Preparation And Properties Of The Polyimide Composite Films With High Dielectric Constant

Dielectric capacitors with high energy storage density and fast charge-discharge response have attracted the interest of many researchers in recent years.As an important energy storage component,capacitors have important applications in the electronic industry,active control system,aerospace,medical equipment,national defense and military fields.Ceramic capacitors usually have high dielectric constant,high dielectric loss and low breakdown field strength.They are difficult to be machined and miniaturized.Polymer capacitors are more applicable in high electric fields because of their high electric breakdown field,low dielectric loss,flexibility and easy processing,but the dielectric constant of polymer is generally low.Therefore,combining the advantages of ceramic dielectric and polymer dielectric,the preparation of polymer based composite films with high dielectric constant,low dielectric loss and high energy storage density has significant application prospect.Polyimide is one of the polymer with the best comprehensive properties at present.Its excellent mechanical properties,good corrosion resistance,simple production process and large-scale industrial production advantages make it widely used in the high-technology field of microelectronics and aerospace.Based on polyimide,the effects of microstructure of various nanofillers,organic-inorganic surface modification and the structure of composite films on dielectric properties and energy storage properties were studied.The main research contents of this paper are as follows:1.The amino modified carbon nanotubes?MWCNTs-NH₂?were obtained by two-step amination,and the MWCNTs coated with polyimide nanofillers?MWCNTs@PI?were prepared by precipitation method.The effect of mass ratio of PI and MWCNTs in MWCNTs@PI on the dielectric properties of PI composite films was studied.As the mass ratio of PI and MWCNTs was 1.5:1,PI composite films exhibit the best dielectric properties with both high dielectric constant and low dielectric loss,and then the effect of the content of MWCNTs in MWCNTs@PI on the dielectric properties of PI composite films was investigated.As the mass fraction of MWCNTs is 15 wt%,the dielectric constant of the composite film is 31.2 at 1 kHz and the dielectric loss is only0.042.2.The BaTiO₃?BT?nanofibers were prepared through electrostatic spinning method,and the BT nanofibers were modified by polydopamine.The effects of the content of BT nanofibers in the upper and lower layers and the content of PI in the middle layer on the dielectric properties and the breakdown strength of the composite films were respectively studied.As the content of BT nanofibers in the upper and lower layers is30 vol%and the intermediate layer is one layer of pristine polyimide,the dielectric constant of the sandwich BT/PI composite film is 14.5,the dielectric loss is only 0.0191 kHz and the breakdown field strength is 204 kV/mm with the maximum energy storage density of 2.62 J/cm³.3.The CaCu₃Ti₄O₁₂@TiO₂?CCTO@TiO₂?nanofibers were prepared by coaxial electrostatic spinning.Compared with CCTO,TiO₂ has a lower dielectric constant.Therefore,the CCTO nanofibers coated with TiO₂ can effectively alleviate the local electric field concentration caused by the large difference in dielectric constant between the ceramic CCTO and PI matrix.The CCTO@TiO₂/PI composite films were prepared through solution casting method.As the content of CCTO@TiO₂ nanofibers is 5 vol%,the dielectric constant of CCTO@TiO₂/PI composite film increases to 5.85,and the breakdown strength increases to 236 kV/mm.As the content of CCTO@TiO₂ is 1 vol%,its energy density is 1.60 J/cm³,which is 14%higher than that of pristine polyimide?1.42 J/cm³?.Meanwhile,all the CCTO@TiO₂/PI composite films show good thermal stability and have no mass loss below 500 ^oC.4.The BT nanofibers were prepared through electrostatic spinning method,and then BT@SiO₂ nanofibers were synthesized by co-precipitation method.The SiO₂ has low dielectric constant and very low dielectric loss.It acts as a dielectric buffer layer between BT and PI.It can inhibit the charge transfer and improve the electric field distribution.As the content of BT@SiO₂ is 5 vol%,the dielectric constant and dielectric loss of BT@SiO₂/PI composite film are 5.05 and 0.015,respectively.As the content of BT@SiO₂ nanofibers is 3 vol%,the breakdown strength of BT@SiO₂/PI composite film is 348 kV/mm,which represents a 12%increment compared with the pristine PI.The PI nanocomposite film exhibits a maximal energy density of 2.31 J/cm³ under the field of 346 kV/mm,which is 62%over the pristine PI?1.42 J/cm³?.The BT@SiO₂/PI composite films have no mass loss below 500 ^oC and show good thermal stability.5.The BT@ZrO₂ nanofibers were prepared through coaxial electrostatic spinning,and BT@ZrO₂/PI composite films were prepared by solution casting.Their dielectric properties,maximum energy storage density and high temperature resistance were studied.The results show that as the content of BT@ZrO₂ is 5 vol%,the dielectric constant and dielectric loss of the BT@ZrO₂/PI composite film are 5.55 and 0.0231,respectively.With the increase of BT@ZrO₂ nanofibers,the breakdown strength and the energy storage density of BT@ZrO₂/PI composite films have been improved.As the content of BT@ZrO₂ nanofibers is 2 vol%,the breakdown strength of BT@ZrO₂/PI composite film is 361 kV/mm,which represents a 17%increment as compared with that of the pristine PI.The energy density of BT@ZrO₂/PI composite film is 2.53 J/cm³,which is 78%higher than that of the pristine PI?1.42 J/cm³?.The electric field distribution of BT/PI composite film and BT@ZrO₂/PI composite film was simulated by COMSOL respectively.The results show that the electric field distribution of BT@ZrO₂/PI composite film is much uniform than that of BT/PI composite film.The ZrO₂ layer could effectively improve the electric field distribution between the PI matrix and BT nanofiber,which is beneficial to improve the breakdown strength of the composite film.