英语学习-《材料科学与工程专业英语》(DOC 38页)

《材料科学与工程专业英语》Unit1MaterialsScienceandMetallurgicalEngineeringMaterialsareproperlymoredeep-seatedinourculturethanmostofusrealize.Trans-portation,housing,clothing,communication,recreationandfoodproduction--virtuallyeverysegmentofoureverydaylivesisinfluencedtoonedegreeoranotherbymaterials.Historically,thedevelopmentandadvancementofsocietieshavebeenintimatelytiedtothemembers'abilitiestoproduceandmanipulatematerialstofilltheirneeds.Infact,earlycivilizationshavebeendesignatedbytheleveloftheirmaterialsdevelopment(i.e.StoneAge,BronzeAge).Theearliesthumanshasaccesstoonlyaverylimitednumberofmaterials,thosethatoccurnaturallystone,wood,clay,skins,andsoon.Withtimetheydiscoveredtechniquesforproducingmaterialsthathadpropertiessuperiortothoseofthenaturalones:thesenewmaterialsincludedpotteryandvariousmetals.Furthermore,itwasdiscoveredthatthepropertiesofamaterialcouldbealteredbyheattreatmentsandbytheadditionofothersubstances.Atthispoint,materialsutilizationwastotallyaselectionprocess,thatis,decidingfromagiven,ratherlimitedsetofmaterialstheonethatwasbestsuitedforanapplicationbyvirtueofitscharacteristic.Itwasnotuntilrelativelyrecenttimesthatscientistscametounderstandtherelationshipsbetweenthestructuralelementsofmaterialsandtheirproperties.Thisknowledge,acquiredinthepast60yearsorso,hasempoweredthemtofashion,toalargedegree,thecharacteristicsofmaterials.Thus,tensofthousandsofdifferentmaterialshaveevolvedwithratherspecializedcharacteristicsthatmeettheneedsofourmodernandcomplexsociety.Thedevelopmentofmanytechnologiesthatmakeourexistencesocomfortablehasbeenintimatelyassociatedwiththeaccessibilityofsuitablematerials.Advancementintheunder--standingofamaterialtypeisoftentheforerunnertothestepwiseprogressionofatechnology.Forexample,automobileswouldnothavebeenpossiblewithouttheavailabilityofinexpensivesteelofsomeothercomparablesubstitutes.Inourcontemporaryera,sophisticatedelectronicdevicesrelyoncomponentsthataremadefromwhatarecalledsemiconductingmaterials.MaterialsScienceEngineeringMaterialsscienceisaninterdisciplinarystudythatcombineschemistry,physics,metallurgy,engineeringandveryrecentlylifesciences.Oneaspectofmaterialsscienceinvolvesstudyinganddesigningmaterialstomakethemusefulandreliableintheserviceofhumankind.Itstrivesforbasicunderstandingofhowstructuresandprocessesontheatomicscaleresultinthepropertiesandfunctionsfamiliarattheengineeringlevel.Materialsscientistsareinterestedinphysicalandchemicalphenomenaactingacrosslargemagnitudesofspaceandtimescales.Inthisregarditdiffersfromphysicsofchemistrywheretheemphasisismoreonexplainingthepropertiesofpuresubstances.Inmaterialssciencethereisalsoanemphasisondevelopingandusingknowledgetounderstandhowthepropertiesofmaterialscanbecontrollablydesignedbyvaryingthecompositions,structures,andthewayinwhichthebulkandsurfacesphasematerialsareprocessed.Incontrast,materialsengineeringis,onthebasisofthosestructurepropertiescorrelations,designingorengineeringthestructureofamaterialtoproduceapredeterminedsetofproperties.Inotherwords,materialsengineeringmainlydealswiththeuseofmaterialsindesignandhowmaterialsaremanufactured.Structureisanebuloustermthatdeservessomeexplanation.Inbrief,thestructureofamaterialusuallyrelatestothearrangementofitsinternalcomponents.Subatomicstructureinvolveselectronswithintheindividualatomsandinteractionswiththeirnuclei.Onanatomiclevel,structureencompassestheorganizationofatomsormoleculesrelativetooneanother.Thenextlargestructuralrealm,whichcontainslargegroupsofatomsthatarenormallyagglomeratedtogether,istermedmicroscopicmeaningthatwhichissubjecttodirectobservationusingsometypeofmicroscope.Finally,structuralelementsthatmaybeviewedwiththenakedeyearetermedmacroscopic.Thenotionofpropertydeserveselaboration.Whileinserviceuse,allmaterialsareexposedtoexternalstimulithatevokesometypeofresponse.Forexample,aspecimensubjecttoforceswillexperiencedeformation;orapolishedmetalsurfacewillreflectlight.Propertyisamaterialtraitintermsofthekindandmagnitudeofresponsetoaspecificimposedstimulus.Generally,definitionsofpropertiesaremadeindependentofmaterialshapeandsize.Virtuallyallimportantpropertiesofsolidmaterialsmaybegroupedintosixdifferentcategories;mechanical,electrical,thermal,magnetic,optical,anddeteriorative.Foreachthereisscharacteristictypeofstimuluscapableofprovokingdifferentresponses.Mechanicalpropertiesrelatedeformationtoanappliedloadorforce:examplesincludeelasticmodulusandstrength.Forelectricalproperties,suchaselectricalconductivityanddielectricconstant,thestimulusisanelectricfiled.Thethermalbehaviorofsolidscanberepresentedintermsofheatcapacityandthermalconductivity.Magneticpropertiesdemonstratetheresponseofamaterialtotheapplicationofamagneticfield.Foropticalproperties,thestimulusiselectromagneticorlightradiation:indexofrefractionandreflectivityarerepresentativeopticalproperties.Finally,deteriorativecharacteristicsindicatethechemicalreactivityofmaterials.Inadditiontostructureandproperties,twootherimportantcomponentsareinvolvedinthescienceandengineeringofmaterials,namelyprocessingandperformance.Wi