Currents In Polyethylene Research Articles

Transient and steady-state currents in polyethylene film under low and high dc voltages

Transient and steady state currents in polyethylene, under low and high dc voltages, are studied. In fact, a bipolar transport physical model is developed in order to characterize the space charge dynamics within an additive-free polyethylene film, under trapping and recombination mechanisms. The numerical computational models, used for the bipolar transport of carriers, are applied to the Poisson's, the continuity and the transport equations with the appropriate physical conditions. The principal and the new numerical results are calculated especially when high dc voltages are applied. Indeed, the net charge density numerical profiles show the appearance of the space charge packets that are revealed for the first time in numerical modelling although they have long been reported in experimental works. These space charge packets induce some new aspects on the external transient current evolutions, produce the oscillations of the conduction and the displacement current before the steady state, and generate a conduction regime that is dominated by the interface electrode-insulator. These numerical results are consistent with those reported in some experimental works. However, under low dc voltage the space charge packets disappear and the space charge limited current aspect appears on the external current profile.

Characterization of polyester films used in capacitors. II. Effects of heat treatments on transient and steady-state charging currents in polyethylene terephthalate thin films

The effects of various heat treatments performed before or after the vacuum deposition of aluminum electrodes on the charging currents flowing through polyethylene terephthalate (PET) thin films (6 and 12 μm) were studied. The amorphous phase of the PET films was characterized by the use of thermally stimulated current/relaxation map analysis spectrometry, allowing precise determination of thermodynamic and thermokinetic parameters. Density measurements were used to calculate the rate of crystallinity achieved after annealing. A correlation was found between the relaxation parameters of the α dipolar relaxation of PET, the rate of crystallinity, and the properties of the charging currents observed from room temperature to 200 °C. Strain-induced crystallization has been put forward to account for the experimental evidences.

Injection-limited trap modulated hopping conduction in carbonyl doped polyethylene

A recently developed theory of steady-state conduction of space charge limited (SCL) currents in polyethylene doped with carbonyl hopping sites, and including detrapping, is modified to apply to the injection limited case, i.e., to a nonzero field F0 at the injecting electrode. Good agreement with experiment was previously obtained for SCL carriers (F0=0) for applied fields up to 0.33 MV/cm. Here, we assume that for higher fields, F0 is proportional to the difference between the applied field and 0.33 MV/cm. Excellent agreement between theory and experimental current-applied field characteristics is then obtained for fields up to 0.5 MV/cm. The field distributions in doped and ‘‘pure’’ polyethylene samples are calculated, and are found to be independent of temperature in the range 49–82 °C for constant field.

The significance of dipolar mechanisms in relation to charge and discharge currents in polyethylene

An attempt has been made to explain the character of the charging and discharging currents in low-density polyethylene in the time range 10-104s at room temperatures in terms of dipole polarisation. The magnitude of these currents as well as their time and voltage dependence are analysed.

Thermally stimulated currents in polyethylene irradiated by γ rays at low temperature

The influence of morphology and thermal history on the thermally stimulated current (TSC) of polyethylene samples irradiated with γ rays at low temperature has been studied. In low density polyethylene (LDPE) and high density polyethylene (HDPE), the TSC maximum occurs at about 50 and 110 °C, respectively, with further peaks observable at lower and higher temperatures. The amount of charge carriers detrapped increases by an order of magnitude when the crystallinity of LDPE changes from 42% to 55%. Annealing polyethylene samples at elevated temperature with subsequent slow cooling results in samples which produce TSC several times larger when compared with samples cooled from elevated temperatures with a high rate. The results are discussed in terms of one mechanism involving trapped ion pairs which undergo geminate recombination on detrapping causing only small TSC at low temperature. A second mechanism involves charges separated sufficiently to undergo volume recombination after detrapping causing the TSC maxima mentioned above. The traps of the latter mechanism are thought to be located mainly at the amorphous crystalline interface.

Non-isothermal currents in polyethylene and the associated space-charge

Experimental data on steady-state isothermal and non-isothermal conduction characteristics of polyethylene are presented. It is shown that in particular, the non-isothermal currents are associated with space-charge build-up in an electrically stressed sample. An outline of a theoretical model to explain the present data is given.

Thermally stimulated currents in polyethylene and ethylene–vinyl-acetate copolymers

Thermally stimulated currents (TSC) from polyethylene and ethylene–vinyl-acetate copolymers with a variety of vinyl-acetate contents have been investigated using a TSC technique with x-ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular-chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl-acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl-acetate unit was determined to be about 1.8 D from a TSC analysis.

Anomalous photo-induced current transients in polyethylene

Phototransient currents in polyethylene have been investigated in the visible and near-infrared region. Their polarities were found to be opposite to those observed by previous workers. No measurable steady-state photoconduction was observed. An explanation has been offered on the basis of redistribution of charge over a finite region at a barrier near the electrode-polymer interface.