Ultrafine Nano-ferroelectric Modified High Energy Density Organic-inorganic Nanocomposite Films

With the rapid development of the power electronics industry,people have higher requirements for energy storage systems.As a physical energy storage method,dielectric energy storage has a wide range of application requirements in the pulse power supply field of the electronics industry and renewable energy field due to its inherent ultra-high power density and rapid charge and discharge capability.However,the lower energy density limits its further development.Aiming at the bottleneck of low energy density faced by dielectric materials and devices,this paper proposes a strategy based on ultrafine-grained nano-ferroelectric modified organic-inorganic composite films,and carried out a series of systematic research.The main research contents of this thesis are as follows:To reveal the size effect of the BT phase in nanocomposite films,and provide a systematic description of how ultrafine nanoparticles affect the performance of nanocomposites compared to coarser particles.Two smaller BT particles of 5.9 nm and 17.8 nm were prepared by hydrothermal method at different temperatures,in contrast with the commercially available BT particles with an average size of 92.3 nm.The BT/PVDF namocomposite films with different sizes of BT particles were prepared by simple casting method.The microstructure,dielectric properties and energy storage density of the films with different particle sizes and different volume fractions were compared.The characterization confirmed that ultrafine BT has a more prominent advantage than coarse BT particles in improving dielectric breakdown strength and energy storage of the nanocomposites.A simple general casting method was chosen to prepare nanocomposite films with coarse BT particles and ultrafine BT nanoparticles,and the two smaller BT nanoparticles had no additional surface treatment.Therefore,the effect of the size effect of the particles on the organic-inorganic nanocomposite films can be more objectively and truly reflected.Based on the interface control,the Stober method was used to prepare SO@BT particles with different fractions of SiO2 on the surface of BT particles by controlling the amount of tetraethyl silicate(TEOS).After a series of tests,it was found that the amorphous SiO2 layer was uniformly and continuously coated on the BT crystal.It was proved that SO@BT particles with excellent ultrafine core-shell structure and good dispersibility were successfully prepared in this experiment.The obtained particles were introduced into a PVDF substrate to prepare nanocomposite films,and the optimum coating amount of SiO2 was determined by testing the properties of the films.The SO@BT particles with fixed 5%BT particles and 0-30wt%SiO2 were used for the preparation of the nanocomposite films.The coated BT particles significantly increase the breakdown and storage density of the nanocomposite films,a maximum discharge energy density of 6.77 J/cm3 was obtained at a breakdown voltage of 323 kV/mm in the films with 6wt%SiO2.The SO@BT particles with ultrafine core-shell structure show remarkable advantages in polymer modification.In theory,phase-field simulation can better understand the electrical breakdown mechanism in nanocomposites filled with uncoated BT particles and ultrafine core-shell SO@BT particles.It is further confirmed that the SiO2 insulating layer can effectively improve the breakdown strength of the nanocomposite,by comparing the breakdown paths of the two nanocomposites,thereby further improving the energy storage.In order to prepare nanocomposite films with high energy storage density,particles with optimum coating amount of SiO2 obtained in the above are used as fillers.The filling volume fraction of SO@BT particles was changed.The microstructure,dielectric properties and energy storage of the films with different volume fractions of SO@BT were characterized.Finally,when the filling volume fraction of the SO@BT particles was 3%,a highest discharge energy density of 11.5 J/cm3 was obtained at 420 kV/mm,and the energy efficiency was 64%.This study provides a promising method for the preparation of nanocomposites with high energy storage density and a simple method.