Polarization And Dielectric Properties Of Pi/nano-TiO₂Composite Films

Due to easy molding, prominent properties and extensive using temperaturerange, polyimide based composites are used generally in the area of aerospace,electric, electronic et al. Polyimide/nano-TiO₂（PI/TiO₂） composite films wereprepared via in-situ dispersive polymerization in this study. The surfacemorphology, composition and microstructure of films were measured by SEM,XRD, SAXS, ultraviolet spectrophotometer and TG. Besides, the trap state,breakdown strength, corona-resistance, conductivity, permittivity and dielectricloss of films were characterized to research the effect of nano-TiO₂on thepolarization and dielectric properties.The results show that rutile type nano-TiO₂particles are homogeneouslydispersed in the PI matrices. With the TiO₂doping concentration increasing, thechanges of the PI diffraction peak value and fractal dimension indicate thevariation of microstructure and compacter composition in the composite filmsafter certain doping. Meanwhile, the more traps, higher ultraviolet absorptivecapacity and heat stability are observed in the composites compared with pure PI.With the TiO₂concentration increasing, permittivity increases firstly （0-3%）,then decreases （3-7%）; conductivity and dielectric loss tangent are effectedgreatly by frequency; in the range of lower frequencies, conductivity, dielectricloss and slope of permittivity are enhanced which indicates that the interfacialpolarization is introduced after doping. Meanwhile, in the range of higherfrequencies, slopes of conductivity and dielectric loss of pure PI are larger thancomposites. We also observed that polyimide/TiO₂composites exhibit lowerrelative permittivity at low TiO₂doping concentration （0-3%） compared to a purepolyimide. Namely, TiO₂doping sites act as pinning mechanism to reducepolymer chain motion in orientation polarization. The interplay of these twopolarizations and nano-TiO₂self-polarization processes determines the dielectricproperty changes that we observed on polyimide/TiO₂composites.With the TiO₂concentration increasing, breakdown strength whose maximum is240kV/mm at1%increases first and then decreases; time-to-breakdown of the composite films which can be lengthened continuously isdirectly proportional to the concentration. The slope is approximately positive6.32. Time-to breakdown of7%composite films is more than15times longerthan that of pure PI. With the corona ageing continuing, PI matrix on the surfaceof films decomposes. Then, a large number of TiO₂particles are graduallyaccumulated and play a role in shielding further erosion. The corona-resistance ofcomposite films is enhanced by synergistic effects which are composed of trapstate in the interface, TiO₂particles properties and film fractal characteristics.The interface and nano-particles dominate the variation of short-long breakdownin the range lower and higher doping, respectively.