Correlations Between the Microstructure and the Insulating Properties of Biaxially Oriented Polypropylene Films
In recent years,the rapid development of power grids put forward higher requirements to power capacitors.The insulation performance of biaxially oriented polypropylene(BOPP)film,as the core dielectric material of capacitors,di-rectly determines the energy storage density and operational reliability of capacitors.In order to investigate the influence of the microstructure of BOPP on its insulating properties,we selected three kinds of BOPP films made from the same PP resin by different manufacturers,and tested and analyzed their surface morphology,crystalline properties,orientation and electrical properties.The results show that the enhancement of crystallinity,lamella thickness,orientation and glass transi-tion temperature will result in higher DC breakdown strength of BOPP films.Further conductivity tests were performed to obtain the macroscopic migration properties of carriers,and dynamic thermo-mechanical analysis tests(DMA)and sur-face potential decay tests(SPD)were used to investigate the carrier migration process.The volume conductivity test shows that the conduction mechanism of BOPP film is dominated by hopping conduction.The calculation results of trap parameters and activation energy of molecular segment elucidate the mechanism of microstructure influence on the insu-lating property of BOPP film.The ordered and regular microstructure decreases the shallow trap density of BOPP film,increases the activation energy of molecular segment movement,and then the migration process of carriers across the chains and along the chains is suppressed,which is finally reflected in the decrease of conductivity and the improvement of breakdown strength.
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