Papers by Author: José N. Marat-Mendes

Abstract: The isothermal charging current and the isothermal discharging current in low mobility materials are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current there is a current related to charge injection or extraction at the metal-dielectric interface and a reverse current related to the charge trapped into the shallow superficial or near superficial states of the dielectric and which can move at the interface in the opposite way that occurring during injection. Two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms which are involved into the transient current and (ii) the height of the potential barrier W0 at the metal-dielectric interface. The experimental data demonstrate that there is no threshold field for electron injection/extraction at a metal-dielectric interface.

Abstract: For the characterization of the new materials and for a better understanding of the connection between structure and properties it is necessary to use more and more sensible methods to study molecular movement at nanometric scale. This paper presents the experimental basis for a new electrical method to study the fine molecular movements at nanometric scale in dielectric materials. The method will be applied for polar and non-polar materials characterization. Traditionally, the electrical methods used to study the molecular movements are based on the movements of the dipoles that are parts of the molecules. We have proposed recently a combined protocol to analyze charge injection/extraction, transport, trapping and detrapping in low mobility materials. The experimental results demonstrate that the method can be used to obtain a complex thermogram which contains information about all molecular movements, even at nanoscopic level. Actually during the charging process we are decorating the structure with space charge and during the subsequent heating we are observing an apparent peak and the genuine peaks that are related to charge de-trapping determined by the molecular movement. The method is very sensitive, very selective and allows to determinate the parameters for local and collective molecular movements, including the temperature dependence of the activation energy and the relaxation time.

Abstract: Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight, but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork, TetraPak® containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork, TetraPak® and paraffin, different granules size, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate, which makes it a better material for charge storage.

Abstract: The thermally stimulated discharge current (TSDC) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. The experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature Tma shifted with about 10 to 30 K above the charging temperature Tc. The shift of Tma with respect to Tc depends on the experimental conditions. The peak width at the half maximum intensity decreases as Tc increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

Abstract: Compensation has been reported for the relaxation parameters: the activation energy W and the pre-exponential factor τ0, determined from the Thermal Sampling of Thermally Stimulated Depolarization Current technique. Below the glass transition it is assumed that the relaxation time follows an Arrhenius equation. In the vicinity of glass transition temperature an experimental thermogram may be analyzed using the Vogel-Fulcher-Tamman-Hesse (VFTH) or the Williams- Landel -Ferry equation. In this article we use the VFTH relationship to study the compensation effect in the range of glass transition. For an elementary peak obtained by TS there is a relationship between the activation energy W, the temperature of the maximum current Tm, the VFTH temperature, the compensation temperature Tc and the compensation time τ c. We employ this relationship for a basic analysis of the compensation effect in the temperature range around Tg. By numerical simulations, and assuming parameters similar to those measured experimentally, we show that it is possible to observe a compensation point in some well defined conditions

Abstract: Electrical properties of natural cork, commercial cork agglomerates (for floor and wall coverings) and a recently developed composite of cork/TetraPak® were studied. Measurements of isothermal charge and discharge currents were made for natural cork samples in different directions (axial, radial and tangential cuts). The isothermal current characteristics and the samples conductivity were investigated under different conditions (electric field, temperature and environmental conditions: in air at ambient relative humidity (RH), dry air and vacuum), also the samples could be or not conditioned (dried in vacuum or in a P2O5 atmosphere at room temperature). From these results the influence of water on the electrical properties of natural cork could be seen. In order to compare the three different cork materials a preliminary study was made. Isothermal charge and discharge currents and conductivity after 1h charging were measured and compared for different electric fields and temperature in air at ambient RH.

Abstract: Cross-linked polyethylene (XLPE) peelings from aged power cables from three different sources were studied using a combined procedure of isothermal and thermo-stimulated current measurements. Different parameters, such as electric field, temperature, charging/discharging times, can be selected in order to make an analysis of the space charge characteristics (such as, relaxation times and activation energies). Three different cables peelings were analyzed: A – electrically aged in the laboratory at high temperature, B – service aged for 18 years and C – thermally aged in the laboratory at high temperature. The results were compared for the different types of samples and also with previous results on laboratory aged and produced films of low-density polyethylene (LDPE) and XLPE.

Abstract: The direct current (dc) electric measurements in dielectrics, especially for highly insulating materials, are strongly influenced by the ambient air, mainly by the water vapors contained by the ambient (humid) air which determine a significant increase of the electric conductivity. Beside this, there is a second effect related to the interaction of the absorbed and adsorbed gas molecules with the polymer matrix that modify both the surface and the bulk conductivities. The aim of the paper is to present the influence of the absorbed and/or adsorbed gas molecules on the electric charge trapping and/or detrapping in polyamide 11. The measurements were carried out under ambient (humid) air, in a vacuum at 8 x 10-3 Torr and in dry nitrogen. Because the trapping/detrapping processes and the molecule diffusion process in polymeric materials are slow processes, a variant of the thermally stimulated discharge current method, namely the final thermally stimulated discharge current method and the final isothermal discharge current method have been used in the temperature range from 20 to 190 0C. The absorbed/adsorbed gas molecules into polymer matrix results in a perturbation of the thermodynamic equilibrium and consequently a charge transfer could occur speeding up the deep level charge detrapping.

Abstract: Thermoelectrets of poly(acrylonitrile) have been prepared for various electric field polarization strengths. The thermally stimulated discharge current technique has been used to study the remanent charge (dipolar and/or trapped charge) stability. The measurements were carried out in a vacuum. Two overlapping peaks and a significant increase of the current at high temperature have been observed. The first peak is centered around 372 K and the second one is centered around the polarization temperature. When the polarizing field increases from 0.47 to 1.2 MV m-1 the apparent activation energy varies from 1.08 to 1.40 eV, revealing a weak polarization field strength dependence, while the relaxation time at infinite temperature τ0 decreases from 2.05 x10 –15 to 1.05 x 10-18 s. The relaxation time at 300 K increases from 3.48 x 106 to 1.7 x 109 s indicating that the trapped electric charge may be stable at room temperature for the given polarization conditions.

Abstract: The electric conductivity σ in highly insulating materials is determined by the equilibrium thermally generated carriers and by the injected carriers. The injected excess electrons will dominate the thermally generated electrons when the total number of injected electrons substantially exceeds the total number of initially empty electron traps existing in the material. Under these circumstances the electrical charge transport mechanism is no longer ohmic. In order to analyze the dependence of σ upon injected/trapped charge, isothermal and non-isothermal currents in Teflon FEP have been investigated at various temperatures, field strengths, in a vacuum or in ambient air conditions. At temperatures below 413 K, for charging times longer than about 10 s but shorter than about 600 s, the electric conductivity is almost electrical field strengths independent proving that the injected charge plays a minor role. For these conditions the charge is mostly trapped in superficial traps. At higher temperatures σ is field dependent. The final thermally stimulate discharge current has a peak around 500 K with a mean apparent activation energy around 1.35 eV. For a well conditioned sample the peak current is strongly dependent on the charging electric field and on the mean trapping depth of the injected charge. The relaxation time of the trapped charge is around 106 s at 523 K, proving that the injected charge is very stable, a fact of significant importance for applications.