Changes

Over the past years, many Ag/CdO contact materials have been replaced by Ag/SnO₂ based materials with 2-14 wt% SnO₂ because of the toxicity of Cadmium. This changeover was further favored by the fact that Ag/SnO₂ contacts quite often show improved contact and switching properties such as lower arc erosion, higher weld resistance and a significant lower tendency towards material transfer in DC switching circuits (<xr id="tab:Contact and Switching Properties of Silver–Metal Oxide Materials"/>)<!--(Table 2.30)-->. Ag/SnO₂ materials have been optimized for a broad range of applications by other metal oxide additives and modification in the manufacturing processes that result in different metallurgical, physical and electrical properties(<xr id="tab:tab2.28"/><!--(Tab. 2.28)--> und and <xr id="tab:tab2.29"/>)<!--(Table 2.29)-->.

Manufacturing of Ag/SnO₂ by ''internal oxidation'' is possible in principle, but during heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxide layers formed on the surface of the material prohibit the further diffusion of oxygen into the bulk of the material. By adding Indium or Bismuth to the alloy, the internal oxidation is possible and results in materials that typically are rather hard and brittle and may show somewhat elevated contact resistance and is limited to applications in relays. Adding a brazable fine silver layer to such materials results in a semifinished material, suitable for the manufacture as smaller weld profiles (<xr id="fig:Micro structure of Ag SnO2 92 8 WTOS F"/>)<!--(Fig. 2.116)-->. Because of their resistance to material transfer and low arc erosion, these materials find for example a broader application in automotive relays (<xr id="tab:Application Examples of Silver–Metal Oxide Materials"/>)<!--(Table 2.31)-->.