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Brief description of electrical contact materials
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(Summary description)Brief description of electrical contact materials 1. Electrical contact materials can be divided into the following categories according to the purpose of use: (1) Silver and silver alloy electrical contacts are used for high conductivity and weak current (low-energy circuits) occasions. (2) Platinum and palladium alloy electrical contacts are used for corrosion resistance, oxidation resistance, and weak current applications. (3) Ag-W, Ag-CdO, Ag-Ni, Ag-SnO and other silver-based alloy material electrical contacts are used for arc-resistant, high-conductivity, and low-load occasions. (4) Ag-Cu, palladium, and gold-based multi-element precious metal alloy electrical contacts are used for sliding contacts with wear resistance and low contact resistance.
Brief description of electrical contact materials
(Summary description)Brief description of electrical contact materials
1. Electrical contact materials can be divided into the following categories according to the purpose of use:
(1) Silver and silver alloy electrical contacts are used for high conductivity and weak current (low-energy circuits) occasions.
(2) Platinum and palladium alloy electrical contacts are used for corrosion resistance, oxidation resistance, and weak current applications.
(3) Ag-W, Ag-CdO, Ag-Ni, Ag-SnO and other silver-based alloy material electrical contacts are used for arc-resistant, high-conductivity, and low-load occasions.
(4) Ag-Cu, palladium, and gold-based multi-element precious metal alloy electrical contacts are used for sliding contacts with wear resistance and low contact resistance.
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Information
Brief description of electrical contact materials
1. Electrical contact materials can be divided into the following categories according to the purpose of use:
(1) Silver and silver alloy electrical contacts are used for high conductivity and weak current (low-energy circuits) occasions.
(2) Platinum and palladium alloy electrical contacts are used for corrosion resistance, oxidation resistance, and weak current applications.
(3) Ag-W, Ag-CdO, Ag-Ni, Ag-SnO and other silver-based alloy material electrical contacts are used for arc-resistant, high-conductivity, and low-load occasions.
(4) Ag-Cu, palladium, and gold-based multi-element precious metal alloy electrical contacts are used for sliding contacts with wear resistance and low contact resistance.
As the contact material, the resistivity must be small first, and the second important thing is the resistance to discoloration of the electrical contact surface. Therefore, it is only limited to very special cases to use the electrical contacts of base metal materials such as aluminum, copper, tungsten, etc., which are easy to form oxide films, and electrical contacts made of carbon or graphite materials that are easy to consume. Usually, the ones with good discoloration resistance are widely used. Precious metals are used as electrical contact materials.
The use of metal composite materials as electrical contacts can save 40%-70% of precious metals and has very obvious economic benefits. Therefore, it has been widely used instead.
Silver can be ranked first among the standard materials used for electrical contacts. It has the highest conductivity among metal materials, with a value of 106% IACS. The disadvantage of silver is that it is easy to be vulcanized to produce a black vulcanized film (Ag2S). The solution is to add precious metals that are not vulcanized. In this case, since it is necessary to add more than 50% (atoms) of other precious metal elements to form an alloy, it is no longer a silver-based alloy. However, due to its excellent physical properties, silver is widely used as an electrical contact for low-energy circuits.
Gold is only used as an electrical contact material under very special purposes. But because of its excellent chemical stability, it has been used more and more widely in the electronic field to electroplate the film on the contact surface of the electrical contact.
Platinum ranks second among the standard materials for electrical contacts and belongs to a class of discoloration-resistant materials, but it is necessary to pay attention to the specific phenomenon of brown powder in an organic atmosphere. Palladium is an element of the platinum group and can be used instead of platinum, but the peculiar phenomenon is the same as platinum.
Rhodium is not corroded by all known chemicals at room temperature, and it also has the advantages of no discoloration and small change in contact resistance. The rhodium plating surface is used for the plating of printed circuit terminals due to its very high hardness.
Second, the selection of electrical contact materials
(1) The influence of surrounding atmosphere and conditions on the surface of electrical contacts
The surrounding atmosphere and conditions have a great influence on the normal operation and service life of the electrical contacts. In addition to the enclosed electrical contacts isolated from the atmosphere, the vicinity of the electrical contacts is generally surrounded by a variety of gases or dust. These surrounding atmospheres include gases emitted from the room or from the constituent materials of the machine itself, and many of them are caused by air pollution. Affected by the surrounding atmosphere, electrical contacts are prone to failure.
Changes in humidity have an effect on the contact resistance of some electrical contacts (such as platinum contacts). For platinum, the presence of extremely low humidity can inhibit the increase in contact resistance. When the humidity is 10%, there is no black powdery friction powder on the contact surface of the electrical contact. The same is true for rhodium. When the humidity exceeds 40% In the above, such an effect is visible, but the influence of humidity is almost invisible to palladium.
Air pollution also affects the surface of electrical contacts. Air pollutants include dust, sulfur dioxide, nitride, chlorine and chloride, hydrogen sulfide, and organic gases. These pollutants cause corrosion and abrasion on the surface of electrical contacts, which directly affect the work and service life of electrical contacts. In addition, as the current increases, the influence caused by the arc intensifies.
(2) Conditions for selecting contact materials
1. The ideal electrical contact material must meet the following conditions
(1) The contact resistance is low and stable; (2) It is not easy to wear and the contact surface is not easy to deform; (3) No welding occurs and the opening and closing action is reliable; (4) The price is moderate.
2. Various electrical contact materials have their own strengths and weaknesses, so it is impossible to meet all conditions with a single material. Therefore, it is actually based on the requirements of various circuits to make full use of the advantages of various materials to select. Especially when used in heavy and weak current situations, the first and most important conditions are stable contact and reliable opening and closing. Therefore, chemically stable, that is, gold and platinum group alloys with good discoloration resistance are widely used.
3. The electrical contact materials are classified as follows when the general electrical conditions are mainly current:
(1) Gold and gold alloy electrical contacts for 30-50 mA or less; (2) Silver alloy electrical contacts, platinum group alloy electrical contacts, and silver oxide electrical contacts for 50 mA-20A; ( 3) 20 Anxin uses silver oxide electrical contacts. As the current increases, the influence caused by the arc intensifies. Therefore, electrical contact materials with excellent welding resistance and wear resistance are required.
4. In addition, there are special phenomena similar to those of platinum group metals in electrical contact materials (see the table below). Therefore, we must understand this and take corresponding measures.
The peculiar phenomenon of the contact; the characteristics of air pollutants and materials are shown in Table 2-01 and 2-02. Table 2-01 The peculiar phenomenon of electrical contacts
Phenomenon Cause Factor Countermeasure
Oxidation and sulfuration reactions When used in the atmosphere, due to the presence of inorganic gases in the surrounding atmosphere, the surface of the electrical contacts generates a film due to chemical reactions, and the contact resistance increases with the increase of the film, causing circuit failure O2, H2S, SO2, etc.
1. Purify the surrounding atmosphere and seal the electrical contacts; 2. Increase the contact pressure; 3. Implement sliding contact; brown powder (polymer generated by friction) In the atmosphere where organisms exist, some metals are mainly platinum group When the electrical contact slides, the organic gas molecules adsorbed on the surface of the electrical contact react, and due to friction and metal catalysis, a brown or dark brown powdery organic compound is generated. 1. The metal that produces the polymer: Pt, Pd, Ru, Ta, Rh, Mo, Cr2, metals that produce brown powder traces: Au, A-Pd alloy 3, metals that do not produce brown powder: Ni, Ag, Fe, Cu, W, V, Zn
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