TEM英文教学课件

4.1.3PumpsandHolders,.VACUUMS,.ROUGHINGPUMPS,Themostcommonroughingpumpisamechanical(rotary)pumpinwhichabelt-driven,eccentricallymountedreciprocatingmechanismsucksairthroughaninletvalveintoachamberandexpelsitthroughanexitvalve,Suchpumpsareveryreliable,relativelyinexpensive,noisyanddirty,andonlylowerthepressureto10-1Pa(10-3Torr).,Figure.Amechanicalpumpforroughingvacuums.TheeccentricmotionofthepumpcreatesavacuumintheRHsidewhenitrotatesandthevacuumsucksairintotheinletvalve.Asthecylinderrotatesfurther,itcutsofftheinletandforcestheairthroughtheoutletontheLHside,creatingavacuumagainontheinletsideasitdoesso.Becauseoftheconstantcontactbetweentherotatingcylinderandtheinsideofthepump,oilisneededtoreducefrictionalheating.,.HIGH/ULTRA-HIGHVACUUMPUMPS,.A.DiffusionPumps,Figure.Principlesofdiffusionpumpoperation.Aheaterplateatthebaseofthepumpboilssyntheticoil.Theexpansionoftheoilvaporonboilingcreatesapressure,whichforcesthevaporupthecentralcolumnandoutofseveralholes.Thestreamofoilvaporpullsgasmoleculesoutofthetopofthepumpdowntothebase,wheretheoilcondensesandtheairispumpedoutofthebasebyamechanicalbackingpump.,Diffusionpumpsarecapableofveryefficientpumpingfrom10-1to10-9Pa(10-11Torr)and,ifproperlytrapped,willprovideacleanUHVsystemthatisveryreliable.,.B.TurbomolecularPumps,Turbomolecularpumps,orturbopumps,asthenameimplies,useaturbinetoforcegasesfromthemicroscope.Theyhavemanypartsmovingathighspeeds(inexcessof20,000-50,000rpmiscommon),sotheyaremoreliabletofailthandiffusionpumps.,Figure.Aturbopump(withandwithoutitscasing)whichisnothingmorethanasmallturbinethatrotatesathighspeed.Likeajetturbineitpullsairinatthefrontendandforcesitoutoftheback.Thebladesaredesignedlikeairfoilstoenhancetheflowofgasthroughthesystem.,.C.IonPumps,Ionpumpsdonotcontainoil,sotheycannotcontaminatetheTEMcolumn.,Theyalsohavenomovingparts,relyingsolelyontheionizationprocesstoremoveair.,Theionpumpemitselectronsfromacathode,andthesespiralinamagneticfieldandionizeairmolecules,whicharethenattractedtothecathode.TheenergeticgasionssputterTiatomsfromthecathodeandtheycondensethroughoutthesystem,mainlyonthecylindricalanode,trappinggasatoms.,Ionpumpsareonlyefficientathighvacuums,sotheyareusuallyswitchedonafteradiffusionpumphasloweredthepressureto10-3Pa(10-5Torr).,ItiscommontoaddionpumpsdirectlytothestageorgunchambersofTEMstofocustheirpumpingactionontheseimportantregions.,Figure.SchematicdiagramshowinghowionpumpstrapionizedgasatomsbylayersofTiatomsatelectrodes.Oncetrapped,theionscannotescapeuntilthepumpisturnedoff.,.D.Cryogenic(Adsorption)Pumps,Asthenameimplies,cryogenicpumps(cryopumps)relyonliquidN2tocoolmolecularsieveswithlargesurfaceareas.Thecoldsurfaceefficientlyremovesairmoleculesfromambientpressuredownto10-4Pa(10-6Torr).,Becausetheyareoil-free,cryopumpsarealsousedtobackoutionpumpsandpreventtheiraccidentalcontaminationthroughbackstreamingfromoil-bearingpumps.,Wealsousecoldsurfacestoenhancevacuumsinthestageofmostnon-UHVTEMs.Suchcoldfingersoranticontaminatorsprovideanalternativesite(toyourspecimen)forcondensationofresidualcomponentsinthevacuum.,Ionpumpsandcryopumpsaretrappingpumps.Theykeeptheairmoleculeswithinthemandreleasethemwhenturnedofforwarmedup,respectively.,.THEWHOLESYSTEM,Figure.TheprinciplesoftheTEMvacuumsystem.Often,theconsoledisplayontheTEMwillshowasimilardiagram.Themechanicalpumpcanpumpthecolumndirectlyorbackoutthediffusionpump,whichisconnecteddirectlytothebaseofthemicroscope.Ionpumpsareofteninterfaceddirectlytothestageandgunareas.Computer-controlledvalvesseparatethepumpsfromthecolumnandfromeachother.,.SPECIMENHOLDERSANDSTAGES,Historically,microscopistshaveusedtwodifferentdesignsandalotofwhatyoullreadhasastronghistoricalbackground.,1.Thetraditionalside-entryholderisarodwithamotorattachedtotiltand/orrotatethespecimenandaleadconnectingittoapowersupplyandcontrolbox,orliquid-N2dewar.,2.Thetraditionaltop-entryholderisacartridgewhichyouloadintotheTEMbutisdetachedfromtheoutsideworldwhenyouusethemicroscope.,.SIDE-ENTRYHOLDERS,Figure.Principalpartsofaside-entryholderthatisheldinthegoniometerstage.Thespecimenisclampedintothecupattheendoftherod.Asmalljewelattheendoftherod(usuallysapphire)fitsintoanotherjewelbearinginthestagetoprovideastablebaseformanipulatingthespecimen.TheO-ringsealstheendoftheholderinsidethevacuum.Manipulatingthespecimenisaccomplishedfromoutsidethecolumnviacontrolswithintherod.,.TOP-ENTRYHOLDERS,Figure.Crosssectionofatop-entryholder.Thecartridgehasaconeshapewhichfitsintothetaperedboreoftheobjectivelenspole-piece.Thespecimensitsinacupatthebaseofacolumnthroughtheconedownwhichtheincidentbeamtravels.Simplemanipulationssuchastiltingorrotatingrequirecomplexmicromechanicaldesign,sincethespecimenisatthebaseofthecartridgeandcompletelysurroundedbythepolepiece.Totilt,e.g.,asshownintheupperdiagram,pushrodsarepressedagainstspringsintwoorthogonaldirections,displacingaringaroundthecolumn(lowerdiagram),thustiltingthespecimencup.,0.DIFFERENTTYPESOFHOLDERS,Single-tiltholder,Quickchangeholder,Multiple-specimenholder,Bulkspecimenholder,Double-tiltholder,Tilt-rotateholder,Low-backgroundholder,Heatingholder,Coolingholder,Cryo-transferholder,Strainingholder,Figure.Examplesofdifferentdesignsfortheside-entryholder.Fromthetop,theyare:arotationholder,aheatingholder,acoolingholder,adouble-tiltholder,andasingle-tiltholder.,Figure.Multiple-specimenholders.(A)Five-specimensingle-tiltand(B)two-specimendouble-tilt.,(A),(B),Figure0.Abulkholderforlargespecimens.,Figure1.Aside-entrycombinedstrainingandheatingholder.Thespecimenlook