Copper And Silicon

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Copper is a chemical element with the symbol Cu (from Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; a freshly exposed surface has a reddish-orange color. It is used as a conductor of heat and electricity, a building material, and a constituent of various metal alloysSony VAIO VPCF135Z1E/B battery.

The metal and its alloys have been used for thousands of years. In the Roman era, copper was principally mined on Cyprus, hence the origin of the name of the metal as сyprium (metal of Cyprus), later shortened to сuprum. Its compounds are commonly encountered as copper(II) salts, which often impart blue or green colors to minerals such as turquoise and have been widely used historically as pigmentsSony VAIO VPCF137HG/BI battery. Architectural structures built with copper corrode to give green verdigris (or patina). Decorative art prominently features copper, both by itself and as part of pigments.

Copper(II) ions are water-soluble, where they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, they are poisonous to higher organisms; at lower concentrations it is an essential trace nutrient to all higher plant and animal life. The main areas where copper is found in animals are liver, muscle and boneSony VAIO VPCF136FG/BI battery.

Characteristics

[edit]Physical

A copper disc (99.95% pure) made by continuous casting and etching.

Copper just above its melting point keeps its pink luster color when enough light outshines the orange incandescence color.

Copper, silver and gold are in group 11 of the periodic table, and they share certain attributes: they have one s-orbital electron on top of a filled d-electron shell and are characterized by high ductility and electrical conductivitySony VAIO VPCF135FG/B battery. The filled d-shells in these elements do not contribute much to the interatomic interactions, which are dominated by the s-electrons through metallic bonds. Contrary to metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. This explains the low hardness and high ductility of single crystals of copperSony VAIO VPCF127HG/BI battery.[2] At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress thereby increasing its hardness. For this reason, copper is usually supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline formsSony VAIO VPCF11Z1E battery.[3]

The low hardness of copper partly explains its high electrical (59.6×106 S/m) and thus also high thermal conductivity, which are the second highest among pure metals at room temperature.[4] This is because the resistivity to electron transport in metals at room temperature mostly originates from scattering of electrons on thermal vibrations of the latticeSony VAIO VPCF11S1E battery, which are relatively weak for a soft metal.[2] The maximum permissible current density of copper in open air is approximately 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively.[5] As with other metals, if copper is placed against another metal, galvanic corrosion will occur.[6]

Together with osmium (bluish), and gold (yellow), copper is one of only three elemental metals with a natural color other than gray or silverSony VAIO VPCF11M1E/H battery.[7] Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light. The same mechanism accounts for the yellow color of goldSony VAIO VPCF11M1E battery.[2]

[edit]Chemical

Unoxidized copper wire (left) and oxidized copper wire (right).

Copper forms a rich variety of compounds with oxidation states +1 and +2, which are often called cuprous and cupric, respectively.[8] It does not react with water, but it slowly reacts with atmospheric oxygen forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by wet air, this oxide layer stops the further, bulk corrosionSony VAIO VPCF11JFX/B battery. A green layer of verdigris (copper carbonate) can often be seen on old copper constructions, such as the Statue of Liberty, the largest copper statue in the world built using repoussé and chasing.[9] Hydrogen sulfides and sulfides react with copper to form various copper sulfides on the surface. In the latter case, the copper corrodes, as is seen when copper is exposed to air containing sulfur compoundsSony VAIO VPCF119FJ battery.[10] Oxygen-containing ammonia solutions give water-soluble complexes with copper, as do oxygen and hydrochloric acid to form copper chlorides and acidified hydrogen peroxide to form copper(II) salts. Copper(II) chloride and copper comproportionate to form copper(I) chloride.[11]

[edit]Isotopes

Main article: Isotopes of copper

There are 29 isotopes of copper. 63Cu and 65Cu are stable, with 63Cu comprising approximately 69% of naturally occurring copper; they both have a spin of 3/2Sony VAIO VPCF119FC/BI battery.[12] The other isotopes are radioactive, with the most stable being 67Cu with a half-life of 61.83 hours.[12] Seven metastable isotopes have been characterized, with 68mCu the longest-lived with a half-life of 3.8 minutes. Isotopes with a mass number above 64 decay by β-, whereas those with a mass number below 64 decay by β+. 64Cu, which has a half-life of 12.7 hours, decays both waysSony VAIO VPCF119FC battery.[13]

62Cu and 64Cu have significant applications. 64Cu is a radiocontrast for X-ray imaging, and complexed with a chelate can be used for treating cancer. 62Cu is used in 62Cu-PTSM that is a radioactive tracer for positron emission tomography.[14]

[edit]Occurrence

Copper can be found as either native copper or as part of minerals. Native copper is a polycrystal, with the largest described single crystal measuring 4.4×3.2×3.2 cm.[15] The largest mass of elemental copper weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US. Sony VAIO VPCF117HG/BI battery There are many examples of copper-containing minerals: chalcopyrite and chalcocite are copper sulfides, azurite and malachite are copper carbonates and cuprite is a copper oxide.[4] Copper is present in the Earth's crust at a concentration of about 50 parts per million (ppm),[16] and is also synthesized in massive stars.[17]

[edit]Production

Chuquicamata in Chile is one of the world's largest open pit copper minesSony VAIO VPCF116FG/BI battery.

World production trend

Copper output in 2005

Copper prices 2003–2011 in USD per tonne

See also: List of countries by copper production

Most copper is mined or extracted as copper sulfides from large open pit mines in porphyry copper deposits that contain 0.4 to 1.0% copper. Examples include Chuquicamata in Chile, Bingham Canyon Mine in Utah, United States and El Chino Mine in New Mexico, United States. According to the British Geological Survey, in 2005, Chile was the top mine producer of copper with at least one-third world share followed by the United StatesSony VAIO VPCF115FG/B battery, Indonesia and Peru.[4] The amount of copper in use is increasing and the quantity available is barely sufficient to allow all countries to reach developed world levels of usage.[18]

[edit]Reserves

See also: Peak copper#Reserves

Copper has been in use at least 10,000 years, but more than 95% of all copper ever mined and smelted has been extracted since 1900. As with many natural resources, the total amount of copper on Earth is vast (around 1014 tons just in the top kilometer of Earth's crust, or about 5 million years worth at the current rate of extraction) Sony VAIO VPCF21ZHJ battery. However, only a tiny fraction of these reserves is economically viable, given present-day prices and technologies. Various estimates of existing copper reserves available for mining vary from 25 years to 60 years, depending on core assumptions such as the growth rate.[19] Recycling is a major source of copper in the modern world.[20] Because of these and other factorsSony VAIO VPCF21Z1E/BI battery, the future of copper production and supply is the subject of much debate, including the concept of Peak copper, analogous to Peak Oil.

The price of copper has historically been unstable,[21] and it quintupled from the 60-year low of US$0.60/lb (US$1.32/kg) in June 1999 to US$3.75 per pound (US$8.27/kg) in May 2006. It dropped to US$2.40/lb (US$5.29/kg) in February 2007, then rebounded to US$3.50/lb (US$7.71/kg) in April 2007Sony VAIO VPCF21AHJ battery.[22] In February 2009, weakening global demand and a steep fall in commodity prices since the previous year's highs left copper prices at US$1.51/lb.[23]

[edit]Methods

Main article: Copper extraction techniques

The concentration of copper in ores averages only 0.6%, and most commercial ores are sulfides, especially chalcopyrite (CuFeS2) and to a lesser extent chalcocite (Cu2S).[24] These minerals are concentrated from crushed ores to the level of 10–15% copper by froth flotation or bioleachingSony VAIO VPCF21AGJ battery.[25] Heating this material with silica in flash smelting removes much of the iron as slag. The process exploits the greater ease of converting iron sulfides into its oxides, which in turn react with the silica to form the silicate slag, which floats on top of the heated mass. The resulting copper matte consisting of Cu2S is then roasted to convert all sulfides into oxidesSony VAIO VPCF21AFJ battery:[24]

2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2

The cuprous oxide is converted to blister copper upon heating:

2 Cu2O → 4 Cu + O2

The Sudbury matte process converted only half the sulfide to oxide and then used this oxide to remove the rest of the sulfur as oxide. It was then electrolytically refined and the anode mud exploited for the platinum and gold it contained. This step exploits the relatively easy reduction of copper oxides to copper metal. Natural gas is blown across the blister to remove most of the remaining oxygen and electrorefining is performed on the resulting material to produce pure copperSony VAIO VPCF219FJ/BI battery:[26]

Cu2+ + 2 e– → Cu

[edit]Recycling

Copper, like aluminium, is 100% recyclable without any loss of quality whether in a raw state or contained in a manufactured product. In volume, copper is the third most recycled metal after iron and aluminium. It is estimated that 80% of the copper ever mined is still in use today.[27] According to the International Resource Panel's Metal Stocks in Society reportSony VAIO VPCF217HG/BI battery, the global per capita stock of Copper in use in society is 35–55 kg. Much of this is in more-developed countries (140–300 kg per capita) rather than less-developed countries (30–40 kg per capita).

The process of recycling copper follows roughly the same steps as is used to extract copper, but requires fewer steps. High purity scrap copper is melted in a furnace and then reduced and cast into billets and ingots; lower purity scrap is refined by electroplating in a bath of sulfuric acid. Sony VAIO VPCF217HG battery

[edit]Compounds

A sample of copper(I) oxide.

See also: Category:Copper compounds

[edit]Binary compounds

As for other elements, the simplest compounds of copper are binary compounds, i.e. those containing only two elements. The principal ones are the oxides, sulfides and halides. Both cuprous and cupric oxides are known. Among the numerous copper sulfides, important examples include copper(I) sulfide and copper(II) sulfideSony VAIO VPCF14ZHJ battery.

The cuprous halides with chlorine, bromine, and iodine are known, as are the cupric halides with fluorine, chlorine, and bromine. Attempts to prepare copper(II) iodide give cuprous iodide and iodine.[8]

2 Cu2+ + 4 I− → 2 CuI + I2

[edit]Coordination chemistry

Copper(II) gives a deep blue coloration in the presence of ammonia ligands. The one used here is tetramminecopper(II) sulfateSony VAIO VPCF14AHJ battery.

Copper, like all metals, forms coordination complexes with ligands. In aqueous solution, copper(II) exists as [Cu(H2O)6]2+. This complex exhibits the fastest water exchange rate (speed of water ligands attaching and detaching) for any transition metal aquo complex. Adding aqueous sodium hydroxide causes the precipitation of light blue solid copper(II) hydroxide. A simplified equation isSony VAIO VPCF14AGJ battery:

Cu2+ + 2 OH− → Cu(OH)2

Aqueous ammonia results in the same precipitate. Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper(II):

Cu(H2O)4(OH)2 + 4 NH3 → [Cu(H2O)2(NH3)4]2+ + 2 H2O + 2 OH−

Many other oxyanions form complexes; these include copper(II) acetate, copper(II) nitrate, and copper(II) carbonate. Copper(II) sulfate forms a blue crystalline pentahydrate, which is the most familiar copper compound in the laboratory. It is used in a fungicide called the Bordeaux mixtureSony VAIO VPCF14AFJ battery.[29]

Ball-and-stick model of the complex [Cu(NH3)4(H2O)2]2+, illustrating the octahedral coordination geometry common for copper(II).

Polyols, compounds containing more than one alcohol functional group, generally interact with cupric salts. For example, copper salts are used to test for reducing sugars. Specifically, using Benedict's reagent and Fehling's solution the presence of the sugar is signaled by a color change from blue Cu(II) to reddish copper(I) oxideSony VAIO VPCF149FJ/BI battery.[30] Schweizer's reagent and related complexes with ethylenediamine and other amines dissolve cellulose.[31] Amino acids form very stable chelate complexes with copper(II). Many wet-chemical tests for copper ions exist, one involving potassium ferrocyanide, which gives a brown precipitate with copper(II) saltsSony VAIO VPCF148FJ/B battery.

[edit]Organocopper chemistry

Main article: Organocopper compound

Compounds that contain a carbon-copper bond are known as organocopper compounds. They are very reactive towards oxygen to form copper(I) oxide and have many uses in chemistry. They are synthesized by treating copper(I) compounds with Grignard reagents, terminal alkynes or organolithium reagents;[32] in particular, the last reaction described produces a Gilman reagentSony VAIO VPCF13ZHJ battery. These can undergo substitution with alkyl halides to form coupling products; as such, they are important in the field of organic synthesis. Copper(I) acetylide is highly shock-sensitive but is an intermediate in reactions such as the Cadiot-Chodkiewicz coupling[33] and the Sonogashira coupling.[34] Conjugate addition to enones[35] and carbocupration of alkynes[36] can also be achieved with organocopper compoundsSony VAIO VPCF13Z0E/B battery. Copper(I) forms a variety of weak complexes with alkenes and carbon monoxide, especially in the presence of amine ligands.[37]

[edit]Copper(III) and copper(IV)

Complexes of copper(III) are frequent intermediates in reactions of organocopper compounds. Dicopper oxo complexes also feature copper(III).[38] Fluoride ligands, being highly basic, stabilize metal ions in high oxidation states; indeed, representative copper(III) and copper(IV) complex are fluorides. These include K3CuF6 and Cs2CuF6.[8] With di- and tripeptidesSony VAIO VPCF13M8E/B battery, purple-colored complexes of copper(III) have been observed, this high oxidation state being stabilized by the deprotonated amide ligands.[39]

History

Copper Age

Main article: Copper Age

A corroded copper ingot from Zakros, Crete, shaped in the form of an animal skin typical for that era.

Copper occurs naturally as native copper and was known to some of the oldest civilizations on record. It has a history of use that is at least 10,000 years old, and estimates of its discovery place it at 9000 BC in the Middle East;[40] a copper pendant was found in northern Iraq that dates to 8700 BCSony VAIO VPCF13AHJ battery.[41] There is evidence that gold and meteoric iron (but not iron smelting) were the only metals used by humans before copper.[42] The history of copper metallurgy is thought to have followed the following sequence: 1) cold working of native copper, 2) annealing, 3) smelting, and 4) the lost wax method. In southeastern Anatolia, all four of these metallurgical techniques appears more or less simultaneously at the beginning of the Neolothic c. 7500 BCSony VAIO VPCF13AGJ battery.[43] However, just as agriculture was independently invented in several parts of the world (including Pakistan, China, and the Americas) copper smelting was invented locally in several different places. It was probably discovered independently in China before 2800 BC, in Central America perhaps around 600 AD, and in West Africa about the 9th or 10th century ADSony VAIO VPCF13AFJ battery.[44] Investment casting was invented in 4500–4000 BC in Southeast Asia[40] and carbon dating has established mining at Alderley Edge in Cheshire, UK at 2280 to 1890 BC.[45] Ötzi the Iceman, a male dated from 3300–3200 BC, was found with an axe with a copper head 99.7% pure; high levels of arsenic in his hair suggest his involvement in copper smeltingSony VAIO VPCF138FJ/BI battery.[46] Experience with copper has assisted the development of other metals; in particular, copper smelting led to the discovery of iron smelting.[46] Production in the Old Copper Complex in Michigan and Wisconsin is dated between 6000 and 3000 BC.[47][48] Natural bronze, a type of copper made from ores rich in silicon, arsenic, and (rarely) tin, came into general use in the Balkans around 5500 BCSony VAIO VPCF138FJ/B battery. Previously the only tool made of copper had been the awl, used for punching holes in leather and gouging out peg-holes for wood joining. However, the introduction of a more robust form of copper led to the widespread use, and large-scale production of heavy metal tools, including axes, adzes, and axe-adzes. Sony VAIO VPCF138FC/BI battery

[edit]Bronze Age

Main article: Bronze Age

Alloying copper with tin to make bronze was first practiced about 4000 years after the discovery of copper smelting, and about 2000 years after "natural bronze" had come into general use. Bronze artifacts from Sumerian cities and Egyptian artifacts of copper and bronze alloys date to 3000 BC.[49] The Bronze Age began in Southeastern Europe around 3700 - 3300 BCSony VAIO VPCF135FG battery, in Northwestern Europe about 2500 BC. It ended with the beginning of the Iron Age, 2000-1000 BC in the Near East, 600 BC in Northern Europe. The transition between the Neolithic period and the Bronze Age was formerly termed the Chalcolithic period (copper-stone), with copper tools being used with stone tools. This term has gradually fallen out of favor because in some parts of the world the Calcholithic and Neolithic are coterminous at both endsSony VAIO VPCF12AHJ battery. Brass, an alloy of copper and zinc, is of much more recent origin. It was known to the Greeks, but became a significant supplement to bronze during the Roman Empire.[49]

Antiquity and Middle Ages

In alchemy the symbol for copper was also the symbol for the goddess and planet Venus.

Chalcolithic copper mine in Timna Valley, Negev Desert, Israel.

In Greece, copper was known by the name chalkos (χαλκός). It was an important resource for the Romans, Greeks and other ancient peoples. In Roman times, it was known as aes Cyprium, aes being the generic Latin term for copper alloys and Cyprium from Cyprus, where much copper was minedSony VAIO VPCF12AGJ battery. The phrase was simplified to cuprum, hence the English copper. Aphrodite and Venus represented copper in mythology and alchemy, due to its lustrous beauty, its ancient use in producing mirrors, and its association with Cyprus, which was sacred to the goddess. The seven heavenly bodies known to the ancients were associated with the seven metals known in antiquity, and Venus was assigned to copperSony VAIO VPCF12AFJ battery.[50]

Britain's first use of brass occurred around the 3rd–2nd century BC. In North America, copper mining began with marginal workings by Native Americans. Native copper is known to have been extracted from sites on Isle Royale with primitive stone tools between 800 and 1600.[51] Copper metallurgy was flourishing in South America, particularly in Peru around 1000 ADSony VAIO VPCF129FJ/BI battery; it proceeded at a much slower rate on other continents. Copper burial ornamentals from the 15th century have been uncovered, but the metal's commercial production did not start until the early 20th century.

The cultural role of copper has been important, particularly in currency. Romans in the 6th through 3rd centuries BC used copper lumps as money. At first, the copper itself was valued, but gradually the shape and look of the copper became more importantSony VAIO VPCF128FJ/B battery. Julius Caesar had his own coins made from brass, while Octavianus Augustus Caesar's coins were made from Cu-Pb-Sn alloys. With an estimated annual output of around 15,000 t, Roman copper mining and smelting activities reached a scale unsurpassed until the time of the Industrial Revolution; the provinces most intensely mined were those of Hispania, Cyprus and in Central Europe. Sony VAIO VPCF127FJ/W battery

The gates of the Temple of Jerusalem used Corinthian bronze made by depletion gilding. It was most prevalent in Alexandria, where alchemy is thought to have begun.[54] In ancient India, copper was used in the holistic medical science Ayurveda for surgical instruments and other medical equipment. Ancient Egyptians (~2400 BC) used copper for sterilizing wounds and drinking waterSony VAIO VPCF11ZHJ battery, and later on for headaches, burns, and itching. The Baghdad Battery, with copper cylinders soldered to lead, dates back to 248 BC to AD 226 and resembles a galvanic cell, leading people to believe this was the first battery; the claim has not been verified.[55]

[edit]Modern period

Acid mine drainage affecting the stream running from the disused Parys Mountain copper mines

The Great Copper Mountain was a mine in Falun, Sweden, that operated from the 10th century to 1992. It produced two thirds of Europe's copper demand in the 17th century and helped fund many of Sweden's wars during that timeSony VAIO VPCF11AHJ battery.[56] It was referred to as the nation's treasury; Sweden had a copper backed currency.[57]

The uses of copper in art were not limited to currency: it was used by Renaissance sculptors, in photographic technology known as the daguerreotype, and the Statue of Liberty. Copper plating and copper sheathing for ships' hulls was widespread; the ships of Christopher Columbus were among the earliest to have this featureSony VAIO VPCF11AGJ battery.[58] The Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in 1876.[59] The German scientist Gottfried Osann invented powder metallurgy in 1830 while determining the metal's atomic mass; around then it was discovered that the amount and type of alloying element (e.g., tin) to copper would affect bell tones. Flash smelting was developed by Outokumpu in Finland and first applied at Harjavalta in 1949Sony VAIO VPCF11AFJ battery; the energy-efficient process accounts for 50% of the world’s primary copper production.[60]

The Intergovernmental Council of Copper Exporting Countries, formed in 1967 with Chile, Peru, Zaire and Zambia, played a similar role for copper as OPEC does for oil. It never achieved the same influence, particularly because the second-largest producer, the United States, was never a member; it was dissolved in 1988Sony VAIO VPCF118FJ battery.[61]

Applications

Assorted copper fittings

The major applications of copper are in electrical wires (60%), roofing and plumbing (20%) and industrial machinery (15%). Copper is mostly used as a metal, but when a higher hardness is required it is combined with other elements to make an alloy (5% of total use) such as brass and bronze.[16] A small part of copper supply is used in production of compounds for nutritional supplements and fungicides in agriculture. Sony VAIO VPCF117FJ battery Machining of copper is possible, although it is usually necessary to use an alloy for intricate parts to get good machinability characteristics.

[edit]Electronics and related devices

Copper electrical busbars distributing power to a large building

The electrical properties of copper are exploited in copper wires and devices such as electromagnets. Integrated circuits and printed circuit boards increasingly feature copper in place of aluminium because of its superior electrical conductivity (see Copper interconnect for main article); heat sinks and heat exchangers use copper as a result of its superior heat dissipation capacity to aluminiumSony VAIO VGN-CS33H battery. Vacuum tubes, cathode ray tubes, and the magnetrons in microwave ovens use copper, as do wave guides for microwave radiation.[63]

[edit]Electric motors

Main article: Copper in energy efficient motors

Copper’s greater conductivity versus other metallic materials enhances the electrical energy efficiency of motors.[64] This is important because motors and motor-driven systems account for 43%-46% of all global electricity consumption and 69% of all electricity used by industry.[65] Increasing the mass and cross section of copper in a coil increases the electrical energy efficiency of the motorSony VAIO VGN-CS33H/Z battery. Copper motor rotors, a new technology designed for motor applications where energy savings are prime design objectives,[66][67] are enabling general-purpose induction motors to meet and exceed National Electrical Manufacturers Association (NEMA) premium efficiency standards.[68]

[edit]Architecture and industry

Copper roof on the Minneapolis City Hall, coated with patina

Old copper utensils in a Jerusalem restaurant

Because of the waterproof nature of copper, it has been used as the roofing material of many buildings since ancient timesSony VAIO VGN-CS33H/B battery. The green color on these buildings is due to a long-term chemical reaction: copper is first oxidized to copper(II) oxide, then to cuprous and cupric sulfide and finally to copper(II) carbonate, also called verdigris, which is highly corrosion-resistant.[69] The copper used in this application is phosphorus deoxidized copper (Cu-DHP).[70] Lightning rods use copper as a means to divert electric current throughout the ground instead of destroying the main structureSony VAIO VGN-CS31Z/Q battery.[71] Copper has excellent brazing and soldering properties and can be welded; the best results are obtained with gas metal arc welding.[72]

[edit]Copper in alloys

See also: List of copper alloys

Numerous copper alloys exist, many with important uses. Brass is an alloy of copper and zinc and bronze usually refers to copper-tin alloys, but can refer to any alloy of copper such as aluminium bronze. Copper is one of the most important constituents of carat silver and gold alloys and carat solders used in the jewelry industry, modifying the color, hardness and melting point of the resulting alloysSony VAIO VGN-CS31S/W battery.[73]

The alloy of copper and nickel, called cupronickel, is used in low-denomination statuary coins, often for the outer cladding. The US 5-cent coin called nickel consists of 75% copper and 25% nickel and has a homogeneous composition. The 90% copper/10% nickel alloy is remarkable by its resistance to corrosion and is used in various parts being exposed to seawaterSony VAIO VGN-CS31S/V battery. Alloys of copper with aluminium (about 7%) have a pleasant golden color and are used in decorations.[16] Copper alloys with tin are part of lead-free solders.[74]

[edit]Antibiofouling applications

Main articles: Copper alloys in aquaculture and Copper sheathing

Copper has long been used as a biostatic surface to line parts of ships to protect against barnacles and mussels. It was originally used pure, but has since been superseded by Muntz metal. Bacteria will not grow on a copper surface because it is biostatic. Similarly, as discussed in copper alloys in aquacultureSony VAIO VGN-CS31S/T battery, copper alloys have become important netting materials in the aquaculture industry for the fact that they are antimicrobial and prevent biofouling even in extreme conditions[75] and have strong structural and corrosion-resistant[76] properties in marine environments.

[edit]Other uses

Copper compounds in liquid form are used as a wood preservative, particularly in treating original portion of structures during restoration of damage due to dry rot. Together with zincSony VAIO VGN-CS31S/R battery, copper wires may be placed over non-conductive roofing materials to discourage the growth of moss. Textile fibers use copper to create antimicrobial protective fabrics,[77] as do ceramic glazes, stained glass and musical instruments. Electroplating commonly uses copper as a base for other metals such as nickel.

Copper is one of three metals, along with lead and silver, used in a museum materials testing procedure called the Oddy testSony VAIO VGN-CS31S/P battery. In this procedure, copper is used to detect chlorides, oxides, and sulfur compounds.

Copper is also commonly found in jewelry, and folklore states that copper bracelets relieve arthritis symptoms, though this is not proven.[78]

[edit]Biological role

Main article: Copper in health

Rich sources of copper include oysters, beef and lamb liver, Brazil nuts, blackstrap molasses, cocoa, and black pepper. Good sources include lobster, nuts and sunflower seeds, green olives, avocados, and wheat branSony VAIO VGN-CS28 battery.

Copper proteins have diverse roles in biological electron transport and oxygen transportation, processes that exploit the easy interconversion of Cu(I) and Cu(II).[79] The biological role for copper commenced with the appearance of oxygen in earth's atmosphere.[80] The protein hemocyanin is the oxygen carrier in most mollusks and some arthropods such as the horseshoe crab (Limulus polyphemus) Sony VAIO VGN-CS28/Q battery.[81] Because hemocyanin is blue, these organisms have blue blood, not the red blood found in organisms that rely on hemoglobin for this purpose. Structurally related to hemocyanin are the laccases and tyrosinases. Instead of reversibly binding oxygen, these proteins hydroxylate substrates, illustrated by their role in the formation of lacquersSony VAIO VGN-CS27 battery.[79]

Copper is also a component of other proteins associated with the processing of oxygen. In cytochrome c oxidase, which is required for aerobic respiration, copper and iron cooperate in the reduction of oxygen. Copper is also found in many superoxide dismutases, proteins that detoxify superoxides, by converting it (by disproportionation) to oxygen and hydrogen peroxideSony VAIO VGN-CS27/W battery:

2 HO2 → H2O2 + O2

Several copper proteins, such as the "blue copper proteins", do not interact directly with substrates, hence they are not enzymes. These proteins relay electrons by the process called electron transfer.[79]

Silicon ( /ˈsɪlɨkən/ sil-ə-kən or /ˈsɪlɨkɒn/ sil-ə-kon) is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbonSony VAIO VGN-CS27/R battery, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table. Controversy about silicon's character dates to its discovery: silicon was first prepared and characterized in pure form in 1824, and given the name silicium (from Latin: silicis, flints), with an -ium word-ending to suggest a metalSony VAIO VGN-CS27/P battery, a name which the element retains in several non-English languages. However, its final English name, suggested in 1831, reflects the more physically similar elements carbon and boron.

Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. Over 90% of the Earth's crust is composed of silicate mineralsSony VAIO VGN-CS27/C battery, making silicon the second most abundant element in the earth's crust (about 28% by mass) after oxygen.[5]

Most silicon is used commercially without being separated, and indeed often with little processing of compounds from nature. These include direct industrial building-use of clays, silica sand and stone. Silica is used in ceramic brick. Silicate goes into Portland cement for mortar and stucco, and when combined with silica sand and gravel, to make concreteSony VAIO VGN-CS26T/W battery. Silicates are also in whiteware ceramics such as porcelain, and in traditional quartz-based soda-lime glass. More modern silicon compounds such as silicon carbide form abrasives and high-strength ceramics. Silicon is the basis of the ubiquitous synthetic silicon-based polymers called silicones.

Elemental silicon also has a large impact on the modern world economy. Although most free silicon is used in the steel refiningSony VAIO VGN-CS26T/V battery, aluminum-casting, and fine chemical industries (often to make fumed silica), the relatively small portion of very highly purified silicon that is used in semiconductor electronics (< 10%) is perhaps even more critical. Because of wide use of silicon in integrated circuits, the basis of most computers, a great deal of modern technology depends on itSony VAIO VGN-CS26T/T battery.

Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals,[6] however various sea sponges need silicon in order to have structure. It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatomsSony VAIO VGN-CS26T/R battery.

Characteristics

[edit]Physical

Silicon crystallizes in a diamond cubic crystal structure

Further information: Monocrystalline silicon

Silicon is a solid at room temperature, with relatively high melting and boiling points of approximately 1,400 and 2,800 degrees Celsius respectively.[7] Interestingly, silicon has a greater density in a liquid state than a solid state. Therefore, it does not contract when it freezes like most substancesSony VAIO VGN-CS26T/Q battery, but expands, similar to how ice is less dense than water and has less mass per unit of volume than liquid water. With a relatively high thermal conductivity of 149 W·m−1·K−1, silicon conducts heat well and as a result is not often used to insulate hot objects.

In its crystalline form, pure silicon has a gray color and a metallic luster. Like germanium, silicon is rather strong, very brittle, and prone to chipping. Silicon, like carbon and germanium, crystallizes in a diamond cubic crystal structure, with a lattice spacing of approximately 0.5430710 nm (5.430710 Å). Sony VAIO VGN-CS26T/P battery

The outer electron orbital of silicon, like that of carbon, has four valence electrons. The 1s,2s,2p and 3s subshells are completely filled while the 3p subshell contains two electrons out of a possible six.

Silicon is a semiconductor. It has a negative temperature coefficient of resistance, since the number of free charge carriers increases with temperatureSony VAIO VGN-CS26T/C battery. The electrical resistance of single crystal silicon significantly changes under the application of mechanical stress due to the piezoresistive effect.[9]

[edit]Chemical

Silicon powder

Silicon is a metalloid, readily either donating or sharing its four outer electrons, allowing for many forms of chemical bonding. Even though it is relatively inert like carbon, silicon still reacts with halogens and dilute alkalis, but most acids (except for some hyper-reactive combinations of nitric acid and hydrofluoric acid) have no known effect on itSony VAIO VGN-CS25H battery. However, having four bonding electrons gives it, like carbon, many opportunities to combine with other elements or compounds under the right circumstances.

[edit]Isotopes

Main article: isotopes of silicon

Naturally occurring silicon is composed of three stable isotopes, silicon-28, silicon-29, and silicon-30, with silicon-28 being the most abundant (92% natural abundance).[10] Out of these, only silicon-29 is of use in NMR and EPR spectroscopy.[11] Twenty radioisotopes have been characterized, with the most stable being silicon-32 with a half-life of 170 yearsSony VAIO VGN-CS25H/W battery, and silicon-31 with a half-life of 157.3 minutes.[10] All of 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.[10] Silicon does not have any known nuclear isomers.[10]

The isotopes of silicon range in mass number from 22 to 44.[10] The most common decay mode of six isotopes with mass numbers lower than the most abundant stable isotope, silicon-28Sony VAIO VGN-CS25H/R battery, is β+

, primarily forming aluminium isotopes (13 protons) as decay products.[10] The most common decay mode(s) for 16 isotopes with mass numbers higher than silicon-28 is β−

, primarily forming phosphorus isotopes (15 protons) as decay products.[10]

[edit]Occurrence

Quartz crystal cluster from Tibet. The naturally-occurring mineral is a network solid with the formula SiO2Sony VAIO VGN-CS25H/Q battery.

See also: Silicate minerals

Measured by mass, silicon makes up 27.7% of the Earth's crust and is the second most abundant element in the crust, with only oxygen having a greater abundance.[12] Silicon is usually found in the form of complex silicate minerals, and less often as silicon dioxide (silica, a major component of common sand). Pure silicon crystals are very rarely found in natureSony VAIO VGN-CS25H/P battery.

The silicate minerals—various minerals containing silicon, oxygen and reactive metals—account for 90% of the mass of the Earth's crust. This is due to the fact that at the high temperatures characteristic of the formation of the inner solar system, silicon and oxygen have a great affinity for each other, forming networks of silicon and oxygen in chemical compounds of very low volatilitySony VAIO VGN-CS25H/C battery. Since oxygen and silicon were the most common non-gaseous and non-metallic elements in the debris from supernova dust which formed the protoplanetary disk in the formation and evolution of the Solar System, they formed many complex silicates which accreted into larger rocky planetesimals that formed the terrestrial planets. Here, the reduced silicate mineral matrix entrapped the metals reactive enough to be oxidized (aluminum, calcium, sodium, potassium and magnesium) Sony VAIO VGN-CS23T/W battery. After loss of volatile gases, as well as carbon and sulfur via reaction with hydrogen, this silicate mixture of elements formed most of the Earth's crust. These silicates were of relatively low density with respect to iron, nickel, and other metals non-reactive to oxygen and thus a residuum of uncombined iron and nickel sank to the planet's core, leaving a thick mantle consisting mostly of magnesium and iron silicates aboveSony VAIO VGN-CS23T/Q battery.

Examples of silicate minerals in the crust include those in the pyroxene, amphibole, mica, and feldspar groups. These minerals occur in clay and various types of rock such as granite and sandstone.

Silica occurs in minerals consisting of very pure silicon dioxide in different crystalline forms, quartz, agate amethyst, rock crystal, chalcedony, flint, jasper, and opal. The crystals have the empirical formula of silicon dioxideSony VAIO VGN-CS23H battery, but do not consist of separate silicon dioxide molecules in the manner of solid carbon dioxide. Rather, silica is structurally a network-solid consisting of silicon and oxygen in three-dimensional crystals, like diamond. Less pure silica forms the natural glass obsidian. Biogenic silica occurs in the structure of diatoms, radiolaria and siliceous spongesSony VAIO VGN-CS23H/S battery.

Silicon is also a principal component of many meteorites, and is a component of tektites, a silicate mineral of possibly lunar origin, or (if Earth-derived) which has been subjected to unusual temperatures and pressures, possibly from meteorite strike.

[edit]Production

Ferrosilicon alloy

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 siliconSony VAIO VGN-CS23H/B battery, providing 4.6 million tonnes (or 2/3 of the world output) of silicon, most of which is 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).[13] Ferrosilicon is primarily used by the steel industry (see below).

Aluminum-silicon alloys are heavily used in the aluminum alloy casting industry, where silicon is the single most important additive to aluminum to improve its casting propertiesSony VAIO VGN-CS23G battery. Since cast aluminum is widely used in the automobile industry, this use of silicon is thus the single largest industrial use of "metallurgical grade" pure silicon (as this purified silicon is added to pure aluminum, whereas ferrosilicon is never purified before being added to steel).[14]

[edit]Metallurgical grade

Elemental silicon not alloyed with significant quantities of other elements, and usually > 95% is often referred to loosely as silicon metalSony VAIO VGN-CS23G/W battery. It makes up about 20% of the world total elemental silicon production, with less than 1 to 2% of total elemental silicon (5–10% of metallurgical grade silicon) ever purified to higher grades for use in electronics. Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal in an electric arc furnace using carbon electrodesSony VAIO VGN-CS23G/Q battery. At temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and the silicon undergo the chemical reaction SiO2 + 2 C → Si + 2 CO. Liquid silicon collects in the bottom of the furnace, which 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) may also form from an excess of carbon in one or both of the following waysSony VAIO VGN-CS23G/P battery: SiO2 + C → SiO + CO or SiO + 2 C → SiC + CO. However, provided the concentration of SiO2 is kept high, the silicon carbide can be eliminated by the chemical reaction 2 SiC + SiO2 → 3 Si + 2 CO.

As noted above, metallurgical grade silicon "metal" has its primary use in the aluminum casting industry to make aluminum-silicon alloy parts. The remainder (about 45%) is used by the chemical industry, where it is primarily employed to make fumed silicaSony VAIO VGN-CS21Z/Q battery.[15]

As of September 2008, metallurgical grade silicon costs about US$1.45 per pound ($3.20/kg),[16] up from $0.77 per pound ($1.70/kg) in 2005.[17]

[edit]Electronic grade

Monocrystalline silicon ingot grown by the Czochralski process

The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Very pure silicon (>99.9%) can be extracted directly from solid silica or other silicon compounds by molten salt electrolysis. Sony VAIO VGN-CS21S/W battery This method, known as early as 1854[20] (see also FFC Cambridge process), has the potential to directly produce solar-grade silicon without any carbon dioxide emission at much lower energy consumption.

Solar grade silicon cannot be used for semiconductors, where purity must be extreme to properly control the process. Bulk silicon wafers used at the beginning of the integrated circuit making process must first be refined to "nine nines" purity (99.9999999%)Sony VAIO VGN-CS21S/V battery, a process which requires repeated applications of refining technology.

The majority of silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) since it is the cheapest method available and it is capable of producing large size crystals. However, single crystals grown by the Czochralski process contain impurities because the crucible containing the melt often dissolvesSony VAIO VGN-CS21S/T battery. Historically, a number of methods have been used to produce ultra-high-purity silicon.

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 IISony VAIO VGN-CS21S/R battery, 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 productSony VAIO VGN-CS21S/P battery.

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 itSony VAIO VGN-CS215J/R battery. 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. Sony VAIO VGN-CS215J/Q battery

A polycrystalline silicon rod made by the Siemens process

At one time, DuPont produced ultra-pure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950 °C, producing silicon by 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 processSony VAIO VGN-CS19/W battery. 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. Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of less than one part per billion. Sony VAIO VGN-CS19/R battery

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.[25] The advantage of fluid bed technology is that processes can be run continuously, yielding higher yields than Siemens Process, which is a batch processSony VAIO VGN-CS19/Q battery.

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 siliconSony VAIO VGN-CS19/P battery.

In addition, there is 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 produces no amorphous silicon dust waste, all of which are drawbacks of the Siemens processSony VAIO VGN-CS17H/W battery.[26] However, there are yet to be any major factories built using this process.

Compounds

PDMS – a silicone compound

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.[27]

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 armorSony VAIO VGN-CS17H/Q battery.

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.[28] 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) Sony VAIO VGN-CS16T/W battery. These are all readily hydrolyzed and are thermally unstable, particularly the heavier members.[29][30]

Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are generally

highly reactive requiring large substituent groups to stabilize them.[31] 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 alkynesSony VAIO VGN-CS16T/T battery.[32]

Tetrahalides, SiX4, are formed with all the halogens.[33] Silicon tetrachloride, for example, reacts with water, unlike its carbon analogue, carbon tetrachloride.[34] 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. Sony VAIO VGN-CS16T/R battery

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.[34] Silica is soluble in water at high temperatures forming a range of compounds called monosilicic acid, Si(OH)4. Sony VAIO VGN-CS16T/Q battery

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. Sony VAIO VGN-CS16T/P battery

With oxides of other elements the high temperature reaction of silicon dioxide can give a wide range of glasses with various properties.[36] 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).[37]

Silicon forms a nitride, Si3N4 which is a ceramic.[38] Silatranes, a group of tricyclic compounds containing five-coordinate silicon, may have physiological properties. Sony VAIO VGN-CS13T/W battery

Many transition metal complexes containing a metal-silicon bond are now known, which include complexes containing SiHnX3−n ligands, SiX3 ligands, and Si(OR)3 ligands.[39]

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. Sony VAIO VGN-CS13H/W battery

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.[41] Polycarbosilane, [(SiMe2)2CH2]n with a backbone containing a repeating -Si-Si-C unit, is a precursor in the production of silicon carbide fibers. Sony VAIO VGN-CS13H/R battery

[edit]History

Attention was first drawn to quartz as the possible oxide of a fundamental chemical element by Antoine Lavoisier, in 1787. 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 productSony VAIO VGN-CS13H/Q battery, nor identify it as a new element. In 1824, 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. He named the product silicium from the Latin silex, silicis for flint, flints, and adding the "-ium" ending because he believed it was a metalSony VAIO VGN-CS13H/P battery. As a result he is usually given credit for element's discovery.[42][43] Silicon was given its present name in 1831 by Scottish chemist Thomas Thomson. He retained part of Berzelius's name but added "-on" because he believed silicon a nonmetal more similar to boron and carbon.

Silicon in its more common crystalline form was not prepared until 31 years later, by Deville.[44] By electrolyzing impure sodium-aluminum chloride containing approximately 10% silicon, he was able to obtain a slightly impure allotrope of silicon in 1854Sony VAIO VGN-CS11Z/T battery.[45] Later, more cost-effective methods have been developed to isolate silicon in several allotrope forms, the most recent being silicene.

Because silicon is an important element in semiconductors and high-technology devices, many places in the world bear its name. For example, Silicon Valley in California, since it is the base for a number of technology-related industriesSony VAIO VGN-CS11Z/R battery, bears the name silicon. Other geographic locations with connections to the industry have since been named after silicon as well. 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, and Silicon Gorge in Bristol, EnglandSony VAIO VGN-CS11S/W battery.

Applications

Compounds

Most silicon is used industrially without being separated into the element, and indeed often with comparatively little processing from natural occurrence. 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 compoundsSony VAIO VGN-CS11S/Q battery, either as the silicate minerals or silica (crude silicon dioxide). For example, silica is an important part of ceramic brick. 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. Sony VAIO VGN-CS11S/P battery

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(Sony VAIO VGN-AW11M/H battery).

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 as silicones. These compounds containing silicon-oxygen and occasionally silicon-carbon bonds have the capability to act as bonding intermediates between glass and organic compounds(Sony VAIO VGN-AW11S/B battery), and to form polymers with useful properties such as impermeability to water, flexibility and resistance to chemical attack. 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(Sony VAIO VGN-AW11Z/B battery).[47] Silly Putty was originally made by adding boric acid to silicone oil.[48]

[edit]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(Sony VAIO VGN-AW170C battery), can be more closely controlled. 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(Sony VAIO VGN-AW19/Q battery). 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.[14] Silicon is an important constituent of electrical steel, modifying its resistivity and ferromagnetic properties.

Metallurgical grade silicon is silicon of 95–99% purity. About 55% of the world consumption of metallurgical purity silicon goes for production of aluminum-silicon alloys for aluminum part casts(Sony VAIO VGN-AW19 battery), mainly for use in the automotive industry. The reason for the high silicon use in these alloys is noted above.[15] Much of the rest of metallurgical-grade silicon is used by the chemical industry for production of the important industrial product fumed silica. The remainder is used in production of other fine chemicals such as silanes and some types of silicones(Sony VAIO VGN-AW21M/H battery).

[edit]Electronics

Main article: Semiconductor device fabrication

Silicon wafer with mirror finish

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). 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. (Sony VAIO VGN-AW21S/B battery) 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.

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 which may be released from atoms within the crystal by heat(Sony VAIO VGN-AW21VY/Q battery), and thus increase silicon's electrical conductance with higher temperatures. Pure silicon has too low a conductance to be used as a circuit element in electronics without being doped with small concentrations of certain other elements. This process greatly increases its conductivity and adjusts its electrical response by controlling the number and charge (positive or negative) (Sony VAIO VGN-AW21XY/Q battery) of activated carriers. Such control is necessary for transistors, solar cells, semiconductor detectors and 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(Sony VAIO VGN-AW21Z/B battery).

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 which 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(Sony VAIO VGN-AW230J/H battery), because of all the elements, silicon is the semiconductor which can withstand the highest powers and temperatures without becoming dysfunctional due to avalanche breakdown, a process in which an electron avalanche is created by a chain reaction process where heat produces free electrons and holes, which in turn produce more current which produces more heat(Sony VAIO VGN-AW235J/B battery). 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 type of fabrication techniques.[49]

Monocrystalline silicon is expensive to produce, and is usually only justified in production of integrated circuits, where tiny crystal imperfections can interfere with tiny circuit paths. For other uses, other types of pure silicon which do not exist as single crystals may be employed(Sony VAIO VGN-AW270Y/Q battery). These include hydrogenated amorphous silicon and upgraded metallurgical-grade silicon (UMG-Si) which are 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 which are either slightly less pure than those used in integrated circuits, or which are produced in polycrystalline rather than monocrystalline form(Sony VAIO VGN-AW31M/H battery), make up roughly similar amount of silicon as are produced for the monocrystalline silicon semiconductor industry, or 75,000 to 150,000 metric tons per year. However, production of such materials is growing more quickly than silicon for the integrated circuit market. By 2013 polycrystalline silicon production, used mostly in solar cells, is projected to reach 200,000 metric tons per year(Sony VAIO VGN-AW31S/B battery), while monocrystalline semiconductor silicon production (used in computer microchips) remains below 50,000 tons/year.[15]

[edit]Biological role

Silica skeletons of radiolaria in false color.

Although silicon is readily available in the form of silicates, very few organisms have a use for it. Diatoms, radiolaria and siliceous sponges use biogenic silica as a structural material to construct skeletons. In more advanced plants, the silica phytoliths (opal phytoliths) are rigid microscopic bodies occurring in the cell; some plants, for example rice(Sony VAIO VGN-AW41MF battery), need silicon for their growth.[50][51][52] Although silicon was proposed to be an ultra trace nutrient, its exact function in the biology of animals is still under discussion. Higher organisms are only known to use it in very limited occasions in the form of silicic acid and soluble silicates.

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) (Sony VAIO VGN-AW41XH/Q battery)."[53][54] Silicon has been shown in university and field studies to improve plant cell wall strength and structural integrity,[55] improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems.[56] 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(Sony VAIO VGN-AW41XH battery).[55]

Hypothetical silicon-based lifeforms are the subject of silicon biochemistry, by analogy with carbon-based lifeforms. Silicon, being below carbon in the periodic table, is thought to have similar enough properties that would make silicon-based life possible, but much different from life as we know it(Sony VAIO VGN-AW41ZF/B battery).

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