Wear-out and breakdown of polymers used as insulating materials are closely related to the dissipation of electrostatic energy in space charge regions. A fraction of this energy is radiated off by electroluminescence which can be considered as a pre-breakdown warning and provides a probe to investigate the mechanisms leading to structural degradation. Electroluminescence emission from two aromatic polyesters is studied - poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate). We investigate the excitation processes of the luminescent centres, the nature of the centres, and the relationship of the emission features to electrical ageing. Sample electroluminescence is excited by dc and ac voltages under a uniform field configuration. The discussion relies on the analysis of the charge transport process in the luminescent regime, the acquisition of electroluminescence and photoluminescence spectra, and the investigation of the field effect on photoluminescence.
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.
The dielectric and mechanical properties of standard (PEI), nanoscale-filled and microscale-filled varnishes are detailed and analyzed in this communication. Fumed silica (10nm and 400nm) have been tested as a filler. A particular attention has been paid to Partial Discharge Inception Voltage, Lifetime under pulse voltage and bonding strength which are crucial for varnish to be used in rotating machines fed by inverters.
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.
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INTRODUCTIONMost experimental studies of electrical ageing have concentrated on semi-crystalline polymers such as those used in cable insulation and capacitors (see for example [1]). Theoretical models [2][3][4] for electrical ageing have been developed on the basis of these studies. The consensus is that ageing involves the formation of lowdensity regions, though the mechanisms responsible are disputed. For example, bond scission by high-energy electrons [2,5], and mechanical deformation have both been suggested. Both of these mechanisms are related to charge injection and the subsequent formation of high local fields. The semi-crystalline polymers studied so far have similar chemistries and almost identical morphologies. They tend, therefore, to show many similarities in, for example, the size of the energy barriers for the ageing reaction, critical ageing levels, and field dependence of ageing [4]. These similarities make it difficult to discriminate between mechanisms. Epoxy resins, however, are network polymers with a different molecular chemistry to that of the semi-crystalline polymers and are thus ideal to evaluate the proposed ageing mechanisms. We have therefore studied an epoxy resin (CY1301) under both uniform field and high divergent field conditions. Uniform field conditions were used to gain baseline characteristics for the properties of the unaged epoxy resin, and also for the effects of electrical ageing in low fields. Studies in high divergent fields were made using an electrode arrangement adapted from that of [6]. A number of wires set approximately 0.5mm apart were embedded, parallel to the flat faces, in thin (∼290 µm ) flat samples. The radius of the wires ranged from 5 µm (gold plated tungsten) to 25 µm (tungsten). Relatively small voltages applied to the wires (≤5 kV DC) therefore produced local fields up to 170 kV/mm depending upon the wire radius chosen. These field levels are high enough to inject space-charge [6] without leading to instantaneous failure. This geometry, therefore, may both inject charge and simulate local stress enhancements arising from charge accumulation. The number of wires is large (∼30) so that the volume affected is big enough to allow changes on ageing to be detectable.
EXPERIMENTAL INVESTIGATIONS
Dielectric ResponseThe dielectric response of the epoxy exhibited a glass transition just below 40 ˚C [7]. At this temperature and above a loss peak was observed at low frequencies that merged into a power law response at higher frequencies and eventually into a high frequency loss peak at ∼3 10 5 Hz. A dc conductivity masking the low frequency side of the alpha peak was observed at T≥70 ˚C, figure 1. An Arrhenius plot of the low frequency peak show the approach to zero frequency typical of the alpha response of a glass transition, figure 2. The power law response is less clear-cut as it is partially obscured by the two...
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