CHARGE ACCUMULATION AND RELAXATION IN FILM AND FIBROUS POLYIMIDES

The processes of accumulation and relaxation of the space charge largely determine the main characteristics of insulating materials: conductivity, polarization, dielectric losses and long-term electrical strength, therefore, the service life of the insulation. The accumulation of the space charge is an undesirable phenomenon, because it distorts the electric field, worsens the electrical strength and reduces the life of the insulation. On the other hand, the value of the space charge and its stability over time determines the ability of sensors and microphones to operate. This paper presents the results of studying the electrophysical properties of thermoplastic polyimides (TP PI) R-BAPB, R-BAPS, obtained on the basis of 1,3-bis- (3,3', 4,4'- dicarboxyphenoxy) benzene (R dianhydride), 4,4'-bis- (4"-aminophenoxy)biphenyl (diamine BAPB) and bis- (4-aminophenoxy)diphenylsulfone (diamine BAPS) and thermosetting polyimide PMDA-ODA (based on pyromellite dianhydride (PMDA) and 4,4'-oxidiphenylenediamine (ODA)). Particular attention was paid to the analysis of the effect of the material structure on its ability to retain charge for a long time (electret effect). The properties of PMDA-ODA materials made in the form of a film obtained by casting and a nonwoven material obtained by electroforming were studied. The stability of the electret potential difference over a long period of time and in a wide temperature range was measured. The characteristics of relaxation processes were determined by thermal activation spectroscopy methods. The mechanisms of charge accumulation and relaxation, the effect of temperature and electric field in the sample on the stability of the space charge were considered. The results of experimental studies were analyzed based on theoretical models and modern concepts of charge relaxation processes.