材料科学与工程专业英语.docx

材料科学与工程专业英语.docx

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材料科学与工程专业英语

2.6semiconductor

Followingthediscussionofintrinsic,elementalsemiconductorswenotethatthefermifunctionindicatesthatthenumberofchargecarriersincreasesexponentiallywithtemperature.Thiseffectsodominatestheconductivityofsemiconductorsthatconductivityalsofollowsanexponentialincreasewithtemperature（anexampleofanarrheniusequation）.Thisincreaseisinsharpcontrasttothebehaviorofmetals.

Weconsidertheeffectofimpuritiesinextrinsic,elementalsemiconductors.DopingagroupIVamaterial（suchasSi）withagroupVaimpurity（suchasP）producesann-typesemiconductorinwhichnegativechargecarriers（conductionelectrons）dominate.The“extea”electronfromthegroupVAadditionproducesadonorlevelintheenergybandstructureofthesemiconductor.Aswithinstrinsicsemiconductors,extrinsicsemiconductionexhibitsarrheniusbehavior.inn-typematerial,thetemperaturespanbetweentheregionsofextrinsicandinsrinsicbehavioriscalledtheexhaustionrange.Ap-typesemiconductorisproducedbydopingagroupIVamaterialwithagroupIIIaimpurity（suchasAl）.ThegroupIIIAelementhasa“missing”electronproducinganacceptorlevelinthebandstuctureandleadingtoformationofpositivechargecarriers（electronholes）.Theregionbetweenextrinsicandinstrinsicbehaviorforp-typesemiconductorsiscalledthesaturationrange.Halleffectmeasurementscandistinguishbetweenn-typeandp-typeconduction.

CompoundsemiconductorsusuallyhaveanMXcompositionwithanaverageoffourvalenceelectronsperatom.TheIII-VandII-VIcompoundsarethecommonexamples.amorphoussemiconductorsarethenon-crystallinematerialswithsemiconductingbehavior.Elementalandcompoundmaterialarebothfoundinthiscategory.Toappreciatetheapplicationsofsemiconductors,wereviewafewdecades.thesolidstaterectifier（ordiode）containsasinglep-njunction.Currentflowsreadilywhenthisjunctionisforwardbiasedbutisalmostcompletelychokedoffwhenreversebiased.thetransistorisadeviceconsistingofapairofnearbypnjunctions.Thenetresultisasolidstateamplifier.Replacingvacuumtubeswithsolidstateelementssuchastheseproducedsubstantialminiaturizationofelectricalcircuits.Furtherminiaturizationhasresultedbytheproductionofmicrocircuisconsistingofpreciseparrernsofn-typeandp-typeregionsonasinglecrystalchip.

Themajorelectricalpropertiesneededtospecifyanintrinsicsemiconductorarebandgap,electronmobility,holemobility,andconductionelectrondensity（=electronholedensity）atroomtemperature.Forextrinsicsemiconductors,oneneedstospecifyeitherthedonorlevel（forn-typematerial）ortheacceptorlevel（forp-typematerial）.

2.7composites

Onecategoryofstructuralengineeringmaterialisthatofcomposites.Thesematerialsinvolvesomecombinationoftwoormorecomponentsfromthe“fundamental”materaltypes.Akeyphilosophyinselectingcompositematerialsisthattheyprovidethe“bestofbothworlds”thatis,attrativepropertiesfromeachcomponent.Aclassicexampleisfiberglass.Thestrengthofsmalldiameterglassfibersiscombinedwiththeductilityofthepolymetricmatrix.Thecombinationofthesetwocomponentsprovidesaproductsuperiortoeithercomponentalone.Manycomposites,suchasfiberglass,involvecombinationsthatcrossovertheboundariesofdifferentkindsofmaterials.Others,suchasconcrete,involvedifferentcomponentfromwithinasinglematerialtype.Ingeneral,weshalluseafairlynarrowdefinitionofcomposites.Weshallconsideronlythodematerialthatacombinedifferentcomponentsonthemicroscopic（ratherthanmacroscopic）scale.Weshallnootincludemultiphasealloysandceramics,whicharetheresultofroutineprocessing.Similarly,themicrocircuitsbediscussedlaterarenotincludebecauseeachcomponentretainsitsdistinctivecharacterinthesematerialsystems.Inspiteoftheserestrictions,weshallfindthiscategorytoincludeatremendouslydiversecollectionofmaterials,fromthecommontosomeofmostsophisticated.

Weshallconsiderthreecategoriesofcompositesmateials.Conveninently,thesecategoriesaredemonstratedbythreeofourmostcommonstructuralmaterial,fiberglass,wood,andconcrete.Fiberglass（orglassfiberreinforcedpolymer）isanexcellentexampleofahumanmadefiberreinforcedcomposite.Theglass-polymersystemisjustoneofmanyimportantexample.Thefiberreinforcementisgenerallyfoundinoneofthreeprimaryconfigutations:

alignedinasingledirection,randomlychopped,orwoveninafabricthatislaminatedwiththematrix.Woodisastucturalanalogoffiberglass,thatis,anaturalfiberreinforcedcomposite.Thefibersofwoodareelongated,biologicalcells.Thematrixcorrespondstoligninandhemicellulosedeposits.concreteisourbestexampleofanaggregatecomposite,inwhichparticlesratherthanfibersreinforceamatrixcommonconcreteisrockandsandinacalciumaluminosilicate（cement）matrix.Whileconcretehasbeenaconstructionmaterialforcenturies,thesearenumerousccompositesdevelopedinrecentdecadesthatuseasimilarparticulatereinforcementconcept.

Theconceptofpropertyaveragingiscentraltounderstandingtheutilityofcompositematerial.animportantexampleistheelasticmodulusofacomposite.Themodulusisasensitivefunctionofthegemetryofthereinforcingcomponent.similarlyimportantisthesrengthoftheinterfacebetweeenthereinforcingcomponentandthematrix.Wesahllconcentrateonthesemechanicalpropertiesofcompositesinregardfortheirwideuseasstructuralmaterials.Socaaled“advanced”compositeshaveprovidedsomeunusuallyattractivefeatures,suchashighstrrenthtoweightratios.Somecareisrequiredincitingtheseproperties,astheycanbehighlydirectionalinnature.

2.6therearenumeroususesofpiezoelectrics.forinstance,platescutfromasinglecrystalcanexhibitaspecificnaturalresonancefrequency（i.e.,thefrequencyofanelectromagneticwavethatcausesittovibratemechanicallyatthesamefrequency）;thesecanbeusedasafrequencystandardinhighlystablecrystalcontrolledclocksandinfixedfrequencycommunicationsdevices.otherresonantapplicationsincludeselectivewavefiltersandtransducers（e.g.,forultrasoniccleaninganddrilling）andnon-resonantdevices（e.g.,accelerometers,pressuregauges,microphonepickups）aredominatedbyceramicpiezoelectrics.

2.7.3fiberglasswasaconvenientandfamiliarexampleoffiberreinforcedcomposites.Similarly,concreteisanexcellentexampleofanaggregatecomposite.Aswithwood,thiscommonconstructionmaterialisusedinstaggeringquantities.Theweightofconcreteusedannuallyexceedsthatofallmetalscombined.

Forconcrete,theterm“aggregate”referstoacombinationofsand（fineaggregate）andgravel（coarseaggregate）.Thiscomponentofconcreteisa“natual”materialinthesamesenseaswood.Ordinarily,thesematerialsarechosenfortheirrelativelyhighdensityandstrength.Atableofaggregatecompositionswouldbecomplexandlargelymeaningless.Ingeneral,aggregatematerialsaregeologicalsilicateschosenfromlocallyavailabledeposits.Assuch,thesematerialsarecomplexandrelativelyimpureexamplesofthecrystallinesilicates.Igneousrocksarecommonexamples.“igneous”meanssolidifiedfromamoltenstate.Forquicklycooledigneousrocks,somefractionoftheresultingmaterialmaybenon-crystalline,correspondingtotheglassysilicates.Therelativeparticlesizesofsandandgravelaremeasured（andcontrolled）bypassingthesematerialsthroughstandardscreens（orsieves）.Thereasonforacombinationoffineandcoarseaggregateinagivenconcretemixisthatthespaceismoreefficientlyfilledbyarangeofparticlesizes.Thecombinationoffineandcoarseaggregateaccountsfor60to75percentofthetotalvolumeofthefinalconcrete.

Modernconcreteusesportlandcement,whichisacalciumaluminosilicate.Therearefivecommontypesofportlandcement.Theyvaryintherelativeconcentrationsoffourcalciumcontainingminerals.Thematrixisformedbytheadditionofwatertotheappropriatecementpowder.Theparticlesizesforthecementpowersarerelativelysmallcomparedtothefinestoftheaggregates.Variationincementparticlesizecanstronglyaffecttherateatwhichthecementhydrates.Asonemightexpectfrominspectingthecomplexcompositionsofportlandcement,thechemistryofthehydrationprocessisequallycomplex.

Inploymertechnology,wenotedseveral“additives”whichprovidedcertaindesirablefeaturestotheendproduct.Incementtechnology,thereareanumbleofadmixtures,whichareadditionsprovidingcertainfeatures.Anycomponentofconcreteotherthanaggregate,cement,orwateris,bydefinition,anadmixture.Oneoftheadmixturesisan“airentrainer”whichremindsusthataircanbethoughtofasafourthcomponentofconcrete.Theairentraineradmixtureincreasestheconcentrationofentrappedairbubbles,usuallyforthepurposeofworkability（duringforming）andincreasedresistancetofreezethawcycles.

Whyconcreteisanimportantengineeringmaterial,alargenumbleofothercompositesystemsarebasedonparticlereinforcement.Particulatecompositesreferspecificallytosystemswithrelativelylargesizedispersedparticles（atleastseveralmicrometersindiameter）,andtheparticlesareinrelativelyhighconcentration（greaterthan25andfrequentlybetween60and90）ofsmalldiameteroxideparticles.Theoxideparticlesstrengthenthemetalbyservingasobstaclestodislocationmotion.

2.7.2likesomanyaccomplishmentsofhumanbeings,thosefiberreinforcedcompositesimitatenature.Commonwoodissuchacomposite,whichservesas