[Advanced Materials] A clear guide to polyimide, one of the "bottleneck" new materials

Polyimide's poor corona resistance seriously shortens its service life under complex operating conditions, and nano-modification technology offers a new direction for developing corona-resistant polyimide films. Zhang Yiyi, Zhao Ziwei, Liu Jiefeng, and Zha Junwei — from the Guangxi Power Transmission and Distribution Network Lightning Protection Engineering Technology Research Center at Guangxi University, the School of Chemistry and Biological Engineering at the University of Science and Technology Beijing, and the Beijing Advanced Innovation Center for Materials Genome Engineering at USTB — published an article in the 2023, Issue 5 edition of Transactions of China Electrotechnical Society. Starting from the mechanism of corona resistance, they first reviewed the aging process of polyimide under corona; then analyzed strategies for improving the corona resistance of composite polyimide based on performance parameters such as dielectric properties, conductivity, trap characteristics, interface effects, and thermal conductivity; and finally summarized the existing problems in current research on corona-resistant polyimide films and offered an outlook on future directions, aiming to achieve the development of polyimide films with excellent corona resistance. With the development of power electronics technology, power electronic transformers that combine power electronics with high-frequency transformers have become key electrical equipment in smart grids, high-speed rail, aerospace, and many other fields. As the core device in power electronic transformers, high-frequency transformers can efficiently achieve voltage level conversion, electrical isolation, power regulation, and bidirectional energy flow. However, because high-frequency transformers operate under complex conditions — especially since the voltage applied to the coil windings has large amplitude, high frequency, and short rise time — and because the internal winding structure is compact, heat is hard to dissipate and temperature builds up. Under this multi-physics coupling, the inter-turn insulation, as the weak point of insulation, easily triggers corona discharge, which accelerates insulation aging and causes corona breakdown in a short time, seriously endangering the safety of electrical equipment. Therefore, developing insulating materials with excellent corona resistance has become a top priority. Polyimide (PI) is a class of polymer materials with imide ring structures. The very stable aromatic heterocyclic structure in the main chain gives it excellent thermal stability, mechanical properties, and insulating properties, so it is widely used as insulating material in high-frequency transformers, variable-frequency motors, wind power equipment, and more. However, polyimide has poor corona resistance under high-frequency voltage, which seriously shortens the service life of electrical equipment. To improve the corona resistance of polyimide films, scholars at home and abroad have carried out extensive research on polyimide composite films using nano-modification technology in recent years, and it has been confirmed that adding an appropriate amount of nanoparticles can effectively improve the corona resistance of polyimide. One of the most representative corona-resistant polyimide films, already in large-scale production, is 100CR developed by DuPont. Its unique "sandwich" structure (top and bottom layers of Al2O3-doped composite PI film with a pure PI film in the middle) gives it a much longer corona resistance life under high-frequency voltage, up to more than 10 times that of pure polyimide film (100HN). Although many scholars have confirmed that doping with inorganic nanoparticles can improve the corona resistance of dielectrics, explanations of the corona resistance mechanism of nanocomposite dielectrics are not yet unified.