Composite powders contact charging

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Contact electrification of insulative materials, predominantly in film form, has been studied in many laboratories. In this paper, electric field dependent charging of polymeric and polymer-carbon black powders in contact with a metallic electrode has been studied. Results show the charging behavior to be strongly dependent on the composition of the powder surface. Carbon black loading, type of carbon black and degree of dispersion are methods used to alter the powder surface. The field dependent contribution to the charge exchange dominates over the zero field values. 

The purpose of these experiments is to study the effects of particle composition, preparation techniques and applied electrode potential on charge exchange with a metallic electrode in a specific contact configuration. The contact area has not been quantitatively defined in this configuration, but is presumed to be independent of powder composition and flow properties, and remain constant in all these measurements. Equilibrium charge exchange is not attained with these techniques. The electric field experienced by the particles is not easily calculated... 

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. 

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