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 long-term electrical stability and dielectric breakdown of
insulating polymers is largely determined by the density and energy
distribution of the conduction and trapping levels. Although these materials
are widely used in technical applications very little is known about the nature
of the deep trapping centres which are responsible for charge storage. We show
that luminescence techniques can be used to gather information on their
nature. The proposed approach relies on photo- and recombination-induced
luminescence experiments. The latter has been specifically developed to record
the optical spectrum of the emission radiated by a deep trapping centre during
a recombination event. The work concerns low-density polyethylene, doped or
undoped, and cross-linked polyethylene. It is shown that the implication of a
given chemical in the trapping process can be probed by using the
recombination-induced luminescence technique.
Time-resolved space charge and electroluminescence measurements are carried out during 50 Hz AC voltage cycles to probe injection, trapping and recombination of charges in polyethylene films. Space charge detection allows defining a field value above which space charge accumulates. This field is well correlated with the onset for electroluminescence. The behavior is consistent with a model of bipolar injection. It is also shown that different polyethylenes behave differently regarding the field above which charge is injected, providing a way to compare the ability of different materials to sustain AC field. through space charge and conduction current measurements", in Proc. ICSD,
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