透射电子显微镜结构及工作原理.ppt

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1、Chapter2TransmissionElectronMicroscope(TEM)TEMElectronopticssystemVacuumsystemPowerSupplyandcontrolsystem(1)illuminationsystem(2)imagingsystem(3)imageviewingandrecordingStructureofTEMIlluminationsystemImagingsystemImageviewingandrecording2.1Theillumin

2、ationsystemCompriseselectrongun(provideselectronsource)condenserlenses(controltheelectronbeam)2.1.1Electrongunprovidessourceofelectronstoilluminatethespecimen.Therearetwotypesofelectronsources:thermionicsourcetungstenfilamentslanthanumhexaboride(LaB6)cr

3、ystalsfield-emissionsource(FEG)finetungstenneedlesThermionicEmissionIfanymaterialtobeheatedtoahighenoughtemperature,theelectronsgainssufficientenergytoovercomethenaturalbarrier(workfunction)thatpreventsthemfromleakingouttoescapefromthesource.Twothermion

4、icsourcesusedinpracticearetungstenandLaB6.WfilamentLaB6crystalThermionicgunAhighvoltageisplacedbetweenthefilament(actingasacathode)andtheanode,modifiedbyapotentialontheWehneltwhichactstofocustheelectronsintoacrossover,withdiameterd0andconvergence/diverg

5、enceanglea0.FieldEmissionTheprinciplebehindfieldemissionisthatthestrengthofanelectricfieldEisconsiderableincreasedatsharppoints,becauseifwehaveavoltageVappliedtoa(spherical)pointofradiusrthenE=V/r.Oneoftheeasiestmaterialstoproducewithafinetipistungstenw

6、ire.Toallowfieldemission,thesurfaceshouldbefreeofcontaminantsandoxide.ThiscanbeachievedbyoperatinginUHV(betterthan10-11Torr)AnFEGtip(fineWneedle)FieldEmissionGun(FEG)InordertogetanFEGtowork,wemakeitthecathodewithrespecttotwoanodes.Afinecrossoverisformed

7、bytwoanodesactingasanelectrostaticlensIdealelectronsourcehighbrightness(highcurrentdensity)bettercoherency(smallenergyspread)smallchromaticaberrationgoodformodernTEMworkgoodstabilitylonglifetimeCharacteristicsofthethreesourcesoperatingat100kVComparisono

8、felectronsourcesTungstensourcetheworstinmostrespects,butforroutineTEMapplicationstheyareexcellent,reliablesourcesandarecheapandeasilyreplaceable.LaB6highbrightness,improvedcoherencyandtheenergyspread,increasedoperatinglifeisareco