Alternative Coatings for Reed Switches

Abstract

A method of coating the contact areas of electrical reed switches was developed which reduced the amount of precious metals required, and allowed the development of alloy coatings not easily produced by electrode position. The method consisted of printing the metals on to the reed blanks in the form of a paste, composed of small metal particles mixed with an organic vehicle. The pastes were then heated to 800°C - 1000°C in an inert atmosphere, to drive off the vehicle and sinter the remaining particles. The sintered coating had up to 80% solid density with an adhesion to the substrate approaching that obtained from electrodeposition. The work was carried out with gold powders, which were produced from aqueous gold chloride solution by various reducing agents. The morphology of the gold powders was studied with the scanning electron microscope and a scheme of morphology changes outlined. This related the morphology changes to alterations in the rate of the reducing reaction. The best gold powders produced were spherical with a diameter of between 1 and 5um. The sintering of the deposits was found to be inhibited by firing in either vacuum or nitrogen 10% hydrogen mixture, and this was considered to be due to the presence of vapour phase transport of gold at the start of the sintering process. Firing in nitrogen or argon produced dense deposits with a surface finish comparable to that of worn electroplated reeds. The electrical resistance was slightly higher than that of electrodeposited metals. Alloys of gold with palladium, palladium and tin, palladium and nickel, iron and copper were examined. The morphology of sintered 1% alloys of nickel, iron and copper with gold were found to be discontinuous.