Surface resistance measurements on epoxy composites: Influence of surrounding gas, pressure, temperature, and analysis of the origin of measured currents

Surface resistance measurements on insulating solids are usually performed by measuring the current induced when an electric field tangential to the surface is applied. The measured current results from the addition of currents flowing along the surface, and also partly in the volume. In this paper, surface current measurements on an alumina-filled epoxy resin are performed under varying conditions of temperature (40-60°C), relative humidity (0-80 %), gas nature (SF₆, N₂, air), pressure (0.1-0.8 MPa). In dry conditions, measured currents do not depend on the gas nature and pressure. In air, when the relative humidity (RH) is varied, a large influence is measured, and the recorded variation of surface currents includes two phases: a rapid one attributed to the immediate change of surface resistance, followed by a much slower variation attributed to the progressive impregnation of the material by water. Numerical simulation shows the dominant influence of surface properties on measured currents, and evidences the existence of a surface layer with a higher conductivity compared to the material volume, even in dry conditions.