Surface charging: Field and photo effects

Charge build-up on the surface of a dielectric is investigated on the injection of charge from electrodes lying flush with the insulator surface. The materials tested were 19 μm PET (polyethylene terephthalate) and 25 μm PI. Some tests were also done on LDPE (low-density polyethylene) and Nylon 6-6. UV (ultraviolet) irradiation was carried out with a 4 W short wavelength tube (254 nm), giving an intensity of about 200 μW/cm² at the sample plane. The results suggest that there is little effect without UV light. In gaseous ambients, there is homocharge injection at the electrode edges, presumably due to photoionization of the plastic near the anode, and of the electrode metal near the cathode. Once launched, negative charges propagate along the surface with a mobility comparable to a bulk mobility typical of ionic motion. In vacuum, a general photoionization of the surface also occurs, and this builds up until the original field at the surface is annulled