Preparation and characterization of Ag-ZnO powders for applications in electrical contact materials

Abstract

Silver-metal oxide composites are used as electrical contact materials in relays, contactors, switches, circuit breakers and other switchgear components [1]. Variety of systems such as Ag-CdO, Ag-SnO2, Ag-CuO, Ag-ZnO, Ag-NiO, etc. are used/being developed as contact materials [2]. Ag-CdO, though pioneer among all these, suffers from a serious drawback of toxicity of cadmium fumes and hence is likely to be completely banned by the end of the century [3, 4]. Many investigators have worked on alternative systems to replace Ag-CdO [5, 6]. The physical, mechanical as well as electrical properties of contact materials strongly depend on the degree of mixing of silver and metal oxide phase and in turn on powder processing routes. A variety of routes such as the conventional powder metallurgy process involving mixing/blending of powders, internal oxidation of alloy powders (IOAP process), coprecipitation technique, freeze-drying, electroless coating method, etc. are being used or are under development [7-10]. In the present work an attempt is made to carry out a comparative study of Powders processed by mechanical mixing, spray-coprecipitation and electroless coating methods for the Ag-ZnO system. In the mechanical mixing method, Ag10 wt%ZnO powder was prepared by milling ARgrade -400mesh silver powder of 99.9% purity with stoichiometric amount of AR-grade zinc oxide powder in a ball mill (denoted as sample M). The spray-coprecipitation technique involved spraying of 1 molar mixed nitrate solution of silver and zinc into 1:1, 1 molar solution of potassium carbonate and sodium carbonate, keeping a larger excess of mixed carbonate solution for precipitation than necessary for stoichiometry. The resulting precipitates of carbonates of silver and zinc were filtered, washed, dried and fired at 550 °C for 90 min. The spray-coprecipitated powder was designated as sample S. The third approach to solving the problem of uniform dispersion of silver on zinc oxide particle surfaces was to make a zinc oxide sol and to completely coat the colloidal particles of zinc oxide with silver in an aqueous solution by an electroless coating process. In order to obtain a dense and