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→Silver-Metal Oxide Materials Ag/CdO, Ag/SnO2, Ag/ZnO
The manufacturing of strips and wires by internal oxidation starts with a molten alloy of silver and cadmium. During a heat treatment below it's melting point in a oxygen rich atmosphere in such a homogeneous alloy the oxygen diffuses from the surface into the bulk of the material and oxidizes the Cd to CdO in a more or less fine particle precipitation inside the Ag matrix. The CdO particles are rather fine in the surface area and are becoming larger further away towards the center of the material <xr id="fig:fig2.83"/> (Fig. 2.83).
During the manufacturing of Ag/CdO contact material by internal oxidation the processes vary depending on the type of semi-finished material. For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followed by wire-drawing to the required diameter <xr id="fig:fig2.77Strain hardening of internally oxidized AgCdO9010"/> (Figs. 2.77) and <xr id="fig:fig2.78Softening of internally oxidized AgCdO9010"/> (Fig. 2.78). The resulting material is used for example in the production of contact rivets. For Ag/CdO strip materials two processes are commonly used: Cladding of an AgCd alloy strip with fine silver followed by complete oxidation results in a strip material with a small depletion area in the center of it's thickness and a Ag backing suitable for easy attachment by brazing (sometimes called “Conventional Ag/CdO”). Using a technology that allows the partial oxidation of a dual-strip AgCd alloy material in a higher pressure pure oxygen atmosphere yields a composite Ag/CdO strip material that has besides a relatively fine CdO precipitation also a easily brazable AgCd alloy backing <xr id="fig:fig2.85"/> (Fig. 2.85). These materials (DODURIT CdO ZH) are mainly used as the basis for contact profiles and contact tips.
During powder metallurgical production the powder mixed made by different processes are typically converted by pressing, sintering and extrusion to wires and strips. The high degree of deformation during hot extrusion produces a uniform and fine dispersion of CdO particles in the Ag matrix while at the same time achieving a high density which is advantageous for good contact properties <xr id="fig:fig2.84"/> (Fig. 2.84). To obtain a backing suitable for brazing, a fine silver layer is applied by either com-pound extrusion or hot cladding prior to or right after the extrusion <xr id="fig:fig2.86"/> (Fig. 2.86).
For larger contact tips, and especially those with a rounded shape, the single tip Press-Sinter-Repress process (PSR) offers economical advantages. Thepowder mix is pressed in a die close to the final desired shape, the “green” tips are sintered, and in most cases the repress process forms the final exact shape while at the same time increasing the contact density and hardness.
Using different silver powders and minor additives for the basic Ag and CdO starting materials can help influence certain contact properties for specialized applications.
<xr id="fig:fig2.77Strain hardening of internally oxidized AgCdO9010"/> Fig. 2.77: Strain hardening of internally oxidized Ag/CdO 90/10 by cold working
<xr id="fig:fig2.78Softening of internally oxidized AgCdO9010"/> Fig. 2.78: Softening of internally oxidized Ag/CdO 90/10 after annealing for 1 hr after 40% cold working
<xr id="fig:fig2.79Strain hardening of AgCdO9010P"/> Fig. 2.79: Strain hardening of Ag/CdO 90/10 P by cold working
<xr id="fig:fig2.80"/> Fig. 2.80: Softening of Ag/CdO 90/10 P after annealing for 1 hr after 40% cold working
<div class="multiple-images">
<figure id="fig:fig2.77Strain hardening of internally oxidized AgCdO9010">
[[File:Strain hardening of internally oxidized AgCdO9010.jpg|left|thumb|<caption>Strain hardening of internally oxidized Ag/CdO 90/10 by cold working</caption>]]
</figure>
<figure id="fig:fig2.78Softening of internally oxidized AgCdO9010">
[[File:Softening of internally oxidized AgCdO9010.jpg|left|thumb|<caption>Softening of internally oxidized Ag/CdO 90/10 after annealing for 1 hr after 40% cold working</caption>]]
</figure>
<figure id="fig:fig2.79Strain hardening of AgCdO9010P">
[[File:Strain hardening of AgCdO9010P.jpg|left|thumb|<caption>Strain hardening of Ag/CdO 90/10 P by cold working</caption>]]
</figure>
</div>
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Over the past years, many Ag/CdO contact materials have been replaced by Ag/SnO<sub>2</sub> based materials with 2-14 wt% SnO<sub>2</sub> because of the toxicity of Cadmium. This changeover was further favored by the fact that Ag/SnO<sub>2</sub> 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:tab2.30"/> (Table 2.30). Ag/SnO<sub>2</sub> 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 <xr id="tab:tab2.29"/> (Table 2.29).
Manufacturing of Ag/SnO<sub>2</sub> 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. To make a ductile material with fine oxide dispersion (SISTADOX TOS F) <xr id="fig:fig2.114"/> (Fig. 2.114) it is necessary to use special process variations in oxidation and extrusion which lead to materials with improved properties in relays. Adding a brazable fine silver layer to such materials results in a semifinished material suitable for the manufacture as smaller weld profiles(SISTADOX WTOS F) <xr id="fig:fig2.116"/> (Fig. 2.116). Because of their resistance to material transfer and low arc erosion these materials find for example a broaderapplication in automotive relays <xr id="tab:tab2.31"/> (Table 2.31).
''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO<sub>2</sub> contact materials. Besides SnO<sub>2</sub> a smaller amount (<1 wt%) of one or more other metal oxides such as WO<sub>3</sub>, MoO<sub>3</sub>, CuO and/or Bi<sub>2</sub>O<sub>3</sub> are added. These
