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1、材料科学与工程专业英语2Solidmaterialsareconvenientlydividedintothreebasictypes:metals,ceramics,andpolymers・Thisclassificationisfirstbasedonchemicalcompositionandatomicstructure,andmostofthematerialfallsintoadistinetcategory,althoughtherearemanyintermediates・Inaddition,ther
2、earethreeotherimportantengineeringmaterials一compositematerials,semiconductormaterialsandbiologicalmaterials・Compositesconsistoftwoormorethantwodifferentmaterials,however,semiconductorsareusedbecauseoftheirunusualelectricalproperties;biologicalmaterialsaretranspl
3、antedintohumanbodies・Asimpleexplanationofthematerialtypeandtheirspecialfeatureswillbegivenlater・Metalmaterialisusuallymadeupofmetalelements・Theyhavealargenumberofirregularlymovingelectrons.Thatis,theseelectronsarenotboundtoaparticularatom.Manyofthepropertiesofme
4、talsaredirectlyattributabletotheseirregularlymovingelectrons・Metalsareverygoodelectricalandheatconductors,whichareopaquetovisiblelight;apolishedmetalsurfacewithabrilliantsurface・Inaddition,metalsareveryhardanddeformable,andthispropertyexplainstheirwidespreadusei
5、nstructuralapplications.Ceramicsarecompoundsbetweenmetalsandnonmetallicelements:theyareusuallyoxides,nitrides,andcarbides・Thewiderangeofmaterialsfallingintothiscategoryincludesceramics,consistingofclayminerals,cementandglass・Thesematerialsaretypicalelectricaldnd
6、thermalinsulators,andtheyaremoreresistanttoheatandharshenvironmentsthanmetalsandpolymers・Asfarasmechanicalperformanceisconcerned,ceramicsarehardbutbrittle・Polymersincludecommonplasticsandrubbermaterials・Mostofthemareorganiccompoundsthatcombinecarbon,hydrogen,and
7、othernonmetallieelementsinachemicalway.Asaresult,theyhaveverylargemolecularstructures・Thesematerialsareusuallyoflowdensityandmaybeverysoft・Manycompositesareusedforengineeringpurposes・Theyconsistofatleastonetypeofmateria1.Glassfiberisafamiliarexampleofglassfibers
8、embeddedinpolymermaterials・Inordertocombinethebestfeaturesofeachcomponentmaterial,acompositematerialisdevised・Glassfibersobtainstrengthfromglassandobtainsoftnessfromp
