Fabrication And Performance Study Of Cascaded Dielectric Capacitors

Capacitors are critical components in devices for storing,filtering,controlling,and manipulating of electric charges.A good dielectric capacitor material should have the following properties:high breakdown electric field,low loss,light weight,fast charge and discharge speed,and low cost.However,the current popular film capacitors have problems such as low dielectric constant and low energy storage density.It is urgent to prepare dielectric capacitors with high energy storage density.poly?vinylidene fluoride?and its copolymers have been proven to have the highest dielectric constant and energy density,which are a typical class of ferroelectric polymer materials.In recent years,new structures such as double-layer structures and sandwich structures have been expected to be used in the field of capacitors,because it can make full use of the advantages of different materials.In this paper,polyvinylidene fluoride copolymer with excellent physical and chemical properties was selected as the base material to prepare composite film by mixing ceramic particles.Then,the structure of the film is improved to prepare a sandwich structure dielectric capacitor,which further improves the energy storage performance of the dielectric film.The main contents are as follows:In this thesis,microstructure and energy storage characteristics of poly?vinylidene fluoride??PVDF?,poly?vinylidene fluoride-chlorotrifluoroethylene??P?VDF-CTFE??,poly?vinylidene fluoride-hexafluoropropylene?P?VDF-HFP?and poly?vinylidene fluoride-trifluoroethylene chlorotrifluoroethylene??P?VDF-TrFE-CTFE??were studied systematically.It is found that P?VDF-TrFE-CTFE?has the highest dielectric constant at low electric field frequency,and the dielectric constant decreases clearly with the increase of electric field frequency,and the loss increases rapidly with the increase of frequency.In addition,for P?VDF-CTFE?,a polymer film crystallized with a high concentration solution has higher charge and discharge efficiency,dielectric constant and breakdown field strength than a polymer film crystallized with a low concentration solution,which is more suitable for application in the field of dielectric capacitors.In order to study the effect of crystal phase structure on the properties of dielectric films,two kinds of P?VDF-CTFE?films were prepared at a low drying temperature?60°C?and at a high drying temperature?200°C?.The result shows that the dielectric film drying at 60°C is mainly formed by?-phase,and the high temperature annealing at 200°C promotes the transformation of the?-phase to the?-phase,which improves the dielectric properties of the film.In order to combine the high dielectric constant of ceramic materials and the high breakdown field strength of polymer materials,composite films with different content of BaTiO₃ were prepared by using P?VDF-CTFE?polymer as the base material.It is found that the dielectric constant of the composite film increases gradually with the increase of BaTiO₃ content.When the content of BaTiO₃ is 40 wt.%,the dielectric constant is 22.45,which is three times as large as P?VDF-CTFE?polymer.At the same time,the mixing of ceramic fillers introduces defects into the composite film.And,the poor interfacial compatibility between ceramic particles and polymer materials makes the breakdown field strength of the composite film significantly lower.In view of the above problems,the surface of BaTiO₃ was modified by dopamine,which show that the dielectric properties of the surface-modified BaTiO₃ composite film are improved.In the first two parts,the effects of processing technology,concentration and surface modification on the performance of dielectric capacitors are analyzed.In order to further improve the energy storage density of dielectric capacitors,this paper designs and studies the effects of sandwich structures on the dielectric properties of thin films.Two different types of sandwich film were prepared by using P?VDF-CTFE?film which has excellent dielectric properties and BaTiO₃ nanoparticles which has high dielectric constant.In the end,this thesis analysised the advantages and disadvantages of hot pressing processes and casting processes.It is found that the sandwich structure film prepared by the casting method has better dielectric properties than the single layer film.The breakdown field strength of the sandwich structure film is about 30%higher than that of the composite film,and the charge and discharge efficiency is higher than 83%.