Porous PTFE space-charge electrets for piezoelectric applications

Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d₃₃ coefficients of up to approximately 600 pC/N or 600 pm/V. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed     

Electric charge transport and trapping in dielectrics deduced from isothermal and nonisothermal measurements

By sequential use of the isothermal charging, the isothermal discharging, the final thermally stimulated discharge current and the final isothermal discharging current techniques, the charge dynamics in highly insulating materials may be investigated. The method is demonstrated for polyethylene terephthalate. The injected charge for a field of 20 MV m/sup -/1 and polarization temperatures up to 110/spl deg/C is almost totally trapped in the material and is released during the heating of the sample at 180/spl deg/C for a sufficiently long time. A significant current at high temperatures, about 90/spl deg/C above the poling temperature, was observed proving that it originates from charge detrapping. The final thermally stimulated discharge current peaks shift to higher temperature when the polarization temperature increases, and are characterized by activation energies in the range from 1.03 to 1.56 eV. They allowed the identification of the glass transition around 114/spl deg/C. The relaxation time of the trapped charge, at 180/spl deg/C, was determined to be about 3780 s, explaining the very good stability of trapped charge.     

Investigations on charge storage and transport in plasma-depositedinorganic electrets

The storage and transport of charges in plasma deposited inorganic electrets with low mechanical stress were investigated under different conditions. Silicon dioxide, nitride and oxynitride were chemically deposited on Si substrates by means of a radio frequency (13.56 MHz) plasma system. The samples were negatively charged by the constant voltage corona method. The surface potential was observed during aging at room temperature, at elevated temperatures of 200, 250 and 300°C and at a relative humidity of 95% at 60°C. The thermal activation process was examined by thermally stimulated discharge (TSC) measurements. Silicon oxynitride possessed the highest charge stability during the different experiments, correlating with the current maximum located at ~400°C. The charge transport in silicon dioxide, with a current maximum located at ~330°C, was described by a theoretical model which considered the drift and the partial retrapping of free charges in the bulk. Despite treatment with hexamethyldisilazane, it was not possible to stabilize the surface potential of silicon nitride samples even at room temperature     

Photostimulated discharge in electret polymers: an alternative approach for investigating deep traps

The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm/sup 2/ were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d/sub 33/ coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 nm could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-)piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets.     

Preparing and polarizing stability of Teflon AF nonlinear opticalpolymer electret double layer film system

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 AF/nonlinear 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     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

Silicondioxide electret films prepared by the sol-gel process

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) experiments were carried out. The results showed that the main peak of the TSD current spectra was situated at ~250°C 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. The authors found activation energies of 1.3 and 0.7 eV for negative and positive corona charged samples. Experimental results suggest that the sol-gel process could be useful in electret and electronic technique applications     

The transport and trapping of electrons in polymers

The conductivity of a polymer is determined by the availability of charge carriers and their mobility, and both terms are dominated by the trapping states in the polymer. The distribution in energy of these states may be studied by populating the states and observing the current that results from the relaxation of the trapped charges. In our experiments the states are populated by a pulse of electrons from an injection beam, and the surface potential of the polymer film is monitored by a second electron beam that passes above the charged surface; we have used this technique to study both charge trapping and the isothermal discharge process that follows a charging pulse. The general case of charge decay from an arbitrary distribution of states with retrapping can be analyzed by using a multiple trapping model, and this analysis yields the distribution of trapping states in the polymer. The analysis takes on a particularly simple form in the limit of negligible retrapping. Experimentally we have concentrated on PS-2 polystyrene and on several mono-dispersed polystyrenes. We find a change in the distribution of trapping states with molecular weight; there is also a change in trap distribution near the glass transition temperature     

Charge storage in corona-charged polypropylene films analyzed by LIPP and TSC methods

Negatively corona-charged 50-/spl mu/m-thick polypropylene (PP) film is measured using laser-induced pressure pulse (LIPP) and thermally stimulated current (TSC) in order to study the charge storage mechanism in the PP film. The LIPP can reveal the space-charge distribution in the depth direction of the PP films and the TSC can be used to measure the energetic depth of the charge trap. The LIPP shows that negative charge is deposited on the charged surface of the sample. Almost all surface charges are removed by soaking the sample in ethanol. However, about 5% of the surface charge is injected into the sample up to a depth of about 7 /spl mu/m from the surface. The injected charge is not removed by the dip-in-ethanol method because the ethanol does not penetrate into the sample. The injection of the surface charge increases with corona-charging temperature. Besides the negative charge injection, the injection of positive charge from the opposite surface is also observed when the sample is charged at higher than 60/spl deg/C. In addition, negative bulk charge is formed when the sample is charged at higher than 80/spl deg/C. The LIPP profile is compared with the TSC spectrum. It is shown that the space charge observed using LIPP disappears when the temperature of the sample exceeds 80/spl deg/C. However, TSC is observed at even higher than 80/spl deg/C. This indicates that the TSC is observed even after the disappearance of the space charge measured using LIPP.     

The influence of elevated humidity on the stability of the electret state in polyimide films

The influence of environmental humidity on the stability of spatial charge storage in polyimide films has been studied. The decay of the electret potential difference under isothermal conditions and the spectra of thermally stimulated depolarization discharge (TSD) currents have been measured. The experimental results obtained show that as the environmental humidity grows, the stored spatial charge stability drops significantly. It has been shown that the spectra TSD currents in films with increased moisture absorption are shifted towards the lower temperature region. These changes are related to growth in the film’s electric conductivity     

Charging of Polymeric Surfaces by Positive Impulse Corona

This paper focuses on analysis of charging of polymeric surfaces by means of impulse corona discharges in air. Internal (space charge densities and electric fields) as well as external (circuit current) characteristics of corona in a point-plane electrode configuration are investigated by means of computer simulations. Two types of onset positive corona modes, namely positive glow corona and burst pulse corona are identified. The developed and verified computer model is further used to study corona charging of a 2 mm thick polymeric material sample. Both the mechanism of charge deposition and distribution of deposited charges on the surface are dependent on the mode of the corona discharge used. In the case of glow corona, charge generation is limited to the anode region and the generated charges move towards the sample surface under applied electric field. Thereafter the deposited charge cloud expands radially along a portion of the surface with fairly constant concentration. In the case of burst pulse corona, series of positive charge clouds start from the anode and move towards the sample surface in a wave-like manner. Each burst contributes to the deposited charge, which spreads over the surface less extensively than that observed during glow corona charging.     

Ultra high vacuum properties of some engineering polymers

Engineering Polymers are very good candidates for applications requiring mechanical properties comparable with metals, chemical inertia, high insulation capability, high temperature operation and ultra high vacuum (UHV) compatibility. The results of a systematic test series, aimed at qualifying the engineering resins VESPEL/spl reg/ SP1, PEEK and CELAZOLE/spl reg/ PBI as UHV seals, are reported. The study of the materials behavior has been carried out over a wide temperature interval, ranging from 20 to 400/spl deg/C. In addition to the tightness and permeation tests, thermal desorption and gas chromatographic-mass spectrometer (GC/MS) analysis have also been performed. The results obtained indicate that CELAZOLE/spl reg/PBI provides the best performance, since it can be operated safely up to 375/spl deg/C, without giving any sign of leak or other drawbacks. PEEK, on the contrary, does not stand temperatures higher than 275/spl deg/C but, below this limit, it remains a very cost effective and reliable alternative. VESPEL/spl reg/ SP1, in its turn, can be operated safely up to 325/spl deg/C but above this temperature its properties start to degrade even if not in an abrupt manner as is the case for the other two resins. The possible applications of some of these polymers in the field of nuclear fusion research are also briefly described.     

Quasistatic and dynamic piezoelectric coefficients of polymer foams and polymer film systems

Piezoelectric properties of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 /spl mu/m are studied. In addition, bilayer or multilayer structures composed of one of these polymers in solid or voided form plus an additional air layer are investigated. In particular, the quasistatic and the dynamic piezoelectric d/sub 33/ coefficients are determined with electro-mechanical, optical, and acoustic methods. The quasistatic coefficients are of the order of 100 to 350 pC/N for the cellular PP and much lower for the porous PTFE. With increasing frequency up to about 50 kHz the coefficient of cellular PP decreases where it starts to rise toward the resonance at approximately 300 kHz. The bilayer or multilayer structures show considerably higher quasistatic coefficients which reach up to 20,000 pC/N for certain combinations. As for the single-layer systems, a decrease toward higher frequencies is observed. The effect is linear as a function of load pressure at relatively low pressures, ranging up to 100 Pa for some air-gap systems and up to several kPa for the cellular PP.     

Charging characteristics and electric field distribution on alumina as affected by triple junctions in vacuum

This paper describes the dependence of the charging characteristics on the electric field distribution on the alumina (Al/sub 2/O/sub 3/) surface as affected by the triple junction in vacuum. For HV electrical insulation design of vacuum interrupter, surface flashover in vacuum is very important problem to be solved. Attention should be paid to the fact that the insulation characteristics on the dielectric surface are strongly influenced by field emission of electrons from the triple junction and the accumulated charges on the dielectric surface. In order to clarify the charging mechanism, we measured the charging characteristics for various types of triple junctions. In particular, we focused on the influence of the electric field distribution along the solid dielectrics and near the cathode triple junction (CTJ) on the charging characteristics. The results confirmed that the electric field distribution strongly affected the 2-dimensional (2D) distribution of the surface charge on the dielectric sample. Consequently, it was found that positive charging was generated on alumina, when the incident angle of the electric line of force on the alumina surface became >60/spl deg/.     

Effect of Barrier Materials on Discharge Properties in Air at Low Pressure

Dielectric barrier discharge(DBD) is widely investigated in order to obtain uniform low-temperature plasma.Many studies have proved that some barrier materials,especially electrets,can improve the uniformity of discharge.It is regarded as an available way to get atmospheric pressure glow discharge(APGD).In this paper,discharge forms with 4 different barrier materials(alumina,quartz,PTFE and PET) are investigated,and the transition of discharge form depending on the air pressure are recorded to estimate the influence of barrier materials on discharge.By using electrets as barrier materials,homogeneous discharges can be obtained in a more wide pressure range.Under the same experimental conditions,discharges with electrets are more uniform or have larger uniform areas due to the storage and desorption of charges on the surface of electrets.The electrons deposited in the surface layer can be released on next half cycle when the polarity of the applied voltage changes,and provide a number of seed electrons,which makes the discharge more homogeneous.The capacitance and the permittivity of barrier materials have no effect on the discharge form directly.     

Triboelectric charging between polytetrafluoroethylene and metals

Measurements were obtained on charge exchange processes occurring between small metal ballistic pellets and a polytetrafluoroethylene (PTFE) tube. Lead pellets of 5.5-mm diameter were propelled at a range of speeds through PTFE tubes of different lengths by compressed carbon dioxide. The charge incurred by the pellets and the tube was compared with the charge observed on copper and lead spheres rolled through the tube, driven by gravity. In all experiments, the charge on the moving pellet was measured with a Faraday cup. The experiments determined the effect of pellet speed on the magnitude of the charge accumulated on the pellet and on the PTFE tube. Sectioned shielding on the PTFE tube allowed the determination of surface charge distribution along its length by means of an electrometer. It was observed that the charge also depended on the work functions of the materials involved (lead, copper, PTFE). The charge on the pellets was found to range from +0.5/spl times/10/sup -8/ to +3.0/spl times/10/sup -8/ C, for pellet speeds from 10 to 80 m/spl middot/s/sup -1/. The reproducibility of results is discussed and comment provided on the degree of charge imbalance observed.     

Characterization of chargeability of biological particulates bytriboelectrification

Triboelectric chargeability of pecan pollen and lycopodium spores, using commercial Teflon and nylon chargers, has been investigated as reported in this paper. The charge-to-mass ratio of lycopodium, measured at different powder feed rates, i.e., 50-500 mg/s, indicated that both Teflon and nylon chargers increased particle charge by approximately elevenfold over background with Teflon charging particles positively to +11.06 mC/kg and nylon negatively to -11.52 mC/kg. Pecan pollen charge increased by approximately 11 times for Teflon charging positively to +7.59 mC/kg, and five times for nylon charging negatively to a value of -3.72 mC/kg. Pollen charging was not affected by carrier-air humidity in the range of 18.1%-47.6% relative humidity (RH) and at a temperature of 22.5°C. Pollen chargeability by Teflon measured as a function of the duration of air flow over the charger surface indicated that triboelectric charging was not degraded, even by 3.5 h continuous flow of humid air (44.2% RH at 19.2°C). Results indicate that tribocharging is a feasible method for imparting charge to pollen in electrostatic pollination technology currently under development     

Polarization and electric field distribution in thermally poledPVDF and FEP

In the present paper, heat wave (LIMM) and pressure step (PPS) measurements of the spatial polarization and electric field distributions in nonuniformly thermally poled α-PVDF and Teflon FEP films are reported. The spatial distributions obtained by the two methods are compared over the entire thickness range, and a satisfactory agreement is found. For PVDF the results of both methods are in accordance with literature data for similarly poled specimens of similar material, the so-called `thermal profile', a polarization peak near the anode. Furthermore, in the LIMM experiments we found a small peak near the cathode, which can be explained by a positive compensation charge layer extending ≈2 μm in depth. For the first time, LIMM spectra of FEP are published. The space charge distribution in the FEP sample is nearly homogeneous inside the sample. Near the anode side an accumulation of negative charges appear. Near both surfaces a positive compensation charge was found with a thickness of ~1 μm. It is demonstrated that by means of our deconvolution the spatial distribution can be determined simultaneously with two thermal parameters, the diffusivity of the sample material and the heat transfer coefficient between sample and sample holder     

Validity of constant voltage stress based reliability assessment of high-/spl kappa/ devices

Charge trapping in high-/spl kappa/ gate dielectrics affects the result of electrical characterization significantly. DC mobility degradation and device threshold voltage instability and C-V and I-V hysteresis are a few examples. The charging effects in high-/spl kappa/ gate dielectric also affect the validity of conventional reliability test methodologies developed for SiO/sub 2/ devices. In this paper, we review high-/spl kappa/ materials specific phenomena that can affect the validity of constant-voltage-stress-based reliability test methods to address the direction of future reliability study on high-/spl kappa/ devices.     

Self-controlled pre-breakdown discharges in planar symmetry

In measurements on Teflon FEP films charged in ⩽50 μm air gaps by microsecond impulse voltages, a uniform charge deposition on the films was observed. A regular increase of the film surface potential from a threshold value of the peak impulse voltage was found. However, for 300 μm air gap it was observed that abrupt charging occurs at a lower threshold voltage, indicating breakdown, and the charge deposition on the film becomes nonuniform. The behavior in air gaps <50 μm is explained here using Townsend's theory of pre-discharges. It is shown how and why the interposed insulating film acts to make the system self-controlled, thus avoiding breakdown in the air gap, despite the large values of the applied peak impulse voltage