Semiconductors Category: Products from resources Where Found Semiconductor materials are found all over the world.. The most frequently used semiconductor ma-terials are composed of crys
Trang 1evaporation basin for the drainage waters of the
west-ern San Joaquin Valley Originally this was surface
water, but by 1981 almost all the water entering the
reservoir was subsurface agricultural drainage water
from irrigated agricultural fields Because of interest
in saving some of Northern California’s disappearing
wetlands, water that entered the reservoir was
di-verted and used to preserve wetlands in the adjacent
Kesterson National Wildlife Refuge in Merced County,
California
By 1983 the incidence of embryo deformity and
mortality among aquatic birds nesting in the
Kesterson Reservoir was alarmingly high No one
im-mediately suspected, when the drainage water was
used in the wetlands, that it contained almost 4.2
mil-ligrams of selenium per liter—a selenium
concentra-tion one thousand times greater than in naturally
oc-curring drainage in the region As a consequence,
phytoplankton in the reservoir accumulated
sele-nium to levels 100 to 2,600 times greater than normal
Since these plankton formed the base of the food
chain in the reservoir, the levels of selenium in the
fish, frogs, snakes, birds, and mammals also increased
to levels 12 to 120 times greater than normal (20 to
170 milligrams of selenium per kilogram) Migratory
birds that fed on plants, invertebrates, and fish in the
reservoir contained up to 24 times the normal level of
selenium in their tissue Between 1983 and 1985 an
es-timated one thousand migratory birds died as a
conse-quence of selenium toxicity To protect the migratory
birds from future selenium exposure, the reservoir
was drained in 1988 and filled with dirt, effectively
burying and isolating the excess selenium
Selenium is a good demonstration of the adage
“the dose is the poison.” Trace quantities of selenium
are nutritionally essential, and blood concentrations
of 0.1 milligram of selenium per liter are nutritionally
sound The minimum lethal concentration of
sele-nium in tissue, however, is only 1.5 to 3.0 milligrams of
selenium per kilogram of body weight Symptoms of
toxicity may occur when dietary intake exceeds 4
mil-ligrams per kilogram of body weight Selenium
toxic-ity leads to the syndromes known as alkali disease and
blind stagger On the other end of the scale,
symp-toms of deficiency may appear if dietary intake is less
than 0.04 milligram of selenium per kilogram of body
weight Selenium deficiency leads to a syndrome
known as white muscle disease In mammals,
includ-ing humans, selenium is an essential component of
the enzyme glutathione peroxidase, found in red
blood cells Glutathione peroxidase is an antioxidant;
it protects tissues against oxidation by destroying hy-drogen peroxide or organic hydroperoxides
History
In 1817, selenium was purified and identified by Jöns Jacob Berzelius However, its environmental influ-ences, particularly its toxic effects, have been known for much longer Marco Polo, for example, described unmistakable signs of selenium toxicity in horses, cat-tle, sheep, and humans during his travels across China
in 1295 Selenium toxicity was described in Colombia
in 1560, in South Dakota in 1857, and in Wyoming in
1908 Selenium was specifically identified as the cause
of the toxicity in alkaline soils in the western United States in 1929 Its essential role in animal nutrition was identified in the 1950’s In the mid-1980’s, the toxic effects of selenium were once more advertised when it was discovered to be the cause of widespread bird mortality at the Kesterson National Wildlife Ref-uge in Northern California
Obtaining Selenium There are no known commercially usable selenium deposits, and the concentration of selenium in soil and water is too dilute to be of economic significance Consequently, most selenium is a by-product extracted from more abundant materials in which it is a contam-inant, particularly during the refining of ores contain-ing metal sulfides such as chalcopyrite Most of the annual selenium production comes from the waste sludge produced during the electrolytic refining of copper
Uses of Selenium Selenium’s industrial uses are varied The principal use is in the glass industry, where it is used to prevent discoloration of glass by iron oxides Ammonium sele-nite is also used as a pigment in making red glass Sele-nium diethyldithiocarbamate is used as a fungicide, but more important, it is used as a vulcanizing agent
by the rubber industry to increase wear resistance Selenium is also incorporated into plastics and paints because it improves resistance to heat, light, weather-ing, and chemical action Selenium’s antioxidant properties cause it to be included in inks, mineral and vegetable oils, and lubricants Cadmium selenide is found in photoelectric cells and photoconductors In addition to its use as a dietary supplement, selenium is used in pharmaceutical remedies for eczema, fungal
Trang 2infections, and dandruff Selenium also plays a
nutritional role and is incorporated into
di-etary supplements for animals, including
hu-mans, although too much selenium in the diet
can have deleterious effects
Mark S Coyne
Further Reading
Adriano, Domy C “Selenium.” In Trace
Ele-ments in Terrestrial EnvironEle-ments:
Biogeochem-istry, Bioavailability, and Risks of Metals 2d ed.
New York: Springer, 2001
Ehrlich, Henry Lutz, and Dianne K Newman
Geomicrobiology 5th ed Boca Raton, Fla.:
CRC Press, 2009
Frankenberger, William T., Jr., and Sally
Ben-son, eds Selenium in the Environment New
York: Marcel Dekker, 1994
Frankenberger, William T., Jr., and Richard A
Engberg, eds Environmental Chemistry of
Sele-nium New York: Marcel Dekker, 1998.
Greenwood, N N., and A Earnshaw
“Sele-nium, Tellurium, and Polonium.” In
Chemis-try of the Elements 2d ed Boston:
Butterworth-Heinemann, 1997
Jacobs, L W., ed Selenium in Agriculture and the
Environment: Proceedings of a Symposium.
Madison, Wis.: American Society of Agronomy, Soil
Science Society of America, 1989
Massey, A G “Group 16: The Chalcogens—Oxygen,
Sulfur, Selenium, Tellurium, and Polonium.” In
Main Group Chemistry 2d ed New York: Wiley, 2000.
Rosenfeld, Irene, and Orville A Beath Selenium:
Geobotany, Biochemistry, Toxicity, and Nutrition New
York: Academic Press, 1964
Surai, Peter F Selenium in Nutrition and Health
Not-tingham, England: Nottingham University Press,
2006
Web Sites
Natural Resources Canada
Canadian Minerals Yearbook, Mineral and Metal
Commodity Reviews
http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
U.S Geographical Survey
Selenium and Tellurium: Statistics and Information
http://minerals.usgs.gov/minerals/pubs/
commodity/selenium
See also: Food chain; Groundwater; Igneous pro-cesses, rocks, and mineral deposits; Irrigation; Leaching; Soil; Wetlands
Semiconductors
Category: Products from resources
Where Found Semiconductor materials are found all over the world The most frequently used semiconductor ma-terials are composed of crystalline inorganic solid ele-ments found in nature, with silicon the most common semiconductor material Standardized semiconduc-tor crystals are grown in laborasemiconduc-tories, with the global semiconductor industry dominated by Taiwan, South Korea, the United States, and Japan
Primary Uses Semiconductors form the basis for how modern tech-nology operates Many different types of
Glass manufacturing 35%
Chemicals
& pigments 20%
Electronics
& photocopier components 12%
Other 33%
Source:
Historical Statistics for Mineral and Material Commodities in the United States
U.S Geological Survey, 2005, selenium statistics, in T D Kelly and G R Matos, comps.,
, U.S Geological Survey Data Series 140 Available online at http://pubs.usgs.gov/ds/ 2005/140/.
U.S End Uses of Selenium
Trang 3tor devices, including radios, diodes,
microproces-sors, computer chips, cellular phones, and power
grids, utilize semiconductor materials Integrated
cir-cuits comprise numerous interconnected
semicon-ductors Current “smart” technology products
com-bine integrated circuits with power semiconductor
technology
Technical Definition
Semiconductors are special materials that conduct
differently under different conditions and are
fre-quently silicon-based Semiconductors can act as a
nonconductor or a conductor, depending on the
po-larity of electrical charge applied to it, thus leading to
the term “semiconductor.” A number of elements are
classified as semiconductors, including silicon, zinc,
and germanium Other materials include gallium
ar-senide and silicon carbide Because silicon is readily
obtained, it is the most widely used semiconductor
material These compounds have the ability to
con-duct electrical current and can be regulated in the
amount of their conductivity Semiconductor devices
operate by utilizing electronic properties of
semicon-ductor materials
Description, Distribution, and Forms
Semiconductor material takes advantage of the
move-ment of electrons between materials with varied
con-ductive properties Semiconductors, special materials
that are frequently silicon-based, have varying
electri-cal conductivity properties depending on specific
conditions Electrical resistance properties of
semi-conductor materials fall somewhere between those of
a conductor and those of an insulator Most
semicon-ductor devices contain silicon chips with impurities
embedded to conduct electricity under some
condi-tions and not others Silicon is the material used most
frequently to create semiconductors Applying an
ex-ternal electrical field to a semiconductor material
changes its resistance The ability of a semiconductor
material to conduct electricity can be changed
dra-matically by adding other elements or “impurities,” a
process known as “doping.” A pure semiconductor
without impurities is called an “intrinsic”
semicon-ductor The amount of impurity, or dopant, added to
a semiconductor determines its level of conductivity
Semiconductors are used to control electricity
flow-ing through a circuit, to amplify a signal, or to turn a
flow of current on or off Semiconductor devices
uti-lize the electronic properties of semiconductor
mate-rials and have replaced vacuum tubes in most applica-tions They utilize conductivity of electricity in the solid state compared with the gaseous state of a vac-uum Semiconductor devices are manufactured both
as discrete devices and as integrated circuits that con-sist of numerous devices, ranging from a few to mil-lions, manufactured and interconnected on a single semiconductor substrate Typical semiconductor cir-cuits include a combination of transistors, diodes, re-sistors, and capacitors that function in switching, reg-ulating, resisting, and storing electricity Combining smaller circuits such as these can be used to produce integrated circuits, sensors, and microcontroller chips These devices are important in a broad spectrum of consumer products and business equipment Devices made from semiconductor materials are the founda-tion of modern electronics, including radios, comput-ers, telephones, solar cells, and many other devices In fact, semiconductors serve as the essential compo-nent in almost every modern electronic device Silicon, which is extracted from sand, is the most common semiconductor material In the 1990’s, there was a tremendous growth in the semiconductor mate-rials industry The increase in production of com-puters increased the need for semiconductors, with major industry centers emerging in South Korea, Tai-wan, Singapore, Malaysia, and Hong Kong
History Semiconductor materials were studied in laboratories
as early as 1830 Over the years, many semiconductor materials have been researched The first materials studied were a group of elements and compounds that were generally poor conductors if heated Shining light on some of them would generate an electrical current that could pass through them only
in one direction
In the electronics field, semiconductors were used for some time before the invention of the transistor
By 1874, electricity was used not only to carry power but also to carry information The telegraph, the tele-phone, and, later, the radio were the earliest devices
in an industry that would later be called electronics
In the early part of the twentieth century, semicon-ductors became common as detectors in radios, used
in a device called a “cat’s whisker.” The cat’s whisker diode was created using the galena crystal, a semicon-ductor material composed of lead sulfide, and was considered the first semiconductor device In the late 1950’s, a process called “planar technology” enabled
Trang 4scientists to diffuse various layers onto the surface of a
silicon wafer to make a transistor with a layer of
pro-tective oxide in the junctions, making commercial
production of integrated circuits possible
Obtaining Semiconductors
Most semiconductor chips and transistors are created
with silicon, because the material is easily obtained
Semiconductors with predictable and reliable
elec-tronic properties are required for commercial
pro-duction of semiconductor devices Because the
pres-ence of even small amounts of impurities can result in
large effects on properties of the material, an
ex-tremely high level of chemical purity is necessary
High crystalline perfection is also necessary because
faults in crystal structure interfere with
semiconduct-ing properties Consequently, most semiconductors
are grown in laboratories as crystals Commercial
pro-duction uses crystal ingots between 10 and 30
centi-meters in diameter These crystals are grown as
cylin-ders up to 2 meters in length and weighing several
hundred kilograms They are sliced thin, into wafers
of standardized dimensions The Czochralski process
is a method for growing single crystals of
semiconduc-tors and results in high-purity crystals
Uses of Semiconductors
Semiconductor substances, commonly composed of
silicon, germanium, or compounds of gallium, are
the basis of integrated circuits controlling computers,
cell phones, and other electronic devices
Semicon-ductors serve as essential components in almost every
electronic device in use From outdated items such as
transistor radios to continuously evolving ones such as
the computer, semiconductors are responsible for
current technology Modern semiconductor devices
include transistors, diodes, resistors, and capacitors
They are found in televisions, automobiles, washing
machines, and computers Automobiles use
semicon-ductors to control air-conditioning, injection
pro-cesses, ignition propro-cesses, sunroofs, mirrors, and
steer-ing Any item that is computerized or uses radio waves
depends on semiconductors in order to function
Power semiconductor devices combine integrated
cir-cuits with power semiconductor technology, devices
often referred to as “smart” power devices
Semicon-ductors serve essential roles in the control of motor
systems by optimizing a wide array of manufacturing
and industrial motor systems responsible for
produc-tion of many diverse goods Semiconductors are also
used in light-emitting diode lighting All items that use sensors or controllers rely on semiconductor ma-terials
Semiconductor-based power electronics are cru-cial tools in the battle for energy efficiency Semicon-ductor technologies have enabled both performance and energy efficiency improvements in telecommuni-cation devices such as radios, televisions, emergency response networks, and networking technology, pro-cesses that require increasingly fast speeds and data-management capabilities Semiconductors have helped increase efficiency of transportation in the United States, with automobiles increasing their fuel economy by more that 70 percent since 1980 Semi-conductor technologies are used in diverse capacities
to enhance home life, business, and personal commu-nications Semiconductor technologies lead to indus-trial productivity and enhanced energy efficiency and use Although there are many modern uses of semi-conductors, their application in future devices ap-pears unlimited
C J Walsh
Further Reading
Anderson, Richard L., and Betty Lise Anderson Fun-damentals of Semiconductor Devices New York:
McGraw-Hill, 2005
Orton, John W The Story of Semiconductors New York:
Oxford University Press, 2009
Singh, Jasprit Semiconductor Devices: Basic Principles.
New York: Wiley, 2000
Turley, Jim The Essential Guide to Semiconductors
Up-per Saddle River, N.J.: Pearson Education, 2003
Yacobi, B G Semiconductor Materials: An Introduction to Basic Principles New York: Kluwer Academic, 2003.
Web Sites Nobel Prize.org Semiconductors http://nobelprize.org/educational_games/
physics/semiconductors/
U.S Geological Survey Mineral Information: Silicon Statistics and Information
http://minerals.usgs.gov/minerals/pubs/
commodity/silicon/
See also: Fuel cells; Gallium; Germanium; Photovol-taic cells; Silicon
Trang 5Sewage disposal See Solid waste
management; Waste management
and sewage disposal
Shale
Category: Mineral and other nonliving resources
Where Found
Shale is found throughout the world It is the most
common of the three principal types of sedimentary
rock, that category of rock formed by consolidation of
rock fragments or by chemical precipitation In the
geologic record, for every approximate five units of
shale known, three units of sandstone and two units of
limestone (the remaining two common categories of
sedimentary rock) are also known
Primary Uses
Shale is used as a filler in numerous construction
ma-terials It is also used in everyday products such as
cos-metics and toothpaste, and as an energy source
Technical Definition
Shale is a fine-grained consolidated rock principally
composed of silt-size (particles between 0.0039 and
0.0625 millimeter in diameter) and clay-size (less than
0.0039 millimeter in diameter) rock detritus Shale is
generally characterized by a tendency to break along
well-defined bedding planes
Description, Distribution, and Forms
The classification of shale is generally based on the
presence or absence of well-defined bedding
(lamina-tion) planes Fine-grained rock lacking this
character-istic is termed mudstone, while a similar rock
com-posed entirely of clay-size material is known as
claystone The ubiquity of shale is explained by its
rep-resenting approximately 75 percent of all
sedimen-tary rock produced throughout the entirety of
geo-logic time
Because of their fine-grained nature, shales cannot
be conveniently examined mineralogically Bulk
chemistry and X-ray studies show, however, that the
average shale is composed principally of the following
oxides: silica (approximately 58 percent), aluminum
(approximately 15 percent), iron (approximately 7
percent), and calcium, potassium, and carbon (each approximately 3 percent)
History Shale rich in organic material deposited by the Missis-sippi River over the past several tens of millions of years caused the Gulf of Mexico to be one of the rich-est hydrocarbon provinces in the world Throughout ten of the eastern United States and three western states, the Chattanooga Shale and the Green River Shale are identified as significant oil shale resources
Obtaining Shale Shale is a prime geologic source of crude oil and natu-ral gas (hydrocarbon) Hydrocarbon originates from organic matter that accumulates in varieties of shale generally deposited under marine conditions The preserved organic matter is converted to petroleum and natural gas by burial and related postdepositional changes through the passage of geologic time The general lack of permeability of shale will later form a barrier to the upper subsurface migration (and thus
to possible loss by surface evaporation) of generated hydrocarbon
Uses of Shale Kaolinite-rich shale supplies the basic material for a wide range of ceramic products, from pottery and fine porcelain to sewer pipe Shale rich in barite is em-ployed in the hydrocarbon industry to prevent oil and natural gas blowouts during the drilling of explor-atory boreholes Clay-rich shales are also employed in the cosmetics, insulator, printing ink, medicine, and toothpaste industries The highly indurated form of shale known as slate is used in the construction indus-try as roofing and paving material One major eco-nomic importance of shale is associated with the worldwide distribution of oil shale, a dark-colored rock containing 5 to more than 25 percent solid or-ganic material, from which oil can be extracted by dis-tillation Shale rich in organic material also acts di-rectly as a primary source of crude oil and natural gas
Albert B Dickas
Web Sites Natural Resources Canada Stone
http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/content/2006/56.pdf
Trang 6U.S Geological Survey
Stone, Dimension
http://minerals.usgs.gov/minerals/pubs/
commodity/stone_dimension/myb1-2007-stond.pdf
See also: Limestone; Oil and natural gas formation;
Oil and natural gas reservoirs; Oil shale and tar sands;
Sandstone; Sedimentary processes, rocks, and
min-eral deposits
Siemens, William
Category: People
Born: April 4, 1823; Lenthe, Prussia (now in
Germany)
Died: November 19, 1883; London, England
Siemens was an inventor whose work included the
steam engine and the regenerative furnace He was
also a part of Siemens Brothers, a company formed
with four of his brothers, which is credited for
ad-vanced work on telegraph cables Late in life, he
pro-posed the use of wind and water to produce electricity.
Biographical Background
Charles William Siemens was born Karl Wilhelm
Sie-mens to Christian Ferdinand SieSie-mens and Eleonore
Deichmann Scientific education was provided at an
industrial school in Magdeburg, Germany, at the
Uni-versity of Göttingen, and at the works of Count
Stol-berg in Magdeburg
Siemens spent most of his life working in successful
collaborative relationships with four of his brothers
His work with oldest brother Werner was often in the
area of electrical discovery, while collaboration with
Frederick led to the regenerative furnace The
sib-lings eventually opened a company called Siemens
Brothers in 1858
Siemens married Anne Gordon on July 23, 1859,
becoming a naturalized British citizen that same year
The couple had no children Siemens died in 1883 of
heart disease, leaving instructions in his will that the
papers pertaining to his scientific work were to be
published Though not all of his experiments had
been successful, Siemens took copious notes that
pro-vide the basis of scientific research in a number of
areas
Impact on Resource Use Siemens’s inventions centered on preserving and us-ing resources produced through natural or estab-lished power sources This work progressed after Sie-mens went to England in 1843 to impart knowledge of his electrical discoveries In 1847, he settled in Man-chester and began work on the steam engine This work suggested the harnessing of energy from heat combustion and recycling it into a working power source In 1850, the Society of Arts awarded him a gold medal for his invention of the regenerative con-denser He also earned the Telford Premium and medal of the Institution of Civil Engineers in 1853 for this work
In the same decade, he reaped financial rewards from the success of his water meter; it sold so well, he was able to live off the royalties The water meter used water energy to power a screw-turned meter Siemens received a patent for the fluid meter on April 15, 1852 The patent also allowed for an application of the water-powered screw to a meter that measured ship speed
Moving to London that year, he became an
William Siemens invented the regenerative furnace (Time & Life
Pictures/Getty Images)
Trang 7pendent civil engineer With Frederick, he continued
working on fine-tuning his steam engine The two
men developed the regenerative furnace, which
Fred-erick patented in 1856 The regenerative furnace was
an expansion on the regenerative condenser
In 1858, the brothers started a small factory, which
eventually became known as Siemens Brothers Here,
the brothers’ work moved in a different direction
Werner’s work on insulation of telegraph wiring was
so successful that the company was given
responsibil-ity for laying many telegraph lines both in England
and abroad William’s major contribution during this
period was his design of a cable-laying ship
Toward the end of his life, Siemens shifted his
in-terest back to electricity, and in 1877 he extended his
earlier work by proposing an expanded use of power
transmitted through water and wind sources As a
re-sult, the family company became known for power
transmission He spent his later years studying,
lectur-ing, and traveling
Theresa L Stowell
See also: Electrical power; Hydroenergy; Steam and
steam turbines; Steam engine; Wind energy
Sierra Club
Category: Organizations, agencies, and programs
Date: Established 1892
The Sierra Club was founded in order to preserve U.S.
natural habitats for future generations From its
incep-tion, the Sierra Club has made its goals the
conserva-tion of nature, the educaconserva-tion of the public concerning
the preservation of nature, and the enjoyment of the
great outdoors.
Background
The Sierra Club was founded in 1892 in San Francisco
by 182 charter members led by John Muir Muir and
the other members incorporated the Sierra Club with
the mission, as stated by Michael Cohen, “to explore,
enjoy, and render accessible the mountain regions of
the Pacific Coast, to publish authentic information
concerning them,” and “to enlist the support and
co-operation of the people and government in
preserv-ing the forests and other natural features of the Sierra
Nevada.”
Impact on Resource Use The Sierra Club has been influential in helping to gain national park status for Yosemite, Mount Rainier, and numerous other important sites Its members have served on important governmental committees and have spurred the enactment of many pieces of legislation designed to conserve natural resources The club also leads expeditions large and small that enable people to experience the wilderness
The Sierra Club continues to work to ensure that the legacy of clean air, water, soil, and wilderness will remain for generations to come It publishes a num-ber of periodicals that help to educate the public con-cerning the need to preserve American natural re-sources
Judy Arlis Chesen
Web Site Sierra Club http://www.sierraclub.org/
See also: Conservation; Izaak Walton League of America; Muir, John; National parks and nature re-serves; National Wildlife Federation; Nature Conser-vancy; Wilderness; Wilderness Society
Silicates
Category: Mineral and other nonliving resources
The two most common elements in the Earth’s crust are silicon and oxygen The prevalence of these two ele-ments and their ability to combine as stable complex ions accounts for the fact that silicate minerals consti-tute a major portion of the minerals in Earth’s crust Their wide range of physical properties leads to many commercial uses.
Definition Silicon and oxygen combine to form stable complex ions composed of one silicon ion surrounded by four oxygen ions The resulting complex ion is a four-sided figure known as a tetrahedron Silicate tetrahedra may exist independently separated by cations or link together by sharing oxygens to form a wide range of structural groupings Tetrahedra groupings act as skeletons in which charge neutrality is attained by the addition of cations between and within silicate
Trang 8tetrahedra Silicate skeletons may exist as isolated
tetrahedran; as two tetrahedra sharing one oxygen
atom; as rings of three, four, or six tetrahedra; as
single and double chains; as sheets; and as continuous
three-dimensional frameworks These skeletal
arrange-ments impart many diverse physical characteristics to
the various silicate minerals
Overview
No generalization describes all silicates, although most
are translucent to transparent, have moderate
spe-cific gravity, and are chemically inert Silicates range
from extremely soft to hard Some display excellent
cleavage, but others are uniformly resistant to
break-ing in all directions
Because silicate minerals exhibit such a wide
varia-tion in physical properties, they have variable
com-mercial uses Talc, a magnesium silicate, is used as a
filler in paint, ceramics, rubber, insecticides, roofing,
and paper Its most familiar form is as talcum powder
Clay minerals (extremely small platy grains of
hy-drous aluminum silicates) are important industrial
minerals They are used in a variety of fired products,
ranging from bricks to fine china and porcelain Clay
is used as a filler in many products, including paper
Montmorillonite, an expandable clay, is widely used
as a sealant Zeolites, which are hydrous silicates with
open tunnels within a framework lattice, are widely
used as molecular sieves and for ion exchange resins;
they are valuable for oil-spill cleanup and wastewater
treatment and are used in water softeners
In many applications, natural silicate minerals have
been replaced by the industrial manufacture of
syn-thetic and substitute materials Muscovite mica, a
sheet structure, was used largely as an electrical
insu-lator in capacitors and electronic tubes Asbestos, a
term describing flexible, fibrous silicate materials, was
widely used for its heat resistance and its ability to
be woven as a fabric in fire-retardant cloth, in
heat-resistant sheets, in blown insulation, and in brake
lin-ings Almost all asbestos use is outlawed in the United
States, as it is considered a carcinogen Quartz (silicon
dioxide) is used as an abrasive, as optical components,
and as thin wafers to control the frequency of radio
and radar transmission Quartz crystals are now grown
by commercial hydrothermal processes
Many semiprecious stones are silicate minerals
Microcrystalline varieties of quartz that are used in
jewelry include fibrous-appearing tiger’s eye, red
jas-per, multicolored agate, and the red-spotted
blood-stone Crystalline varieties of quartz that serve as semi-precious stones include yellow citrine and violet amethyst Topaz, jade, garnet, opal, and peridot are silicates, as is emerald, which is gem-quality beryl
René A De Hon
See also: Asbestos; Clays; Feldspars; Mica; Minerals, structure and physical properties of; Orthosilicate minerals; Quartz; Silicon; Talc
Silicon
Category: Mineral and other nonliving resources
Where Found Silicon makes up 25.7 percent of the Earth’s crust and
is the second most abundant element after oxygen It
is not found in its elemental form, but rather occurs in compounds such as oxides and various silicate miner-als Silicon is a trace element participating in the metabolism of higher animals, and siliceous struc-tures are found in many biological systems in the form
of cell walls, scales, and other skeletal features
Primary Uses Silicon metal and alloys, including ferrosilicon, are used mainly by producers of aluminum, aluminum al-loys, and chemicals Very pure silicon is an essential component of semiconductors and has given its name
to the “silicon age,” a term that came into prominence during the 1990’s
Technical Definition Silicon (abbreviated Si) is the fourteenth element of the periodic table, with an atomic number of 28 With carbon, germanium, and tin, it belongs to Group IVA
of the periodic table and resembles germanium (Ge) most strongly in its physical, chemical, and electronic properties Pure silicon is a hard, gray solid with a me-tallic luster and a cubic crystalline structure similar to that of carbon in diamond form It has eight isotopes, the most abundant of which are Si28(92.23 percent),
Si29(4.67 percent), and Si30(3.10 percent) Its density
is 2.329 grams per cubic centimeter, and it has a melt-ing point of 1,410° Celsius and a boilmelt-ing point of 2,355° Celsius While the single-crystal form of silicon has been most extensively studied from both basic and practical viewpoints, the polycrystalline and
Trang 9amorphous forms of silicon have also become
ex-tremely important: Polycrystalline silicon has been
applied in the construction of solar panels and
cen-tral processing units of computers Amorphous
sili-con has been used in thin-film transistors and solar
cells
Description, Distribution, and Forms Silicon is widely available in oxides and silicates The oxide forms include sand, quartz, rock crystal, ame-thyst, agate, flint, and opal Granite, feldspar, clay, and mica are some of the common forms of silicates A ba-sic requirement of silicon in all its preeminent
elec-Data from the U.S Geological Survey, U.S Government Printing Office, 2009.
39,000
340,000
640,000
140,000
78,000
85,000
166,000
60,000
Metric Tons of Silicon Content
3,500,000 3,000,000
2,500,000 2,000,000
1,500,000 1,000,000
500,000 Venezuela
Spain
South Africa
Russia
Norway
Macedonia
Ukraine
United States
Other countries
66,000
3,300,000
160,000
74,000
39,000
68,000
Iceland
France
China
Canada
Brazil
India
Kazakhstan
180,000 270,000
Silicon: World Production, 2008
Trang 10tronic applications is extreme purity—to levels much
better than parts per billion (ppb)
The single-crystal form of silicon, while essential
for computer chips, has cost and size limitations for a
host of other potentially high-volume applications
Hence silicon is also produced in polycrystalline and
amorphous forms by techniques such as casting and
thin-film deposition Polycrystalline forms (poly-Si)
contain crystalline grains separated by grain
bound-aries, while amorphous silicon lacks the long-range
crystalline order completely However, both have
use-ful semiconducting properties and have been widely
developed for a range of uses
The interesting and extremely useful electronic
and optoelectronic properties of silicon stem from its
tetrahedral bonding and diamond cubic structure
Replacing a host silicon atom with a Group V element
(such as phosphorus) or Group III element (such as
boron) adds a free electron or “hole” (an electron
va-cancy that behaves like a positively charged free
parti-cle Thus the electrical conductivity of silicon can be
changed over several powers of ten simply by
control-ling the trace quantities of phosphorus or boron The
bandgap separating the electron and hole states has a
value of 1.12 electron volts for silicon, making it a
nearly ideal choice for devices as varied as transistors,
diodes, solar cells, and various types of sensors
Optically, silicon is transparent to infrared
wave-lengths above 1.1 micrometers while it absorbs the
vis-ible spectrum Silicon is brittle, but its highly
direc-tional bonds enable easy “scribing” of the silicon
wafer into individual computer chips under properly
chosen crystal orientations The intricate chemical
properties of silicon enable deployment of a variety of
fabrication techniques, with individual feature sizes
falling into the submicron regime The modest
ther-mal conductivity of silicon places some restraints on
thermal dissipation in computer chips
History
Although many chemists recognized silicon as an
ele-ment by the early nineteeth century, its tight bonding
with oxygen made it difficult to isolate as a separate
el-ement Jöns Jacob Berzelius achieved the isolation of
silicon in 1823 using a method similar to one
devel-oped by Sir Humphy Davy, who earlier had tried but
failed to isolate silicon The newly isolated element
was named for the Latin word for flint, silex, and
subse-quently was investigated by German chemist Friedrich
Wöhler and others
Obtaining Silicon Semiconductor-grade silicon requires conversion of raw silicon obtained from reducing silica (SiO2) into gaseous compounds such as chlorosilanes Multiple fractional distillation of the latter leads to high-purity silicon rods These rods are subsequently melted and grown into dislocation-free single crystals by either the Czochralski (CZ) crystal pulling process or the float zone (FZ) process Necessary dopants such as boron (for p-type silicon) and phosphorus (for n-type silicon) are added to the melt CZ silicon ingots are probably the largest single crystals ever produced— more than 3 meters long, with diameters as large as
300 millimeters Wafers, about a millimeter thick, sliced from the ingots serve as the starting material for the batch fabrication of microelectronic chips, each containing up to a few million transistors
Silicon by itself is inert, but a number of source gases and reagents used in manufacturing it are highly toxic, so extreme care must be exercised in waste dis-posal and protection of assembly workers Silicon has been implicated in silicotic lung diseases and certain cancers
Uses of Silicon The principal applications of high-grade silicon are
in microelectronics The atomic structure of crystal-line silicon makes it the most important semiconduc-tor Silicon in its highly purified form, when “doped” with elements such as boron and phosphorus, be-comes the basic element of computer chips, transis-tors, diodes, and various other electronic switching and control devices The enormous success of the sili-con transistor, the basic electronic amplifying device, was made possible by an extremely pristine interface with silicon dioxide (an insulator readily grown on sil-icon by heating in oxygen) and by the continual scal-ing down of transistor feature size, which translates di-rectly to faster computer speed and higher memory capacity
The field of giant microelectronics, exemplified by portable computer displays and flat-screen television, uses silicon in its polycrystalline or amorphous forms Another area of great impact for silicon is in terres-trial solar cells, for which extremely large volumes at low cost are necessary Here computer-grade single crystals are not cost-effective; large-grain polycrys-talline silicon holds the key for this crucial renewable energy application
A late-twentieth century silicon technology