Institute for Electroscoustics, Technical University of Darmstadt, Darmstadt, Germany Sol-gel silica electret films were coated on silicon substrates by a spin coating technique. The process of preparing the films is described and the electret behavior of the films is discussed. The films were corona charged at room temperature. Then, isothermal decay of the surface potential at room temperature and open-circuit thermally stimulated discharge (TSD) exper-iments were carried out. The results showed that the main peak of the TSD current spectra was situated at ～250℃ and charge deposited in the film indicated good stability for the negative charged sample. After storage at room temperature for 50 days, the surface potential of the sample still retained at ～90% of the initial value. We found activation energies of 1.3 and 0.7eV for negative and positive corona charged samples.Our experimental results suggest that the sol-gel process could be useful in electret and electronic technique applications.

Nonlinear optical activity of an nonlinear optical (NLO) polymer electret originates from the preferential orientation of dipoles doped or chemically attached to the polymer matrix. In this paper, the idea of fabricating a Teflon AFlnonlinear optical polymer electret double layer film system is proposed to improve the orientation stability of the dipole in the polymer electret. The polar stability has been investigated by means of measuring the open-circuit thermally stimulated depolarization (TSD) spectrum, the electro-optical coefficient, and the isothermal surface potential decay. The results indicate that the double layer film system may markedly increase the orientation stability of the dipole in the NLO polymer layer. Because the Teflon AF layer has high charge storage stability, the dipole orientation can be bound by the electric field generated by the space charge in Teflon AF layer.

Single-film bipolar electrets of porous polytetrafluoroethylene are generated by means of a two-step coronacharging process at elevated temperatures. Quasi-static direct piezoelectric coefficients of up to 0.15nC=N have been observed on these films. In addition, multiple-layer stacks of porous and non-porous polytetrafluoroethylene films with monopolar charge were also investigated. While the piezoelectric responses of the stacks were not as high as those of the single films, the multiple-film arrangements may have other advantages such as better electrical shielding or tunable mechanical properties and adjustable acoustical impedance. Our new results are discussed in the context of the emerging field of porous polymer electrets with many potential device applications.

In this paper, the influence of porosity on the stability of charge andpiezoelectric d33 coefficient for porous PTFE electrets is studied by using measurements of isothermal surface potential decay, quasi-static direct piezoelectric coefficient, elastic (compression) modulus and a leakage current through porous andnon-porous PTFE samples during corona charging, analyses of open-circuit thermally stimulated discharge current spectra, X-ray diffraction spectra andd ifferential scanning calorimetry. The results point out that with the increase of porosity, the stability of charge deposited in the film and the piezoelectricity of the film are improvedin a certain range of porosity. The reasons for the influence of porosity on the thermal stability of the charge storage andpiezoelectricity are also investigated. Our results also demonstrate that the improvement of the thermal stability of the piezoelectric coefficient at increasing porosity is due to improvedthermal charge stability andthe improvedstability of the mechanical properties of the material. At the same time, the limitation of the improvement of charge storage and piezoelectricity of a PTFE film with very high porosity is discussed.

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

The well-known thermal-pulse technique is based on the temporal change of the temperature distribution in an electret sample; the resulting non-uniform thermal expansion in the sample volume leads to an electrical signal which contains information on the charge or polarization profile. Here, a new method is proposed in which known charge profiles (e.g. electron-beam-deposited charge layers) serve as probes for the temperature distribution as it develops after heat-pulse excitation of one of the sample electrodes. The principle of operation is described for both single- and double-side metalized samples and first experiments on electron-beam-charged Teflon FEP films are reported.

In this paper, the outstanding features and the charge stability achieved by negative and positive charging tetrafluoroethyleneperfluoromethoxyethylene copolymer (PFA) at elevated temperature have been investigated by means of corona charging and thermally stimulated current (TSC) experiments. Optimal charging temperatures for negative and positive charging have been determined. Activation energies, which are obtained from the initial rise of the TSC peaks and by the peak cleaning technique, are 0.7, 1.0, 1.3 and 1.1eV at room temperature (RT), 1.4eV at 200℃ for negative charging. Mean charge depths and charge distributions were measured at various stages of open-circuit TSC measurements on corona charged PFA by means of the laser-induced pressure pulse (LIPP) method. The spatial charge distribution of the different energy levels along the thickness direction of the sample has been determined by means of electron beam charging in combination with TSC measurements. The charged state of some virgin PFA films is discussed. Some electrical and mechanical properties of PFA are compared with those of FEP.

25μm thick films of Teflon PFA were charged positively and negatively with a constant-current corona at different temperatures and subsequently discharged. The time behavior of surface potential, total current, and conduction current during constant-current and constant-voltage charging was compared. The mean charge depth was found to increase with negative constant-current and constant-voltage charging at elevated temperatures. Compared with constant-voltage charging, constant-current charging leads to a more prominent meancharge depth shift towards the back electrode and thus also to an improved charge stability in Teflon PFA, since a large amount of the deeper bulk traps is being filled.

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, non-porous PTFE (Teflon" PTFE) and non-porous FEP (Teflon" FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200℃ slow retrapping plays a dominant role: from about 200℃ to 300℃ fast retrapping controls the transport.

The discharging behavior of Mylar polyethyleneterephthalate (PETP) films is investigated and discussed in terms of dipole relaxation at 85℃ as well as conductivity and trap dynamics at higher temperatures. For comparison, the low-temperature peak due to positive real charge in nonpolar fluoropolymer Teflon FEP is also described. The shift of the mean charge depth in PETP samples charged at elevated temperatures, the transport of detrapped real charge in the bulk of PETP, and the compensation of detrapped charge in the bulk during thermally stimulated discharge are studied by means of heat-pulse experiments as well as measurements of the steady-state conduction current and the bulk conductivity glow curve.