Study of charge injection phenomenon at the metal/insulating liquid interfaces

The goal of this paper is to study the transport phenomena occurring in mineral oils used as power transformers insulation. The main issue is to understand and predict the insulation behavior under different operating conditions. It is known that conduction phenomenon is explained through ionic carriers provided by dissociation phenomenon in the bulk of the insulating liquid when it is submitted to high voltage, but the problem is more complex at the metal/insulating liquid interfaces. For example, formation of the multilayer regions of space charge near the electrodes affects strongly the transport phenomenon, charge carriers injection from the metal into the dielectric liquid may be also possible. Using the isothermal charging currents measurements, the aim of the work is to study different metal/insulating liquid interfaces. We have associated various kinds of metallic electrodes (stainless steel, aluminium and brass) with the Borak22 mineral oil. The metal/liquid/metal structures were submitted to a DC voltage and the electrical charging current was measured with high sensitive electrometer. The results show clearly the effect of the barrier injection high for the three interfaces under high or low applied electric fields. Under high ones, this means that in spite of the formation of the Helmholtz and the diffusive layers, the injection phenomenon still exists but measurements under low applied fields allow announcing that charge carriers are injected at metal/insulating liquid interface according to the schottky low, so the electronic conduction remain possible.