PREPARATION OF ELECTRET TRANSDUCER ELEMENTS BY APPLICATION OF CONTROLLED BREAKDOWN ELECTRIC FIELD

1363717 Electrets WESTERN ELECTRIC CO Inc 26 Oct 1971 [2 Nov 1970] 49605/71 Heading H1M In a method of manufacturing an electret from a polymer foil, the foil 10, optionally metallized at 12, is placed, with an insert 23 of dielectric material, between electrodes 21, 22 and a potential difference is applied, sufficient to cause partial breakdown in the foil 10 but insufficient to cause breakdown in insert 23. Preferably the thickness and resistivity of the material of layer 23 are chosen so that the resistance of the-layer is greater than that of foil 10, and the voltage is chosen to give a current density of 10-8 to 10-6A/cm.2. Layer 23 may be soda-lime glass of resistance 109 to 1012 ohms for an area of 10 to 100 cm.2. The polymer may be polyethylene terephthalate, P.T.F.E., or a, fluorinated ethylene polymer. The process may be carried out at room temperature and atmospheric pressure. The electret formed has a homocharge and is suitable for use in an electroaccoustic transducer (Fig. 1, not shown).