The Fabrication And Electric Properties Of Poly(Vinylidene Fluoride)-based Ferroelectric Composites

Due to the outstanding dielectric,ferroelectric,piezoelectric and electrocaloric properties,poly（vinylidene fluoride）（PVDF）based ferroelectric polymers have been considered to have great potential for a broad range of applications,such as high charge storage capacitors,ferroelectric memory devices,sensors and solid cooling decvices.The PVDF based polymers are semi-crystalline and exhibit various crystalline phases.The microstructures of PVDF based polymers are dependent on the processing history,which will in turn affect the electric performance of PVDF based polymers.Thus,it is possible for us to tune the mircostructures and electric properties of PVDF based polymers if we can tune the processing history including thermal history,mechanical history and electric filed history.In this thesis,we aimed to tune the microstructure and optimize the energy strorage,piezoelectric,electrocaloric performance of PVDF-based polymers by tuning the fabrication conditions and employing a nanocomposites stragegy,which involved the use of carbon nanoparicles.The main results are listed as follows:1)A novel method was developed here to fabricate PVDF films with excellent piezoelectric properties.A systemtic work was carried out to study the effect of solution casting conditions and rolling conditions on the microstructure of PVDF.By the combination of solution casting and rolling methods,it was possible to fabricate PVDF films with high piezoelectric properties.The obtained maximum piezoelectric coefficient d₃₃ in PVDF films was 27 pC/N while the d₃₃ of commercialized piezoelectric PVDF films was 18-22 pC/N.2)In aiming to improve the piezoelectric properties of PVDF films,a nanocomposites stragegy which involved the employment of carbon nanotubes（MWNTs）was proposed.First,to enable the uniform dispersion of MWNTs in PVDF,MWNTs were treated by the nitrid acid through a simple hydrothermal method.Afterwards,different fabrication methods including solution casting and melting were adopted to prepare nanocomposites.The effect of fabrication methods on the microstucture and electric properties of PVDF-based nanocomposites were studied.Due to the incorporation of conductive MWNTs,the breakdown strength of MWNTs/PVDF nanocomposites was significantly reduced,thus it was impossible for the MWNTs/PVDF nanocomposites to be poled under high electric field to obtain high piezoelectric performance.In addressing this issue,a rolling strategy was proposed and a detailed work was performed to study the effect of rolling on the microstrure and electric properties of nanocomposites.It was found that the rolling process could induce the orientation of MWNTs along the rolling direction and give rise to a more dense structure,which are favorable for the improvement of breakdown strength of nanocomposites.With higher breakdown strength,the rolled films could be efficiently poled to yield a high piezoelectric coefficient up to 33 pC/N,which was 50%higher than that of pure PVDF when being poled under the same electric field.3)A novel conductor-semiconductor-insulator model was designed and developed to address the issue of low breakdown strength in conductor/insulator nanocomposites represented by MWNTs/PVDF.By a simple hydrothermal process,the MWNTs were coated with a continuous TiO₂ layer,forming a core-shell structured nanoparticle MWNTs@TiO₂.The TiO₂ can prevent the MWNTs from direct contact in PVDF and act as the buffer layer to lower the electric field concentrated inside the polymer matrix,which is beneficial for the enhancement of breakdown strength in nanocomposites.Meanwhile,the addition of MWNTs@TiO₂ can also induce significantly improved dielectric properties.With the enhancement of breakdown strength,the MWNTs@TiO₂/PVDF nanocomposites can be applied under high electric field,resulting in greatly improved energy storage（discharged energy density 6.5 J/cm³）,ferroelectric and piezoelectric performance(d₃₃ 39 pC/N).4)To study the electrocaloric effect（ECE）in PVDF-based polymers,the relaxor ferroelectric poly（vinylidene fluoride-trifluoriethylene-cholofluoroethylene）terpolymer（P（VDF-TrFE-CFE））was investigated here.As a promising electrocaloric materials,the P（VDF-TrFE-CFE）has been demonstrated to exhibit giant ECE.However,a rather high electric field was required to induce the giant ECE performance of P（VDF-TrFE-CFE）,which largely limited its practical application.In order to enhance the ECE performance of P（VDF-TrFE-CFE）under low electric fields,a percolative nanocomposites which utilized the graphene was adopted here.Making use of the conductivity and the interface effect induced by graphene,the ECE performance of P（VDF-TrFE-CFE）based nanocomposites can be dramatically increased under low electric field,i.e.a high temperature changeΔT of 5.2 K and entropy changeΔS of 24.8 Jkg^-1K^-11 can be obtained under 40 MV/m.Moreover,a simple and cost-effective method was developed to fabricate graphene/P（VDF-TrFE-CFE）nanocomposites with giant ECE(i.e.a highΔT of 4 K andΔS of 19.4 Jkg^-1K^-11 under 50 MV/m).Instead of incorporating graphene into graphene directly,graphene oxide nanoparticles are used as the precursor and they could be partly reduced to graphene during the preparation of nanocomposties,which enabled the uniform dispersion of nanofillers and eliminated the needs for a complex reduction and chemical modifications route.