High temperature breakdown strength and voltage endurance characterization of nanofilled polyamideimide

For high temperature wire enamel application, the dielectric properties of nanoscale silica and alumina filled polyamideimide were characterized as a function of filler loading, and temperature. It was found that although the dc breakdown strength improved for both composite systems, only the alumina also exhibited an increase in ac breakdown strength. In addition, the voltage endurance at elevated temperature in both a needle/plate and twisted wire geometry was improved over a narrow range of filler loading (5-7.5 wt%). At room temperature, pulsed electroacoustic analysis (PEA) data does not indicate any difference in space charge buildup in the composite for fields up to 25 kV/mm over a period of 2 h. This is in contrast to other nanocomposite systems, which tend to show mitigation of space charge buildup with the addition of nanoparticles. Dielectric spectroscopy shows that the addition of nanoparticles increases the real permittivity above that expected for volumetric mixing rules. This enhancement is associated with high effective permittivity interfacial areas. No systematic changes in losses in the frequency range between 10^-4 and 10⁵ Hz were seen. The results suggest that carrier scattering is the primary mechanism leading to the improved breakdown strength.