A n experimental study has shown that the AC short-term dielectric breakdown potential of material specimens consisting of polyhedral oligomeric silsesquioxane (POSS) nanoparticles dispersed within an epoxy matrix exceeds the AC short-term dielectric breakdown potential of equal-thickness specimens of the neat epoxy. This finding is consistent with results of prior studies in which it was found that inorganic nanoparticle fillings yielded improvements in electrical, mechanical, or thermal properties.

The epoxy matrix resin (Epon 828 resin) in this study was used with an aliphatic diamine curing agent (Jeffamine D400). Specimens consisting, variously, of the neat epoxy or the epoxy filled with 5 weight percent of nanoparticles of trisilanolphenyl- POSS were fabricated by use of a fixture designed specifically for this purpose. In order to make the dispersion of the nanoparticles in the epoxy as nearly uniform as possible, it was necessary to employ a combination of thermal and mechanical agitation techniques during the blending of the nanoparticles into the epoxy. All specimens were cured by heating for two hours at a temperature of 80 ºC, then three hours at 120 ºC. The specimens had an average thickness of 0.17 mm.

Breakdown Potentials were measured in tests of 0.17-mm-thick specimens of neat epoxy and of epoxy filled with 5 weight percent of POSS nanoparticles.
Short-term dielectric-breakdown tests were performed on the specimens by use of electrodes specified in standard D149 of the American Society for Testing and Materials (ASTM). The electrodes were mounted in a fixture designed to hold a specimen and inhibit surface flashover (without need for an insulating gas or liquid) while enabling bulk breakdown to occur. The electrodes were connected into a circuit that limited the breakdown current to about 40 mA. In each test, AC potential at a frequency of 60 Hz was applied to the electrodes at a rate of about 1 kV/s. The average breakdown potential of the epoxy-filled specimens was found to be about 34 percent greater than that of the neat epoxy specimens (see figure).

High-resolution scanning electron microscopy of fracture surfaces of randomly selected POSS-containing specimens showed that the POSS nanoparticles were well dispersed, down to a length scale between 10 and 100 nm, limited by agglomeration of the nanoparticles into clumps 10 to 20 nm wide. The high degree of uniformity of dispersion appeared to be critical to the increase in the dielectric-breakdown potential.

This work was done by John C. Horwath, Daniel L. Schweickart, and Guido Garcia of the Air Force Research Laboratory; Donald Klosterman, Mary Galaska, and Amanda Schrand of the University of Dayton Research Institute; and Lawrence C. Walko of Innovative Scientific Solutions, Inc.


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Improved Dielectric-Breakdown Property of POSS-Filled Epoxy

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This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine.

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