Fundamental and applied aspects of contact electrification
Apparatus has been developed, which permits the measurement of electrification produced by rolling or sliding contact between a spherical specimen, and a plane dielectric sample. Measurements of the charge transferred to metal contact spheres, and the two-dimensional dielectric surface charge distribution, have been carried out under controlled conditions. The dissipation processes of air breakdown and bulk condution have been studied. The importance of metal work function, dielectric material, the normal force during contact, the mode of contact, and the time of contact has also been investigated. Contact electrification and charge dissipation occurring in a vibrated bed have been studied. The variables of interest were the bulk particle resistivity, the amplitude of vibration, and the. system electrode geometry. Theoretical models have been developed for the processes occurring, and the models could be used to interpret experimental observations. The relevance of the results to previous particle electrification studies, and to general particle handling situations has been discussed. Two new applications of particle charging have been examined. The first utilised particle contact electrification occurring naturally in a fluidised bed. By applying a suitable alternating electric field normal to an immersed heater surface, considerable increases in the heat transfer coefficient have been obtained, as a result of the imposed particle movement. The influence on the heat transfer coefficient, of the electric field magnitude and frequency, together with the fluidising air velocity, has been studied. The second application involved devising a novel means of electrostatic separation, using induction charging applied to the particles on the surface of a vibrated bed. A theoretical model for predicting particle collection rates has been developed, and verified by comparison with experimental observations. An efficient separation has been demonstrated on a combined size/resistivity basis.
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