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FABRICATIONOFPIEZOELECTRICCERAMlClPOLYMERCOMPOSITESBYINJECTIONMOLDING.LeslieJ.BowenandKennethW.French,MaterialsSystemsInc.53HillcrestRoad,Concord,MA01742AbstractResearchattheMaterialsResearchLaboratory,PennsylvaniaStateUniversityhasdemonstratedthepotentialforimprovinghydrophoneperformanceusingpiezoelectricceramic/polymercomposites.AspartofanONRfundedinitiativetodevelopcosteffectivemanufacturingtechnologyforthesecomposites,MaterialsSystemsispursuinganinjectionmoldingceramicfabricationapproach.Thispaperbrieflyoverviewskeyfeaturesoftheceramicinjectionmoldingprocess,thendescribestheapproachandmethodologybeingusedtofabricatePZTceramic/polymercomposites.PropertiesandapplicationsofinjectionmoldedPZTceramicsarecomparedwithconventionallyprocessedmaterial.IntroductionPiezoelectricceramic/polymercompositesofferdesignversatilityandperformanceadvantagesoverbothsinglephaseceramicandpolymerpiezoelectricmaterialsinbothsensingandactuatingapplications.ThesecompositeshavefounduseinhighresolutionmedicalultrasoundaswellasdevelopmentalNavyapplications.Manycompositeconfigurationshavebeenconstructedandevaluatedonalaboratoryscaleoverthepastthirteenyears.Oneofthemostsuccessfulcombinations,designated13compositeinNewnhamsnotationl1,hasaonedimensionallyconnectedceramicphasePZTfiberscontainedwithinathreedimensionallyconnectedorganicpolymerphase.Hydrophonefiguresofmeritforthiscompositecanbemadeover10,000timesgreaterthanthoseofsolidPZTceramicbyappropriatelyselectingthephasecharacteristicsandcompositestructure.ThePennStatecompositeswerefabricatedlbyhandaligningextrudedPZTceramicrodsinajigandencapsulatinginepoxyresin,followedbyslicingtotheappropriatethicknessandpolingtheceramic.Asidefromdemonstratingtheperformanceadvantagesofthismaterial,thePennStateworkhighlightedthedifficultiesinvolvedinfabricating13compositesonalargescale,orevenforprototypepurposes.Theseare11TherequirementtoalignandsupportlargenumbersofPZTfibersduringencapsulationbythepolymer.2Thehighincidenceofdielectricbreakdownduringpolingarisingfromthesignificantprobabilityofencounteringoneormoredefectivefibersinatypicallargearray.Overthepastfiveyearsseveralattemptshavebeenmadetosimplifytheassemblyprocessfor13transducerswiththeintentionofimprovingmanufacturingviabilityandloweringthematerialcost.EarlyattemptsinvolveddicingsolidblocksofPZTceramicintothedesiredconfigurationandbackfillingthespaceswithapolymerphase.Thistechniquehasfoundwideacceptanceinthemedicalultrasoundindustryformanufacturinghighfrequencytransducers2.Morerecently,FiberMaterialsCorp.hasdemonstratedtheapplicabilityofitsweavingtechnologyforfiberreinforcedcompositestotheassemblyofpiezoelectriccomposites31.Anotherexploratorytechniqueinvolvesreplicatingporousfabricshavingtheappropriateconnectivity41.Forextremelyfinescalecomposites,fibershavingdiametersintheorderof25to100pnandaspectratiosinexcessoffivearerequiredtomeetdeviceperformanceobjectives.Asaresult,thesedifficultiesarecompoundedbytheadditionalchallengeofformingandhandlingextremelyfinefibersinlargequantitieswithoutdefects.Recently,researchersatSiemensCorp.haveshownthatveryfinescalecompositescanbeproducedbyafugitivemoldtechnique.However,thismethodrequiresfabricatinganewmoldforeverypart51.Thispaperdescribesanewapproachtopiezoelectriccompositefabrication,vizCeramicinjectionmolding.CeramicinjectionmoldingisacosteffectivefabricationapproachforbothNavypiezoelectricceramic/polymercompositesandforthefabricationofultrafinescalepiezoelectriccomposites,suchasthoserequiredforhighfrequencymedicalultrasoundandnondestructiveevaluation.Theinjectionmoldingprocessovercomesthedifficultyofassemblingorientedceramicfibersintocompositetransducersbynetshapepreformingceramicfiberarrays.Asidefromthisadvantage,theprocessmakespossibletheconstructionofcompositetransducershavingmorecomplexceramicelementgeometriesthanthosepreviouslyenvisioned,leadingtogreaterdesignflexibilityforimprovedacousticimpedancematchingandlateralmodecancellation.ProcessDescriotionInjectionmoldingiswidelyusedintheplasticsindustryasameansforrapidmassproductionofcomplexshapesatlowcost.Itsapplicationtoceramicshasbeenmostsuccessfulforsmallcrosssectionshapes,e.g.threadguides,andlarge,complexshapeswhichdonotrequiresinteringtohighdensity,suchasturbinebladecastinginserts.Morerecently,theprocesshasbeeninvestigatedasaproductiontechnologyforheatengineturbinecomponents6,71.TheinjectionmoldingprocessusedforPZTmoldingisshownschematicallyinFigure1.Byinjectingahotthermoplasticmixtureofceramicpowderandorganicbinderintoacooledmold,complexshapescanbeformedwiththeeaseandrapiditynormallyassociatedwithplasticsmolding.Precautions,suchashardfacingthemetalcontactsurfaces,areimportanttominimizemetalliccontaminationfromthecompoundingandmoldingmachinery.Forceramics,thebindermustberemovednondestructively,necessitatinghighsolidsloading,carefulcontrolofthebinderremovalPowderProcessingr4CERAMICPREFORMOrganicBinderIGranulateIPREFORMLAYUPTOFORMLARGERARRAYS_Lui_11andapplyelectrodesFigure1InjectionMoldingProcessRoute.process,andproperfixturing.Oncethebinderisremoved,thesubsequentfiring,polingandepoxyencapsulationprocessesaresimilartothoseusedforconventionalPZTipolymercomposites11I.Thus,theprocessoffersthefollowingadvantagesoveralternativefabricationroutesComplex,nearnetshapecapabilityforhandlingmanyfiberssimultaneouslyrapidthroughputtypicallysecondsperpartcompatibilitywithstatisticalprocesscontrollowmaterialwasteflexibilitywithrespecttotransducerdesignallowsvariationinPZTelementspacingandshapeandlowcostinmoderatetohighvolumes.Ingeneral,becauseofthehighinitialtoolingcost,theceramicsinjectionmoldingprocessisbestappliedtocomplexshapedcomponentswhichrequirelowcostinhighvolumes.ComoositeFabricationandEvaluationTheapproachtakentofabricate13piezoelectriccompositesisshowninFigure2a,whichillustratesaPZTceramicpreformconceptinwhichfiberpositioningisachievedusingacomoldedintegralceramicbase.Afterpolymerencapsulationtheceramicbaseisremovedbygrinding.Asidefromeaslngthehandlingofmanyfibers,thispreformapproachallowsbroadlatitudeintheselectionofpiezoelectricceramicelementgeometryforcompositeperformanceoptimization.Tooldesignisimportantforsuccessfulinjectionmoldingofpiezoelectriccomposites.TheapproachshowninFigure2busesshapedtoolinsertstoallowchangesinpartdesignwithoutincurringexcessiveretoolingcosts.Figure2cshowshowindividualpreformsareconfiguredtoformlargerarrays.Figure2aPreformConfigurationApprox.400ceramicelementsREMOVABLEINSERTCAVITYTOOLBODYUSPRUEFigure2bInjectionMoldingToolConfigurationFigure2cLargeAreaCompositeArraysmadefromPreformsFigure2PreformApproachtoCompositeFabrication.Inpractice,materialandmoldingparametersmustbeoptimizedandintegratedwithinjectionmoldingtooldesigntorealizeintactpreformejectionaftermolding.KeyparametersincludePZT/binderratio,PZTelementdiameterandtaper,PZTbasethickness,toolsurfacefinish,andthemoldedpartejectionmechanismdesign.Inordertoevaluatetheseprocessparameterswithoutincurringexcessivetoolcost,atooldesignhavingonlytworowsof19PZTelementseachhasbeenadoptedforexperimentalpurposes.Eachrowcontainselementshavingthreetaperangles0,1and2degreesandtwodiameters0.5andlmm.Toaccommodatemoldingshrinkage,thesizeofthepreformismaintainedat5Ox50mmtominimizethepossibilityofshearingofftheoutermostfibersduringthecoolingportionofthemoldingcycle.Figure3InjectionMolded13CompositePreforms.161Figure3showsgreenceramicpreformsfabricatedusingthistoolconfiguration.NotethatallofthePZTelementsejectedintactaftermolding,includingthosehavingnolongitudinaltaperingtofacilitateejection.Slowheatinginairhasbeenfoundtobeasuitablemethodfororganicbinderremoval.Finally,theburnedoutpreformsaresinteredinaPbOrichatmosphereto9798ofthetheoreticaldensity.Noproblemshavebeenencounteredwithcontrollingtheweightlossduringsinteringofthesecompositepreforms,evenforthosefinescale,highsurfaceareapreformswhichareintendedforhighfrequencyultrasound.........L..Figure4ScanningElectronMicrographsofAsmoldedUpperandAssinteredLowerSurfacesofPZTFibers.Figure4illustratesthesurfacesofasmoldedandassinteredfibers,showingthepresenceofshallowfoldlinesapproximately10pmwide,whicharecharacteristicoftheinjectionmoldingprocess.Thefibersexhibitminorgroovingalongtheirlengthduetoejectionfromthetool.Figure5showsthecapabilityofnearnetshapemoldingforfabricatingveryfinescalepreformsPZTelementdimensionsonly30pmwidehavebeendemonstrated.TheassinteredsurfaceoftheseelementsindicatesthatthePZTceramicmicrostructureisdenseanduniform,consistingofequiaxedgrains23pmindiameter.Figure5Finescale22CompositeformedbyNearNetshapeMoldingUpperMicrograph.AssinteredSurfaceLowerMicrograph.Inordertodemonstratethelayupapproachforcompositefabrication,compositesofapproximately10volumepercentPZT5HfibersandSpurrsepoxyresinwerefabricatedbyepoxyencapsulatinglaiduppairsofinjectionmoldedandsinteredfiberrowsfollowedbygrindingawaythePZTceramicstockusedtomoldthecompositepreform.Figure6showscompositesamplesmadefromfreshlycompoundedPZT/bindermixtureandfromreusedmaterial.Recyclingofthecompoundedandmoldedmaterialappearstobeentirelyfeasibleandresultsingreatlyenhancedmaterialutilization.Table1comparesthepiezoelectricanddielectricpropertiesofinjectionmoldedPZTceramicspecimenswiththosereportedforpressedPZT5Hsamplespreparedbythepowdermanufacturer.WhenthesinteringconditionsareoptimizedforthePZT5Hformulation,thepiezoelectricanddielectricpropertiesarecomparableforbothmaterials.SincethedonordopedPZT5Hformulationisexpectedtobeparticularlysensitivetoironcontaminationfromtheinjectionmoldingequipment,theimplicationofthesemeasurementsisthatsuchcontaminationisnegligibleinthisinjectionmoldedPZTmaterial.PowdersuppliedbyMorganMatroc,Inc.,Bedford,OhioLot105A.162Table1PropertiesofInjectionMoldedPiezoelectricCeramics.SpecimenRelativeDielectricd33TYPePermittivityLoss1kHz1pC/NDiePressed35840.018745Inj.Molded35880.018755Aged24hoursbeforemeasuremegt.Polingconditions2.4kV/mm,60C,2minutes.Figure6InjectionMoldedPZTFiber/EpoxyResinCompositespreparedbythePreformLayupMethod.SummarvCeramicinjectionmoldinghasbeenshowntobeaviableprocessforfabricatingbothPZTceramicsandpiezoelectricceramic/polymertransducers.TheelectricalpropertiesofinjectionmoldedPZTceramicsarecomparablewiththosepreparedbyconventionalpowderpressing,withnoevidenceofdeleteriouseffectsfrommetalliccontaminationarisingfromcontactwiththecompoundingandmoldingequipment.Byusingceramicinjectionmoldingtofabricatecompositepreforms,andthenlayingupthepreformstoformlargercompositearrays,anapproachhasbeendemonstratedfornetshapemanufacturingofpiezoelectriccompositetransducersinlargequantities.AcknowledaementsThisworkwasfundedbytheOfficeofNavalResearchunderthedirectionofMr.StephenE.Newfield.TheauthorswishtothankMs.HongPhamfortechnicalassistance,andDr.ThomasShroutoftheMaterialsResearchLaboratory,Penn.StateUniversityforelectricalmeasurements.ReferenceslR.E.Newnhametal,CompositePiezoelectricTransducers,MaterialsinEngineering,Vol.2,pp.93106,Dec.1980.21C.Nakayaetal,IEEEUltrasonicsSymposiumProc.,Oct.1618,1985,p634.131S.D.Darrahetal,LargeAreaPiezoelectricComposites,Proc.oftheADPAConferenceonActiveMaterialsandStructures,Alexandria,Virginia,Nov.48,1991,Ed.G.Knowles,InstituteofPhysicsPublishing,pp139142.A.SafariandD.J.Waller,FineScalePZTFiber/PolymerComposites,presentedattheADPAConferenceonActiveMaterialsandStructures,Alexandria,Virginia,Nov.4148,1991.5U.Bast,D.CramerandA.Wolff,ANewTechniquefortheProductionofPiezoelectricCompositeswith13Connectivity,Proc.ofthe7thCIMTEC,Montecatini,Italy,June2430,1990,Ed.P.Vincenzini,Elsevier,pp20052015.G.BandyopadhyayandK.W.French,FabricationofNearnetShapeSiliconNitridePartsforEngineApplication,J.Eng.forGasTurbinesAndPower,108,J.Greimetal,InjectionMoldedSinteredTurbochargerRotors,Proc.3rd.Int.Symp.onCeramicMaterialsandComponentsforHeatEngines,LasVegas,Nev.,pp.13651375,Amer.Cer.Soc.1989.61pp536539,1986.171163
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