| Polymer dielectric materials are widely used in flexible electronic devices,pulsed power devices and electric power systems owing to their merits of excellent flexibility,light weight,good formability and low cost.To enhance the dielectric constant of polymer dielectric materials,incorporating ceramic fillers with intrinsically high dielectric constant into the polymer matrix is proved to be an effective approach.However,to obtain a high dielectric constant,a large adding amount(>50 vol%)of ceramic fillers is usually required,which would lead to the deterioration of flexibility and mechanical properties.Therefore,it is a worth-studying problem to achieve high dielectric properties with low filler content.In this thesis,various kinds of composite fillers were fabricated.Their influences on the properties of polymer matrix were studied and the mechanism was revealed.By combining microstructure design of fillers with macrostructure assembly of composite films,the comprehensive dielectric properties of polymer composites were synergistically improved.The main contents and conclusions of this thesis are as follows:(1)One-dimensional Ni@PDA@BT NWs were fabricated via polymerization of dopamine and reduction of nickel chloride.The nanowires were then used as fillers for P(VDF-HFP)matrix and Ni@PDA@BT NWs/P(VDF-HFP)composites were prepared,The crystallization process of P(VDF-HFP)matrix is hindered by Ni@PDA@BT NWs and the crystallization temperature(Tc)and crystallinity(?c)of the composite films are decreased.Due to synergistic influences of interfacial polarization effect and microcapacitor effect,the dielectric properties of composite films are largely enhanced and they can be improved by increasing the loading amount of Ni nanoparticles.With 20 vol%Ni@PDA@BT NWs added,the dielectric constant of NPB2/P(VDF-HFP)composite film reaches 48.4 at 1 kHz,which is 1.6 times larger than that of 20 vol%BT NWs and 4.6 times larger than pristine P(VDF-HFP).The dielectric loss still remains at a low level(0.23).The breakdown strength decreases with the increase of filler content and the loading amount of Ni nanoparticles.(2)Satellite-core structured Fe2O3@BT nanoparticles were fabricated from ferric nitrate.The nanoparticles were then used as fillers for P(VDF-HFP)matrix and Fe2O3@BT/P(VDF-HFP)composites were prepared.Because of the hindering effect of Fe2O3@BT on the crystallization process of P(VDF-HFP)matrix,the crystallization temperature and crystallinity of the composite films decrease with the increase of filler content.The interfacial polarization effect is enhanced by the loading of Fe2O3 nanoparticles and the dielectric constant of composite films are thus improved.Besides,the satellite-core structure prevents Fe2O3 nanoparticles from directly contacting with each other,so the dielectric loss is maintained at a quite low level.With 20 vol%Fe2O3@BT nanoparticles added,the dielectric constant of Fe2O3@BT/P(VDF-HFP)composite film reaches 31.7 at 1 kHz,which is 1.8 times larger than that of 20 vol%BT and 3.0 times larger than pristine P(VDF-HFP).The dielectric loss is only 0.05.The satellite-core structured Fe2O3@BT nanoparticles impede the formation of conductive pathways and keep the breakdown strength of composite films at a high level(>150 MV/m).(3)Core-shell structured BT@Ag nanoparticles were fabricated via reduction of silver nitrate.The nanoparticles were then used as fillers for P(VDF-HFP)matrix and BT@Ag/P(VDF-HFP)composites were prepared.BT@Ag nanoparticles exhibit dual effects on the crystallization process of P(VDF-HFP)matrix.When the filler content is small,the crystallization temperature is raised due to heterogeneous nucleation;when the filler content rises,the hindering effect plays a leading role during crystallization,resulting in the decrease of Tc.Consequently,the final X,decreases because of the accumulation of hindering effect.Owing to the synergistic influences of interfacial polarization effect and microcapacitor effect,the dielectric properties of composite films are largely enhanced.With 20 vol%BT@Ag nanoparticles added,the dielectric constant of BT@Ag/P(VDF-HFP)composite film reaches 26.2 at 1 kHz,which is 1.5 times larger than that of 20 vol%BT and 2.5 times larger than pristine P(VDF-HFP).The dielectric loss is only 0.05.The breakdown strength decreases with the increase of filler content.(4)Core-shell structured BT@PDA@PANI nanoparticles were fabricated via in-situ polymerization of dopamine and aniline.The nanoparticles were then used as fillers for P(VDF-HFP)matrix and BT@PDA@PANI/P(VDF-HFP)composites were prepared.On this basis,P(VDF-HFP)-based BT-BT@PDA@PANI-BT sandwich structured composites were prepared by employing BT@PDA@PANI/P(VDF-HFP)film as central high-D layer and 3 vol%BT/P(VDF-HFP)as outer high-Eb layer.Due to synergistic influences of interfacial polarization effect and microcapacitor effect induced by BT@PDA@PANI nanoparticles,the dielectric properties of BT@PDA@PANI/P(VDF-HFP)composite films are largely enhanced.But the breakdown strength decreases because of the electric field mismatch between polymer matrix and filler particles.With 20 vol%BT@PDA@PANI nanoparticles added,the dielectric constant of BT@PDA@PANI/P(VDF-HFP)composite film reaches 53.5 at 1 kHz,which is 5.0 times larger than that of pristine P(VDF-HFP).The dielectric loss is 0.57.To further improve the dielectric performance,BT-BT@PDA@PANI-BT sandwich films are prepared.The interfaces between film layers impede the growth of conductive path,results in the enhancement of breakdown strength.The dielectric constant and breakdown strength of sandwich film is synergistically improved and the discharged energy density is also increased.The dielectric constant of S 3-20-3 sandwich film(the numbers are volume fractions of fillers in each layer)reaches 20.3 at 1 kHz,which is 1.9 times larger than that of pristine P(VDF-HFP).The dielectric loss is only 0.07 and the breakdown strength is raised to 206.0 MV/m.The discharged energy density at 140 MV/m is 2.4 J/cm3,which is 1.8 times larger than pristine P(VDF-HFP). |
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