Nanoindentation response of zinc titanate thin films deposited by co-sputtering process

(1)

ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

j o ur na l ho me p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Nanoindentation

response

of

zinc

titanate

thin

ﬁlms

deposited

by

co-sputtering

process

Shyh-Chi

Wu

a,b

_,

_Yeau-Ren

_Jeng

c

_,

_Wei-Hung

_Yau

d

_,

_Kuan-Te

_Wu

c

_,

_Chien-Huang

_Tsai

e,∗

_,

_Chang-Pin

_Chou

a

a_Department_of_Mechanical_Engineering,_National_Chiao_Tung_University,_Hsinchu_300,_Taiwan,_ROC b_Chung_Shan_Institute_of_Science_and_Technology_(CSIST),_Taoyuan_325,_Taiwan,_ROC

c_Department_of_Mechanical_Engineering,_National_Chung_Cheng_University,_Chia-Yi_621,_Taiwan d_Department_of_Mechanical_Engineering,_Chin-Yi_University_of_Technology,_Taichung_400,_Taiwan,_ROC e_Department_of_Automation_Engineering,_Nan_Kai_University_of_Technology,_Nantou_54243,_Taiwan,_ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received4October2011

Receivedinrevisedform16February2012 Accepted16February2012

Available online 21 March 2012 Keywords:

Radiofrequencymagnetronco-sputtering Hardness

Atomicforcemicroscopy X-rayphotoelectronspectroscopy

a

b

s

t

r

a

c

t

Inthisstudy,ZnTiO3ﬁlmsweregrownbyradiofrequencymagnetronco-sputteringusingasintered

ceramictargetonsiliconsubstrates,weusednanoindentertechniquesunderaCSMmodetoevaluate

thehardness(H)andelasticmodulus(E)oftheﬁlmsafterannealingintemperaturerangeof520–820◦_C.

ThemeasuredvaluesofhardnessandelasticmodulioftheZnTiO3ﬁlmswereintherangefrom8.5±0.4to

5.6±0.4GPaandfrom171±2.3to155±2.5GPa,respectively.Itisevidentthatanincreaseinthe

rough-nessduetohighannealingtemperatureusingatomicforcemicroscopy.TheXRDpatternswereobserved

thatas-depositedﬁlmsaremainlyamorphous,however,thehexagonalZnTiO3phasewasobservedwith

theZnTiO3(104),(110),(116),and(214)peaksfrom620to820◦C,indicatingthatthereishighly

(104)-orientedZnTiO3onthesiliconsubstrate.TheX-rayphotoelectronspectroscopycorelevel

analy-sisoftheZnTiO3ﬁlmshavebeenmeasuredforO1sthatcanbeattributedtheweakerbondsandlower

resistanceattheﬁlmbasedonthehigherannealedtemperature.TheH,M,Rms,andRawerealtereddue

tothegraingrowthandrecoverytoresultinarelaxcrystallinityatZnTiO3ﬁlms.

1. Introduction

ZnO–TiO2systemisattractivebecauseoftheiroutstanding

per-formanceasaninorganiccompound.Aboveofthem,ZnTiO3ﬁlm

wasfoundtobezincmetatitanatewithahexagonalstructureand

titaniumdioxidewhiletheﬁlmofZn/Tiratioislowerthan1.The

zinctitanatephasesfoundintheﬁlmsdependprimarilyonthe

ﬁlmstoichiometry.The introductionof threecompounds

exist-ingintheZnO–TiO2systemhasbeenreportedinpreviousstudy

[1], suchas-Zn2TiO4 (cubic), ZnTiO3 (hexagonal),and Zn2Ti3O8

(cubic).Fundamentalstudiesisconcernedbecauseofitsdiverse

electricalandchemicalproperties,leadingtothecommercial

appli-cations,suchaswhitepigment,catalyticsorbent[2–4],microwave

dielectricmaterials[5–7],gassensor[8],andhighperformance

cat-alysts[9–11].Beside,theluminescentapplicationcouldbeinduced

by the ZnTiO3 doped with some transition metal ions [12,13].

ZnTiO3,preparationbysolidstatereaction,wasreportedbymeans

of sol–gel[12,14,15],bulk manufacture [16,7,17], and chemical

vapordeposition[18].However theirstudies arelimitedtothe

∗ Correspondingauthor.

E-mailaddress:[email protected](C.-H.Tsai).

analysesof phase, composition,and microstructure oftheﬁlm.

TherearefewreportsconcerningthenanohardnessofZnTiO3

sys-teminliteratures,excepttheseproperties–structures,grainsizes,

and morphologiesthatcanbeenhanced throughtheirextrinsic

properties.

Inthisstudy,theauthorshaveattemptedtosynthesizeZnTiO3

ﬁlmbyRFmagnetronco-sputteringprocess.Weemployed

nanoin-dentation techniques to discover the response of ZnTiO3 ﬁlm;

their surface roughness, crystallization, and chemical bonding

wereanalyzedbyusingatomicforcemicroscopydiffraction,X-ray

diffraction,andX-rayphotoelectronspectroscopy.Thequalitiesof

theresultingZnTiO3ﬁlms,intermsofphasepresentunder

anneal-ingtreatment,wereexamined.

2. Experimentaldetails

TheZnTiO3ﬁlmswerepreparedbyRFmagnetronco-sputtering

system using 4-inch-diameter sintered ceramic target (Zn and

Ti).In ordertoobtainstoichiometricdepositZnTiO3 ﬁlmsonSi

substrate,Ar–O2ﬂow(8:2)withapurityof99.999gasof50sccm

wasintroducedintothechamberwithmassﬂowcontrollersand,

RFpowersat200Wwereused.Inaddition,thesputteringpressure

was2×10−2, thesubstratetemperature wassetat250◦C, and

(2)

Table1

RFsputterconditionsforthezinctitanateﬁlms.

Target ZnTiO3

Targetsizeindiameter(in.) 4

Targettosubstratedistance(mm) 90

RFpower(W) 200

Chamberpressure(Torr) 5×10−6

Workingpressure(Torr) 2×10−6

Sputteringgas Ar

Ar–O2ﬂow(seem) 50(8:2)

Substratetemperature(◦_C) ₂₅₀

Depositiontime(min) 120

thedurationofthedepositionwas1h.Toremovethe

contami-nantsformedonthetargetsurfaceandtostabilizethesputtering

conditions,itwasnecessaryfor pre-sputteringtobeperformed

for5minpriortoeachdepositionprocess.Thosespecimenswere

subsequentlysubjectedtoexsituthermaltreatmentinafurnace

underN2 gasfor2hwithaheatingrateof5◦C/minat520,620,

720,and820◦C,respectively.Thedetaileddepositionconditions

oftheZnTiO3ﬁlmsarelistedinTable1.

After deposition, the surface roughness and microstructure

areanalyzedusingAFM(VeecoDimension5000,ScanningProbe

Microscopy,D5000).ThevaluesofHandMof theZnTiO3 ﬁlms

were determined using a Nano Indenter XP instrument (MTS

Cooperation,NanoInstrumentsInnovationCenter,TN,USA).The

nanoindentationwasperformedusingadiamondBerkovich

inden-tertip(tipradius:ca.50nm);plasticdeformationwasgenerated

atverysmallloads.Thecontinuouscontactstiffnessmeasurement

(CSM)mode,whichisexecutedbysuperimposingsmall

oscilla-tionsontheforcesignaltomeasuredisplacementresponses,offers

adirectmeasurementofdynamiccontactstiffnessduringthe

load-ingprocessintheindentationtest[19].Chemicalbondingstates

andchemicalcompositionsoftheﬁlmswereanalyzedbyX-ray

photoelectron spectroscopy (XPS, VG Scientiﬁc Microlab 310F).

Inductivelycoupledplasma-massICPspectrometerwasappliedto

determinetheelementsconcentrationsintheﬁlms,which

con-ﬁrmedthestoichiometryofZnTiO3.Crystallinityoftheﬁlmswere

analyzedbyX-raydiffraction(PANalyticalX’PertPro(MRD),with

CuK␣(=0.154nm)radiationfor2from20◦to80◦atascanspeed

of2◦min−1,andagrazingangleof0.5◦under30kVand30mA.

3. Resultsanddiscussion

Fig.1showsAFMinspectionsofZnTiO3ﬁlmsdepositedwith

sil-iconsubstrate.ThesurfaceroughnessofZnTiO3ﬁlmsisincreased

from3,3,3.9,and5.8to20.6nmastheannealingtemperature

increasedfromRTto820◦C.Clearly,astheannealingtemperature

wasincreased,surfacemorphologiesofthesamplesbecamerough

andgrainsgrew.ThecorrespondingAFMobservationsdemonstrate

thattherearemanysmallcavitiesandbrightparticlesintheﬁlm

afterannealingtreatment,particularstartat620◦C.Fromprevious

articles[20–22],thesimilarphenomenonwasobservedthatthe

cavitiesareduetotheevaporationofZn,andthewhiteparticles

areTiO2.Furthermore,itappearsthatahighannealingtemperature

(3)

favorsaparticlewithlargergrainsizesat820◦Candmoredensely

thanthatoflowannealingtemperatures.Theexperimentsclearly

demonstratedthatthecrystallinestructureandsurfaceroughness

ofZnTiO3ﬁlmcanbeclearlychanged.Itsuggeststhatahigh

anneal-ingtemperatureenhancedtheatomicmobilityandcausedthegrain

recoverytoresultinarelaxcrystallinity.Thesubsequentresultsof

XRDwillbediscussedaswell.

Tostudy thenanomechanical properties of theZnTiO3 ﬁlm,

nanoindentationwasemployedtomeasurethevaluesofHandE

uponvaryingconditions.Fig.2(a)and(b)displaythevariationin

nanohardnessH(GPa)andYoung’smodulusE(GPa)withrespect

totheZnTiO3ﬁlm.ItisdeterminedthatthevaluesofHandEof

theZnTiO3 ﬁlmwithrespecttotheindentationdepth,following

themethodproposedbyOliverand Pharr[20]. Forindentation

depthsupto5nm,thevaluesofHincreaseduponincreasingthe

indentationdepth;thiscanbeattributedtothetransitionbetween

purelyelasticand elastoplasticcontact, whereas thevalueofH

isactuallyequaltothecontactpressure[16].Whilethe

indenta-tiondepthsover50nm,theHtendstoconstant.Werespectthat

astrengtheningeffectisdominatedfromthestrainorstrainrate

hardening.The HandEwerecalculated byaveraging

measure-mentsatindentationdepthsrangingfrom80to180nm.Thedepths

oftheﬁlmthicknessnotexceeding20%thatcanbeattributedto

accessafullyplasticzoneandtoavoidthesubstrateeffect[19].

ForthermaltreatmentatRT,520,620,720, and820◦C,the

val-uesofHoftheZnTiO3 ﬁlmwere8.5±0.4,11.4±0.4,10.5±0.4,

9.6±0.5, and 5.6±0.4GPa, respectively, while those of Ewere

171±2.3,178±2.4,202±3.5,198±3.2,and155±2.5GPa,

respec-tively. Markedly, in metal oxides the intrinsic defects such as

oxygenvacanciesandmetalioninterstitialsusuallycoexist.We

concludethatthelowervaluesofHandEoftheZnTiO3ﬁlmcanbe

inducedbysomereason,includingthepackingfactor,

stoichiom-etry,residualstress,preferredorientation,andgrainsize[23].A

lowvalueofRmsisbeneﬁcialtoaZnTiO3ﬁlmbecauseitcanbe

usedtoprovideinformationregardingitsmorphology;thesurfaces

morphologiesofourﬁlmswererelativelyroughandnon-uniformly

distributed.Inaddition,thegrainsizeintheZnTiO3ﬁlmincreases

obviouslywhenannealingtemperatureishigher,casingmanygaps

betweengrainboundariesduetotheabnormalgraingrowth.We

indicatethatthestructureofZnTiO3 ﬁlmschangeswith

anneal-ingatmosphere,whichinducesthevariationoftheconcentration

ofintrinsicdefects.Fig.2(c)presentstheload/displacementP–h

curverecordedduringtheindentationprocess;itreacheda

maxi-mumindentationloadof9mN.Weobtaineddiscordantcurvesand

irregularitiesappearedinthecourseofplasticdeformation,which

ischaracterizedbythecontinuitiesatpenetrationdepths,forthe

typicalloading/unloadingprocess.Itis suggestedthatthecurve

impliesatrendofgradualelasticdeformation.Wealsoobserved

a larger deviation in the P–h curves of the ZnTiO3 ﬁlms that

hadbeensubjectedtovariousloading/unloading.Nanoindentation

resultrevealsavariationofcompressivestresstransitionaround

theindentationregionsoftheZnTiO3ﬁlms.Theinitial

deforma-tion,elasticdeformation,andresidualdeformationoftheZnTiO3

ﬁlmswereevaluatedbasedonthethermaltreatment.Theelastic

modulusisafundamentalpropertythatisinﬂuencedbybonding

betweenatoms.Thestiffnessorelasticmodulusofaﬁlmdepends

upontheinteratomicdistances[23–25].

TheconstituentelementsoftheZnTiO3ﬁlmsweremeasured.

Thechemicalcompositionsoftheﬁlmsdepositedatvarious

anneal-ingtemperaturewereshowninFig.3.Jungetal.[28]conductedthat

amainphaseZnTiO3intoZn2TiO4andTiO2occurredinassociation

withtheincreaseofannealingtemperature.Fromearlierreports

[27,28],theceramicpowder reactioncanbe displayedthatthe

ZnTiO3phasedecomposedtofromZn2TiO4andrutileTiO2above

945◦Cwithincreasingtheannealingtemperature.Thetrendofthe

increaseinoxygencontent(65–70%)issimilarinourresultswhile

Fig.2. (a)Hardness,(b)elasticmoduli,and(c)displacementsofloading–unloading curvesofZnTiO3ﬁlmssampleswithrespecttothevariationofannealingcondition:

(4)

Fig.3.ThestoichiometricchangeofZnTiO3 thinﬁlmstreatedbyconventional

annealingcondition:RT,520,620,720,and820◦C.

chemicalcompositionsarechangedbasedonannealing

tempera-ture.Atthesametime,chemicalbondingstatesintheZnTiO3ﬁlms

werecharacterizedbyXPSmeasurement,asshowninFig.4.The

XPSspectrahavebeenchangedcorrectedtotheadventitiousO1s.

ThebindingenergyoftheO1sstateislocatedfrom530to538eV,

whichdependedontheannealingtemperaturethatsplitsintheir

peaksundervariousgrowthproceduresofZnTiO3ﬁlmsareclearly

examined.OnecanobserveapartofO Znbondformation(binding

energyat529.4eVand531.5eV)attheannealingtemperaturefrom

RTto520◦Cthatthepeakpositionsareingoodagreementwiththe

Zn ObondandsomeTi Obond[1,29].Apparently,theannealing

temperaturefrom620to720◦Cisevidencethatthemains

chemi-calbondingmaybecoexistingasZn O(bindingenergyat531.5eV)

andTi O(bindingenergyat533and536eV).WecancompareO

1sstateatthe820◦C,thecurveissigniﬁcantshiftintootherzone

duetomorechemicalimpurityfromZn OandTi Obond.In

addi-tion,O1sstateattheRTto520◦C,thepeakat530.1eVmaybe

duetoZn Obondsandsigniﬁcantpeakat531.8eVisattributed

asMnCl2.Thebroadbindingenergyat532–534eVseeninO1s

spectrumisconsideredtobeduetoresidualoxygenattheﬁlm

sur-face,ofwhichthebindingenergyisreportedtobe532.8eV[29,30].

Phanietal.[31]hasdiscussedthatstoichiometricchangeofZnTiO3

thinﬁlmscouldbetreatedbyconventionalannealing.Theannealed

Fig.4. XPScorelevelspectra(O1s)ofZnTiO3ﬁlmsonSisubstratewithrespectto

thevariationofannealingcondition:RT,520,620,720,and820◦_C.

Fig.5.X-raydiffractionpatternsoftheZnTiO3ﬁlms(a)as-depositedatRT,and

differentthermaltemperatures(b)620and(c)820◦C.

ﬁlmshaveshownZnTiO3cubicphaseformationatandabove620◦C

substratetemperature.Thechemical532.0eVpeakforZnTiO3ﬁlms

areattributedtoO HbondsduetoabsorbedH Omoleculesonthe

ﬁlms.Inaddition,thepeakappearedat533.1eVhasbeenassigned

toO1sinSiO2thatpresentedonthesurfaceoftheannealedﬁlms

onSisubstrate.

InFig.5,theXRDpatternsoftheZnTiO3ﬁlms:(a)as-deposited

andtheannealingtreatmentwas(b)620◦Cand(c)820◦C.Itwas

observedthattherewerenodiffractionpeaksintheXRDpattern

foras-depositedﬁlms(Fig.3(a)).Itissuggestedthattheﬁlm

struc-tureismainlyamorphous[16].Herein,astheannealingtreatment

samplewas620◦C,thehexagonalZnTiO3phasewasobservedwith

theZnTiO3(104),(110),(116),and(214)peaks(Fig.5(b)).The

intensitiesof the (104)peak was higherthan the otherpeaks

ofZnTiO3 ﬁlmsat820◦C, indicatingthatthere ishighly(10

4)-orientedZnTiO3onthesiliconsubstrate.Fromrecentreports[16],

thepreferredorientationtendstoreduceitsfreeenergytoreach

astablestate,thisisthesamecaseinourexperimentaldata.The

ZnTiO3 structureisalsocomparablewiththeXPSthatZn-Oand

Ti-Obindingenergyat533and536eVisstronglyastheannealing

treatmentsamplewas620◦C.However,thelowZn OandTi O

bindingenergyisdisplayedduetomorechemicalimpurityfrom

XPSanalysis;thustheZnTi(110)structurehasaweakdiffraction

peakintheXRDpatternatthe820◦C.Weattributedthatthe

hexag-onalZnTiO3decomposedintocubicZn2TiO4andTiO2(rutile),even

theZnTiO3 phaseremainedstableatthetemperaturesof820◦C

[34].

Table1liststhesummariesofH,M,Rms,andRa.ZnTiO3 ﬁlms

oftenhavedifferentthermalproperties,resultinginstrainwhen

theﬁlmsareannealedunderelevatedtemperatures.Thecrystal

textureofZnTiO3ﬁlmscaninﬂuencetheirmechanicalproperties

atdifferentannealingtreatmentstage;thecomparisonofphase

transformationofZnTiO3ﬁlmswasperformedbyXRDpatternas

well.Graingrowthandstrainenergyareprocessedattheinner

ﬁlmthatcanoftenbeadominantfactor.Althoughweobtained

appropriateinformationfromXPSanalysis,thedeposition

condi-tionsaffectednotonlydependedonannealedtemperaturebutalso

theTi,Zn,andOcompositionsduringthegrowthbyRFmagnetron

Table2

ThesummariesofZnTiO3ﬁlmwithrespecttotheH,M,Rms,andRa.

ZnTiO3 Modulus(GPa) Hardness(GPa) Rms Ra

As-deposited 171±2.3 8.5±0.4 3 2.1

520◦_C ₁₇₈_±_2.4 _11.4_±_0.4 ₃ _2.2

620◦_C ₂₀₂_±_3.5 _10.5_±_0.4 _3.9 _3.1

720◦_C ₁₉₈_±_3.2 _9.6_±_0.5 _5.8 _4.6

(5)

co-sputteringprocess.Weattributedthatoxygenchemisorbswith

metalbondandcasesasurfacelayerofadsorbedoxygen(Table2).

4. Conclusion

We have characterized RF magnetron co-sputtering ZnTiO3

ﬁlmsusingAFMandnanoindentertechniques.Themeasured

val-uesofhardnessandelasticmoduliof theZnTiO3 ﬁlmswerein

the range from 8.5±0.4 to 5.6±0.4GPa and from171±2.3 to

155±2.5GPa,respectively. Slightoscillationsand discontinuous

phenomenaineachhardnesscurveduetotherelaxinginZnTiO3

structure.TheXRDpatternsoftheZnTiO3ﬁlmsobservedthatthere

werenodiffractionpeaksintheXRDpatternforas-depositedﬁlms

due to theﬁlm structure is mainlyamorphous. The hexagonal

ZnTiO3phasewasobservedwiththeZnTiO3(104),(110),(116),

and(214)peaksat620◦C.Theintensitiesofthe(104)peakwas

higherthantheotherpeaksofZnTiO3ﬁlmsat820◦C,indicatingthat

thereishighly(104)-orientedZnTiO3onthesiliconsubstrate.The

XPScorelevelanalysisoftheZnTiO3ﬁlmshavebeenmeasuredfor

O1s,thechangeduetoZn–TicomplexcompoundthataltertheH,

M,Rms,andRafromannealingtreatment.Theaverage

nanoinden-tationdepthincreasedsigniﬁcantlyuponincreasingtheannealing

temperature,implyingthathigherannealingtemperatureresulted

inweakerbonds,lowerresistance,anddecreasedinstrainenergy

attheﬁlm.

References

[1]F.H.Dulin,D.E.Rase,J.Am.Ceram.Soc.43(1960)125. [2]S.F.Bartram,R.A.Slepetys,J.Am.Ceram.Soc.44(10)(1961)493. [3]H.W.Yao,R.B.Feng,Min.Saf.Environ.Prot.28(2001)20.

[4]S.F.Wang,M.K.LDu,F.Gu,C.F.Song,D.Xu,D.R.Yuan,G.J.Zhou,Y.X.Qi,Inorg. Chem.Commun.6(2003)185.

[5]R.P.Gupta,S.K.Gangwal,S.C.Jain,U.S.Patent5,714,431(1998). [6]H.T.Kim,Mater.Res.Bull.33(1998)963.

[7]A.Golovchansky,H.T.Kim,Y.H.Kim,J.KoreanPhys.Soc.32(1998)1167. [8]H.T.Kim,S.Nahm,J.D.Byun,J.Am.Ceram.Soc.82(1999)3476. [9]H.Obayashi,Y.Sakurai,T.Gejo,J.SolidStateChem.17(1976)299. [10] Z.X.Chen,A.Derking,W.Koot,M.P.vanDijk,J.Catal.161(1996)730. [11]K.H.Yoon,J.Cho,D.H.Kang,Mater.Res.Bull.34(1999)1451.

[12] E.Hosono,S.Fujihara,M.Onuki,T.Kimura,J.Am.Ceram.Soc.87(2004)1785. [13]S.F.Wang,M.K.Lu,F.Gu,C.F.Song,D.Xu,D.R.Yuan,S.W.Liu,G.J.Zhou,Y.X.Qi,

Inorg.Chem.Commun.6(2003)185.

[14]S.F.Wang,F.Gu,M.K.Lu,C.F.Song,D.Xu,D.R.Yuan,S.W.Liu,Chem.Phys.Lett. 373(2003)223.

[15]H.T.Kim,Mater.Res.Bull.33(1998)975.

[16] Y.S.Chang,Y.H.Chang,I.G.Chen,G.J.Chen,Y.L.Chai,J.Cryst.Growth243(2002) 319.

[17] Y.S.Chang,Y.H.Chang,J.AlloysCompd.354(2003)303. [18]Y.S.Chang,Y.H.Chang,SolidStateCommun.123(2003)203.

[19] Z.X.Chen,J.Eyden,W.Koot,R.Berg,J.Mechelen,A.Derking,J.Am.Ceram.Soc. 78(1995)2993.

[20]W.C.Oliver,G.M.Pharr,J.Mater.Res.7(1992)1564. [21]M.P.Pechini,USPatentNo.3231(1996),p.328.

[22]G.A.Hutchins,G.H.Maher,S.D.Ross,Am.Ceram.Soc.Bull.66(4)(1987)681. [23]P.F.Yang,H.C.Wen,Y.S.Lai,C.T.Wang,S.Wu,R.S.Chen,Microelectron.Reliab.

48(2008)389.

[24]D.SreekanthaReddy,B.Kang,S.C.Yu,Y.DwarakanadhaReddy,S.K.Sharma, K.R.Gunasekhar,K.N.Rao,P.SreedharaReddy,Curr. Appl.Phys.9(2009) 431.

[25]M.H.Lin,H.C.Wen,Y.R.Jeng,C.P.Chou,NanoscaleRes.Lett.5(11)(2010)1812. [27]Y.M.Chang,H.C.Wen,C.S.Yang,D.Lian,C.H.Tsai,J.S.Wang,W.F.Wu,C.P.Chou,

Microelectron.Reliab.50(2011)1111.

[28]J.S.Jung,Y.H.Kim,S.K.Gil,D.H.Kang,J.Electroceram.23(2009)272. [29]O.Yamaguchi,M.Morimi,H.Kawabata,K.Shimizu,J.Am.Ceram.Soc.70(1987)

97.

[30] B.R.Strohmeier,D.M.Hercules,J.Catal.86(1984)266.

[31]A.R.Phani,M.Passacantando,S.Santucci,J. Phys.Chem.Solids68(2007) 317.