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industrial controls, motor controls, and protective devices (Table 2.13).
*===Silver-cadmium oxide (DODURIT CdO) materials===
Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are produced
b) parallel to extrusion direction
*===Silver–tin oxide(SISTADOX)materials===Over the past years, many Ag/CdO contact materials have been replaced byAg/SnO based materials with 2-14 wt% SnO 2 2 because of the toxicity ofCadmium. This changeover was further favored by the fact that Ag/SnO2contacts quite often show improved contact and switching properties such aslower arc erosion, higher weld resistance, and a significant lower tendencytowards material transfer in DC switching circuits (Table 2.30). Ag/SnO2materials have been optimized for a broad range of applications by other metaloxide additives and modification in the manufacturing processes that result indifferent metallurgical, physical and electrical properties (Table 2.29). Manufacturing of Ag/SnO2 by ''internal oxidation'' is possible in principle, butduring heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxidelayers formed on the surface of the material prohibit the further diffusion ofoxygen into the bulk of the material. By adding Indium or Bismuth to the alloy theinternal oxidation is possible and results in materials that typically are rather hardand brittle and may show somewhat elevated contact resistance and is limitedto applications in relays. To make a ductile material with fine oxide dispersion(SISTADOX TOS F) (Fig. 2.114) it is necessary to use special process variationsin oxidation and extrusion which lead to materials with improved properties inrelays. Adding a brazable fine silver layer to such materials results in a semifinishedmaterial suitable for the manufacture as smaller weld profiles(SISTADOX WTOS F) (Fig. 2.116). Because of their resistance to materialtransfer and low arc erosion these materials find for example a broaderapplication in automotive relays (Table 2.31). ''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO2contact materials. Besides SnO2 a smaller amount (<1 wt%) of one or moreother metal oxides such as WO3, MoO3, CuO and/or Bi2O3 are added. Theseadditives improve the wettability of the oxide particles and increase the viscosityof the Ag melt. They also provide additional benefits to the mechanical andarcing contact properties of materials in this group ''(Table 2.26)''. In the manufacture the initial powder mixes different processes are appliedwhich provide specific advantages of the resulting materials in respect to theircontact properties ''(Figs. 2.87 – 2.119)''. Some of them are described here asfollows::a) Powder blending from single component powders:In this common process all components including additives that are partof the powder mix are blended as single powders. The blending isusually performed in the dry stage in blenders of different design. b) Powder blending on the basis of doped powdersFor incorporation of additive oxides in the SnO powder the reactive spray 2process (RSV) has shown advantages. This process starts with a waterbasedsolution of the tin and other metal compounds. This solution isnebulized under high pressure and temperature in a reactor chamber.Through the rapid evaporation of the water each small droplet is convertedinto a salt crystal and from there by oxidation into a tin oxide particle in whichthe additive metals are distributed evenly as oxides. The so created dopedAgSnO powder is then mechanically mixed with silver powder.c) Powder blending based on coated oxide powdersIn this process tin oxide powder is blended with lower meting additive oxidessuch as for example Ag MoO and then heat treated. The SnO particles are 2 4 2coated in this step with a thin layer of the additive oxide.d) Powder blending based on internally oxidized alloy powderse) Powder blending based on chemically precipitated compoundpowders A combination of powder metallurgy and internal oxidation this process startswith atomized Ag alloy powder which is subsequently oxidized in pureoxygen. During this process the Sn and other metal components aretransformed to metal oxide and precipitated inside the silver matrix of eachpowder particle.A silver salt solution is added to a suspension of for example SnO together 2with a precipitation agent. In a chemical reaction silver and silver oxiderespectively are precipitated around the additive metal oxide particles whoact as crystallization sites. Further chemical treatment then reduces the silveroxide with the resulting precipitated powder being a mix of Ag and SnO .
