José A. Giacometti

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. >

Point‐to‐plane corona: Current‐voltage characteristics for positive and negative polarity with evidence of an electronic component

A series of measurements carried out with the point‐to‐plane symmetry has shown that positive corona confirms the Townsend relation while for the negative corona this happens only at large point‐to‐plane distances (greater than 1.5 cm). In this case, the ratio of the prefactors in Townsend relation, for the same geometry, approaches the expected constant (the ratio of the mobilities) at larger distances, around 6 cm. For distances below 1.5 cm, the square root of the current is linear with the voltage, with an intercept of ∼1.4 kV. The results are interpreted as giving evidence of an electronic component at small distances, as suggested by Sigmond. The characteristics change to Townsend at higher distances when the electronic component becomes small as compared with the ionic one.

Electric field distribution and near-surface modifications in soda-lime glass submitted to a dc potential

Abstract The laser-induced pressure pulse method (LIPP) is employed to investigate the electric field distribution in soda-lime glass samples after application of a dc potential (dc poling). The modifications of near-surface composition resulting from the dc polarization procedure were determined by means of three nuclear techniques for near-surface analysis: Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERD) and nuclear reaction analysis. An alkali-depleted layer is formed near the anode. This depletion layer persists for a long time after the dc poling. The electric field in this region is of the order of the intrinsic dielectric breakdown, and its configuration depends on the penetration, or not, of hydrogen. Based on these results, the conduction mechanism in the alkali-depleted layer is discussed. It is suggested that when hydrogen blocking electrodes are used ionized electrons are probably the charge carriers in the alkali-depleted layer.

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.

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.

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.

A novel method for electret production using impulse voltages

A novel method for producing electrets is proposed in which an electric discharge is induced by a negative impulse voltage in a small air gap. The dielectric foil to be charged is placed between two planar electrodes, being in contact with one of them and separated from the other by the air gap. The electric discharge in the gap causes electric charges to deposit onto the dielectric surface where they are trapped, thus forming the electret. Measurements carried out with Teflon FEP (fluoroethylene propylene) show a very uniform deposition, and that the sample surface potential increases linearly with increasing impulse peak voltage. In principle, this method compares with the corona discharge for the production of electrets with regard to high turnout and low cost. >

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.

On optical phase conjugation in polystyrene films containing the azobenzene dye Disperse Red 1

Optical phase conjugation in polystyrene films containing the commercially available azobenzene dye Disperse Red 1 was measured using CW laser radiation generated by a frequency-doubled YAG laser of total power 50 mW. The degenerate four-wave mixing (DFWM) experiment allowed for measurement of the build-up time of the phase-conjugate signal and phase-conjugate reflectivity as a function of dye concentration and input beam intensities. The phase-conjugate reflectivities reached 0.1% in polystyrene films containing 2.5 wt% of the dye, with the phase-conjugate signal build-up time constants amounting to about 100 ms. The appearance of a phase-conjugate signal is linked with a build-up of absorption-induced gratings of a mixed (phase and amplitude) nature. Interesting thermally induced effects such as lensing effects and nonlinear chaotic behaviour of the phase-conjugate signal were observed.

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. >