Electrification of fine particles by impact on a polymer film target

Impact charging between fine particles and polymer film has been investigated both theoretically and experimentally. A powder, fly-ash of 12 mum mass median diameter, was dispersed and accelerated in air flow, and then the particles were impacted on a target whose frontal surface was covered with a polymer film and the rear side was a metal plate. The films used were in the range of 25-250 mum thick. The rear metal plate was grounded and the electric currents caused by impact charging were measured with an electrometer. Two types of currents were detected, i.e. induction current and conduction current. The induction current was due to the charge accumulating on the film, whereas the conduction current was due to the dielectric breakdown arising from the particle impact. Using a film with small artificial holes, the conduction current was detected. This fact shows that insulating material can be utilized as an effective charging wall as long as the charge accumulation on the material is well suppressed.