aim雅虎邮箱停止服务

InternationalJournalofAutomotiveandMechanicalEngineering(IJAME)ISSN:2229-8649(Print);ISSN:2180-1606(Online);Volume7,pp.
840-849,January-June2013UniversitiMalaysiaPahangDOI:http://dx.
doi.
org/10.
15282/ijame.
7.
2012.
4.
0069840EFFECTOFTa2O5DOPINGONTHEMICROSTRUCTUREANDDIELECTRICPROPERTIESOFBaTiO3BASEDCERAMICSAdnanMousharrafandMd.
FakhrulIslamDepartmentofMaterialsandMetallurgicalEngineeringBangladeshUniversityofEngineeringandTechnology,Dhaka-1000,BangladeshEmail:addumos@yahoo.
comPhone:+8801714939276.
ABSTRACTThemainfocusoftheresearchwastocorrelatecompositionandsinteringparameterswiththemicrostructureanddielectricpropertiesofTa2O5dopedBaTiO3ceramics.
Thesamplesweresinteredusingbothsingleandtwo-stagesinteringtechniques.
ThereafterSEMandXRDtechniqueswereusedtoexaminethestructureofthesampleswithaparticularfocusontheincorporationofTa5+ionsintotheBaTiO3crystallattice.
TheSEManalysisfocusedonmeasuringthegrainsizeandinvestigatingthegrainsizedistributionofthesinteredsamples.
Finally,thedielectricpropertieswereanalysedandtherelationshipbetweenthepropertiesandstructureofthedopedBaTiO3wasestablished.
Fromtheresearchitcanbestatedthattwo-stagesinteringyieldedthebestdielectricproperties.
Thebeststablevalueoftheroomtemperaturedielectricconstant(k)of19000wasobtainedforthe1.
5mole%Ta2O5dopedBaTiO3samplesinteredat13200Cfor0hrsand12800Cfor6hrs,duetothecombinationofahighpercenttheoreticaldensity(%TD)andoptimumgrainsize.
Atatemperaturerangeof300to600C,thiscombinationofcompositionandsinteringparametersyieldedadielectricconstantintherangeof18000–19000.
Keywords:Bariumtitanate;tantalaumoxide;doping;ceramic;double-stagesintering.
INTRODUCTIONBaTiO3,aferroelectricceramicmaterial,hasattractedconsiderableinterestforapplicationinavarietyoffields,suchascapacitors,transducers,actuators,electro-opticdevices.
Thereareseveralreasonsforitsextensivepracticalapplication.
Firstlybecauseitischemicallyandmechanicallyverystable;secondlyitexhibitsferroelectricpropertiesatandaboveroomtemperature,andfinallybecauseitcanbeeasilypreparedandusedintheformofceramicpolycrystallinesamples.
However,pureBaTiO3withanaveragegrainsizeofaround1mexhibitsadielectricconstantof3000–5000atroomtemperature(Amarandeetal.
,2007;Arltetal.
,1985;Sivakumaretal.
,2011;Jingkunetal.
,1999).
Dielectricconstantsashighas5800havealsobeenreported(Amarandeetal.
,2007).
ForpureBaTiO3,thedielectricconstantvalueincreaseswithadecreaseingrainsizeandageneralbroadeningofthetransitionpeakresults(KinoshintaandYamaji,1976).
Moreover,thesuppressionofthetransitionpeaksignificantlyincreasestheroomtemperaturedielectricconstantofpureBaTiO3(BurfootandMartirena,1974).
Despiteallthesefindings,theindustrialapplicationofpureBaTiO3isstillsomewhatlimitedduetoitslowdielectricproperties.
InordertouseBaTiO3onanindustrialscale,BaTiO3hasbeendopedwithseveraldopantslikeMgO,ZrO2,Nd2O3,Ta2O5,Nb2O5,andmanymoretocontrolitsgrainsizeEffectofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3basedceramics841andtoimproveitsdielectricproperties(Armstrongetal.
,1989;Kelvinetal.
,2010;Chanetal.
,1986;Chaoetal.
,2008;Hwangetal.
,2000;Kelvinetal.
,2010;Mastelaroetal.
,2004).
Amongthesedopants,pentavalentoxidessuchasTa2O5,Nb2O5haveastronginfluenceonthedielectricpropertiesofBaTiO3.
TheyinfluencethedielectricpropertiesofpureBaTiO3bycontrollingthegrainsize,improvingthedensityandshiftingtheCurietemperature.
Underoptimumsinteringconditionsanddopinglevels,thedielectricpropertiesofpureBaTiO3canbesignificantlyenhancedwithTa2O5,Nb2O5(Ahnetal.
,2009).
Judgingthepotentialofpentavalentoxidesusedasdopants,thecurrentstudyextensivelyexaminedtheeffectsofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3ceramics.
Theultimateaimofthestudywastodevelopanenvironmentally-friendlyceramiccapacitorforelectronicindustries.
EXPERIMENTALReagent-gradenano-sizedoxidepowderofBaTiO3withapuritybetterthan99%wasusedasthestartingrawmaterials.
ThepowderwasdopedwithTa2O5atconcentrationsrangingfrom0.
5–1.
5mole%.
Bothpowdersweremixedandmilledfor18–20hrs.
Followingthat,thepowdersweredriedandabinderPVAwasadded.
Thenthepowderswerepressedintopellets,approximately5mmthickand12mmindiameter,atapressureofaround150MPausingahydraulicpress(Figure1(a)).
Subsequently,thepressedsamplesweresinteredinahightemperaturefurnace(Figure1(b)).
Foraparticularsinteringcycle,thesampleswereatfirstheatedto5500Cforonehourtoremovethebinderandwerethenheatedtothedesiredsinteringtemperature.
Singlestagesinteringwascarriedoutintherangeof1250–13000Cfortwohours.
Whereas,two-stagesinteringwascarriedoutbykeepingthe1ststagesinteringconditionfixedat13200Cforzerohoursandonlyvaryingtheholdingtimeduring2ndstagesinteringfrom4–6hrsataconstanttemperatureof12800C.
Aftersintering,X-raydiffraction(XRD)wasperformedinordertodeterminethepercenttheoreticaldensity(%TD),phaseandstructuralanalysis.
Moreover,micro-structuralanalysiswasperformedusingafieldemissionscanningelectronmicroscope(FESEM)(Figure2(a)),andthetemperaturedependenceofthedielectricconstant(k)wasmeasuredatvariousfrequencies(10to500kHz)ataheatingrateof4oC/minusinganImpedanceanalyser(Figure2(b)).
Figure1.
Experimentalsetupforsamplepreparation:(a)Hydraulicpressforpelletizing;(b)Hightemperaturefurnaceforsintering.
(b)(a)MousharrafandIslam/InternationalJournalofAutomotiveandMechanicalEngineering7(2013)840-849842Figure2.
Experimentalsetupforcharacterisationanddielectricpropertymeasurement:(a)FE-SEMformicrostructuralanalysis;(b)Impedanceanalyserfordielectricpropertymeasurement.
RESULTSANDDISCUSSIONNumericaldataontheeffectofsinglestagesinteringonthepercenttheoreticaldensity(%TD),grainsizeanddielectricconstant(k)ofTa2O5dopedBaTiO3samplesaretabulatedinTable1.
Table1showsthatforaparticularmole%ofTa2O5doping,thepercenttheoreticaldensityoftheTa2O5dopedBaTiO3samplesincreasedwithincreasingtemperature.
However,atacertaintemperature,anincreaseintheTa2O5mole%resultedinaloweringofpercenttheoreticaldensityofthesamples.
Table1.
Percenttheoreticaldensity(%TD),grainsizeanddielectricconstant(k)ofsinglestagesinteredTa2O5dopedBaTiO3samplesSINo:Sinteringrate(0C/min)Sinteringtemperature(0C)Holdingtime(Hours)Coolingrate(0C/min)Dopingmole%%TDGrainsize(m)Dielectricconstant(k)atroomtemperature1512502hrs30.
591.
70.
301700–20002512502hrs31.
090.
50.
281700–20003512502hrs31.
588.
00.
251700–20004512752hrs30.
592.
00.
351700–20005512752hrs31.
091.
60.
301700–20006512752hrs31.
590.
00.
261700–20007513002hrs30.
593.
01.
10130008513002hrs31.
092.
00.
401700–20009513002hrs31.
591.
50.
381700–2000(a)(b)EffectofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3basedceramics843ItisalsoobservedfromTable1thatforallTa2O5dopedBaTiO3samplessinteredwithinthetemperaturerangeof1250–12750C,2hrsofholdingtimeprovedtobeinsufficient,resultinginsampleswithafinegrainsize.
Duetotheirfinegrainsize,allthesesamplesexhibitedapoordielectricconstantinspiteofhavingahighpercenttheoreticaldensity.
However,at13000Cthe2hrsoftheholdingtimeprovedtobesufficientforthe0.
5mole%Ta2O5dopedBaTiO3sample.
Asaresult,thesampleexhibitedahighpercenttheoreticaldensity,satisfactorygrainsize,andmoderatedielectricconstant.
For1.
0–1.
5mole%Ta2O5dopedBaTiO3samples,theholdingtimeagainprovedtobeinadequateandresultedinsamplesoffinegrainsize.
Asaconsequence,thesesamplesagainpresentedapoordielectricconstant.
AlltheseresultsareconsistentwiththefindingsofManalertandRahaman(1998),whoconcludedthattheamountofpentavalentoxidedopantscontrolsthemicrostructureofTa2O5dopedBaTiO3samples.
Forthepenta-valancedonorcations,thegrainboundarymobilityinitiallyincreaseswithcationconcentrationbutthendecreasessignificantlyaboveadopingthresholdof0·3–0·5mole%.
Generallydopantsneedverylowtoalmostnoenergytoconcentrateatthegrainboundaries.
However,energyisrequiredtoincorporateadopantionintoanindividuallatticesiteincomplexoxides.
Theamountofenergyrequiredisrelatedtothedistortions,i.
e.
differenceinionicradii,andtheformationofcompensatingdefectsduringtheincorporationofaliovalentionsthathavedifferentvalencestates(Ahnetal.
,2009).
Thus,moreenergywasrequiredinourresearchforthediffusionofthedopantstomoveinwardsfromthegrainboundaryintothelatticeandtoreducethepinningeffect.
Therequiredenergycouldhavebeensuppliedbyincreasingtheholdingtimeofthesinglestagesintering.
Butthisroutewasassociatedwithahighcostandwouldmaketheprojectlessviableforindustrialapplication.
Asaresult,furtherresearchwasfocusedontwo-stagesintering.
Numericaldataoftheeffectoftwo-stagesinteringonpercenttheoreticaldensity(%TD),grainsizeanddielectricconstant(k)ofTa2O5dopedBaTiO3samplesaretabulatedinTable2.
Table2.
Percenttheoreticaldensity(%TD),grainsizeanddielectricconstant(k)oftwo-stagesinteredTa2O5dopedBaTiO3samples.
SINo:Sinteringrate(0C/min)Sinteringtemperatureandholdingtime(1ststage)Sinteringtemperatureandholdingtime(2ndstage)Coolingrate(0C/min)Dopingmole%%TDGrainsize(m)Dielectricconstant(k)atroomtemperature1513200Cfor0hrs12800Cfor4hrs30.
595.
01.
8095002513200Cfor0hrs12800Cfor4hrs31.
093.
01.
30165003513200Cfor0hrs12800Cfor4hrs31.
591.
01.
10179004513200Cfor0hrs12800Cfor6hrs30.
593.
51.
9089005513200Cfor0hrs12800Cfor6hrs31.
092.
11.
40137006513200Cfor0hrs12800Cfor6hrs31.
592.
91.
2019000MousharrafandIslam/InternationalJournalofAutomotiveandMechanicalEngineering7(2013)840-849844Table2showsthatfora0.
5mole%Ta2O5dopedBaTiO3sample,4hrsofholdingtimeduring2ndstagesinteringdecreasedthedielectricconstantvalueduetoanexcessiveincreaseingrainsize.
ThehighgraingrowthofthesamplemayhaveresultedfromthecompletediffusionofTa2O5intothebulkmaterial.
ThisphenomenonisclearlyevidentfromtheSEMmicrographofFigure3(a),whichshowsnoevidenceofbimodalgrainsizedistributionorthepinningeffect.
However,fora1.
0mole%Ta2O5dopedBaTiO3sample,4hrsofholdingtimeproducedcontrolledgraingrowthandincreasedthedielectricconstantvaluetoamoderatelevel,whichisshownintheSEMmicrographofFigure3(b).
Forthe1.
5mole%Ta2O5dopedBaTiO3sample,4hrsofholdingtimemightnothavebeensufficientforcompletediffusionofthedopantsintothelattice,andasaresulttherewasevidenceofthepinningeffect.
TheSEMmicrographofFigure3(c)clearlyshowsthepresenceofbimodalgrainsizedistribution.
However,thecombinationofamoderatepercenttheoreticaldensityandsuitablegrainsizeof1.
5mole%Ta2O5dopedBaTiO3sampleresultedinthebestdielectricpropertiesfor4hrsofholdingtime.
Figure3.
SEMmicrographs(X50,000)of:(a)0.
5mole%;(b)1.
0mole%;and(c)1.
5mole%Ta2O5dopedBaTiO3samplessinteredat13200C(0hrs)and12800C(4hrs).
(b)(a)(c)EffectofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3basedceramics845Table2alsoshowsthatfor6hrsofholdingtime,the0.
5mole%Ta2O5dopedBaTiO3sampleshowedevenmoreexaggeratedgraingrowthincomparisontothesamplesinteredat4hrsofholdingtime(Figure4(a)).
Thisexcessivegraingrowthresultedinadecreaseinpercenttheoreticaldensityofthesampleandalsolowereditsdielectricconstant.
However,underthesamesinteringconditions,the1.
0mole%Ta2O5dopedBaTiO3sampleshowedcontrolledgraingrowthbutlargerthanthesamplesinteredunderprevioussinteringconditions(Figure4(b)).
Forthe1.
5mole%Ta2O5dopedBaTiO3sample,even6hrsofholdingtimeshowedsignsofthepinningeffect.
Butthegrainsizeofthesamplewasslightlyhigherthanthesamplesinteredat4hrsofholdingtime(Figure4(c)).
Duetothecombinationofhighpercenttheoreticaldensityandoptimumgrainsize,the1.
5mole%Ta2O5dopedBaTiO3sampleexhibitedthebestdielectricpropertiesfor6hrsofholdingtime.
Figure4.
SEMmicrographs(X50,000)of:(a)0.
5mole%;(b)1.
0mole%;and(c)1.
5mole%Ta2O5dopedBaTiO3samplessinteredat13200C(0hrs)and12800C(6hrs).
TheXRDpatternsofFigure5indicatetheformationofbothtetragonalandcubicphasesofBaTiO3.
TheXRDpatternforthe0.
5mole%Ta2O5dopedBaTiO3samplesinteredat13200Cfor0hrsand12800Cfor4hrsshowsthepresenceoftwinpeaks(b)(a)(c)MousharrafandIslam/InternationalJournalofAutomotiveandMechanicalEngineering7(2013)840-849846similartopureBaTiO3powder(Figures5(a)and5(b)).
Soitcanbeconcludedthatfor0.
5mole%Ta2O5doping,theBaTiO3retainedthetetragonalperovskitestructure.
However,theXRDpatternforthe1.
5mole%Ta2O5dopedBaTiO3samplessinteredat13200Cfor0hrsand12800Cfor4hrsshowsnoevidenceoftwinpeaks,whichindicatesthat1.
5mole%Ta2O5reducedthetetragonalityandstabilisedthecubicphaseofBaTiO3(Figure5(c)).
Thisphenomenonisalsoconsistentinthe1.
5mole%Ta2O5dopedBaTiO3samplessinteredat13200Cfor0hrsand12800Cfor6hrs(Figure5(d)).
Soitcanbestatedthat1.
5mole%Ta2O5stabilisedthecubicphaseofBaTiO3.
Figure5.
XRDplotsfor:(a)PureBariumTitanatepowder;(b)0.
5mole%;and(c)1.
5mole%Ta2O5dopedBaTiO3sinteredat13200C(0hrs)and12800C(4hrs);and(d)1.
5mole%Ta2O5dopedBaTiO3sinteredat13200C(0hrs)and12800C(6hrs).
EffectofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3basedceramics847ThefindingsfromtheXRDpatternsarealsoconsistentwiththeresultsofthetemperaturedependenceofthedielectricconstant(k).
AlthoughtheCurietemperature(Tc)forpureBaTiO3is1200C,theCuriepointofthe1.
5mole%Ta2O5dopedBaTiO3sinteredat13200Cfor0hrsand12800Cfor4hrsshiftedto840C(Figure6),andtheCuriepointofthe1.
5mole%Ta2O5dopedBaTiO3sinteredat13200Cfor0hrsand12800Cfor6hrsshiftedto800C(Figure7).
Accordingtotheory,atafixedtemperatureanincreaseinfrequencyresultsinadecreaseindielectricconstant(Bowenetal.
,1976;CarterandNorton,2007;Richerson,1992).
ThiseffectoftfrequencyonthedielectricconstantisclearlyevidentinFigure8.
Thebeststablevalueofdielectricconstantasafunctionoftemperaturewasobtainedaround18000forthe1.
5mole%Ta2O5dopedBaTiO3samplesinteredat13200Cfor0hrsand12800Cfor6hrs(Figure7).
Figure6.
Variationindielectricconstantwithtemperature1.
5mole%Ta2O5dopedBaTiO3sinteredat13200C(0hrs)and12800C(4hrs).
Figure7.
Variationindielectricconstantwithtemperatureof1.
5mole%Ta2O5dopedBaTiO3sinteredat13200C(0hrs)and12800C(6hrs).
MousharrafandIslam/InternationalJournalofAutomotiveandMechanicalEngineering7(2013)840-849848Figure8.
Roomtemperaturedielectricconstantofsamplessinteredatoptimumsinteringcycles.
CONCLUSIONAnaveragegrainsizeofaround1mnormallyprovidesagoodconditionforthedielectricpropertiesofdopedBaTiO3.
Inthisresearch,grainsizesintherangeof0.
8–1.
2mshowedhighvaluesofdielectricconstant,whileafurtherincreaseingrainsizedeterioratedthisproperty.
Itcanalsobeconcludedthatupto0.
5mole%Ta2O5doping,BaTiO3retaineditstetragonalperovskitestructure.
However,1.
5mole%Ta2O5stabilisedthecubicphaseofBaTiO3andshiftedtheCurietemperaturetowardsroomtemperature.
ACKNOWLEDGEMENTSTheauthorsacknowledgeMr.
TanTeckSiong,JEOLAsiaPteLtdforhisassistanceintheSEManalysisandBCSIRfortheirsupportintheXRDanalysis.
REFERENCESAhn,Y.
H.
,Hyun,J.
W.
,Kim,H.
S.
,Kim,Y.
J.
,Lee,J.
H.
,Noh,S.
J.
andYun,M.
Y.
2009.
MicrostructuralCharacterizationandDielectricPropertiesofBariumTitanateSolidSolutionswithDonorDopants.
BulletinoftheKoreanChemicalSociety.
30(6):1267-1273.
Amarande,L.
,Cioangher,M.
,Gheorghiu,A.
,Miclea,C.
,Miclea,C.
F.
,Miclea,C.
T.
,Spanulescu,I.
andTanasoiu,C.
2007.
MicrostructureandPropertiesofBariumTitanateCeramicsPreparedbyMechanochemicalSynthesis.
RomanianJournalofInformationScienceandTechnology.
10(4),335-345.
Arlt,G.
,DeWith,G.
andHennings,D.
1985.
DielectricPropertiesofFine-GrainedBariumTitanateCeramics.
JournalofAppliedPhysics.
58(41):1619-1625.
EffectofTa2O5dopingonthemicrostructureanddielectricpropertiesofBaTiO3basedceramics849Armstrong,T.
R.
,Buchanan,R.
C.
,Maurice,A.
K.
andMorgens,L.
E.
1989.
EffectsofZirconiaonMicrostructureandDielectricPropertiesofBariumTitanateCeramics.
JournalofAmericanCeramicSociety.
72(4):605-611.
Bowen,H.
K.
,Kingery,W.
D.
andUhlman,D.
R.
1976.
IntroductiontoCeramics.
Singapore:JohnWilleyandSons.
Burfoot,J.
C.
andMartirena,H.
T.
1974.
Grain-SizeEffectsonPropertiesofSomeFerroelectricsCeramics.
JournalofPhysicsC:SolidStatePhysics.
7:3182-3192.
Carter,C.
B.
andNorton,M.
G.
2007.
CeramicMaterialsScienceandEngineering.
NewYork:Springer.
Chan,H.
M.
,Harmer,M.
P.
andSmyth,D.
M.
1986.
CompensatingDefectsinHighlyDonor-DopedBaTiO3.
JournalofAmericanCeramicSociety.
69(6):507-510.
Chao,M.
,Liu,H.
,Liu,Y.
,Liu,Y.
,Liu,Y.
,Shen,Z.
,Yao,Z.
andWu,Z.
2008.
StructureandDielectricBehaviorofNd-DopedBaTiO3Perovskites.
MaterialsChemistryandPhysics.
109:475-481.
Hwang,H.
J.
,Iijima,K.
,Niihara,K.
,Nagai,T.
,Sando,M.
andSekinoand,T.
2000.
EffectofMgODopingonThePhaseTransformationsofBaTiO3.
JournalofAmericanCeramicSociety.
83(1):107-112.
Jingkun,G.
,Lian,G.
andWeiling,L.
1999.
SinteringandDielectricPropertiesofFine-GrainedBaTi03ceramics.
ScienceinChina(SeriesE).
42(5):554-560.
Kelvin,H.
,Ramesh,S.
,Tan,C.
Y.
andTeng,W.
D.
2010.
Phaseanalysisanddensificationofsteatite-basedceramics.
InternationalJournalofAutomotiveandMechanicalEngineering,1:38-45.
Kinoshinta,K.
andYamaji,A.
1976.
Grain-SizeEffectsonDielectricPropertiesinBariumTitanate.
JournalofAppliedPhysics.
47(1):371-374.
Manalert,R.
andRahaman,M.
N.
1998.
GrainBoundaryMobilityofBaTiO3DopedWithAliovalentCations.
JournaloftheEuropeanCeramicSociety.
18(8):1063-107.
Mastelaro,V.
R.
,Stojanovi,B.
D.
,Santos,C.
O.
P.
andVarela,J.
A.
2004.
StructureStudyofDonorDopedBariumTitanatePreparedfromCitrateSolutions.
ScienceofSintering.
36:179-188.
Richerson,D.
W.
1992.
ModernCeramicEngineering.
NewYork:MarcelDekker.
Sivakumar,S.
,Ramesh,S.
,Chin,K.
L.
,Tan,C.
Y.
andTeng,W.
D.
2011.
EffectofsinteringprofilesonthepropertiesandageingresistanceofY-TZPceramic.
InternationalJournalofAutomotiveandMechanicalEngineering,4:405-413.
Kelvin,H.
,Ramesh,S.
,Tan,C.
Y.
andTeng,W.
D.
2010.
Phaseanalysisanddensificationofsteatite-basedceramics.
InternationalJournalofAutomotiveandMechanicalEngineering,1:38-45.