Study On Dielectric Properties Of PI/TiO₂ And PI/SiO₂ Based On Photo-stimulated Discharge

Nowadays,the study of the influence of different nanomaterials on the structure and properties of polyimide?PI?composite films has become a hotspot.A great deal of research shows that the incorporation of inorganic nanoparticles can improve the dielectric properties of PI composite films.In this thesis,PI/TiO₂ and PI/SiO₂ composite films were prepared.The effects of particle kinds and doping concentrations on the structure and dielectric properties of PI composite films were investigated.In this thesis,PI/TiO₂ and PI/SiO₂ composite films were prepared by in-situ polymerization and the diameters of nanoparticles are both 20 nm.The doping concentration of composite films includes 5wt%,7wt% and 10wt%.The surface and fracture morphology,fractal characteristics,particle size distribution and chemical bonds were characterized by SEM,SAXS and FTIR techniques.The traps distribution,dielectric constant,dielectric loss,electrical conductivity,corona-resistant lifetime and breakdown strength of composite films were measured by the method of photo-stimulated discharge,dielectric spectra,corona aging resistance and AC breakdown.The key factors affecting the dielectric properties of the composite films were investigated based on the trap energy level and structural characteristics.The results show that compared with SiO₂ particles,TiO₂ particles were more closely wrapped with the PI matrix.Combining with the SAXS results,the size of two kinds of particles in the composite film was close to the original diameter of particles,and there was no obvious cluster phenomenon.This indicates that TiO₂ and SiO₂ particles have good dispersion in the PI matrix.In the low-frequency dielectric loss,the loss of PI/TiO₂ was dominated by the interface polarization and the conduction loss was main in the loss of PI/SiO₂.Compared with pure PI,both nanoparticles introduced a large number of deep traps in the PI matrix,but their trap level was shallower.The trap density of PI/TiO₂ films was larger than that of PI/SiO₂ films,while PI/SiO₂ films had relatively deeper trap level.With the increase of the doping concentration,the breakdown strength of PI composite film decreased,but still met the actual needs.Moreover,the breakdown field strength of PI/SiO₂ films was higher than the PI/TiO₂ films.The comprehensive results show that TiO₂ and SiO₂ nanoparticles can effectively improve the corona resistance of PI composite thin films due to the deep traps introduced.Compared with PI/SiO₂ films,the trapping density of PI/TiO₂ films is larger,but the trap level is lower,so that the lifetime of space charge in the traps is shorter,and the trapping and detrapping of charge is relatively easier in the PI/TiO₂ films,which increases the dielectric loss angle.Therefore,PI/SiO₂ films had better corona-resistant performance than the PI/TiO₂ films.