Comprehensive Optimization For Service Performance Of PVDF Composite Dielectric Materials

Dielectric capacitors have outstanding advantages in the pulse energy storage scenario requering extremely high power and ultra-fast response.The continuous development of dielectric capacitors has put forward new requirements for the service performance of dielectric materials.Polymer dielectric has high breakdown field strength,light weight,good machining performance and high chemical tolerance,but the dielectric constant of polymer is usually very low,which greatly limits the improvement of energy storage density.In addition,the intrinsic thermal conductivity of the polymer is very low,and a large amount of heat will accumulate inside the polymer in the high frequency and high energy charge/discharge process,which will affect the normal operation of the capacitor.Based on this,using poly(vinylidene fluoride)(PVDF)as matrix,2D transition metal carbon compounds(MXene-Ti3G2Tx)and boron nitride nanosheets(BNNSs)were doped respectively to improve the dielectric property and thermal conductivity of PVDF in this thesis.And molecular simulation was used to reveal the underlying mechanism of material performance improvement.Finally,through the arrangement of multilayer structure,PVDF/MXene and PVDF/BNNSs were used to prepare three kinds of multilayer films including film with interlayer arrangement(IPIP),film with polarized surface(PIIP)and film with insulating surface(IPPI).Based on AHP-TOPSIS multi-objective decision analysis method,the service performance of different multilayer films was comprehensively evaluated to screen out the optimal design scheme of PVDF composite dielectric materials.The main research results are as follows:(1)MXene can significantly improve the dielectric constant of PVDF matrix and reduce its dielectric loss to a certain extent.The dielectric constant of composites increased with the increase of MXene content while the composite system showed very high dielectric loss at low frequency band when the content of MXene was too high.The relative dielectric constant of PVDF/MXene-1.5wt%system can reach 14.57 at 100Hz,which is 56.43%higher than that of pure PVDF.MXene with no surface reactive groups had the most significant improvement on the dielectric constant of PVDF.The presence of surface reactive groups on MXene can inhibit the improvement of the dielectric constant of the composite system to a certain extent.The narrow band gap of MXene leads to the increase of the conductivity loss of the composite when it is doped in PVDF.However,Mxene has a certain agglomeration effect on the long-chain molecules of PVDF,which reduced the polarization loss of PVDF.Moreover,the existence of surface reactive groups can further amplify this loss inhibition effect.In addition,MXene doping can also improve the thermal conductivity and mechanical properties of the composite system to a certain extent,but reduced the breakdown field strength of the composite systems.(2)The doping of BNNSs can effectively improve the thermal conductivity and thermal stability of the composite material.When the filling ratio of BNNSs reached 15wt%,the thermal conductivity of the composite system can be improved by 219.01%compared with that of pure PVDF.Meanwhile,BNNSs improved the initial decomposition temperature of PVDF matrix,significantly inhibiting the decomposition of PVDF at high temperature.BNNSs has a high aspect ratio and surface energy,which can not only form thermal conduction pathway inside PVDF but enhance the binding strength of PVDF matrix,thus the thermal conductivity of PVDF can be improved.At the same time,the presence of BNNSs improved the reaction stability of PVDF molecules and inhibited the diffusion of O2 in the PVDF matrix,which inhibited the self-pyrolysis and oxidative pyrolysis of PVDF.Doping BNNSs properly can improve the insulating and mechanical properties of the composite materials.The breakdown field strength of the composite system increased to a certain extent when the BNNSs filling ratio was low,and BNNSs can significantly improve the tensile strength and elastic modulus of PVDF matrix.However,with the increase of filler content,the defects in the composite system increased and the mechanical/insulating properties of the composites began to decline.(3)The multilayer-structure can make the superior properties of each single layer cooperate with each other,and finally improve the comprehensive performance of the multilayer film.Due to the interlayer polarization,the dielectric properties of the multilayer composite films were significantly improved.The dielectric constant of IPIP film was up to 14.71 at 100Hz,which was 63.24%higher than that of pure PVDF.Due to the coordination of MXene and BNNSs,the thermal conductivity of the multilayer films was improved significantly.Compared with pure PVDF,the thermal conductivity of PIIP system was improved by 91.45%.When PVDF/MXene was arranged side-byside with PVDF/BNNSs,the presence of BNNSs will form a structure similar to"barrier" on one side of MXene,reducing the formation probability of discharge channel,and thereby slightly increasing the breakdown field strength of multilayer thin films.Compared with pure PVDF,the discharge efficiency and maximum discharge density of multilayer films were significantly improved.The tensile strength of the multilayer films was improved but the elastic modulus declined to some extent.Finally,through quantitative evaluation,the comprehensive performance of IPPI multilayer film and the optimal solution has the highest fitting degree,about 0.87,indicating that it has the best comprehensive performance.In this thesis,based on experimental means and molecular simulation technology,the macro-performance evaluation and micro-mechanism analysis were combined to achieve the improvement of various properties of polymer-based dielectrics.A comprehensive evaluation system of dielectric material service performance was constructed.Finally,the whole process of experimental modification,test characterization,performance improvement mechanism analysis and comprehensive performance optimization evaluation of dielectric materials was realized,which provided a new idea for further research and development of high-performance polymer-based dielectric materials.