Factors influencing the permittivity of polymer/ceramic composites for embedded capacitors

The effect of the polymer permittivity on the permittivity of the polymer/lead magnesium niobate-lead titanate (PMN-PT) composite was investigated using epoxies, polyimides, polycarboxylic acids, and poly(methyl methacrylate) (PMMA). The permittivity of the polymers (without ceramic loading) ranged from 2.8-4.6. The permittivity of the composites, except for epoxies, was found to agree with the value predicted by the Smith equation. Polyacrylic acid (PAA) was found to bond to the PMN-PT surface due to an acid-base reaction. Novolac epoxy was not found to interact strongly with PMN-PT. The results suggest that polycarboxylic acids can act as a surfactant to deagglomerate PMN-PT powder while epoxies might not. Hence, the lower permittivity found in the epoxy based composites may be due to particle agglomeration. Three out of eight phosphated ester type surfactants were shown to increase the composite permittivity, indicating that the most effective surfactants decreased agglomeration of the ceramic powder and improved the dispersion in the polymer matrix. The particle size of the PMN-PT in the same solvent as used in the composite experiment was reduced by the use of ball-milling with the aid of the surfactant. Assuming that agglomerated particles act as a larger particle, an electrostatic field simulation was used to determine that the reduction in particle size due to deagglomeration increased the permittivity of the composite. These results were confirmed by experiment.