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===2.4.3.1 Silver-Nickel (SINIDUR) Materials===
Since silver and nickel are not soluble in each other in solid form and in the liquid
phase have only very limited solubility silver nickel composite materials with
higher Ni contents can only be produced by powder metallurgy. During extrusion
of sintered Ag/Ni billets into wires, strips and rods the Ni particles embedded in
the Ag matrix are stretched and oriented in the microstructure into a pronounced
fiber structure (Figs. 2.75. and 2.76)
The high density produced during hot extrusion aids the arc erosion resistance
of these materials (Tables 2.21 and 2.22). The typical application of Ag/Ni
contact materials is in devices for switching currents of up to 100A (Table 2.24).
In this range they are significantly more erosion resistant than silver or silver
alloys. In addition they exhibit with nickel contents <20 wt% a low and over their
operational lifetime consistent contact resistance and good arc moving
properties. In DC applications Ag/Ni materials exhibit a relatively low tendency
of material transfer distributed evenly over the contact surfaces (Table 2.23).
Typically Ag/Ni (SINIDUR) materials are usually produced with contents of 10-40
wt% Ni. The most widely used materials SINIDUR 10 and SINIDUR 20- and also
SINIDUR 15, mostly used in north america-, are easily formable and applied by
cladding (Figs. 2.71-2.74). They can be, without any additional welding aids,
economically welded and brazed to the commonly used contact carrier
materials.
The (SINIDUR) materials with nickel contents of 30 and 40 wt% are used in
switching devices requiring a higher arc erosion resistance and where increases
in contact resistance can be compensated through higher contact forces.
The most important applications for Ag/Ni contact materials are typically in
relays, wiring devices, appliance switches, thermostatic controls, auxiliary
switches, and small contactors with nominal currents >20A (Table 2.24).
Table 2.21: Physical Properties of Silver-Nickel (SINIDUR) Materials
Table 2.22: Mechanical Properties of Silver-Nickel (SINIDUR) Materials
Fig. 2.71:
Strain hardening
of Ag/Ni 90/10 by cold working
Fig. 2.72:
Softening of Ag/Ni 90/10
after annealing
for 1 hr after 80% cold working
Fig. 2.73:
Strain hardening
of Ag/Ni 80/20 by cold working
Fig. 2.74:
Softening of Ag/Ni 80/20
after annealing
for 1 hr after 80% cold working
Fig. 2.75: Micro structure of Ag/Ni 90/10 a) perpendicular to the extrusion direction
b) parallel to the extrusion direction
Fig. 2.76: Micro structure of Ag/Ni 80/20 a) perpendicular to the extrusion direction
b) parallel t o the extrusion direction
Table 2.23: Contact and Switching Properties of Silver-Nickel (SINIDUR) Materials
Table 2.24: Application Examples and Forms of Supply
for Silver-Nickel (SINIDUR) Materials
===2.4.3.2: Silver-Metal Oxide Materials Ag/CdO, Ag/SnO , Ag/ZnO===
The family of silver-metal oxide contact materials includes the material groups:
silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzinc
oxide (DODURIT ZnO). Because of their very good contact and switching
properties like high resistance against welding, low contact resistance, and high
arc erosion resistance, silver-metal oxides have gained an outstanding position
in a broad field of applications. They mainly are used in low voltage electrical
switching devices like relays, installation and distribution switches, appliances,
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
by both, internal oxidation and powder metallurgical methods (Table 2.25).
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 (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 (Figs. 2.77 and 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 (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
(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
(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. The
powder 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.
Fig. 2.77:
Strain hardening of internally oxidized
Ag/CdO 90/10 by cold working
Fig. 2.78:
Softening of internally oxidized
Ag/CdO 90/10 after annealing
for 1 hr after 40% cold working
Table 2.25: Physical and Mechanical Properties as well as Manufacturing Processes and
Forms of Supply of Extruded Silver Cadmium Oxide
(DODURIT CdO) Contact Materials
Fig. 2.79:
Strain hardening of
Ag/CdO 90/10 P by cold working
Fig. 2.80: Softening
of Ag/CdO 90/10 P after annealing
for 1 hr after 40% cold working
Fig. 2.81:
Strain hardening
of Ag/CdO 88/12 WP
Fig. 2.82:
Softening of Ag/CdO 88/12WP after annealing
for 1 hr after different degrees of
cold working
Fig. 2.83: Micro structure of Ag/CdO 90/10 i.o. a) close to surface
b) in center area
Fig. 2.84: Micro structure of Ag/CdO 90/10 P:
a) perpendicular to extrusion direction
b) parallel to extrusion direction
Fig. 2.85:
Micro structure of Ag/CdO 90/10 ZH:
1) Ag/CdO layer
2) AgCd backing layer
Fig. 2.86: Micro structure of AgCdO 88/12 WP: a) perpendicular to extrusion direction
b) parallel to extrusion direction
*===Silver–tin oxide(SISTADOX)materials===
