FABRICATIONOFPIEZOELECTRICCERAMlClPOLYMERCOMPOSITESBYINJECTIONMOLDING.LeslieJ.BowenandKennethW.French,MaterialsSystemsInc.53HillcrestRoad,Concord,MA01742AbstractResearchattheMaterialsResearchLaboratory,PennsylvaniaStateUniversityhasdemonstratedthepotentialforimprovinghydrophoneperformanceusingpiezoelectricceramic/polymercomposites.AspartofanONR-fundedinitiativetodevelopcost-effectivemanufacturingtechnologyforthesecomposites,MaterialsSystemsispursuinganinjectionmoldingceramicfabricationapproach.Thispaperbrieflyoverviewskeyfeaturesoftheceramicinjectionmoldingprocess,thendescribestheapproachandmethodologybeingusedtofabricatePZTceramic/polymercomposites.PropertiesandapplicationsofinjectionmoldedPZTceramicsarecomparedwithconventionallyprocessedmaterial.IntroductionPiezoelectricceramic/polymercompositesofferdesignversatilityandperformanceadvantagesoverbothsinglephaseceramicandpolymerpiezoelectricmaterialsinbothsensingandactuatingapplications.ThesecompositeshavefounduseinhighresolutionmedicalultrasoundaswellasdevelopmentalNavyapplications.Manycompositeconfigurationshavebeenconstructedandevaluatedonalaboratoryscaleoverthepastthirteenyears.Oneofthemostsuccessfulcombinations,designated1-3compositeinNewnhamsnotationl1,hasaone-dimensionallyconnectedceramicphase(PZTfibers)containedwithinathree-dimensionallyconnectedorganicpolymerphase.Hydrophonefiguresofmeritforthiscompositecanbemadeover10,000timesgreaterthanthoseofsolidPZTceramicbyappropriatelyselectingthephasecharacteristicsandcompositestructure.ThePennStatecompositeswerefabricatedlbyhand-aligningextrudedPZTceramicrodsinajigandencapsulatinginepoxyresin,followedbyslicingtotheappropriatethicknessandpolingtheceramic.Asidefromdemonstratingtheperformanceadvantagesofthismaterial,thePennStateworkhighlightedthedifficultiesinvolvedinfabricating1-3compositesonalargescale,orevenforprototypepurposes.Theseare:11TherequirementtoalignandsupportlargenumbersofPZTfibersduringencapsulationbythepolymer.2)Thehighincidenceofdielectricbreakdownduringpolingarisingfromthesignificantprobabilityofencounteringoneormoredefectivefibersinatypicallargearray.Overthepastfiveyearsseveralattemptshavebeenmadetosimplifytheassemblyprocessfor1-3transducerswiththeintentionofimprovingmanufacturingviabilityandloweringthematerialcost.EarlyattemptsinvolveddicingsolidblocksofPZTceramicintothedesiredconfigurationandback-fillingthespaceswithapolymerphase.Thistechniquehasfoundwideacceptanceinthemedicalultrasoundindustryformanufacturinghighfrequencytransducers2.Morerecently,FiberMaterialsCorp.hasdemonstratedtheapplicabilityofitsweavingtechnologyforfiber-reinforcedcompositestotheassemblyofpiezoelectriccomposites31.Anotherexploratorytechniqueinvolvesreplicatingporousfabricshavingtheappropriateconnectivity41.Forextremelyfinescalecomposites,fibershavingdiametersintheorderof25to100pnandaspectratiosinexcessoffivearerequiredtomeetdeviceperformanceobjectives.Asaresult,thesedifficultiesarecompoundedbytheadditionalchallengeofformingandhandlingextremelyfinefibersinlargequantitieswithoutdefects.Recently,researchersatSiemensCorp.haveshownthatveryfinescalecompositescanbeproducedbyafugitivemoldtechnique.However,thismethodrequiresfabricatinganewmoldforeverypart51.Thispaperdescribesanewapproachtopiezoelectriccompositefabrication,viz:Ceramicinjectionmolding.Ceramicinjectionmoldingisacost-effectivefabricationapproachforbothNavypiezoelectricceramic/polymercompositesandforthefabricationofultrafinescalepiezoelectriccomposites,suchasthoserequiredforhighfrequencymedicalultrasoundandnondestructiveevaluation.Theinjectionmoldingprocessovercomesthedifficultyofassemblingorientedceramicfibersintocompositetransducersbynet-shapepreformingceramicfiberarrays.Asidefromthisadvantage,theprocessmakespossibletheconstructionofcompositetransducershavingmorecomplexceramicelementgeometriesthanthosepreviouslyenvisioned,leadingtogreaterdesignflexibilityforimprovedacousticimpedancematchingandlateralmodecancellation.ProcessDescriotionInjectionmoldingiswidelyusedintheplasticsindustryasameansforrapidmassproductionofcomplexshapesatlowcost.Itsapplicationtoceramicshasbeenmostsuccessfulforsmallcross-sectionshapes,e.g.threadguides,andlarge,complexshapeswhichdonotrequiresinteringtohighdensity,suchasturbinebladecastinginserts.Morerecently,theprocesshasbeeninvestigatedasaproductiontechnologyforheat-engineturbinecomponents6,71.TheinjectionmoldingprocessusedforPZTmoldingisshownschematicallyinFigure1.Byinjectingahotthermoplasticmixtureofceramicpowderandorganicbinderintoacooledmold,complexshapescanbeformedwiththeeaseandrapiditynormallyassociatedwithplasticsmolding.Precautions,suchashard-facingthemetalcontactsurfaces,areimportanttominimizemetalliccontaminationfromthecompoundingandmoldingmachinery.Forceramics,thebindermustberemovednondestructively,necessitatinghighsolidsloading,carefulcontrolofthebinderremovalPowderProcessing-r-4CERAMICPREFORMOrganicBinder-IGranulateI-PREFORMLAY-UPTOFORMLARGERARRAYS-_Lu-i_-1-1andapplyelectrodesFigure1:InjectionMoldingProcessRcess,andproperfixturing.Oncethebinderisremoved,thesubsequentfiring,polingandepoxyencapsulationprocessesaresimilartothoseusedforconventionalPZTipolymercomposites11I.Thus,theprocessoffersthefollowingadvantagesoveralternativefabricationroutes:Complex,nearnet-shapecapabilityforhandlingmanyfiberssimultaneously；rapidthroughput(typicallysecondsperpart)；compatibilitywithstatisticalprocesscontrol；lowmaterialwaste；flexibilitywithrespecttotransducerdesign(allowsvariationinPZTelementspacingandshape)；andlowcostinmoderatetohighvolumes.Ingeneral,becauseofthehighinitialtoolingcost,theceramicsinjectionmoldingprocessisbestappliedtocomplex-shapedcomponentswhichrequirelowcostinhighvolumes.ComoositeFabricationandEvaluationTheapproachtakentofabricate1-3piezoelectriccompositesisshowninFigure2a,whichillustratesaPZTceramicpreformconceptinwhichfiberpositioningisachievedusingaco-moldedintegralceramicbase.Afterpolymerencapsulationtheceramicbaseisremovedbygrinding.Asidefromeaslngthehandlingofmanyfibers,thispreformapproachallowsbroadlatitudeintheselectionofpiezoelectricceramicelementgeometryforcompositeperformanceoptimization.Tooldesignisimportantforsuccessfulinjectionmoldingofpiezoelectriccomposites.TheapproachshowninFigure2busesshapedtoolinsertstoallowchangesinpartdesignwithoutincurringexcessiveretoolingcosts.Figure2cshowshowindividualpreformsareconfiguredtoformlargerarrays.Figure2a:PreformConfiguration(Approx.400ceramicelements)REMOVABLEINSERT:CAVITYTOOLBODYU*SPRUEFigure2b:InjectionMoldingToolConfigurationFigure2c:LargeAreaCompositeArraysmadefromPreformsFigure2:PreformApproachtoCompositeFabrication.Inpractice,materialandmoldingparametersmustbeoptimizedandintegratedwithinjectionmoldingtooldesigntorealizeintactpreformejectionaftermolding.Keyparametersinclude:PZT/binderratio,PZTelementdiameterandtaper,PZTbasethickness,toolsurfacefinish,andthemoldedpartejectionmechanismdesign.Inordertoevaluatetheseprocessparameterswithoutincurringexcessivetoolcost,atooldesignhavingonlytworowsof19PZTelementseachhasbeenadoptedforexperimentalpurposes.Eachrowcontainselementshavingthreetaperangles(0,1and2degrees)andtwodiameters(0.5andlmm).Toaccommodatemoldingshrinkage,thesizeofthepreformismaintainedat5Ox50mmtominimizethepossibilityofshearingofftheoutermostfibersduringthecoolingportionofthemoldingcycle.Figure3:InjectionMolded1-3CompositePreforms.161Figure3showsgreenceramicpreformsfabricatedusingthistoolconfiguration.NotethatallofthePZTelementsejectedintactaftermolding,includingthosehavingnolongitudinaltaperingtofacilitateejection.Slowheatinginairhasbeenfoundtobeasuitablemethodfororganicbinderremoval.Finally,theburned-outpreformsaresinteredinaPbO-richatmosphereto97-98%ofthetheoreticaldensity.Noproblemshavebeenencounteredwithcontrollingtheweightlossduringsinteringofthesecompositepreforms,evenforthosefine-scale,high-surfaceareapreformswhichareintendedforhighfrequencyultrasound.-.-.L.Figure4:ScanningElectronMicrographsofAs-molded(Upper)andAs-sintered(Lower)SurfacesofPZTFibers.Figure4illustratesthesurfacesofas-moldedandas-sinteredfibers,showingthepresenceofshallowfoldlinesapproximately10pmwide,whicharecharacteristicoftheinjectionmoldingprocess.Thefibersexhibitminorgroovingalongtheirlengthduetoejectionfromthetool.Figure5showsthecapabilityofnearnet-shapemoldingforfabricatingveryfinescalepreforms；PZTelementdimensionsonly30pmwidehavebeendemonstrated.Theas-sinteredsurfaceoftheseelementsindicatesthatthePZTceramicmicrostructureisdenseanduniform,consistingofequiaxedgrains2-3pmindiameter.Figure5:Fine-scale2-2CompositeformedbyNearNet-shapeMolding(UpperMicrograph).As-sinteredSurface(LowerMicrograph).Inordertodemonstratethelay-upapproachforcompositefabrication,compositesofapproximately10volumepercentPZT-5HfibersandSpurrsepoxyresinwerefabricatedbyepoxyencapsulatinglaid-uppairsofinjectionmoldedandsinteredfiberrowsfollowedbygrindingawaythePZTceramicstockusedtomoldthecompositepreform.Figure6showscompositesamplesmadefromfreshly-compoundedPZT/bindermixtureandfromreusedmaterial.Recyclingofthecompoundedandmoldedmaterialappearstobeentirelyfeasibleandresultsingreatlyenhancedmaterialutilization.Table1comparesthepiezoelectricanddielectricpropertiesofinjectionmoldedPZTceramicspecimenswiththosereportedforpressedPZT-5Hsamplespreparedbythepowdermanufacturer.WhenthesinteringconditionsareoptimizedforthePZT-5Hformulation,thepiezoelectricanddielectricpropertiesarecomparableforbothmaterials.Sincethedonor-dopedPZT-5Hformulationisexpectedtobeparticularlysensitivetoironcontaminationfromtheinjectionmoldingequipment,theimplicationofthesemeasurementsisthatsuchcontaminationisnegligibleinthisinjectionmoldedPZTmaterial.*PowdersuppliedbyMorganMatroc,Inc.,Bedford,Ohio；Lot105A.162Table1:PropertiesofInjectionMoldedPiezoelectricCeramics.SpecimenRelativeDielectricd33TYPePermittivityLoss(1kHz1(pC/N)Die-Pressed35840.018745Inj.Molded*35880.018755*Aged24hoursbeforemeasuremegt.*Polingconditions:2.4kV/mm,60C,2minutes.Figure6:InjectionMoldedPZTFiber/EpoxyResinCompositespreparedbythePreformLay-upMethod.SummarvCeramicinjectionmoldinghasbeenshowntobeaviableprocessforfabricatingbothPZTceramicsandpiezoelectricceramic/polymertransducers.TheelectricalpropertiesofinjectionmoldedPZTceramicsarecomparablewiththosepreparedbyconventionalpowderpressing,withnoevidenceofdeleteriouseffectsfrommetalliccontaminationarisingfromcontactwiththecompoundingandmoldingequipment.Byusingceramicinjectionmoldingtofabricatecompositepreforms,andthenlayingupthepreformstoformlargercompositearrays,anapproachhasbeendemonstratedfornet-shapemanufacturingofpiezoelectriccompositetransducersinlargequantities.AcknowledaementsThisworkwasfundedbytheOfficeofNavalResearchunderthedirectionofMr.StephenE.N