Influence of low pressure on electrets behaviour at various air gaps

Polypropylene samples, 20 μm thick, charged in a negative corona were studied. Each sample was placed on a metal pad. The samples were charged to different initial surface potentials, by means of a conventional corona triode system. The negative corona voltage was 5 kV and the grid voltages of the same polarity were 350 V, 500 V, 650 V, 800 V and 950 V. After measuring the surface potential, the samples, together with their metal pads, were placed for 30 min between two short circuited plate electrodes in a vacuum chamber under various low pressures from 10 Pa to 1 × 105 Pa. The air gap thicknesses between the charged surface of electrets and the upper electrode were 0.28 mm, 0.84 mm, 1.69 mm and 3.00 mm, respectively. The results obtained show that there is a pressure range within which the surface potential sharply decays. The sharp surface potential decay depends on the pressure-to-initial surface potential ratio and it is different for different air gap thicknesses and for different initial surface potentials. A spark breakdown in the air gap according to the Paschen’s law and processes of desorption from the electret surface are assumed to occur.