Title: Development of, Cadmium free, Silver based Electrical Contact Materials
1Development of, Cadmium free, Silver based
Electrical Contact Materials
Satyanarayana Emani
emansat_at_iit.edu
Philip Nash
nashp_at_iit.edu
Engineered materials solutions
www.emsclad.com
2Objective
-
- Develop cadmium-free silver based internally
oxidized cost effective electrical contact
material with good performance.
3Manufacturing Procedure
- Melting and Casting Induction Melting in a
graphite crucible. -
Horizontal continuous cast. - Forming Cold rolling ( 60
70 reduction).
- Oxidation Internally
oxidized at 900 F under 3 atm -
pressure of Oxygen for one week. - Cold rolling
- Stamping
-
4Necessary properties for Electrical Contact
Materials
- High conductivity Conduct current.
-
Prevent resistive current rise. -
Dissipate maximum heat. - High melting point Resists contact welding.
-
Resists arc erosion. - High hardness Resists contact
welding(sticking). -
Resists mechanical wear. - Corrosion resistance No films to increase
electrical- -
-resistance.
5Application
- Medium duty appliance controls and switches
(10 60 amps / 12 120 V a.C). - Ex coffee makers, Owens.
- General modes of failure of contacts
- 1) Welding Melting of the surface caused by
the arc. - Sticking of the contact
in the make operation. - 2) Erosion Stirring action of the arc.
- Splatter caused by the
stirring.
6Why not pure Silver?
- 1) Easily welded
- 2) High erosion.
- Due to low hardness and low melting point.
- Reasons for alloying and oxidation
- 1) Increase hardness - Decrease in welding and
wear - 2) Increase in viscosity - Decrease in erosion
- Why not Ag-CdO ?
- 1) Dissociation of CdO into Cd and O2.
- 2) Vaporization of Cadmium.
- 3) Cadmium vapors detrimental to the
environment.
7Properties of Ag-CdO contacts
- Low erosion rate.
- Low welding tendency.
- Maintains stable electrical resistance.
- Easily produced by internal oxidation.
- Good formability.
8Mechanisms by which properties are achieved in
Ag-CdO contacts
- Arc-quenching effect dissociation of CdO.
- Splatter reduction effect increase in
viscosity. - Anti welding effect ceramic nature of CdO.
- Self cleaning effect vaporization of Cd.
- Greater affinity for oxygen than silver.
- High diffusion of oxygen in Cd than in Ag.
- Uniform particle distribution.
9Process parameters governing alloy development
- Material must be developed by wrought alloy
process( powder metallurgy is not to be used). - Material developed must have good ductility so as
to permit the use of metal forming operations
such as cold rolling (60 70 reduction) and
stamping. - Material developed must not be hazardous to the
environment - Material developed must be produced at a cost
that is comparable to or less than the existing
costs for silver cadmium oxide. - Material developed must have performance
comparable to or better than Ag-CdO contacts.
10Possible replacements
- Silver - Tin oxide
- Silver - Tin oxide - ternary additives
-
Indium oxide -
Zinc oxide -
Gallium oxide -
Copper oxide -
Nickel Oxide -
Iron Oxide -
Magnesium Oxide - Silver - Nickel
- Silver - Zinc
- Silver - Carbon
- Silver - Tungsten carbide
-
11Disadvantages of Ag-SnO2 Contacts
- Very high diffusion of Tin in Silver.
- Additions above 4 wt of tin cause external
oxidation. - Formation of resistive layer on oxidation, on the
surface, attributed too high thermal stability of
the tin oxide. - Formation of precipitate free pure silver layer
on the surface. - Poor wettability of SnO2 by molten silver.
- Low ductility.
-
12Role of ternary additives
- Preferentially oxidize over Tin and act as
nucleation sites for Tin oxide formation, there
by improving internal oxidation and dispersion of
oxides. - Reduce the diffusion of Tin in Silver.
- Improve the wettability of Tin Oxide by molten
Silver and prevent the oxide slag formation.
(Which ever oxide wets the silver easily will
enhance the wetting angle of Tin Oxide). - Improve the ductility by changing the morphology
of the size and shape of the oxide precipitate. - Render the oxide particles fine and improve the
dispersion. -
13Determination of wetting angles
part 1
- Electroplating of metal sheet which has a higher
melting point than silver with various alloying
elements. - Electrolyte 5 HCl aqueous
solution. - Anode Metal
sheet(Ni-Fe-Mo). - Cathode Alloying element(Sn,
Zn, In). - Oxidation of electroplated sheet
- Furnace Muffle furnace.
- Temperature range 500 800 C.
- Atmosphere Air
- Fe, Cu, Ni oxides are directly prepared by
heating the metal sheets in the furnace in
atmosphere air. -
14Determination of wetting angles
part 2
- The oxidized sheet is kept in the tube furnace at
1000 C in vacuum. - A small silver bead is kept on the oxide layer.
- The furnace is closed with a Quartz window on one
side for inside vision. - After the silver melts pictures of the molten
silver are taken by using a Traveling Microscope
with an attached camera. - The pictures are analyzed and the wetting angles
are determined.
15Determination of wetting angles
Tube furnace
Fused silica glass window
Oxide substrate
Liquid metal
Image analyzer
Traveling microscope
thermocouple
Wetting angle ?
Computer
Oxide substrate
Liquid metal
16Work Done
- Sn, In, Zn oxide substrates are prepared.
- The alloys with the following composition which
will yield 12 percent volume of oxide upon
oxidation are prepared. - Ag - 4.44 Sn - 2 Cu (wt)
- Ag - 4.20 Sn - 2 Zn (Wt)
- Ag - 4.75 Sn - 2 In (wt)
- Melting is done in a graphite crucible in a
controlled Nitrogen atmosphere in a muffle
furnace. - Experimental instruments necessary (Tube furnace,
Fused silica window, Traveling microscope,
computer aided image analyzer) to perform the
sessile drop Experiments are set up. -
17Zn
18In
19Sn
20SnO2
21ZnO
22Work in progress
- Preparation of Fe, Ni, Cu oxide substrates.
- Microstructure analasys of the prepared alloys.
- Preparation of Ag-Sn-Fe, Ag-Sn-Ni, Ag-Sn-Mg,
Ag-Sn-Ga alloys. - Forming (cold rolling) of the alloys prepared.
- Internal oxidation of the alloys.
- Determination of the wetting angles for the
oxides prepared.
23Future plan of work
- Determine the wetting angles of different metal
oxides by Silver melt by using sessile drop
technique. And understand the mechanisms by which
the wettability is improved. - Metallurgical analysis (Metallography, S.E.M,
T.E.M,X-Ray) to characterize diffusion and
oxidation properties. - Understand the mechanism by which Tin imparts
brittleness to Silver and find processes by which
it can be reduced. - Find processes by which the diffusion of Tin in
Silver is reduced. - Determine the erosion characteristics and weld
force of the alloys developed by using endurance
tests. -
24THANK YOU