Silicon

Allotropic forms

Discovery and Naming

Silicon is an abundant element

Silicates

Silicones

Other uses of silicon

Resources

Silicon is the chemical element of atomic number 14, symbol Si and atomic weight 28.085. The seventh most abundant element in the universe, silicon is the second element in Group 14 of the periodic table. In its crystalline form of dark gray crystals, it has a specific gravity of 2.42 at 68°F (20°C), a melting point of 2,588°F (1,420°C) and a boiling point 5,936°F (3,280°C). It exists also in two amorphous (shapeless) forms, a brown powder and a black crystal. Silicon consists of three stable isotopes of mass numbers 28, 29 and 30.

Silicon, is a key component of microchips and microprocessors that allow the construction of inexpensive digital wristwatch to worldwide networks of computers. The conductive properties of silicon allow micro-devices to perform millions of calculations per second.

In terms of weight, silicon is the second most abundant element in the crust of Earth at 27.7%— second only to oxygen (46.6%). In rough terms, the Earth is essentially a spheroid of iron (the core) surrounded by layers (the mantle and the crust) of silicon and oxygen dominated compounds that include the other elements.

Earth was originally a molten ball of mostly iron, oxygen, silicon and aluminum that cooled. While still molten lighter atoms—including silicon and oxygen (atomic weights 28 and 16), moved outward from the core region, while the heavier iron atoms (atomic weight 56) dominated the central core. By about 3.5 billion years ago, the outermost layer had cooled to a crustal surface. The crustal composition is three-quarters oxygen and silicon.

Allotropic forms

Silicon exists in two allotropic forms, one of which consists of shiny, grayish black needle-like or crystal plates. The other allotrope is an amorphous brown powder. The melting point of the crystalline allotrope is about 2,570°F (1,410°C), its boiling point is about 4,270°F (2,355°C), and its density is 1.35 ounces per cubic inch (2.33 grams per cubic centimeter). It is a solid at room temperature. Silicon is a relatively hard element with a hardness of 7 on the Mohs scale. Classified as a semi-metal, silicon is a semiconductor, a property that determines some of its most important uses.

As its two allotropic forms might suggest, silicon is a metalloid. It is relatively inactive at room temperature, and resists attack by water and most acids. At higher temperatures, it reacts with many metals, oxygen, nitrogen, sulfur, phosphorus, and the halogens. It also forms a number of alloys in the molten state.

Discovery and Naming

The discovery of silicon as an element evaded chemists for many years because of the stability of most silicon compounds. Most chemists had little reason to believe that a new element existed in sand, silicates, and other earthy materials. Even if they did, scientists did not have the technology to extract the element from its compounds. One researcher with perhaps the greatest reason to hope for success in producing silicon was English chemist and physicist Sir Humphry Davy (1778–1829). Davy had developed a technique by which unusually stable compounds could be decomposed into their constituent elements. He used this method to prepare sodium, potassium, calcium, and other elements for the first time. Davy was unsuccessful, however, in producing silicon by the same method.

The first successful effort in the search for silicon was achieved by Swedish chemist Jons Jakob Berzelius (1779–1848). In 1823, Berzelius electrolyzed a molten mixture of potassium metal and potassium silicon fluoride (K₂SiF₆) and obtained a small sample of pure silicon: 4K + K₂SiF₆ —heat and electricity → 6KF + Si. The new element was named by Scottish chemist Thomas Thomson (1773–1852) because of the element’s presence in the mineral flint (silex or silicis in Latin ). He added the ending -on because of the element’s similarity to carbon.

Silicon is an abundant element

Silicon exists in the sun, stars, and in meteorites. It is found in plants and in animal bones. In Earth’s crust, there are at least 500 minerals—substances with definite chemical compositions and crystal forms. More than a third of these compounds contain silicon and oxygen.

Silicon and oxygen form silicon dioxide, SiO₂, (as known as silica). Sand is mostly silica with some contributions from shells and corals. When mixed with lime (calcium oxide, CaO), soda (sodium carbonate, Na₂CO₃) and trace substances, then melted in a furnace, silica become the key component of glass.

The purest form of silica, SiO₂, is quartz, a common mineral that is found as nearly colorless crystals. Slightly impure quartz makes crystals of amethyst (purple or violet), opal (translucent, milky) and agate (striped), all of which are prized for their aesthetic value.

Practically all the rocks and clays contain silicon and oxygen combined chemically with metallic elements in compounds called silicates. A common exception is limestone, which is calcium carbonate.

Silicates

The atoms of carbon can bond to each other to make long chains that include branches, and rings of carbon atoms onto which atoms of hydrogen and several other elements (including oxygen) can bond. The entire field of organic chemistry, with its millions of different organic compounds, is based on this ability of the carbon atom.

Silicon also has increased bonding abilities. On the periodic table, silicon is directly beneath carbon in group 14, which means that it, like carbon, has four electrons in its outermost shell that are available to share in chemical bonds with other elements. Like carbon, it can share those electrons with other silicon atoms. Because silicon atoms are about one and a half times larger in diameter that carbon atoms, however, the atoms can not pack as tightly and therefore can not to bond into long -Si-Si-Si-Si- chains that allow as much access as do carbon chains. Oxygen atoms can act as separators, or bridges, between the Si atoms to make -Si-O-Si-O-Si-O-Si- chains. Oxygen has a valence of two, and it can bond to two silicon atoms to bridge a chain. Such bridged structures open up the possibility of vast networks of silicon and oxygen based silicates.

The network in a quartz crystal consists of silicon and oxygen atoms. Each silicon atom is bonded to four oxygen atoms. Each silicon atom has only half possession of the four oxygen atoms surrounding it, so the overall formula is SiO2, not SiO4. Half of four oxygen atoms per silicon atom equal two oxygen atoms per silicon atom. In other silicate minerals, this network incorporates the presence of other atoms such as aluminum, iron, sodium, and potassium, that allow crystals to take on different shapes and properties.

Talc is a silicate mineral whose silicon and oxygen atoms are bonded together in sheets rather than in quartz-like three-dimensional solid crystals. These thin sheets can slide over one another. The low friction of talcum powder (ground-up talc) results from this sheet like configuration. Asbestos is a silicate mineral with silicon and oxygen atoms are bonded in long strings. Asbestos is therefore a mineral rock that can be shredded into fibers.

A silicate material widely used in industry is cement. Recent estimates place use of this cement at more than 100 million tons in the United States each year. Cement is manufactured from two minerals: clay or shale (both aluminum silicates) plus limestone (calcium carbonate, CaCO₃). These minerals are mixed, then heated together at a temperature of 2,732°F (1,500°C). At this temperature, the limestone converts to lime, CaO. The mixture is then cooled, and it is ground to a very fine, gray powder. When this cement powder is mixed with sand, gravel, and water, it sets into concrete. Accordingly, although the terms are sometimes inappropriately used synonymously, concrete is actually an aggregate material containing cement. Concrete is a very hard and strong material, largely because strong Si-O-Si bridges in the clay.

Silicones

Like silicates, silicones are a family of compounds held together by strong Si-O-Si bridges. But where silicates have two additional, non-bridging oxygen atoms attached to each silicon atom, the silicones have organic groups for example, two methyl groups, CH₃. The resulting (CH₃)₂SiO- groups can build up into long chains, just as the silicates. In contrast, however, are organic groups in the chains, that allow the compounds to resemble organic materials such as oils, greases, and rubbers.

As with organic compounds, a variety of silicone compounds can be composed of various-length silicon-oxygen chains with organic groups attached. The smaller molecules are the basis of silicone oils that, as with the all-organic petroleum oils, are used as lubricants, which resist decomposition at higher temperatures. Very large silicone molecules make silicone rubbers with high compression elasticity. These compounds are incorporated into ranging from super-bouncing balls to high impact bumpers. The first human footprint on the moon was made with a silicone-rubber-soled boot.

Between the oils and rubbers are hundreds of kinds of silicones that are used in electrical insulators, rust preventives, soaps, fabric softeners, hair sprays, hand creams, furniture and auto polishes, paints, adhesives, and chewing gum. Silicones are also used in surgical implants because they less prone that organic material to rejection by the immune system.

Other uses of silicon

On the periodic table, silicon lies on the borderline between the metals and nonmetals. Silicon is essentially a semi-metal (i.e., has some metallic properties such as metallic conductivity) that allows it to be used in semiconductor devices (i.e., silicon is a semiconductor). Thin slices of ultra-pure silicon crystals, generally known as chips, can have as many as half a million microscopic, interconnected electronic circuits etched into them. These circuits can act as electron gates and perform incredibly complex manipulations of voltages, that can be treated as binary numbers (e.g., voltage on = 1, voltage off = 0).

Silica gel is a porous form of silica, SiO₂, that absorbs water vapor from the air. In its most common form, silica gel is manufactured for use as a drying agent and small packages of silica gel are often packed with shipped products such as electronics that may be sensitive to moisture. Absorption by silica acts to maintain the humidity levels in a package as the package undergoes temperature changes.

Silicon carbide (SiC), is an extremely hard crystalline material, manufactured by fusing sand (SiO2) with coke (C) in an electric furnace at a temperature above 3,992°F (2,200°C). Silicon carbide, also known by its trade name, Carborundum, is often used as an abrasive, By attaching an ultrasonic impact grinder to a magnetostrictive transducer and using an abrasive liquid containing silicon carbide, holes of practically any shape can be drilled in hard, brittle materials such as tungsten carbide or precious stones.

Silicon based semiconductors are also used in the search for weapons of mass destruction, especially nuclear materials. The interactions of radiation with semiconducting crystals such as silicon can also be measured and semiconducting radiation detectors have the advantages of small size, high sensitivity, and high accuracy. Silicon chips also are key components of hand-held advanced nucleic acid analyzers (HANAA) that allow real-time polymerase chain reaction (PCR) based tests for pathogens (disease-causing organisms) that can be used by potential bioterrorists.

Silicon is also used in chips to which DNA (deoxyribonucleic acid) binds during hybridization procedures.

Resources

BOOKS

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford, UK: Oxford University Press, 2003.

Oxtoby, David W., et al. The Principles of Modern Chemistry. 5th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

PERIODICALS

Bennewitz, R., et al. “Atomic Scale Memory at a Silicon Surface.” Nanotechnology, 13 (2000): 499-502.

Cao, Y.W.C, R. Jin, C.A. Mirkin. “Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection.” Science, no. 5586 (2002): 1536-1540

Robert L. Wolke

K. Lee Lerner

SILICON (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Silicon is a member of Group 14 (IVA) in the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Silicon is also part of the the carbon family. Other carbon family elements include carbon, germanium, tin, and lead. Silicon is a metalloid, one of only a very few elements that have characteristics of both metals and non-metals.

Silicon is the second most abundant element in the Earth's crust, exceeded only by oxygen. Many rocks and minerals contain silicon. Examples include sand, quartz, clays, flint, amethyst, opal, mica, feldspar, garnet, tourmaline, asbestos, talc, zircon, emerald, and aquamarine. Silicon never occurs as a free element. It is always combined with one or more other elements as a compound.

SYMBOL
Si

ATOMIC NUMBER
14

ATOMIC MASS
28.0855

FAMILY
Group 14 (IVA)
Carbon

PRONUNCIATION
SIL-i-con

By the early 1800s, silicon was recognized as an element. But chemists had serious problems preparing pure silicon because it bonds (attaches) tightly to oxygen. It took chemists many years to find out how to separate silicon from oxygen. That task was finally accomplished in 1823 by Swedish chemist Jons Jakob Berzelius (1779-1848).

Silicon's most important application is in electronic equipment. Silicon is one of the best materials from which to make transistors and computer chips. The total weight of silicon used for this purpose is relatively small. Much larger amounts are used, for example, to make metal alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals.

Discovery and naming

In one sense, humans have always used silicon. Nearly every naturally occurring rock or mineral contains some silicon. So when ancient peoples built clay huts or sandstone temples, they were using compounds of silicon.

But no one thought about silicon as an element until the nineteenth century. Then, a number of chemists tried to separate silicon from the other elements with which it is combined in the earth. English scientist Sir Humphry Davy (1778-1829) developed a technique for separating elements that tightly bond to each other. He melted these compounds and passed an electric current through them. The technique was successful for producing free or elemental sodium, potassium, calcium, and a number of other elements for the first time. But he failed with silicon. (See sidebar on Davy in the calcium entry in Volume 1.)

Berzelius also tried to isolate silicon using a method similar to that of Davy's. He mixed molten (melted) potassium metal with a compound known as potassium silicon fluoride (K₂SiF₆). The result was a new element—silicon.

Scottish chemist Thomas Thomson (1773-1852) suggested the name silicon, based on the Latin word for "flint," silex (or silids). He added the ending -on because the new element was so much like boron and carbon. Thus, the new element's name was accepted as silicon.

Some interesting studies were done on silicon over the next few years. German chemist Friedrich Wohler (1800-82) produced a series of compounds known as silanes. These compounds contain silicon, hydrogen, and, sometimes, other elements. The simplest silane is silicon tetrahydride (SiH₄). This compound is also called silane.

A group of compounds known as the siloxanes were produced at about the same time. The siloxanes are made up of silicon, oxygen, and an organic group. Organic compounds contain carbon.

Silanes and siloxanes were not discovered in the search for the answer to any practical question. Chemists were just curious about the kinds of compounds they could make with silicon. But many years later, chemists made some interesting discoveries. Both groups of compounds do have some very important practical uses. For example, the compounds known as silicones are a form of the siloxanes.

Physical properties

Silicon is a metalloid, an element with properties of both metals and non-metals. Silicon exists in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope is in the form of shiny, grayish-black, needle-like crystals, or flat plates. The second allotrope has no crystal structure and usually occurs as a brown powder.

The melting point of silicon is 1,410°C (2,570°F) and the boiling point is 2,355°F (4,270°F). Its density is 2.33 grams per cubic centimeter. Silicon has a hardness of about 7 on the Mohs scale. The Mohs scale is a way of expressing the hardness of a material. It runs from 0 (for talc) to 10 (for diamond).

Silicon is a semiconductor. A semiconductor is a substance that conducts an electric current better than a non-conductor—like glass or rubber—but not as well as a conductor—like copper or aluminum. Semiconductors have important applications in the electronics industry.

Chemical properties

Silicon is a relatively inactive element at room temperature. It does not combine with oxygen or most other elements. Water, steam, and most acids have very little affect on the element. At higher temperatures, however, silicon becomes much more reactive. In the molten (melted) state, for example, it combines with oxygen, nitrogen, sulfur, phosphorus, and other elements. It also forms a number of alloys very easily in the molten state.

Occurrence in nature

Silicon is the second must abundant element in the Earth's crust. Its abundance is estimated to be about 27.6 percent of the crust. It ranks second only to oxygen. Some authorities believe that more than 97 percent of the crust is made of rocks that contain compounds of silicon and oxygen.

Silicon has been detected in the Sun and stars. It also occurs in certain types of meteorites known as aerolites or "stony meteorites." Meteorites are rock-like chunks that fall to the Earth's surface from outside the Earth's atmosphere.

Silicon never occurs as a free element in nature. It always occurs as a compound with oxygen, magnesium, calcium, phosphorus, or other elements. The most common minerals are those that contain silicon dioxide in one form or another. These are known as silicates.

Silicon has been detected in the Sun and stars. It also occurs in certain types of meteorites.

Isotopes

There are three naturally occurring isotopes of silicon: silicon-28, silicon-29, and silicon-30. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Five radioactive isotopes of silicon are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

None of the radioactive isotopes of silicon has any commercial use.

Extraction

Silicon is prepared by heating silicon dioxide with carbon. Carbon replaces the silicon in the compound. The silicon formed is 96 to 98 percent pure.

Many applications of silicon require a very pure product. Methods have been developed to produce silicon that is at least 99.97 percent pure silicon. This form of silicon is called hyper-pure silicon.

Uses

Perhaps the best known use of silicon is in electronic devices. Hyperpure silicon is used in transistors and other components of electronic devices. It is also used to make photovoltaic (solar) cells, rectifiers, and parts for computer circuits. A photovoltaic cell is a device that converts sunlight into electrical energy. A rectifier is an electrical device for changing one kind of electric current (alternating current, or AC) into another kind of electric current (direct current, or DC).

Almost without exception, all glass contains silicon dioxide.

The largest single use of silicon, however, is in making alloys. The most important silicon alloys are those made with iron and steel, aluminum, and copper. When silicon is produced, in fact, scrap iron and metal is sometimes added to the furnace. As soon as the silicon is produced, it reacts with iron and steel to form ferrosilicon. Ferrosilicon is an alloy of iron or steel and silicon. It is used for two major purposes. First, it can be added to steel to improve the strength and toughness of the steel. Second, it can be added during the steel-making process to remove impurities from the steel that is being made.

The aluminum industry uses large amounts of silicon in alloys. These alloys are used to make molds and in the process of welding. Welding is a process by which two metals are joined to each other. Alloys of silicon, aluminum, and magnesium are very resistant to corrosion (rusting). They are often used in the construction of large buildings, bridges, and transportation vehicles such as ships and trains.

Compounds

A number of silicon compounds have important uses. Silicon dioxide (sand) is used in the manufacture of glass, ceramics, abrasives, as a food additive, in water filtration systems, as an insulating material, in cosmetics and Pharmaceuticals (drugs), and in the manufacture of paper, rubber, and insecticides. Each of these applications could be the subject of a very long discussion in and of itself. For example, humans have made glass for thousands of years. Today, dozens of different kinds of glass are produced, each with special properties and uses. But almost without exception, they all contain silicon dioxide.

Another important compound is silicon carbide (SiC). Silicon carbide is also known as carborundum. It is one of the hardest substances known, with a hardness of about 9.5 on the Mohs scale. Carborundum is widely used as an abrasive, a powdery material used to grind or polish other materials. Carborundum also has refractory properties. A refractory material can withstand very high temperatures by reflecting heat. Refractory materials are used to line the inside of ovens used to maintain very high temperatures.

Another important silicon group is the silicones. The silicones have an amazing range of uses. These include toys (Silly Putty and Superballs), lubricants, weatherproof!ng materials, adhesives (glues), foaming agents, brake fluids, cosmetics, polishing agents, electrical insulation, materials to reduce vibration, shields for sensitive equipment, surgical implants, and parts for automobile engines.

Health effects

Information on the health effects of silicon is limited. Some studies show that silicon may be needed in very small amounts by plants and some animals. One study showed, for example, that chickens that did not receive silicon in their diet developed minor health problems. Overall, silicon probably has no positive or negative effects on human health.

However, a serious health problem called silicosis is associated with silicon dioxide (SiO₂). Silicon dioxide occurs in many forms in the earth. Ordinary sand is nearly pure silicon dioxide.

In some industries, sand is ground up into a very fine powder that gets into the air. As workers inhale the dust, it travels through their mouths, down their throats, and into their lungs. Silicon dioxide powder can block the tiny air passages in the lungs through which oxygen and carbon dioxide pass. When this happens, silicosis results.

Silicosis is similar to pneumonia. The person finds it difficult to breathe. The longer one is exposed to silicon dioxide dust, the worst the problem gets. In the worst cases, silicosis results in death because of the inability to breathe properly.

Silicon

Silicon is the chemical element of atomic number 14, symbol Si and atomic weight 28.085. In its crystalline form of dark gray crystals, it has a specific gravity of 2.42 at 68°F (20°C), a melting point of 2,588°F (1,420°C) and a boiling point 5,936°F (3,280°C). It exists also in an amorphous (shapeless) form, a brown powder. Silicon consists of three stable isotopes of mass numbers 28, 29 and 30.

Silicon, is a key component of microchips and microprocessors that allow the construction of inexpensive digital wristwatch to worldwide networks of computers. The conductive properties of silicon allow micro-devices to perform millions of calculations per second.

In terms of weight, silicon is the second most abundant element in the crust of Earth at 27.7%—second only to oxygen (46.6%). In rough terms, Earth is essentially a spheroid of iron (the core) surrounded by layers (the mantle and the crust) of silicon and oxygen dominated compounds that include the other elements.

Earth was originally a molten ball of mostly iron, oxygen, silicon and aluminum that cooled. While still molten lighter atoms—including silicon and oxygen (atomic weights 28 and 16), moved outward from the core region, while the heavier iron atoms (atomic weight 56) dominated the central core. By about 3.5 billion years ago, the outermost layer had cooled to a crustal surface. The crustal composition is three-quarters oxygen and silicon.

Silicon is an abundant element

Silicon exists in the Sun, stars, and in meteorites. It is found in plants and in animal bones. In the Earth's crust, there are at least 500 minerals—substances with definite chemical compositions and crystal forms. More than a third of these compounds contain silicon and oxygen.

Silicon and oxygen form silicon dioxide, SiO2, (as known as silica). Sand is mostly silica with some contributions from shells and corals. When mixed with lime (calcium oxide, CaO), soda (sodium carbonate, Na2CO3) and trace substances, then melted in a furnace, silica become the key component of glass .

The purest form of silica, SiO2, is quartz, a common mineral that is found as nearly colorless crystals. Slightly impure quartz makes crystals of amethyst (purple or violet), opal (translucent, milky) and agate (striped), all of which are prized for their aesthetic value.

Practically all the rocks and clays contain silicon and oxygen combined chemically with metallic elements in compounds called silicates. A common exception is limestone, which is calcium carbonate.

Silicates

The atoms of carbon can bond to each other to make long chains that include branches, and rings of carbon atoms onto which atoms of hydrogen and several other elements (including oxygen) can bond. The entire field of organic chemistry , with its millions of different organic compounds, is based on this ability of the carbon atom.

Silicon also has increased bonding abilities. On the periodic table, silicon is directly beneath carbon in group 14, which means that it, like carbon, has four electrons in its outermost shell that are available to share in chemical bonds with other elements. Like carbon, it can share those electrons with other silicon atoms. Because silicon atoms are about one and a half times larger in diameter that carbon atoms, however, the atoms can not pack as tightly and therefore can not to bond into long -Si-Si-Si-Si-chains that allow as much access as do carbon chains. Oxygen atoms can act as separators, or bridges, between the Si atoms to make -Si-O-Si-O-Si-O-Si-chains. Oxygen has a valence of two, and it can bond to two silicon atoms to bridge a chain. Such bridged structures open up the possibility of vast networks of silicon and oxygen based silicates.

The network in a quartz crystal consists of silicon and oxygen atoms. Each silicon atom is bonded to four oxygen atoms. Each silicon atom has only half possession of the four oxygen atoms surrounding it, so the overall formula is SiO2, not SiO4. Half of four oxygen atoms per silicon atom equal two oxygen atoms per silicon atom. In other silicate minerals, this network incorporates the presence of other atoms such as aluminum, iron, sodium, and potassium, that allow crystals to take on different shapes and properties.

Talc is a silicate mineral whose silicon and oxygen atoms are bonded together in sheets rather than in quartz-like three-dimensional solid crystals. These thin sheets can slide over one another. The low friction of talcum powder (ground-up talc) results from this sheet like configuration. Asbestos is a silicate mineral with silicon and oxygen atoms are bonded in long strings. Asbestos is therefore a mineral rock that can be shredded into fibers.

A silicate material widely used in industry is cement. Recent estimates place use of this cement at more than 100 million tons of in the United States each year. Cement is manufactured from two minerals: clay or shale (both aluminum silicates) plus limestone (calcium carbonate, CaCO3). These minerals are mixed, then heated together at a temperature of 2,732°F (1,500°C). At this temperature the limestone converts to lime, CaO. The mixture is then cooled and ground to a very fine, gray powder. When this cement powder is mixed with sand, gravel, and water, it sets into concrete. Accordingly, although the terms are sometimes inappropriately used synonymously, concrete is actually an aggregate material containing cement. Concrete is a very hard and strong material, largely because strong Si-O-Si bridges in the clay.

Silicones

Like silicates, silicones are a family of compounds held together by strong Si-O-Si bridges. But where silicates have two additional, non-bridging oxygen atoms attached to each silicon atom, the silicones have organic groups-for example, two methyl groups, CH3. The resulting (CH3)2SiO-groups can build up into long chains, just as the silicates. In contrast, however, are organic groups in the chains, that allow the compounds to resemble organic materials such as oils, greases, and rubbers.

As with organic compounds, a variety of silicone compounds can be composed of various-length silicon-oxygen chains with organic groups attached. The smaller molecules are the basis of silicone oils that, as with the all-organic petroleum oils, are used as lubricants but which better resist decomposition at higher temperatures. Very large silicone molecules make silicone rubbers with high compression elasticity. These compounds are incorporated into ranging from super-bouncing balls to high impact bumpers. The first human footprint on the moon was made with a silicone-rubber-soled boot.

Between the oils and rubbers are hundreds of kinds of silicones that are used in electrical insulators, rust preventives, soaps, fabric softeners, hair sprays, hand creams, furniture and auto polishes, paints, adhesives, and chewing gum. Silicones are also used in surgical implants because they less prone that organic material to rejection by the immune system.

Other uses of silicon

On the periodic table , silicon lies on the borderline between the metals and nonmetals. Silicon is essentially a semi-metal (i.e., has some metallic properties such as metallic conductivity) that allows it to be used in semi-conductor devices (i.e., silicon is a semiconductor). Thin slices of ultra-pure silicon crystals, generally known as chips, can have as many as half a million microscopic, interconnected electronic circuits etched into them. These circuits can act as electron gates and perform incredibly complex manipulations of voltages, that can be treated as binary numbers (e.g., voltage on = 1, voltage off = 0).

Silica gel is a porous form of silica, SiO2, that absorbs water vapor from the air. In its most common form, silica gel is manufactured for use as a drying agent and small packages of silica gel are often packed with shipped products such as electronics that may be sensitive to moisture. Absorption by silica acts to maintain the humidity levels in a package as the package undergoes temperature changes.

Silicon carbide (SiC), is an extremely hard crystalline material, manufactured by fusing sand (SiO2) with coke (C) in an electric furnace at a temperature above 3,992°F (2,200°C). Silicon carbide, also known by its trade name, Carborundum, is often used as an abrasive, By attaching an ultrasonic impact grinder to a magnetostrictive transducer and using an abrasive liquid containing silicon carbide, holes of practically any shape can be drilled in hard, brittle materials such as tungsten carbide or precious stones.

Silicon based semiconductors are also used in the search for weapons of mass destruction, especially nuclear materials. The interactions of radiation with semi-conducting crystals such as silicon can also be measured and semiconducting radiation detectors have the advantages of small size, high sensitivity, and high accuracy. Silicon chips also are key components of hand-held advanced nucleic acid analyzers (HANAA) that allow real-time polymerase chain reaction (PCR) based tests for pathogens (disease-causing organisms) that can be used by potential bioterrorists.

Silicon is also used in chips to which DNA binds during hybridization procedures.

Resources

books

Oxtoby, David W., et al. The Principles of Modern Chemistry. 5th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

periodicals

Bennewitz, R., et al. "Atomic Scale Memory at a Silicon Surface." Nanotechnology, 13 (2000): 499–502.

Cao, Y.W.C., R. Jin, C.A. Mirkin. "Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection." Science no. 5586 (2002): 1536–1540

other

Ronald Koopman, et al. "HANAA: Putting DNA Identification in the Hands of First Responders" [cited 15 January 2003]. <http://coffee.phys.unm.edu/BTR/2001%20Conference/pdf/Koopman_Ronald.pdf>.

Robert L. Wolke
K. Lee Lerner

Silicon

Background

Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates. Silicon's compounds are also found in water, in the atmosphere, in many plants, and even in certain animals.

Silicon is the fourteenth element of the periodic table and is a Group IVA element, along with carbon germanium, tin and lead. Pure silicon is a dark gray solid with the same crystalline structure as diamond. Its chemical and physical properties are similar to this material. Silicon has a melting point of 2570° F (1410° C), a boiling point of 4271° F (2355° C), and a density of 2.33 g/cm3.

When silicon is heated it reacts with the halogens (fluorine, chlorine, bromine, and iodine) to form halides. It reacts with certain metals to form silicides and when heated in an electric furnace with carbon, a wear resistant ceramic called silicon carbide is produced. Hydrofluoric acid is the only acid that affects silicon. At higher temperatures, silicon is attacked by water vapor or by oxygen to form a surface layer of silicon dioxide.

When silicon is purified and doped with such elements as boron, phosphorus and arsenic, it is used as a semiconductor in various applications. For maximum purity, a chemical process is used that reduces silicon tetrachloride or trichlorosilane to silicon. Single crystals are grown by slowly drawing seed crystals from molten silicon.

Silicon of lower purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, alumiinum, and bronze. When small amounts of silicon are added to aluminum, aluminum becomes easier to cast and also has improved strength, hardness, and other properties. In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents.

In 1999, world production was around 640,000 metric tons (excluding China), with Brazil, France, Norway and the United States major producers. This is a continued decline compared to the last several years (653,000 tons in 1998 and 664,000 in 1997). Though data is not available, China is believed to be the largest producer, followed by the United States. One estimate puts China's production capacity as high as 400,000 metric tons per year, with over 400 producers. Exports from this country have increased in recent years.

Consumption of silicon metal in the United States was roughly 262,000 metric tons, at a cost of 57 cents per pound. The annual growth rate during 1980-1995 was about 3.5% for silicon demand by the aluminum industry and about 8% by the chemical industry. Demand by the chemical industry (mainly silicones) was affected by the Asian economic crisis of the late 1990s.

History

Silicon was first isolated and described as an element in 1824 by a Swedish chemist, Jons Jacob Berzelius. An impure form was obtained in 1811. Crystalline silicon was first produced in 1854 using electrolysis.

The type of furnace now used to make silicon, the electric arc furnace, was first invented in 1899 by French inventor Paul Louis Toussaint Heroult to make steel. The first electric arc furnace in the United States was installed in Syracuse, New York in 1905. In recent years, furnace technology, including the electrodes used for heating elements, has improved.

Raw Materials

Silicon metal is made from the reaction of silica (silicon dioxide, SiO2) and carbon materials like coke, coal and wood chips. Silica is typically received in the form of metallurgical grade gravel. This gravel is 99.5% silica, and is 3 x 1 or 6 x 1 in (8 x 3 cm or 15 x 3 cm) in size. The coal is usually of low ash content (1-3% to minimize calcium, aluminum, and iron impurities), contains around 60% carbon, and is sized to match that of the gravel. Wood chips are usually hardwood of 1/2 x 1/8 inch size (1 x. 3 cm size). All materials are received as specified by the manufacturer.

The Manufacturing Process

The basic process heats silica and coke in a submerged electric arc furnace to high temperatures. High temperatures are required to produce a reaction where the oxygen is removed, leaving behind silicon. This is known as a reduction process. In this process, metal carbides usually form first at the lower temperatures. As silicon is formed, it displaces the carbon. Refining processes are used to improve purity.

The Reduction Process

• 1 The raw materials are weighed and then placed into the furnace through the top using the fume hood, buckets, or cars. A typical batch contains 1000 lb (453 kg) each of gravel and chips, and 550 lb (250 kg) of coal. The lid of the furnace, which contains electrodes, is placed into position. Electric current is passed through the electrodes to form an arc. The heat generated by this arc (a temperature of 4000° F or 2350 ° C) melts the material and results in the reaction of sand with carbon to form silicon and carbon monoxide. This process takes about six to eight hours. The furnace is continuously charged with the batches of raw materials.
• 2 While the metal is in the molten state, it is treated with oxygen and air to reduce the amount of calcium and aluminum impurities. Depending on the grade, silicon metal contains 98.5-99.99% silicon with trace amounts of iron, calcium and aluminum.

Cooling/Crushing

• 3 Oxidized material, called slag, is poured off into pots and cooled. The silicon metal is cooled in large cast iron trays about 8 ft (2.4 m) across and 8 in (20 cm) deep. After cooling, the metal is dumped from the mold into a truck, weighed and then dumped in the storage pile. Dumping the metal from the mold to the truck breaks it up sufficiently for storage. Before shipping, the metal is sized according to customer specifications, which may require a crushing process using jaw or cone crushers.

Packaging

• 4 Silicon metal is usually packaged in large sacks or wooden boxes weighing up to 3,000 lb (1,361 kg). In powder form, silicon is packaged in 50-lb (23-kg) plastic pails or paper bags, 500-lb (227-kg) steel drums or 3,000-lb (1,361-kg) large sacks or boxes.

Quality Control

Statistical process control is used to ensure quality. Computer-controlled systems are used to manage the overall process and evaluate statistical data. The two major process parameters that must be controlled are amounts of raw materials used and furnace temperatures. Laboratory testing is used to monitor the chemical composition of the final product and to research methods to improve the composition by adjusting the manufacturing process. Quality audits and regular assessments of suppliers also ensure that quality is maintained from extraction of raw materials through shipping of the final product.

Byproducts/Waste

With statistical process control, waste is kept to a minimum. A byproduct of the process, silica fume, is sold to the refractory and cement industries to improve strength of their products. Silica fume also is used for heat insulation, filler for rubber, polymers, grouts and other applications. The cooled slag is broken down into smaller pieces and sold to other companies for further processing. Some companies crush it into sandblasting material. Because electric arc furnaces emit particulate emissions, manufacturers must also comply with the Environmental Protection Agency's (EPA) regulations.

The Future

Though industry analysts predicted demand for chemical-grade silicon by Western countries would increase at an annual average rate of about 7% until 2003, this growth may be slower due to recent economic declines in Asia and Japan. If supplies continue to outpace demand, prices may continue to drop. The outlook for the automotive market is positive, as more car makers switch to an aluminum-silicon alloy for various components.

Other methods for making silicon are being investigated, including supercooling liquid to form bulk amorphous silicon and a hydrothermal method for making porous silicon powder for optical applications.

Where to Learn More

Books

Kirk-Othmer. Encyclopedia of Chemical Technology. New York: John Wiley & Sons, Inc. 1985.

Periodicals

Bendix, Jeffrey. "The Heart of Globe is in Cleveland." Cleveland Enterprise (Fall 1991).

Ward, Patti. "Heroult Electric Arc Furnace Stands the Test of Time." Iron and Steelmaker 26, no. 11 (November 1999). http://www.issource.org/magazine/Web/9911/Ward-9911.htm.

Other

Annual Minerals Review: Silicon. U.S. Geological Survey, 1998.

Mineral Commodity Summaries: Silicon. U.S. Geological Survey, February 2000.

Mineral Industry Surveys: Silicon in February 2000. U.S. Geological Survey, May 2000.

—LaurelM.Sheppard

Silicon

melting point: 1,410°C
boiling point: 2,355°C
density: 2.329 g/cm³
most common ions: Si⁴⁺, H₃SiO₄⁻, H₂SiO₄²⁻, HSiO₄³⁻, SiO₄⁴⁻

Silicon is the second most abundant element in Earth's crust and mantle, after oxygen. It is the seventh most abundant element in the universe. It was first obtained in elemental form by Jöns Jakob Berzelius in 1823, from reduction of a complex fluoride, K₂SiF₆, by potassium. It has a strong chemical affinity for electronegative elements such as oxygen and fluorine. It is always found in nature in bound form, as the oxide, SiO₂, or in silicate minerals such as olivine, (Mg, Fe)₂SiO₄. The principal hydride is silane (SiH₄), a pyrophoric gas, and the halides (e.g., SiF₄, SiCl₄) are gases or liquids. Silicate minerals form in a wide variety of crystallographic structures, i.e., a mineral's internal (repeating) structure; many of these minerals are important ceramics (e.g., heated mica produces clays used in pottery; calcium silicates are the primary components of cement). Amorphous (noncrystalline) silicates form glasses used in windows and containers. Silicon nitride, Si₃N₄, is an important ceramic used in turbine engines. Silicon carbide, SiC, is an ultrahard solid used in abrasives.

Silicon is extracted from quartz (SiO₂) sand by high temperature reduction with carbon. The crystalline element is a shiny gray semiconducting solid with the tetrahedral diamond structure. Usually doped with boron, arsenic , or phosphorus, it conducts electricity via the diffusion of electrons or positive "holes" (electron vacancies). Polymorphs of silicon formed under high pressure conditions are metallic and contain octahedral silicon; the liquid is metallic also. Amorphous silicon formed by vapor phase decomposition of silane is used to transform light into electricity in solar cells. Silicones (organic silicon compounds containing Si-O-Si linkages) are prepared from silane, silicon halides, or metal -Si alloys by hydrolysis of the silicone material to give oils, greases, synthetic rubbers, and adhesives.

see also Semiconductors; Solar Cells.

Paul F. McMillan

Bibliography

Greenwood, N. N., and Earnshaw, A. (1984). Chemistry of the Elements. New York: Pergamon Press.

Iler, Ralph K. (1979). The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. New York: Wiley.

Liu, Lin-gun, and Bassett, William A. (1986). Elements, Oxides, and Silicates: High-Pressure Phases with Implications for the Earth's Interior. New York: Oxford University Press.

Riordan, Michael, and Hoddeson, Lillian (1997). Crystal Fire: The Birth of the Information Age. New York: Norton.