材料科學與工程基礎課件

Fundamentalsofmaterialsscienceandengineering材料科學與工程基礎

IntroductionChapter1Historically,thedevelopmentandadvancementofsocietieshavebeenintimatelytiedtothemembers’abilitytoproduceandmanipulatematerialstofilltheirneeds.Infact,earlycivilizationshavebeendesignatedbytheleveloftheirmaterialsdevelopmentMaterialsareprobablymoredeep-seatedinourculturethanmostofusrealize.1.1HistoricalPerspectiveStoneageBronzeageIronageSteelageAdvancedmaterialageActually,ItisamaterialsphylogenythathumanhistoryevolvedStoneBronze(Cu-Snalloy)

中國商代司母戊大方鼎

(公元前14~11世紀)IronStructuresMaterialssciencePropertiesinvestigatingrelationshipsDesigningorengineeringthestructureofamaterialtoproduceapredeterminedsetofpropertiesOnthebasisofthesecorrelationsMaterialsengineering1.2MaterialsScienceandEngineeringSubatomicstructure(原子內(nèi))

involveselectronswithintheindividualatomsandinteractonswiththeirnucleiAtomiclevelstructure(原子間)

encompassestheorganizationofatomsormoleculesrelativetooneanother“Structure”ofamaterialusuallyrelatestothearrangementofitsinternalcomponents.Microscopicstructure

largerstructuralrealmthanatom

containslargegroupsofatomsthatarenormallyagglomeragedtogether

besubjecttodirectobservationusingsometypeofmicroscopeMacroscopicstructure

thestructuralelementsthatmaybeviewedwiththenakedeye“Property”:whileinserviceuse,allmaterialsareexposedtoexternalstimulationthatevokesometypeofresponse.Forexample:SubjectedtoforcedeformationPolishedmetalsurfacereflectlightGenerally,definitionsofpropertiesaremadeindependentofmaterialshapeandsizeMechanicalpropertiesElectricalpropertiesThermalbehaviorMagneticpropertiesOpticalpropertiesDeteriorativecharacteristicsSixdifferentimportantpropertiesStimulusResponseAppliedloadorforceelasticmodulusandstrength…ElectricfieldElectricalconductivityDielectricconstant…TemperatureHeatcapacityThermalconductivity…MagneticfieldMagnetismElectromagneticorlightradiationIndexofrefractionReflectivity…ServiceconditionChemicalreactivityofmaterialsThereisacharacteristictypeofstuimuluscapableofprovokingdifferentresponsesStructure

組織結構Processing

合成與加工Properties

性質(zhì)Performance

使用性能ThefourcomponentsofthedisciplineofmaterialsscienceandengineeringandtheirlinerinterrelationshipThestructureofamaterialwilldependonhowitisprocessed.Amaterial’sperformancewillbeafunctionofitsproperties.

Wedrawattentiontotherelationshipsamongthesefourcomponentsintermsofthedesign,production,

andutilization

ofmaterials.Figure1.2Photographshowingthelighttransmittanceofthreealuminumoxidespecimens.Fromlefttoright:

singlecrystalmaterial(sapphire),whichistransparent;

apolycrystallineandfullydense(nonporous)material,whichistranslucent;

apolycrystallinematerialthatcontainsapproximately5%porosity,whichisopaque.EachmaterialwasproducedusingadifferentprocessingtechniqueManyanapplied(mechnaical,civil,chemical,orelectrical)scientistorengineerwillatonetimeoranotherbeexposedtoadesignprobleminvolvingmaterials.Example:transmissiongear,integratedcircuitchip……1Materialsscientistsandengineers—

investigationanddesignofmaterials1.3WhyStudyMaterialsScienceandEngineeringManytime,amaterialsproblemisoneofselectingtherightmaterialfromthemanythousandsthatareavailable.Severalcriteria:Firstofall,thein-serviceconditionsmustbecharacterized,forthesewilldictatethepropertiesrequiredofthematerial.2tradeoffonecharacteristicforanother.strengthandductilityTheclassicexample:Asecondselectionconsiderationisanydeteriorationofmaterialpropertiesthatmayoccurduringserviceoperation.Forexample:ExposuretoelevatedtemperaturesorcorrosiveenvironmentsresultinsignificantreductionsinmechanicalstrengthSomecompromiseisinevitableFinally,probablytheoverridingconsiderationisthatofeconomics.AmaterialhasidealsetofpropertiesisprohibitivelyexpensiveWhatwillthefinishedproductcost?Alsoincludesanyexpenseincurredduringfabricationtoproducethedesiredshape.1.4ClassificationofMaterialsAfamiliaritemthatisfabricatedfromthreedifferentmaterialtypesisthebeveragecontainer.

Beveragesaremarketedin

aluminum(metal)cans,glass(ceramic)bottles,plastic(polymer)bottlesbasedprimarilyonchemicalmakeupandatomicstructure:metals,ceramics,polymers

金屬陶瓷聚合物mostmaterialsfallintoonedistinctgroupingoranother,althoughtherearesomeintermediatesbasedonengineering:composites,semiconductors,biomaterials

復合材料半導體材料生物材料Manyproperties:MetalscombinationsofmetallicelementsChemicalmakeup:havelargenumbersofnonlocalizedelectrons（自由電子）Structurecharacteristic:extremelygoodconductorsofelectricityandheatnottransparenttovisiblelightlustrousappearancestrong,deformableCeramicscompoundsbetweenmetallicandnonmetallicelementsChemicalmakeup:Compoundsaremostfrequently

oxides,nitrides,andcarbides.Widerangeofmaterials—arecomposedofclayminerals,

cement,andglass.(粘土礦，水泥，玻璃)(氧化物，氮化物，碳化物)1typicallyinsulativetothepassageofelectricityandheat(電和熱的絕緣體)3moreresistanttohightemperaturesandharshEnvironments(耐高溫和耐惡劣環(huán)境)2Ceramicsarehardbutverybrittle.(硬而脆)

SomeporpertiesPolymersChemicalmakeup:Manyofthemareorganiccompoundsthatarechemicallybasedoncarbon,hydrogen,andothernonmetallicelementsPlasticandrubberhavelowdensitiesmaybeextremelyflexiblehaveverylargemolecularstructure(塑料,橡膠)Include:Composites

consistofmorethanonematerialtype.glassfibers(acquiresstrength)areembeddedwithinapolymeric(acquiresflexibility)materialfiberglass—

bedesignedtodisplayacombinationofthebestcharacteristicsofeachofthecomponentmaterials（復合材料）Semiconductors

haveelectricalpropertiesthatareintermediatebetweentheelectricalconductorsandinsulators.

electricalcharacteristicsareextremelysensitivetothepresenceofminuteconcentrationsofimpurityatoms.

havemadepossibletheadventofintegratedcircuitryBiomaterialsBiomaterialsareemployedincomponentsimplantedintothehumanbodyforreplacementofdiseasedordamagedbodyparts.

Berequired:mustnotproducetoxic,compatiblewithbodytissues.1.5AdvancedmaterialsMaterialsthatareutilizedinhigh-technologyapplicationsaresometimestermedadvancedmaterials.Electronicequipment(VCRs,CDplayers,etc.),computers,fiber-opticsystems,spacecraft,aircraft,andmilitaryrocketry.Examplesofhigh-technologyapplications:whosepropertieshavebeenenhancedAdvancedmaterialsareeithertraditionalmaterialshigh-performancematerialsnewlydeveloped,Advancedmaterialsmaybeofallmaterialstype;arenormallyrelativelyexpensive

Lasers,integratedcircuits,

Magneticinformationstorage,

Liquidcrystaldisplays,

Fiberoptics,

ThermalprotectionsystemForexample:1.6Modernmaterials’needsConsiderationoftheenvironmentalimpactofmaterialsproductionTechnologicalchallenge:DevelopmentofevenmoresophisticatedandspecializedmaterialsMaterialwillplayasignificantroleinfollowingsomeaspect

fuels

containmentstructures

facilitiesforthedisposalofradioactivewasteTransportationNuclearenergy:12

Reducingtheweightoftransportationvehicles,increasingengineoperatingtemperatures,Newhighstrength,low-densitystructuralmaterialshigher-temperaturecapabilityforuseinenginecomponentsDirectconversionofsolarintoelectricalenergy3Materialsforsolarcellsarehighlyefficientinthisconversionprocessyetlesscostlymustbedeveloped

Tocontrolairandwaterpollution4

Pollutioncontroltechniquesemployvariousmaterials.

Lesspollution,lessdespoilageofthelandscape,produceoftoxicsubstance,ecologicalimpactetc.Nonrenewableresourcesaregraduallybecomingdepleted,51、thediscoveryofadditionalreserves2、thedevelopmentofnewmaterials.3、increasedrecyclingwhichnecessitates:Thismicrograph,whichrepresentsthesurfaceofagoldspecimen,wastakenwithasophisticatedatomicforcemicroscope(AFM).Individualatomsforthis(111)crystallographicsurfaceplaneareresolved.Alsonotethedimensionalscale(inthenanometerrange)belowthemicrograph.2.2FundamentalConceptsatomnucleuselectronsprotonsneutronsElectricallycharged1.60

10-19C1.67

10-27kg9.11

10-31kgthenumberofprotonscharacterizeEachchemicalelementtheatomicnumber(Z):Forachemicalelement,thenumberofprotonswithintheatomicnucleus.=thenumberofelectrons(foranelectricallyneutralorcompleteatom)質(zhì)子中子原子序數(shù)Theatomicnumberrangesinintegralunitsfrom1forhydrogento92foruranium,Isotopes:Atomsofthesameelementthathavedifferentatomicmasses.

thenumberofneutrons(N)maybevariable.Theatomicmass(A)ofaspecificatommaybeexpressedasthesumofthemassesofprotonsandneutronswithinthenucleus.Theatomicweight

(A)Theweightedaverageoftheatomicmassesofanatom’snaturallyoccurringisotopes.Theatomicmassunit(amu):

Ameasureofatomicmass;onetwelfthofthemassofanatomofC12,carbon12(12C)(A

12.00000).A

Z+

N鈾同位素原子量原子量單位Mole:

Thequantityofasubstancecorrespondingto6.023

1023atomsormolecules.1amu/atom(ormolecule)=1g/mol2.3ElectronsinAtomsAtomicModelsQuantummechanics.Abranchofphysicsthatdealswithatomicandsubatomicsystems;itallowsonlydiscretevaluesofenergythatareseparatedfromoneanother.Bycontrast,forclassicalmechanics,continuousenergyvaluesarepermissible.Forexample,theatomicweightofironis55.85amu/atom,or55.85g/mol.“A”maybeexpressedintermsofatomicmassunits(onanatomicbasis),orthemasspermoleofatoms.duringthelatterpartofthenineteenthcentury摩爾不連續(xù)的量子力學Bohratomicmodel:Anearlyatomicmodel,inwhichelectronsareassumedtorevolvearoundthenucleusindiscreteorbitals.OneearlyoutgrowthofquantummechanicsFig.2.1SchematicrepresentationoftheBohratom.twoimportantprincipleelectrondiscreteorbitalsEnergiesarequantizedandmaychangeenergy.energylevelsorstateswithabsorptionofenergywithemissionofenergy波爾原子模型分立軌道Figure2.2and2.3comparesBohrandwave-mechanicalmodelsforthehydrogenatom.Wave-mechanicalmodel.Atomicmodelinwhichelectronsaretreatedasbeingwavelike.inwhichtheelectronisconsideredtoexhibitbothwavelikeandparticle-likecharacteristics.anelectronnoasaparticlemovinginadiscreteorbitalpositionisdescribedbyaprobabilitydistributionorelectroncloud.波動力學模型FIGURE2.2(a)ThefirstthreeelectronenergystatesfortheBohrhydrogenatom.(b)Electronenergystatesforthefirstthreeshellsofthewaveme-chanicalhydrogenatom.Bohrmodelwaseventuallyfoundtohavesomesignificantlimitationsbecauseofitsinabilitytoexplainseveralphenomenainvolvingelectrons.FIGURE2.3Comparisonofthe(a)Bohrand(b)wavemechanicalatommodelsintermsofelectrondistribution.

thestatisticpropertyofelectron-electroncloudQUANTUMNUMBERSquantumnumbers:Asetoffournumbers,thevaluesofwhichareusedtolabelpossibleelectronstates.Threeofthequantumnumbersareintegers,whichalsospecifyofavacuum.thesize,shape,andspatialorientationofanelectron’sprobabilitydensity;thefourthnumberdesignatesspinorientation.Bohrenergylevelsseparateintoelectronsubshells,andquantumnumbersdictatethenumberofstateswithineachsubshell.整數(shù)自旋方向空間位向UsingwavemechanicsPrincipalquantumnumber(n):specifyshells-integralvaluessometimestheseshellsaredesignatedbythelettersK,L,M,N,O,andsoon,correspond,respectively,ton=1,2,3,4,5……(Thisquantumnumberisrelatedtothedistanceofanelectronfromthenucleus,oritsposition.)Thesecondquantumnumber(l):signifiesthesubshell,lowercaseletter—ans,p,d,orf

l=0,1,…,(n-1)(itisrelatedtotheshapeoftheelectronsubshell)主量子數(shù)角量子數(shù)spds(l=0),p(l=1),d(l=2),orf(l=3)…;thirdquantumnumber(ml):Foranssubshell,thereisasingleenergystate,whereasforp,d,andfsubshells,three,five,andsevenstatesexist,respectively.Ml=0,±1,±2,..,±l(Intheabsenceofanexternalmagneticfield,thestateswithineachsubshellareidentical.However,whenamagneticfieldisappliedthesesubshellstatessplit,eachstateassumingaslightlydifferentenergy.)磁量子數(shù)l=0,ml=0

asingleenergystate

l=2,ml

=

2,

1,0,+1,+2

denotingfivespatialorientationl=1,ml

=1,0,1

threestatesexist,denotingthreespatialorientationpxpypzdxzdxydyzSpatialorientationshapesizeofelectron’sprobabilitydensityThreeofthesequantumnumbersn,l,mldescribeTheBohrmodelwasfurtherrefinedbywavemechanics,inwhichtheintroduction

ofn,l,mlgivesrisetoelectronsubshellswithineachshell.fourthquantumnumber(ms):

Associatedwitheachelectronisaspinmoment,whichmustbeorientedeitherupordown.ms=自旋磁量子數(shù)afterthen,landmltobeusedhavebeendetermined,assignthevalue+1/2tooneelectron,thenassign-1/2tothenextelectron,whileusingthesamen,landmlvalues.Theruleforselectingmsisasfollows:FIGURE2.4Schematicrepresentationoftherelativeenergiesoftheelectronsforthevariousshellsandsubshells.SeveralFeaturesoftheDiagramareWorthNoting:1.thesmallertheprincipalquantumnumber,thelowertheenergylevel;Forexample,1s<2s<3s2.withineachshell,theenergyofasubshelllevelincreaseswiththevalueofthelquantumnumber.Forexample,3d>3p>3s.3.theremaybeoverlapinenergyofastateinoneshellwithstatesinanadjacentshell,whichisespeciallytrueofdandfstates;Forexample,3d>4sELECTRONCONFIGURATIONSPauliexclusionprinciple:Todeterminethemannerinwhichthesestatesarefilledwithelectrons.Thisprinciplestipulatesthateachelectronstatecanholdnomorethantwoelectrons,whichmusthaveoppositespins.Thus,s,p,d,andf—atotalof2,6,10,and14electrons;electronstates:Oneofasetofdiscrete,quantizedenergiesthatareallowedforelectrons.Intheatomiccaseeachstateisspecifiedbyfourquantumnumbers.theelectronsfillupthelowestpossibleenergystatesintheelectronshellsandsubshells,twoelectrons(havingoppositespins)perstate.電子態(tài)Pauli不相容原理能量最低原理FIGURE2.5Schematicrepresentationofthefilledenergystatesforasodiumatom.groundstate:Whenalltheelectronsoccupythelowestpossibleenergiesinaccordwiththeforegoingrestrictions.electrontransitions:electronfromlowerenergystates

tohigherenergystates.Thedenotationof

electronconfiguration:Forexample:hydrogen—1s1helium—1s2sodium—1s22s22p63s1（2+9）基態(tài)電子躍遷電子結構的表示valenceelectrons:arethosethatoccupytheoutermostfilledshell.Theyparticipateinthebondingbetweenatomstoformatomicandmolecularaggregates.Furthermore,manyofthephysicalandchemicalpropertiesofsolidsarebasedonthesevalenceelectrons.stableelectronconfigurations:thestateswithintheoutermostorvalenceelectronshellarecompletelyfilled.occupationofjustthesandpstatesfortheoutermostshellbyatotalofeightelectrons.asinneon,argon,andkrypton;價電子穩(wěn)定電子結構氖氬氪Ne,Ar,Kr,andHe:aretheinert,ornoble,gases.Someatomshaveunfilledvalenceshellsgainingorlosingelectronschargedions,orsharingelectronswithotheratoms.惰性(稀有)氣體Underspecialcircumstances,thesandporbitalscombinetohybridspn

orbitals,(wheren

indicatesthenumberofporbitalsinvolved,whichmayhaveavalueof1,2,or3.)雜化軌道氦Thedrivingforceisalowerenergystateforthevalenceelectrons.Forcarbonthesp3

hybridisofprimaryimportanceinorganicandpolymerchemistries.Theshapeofthesp3

hybridiswhatdeterminesthe109o

(ortetrahedral)anglefoundinpolymerchains.四面體TheIIIA,IVA,andVAgroupelementsmostoftenthesehybridssp3hybridorbitals—e.g.CH4hybridizesorbitalporbitalssp3

hybrid

orbitals(tetrahedral109o)promotehybridizeC:1s2p2s1s2s2p1ssp3HHHH2.4ThePeriodicTable1stPeriod

1s1———2elements—————

1s22ndPeriod2s1—2s2——8elements—2p1—

2p63rd

Period3s1——8elements—————

3p64thPeriod4s1—3d1-10—18elements———4p65th

Period5s1—4d1-10—18elements———5p66th

Period6s1—5d1—4f1-14—5d10———

6p67th

Period7s1—6d1—5f1-14——6d10periodictable:Thearrangementofthechemicalelementswithincreasingatomicnumberaccordingtotheperiodicvariationinelectronstructure.Nonmetallicelementsarepositionedatthefarright-handsideofthetable.Periods:insevenhorizontalrows.columnorgroup:havesimilarvalenceelectronstructures,aswellaschemicalandphysicalproperties.inertgases:therightmostgroup,positionedinGroup0,whichhavefilledelectronshellsandstableelectronconfigurations.Halogens:TheGroupVIIAelements(F,Cl,Br,I,andAt)周期族惰性氣體鹵素氟、氯、溴、碘、砹Thealkaliandthealkalineearthmetals:GroupsIAandIIAoneandtwoelectronsinexcessofstablestructures(Li,Na,K,Be,Mg,Ca,etc.)（堿金屬和堿土金屬）transitionmetals:Theelementsinthethreelongperiods,GroupsIIIBthroughIIBwhichhavepartiallyfilleddelectronstatesandinsomecasesoneortwoelectronsinthenexthigherenergyshell過渡金屬intermediatebetweenthemetalsandnonmetals:GroupsIIIA,IVA,andVA(B,Si,Ge,As,etc.)半金屬Electropositive.Foranatom,havingatendencytoreleasevalenceelectrons.Also,atermusedtodescribemetallicelements.(mostoftheelementsreallycomeunderthemetalclassification.)theyarecapableofgivinguptheirfewvalenceelectronstobecomepositivelychargedions.Electronegative.Foranatom,havingatendencytoacceptvalenceelectrons.Also,atermusedtodescribenonmetallicelements.(theelementssituatedontheright-handsideofthetable)Theyreadilyacceptelectronstoformnegativelychargedions.orsometimestheyshareelectronswithotheratoms.負電性正電性Fig.2.7Theelectronegativityvaluesfortheelements.AddYourTextAddYourTextAddYourTextAddYourText2.5BondingForcesAndEnergiesphysicalpropertiesofmaterialsinteratomicforcesAtlargedistancesinteractionsarenegligibleexertsforceseachotherastheatomsapproachattractivetwoisolatedatomsrepulsivemagnitudeFunctionofInteratomicdistanceFN=

FA+

FR

(2.2)FN:netforceFA:attractivecomponentsFR:repulsivecomponents孤立原子WhenFA+

FR=

0(2.3)Thereisnonetforce.Thecentersofthetwoatomswillremainseparatedbytheequilibriumspacingr0Formanyatoms,r0isapproximately0.3nm(3?).potentialenergiesforcesMathematically:(2.4)(2.5

2.7)平衡距離勢能FIGURE2.8Thedependenceofrepulsive,attractive,andnetforcesoninteratomicseparationfortwoisolatedatoms.(b)Thedependenceofrepulsive,attractive,andnetpotentialenergiesoninteratomicseparationfortwoisolatedatoms.potentialenergytroughorwellminimumofthepotentialenergycurver0bondingenergy(E0):representstheenergythatwouldberequiredtoseparatethesetwoatomstoaninfiniteseparation.asimilaryetmorecomplexcondition:aE0existanumberofmaterialpropertiesdependonE0,thecurveshape,andbondingtype.勢阱結合能Forexample:bondingenergies

meltingtemperatures

;andSolid>liquids>gaseousstateSecondaryBondingPrimaryInteratomicBondsIoniccovalentmetallicvanderwaalsbondinghydrogenbondingThetypeofBonds第一類(主)鍵第二類鍵Chemicalbondphysicalbond2.6PrimaryInteratomicBonds1.IonicBonding:Acoulombicinteratomicbondthatexistsbetweentwoadjacentandoppositelychargedions.Alwaysbetweenmetallicandnonmetallicelements,metallicelementvalenceelectronsgiveuptononmetallicatomsacquirestableorinertgasconfigurationsbecomeions(Chemicalbonds)離子鍵SchematicforformationofIonicbondinginsodiumchloride(NaCl)FIGURE2.9Schematicrepresentationofionicbondinginsodiumchloride(NaCl).theattractiveenergy

2.8repulsiveenergy2.9Ionicbondingistermednondirectional,(ceramicmaterials)2.Bondingenergies:whichgenerallyrangebetween600and1500kJ/mol(3and8eV/atom),arerelativelylarge,asreflectedinhighmeltingtemperatures.3.characteristicallyhardandbrittlefurthermore,electricallyandthermallyinsulative.無方向性絕緣Character:2.CovalentBondingAprimaryinteratomicbondthatisformedbythesharingofelectronsbetweenneighboringatoms.FIGURE2.10Schematicrepresentationofcovalentbondinginamoleculeofmethane(CH4).共價鍵甲烷Manynonmetallicelementalmolecules(H2,Cl2,F2,etc.)Moleculescontainingdissimilaratoms,suchasCH4,H2O,HNO3,andHF,elementalsolids:carbon,silicon,germaniumsolidcompounds:(GaAs),(InSb),and(SiC).anatomcancovalentlybondwithatmost8-

N’

otheratoms.N’=numberofvalenceelectrons.Forexamples:Cl2(N’=7);

Diamond(N’=4).碳、硅、鍺砷化鎵，銻化銦，碳化硅Covalentbondsmaybeverystrong

—diamond,Tm>3550oCtheymaybeveryweak,too.—bismuth,Tm

270oCPolymericmaterials:molecularstructurebeingalongchainofcarbonatomsthatarecovalentlybondedtogetherwithtwooftheiravailablefourbondsperatom.Theremainingtwobondsnormallyaresharedwithotheratoms,whichalsocovalentlybond.鉍氯氣甲烷乙烯氟分子金剛石interatomicbondspartiallyionicandpartiallycovalent,compounddifferenceofelectronegativitieslowerlefttotheupper-right-handcornerthemoreionicthebondtheclosertheatomsaretogether(inperiodictable)thegreaterthedegreeofcovalencyapproximatedbytheexpression:%ioniccharacter={1-

exp[-(0.25)(XA-

XB)2]}

100(2.10)whereXAandXBaretheelectronegativitiesfortherespectiveelements.3.MetallicBondingAprimaryinteratomicbondinvolvingthenondirectionalsharingofnonlocalizedvalenceelectrons(‘‘seaofelectrons’’)thataremutuallysharedbyalltheatomsinthemetallicsolid.FIGURE2.11Schematicillustrationofmetallicbonding.Metallicmaterialshaveone,two,oratmost,threevalenceelectrons.freeelectronsioncoresseaofelectronsorelectroncloud+‘‘glue’’離子實Character:1.Nondirectional.2.Bondingmaybeweakorstrong;energiesrangefrom68kJ/mol(0.7eV/atom)formercuryto850kJ/mol(8.8eV/atom)fortungsten.Theirrespectivemeltingtemperaturesare-39and3410oC.3.ForgroupIAandIIAelementsandallelementalmetals.無方向性汞鎢Somegeneralbehaviorsofthevariousmaterialtypesmaybeexplainedbybondingtype.Forexample,metalsaregoodconductorsofbothelectricityandheat,asaconsequenceoftheirfreeelectrons.Bywayofcontrast,ionicallyandcovalentlybondedmaterialsaretypicallyelectricalandthermalinsulators,duetotheabsenceoflargenumbersoffreeelectrons.2.7SecondaryBondingorVanDerWaalsBonding(physicalbonds)SecondarybondingforcesdipolesApairofequalyetoppositeelectricalchargesthatareseparatedbyasmalldistance.FIGURE2.12SchematicillustrationofvanderWaalsbondingbetweentwodipoles.coulombicattractionbetweenthepositiveendofonedipoleandthenegativeregionofanadjacentone偶極Secondarybondingisevidencedfortheinertgases,whichhavestableelectronstructures,and,inaddition,betweenmoleculesinmolecularstructuresthatarecovalentlybonded.Hydrogenbonding:Astrongsecondaryinteratomicbondthatexistsbetweenaboundhydrogenatom(itsunscreenedproton)andtheelectronsofadjacentatoms.氫鍵FIGURE2.13Schematicrepresentationsof(a)anelectricallysymmetricatomand(b)aninducedatomicdipole.FluctuatingInducedDipoleBondsInstantaneousandshort-liveddistortions.smallelectricdipolesforcesaretemporaryandfluctuatewithtime.Thebondsaretheweakest.感應偶極子結合electricallysymmetricvibrationalmotion瞬間短時變形PolarMolecule-inducedDipoleBondspolarmolecules:Amoleculeinwhichthereexistsapermanentelectricdipolemomentbyvirtueoftheasymmetricaldistributionofpositivelyandnegativelychargedregions.FIGURE2.14Schematicrepresentationofapolarhydrogenchloride(HCl)molecule.Itcaninducedipolesinadjacentnonpolarmolecules,Thenattractivetwomolecules極性分子引起偶極結合FIGURE2.15Schematicrepresentationofhydrogenbondinginhydrogenfluoride(HF).PermanentDipoleBondsThestrongestsecondarybondingtype,hydrogenbond,Forexample:HF,H2OandNH3永久的偶極結合2.8MoleculesMolecule:Agroupofatomsthatareboundtogetherbystrongprimaryinteratomicbonds.entiretyofionicandmetallically