[工学]材料科学基础英文版2.ppt

Chapter 1 Atomic Structure and Interatomic Bonding 11.IntroductionSomeoftheimportantpropertiesofsolidmaterialsdependongeometricalatomicarrangements.Thepropertiesofmaterialsarecontrollableandcanbetailoredtotheneedsofagivenapplicationbycontrollingtheirstructureandcomposition.Wecanexamineanddescribethestructureofmaterialsatdifferentlevels.2Subatomic level:Electronicstructureofindividualatomsthatdefinesinteractionamongatoms(interatomicbonding).Atomic level:Arrangementofatomsinmaterials.Nanostructure:thestructureofmaterialatalength-scaleof100nm.Microstructure:thestructureofmaterialatalength-scaleof10to1000nm.Macrostructure:thestructureofamaterialatamacroscopiclevelwherethelength-scaleis1000,000nm.3Length-scalesAngstrom=1=1/10,000,000,000meter=10-10mNanometer=10nm=1/1,000,000,000meter=10-9mMicrometer=1m=1/1,000,000meter=10-6mMillimeter=1mm=1/1,000meter=10-3mInteratomicdistanceafewAhumanhairis50mElongatedbumpsthatmakeupthedatatrackonCDare0.5mwide,minimum0.83mlong,and125nmhigh45Thismicrograph,whichrepresentsthesurfaceofagoldspecimen,wastakenwithaatomicforcemicroscope(AFM).Individualatomsforthis(111)crystallographicsurfaceplaneareresolved.6Amorphous:lackalong-rangeorderingofatomsorions.Crystalline:exhibitperiodicarrangementsofatomsorions.Thelong-range atomic orderisintheformofatomsorionsarrangedinathreedimensionalpatternthatrepeatsovermuchlargerdistances(from100nmtouptofewcm).Short-range atomic arrangements:theatomsofionsshowaparticularorderonlyoverrelativelyshortdistances.72.Atomic Structure Atomicstructureinfluenceshowatomsarebondedtogether.Anunderstandingofthishelpscategorizematerialasmetals,semiconductors,ceramics,orpolymers.Atoms=nucleus(protonsandneutrons)+electrons8Charges:Electronsandprotonshavenegativeandpositivechargesofthesamemagnitude,1.610-19Coulombs.Neutrons areelectricallyneutral.Masses:ProtonsandNeutronshavethesamemass,1.6710-27kg.Mass of an electronismuchsmaller,9.1110-31kgandcanbeneglectedincalculationofatomicmass.The atomic mass(A)=massofprotons+massofneutrons#protonsgiveschemicalidentificationoftheelement#protons=atomic number(Z)#neutronsdefinesisotopenumber9Thereare92naturallyoccurringelements,eachidentifiedbytheatomic number(numberofprotons)andatomic weight(whichincludestheweightoftheneutrons)andrepresentsanaverageoverthevariousisotopesthatmayexist).Theatomicweighthasunitsofgramspermole.Amoleistheamountofmaterialthatcorrespondstotheatomicweight.A mole istheamountofmatterthathasamassingramsequaltotheatomicmassinamuoftheatoms(Amoleofcarbonhasamassof12grams).ThenumberofatomsinamoleiscalledtheAvogadro number,Nav=6.0231023.10THE PERIODIC TABLE113.Electronic in AtomsBohr atomic modelOrbital electron12Quantumnumbersarethenumbersinanatomthatassignelectronstodiscreteenergylevels.Principal quantum number n:isassignedintegralvalues1,2,3,4,5thatrefertothequantumshelltowhichtheelectronbelongs.Orbital(Azimuthal)quantum number l:describetheenergylevelsineachquantumshell.l=0,1,2,n-1.Magnetic quantum number ml:describesthenumberofenergylevelsforeachorbitalquantumnumber.l,+lSpin quantum number ms:assignedvalues+1/2and-1/2andreflectsthedifferentelectronicspinsThenumberofpossibleenergylevelsisdeterminedbythefirstthreequantumnumbers.13Thecompletesetofquantumnumbersforeachofthe11electronsinsodium.3s1electron11n=3,l=0,ml=0,ms=+1/2or-1/2electron10n=2,l=1,ml=+1,ms=-1/2electron9n=2,l=1,ml=+1,ms=+1/22p6electron8n=2,l=1,ml=0,ms=-1/2electron7n=2,l=1,ml=0,ms=+1/2electron6n=2,l=1,ml=-1,ms=-1/2electron5n=2,l=1,ml=-1,ms=+1/22s2electron4n=2,l=0,ml=0,ms=-1/2electron3n=2,l=0,ml=0,ms=+1/21s2electron2n=1,l=0,ml=0,ms=-1/2electron1n=1,l=0,ml=0,ms=+1/214Thepatternusedtoassignelectronstoenergylevelsl=0(s)l=1(p)l=2(d)l=3(f)l=4(g)l=5(h)n=1(K)2n=2(L)26n=3(M)2610n=4(N)261014n=5(O)26101418n=6(P)2610141822Note:2,6,10,14,refertothenumberofelectronsintheenergylevel.15Thissequenceemphasizestherelativeenergylevelsoftheshellssothattheouter,higherenergyandmoreasymmetricdlevelsmayfillaftertheinnerslevelofthenextshellandmoreasymmetriclevelsmayfillaftertheinnerpandeventhedlevelofthenextshell.16havecompletesandpsubshellstendtobeunreactive.Stableelectronconfigurations.Stable Electron Configurations17Valence:Thenumberofelectronsinanatomthatparticipateinbondingorchemicalreactions.Usually,thevalenceisthenumberofelectronsintheoutersandpenergylevels.Mg:1s22s22p6 valence=2Al:1s22s22p6 valence=3Ge:1s22s22p63s23p63d10valence=4Ifanatomhasavalenceofzero,theelementisinert(non-reactive).1s22s22p6 Valencealsodependsontheimmediateenvironmentsurroundingtheatomortheneighboringatomsavailableforbonding.ForExample,P2O3,PH3.3s23s23p14s24p23s23p618Electronegativityisthequantitativedescriptionofanatomsdesiretogainorloseanelectron.Itisafunctionofthenumberofelectronsinanatomsvalenceshell,andthedistanceoftheshellfromthenucleus.Forexample,chlorine,with7valenceelectrons,isveryeagertogainanelectrontofillitsoutershell,whilesodiumwilleasilygiveupits1valenceelectron.4.The Periodic Table19The Periodic Table20Electronegativityincreasesasyougorightandupontheperiodictable.21Columns:SimilarValenceStructureElectropositiveelements:Readilygiveupelectronstobecome+ions.Electronegativeelements:Readilyacquireelectronstobecome-ions.The Periodic TableHeNeArKrXeRninertgases accept1e accept2e giveup 1egiveup 2egiveup 3eFLiBeMetalNonmetal Intermediate HNaClBrIAtOSMgCaSrBaRaKRbCsFrScYSeTePo225.Atomic Bonding in solidTheattractiveforcevariesinverselywiththesquareofthedistancebetweentheatomsforallofthevarioustypesofbonds.Thisforcepullstheatomstogetherwithagreaterforceastheygetclosertogether.Arepulsiveforceariseswhentheelectroncloudsofnegativechargemeet.Thisforceincreasesmuchfasterwithdistance,withanexponentintherange6-9thatdependsontheparticularatomanditselectronshells.Whenthesumoftheseforcesiszero,thedistancebetweentheatomsisattheequilibriumvalue23Boththeattractiveandrepulsiveforcesincreaseastheatomsarebroughtclosertogether,andthesumofthetwoiszeroattheequilibriumpoint.Theslopeoftheforcevs.distancecurveattheequilibriumpointdefinestheforceneededtopulltheatomsslightlyapartandistheslopeofthestressvs.straincurve.Thisslopeisthemodulusofelasticity.24TheBondenergyvs.distancecurveistheintegralofthebondforcecurve.Thelowestenergyistheequilibriumpoint.25Thedepthoftheminimumisthetotalbondstrengthwhichreflectstheenergyrequiredtopulltheatomscompletelyapart.Thedeeperthepotentialminimum,thehigherthemeltingtemperature.Thebondenergycurveisasymmetric.Thisiswhymostmaterialsexpandwhenheated.Becauseoftheasymmetryofthebondenergycurve,theaveragedistancebetweenatomsincreaseswithtemperature.Thenarrowerthepotentialminimum,thelowertheexpansioncoefficient.26Atomic Bonding in solidTherearefourprincipalkindsofbondsthatformbetweenatoms:Metallic bond Ionic bond Covalent bond Van der Waals bondStrongBondWeakBondTheforcesbetweenatomsareelectrostaticdependentdirectlyontheelectronsthatsurroundtheatoms.Thedifferentbondtypesarecharacterizedbyhowelectronsaresharedwhichcontrolsthegeometryoftheatompacking,andbytherelativestrengthofthebond.about1/100thasstrong27Arisesfromaseaofdonatedvalenceelectrons(1,2,or3fromeachatom).PrimarybondformetalsandtheiralloysMetallic Bonding28The Metallic Bond:sharedvalenceelectronsformingahighlymobileelectronsea.Themetallicbondisformedbetweenatomsthathavealowvalueofelectronegativityandeasilygiveuptheirouter(valence)electrons.Theseformanelectronseathatgluesthepositiveioncores(nucleiandinnerelectrons)togetherandshieldsthepositivecoresfromeachother.29Themetallicbondhasthefollowingcharacteristics:1.Electronssharedamongallatoms2.Nodirectionality-desireforthelargestnumberofnearestneighbors3.Highstrength(slightlylessthancovalentorionic;25-200kcal/mol)4.Formsbetweenatomswithlowelectronegativity30RequiressharedelectronsExample:CH4C:has4valencee,needs4moreH:has1valencee,needs1moreElectronegativitiesarecomparable.Covalent Bonding31The Covalent bond:isformedbyasharingofvalenceelectronsbetweentwoadjacentatoms.Conventionshowsthisasdiscreteelectronsintheirrespectiveorbitslocatedbetweentheatoms.Amorerealisticmodelshowstheelectronsasacloudofnegativechargebetweentheatoms.32MoleculeswithnonmetalsMoleculeswithmetalsandnonmetalsElementalsolids(RHSofPeriodicTable)Compoundsolids(aboutcolumnIVA)AdaptedfromFig.2.7,Callister 6e.(Fig.2.7isadaptedfromLinusPauling,The Nature of the Chemical Bond,3rdedition,Copyright1939and1940,3rdedition.Copyright1960byCornellUniversity.Examples:Covalent Bonding33Sincetheyarenegative,theCovalentbondsrepeleachother,andthisresultsintheirstayingasfarapartaspossible,producingthecharacteristicdihedralanglein3D.Dihedral Angle:Becausetheelectroncloudslocalizedinthecovalentbondsrepeleachotherinsteadoflyinginaplane,theycangetfartherapartinthreedimensionsbypointingtooppositecornersofacube.34Thelengthofthefacediagonalinthiscubeis.Inthediagonalplanethroughthecube,thesidesoftherighttriangledefinetheangleasshown.Consequently,thedihedralanglebetweenthebondsis109.5degrees.35Covalentbondsareverystrong.Asaresult,covalentlybondedmaterialsareverystrongandhard.Theelectricalconductivityofmanycovalentlybondedmaterials(i.e.,silicon,diamond,andmanyceramics)isnothighsincethevalenceelectronsarelockedinbondsbetweenatomsandarenotreadilyavailableforconduction.36Thecovalentbondhasthefollowingcharacteristics:1.Electronssharedamongtwoadjacentatoms2.Strongdirectionality-numberofneighborslimited3.Highstrength(slightlylessthanionic;125-300kcal/mol)4.Formsbetweenatomswithhighelectronegativity37Occursbetween+and-ions.Requireselectrontransfer.Largedifferenceinelectronegativityrequired.Example:NaClIonic Bonding38The Ionic Bond:valenceelectronstransferredbetweentwoatoms.TheIonicbondisanelectrostaticattractionbetweenpositivelyandnegativelychargedions.Thesebondsareformedbythetransferofelectronsfromoneatomwithalowelectronegativitytoadifferentatomwithahighelectronegativity.39Theexampleshowssodium(Na11)andchlorine(Cl17).TransferringtheelectronfromNatoClproducespositiveandnegativelychargedionswhicharesmallerandlarger,respectively,thantheoriginalneutralatoms.Na40PredominantbondinginCeramicsGiveupelectronsAcquireelectronsExamples:Ionic Bonding41Theelectrostaticattractiveforcebetweentheatomsisnon-directional,andsotheatompackinginanionicmaterialattemptstoarrangeasmanypositiveionsaroundeachnegativeoneaspossible,andviceversa.22inverse r242Whenvoltageisappliedtoanionicmaterial,entireionsmustmovetocauseacurrenttoflow.Ionmovementisslowandtheelectricalconductivityispoor.43Theionicbondhasthefollowingcharacteristics:1.Electronstransferredbetweenatoms,producingions2.Nodirectionality-eachionsurroundedbymaximumnumberofoppositesign3.Highstrength(150-370kcal/mol)4.Formsbetweenatomsofdifferentelectronegativityvalues(onehigh,onelow)44ArisesfrominteractionbetweendipolesPermanentdipoles-moleculeinducedFluctuatingdipoles-generalcase:-ex:liquidHCl-ex:polymerAdaptedfromFig.2.13,Callister 6e.Secondary Bonding45The Van der Waals bond:polarizationduetobondstructurecausesattractiveandrepulsiveforcebetweenmolecules.TheVanderWaalsbondisaweakbondthatformsbyelectrostaticattractionbetweenmolecules.Amoleculewithcovalentbondsbetweentheatomslocalizestheelectronsintheregionofthebond.Thefractionalamountofthechargeismuchlessthanproducedbyionizationthatremovesoraddsawholeelectron.46TheHydrogenbondisacommonexampleofaVanderWaalsbond.TheH2Omoleculesinwaterhavepolarizationchargethatispositiveontheexposedtipsofthehydrogenatomsandnegativewherethevalenceelectronsarelocalized.ThisproducesaVanderWaalsforcebetweenthemolecules.-+-+OxygenOxygenHHHH47Sincethebondangleis109.5,closeto120,thisresultsinaroughlyhexagonalarrangementofmolecules.Indeed,whenwaterfreezestomakeice,ithasahexagonalstructureduetothesesamebondforces.Thehexagonalgeometryoftheicehasalowerdensitythanthewaterwhichiswhyicefloats.VanderWaalsbondsaresecondarybonds,theatomswithinthemoleculeorgroupofatomsarejoinedbystrongcovalentorionicbonds.48TheVanderWaalsbondhasthefollowingcharacteristics:1.Polarizationproducesslightelectrostaticchargebetweenmolecules2.Notdirectionalbutaffectsregionsofmolecules3.Weakbond(1/100thofstrongbonds;10kcal/mol)4.Hydrogenbondisanexample49Metallic bondIonic bondCovalent bondVan der Waals bondElectronssharedamongallatomsElectronstransferredbetweenatoms,producingionsElectronssharedamongtwoadjacentatomsPolarizationproducesslightelectrostaticchargebetweenmoleculesNodirectionalityNodirectionalitydirectionalityNotdirectionalbutaffectsregionsofmoleculesHighstrength(25-200kcal/mol)Highstrength(150-370kcal/mol)Highstrength(125-300kcal/mol)Weakbond(1/100thofstrongbonds;10kcal/mol)FormsbetweenatomswithlowelectronegativityFormsbetweenatomsofdifferentelectronegativityvalues(onehigh,onelow)FormsbetweenatomswithhighelectronegativityHydrogenbondisanexample50TypeIonicCovalentMetallicSecondaryBondEnergyLarge!Variablelarge-Diamondsmall-BismuthVariablelarge-Tungstensmall-MercurysmallestCommentsNondirectional(ceramics)Directionalsemiconductors,ceramicspolymerchains)Nondirectional(metals)Directionalinter-chain(polymer)inter-molecularSummary:Bonding51Mixed BondsInmostmaterials,bondingbetweenatomsisamixtureoftwoofmoretypes.TheIonicandcovalentbondtypesrepresenttheextremesoftransferandsharing ofelectrons.Realbondsareamixtureoftheseextremes,dependingontheelectronegativitiesoftheelementsinvolved.Compoundsformedfromtwoormoremetalsmaybebondedbyamixtureofmetallicandionicbonds,particularlywhenthereisalargedifferenceinelectronegativitybetweentheelements.52Itistheelectronegativitiesofatomsthatdeterminewhatbondtypeforms.Iftheatomshavehighelectronegativity,theyshareatomsinacovalentbondbetweenindividualpairsofatoms.Iftheatomshavelowelectronegativity,theyshareatomsamongalloftheatomsinametallicbond.Ifsomeoftheatomshavehighandsomelowelectronegativity,theelectronsaretransferredfromoneatomtoanother,resultinginanionicbond.Electronegativityandbonding53TheformulaisFractionCovalent=e-0.25E2Eisthedifferenceintheelectronegativitiesoftheatoms.ForSiO2,theelectronegativityofSiis1.8andofOis3.5,sothefractionis0.486andthebondis48.6%covalent.Similarly,forAl2O3,thebondis37%covalentandforCuOis53%covalent.Themorecovalentthebond,themoredirectionalityispresentandthelowerthetendencyfortheionstopackdenselyaroundonesoftheoppositesign.54Materialshavingahighbindingenergyalsohaveahighstrengthandahighmeltingtemperature.Ionicallybondedmaterialshaveaparticularlylargebindingenergy.Metalshavelowerbindingenergies.BondBinding Energy(kcal/mol)Ionic 150-370Covalent125-300Metallic25-200Van der Waals10Table Binding energies for the four bonding mechanisms55Bondlength,rBondenergy,EoMeltingTemperature,TmTmislargerifEoislarger.Properties From Bonding:TM56Elasticmodulus,EEcurvatureatroEislargerifEoislarger.Properties From Bonding:E57Coefficientofthermalexpansion,aasymmetryatroaislargerifEoissmaller.Properties From Bonding:a58Ceramics(Ionic&covalentbonding):Metals(Metallicbonding):Polymers(Covalent&Secondary):LargebondenergylargeTmlargeEsmallaVariablebondenergymoderateTmmoderateEmoderateaDirectionalPropertiesSecondarybondingdominatessmallTsmallElargeaSummary:Primary Bonds5960