工学]材料科学基础英文版

1 Chapter1AtomicStructureandInteratomicBonding 2 1 Introduction Someoftheimportantpropertiesofsolidmaterialsdependongeometricalatomicarrangements Thepropertiesofmaterialsarecontrollableandcanbetailoredtotheneedsofagivenapplicationbycontrollingtheirstructureandcomposition Wecanexamineanddescribethestructureofmaterialsatdifferentlevels 3 Subatomiclevel Electronicstructureofindividualatomsthatdefinesinteractionamongatoms interatomicbonding Atomiclevel Arrangementofatomsinmaterials Nanostructure thestructureofmaterialatalength scaleof 100nm Microstructure thestructureofmaterialatalength scaleof 10to1000nm Macrostructure thestructureofamaterialatamacroscopiclevelwherethelength scaleis 1000 000nm 4 Length scalesAngstrom 1 1 10 000 000 000meter 10 10mNanometer 10nm 1 1 000 000 000meter 10 9mMicrometer 1 m 1 1 000 000meter 10 6mMillimeter 1mm 1 1 000meter 10 3mInteratomicdistance afew Ahumanhairis 50 mElongatedbumpsthatmakeupthedatatrackonCDare 0 5 mwide minimum0 83 mlong and125nmhigh 5 6 Thismicrograph whichrepresentsthesurfaceofagoldspecimen wastakenwithaatomicforcemicroscope AFM Individualatomsforthis 111 crystallographicsurfaceplaneareresolved 7 Amorphous lackalong rangeorderingofatomsorions Crystalline exhibitperiodicarrangementsofatomsorions Thelong rangeatomicorderisintheformofatomsorionsarrangedinathreedimensionalpatternthatrepeatsovermuchlargerdistances from 100nmtouptofewcm Short rangeatomicarrangements theatomsofionsshowaparticularorderonlyoverrelativelyshortdistances 8 2 AtomicStructure Atomicstructureinfluenceshowatomsarebondedtogether Anunderstandingofthishelpscategorizematerialasmetals semiconductors ceramics orpolymers Atoms nucleus protonsandneutrons electrons 9 Charges Electronsandprotonshavenegativeandpositivechargesofthesamemagnitude 1 6 10 19Coulombs Neutronsareelectricallyneutral Masses ProtonsandNeutronshavethesamemass 1 67 10 27kg Massofanelectronismuchsmaller 9 11 10 31kgandcanbeneglectedincalculationofatomicmass Theatomicmass A massofprotons massofneutrons protonsgiveschemicalidentificationoftheelement protons atomicnumber Z neutronsdefinesisotopenumber 10 Thereare92naturallyoccurringelements eachidentifiedbytheatomicnumber numberofprotons andatomicweight whichincludestheweightoftheneutrons andrepresentsanaverageoverthevariousisotopesthatmayexist Theatomicweighthasunitsofgramspermole Amoleistheamountofmaterialthatcorrespondstotheatomicweight Amoleistheamountofmatterthathasamassingramsequaltotheatomicmassinamuoftheatoms Amoleofcarbonhasamassof12grams ThenumberofatomsinamoleiscalledtheAvogadronumber Nav 6 023 1023 11 THEPERIODICTABLE 12 3 ElectronicinAtoms Bohratomicmodel Orbitalelectron 13 Quantumnumbersarethenumbersinanatomthatassignelectronstodiscreteenergylevels Principalquantumnumbern isassignedintegralvalues1 2 3 4 5 thatrefertothequantumshelltowhichtheelectronbelongs Orbital Azimuthal quantumnumberl describetheenergylevelsineachquantumshell l 0 1 2 n 1 Magneticquantumnumberml describesthenumberofenergylevelsforeachorbitalquantumnumber l lSpinquantumnumberms assignedvalues 1 2and 1 2andreflectsthedifferentelectronicspinsThenumberofpossibleenergylevelsisdeterminedbythefirstthreequantumnumbers 14 Thecompletesetofquantumnumbersforeachofthe11electronsinsodium 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 2 15 Thepatternusedtoassignelectronstoenergylevels Note 2 6 10 14 refertothenumberofelectronsintheenergylevel 16 Thissequenceemphasizestherelativeenergylevelsoftheshellssothattheouter higherenergyandmoreasymmetricdlevelsmayfillaftertheinnerslevelofthenextshellandmoreasymmetriclevelsmayfillaftertheinnerpandeventhedlevelofthenextshell 17 havecompletesandpsubshells tendtobeunreactive Stableelectronconfigurations StableElectronConfigurations 18 Valence Thenumberofelectronsinanatomthatparticipateinbondingorchemicalreactions Usually thevalenceisthenumberofelectronsintheoutersandpenergylevels Mg 1s22s22p6valence 2Al 1s22s22p6valence 3Ge 1s22s22p63s23p63d10valence 4Ifanatomhasavalenceofzero theelementisinert non reactive 1s22s22p6Valencealsodependsontheimmediateenvironmentsurroundingtheatomortheneighboringatomsavailableforbonding ForExample P2O3 PH3 3s2 3s23p1 4s24p2 3s23p6 19 Electronegativityisthequantitativedescriptionofanatom sdesiretogainorloseanelectron Itisafunctionofthenumberofelectronsinanatom svalenceshell andthedistanceoftheshellfromthenucleus Forexample chlorine with7valenceelectrons isveryeagertogainanelectrontofillitsoutershell whilesodiumwilleasilygiveupits1valenceelectron 4 ThePeriodicTable 20 ThePeriodicTable 21 Electronegativityincreasesasyougorightandupontheperiodictable 22 ThePeriodicTable 23 5 AtomicBondinginsolid Theattractiveforcevariesinverselywiththesquareofthedistancebetweentheatomsforallofthevarioustypesofbonds Thisforcepullstheatomstogetherwithagreaterforceastheygetclosertogether Arepulsiveforceariseswhentheelectroncloudsofnegativechargemeet Thisforceincreasesmuchfasterwithdistance withanexponentintherange6 9thatdependsontheparticularatomanditselectronshells Whenthesumoftheseforcesiszero thedistancebetweentheatomsisattheequilibriumvalue 24 Boththeattractiveandrepulsiveforcesincreaseastheatomsarebroughtclosertogether andthesumofthetwoiszeroattheequilibriumpoint Theslopeoftheforcevs distancecurveattheequilibriumpointdefinestheforceneededtopulltheatomsslightlyapartandistheslopeofthestressvs straincurve Thisslopeisthemodulusofelasticity 25 TheBondenergyvs distancecurveistheintegralofthebondforcecurve Thelowestenergyistheequilibriumpoint 26 Thedepthoftheminimumisthetotalbondstrengthwhichreflectstheenergyrequiredtopulltheatomscompletelyapart Thedeeperthepotentialminimum thehigherthemeltingtemperature Thebondenergycurveisasymmetric Thisiswhymostmaterialsexpandwhenheated Becauseoftheasymmetryofthebondenergycurve theaveragedistancebetweenatomsincreaseswithtemperature Thenarrowerthepotentialminimum thelowertheexpansioncoefficient 27 AtomicBondinginsolid Therearefourprincipalkindsofbondsthatformbetweenatoms MetallicbondIonicbondCovalentbondVanderWaalsbond StrongBond WeakBond Theforcesbetweenatomsareelectrostaticdependentdirectlyontheelectronsthatsurroundtheatoms Thedifferentbondtypesarecharacterizedbyhowelectronsaresharedwhichcontrolsthegeometryoftheatompacking andbytherelativestrengthofthebond about1 100thasstrong 28 MetallicBonding 29 TheMetallicBond sharedvalenceelectronsformingahighlymobileelectronsea Themetallicbondisformedbetweenatomsthathavealowvalueofelectronegativityandeasilygiveuptheirouter valence electrons Theseforman electronsea thatgluesthepositiveioncores nucleiandinnerelectrons togetherandshieldsthepositivecoresfromeachother 30 Themetallicbondhasthefollowingcharacteristics ElectronssharedamongallatomsNodirectionality desireforthelargestnumberofnearestneighborsHighstrength slightlylessthancovalentorionic 25 200kcal mol Formsbetweenatomswithlowelectronegativity 31 CovalentBonding 32 TheCovalentbond isformedbyasharingofvalenceelectronsbetweentwoadjacentatoms Conventionshowsthisasdiscreteelectronsintheirrespectiveorbitslocatedbetweentheatoms Amorerealisticmodelshowstheelectronsasacloudofnegativechargebetweentheatoms 33 Moleculeswithnonmetals Moleculeswithmetalsandnonmetals Elementalsolids RHSofPeriodicTable Compoundsolids aboutcolumnIVA AdaptedfromFig 2 7 Callister6e Fig 2 7isadaptedfromLinusPauling TheNatureoftheChemicalBond 3rdedition Copyright1939and1940 3rdedition Copyright1960byCornellUniversity Examples CovalentBonding 34 Sincetheyarenegative theCovalentbondsrepeleachother andthisresultsintheirstayingasfarapartaspossible producingthecharacteristicdihedralanglein3D DihedralAngle Becausetheelectroncloudslocalizedinthecovalentbondsrepeleachotherinsteadoflyinginaplane theycangetfartherapartinthreedimensionsbypointingtooppositecornersofacube 35 Thelengthofthefacediagonalinthiscubeis Inthediagonalplanethroughthecube thesidesoftherighttriangledefinetheangleasshown Consequently thedihedralanglebetweenthebondsis109 5degrees 36 Covalentbondsareverystrong Asaresult covalentlybondedmaterialsareverystrongandhard Theelectricalconductivityofmanycovalentlybondedmaterials i e silicon diamond andmanyceramics isnothighsincethevalenceelectronsarelockedinbondsbetweenatomsandarenotreadilyavailableforconduction 37 Thecovalentbondhasthefollowingcharacteristics ElectronssharedamongtwoadjacentatomsStrongdirectionality numberofneighborslimitedHighstrength slightlylessthanionic 125 300kcal mol Formsbetweenatomswithhighelectronegativity 38 IonicBonding 39 TheIonicBond valenceelectronstransferredbetweentwoatoms TheIonicbondisanelectrostaticattractionbetweenpositivelyandnegativelychargedions Thesebondsareformedbythetransferofelectronsfromoneatomwithalowelectronegativitytoadifferentatomwithahighelectronegativity 40 Theexampleshowssodium Na11 andchlorine Cl17 TransferringtheelectronfromNatoClproducespositiveandnegativelychargedionswhicharesmallerandlarger respectively thantheoriginalneutralatoms 41 Examples IonicBonding 42 Theelectrostaticattractiveforcebetweentheatomsisnon directional andsotheatompackinginanionicmaterialattemptstoarrangeasmanypositiveionsaroundeachnegativeoneaspossible andviceversa inverser2 43 Whenvoltageisappliedtoanionicmaterial entireionsmustmovetocauseacurrenttoflow Ionmovementisslowandtheelectricalconductivityispoor 44 Theionicbondhasthefollowingcharacteristics Electronstransferredbetweenatoms producingionsNodirectionality eachionsurroundedbymaximumnumberofoppositesignHighstrength 150 370kcal mol Formsbetweenatomsofdifferentelectronegativityvalues onehigh onelow 45 Arisesfrominteractionbetweendipoles Permanentdipoles moleculeinduced Fluctuatingdipoles generalcase ex liquidHCl ex polymer AdaptedfromFig 2 13 Callister6e SecondaryBonding 46 TheVanderWaalsbond polarizationduetobondstructurecausesattractiveandrepulsiveforcebetweenmolecules TheVanderWaalsbondisaweakbondthatformsbyelectrostaticattractionbetweenmolecules Amoleculewithcovalentbondsbetweentheatomslocalizestheelectronsintheregionofthebond Thefractionalamountofthechargeismuchlessthanproducedbyionizationthatremovesoraddsawholeelectron 47 TheHydrogenbondisacommonexampleofaVanderWaalsbond TheH2Omoleculesinwaterhavepolarizationchargethatispositiveontheexposedtipsofthehydrogenatomsandnegativewherethevalenceelectronsarelocalized ThisproducesaVanderWaalsforcebetweenthemolecules 48 Sincethebondangleis109 5 closeto120 thisresultsinaroughlyhexagonalarrangementofmolecules Indeed whenwaterfreezestomakeice ithasahexagonalstructureduetothesesamebondforces Thehexagonalgeometryoftheicehasalowerdensitythanthewaterwhichiswhyicefloats VanderWaalsbondsaresecondarybonds theatomswithinthemoleculeorgroupofatomsarejoinedbystrongcovalentorionicbonds 49 TheVanderWaalsbondhasthefollowingcharacteristics PolarizationproducesslightelectrostaticchargebetweenmoleculesNotdirectionalbutaffectsregionsofmoleculesWeakbond 1 100thofstrongbonds 10kcal mol Hydrogenbondisanexample 50 51 Type Ionic Covalent Metallic Secondary BondEnergy Large Variable large Diamond small Bismuth Variable large Tungsten small Mercury smallest Comments Nondirectional ceramics Directional semiconductors ceramics polymerchains Nondirectional metals Directional inter chain polymer inter molecular Summary Bonding 52 MixedBondsInmostmaterials bondingbetweenatomsisamixtureoftwoofmoretypes TheIonicandcovalentbondtypesrepresenttheextremesoftransferandsharingofelectrons Realbondsareamixtureoftheseextremes dependingontheelectronegativitiesoftheelementsinvolved Compoundsformedfromtwoormoremetalsmaybebondedbyamixtureofmetallicandionicbonds particularlywhenthereisalargedifferenceinelectronegativitybetweentheelements 53 Itistheelectronegativitiesofatomsthatdeterminewhatbondtypeforms Iftheatomshavehighelectronegativity theyshareatomsinacovalentbondbetweenindividualpairsofatoms Iftheatomshavelowelectronegativity theyshareat