Resorption Currents Research Articles

Charge injection and conduction on the surface of insulators

We have calculated numerically the flow of charge onto an insulator surface by injection from an electrode touching the surface, using conformal mapping in conjunction with the boundary element method. We have assumed that the driving fields are due to the electrodes and to the surface charge itself, while the natural conductivity of the surface is negligible. We have considered three geometries used experimentally. In general, we find that for strong injection, the surface charge is confined to the region close to the injecting electrode, that the absorption current behaves as I∝t−γ,γ∼1/3, and that the absorption and resorption currents do not exhibit mirror symmetry. Furthermore, if the active electrode can inject charges of either sign, then on shorting the electrodes a counter charge is injected, which leads to a more rapid discharge at early times but does not give rise to a current reversal (anomalous current). Materials of higher dielectric constant store more surface charge. We compare our results with previous calculations and with existing experimental work.

Electrical conduction of polyethylene below and above its melting point

The electrical conduction of low-density polyethylene (LDPE) has been investigated between 20 degrees C and 150 degrees C and up to 1 MV/cm. Special molded specimens and a self-compensating test cell were used; particular care has been taken in the control of experimental conditions. Absorption currents show broad maxima, while resorption currents display polarity reversal. Steady-state currents increase with a power law of the electrical field, and they tend to saturate at high fields and temperatures. Results are discussed in the framework of space-charge-limited conduction and dispersive hopping transport models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Absorption current and surface leakage in polypropylene

Transient absorption and resorption currents in low-loss polypropylene at fields in the MV/cm range have been resolved into bulk and surface components. The conduction component of the bulk current was almost ohmic, whereas the total measured current varied non-linearly with field. No evidence of contact effects or injected charge was found. Conduction is attributed to impurity ions.

Drift-diffusion model for absorption and resorption currents in polymers

Considering the lack of explanations for absorption and resorption currents in polymers, we propose to reformulate the latest and most comprehensive model based on surface ohmic conduction. This model, carried out by numerical simulation, contained oversimplifications. We also analyze the diffusion of carriers from the electrode edge. Finally, and in view of the negative results obtained by both models, we propose a new model based on a mechanism of drift diffusion which allows us to explain the characteristics of absorption and resorption currents in polymers. The simulation carried out for the three models is very precise and employs the integral equation method.

Absorption/resorption currents and thermally stimulated resorption in polycarbonate provided with gold and/or reticulate CT complex electrodes

Absorption and resorption currents and thermally stimulated resorption in polycarbonate samples provided with reticulate charge-transfer complex electrodes and/or conventional evaporated gold electrodes were investigated. Using electrodes of different geometries bulk and surface components of the absorption/resorption currents were calculated in each case. It was found that both components are higher for reticulated electrodes and for the bulk component a polarity dependence was observed. It is concluded that the mechanism responsible for the surface component is the same for different electrodes (high-field injection) while in the case of the bulk component much more efficient charge carrier injection through the non-contaminated polymer CT complex contact occurs. Thermally stimulated resorption reveals that different mechanism is reflected also in different trapping/detrapping phenomena.

Surface component of absorption and resorption currents in polymethyl methacrylate and polystyrene in different conditions: Thermally stimulated resorption

Surface and bulk components of absorption and resorption currents and thermally-stimulated resorption (TSR) in polymethyl methacrylate (PMMA) and polystyrene (PS) were investigated in different conditions. Calculations performed for pairs of suitable electrodes which differ in surface/perimeter ratio, evaporated on the same samples yield current density for both components in both polymers and allow selection of samples with high and determined contribution of either component. The surface component coitribution in total current density changes from 2% to 70% in the case of PMMA and from 50% to 98% in the case of PS depending on the electrode preparation procedure. The comparison of the time dependence of the currents and TSR spectra obtained in different conditions for different samples shows that the most likely explanation of the origin of the surface component in absorption and resorption currents seems to be the field injection on the electrode edge.

Surface component of vacuum absorption and resorption currents in polymers. II. Surface charge accumulation

For pt.I see ibid., vol.13, p.625 (1980). Approximate relative magnitudes of the bulk and surface components of absorption/resorption currents in six organic polymers have been determined in vacuum, using a twin electrometer technique. The surface components approach 100% of the total current in polytetrafluoroethylene and polyethylene, 60% in polystyrene and a co-polymer of hexafluoropropylene and tetrafluoroethylene, but are very small in polyvinylchloride and polyethyleneterephthalate. The accumulation of charge on the polymer surface adjacent to the electrode edge has been monitored directly, using a series of shielded probes of varying sizes. Although the strength of the electric field applied for periods up to 12 h exceeded 106 V m-1, the accumulated charge on the surface of a polyethylene sample did not extend more than 18 mu m beyond the edge of an aluminium electrode in the form of a thin strip. It seems likely that, when the samples are short-circuited, the resorption current is due to the return of the accumulated charge to the electrode, under its self-field. The observed t-1 time-dependence of the absorption current is incompatible with charge transport on the surface via a pure diffusion process.

Surface component of vacuum absorption and resorption currents in polymers. I. Origin and magnitude

Absorption and resorption currents have been studied in several polymers, the measurements being made in vacuum and usually at room temperature. It is shown that lateral charge transfer between metallic electrode edges and the immediately adjacent polymer surface accounts for almost all of the measured absorption/resorption current flow in polytetrafluoroethylene (PTFE), and at least half that in polyethylene, polystyrene, and a widely used co-polymer of tetrafluoroethylene and hexafluoropropylene; its contributions in polyvinylchloride (PVC) and polyethyleneterephthalate (PET) are very small. This surface varies approximately as t-1, where t is the time since the external voltage was applied or removed. The exponent varies little from polymer to polymer. The component of the absorption/resorption current flowing in the sample bulk in the co-polymer probably originates in dipole orientation or hopping motion of the charge carriers.

Surface component of vacuum absorption and resorption currents in polymers. III. A source of dielectric loss

For pt.II see ibid., vol.13, p.639 (1980). It is shown that, for a commercial co-polymer of hexafluoropropylene and tetrafluoroethylene, lateral charge transfer between the aluminium electrode edge and the immediately adjacent polymer surface dominates the dielectric loss currents in the frequency range 32 Hz-3.2 kHz. As a result the measured tan delta values vary with the electrode diameter. The variation decreases if the sample is allowed to age in air before dielectric loss measurements are made. This could be due to the formation of a thin oxide film on the aluminium electrode, and a decreasing probability of electron/hole tunnelling through the film as its thickness increases.

Origin of absorption and resorption currents in the co-polymer poly(hexafluoropropylene-tetrafluoroethylene)

A range of experimental evidence is presented showing that absorption and resorption currents in 125 mu m thick samples of the co-polymer(hexafluoropropylene-tetrafluoroethylene) are dominated by lateral charge transfer between the edge of the evaporated aluminium electrode and the surrounding uncoated polymer surface, rather than by bulk conductivity processes. These currents were found to increase proportionally with the electrode perimeter, rather than with the electrode area as would be expected if bulk conductivity processes were dominant. The surface conductivity mechanism accounts for at least 90% of the charge under the current-time plots.

Étude électrique de couches minces de polymère obtenues dans une décharge luminescente I: Caractérisation électrique sous champ continu

The behaviour of thin polystyrene layers biased by a d.c. electric field has been studied. These thin polymer layers, obtained by glow discharge polymerisation of styrene, have thickness between 1000 and 10 000 Å. The transient mode which appears when a step voltage is applied to a metal-polymer-metal (MPM) structure has been studied. Absorption and resorption currents are explained by the standard Cole and Cole formalism which is in good agreement with the experimental results. The study of the permanent mode and its variation with the amplitude of the step voltage allows the polymer conduction mechanism to be analysed. Study of the conductivity variation with the polymer thickness and with its temperature shows that only the Poole-Frenkel effect can explain the experimental results, the depth of the corresponding potential well being 0.85 eV.

Influence of interfacial tunnel exchange on dielectric losses in thin amorphous insulating films

Charge storage kinetics (absorption and resorption currents) and harmonic response of amorphous dielectric thin films have been interpreted with account of the tunnel exchange between the electrons of the metal and the traps in the insulator. The space distribution of traps in the insulator monitors the behaviour of charge storage and consequently the time variation of the currents. Simple space distributions of traps have been analysed and their consequences have been satisfactorily compared with former experimental results. Furthermore, the influence of the energy distribution of the traps and of the sample temperature on the magnitude of the transient response have also been discussed.