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Electrical ContactsSilver ContactsCMW manufactures silver contacts from a variety of of silver alloy and silver composite materials. Commonly specified materials are Coin Silver, Silver Nickel, Silver Graphite, CMW® D54F and CMW® D55F Silver Cadmium Oxide alloys to mention only a few. Below, you will find more information and links to the material’s typical properties.
Coin SilverCMW® Coin Silver (90% Ag / 10% Cu) and Elkonium®1 (75% Ag / 24.5% Cu / 0.5% Ni) Coin Silver and Elkonium® 1 alloys are the most widely used materials of this group for electrical contacts. Elkonium® 1 material sold by CMW displays low transfer tendencies and high current switching capability in a number of automotive applications. Coin silver also has been used in a number of relays and switches because it is more economical than fine silver. It has been used in rotary switches and other sliding contact applications because it has good resistance to mechanical wear. Both coin silver and Elkonium® 1 materials are used in high contact force applications where fine silver has failed by mechanical wear or deformation due to its lower hardness. Availability:
Fine SilverAg Fine Silver Typical Properties Fine silver is a silver contact used extensively for make and break contacts in the moderate current range. It has the highest electrical and thermal conductivity of all of the metals. These properties enable fine silver to have a low contact resistance and high heat dissipation capability. It is relatively tarnish resistant and does not form stable oxides under normal conditions. Disadvantages are that it forms sulfide films, which can affect the contact resistance adversely, especially where the contact closing force, current or voltage is low. The migration of silver ions across or through certain insulating materials occurs where there is a voltage difference across the insulation and it is in contact with silver. This effect accelerates with increasing humidity. It can cause dielectric failures and must be guarded against by device design. Fine silver contacts are found in a variety of switches and relays used in the appliance, automotive, aviation and industrial markets. They are used in both AC and DC circuits and at currents ranging from about 0.5 to 15 amperes. The oxidation resistance of the material permits it to be used with low contact forces ranging from about 1/2 ounce to a few ounces. It is often used in applications where the contacts remain closed for long periods of time. CMW furnishes fine silver with a minimum purity of 99.90% silver. It is a ductile material and can readily be formed into many shapes. Contact designs include solid rivets, composite rivets, composite weld buttons, solid weld buttons, composite screws, round or rectangular discs and assemblies. Availability:
Silver Cadmium OxideAg/CdO Silver Cadmium Oxide Typical Properties CMW® D54X (90% Silver / 10% Cadmium Oxide) CMW Type “F” and Type “X” Silver Cadmium Oxide contact materials are produced as true alloys. The "X" is appended to the material names to denote that the material is cast and formed as an alloy of Silver and Cadmium. The cadmium within the contact is then "post-oxidized" by heating in the presence of oxygen to convert it to cadmium oxide. Because of the kinetics of the process (which involves both migration of oxygen and cadmium) there is a resulting high concentration of fine Cadmium Oxide phase near the surface of the contact, and a lower concentration of courser Cadmium Oxide as one approaches the core of the item. Sometimes this "core" is referred to as a "depleted zone" or "silver-rich zone" due to the fact that the cadmium oxide content at this point is in much lower concentration. This should not be confused with an "un-oxidized core" (failure of sufficient oxidation time to reach cadmium at the core) or "reduced layer" (a surface condition where cadmium oxide has been re-reduced to cadmium because of contact with a "reducing" atmosphere). In the latter two cases, the cadmium is still present, but not in its "oxidized" form. The kinetics of the situation, (and processing techniques at CMW) limit the "center-to-surface distance" for this type of material to about 1/16 inch. Contacts thicker than 1/8 inch will likely have an "un-oxidized core". Of Ag/CdO materials, the "post-oxidized type" has the least tendency to for early erosion. It is the material of choice for arcing contacts of medium-low to medium-high current rating both where early "erosion" primary concern and anti-welding is also a high priority. Magnetic contactors, relays, and industrial controls are common applications. It is often chosen for its manufacturing ease and economics. Prior to oxidation, the silver cadmium alloy is readily cold worked, and can be headed from wire (to make "rivet contacts"), or rolled and formed to exotic shapes from strip with very good manufacturing economics for high volume applications. It is not used in "high reliability - low current" applications (where CMW Elkonium® materials are best), nor in "high current circuit breaking" (where Elkonite® materials are superior). Generally this makes it a choice for "high-end" household and "low-end" industrial relay and contactor applications. CMW® D54F (90% Silver / 10% Cadmium Oxide) The "F" is appended to the material names to denote that the material is cast as a grain alloy of Silver and Cadmium. This grain is then oxidized to become Silver-Cadmium oxide grain. After oxidation the grain is subsequently hot and cold worked into wire, bar, or strip, and ultimately headed, or blanked, or coined into contacts. CMW refers to this processing as "Pre-oxidized", because conversion to cadmium oxide "precedes" formation of the contacts. Generally pre-oxidized ("F") AgCdO material is quite similar in properties to post-oxidized ("X") material with the exception of microstructure. Pre-oxidized ("F") material has a uniform distribution of fine and coarse cadmium oxide phase, while the post-oxidized ("X") material does not (see above). Because of this, there is no fear of reduced resistance to sticking as one wears down the contact and approaches its core region. Pre-oxidized Silver Cadmium Oxide ("F") materials cannot be cold worked to the degree that post-oxidized "X" type material. There are definite limitations on the heading of wire to form rivets, as well as rolling and coining. Compositions greater than 13.3% CdO are not considered to be "headable" at all. Conversely, there is no "1/8 inch thickness" limitation on contacts with "F" materials, since kinetics of oxidation at the contacts geometry are not a consideration. Thus heavier duty contacts can be blanked from thick "F" type strip. In strip form, fine silver backing is recommended as a brazing surface to eliminate brazing problems that occur if it is omitted. This backing is readily applied during the hot rolling of the strip. Of Ag/CdO materials, the "pre-oxidized type" has the superior arcing properties for everything except "anti-sticking" where the "powder metal" type is superior. It is the material of choice for arcing contacts of medium-low to medium-high current rating both where economics for a heavier and high volume application is of primary concern. Magnetic contactors, relays, and industrial controls are common applications. It is not used in "high reliability - low current" applications (where CMW Elkonium®s are best), nor in "high current circuit breaking" (where Elkonite® materials are superior). Generally this makes it a choice for "high-end" household and "low-end" industrial relay and contactor applications. The cost of Ag/CdO materials are directly tied to the cost of silver and the silver content of the material. As would be expected, the conductivity (and therefore resistance to temperature rise) is better with higher silver content.
Silver GraphiteAg/C Silver Graphite Typical Properties The addition of graphite to silver provides a family of materials which is useful as make and break and brush contacts. The graphite provides good lubrication properties and, therefore, reduces the tendency for galling or scoring in sliding silver contacts, hinge joints, and slip ring applications. It also has high resistance to welding or sticking. CMW® D582F (99.75% Silver / 0.25% Graphite) CMW® D581F (99.50% Silver / 0.50% Graphite) The materials containing the lower percentages of graphite such as CMW® D582F, D581F, D583F and D158F can be fabricated into wire form and headed into rivets over a limited size range. These silver contacts are used in both AC and DC circuits and has been used primarily to reduce sticking. However, they are also used as brush contacts in some light force sliding applications. CMW also produces Silver Graphite materials using powder metallurgy techniques, allowing for a additional varieties of shapes and configurations.
Silver NickelAg/Ni Silver Nickel Typical Properties CMW® D505F (95.0% Silver / 5% Nickel) and CMW® D510F (90.0% Silver / 10% Nickel) This group of silver nickel compositions is made from metal powder. The lower nickel compositions, CMW® D505F and CMW® D510F, have good ductility and are fabricated into wire and strip. These compositions can also be headed into rivets, discs, or solid weld buttons. CMW® D505F is used in some appliance switches where a higher resistance to wear than fine silver would afford is required. CMW® D510F is used in both AC and DC applications. It is used in the lower rating line starters operating against fine silver. CMW® D510F is also used in polarized low voltage DC circuits as a positive contact operating against Elkonium®45. CMW® D50F (85.0% Silver / 15.0% Nickel) and CMW® D50 (85.0% Silver / 15.0% Nickel) CMW® D50F can also be fabricated into wire, rod and strip; however, the headability of CMW® D50F is limited to a narrow range. It is used for main contacts in circuit breakers and switching devices where auxiliary contacts are used. This type of material has good resistance to mechanical wear under high contact forces, and maintains a stable surface resistance. CMW® D50 is the same composition as CMW D50F, but it is furnished in the form of discs, rectangles, and special shapes produced by conventional powder metallurgy techniques.
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