Insulator charging by contact with metals

Sammanfattning: In spite of the fact that charge transfer is one of the oldest physical phenomena known, the basic mechanisms involved are not yet quite understood. For experimental investigations of the charge transfer to insulators it is still informative to scrutinize electrification systems consisting of as simple components as possible. The choice of one metallic component is justified by the well-known electronic properties of metals.The charge transferred on (100) oriented surfaces of NaCl crystals from a sliding gold electrode has been measured as a function of sliding velocity and normal force between the electrode and the insulator surface. Besides (100) surfaces other crystallographic orientations of NaCl have been investigated with electrodes of different metals. A series of experiments comparing the sliding and the rolling modes of separation has also been carried out on (100) surfaces and it is shown that in broad outline the different modes of separation are consistent. The results from the investigations on metal-NaCl systems suggest that electrons are transferred into trapping levels in the forbidden energy range of the NaCl crystal.Electrification of insulators by metals is currently explained by uniform carrier injection to a constant depth for each particular material or by the formation of a strictly two-dimensional surface charge. Calculations concerning a more general model are presented, which includes both alternatives as special cases. An expression for the effective surface charge is obtained for the case where an electric field is applied across the insulator. Electrification experiments with polyethylene and various metals have been carried out, and the results are compared with the theoretical predictions. It is found that the assumption of a two-dimensional charge distribution leads to contradiction, and must be rejected. It is shown that the depth of injection in the bulk charge model ma^y be obtained from the experimental data and the formalism developed. The possibility of a non-uniform space charge density is also discussed.By means of special equipment which permits measurements of charging of capacitors due to a step voltage after the capacitors are classically fully charged, the electron injection has been studied as a function of time.This has been done on commercial polymeric capacitors as well as on gold evaporated foils of Teflon (PTFE) or polyethylene. The findings in the case3of polyethylene are correlated with the results on the field dependence of electrostatic charging and support the model of electron injection to a finite depth. An appendix will give some brief considerations concerning the release of electrification charge by exposing the insulator surface to light in the UV-range.