TEM英文教学课件3

4.1.3PumpsandHolders4.1.3.1.VACUUMS4.1.3.2.ROUGHINGPUMPSThemostcommonroughingpumpisamechanical(rotary)pumpinwhichabelt-driven,eccentricallymountedreciprocatingmechanismsucksairthroughaninletvalveintoachamberandexpelsitthroughanexitvalveSuchpumpsareveryreliable,relativelyinexpensive,noisyanddirty,andonlylowerthepressureto~10-1Pa(~10-3Torr).Figure4.1.3.1.Amechanicalpumpforroughingvacuums.TheeccentricmotionofthepumpcreatesavacuumintheRHsidewhenitrotatesandthevacuumsucksairintotheinletvalve.Asthecylinderrotatesfurther,itcutsofftheinletandforcestheairthroughtheoutletontheLHside,creatingavacuumagainontheinletsideasitdoesso.Becauseoftheconstantcontactbetweentherotatingcylinderandtheinsideofthepump,oilisneededtoreducefrictionalheating.4.1.3.3.HIGH/ULTRA-HIGHVACUUMPUMPS4.1.3.3.A.DiffusionPumpsFigure4.1.3.2.Principlesofdiffusionpumpoperation.Aheaterplateatthebaseofthepumpboilssyntheticoil.Theexpansionoftheoilvaporonboilingcreatesapressure,whichforcesthevaporupthecentralcolumnandoutofseveralholes.Thestreamofoilvaporpullsgasmoleculesoutofthetopofthepumpdowntothebase,wheretheoilcondensesandtheairispumpedoutofthebasebyamechanicalbackingpump.Diffusionpumpsarecapableofveryefficientpumpingfrom~10-1to~10-9Pa(10-11Torr)and,ifproperlytrapped,willprovideacleanUHVsystemthatisveryreliable.4.1.3.3.B.TurbomolecularPumpsTurbomolecularpumps,orturbopumps,asthenameimplies,useaturbinetoforcegasesfromthemicroscope.Theyhavemanypartsmovingathighspeeds(inexcessof20,000-50,000rpmiscommon),sotheyaremoreliabletofailthandiffusionpumps.Figure4.1.3.3.Aturbopump(withandwithoutitscasing)whichisnothingmorethanasmallturbinethatrotatesathighspeed.Likeajetturbineitpullsairinatthefrontendandforcesitoutoftheback.Thebladesaredesignedlikeairfoilstoenhancetheflowofgasthroughthesystem.4.1.3.3.C.IonPumpsIonpumpsdonotcontainoil,sotheycannotcontaminatetheTEMcolumn.Theyalsohavenomovingparts,relyingsolelyontheionizationprocesstoremoveair.Theionpumpemitselectronsfromacathode,andthesespiralinamagneticfieldandionizeairmolecules,whicharethenattractedtothecathode.TheenergeticgasionssputterTiatomsfromthecathodeandtheycondensethroughoutthesystem,mainlyonthecylindricalanode,trappinggasatoms.Ionpumpsareonlyefficientathighvacuums,sotheyareusuallyswitchedonafteradiffusionpumphasloweredthepressureto~10-3Pa(10-5Torr).ItiscommontoaddionpumpsdirectlytothestageorgunchambersofTEMstofocustheirpumpingactionontheseimportantregions.Figure4.1.3.4.SchematicdiagramshowinghowionpumpstrapionizedgasatomsbylayersofTiatomsatelectrodes.Oncetrapped,theionscannotescapeuntilthepumpisturnedoff.4.1.3.3.D.Cryogenic(Adsorption)PumpsAsthenameimplies,cryogenicpumps(cryopumps)relyonliquidN2tocoolmolecularsieveswithlargesurfaceareas.Thecoldsurfaceefficientlyremovesairmoleculesfromambientpressuredownto~10-4Pa(10-6Torr).Becausetheyareoil-free,cryopumpsarealsousedtobackoutionpumpsandpreventtheiraccidentalcontaminationthroughbackstreamingfromoil-bearingpumps.Wealsousecoldsurfacestoenhancevacuumsinthestageofmostnon-UHVTEMs.Suchcoldfingersoranticontaminatorsprovideanalternativesite(toyourspecimen)forcondensationofresidualcomponentsinthevacuum.Ionpumpsandcryopumpsaretrappingpumps.Theykeeptheairmoleculeswithinthemandreleasethemwhenturnedofforwarmedup,respectively.4.1.3.4.THEWHOLESYSTEMFigure4.1.3.5.TheprinciplesoftheTEMvacuumsystem.Often,theconsoledisplayontheTEMwillshowasimilardiagram.Themechanicalpumpcanpumpthecolumndirectlyorbackoutthediffusionpump,whichisconnecteddirectlytothebaseofthemicroscope.Ionpumpsareofteninterfaceddirectlytothestageandgunareas.Computer-controlledvalvesseparatethepumpsfromthecolumnandfromeachother.4.1.3.7.SPECIMENHOLDERSANDSTAGESHistorically,microscopistshaveusedtwodifferentdesignsandalotofwhatyou'llreadhasastronghistoricalbackground.1.Thetraditionalside-entryholderisarodwithamotorattachedtotiltand/orrotatethespecimenandaleadconnectingittoapowersupplyandcontrolbox,orliquid-N2dewar.2.Thetraditionaltop-entryholderisacartridgewhichyouloadintotheTEMbutisdetachedfromtheoutsideworldwhenyouusethemicroscope.4.1.3.8.SIDE-ENTRYHOLDERSFigure4.1.3.6.Principalpartsofaside-entryholderthatisheldinthegoniometerstage.Thespecimenisclampedintothecupattheendoftherod.Asmalljewelattheendoftherod(usuallysapphire)fitsintoanotherjewelbearinginthestagetoprovideastablebaseformanipulatingthespecimen.TheO-ringsealstheendoftheholderinsidethevacuum.Manipulatingthespecimenisaccomplishedfromoutsidethecolumnviacontrolswithintherod.4.1.3.9.TOP-ENTRYHOLDERSFigure4.1.3.7.Crosssectionofatop-entryholder.Thecartridgehasaconeshapewhichfitsintothetaperedboreoftheobjectivelenspole-piece.Thespecimensitsinacupatthebaseofacolumnthroughtheconedownwhichtheincidentbeamtravels.Simplemanipulationssuchastiltingorrotatingrequirecomplexmicromechanicaldesign,sincethespecimenisatthebaseofthecartridgeandcompletelysurroundedbythepolepiece.Totilt,e.g.,asshownintheupperdiagram,pushrodsarepressedagainstspringsintwoorthogonaldirections,displacingaringaroundthecolumn(lowerdiagram),thustiltingthespecimencup.4.1.3.10.DIFFERENTTYPESOFHOLDERSSingle-