Millisecond time-range analysis of space-charge distribution and electroluminescence in insulating polymers under transient electric stress

Abstract

The combined analysis of the space charge and electroluminescence (EL) in insulating polymers provides useful information on charge transport and energy dissipation. We are particularly interested in defining critical transport regimes leading to energy releases potentially harmful for the material. These critical regimes can be characterized by investigating the internal charge distribution when electroluminescence is detected. The dc case has been adequately covered: however, very few results are reported involving transient electric stress because its investigation needs a very fast space-charge detection setup. We have carried out such an analysis in the case of low-density polyethylene and polyethylene naphthalate (PEN) films by using fast acquisition techniques for both, the space-charge profile (resolution of 40μs in time and 15μm in depth) and the electroluminescence acquisition (250μs time resolution). The films were subjected to a voltage ramp up to a targeted level followed by a stabilization and a short circuit, the whole wave form lasting for 50 ms. Voltage rise rates from 300 to 3000kV∕s and fields up to 200kV∕mm were used as well as ac stress in a frequency range from 0.2 to 50 Hz. Mobile charges following the voltage application were in evidence in PEN. Two contributions have been distinguished in the EL excitation mechanisms: one is due to hot carrier impact processes driven by dV∕dt, and the other is linked with recombination and controlled by the dc transport properties of the material.