Conductor materials. Contact materials. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Electrician

Conductor materials. Contact materials. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrical materials

Distinguish according to the type of work three types of contacts: fixed, commuting and sliding.

Fixed contacts - clamps, bolted and screw connections, twists, soldered and welded contacts. The quality of clamp contacts is determined by their contact resistance, which occurs at the points of direct contact. Surface improvement and protection of contacts against corrosion is achieved by soldering, welding or coating with corrosion-resistant, highly conductive metals.

In air at temperatures up to 75 ° C, all conductive metals give fairly stable transient resistances. The most important condition in this case is to ensure the necessary specific pressures on the contact surface.

General pattern for all types of solderless contacts, other things being equal, there is an inverse dependence of the contact resistance on pressing. With an increase in temperature due to the acceleration of the corrosion process, the contact resistance increases sharply, so copper, aluminum and steel contacts are coated with corrosion-resistant metals.

At a temperature of 100-120 ° C, tin-plated, silver-plated or cadmium-plated contacts work well. Contacts made of steel must be galvanized or cadmium-plated.

Bus contacts (usually in the form of strips), especially when using aluminum, it is recommended to clean with a glass skin under a layer of petroleum jelly; copper and steel require tin-lead soldering or pure tin.

switching contacts - materials of discontinuous electrical contacts - must have a low resistivity and a sufficiently low and especially stable transient resistance, high resistance to oxidation, welding and erosion, good wear resistance and a number of technological properties.

For the manufacture of low power breaking contacts, used mainly in low-current technology, use:

platinum group metals;
gold and its alloys;
silver and its alloys;
tungsten, molybdenum and their alloys.

Of electrodeposited contacts in the form of thin galvanic coatings that work in the absence of an arc, silver, gold, platinum, palladium, and especially rhodium, which combine a relatively low resistivity and very high hardness, should be noted.

For the manufacture of powerful discontinuousand precision contacts in modern technology, various cermet compositions are used, since the use of metals and their alloys does not give satisfactory results. Ceramic-metal contacts are made from metal powders by pressing from a mixture of a given composition in the form of a finished product, followed by sintering the compacts, re-pressing and annealing.

All brands of contacts made of cermet compositions can be break into groups.

Contacts made of compositions "silver - cadmium oxide" are widely used in low-voltage apparatus engineering, they are reliable at high current loads and moderate pressure on the contact. They have high wear resistance, low and stable contact resistance and increased arc resistance, but in the latter case they are inferior to contacts made of compositions with tungsten additives. Are issued for the soldering and welding with an underlayer of silver.

Contacts from compositions "silver - copper oxide" have low and stable transitional resistance, high electrical wear resistance and resistance to welding. At high current loads, they are more preferable than silver-cadmium oxide contacts. Are issued for the soldering and welding with an underlayer of silver.

Contacts from compositions "silver - nickel" resistant to electrical wear, have a low and stable contact resistance and are used in low-voltage DC and AC equipment with moderate loads. They are inferior to contacts of the "silver - cadmium oxide" and "silver - copper oxide" type in terms of resistance to welding, but are more resistant than pure silver. They allow soldering and welding without silver underlayer.

Contacts from compositions "silver - nickel - graphite". Addition of graphite improves arc resistance and welding resistance and allows these contacts to be used in low-voltage equipment with significant loads, as well as in air circuit breakers, usually paired with silver-nickel contacts.

Contacts from compositions "silver - graphite" have high arc resistance, resistance to welding and resistance to mechanical abrasion. Electrical resistance and mechanical strength are relatively low. Used in conjunction with silver-nickel contacts.

Contacts from compositions "silver - tungsten" highly resistant to melting, however, they have an increased transient resistance, which increases with an increase in the tungsten additive. They are used in air high-voltage switches in the form of overlays on the surface of copper contacts.

Contacts from compositions "silver - cadmium - nickel" have a higher dielectric strength than silver contacts and are characterized by a particularly stable and low contact resistance. Used for high voltage circuits.

Contacts from compositions "copper - tungsten" have high resistance to wear, welding and oxidation at high current loads. Due to the increased transient resistance, they have found application in high-voltage, mainly in oil switches, under conditions of strong arcing.

Contacts from compositions "copper - graphite" are used for contacts breaking currents of 30-80 kA. In order to guarantee against welding, these contacts are made porous; they have low strength, are designed for a small number of trips and are made with a copper sublayer.

Author: Koryakin-Chernyak S.L.

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