Investigation of charge trapping in thin Kapton® polyimide films by means of photo-stimulated discharge spectroscopy
Photo-stimulated discharge-current spectroscopy was performed on 50 μm Kapton® 200HN polyimide thermo- and photoelectrets formed in the temperature range between −40ºC and +130ºC. Films were either charged in the DC electric field of 20 MV/m or photo-charged by illumination with light at the wavelength of 420 nm under the electric field of the same strength. Photocurrent spectra scanned in the wavelength interval 600–350 nm demonstrate a pronounced peak at approximately 470 nm revealing the presence of one charge-trapping level. The long-wavelength cut-off of the peak is at 550 nm corresponding to 2.3 eV trap depth.
Photo-stimulated discharge current in polyimide films
Photo-stimulated discharge (PSD) method is one of the ways to measure the trap level of space charge in the dielectric materials. Compared with the thermally stimulated discharge (TSD) method, it has no observable destroy to the sample's microstructure during the procedure of the light scanning and can therefore be considered to be a more accurate measuring method. However, the light scanning procedure may possibly lead to the occurrence of the external photoelectric effect between the electrode-dielectric interfaces or the internal photoelectric effect in the sample bulk, where the extra current due to the two photoelectric effects can seriously distort the PSD current. In this paper, the total current excited by the scanning light on the polarized or being polarized polyimide films are studied. The influence of the applied field intensity, the electrode structures, the evaporated electrode thickness and the isothermal decaying current of the polarized films on the stimulated current is evaluated, which is comprehensively analyzed to extract the space charge dependent PSD current from the distorted current. The charge trap level based on the PSD method is finally obtained.
Thermally stable dynamic piezoelectricity in sandwich films of porous and nonporous amorphous fluoropolymer
Porous amorphous fluoropolymer films of very low stiffness were produced by a solvent evaporation technique. Corona-charged sandwich films consisting of a porous and a nonporous layer exhibit piezoelectric thickness-extension resonances in their dielectric spectrum, through which the temperature dependence of their dynamic stiffness, coupling factor, and piezoelectric coefficient could be determined. Their strong piezoelectricity with coefficients of up to 600 pC/N at temperatures of at least 120 °C could make these polymer electret films interesting candidates for sensor and actuator applications in elevated temperature environments.