The Dielectric Properties of Silicone Fluids

The dielectric properties of polydimethylsiloxane fluids have been investigated under dc and ac fields as a function of field, frequency, temperature, and water content. The results of the ac studies have shown that the dielectric constant and dielectric loss of the fluid exhibit several transitions as a function of temperature at low temperatures. These transitions correlate reasonably well with structural changes in the fluid. The dielectric loss is also thermally activated and decreases rapidly with increasing frequency at ambient temperature. It is also weakly dependent on the applied field and the relative humidity. The dc properties exhibit a strong dependence of the conductivity on relative humidity. However, the dependence of the conductivity on the applied field is a complex one, and changes from an ohmic behavior at low fields, to a region of saturation at intermediate fields, and finally to an "ionization" region at high fields. The polydimethylsiloxane fluid also exhibits a reversal in the direction of the discharge current above a certain field, which is reminiscent of the electret effect observed in some polymeric solids. A tentative theoretical discussion of these properties is presented.