Laser surface modifications of polymeric electrets for efficient charge trapping

Abstract

In this study, poly(ethylene terephthalate) (PET) films were irradiated by KrF pulsed excimer laser of wavelength 248 nm with different number of pulses at IHz. The PET samples were then electrified by the negative corona discharging method. The electret properties of the charged samples were studied by various techniques. A significant increase of surface potential was obtained for the PET films irradiated of more than 15 pulses with a laser fluence of 86 mJ/cm2. The surface potential of those films irradiated for more than 20 pulses did not increase further but dropped to the same level as untreated films. From the SEM pictures, melt ripple tips were observed in the laser irradiated area on the PET film surface and some of them were even fused together. This layer of melt polymer led to an increase of charge storage, which the surface potentials of the samples were enhanced. It suggested the possibility of enhancing the charge trapping capability of polymers by altering the surface morphology with the laser ablation process. On the other hand, the charge retention capabilities of the PET films electrified with different corona voltages were studied. The results showed that the surface potential of the electrets would decay and reach a steady value whatever the initial charging condition and the corona voltage were different. This steady value was depending on the sample thickness. Using the open-circuit Thermally Stimulated Depolarization Current (TSDC) with a Polytetrafluoroethylene (PTFE) spacer, the characteristics of the PET electrets obtained in various conditions were studied. The polarization and space charges peaks were distinguished clearly in the TSDC thermograms. TSDC with different heating rates were conducted. From the current magnitudes and peak positions obtained, the activation energy of the space charges in PET can be estimated. Finally, a prototype electret microphone was fabricated to examine the laser irradiated PET film with respect to its frequency responses and signal sensitivity as the microphone diaphragm. It can be seen that the performance of the laser irradiated PET film was better than the untreated film and was comparable to the commercial Teflon diaphragm and even better at frequencies over 5 kHz.