电气博士论文论文

1AsimplesynthesisofFe3O4powdersandtheirStructure,magneticandelectricaltransportpropertiesK.L.Liu*,Y.D.Du,C.Y.LiDepartmentofPhysics,ZhenzhouelecticlpowercollagezhanghaiyangPeople’sRepublicofChina.ABSTRACTFe3O4powderswithhighlycrystallinehavebeensynthesizedbyasol-gelautocombustionsynthesismethodaccompanyingthermalprocess.Thephasestructures,morphologiesofFe3O4havebeencharacterizedbyx-raydiffraction(XRD)andscanningelectronmicroscopy(SEM)equippedwithenergydispersivex-rayspectrometer(EDX).Theresultsshowthatcompleteandhighlycrystallinenaturemagnetitepowderscanbeobtained.Furthermore,theroom-temperaturemagnetizationsandtheresistanceofFe3O4powderswerealsomeasured.ThemaximummagnetizationofsampleS1,S2andS3is88.3,87.32and89.4emu/gattheappliedmagneticfield5kOe,respectively,whicharelargerthanthesaturationmagnetizationofthebulkγ-Fe2O3(76emu/g).ItisprovedfurtherthatFe3O4powderscanbesynthesizedbythecitrate-nitratesol-gelautocombustionreactionmethod.Thepreliminaryreactionmechanismisalsodiscussed.Keywords:Fe3O4;Magneticmaterials;Sol-gelpreparation;Halfmetallic1.IntroductionMagnetite,Fe3O4,isferromagneticinversespinelstructurewithananomaloushighCurietemperatureof860Kandanexceptionallyhighconductivity[1].Inaddition,theband-structurecalculationsformagnetitehaveindicatedthatFe3O4ishalf-metallicandtheconductionelectrons*Correspondingauthor.Address:DepartmentofPhysics,ZhoukouNormalUniversity,Zhoukou466000,PRChina.Tel:[phone];E-mailaddress:[e-mail]2are100%spinpolarized[2].Thesepropertiesmakemagnetitetobeappliedextensivelyinmagnetoresistivedevices,magneticstorageandbiomedicine[3-7].Thenanocrystallinemagnetitewithauniformparticlesizehasbeensuccessfullysynthesizedbychemicalco-precipitation[8,9],thermaldecomposition[10],hydrothermalreaction[11]andsol–gel[12]andothermethod[13].Inthiswork,uniformFe3O4powdersweresynthesizedbythecitrate-nitratesol-gelautocombustionreactionmethod.Thesol-gelautocombustionsynthesismethodforpreparingnanoparticlesofferritespinelssuchasMFe2O4(M=Ni,Zn,Co)[14-18]hasbeendevelopedsuccessfully,whichcanbeanalternativemethodtothoselistedabove.Inthismethod,intermediatedecompositionandcalciningstepsarenotinvolved.Moreover,itiseasytocontrolpreciselythestoichiometriccompositionandcrystallitesize,whichhaveimportanteffectsonthemagneticpropertiesandapplicationsofthemagnetite.Here,forthefirsttimethesynthesisofferritespinelFe3O4powders,usingcitricacidasreducingagentandironnitrateasstartingmaterial,isreported,andthemagnetizationsandresistancesofthesamplesarealsocarriedout.2.ExperimentalprocedureThechemicalmaterialsFe(NO3)3·9H2OandcitricacidwithanalyticalgradewereemployedasrawmaterialstoprepareFe3O4ferrite.Anaqueoussolutioncontainingferricnitrate(0.8M)waspreparedfirst.Withconstantstirring,anappropriateamountofcitricacidwasaddedtothissolutiontoproduceclearcationicsolution.InordertopreparetheperfectFe3O4powders,thedifferentmolarratioofironnitratetocitricacidwasintroduced.S1,S2,andS3wereusedtopresentthesamplessynthesizedatthedifferentmolarratioof1:1,2:1and3:1,respectively.Theresultingsolutionwasevaporatedat348Kandstirredtoformatransparentsol.Then,thetransparentsolwasdriedat403Kforhalfanhourtofurtherremovewater.Thesolturnedintoa3viscousbrowngel.Finally,theobtainedgelwasnotheateduntilignitedpointwasobserved.Thedriedgelwasburntinaself-propagatingcombustionmanneruntilallthegelwasburntoutcompletelytoswellintoafluffymass,whichtransformedintopowderattheslightesttouch.Theas-preparedchocolatebrownpowderwasthencalcinedat923Kfor10hinargonatmosphereandtheresultingblackFe3O4powderswereobtained.Phasecompositionofthesampleswasperformedbyx-raypowderdiffraction(XRD,PhilipsX’pertpro).Themorphologyandcompositionofthesampleswereexaminedbyscanningelectronmicroscopy(SEM)equippedwithenergydispersivex-rayspectrometer(EDX).TheresistancesandthemagnetizationsofallsamplesweremeasuredinacommercialPhysicalPropertyMeasurementsSystem(QuantumDesignPPMS).3.ResultsanddiscussionFig.1displaysXRDpatternsofthethreesamplespreparedbythecombustionreactionmethodatthedifferentmolarratioofironnitratetocitricacid.ThepositionandrelativeintensityofalldiffractionpeaksmatchwellwiththecubicphasemagnetiteofFe3O4(JCPDC19-0629)withlatticeparameterofa=8.3960Å.NootherphaseisdetectedinFig.1,whichindicatesthatthefinalproductsconsistofhighlycrystallineandsingle-phaseFe3O4.Butthemaghemite(γ-Fe2O3)4iseasilyformedduetooxidationduringtheformationofFe3O4andithasalmostthesameXRDpatternofFe3O4,whichgivesrisetodifficultytoidentifythekindoftheoxideonlybytheXRDpatternsduetothesimilarityinlatticeconstantvaluebetweenthem.Inordertoclarifythephasesexhibitedinthepowders,spotenergydispersiveX-rayspectroscopy(EDX)analysis(Fig.2(a))wasusedtoconfirmthepurenatureoftheFe3O4product:onlyFeandOelementsweredetectedfromtheproduct,theatomratiooftheseelementsFe:Ois45.91:54.09,verysimilartothetheoryratio3:4.TheEDXdateisconsistentwiththeXRDanalysisandfurtherprovestheformationofpurecubicphaseofFe3O4product.5ThemorphologiesandmicrostructuresoftheFe3O4werefurthercharacterizedbySEM.Fig.2showsatypicaloverallmorphol