Saturday, October 26, 2013

Early silicon purification techniques

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. Dell MF60100V1-Q000-G99 CPU Fan 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. DELL R859C CPU Fan 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. SONY Vaio VGN-FW140E CPU Fan 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. At one time, DuPont produced ultra-pure silicon by reacting silicon tetrachloride with high-purity zincvapors at 950 °C, producing silicon by SiCl4 + 2 Zn → Si + 2 ZnCl2. ACER Aspire 5730Z CPU Fan 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. 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 because 2 HSiCl3 → Si + 2 HCl + SiCl4. SONY Vaio VPC-EB4S1E/WI CPU Fan Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of less than one part per billion.[24][25][26] In 2006 REC announced construction of a plant based on fluidized bed (FB) technology using silane: 3 SiCl4 + Si + 2 H2 → 4 HSiCl3, 4 HSiCl3 → 3 SiCl4 + SiH4, SiH4 → Si + 2 H2.[27] HP Pavilion dv9580ed CPU Fan The advantage of fluid bed technology is that processes can be run continuously, yielding higher yields than Siemens Process, which is a batch process. 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. Toshiba Satellite L305D-S5934 CPU Fan 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 addition, there is the Schumacher process, which utilizes tribromosilane in place of trichlorosilane and fluid bed technology. SONY Vaio VGN-CR31S/P CPU Fan It requires lower deposition temperatures, lower capital costs to build facilities and operate, no hazardous polymers nor explosive material, and produces no amorphous silicon dust waste, all of which are drawbacks of the Siemens process.[28] However, there are yet to be any major factories built using this process. HP Pavilion dv5-1210et CPU Fan
  • 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.[29]
Silicon carbide, SiC (carborundum) is a hard, high melting solid and a well known abrasive. It may also be sintered into a type of high-strength ceramic used in armor. SONY Vaio VGN-FW465J/B CPU Fan 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.[30] There is a range of catenated silicon hydrides that form a homologous series of compounds, SinH2n+2 where n = 2–8 (analogous to the alkanes). HP Envy 15 CPU Fan
  • These are all readily hydrolyzed and are thermally unstable, particularly the heavier members.[31][32]
Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are generally highly reactive requiring large substituent groups to stabilize them.[33] 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.[34] HP Pavilion dv7-1190er CPU Fan Tetrahalides, SiX4, are formed with all the halogens.[35] Silicon tetrachloride, for example, reacts with water, unlike its carbon analogue, carbon tetrachloride.[36] 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.[32] Toshiba Satellite A105-S4074 CPU Fan Silicon dioxide is a high melting solid with a number of 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.[36] Silica is soluble in water at high temperatures forming a range of compounds called monosilicic acid, Si(OH)4.[37]  IBM MCF-208AM05-1 CPU Fan

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 silicate minerals and are called polysilicic acids {Six(OH)4–2x}n.[37]

With oxides of other elements the high temperature reaction of silicon dioxide can give a wide range of glasses with various properties.[38] IBM MCF-217PAM05 CPU Fan
  • 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).[39]
Silicon forms a nitride, Si3N4 which is a ceramic.[40] Silatranes, a group of tricyclic compounds containing five-coordinate silicon, may have physiological properties.[41]  HP GB0507PGV1-A CPU Fan
  • 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)3ligands.[41]
Silicones are large group of polymeric compounds with an (Si-O-Si) backbone. An example is the silicone oil PDMS (polydimethylsiloxane). IBM MCF-217PAM05 CPU Fan

These polymers can be crosslinked to produce resins and elastomers.[42]

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 (SiMe 2)n.[43] HP Pavilion dv6-3385la CPU Fan

Polycarbosilane, [(SiMe2)2CH2]n with a backbone containing a repeating -Si-Si-C unit, is a precursor in the production of silicon carbide fibers.

Attention was first drawn to silica as the possible oxide of a fundamental chemical element by Antoine Lavoisier, in 1787.[44] After an attempt to isolate silicon in 1808, Sir Humphry Davy proposed the name "silicium" for silicon, from the Latin silexsilicis for flint, flints, and adding the "-ium" ending because he believed it was a metal.[45] HP Pavilion tx2510us  CPU  Fan In 1811, Gay-Lussac and Thénard are thought to have prepared impure amorphous silicon, 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.[46] Silicon was given its present name in 1817 by Scottish chemist Thomas Thomson. HP G62-b55EV CPU Fan He retained part of Davy's name but added "-on" because he believed that silicon was anonmetal similar to boron and carbon.[47] In 1823, Berzelius prepared amorphous silicon using approximately the same method as Gay-Lussac (potassium metal and potassium fluorosilicate), but purifying the product to a brown powder by repeatedly washing it.[48] As a result he is usually given credit for the element's discovery.[49][50] HP Pavilion dv6-2145sf CPU Fan Silicon in its more common crystalline form was not prepared until 31 years later, by Deville.[51][52] By electrolyzing impure sodium-aluminum chloridecontaining approximately 10% silicon, he was able to obtain a slightly impure allotrope of silicon in 1854.[53] Later, more cost-effective methods have been developed to isolate silicon in several allotrope forms, the most recent being silicene. HP Pavilion dv7-1120ew CPU Fan Because silicon is an important element in semiconductors and high-technology devices, many places in the world bear its name. For example, Silicon Valley inCalifornia, since it is the base for a number of technology-related industries, bears the name silicon. Other geographic locations with connections to the industry have since been named after silicon as well. HP 538340-001 CPU Fan Examples include Silicon Forest in Oregon, Silicon Hills in Austin, Texas, 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, andSilicon Gorge in Bristol, England. Most silicon is used industrially without being separated into the element, and indeed often with comparatively little processing from natural occurrence. HP 535766-001 CPU Fan Over 90% of the Earth's crust is composed of silicate minerals. 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). For example, silica is an important part of ceramic brick. HP 535442-001 CPU Fan Silicates are used in making Portland cement which is used in building mortar and stucco, but more importantly combined with silica sand, and gravel (usually containing silicate minerals like granite), to make the concrete that is the basis of most of the very largest industrial building projects of the modern world. [54] HP 535441-001 CPU Fan Silicate minerals are also in whiteware ceramics, an important class of products usually containing various types of fired clay (natural aluminum silicate). An example is porcelain which is based on silicate mineral kaolinite. Ceramics include art objects, and domestic, industrial and building products. Traditional quartz-based soda-lime glass also functions in many of the same roles. HP 535439-001 CPU Fan More modern silicon compounds also function as high-technology abrasives and new high-strength ceramics based upon (silicon carbide), and in superalloys. Alternating silicon-oxygen chains with hydrogen attached to the remaining silicon bonds form the ubiquitous silicon-based polymeric materials known assilicones. HP 535438-001 CPU Fan These compounds containing silicon-oxygen and occasionally silicon-carbon bonds 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. HP 533736-001 CPU Fan Silicones are often 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.[55] Silly Putty was originally made by adding boric acid tosilicone oil. IBM 13R2760 CPU Fan 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. ACER LU.S040A.067 CPU Fan Ferrosilicon production and use is a monitor of the steel industry, and although this form of elemental silicon is impure, it accounts for 80% of the world's use of free silicon. The properties of silicon itself can be used to modify alloys. Silicon's importance in aluminum casting is that a significantly high amount (12%) of silicon in aluminum forms a eutectic mixture which solidifies with very little thermal contraction. ACER LU.S050B.078 CPU Fan 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 aluminum.[16] Silicon is an important constituent ofelectrical steel, modifying its resistivity and ferromagnetic properties. Metallurgical grade silicon is silicon of 95–99% purity. DELL Studio 1535 CPU Fan About 55% of the world consumption of metallurgical purity silicon goes for production of aluminum-silicon alloys for aluminum part casts, mainly for use in the automotive industry. The reason for the high silicon use in these alloys is noted above.[17] Much of the rest of metallurgical-grade silicon is used by the chemical industry for production of the important industrial product fumed silica. HP Pavilion dv6736nr CPU Fan The remainder is used in production of other fine chemicals such as silanes and some types of silicones. Since most elemental silicon produced remains as ferrosilicon alloy, only a relatively small amount (20%) of the elemental silicon produced is refined to metallurgical grade purity (a total of 1.3–1.5 million metric tons/year). HP Envy 15-1050ca CPU Fan The fraction of silicon metal which is further refined to semiconductor purity is estimated at only 15% of the world production of metallurgical grade silicon.[17]However, the economic importance of this small very high-purity fraction (especially the ~ 5% which is processed to monocrystalline silicon for use in integrated circuits) is disproportionately large. HP Pavilion dv7-6100et CPU Fan Pure monocrystalline silicon is used to produce silicon wafers used in the semiconductor industry, in electronics and in some high-cost and high-efficiency photovoltaic applications. In terms of charge conduction, pure silicon is an intrinsic semiconductor which means that unlike metals it conducts electron holes and electrons that may be released from atoms within the crystal by heat, APPLE 661-4951 CPU Fan and thus increase silicon's electrical conductivity with higher temperatures. Pure silicon has too low a conductivity (i.e., too high aresistivity) to be used as a circuit element in electronics. In practice, pure silicon is doped with small concentrations of certain other elements, a process that greatly increases its conductivity and adjusts its electrical response by controlling the number and charge (positive or negative) of activated carriers. IBM 42W2403 CPU Fan Such control is necessary for transistors, solar cells, semiconductor detectorsand other semiconductor devices, which are used in the computer industry and other technical applications. For example, in silicon photonics, silicon can be used as a continuous wave Raman laser medium to produce coherent light, though it is ineffective as an everyday light source. ACER Aspire 9304WSMi CPU Fan 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 by exposing the element to oxygen under the proper conditions. Silicon has become the most popular material to build both high power semiconductors and integrated circuits. HP 610773-001 CPU Fan The reason is that silicon is the semiconductor that can withstand the highest temperatures and electrical powers without becoming dysfunctional due to avalanche breakdown (a process in which an electron avalanche is created by a chain reaction process whereby heat produces free electrons and holes, which in turn produce more current which produces more heat). IBM ThinkPad R50p 1830 CPU Fan

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