Utilisateur:Rhadamante/silicium

Le silicium a été identifié la première fois par Antoine Lavoisier en 1787 (nom tiré du latin silex ou silicis, caillou)et fut pris par erreur par Humphry Davy en 1800 pour un composé. En 1811 Gay-Lussac et Thénard ont probablement préparé du silicium amorphe en chauffant du potassium avec du tétrafluorure de silicium. Berzelius est généralement crédité de la découverte de l'élément silicium en 1824, pour avoir préparé du silicium amorphe selon à peu près la même méthode que Gay-Lussac. Berzelius l'a aussi purifié en le lavant à répétition^[3],[4].

Le silicium sous sa forme cristalline ne fut cependant préparé que 31 ans plus tard par Henri Sainte-Claire Deville^[5]. Il a obtenu par électrolyse de chlorure de sodium-aluminium très impur contenant environ un dixième de silicium une forme allotrope faiblement impure de silicium, en 1854.^[6].

En terme de masse, le silcium représente 27,7% de la croûte terrestre et en est le second élément le plus abondant après l'oxygène. Le silicium pur est très rare dans la nature ; on en trouve sous forme d'iinclusions dans l'or et les exhalaisons volcaniques. On trouve principalement le silicium sous forme de dioxyde de silicium (appelé silice) ou d'autres silicates plus complexes.

La silice présente dans les minéraux est en général du dioxyde de silicium pur sous différentes formes cristallines. L'améthyste, l'agate, le quartz, la calcédoine, le silex, le jaspe et l'opale sont des exemples de minéraux composés de [[dioxyde de silicium. On trouve de la silice biogène chez les diatomées, les radiolaires et les éponges siliceuses.

le silicium est présent sous la forme de silicates minérales (minéraux variés contenant du silicium, de l'oxygène et un ou plusieurs autres métaux), comme par exemple le groupe des feldspaths. ces minéraux sont présents dans l'argile, le sable et de nombreux types de roches tels que le granite et le grès. Le feldspath, le pyroxène, l'amphibole et le mica sont quelques uns des groupes de silicates les plus communs.

Le silicium est l'un des composés principaux des météorites, ainsi qu'un des composés de l'obsidienne et des tectites, des fromes naturelles de verre.

Le silicium a de nombreux isotopes avec un nombre de masse compris entre 22 et 44. Il possède trois isotopes stables, ²⁸Si (l'isotope le plus abondant, représentant 92,23%), ²⁹Si (4,67%) et ³⁰Si (3,1%)^[7]. Seulement un parmi ces trois, ²⁹Si est utilisé en spectroscopie RMN^[8]. Cet élément ne possède pas d'isotope métastable connu et est d'ailleurs l'un des seuls élément du 3^e das cas, avec le phosphore et le magnésium.

Le silicium a aussi 18 isotopes radioactifs, avec seulement un radioisotope, ³²Si, produit naturellement par spallation des rayons cosmiquesspallation des rayons cosmiques de l'argon^[9]. Sa demi-vie est d'environ 170 ans (0,21 MeV) et se dégrade par désintégration β en ³²P (qui a une demi-vie de 14,28 jours)^[10], et en ³²S. Tous les autres radioisotopes sont synthétiques, l'isotope ³¹Si étant celui avec la plus grande demi-vie, 153,3 min, alors que la majorité des autres isotopes ont une demi-vie de moins d'une seconde. Tous les isotopes du silicium avec une masse atomique inférieure à celle de l'isotope stable le plus léger, ²⁸Si, se dégradent, au moins partiellement, par désintégration β⁺, et tous les isotopes avec une masse atomique supérieure à celle de l'isotope stable le plus lourd, ³⁰Si, se dégradent, au moins partiellement, par désintégration β^-.u

Le silicium à une structure électronique annalogue à celle du carbone (couche de valence NsNp à moitié remplie, quatre électrons de valence) et les deux éléments ont parfois des propriétés chimiques similaires. Même s'il est un élément relativement inerte, le silicium réagit avec les halogènes et les alcalins dilués, mais résiste à la plupart des acides (à l'exception de mélanges hyper-réactifs deacide nitrique et d'acide fluorhydrique). Ses quatre électrons de valence lui donnent la possibilité, comme au carbone, de se lier avec de nombreux autres éléments ou composés.

Le silicium, comme les autres éléments de la colonne IV (germanium, carbone sous certaines formes), est un semi-conducteur, son gap étant de 1,12 eV. Comme chez les autre semi-conducteurs, la résistivité du silicium décroit fortement avec l'augmentation de la température, une température plus élevée augmentant le nombre de porteurs de charge libres. La résistance électrique d'un monocristal de silicium change de façon significative sosu l'application d'un stress mécanique du à l'effet piézorésistif.

Dans sa forme cristalline pure, le silicium a une couleur gris-noir avec un éclat métallique.

Le silicium est l'une des rares substances (comme l'eau, le bismuth et la gallium) dont la masse volumique est plus grande à l'état liquide qu'à l'état solide, et dont le volume augmente donc lors de la solidification.

La Chine est le premier fournisseur mondial de silicium et de ferrosilicium avec 4,6 million de tonnes produite en 2010 (2/3 de la production mondiale). Elle est suivie par la Russie (610 000 t), la Norvège (330 000 t), le Brésil (240 000 t) et les États-Unis (170 000 t), le ferrosilicium représentant environ 80% de cette production^[11].

Le silicium est produit industriellement par réaction entre de la silice ultra-pure, du bois, du charbon de bois et du charbon dans un four à arc électrique en utilisant des électrodes de carbone. À des températures supérieures à 1 900 °C, le carbone réduit la silice en silicium, selon l'équation :

SiO₂ + 2 C → Si + 2 CO

Le silicium à l'état liquide est recueilli à la sortie du four, égoutté et refroidi. Le silicium ainsi produit est appelé « silicium de qualité métallurgique » (« metallurgical grade silicon »), noté MG-silicium et est pur à 99%. Par cette méthode, on peut aussi produire du carbure de silicium (SiC) à cause d'un excès de carbone :

SiO₂ + C → SiO + CO

SiO + 2 C → SiC + CO

Cependant, si la concentration en SiO₂ est maintenue haute, il est possible d'éliminer le carbure de silicium :

2 SiC + SiO₂ → 3 Si + 2 CO

En septembre 2008, le silicium de qualité métallurgique coûtait environ 3,20 $/ kg^[12], contre 1,70$/kg en 2005^[13].

Du silicium pur (>99,9%) peut être extrait directement de silice solide ou d'autres composés du silicium par électrolyse avec des sels fondus^{[14],[15],[16],[17]}. Cette méthode, connue depuis 1854^[18] (voir aussi procédé FFC Cambridge) peut potentiellement produire de façon directe du silicium de « qualité solaire » (pureté 99,999 9 %) sans émission de CO₂ et avec une consommation d’énergie réduite.

Le silicium, tout comme le carbone (sous forme de diamant) et le germanium cristallise selon une structure cristalline de type diamant avec un paramètre de maille de 0,543 071 0 nm (ou 5,430 710 Å)^[19].L’essentiel des cristaux de silicium est produit par le procédé de Czochralski (CZ-Si) car c'est la méthode la moins onéreuse et qu'elle permet de produire des cristaux de grande taille. Cependant les monocristaux ainsi produits contiennent des impuretés car le creuset qui contient le silicium liquide peut se dissoudre. Pour certaines applications électroniques, en particulier celles requérant une puissance électrique élevée, le silicium produit par la méthode de Czochralski n'est pas assez pur. Pour ces applications, on peut utiliser du float-zone silicon (en) (FZ-Si).

With the CZ-Si method the seed is dipped into the silicon melt, and the growing crystal is pulled upward, whereas with the FZ-Si method the thin seed crystal 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 300 mm or larger are grown by the Czochralski method with purity level significantly improved.

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 is desired.

Chemical methods

Today, silicon is purified by converting it to a silicon compound that can be more easily purified by distillation than in its original state, 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.

Zinc Process

At one time, DuPont produced ultra-pure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950 °C, producing silicon:

SiCl₄ + 2 Zn → 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.

Siemens Process

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:

2 HSiCl₃ → 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.

Fluid Bed Technology

In 2006 REC announced construction of a plant based on fluidized bed (FB) technology using silane:^[20]

3 SiCl₄ + Si + 2 H₂ → 4 HSiCl₃

4 HSiCl₃ → 3 SiCl₄ + SiH₄

SiH₄ → Si + 2 H₂

The advantage of fluid bed technology is that processes can be run continuously, yielding higher yields than Siemens Process, which is a batch process.

Schumacher Process

There is also the Schumacher Process, which utilizes tribromosilane in place of trichlorosilane and fluid bed technology; it requires lower deposition temperatures, lower capital costs to build facilities and operate, no hazardous polymers nor explosive material, and no waste amorphous silicon dust waste, all of which are drawbacks of the Siemens Process.^[21] However, there are yet to be any major factories built on this process.

Silicon wafer with mirror finish	Silicon powder	Nanocrystalline silicon powder

There is a color change in silicon nanopowder caused by the quantum effects which occur in particles of nanometric dimensions. (See also potential well, quantum dot, and nanoparticle).

Silicon forms binary compounds called silicides with many metallic elements whose properties range from reactive compounds, e.g. magnesium silicide, Mg₂Si through high melting refractory compounds such as molybdenum disilicide, MoSi₂.(Greenwood 1997)</ref>Silicon carbide, SiC (carborundum) is a hard, high melting solid and a well known abrasive.Silane, SiH₄, is a pyrophoric gas with a similar tetrahedral structure to methane, CH₄. When pure, it does not react with pure water or dilute acids; however, even small amounts of alkali impurities from the laboratory glass can result in a rapid hydrolysis.(Greenwood 1997, p. 339) There is a range of catenated silicon hydrides that form a homologous series of compounds, Si_nH_2n+2 where n = 2–8 (analogous to the alkanes). These are all readily hydrolyzed and are thermally unstable, particularly the heavier members.(Greenwood 1997, p. 337)^[22]Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are generally highly reactive requiring large substituent groups to stabilize them.^[23] A disilyne with a silicon-silicon triple bond was first isolated in 2004; although as the compound is non-linear, the bonding is dissimilar to that in alkynes.^[24]Tetrahalides, SiX₄, are formed with all of the halogens.(Greenwood 1997) Silicon tetrachloride, for example, reacts with water, unlike its carbon analogue, carbon tetrachloride.(Greenwood 1997, p. 342) Silicon dihalides are formed by the high temperature reaction of tetrahalides and silicon; with a structure analogous to a carbene they are reactive compounds. Silicon difluoride condenses to form a polymeric compound, (SiF₂)_n.^[22]Silicon dioxide is a high melting solid with a number of different crystal forms; the most familiar of which is the mineral quartz. In quartz each silicon atom is surrounded by four oxygen atoms that bridge to other silicon atoms to form a three dimensional lattice.(Greenwood 1997, p. 342) Silica is soluble in water at high temperatures forming a range of compounds called monosilicic acid, Si(OH)₄.(Greenwood 1997, p. 346)

Under the right conditions monosilicic acid readily polymerizes to form more complex silicic acids, ranging from the simplest condensate, disilicic acid (H₆Si₂O₇) to linear, ribbon, layer and lattice structures which form the basis of the many different silicate minerals and are called polysilicic acids {Si_x(OH)_4–2x}_n.(Greenwood 1997, p. 346) Silicates are also important constituents of concretes.(Greenwood 1997, p. 356)With oxides of other elements the high temperature reaction of silicon dioxide can give a wide range of glasses with various properties.(Greenwood 1997, p. 344) Examples include soda lime glass, borosilicate glass and lead crystal glass.Silicon sulfide, SiS₂ is a polymeric solid (unlike its carbon analogue the liquid CS₂).(Greenwood 1997)Silicon forms a nitride, Si₃N₄ which is a ceramic.(Greenwood 1997, p. 360) Silatranes, a group of tricyclic compounds containing five-coordinate silicon, may have physiological properties.^[25]Many transition metal complexes containing a metal-silicon bond are now known, which include complexes containing SiH_nX_3−n ligands, SiX₃ ligands, and Si(OR)₃ ligands.^[25]Silicones are large group of polymeric compounds with an (Si-O-Si) backbone. An example is the silicone oil PDMS (polydimethylsiloxane). These polymers can be crosslinked to produce resins and elastomers.(Greenwood 1997)Many organosilicon compounds are known which contain a silicon-carbon single bond. Many of these are based on a central tetrahedral silicon atom, and some are optically active when central chirality exists.Long chain polymers containing a silicon backbone are known, such as polydimethysilylene (SiMe₂)_n.^[26] Polycarbosilane, [(SiMe₂)₂CH₂]_n with a backbone containing a repeating -Si-Si-C unit, is a precursor in the production of silicon carbide fibers^[26].

As the second most abundant element in the Earth's crust, silicon is vital to the construction industry as a principal constituent of natural stone, glass, concrete and cement. Silicon's greatest impact on the modern world's economy and lifestyle has resulted from silicon wafers used as substrates in the manufacture of discrete electronic devices such as transistors, and in the development of integrated circuits such as computer chips.

Alloys

The largest application of metallurgical grade silicon, representing about 55% of the world consumption, is in the manufacture of aluminium-silicon alloys to produce cast parts, mainly for the automotive industry.^[27] Silicon is an important constituent of electrical steel, modifying its resistivity and ferromagnetic properties. Silicon is added to molten cast iron as ferrosilicon or silicocalcium alloys to improve its performance in casting thin sections, and to prevent the formation of cementite at the surface.

Electronics

Pure silicon is used to produce ultra-pure silicon wafers used in the semiconductor industry, in electronics and in photovoltaic applications. Ultra-pure 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. In silicon photonics, it can be used as a continuous wave Raman laser medium to produce coherent light, though it is ineffective as a light source. Hydrogenated amorphous silicon and upgraded metallurgical-grade silicon (UMG-Si)^[27] are used in the production of low-cost, large-area electronics in applications such as LCDs, and of large-area, low-cost thin-film solar cells.

Silicones

The second largest application of silicon (about 40% of world consumption) is as a raw material in the production of silicones, compounds containing silicon-oxygen and silicon-carbon bonds that have the capability to act as bonding intermediates between glass and organic compounds, and to form polymers with useful properties such as impermeability to water, flexibility and resistance to chemical attack. Silicones are used in waterproofing treatments, molding compounds and mold-release agents, mechanical seals, high temperature greases and waxes, caulking compounds and even in applications as diverse as breast implants, contact lenses, explosives and pyrotechnics^[28].

• 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 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 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.
• Silly Putty was originally made by adding boric acid to silicone oil.^[29]

Although silicon is readily available in the form of silicates, biology only uses it in very limited occasions in the form of silicic acid and soluble silicates. Diatoms, radiolaria and siliceous sponges use biogenic silica as a structural material to construct skeletons. In higher plants the silica phytoliths (opal phytoliths) are rigid microscopic bodies occurring in the cell.^[30][31][32] Although silicon was proposed to be a ultra trace nutrition its exact function in the biology of animals is still under discussion. It seems more clear that some plants, for example rice, need silicon for their growth^{[2],[31],[32]}.

Silicon is currently under consideration by the Association of American Plant Food Control Officials (AAPFCO) for elevation to the status of a "plant beneficial substance."^[33][34] Silicon has been shown in university and field studies to improve plant cell wall strength and structural integrity,^[35] improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems.^[36] Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields^[35].

Because silicon is an important element in semiconductors and high-technology devices, Silicon Valley in California is named after this element since it is the base for a number of technology related industries. Other geographic locations with connections to the industry have since characterized themselves as Siliconia as well, for example Silicon Forest in Oregon, Silicon Hills in Austin, Silicon Saxony in Germany, Silicon Valley in India, Silicon Border in Mexicali and Silicon Fen in Cambridge, England.

Modèle:Colbegin

• Black silicon
• Covalent superconductors
• Monocrystalline silicon
• Polycrystalline silicon
• List of silicon producers
• Printed silicon electronics
• Silicon (journal)

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