Constant Current Corona Research Articles

Constant-current corona triode adapted and optimized for the characterization of thin dielectric films.

This work describes an enhanced corona triode with constant current adapted to characterize the electrical properties of thin dielectric films used in organic electronic devices. A metallic grid with a high ionic transparency is employed to charge thin films (100 s of nm thick) with a large enough charging current. The determination of the surface potential is based on the grid voltage measurement, but using a more sophisticated procedure than the previous corona triode. Controlling the charging current to zero, which is the open-circuit condition, the potential decay can be measured without using a vibrating grid. In addition, the electric capacitance and the characteristic curves of current versus the stationary surface potential can also be determined. To demonstrate the use of the constant current corona triode, we have characterized poly(methyl methacrylate) thin films with films with thicknesses in the range from 300 to 500 nm, frequently used as gate dielectric in organic field-effect transistors.

Influence of constant-current corona charging on electret state of porous PTFE film electret

The influence of constantcurrent and constantvoltage corona charging technique on the electret state of porous polytetrafluoroethylene(PTFE) film electret has been studied by constant current and constantvoltage corona charging at room temperature, is othermal surface potential decay, thermally stimulated discharge(TSD) and SEM(sc anning electron microscope), etc. Compared to the constantvoltage corona charging, the current through films is constant during constantcurrent charging, the r efore, the trapping possibility along the thickness direction of porous structur e increases and the charge density goes up, which improves the charge storage ab ilit y of the electret. However, the charges trapped in the dielectric surfaces of the interfaces with different depths along the thickness of the sample are externally stimulated during storage or application, their transport paths to the back electrode from the trapping positions are relatively shorter according to the hopping model of the charges, therefore the charge decay is accelerated and the charge sta bility is decreased.

Corona charging of polymers: recent advances on constant current charging

This paper contains a brief overview on the recent developments of corona charging of polymers, with emphasis on the current corona triode. This latter method, which has been successfully applied to several types of polymer, is a legacy from Prof. Bernhard Gross' work in Sao Carlos, Brazil. Following a short introduction to corona charging, the experimental setups are described, especially with regard to the advantages in the constant current method. A few examples are given of the use of the constant current corona triode in the investigation of electrical properties of nonpolar and ferroeleectric polymers. The application of corona charging to pole nonlinear optic (NLO) polymers is discussed, including the perspectives for the constant current charging method for the NLO field.

Study of poling behavior of biaxially stretched poly(vinylidene fluoride) films using the constant-current corona triode

This article reports systematic results of corona poling measurements obtained on biaxially stretched polyvinylidene fluoride films charged in dry air. Charging was performed using the constant current corona triode. The dependence of the poling process on the sample thickness, charging current, and successive charging processes was investigated. Phase transitions from α to δ and to β phases were observed when virgin samples were corona charged. The thermal pulse technique showed that the polarization profiles during charging can be made consistently almost uniform and that the ferroelectric reorientation can be associated with the rising plateau region displayed on potential buildup curves.

Surface potentials of corona-charged ferroelectric P(VDF-TrFE) copolymer samples

Surface potential measurements are presented as a function of time for negatively corona charged P(VDF-TrFE) samples in a constant current corona triode kept at low humidity levels. The surface potential curves roughly display three time regions, the middle one being a plateau at the coercive field where dipole switching occurs, preceded and followed by capacitive charging regions. This behavior is characteristic of ferroelectric polymers, as has already been reported for PVDF. The experimental results are interpreted by neglecting the conduction through the sample, assuming a dynamics for the electrical polarization which depends on the time only through the changing electric field, and also by assuming a distribution of coercive fields. This analysis allows the determination of a mean coercive field and remanent polarization of about 43 mC/m2 and 56 MV/m, respectively.

Electric-field-induced phase changes in polyvinylidene fluoride: Effects from corona polarity and moisture

The constant-current corona triode has been used to charge and monitor the surface potential buildup of polyvinylidene fluoride samples in α form. Under dry air and positive polarity it is possible to pole the samples to sufficiently high electric fields so as to cause a phase change from the nonpolar to the polar δ form. This phase transition is confirmed by x-ray diffractograms. Surface potential measurements on δ samples display a ferroelectric behavior in which a plateau appears in the surface potential buildup curve due to the dipole reorientation. The values of the remanent polarization and the coercive field were estimated to be ∼50 mC/m2 and ∼65 MV/m, respectively.

Constant current corona charging as a technique for poling organic nonlinear optical thin films and the effect of ambient gas

Poling, which involves the alignment of nonlinear optical (NLO) dipolar molecules in the direction of the applied electric field, was carried out using constant current charging, a recently developed corona charging technique. Using this technique, the sample surface voltage buildup, and hence the efficiency of the dipole alignment, can be monitored during the poling process. In addition, it will be shown that this capability also provides a way to detect the end point of the dipolar orientation. Corona charging of the organic NLO film used in this study, however, needs to be done in an inert atmosphere. Results from in situ IR spectroscopy measurement during corona charging of the NLO film, ultraviolet and visible absorption spectroscopy and electron scattering for chemical analysis of this film before and after poling, indicated that exposing it to corona discharge in air ambient caused oxidation of the polymer surface. Various types of nitrogen oxides and carboxylic acids were found to be the products of this oxidation reaction. Finally, we also obtained evidence showing that the surface charging process of the organic NLO film is affected by this newly grown oxidized layer.

Constant-current corona charging of biaxially stretched PVDF films in humidity-controlled atmospheres

A special version of a constant current corona triode has been developed for charging samples in controlled atmospheres. Measurements performed on biaxially stretched polyvinylidenefluoride (PVDF) samples show that the buildup of the surface potential is dependent on air humidity. In a dry air atmosphere, the polarization clearly shows the ferroelectric behavior of the PVDF sample. For samples charged under humid conditions, the surface potential vs. time curves pass through a maximum value which decreases with increasing relative humidity. A tentative explanation for the surface potential dependence on the humidity is given, based on the increase in conductivity originating from the dissociated water molecules absorbed into the sample. >

Corona charging of polymers

A review of the corona charging of polymers is presented. After providing a brief account of the corona discharge process and of its application for charging materials, the paper focuses on the evolution of corona triodes that allow the sample surface potential to be monitored during the charging process. Particular emphasis is given to the main contributions arising from research, basically on Teflon FEP and ferroelectric polymers, in which a constant-current corona triode was used. Such a triode system has become a powerful tool in the study of charge transport and ferroelectric properties of polymers, because keeping the charging current constant facilitates matching of the experimental results to a number of theoretical models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measuring hysteresis loops of ferroelectric polymers using the constant charging current corona triode

A novel method is proposed for measuring the hysteresis loops of ferroelectric polymers. The samples need to have only one electrode and are poled by corona discharge in a constant current corona triode. It is shown how the sample surface potential and the charging current are related to the remanent polarization and coercive field, so that the hysteresis loops can be obtained. An illustrative example is given for samples of β-PVDF for which the hysteresis cycles were remarkably close to those obtained with the traditional Sawyer–Tower circuit. Values of 80 MV/m and 70 mC/m2 were estimated for the coercive field and remanent polarization, respectively.

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. >

Constant current corona triode with grid voltage control. Application to polymer foil charging

A new version of a corona triode setup, allowing charging dielectric foils with a constant current, was developed. The charging current is kept constant by controlling the grid voltage. We show that the sample surface potential can be inferred from the grid voltage and present results of the uniformity of charge deposition during the charging procedure. Sample charging currents could be varied from 1 to 300 nA/cm2 and the foils could be charged up to ±6 kV. The method was applied to measure the potential buildup on Teflon FEP and PVDF samples and these results are discussed. They showed that the method can be successfully applied to study the polarization buildup of β-PVDF foils under corona charging.

Deep trapping of injected carriers in ferroelectric polymer

Constant-current corona charging method is applied to study the processes of space charge formation and polarization build-up in a ferroelectric polymer such as uniaxially stretched polyvinylidenefluoride (PVDF). It is shown that the charging process consists of three stages which are analysed considering the important role of injected and deeply trapped charge carriers. In the phenomenological model are also considered the non-linear field dependence of polarization as well as the field and polarization non-uniformity through the sample thickness. The analytical expression is obtained for the electret potential-time dependence which is employed to compare experimental and calculated data and to find the coercive field and several other parameters of PVDF. [Russian text Ignored]

Constant schubweg for hole transport in corona charged fluorethylenepropylene

Thin (25 μm) foils of Teflon® FEP are charged positively in a constant-current corona triode with currents between 1–6×10−9 A to a voltage of 3 kV. Experimental results give the voltage as a function of time during charging and during the discharge which occurs after the corona current has been turned off. Results can be interpreted in terms of a theory which introduces shallow surface and deep bulk traps and assumes that the field-induced carrier drift is characterized by a constant schubweg.

Constant current corona charging of PVF2

Conduction of polyvinylidene fluoride is investigated by the method of constant current corona charging. During poling the surface voltage approaches a constant value, proportional to the current, if this is sufficiently small. When the corona is turned off it decreases to zero. Measurements with single and double electroded samples, for both polarities, give identical results, seeming to indicate conduction due to preexisting charges. Measurements in air of different relative humidity and in argon show that the conductivity depends on the presence of water vapor in the surrounding gas. Results are tentatively analyzed in terms of an equivalent circuit in which the sample is represented by a parallel connection of a capacitor and a resistor.