**1. Introduction**

Polyimide film (PI) is widely applied in reactor extra-high voltage (EHV) winding insulation, wind turbine winding, and variable frequency motor winding insulation due to its excellent physical, chemical, and heat resistance properties, with the wide range of applications in electronics and aerospace. During the operation of electrical equipment, the corona and partial discharge will inevitably happen and be accompanied by temperature rise [1], making insulation materials of EHV equipment in a complex environment where high temperature, high voltage, corona, and partial discharge exist together for a long time. A large amount of surface and space charge will inject into insulation materials, leading to the accumulation of local charges in the insulation and electrical field distortion, which will reduce the life of

### *Polyimide for Electronic and Electrical Engineering Applications*

equipment. Therefore, it is vital to study the surface charge and space charge transport mechanism of polyimide to improve the electrical properties of the film.

Nanocomposite dielectrics have shown excellent insulation performance due to the special size effect of nanoparticles, especially in corona resistance. Plenty of researchers have added inorganic nanoparticles to the polymer matrix to prepare nanocomposite dielectrics and achieved some results in practical applications [2–8]. Zhong added the silane coupling agent when preparing the SiO2/PI composite films, the results showed that the coupling agent improved the dispersion of the nanoparticles and the interface morphology [9]. Zha Junwei et al. prepared PI/ZnO composite films via in-situ polymerization, with studying the material's electrical resistance, volume resistivity, dielectric constant, and dielectric loss, the better corona resistance of the composite films is acquired. However, both the dielectric loss and the dielectric constant increase with the growth of the ZnO content [10]. The Al2O3/PI composite films were prepared using in-situ polymerization. The corona resistance testing results showed that the corona resistance improved when the mass fraction of nano-Al2O3 increased, and the corona resistance reaches the peak when in 20 wt%. Nevertheless, the agglomeration phenomenon becomes more and more obvious with the mass fraction gradually starts from 5 wt%, observed by the electron microscope [11]. Based on the above studies, it can be found that some achievements have been obtained in the nano-doped polyimide films, but its electrical resistance is not ideal. The research status of nano-composite polyimide films is still lingering in pursuing higher content of inorganic nanoparticles, better compatibility, and the better physical and chemical structure of nanocomposites. The surface and space charges have not been studied.

On the other hand, the electrical properties of fluoropolymers have not attracted the attention of researchers. In fact, the molecular structure modification (fluorination), as a kind of polymer surface chemical structure that changes the chemical composition of the polymer surface layer, has mature applications in the chemical industry [12, 13]. A Zhenlian found that the fluorocarbon layer is formed on the surface of polyethylene by fluorination, and the space charge characteristics have been studied [14]. It was found that the fluorinated layer can suppress the space charge injection. However, the current research has not carried out on the dynamic characteristics of surface and space charge accumulation as well as dissipation and on the effect of the introduction of fluorine on the depth of dielectric traps. So far, the application of surface fluorination is mainly used to improve the adhesion properties, barrier properties, and anti-permeability properties of polymers. There are few reports on its application in electrical insulation [15]. In the chapter, the molecular structure modification is used to control the surface structure of polyimide film and nano-composite polyimide film, and the surface and space charge dynamic characteristics of the film are studied to develop new polymer dielectrics, which is important in the field of engineering dielectric.
