Dielectric Permittivity, Conductivity, and Breakdown Characteristics of Polymer-MICA Composites

Novel and economically promising polyethylene (PE) mica composites containing up to 50% mica by weight have been produced by the following procedure: Highaspect-ratio mica flakes (-40 +70 mesh) were treated in a "cold" microwave plasma in ethylene vapor, then blended with PE powder and compression molded. Improvements in mechanical and rheological properties of plasma-treated composites have prompted us to carry out "diagnostic" measurements of the interfacial polarization [Maxwell-Wagner (M-W) effect]. As the major axes of the mica "ellipsoids" are well aligned by the molding process, Sillars' modification of M-W theory should describe the dielectric permittivity of the composites adequately: satisfactory agreement between experimental and theoretical data support this interpretation. The high-voltage insulating properties of the present composite materials have been evaluated by means of dc conductivity and breakdown measurements. A novel high-voltage endurance test based on a cylinder-plane geometry has been devised, where the cylinder is a thin wire electrode embedded beneath one surface of a sample plaque. This procedure has the advantage of providing an infinite number of possible initiation sites for breakdown, as required by the theory of statistical life testing. Experimental Weibull plots indicate, firstly, that breakdown characteristics are extremely sensitive to sample quality (e.g. residual porosity) and, secondly, that plasma treatment of the mica substantially enhances high-voltage endurance relative to that encountered with untreated mica. The ultimate value of mica as a filler in PE-based high-voltage insulation is, as yet, uncertain.