An Air Filter Made from an Electret and a Conductor

Electrically charged filter fibers, and, in particular, electret fibers, can be used in air filters. Planar electrets have a high internal and a low external electric field; but only external fields contribute to filtration, and so if electrets are used in filters, the fundamental problem is to increase the external field. In a conventional electret filter this problem is solved simply by reducing the width of the electret sheet by making it into fibers, but an alternative solution is to place conductors close to the surface of the electret. A filter with a low resistance to airflow can be produced by this means, but the level of charge that can be sustained on the electret is limited by dielectric breakdown in the air between dielectric and conductor. The total charge on such a filter has been measured microscopically by X-irradiation, and macroscopically with a probe; the two methods give similar results. The electric field inside the filter has been calculated by a self-consistent method. Specimen results for filtration efficiency are given.