Structural Design And High Temperature Energy Storage Characteristics Of Polycarbonate Composite Dielectric

As an energy storage device with high power density,high reliability and high operating voltage,dielectric capacitor is an important device in high power energy storage systems such as new energy vehicles and advanced electromagnetic weapons,which requires its stable operation in an environment far higher than room temperature.In the high temperature environment,due to a large number of carrier injection,the conduction loss increases,heat generation increases,and a vicious cycle occurs,which seriously affects the energy storage characteristics of dielectrics.This can be attributed to the deterioration of the insulation performance of the energy storage dielectric at high temperatures.In addition,high temperature resistant polymers tend to be paraelectric dielectrics with low relative permittivity,which also limits the possibility of obtaining excellent energy storage density.To solve the above problems,high temperature resistant polymer polycarbonate(PC)is used as research matrix,and a series of structural designs are carried out to improve the dielectric properties and high temperature insulation of energy storage dielectric,so as to obtain an ideal high temperature energy storage characteristics.In order to solve the problems of low relative permittivity of paramagnetic polymers such as PC and poor compatibility of organic-inorganic materials,polyvinylidene fluoride(PVDF),a ferroelectric polymer with high relative permittivity,is introduced and synthesized with PC to prepare all-organic composite dielectric with sandwich structure.PC and PVDF have different band structures,which have an important effect on the carriers injection and the interfacial charge accumulation,thus affecting the dielectric properties of composite dielectrics.In addition,due to the characteristics that the relative permittivity of PC and PVDF vary with temperature,the electric field distribution of multilayer composite dielectric can be self-regulated with temperature.By adjusting the relative position of PC and PVDF,the synergistic optimization of the polarization characteristics and dielectric strength of the composite dielectric is realized.When the PC is in contact with the electrode,the composite dielectric can withstand an electric field strength of 500 MV/m,and obtain an energy storage density of 6.35 J/cm~3,with a charge-discharge efficiency of 77.21%.Its energy storage density and charge-discharge efficiency are respectively 93.52%and 91.31%of room temperature,and have excellent temperature stability.In order to improve the high-temperature insulation of the composite dielectric,aiming at the problem of carrier injection at high-temperature and high-electric field,a sandwich structural composite dielectric(h-BN/PC/h-BN)with hexagonal boron nitride(h-BN)inorganic barrier layer on the surface of PC is prepared by using electrostatic spinning and hot pressing technology.The large band gap width of h-BN is used to increase the carrier injection barrier and suppress the carrier injection.The maximum electric field strength that the composite dielectric can withstand at 100°C is 500 MV/m,and obtaining an energy storage density of 4.4 J/cm~3 and a charge-discharge efficiency of 87.8%.By plasma enhanced chemical vapor deposition(PECVD),a nano-thick and dense inorganic barrier layer of BN or Si O₂ is grown in-situ on the surface of the PC film,and a sandwich structure composite dielectric containing an inorganic barrier layer on the surface is also obtained.The barrier with good density can effectively inhibit the carrier injection at the electrode.By studying the effects of BN and Si O₂inorganic barrier layers obtained by this method on the electrical properties of the composite dielectric,it is found that the Si O₂ barrier layer with a wider gap has a more significant inhibition effect on the carrier injection at the electrode,thus realizing the optimization of the insulation performance of the composite film in a wide temperature range.The high temperature insulation performance of the PC-Si O₂-2 composite dielectric containing Si O₂ barrier layer is significantly improved compared with that of pure PC,and the ideal high temperature energy storage characteristics is obtained.PC-Si O₂-2 can withstand an electric field strength of up to 630 MV/m at 100°C,and obtain an energy storage density of 6.94 J/cm~3,and the charge-discharge efficiency can be maintained at 95.05%.In order to further improve the insulation performance of dielectric at high temperature,multilayer composite dielectric with BN/Si O₂ heterojunction is prepared.By adjusting the structure of the heterojunction,the direction of the built-in electric field at the interface of the heterojunction can be freely regulated,and the acceleration or inhibition of carrier transport can be adjusted.When Si O₂ is in contact with the electrode(PC-BNSi),it realizes the synergistic suppression of carrier injection and transport,and significantly improves the insulation performance of the composite dielectric in a wide temperature range,and obtains excellent high-temperature energy storage characteristics.The PC-BNSi composite dielectric can withstand an electric field strength of up to 760 MV/m at 100°C,and obtains an excellent energy storage density of 8.32 J/cm~3 and a charge-discharge efficiency of 89.65%.This thesis mainly carried out a series of structural designs on PC,clarified the mechanism of the influence of heterojunction on carrier transport,realized the synergistic suppression of carrier injection and transport,and effectively improved the high-temperature insulation of PC and obtains excellent high-temperature energy storage characteristics,which is of great significance to the development of high-temperature energy storage.