During the use of AgWC12C3 contacts material, found that there are also problems of performance and quality, at present, the scientific and technological workers and researchers at home and abroad have adopted a lot of methods to improve part performance of AgWC12C3 contacts, for example, add the second phase elements (Co, Ni, Fe, etc.); use a secondary sintering multi-pressure process or chemical coating technologies to achieve this goal. However, because wettablity of Ag and WC, Ag and C is relatively poor, the connection of Ag and WC interface is not firm, but also because it is impossible to get mechanical bonding interface due to plastic deformation when they are under pressure, so the function of adjusting material composition and improving process on performance improvement of contacts material is limited.

In recent years, studies have shown that anti-welding properties, arc erosion resistance and other physical properties of the contacts material have a close relation with its microstructure, when the second phase presents fiber distribution, contacts material performance can be greatly improved.

 

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1. The dark brown gum on tungsten contacts surface is --- H₂WO₄ (which can be dissolved in water, its aqueous solution is gray, after dewatering, it would turn into brown oxide dura --- WO₃) is a compound which is generated by electrochemical corrosion under high temperatures and high humidity conditions.
2. The root causes of electrochemical corrosion: the potential difference (0.54V) between contact wafer (tungsten) and the base (copper) is larger.
Electrochemical corrosion refers to a corrosion galvanic cell, in which metal and electrolyte act as an electrode composition.
Deoxygenating methods: the common deoxidize methods of vacuum, heating powder, iron scrap, resin and chemical agent methods.      
3. In order to improve the reliability of the contact of tungsten contacts
(1) Minimize the electrode potential difference between the contact wafer and the copper base. Under normal weather conditions, the contact potential difference between the electrodes can not be more than 0.5V; in bad weather, can not be greater than 0.25V.
Electrode potential refers to, in the electrolyte solution (water), there are potential difference generated between the metal (tungsten) surface and the solution, that is, the electrode potential of the metal in the solution.
(2) The combination of W and Ni or tungsten-rhenium 10 - nickel, then weld them into contact, can improve the reliability of tungsten contacts, which can prevent the formation of oxide film on tungsten contacts surface.
4. When the load current is not very large, use other materials, such as silver and copper, to take the place of tungsten as a contact.



 

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Ag-WC-C contacts material has been gradually replaced AgC5 contact material, because its excellent physical properties is better than the latter’s and it can reduce the use of silver material and saving cost. Ag-WC-C contacts material is mainly making use of the properties of good thermal conductivity and electrical conductivity of Ag and the properties of high hardness, high melting points and wear resistance of WC, which has been promoting the use, mainly used in electrical switches, such as, large capacity circuit breakers, molded case circuit breakers and automatic switches.

Traditional Ag-WC-C contacts preparation method is usually adopting powder metallurgy method --- pressing the mixing powder directly in the mold after mixing silver powder, tungsten carbide powder and additives, and then the contacts can get by sintering and forging and other processes. The advantages of using the process to prepare the kind of contacts are: simple process, easy operation and low requirements for equipment and lower production costs; disadvantages are: the contact material is defective prepared by this process, for example, its density is poor, organization is not even, and arc erosion resistance is poor and so like.

In the on-off process, Ag-WC-C contacts often due to material melting, arc erosion or splashing, making the generation of tungsten trioxide, silver tungstate and some other non-conductive compounds on its surface, leading to the increase of contact resistance, resulting in contact surface temperature increasing greatly, and ultimately affect the on-off performance of contact.



 

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Tungsten polymer is a composition of various resins and tungsten powder which are mixed together through special metallurgical technology, which is injection molded. As its special high density of 11.34 g/cm3 min., tungsten polymer has a perfect radiation shielding performance comparable to lead for anti-radiation shielding.

There has been conducted with as-received, agglomerated tungsten powder and rod milled tungsten powder to study the effect of mixing on the homogeneity of tungsten polymer injection molding. An increase in the mixing shear intensity increases the hydrodynamic force and contributes to the agglomerate size reduction. Deagglomeration during mixing improves the packing efficiency of the particles promotes homogeneity. Viscosity is a good indicator of the homogeneity of the feedstock. An increase in the homogeneity of the feedstock is reflected as a decrease in its viscosity. A perfectly homogeneous mixture will have the least viscosity. It is observed that with an increase in the intensity of mixing, the viscosity of the feedstock composed of as-received powder approaches that of the deagglomerated, rod milled powder.

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