Particle size effect of nano-ZnO fillers on the dielectric and energy storage properties of PI/ZnO composite films

Three series of polyimide (PI)/ZnO composite films were prepared and how their dielectric and energy storage performance is affected by particle size of the nano-ZnO fillers for the films has been systematically investigated. The EDS findings indicate that nano-ZnO particles tend to aggregate in the PI matrix with the increasing ZnO-doping amount, and the nano-ZnO particles with smaller size are more likely to aggregate with the same doping amount due to their higher activity. Profiting from the interface polarization effect between PI matrix and nano-ZnO fillers, the dielectric constant can be improved by incorporating an appropriate number of nano-ZnO fillers, but reduced by excessive doping due to the aggregation of nano-ZnO particles. The experimental results show that the dielectric properties and breakdown strength of these films are closely related to nano-ZnO particle size. For instance, the dielectric constant reaches the maximum value when the content of ZnO-30nm fillers is 0.1  wt %, but mounts to the maximum values as the fillers content for ZnO-50nm and ZnO-90nm is 0.2  wt %. The breakdown strength of PI/ZnO composite films shows a decreasing trend with increasing ZnO-30nm fillers content, while a trend of first increasing and then decreasing with increasing ZnO-50nm and ZnO-90nm fillers content, reaching the highest value when the doping amount is 0.2 wt%, respectively. The PI/ZnO-90nm films exhibit the optimal energy storage performance, and the film with 0.2  wt % ZnO content shows an U d of 4.05 J·cm −3 with an energy efficiency of 75 % under electric field strength of 425 kV/m. Positron annihilation lifetime spectroscopy (PALS) has been applied to study the free volume characteristics, and the correlation between interface free volume and electrical breakdown performance of PI/ZnO films has been explained.