Embedded sphere method for measuring dielectric breakdown in polymers and polymer composites

This work describes a new approach, the embedded sphere method, for measuring the dielectric breakdown field strength of pure polymers and polymer composites. In this method, a polished metal sphere is pressed into a polymer film heated above its glass transition temperature, resulting in partial embedment of the sphere. The degree of embedment can be varied by controlling the initial film thickness and the heating time and temperature. After cooling, we measure pre-breakdown leakage current and the breakdown voltage. Then we remove the sphere, confirm that breakdown occurred at the center of the indentation left by the sphere, and measure the minimum thickness at the center of the indentation. The embedded sphere method minimizes incidence of premature breakdown due to air near the point of contact. Scatter in breakdown field strength data is reduced when the indentation depth is greater than the average surface roughness of the film. We show breakdown field strength results for spin cast polystyrene films and composites.