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.
• 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, Ovens.
• 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 the 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
• 
  Bismuth oxide
• 
  Gallium oxide
• 
  Antimony oxide
• 
  Copper oxide
• 
  Tungsten 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.

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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.
• 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