Mechanism Study Of Functional Fillers Influence On Microstructure And Dielectric Properties In Polyvinylidene Fluoride-Matrix Composites

The rapid development of the electronics industry needs high dielectricconstant materials to meet the needs of electronic components with highperformance and miniaturization. Ceramic materials have high dielectricconstant, but its high process energy consumption, complex molding processand large density limit their applications. Many kinds of polymer materialsusually have high dielectric breakdown strength and they are easy to process,but their dielectric constant is small. As a result, researchers hope to fabricatea kind of polymer based composites to meet the demand of the practicalapplication. Introducing inorganic particles with high dielectric constant orconductive particles into the polymer matrix is the two most important way torealize the polymer matrix high-dielectric-constant composites. Manyresearchs have proved it is difficult to realize excellent dielectric properties byjust merely change the kinds of filler and polymer matrix in the composites.Chang the microstructure of the composite is a promising way to improve thedielectric properties of the composites. Better understand the relationship between the microstructure and the dielectric properties is badly needed. Inthis thesis, polyvinylidene fluoride (PVDF) is employed as the polymer matrixbecause of its high dielectric constant. Conductive particles and inorganicparticles with its surface be modified were introduced into the polymer matrixto prepare the composites. This study focuses on the influence of interface andmicrostructure on the dielectric constant in the composite. The specificinformation is given as follows:(1) Hydroxylated the surface of barium titanate (BT) particles andfabricated the BT/PVDF composite by solution method. Results showcompared with the composites filled with crude BT, composites filled withmodified BT particles have less void observed in SEM image of the fracturedcross section. At same time, the loss tangent peak of the composites filled withmodified BT move to a high temperature. Both above phenomenons provesthe improved the compatibility between the BT particles and PVDF matrix.Moreover, the improved junction between the filler and polymer matrix endowthe composites with weaker frequency dependence and temperaturedependence, and the loss tangent of the composites also decreased. It isbelieved the formation of the hydrogen bonds between the BT particles are thereasons for the changes.(2) Multi-walled carbon nanotubes (MWNTs) with emeraldine base (EB)coating on its surface was realized by in situ polymerization. This filler, denoted as MEB, have a core-shell structure. Compared with MWNTs/PVDFcomposites, MEB/PVDF composites could achieved lower loss factor,smallerconductivity and larger dielectric constant. It is believed the existing ofinsulation shell on the surface of the MWNTs is the reason for the improvedthe dielectric properties of the composites. Raman spectra proved theinteraction between the PVDF molecular and the fillers. The interaction lowerthe mobility of the PVDF molecular and weaker the temperature dependenceof the dielectric constant of the composites(3) In order to low the percolation threshold, reduced graphene oxide(RGO), a kind of two-dimension nano fillers, was empolyed as the filler. Wesuccessfully coated reduced the graphene oxide (RGO) with EB, and the fillerwas denoted as REB. The water contact angle and the sedimentation test showthe REB could well dispersed in the solution and have better compatibilitywith the PVDF matrix. The dielectric properties shows the REB/PVDFcomposites have a lower percolation threshold than that of the RGO/PVDFcomposites.(4) BT and REB fillers were introduced into the polymer matrix together.Results show increasing the filer consentration could increase the dielectricconstant of the composites. The addition of BT particles within the compositescould lead the percolation phenomenon in the composites. It is believed thereis synergistic effect show in the composites. Compared the REB/PVDF composites, BT-REB/PVDF have a larger dielectric constants.