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Silicon

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                14            aluminium ← silicon → phosphorus
                 C
                ↑
                Si
                ↓
                Ge

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number silicon, Si, 14
                                                Chemical series metalloids
                                             Group, Period, Block 14, 3, p
                                        Appearance dark gray, bluish tinge
                                             Atomic mass 28.0855 (3) g/mol
                                     Electron configuration [Ne] 3s^2 3p^2
                                               Electrons per shell 2, 8, 4
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 2.33 g·cm^−3
                                    Liquid density at m.p. 2.57 g·cm^−3
                                                     Melting point 1687  K
                                                    (1414 ° C, 2577 ° F)
                                                      Boiling point 3538 K
                                                    (3265 ° C, 5909 ° F)
                                         Heat of fusion 50.21 kJ·mol^−1
                                     Heat of vaporization 359 kJ·mol^−1
                         Heat capacity (25 °C) 19.789 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 1908 2102 2339 2636 3021 3537

                                                         Atomic properties
                                         Crystal structure Diamond Lattice
                                                        Oxidation states 4
                                                       ( amphoteric oxide)
                                    Electronegativity 1.90 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 786.5 kJ·mol^−1
                                                  2nd: 1577.1 kJ·mol^−1
                                                  3rd: 3231.6 kJ·mol^−1
                                                      Atomic radius 110 pm
                                              Atomic radius (calc.) 111 pm
                                                    Covalent radius 111 pm
                                               Van der Waals radius 210 pm
                                                             Miscellaneous
                                             Magnetic ordering nonmagnetic
                        Thermal conductivity (300 K) 149 W·m^−1·K^−1
                        Thermal expansion (25 °C) 2.6 µm·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 8433 m/s
                                                    Young's modulus 47 GPa
                                                      Bulk modulus 100 GPa
                                                         Mohs hardness 6.5
                                             CAS registry number 7440-21-3
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of silicon

                                 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    132 y     β^- 13.020    ^32P

                                                                References

          Not to be confused with Silicone.

   Silicon ( IPA: /ˈsɪlikən/, Latin: silicium) 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 (practically) pure silicon
   dioxide in different crystalline forms (quartz, chalcedony, opal) and
   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 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 colour 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 (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 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.

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

     * The largest application of pure silicon (metallurgical grade
       silicon) is in aluminium - silicon alloys, often called "light
       alloys", to produce cast parts, mainly for automotive industry
       (this represents about 55% of the world consumption of pure
       silicon).
     * The second largest application of pure silicon is as a raw material
       in the production of silicones (about 40% of the world consumption
       of silicon)
     * 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 ( positive or negative) 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 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.

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 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 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 rarely found in nature; natural silicon is usually found
   in the form of silicon dioxide (also known as silica) 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 (they are known
   as " lithogenic", as opposed to " biogenic", silicas). 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.

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

          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 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 + SiO[2] → 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

   Silicon wafer with mirror finish (NASA)
   Enlarge
   Silicon wafer with mirror finish (NASA)

   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 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 less than 10^-9.

   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 favour 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 (FZ-Si) can
   be used instead.

Different forms of silicon

   Granular silicon

                   Polycrystal silicon

                                      Silicon monocrystal

                                                         Silicon nanopowder

   Silicon Ingot

   One can notice the colour 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 Nanoparticle

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.

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 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.

Compounds

   For examples of silicon compounds see silicate, silane (SiH[4]),
   silicic acid (H[4]SiO[4]), silicon carbide (SiC), silicon dioxide
   (SiO[2]), silicon tetrachloride (SiCl[4]), silicon tetrafluoride
   (SiF[4]), and trichlorosilane (HSiCl[3]).
   Retrieved from " http://en.wikipedia.org/wiki/Silicon"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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