Community Research and Development Information Service - CORDIS

New polyester film for electrical capacitor manufacture

The trend to miniaturization means that film capacitors must use thinner and thinner films as dielectrics which increases the relative importance of the interface. One major question is the effect of a proportionally larger interface on dielectric behaviour or on leakage currents. The leakage behaviour of a material is characterized by its insulation resistance which is the ration of applied voltage to leakage current. A comprehensive analysis of conduction mechanisms in metallized polyester was carried out to improve the polyester's IR, and, specifically, to explain observed IR variations after heat and pressure treatments.

Standard ASTM methods for measuring these currents were not applicable in the case of ultrathin films and new measuring techniques have been developed. It was shown that, for standard capacitor geometries, the volume currents dominate the surface currents by roughly two orders of magnitude and thus determine the capacitor's insulation resistance.

The conduction at the interface has been shown to proceed via carrier injection at a Richardson-Schottky barrier. Contrary to expectations, however, the nature of the interface does not influence the volume charging currents. For temperatures below the polymer's glass transition, they are largely determined by the film's processing history, that is by bulk phenomena such as free volume relaxations in the amorphous phases of the polymer. This recognition has led to a modification in the processing conditions and thus to improvements in the insulation resistance of the final components, eliminating one of the problems which plagued the capacitor industry for a long time.

Reported by

Du Pont de Nemours SA
2984 Luxembourg
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