J.N. Marat-Mendes

Analysis of the thermally stimulated discharge current around glass-rubber transition temperature in polyethylene terephthalate

The nature of the thermally stimulated discharge current (TSDC) for polyethylene terephthalate samples in the temperature range from room temperature to above glass-rubber transition temperature of the amorphous phase is analyzed. The well conditioning of the sample is strictly necessary in order to have a good reproducibility and accuracy of results. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows through the sample during polarization. The peak position changes as well, if the sample is polarized in air or in oxygen and the nature of change is more important in the case of oxygen. The shape of the peak is complex and at least four shoulders have been identified around 85, 90, 105, and 125 °C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the glass-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For...

Recent advances in ceramic-polymer composite electrets

Recent work on ceramic-polymer composites for piezoelectric and pyroelectric applications is presented with special regard to the production and characterisation of new composite materials as well as two new applications of these composite materials. One of these composites is made using ceramic powders obtained using the sol-gel technique. This technique allows a better control of the stoichiometry as well as a lower temperature of crystallisation as compared with the conventional mixed oxides route. A better control of powder morphology also produces ceramic grains in the submicron range enabling the production of nanocomposites with electroactive properties. A second type of composite is reported using high temperature polymer PEEK, thus extending the temperature range of common electroactive composite materials. Finally the use of these materials is demonstrated in two piezoelectric applications, an angular acceleration accelerometer and an acoustic emission sensor.

Study of poling behavior of biaxially stretched poly(vinylidene fluoride) films using the constant‐current corona triode

This article reports systematic results of corona poling measurements obtained on biaxially stretched polyvinylidene fluoride films charged in dry air. Charging was performed using the constant current corona triode. The dependence of the poling process on the sample thickness, charging current, and successive charging processes was investigated. Phase transitions from α to δ and to β phases were observed when virgin samples were corona charged. The thermal pulse technique showed that the polarization profiles during charging can be made consistently almost uniform and that the ferroelectric reorientation can be associated with the rising plateau region displayed on potential buildup curves.

Nonisothermal and isothermal discharging currents in polyethylene terephthalate at elevated temperatures

The thermally stimulated discharge current and the isothermally final discharging current have been measured, in vacuum and in different ambient gases for “as-received” polyethylene terephthalate specimens, in order to understand the nature of the origin of the released current in the temperature range from glass–rubber transition temperature up to 220 °C. The behavior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized states in the material. The current spectrum is determined by the space-charge existing in the as-received sample, and by the adsorbed and/or absorbed gases and water vapors. The movement of the ions, resulting from the interaction of the adsorbed and absorbed gases with the parasitic space charge, in the field produced by the space charge, is responsible for observed change in polarity of the current during nonisothermal and/or isothermal measurements and for the appearance of the ρ or space-charge p...

The fractal analysis of water trees: an estimate of the fractal dimension

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 /spl mu/m), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (/spl sim/3 /spl mu/m). Each photograph was analyzed to compare regions with and without water trees.

Combined Isothermal and Non-Isothermal Techniques to Analyze Charge Trapping and Stability in Insulating Materials

Taking into account the time-consuming trapping and detrapping of electric charge in insulating materials, a sequential protocol for the investigation the trapping sites and trapped charge stability is proposed. The method is demonstrated for Teflon FEP-A. Elemental peaks are obtained whose maxima shift with increasing field from 456 to 486 K and are characterized by activation energies that increase from 1.1 to 1.9 eV. The relaxation time of the trapped charge, at 523 K, increases as the amount of charge stored in the material decreases and explains the good charge stability for Teflon FEP-A electrets.

Effects of a corona discharge on the charge stability of Teflon FEP negative electrets

Measurements of TSD together with surface potential were used to study negative corona discharge effects on the charge stability of Teflon FEP electrets. During charging, liquid formation over the sample surface (clearly visible for long-time corona irradiation) was concluded to be responsible for a decreasing in the thermal charge stability. The authors also discuss the charge stability at room temperature, while the sample is exposed to the neutral activated species from a corona discharge source. A decay in the potential was observed and this was attributed to a surface charge compensation by positive ions, and not to neutral activated molecules coming from the corona tip as suggested previously.

Electrical conduction and space charge trapping in highly insulating materials

The electrical conduction in low-density polyethylene (LDPE) was investigated in the temperature range from 20?C to 95?C and for applied fields up to 12?MV?m?1. The objective was to study the effects of the space charge (SC) accumulated in the material on the low and medium field quasi-steady-state conductivity in LDPE. Very long time (a week or longer) isothermal current measurements were carried out to focus on the situation when the SC effects are dominant. Complementary results were obtained using the isothermal discharge current, the final thermally stimulated discharge current and the isothermal final discharge current measurements. The results are explained by taking into account the constraint imposed by the trapped SC on charge injection, trapping and transport processes.

Space-charge-controlled conductivity in low-density polyethylene

We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.

Constant-current corona charging of biaxially stretched PVDF films in humidity-controlled atmospheres

A special version of a constant current corona triode has been developed for charging samples in controlled atmospheres. Measurements performed on biaxially stretched polyvinylidenefluoride (PVDF) samples show that the buildup of the surface potential is dependent on air humidity. In a dry air atmosphere, the polarization clearly shows the ferroelectric behavior of the PVDF sample. For samples charged under humid conditions, the surface potential vs. time curves pass through a maximum value which decreases with increasing relative humidity. A tentative explanation for the surface potential dependence on the humidity is given, based on the increase in conductivity originating from the dissociated water molecules absorbed into the sample. >