Study On Preparation And Properties Of Polymer-based Barium Titanate Nanocomposites With High Dielectric Constant

The polymer composites with high dielectric constant are important for the electronic，powder， and energy storage industries and they can be widely used in the embeddedthin-film capacitors， electric stress control and energy storage devices. The fabrication ofthese new materials has attracted intensive interest over the world， because thedevelopment of high dielectric materials could improve the performance of equipments inindustries，even in the weapons for the national security. Although a lot of effort has beenpaid to the new materials and some developments have been achieved， there are stillchallenges in the fabrication of desirable polymer composites with high dielectric constant.In this thesis， the influence of interface structure on the dielectric properties and thepolarization mechanism in the polymer nanocomposites were studied in order to fabricatebarium titanate(BT)/polymer nanocomposites with high dielectric constant and lowdielectric loss. Special interface structures were designed and prepared via chemicalreactions.Firstly， in order to increase the dielectric constant， the BT nanoparticles werecoated with hyperbranched polyamide(HBP) which possess relatively high electricconductivity and dielectric constant. Then the coated BT nanoparticles were blended withpoly（vinylidene fluoride-trifluoroethylene-chlorofluoroethylene）(PVDF-TrFE-CFE) tofabricate BT@HBP/PVDF-TrFE-CFE with superhigh dielectric constant. The FTIR，1HNMR， TGA， TEM， XPS and dynamic light scattering were used to confirm thesuccessful graft of HBP onto the BT nanoparticle surface. The dielectric responses werecarefully studied by broadband dielectric spectrum and the results show that the dielectricconstant of polymer nanocomposite increases quickly as the BT loading increases. The coating of BT with HBP can increase the electric conductivity of the polymernanocomposites and then enhance the interfacial polarization and increase the dielectricconstant largely. For example， when the BT loading is40vol%， the dielectric constantof BT@HBP-40at1kHz is1485.5. In addition， the dielectric constant of BT@HBP/PVDF-TrFE-CFE nanocomposites is much higher when compared with the BT/PVDF-TrFE-CFE nanocomposites prepared via traditional solution-blend method.Secondly， we used three methods to reduce the dielectric loss of polymernanocomposites. One is grafting the PMMA chains to the BT nanoparticle surface viacovalent bond controllably by the in situ surface-initiated ATRP method， forming stronginterface interaction to improve the dispersion of BT nanoparticles. The dielectricmeasurement results show that the dielectric constant of BT@PMMA nanocompositesincreases as the BT loading increases， while the dielectric loss stays at a low level as thepure PMMA. Compared with the traditional solution-blend BT/PMMA nanocomposites，the BT@PMMA nanocomposites possess not only low dielectric loss， but also the higherthermal conductivity. The other is grafting the functional PVDF-HFP to the BTnanoparticles treated with aminopropyl triethoxysilane. The reaction between the BT-APSnanoparticles and PVDF-HFP-GMA enhances the interface interaction and improve theBT nanoparticle dispersion， regardless of the poor compatibility between them. Theintroduction of BT nanoparticles to PVDF-HFP-GMA will increase the dielectric constant，energy storage density and thermal conductivity，while reduce the dielectric loss. Anotheris decorating the BT nanoparticles with nano-Ag particles， forming the core-satelliteAg@BT nanoassemblies. The coulomb blockade effect of nano-Ag particles will hinderthe movement of charges， reduce the electric current and dielectric loss， and improvethe breakdown strength.Thirdly， based on the above study， we prepared core@double-shell structuredpolymer nanocomposites with high dielectric constant and low dielectric loss. The core isBT nanoparticle， the first shell is HBP and the second shell is PMMA. The core， thefirst and second shell are connected by covalent bond. The HBP with high dielectric constant and high electric conductivity can enhance the interfacial polarization andincrease the dielectric constant. The PMMA with low dielectric constant and low electricconductivity can hinder the movement of charges， reduce the electric current anddielectric loss. Compared with solution-blend BT/PMMA nanocomposites， theBT@HBP@PMMA nanocomposites have higher dielectric constant and energy storagedensity， lower dielectric loss.