Charge injection, electroluminescence, and ageing of an epoxy resin in high divergent fields

Abstract

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link: 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...

Internal charge behaviour of nanocomposites

“…In the frequency range 0.1-100 Hz, the base-resin and nanocomposite behave in a similar way. There is possibly a small dielectric relaxation in this range, consistent with the findings of Griseri [15], shown by the real permittivity increasing with decreasing frequency. For the micro-filled particles this has an activation energy of 1.7 eV whereas, for the nano-filled material, this drops to 1.4 eV.…”

Re-examination of the dielectric spectra of epoxy resins: bulk charge transport and interfacial polarization peaks

“…A similar type of response has been observed in [4,5] for the same epoxy resin system. At sample temperatures of 20 and 40 °C, well below the glass transition temperature T g of 50 °C, the real part of the permittivity, ε', is constant over the measured frequency range and the imaginary component, ε", is dominated by instrumental noise at frequencies below 100 Hz.…”

“…The dielectric properties of diglycidyl ether of bisphenol-A (DGEBA) epoxy systems (in particular Araldite CY1301) have been studied previously using frequency-domain spectroscopy and several processes have been found to contribute to the overall spectrum [4,5]. These processes were classified as low-, mid-and high frequency according to their characteristic frequency in the range 10 -2 to 10 5 Hz.…”

“…These processes were classified as low-, mid-and high frequency according to their characteristic frequency in the range 10 -2 to 10 5 Hz. In [4] the low frequency process was attributed to DC conduction, the mid-frequency dispersion was related to the glass transition temperature and recognised as α-relaxation and no particular interpretation was given to the high frequency process. However, no account was taken of the level of absorbed moisture in the samples used in this work.…”

Electroluminescence excitation mechanisms in an epoxy resin under divergent and uniform field

“…Dielectric spectroscopy [31] has revealed the contribution of three polarization processes, one of which has been identified in the time dependence of the current. The contribution of dipolar orientation to the PEA response corresponds to a pure capacitive signal (only surface charge) as far as the dipolar orientation can be considered as uniform throughout the depth of the sample.…”