Mean Charge Depth Research Articles

Electret properties and chemical modification of cellular polypropylene films

The electret properties of virginal cellular polypropylene (PP) films and chemically modified cellular PP films by extraction from CH 2Cl 2 solution, oxidation in a mixture solution of H 2SO 4, CrO 3 and H 2O and fluorination in a hydrofluoric acid (HF) solution, were systematically studied by measuring the open-circuit thermally stimulated discharge (TSD) current spectra, charge TSD spectra and isothermal charge decay. The results point out that there are more deep traps than shallow traps in the surface region while the contrary case occurs in the bulk region. The thermal stability of charge storage of the chemically modified cellular PP film is significantly improved in comparison with that of the virginal one. Light irradiation or reacting at elevated temperature has remarkable promotion effect on the reaction of HF with the extracted and oxidized film. Moreover, a method for investigating the dynamic changes of mean charge depth relative to its initial value during heating was proposed.

Chemical surface treatment and charge stability of polypropylene cellular electret film

The charge stability has been systematically studied by measuring open-circuit thermally stimulated discharge (TSD) current spectra, in situ TSD charge spectra and isothermal charge decay for the polypropylene (PP) cellular films after chemical treatments ( extraction, oxidation and HF solution treatment). The results showed that a very strong new current peak, which the virginal film does not have, appeared at about 184℃ in the TSD current spectrum of the PP cellular film properly oxidized and then HF-treated at room temperature (RT), and that its charge storage thermal stability is significantly improved compared with that of the virginal PP film, and is enhanced further by high temperature charging method. The reason that charge stability of the HF-treated PP films is improved has been clarified by attenuated total reflection infrared (ATR-IR) spectroscopy. The charge stability of treated films can be improved by appropriately prolonging either HF treatment time at RT or oxidation time. The theory of in situ TSD charge indicates that an in situ TSD charge spectrum contains combined information on the changes of the mean charge depth of the deposited charge layer and the deposited charge value. Combining the TSD current spectrum and the initial mean charge depth of a PP cellular film at RT, the change of the mean charge depth can be in situ investigated in heating process.

Influence of thickness on charge storage and dynamic properties of nonpolar film electrets

In this paper, taking commercial Teflon FEP films of Du Pont Co. as an example, the influence of sample thickness on deposited charge density, the internal electrical field of film electrets charged at room and elevated temperatures, and on their bulk conductivity and c harge storage stability is discussed by means of heat pulse technique, measurement of isothermal surface potential decay and analysis of open circuit thermally stimulated discharge current spectra. The influence of the thickness on the shift of mean charge depth of the deposited charge layer under different temperature conditions is also investigated by the measurements of heat pulse technique in combination with conductivity glow curve. The results indicate that under the same charging conditions and with the decrease of the sample thickness, charge density increases, but at the same time charge storage stability is decreased gradually. Therefore, the charge storage capability and charge stability can be optimized by reasonably regulating the thickness of the sample.

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

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, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ 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/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.

Charge storage and transport in polymethylmethacrylate (PMMA) film

In this paper, the PMMA films were prepared by spin-coating method. The films were charged by means of constant voltage corona charging. The charge storage stability and the transport of detrapping charges in the PMMA electrets were investigated by isothermal surface potential decay, open-circuit TSD current spectrum analysis. The shifts of mean charge depth for PMMA at different environment humidity and charging temperature were measured by heat pulse technique. The experimental results indicated that the charge storage stability of PMMA materials was significantly influenced by relative humidity. Charging at elevated temperature could improve charge storage stability in PMMA.

Charge dynamics in Mylar films corona-charged at various temperatures

The discharging behavior of Mylar polyethyleneterephthalate (PETP) films is investigated and discussed in terms of dipole relaxation at 85 degrees C 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. >

An experimental study of charge distributions in electron-beam irradiated polypropylene films

Mean depths and profiles of charge distributions in 34 to 40 mu m thick polypropylene films during and after injection of 10 to 40 keV electrons are investigated with the split Faraday cup and the laser-induced pressure pulse methods. For an open front (irradiated) electrode, the mean charge depth increases with time during an after irradiation and eventually attains a stationary value. This value depends on the 1.7-th power of the electron beam energy. The charge depth also increases with increasing injected charge density, but does not significantly depend on current density at the same injected charge density. If the front and rear electrodes are short circuited after charging in open circuit, the motion of the charge is halted. For relatively small injected charge densities, the charge eventually is trapped in a narrow region just beyond the continuous slowing down approximation range of the electrons. For larger charge densities, the distribution broadens toward the rear electrode. The charge dynamics are caused by the space charge field, the radiation-induced conductivity, and the limited trap density.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Charge build-up in polypropylene thin films

Irradiation of insulating polymer films with nonpenetrating electron beams results in lasting space charge. The charge-storage properties of polymer electrets are the most important for their application. The location of the charge and the radiation-induced conductivity are interesting both for production of foil electrets and for the investigation of charge phenomena in such materials. In this paper, charge buildup in 30 micrometers thick films of polypropylene irradiated with monoenergetic electron beams of 10 to 45 KeV energy is investigated. Using the split-Faraday cup, the mean charge depth at the beginning of electron injection, at the termination of injection, and 10 minutes after the injection are measured. It is found that the depth increase with time during charging, but decreases a little after the injection of electron beams. The charge depth also increases with the amount of injected charge. From the dynamics of charge motion during irradiation, the radiation-induced conductivity is determined as a function of dose rate.

CORONA CHARGING AT ELEVATED TEMPERATURE AND CHARGE TRANSPORT FOR MYLAR PETP FOILS

In this paper, charge storage stability of constant voltage corona charged Mylar PETP at room and elevated temperature is discussed. Significant difference between influence of dipole peak at low temperature for Mylar PETP and that of real charge peak at lower temperature for nonpolarity fluoropolymers on charge stability is analyzed. The influence of bulk conduetivity of Mylar PETP charged at RT and elevated temperature on mean charge depth and the charge transport model in the bulk during TSI) after charging at RT for Mylar PETP are studied by means of conductivity glow curve measurement and heat pulse technique.

Constant-current corona charging of Teflon PFA

Experiments in which 25- mu m-thick films of tetrafluoroethylene-per-fluoromethoxyethylene copolymer (Teflon PFA) were charged positively and negatively with a constant-current corona at different temperatures and subsequently discharged are discussed. The time behaviors of surface potential, total current, and conduction current during constant-current and constant-voltage charging are compared. The mean charge depth is found to increase with negative constant-current and constant-voltage charging at elevated temperatures. It is shown that, compared with constant-voltage charging, constant-current charging leads to a more prominent mean-charge-depth shift towards the back electrode and to an improved charge stability in Teflon PFA, since a large amount of the deeper bulk traps is being filled. >

Modeling of thermally stimulated currents in polytetrafluoroethylene

A phenomenological model has been derived for thermally stimulated open-circuit current measured on negatively charged polytetrafluoroethylene electret film. The model is based on trap controlled release of negative charge carriers using approximations justified by thermally stimulated current and mean charge depth results. To describe the open-circuit current, a minimum of five traps was necessary including surface and bulk traps with activation energies of 1.3 (2), 1.4, 1.5, and 1.7 eV. In addition, attempt-to-escape frequencies were calculated and found in the range 1011–1013 s−1.

Mean charge depth in non-metallized electrets

Mean charge depth in non-metallized electrets

Studies of thermally stimulated currents and surface charge in polystyrene thin films

A careful study was undertaken of the initial charge developed on the non-metallized surface and of short-circuit thermally stimulated currents (TSCs) in unilaterally metallized polystyrene (PS) films 25 microm thick. The films had been negatively charged by Townsend breakdown at a voltage of −5 kV at various temperatures ranging from 90 to 130°C. The surface charge densities observed were of the order of 10 -8 C cm -2, corresponding to full trap densities of the order of 10 14 cm −3. The short-circuit TSC spectra of newly charged samples show a single peak at a temperature between 98 and 102°C. The thermal activation energy associated with this peak was found to be 1.2 eV. An analysis of TSCs indicates that electrons are subject to fast retrapping in PS. The ratio of the mean charge depth to the sample thickness is found to increase from 0.069 to 0.12 with the increase in the temperature of polarization, indicating a higher charge accommodation at higher temperatures of polarization. The trap-modulated mobility values for electrons at the TSC peak temperatures are the order of 10 −12 cm 2 V −1s −1. Such low mobility values at peak temperatures are considered to indicate the excellent charge storage properties of PS.

Studies of spatial depth and density of charge in breakdown-charged polystyrene foils

A detailed study of the initial charge developed on the nonmetallized surface, coupled with that of short-circuit thermally stimulated currents (TSC), has been undertaken in breakdown-charged one-sided metallized 25 μm polystyrene foils. The observed surface charge densities are of the order of 10 −8 C cm −2. Short-circuit TSC spectra of freshly prepared electrets show a dominant high-temperature peak and two other peaks of small magnitude at lower temperature. The polymer does not become totally discharged before reaching its softening point at 125°C. Analysis of thermally stimulated currents indicates that the electrons are subject to fast retrapping in this polymer. The largest ratios of the mean charge depth to the sample thickness, obtained for electrets depolarized soon after charging and 24 h after changing, are found to be 0.08 and 0.17, respectively. This increase is considered to be due to electron retrapping at various energy levels. The observed initial released charge densities correspond to full-trap densities of the order of 10 15 cm −3.