Influence of gas media on the stability of corona charged polyethylenterephtalate films stored at low pressure

The influence of the gas media in which electrets have been obtained on the electrets behaviour of samples stored at low pressures was investigated. Non-metalized samples of poly(ethylene terephtalate) (PET) films were studied. Initially the PET films were cleaned with alcohol in an ultrasonic bath for 4 minutes, washed in distilled water, and dried under room conditions. The samples were charged in corona discharge in different gas media by a trielectrode system. The used gases were O₂, N₂, Ar, CO₂ and dry air. Negative or positive voltage of 5 kV was applied to the corona electrode, and negative or positive voltages of 800 V were applied to the grid. Electrets surface potential was measured by the method of a vibrating electrode with compensation. After charging the initial surface potential V₀ was measured out of the chamber. Then the samples, together with their metal pads, were placed again into a vacuum chamber. Then the pressure was decreased step by step from 0.1 MPa to 10 Pa (1 bar to 0.1 mbar). At each pressure the samples were stored for 5 minutes. After each step the electrets were removed from the vacuum chamber, the surface potential V was measured again and the normalized surface potentials V/V₀ were calculated. Charge thermally stimulated discharge method was also used to investigate the electrets obtained in the presence of different gas media. The surface potentials of two kinds of samples, before and after subjecting to low pressure, were measured.