a working device, before eventually
working with germanium instead. The first working transistor was a
point-contact transistor built by John Bardeen and Walter Brattain
later that year while working under Shockley. In 1954, physical
chemist Morris Tanenbaum fabricated the first silicon junction
transistor at Bell Labs. In 1955, Carl Frosch and Lincoln Derick at
Bell Labs accidentally discovered that silicon dioxide () could be
grown on silicon. By 1957 Frosch and Derick published their work on
the first manufactured semiconductor oxide transistor: the first
planar transistors, in which drain and source were adjacent at the
same surface. In 1959, Robert Noyce developed the first silicon-based
integrated circuit at Fairchild Semiconductor, building on prior work
by Jack Kilby that relied on germanium as the semiconductor.
Silicon Age
=============
The "Silicon Age" refers to the late 20th century to early 21st
century. This is due to silicon being the dominant material used in
electronics and information technology (also known as the Digital Age
or Information Age), similar to how the Stone Age, Bronze Age and Iron
Age were defined by the dominant materials during their respective
ages of civilization.
Because silicon is an important element in high-technology
semiconductor devices, many places in the world bear its name. For
example, the Santa Clara Valley in California acquired the nickname
Silicon Valley, as the element is the base material in the
semiconductor industry there. Since then, many other places have been
similarly dubbed, including Silicon Wadi in Israel; Silicon Forest in
Oregon; Silicon Hills in Austin, Texas; Silicon Slopes in Salt Lake
City, Utah; Silicon Saxony in Germany; Silicon Valley in India;
Silicon Border in Mexicali, Mexico; Silicon Fen in Cambridge,
England; Silicon Roundabout in London; Silicon Glen in Scotland;
Silicon Gorge in Bristol, England; Silicon Alley in New York City; and
Silicon Beach in Los Angeles.
Physical and atomic
=====================
A silicon atom has fourteen electrons. In the ground state, they are
arranged in the electron configuration [Ne]3s23p2. Of these, four are
valence electrons, occupying the 3s orbital and two of the 3p
orbitals. Like the other members of its group, the lighter carbon and
the heavier germanium, tin, and lead, it has the same number of
valence electrons as valence orbitals: hence, it can complete its
octet and obtain the stable noble gas configuration of argon by
forming sp3 hybrid orbitals, forming tetrahedral derivatives where
the central silicon atom shares an electron pair with each of the four
atoms it is bonded to. The first four ionisation energies of silicon
are 786.3, 1576.5, 3228.3, and 4354.4 kJ/mol respectively; these
figures are high enough to preclude the possibility of simple cationic
chemistry for the element. Following periodic trends, its single-bond
covalent radius of 117.6 pm is intermediate between those of carbon
(77.2 pm) and germanium (122.3 pm). The hexacoordinate ionic radius of
silicon may be considered to be 40 pm, although this must be taken as
a purely notional figure given the lack of a simple cation in
reality.
Electrical
============
At standard temperature and pressure, silicon is a shiny semiconductor
with a bluish-grey metallic lustre; as typical for semiconductors, its
resistivity drops as temperature rises. This arises because silicon
has a small energy gap (band gap) between its highest occupied energy
levels (the valence band) and the lowest unoccupied ones (the
conduction band). The Fermi level is about halfway between the valence
and conduction bands and is the energy at which a state is as likely
to be occupied by an electron as not. Hence pure silicon is
effectively an insulator at room temperature. However, doping silicon
with a pnictogen such as phosphorus, arsenic, or antimony introduces
one extra electron per dopant and these may then be excited into the
conduction band either thermally or photolytically, creating an n-type
semiconductor. Similarly, doping silicon with a group 13 element such
as boron, aluminium, or gallium results in the introduction of
acceptor levels that trap electrons that may be excited from the
filled valence band, creating a p-type semiconductor. Joining n-type
silicon to p-type silicon creates a p-n junction with a common Fermi
level; electrons flow from n to p, while holes flow from p to n,
creating a voltage drop. This p-n junction thus acts as a diode that
can rectify alternating current that allows current to pass more
easily one way than the other. A transistor is an n-p-n junction, with
a thin layer of weakly p-type silicon between two n-type regions.
Biasing the emitter through a small forward voltage and the collector
through a large reverse voltage allows the transistor to act as a
triode amplifier.
Crystal structure
===================
Silicon crystallises in a giant covalent structure at standard
conditions, specifically in a diamond cubic crystal lattice (space
group 227). It thus has a high melting point of 1414 °C, as a lot of
energy is required to break the strong covalent bonds and melt the
solid. Upon melting silicon contracts as the long-range tetrahedral
network of bonds breaks up and the voids in that network are filled
in, similar to water ice when hydrogen bonds are broken upon melting.
It does not have any thermodynamically stable allotropes at standard
pressure, but several other crystal structures are known at higher
pressures. The general trend is one of increasing coordination number
with pressure, culminating in a hexagonal close-packed allotrope at
about 40 gigapascals known as Si-VII (the standard modification being
Si-I). An allotrope called BC8 (or bc8), having a body-centred cubic
lattice with eight atoms per primitive unit cell (space group 206),
can be created at high pressure and remains metastable at low
pressure. Its properties have been studied in detail.
Silicon boils at 3265 °C: this, while high, is still lower than the
temperature at which its lighter congener carbon sublimes (3642 °C)
and silicon similarly has a lower heat of vaporisation than carbon,
consistent with the fact that the Si-Si bond is weaker than the C-C
bond.
It is also possible to construct silicene layers analogous to
graphene.
Isotopes
==========
Naturally occurring silicon is composed of three stable isotopes, 28Si
(92.23%), 29Si (4.67%), and 30Si (3.10%). Out of these, only 29Si is
of use in NMR and EPR spectroscopy, as it is the only one with a
nuclear spin ('I' =). All three are produced in Type Ia supernovae
through the oxygen-burning process, with 28Si being made as part of
the alpha process and hence the most abundant. The fusion of 28Si with
alpha particles by photodisintegration rearrangement in stars is known
as the silicon-burning process; it is the last stage of stellar
nucleosynthesis before the rapid collapse and violent explosion of the
star in question in a type II supernova.
Twenty-two radioisotopes have been characterized, the two stablest
being 32Si with a half-life of about 150 years, and 31Si with a
half-life of 2.62 hours. All the remaining radioactive isotopes have
half-lives that are less than seven seconds, and the majority of these
have half-lives that are less than one-tenth of a second. Silicon has
one known nuclear isomer, 34mSi, with a half-life less than 210
nanoseconds. 32Si undergoes low-energy beta decay to 32P and then
stable 32S. 31Si may be produced by the neutron activation of natural
silicon and is thus useful for quantitative analysis; it can be easily
detected by its characteristic beta decay to stable 31P, in which the
emitted electron carries up to 1.48 MeV of energy.
The known isotopes of silicon range in mass number from 22 to 46. The
most common decay mode of the isotopes with mass numbers lower than
the three stable isotopes is β+ decay, primarily forming aluminium
isotopes (13 protons) as decay products. The most common decay mode
for the heavier unstable isotopes is beta decay, primarily forming
phosphorus isotopes (15 protons) as decay products.
Silicon can enter the oceans through groundwater and riverine
transport. Large fluxes of groundwater input have an isotopic
composition which is distinct from riverine silicon inputs. Isotopic
variations in groundwater and riverine transports contribute to
variations in oceanic 30Si values. Currently, there are substantial
differences in the isotopic values of deep water in the world's ocean
basins. Between the Atlantic and Pacific oceans, there is a deep water
30Si gradient of greater than 0.3 parts per thousand. 30Si is most
commonly associated with productivity in the oceans.
Chemistry and compounds
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C-X and Si-X bond energies (kJ/mol)
C Si H F Cl Br I O- N<
368 360 435 453 351 293 216 ~360 ~305
360 340 393 565 381 310 234 452 322
Crystalline bulk silicon is rather inert, but becomes more reactive at
high temperatures. Like its neighbour aluminium, silicon forms a thin,
continuous surface layer of silicon dioxide () that protects the
material beneath from oxidation. Because of this, silicon does not
measurably react with the air below 900 °C. Between 950 °C and 1160
°C, the formation rate of the vitreous dioxide rapidly increases, and
when 1400 °C is reached, atmospheric nitrogen also reacts to give the
nitrides SiN and . Silicon reacts with gaseous sulfur at 600 °C and
gaseous phosphorus at 1000 °C. This oxide layer nevertheless does not
prevent reaction with the halogens; fluorine attacks silicon
vigorously at room temperature, chlorine does so at about 300 °C, and
bromine and iodine at about 500 °C. Silicon does not react with most
aqueous acids, but is oxidised and complexed by hydrofluoric acid
mixtures containing either chlorine or nitric acid to form
hexafluorosilicates. It readily dissolves in hot aqueous alkali to
form silicates. At high temperatures, silicon also reacts with alkyl
halides; this reaction may be catalysed by copper to directly
synthesise organosilicon chlorides as precursors to silicone polymers.
Upon melting, silicon becomes extremely reactive, alloying with most
metals to form silicides, and reducing most metal oxides because the
heat of formation of silicon dioxide is so large. In fact, molten
silicon reacts virtually with every known kind of crucible material
(except its own oxide, ). This happens due to silicon's high binding
forces for the light elements and to its high dissolving power for
most elements. As a result, containers for liquid silicon must be made
of refractory, unreactive materials such as zirconium dioxide or group
4, 5, and 6 borides.
Tetrahedral coordination is a major structural motif in silicon
chemistry just as it is for carbon chemistry. However, the 3p subshell
is rather more diffuse than the 2p subshell and does not hybridise so
well with the 3s subshell. As a result, the chemistry of silicon and
its heavier congeners shows significant differences from that of
carbon, and thus octahedral coordination is also significant. For
example, the electronegativity of silicon (1.90) is much less than
that of carbon (2.55), because the valence electrons of silicon are
further from the nucleus than those of carbon and hence experience
smaller electrostatic forces of attraction from the nucleus. The poor
overlap of 3p orbitals also results in a much lower tendency toward
catenation (formation of Si-Si bonds) for silicon than for carbon, due
to the concomitant weakening of the Si-Si bond compared to the C-C
bond: the average Si-Si bond energy is approximately 226 kJ/mol,
compared to a value of 356 kJ/mol for the C-C bond. This results in
multiply bonded silicon compounds generally being much less stable
than their carbon counterparts, an example of the double bond rule. On
the other hand, the presence of radial nodes in the 3p orbitals of
silicon suggests the possibility of hypervalence, as seen in five and
six-coordinate derivatives of silicon such as and . Lastly, because
of the increasing energy gap between the valence s and p orbitals as
the group is descended, the divalent state grows in importance from
carbon to lead, so that a few unstable divalent compounds are known
for silicon; this lowering of the main oxidation state, in tandem with
increasing atomic radii, results in an increase of metallic character
down the group. Silicon already shows some incipient metallic
behavior, particularly in the behavior of its oxide compounds and its
reaction with acids as well as bases (though this takes some effort),
and is hence often referred to as a metalloid rather than a nonmetal.
Germanium shows more, and tin is generally considered a metal.
Silicon shows clear differences from carbon. For example, organic
chemistry has very few analogies with silicon chemistry, while
silicate minerals have a structural complexity unseen in oxocarbons.
Silicon tends to resemble germanium far more than it does carbon, and
this resemblance is enhanced by the d-block contraction, resulting in
the size of the germanium atom being much closer to that of the
silicon atom than periodic trends would predict. Nevertheless, there
are still some differences because of the growing importance of the
divalent state in germanium compared to silicon. Additionally, the
lower Ge-O bond strength compared to the Si-O bond strength results in
the absence of "germanone" polymers that would be analogous to
silicone polymers.
Occurrence
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Silicon is the eighth most abundant element in the universe, coming
after hydrogen, helium, carbon, nitrogen, oxygen, iron, and neon.
These abundances are not replicated well on Earth due to substantial
separation of the elements taking place during the formation of the
Solar System. Silicon makes up 27.2% of the Earth's crust by weight,
second only to oxygen at 45.5%, with which it always is associated in
nature. Further fractionation took place in the formation of the Earth
by planetary differentiation: Earth's core, which makes up 31.5% of
the mass of the Earth, has approximate composition ; the mantle makes
up 68.1% of the Earth's mass and is composed mostly of denser oxides
and silicates, an example being olivine, ; while the lighter siliceous
minerals such as aluminosilicates rise to the surface and form the
crust, making up 0.4% of the Earth's mass.
The crystallisation of igneous rocks from magma depends on a number of
factors; among them are the chemical composition of the magma, the
cooling rate, and some properties of the individual minerals to be
formed, such as lattice energy, melting point, and complexity of their
crystal structure. As magma is cooled, olivine appears first, followed
by pyroxene, amphibole, biotite mica, orthoclase feldspar, muscovite
mica, quartz, zeolites, and finally, hydrothermal minerals. This
sequence shows a trend toward increasingly complex silicate units with
cooling, and the introduction of hydroxide and fluoride anions in
addition to oxides. Many metals may substitute for silicon. After
these igneous rocks undergo weathering, transport, and deposition,
sedimentary rocks like clay, shale, and sandstone are formed.
Metamorphism also may occur at high temperatures and pressures,
creating an even vaster variety of minerals.
There are four sources for silicon fluxes into the ocean: chemical
weathering of continental rocks, river transport, dissolution of
continental terrigenous silicates, and the reaction between submarine
basalts and hydrothermal fluid which release dissolved silicon. All
four of these fluxes are interconnected in the ocean's biogeochemical
cycle as they all were initially formed from the weathering of Earth's
crust.
Approximately 300-900 megatonnes of aeolian dust is deposited into the
world's oceans each year. Of that value, 80-240 megatonnes are in the
form of particulate silicon. The total amount of particulate silicon
deposition into the ocean is still less than the amount of silicon
influx into the ocean via riverine transportation. Aeolian inputs of
particulate lithogenic silicon into the North Atlantic and Western
North Pacific oceans are the result of dust settling on the oceans
from the Sahara and Gobi Desert, respectively. Riverine transports are
the major source of silicon influx into the ocean in coastal regions,
while silicon deposition in the open ocean is greatly influenced by
the settling of aeolian dust.
Production
======================================================================
Silicon of 96-99% purity is made by carbothermically reducing
quartzite or sand with highly pure coke. The reduction is carried out
in an electric arc furnace, with an excess of used to stop silicon
carbide (SiC) from accumulating:
: + 2 C → Si + 2 CO
:2 SiC + → 3 Si + 2 CO
This reaction, known as carbothermal reduction of silicon dioxide,
usually is conducted in the presence of scrap iron with low amounts of
phosphorus and sulfur, producing ferrosilicon. Ferrosilicon, an
iron-silicon alloy that contains varying ratios of elemental silicon
and iron, accounts for about 80% of the world's production of
elemental silicon, with China, the leading supplier of elemental
silicon, providing 4.6 million tonnes (or two-thirds of world output)
of silicon, most of it in the form of ferrosilicon. It is followed by
Russia (610,000 t), Norway (330,000 t), Brazil (240,000 t), and the
United States (170,000 t). Ferrosilicon is primarily used by the iron
and steel industry (see below) with primary use as alloying addition
in iron or steel and for de-oxidation of steel in integrated steel
plants.
Another reaction, sometimes used, is aluminothermal reduction of
silicon dioxide, as follows:
:3 + 4 Al → 3 Si + 2
Leaching powdered 96-97% pure silicon with water results in ~98.5%
pure silicon, which is used in the chemical industry. However, even
greater purity is needed for semiconductor applications, and this is
produced from the reduction of tetrachlorosilane (silicon
tetrachloride) or trichlorosilane. The former is made by chlorinating
scrap silicon and the latter is a byproduct of silicone production.
These compounds are volatile and hence can be purified by repeated
fractional distillation, followed by reduction to elemental silicon
with very pure zinc metal as the reducing agent. The spongy pieces of
silicon thus produced are melted and then grown to form cylindrical
single crystals, before being purified by zone refining. Other routes
use the thermal decomposition of silane or tetraiodosilane (). Another
process used is the reduction of sodium hexafluorosilicate, a common
waste product of the phosphate fertilizer industry, by metallic
sodium: this is highly exothermic and hence requires no outside energy
source. Hyperfine silicon is made at a higher purity than almost any
other material: transistor production requires impurity levels in
silicon crystals less than 1 part per 1010, and in special cases
impurity levels below 1 part per 1012 are needed and attained.
Silicon nanostructures can directly be produced from silica sand using
conventional metalothermic processes, or the combustion synthesis
approach. Such nanostructured silicon materials can be used in various
functional applications including the anode of lithium-ion batteries
(LIBs), other ion batteries, future computing devices like memristors
or photocatalytic applications.
Compounds
===========
Most silicon is used industrially without being purified, often with
comparatively little processing from its natural form. More than 90%
of the Earth's crust is composed of silicate minerals, which are
compounds of silicon and oxygen, often with metallic ions when
negatively charged silicate anions require cations to balance the
charge. Many of these have direct commercial uses, such as clays,
silica sand, and most kinds of building stone. Thus, the vast majority
of uses for silicon are as structural compounds, either as the
silicate minerals or silica (crude silicon dioxide). Silicates are
used in making Portland cement (made mostly of calcium silicates)
which is used in building mortar and modern stucco, but more
importantly, combined with silica sand, and gravel (usually containing
silicate minerals such as granite), to make the concrete that is the
basis of most of the very largest industrial building projects of the
modern world.
Silica is used to make fire brick, a type of ceramic. Silicate
minerals are also in whiteware ceramics, an important class of
products usually containing various types of fired clay minerals
(natural aluminium phyllosilicates). An example is porcelain, which is
based on the silicate mineral kaolinite. Traditional glass
(silica-based soda-lime glass) also functions in many of the same
ways, and also is used for windows and containers. In addition,
specialty silica based glass fibers are used for optical fiber, as
well as to produce fiberglass for structural support and glass wool
for thermal insulation.
Silicones often are used in waterproofing treatments, molding
compounds, mold-release agents, mechanical seals, high temperature
greases and waxes, and caulking compounds. Silicone is also sometimes
used in breast implants, contact lenses, explosives and pyrotechnics.
Silly Putty was originally made by adding boric acid to silicone oil.
Other silicon compounds function as high-technology abrasives and new
high-strength ceramics based upon silicon carbide. Silicon is a
component of some superalloys.
Alloys
========
Elemental silicon is added to molten cast iron as ferrosilicon or
silicocalcium alloys to improve performance in casting thin sections
and to prevent the formation of cementite where exposed to outside
air. The presence of elemental silicon in molten iron acts as a sink
for oxygen, so that the steel carbon content, which must be kept
within narrow limits for each type of steel, can be more closely
controlled. Ferrosilicon production and use is a monitor of the steel
industry, and although this form of elemental silicon is grossly
impure, it accounts for 80% of the world's use of free silicon.
Silicon is an important constituent of transformer steel, modifying
its resistivity and ferromagnetic properties.
The properties of silicon may be used to modify alloys with metals
other than iron. "Metallurgical grade" silicon is silicon of 95-99%
purity. About 55% of the world consumption of metallurgical purity
silicon goes for production of aluminium-silicon alloys (silumin
alloys) for aluminium part casts, mainly for use in the automotive
industry. Silicon's importance in aluminium casting is that a
significantly high amount (12%) of silicon in aluminium forms a
eutectic mixture which solidifies with very little thermal
contraction. This greatly reduces tearing and cracks formed from
stress as casting alloys cool to solidity. Silicon also significantly
improves the hardness and thus wear-resistance of aluminium.
Metallurgical grade silicon is made by melting quartz or quartzite in
a large arc furnace, in a carbothermal reduction process with
carbon-containing material such as coal, coke or charcoal and
woodchips for gas circulation. This production technique without iron
is often used for polysilicon production for photovoltaics and also
semiconductors.
Electronics
=============
Most elemental silicon produced remains as a ferrosilicon alloy, and
only approximately 20% is refined to metallurgical grade purity (a
total of 1.3-1.5 million metric tons/year). An estimated 15% of the
world production of metallurgical grade silicon is further refined to
semiconductor purity. This typically is the "nine-9" or 99.9999999%
purity, nearly defect-free single crystalline material.
Monocrystalline silicon of such purity is usually produced by the
Czochralski process, and is used to produce silicon wafers used in the
semiconductor industry, in electronics, and in some high-cost and
high-efficiency photovoltaic applications. Pure silicon is an
intrinsic semiconductor, which means that unlike metals, it conducts
electron holes and electrons released from atoms by heat; silicon's
electrical conductivity increases with higher temperatures. Pure
silicon has too low a conductivity (i.e., too high a resistivity) to
be used as a circuit element in electronics. In practice, pure silicon
is doped with small concentrations of certain other elements, which
greatly increase its conductivity and adjust its electrical response
by controlling the number and charge (positive or negative) of
activated carriers. Such control is necessary for transistors, solar
cells, semiconductor detectors, and other semiconductor devices used
in the computer industry and other technical applications. In silicon
photonics, silicon may be used as a continuous wave Raman laser medium
to produce coherent light.
In common integrated circuits, a wafer of monocrystalline silicon
serves as a mechanical support for the circuits, which are created by
doping and insulated from each other by thin layers of silicon oxide,
an insulator that is easily produced on Si surfaces by processes of
thermal oxidation or local oxidation (LOCOS), which involve exposing
the element to oxygen under the proper conditions that can be
predicted by the Deal-Grove model. Silicon has become the most popular
material for both high power semiconductors and integrated circuits
because it can withstand the highest temperatures and greatest
electrical activity without suffering avalanche breakdown (an electron
avalanche is created when heat produces free electrons and holes,
which in turn pass more current, which produces more heat). In
addition, the insulating oxide of silicon is not soluble in water,
which gives it an advantage over germanium (an element with similar
properties which can also be used in semiconductor devices) in certain
fabrication techniques.
Monocrystalline silicon is expensive to produce, and is usually
justified only in production of integrated circuits, where tiny
crystal imperfections can interfere with tiny circuit paths. For other
uses, other types of pure silicon may be employed. These include
hydrogenated amorphous silicon and upgraded metallurgical-grade
silicon (UMG-Si) used in the production of low-cost, large-area
electronics in applications such as liquid crystal displays and of
large-area, low-cost, thin-film solar cells. Such semiconductor grades
of silicon are either slightly less pure or polycrystalline rather
than monocrystalline, and are produced in comparable quantities as the
monocrystalline silicon: 75,000 to 150,000 metric tons per year. The
market for the lesser grade is growing more quickly than for
monocrystalline silicon. By 2013, polycrystalline silicon production,
used mostly in solar cells, was projected to reach 200,000 metric tons
per year, while monocrystalline semiconductor grade silicon was
expected to remain less than 50,000 tons per year.
Quantum dots
==============
Silicon quantum dots are created through the thermal processing of
hydrogen silsesquioxane into nanocrystals ranging from a few
nanometers to a few microns, displaying size dependent luminescent
properties. The nanocrystals display large Stokes shifts converting
photons in the ultraviolet range to photons in the visible or
infrared, depending on the particle size, allowing for applications in
quantum dot displays and luminescent solar concentrators due to their
limited self absorption. A benefit of using silicon based quantum dots
over cadmium or indium is the non-toxic, metal-free nature of silicon.
Another application of silicon quantum dots is for sensing of
hazardous materials. The sensors take advantage of the luminescent
properties of the quantum dots through quenching of the
photoluminescence in the presence of the hazardous substance. There
are many methods used for hazardous chemical sensing with a few being
electron transfer, fluorescence resonance energy transfer, and
photocurrent generation. Electron transfer quenching occurs when the
lowest unoccupied molecular orbital (LUMO) is slightly lower in energy
than the conduction band of the quantum dot, allowing for the transfer
of electrons between the two, preventing recombination of the holes
and electrons within the nanocrystals. The effect can also be achieved
in reverse with a donor molecule having its highest occupied molecular
orbital (HOMO) slightly higher than a valence band edge of the quantum
dot, allowing electrons to transfer between them, filling the holes
and preventing recombination. Fluorescence resonance energy transfer
occurs when a complex forms between the quantum dot and a quencher
molecule. The complex will continue to absorb light but when the
energy is converted to the ground state it does not release a photon,
quenching the material. The third method uses different approach by
measuring the photocurrent emitted by the quantum dots instead of
monitoring the photoluminescent display. If the concentration of the
desired chemical increases then the photocurrent given off by the
nanocrystals will change in response.
Biological role
======================================================================
Although silicon is readily available in the form of silicates, very
few organisms use it directly. Diatoms, radiolaria, and siliceous
sponges use biogenic silica as a structural material for their
skeletons. Some plants accumulate silica in their tissues and require
silicon for their growth, for example rice. Silicon may be taken up by
plants as orthosilicic acid (also known as monosilicic acid) and
transported through the xylem, where it forms amorphous complexes with
components of the cell wall. This has been shown to improve cell wall
strength and structural integrity in some plants, thereby reducing
insect herbivory and pathogenic infections. In certain plants, silicon
may also upregulate the production of volatile organic compounds and
phytohormones which play a significant role in plant defense
mechanisms. In more advanced plants, the silica phytoliths (opal
phytoliths) are rigid microscopic bodies occurring in the cell.
Several horticultural crops are known to protect themselves against
fungal plant pathogens with silica, to such a degree that fungicide
application may fail unless accompanied by sufficient silicon
nutrition. Silicaceous plant defense molecules activate some
phytoalexins, meaning some of them are signalling substances producing
acquired immunity. When deprived, some plants will substitute with
increased production of other defensive substances.
Life on Earth is largely composed of carbon, but astrobiology
considers that extraterrestrial life may have other hypothetical types
of biochemistry. Silicon is considered an alternative to carbon, as it
can create complex and stable molecules with four covalent bonds,
required for a DNA-analog, and it is available in large quantities.
Marine microbial influences
=============================
Diatoms use silicon in the biogenic silica (bSi) form, which is taken
up by the silicon transport protein (SIT) to be predominantly used in
the cell wall structure as frustules. Silicon enters the ocean in a
dissolved form such as silicic acid or silicate. Since diatoms are
one of the main users of these forms of silicon, they contribute
greatly to the concentration of silicon throughout the ocean. Silicon
forms a nutrient-like profile in the ocean due to the diatom
productivity in shallow depths. Therefore, concentration of silicon is
lower in the shallow ocean and higher in the deep ocean.
Diatom productivity in the upper ocean contributes to the amount of
silicon exported to the lower ocean. When diatom cells are lysed in
the upper ocean, their nutrients such as iron, zinc, and silicon, are
brought to the lower ocean through a process called marine snow.
Marine snow involves the downward transfer of particulate organic
matter by vertical mixing of dissolved organic matter. It has been
suggested that silicon is considered crucial to diatom productivity
and as long as there is silicic acid available for diatoms to use, the
diatoms can contribute to other important nutrient concentrations in
the deep ocean as well.
In coastal zones, diatoms serve as the major phytoplanktonic organisms
and greatly contribute to biogenic silica production. In the open
ocean, however, diatoms have a reduced role in global annual silica
production. Diatoms in North Atlantic and North Pacific subtropical
gyres only contribute about 5-7% of global annual marine silica
production. The Southern Ocean produces about one-third of global
marine biogenic silica. The Southern Ocean is referred to as having a
"biogeochemical divide" since only minuscule amounts of silicon are
transported out of this region.
Human nutrition
=================
There is some evidence that silicon is important to human health for
their nail, hair, bone, and skin tissues, for example, in studies that
demonstrate that premenopausal women with higher dietary silicon
intake have higher bone density, and that silicon supplementation can
increase bone volume and density in patients with osteoporosis.
Silicon is needed for synthesis of elastin and collagen, of which the
aorta contains the greatest quantity in the human body, and has been
considered an essential element; nevertheless, it is difficult to
prove its essentiality, because silicon is very common, and hence,
deficiency symptoms are difficult to reproduce.
Silicon is currently under consideration for elevation to the status
of a "plant beneficial substance by the Association of American Plant
Food Control Officials (AAPFCO)."
Safety
======================================================================
People may be exposed to elemental silicon in the workplace by
breathing it in, swallowing it, or having contact with the skin or
eye. In the latter two cases, silicon poses a slight hazard as an
irritant. It is hazardous if inhaled. The Occupational Safety and
Health Administration (OSHA) has set the legal limit for silicon
exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3
respiratory exposure over an eight-hour workday. The National
Institute for Occupational Safety and Health (NIOSH) has set a
recommended exposure limit (REL) of 10 mg/m3 total exposure and 5
mg/m3 respiratory exposure over an eight-hour workday. Inhalation of
crystalline silica dust may lead to silicosis, an occupational lung
disease marked by inflammation and scarring in the form of nodular
lesions in the upper lobes of the lungs.
See also
======================================================================
* Amorphous silicon
* Black silicon
* Covalent superconductors
* List of countries by silicon production
* List of silicon producers
* Monocrystalline silicon
* Polycrystalline silicon
* Printed silicon electronics
* Silicene
* Silicon nanowire
* Silicon tombac
* Silicon Valley
* Transistor
License
=========
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License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Silicon
