J. Lewiner

Electrical properties of electrode/polyethylene/electrode structures

The pressure-wave propagation (PWP) method allows for the nondestructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyethylene/electrode structures such as those involved in high-voltage cables. The influence of the composition of the insulating resin itself and that of the electrodes are analyzed in polyethylene samples. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both is observed. This application of the PWP method is of particular interest, since it allows for a suitable choice of the materials and structures involved in insulator/conductor interfaces. >

Pyroelectricity induced by space‐charge injection in polymer electrets

Corona‐charged polypropylene‐film electrets are reported to give a reversible short‐circuit electrical current when subjected to a change in temperature. This current varies as the time derivative of the temperature; we point out a linear dependence of the signal amplitude on the charge of the electret. We show that without specific assumptions concerning charge trapping, the sample expansion cannot account for this pyroelectric behavior. The results can be explained in terms of either a dipolar distribution or a noncentrosymmetry of the trapping centers.

Space charge injection and extraction in high divergent fields

The pressure wave propagation method has previously been used to obtain a direct measurement of space charge injection into an epoxy resin at high divergent electric fields. Here we report the results of a series of experiments designed to investigate the extraction dynamics of such injected charge by following the decay of the measured charge subsequent to voltage removal. The investigated injection conditions covered a range of voltages (10 kV to 30 kV) equivalent to maximum applied fields of 190 MV/m to 590 MV/m, and times of application (5 to 100 ms). A modification of Lindmayers model has been used to relate the observed charge decay plots to the energy range of filled traps. The variation of the trap range with different injection conditions, together with the observed changes in the depth of penetration allow us to make qualitative statements about the way in which charge injection proceeds in high divergent fields.

Evidence of mechanical instabilities of electrets in electric fields

Evidence of mechanical instabilities of an electret in an electric field is reported. It is shown, by a simple calculation of the forces acting on a surface‐charged electret, that the electret may have monostable or bistable behaviors, depending on its charge densities and on the geometrical characteristics of the system. The threshold voltages allowing the switching are calculated. A comparison is made between theory and experiment.

Can non-destructive space charge measurement techniques have fallout in other fields?

For more than 20 years, non-destructive space charge distribution measurement techniques have been successfully used to study electrical phenomena in insulators and dielectrics. Where other experimental techniques would require various hypotheses, they give directly spatial information. The implementation is carried out by producing a perturbation within the sample which may involve thermal diffusion, elastic wave propagation or electrical stress. A recent physical study of these methods gives a vision that offers new types of applications. In this paper the question of complex geometry sample and of free charge detection is considered.

Measurement of charge distributions in coaxial structures-application to high voltage cables

The authors present direct measurements of charge distributions in coaxial cables conducted during the application of a high voltage, using the pressure wave propagation (PWP) method. It is shown that, as could be expected, the main effects take place near the inner electrode, where the applied electric field is the largest. Near this electrode, charges are transferred which lead to the creation of homocharges and to a decrease of the interfacial electric field. Heterocharges build up with time and compensate the effect of the homocharges on this electric field. A stabilized state is reached at 50 degrees C after approximately 120 hours. Heterocharges of two types have been observed, some localized very close to the inner electrode, the others extending over a quarter of the insulator. The origin of the first type can be attributed to the diffusion in the insulator of impurities present in the inner semiconducting electrode impurities dissociated by the local electric field. These results demonstrate that the PWP method can be applied to the study of high-voltage cables.<<ETX>>

Spatial distribution of polarization and space charge in barium strontium titanate ceramics

Ferroelectric films of BaSrTiO3 (BST) are identified as a potential alternative to silicon dioxide in the silicon-based microelectronics industry owing to their very high dielectric constant. In this paper the distribution of space charge and polarization in BST ceramics is studied at room temperature with the laser induced pressure pulse method. Under voltage, dipoles orient rapidly but are subjected to the material nonuniformities. At longer time the ceramic becomes uniformly poled, indicating a transformation of the material. The remnant polarization remains uniform and stable for at least two days once the sample shorted. Long stress cycles and annealing have shown similar effects on the electrical behavior owing to similar microregion rearrangements.

Direct observation of injected charges at insulator‐electrode interfaces in strongly diverging fields

Various theoretical models have been proposed to explain the behavior of insulator‐electrode interfaces in the case of strongly diverging electric fields. Until now, direct measurement of the charge transferred at these interfaces could not be performed. In this letter, we propose a novel structure which allows for such measurements. An application is presented in which charge transfer around 30 μm diameter wires embedded in an insulator is directly observed.

Effect of the thickness of insulators on the build up of a space charge distribution

When an electric stress is applied to an insulator, a space charge distribution may develop and have a strong effect on the electric strength of the material. It has already been reported that the thickness of the insulator may have an effect on the observed phenomena. In this paper we have investigated this effect, using the pressure wave propagation (PWP) method, and present experimental results which have been obtained on pure polyethylene samples of different thicknesses with similar electrodes, submitted to the same applied electric field. These results show that the distribution of space charge depends on the thickness for a given material, but not the maximum value of the charge density. The effect of dopant is also considered by comparison with already reported data.

A New Principle for Radiation Dosimetry and Control

A new principle which can be used for radiation dosimetry and control is described. It is based on the behaviour of a moving electrode subjected to two opposite forces ; one of them is created by the field of an electret, which decreases during irradiation the other force, of mechanical or magnetic origin for instance, is independent of the irradiation dose. A small, light weight, individual dosimeter is described ; it gives a sound alarm when a pretedermined dose is reached ; preliminary experimental results are presented.