聚合物材料的介电性能

TECHNICALWHITEPAPERDielectricPropertiesofPolymersIntroductionUnderstandingthestructureofplastics(andinparticular,thefluoropolymers)notonlygivesabetterunderstandingofchemicalresistancebutalsooftheelectricalproperties.AlbertEinsteinsaidthat‘Goddoesnotplaydicewiththeuniverse,’andthisisjustastrueonthe‘micro’scaleasitisonthe‘macro’scale.Structuredeterminespropertiesatalllevels.Mostplasticsaredielectricsorinsulators(poorconductorsofelectricity)andresisttheflowofacurrent1.Thisisoneofthemostusefulpropertiesofplasticsandmakesmuchofourmodernsocietypossiblethroughtheuseofplasticsaswirecoatings,switchesandotherelectricalandelectronicproducts.Despitethis,dielectricbreakdowncanoccuratsufficientlyhighvoltagestogivecurrenttransmissionandpossiblemechanicaldamagetotheplastic.Theapplicationofapotentialdifference(voltage)causesinthemovementofelectronsandwhentheelectronsarefreetomovethereisaflowofcurrent.Metalscanbethoughtofasacollectionofatomicnucleiexistingina‘seaofelectrons’andwhenavoltageisappliedtheelectronsarefreetomoveandtoconductacurrent.Polymersandtheatomsthatmakethemuphavetheirelectronstightlyboundtothecentrallongchainandsidegroupsthrough‘covalent’bonding.Covalentbondingmakesitmuchmoredifficultformostconventionalpolymerstosupportthemovementofelectronsandthereforetheyactasinsulators.PolarandNon-PolarPlasticsNotallpolymersbehavethesamewhensubjectedtovoltageandplasticscanbeclassifiedas‘polar’or‘non-polar’todescribetheirvariationsinbehavior.Thepolarplasticsdonothaveafullycovalentbondandthereisaslightimbalanceintheelectronicchargeofthemolecule.Asimpleexampleofthistypeof1Actuallytherearesomepolymersthatconductelectricityverywellandtheseareperhapsevenmoreexcitingintermsofwhatthefuturemayholdforplastics.These‘conductive’polymersarebeingdevelopedforapplicationsasdiverseasprintedelectronics,newdisplaytechnologies,flexibleelectronicsandphotovoltaicproducts.Excitingstuffthatwehopetolookatinthefuturebutwe’lldealwiththebasicsfirst.Page1of7Copyright©2005ZeusIndustrialProducts,Inc.Page2of7Copyright©2005ZeusIndustrialProducts,Inc.behaviorwouldbethatofthewatermolecule(H2O).Theconventionalrepresentationofthemoleculeisthatshownatright.Thetwohydrogenatomsareattachedtotheoxygenatomandtheoverallmoleculehasnocharge.Inreality,theelectronstendtobearoundtheoxygenatommorethanaroundthehydrogenatomsandthisgivestheoxygenaslightlynegativechargeandthehydrogenatomsaslightlypositivecharge.Thisisshowninthediagramatrightwherethegreyareasshowwheretheelectronsaremoreoftenfound.Theoverallwatermoleculeisneutralanddoesnotcarryachargebuttheimbalanceoftheelectronscreatesa‘polar’molecule.This‘polardipole’willmoveinthepresenceofanelectricfieldandattempttolineupwiththeelectricfieldinmuchthesamewayasacompassneedleattemptstolineupwiththeearth’smagneticfield.Inpolarplastics,dipolesarecreatedbyanimbalanceinthedistributionofelectronsandinthepresenceofanelectricfieldthedipoleswillattempttomovetoalignwiththefield.Thiswillcreate‘dipolepolarization’ofthematerialandbecausemovementofthedipolesisinvolvedthereisatimeelementtothemovement.ExamplesofpolarplasticsarePMMA,PVC,PA(Nylon),PCandthesematerialstendtobeonlymoderatelygoodasinsulators.Thenon-polarplasticsaretrulycovalentandgenerallyhavesymmetricalmolecules.Inthesematerialstherearenopolardipolespresentandtheapplicationofanelectricfielddoesnottrytoalignanydipoles.Theelectricfielddoes,however,movetheelectronsslightlyinthedirectionoftheelectricfieldtocreate‘electronpolarization’,inthiscasetheonlymovementisthatofelectronsandthisiseffectivelyinstantaneous.Examplesofnon-polarplasticsarePTFE(andmanyotherfluoropolymers),PE,PPandPSandthesematerialstendtohavehighresistivitiesandlowdielectricconstants.Thestructureofthepolymerdeterminesifitispolarornon-polarandthisdeterminesmanyofthedielectricpropertiesoftheplastic.MeasurementsofElectricalPropertiesDielectricConstant(alternatingcurrent)Thedielectricconstantisameasureoftheinfluenceofaparticulardielectriconthecapacitanceofacondenser.Itmeasureshowwellamaterialseparatestheplatesinacapacitorandisdefinedastheratioofthecapacitanceofasetofelectrodeswiththedielectricmaterialbetweenthemtothecapacitanceofthesameelectrodeswithavacuumbetweenthem.Thedielectricconstantforavacuumis1andforallothermaterialsitisgreaterthan1.Page3of7Copyright©2005ZeusIndustrialProducts,Inc.+Current-++++++++-------VoltageCondenseronly+Current-++++++++--------------++++++++VoltageCondenserwithdielectricinsertedForpolarplasticsthealternatingcurrentfrequencyisanimportantfactorbecauseofthetimetakentoalignthepolardipoles.Atverylowfrequenciesthedipoleshavesufficienttimetoalignwiththefieldbeforeitchangesdirectionandthedielectricconstantishigh.Atveryhighfrequenciesthedipolesdonothavetimetoalignbeforethefieldchangesdirectionandthedielectricconstantislower.Atintermediatefrequenciesthedipolesmovebuthavenotcompletedtheirmovementbeforethefieldchangesdirectionandtheymustrealignwiththechangedfield.Polarplasticsatlowfrequencies(60Hz)generallyhavedielectricconstantsofbetween3and9andathighfrequencies(106Hz)generallyhavedielectricconstantsofb