Charge Injection Phenomena Research Articles

Electrical measurements for the characterization of electret properties of cellular polymers

Cellular polymeric materials may show interesting quasi-piezoelectric properties, upon charging under specific conditions of electrical field, temperature, pressure and environment. Electrical charging is the trigger of electret properties. It has been proved that the occurrence of electrical avalanches in the cavities which are present in a material is associated with the appearance of electret behavior, but the precise nature of the phenomena involved in this transformation is still under investigation. This paper provides a contribution to the study of these materials focusing on the relation between the appearance of electret properties and the presence of charge injection and partial discharge phenomena. For this purpose, space charge, partial discharges and low-frequency polarization measurements were carried out on polypropylene (PP) cellular materials, at different fields and temperatures. The existence of a threshold field for the start up of electret behavior that is larger than the threshold for space charge injection and partial discharge inception was observed. Steady charge is accumulated in the material, thus generating electret behavior, only if partial discharge repetition rate becomes sufficiently high and space charge is injected from electrodes. Space charge measurements were performed through the pulsed electroacoustic technique, in spite of significant attenuation of acoustic waves due to the cellular nature of the materials. Partial discharge measurements were carried out by an advanced system recording each discharge pulse, thus allowing observation of single and multiple pulses, besides achieving statistics of discharge pulse amplitude and phase. By such approach, space charge and partial discharge measurements proved to constitute a valuable tool to investigate the phenomena leading to piezoelectric behavior and, also, to infer electret time stability and optimize charging methodologies.

Effective ionic charge polarization using typical supporting electrolyte and charge injection phenomena in molecularly doped polymer light-emitting diodes

An effective method of enhancing charge injection and electroluminescence efficiency of polymer-based light-emitting diodes is introduced. Spin-coated films of poly (N-vinylcarbazole) blended with electron-transport material (Bu–PBD), laser dye (Coumarin6), and the typical supporting electrolyte [tetraethylammonium perchlorate (TEAP)] were examined and it was found that the injection current and luminance of the light emitting diodes doped with TEAP were enhanced dramatically after heat treatment at 80 °C and appropriate biasing in an external electric field of 1.5×108 V/m at this temperature. A charge injection model based on Fowler–Nordheim tunneling is proposed, taking into account electric field distortion due to the accumulation of ionic space charges at the electrode/film interface. The relaxation time of ionic polarization is found to be related to the cation size of the electrolyte.

Comparative study of theoretical Kerr electro-optic fringe patterns in 2-D and axisymmetric electrode geometries

Kerr electro-optic fringe patterns have long been used to study space charge injection and transport phenomena in highly birefringent materials such as nitrobenzene, Most past experimental work has been limited to 1 or 2-dimensional geometries where the electric field magnitude and direction have been constant along the light path such as two concentric or parallel cylinders or parallel plate electrodes. For these geometries the extrema in the fringe patterns can be used directly to find the electric field magnitude and direction. In this work we extend the fringe based Kerr electro-optic measurement technique to a point/plane electrode geometry which often is used in HV research to create large electric fields for charge injection at known location and at reasonable voltages. We calculate theoretical Kerr electro-optic fringe patterns for this point/plane electrode geometry with and without space charge distributions, for which the electric field magnitude and direction vary along the light path. We particularly compare the calculated space charge free optical patterns for the point/plane electrodes to the optical patterns of the 2-dimensional analog blade/plane geometry. We underline the differences and study how these fringe patterns can be used to reconstruct the axisymmetric electric field components in practice.

Charge distribution and transport in polymers

Experimental work on charge distributions and charge transport in polymer films is reviewed. For studying the charge distributions in the thickness dimension, thermal or acoustic techniques are used with resolutions down to /spl sim/1 /spl mu/m. The polymer samples usually are charged with corona, electron beam or thermal methods. Observation of the change of charge and polarization distributions with time allows one to draw conclusions about charge transport and charge injection phenomena. In some cases theoretical models have been established, based on excess-charge drift due to a finite carrier mobility and on charge compensation due to intrinsic or radiation-induced conductivity. Other models are concerned with the interaction of injected or intrinsic space charge with a dipole polarization. Comparisons of measured and calculated charge distributions and their change with time show the applicability of these models and shed new light on charge trapping and charge transport phenomena.

Improvement of dielectric properties of polymers by carbon coatings

The dielectric properties of insulating polymers are affected by the nature of the materials which act as associated electrodes. In this paper, we report a series of experiments which show that the voltage which can be endured by a polymer is increased when vacuum are deposits of carbon are used as electrodes instead of the usual aluminium deposits. A plausible explanation for the improvement of the dielectric properties of polymers by carbon coatings is proposed based on the reduction in the charge injection and charge migration phenomena.

Electrical conduction in impregnants for all-film power capacitors

The conduction properties of some impregnants (monodibenzyltoluene and phenylxylylethane) used in the all-film capacitors technology are investigated. The intent of this research contribution is to perform detailed investigations of these properties in a substantial range of electric fields (from 10−2 to 102 V μm−1). Measurements of the macroscopic charge transport properties (conductivity, mobility, and viscosity) at low field are reported for the temperature range 293–373 K. Evidence of ionic conduction is found from these results. It is possible to evaluate the ionic dissociation parameter of ionic species [e.g., TBAP (tetrabutylammonium picrate)] by studying solutions of known electrolytes in well-purified solvents. Combining this information with the corresponding investigation in well-purified liquids, it is found that the residual ionic conductivity of these fluids corresponds to 10−10–10−11 mol/l equivalent TBAP. In the high-field region, particular emphasis is given to the quantitative interpretation of the current (electrical losses) versus applied field characteristics. Bulk effects associated with the field-enhanced ionic dissociation (Onsager theory) are observed when the impregnant contains an ionic surfactant such as sodium di-(2-ethylhexyl) sulphosuccinate, while unipolar injection in the neighborhood of the metallic electrodes is evidenced in solutions containing small amounts of ammonium picrate electrolytes (TBAP, triisoamylammonium picrate). It is found here that the high-field conduction mechanisms in purified liquids can be attributed to a charge injection phenomena. A physical analysis that satisfactorily predicts the high-field conduction behavior of these fluids is further proposed and discussed in connection with recent numerical simulations.

Charge injection into monolayers

Charge injection phenomena in lipid—iodine systems are studied by the monolayer technique. Surface potential measurements indicate that charge separation takes place during iodine adsorption at the lipid/water interface and also when a charge-transfer complex is formed between lipid and iodine. In both cases the lecithin monolayer becomes more positive. Negative charges are injected into the monolayer of various lipids when the subphase contains triiodide ions.

Charge injection in metal-ZnO-SiO2-Si structures

The charge injection phenomenon occurring in metal-ZnO-SiO2-Si (MZOS) structures is quantitatively analyzed yielding a detailed model of the injection process. Theoretical capacitance-voltage and charge-voltage relationships describing the operational behavior of the gated MZOS-C structure are first established to provide a firm foundation for the interpretation of experimental data. Key experimental results, arranged so as to retrace the logical sequence of conclusions leading to the adopted charge injection model, are next described and discussed. In negatively biased illumination-relaxed structures all of the injected charge is shown to be located in deep-level interfacial traps and a dipole-like layer or surface barrier is found to exist in the near vicinity of the ZnO-SiO2 interface. Finally, a discussion is presented detailing both the stationary and transient aspects of the charge injection model formulated in accordance with experimental observations.

On the interpretation of electrical measurements on the GaAs-MOS system

Detailed evidence is presented which shows that previously published analyses of various investigations regarding the electrical characteristics of the GaAs MOS system lead to incorrect conclusions. Primarily the results were interpreted in terms of models which were developed to fit the Si-SiO 2 interface properties. However, detailed I–V and admittance measurements show that there are basic differences between the two systems, as the one with GaAs exhibits a charge injection phenomenon across the interface barrier, oxide conduction and a complicated (up to now insufficiently understood) interface behaviour. All these effects prevent the determination of interface state densities by conventional methods. The conclusion, that simply because of a high density of interface states neither accumulation nor inversion are possible, is not justified.

ポリエチレンの電荷注入特性に及ぼす極性基の効果

ポリエチレンの電荷注入特性に及ぼす極性基の効果

Electrical Properties: Charge Injection Phenomena

Electrical Properties: Charge Injection Phenomena