Preparation And Research Of Energy Storage Properties Of Ba(Fe_0.5Ta_0.5)O₃/PVDF Composites

With the rapid development of electronic devices to an aim toward miniaturization,multifunction,high energy storage and portability,it is an enormous challenge for researchers to develop excellent dielectric property,processability and machinability.In this situation,ceramic/polymer nanocomposites have emerged as time has required.Utilizing the properties of both,polymers exhibit high breakdown strength and processability,while the ceramics have a high dielectric constant.In this thesis,nano-sized Ba(Fe_0.5Ta_0.5)O₃（BFT）ceramics with giant dielectric constant,broad temperature and frequency interval,are an excellent candidate of the filler in high dielectric constant ceramic/polymer nanocomposites.Semicrystalline poly（vinylidene fluoride）（PVDF）with high breakdown strength is employing as the polymer host.According to different preparation methods,we hope to develop of a new type of polymer matrix nanocomposites with high energy storage density under low electric field.The main researching content is as follows.BFT/PVDF bulk nanocomposites are fabricated via a physical mixing method followed by heat treatment.The BFT particles in nanocomposites exhibit excellent dispersion and compatibility.The enhanced dielectric constant and loss have been obtained,which is attributed to the giant dielectric constant of BFT and the enhanced interfacial polarization.An energy storage density up to 2.18J/cm³ is achieved in the nanocomposites with a BFT filler content of 2 vol%at a low electric field of 50 MV/m.This value of energy storage density at the same electric field is much higher than other polymer-based nanocomposites.BFT/PVDF flexible nanocomposite films are fabricated by tape casting using dopamine（DA）modified BFT nanopowders and PVDF as a matrix polymer.After a surface modification of installing a DA layer with a thickness of5 nm,the interfacial couple interaction between BFT and PVDF is enhanced,resulting in less hole defects at the interface.Then the dielectric constant（ε′）,loss tangent（tanδ）,and AC conductivity of nanocomposite films are reduced.Meanwhile,the value of the reduced dielectric constant（Δε′）and the strength of interfacial polarization（k）are introduced to illustrate the effect of DA on the dielectric behavior of nanocomposite films.Δε′can be used to calculate the magnitude of interfacial polarization,and the strength of the dielectric constant contributed by the interface can be expressed as k.Most importantly,the energy storage density and energy storage efficiency of nanocomposite films with a small BFT@DA filler content of 1 vol%at a low electric field of 150 MV/m are enhanced by about 15%and 120%,respectively,after DA modification.The high energy storage density of 1.81 J/cm³ is obtained in the sample.This value is much larger than the reported polymer-based nanocomposite films.In addition,the outstanding cycle and bending stability of the nanocomposite films make it a promising candidate for future flexible portable energy devices.Sandwich-structured BFT@DA/PVDF-PVDF-BFT@DA/PVDF flexible nanocomposite films are fabricated by tape casting method.According to SEM image,the thickness of the film is about 15μm and the each layer is about 5μm.In comparison with single layer BFT@DA/PVDF nanocomposite films,sandwich-structured nanocomposite films exhibit a high dielectric constant,low loss tangent and low AC conductivity.The high dielectric constant is attributed to the enhanced interfacial polarization between out layer and inner layer.The low loss tangent and low AC conductivity are due to avoid the formation of conductive paths by PVDF layer.The energy storage density of sandwich-structured nanocomposite films with a small filler content of 1 vol%at a low electric field of 150 MV/m is 1.93 J/cm³.The maximal energy storage density up to 4.87 J/cm³ is also achieved under the breakdown strength of 250MV/m in the sample.The sandwich-structured BFT@DA/PVDF nanocomposite films with excellent energy storage performance in this work can be applied in future portable applications.