Contact charging mechanism

Electrostatic-Separation Machines The first electrostatic machines to be used commercially employed the principle of contact elec trification. These were free-fall devices incorporating large vertical plates between which an electrostatic field was maintained. Tribo-elec tric separation (contact charging) has experienced an increase in apphcations due to advances in mechanical self-cleaning and electrical design as well as the development of efficient precharging techniques. 

It is clear that the presence of electrostatic charges, whether due to contact charging, fractoemissions, or some other mechanism, will affect particle adhesion. However, to date there has been no satisfactory attempt made at properly integrating electrostatic forces into partiele adhesion theory. 

The ethylenediamine derivative [31] possesses higher promoting activities than other diamines. This phenomenon may be ascribed to the copromoting effect of the two amino groups on the decomposition of persulfate through a CCT (contact charge transfer complex) formation. So we proposed the initiation mechanism via CCT as the intimate ion pair and deprotonation via CTS (cyclic transition state) as follows ... 

Charging by contact electrification is an active mechanism whenever dissimilar particles make and break contact with each other, or whenever they slide over a chute or an electrode. This charging mechanism is most frequently used to charge selectively and obtain an electrostatic separation of two species of dielectric materials as realized in a free fall electrostatic separator. 

Mercury is a special type of liquid and behaves essentially as a metal for which charging mechanisms are discussed in Section V, F, 3. Dodd (D5) determined the charge acquired by mercury drops in contact with glass and various insulators. The mercury was always positively charged with ps values of the order of 0.01 V/micron. 

In this section we discuss five different materials as examples with different charging mechanisms mercury, silver iodide, oxides, mica, and semiconductors. Mercury is one example of an inert metal. Silver iodide is an example of a weakly soluble salt. Oxides are an important class of minerals. For most biological substances like proteins or lipids a similar charging process dominates. Mica is an example for a clay mineral. In addition, it is widely used as a substrate in surface force measurements and microscopy. We also included a general discussion of semiconductors because the potential in the semiconductor can be described similarly to the diffuse layer in electrolytes and there is an increasing effort to make a direct contact between a liquid or a living cell and a semiconductor. 

One of the important characteristics of gas-solid multiphase flows is concerned with the electrostatic effect. Particles can be charged by surface contact in a collision, by corona charging and scattering in an ionized gas, by thermionic emission in a high-temperature environment, and by other charging mechanisms such as colloidal propulsion... 

The actual mechanism for charging by surface contact is complicated, and many causes remain unresolved. However, it is clear that electron and/or ion transfers are principal factors responsible for the contact charging. More information is available in Montgomery (1959), Harper (1967), and Cross (1987). 

Chains, Stokes law with, 68-70 Chan, T., 120 Chapman, S. J., 168-169 Characteristic charge, 187 Characteristic length and diffusion, 155 Charging mechanisms, 179 collisions with ions, 185 contact electrification, 182-183 corona discharge, 195-198 diffusion, 185-189,195 electric, 179-183 equilibrium with, 200-201 steady-state theory of, 201-207 transient approach to, 207-208 field charging, 185,189-195 flame ionization, 184-185 and force, 179-180 frictional, 184... 

FIG. 19-55 Schematic representation of charging mechanisms. (A) Contact electrification. (B) Conductive induction. (C) Ion bombardment. Cond. = conductor particle diel. = dielectric particle = high-voltage dc electrode 0 = ions from corona discharge at high-voltage electrode. 

Although there is a body of theory to account for contact electrification and a large literature on the subject, it is sufficient here to emphasize that contact charging is a common phenomenon, probably impossible to avoid, and that it is intimately related to composition, electrical conductivity, and the mechanics of contact of surfaces. Because even monomolecular contamination can markedly affect both the amount and the sign of the charge, experimental investigations are notoriously erratic in their observations and conclusions. 

In the usual space-charge limited theory, electrons are injected into the insulator conduction band, and some of these electrons are immobilized in localized defect states. We have considered an alternate mechanism more appropriate to the polymer structure. Contact charge transfer studies in Polyethylene Terephthalate (PET) and other polymers (15-16) suggest that the electronic states accessible from metal contacts are localized molecular-ion states located deep in the forbidden energy gap. Charge transport is by hopping between localized states. 

The dielectric polymer particles can be electrostatically charged in the outer electric field, in the gas discharge, following the contact electrification mechanism, or through triboelectrization. The presence of Cl, whose electric conductivity differs from that of the polymer, in the powder composition considerably affects these mechanisms. 

Powder particles may be charged by the contact electrification mechanism, when they acquire their charge on contact with the electrodes, or by the ion absorption mechanism, when air is ionized under the gas discharge effect followed by charge transfer from ions to powder particles. 

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