High field conduction in biaxially oriented polypropylene at elevated temperature

Abstract

Biaxially oriented polypropylene (BOPP) thin film is the predominant dielectric material used in film capacitive energy storage for pulsed power engineering and power conversions due to its remarkable high dielectric strength and low conduction loss. However, the design rating of BOPP film capacitors in high power density conversion systems operated also under high temperature is still based on the empirical criteria due to the lack of systematic mechanism studies at elevated temperature. In this work, the temperature-dependent electrical conduction in tenter and bubble BOPP films up to their breakdown strength was systematically studied using a specialised circuitry featuring dynamic gain-controlled capacitive current cancellation. Both tenter and bubble BOPP films exhibit an extended trap-limited conduction region at the high electric field, followed subsequently with a trap-filled limited conduction until breakdown. This trap-filled-limited conduction presents characteristics of carriers transport with detrimental high mobility and soaring conduction loss. Overall, the shallow localised states revealed by the Arrhenius analysis, the large bandgap, and high barrier height of BOPP film together render its exceptional electrical integrity. In comparison, the enhanced crystallinity and larger crystallite sizes in tenter BOPP produced by the sequential stretching result in a higher upper operational temperature and slightly higher breakdown strength than bubble BOPP, suggesting the important role of processing induced enhancements to intrinsic properties of molecular origin. This study provides insights into the high-field characteristics of BOPP films at elevated temperature with promising learning outcomes useful to the expedited designs of the next generation polymer films for capacitive energy storages.