搜索
Thethermalconductivityandmechanicalpropertiesofhighlyporouszirconiafelt/ZrO2compositesbyzirconiumoxideprecursorimpregnationJianpingAia,b,GuohongZhoua,*,ZhengjuanWanga,b,HailongZhanga,XianpengQina,b,PengLiua,ShiweiWanga,*aTheStateKeyLaboratoryofHighPerformanceCeramicsandSuperfineMicrostructures,ShanghaiInstituteofCeramics,ChineseAcademyofSciences,1295DingxiRoad,Shanghai200050,PRChinabUniversityofChineseAcademyofSciences,Beijing100049,PRChinaAbstractThevacuumpressureimpregnationmethodwasadoptedtomakebothlowthermalconductivityandhigh-temperatureresistanceporouszirconiafelt/ZrO2composites.Incorporatingzirconiafeltintoporouszirconiamatrixtoimprovethemechanicalstrength.Meanwhile,lanthanumnitrateasanadditivetoyttrium-stabilizedzirconiatoreducethethermalconductivityofthematrix.Theeffectofsinteringtemperatureoncompressivestrengthandthermalconductivitywereinvestigatedonthebasisofporosity,microstructureandcomposition.Thetetragonalphaseandthecubicphaseco-existedat1450℃,whilethecubicphasepredominatedupon1450℃aszirconiaionswerereplacedbylanthanumandyttriumions.Andtheopenporosityandsizeofsmallporesdecreasedasthesinteringtemperatureincreasedfrom1450to1600℃duetothezirconiagrainsgrewup,adecreaseofopenporosityfrom48.1%to42.5%.Comparedwithothertemperaturesinteredsamples,the1600℃-sinteredsamplesalwaysownedhigherthermalconductivityvalueof0.70~0.78W/(m•K)atlowertemperature,inversely,possessedlowerthermalconductivityafter800℃.1.IntroductionZirconia-basedceramicshavehighmeltingpoint,excellentmechanicalstrength,lowthermalconductivitycomparedwithotherceramics[1-3].Itisthereforeofinteresttoconsiderthemasthermalprotections.Zirconia-basedceramicsshowpromiseasthermalbarriercoatings(TBCs)[4-6].TBCsofferadvantageinthermalinsulating,however,theycannotmeetthestrengthrequirementofthermalprotectionswhichrequirenotonlylowthermalconductivitybutalsohighstrength.Therefore,itissignificanttoresearchzirconia-basedceramicsasbulkmaterialswhichmeetboththermalandmechanicalrequirements.Ithasbeenconfirmedthatthermalconductivityofzirconia-basedceramicsisloweredbyaddinglanthanumoxidetoYSZandintroducingporestoformporousceramics[7-9].Meanwhile,lanthanumoxideaddedzirconiaceramicsshowsmallermechanicalstrengthcomparedwiththeotheroxideaddedones[10],andthestrengthofporousceramicsdecreasessignificantlywithincreasingporosity[11].Thereiscurrentlywidespreadinterestinfabricatingporouszirconiaceramicswithlowthermalconductivityfortheirapplicationsinbulkthermalprotections.Thefabricationmethodsincludetemplatemethods[12-14],employingporeformingagents[15,16],impregnationofopen-cellpolyurethanefoams[17],partialsintering[18].However,fabricationofporousceramicswithbothhighporosityandhighstrengthwererarelyreported.Inthepresentstudy,weemployedlanthanumnitrateasanadditivetomakeporousYSZ-basedceramicssuitableasthermalinsulators.Atthistime,ourinnovatedzirconiaprecursorconversionmethodwasadoptedtoimprovethemechanicalstrengthbyreinforcingporouszirconiamatrixwithzirconiafelt.Incorporatingzirconiafeltintootherporousmaterials,however,wasrarelyreported.TheaddedlanthanumionssubstitutedforzirconiumonesandtransformedtheYSZceramicfromatetragonaltoacubicstructurehavinglowerthermalconductivity.Theeffectofsinteringtemperatureoncompressivestrengthandthermalconductivitywereinvestigatedonthebasisofporosity,microstructureandcomposition.2Experimentalprocedure2.1.SamplepreparationCommerciallyavailablezirconiafelt(NanjingYulongnewmaterialcorporation,Nanjing,China)wasusedasstartingmaterial.Thetotalporosityanddensityofthiszirconiafeltarearound80%and0.96g/cm3.XRDpatternwasshowninFig.1forzirconiafelt,asmallamountofmonoclinicphaseremainedinzirconiafelt,whilethecubicphasepredominated.Theaveragediameterofchoppedzirconiafiberisaround20μm,asshowninFig.2.Fig.1.XRDpatternsofrawmaterialaboutzirconiafelt.Fig.2.SEMmicrographofzirconiafelt.TheprocedureforthepreparationofZrO2precursoremulsionwasasfollows:first,inorganicsaltZrOCl2•8H2O(analyticalpure),yttriumnitratehexahydrate(analyticalpure),lanthanumnitratehexahydrate(analyticalpure)werefullydissolvedindeionizedwaterbymagneticstirrer,andthemoleratioofatomsLa,YandZris3:5:92.Subsequently,polyethyleneglycol(chemicalpure)asadispersantwasaddedtothissolution.NH4OH(concentrationis2mol/L)asacatalystwasaddedintothesolutionwhilevigorouslystirring.Finally,thesolutionwasevaporatedacertaincontentofwaterwhilestirring,thenwascooledtoroomtemperature.Sothepreparationof3La5YSZprecursoremulsionwascomplete.Fig.3.Aschematicdiagramofpreparationofporouszirconiafelt/ZrO2composites.Thepreparationofporouszirconiafelt/ZrO2compositesincludedthreestages,asshowninFig.3.Inthestage1,3La5YSZprecursoremulsionmixedwithethanolandpolyvinylalcoholsolutiontopreparesuitableviscosityZrO2precursormixture,andthenpreformswereinfiltratedwithZrO2precursormixtureinvacuum.Afterhalfanhour,impregnationtankwasfilledN2to2MPaandmaintained2MPapressurefor9h.Inthestage2,thepreformsfilledwithprecursorweredriedat60~90℃for12h.Inthestage3,thedriedpreformswerepyrolyzedinairat970℃for2h.Inordertodensifythecomposites,theothereleve