The importance of studying the electrostatic charging problem of particles, especially in dry powder processes, is self-evident for practitioners in particle technology.
In this review, the problems of contact or impact electrification of particulate materials are focused and described in detail. The major subject or the initial problem of this review could be specified here as a question of “What process dominates the amount of electrostatic charge? (at least for atmospheric conditions)” This was based on the fact that the typically observed charge density is comparable to that by which gaseous discharge in the atmosphere occurs. If gaseous discharge takes place, there is a possibility that the charges due to impact with a metal plate by particulate materials are observed as residual charges which as a result of charge relaxation during the separation process of the contact surfaces . This charge determining scheme is called “The CHARGE RELAXATION MODEL,” and to review the model in detail was the major object of this review.
Section 2 shows the principal results of the “impact charging experiments.” They are (i) the impact charge depends proportionally on the initial charge, (ii) the equilibrium charge, which is the initial charge when no net charge transfer occurs, is independent of the impact conditions such as the impact velocity and angle, and (iii) the impact charging is influenced not only by the impact velocity but also the impact angle. Hereafter, the models or schemes determining the impact charge should be tested by whether they can account for these results.
Section 3 describes the “simple condenser model” in detail. This model is the traditional and most simple understanding of the generation of contact electrification. It provides a first physical insight, and shows us qualitatively that there is a linear dependency of the impact charge upon the initial charge. However, the model has some essential difficulties, e.g., a quantitative prediction for the impact charging cannot be provided by this model.
Section 4 reviews the “charge relaxation model” in detail. This is a new model proposed as the mechanism dominating the charge generated on a particle due to impact. In the model, the charge relaxation process due to gaseous discharge dominates the impact charging of a particle in atmospheric conditions, and as a result, some difficulties of the simple condenser model are avoided. The conclusions derived from the model are summarized as follows:
(1) The model predictions are in good agreement with the “equilibrium charges” given as the results of impact charging experiments. Furthermore, the impact i charging experiments conducted in Ar gas indicated that the impact charging characteristics change corresponding to the breakdown limit potential of the environmental gas; this was also predicted quantitatively by the model.
(2) It was also shown that the model explains not only the “equilibrium charge,” but also the linear dependency of the impact charge on the initial charge.
(3) The measured impact charge increased and assumed the saturation value with increasing impact velocity and angle. This saturation value fell within the range around 110% of that predicted by the model.
(4) The two-dimensional relaxation on the particle surface in the discharge relaxation process predicted that charge would relax completely in the case of impact charging between a conducting particle and a metal plate. This was in accord with experimental results.
It should be emphasized here that the model provided good agreement with the experimental results without recourse to any empiricism.
Section 5 briefly reviews the mechanism of the charge generation. The charge generation process at contact and charge relaxation process which determines the actual impact charge needtobediscussedindependently. Ourreviewonthemechanismofthechargegeneration shows that there are many models which are applied to this problem, without the problem being clearly understood at present.
Section 6 is the summary section of this review, and a direction for the additional research which is recommended.
Additionally the Appendices provide, in detail, the precise mathematical procedures for the potential of a charged spherical particle located in the vicinity of a conducting plane.