Development of, Cadmium free, Silver based Electrical Contact Materials

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Development 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
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Objective

• Develop cadmium-free silver based internally
  oxidized cost effective electrical contact
  material with good performance.

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Manufacturing 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
• 
  
  

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Necessary 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.

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Application

• 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.

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Why 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.

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Properties of Ag-CdO contacts

• Low erosion rate.
• Low welding tendency.
• Maintains stable electrical resistance.
• Easily produced by internal oxidation.
• Good formability.

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Mechanisms 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.

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Process 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.

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Possible 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

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Disadvantages 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.

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Role 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.

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Determination 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.

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Determination 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.

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Determination 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
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Work 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.

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Zn
18
In
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Sn
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SnO2
21
ZnO
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Work 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.

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Future 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.

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THANK YOU