D. Mary

Ac and dc electroluminescence in insulating polymers and implication for electrical ageing

Electroluminescence detected in many different polymers under uniform 50 Hz ac and dc fields exhibits a threshold-like character, i.e. the light emission increases supralinearly with the applied voltage above a critical value called the emission threshold. The significance of this threshold is not straightforward. It could be a true physical threshold corresponding to the onset of the luminescence excitation as opposed to a sensitivity-limited threshold without specific significance. This paper discusses this issue on the basis of electroluminescence analysis in several polymers used in electrical engineering, such as polyethylenes, polyesters and polyimides. The analysis relies on the consideration of the field dependence of the electroluminescence and its imaging in the sample plane. It is shown that the threshold-like character under a dc field reveals the onset of the electroluminescence excitation whereas it has no specific meaning under ac stress where the detection is sensitivity-limited. The significance of the luminescence-voltage characteristic, as regards the electrical ageing and its possible use for diagnostic purposes, is further considered.

Identification of the components of the electroluminescence spectrum of PE excited in uniform fields

The electroluminescence (EL) of insulating polymers is a subject of great interest because it gives insight into aging and breakdown processes. While most of the studies performed on polyolefins are under divergent field configurations, a uniform field is used more and more. However, a convincing interpretation of the emission spectrum has not been given up to now, and some discrepancies have appeared between the spectra recorded under divergent and uniform fields. Wavelength-resolved EL spectra are carried out on metalized polymeric films biased under different ac and dc voltage levels. We show that the light emission consists of two different components with typical field dependence and spectral distribution. One is associated with the luminescence of the polymer, the other is due to photon emission from the metallic layers through the radiative decay of surface plasmons excited by the injection current. The proposed interpretation reconciles the observations made under divergent and uniform fields and allows us to discuss the nature of the chromophores involved in EL.

Analysis of the luminescence decay following excitation of polyethylene naphthalate films by an electric field

Electroluminescence is excited in poly(ethylene 2,6-naphthalate) films submitted to a d.c. field. Kinetic and spectral analysis of the light emitted after field removal, so-called delayed luminescence, is carried out in order to understand the phosphorescence enhancement observed in electroluminescence. It is shown that light emission can be interpreted in terms of tunnelling recombination of deeply trapped electrons to the luminescent centres formed by the ionized molecules. The phosphorescence enhancement is due to direct coupling between the trapping state and the triplet state of the molecule.

Effect of filler concentration on dielectric behaviour and on charge trapping in PP/clay nanocomposites

Nanocomposites obtained from polypropylene base materials and organophilic layered silicate as nanofiller might be a great source of innovation for the development of advanced electrical devices. Previous investigations on dielectric and electrical properties of such nanocomposites with either isotactic (iPP) or syndiotactic (sPP) polypropylene as matrix have shown that trapping properties are highly modified by the presence of nanofillers. A reduction of space charge accumulation under DC stress has been shown. In this communication, we report on electrical properties using dielectric and photoluminescence spectroscopy. Dielectric spectroscopy has been used to characterize ionic conduction and to evaluate the effect of the filler concentration on the dielectric permittivity. A relaxation process has been observed at high filler concentration, high temperature and low frequency. Spectral analysis of photoluminescence was performed with the aim of inferring the nature of trapping centers. It appears that nanocomposites made of iPP and sPP exhibit different behaviours.

Silica nanofilled varnish designed for electrical insulation of low voltage inverter-fed motors

The resistance to partial discharges of pure, microfilled and nanofilled Polyetherimide varnish for low voltage motors fed by inverters is presented. Micro and nano-sized Silica particles (1.5 % wt) are used as fillers. Experimental results of lifetime versus temperature (-55 to 180°C), frequency (5 to 15 kHz) and level of the applied square voltage (+/- 1 kV to +/- 3 kV) clearly demonstrate that nanofillers are more efficient than microfillers. The lifetime of nanofilled polyetherimide varnish is found to be up to thirty times higher than the standard one. Crucial properties for a varnish such as its electrical conductivity, its dc and ac dielectric strengths and its bonding strength are measured. Due to the presence of aggregates having a micrometric size, the microfilled varnish exhibits the worst characteristics, except for the ac dielectric strength which remains unchanged. These aggregates act as bridges for charge carriers, increasing the conduction current and reducing the resulting dc dielectric breakdown field. They also reduce the mechanical properties of the varnish.

Dependence of electroluminescence intensity and spectral distribution on ageing time in polyethylene naphthalate as modelled by space charge modified internal field

Wavelength-resolved electroluminescence and current measurements have been performed on polyethylene naphthalate thin films submitted to high direct current fields (3.4 MV cm-1) for several days. The change in the shape of the emission spectrum over time resembles that observed in short-term experiments when the field is increased. This behaviour provides the basis for a model in which the time evolution of the spectrum shape is attributed to the progressive charging of the material, through homopolar injection. The net result is an increase of the internal field in the charge-free region where luminescence occurs.

Influence of frequency, electrode material and superimposed dc on ac electroluminescence in polymer films

Summarizes new results on the influence of some experimental parameters on ac electroluminescence (EL) of polymers used as insulating materials in electrical engineering. Electroluminescence under a uniform ac field is investigated in low density polyethylene (PE) and polyethylene naphthalate (PEN) films with semi-transparent electrodes on both sides. Particular emphasis is given to polyethylene. The dependence of EL on the voltage frequency is investigated in the range of 10/sup -2/ Hz to 10/sup +2/ Hz with the help of phase-resolved measurements. The influence of the electrode/polymer contact is investigated by changing the electrode material (gold, silver, and calcium). The effect of a superimposed dc bias on the ac voltage is also reported.

Investigation of electrical and luminescent properties of polypropylene-based nanocomposite materials

Nano-structured polypropylene (PP) materials are investigated resorting to electrical and luminescence measurements, by means of several experimental tools, in order to evaluate their potentiality for application as electrical insulation and to understand the mechanisms of charge trapping relevant to the nano-structuration process. Both matrix compatibilization process and filler content appear as highly influential factors for dielectric properties.

Current conduction instabilities in polyethylene during heat cycles

Both low density and cross-linked polyethylene 1.5 mm-thick virgin samples have been submitted to a HVDC (20 kV) at room temperature. When the steady-state current was reached, the temperature was increased up to 70degC (ramp: 0.5degC/min). After a stay at 70degC, the temperature was then reduced. During this heat treatment, the external current was recorded. Analysis of this external current revealed instabilities (peak current); these instabilities has been correlated with some changes occurring in the structure of the studied polymers and revealed by TSC.

Electroluminescence in saturated polyesters: Temperature dependence and correlation with space charge measurements

This paper reports experimental investigations of poly(ethylene terephthalate) dealing with the electro-luminescence (EL) measurements at variable temperature under both DC and AC stress, with an over-pressure of nitrogen. The changes in the emission yield are discussed based on the temperature dependencies of photoluminescence and transport properties of these materials in correlation with space charge measurement.