Design,Preparation And Dielectric Properties Of TiO₂/Polymer Composites

Composites containing inorganic fillers embedded in polymer matrices have brought about considerable attention.Recent contributions have shown their potential applications in integrated capacitors,energy harvesting/storage devices and flexible displays due to their inherent advantages of good dielectric properties,light-weight,and easy processing.Among the class of inorganic fillers,titanium dioxide,which features advantages such as moderate dielectric constant,corrosion stability and environmental benignity,has captured considerable attention.However,a high volume fraction of fillers is usually required to achieve a high dielectric permittivity,which would inevitably deteriorate the flexibility of the composite film.In this thesis,we designed and synthesized various interface structures.The methods of controllable preparation of self-assembled nanostructures with different morphologies and sizes were explored.Four series of TiO2-based polymer composites with excellent dielectric properties at low filler contents were successfully fabricated.(1)In order to improve the the dielectric constant by enhancing Maxwell-Wagner-Sillars polarization,flower-like TiO2 particles with sophisticated morphology were designed and prepared via a facile solvothermal process.The results reveal that flower-like TiO2 particles(F-TiO2)are more effective in increasing the properties of P(VDF-HFP)when compared with commercial TiO2.The improvement could be attributed to the enhancement of MWS polarization because there are more interaction zones at the interfaces of F-TiO2 and the polymer matrix.Typically,the dielectric constant of the P(VDF-HFP)composite filled with 20 vol%flower-like TiO2 reaches 83.1 at 100 Hz,in contrast to 43.4 for the composite filled with 20 vol%commercial Ti02 and 11.3 for pristine P(VDF-HFP).Also,the flower-like TiO2-filled composites exhibit similar characteristic breakdown strengths to their commercial TiO2-filled counterparts.(2)Carbon was introduced into the three-dimensional flower-like TiO2.TiO2/carbon(TiO2/C)nanostructures were in-situ synthesized via a solvothermal method involving calcination of organic precursor under inert atmosphere.The results reveal that loading the fillers with a small amount of carbon is an effective way to improve the dielectric constant and suppress the dielectric loss.In addition,TiO2/C particles with higher carbon contents exhibit superiority in enhancing the dielectric constants of composites in comparison with their noncarbon counterparts.Typically,the highest dielectric constant(330.6)of the TiO2/C composites is 10 times over that of noncarbon-TiO2-filled ones at the same filler volume fraction,and 32 times over that of pristine P(VDF-HFP).The promotion in the dielectric constant can be attributed to MWS polarization and the formation of a large network,which is composed of local micro-capacitors with carbon particles as electrodes and TiO2 as the dielectric in between.(3)In order to inhibit the direct contact among conductive fillers and lower the percolation threshold,Ag@TiO2 core-shell nanowires were prepared via a solvothermal method.Due to the combined effect of different mechanisms(i.e.micro-capacitors and Maxwell-Wagner-Sillars polarization),the composites filled with Ag@TiO2 nanowires achieve high dielectric constants at low volume fractions of fillers.For instance,the dielectric constants of composites filled with 3.5 vol.%and 5.7 vol.%Ag@TiO2 nanowires are 1.5 and 2 times over that of pristine P(VDF-HFP),respectively.In addition,the insulating TiO2 shells isolate Ag cores from each other,resulting in suppressed dielectric losses and low conductivities.(4)Semiconductive SnO2 nanoparticles were in-situ synthesized onto the surface of TiO2 nanowires via a precipitation method.Due to the combination of quantum size effect and coulomb blockade effect,a small amount of SnO2 nanoparticles can inhibit the the transport of carriers.As a consequence,the composites exhibit improved breakdown strengths/energy storage behaviors and suppressed dielectric losses.With increasing SnO2 loading,the carriers can transport among nanoparticles by tunneling effect.Moreover,the interfacial polarization is enhanced,leading to the significant improvement in the dielectric constant.Typically,the composites can achieve a high dielectric constant(133.4)at a low filler content(10 vol.%).In addition,the existence of TiO2 nanowires contributes to the homogeneous dispersion of SnO2 nanoparticles,thus endowing the composites with excellent insulating property.