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Surface&CoatingsTechnology
journalhomepage:www.elsevier.com/locate/surfcoat
CharacterizationofAl2O3–TiO2nanoporoussolarabsorbersderivedviaMAO/sol-gelhybridprocess
M.R.Bayatia,⁎,H.R.Zargarb,AliTalimiana,AhmadZiaeea,RoyaMolaeiaabSchoolofMetallurgyandMaterialsEngineering,IranUniversityofScienceandTechnology,P.O.Box:16845-161,Tehran,IranDepartmentofMetalsandMaterialsEngineering,UniversityofBritishColumbia,Vancouver,BCV6T1Z4,Canada
articleinfoabstract
Microarcoxidationcombinedwithsol-gelprocesswasusedtodevelopsolarselectiveabsorbersystemsforthefirsttime.PhasestructureandchemicalcompositionofthelayerswerestudiedbyXRDandXPStechniques.Thelayersconsistedofanatase,rutile,α-alumina,γ-alumina,andAl2TiO5phaseswithvaryingfractionswiththegrowthconditions.SEMandAFMinvestigationsrevealedaporousstructure(poresize:60–550nm)witharoughsurfacewhereporesizeandsurfaceroughnessincreasedwiththeappliedvoltage.SolarabsorptionandemissioncoefficientsweremeasuredusingUV–Vis–NIRandIRspectrophotometry.Inspiteofbeingsothin,thefabricatedsamplesrevealedahighsolarabsorptivityofα=0.99andlowthermalemissivityofε=0.06.Aformationmechanismwasproposedbasedontheelectrochemicalfoundationsaswell.
©2010ElsevierB.V.Allrightsreserved.
Articlehistory:
Received5March2010
Acceptedinrevisedform25September2010Availableonline1October2010Keywords:
MicroarcoxidationSol-gelCoatingSelectiveSurfaceComposite
1.Introduction
Economicalandeffectiveutilizationofsolarenergyrequiresanefficientabsorberlayerwithproperopticalproperties[1].Anefficientabsorberisdefinedashavingahighabsorptancecoefficientwithinthewavelengthrangeofsolarradiations(0.3–2.5μm)andlowemittancecoefficientforlongwavelengthradiations.Thispropertyisnamedselectivity[2].Selectivesolarabsorbersareoneofthevitalcomponentsinsolarthermalsystems.Therequiredcharacteristicsforthemarehighsolarabsorbanceα,lowthermalemittanceε,highmoistureresistance,adhesion,scratchresistance,cost,simplicityofproductiontechniques,andsoon.Toachievehighphotothermalconversionefficiency,highαisessentialregardlessofoperatingtemperatures,whereascontributionoflowεbecomesmoresignifi-cantwithincreasingtemperaturesincethermalradiationlossisproportionaltoεT4[3,4].
Titaniumandaluminumoxidesaretwoofmostimportantmetaloxideswhicharewidelyusedasasolarthermalabsorbersurface[5–7]whichhavebeengrownviadifferentmethods.Microarcoxidation(MAO)isanin-situgrowthprocessforproducingoxidefilmsonthesurfaceofnonferrousmetalssuchasaluminum,titanium,magnesiumandzirconium.Itisanewsurfacetreatmenttechniquebasedon
⁎Correspondingauthor.Tel./fax:+982177240291.E-mailaddress:bayati@iust.ac.ir(M.R.Bayati).
0257-8972/$–seefrontmatter©2010ElsevierB.V.Allrightsreserved.doi:10.1016/j.surfcoat.2010.09.046
conventionalanodicoxidizationtechnology.Itisasimpleandpromisingapproachforfabricationofdifferentcategoriesofoxidelayers.MAOisanelectrochemicaltechniqueforformationofanodicfilmsbyspark/arcmicro-dischargeswhichmoverapidlyonthevicinityoftheanodesurface[8–12].Itischaracterizedbyhighproductivity,economicefficiency,ecologicalfriendliness,highhardness,goodwearresistance,andexcellentbondingstrengthwiththesubstrate[13–15].Thisprocessiscarriedoutatvoltageshigherthanthebreakdownvoltageofthegaslayerenshroudingtheanode.Sincethesubstrateisconnectedtopositivepoleoftherectifier(anode),thegaslayerconsistsofoxygen.Whenthedielectricgaslayercompletelycoverstheanodesurface,electricalresistanceoftheelectrochemicalcircuitsurgesandtheprocesscontinuesprovidingthattheappliedvoltageovercomesthebreakdownvoltageofthegaslayer.Applyingsuchvoltagesleadstoformationofelectricaldischargesviawhichelectricalcurrentcouldpassthegaslayer.MAOprocessischaracterizedbytheseelectricalsparks[10,16,17].
Solgelprocessingisoneofthemostcommonmethodstoproducemetaloxidesinbothformsofcoatingsandpowder.Itprovidesmanyadvantagesovertheconventionalmethodsincludingthepossibilitytocontrolanumberofdeterminingparametersofthefinalproduct:homogeneity,purity,microstructure,andsinteringtemperatureareamongthemanyadvantagesofsolgelprocessingmethods.Itisalowcostandlowtemperatureprocessaswell.Thefinalproductcouldbeshapedintovariousformssuchasfibers,monoliths,thinandthickfilmcoatings,andpowders.[18–24].
Inourpreviousworks,wesynthesizedV2O5–TiO2andWO3–TiO2combinedlayerssuitableforphotocatalyticapplicationsemployingMAOtechnique[25–29].Inthepresentstudy,Al2O3–TiO2composite
2484
Table1
Chemicalcompositionofthetitaniumsubstrates.ElementContent(wt.%)
Al0.25
Vb0.05
Crb0.01
Cub0.02
M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–2489
Fe0.04
Mn0.01
Others0.11
TiBalance
layersweregrownasasolarselectiveabsorberviaahybridmethodincludingMAOandsol-gelprocessesforthefirsttime.Theeffectofsynthesisparametersonsurfacemorphology,topography,chemicalcomposition,andopticalpropertiesofthegrowncompositelayersisstudiedandaformationmechanismisputforwardwithemphasisonthechemicalandelectrochemicalbases.
0.1molofacetylacetoneand0.1molofwaterwereaddedtothealcoholsolution.Themixturewasvigorouslyagitatedfor1hbyamagnetstirrer.Thecontainerwassealedandstoredatroomtemperaturefor1htocompletethehydrolysisprocess.ThechemicalcompositionoftheresultingTiO2solwas1:1:32:1forTTIP/acetylacetone/EtOH/H2O.pHofthesolwasmeasuredas3.Thesolutionwasleftinorderthatthesuspendedsolidscouldsediment,andclearsolutioncouldbeobtained.TheMAO-fabricatedsamplesweredippedintothissolutionemployingadip-coatermachine.Thewithdrawalspeedwasfixedat0.5mm/sbyanelectricmotorcontrolledbyacomputer.Allofthesampleswerecalcinedat500°Cfor1h.Aftercalcination,thesampleswerecooledinatmospheretotheroomtemperature.2.2.Characterizations
Surfacemorphologyandtopographyofthelayerswereexaminedbyscanningelectronmicroscopy(TESCAN,VegaII)andatomicforcemicroscopy(Veecoautoprobe)inairwithasilicontipof10nmradiusincontactmode.X-raydiffraction(Philips,PW3710)andX-rayphotoelectronspectroscopy(VGMicrotech,Twinanode,XR3E2x-raysource,equippedwithAlKαX-raysourceatenergyof1486.6eV),utilizinghemisphereenergyanalyzer,techniqueswereusedtostudyphasestructureandchemicalcompositionofthesynthesizedlayers.Meanwhile,solarabsorptanceandemmitancecoefficientsweredeterminedemployingaUV–Vis–NIRspectrophotometer(Cary500)andanIRspectrophotometer(PerkinElmer599),respectively.3.Resultsanddiscussion3.1.Microstructureandtopography
SEMtop-viewoftheMAO-grownlayersisdepictedinFig.1whereaporousstructureisobserved.Theporesaregeneratedbytheelectricalsparksoccurringatappliedvoltagesabovethebreakdownvoltage(Vb)ofthegaslayerenshroudingtheanode.Onlyfewporesareobservedonthesurfaceofthelayersynthesizedunderthevoltageof300Vandporedensityincreaseswiththevoltage.Itiswellestablishedthattheelectricaldischargesoccurattheregionswheretheappliedvoltageontheanodesurfaceismorethanthebreakdownpotential(Vb)ofthegaslayer(VaNVb).Sincethevoltage,appliedtotheelectrochemicalcell,islow,theanodevoltageisalsolow,and,hence,canprevailVbatlimitedpoints;consequently,fewelectricavalanchestakeplaceonthevicinityoftheanode.Asaresult,afewstructuralporesforminthegrowingoxidelayer.Itisalsoobserved
2.Experimentaldetails2.1.Samplespreparation
Ahome-maderectifierwithamaximumoutputof600V/30A,abletosupplyAC,DCandpulse-DC,wasusedascurrentsource.3cm×3cm×0.5mmcommerciallypuretitaniumpieces,whosetypicalchemicalcompositionislistedinTable1,wereusedassubstrate.Theywereconnectedtothepositivepoleofthepowersupplyasanode.AnASTM316stainlesssteelcylindricalcontainer,surroundingthesubstrates,wasalsousedascathode.Priortolayersfabrication,substratesunderwentacleaningprocessincludingmechanicalpolishingfollowedbywashingindistilledwater.Afterward,thetitaniumpieceswerechemicallyetchedindilutedHFsolution(HF:H2O=1:20Vol.%)atroomtemperaturefor30storemoveanyprobablesurfaceoxidelayer.Then,theywerewashedindistilledwateragain.Finally,thesubstrateswereultrasonicallycleanedinacetonefor15minandwashedbydistilledwater.
Theutilizedelectrolytessimultaneouslyconsistedofsodiumphosphate(10.0gl−1)andaluminate(2.0gl−1)salts.Electrolytetemperaturewasfixedat70±3°Cemployingawatercirculatingsystem.Thevoltagesof300,450,and600Vwereapplied.Moreover,currentfrequencyanddutycyclewerefixedat250Hzand5%,respectively.MAOprocesswascarriedoutfor2minineachrun.AfterMAOtreatment,thesampleswerecompletelywashedindistilledwateranddriedbyanitrogengunfor5mintocompletelyemptyoutthewatertrappedinthepores.
Sol-gelmethodwasperformedtodepositanadditionalTiO2layerintothepores,andfillthem.Oursolconsistedoftitaniumtetraisopropoxide(TTIP)asthetitaniumsource.0.1molofTTIPwasdissolvedin3.2molofabsoluteethanolundervigorousstirring.About
Fig.1.SEMmorphologyoftheMAOgrowncompositelayersatvariousappliedvoltages:(a)300,(b)450,(c)600V.
M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–24892485
Fig.2.SEMmorphologyoftheMAO/Sol-Gelgrowncompositelayersatvariousappliedvoltages:(a)300,(b)450,(c)600V.
Table2
Thicknessoftheoxidelayersbeforeandaftersol-gelstage.Appliedvoltage(V)300450600
Thicknessbeforesolgelstage(μm)0.801.301.50
Thicknessaftersolgelstage(μm)0.951.401.70
localmeltingandsolidifyingofthegrowinglayerintheadjacentelectrolyte[16].Thisphenomenonmakesthelayersroughened.Furthermore,itisobviousthatthesurfaceroughnessdecreasesaftersol-gelstagewhichconfirmsdepositionofnewlayersoverthepreviouslayers.AverageroughnessnumbersarelistedinTable3.3.1.Phasestructureandchemicalcomposition
XRDpatternsofthesamplesbeforeandaftersol-gelstage,showninFigs.5and6,illustratethatboththetitaniumoxides(intheformsofanataseandrutilephases)andthealuminumoxides(intheformsofα-aluminaandγ-aluminaphases)existinthelayers.Relativefractionsoftheanataseandrutilephaseswerecalculatedusingtheformulaf=Ii/(ΣIj)wheref,Ii,andΣIjaretherelativecontent,normalizedXRD-peakintensityofaparticularphase,andsummationofthenormalizedXRD-peakintensitiesofallphases,respectively.TheresultsarepresentedinFig.7.Asisseen,therelativecontentofthetitaniaincreasedaftersol-gelprocessdemonstratingthatanaddi-tionalamountoftitaniumoxidewasdepositedonthevicinityofsamplesafterthisstage.Meanwhile,itisobservedthattherutilerelativecontentincreasescontinuallywiththeappliedvoltage,whiletheanataserelativecontentreachesitsmaximumvalueatinterme-diateappliedvoltagesand,then,decreasesathighervoltages.Thereasonisthatapplyinghighervoltagesresultsinincreasingtheelectricalcurrentpassingthecell,and,consequently,warminguptheanodeaswellasthegrowinglayer.Themetastableanatasephasetransformstorutilewhichisathermodynamicallystablephaseatalmostalltemperaturesduetothisgeneratedheat.
Fig.8showsXPSspectraofthelayergrownundertheappliedvoltageof300V.Itistobenotedthatallbindingenergieswere
thattheporesizeincreaseswiththeappliedvoltage.Thereasonisthatapplyinghighervoltagescauseselectricalsparkswithhigherenergiesduetohigherelectricalcurrentpassingthroughtheelectrochemicalcell.Strongeravalanchesresultinformationofthewiderpores.Fig.2showsthemicrostructureofthesamplesaftersol-gelstage.Thestructuralpores,clearlyobservedinFig.1,arenotseen.ItrevealsthatanadditionalTiO2layerhasbeendepositedintothepores.Thisfindingwasconfirmedbythefilmthicknessestimations,performedbySEMcrosssectionimaging,resultsofwhicharepresentedinTable2.Nosignificantincreaseinfilmthicknesswasobservedaftersol-gelstage.Itshowsthattheadditionaltitaniawasmainlydepositedintothestructuralporesandnotoverthepreviouslygrownlayers.
SurfacetopographyofthelayerswasinvestigatedbyAFMwhoseresultsareexhibitedinFigs.3and4.TheresultsdepictaroughsurfacewhichisusualforMAO-synthesizedlayers.Ourstatisticalanalysisshowsthatthesurfaceroughnessofthelayersincreasedwiththeappliedvoltage;thatis,becausetheelectricalcurrent,passingthoroughtheelectrochemicalcell,increasedwiththeappliedvoltage,strongerelectronavalanchestakeplaceatsuchvoltagesresultingin
Fig.3.AFMtopographyoftheMAOgrowncompositelayersatvariousappliedvoltages:(a)300,(b)450,(c)600V.
2486M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–2489
Fig.4.AFMtopographyoftheMAO/sol-gelgrowncompositelayersatvariousappliedvoltages:(a)300,(b)450,(c)600V.
Table3
Averageroughnessnumbers(nm)beforeandaftersol-gelstage.Process
Appliedvoltage(V)300
MAO
MAO/sol-gel
12.310.0
45013.511.1
60015.212.9
referencedtotheC(1s)corelevelatbindingenergyof285.0eV.TheO(1s)corelevelbindingenergy,depictedinFig.8a,isdeconvolutedtothreedistinctcomponentsusingXPSoriginalsoftware.Itdemonstrates
thattherearethreedifferentO-bindingsinthelayers.ThepeakA,locatedatthebindingenergyof530.7eV,isassignedtothecrystallatticeoxygen(O-TiandO-Al).ThepeakB,atthebindingenergyof531.8eV,correspondstotheoxygeninthehydroxylgroups(−OH).Metaloxidefreesurfacescontactingwiththeatmospherearealwayshydrated,i.e.containwatermoleculesandhydroxylgroups.Finally,thepeakC,havingabindingenergyof532.9eV,isassignedtotheO-bondinginwatermoleculeswhichisusualforporouslayersgrowninaqueoussolutions.Fig.8b,whichdepictstheTi(2p3/2)corelevelatbindingenergyof458.4eV,assertstheexistenceoftitaniumintheform
Fig.5.XRDspectraoftheoxidelayerssynthesizedunderdifferentappliedvoltage:(a)beforesol-gelstageand(b)aftersol-gelstage.
M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–24892487
Fig.6.Anataseandrutilephasesrelativecontentasafunctionoftheappliedvoltagebeforeandaftersol-gelstage.
Fig.7.XPSspectraofthealumina-titaniacompositelayersgrownunderthevoltageof300V:(a)O(1s)corelevel,(b)Ti(2p)corelevel,and(c)Al(2p)corelevelbindingenergies.
ofTi4+statemerely.Fig.8cshowstheAl(2p)corelevelbindingenergywhichcanberesolvedintothreedistinctcomponents.ItconfirmsthatthreekindsofAl-bindingstatescanbefoundinthelayers.ThepeaksA,
B,andC,locatedatthebindingenergiesof74.3,74.1,and73.9eVcorrespondtoAl(OH)3,α-Al2O3,andγ-Al2O3compounds,respectively.3.2.Formationmechanism
Basedonthechemicalandelectrochemicaltheoreticalfounda-tions,aprobableformationmechanismisproposed.Atfirst,OH−andAlO−2anionsaregeneratedduetoionizationofthesodiumthree-phosphateandaluminatesaltsinthewaterasshownbelow:Na3PO4→3Na+PO4PO4
3−
þ
3−
ð1Þ
−
+H2O→HPO4
þ
2−
+OH
ð2Þð3Þ
NaAlO2→Na+AlO2
−
Table4
Absorptionandemissioncoefficientsbeforesol-gelstage.Appliedvoltage(V)300450600
Averageα(0.3~2.0μm)0.920.940.94
Averageε(2.0~20μm)0.050.060.06
Selectivity(α/ε)18.415.615.6
Fig.8.Sequentialabsorptionofthelightspectrumstrappedwithinthesurfacepores.
2488M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–2489
Table5
Absorptionandemissioncoefficientsaftersol-gelstage.Appliedvoltage(V)300450600
Averageα(0.3–2.0μm)0.980.980.99
Averageε(2.0–20μm)0.060.060.06
Selectivity(α/ε)19.616.316.5
Consequently,aluminumoxideparticlesprecipitatewithinthesurfaceporesrandomlydistributedontheoxidelayersurface.3.4.Solarindexes
Tables4and5showresultsofabsorptanceandemmitancecoefficientsofthecompositelayersbeforeandaftersol-gelstage,respectively.Itisclearthatthesolarabsorptivityincreasedaftersol-gelstage.Threemechanismsaresuggestedforsuchanopticalbehaviorasbelow:
3.4.1.MechanismI
Opticalwavesaretrappedbystructuralpores.Whenlightspectrumsencountertheabsorbersurfacesomeofthementerintothefilmandsomeofthemarereflected.Whenlightpropagatesinamediumotherthanvacuum,itsintensitydecreaseswithdistance.Assumethatthereisonlyanabsorberfilmwithoutasubstrate.Thelightintensitypenetratingjustthroughthefrontsurfaceofthefilm(Io)attenuatestoIthroughthefilm.Ifscatteringcanbeneglected,theattenuationoflightdependsontheintrinsicabsorptionwithinthefilm.Anormalizedattenuationofthelightintensityisdefinedasbelow:
I=Io=expð−4πkd=λÞ
ð10Þ
Whenthepotentialisapplied(atthebeginningtimeoftheMAOprocess),titaniumsubstrate,whichisconnectedtothepositivepoleoftherectifierasanode,involvesanoxidationreactionovertheanodesurfaceasbelow:Ti→Ti
4+
+4e
−
ð4Þ
Simultaneously,theOH−anionsmovetowardtheanodesurfacebecauseofthestrongelectricalfieldbetweenanodeandcathode,andreactwiththeTi4+cationsonthevicinityoftheanode:Ti
4+
+4OH→TiO2+2H2O
−
ð5Þ
Inthecaseofusingsodiumaluminateasanadditive,thereducedwaterinalkalinesolutionformseithermono-orpolyhydroxyanions,
(n+2)-[30].Asmentionedbefore,thee.g.Al(OH)−4orAln(OH)4n+2negativelychargedionssuchashydroxylanionsapproachtheanodesurfaceduetoelectricalfieldandtheirnegativecharge.TheytakepartinaseriesofreactionstoformTiO2andAl2O3asfollows:TiTi
4+
+4AlO2→TiO2+2Al2O3
+4AlðOHÞ4→TiO2+Al2O3+2AlðOHÞ3+5H2O
−
−
−
−
ð6Þð7Þð8Þ
4+
2AlðOHÞ4→Al2O3+2OH+3H2O
Moreover,athermalchemicalreactiontookplacesimultaneouslywiththeabovereactionsasbelow:Al2O3+TiO2→Al2TiO5
ð9Þ
Wherekistheextinctioncoefficient,disthefilmthickness,andλisthewavelength.Thecorrespondingabsorptioncoefficientisα=4πk/λ.Thus,1−I/Ioreflectstheintrinsicabsorption.Thismechanismhasbeennamednormalabsorption[31].Simultaneously,thereflectedportionofthelightwavesreturntotheenvironment.AsitisshowninFig.9,whenporousabsorberlayersareused,thereflectedwavesconsecutivelyencounterthewallsoftheporesandenterintotheabsorberlayeragain.Underthiscircumstance,thewavesenergyispartiallyabsorbedduringeachcontactresultinginhighersolarabsorptivity.Thismechanismisnamedmultipleabsorp-tions[32,33].
3.4.2.MechanismII
Thesynthesizedoxidefilmscanbeassumedasamultilayersystemduetothedoublestagedepositionprocess.AsitisschematicallyshowninFig.9b,interfacesbetweenabsorberlayersactasareflectorbecausethelayershavedifferentphysicalproperties.Theenergyofthewavesisalsoabsorbedinreturningpathinadditiontonormalpath.Thismechanismisnameinterferenceabsorption.
3.4.3.MechanismIII
TheAl2O3particles,randomlydistributedintheTiO2matrix,actasareflectoraswell.Thewavesarereflectedwhentheycontactthe
Atthesametime,alargeamountofO2andH2bubblesappearsonthesurfaceoftheanodeandcathode,respectively.Astheappliedpotentialincreases,thebubblesonthesurfaceoftheanodebreak,generatinglargeamountofenergyandlowtemperatureplasma.Thehigh-energyplasmaacceleratestheelectrochemicalreaction,andfacilitatestheMAOprocess.Sincetheanionsaredrawnintotheporesduetoelectricalfieldtoattendtheabovereactions,itisconcludedthatAl2O3particlesarerandomlydistributedintheTiO2matrix,[16].
Fig.9.(a)Schematicillustrationofthefinalstructureofthecompositelayersand(b)schematicillustrationofinterferenceinmultilayerabsorbers.
M.R.Bayatietal./Surface&CoatingsTechnology205(2010)2483–24892489
dispersedparticlesandinterferenceabsorptionoccurs.Asaresult,theirpathwithintheabsorberlayerlengthensandtheirenergyisfurtherabsorbed.
Tosumup,allofthesuggestedmechanismsaredominatinginourlayerswhichresultinhighabsorptancecoefficients,inspiteofbeingthin.Normalabsorptionalwaysoccursintheabsorberlayersandothersuggestedmechanismsenhanceit.Sincethelightcanonlypenetratetheporeslargerthanthelightwavelength,themultipleabsorptionhastrivialinfluenceontheabsorptivityofthesystem[34,35].Itistobenotedthatthewavelengthrangeofthesolarradiationis0.3–2.5μm[2].WebelievethatthenormalabsorptionisfurtheraffectedbythemechanismsIIandIII.Theemissioncoefficientsofthelayers,listedinTables4and5,arelowduetotheirsmallthicknesses.Whenthicknessoftheabsorberlayerissmallerthanthewavelengthofthesolarspectrums,metallicsubstrateisabletoexhibititsopticalproperties,namelylowemittance[34].Itistonotethatallmetalsnaturallyhavelowthermalemissioncoefficients.Usingathinabsorberlayerdepositedoverametallicsubstrateresultsinalowsolaremissivity[35].4.Conclusion
Alumina-titaniacompositesolarselectiveporousabsorbersweredevelopedviaMAOmethodfollowedbysol-gelprocessforthefirsttime.Thelayersconsistedofanatase,rutile,α-alumina,γ-alumina,andAl2TiO5phaseswhosefractionsdependedongrowthconditions.Microstructuralandtopographicalinvestigationsrevealedanano/micro-sizedporousstructurewitharoughsurface.Althoughthelayersweresothin,theyexhibitedahighsolarabsorptivityof0.99andalowthermalemissivityof0.06.Acknowledgement
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