CuO-based sintering aids for low temperature sintering of BaFe12O19 ceramics

JournalofAsianCeramicSocieties

Journal

of

Asian

Ceramic

Societies

j o ur na l h o me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / j a s c e r

CuO-based

sintering

aids

for

low

temperature

sintering

of

BaFe

12

O

19

ceramics

Hung

Vu

a

_,

_Dieu

_Nguyen

a

_,

_John

_G.

_Fisher

a,∗

_,

_Won-Ha

_Moon

b

_,

_Seok

_Bae

b

_,

_Hee-Gyum

_Park

c

_,

Byong-Guk

Park

c

a_{SchoolofMaterialsScienceandEngineering,ChonnamNationalUniversity,Gwangju500-757,RepublicofKorea}

b_{LGComponentsR&DCenter,LGInnotekCo.Ltd.,Ansan-si,Gyeonggi-do426-791,RepublicofKorea}

c_{Dept.ofMaterialsScienceandEngineering,KoreaAdvancedInstituteofScienceandTechnology,Daejon305-701,RepublicofKorea}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received8March2013

Receivedinrevisedform6May2013

Accepted6May2013

Availableonline23May2013

Keywords: BaFe12O19

Liquidphasesintering

CuO

Graingrowth

Magneticproperties

a

b

s

t

r

a

c

t

Thispaperdescribestheeffectofadditionof2wt%ofCuO,30mol%BaO–70mol%CuOandBaCuO2liquid

phasesinteringaidsonthedensification,microstructureandmagneticpropertiesofBaFe12O19ceramics.

Additionofthesinteringaidsenabledreductionofthesinteringtemperaturefrom1250◦Cto1100◦C. Thesinteringaidscausedabnormalgraingrowthinceramicssinteredat1100◦_{C,withCuOhavingthe}

strongesteffect.Allsamplessinteredat1250◦Cshowedabnormalgraingrowth.AdditionofCuOand BaO–CuOcausedthegrainsizedistributiontoshifttolargervaluescomparedtothesamplewithout sinteringaid.Theeffectofthesinteringaidsongraingrowthbehaviorisexplainedusingthe interface-reactioncontroltheoryofgraingrowth.Theincreaseingrainsizecausedareductionincoercivityofthe sampleswithsinteringaidaddition,particularlyinthesamplessinteredat1100◦C.

©2013TheCeramicSocietyofJapanandtheKoreanCeramicSociety.Productionandhostingby ElsevierB.V.Allrightsreserved.

1. Introduction

The M-type hexaferrites can be generally represented as (MeO·6Fe2O3)whereMeisametalsuchasBa,Sr,Pboramixture. Thehexagonalbariumhexaferrite(BaFe12O19)isknownasahard magneticmaterialwithhighcoercivityandlargesaturation mag-netization[1].Itisusedinhighfrequencymicrowavetechnology, magneticrecordingmediaandasapermanentmagneticmaterial [2].Althoughthismaterialhasawiderangeofcommercial appli-cations,itisdifficulttosintertohighdensityand needsa long sinteringtime.Commercially,thesinteringtemperatureshouldbe reducedwhilemaintaininghighdensityandgoodmagnetic prop-erties.

Thesinteringtemperaturecanbereduced bytheadditionof sinteringaidswhichmelttoformaliquidphase.Foraliquidphase sinteringaidtobeeffective,thesolidphaseshouldbepartially sol-ubleintheliquidphasetoaidmasstransport.Wettingoftheliquid

∗ Correspondingauthor.Tel.:+82625301702;fax:+82625301699.

E-mailaddress:johnfi[email protected](J.G.Fisher).

PeerreviewunderresponsibilityofTheCeramicSocietyofJapanandtheKorean

CeramicSociety.

2187-0764©2013TheCeramicSocietyofJapanandtheKoreanCeramicSociety.

ProductionandhostingbyElsevierB.V.Allrightsreserved.

http://dx.doi.org/10.1016/j.jascer.2013.05.002

onthesolidgrainsprovidesacapillaryforcethatpullsthegrains togetherandpromotesdensification.Ahighdiffusionrateofthe solidphaseintheliquidcangivefastsinteringorlowersintering temperatures.

Bi2O3 [3],B2O3 [4]andBaCu(B2O5)[5]havepreviouslybeen usedasliquidphasesinteringaidsforbariumhexaferrite.Inthe presentstudy, low temperaturesinteringof barium hexaferrite wascarriedoutbytheadditionofdifferentCuO-basedsintering aids(CuO, BaCuO2, BaO–CuOeutecticcompound) toBaFe12O19. Thesesinteringaidshavelowermeltingpointsthanthesintering temperatureofsinglephasebariumferrite[6,7].Therefore,they easilyforma liquid phase duringsintering that couldpromote materialdiffusionand densification.CuOwaspreviously added asasolid statesintering aidfor M-typehexaferritesofformula Ba_1−zSrzCuxFey−xO19−ı(z=0or0.5,x=0–1.0,andy=11–12)[8].The CuOenhancedlow-temperaturesinteringthroughtheformationof aliquidphase.TheBaO–CuOeutecticcompound haspreviously beenused successfullyas a sintering aidfor BaTiO3 [9]. Inthe presentstudy,theeffectofsinteringaidadditiononthedensities, phaseidentification,microstructureandmagneticpropertiesofthe sampleswereexamined.

2. Experimental

M-typebariumhexaferriteBaFe12O19powder(99.2%,supplied byLGInnotekCo.Ltd.,LGComponentsR&DCenter) wasmixed with2wt%ofthefollowingsinteringaids:CuO(AlfaAesar99.7%), a 30mol% BaO–70mol% CuO eutectic compound and BaCuOx (AlfaAesar, BaO–CuO1:1ratio compound, 99.9%).The eutectic

compoundwasformedbymixingappropriateamountsofBaCO3 (AlfaAesar99.8%)andCuO,followedbycalcinationat850◦Cfor 10h.CuOformsaeutecticwithBaOatapproximately900–930◦C [6,7].Mixingwascarriedoutbyballmillinginethanolwith stain-lesssteelmediainpolypropylenejarsfor24h.Afterevaporating theethanolwithahotplateandstirrer,thepowderwasground inanagatemortarandpestleandsievedtopassa180␮mmesh. TheBaFe12O19powderwithoutsinteringaidswasalsoball-milled asabove.Powderswerepressedinastainlesssteeldieof10mm diametertoformpellet-shapedsamples.Thesampleswerethen coldisostaticallypressedatapressureof∼147MPa(1500kgf).

Samplesweresinteredinthetemperaturerange900–1300◦C for3hwithheatingandcoolingratesof5◦Cmin−1.ToreduceBa volatilization,sampleswereburiedinBaFe12O19(Kojundo,99%) packingpowderinadoublealuminacruciblewithlids.The den-sityofsinteredsampleswasmeasuredbyusingtheArchimedes principle.Thecrystallineandphasestructuresofsinteredsamples wereinvestigatedbypowderX-raydiffraction(XRD)withCuK␣ radiation(PANalyticalX’PertPRO,PANanalyticalBV,Alemo,The Netherlands)withthediffractionangle2intherangefrom10◦ to90◦,a0.02◦stepsizeandscanspeedof3◦min−1.For measure-mentofunitcellparameters,Si(99.9%AlfaAesar)wasaddedto thepowder samplesasaninternalstandard.XRDwasrepeated (D/MAXUltimaIII,Rigaku,Tokyo,Japan)intherangefrom10◦to 90◦,a0.02◦stepsizeandscanspeedof1◦min−1.Thesamples sin-teredat1100◦Cand1250◦Cwereselectedforpolishingtoa1␮m finishandwerethermallyetched.ThemicrostructureofBaFe12O19 sampleswasanalyzedusingscanningelectronmicroscopy(Hitachi S-4700 FESEM, Hitachi High-Tech, Tokyo,Japan) fitted withan energydispersiveX-rayspectrometerwithstandardless quantifi-cation (EMAXEnergy EX-200,Horiba, Kyoto,Japan). Grainsize distributionsofthesampleswereevaluatedfromtheSEM micro-graphsusinganimageanalysisprogram(UTHSCSAImageTool,The UniversityofTexasHealthScienceCentreinSanAntonio,USA). Themagneticmeasurementswerecharacterizedatroom temper-atureusingavibratingsamplemagnetometer(VSM,RikenDenshi, Japan).

3. Results

3.1. Phaseidentification

TheXRDpatternsoftheBaFe12O19samplesbothwithoutand withsinteringaidssinteredat1100◦Cand1250◦Cfor3hareshown inFig. 1.Allthesamplescontain BaFe12O19 asthemain phase andcanbeindexedwithICDScard#74-1121.BaFe12O19hasthe hexagonalcrystalstructure(spacegroupP63/mmc).Allthesamples sinteredat1100◦Caresinglephase(Fig.1(a)).Thesintering aid-freesamplesinteredat1250◦Ccontainsaminorpeakat56.75◦ whichmaybelongtoasecondary phaseof magnetiteFe2.957O4 (ICDScard#86-1349).Allthesamplessinteredat1250◦C con-tainaminorpeakjustabove30◦.Inthesamplewithoutsintering aidandtheCuO-addedsample,thispeakcanbeindexedwithICDS card#27-1030forBaFe2O4.ForthesampleswithBaO–CuOand BaCuO2addition,itisnotclearifthispeakbelongstoBaFe2O4or BaFe12O19.Therearesmallshiftsinthepeakpositionsofthe sam-pleswithsinteringaidaddition,indicatinga changeinunitcell parameters.

Fig. 2 shows the change in unit cell parameters for sam-ples sintered at 1100◦C without and with sintering aids. The a unit cell parameter decreases with addition of the CuO and BaO–CuOsinteringaids,andthenincreasesagainwithaddition ofBaCuO2.Thecunitcellparameterdecreaseswithadditionof CuO,andthenincreasesforthesampleswithBaO–CuOandBaCuO2

Fig.1.X-raydiffractionpatternsofBaFe12O19withoutandwithadditionofsintering

aidsaftersinteringat(a)1100◦_Cand(b)1250◦_Cfor3h.

addition.Thechanges inunitcellparameters indicatethatCuO and BaO are forming a solid solutionin theBaFe12O19 lattice. Fromacomparisonofionicradii,CuprobablysubstitutesforFein thelattice(Ba2+_r

6=0.135nm,Fe3+r6=0.065nm,Fe3+r4=0.049nm, Cu2+_r

6=0.073nm, Cu2+r4=0.057nm, Cu3+r6=0.054nm[10]).Cu cansubstituteforFeaccordingtothefollowingdefectreaction:

2CuO→2Cu_Fe+2OO+V••O (1)

Aslightexpansioninthelatticewouldbeexpectedforthe sam-plewithCuOaddition,butareductionintheunitcellparameters

2.3195 2.3200 2.3205 2.3210 2.3215 2.3220 0.5887 0.5888 0.5889 0.589 0.5891 0.5892 0.5893

No sintering aid CuO BaO-CuO BaCuO₂

c ( n m ) a ( n m ) a c

is observed. This may bedue to the formation of the charge-compensatingoxygenvacanciesortooxidationofCu2+ _toCu3+ accordingtothefollowingreaction:

2Cu_Fe+V••_O+1₂O2(g)→2CuFe+OO (2) Addition of excess BaO is expected to cause an increase in theunit cellparametersdue toincorporationof BaOintolarge intersticesintheunitcell[11].Additionof30mol%BaO–70mol% CuOsinteringaidcausesanincreaseinthecunitcellparameter (comparedtothesamplewithCuOaddition)buttheaunitcell parametercontinuestodecrease.OnlywithadditionofBaCuO2do bothaandcunitcellparametersincrease.For thesamplewith BaCuO2addition,thecunitcellparameterisalmostequaltothat ofthesamplewithoutsinteringaidaddition,buttheaunitcell parameterislower.ThisindicatesthatBaOandCuOenterthe lat-ticetogetherandthattheeffectofCuOonthelatticeparametersis partiallycancelledoutbytheeffectofBaO.

Therearenoticeablechangesinthepeakintensitiesofthe dif-ferentsamples.Thismaybeanartifactcausedbypressingofthe powder samplesfor XRD. Asthe grainshave a plate-like mor-phology,theymayalignalongcertaindirectionsduringpressing, leadingtoincreasedpeakintensities.

3.2. Density

Fig.3showsthedensityofsampleswithoutandwith sinter-ingaidsasafunctionofsinteringtemperaturebetween900◦Cand 1300◦C.Eachdatapointrepresentstheaveragevalueofnine mea-surements(3samplesforeachsinteringcondition,eachsample measuredthreetimes). Thestandarddeviationof the measure-mentsisshownaserrorbars.Inallcases,thestandarddeviationis <1%andtheerrorbarsaresmallerthanthesymbols.Inthe tem-peraturerange900–1200◦C,itwasfoundthatthedensityofthe

50 55 60 65 70 75 80 85 90 95 100 900 1000 1100 1200 1300 % th eo re  cal d e ns ity Sintering temperature (ºC) No sintering aid 2 wt % CuO 2 wt % BaO-CuO 2 wt % BaCuO₂

Fig.3.ArchimedesdensitiesofsinteredBaFe12O19samples.

sampleswithoutsinteringaidwaslowerthanthatofthe sinter-ingaid-addedsamples.Forexampleat1100◦C,sampledensityfor BaFe12O19withoutsinteringaidwas79.1%whilethedensityof thesamplewithBaO–CuOadditionreached96.7%theoretical den-sity.ThesampleswithCuOandBaCuO2additionhadslightlylower density(95%theoreticaldensityat1100◦C).However,whenthe sinteringtemperaturewashigherthan1200◦C,thesintering aid-freesampleshadahigherdensitythanthesampleswithsintering aidaddition.

3.3. Microstructure

SEMmicrographsofsampleswithoutandwithsinteringaid sin-teredat1100◦CareshowninFig.4.Grainsizedistributionsare

0 10 20 30 40 50 60 1 6 11 16 21 26 31 %

Equivalent spherical 2D radius (μm)

0 5 10 15 20 25 30 35

100%

No sintering aid

0 10 20 30 40 50 60 1 6 11 16 21 26 31 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 CuO 0 10 20 30 40 50 60 1 6 11 16 21 26 31 0 5 10 15 20 25 30 35 BaO + CuO 0 10 20 30 40 50 60 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 66% 0 5 10 15 20 25 30 35 BaCuO2

Fig.5. GrainsizedistributionsofBaFe12O19sinteredat1100◦_Cfor3h.

showninFig.5.ThesamplewithoutsinteringaidinFig.4(a) dis-playsasmallgrainsize(∼0.5–1␮mradius).TheCuO-addedsample inFig.4(b)showsabnormalgraingrowthwithmanylargegrains (radiusupto∼35␮m(Fig.5))aswellassmallmatrixgrains.The samplewithBaO–CuOadditionshowslessabnormalgraingrowth comparedtotheCuO-addedsample(Figs.4(c)and5).Inaddition, thematrixgrainshavegrowntoa radiusof∼2␮m.Thesample withBaCuO2additionissimilartothesamplewithBaO–CuO,but showsfewerabnormalgrains(Figs.4(d)and5).Thesamplewith BaCuO2additioncontainssmallamountsofasecondphase sev-eralmicronsindiameter(notshowninFig.4(d)).Thisphasewas examinedbyEDSandfoundtohaveanapproximatecomposition ofBaFe6O25(Table1).AphasewiththeBaFe6O25compositiondoes notexistintheBaO–Fe2O3phasediagram[12]buttheinteraction volumeforEDScanbeseveralmicronsindiameter[13]andsothe chemicalanalysisofthissecondphasemayincludesignalsfromthe regionbelowthesecondphase.EDSofanabnormalgrainshowed ittohavetheBaFe12O19composition.EDSofamatrixgrainalso showedittohavetheBaFe12O19composition,withinthelimitsof errorforEDSanalysis[13].

Asshown in Fig.6, all samples sinteredat 1250◦C showed abnormal grain growth where some large grains grow rapidly ina matrixoffinegrains.Grainsizedistributions areshownin Fig.7.AdditionofCuOincreasesthesizeoftheabnormalgrains (Fig.6(b)),broadeningthegrainsizedistribution(Fig.7).BaO–CuO

Table1

EDSanalysisofBaCuO2-addedBaFe12O19samplesinteredat1100◦_Cfor3h.

Region Composition(at%) BaFe12O19nominal

composition(at%) Secondphase 3%Ba 3%Ba 18%Fe 38%Fe 79%O 60%O Abnormalgrain 3%Ba 3%Ba 35%Fe 38%Fe 62%O 60%O Matrixgrain 4%Ba 3%Ba 32%Fe 38%Fe 65%O 60%O

additiondoesnothave mucheffect ontheabnormalgrainsize andgrainsizedistributioncomparedwiththeCuO-addedsample (Figs.6(c)and7),butadditionofBaCuO2decreasestheabnormal grainsize(Fig.6(d)),makingthegrainsize distributionslightly narrower(Fig.7).

3.4. Magneticproperties

ByusingaVSMwithamagneticfieldof15kOe,themagnetic hysteresisloopsofsamplessinteredat1100◦Cand1250◦Chave beencharacterizedandshowninFig.8.Measurementshavebeen takenwiththemagneticfieldperpendiculartothesampleface. In addition, thecoercivity, saturationmagnetizationand rema-nentmagnetizationareshowninTable2.Forthehysteresisloops ofthesamplessinteredat1100◦C,thesamplewithoutsintering aidaddition displaysa typicalbroadhysteresisloop (Fig. 8(a)). TheCuO-addedsampleshowsaverynarrowloopwhichis sim-ilartothebehaviorofsoftmagneticmaterials.TheBaO–CuOand BaCuO2-addedsampleshavewiderloopsthattheCuO-added sam-ple, although not as wide as the sample withoutsintering aid addition.Allthesampleswithsinteringaidshavepinched hystere-sisloops;thispinchingismorepronouncedfortheBaO–CuOand BaCuO2-addedsamples.Allthesampleshavesimilarvaluesof satu-rationmagnetization.Remanentmagnetizationdecreasessharply fortheCuO-addedsample,thenincreasesagainfortheBaO–CuO andBaCuO2-addedsamples.

Table2

MagneticpropertiesofBaFe12O19samplessinteredat1100◦_Cand1250◦_Cwithand

withoutsinteringaids.

Sample HC(Oe) MS(emu/g) MR(emu/g)

Nosinteringaid,1100◦C 4295 61 37

CuOsinteringaid,1100◦_{C 329 60 7}

BaO–CuOsinteringaid,1100◦_{C 1504 61 18}

BaCuO2sinteringaid,1100◦C 1874 59 19

Nosinteringaid,1250◦_{C 481 58 9}

CuOsinteringaid,1250◦_{C 155 52 3}

BaO–CuOsinteringaid,1250◦C 247 59 5

Fig.6.SEMimagesofBaFe12O19sinteredat1250◦Cfor3h:(a)nosinteringaid,(b)CuOsinteringaid,(c)BaO–CuOsinteringaidand(d)BaCuO2sinteringaid. 0 5 10 15 20 25 0 5 10 15 20 25 30 35 40 45 %

Equivalent spherical 2D radius (μm)

0 5 10 15 20 25 30 35 40 45 50 No sintering aid 0 5 10 15 20 25 1 3 5 7 9111315171921232527293133353739414345474950 0 5 10 15 20 25 30 35 40 45 CuO 0 5 10 15 20 25 1 3 5 7 91113151719212325272931333537394143454749 0 5 10 15 20 25 30 35 40 45 50 BaO + CuO 0 5 10 15 20 25 1 3 5 7 91113151719212325272931333537394143454749 0 5 10 15 20 25 30 35 40 45 50 BaCuO₂

Fig.7.GrainsizedistributionsofBaFe12O19sinteredat1250◦Cfor3h.

All the samples sintered at 1250◦C have narrow hystere-sis loops(Fig.8(b)). Fromthe insetin Fig.8(b), it canbe seen that the sample without sintering aid addition hasthe widest and thatthesamplewithCuOadditionhasthenarrowest hys-teresis loop. As CuO content in the sintering aid decreases,

the hysteresisloops become wider. The saturation magnetiza-tion of the CuO-added sample is noticeably smaller than that of the other samples, which have similar values of MS. MR follows similar behavior to that of the samples sintered at 1100◦C.

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 -15000 -10000 -5000 0 5000 10000 15000 M ( e m u /g ) H (Oe)

Undoped CuO BaO-CuO BaCuO₂

(a)

(b)

-10 -5 0 5 10 -1000 0 1000 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 -15000 -10000 -5000 0 5000 10000 15000 M ( e m u /g ) H (Oe)

Undoped CuO BaO-CuO BaCuO₂

Fig.8. M–Hloopsofsamplessinteredat(a)1100◦_{C(Hperpendicular)and(b)1250}◦_{C(Hperpendicular).Insetin(b)showsmagnifiedregionattheorigin.}

4. Discussion

4.1. Densification

FromFig.3itisclearthatadditionofCuOisveryeffectiveat increasingthedensityofBaFe12O19 ceramicsforsintering tem-peraturesaslowas1050◦C.CuOmeltsat1326◦C,butreactswith BaOtoformaliquidphaseat932◦C[7].TheCuOmayreactwith BaFe12O19toformaliquidphaseduringsintering.CuO-based liq-uidsareknowntobeeffectiveatpenetratinggrainboundariesand promotingsinteringinoxidesystems[14–16].Theliquidphase maypromotemasstransportanddensificationbythecontact flat-teningorporefillingmechanisms[17,18].Recentworkhasalso shown that abnormal grain growth mayenhance densification byporefillingduringliquidphasesintering[19].Additionofthe BaO–CuOsinteringaidcausedasmallenhancementin densifica-tionat1100◦CcomparedtoCuO,butadditionofBaCuO2hadno effectrelativetoCuO.Matrixgraingrowthduringliquidphase sin-teringispredictedtopromotedensificationbyporefilling[17,18]. Oncooling,theliquidphasemaycrystallizetoformasecondary phaseormaysolidifytoformanamorphousphase atthegrain boundariesandtriplejunctions.Nosecondaryphasewasdetected byXRDin thesamplessinteredat1100◦C (Fig.1(a))butsmall amountsofasecondaryphasewasidentifiedbySEM(Fig.4), indi-catingthattheliquidphasemayhavecrystallizedoncooling.In addition,thechangeinunitcellparameters(Fig.2)showsthatat leastalittleofthesinteringaidhasenteredtheBaFe12O19crystal latticetoformasolidsolution.

Accordingtothesedensityresults,CuO,BaO–CuOandBaCuO2 sinteringaidsweresuitableforloweringthesinteringtemperature ofbariumferriteto1100◦C.However,forthesamplessinteredat >1200◦C,additionofthesinteringaidshasadetrimentaleffecton density(Fig.6).Thismaybeduetovolatilizationofthesintering aidsathighertemperaturesandexcessiveabnormalgraingrowth. Porescanbecometrappedinside therapidlygrowingabnormal grainsandarethenverydifficulttoremove[20].Porecoalescence duringgraingrowthcancauseporeswelling,furtherreducing den-sity[21].Inaddition,thelargeplate-shapedgrainsdonotfillspace efficiently.

4.2. AbnormalgraingrowthinBaFe12O19

FromFigs.4and6,itcanbeseenthatthegrainshavefaceted boundaries. When grainboundaries of materials were faceted, abnormalgraingrowthoccurred[22–25].Forasystemwithfaceted boundaries,Kangproposedthatthemobilityofafacetwasnot con-stantbutvariedwiththedrivingforceforgraingrowthandthat

abnormalgraingrowthcouldbeexplainedbytheinterfacereaction controlledgrowthmechanism[26,27].Forasinglecrystal grow-ingfromasolutionbyinterfacereactioncontrolledgrowth,solute atomscanonlyattachonalow-energysitesuchasa2Dnucleus orscrewdislocation.Forgrowthby2Dnucleationandgrowth,the growthrateRisgivenby[28]:

R∼= stexp



−Vm2 6gvhkT



(3) where



st=step velocityof thegrowingnucleus, Vm themolar volume of thesolid phase,=edgefree energy of thenucleus, gv=thevolumetricdrivingforce,h=stepheightofthenucleus,

k=Boltzman’s constant and T=absolute temperature.For a sit-uation where grains aregrowing in a liquid phase by Ostwald ripening,gvcanbereplacedbythedrivingforceforgraingrowth

[29,30]: g=2Vm



₁ r∗− 1 r



(4) where isthesolid–liquidinterfacialenergy,somer*theradius ofacriticalgrainwhichisneithergrowingnorshrinking(usually takentobethemeangrainradius)andrtheradiusofthegrainof interest.

AccordingtoEqs.(3)and(4),thegrowthrateofagrainwillbe veryslowbelowacriticaldrivingforcegCandthenwillincrease exponentiallyforgrainswithdrivingforces>gC(Fig.9).Forgrains withg>gC,thegraingrowthrateisnowlimitedbydiffusionof soluteatomsthroughtheliquidphasetothegrowinggrain,and thegraingrowthratebecomesalinearfunctionofg.Thecritical drivingforcegCisgivenby[28]:

gC=  2

3kTh (5)

ThevalueofstepfreeenergyandhencethevalueofgCcanbe changedbychangingtemperature[31,32],dopantaddition[22,25] or bychangingthesintering atmosphere[22,25,33]. Depending ontherelativevaluesofgCandthedrivingforcegmaxforthe largestgraininthesystem,stagnant,abnormalorpseudo-normal graingrowthcantakeplace[26,27].Thetypeofgraingrowthcan alsochangewithsinteringtimeduetochangesinthemeangrain sizeandgmax.Similarbehavioralsooccursinsolid-statesintered systems[27,34].

Forthesinteringaid-freeBaFe12O19samplesinteredat1100◦C, thegrainsareallmicron-sized andsignificantgraingrowthhas nottakenplace(Fig.4(a)).Noneofthegrainsarelargeenoughto haveg>gCandsocannotgrowrapidly(thecurvemarked“High ␴”inFig.9).WhenCuOisaddedtoBaFe12O19itmayreactwith

Δg

C(l)

Δg

C(m)

Medium

σ

Driving force

Δg

Gr

ow

th r

a

te

ν

R

Δg

C (h)

0

Δg

_max

Hig

h

σ

Lo

w

σ

Fig.9.Graingrowthratevs.drivingforcefor2Dnucleation-controlledgrowth[33].

BaFe12O19toformaCuO-richliquidphase[7].Thisliquidphase maypromoterapidgraingrowthbypromotingdiffusionacross thegrainboundaries[35]andbyloweringthestepfreeenergyand gC (Eq.(5)andthecurvemarked“Medium␴”inFig.9).Some ofthegrainswillnowhaveg>gC andsocangrowrapidlyto formabnormalgrains.Thesegrainscangrowuntiltheyimpingeon anotherabnormalgrain,inwhichcasegwilldecreaserapidlyto belowgCandgraingrowthwillstop.

Additionofthe30mol%BaO–70mol%CuOeutecticmixturewill alsoproducealiquidphase,asthemixturemeltsat∼900–930◦_C [7].TheincreasedBaOcontentintheliquidphaseappearstocause thestepfreeenergytodecreasefurther.Thestepfreeenergyisnow lowenoughforalargenumberofgrainstohaveg>gC.Many grainsgrowandthegraingrowthbehaviorchangesfromabnormal topseudo-normal(Fig.4(c)andthecurvemarked“Low␴”inFig.9). Asgraingrowthtakesplace,themeangrainsizer*increases, caus-ingadecreaseingforthegrowinggrains(Eq.(4)).Asthevalue ofgforeachgrowinggraindropsbelowgC,itstopsgrowing. Eventually,mostofthematrixgrainsstopgrowing.Afewgrains arelargeenoughtostillhaveg>gCandthesegrainscannow growtoformabnormalgrains.Thistypeoftime-delayedabnormal graingrowthhasbeenpredicted[26,27]andhasbeenobserved inthe(K0.5Na0.5)NbO3 [33]and(Na0.5Bi0.5)TiO3–BaTiO3 systems [36].AdditionofBaCuO2reducesthestepfreeenergystillfurther, furtherdelayingthepointatwhichabnormalgraingrowthstarts (Fig.4(d)).

Increasingthesinteringtemperatureto1250◦Ccausesa reduc-tioninstepfreeenergyandhencegC[31,32].Eveninthesintering aid-free BaFe12O19, there will now be grains large enough to haveg>gCwhichcangrowrapidlytoformabnormalgrains (Fig.6(a)).AdditionofCuOfurtherpromotesrapidgraingrowthby themechanismsdiscussedabove(Fig.6(b)).TheBaO–CuO-added sample has a similar microstructure to theCuO-added sample (Fig.6(c))buttheBaCuO2-addedsamplehasslightlysmallergrains than the other samples sintered at 1250◦C (Figs. 6(d) and 7).

ThereducedstepfreeenergycomparedtotheCuO-addedsample allowsmoregrainstogrowabnormally,limitingtheirfinalsize. 4.3. Theeffectofsinteringaidsonthemagneticpropertiesof BaFe12O19

BaFe12O19hashexagonalcrystalsymmetry,withamajorc-axis and a minor a-axis [2]. Thismaterial has a high magnetocrys-tallineanisotropy,withtheeasydirectionofmagnetizationlying alongthec-axis[37].Changesinmagnetizationinapolycrystalline magneticmaterialarecausedbyrotationofdomain magnetiza-tiontoalignwiththeappliedmagneticfieldorbydomainwall motion[2].Duetothehighmagnetocrystallineanisotropy, rota-tionofdomainmagnetizationwillrequireconsiderableenergyif thedomainmagnetizationhastomovefromthec-axistoana-axis. Domainwallmovementismorefavorable.Domainswith magne-tizationdirections closetothatoftheappliedfield grow,while domainswithmagnetizationdirections pointingaway fromthe appliedfieldshrink.

ThesizeofasingledomaininBaFe12O19is∼1␮m[2].Inthe sinteringaid-freeBaFe12O19samplesinteredat1100◦C,thegrain sizeis∼1␮m,soeachgrainconsistsofasingledomain(Fig.4(a)). Inthiscase,changesinmagnetizationcanonlytakeplaceby rota-tionofdomainmagnetizationandsotheceramicismagnetically hardwithatypicalbroadM–Hloop(Fig.8(a)).Inthesamplewith CuOaddition,manylargeabnormalgrainswithmultipledomains exist(Fig.4(b)).Changesinmagnetizationbydomainwall move-mentcantakeplacemoreeasilyinthesegrainsandsotheM–H loopbecomes much narrower(Fig.8(a)). Lisjakalsofoundthat CuOadditioncausedreducedcoercivity[8].IntheBaO–CuOand BaCuO2-addedsamples(Fig.4(c)and(d)),thenumberofabnormal grainsisreducedandsotheM–Hloopsbecomebroaderagain.The pinchingintheloopsispossiblycausedbythecoexistenceofsmall single-domainmatrixandlargemulti-domainabnormalgrains.

Inallthesamplessinteredat1250◦C,thegrainsizeisverylarge andtheM–Hloopsareverynarrow(Figs.6and 8(b)).It iswell knownthatthecoerciveforceHCinferritesdecreaseswhenthe grainsizeincreases[2,38,39].Thisisduetopinningofthedomains bygrainboundaries.Thelargergrainsizemeanslessgrain bound-aryareaandlesspinning.TheCuOandBaO–CuO-addedsamples havethelowestcoercivitiesduetotheirlargergrainsize(Table2). Thesintering aid-free and BaCuO2-added sampleshave similar grainsizeandcoercivity.

Thesample sintered at 1250◦C with additionof CuO has a noticeablysmallersaturationmagnetizationthantheother sam-plessintered at 1250◦C (Table 2).Althoughwe only measured theunit cellparameters for thesamplessinteredat 1100◦C, it islikelythatCu2+_{alsoentersthelatticeat1250}◦_C.Cu2+_hasthe [Ar]3d9_{electronconfiguration.Ithasoneunpairedd-shellelectron,} comparedtothe5unpairedd-shellelectronsofFe3+_.WhenCu2+ substitutesforFe3+_{inthelattice,itthereforedilutesthemagnetic} moment,leadingtoreducedMS.ForthesampleswithBaO–CuO andBaCuO2addition,lessCuisavailabletoenterthelattice,andso thedilutingeffectdoesnotappear.

5. Conclusion

The addition of 2wt% of sintering aids (CuO, BaCuO2, and BaO–CuO)toBaFe12O19allowedareductioninthesintering tem-perature to1100◦C while maintaininghightheoretical density (95–97%).Inaddition,thepresenceofsinteringaidscaused abnor-malgraingrowthinsamplessinteredat1100◦C.Theadditionof thesinteringaidscausesareductioninstepfreeenergy,leadingto achangeingraingrowthbehaviorfromstagnanttoabnormalthen

pseudo-normalfollowedbyabnormalgrowth.Theabnormalgrain growthcausedareductionincoercivityandpinchedM–Hloops.All samplesdisplayedextensiveabnormalgraingrowthaftersintering at1250◦C.Samplessinteredat1250◦Cshowedafurtherreduction inHCandnarrowM–Hloopsduetothecoarsegrainsize.

Acknowledgements

This work was supported by Chonnam National University, 2012,andbytheChonnamNationalUniversityLGInnotekCentre (projectno.2011-2296).TheauthorswouldliketothankJin-Hee ParkandHye-JeongKimforoperatingtheXRDandSEM respec-tively.

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