Electrical properties of filled silicone rubber

Rubber materials for high-voltage outdoor applications have been studied. T hey were prepared as mixtures of silicone polymer with different concentrat ions of powdered aluminium trihydrate as a filler. The dielectric propertie s, chi*(omega) = chi' - i chi ", were measured at different temperatures us ing a low-frequency dielectric spectroscopy technique. Without any filler a classical interfacial relaxation was pronounced with dominating direct-cur rent (dc) conduction in the losses, chi ". On increasing the concentration of filler, a low-frequency dispersion mechanism started to dominate: the re laxation behaviour. On adding the filler, the de conduction first decreased and then increased again after a certain concentration level was passed. T his behaviour showed that the filler particles acted like scattering centre s or traps for conduction when the concentration was low. As the concentrat ion increased, the conductivity of the filler particles and of the interfac e (between the filler particles and the polymer matrix) started to dominate the conduction. Master curve shifts of the data showed an Arrhenius type o f activation. The data were modelled by superposition of three different pr ocesses, Havriliak-Negami dipolar relaxation, de conduction and low-frequen cy dispersion (hopping conduction) contributions, using a nonlinear least-s quares fitting method.