The Influence of the Ambient Gas Environment on the Non-Isothermal DC Electric Measurements in Polymers

The direct current (dc) electric measurements in dielectrics, especially for highly insulating materials, are strongly influenced by the ambient air, mainly by the water vapors contained by the ambient (humid) air which determine a significant increase of the electric conductivity. Beside this, there is a second effect related to the interaction of the absorbed and adsorbed gas molecules with the polymer matrix that modify both the surface and the bulk conductivities. The aim of the paper is to present the influence of the absorbed and/or adsorbed gas molecules on the electric charge trapping and/or detrapping in polyamide 11. The measurements were carried out under ambient (humid) air, in a vacuum at 8 x 10-3 Torr and in dry nitrogen. Because the trapping/detrapping processes and the molecule diffusion process in polymeric materials are slow processes, a variant of the thermally stimulated discharge current method, namely the final thermally stimulated discharge current method and the final isothermal discharge current method have been used in the temperature range from 20 to 190 0C. The absorbed/adsorbed gas molecules into polymer matrix results in a perturbation of the thermodynamic equilibrium and consequently a charge transfer could occur speeding up the deep level charge detrapping.