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Glass is a state of matter. It is a solid produced by cooling molten material so that the internal arrangement

of atoms, or molecules, remains in a random or disordered state, similar to the arrangement in a liquid. Such

a solid is said to be amorphous or glassy. Ordinary solids, by contrast, have regular crystalline structures.

The difference is illustrated in Figure 1.

any materials can be made to e!ist as glasses. "ard candies, for e!ample, consist primarily of sugar in the

glassy state. #hat the term $glass$ means to most people, ho%ever, is a product made from silica &SiO ' (.

The

Figure 1. Structures of a typical solid (l.) and glass (r.).

common form of silica is sand, but it also occurs in nature in a crystalline form )no%n as quart*.

+ure silica can produce an e!cellent glass, but it is very highmelting &1,-' o /, or ,1 o F(, and the melt is

so e!tremely viscous that it is difficult to handle. 0ll common glasses contain other ingredients that ma)e

the silica easier to melt and the hot liquid easier to shape.Natural Glass

+robably as early as -,222 3./.4. , long before human beings had learned ho% to ma)e glass, they had

used natural glass to fashion )nives, arro%heads, and other useful articles. The most common natural glass is

obsidian, formed %hen the heat of volcanoes melts roc)s such as granite, %hich then become glassy upon

cooling. Other natural glasses are pumice, a glassy foam produced from lava5 fulgurites, glass tubes formed

 by lightning stri)ing sand or sandy soil5 and te)tites, lumps or beads of glass probably formed during

meteoric impacts.

Manmade (Synthetic) Glass

#hen, %here, or ho% human beings discovered ho% to ma)e glass is not )no%n. 6ery small dar)colored

 beads of glass have been dated bac) to 7222 3./.4. These may %ell have been byproducts of copper

smelting or pottery gla*ing. 3y '22 3./.4. small pieces of true synthetic glass appeared in areas such as

esopotamia, but an actual glass industry did not appear until about 122 3./.4. in 4gypt. 3y this time

various small vases, cosmetic 8ars, and 8e%elry items made of glass had begun to appear.

0ll the ancient glasses %ere based on silica &sand(, modified %ith considerable amounts of various metal

o!ides, mainly soda &9a ' O( and lime &/aO(. This is still the most common glass being used today. It is

)no%n as soda lime glass. "o%ever, the ancient glass %as usually colored and opaque due to the presence of 

various impurities, %hereas most modern glass has the useful property of transparency.

"undreds of thousands of different glass compositions have been devised, and they have been used in

different %ays. uch has been learned about %hich combination of chemicals %ill ma)e the best glass for a

 particular purpose. For e!ample, in 1::7 an 4nglishman named ;avenscroft found that adding lead o!ide

&+bO( to a glass melt produced a brilliant glass that %as much easier to melt and to shape. Since that timelead glass has been used to ma)e fine crystal bo%ls and goblets and many )inds of art glass.

0n important )ind of glass %as developed in the early 1<22s to solve a serious problem=the inability of

glass to %ithstand temperature shoc). This failure resulted in tragic accidents in the early days of the

railroads. Glass lanterns used as signals %ould get very hot, and then, if it started to rain, the rapid cooling

%ould sometimes cause the glass to brea) and the signal to fail. The problem %as solved by replacing much

of the soda in the glass %ith boron o!ide &3 ' O (. The resulting glass, called borosilicate, contains about 1'

 percent boron o!ide and can %ithstand a temperature variation of '22 o / &<' o F(. It also has

greater chemical durability than soda lime glass. Today borosilicate glass is used in most laboratory

glass%are &bea)ers, flas)s, test tubes, etc.( and in glass )itchen ba)e%are.

For even greater heat shoc) resistance and chemical durability, alumina &0l ' O ( can be used instead of

 boron o!ide. The resultant aluminosilicate glass has such resistance to heat shoc) that it can be used directlyon the heating element of the )itchen stovetop. It is also used to ma)e the special bottles used for liquid

 pharmaceutical prescriptions, and to produce the glass thread that is %oven into fiberglass fabric.

"igh silica glass &<:.>122? silica( remains difficult to ma)e because of the very high melting point of pure

silica. "o%ever, it is made for special purposes because of its outstanding durability, e!cellent resistance to

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thermal shoc) or chemical attac), and ability to transmit ultravioletlight &an ability that ordinary glass does

not have(. Spacecraft %indo%s, made of 122 percent silica, can %ithstand temperatures as high as 1,'22 o /

&',1<' o F(. Table 1 lists the five ma8or types of glass along %ith properties and uses.

Glass Composition. The ma)ing of glass involves three basic types of ingredients@ formers, flu!es, and

stabili*ers. The glass former is the )ey component in the structure of a glassy material. The former used in

most glasses is silica &SiO ' (. +ure silica is difficult to melt because of its e!tremely high melting point

&1,-' o /, or ,1 o F(, but flu!es can be added to lo%er the melting temperature. Other glass formers %ith

much lo%er melting points &722 o />:22 o /, or -'>1,11' o F( are boric o!ide &3 ' O ( and phosphorus

 pento!ide &+ ' O (. These are easily melted, but because their glass products dissolve in %ater, they have

limited usefulness.

ost silica glasses contain an added flu!, so that the silica can be melted at a much lo%er temperature

&A22 o /><22 o /, or 1,7-'>1,:' o F(. Standard flu!es include soda &9a ' O(, potash &B ' O(, and lithia

&Ci ' O(. Frequently the flu! is added as a carbonate substance &e.g., 9a ' /O (, the /O ' being driven off

during heating. Glasses containing only silica and a flu!, ho%ever, have poor durability and are often %ater

soluble.

To ma)e glasses stronger and more durable, stabili*ers are added. The most common stabili*er is lime

&/aO(, but others are magnesia &gO(, baria &3aO(, and litharge &+bO(. The most common glass, made in

largest amounts by both ancient and modern glassma)ers, is based on silica as the glass former, soda as the

flu!, and lime as the stabili*er. It is the glass used to ma)e %indo%s, bottles, 8ars, and lightbulbs.Colored Glass. The natural glasses used by the ancients %ere all dar) in color, usually ranging from olive

green or bro%n to 8et blac). The color %as

MAJ! G"ASS #$%&S AN' #&! *S&S

Glass #ype %roperties "imitations *ses

Soda lime

Ine!pensive5 easy to melt

and shape5 most %idely

used glass

+oor durability5 not

chemically resistant5 poor

thermal shoc) resistance

#indo%s5 bottles5 light bulbs5

 8ars

Cead glass &often '2> 

2? +b o!ide(

"igh density5 brilliant5

very easy to melt, shape,

cut, and engrave

+oor durability5 easily

scratched

Fine crystal radiation

%indo%s5 T6 tube parts

3orosilicate &usually

>1? 3 ' O (

6ery good thermal shoc)

resistance and chemical

durability5 easy to

 9ot suitable for longterm

high temperature use melt

and shape

Cab%are5 )itchen%are5 special

light bulbs5 glass pipe5 sealed

 beam headlights

0luminosilicate

&usually >12?

0l ' O (

4!cellent thermal

resistance5 durability

ore difficult to melt and

shape than borosilicate

Topofstove coo)%are5 high

quality fiberglass

"igh silica &6ycor

<:.?5 fused quart*

122?(

Outstanding thermal

resistance

Difficult to ma)e5 very

e!pensive

Spacecraft %indo%s5 lab%are5

fiber optics

due to the presence of significant amounts of metal impurities, especially iron. 4ven today the ubiquitous

 presence of iron in nature causes most ordinary glass to have a slight greenish cast.any standard glass colorants are o!ides of metals such as cobalt &blue(, chromium &green(,

and manganese &violet(. Eello% glass is usually made %ith cadmium sulfide, and red or pin) glass usually

contains selenium, although some rubycolored glass has had gold added. The coloring of glass is not a

simple sub8ect. Glass color depends not only on %hich elements are added, but also on the composition of

the glass, and on %hether the furnace used %as in an o!idi*ing or reducing mode. /opper, for e!ample, can

 produce blue, green, or opaque red glass, depending on melting conditions.

The 4gyptians of 122 3./.4. )ne% that they could ma)e brightly colored glasses by adding certain metals

&or their compounds( to the glass melt. The ancient ;omans continued the science of ma)ing colored glass

and e!panded it. 3y the fourth century /.4. the ;omans had learned ho% to produce a dichroic &t%ocolor(

glass. The most famous dichroic glass article left by the ;omans is the Cycurgus /up &no% at the 3ritish

useum(. It is green in reflected light &%ith the lamp in front of the cup(, but red in transmitted light &thelamp behind the cup(. This unusual glass contains microscopic particles of gold and silver.

Glass Forming

Ancient Methods. Shaping hot, molten glass into useful articles has long been a challenge. olten glass is

e!tremely hot, caustic, stic)y, and difficult to handle. In the period e!tending from about '222 3./.4. to

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2 3./.4. , there %ere three basic methods used to form glass. One of the earliest and most %idely used %as

core forming. This involved distributing molten glass around a clay core on a metal rod. The rod %ith the

clay core could either be dipped into molten glass, or the hot liquid glass could simply be poured over it. The

outer glass coating %as then rolled &marvered( on a flat stone surface to smooth it. Often the ob8ect %as

decorated by dribbling more glass, sometimes of a different color, onto its surface. The hot glass %as then

annealed &cooled slo%ly so as to relieve thermal stress(, and the metal rod %as removed and the clay core

scraped out.

0 second method involved sagging and fusing. It called for ta)ing preformed glass rods or canes &%hich

%ere often of different colors(, placing them in or on top of a mold, and then heating the canes until they

sagged and fused together and conformed to the shape of the mold. &Sheets of glass could also be sagged

over shaped clay molds.(

The third method %as casting, %hich called for pouring hot, molten glass into a mold. 0 variation on cast

glass %as faience, made from po%dered quart* blended into molten glass. The mi!ture might be pressed

 bet%een t%o molds to ma)e a cast vessel such as a bo%l.

0ll three of these methods %ere slo%, and they generally produced small items that %ere rather thic). Glass

 pieces tended to be quite e!pensive, and, in antiquity, %ere affordable only by the very %ealthy.

Glass+lo,ing. It %as probably in the iddle 4ast during the first century 3./.4. that the important

technique of glassblo%ing %as discovered. 0 hollo% metal rod &or pipe( %as used to pic) up a gob of molten

glass5 the act of blo%ing into the pipe generated a bubble of glass. If the bubble %ere blo%n into a mold, themolten glass could be given a desired shape. #ooden paddles and pincers %ere used to refine the shape even

further. The blo%ing procedure %as used to ma)e glass ob8ects that %ere larger and thinner than those that

had been made previously, and it %as much faster than previous glassforming methods. 0s glass pieces

 became easier to ma)e, they became cheaper and more available. The ancient ;omans became particularly

s)illful at glassblo%ing. ore glass %as produced and used in the ;oman %orld than in any other

civili*ation of antiquity. During the iddle 0ges, there %as a great e!pansion of glassblo%ing activity,

especially in 6enice, the iddle 4ast, and 4uropean countries such as Spain and Germany.

Some Modern Methods. Since the nineteenth century, many centuriesold glassforming methods have been

mechani*ed, greatly increasing the rate of production of glass ob8ects, and lo%ering the prices of these

ob8ects. For e!ample, the $ribbon machine,$ developed in the 1<'2s for the automatic glassblo%ing of

lightbulbs, is a milestone of mechanical glass forming. In the ribbon machine, puffs of air blo% glass bubbles from a rapidly moving ribbon of molten glass into a moving stream of molds that give the bulbs

their shape and then release them. Small lightbulb blan)s can be made at the rate of 1,222 per minute.

#ith so many millions of %indo%s in buildings and vehicles every%here, %e tend to ta)e sheets of flat glass

for granted. Throughout most of human history, ho%ever, there %ere no sheets of flat, transparent glass.

4ven as recently as the eighteenth century, glass %indo%s %ere quite uncommon.

In a very limited %ay glass %indo%s did start to appear in the ;oman %orld during the third century, but

they %ere generally small glass fragments set in bron*e or %ooden frames. In that era most %indo%s %ere

not glass, but %ere thin sheets of translucent horn or marble, or perhaps panes of mica &isinglass(. 0round

:22 /.4. , during the 3y*antine period, glass %indo%s &usually made of small pieces of colored glass( began

to appear in the large churches, but glass %indo%s in houses and other secular buildings remained quite rare

until the end of the eighteenth century.The principal method for ma)ing flat glass during the 1-22s called for blo%ing a hot glass bubble, securing

an iron rod to the bubbles other side, and then cutting the bubble free from the blo%ing pipe. The tulip

shaped hot glass %as then rotated rapidly around the iron rod a!is until the centrifugal force forced the glass

tulip to open up and form a dis). The rod %as then removed from the glass &leaving a spot in the middle of

the glass dis) that loo)ed rather li)e a bulls eye(. This method %as the source of the old $bullseye$

%indo%s that can still sometimes be found in 4nglish pubs. The %indo%s %ere limited in si*e and poor in

optical quality &besides having a bullseye at their centers(.

The chief method for ma)ing flat glass during the 1A22s %as the cylinder method. The first step %as to blo%

a large glass bubble &compressed air %as often used(5 it %ould then be s%ung bac) and forth until the bubble

 became elongated and acquired a cylindrical shape5 finally the cylinder %as split length%ise, reheated, and

allo%ed to flatten on an iron table. The resulting pane of glass %as not really flat, and it had a lot of opticaldistortion, but the method %as used %idely to ma)e sheet glass. For e!ample, it %as used to produce the

22,222 panes of glass that %ere used to build Condons /rystal +alace, the huge greenhouse constructed for 

the Condon #orlds Fair 4!hibition of 1A1.

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3y the t%entieth century these methods %ere replaced by an innovative technique invented by a 3elgian

named Foucault, %ho had learned ho% to dra% up continuous sheets from a tan) filled %ith molten glass.

4ven this glass %as of nonuniform thic)ness and had some roughness at its surface, therefore, for high

quality flat glass, it had to be ground and polished.

Then, in 1<<, the +il)ington Glass #or)s in 3ritain introduced the $float glass$ process. In the float

 process, molten glass is allo%ed to flo% continuously onto a mirrorli)e surface of molten tin at 1,222 o /

&1,A' o F(. 0t this temperature the glass spreads out and becomes a layer that is about : millimeter &17

inch( thic). If the layer is stretched as it cools, a thic)ness of ' millimeter &2.2A inch( can be achieved. The

glass is allo%ed to advance on the hot liquid tin until, at :22 o / &1,11' o F(, it becomes solid enough to be

lifted off the molten tin surface. It is then annealed &heated to relieve any strain( before being cut into

desired sheet lengths. The float glass method rapidly replaced the Foucault dra%ing process, and today it is

the standard method for ma)ing flat glass. 0 large modern float glass plant can produce ,222 tons of glass

sheet per %ee), and it can be operated '7 hours a day, : days a year, for several years before serious

repairs are apt to be needed. Float glass has uniform thic)ness and bright firepolished surfaces that need no

grinding or polishing.

The dra%ing of glass fibers had long been of interest, but glass fibers found little use until the t%entieth

century. 0rticles such as %edding go%ns made from glass fiber cloth %ere largely curiosities, made for sho%

rather than use. In the 1<2s glass researchers learned to feed molten glass into platinum bushings having

hundreds of tiny holes. Fine glass filaments of 12 to 2 microns %ere rapidly dra%n do%n%ard andassembled as bundles or strands of glass fiber. Today a ma8or use of glass cloth or filaments is to strengthen

the plastics used to ma)e fiberglassreinforced composites. These composites are %idely used in ma)ing

 boats, from canoes to yachts, and bodies for cars, such as the /orvette.

0n even larger poundage mar)et is that of glass %ool insulation. In a process much li)e that used to ma)e

cotton candy, fine glass fibers are spun, sprayed %ith an organic bonding agent, and then heatcured and cut

into mats of various si*es, to be used for insulating buildings and appliances.

Surely the most significant glass fiber development in recent times is fiber optics, or optical %ave guides.

These ultrapure, very fine glass fibers are a most crucial part of modern communications technology,

%herein glass fibers lin) telephones, televisions, and computers. 0 single strand of glass optical fiber that

has a protective plastic coating loo)s much li)e a human hair. The glass fiber has an inner core of ultrapure

fused silica, %hich is coated %ith another silica glass that acts as a lightrefractive barrier. Casers are used toconvert sound %aves and electrical impulses to pulses of light that are sent, staticfree, through the inner

glass core. Glass fibers can transmit many times more information than can be carried by charges moving in

a copper %ire. In fact, one pound of glass optical %ave guides can transmit as much information as can be

transmitted via '22 tons of copper %ire. Today millions of miles of optic fibers are crisscrossing not only the

Hnited States, but also the entire planet.

#indo%s need to be cleaned. In '222 a ne% glass that largely cleans itself %hen it comes into contact %ith

rain %as introduced. This lo%maintenance glass %as developed by +il)ington Glass #or)s, the company

that invented the float process. It is made by depositing a microscopically thin coating of titanium dio!ide

&TiO ' ( on hot sheet glass during its manufacture in the float process. 0s dirt collects on the %indo%, the

Suns ultraviolet rays promote a catalytic reaction at the glass surface that brea)s do%n and loosens surface

dirt. Kenneth E. Kolb

-i+liography

3roo)s, ohn 0. &1<-(. Glass. 9e% Eor)@ Golden +ress.

Douglas, ;. #., and Susan Fran) &1<-'(. A History of Glassmaking. O!fordshire, 4ngland@ G. T. Foulis J

/o.

Bampfer, Frit*, and 3eyer, Blaus G. &1<::(. Glass: A World History. Condon@ Studio 6ista.

Bolb, Benneth 4., and Bolb, Doris B. &1<AA(. Glass: Its Many Faets. "illside, 9@ 4nslo%.

+hillips, +hoebe &1<A1(. !he Enylopedia of Glass. 9e% Eor)@ /ro%n +ublishers.

;ogers, Frances, and 3eard, 0lice &1<7A(. "### $ears of Glass. 9e% Eor)@ Cippincott.

/eramics can be defined as heatresistant, nonmetallic, inorganic solids that are &generally( made up of

compounds formed from metallic and nonmetallic elements. 0lthough different types of ceramics can have

very different properties, in general ceramics are corrosionresistant and hard, but brittle. ost ceramics are

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also good insulators and can %ithstand high temperatures. These properties have led to their use in virtually

every aspect of modern life.

The t%o main categories of ceramics are traditional and advanced. Traditional ceramics include ob8ects

made of clay and cements that have been hardened by heating at high temperatures. Traditional ceramics are

used in dishes, croc)ery, flo%erpots, and roof and %all tiles. 0dvanced ceramics include carbides, such as

silicon carbide, Si/5 o!ides, such as aluminum o!ide, 0l ' O 5 nitrides, such as silicon nitride, Si  9 7 5 and

many other materials, including the mi!ed o!ide ceramics that can act as superconductors. 0dvancedceramics require modern processing techniques, and the development of these techniques has led to

advances in medicine and engineering.

Glass is sometimes considered a type of ceramic. "o%ever, glasses and ceramics differ in that ceramics have

a crystalline structure %hile glasses contain impurities that prevent crystalliation . The structure of glasses

is amorphous, li)e that of liquids. /eramics tend to have high, %elldefined melting points, %hile glasses

tend to soften over a range of temperatures before becoming liquids. In addition, most ceramics are opaque

to visible light, and glasses tend to be translucent. Glass ceramics have a structure that consists of many tiny

crystalline regions %ithin a noncrystalline matri!. This structure gives them some properties of ceramics and

some of glasses. In general, glass ceramics e!pand less %hen heated than most glasses, ma)ing them useful

in %indo%s, for %ood stoves, or as radiant glassceramic coo)top surfaces.

CompositionSome ceramics are composed of only t%o elements. For e!ample, alumina is aluminum o!ide, 0l ' O 5

*irconia is *irconium o!ide, KrO ' 5 and quart* is

/eramics are good insulators and can %ithstand high temperatures. 0 popular use of ceramics is in art%or).

silicon dio!ide, SiO ' . Other ceramic materials, including many minerals, have comple! and even variable

compositions. For e!ample, the ceramic mineral feldspar, one of the components of granite, has the formula

B0lSi O A .

The chemical bonds in ceramics can be covalent, ionic, or polar covalent, depending on the chemical

composition of the ceramic. #hen the components of the ceramic are a metal and a nonmetal, the bonding

is primarily ionic5 e!amples are magnesium o!ide &magnesia(, gO, and barium titanate, 3aTiO . Inceramics composed of a metalloid and a nonmetal, bonding is primarily covalent5 e!amples

are boron nitride, 39, and silicon carbide, Si/. ost ceramics have a highly crystalline structure, in %hich a

threedimensional unit, called a unit cell, is repeated throughout the material. For e!ample, magnesium

o!ide crystalli*es in the roc) salt structure. In this structure, g 'L ions alternate %ith O 'M ions along

each perpendicular a!is.

Manufacture of #raditional Ceramics

Traditional ceramics are made from natural materials such as clay that have been hardened by heating at

high temperatures &driving out %ater and allo%ing strong chemical bonds to form bet%een the fla)es of

clay(. In fact, the %ord $ceramic$ comes from the Gree) keramos , %hose original meaning %as $burnt

earth.$ #hen artists ma)e ceramic %or)s of art, they first mold clay, often mi!ed %ith other ra% materials,

into the desired shape. Special ovens called )ilns are used to $fire$ &heat( the shaped ob8ect until it hardens./lay consists of a large number of very tiny flat plates, stac)ed together but separated by thin layers of

%ater. The %ater allo%s the plates to cling together, but also acts as a lubricant, allo%ing the plates to slide

 past one another. 0s a result, clay is easily molded into shapes. "igh temperatures drive out %ater and allo%

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 bonds to form bet%een plates, holding them in place and promoting the formation of a hard solid. 3inders

such as bone ash are sometimes added to the clay to promote strong bond formation, %hich ma)es the

ceramic resistant to brea)age. The common clay used to ma)e flo%erpots and roof tiles is usually redorange

 because of the presence of iron o!ides. #hite ceramics are made from rarer &and thus more e!pensive( %hite

clays, primarily )aolin.

The oldest )no%n ceramics made by humans are figurines found in the former /*echoslova)ia that are

thought to date from around '-,222 3./.4. It %as determined that the figurines %ere made by mi!ing clay

%ith bone, animal fat, earth, and bone ash &the ash that results %hen animal bones are heated to a high

temperature(, molding the mi!ture into a desired shape, and heating it in a domed pit. The manufacture of

functional ob8ects such as pots, dishes, and storage vessels, %as developed in ancient Greece and 4gypt

during the period <222 to :222 3./.4.

0n important advance %as the development of %hite porcelain. +orcelain is a hard, tough ceramic that is

less brittle than the ceramics that preceded it. Its strength allo%s it to be fashioned into beautiful vessels %ith

%alls so thin they can even be translucent. It is made from )aolin mi!ed %ith china stone, and the mi!ture is

heated to a very high temperature &1,22N/, or ',-'NF(. +orcelain %as developed in /hina around /.4. :22

during the Tang dynasty and %as perfected during the ing dynasty, famous for its blue and %hite

 porcelain. The porcelain process %as introduced to the 0rab %orld in the ninth century5 later 0rabs brought

 porcelain to Spain, from %here the process spread throughout 4urope.

3one china has a composition similar to that of porcelain, but at least 2 percent of the material is finely po%dered bone ash. Ci)e porcelain, bone china is strong and can be formed into dishes %ith very thin,

translucent %alls. Stone%are is a dense, hard, gray or tan ceramic that is less e!pensive than bone china and

 porcelain, but it is not as strong. 0s a result, stone%are dishes are usually thic)er and heavier than bone

china or porcelain dishes.

Manufacture of Ad/anced Ceramics

The preparation of an advanced ceramic material usually begins %ith a finely divided po%der that is mi!ed

%ith an organic binder to help the po%der consolidate, so that it can be molded into the desired shape.

3efore it is fired, the ceramic body is called $green.$ The green body is first heated at a lo% temperature in

order to decompose or o!idi*e the binder. It is then heated to a high temperature until it is $sintered,$ or

hardened, into a dense, strong ceramic. 0t this time, individual particles of the original po%der fuse together

as chemical bonds form bet%een them. During sintering the ceramic may shrin) by as much as 12 to 72 percent. 3ecause shrin)age is not uniform, additional machining of the ceramic may be required in order to

obtain a precise shape.

Solgel technology allo%s better mi!ing of the ceramic components at the molecular level, and hence yields

more homogeneous ceramics, because the ions are mi!ed %hile in solution. In the solgel process, a solution

of an organometallic compound is hydroly*ed to produce a $sol,$ a colloidal suspension of a solid in a

liquid. Typically the solution is a metal al)o!ide such as tetrametho!ysilane in an alcohol solvent. The sol

forms %hen the individual formula units polymeri*e &lin) together to form chains and net%or)s(. The sol

can then be spread into a thin film, precipitated into tiny uniform spheres called microspheres, or further

 processed to form a gel inside a mold that %ill yield a final ceramic ob8ect in the desired shape. The many

crosslin)s bet%een the formula units result in a ceramic that is less brittle than typical ceramics.

0lthough the solgel process is very e!pensive, it has many advantages, including lo% temperaturerequirements5 the ceramists ability to control porosity and to form films, spheres, and other structures that

are difficult to form in molds5 and the attainment of speciali*ed ceramic compositions and high product

 purity.

+orous ceramics are made by the solgel process. These ceramics have spongeli)e structures, %ith many

 poreli)e lacunae, or openings, that can ma)e up from ' to -2 percent of the volume. The pore si*e can be

large, or as small as 2 nanometers &' 12 M: inches( in diameter. 3ecause of the large number of pores,

 porous ceramics have enormous surface areas &up to 22 square meters, or ,A' square feet, per gram of

ceramic(, and so can ma)e e!cellent catalysts. For e!ample, *irconium o!ide is a ceramic o!ygen sensor that

monitors the airtofuel ratio in the e!haust systems of automobiles.

0erogels are solid foams prepared by removing the liquid from the gel during a solgel process at high

temperatures and lo% pressures. 3ecause aerogels are good insulators, have very lo% densities, and do notmelt at high temperatures, they are attractive for use in spacecraft.

%roperties and *ses

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For centuries ceramics %ere used by those %ho had little )no%ledge of their structure. Today, understanding

of the structure and properties of ceramics is ma)ing it possible to design and engineer ne% )inds of

ceramics.

ost ceramics are hard, chemically inert , refractory &can %ithstand very high heat %ithout deformation(,

and poor conductors of heat and electricity. /eramics also have lo% densities. These properties ma)e

ceramics attractive for many applications. /eramics are used as refractories in furnaces and as durable

 building materials &in the form of bric)s, tiles, cinder bloc)s, and other hard, strong solids(. They are also

used as common electrical and thermal insulators in the manufacture of spar) plugs, telephone poles,

electronic devices, and the nose cones of spacecraft. "o%ever, ceramics also tend to be brittle. 0 ma8or

difficulty %ith the use of ceramics is their tendency to acquire tiny crac)s that slo%ly become larger until the

material falls apart. To prevent ceramic materials from crac)ing, they are often applied as coatings on

ine!pensive materials that are resistant to crac)s. For e!ample, engine parts are sometimes coated %ith

ceramics to reduce heat transfer.

/omposite materials that contain ceramic fibers embedded in polymer matrices possess many of the

 properties of ceramics5 these materials have lo% densities and are resistant to corrosion, yet are tough and

fle!ible rather than brittle. They are used in tennis rac)ets, bicycles, and automobiles. /eramic composites

may also be made from t%o distinct ceramic materials that e!ist as t%o separate ceramic phases in the

composite material. /rac)s generated in one phase %ill not be transferred to the other. 0s a result, the

resistance of the composite material to crac)ing is considerable. /omposite ceramics made from diboridesandor carbides of *irconium and hafnium mi!ed %ith silicon carbide are used to create the nose cones of

spacecraft. 3rea)resistant coo)%are &%ith outstanding thermal shoc) resistance( is also made from ceramic

composites.

0lthough most ceramics are thermal and electrical insulators, some, such as cubic boron nitride, are good

conductors of heat, and others, such as rhenium o!ide, conduct electricity as %ell as metals. Indium tin o!ide

is a transparent ceramic that conducts electricity and is used to ma)e liquid crystal calculator displays. Some

ceramics are semiconductors, %ith conductivities that become enhanced as the temperature increases. For

e!ample, silicon carbide, Si/, is used as a semiconductor material in high temperature applications.

"igh temperature superconductors are ceramic materials consisting of comple! ionic o!ides that become

superconducting %hen cooled by liquid nitrogen. That is, they lose all resistance to electrical current. One

e!ample is the material E3a ' /u O -M % , %hich crystalli*es to form $sheets$ of copper and o!ygen atomsthat can carry electrical current in the planes of the sheets.

Some ceramics, such as barium ferrite or nic)el *inc ferrites, are magnetic materials that provide stronger

magnetic fields, %eigh less, and cost less than metal magnets. They are made by heating po%dered ferrite in

a magnetic field under high pressure until it hardens. /eramic magnets are brittle, but are often used in

computers and micro%ave devices.

The properties of pie*oelectric ceramics are modified %hen /oltage is applied to them, ma)ing them useful

as sensors and bu**ers. For e!ample, lead *irconium titanate is a pie*oelectric ceramic used to provide

$muscle action$ in robot limbs in response to electrical signals.

Some ceramics are transparent to light of specific frequencies. These optical ceramics are used as %indo%s

for infrared and ultraviolet sensors and in radar installations. "o%ever, optical ceramics are not as %idely

used as glass materials in applications in %hich visible light must be transmitted. 0n electrooptic ceramicsuch as lead lanthanum *irconate titanate is a material %hose ability to transmit light is altered by an applied

voltage. These electrooptic materials are used in color filters and protective goggles, as %ell as in memory

storage devices.

Still other ceramics are important in medicine. For e!ample, they are used to fabricate artificial bones and to

cro%n damaged teeth. The fact that many ceramics can be easily sterili*ed and are chemically inert ma)es

ceramic microspheres made of these materials useful as biosensors. Drugs and other chemicals can be

carried %ithin microsphere pores to desired sites in the body.

 &oretta &. 'ones

-i+liography

3all, +hilip &1<<-(. Made to Measure: (e) Materials for the !)enty*First +entury. +rinceton, 9@ +rinceton

Hniversity +ress.3arsoum, ichael #. &1<<:(. Fundamentals of +eramis. 9e% Eor)@ cGra%"ill.

3rin)er, /. effrey, and Scherer, George #. &1<<2(. ,ol*Gel ,iene: !he -hysis and +hemistry of ,ol*Gel

 -roessing. 3oston@ 0cademic +ress.

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/alvert, +aul &'222(. $0dvanced aterials.$ In !he (e) +hemistry , ed. 9ina "all. 9e% Eor)@ /ambridge

Hniversity +ress.

Bingery, #. D.5 3o%en, ". B.5 and Hhlmann, D. ;. &1<-:(.  Introdution to +eramis , 'nd edition. 9e%

Eor)@ #iley.

;icherson, David #. &1<<'(. Modern +erami Engineering: -roperties -roesses and /se in 0esign , 'nd

edition, revised and e!panded. 9e% Eor)@ arcel De))er. ;icherson, David #. &'222(. !he Magi of

+eramis. #esterville, O"@ 0merican /eramic Society.

Shac)leford, ames F., ed. &1<<A(. 1ioeramis: Appliations of Glass and +erami Materials in

 Mediine. Kurich@ TransTech +ublications.

#achtman, ohn 3., r., ed. &1<<<(. +erami Inno2ations in the 3#th +entury. #esterville, O"@ 0merican

/eramic Society.

inerals are the building bloc)s of roc)s. 0 mineral may be defined as any naturally occurring inorganic

solid that has a definite chemical composition &that can vary only %ithin specified limits( and possesses

a crystalline structure. The study of minerals is )no%n as mineralogy, %hich dates bac) to prehistory. The

use of minerals in the construction of primitive %eapons and as suppliers of color for ancient artists ma)es

mineralogy one of the oldest of the human arts.

inerals may be characteri*ed by the fundamental patterns of their crystal structures. 0 crystal structure is

commonly identified by its fundamental repeating unit, %hich upon protraction into three dimensionsgenerates a macroscopic crystal. /rystal structures can be divided into crystal systems, %hich can be further

subdivided into crystal classes=a total of thirtyt%o crystal classes, %hich are sometimes referred to as

 point classes.

ore commonly, minerals are described or classified on the basis of their chemical composition. 0lthough

some minerals, such as graphite or diamond, consist primarily of a single element &in this instance, carbon(,

most minerals occur as ionic compounds that consist of orderly arrangements of cations and anions and

have a specific crystalline structure determined by the si*es and charges of the individual ions. /ations

&positively charged ions( are formed by the loss of negatively charged electrons from atoms. 0nions consist

of a single element, the atoms of %hich have become negatively charged via the acquisition of electrons, or

they consist of several elements, the atoms bound together by co/alent +onds and bearing an overall

negative charge. +yrite &FeS ' ( is a mineral that contains a sulfide ion as its anion. Gypsum P/aSO 7 > '&" 'O(Q contains the polyatomic anion )no%n as sulfate &SO 7

'M ( as %ell as t%o %aters of hydration &%ater

molecules that are part of the crystalline structure(.

It has been noted that the chemical composition of minerals could vary %ithin specified limits. This

 phenomenon is )no%n as solid solution. For e!ample, the chemical composition of the mineral dolomite is

commonly designated as /ag &/O ( ' , or as &/a, g(/O . This does not mean that dolomite has calcium

and magnesium e!isting in a onetoone ratio. It signifies that dolomite is a carbonate mineral that has

significant amounts of 

This seacliff in #ales sho%s strata of banded liassic limestone and shale.

 both cations &calcium and magnesium ions( in an infinite variety of proportions. #hen minerals form, ionsof similar si*e and charge, such as calcium and magnesium ions, can substitute for each other and %ill be

found in the mineral in amounts that depend on the proportions that %ere present in solution, or in the melt

&liquid magma( from %hich the mineral formed. Thus, many minerals can e!ist in solid solution. #hen solid

solutions e!ist, names are often given to the endmembers. In the case of the calcium and magnesium

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carbonates, one endmember, /a/O is named calcite or aragonite, depending on the crystalline symmetry,

%hereas the other endmember, g/O , is referred to as magnesite.

3ecause minerals are naturally occurring substances, the abundance of minerals tends to reflect the

abundance of elements as they are found in 4arths crust. 0lthough about 7,222 minerals have been named,

there are forty minerals that are commonly found and these are referred to as the roc)forming minerals.

The most abundant element in 4arths crust is o!ygen, %hich ma)es up about 7 percent of the crust by

mass. The second most abundant element is silicon, %hich accounts for another '- percent by mass. The

ne!t si! most abundant elements, in order of abundance, are aluminum, iron, calcium, magnesium, sodium,

and potassium, %hich collectively comprise about ': percent, leaving only about ' percent for all other

elements. If one classifies minerals according to the commonly accepted system that is based on their

anions, it is not surprising that silicates &having anions that are polyatomic combinations of o!ygen and

silicon( are the most common mineral group.

Silicates

In order to understand the chemical structures and formulas of the silicate minerals, one must begin %ith the

 basic building bloc) of all silicates@ the silica tetrahedron. 0 silica tetrahedron is an anionic species, %hich

consists of a silicon atom covalently bound to four o!ygen atoms. The silicon atom is in the geometric center 

of the tetrahedron and at each of the four points of the

The natural matri! of the Bimberlite diamond.

tetrahedron is an o!ygen atom. The structure has an overall charge of negative four and is represented as

SiO 77M . The mineral olivine, a greencolored mineral as the name suggests, has the formula &g,

Fe( ' SiO 7 . #hen olivine is a gemquality crystal it is referred to as peridot. 0s the formula suggests, olivine

is really a group of minerals that vary in composition, from almost pure endmember forsterite &g ' SiO 7 (

to almost pure fayalite &Fe ' SiO 7 (.

0ll of the silicate minerals arise from various combinations of silica tetrahedra and a sense of their variety

may be gleaned from the understanding that the o!ygen atoms at the tetrahedral vertices may be shared by

ad8acent tetrahedra in such a %ay as to generate larger structures, such as single chains, double chains,

sheets, or threedimensional net%or)s of tetrahedra. 6arious cations occurring %ithin solid solutions

neutrali*e the negative charges on the silicate bac)bone. The variation in geometric arrangements generatesa da**ling array of silicate minerals, %hich includes many common gemstones.

The pyro!ene group and the amphibole group, respectively, are representatives of silicate minerals having

singlechain and doublechain tetrahedral net%or)s. +yro!enes are believed to be significant components of

4arths mantle, %hereas amphiboles are dar)colored minerals commonly found in continental roc)s.

/lays have sheet structures, generated by the repetitious sharing of three of the four o!ygen atoms of each

silica tetrahedron. The fourth o!ygen atom of the silica tetrahedron is important as it has a capacity for

cation e!change. /lays are thus commonly used as natural ione!change resins in %ater purification and

desalination. /lays can be used to remove sodium ions from sea%ater, as %ell as to remove calcium and

magnesium ions in the process of %ater softening. 3ecause the bonds bet%een ad8acent sheets of silicon

tetrahedra are %ea), the layers tend to slip past one another rather easily, %hich contributes to the slippery

te!ture of clays.

/lays also tend to absorb &or release( %ater. This absorption or release of %ater significantly changes clay

volume. /onsequently, soils that contain significant amounts of %aterabsorbing clays are not suitable as

 building construction sites.

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/lays are actually secondary minerals=meaning that they are formed chiefly by the %eathering of primary

minerals. +rimary minerals are those that form directly by precipitation from solution or magma, or by

deposition from the vapor phase . In the case of clays their primary or parent minerals are feldspars, the

mineral group %ith the greatest abundance in 4arths crust. Feldspars and clays are actually aluminosilicates.

The formation of an aluminosilicate involves the replacement of a significant portion of the silicon in the

tetrahedral bac)bone by aluminum.

The feldspar minerals have internal arrangements that correspond to a threedimensional array of silica

tetrahedra that arises from the sharing of all four o!ygen atoms at the tetrahedral vertices, and are sometimes

referred to as frame%or) silicates. Feldspars, rich in potassium, typically have a pin) color and are

responsible for the pin)ish color of many of the feldsparrich granites that are used in building construction.

The feldspathoid minerals are similar in structure to feldspars but contain a lesser abundance of silica. Capis

la*uli, no% used primarily in 8e%elry, is a mi!ture of the feldspathoid la*urite and other silicates, and %as

formerly used in granulate form as the paint pigment ultramarine.

Keolites are another group of frame%or) silicates similar in structure to the feldspars. Ci)e clays they have

the ability to absorb or release %ater. Keolites have long been used as molecular sieves, due to their ability to

absorb molecules selectively according to molecular si*e.

One of the most %ell)no%n silicate minerals is quart* &SiO ' (, %hich consists of a continuous three

dimensional net%or) of silica and o!ygen %ithout any atomic substitutions. It is the second most abundant

continental mineral, feldspars being most abundant. The net%or) of covalent bonds &bet%een silicon ando!ygen( is responsible for the %ell)no%n hardness of quart* and its resistance to %eathering. 0lthough pure

quart* is clear and %ithout color, the presence of small amounts of impurities may result in the formation of

gemstones such as amethyst.

Nonsilicate Minerals

0lthough minerals of other classes are relatively scarce in comparison to the silicate minerals, many have

interesting uses and are important economically. 3ecause of the great abundance of o!ygen in 4arths crust,

the o!ides are the most common minerals after the silicates. Citharge, for e!ample, is a yello%colored o!ide

of lead &+bO( and is used by artists as a pigment. "ematite &Fe ' O (, a reddishbro%n ore, is an iron o!ide

and is also used as a pigment. Other important classes of nonsilicate minerals include sulfides, sulfates,

carbonates, halides, phosphates, and hydro!ides. Some minerals in these groups are listed in Table 1.

0lthough minerals are often identified by the use of sophisticated optical instruments such as the  polari*ingmicroscope or the !ray diffractometer, most can be identified using much simpler and less e!pensive

methods. /olor can be very helpful in identifying minerals &although it can also be misleading(. 0 very pure

sample of the mineral carborundum &0l ' O ( is colorless but the presence of small amounts of impurities in

carborundum may yield the deep red gemstone ruby or the blue gemstone sapphire. The strea) 

&0AM%"&S F CMMN NNS"CA#& MN&!A"S AN' #&! *S&S

S*!C& Tarbuc), 4d%ard ., and Cutgens, Frederic) B. &1<<<(. Earth: An Introdution to -hysial

Geology , :th edition. Hpper Saddle ;iver, 9@ +rentice "all.

Mineral Formula &conomic *se

+yrite FeS ' sulfuric acid production

0nhydrite /aSO 7  plaster /alcite /a/O lime

"alite  9a/l table salt

Turquoise /u0l : &+O 7 ( 7 &O"( A gemstone

3au!ite 0l&O"( .n" ' O aluminum ore

;utile TiO '  8e%elry, semiconductor 

of a mineral &the color of the po%dered form( is actually much more useful in identifying a mineral than is

the color of the entire specimen, as it is less affected by impurities. The strea) of a mineral is obtained by

simply rubbing the sample across a strea) plate &a piece of ungla*ed porcelain(, and the color of the po%der

is then observed. 6irtually all mineral inde!es used to identify minerals, such as those found in 0ana4s

 Manual of Mineralogy list strea)s of individual minerals.

Strea) is used along %ith other rather easily determined mineral properties, such as hardness, specific

gravity, cleavage, double refraction, the ability to react %ith common chemicals, and the overall appearance,

to pinpoint the identity of an un)no%n mineral. ineral hardness is determined by the ability of the sample

to scratch or be scratched by readily available ob8ects &a )nife blade, a fingernail, a glass plate( or minerals

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of )no%n hardness. "ardness is graded on the ohs scale of hardness, %hich ranges from a value of one

&softest( to ten &hardest(. The mineral talc &used in talcum po%der( has a hardness of one, %hereas diamond

has a hardness of ten. 0 fingernail has a hardness of '.5 therefore quart*, %hich has a hardness of seven,

%ould be able to scratch talc or a fingernail, but quart* could not scratch diamond or topa*, %hich has a

hardness of eight. /onversely, topa* or diamond %ould be able to scratch quart*. Specific gravity is the ratio

of the %eight of a mineral to the %eight of an equal volume of %ater and is thus in concept similar to

density. The cleavage of a mineral is its tendency to brea) along smooth parallel planes of %ea)ness and is

dependent on the internal structure of the mineral. 0 mineral may e!hibit double refraction. That is, the

double image of an ob8ect %ill be seen if one attempts to vie% that ob8ect through a transparent bloc) of the

mineral in question. /alcite is a mineral that e!hibits double refraction. Some minerals react spontaneously

%ith common chemicals. If a fe% drops of hydrochloric acid are placed on a freshly bro)en surface of

calcite, the calcite %ill react vigorously. &ffer/escence , caused by reaction of the calcite %ith hydrochloric

acid to form the gas carbon dio!ide, is observed. In contrast, dolomite %ill effervesce in hydrochloric acid

only upon the first scratching the surface of the dolomite.

inerals are a part of our daily lives. They comprise the ma8or part of most soils and provide essential

nutrients for plant gro%th. They are the basic building bloc)s of the roc)s that compose the surface layer of

our planet. They are used in many types of commercial operations, and the mining of minerals is a huge

%orld%ide commercial operation. They are also used in %ater purification and for %ater softening. Finally,

minerals are perhaps most valued for their great beauty. Mary &. ,ohn

-i+liography

Dana, ames D.5 revised by /ornelius S. "ulburt r. &1<<(. 0ana4s Manual of Mineralogy 1-th edition.

 9e% Eor)@ #iley.

Dietrich, ;ichard 6., and S)inner, 3rian . &1<-<(. 5oks and 5ok Minerals. 9e% Eor)@ #iley.

Tarbuc), 4d%ard ., and Cutgens, Frederic) B. &1<<<(. Earth: An Introdution to -hysial Geology :th

edition, Hpper Saddle ;iver, 9@ +rentice "all.

There are several thousand )no%n minerals in nature &%ith estimates ranging from ',222 to -,222(, but

fe%er than a hundred are considered gem minerals. Of these, only about a do*en or so are actually valuableenough to be important gemstones on the %orld mar)et. In order to be considered a gemstone, a mineral

must first of all be beautiful. In addition, it must be hard and durable. Its value increases if it is also rare.

The beauty of a gem is measured in terms of its clarity, brilliance, and color. Its natural beauty can be

enhanced by the %ay it is cut. There are t%o basic )inds of gem cuts@ faceted and cabochon. The faceted cut

has many flat cut surfaces &facets( %ith an overall shape that might be round, oval, square, rectangular, or

 pearshaped. Faceted cuts are preferred for brilliant transparent stones such as diamond. The cabochon cut

has a smooth rounded top, usually %ith a flat base, and it is mainly used for opaque or translucent stones.

"ardness is measured using the ohs scale, on %hich 12 is hardest. &Diamond has a hardness of 12.(

Gemstones should have a ohs hardness of : or more. 0 really durable gem should have a hardness of atleast -, %hich is the hardness of quart*. Table 1 sho%s the hardness of some familiar minerals on the ohs

scale.

The value of a gemstone depends on its beauty and its rarity, but also the si*e of the stone. Si*e is measured

in terms of %eight using the carat as a unit. 0 carat is 2.' grams &2.22- ounces(. &0 12carat diamond %eighs

' grams, or 2.2- ounces.( There are 122 points in a carat, so a 2point diamond %eighs 2. carat, or 2.2:

grams &2.22' ounces(. Since gemstones vary in density &%eight per unit volume(, several different 1carat

stones may vary in si*e, the stones %ith the greatest density being smaller than the others.

Some mportant Gemstones

Diamond is the hardest substance )no%n to occur in nature, measuring 12 on the ohs scale. It is pure

carbon in a tightly pac)ed cubic structure. Diamonds are usually graded on the basis of four /s@ carat, cut,

clarity, and color. /arat refers to the stones %eight and degree of fla%lessness. 0s for color, diamonds areusually colorless, but sometimes they do e!hibit color. The famous "ope diamond, for e!ample, is blue.

ost diamonds come from mines in 0frica, especially southern 0frica, although ;ussia and 0ustralia also

have diamond mines. Industrialgrade diamonds have even been made synthetically at very high pressures

and temperatures. 0 number of other softer colorless stones are often sold as imitation diamonds.

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;uby is a variety of corundum &0l ' O ( %ith a ohs hardness of <. Its red color results %hen chromic ions

&/r L ( replace some of the aluminum ions in the crystal. The finest rubies come from yanmar &formerly

3urma( or Benya. Star rubies are stones %ith a special starli)e appearance. They usually have a cabochon

cut and appear to sho% a :pointed star due to microscopic inclusions. ;ubies of very high quality are

sometimes made synthetically.

Sapphire, li)e ruby, is made of corundum &0l ' O ( and has a hardness of <. "o%ever, instead of /r

impurities, the crystal contains iron and titanium

Mohs2 A!'N&SS SCA"&

Mineral Chemical Composition ardness

Talc g Si 7 O 12 &O"( ' 1

Gypsum /aSO 7 '

/alcite /a/O

Fluorite /aF ' 7

0patite /a &+O 7 ( ' R/aF '

Feldspar B ' 0l ' Si : O 1: :

uart* SiO ' -

Topa* 0l ' SiO 7 F ' A

/orundum 0l ' O <Diamond / 12

ions, %hich produce a blue color, ranging from very pale to very dar) blue. Sapphires are found in

/ambodia and other places in Southeast 0sia and 0ustralia, as %ell as ontana in the Hnited States. 0lso, as

in the case of rubies, there are star sapphires, %hich e!hibit a :pointed star. +ure corundum &%hite sapphire(

%as the first gem to be produced synthetically. It %as a poor substitute for diamond, ho%ever, because of its

lo% refractive inde!. 0dding about 2.1 percent chromium, ho%ever, produces rubies of e!cellent quality5

and the addition of iron and titanium yields beautiful blue sapphires. 4ven star sapphires and rubies that rival

natural stones can be made synthetically.

4merald is a variety of beryl, a beryllium silicate, %ith a hardness of -. to A. It has a beautiful deep green

color, and it is one of the most e!pensive gems, sometimes outran)ing diamond in value. The green color

results from small amounts of chromic o!ide &/r ' O (. The oldest emerald mines %ere in 4gypt near the;ed Sea, but the best emerald mines today are in /olombia. There are others in 3ra*il, +a)istan, and 0frica5

synthetic emeralds of e!cellent quality have also been manufactured.

0quamarine, li)e emerald, is a transparent variety of beryl, or beryllium silicate. Its light blue to bluegreen

color results from small amounts of iron in the crystal. Ci)e most beryl stones, it measures -. to A on the

ohs hardness scale. ost aquamarine gemstones come from 3ra*il.

Topa* is a rather rare silicate mineral %ith a ohs hardness of A. It comes in many colors from yello% to

 pin) to purple to blue, depending on %hat ions are present in the crystal. It can even be colorless. "o%ever,

the favorite variety is orange to bro%n in color and called $imperial topa*.$ uch of the best topa* comes

from 3ra*il. The gem called Condon blue topa* can be made from the colorless variety by treatment %ith

heat and radiation.

/ubic *irconia &KrO ' (, %ith a ohs hardness of A, is a beautiful, usually colorless, stone that is made

synthetically. 0lthough not as hard as diamond, cubic *irconia has much fire and brilliance, and it is popular

as an imitation diamond. Kirconia normally has a monoclinic crystalline structure at room temperature, but

%hen heated to about ',22N/ &7,1-'NF(, it ta)es on a cubic structure. Ordinarily, it %ould revert to the

monoclinic structure on cooling, but the addition of yttrium o!ide &E ' O ( or calcium o!ide &/aO(

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0 mi!ture of natural and synthetic diamonds. Diamonds are the hardest natural substance )no%n.

can stabili*e the *irconia so that it retains the cubic structure at room temperature. /ubic *irconia has optical

 properties very close to those of diamond, and it is clearly the best of all the various diamond imitations.

Kircon is native *irconium silicate &KrSiO 7 ( that e!hibits beautiful transparent crystals and a ohs hardness

of -.. The tetragonal crystals are usually bro%nish yello% in color. 0lso )no%n as 8argon or 8argoon, *ircon

is a stable and durable silicate crystal. Small crystals of *ircon are among the oldest mineral grains everfound on 4arth.

Opal is a hydrous silica &SiO ' (, sometimes thought of as an amorphous silica gel. It is a fairly soft gem,

measuring only to : on the ohs scale. It is relatively common in nature e!cept in its $precious$ form,

%hich comes mainly from 0ustralia. In S%it*erland, since 1<-2, opal of precious quality has been made

synthetically. Hsually cut in the cabochon shape to permit its rainbo%li)e display of color, opals come in

%hite, blac), and fire varieties. $3lac)$ opals are dar) gray to blue, and fire opals, %hich are more

transparent than other opals, are usually orangered in color.

0methyst is a variety of quart* &SiO ' ( that is violet to purple in color, probably because of iron

and manganese impurities. It measures - on the ohs hardness scale and is obtained from many places, but

mainly from India and 3ra*il. It should not be confused %ith oriental amethyst, %hich is a purple native

variety of alumina &0l ' O (.Spinel is a colorless magnesium aluminate &g0l ' O 7 ( of cubic structure. It is hard and durable, but, li)e

%hite sapphire, it is not a good diamond substitute because it has a lo% refractive inde! and lac)s brilliance.

"o%ever, it is readily doped to produce other gems of various colors. 0rtificial ruby, for e!ample, is often

natural red spinel, and most synthetic blue sapphires on the mar)et are actually blue spinel.

+eridot is the gem variety of olivine, a magnesium silicate containing iron &about < g atoms for every Fe

atom(. +eridot is usually transparent,

0 handful of a variety of gemstones. The beauty of gemstones is measured by their clarity, brilliance, and

color.

%ith a color ranging from greenish yello% to bro%nish green. uch peridot comes from an island in the ;ed

Sea, but it is also found in yanmar and an 0pache reservation in 0ri*ona.

Garnet is actually a group of related silicates containing various amounts of magnesium, calcium, aluminum,

iron, manganese, and chromium. Garnets have a hardness of :. to -., depending on their composition, and

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their color also varies along %ith their composition. The popular dar) red garnets are found in many gem

sites in the /*ech ;epublic, but garnets of many colors are also found in other parts of the %orld, such as

India, Tan*ania, ;ussia, and 3ra*il.

Tourmaline is a highly complicated silicate, %ith a %ide range of compositions and colors. It probably

e!hibits more colors than any other )ind of gemstone. Sometimes there are several different colors in the

same crystal. $#atermelon$ tourmaline, for e!ample, is green on the outside but red in the middle.

Capis la*uli is a deep blue gemstone that is a comple! copper silicate mineral varying %idely in

composition. It often contains spar)les of iron pyrite or calcite. The best source is probably 0fghanistan. 0

 pale blue variety is found in /hile. Some material sold as lapis la*uli is actually artificially colored 8asper

from Germany.

Ony! is a striped variety of the common silicate mineral called agate, %ith alternating blac) and %hite

 bands. It comes mainly from India and South 0merica. Sardony! is a variety of ony! %ith bro%n and %hite

 bands.

3rganic3 Gems

The aforementioned gemstones, unless they are synthetic, usually occur in underground deposits from %hich

they are mined. "o%ever, there are some gems that come from once living material.

+earls are little spheres of calcium carbonate &/a/O ( that form in mollus)s & in/erte+rate shellfish( such

as oysters, usually because of some sort of irritation. They are normally %hite or off%hite in color, but they

can have bluish or pin) tints, and sometimes they are dar) gray. 0lthough many pearls form naturally, pearl production has been greatly increased by the $cultured$ pearl industry, %hich raises beds of oysters into

%hich irritants are routinely introduced. The irritants are usually bits of motherofpearl, the lining that

forms inside oyster shells.

0mber is fossili*ed tree resin that hardened over millions of years and no% is valued as a gem. 3altic amber

is thought to be hardened sap from pine trees. It is normally yello%bro%n in color, but the shades vary from

almost %hite to almost blac). 0lthough sometimes completely clear, amber often contains inclusions of

insects or other matter, often considered desirable. uch amber is obtained along the shores of the 3altic

Sea, but it is also found along the coasts of Sicily, ;omania, and yanmar.

/oral, li)e pearls, is calcium carbonate &/a/O ( derived from living matter. It is the outer shells of small

marine animals. It occurs in many colors, from %hite to deep pin) and red. The greatest demand is for red

coral. The best coral comes from the editerranean Sea, along the coasts of 0lgeria and Tunisia.Ivory is a boneli)e material that comes from the tus)s of animals &elephant, %alrus, hippopotamus(. It has

 become such a highly pri*ed material that there are unscrupulous poachers %ho )ill these animals in order to

steal their tus)s. The pale cream color of ne% ivory becomes dar)er %ith age and turns yello%. Ivory is

 brittle, but it does not peel as do its plastic substitutes.

et is actually 8ust a very hard and dense )ind of lignite coal. It %as probably plant material millions of years

ago that has become fossili*ed and blac)ened over time. It often comes from northeast 4ngland, %here it is

derived from fossil drift%ood buried under the sea. Its primary dra%bac) as a gemstone is that it %ill burn

&since it is basically 8ust highly polished coal(.

AM-&! AN' &"&C#!C#$

0ncient Gree)s called amber ele)tron. #hen they rubbed it %ith a cloth, it became charged and attracted

 bits of paper. The %ord $electricity$ derives from the Gree) %ord ele)tron. 0oris K. Kolb

-i+liography

Delins)y, 3arbara &'221( Gemstone. 9e% Eor)@ "arper Torch.

"all, /ally &1<<7(. Gemstones. 9e% Eor)@ Dorling Bindersley.

Braus, 4d%ard "enry &1<1(. Gems and Minerals. 9e% Eor)@ cGra%"ill.

atlins, 0ntoinette C., and 3onanno, 0. /. &1<<-(. Gem Identifiation Made Easy 'nd edition. #oodstoc),

6T@ Gemstone +ress.

+ellant, /hris, and ;ussell, "enry &'221(. Enylopedia of 5oks Minerals and Gemstones. San Diego,

/0@ Thunder 3ay +ress.

Schumann, #alter &1<<-(. Gemstones of the World revised edition. 9e% Eor)@ Sterling.

Smith, ;ichard Thomas &1<A(. Gemstones. 9e% Eor)@ +itman.Spencer, Ceonard ames &1<<(. A Key to -reious ,tones. 9e% Eor)@ 4merson.

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The development of semiconductors is clearly among the most significant technological achievements to

evolve from the study of solidstate chemistry and physics. 0side from their %ell)no%n applications in

computers and electronics, semiconductors are also used in a %ide variety of optical devices such as lasers,

lightemitting diodes, and solar panels. The diversity of applications can be readily understood %ith only a

 basic understanding of the theory behind these materials.

#heoryThe operation of semiconductors is best understood using band theory. 0t its most fundamental level, band

theory can be e!tremely comple!, requiring relatively advanced mathematics and physics. #hen a large

number of atoms combine to form a solid, the electrons e M in the solid are distributed into energy bands

among all the atoms in the solid. 4ach band has a different energy, and the electrons fill these bands from the

lo%est energy to the highest, similar to the %ay electrons occupy the orbitals in a single atom. The variation

in properties bet%een electrical insulators, conductors & metals (, and semiconductors stems from differences

in the band structures of these materials &see Figure 1(. For this discussion, three terms must be defined. The

highest energy band that contains electrons is called the /alence band, %hereas the lo%est energy empty

 band is called the conduction band. The band gap is the difference in energy bet%een the valence and

conduction bands. The la%s of quantum mechanics forbid electrons from being in the band gap5 thus, an

electron must al%ays be in one of the bands.

In a metal &e.g., copper or silver(, the valence band is only partially filled %ith electrons &Figure 1a(. This

means that the electrons can access empty areas %ithin the valence band, and move freely across all atoms

that ma)e up the solid. 0 current can therefore be generated %hen a/oltage is applied. In general, for

electrons to flo% in a solid, they must be in a partially filled band or have access to a nearby empty band. In

an electrical insulator, there is no possibility for electron flo% &Figure 1b(, because the valence band is

completely filled %ith electrons, and the conduction band is too far a%ay in energy to be accessed by these

electrons &the band gap is too large(. 0 semiconductor &Figure 1c( is a special case in %hich the band gap is

small enough that electrons in the valence band can 8ump into the conduction band using thermal energy.

That is, heat in the material

0 conventional tube amplifier, at left, and a solidstate memory cell, at right. The si*e of such

semiconductors allo%s for the manufacturing of smaller devices.

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Figure 1. Schematic of the electronic band structures of different types of solids. 4lectrons are represented

 by shaded areas.

&even at room temperature( gives some of the electrons enough energy to travel across the band gap. Thus,

an important property of semiconductors is that their conductivity increases as they are heated up and more

electrons fill the conduction band. The most %ell)no%n semiconductor is silicon &Si(,

although germanium &Ge( and gallium arsenide &Ga0s( are also common.

To complete the development of semiconductor theory, the concept of doping must be described &see Figure

'(. In principle, the idea is to introduce a different )ind of atom into a semiconductor in order to modify its

electronic structure. /onsider, for e!ample, adding a small amount of phosphorus, +, into a silicon host.

+hosphorus is one column to the right of silicon in the +eriodic Table, so it contains one additional electron.

This means that doping + into Si has the effect of introducing additional electrons to the material, such that

some e M must go into the conduction band. 3ecause e!tra negatively charged electrons are added to the

system, phosphorusdoped Si is called an n* type semiconductor, and phosphorus is described as a donor &of

electrons(. Similarly, a p* type semiconductor can be fabricated by adding an element to the left of Si in the

+eriodic Table. 3oron, 3, is a common dopant for a p* type. In this case, the valence band %ill be missing

electrons. These empty locations in a p* type semiconductor are also referred to as holes. Since holes

represent the absence of an electron, they carry a positive charge. In  p* type semiconductors, boron is

referred to as an acceptor &of electrons(. From Figure ', it can be seen that both n* and p* type materials

create partially filled bands, allo%ing for electrical conduction. Dopant concentrations are fairly small,around 12 1: atomscm , constituting only about tenbillionths of the total mass of the material.

If p* and n* type materials are layered together, a p*n 8unction results &Figure 'c(. ;ight at the interface,

some of the e!cess electrons from the n* type combine %ith holes from the p* type. The resulting charge

separation creates an energy barrier that impedes any further movement of electrons. In most technological

applications, the important properties of semiconductors are the result of the band structure of the  p*

n 8unction. 0 single

Figure '. Schematic diagrams of the band structures of &a(  p type semiconductors, &b( n type

semiconductors, and &c( a p*n 8unction.

 8unction based on the same host material &e.g., one interface of p* and n* doped silicon( is called a

homo8unction. The homo8unction model is used here to describe the properties of many devices that are

 based on semiconductors. "o%ever, it should be noted that real systems are typically composed ofmultiple p*p n*n and p*n 8unctions, called hetero8unctions. Such configurations greatly improve the

 performance of these materials5 in fact, the development of hetero8unction devices %as critical to the

%idespread practical application of this technology.

Semiconductors in &lectronics

Semiconductors are used e!tensively in solidstate electronic devices and computers. The ma8ority of

materials for these applications are based on doped silicon. 0n important property of p*n 8unctions is that

they allo% electron flo% only from the n side to the p side. Such one%ay devices are called diodes.

/onsider Figure 'c again. If a positive voltage &also called a for%ard bias( is applied that lo%ers the energy

 barrier bet%een n and p then the electrons in the conduction band on the n side can flo% across the 8unction

&and holes can flo% from p to n (. 0 reverse bias, ho%ever, raises the height of the barrier and increases the

charge separation at the 8unction, impeding any flo% of electrons from p to n.Diodes have several important applications in electronics. The po%er supplied by most electrical utilities is

typically alternating current &0/(5 that is, the direction of current flo% s%itches bac) and forth %ith a

frequency of si!ty cycles per second. "o%ever, many electronic devices require a steady flo% of current in

one direction &direct current or D/(. Since a diode only allo%s current to flo% through it in one direction, it

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can be combined %ith a capacitor to convert 0/ input to D/ output. For half the 0/ cycle, the diode passes

current and the capacitor is charged up. During the other half of the cycle, the diode bloc)s any current from

the line, but current is provided to the circuit by the capacitor. Diodes applied in this %ay are referred to as

rectifiers.

The by far most important application of semiconductors is as logic gates and transistors in computers.

Cogic gates, such as O; and 09D gates, ta)e advantage of the one%ay nature of diodes to compare the

 presence or 

icro%ires bonded on a silicon chip.

absence of current at different locations in a circuit. ore comple! solidstate transistors are composed

of npn or pnp 8unctions. The device geometry is slightly more complicated than that observed in a diode, but

the result is materials that allo% for the generation of the *eros and ones required for the binary logic used

 by computers.

ptoelectronic 'e/ices

Optoelectronic materials are a special class of semiconductors that can either convert electrical energy into

light or absorb light and convert it into electrical energy. Cightemitting diodes &C4Ds(, for e!ample, are

commonly used for information display and in automotive interior lighting applications. In an C4D, a

for%ard bias applied across the 8unction moves electrons in the conduction band over holes in the valence

 band. The electron and hole combine at the 8unction, and the energy created by this process is conserved via

the emission of light &Figure a(. The %avelength of emitted light %ill depend on the band gap of the

material5 larger band gaps lead to shorter %avelengths of light. Only certain )inds of semiconductors, called

direct gap semiconductors, e!hibit this behavior. Ga0s is an e!ample of a direct gap semiconductor used in

these applications. Silicon is an indirect gap material, and electrons and holes combine %ith the generation

of heat instead of light.

0 diode laser  operates in essentially the same fashion as an C4D. T%o additional requirements must be met

for a direct gap semiconductor to be an efficient laser. The first is that larger for%ard bias currents are

needed for a laser than for an C4D, because lasers require a higher degree of population inversion=a largenumber of electrons in the conduction band above empty levels in the valence band. Casers also require an

optical cavity5 light bounces bac) and forth %ithin the cavity, building up intensity. In a diode

Figure . +rinciple of operation of &a( a lightemitting diode or diode laser and &b( a photodetector or solar

cell.laser, this can be achieved by cleaving and polishing opposite faces of the diode. The smooth faces act li)e

 partially reflecting mirrors. This )ind of laser is used to read information on compact dis)s and is also used

in laser pointers.

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The most common materials for lasers and C4Ds are hetero8unctions based on Ga0s. ore comple! systems

containing Ga, 0s, +, 0l, and 9 are also used. The band gap of these materials can be tailored to create

emission from infrared to yello%. In optical data storage systems, such as compact dis)s, the amount of

information that can be stored is dependent in part on the %avelength of light being used to read the dis)= 

shorter %avelengths allo% for denser information storage. Thus, there has been considerable interest in

developing larger band gap C4Ds and lasers that emit in the blue. This has been achieved in semiconductors

 based on Ga9 &gallium nitride(. Further refinement of these materials %ill no doubt lead to significant

advances in optoelectronic technology in the coming years.

0 final important class of optoelectronic devices based on semiconductors is photovoltaics, such as

 photodetectors and solar cells. In some respects, these can be regarded as C4Ds operating in reverse. Cight

energy incident on the p*n 8unction is absorbed by an electron, %hich then 8umps to the conduction band

&Figure b(. Once in the conduction band, the electron travels do%nhill &energetically( to the n side of the

 8unction, %ith a hole migrating to the p side. This creates a flo% of current that is the reverse of %hat is seen

in a for%ard biased diode. The result is the conversion of light energy to electrical energy. These devices can

therefore be used to detect light, as in digital imaging systems or miniature cameras5 or the electrical energy

can be stored, as in solar cells. /ommercial photovoltaics are based on a variety of host materials, including

Si, 0lGa0s, and In0l0s.

Fa+rication

The industrial fabrication of semiconductors can be e!tremely comple!, involving highpurity materials,sophisticated equipment, and hundreds of steps. ost processes begin %ith the gro%th of a large single

crystal of n* type Si, called a %afer. 0 dopant &e.g., phosphorus( is added to highpurity molten silicon, and a

crystal is then slo%ly e!tracted from this melt. The polished %afer is '2 to 2 centimeters &-.<>11.A inches(

in diameter.

The rest of the processing %ill depend on the nature of the device being produced. 0 simple p*n 8unction is

usually fabricated via photolithography and etching processes. In this method, a layer of silicon dio!ide,

SiO ' , is created on the surface of the %afer by heating it in the presence of 

0 %or)er is testing silicon %afers at the atsushita Semiconductor plant in +uyallup, #ashington.

Semiconductors are used in many different electronic products, such as computers, lasers, and solar panels.

o!ygen. Some of the SiO ' is then chemically stripped a%ay, or etched, e!posing only a portion of the Si

%afer. This e!posed part of the %afer is made into p* type material by bombarding it %ith boron ions. 0s

these ions diffuse into the Si %afer, p* type Si is formed. Since the original %afer %as n* type, a p*n 8unction

forms %here the diffusion of boron stops. etal contacts can then be added to each side of the 8unction to

create a simple homo8unction device.

Fabrication of more complicated devices is achieved via combinations of etching, deposition, and ion

implantation steps. In the production of integrated circuits for computers, about 722 chips can be

synthesi*ed on a single 2centimeter &11.Ainch( %afer. 4ach chip may contain as many as 2 million

transistors in a space barely more than 1 centimeter &2.< inches( on a side=a truly remar)able

technological achievement. 0s faster and faster systems are developed, the demand for smaller and smaller

features increases. Such miniaturi*ation is the most significant challenge facing the semiconductor industrytoday.

Semiconductors are used in a %ide variety of electronic and optoelectronic applications. The useful

 properties of semiconductors arise from the unique behavior of doped materials, the special control of

electron flo% provided by p*n 8unctions, and the interaction of light energy %ith electrons at these 8unctions.

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The industry continues to gro%, and research in this and related areas &i.e., organic semiconductors and

molecular transistors( is occurring at academic institutions around the %orld.

 Anthony 0ia6 

-i+liography

3hattacharya, +allab &1<<7(. ,emiondutor 7ptoeletroni 0e2ies . 4ngle%ood /liffs, 9@ +rentice "all.

ay, Gary S., and S*e, Simon . &'22(. Fundamentals of ,emiondutor Fabriation. 9e% Eor)@ #iley.

iessler, Gary C., and Tarr, Donald 0. &1<<<(. Inorgani +hemistry , 'nd edition, Hpper Saddle ;iver, 9@

+rentice "all.

yers, ". +. &1<<-(. Introdutory ,olid ,tate -hysis , 'nd edition. +hiladelphia@ Taylor J Francis.

Svelto, Ora*io &1<A<(. -riniples of &asers , rd edition, tr. and ed. David /. "anna. 9e% Eor)@ +lenum.

Te!as Instruments Cearning /enter &1<-'(. /nderstanding ,olid ,tate Eletronis , 'nd edition. Dallas@

Te!as Instruments Inc.

#old, 0aron, and D%ight, Birby &1<<(. ,olid ,tate +hemistry . 9e% Eor)@ /hapman J "all.

Superconductivity, %hich is defined as the absence of resistance in a conducting material to a continuously

flo%ing electric current, is a special property that a si*able number of substances attain suddenly at very lo%

temperatures. The substances &called superconductors( include elements, alloys , compounds, and

nonstoichiometric ceramic materials. Superconductors also e!hibit perfect diamagnetism5 that

is, magnetic fields cannot penetrate them &the eissner effect(, and small po%erful magnets actually float

&levitate( above flat superconductor surfaces. 0 superconductors critical transition temperature, ! / , is the

temperature above %hich no superconductivity can be obtained. For elements, alloys, and simple

compounds, very lo% critical transition temperatures & ! / ' B( mean that the cooling effects of liquid

helium & 3.+. 7 B( are needed to bring about and to maintain their superconductivity. The discovery in

1<A: that nonstoichiometric ceramics containing copper and o!ygen can have much higher ! / values has

 provided a ne% impetus for developing superconducting materials.

igh #emperature SuperconductorsIn 0pril 1<A:, B. 0le! Uller and . Georg 3ednor* &%ith I3 in S%it*erland( reported the

superconductivity of a nonstoichiometric ceramic o!ide of lanthanum, barium, and copper,

Ca 'M % 3a  % /uO 7M y , %ith the then record high ! / of B. Further e!periments conducted by Uller,

3ednor*, and others sho%ed that slight modifications made to Ca 'M % 3a  % M /uO 7M y & % 2.' and y is even

smaller( could yield materials having ! / s of 2 B. 3y early 1<A-, +aul /. #. /hu &at the Hniversity of

"ouston(, a%Buen #u &at the Hniversity of 0labama(, and their co%or)ers synthesi*ed another ceramic

o!ide material, E3a ' /u O -M y , and observed that superconductivity in the material %as attainable by

cooling it %ith liquid nitrogen & 3.+. -- B(. This $high temperature superconductor$ made possible

0 magnet is hovering over a superconductor, demonstrating that magnetic fields cannot penetrate thesuperconductor, )no%n as the eissner effect.

superconductor applications that %ere impractical %ith the lo% temperature superconductors. &See Figure 1.(

Other nonstoichiometric ceramic o!ides that contain copper in nonintegral o4idation states have been

synthesi*ed and evaluated. Several of these materials have even higher ! / s than that of E3a ' /u O -M y .

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The 3S//O series 3i ' Sr ' /a n M 1/u n O ' n L7L y &for n 1 to 7( reaches a ! / ma!imum of 112 B for

3i ' Sr ' /a ' /u O 12L y 5 a similar Tl ' 3a ' /a n M1/u n O ' n L7L y series reaches a ma!imum of 1'' B for

Tl ' 3a ' /a ' /u O 12L y 5 "g3a ' /a ' /u O AL y has a ! / of 1 B at ambient pressure5 and

"g 2.A Tl 2.' 3a ' /a ' /u O A. has a ! /of 1A B. 0lso, the ! / of "g3a ' /a ' /u O AL y has been reported to

increase to 1 B at a pressure of 12,222 atmospheres and to 1:2 B at 'A2,222 atmospheres. 4ven

higher ! / values have been claimed for portions of multiparticle ceramics, but no macroscopic material has

sho%n unambiguous superconductivity at these higher temperatures &above 1:2 B(.

Other ne% classes of superconductors that are being investigated include intermediate temperature range

superconductors, such as magnesium diboride & ! / < B(, al)alidoped / :2 & / :2 has a ! / of B(, and

holedoped / :2 & ! / ' B(. The latter result led an "endri) Schon, /hristian Bloc, and 3ertram 3atlogg

&of 3ell Cabs( to the ne%er haloformintercalated, high temperature / :2 superconductors / :2 V '/"/l and

/ :2 V '/"3r , %ith ! / values of A2 B and 11- B, respectively.

The theoretical interpretation of the high temperature superconductors is still under development. The

copper o!ide ceramic superconductors obtain their paired conducting electrons from copper in mi!ed

o!idation states of I and II or II and III, depending on the particular system. The paired conducting electrons

are called /ooper pairs, after Ceon 9. /ooper. /oopers name also gives us the / of 3/S5 the 3/S theory is

an interpretation of superconductivity for lo% temperature superconductors &having ! / s of less than 72 B(.

M&SSN&! &FF&C#

The eissner effect is the repulsion of a magnetic field from the interior of a superconductor belo% itscritical temperature. #hereas a %ea) magnetic field is totally e!cluded from the interior of a

superconductor, a very strong magnetic field %ill penetrate the material and concurrently lo%er the critical

transition temperature of the superconductor. #. eissner and ;. Ochsenfeld discovered the eissner effect

in 1<.

Figure 1. The crystal structure of E3a ' /u O -M y . ;edra%n from 9aval ;esearch Caboratory. 0vailable

from http@cst%%%.nrl.navy.millatticestru).picts.

Applications

0pplications for superconducting materials include strong superconducting magnets %ithout iron cores,%hich in turn have a variety of uses. These superconducting magnets are used in particle

accelerators, nuclear magnetic resonance and magnetic circular dichroism instruments, magnetic resonance

imaging devices in medicine, levitating trains, magnetic refrigerators, magnetic energy storage, and

SHIDS &superconducting quantum interference devices( for very sensitive magnetic field measurements

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&including biomedical magnetoencephalography(. ost magnetic applications %ere developed %ith and still

use the older lo% temperature superconductors, such as niobiumtitanium alloy superconducting %ire, %hich

must be cooled %ith liquid helium. For e!ample, the Fermilab Tevatron &1<A( uses 1,222 liquid helium> 

cooled superconducting magnets in its fourmile &circumference( protonantiproton collider. The sudden

quenching of superconducting magnets by vibrations, e!ternal fields, or accidental %arming can be a serious

 problem, and the associated heat can cause e!tensive helium loss.

/ooling %ith liquid nitrogen rather than liquid helium is much more economical. The difficulty in molding

the high temperature superconductors into strong and fle!ible forms &e.g., filaments or %ires( and the

greater ! / lo%ering that accompanies greater magnetic field strength have limited their use up to the present

time. Several firms have developed methods to improve the transfer of charge among superconducting

 particles5 and it appears that the best superconductors may be impure ones that allo% &the more disordered(

ceramic glass formation rather than ceramic crystallite formation. 3S//O superconducting transmission

lines are being manufactured &by 0merican Superconductor, +irelli, and Intermagnetics General( but are

currently competitive only %hen space or %eight limitations are important. Superconducting filters for

cellular communications, in %hich the lac) of resistance provides filters that have minimal signal loss and

are more discriminating in frequency tuning, are being mar)eted &by Superconductor Technologies, Illinois

Superconductor PIS/OQ, and /onductus(, and superconducting motors &0merican Superconductor( and

generators &General 4lectric( are under development.

-CS #&!$3/S theory, developed by ohn 3ardeen, Ceon /ooper, and ;obert Schrieffer, provides complicated

mathematical equations that satisfactorily e!plain the superconductivity of the classical lo% temperature

superconductors %ith critical transition temperatures belo% 72 B. The theory includes /ooper pairs of

electrons but does not e!plain the high critical transition temperatures of the ne%er ceramic

superconductors.

 5onald 0. Arher 

-i+liography

/ava, ;obert . &1<<2(. $Superconductors 3eyond 1>'>.$ ,ientifi Amerian ':&'(@ 7'>7<.

/hu, +aul /. #. &1<<(. $"ighTemperature Superconductors.$ ,ientifi Amerian '-&(@ 1:'>1:.

Dinnebier, ;obert 4.5 Gunnarsson, Olle5 3rumm, "olger5 et al. &'22'(. $Structure of "aloform Intercalated

/ :2 and Its Influence on Superconductive +roperties.$ ,iene '<:&1(@ 12<>11.Uller, B. 0le!, and 3ednor*, . Georg &1<A-(. $The Discovery of a /lass of "ighTemperature

Superconductors.$ ,iene '-@ 1,1>1,1<.

Schechter, 3ruce &'222(. $9o ;esistance@ "ighTemperature Superconductors Start Finding ;eal#orld

Hses.$ ,ientifi Amerian 'A&'(@ '>.

#ols)y, 0lan .5 Giese, ;obert F.5 and Daniels, 4d%ard . &1<A<(. $The 9e% Superconductors@ +rospects

for 0pplications.$ ,ientifi Amerian ':2&'(@ :2>:<.

/ryogenics is the science that addresses the production and effects of very lo% temperatures. The %ord

originates from the Gree) %ords kryos meaning $frost$ and geni meaning $to produce.$ Hnder such a

definition, it could be used to include all temperatures belo% the free*ing point of %ater &2N/(. "o%ever,

+rofessor "ei)e Bamerlingh Onnes of the Hniversity of Ceiden in the 9etherlands first used the %ord in

1A<7 to describe the art and science of producing much lo%er temperatures. "e used the %ord in reference

to the li5uefaction of permanent gases such as o!ygen, nitrogen, hydrogen, and helium. O!ygen had been

liquefied at M1AN/ a fe% years earlier &in 1AA-(, and a race %as in progress to liquefy the remaining

 permanent gases at even lo%er temperatures. The techniques employed in producing such lo% temperatures

%ere quite different from those used some%hat earlier in the production of artificial ice. In particular,

efficient heat e!changers are required to reach very lo% temperatures. Over the years the term $cryogenics$

has generally been used to refer to temperatures belo% appro!imately M12N/.

0ccording to the la%s of thermodynamics, there e!ists a limit to the lo%est temperature that can be

achieved, %hich is )no%n as absolute *ero. olecules are in their lo%est, but finite, energy state at absolute

*ero. Such a temperature is impossible to reach because the input po%er required approaches infinity.

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"o%ever, temperatures %ithin a fe% billionths of a degree above absolute *ero have been achieved. 0bsolute

*ero is the *ero of the absolute or thermodynamic temperature scale. It is equal to M'-.1N/ or M7<.:-NF.

The metric or SI &International System( absolute scale is )no%n as the Belvin scale %hose unit is the )elvin

&not Belvin(, %hich has the same magnitude as the degree /elsius. The symbol for the Belvin scale is B, as

adopted by the Thirteenth General /ouncil on #eights and easures &/G+( in 1<:A, and not NB. Thus,

2N/ equals '-.1 B. The 4nglish absolute scale, )no%n as the ;an)ine scale, uses the symbol N; and has

an increment the same as that of the Fahrenheit scale. In terms of the Belvin scale, the cryogenic region is

often considered to be that belo% appro!imately 1'2 B &M1N/(. The common permanent gases referred to

earlier change from gas to liquid at atmospheric pressure at the temperatures sho%n in Table 1, called the

normal boiling point &93+(. Such liquids are )no%n as cryogenic liquids or cryogens. #hen liquid helium is

cooled further to '.1- B or belo%, it becomes a superfluid %ith very unusual properties associated %ith

 being in the 5uantum mechanical ground state. For e!ample, it has *ero viscosity and produces a film that

can creep up and over the %alls of an open container, such as a bea)er, and drip off the bottom as long as the

temperature of the container remains belo% '.1- B.

The measurement of cryogenic temperatures requires methods that may not be so familiar to the general

 public. 9ormal mercury or alcohol thermometers free*e at such lo% temperatures and become useless. The

 platinum resistance thermometer has a %elldefined behavior of electrical resistance versus temperature and

is commonly used to measure temperatures accurately, including cryogenic temperatures do%n to about '2

B. /ertain semiconducting materials, such as doped germanium, are also useful as electrical resistancethermometers for temperatures do%n to 1 B and belo%, as long as they are calibrated over the range they are

to be used. Such

N!MA" -"NG %N# S F CMMN C!$G&NC F"*'S

Cryogen (6) (7C) (7!) (7F)

ethane 111.- M1:1. '21.1 M'A.:

O!ygen <2.' M1A.2 1:'.7 M'<-.

 9itrogen --.7 M1<.A 1<. M'2.7

"ydrogen '2. M''.< :. M7'.'

"elium 7.' M':<.2 -.: M7'.1

0bsolute *ero 2 M'-.1 2 M7<.:-secondary thermometers are calibrated against primary thermometers that utili*e fundamental la%s of

 physics in %hich a physical variable changes in a %ell)no%n theoretical %ay %ith temperature.

The production of cryogenic temperatures almost al%ays utili*es the compression and e!pansion of gases. In

a typical air liquefaction process the air is compressed, causing it to heat, and allo%ed to cool bac) to room

temperature %hile still pressuri*ed. The compressed air is further cooled in a heat e!changer before it is

allo%ed to e!pand bac) to atmospheric pressure. The e!pansion causes the air to cool and a portion of it to

liquefy. The remaining cooled gaseous portion is returned through the other side of the heat e!changer %here

it precools the incoming highpressure air before returning to the compressor. The liquid portion is

usually distilled to produce liquid o!ygen, liquid nitrogen, and liquid argon. Other gases, such as helium, are

used in a similar process to produce even lo%er temperatures, but several stages of e!pansion are necessary.

0 multicelled human embryo, 'W days after its removal from a %omb. It %as cryogenically stored at the

3ourn "all Fertility /linic, /ambridgeshire, 4ngland.

/ryogenics has many applications. /ryogenic liquids li)e o!ygen, nitrogen, and argon are often used inindustrial and medical applications. The electrical resistance of most metals decreases as temperature

decreases. /ertain metals lose all electrical resistance belo% some transition temperature and become

superconductors. 0n electromagnet %ound %ith a %ire of such a metal can produce e!tremely high magnetic

fields %ith no generation of heat and no consumption of electric po%er once the field is established and the

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metal remains cold. These metals, typicallyniobium alloys cooled to 7.' B, are used for the magnets of

magnetic resonance imaging &;I( systems in most hospitals. Superconductivity in some metals %as first

discovered in 1<11 by Bamerlingh Onnes, but since 1<A:, another class of materials, )no%n as high

temperature superconductors, have been found to be superconducting at much higher temperatures, currently

up to about 17 B. They are a type of ceramic, and because of their brittle nature, they are more difficult to

fabricate into %ires for magnets.

Other applications of cryogenics include fast free*ing of some foods and the preservation of some biological

materials such as livestoc) semen as %ell as human blood, tissue, and embryos. The practice of free*ing an

entire human body after death in the hope of later restoring life is )no%n as cryonics, but it is not an

accepted scientific application of cryogenics. The free*ing of portions of the body to destroy un%anted or

malfunctioning tissue is )no%n as cryosurgery. It is used to treat cancers and abnormalities of the s)in,

cervi!, uterus, prostate gland, and liver.

 5ay 5adebaugh

-i+liography

3arron, ;andall &1<A(. +ryogeni ,ystems. O!ford@ O!ford +ress.

Flynn, Thomas &1<<-(. +ryogeni Engineering. 9e% Eor)@ arcel De))er.

Scurloc), ;alph G., ed. &1<<(. History and 7rigins of +ryogenis. O!ford@ /larendon +ress.

Seeber, 3ernd, ed. &1<<A(. Handbook of Applied ,uperonduti2ity. 3ristol@ Institute of +hysics +ublishing.

Shachtman, Tom &1<<<(. Absolute 8ero and the +on9uest of +old. 3oston@ "oughton ifflin /ompany.#eisend, ohn G., II, ed. &1<<A(. Handbook of +ryogeni Engineering. +hiladelphia@ Taylor and Francis.