Dielectric properties of polymer-based microheterogeneous insulator

The study of the dielectric behavior of polymer-based insulating materials used in high voltage equipment is presented in this work. The materials tested are microheterogeneous in nature with mineral particles (calcium fluoride) dispersed in a semicrystalline polymeric matrix (polytetrafluoroethylene). The dielectric properties are analyzed in a wide frequency (or time) range by a combination of two techniques: the short time response is assessed by measuring the complex permittivity in the frequency domain (20 Hz-1 MHz), and the long term response is studied by measuring the polarization current in the time domain (1-600 s) thus covering more than eight decades in frequency or time. The modelling used to describe microheterogeneous dielectrics is shown to fit the experimental data. However, exposure to moisture is shown to induce strong relaxational effects. These effects vanish in a reversible way by drying treatment (i.e. removal of water molecules). These additional relaxations are discussed in term of conductivity effects occurring at the interface matrix-filler particles. When mechanical and dielectric spectroscopy studies are compared, it is to be underlined: (i) that the presence of water molecules has no significant effects on internal friction spectra; and (ii) solid state transformations in the non polar polymeric matrix induce strong mechanical and non significant dielectric relaxations. This is an example of complementary relaxation spectroscopy techniques when the coupling between the atomic motions and the excitation is not the same.