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*Silver–tin oxide(SISTADOX)materials
Over the past years, many Ag/CdO contact materials have been replaced by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 2 </sub> because of the toxicity ofCadmium. This changeover was further favored by the fact that Ag/SnO2SnO<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 ''(Table 2.30)''. Ag/SnO2SnO<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 ''(Table 2.29)''.
Manufacturing of Ag/SnO2 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
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) ''(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) ''(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 ''(Table 2.31)''.
''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO2SnO<sub>2</sub>contact materials. Besides SnO2 SnO<sub>2</sub> a smaller amount (<1 wt%) of one or moreother metal oxides such as WO3WO<sub>3</sub>, MoO3MoO<sub>3</sub>, CuO and/or Bi2O3 Bi<sub>2</sub>O<sub>3</sub> are added. These
additives improve the wettability of the oxide particles and increase the viscosity
of the Ag melt. They also provide additional benefits to the mechanical and
:'''a) Powder blending from single component powders''' <br> In this common process all components including additives that are part of the powder mix are blended as single powders. The blending is usually performed in the dry stage in blenders of different design.
:'''b) Powder blending on the basis of doped powders''' <br> For incorporation of additive oxides in the SnO2 SnO<sub>2</sub> powder the reactive spray process (RSV) has shown advantages. This process starts with a waterbased solution of the tin and other metal compounds. This solution is nebulized under high pressure and temperature in a reactor chamber. Through the rapid evaporation of the water each small droplet is converted into a salt crystal and from there by oxidation into a tin oxide particle in which the additive metals are distributed evenly as oxides. The so created doped AgSnO2 powder is then mechanically mixed with silver powder.
:'''c) Powder blending based on coated oxide powders''' <br> In this process tin oxide powder is blended with lower meting additive oxides such as for example Ag2 MoO4 Ag<sub>2</sub> MoO<sub>4</sub> and then heat treated. The SnO2 SnO<sub>2</sub> particles are coated in this step with a thin layer of the additive oxide.
:'''d) Powder blending based on internally oxidized alloy powders''' <br> A combination of powder metallurgy and internal oxidation this process starts with atomized Ag alloy powder which is subsequently oxidized in pure oxygen. During this process the Sn and other metal components are transformed to metal oxide and precipitated inside the silver matrix of each powder particle.
:'''e) Powder blending based on chemically precipitated compound powders''' <br> A silver salt solution is added to a suspension of for example SnO2 SnO<sub>2</sub> together with a precipitation agent. In a chemical reaction silver and silver oxide respectively are precipitated around the additive metal oxide particles who act as crystallization sites. Further chemical treatment then reduces the silver oxide with the resulting precipitated powder being a mix of Ag and SnO2SnO<sub>2</sub>.
Further processing of these differently produced powders follows the
Fig. 2.87:
Strain hardening of
Ag/SnO <sub>2</sub> 92/8 PE by cold working
Fig. 2.88:
Softening of
Ag/SnO <sub>2</sub> 92/8 PE after annealing
for 1 hr after 40% cold working
Fig. 2.89:
Strain hardening of
Ag/SnO <sub>2</sub> 88/12 PE by cold working
Fig. 2.90:
Softening of Ag/SnO <sub>2</sub> 88/12 PE
after annealing for
1 hr after 40% cold working
Fig. 2.91:
Strain hardening of oxidized
Ag/SnO <sub>2</sub> 88/12 PW4 by cold working
Fig. 2.92:
Softening of Ag/SnO <sub>2</sub> 88/12 PW4 after
annealing for 1 hr
after 30% cold working
Fig. 2.93:
Strain hardening of
Ag/SnO <sub>2</sub> 98/2 PX
by cold working
Fig. 2.94:
Softening of
Ag/SnO <sub>2</sub> 98/2 PX
after annealing
for 1 hr after 80%
Fig 2.95:
Strain hardening
of Ag/SnO <sub>2</sub> 92/8 PX
by cold working
Fig. 2.96:
Softening of
Ag/SnO <sub>2</sub> 92/8 PX
after annealing for 1 hr
after 40% cold working
Strain hardening of internally
oxidized
Ag/SnO <sub>2</sub> 88/12 TOS F
by cold working
Fig. 2.98:
Softening of
Ag/SnO <sub>2</sub> 88/12 TOS F after
annealing for 1 hr after 30%
cold working
Strain hardening of
internally oxidized
Ag/SnO <sub>2</sub> 88/12P
by cold working
Fig. 2.100:
Softening of
Ag/SnO <sub>2</sub> 88/12P
after annealing for 1 hr after
40% cold working
Fig. 2.101:
Strain hardening of
Ag/SnO <sub>2</sub> 88/12 WPC
by cold working
Fig. 2.102:
Softening of Ag/SnO <sub>2</sub> 88/12 WPC after annealing
for 1 hr after different degrees of cold working
Fig. 2.103:
Strain hardening of
Ag/SnO <sub>2</sub> 86/14 WPC
by cold working
Fig. 2.104:
Softening of Ag/SnO <sub>2</sub> 86/14 WPC after annealing
for 1 hr after different degrees of cold working
Fig. 2.105:
Strain hardening of
Ag/SnO <sub>2</sub> 88/12 WPD
by cold working
