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Influence_0Influe

    Influence

    ationalEngineoingResearchCenterforRareEarthMaterialsGeneralResearchInstituteforNonferrousMetals,andGriremAdvancedMaterialsCoLtd..Beijing ;100088,China

    ;Received5September2007;receivedinrevisedform27October2007;accepted8November2007

    ;Abstract

    ;ThelongafterglowphosphorCaA1,O4:Eu2+

    ;,

    ;Nd3waspreparedbvthehightemperaturesolidstatereactionmethod,

    ;andtheinfluenceofLa~’

    ;andDyonthepropertiesofthelongafterglowphosphorwasstudiedbyX—raydiffi’action(

    xRD),photoluminescence(PL),andthermolu

    ;minescencefTL).ThexRDpattemshowsthehostphaseofCaA1204isproducedandnoimpurityphaseappears.Thepeakwavelengthof

    ;thephosphordoesnotvarywithLaandDydoping.Itimpliesthatthecrystalfield,whichaffectsthe5delectronstatesofEu,isnot

    ;changeddramaticallyafterdopingofLa

    andDyTheTLspectraindicatethatthephosphordopedwithLaorDyproducesdifierent ;depthsoftrapenergyleve1.Inthemechanismoflongafterglowluminescence,itisconsideredthatLaorDy”worksastrapenergyleve1.

    ;Thedecaytime1iesonthenumberofelectronsinthetrapenergv1eve1andtherateoftheelectronsreturningtotheexcitationleve1.

    ;Ke)words:rareearth;aluminate;longafterglow;phosphor;electronflaps ;1.IntrOductiOn

    ;Thealuminatelongafterglowphosphorcanstoreenergy

    ;aftershorttimeirradiationbysunlightorfluorescentlamp,

    ;andthenemitvisiblelightinthedarkness.Furthermore,the

    ;aluminatelongafterglowphosphorcanovercomethedis

    ;advantagesofsulfidephosphors,andhascharacteristicsof

    ;highluminescentbrightness,longafterglowtime,good

    ;chemicalstabilityandwithoutpollutionandsoon.Thusthe

    ;aluminatelongafterglowphosphoriswidelyappliedtolu

    ;minescentpaint,nightillumination,instrumentaldisplayetc. ;l4].

    ;Studiesonthealuminatesystemhavebeenmainlyfo

    ;cusedonthedopingwiththesecondactivatorexceptEu

    ;since1990s,forexample,DyandNd.Itisexpectedthatthe

    ;phosphorcanformthepropertrapenergylevelbyimporting

    ;microelements.Sotheluminescentbrightnessandafterglow

;timeareenhanced5.10].Tangeta1.studiedthelumines

    ;centcharacteristicofSrAl2O4:Euin19953].Yamamoto

    ;a1.reportedthelongafterglowcharacteristicofSrAl204: ;Eu,DyandCaAI2O4:Eu,Dyin1997[6].

    ;Inthisarticle.theinfluenceOfLa”andDy”onthe

    ;propertiesofthelongafterglowphosphorCaAl204:Eu, ;Correspondingauthor:ZHUANGWeidongEmail:wdzhuang@126corn ;Ndwasstudied.ItisprovedthattheLaandDyworks ;astrapenergyleve1.Theafterglowtimeliesonthenumber ;ofelectronsinthetrapenergylevelandtheratethatthe ;electronsretumtotheexcitationleve1.

    ;2.Experimental

    ;2.1.Preparation

    ;ThelongafterglowphosphorCaAl204:Eu,Nd,RE”

    ;(RE=LaorDy)waspreparedbythehightemperature ;solidstatereactions.TherawmaterialswereCaCO3(3N), ;AI203(4N),Eu203(4N),Nd203(4N),La203(4N),Dy203(4N) ;andHBOisaddedasflux.Themixtureofrawmaterials ;wasgroundandthensinteredinreducingatmosphereat ;1300.Cforabout3h.Aftercoolingandgrinding,thesam

    ;pieofthealuminatelongafterglowphosphorCaAl204:Eu2, ;N.RE”wasobtained.

    ;2.2.Measurements

    ;Theluminescentbrightnessandtheafterglowtimeofthe ;sampleweremeasuredusingaST900PMphotometer.The

    ;excitationandemissionspectraofthesampleweremeas

    ;uredusingFluoroMaxIIspectrofluorometermadeinthe ;

    ;

    ;336

    ;USA.Thecrystalstructureofthesamplewascheckedusing ;aMXP2NAHFX-mydiffractometer.TheTLspectrawere ;measuredbytheLab-made1100AUTOMATEDTLSYS. ;.

    ;3.1suitsanddiscussion

    ;3.1.ExcitationandemissionspectraofCaAI2O4:Eu+, ;Nds+

    ;,REphosphor

    ;Fig.1showstheemissionspectraofCaA12O4:Eu2+, ;Nd3,

    ;Iphosphor.111eemissionspectra(=365nm)showa ;broadbandfromEaboutat440nn’1.Thisemissionisdue

    ;tothetransiti0IlofEthatoriginatesfrom4f65dexcitation ;stateto4groundstate.Thepeakwavelengthofthephos

    ;phordoesnotvarywiththedopedRE3.Itimpliesthatthe ;crystalfield,whichaffectsthe5delectronstatesofEu2+. ;is

    ;notchangeddramaticallybythevariationsofthedoped ;RE3.

    ;Wavelength/nrn

    ;Fig.1.EmissionspectraofCaAlzO4:Eu2+,

    ;Nd3+

    ;,RE:(a)

    ;CaAIz04:Euz+,NdS+;(b)CaAlzO4:Eu,NdLa;(c)

    ;CaAlzO4:Euz+,NdDy3+;(d)CaAI204:Euz+,Nd3+,La3+,Dy3+. ;Fig.2showstheexcitationspectraofthephosphor.There ;aretwoexcitationpeaksat275nnland338nlTIintheexci

    ;tationspectra?=440nm),whichisconsistentwiththe

    ;absorptionofthehostC8A1204andthetransitionofEu2+ ;thatisfrom4iato4f65d,respectivelyI11].

    ;3.2.X-raydiffractionanalysis

    ;Fig.3showstheXRDpaaemsofCaA1204:Eu2+,

    ;Nd3,

    ;REsamples.Ithasamonocliniccrystalstructure.Itslat.

    =0.8698rllTl9b=0.8092nlTlandc= ;riceparametersare

    ;1.5208nm,0CPDS:23l036).TheXRDpaRernsshowthat

    ;thehostphaseofCaA1204isformedforthesampleswith ;differentdopedR.Itisconsistentwiththe?DpaRem

    ;ofthephosphorwithoutRE3doping.Thereisonlyasingle ;phasethatbelongstoCaAI204andnoimpurityphaseap

    ;pears.Itisarguedthatthissinglephasehasbeendeveloped ;fullythroughthepreparationprocedure.

    ;RAREMETALS,VoL27,No.Aug2008

    ;Wavelength/rim

    ;Fig.2.ExcitationspectraofCaAlz04:Eu,Ndlu:(a)

    ;CaA1204:Eu,Nd3+;(b)CaAlzO4:Euz+,Nd3+,La3+;(c) ;CaAlz04:Euz+,Nd3+,Dy3+;(d)CaAI204:Euz+,Nd3+,La3+,D ;20/(o)

    ;Fig.3.XRDpatternsofCaAlz04:Eu,NdRE:(a)

    ;CaAlz04:Euz+,Nd3+;(b)CaAlzO4:EuN,L(c)

    ;CaAlz04:Euz+’NDye;(d)CaAlz04:Euz+,Nd3+,LaDy3+.

    ;Tablelshowsthecrystallatticeparametersofthelong ;aflerglowphosphordopedwithdifferentRE3.Theelectric ;chargeandradiusofREaredifferent_0mthoseofC.

    ;?,11ellLareplacesCa.thecrystallatticewillexpandbe. ;causetheradiusofLa3islargerthanthatofCa2+.Onthe ;contrary,whenofreplacesCa2+,thecrystallatticewill ;shrinkbecausetheradiusofD37+issmallerthanthatofC.

    ;,ellLajandDvjreplaceCa2+simultaneously.thepa- ;rameterchangesarenotmuch.

    ;

    ;

    ;x

    ,lnfluence0fLaandDyontheproperties0fthelongaftergl0wph0sph0rCaAI2O4:Eu2+,N

    d337

    ;Table1.Crystallatticeparametersofthephosphor ;3.3.TLspectraanalysis

    ;Ingenera1.theinitialbrightnessandafterglowtimeare ;relatedtothedensityanddepthofthetrapenergylevelbe

    ;sidesthedensityofluminescentcenter.TheTLspectraare ;measuredtoknowthedensityanddepthofthetrapenergy ;leve1.Fig.4showstheTLspectraofCaA12O4:Eu,Nd”,

    ;IE”.Thespectraareallunitary.Theintensityofthether.

    ;malspectrumpeakstandsforthenumberofelectronscap? ;turedbythetrapenergylevelorthenumberoftrapenergy ;leve1.TheCaA1,O4:Eu,Ndj,I”phosphorsallhave

    ;differentdepthsoftrapenergylevel(Table2).Thedepthof ;CaA1,O4:Eu,Nd”phosphoristhelowest,andthe

    ;CaA12O4:Eu,Nd”dopedwithLajandDy”isthehighest

    ;Thedeeperthetrapenergyleve1.theslowertherateofthe ;electronsreleasingfromthetrapssotheafterglowtimeof ;CaA12O4:Eu,Ndj,I”becomelonger.Itisconsistent

    ;withtheoreticalanalysis.

    ;100

    ;8O

    ;_

    ;60

    ;;

    ;.40

    ;

    ;2O

    ;0

    ;JC

    ;Fig.4.TLspectraofCaAI2O4:Eu,Nd,RE:(a)CaAI2od:

    ;Eu,Nd;Co)CaAI2O4:Eu,Nd,La3;(c)CaAI2O4:Eu, ;Nd,Dy3;(d)CaAI2O4:Eu,Nd,La3,Dy.

    ;Table2.TrapenergylevelsofCaAI2O4:EuNd,RE ;Thecalculationofthetrapenergyadoptstheempirical ;formula,whichwasfoundin1969byChen[121. ;E=G()/rB(2kTm),

    ;where,f=Tm,=T2Tm,co=T2T1,g=O/co,k=Tm

    ;+273.15;TmisthetemperatureoftheTLspectrapeak;

    ;andT2isthetemperatureofthehalfhighpeak,furthermore ;T1<T2;eandBfaretheconstant.

    ;3.4.InfluenceofLaandDy3dopingontheproperties ;oflongafterglowphosphor

    ;Table3andFig.5showtheinitialbrightnessandthede

    ;caycurveofCaA12O4:Eu,Nd,RE,respectively. ;PhosphordopingLaandDynotonlyenhancesitslumi

    ;nescentbrightnessbutalsogreatlyprolongsitsafterglow

    ;time.

    ;Table3.ComparisonofthebrightnessofCaAI2O4:Eu,

    ;Nd,

    ;RE3

    ;PhosphorRelativebrightness

    ;CaA12O4:Eu,Nd

    ;CaA12O4:Eu,Nd,La3

    ;CaA12O4:Eu,Nd,Dy

    ;CaA1204:Eu,Nd,La,Dy

    ;71.1

    ;89.0

    ;83.2

    ;100.0

    ;1OO

    ;

    ;\

    ;,1O.

    ;C

    ;

    ;Tirne|s

    ;Fig.5.LongdecaycurvesofCaAI2O4:Eu

    ;,

    ;Nd

    ;,

    ;RE3:(a)

    ;CaAI2O4:Eu,Nd;Co)CaAI2O4:Eu,Nd,La3;(c) ;CaAI2O4:Eu,Nd,Dy3+;(d)CaAI2O4:Eu,Nd,La3,Dy3.

    ;Ingeneral,thealuminatelongafterglowphosphoracti? ;vatedbyEucanproducetraDenergylevelwithsome ;quantityanddepthduetodopingwithotherrareearthions, ;andthenprolongtheluminescenttimef1314].whenthe

    ;phosphordopedwithLaorDy”.unequalreplacementis

    ;producedbetweenLa”.D/andCa.Thusmoretrapen—

    ;ergylevelisproducedandthedepthoftheenergylevelis ;deeper.Otherwise.LajorDy”possessesproperaffinity

    ;towardtotheelectronsintheenergylevel,whichcanrelease

    ;electronsslowlyandproducelongafterglowluminescence. ;Themorethetrapenergyleve1.themoreelectronsarestored ;intheenergyleve1.Thedeepertheenergyleve1.theslower ;therateofelectronrelease.ThereforetheLaorDy”addi—

    ;

    ;338

    ;tionenhancestheinitialbrighmessandprolongsthelumi

    ;nescenttime.

    ;3.5.MechanismoRlongafterglowluminescenceof ;CaAI204:Eu2+’Nd3+,

    ;Manyresearcheshavebeenconductedonthelongafter- ;glowluminescenceaboutthealuminatesphosphor.TIle ;typicalmechanismisduetodopingwithco.activator,anda ;newenergylevelnamedtrapenergylevelisproducedinthe ;system[6.Fig.6showsthemechanismoflongafterglow ;luminescenceofCaAI204:Eu2_Nd”.RE”.Thelumines-

    ;conceisproducedbythe4f---95dtransitionofEu.TheE ;iORShaveinfluencesonthestructureofhostcrystalsincethe ;radiusof}IJislargerthanthatofC.Thephosphorpro. ;duceselistortionofthehostcrystalandchangesthestructure ;ofthecrysta1.WhenthephosphorisdopedwitllLajor ;Dyj.thenewtrapenergylevelisproducedduetothedif- ;ferenceradiibetweenREandCa2.Theenergylevellies ;betweentheexcitationlevelandgroundlevelofEu2.?ell

    ;thephosphorisexcited,apartoftheelectronsreturntothe ;lowerenergylevelandproduceluminescence.Theother ;electronsarestoredinthetrapenergylevelviarelaxation ;process.Whentheelectronsinthetrapenergylevelabsorb ;energy,theyreturntotheexcitationstateagain,andthen ;transfertothegroundstateandproduceluminescence. ;Theescapeofcapturedelectronsisacontinuousprocess; ;thusthephosphorexhibitsthecharacteroflongafterglow ;luminescence.edepthofthetrapenergylevelisimpor- ;rantforthelongat~’glowphosphor;iftheenergylevelis

    ;tooshallow,theelectronseasilyescapefromthetrapsat ;roomtemperature.andtheafterglowtimeistooshortorthe ;luminescencecannotbeobservedatall;iftheenergylevelis ;toodeep,thenumberofelectronsescapingfromthetrapis ;tooless.sometimesitdoesnotexist.Botharenotbeneficial ;tolongafterglowluminescence.

    ;Conductionband

    ;4f

    ;卜一

    ;——

;UTraps

    ;Valenceband

    ;6.MechanismoflongafterglowluminescenceinCaAI204: ;Eu2+,NdRE.

    ;Theafterglowtimeisalsoconcernedwiththeabsorbed ;energiesandtheelectronsstoredinthetrapenergylevel:the ;moretheelectronsinthetrapenergylevel,thelongerthe ;RAREMETALS,VoL27,No.Aug2008

    ;afterglowtimeis;themoretheabsorbedenergyis,theeasier ;theelectronsconquertheenergydistancebetweenthetrap ;energylevelandexcitationleve1.Thusthephosphorpro- ;ducesthephenomenonoflongafterglowluminescence,but ;itdoesnotmeantoprolongtheluminescenttimewhenthe ;absorbedenergiescontinuouslyincrease.Iftheadequateell- ;ergiescausetheelectronstoreturntotheexcitationlevel ;once,itdoesnothelptoprolongtheluminescenttime.Con- ;trarily,iftheenergiesaretoolow.itdoesnotcausetheelec. ;tronsreturntotheexcitationleve1.andtheafterglowlumi. ;nescenceisnotobserved.ustheafterglowtimeliesonthe ;numberofelectronsinthetrapenergylevelandtherateat ;whichtheelectronsreturntotheexcitationleve1.TIleinten- ;sityoftheafterglowliesintherateatwhichtheelectronsin ;trapenergylevelreturntotheexcitationlevelinunittime. ;4.Conclusions

    ;(1)Thepeakwavelengthofthephosphordoesnotvary ;withtheREdoping.Itimpliesthatthecrystalfield,which ;affectsthe5delectronstatesofEu2+,isnotchangeddra- ;maticallybythevariationsofRE3

    ;(2)TheresultofXRDanalysisindicatesthatthehost ;phaseofCaAl204formswithtlleLajorDyjdoping.No ;impurityphaseappears.

    ;(3)TheTLspectraindicatethatthephosphordopedwith ;La3orD),jproduceadeepertrapenergylevelthanthe ;othersamples.

    ;(4)ThephosphordopedwithLa3orDywillnotonly

    ;enhanceitsluminescentbrightnessbutalsogreatlyprolong ;itsafterglowtime.TheLa3orD),jworksastrapenergy ;leve1.Theafterglowtimeliesonthedepthofthetrapenergy ;levelandtherateatwhichtheelectronsreturntotheexcita- ;tionleve1.

    ;Acknowledgements

    ;ThisstudyisfinanciallysupportedbytheNationalNatu- ;ralScienceFoundationofChina(No.50204002)andthe ;NationalHighTechResearchandDevelopmentProgramof

;China(No.2001AA324080).

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