Tarnish

Abstract: An electrical conduction model for silver filled isotropically conductive adhesives (ICA) was developed The model combines the microscopic resistance of the bulk silver particles and the contact between silver flakes with the macroscale resistor network calculation by percolation theory The resistivities of the composites were calculated by resistor network simulations considering both contact effects and particle size effects Three different types of film typically exist on the silver surface: residual organic films; tarnish films; and a thin epoxy layer The contact resistance between silver flakes can be due to a constriction resistance, to the tunneling resistance through insulating films, or to the resistance of more conductive layers The constriction resistance is produced by the restriction of the current flow by small contact spots and is controlled by the actual contact spot area (metallic contact), which is dependent on the contact force between flakes The tunneling resistance is caused by the very thin layer which may reside on the silver flakes between the metallic contact spots, and is dependent on a barrier film thickness and potential Oxide and sulfide tarnish films are typically degenerate semiconductors Two- and three-dimensional (2-D and 3-D) computer simulations were performed to predict the effects of particle sizes, shapes, and distribution on the percolation conduction thresholds and cluster sizes The model predicts that the percolation threshold decreases with broad particle size distributions and high aspect ratio particles The effective resistivity of the adhesive depends on the thickness dimension of the adhesive pad geometry, with very thin layers resulting in high percolation thresholds and high resistivities Resistivity does not change with the pad thicknesses greater than a certain thickness level Silver flake orientation on the surface increases the resistivity of the conductive adhesive pads, but in the same magnitude range The resistivities of the materials are controlled by silver flake sizes and the nature of the contacts

Abstract: The structure and mechanism of formation of the tarnished layer formed on αj-brass when it is immersed in a copper sulphate–ammonium sulphate–ammonium hydroxide solution of pH 7·2 has been studied. The mode of formation of cracks in this layer when the brass is deformed has been investigated and the various shapes of crack observed are explained in terms of the orientation of the slip vector relative to the surface. It is shown that these cracks localize further chemical reactions when the specimens are subsequently re-immersed in the solution and that, by a process of repeatedly and separately deforming the specimen in tension and re-immersing it in the solution, small, transgranular, crack-like penetrations can be propagated into the brass. The relation of these observations to the phenomenon of stress corrosion of α-brass is discussed, and the general importance of passive, embrittled, surface layers to the stress-corrosion of other systems is indicated.