Adopt solid phase sintering method to prepare Ag-WC-C contacts (Material: silver powder, plate-like grains of tungsten carbide and graphite powder), wherein milling method can be chemical coating powder method.
Preparation process:
1. According to percentage of their content to weigh silver powder, tungsten carbide powder, graphite powder and metal additive powder, such as cobalt (Co), nickel (Ni) or zirconium (Zr). Among which,
Silver accounts for 84-85%; tungsten carbide powder of 12%; graphite powder of 3%; metal additive powder accounting for 0-1%.
Use stainless steel wire mesh to sieve the above-mentioned materials, and then pour them into a blender after mixing.
2. The preparation of silver nitrate solution. Silver should be fully dissolved in concentrated nitric acid.
3. The prepared silver nitrate solution is poured into the reaction vessel, then, complex it with ammonia, until the PH value rise to 10. Then adding the mixing powder to reaction vessel and stirring.
4. Continue stirring, use atomization device to add the reducing agent hydrazine hydrate to the reaction vessel in liquid spray-way until the reaction is sufficient.
Hydrazine hydrate, a colorless transparent oily liquid owns strong basicity and strong reducibility, which can act as a reducing agent and an antioxidant.
5. Washing the mixing powder with distilled water after processing by the above steps until it is neutral, then dry it in a dryer, the temperature is controlled at 90 ℃, dry mixing powder of silver coating can be available.
6. Pressing the dry mixing powder of silver coating (cold pressing), to form its shape, then, sintering it in a sintering furnace, in hydrogen atmosphere, temperature of 750-890 ℃, sintering time of 3-5h, and then re-pressing at a pressure of 10-20T / Cm2 and, ultimately, Ag-WC12-C3 contacts is prepared.



 

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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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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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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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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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Cadmium borotungstate is a heavy white crystal, soluble in water and the solution is yellow or lilac, besides, it will break down in sunlight. Its chemical formula is Cd5 (BWl2O40) • 18H2O. On the other hand, it can be produced by the thermal reaction of tungsten trioxide, cadmium oxide and borax taken.
As below will introduce using extraction method to produce cadmium borotungstate:
1.Taking 396g Na2WO4H2O dissolved in 300ml water, besides, adding HCl with stirring and heated to 85 ℃, heat preservation for 1-2 hours, PH is maintained at about 3. Then added 18.2g H3BO3 maintained at 85 ℃ for 30 minutes, then add the acid to adjusted PH value to 2, maintained at 85℃ 3-4 hours, cooled and filtered. The filtrate contains 12-boron sodium tungstate.
2. The solution was concentrated placed in a separatory funnel and extracted by ether. At the same time, adding 2-3 volumes of 33% H2SO4 for three times, oscillating for 5 minute to layered, obtained third-phase of heavy oil shaped and then separated the bottom of the third phase. Adding a small amount of water to remove Et2O obtain tungstoboric acid.
3. Adding excess CdO to the tungstoboric acid solution, and then filtered to give claybank solution after concentrated, cooling and crystallized to give an off-white cadmium borotungstate.

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Tungsten ore recycling is an important factor in the world's tungsten ore supply. It is estimated that today some 30% is recycled, and the tungsten ore processing industry is able to treat almost every kind of tungsten-containing scrap and waste to recover tungsten ore and, if present, other valuable constituents.

Contaminated cemented carbide scrap, turnings, grindings and powder scrap are oxidized and chemically processed to APT in a way similar to that used for the processing of tungsten ores. If present, cobalt, tantalum and niobium are recovered in separate processing lines. Other tungsten ore containing scrap and residues might require a modified process. 





 

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Cadmium tungstate is a white or yellow fluorescent crystals, with wolframite structure, its physical properties are as follows: chemical formula: CdWO4, molecular weight: 360.26, melting point: 1105 ℃, density: 7.9g / cm, solubility: soluble in ammonium hydroxide and alkali chloride solution, almost insoluble in water or dilute acid and at cold water solubility is 0.05g/100ml. CdWO4 commonly used in fluorescent paint, X-ray screen, a scintillation counter, phosphor and organic catalysts. Besides, cadmium tungstate have some irritation, when operation shlould to wear some protective equipment, such as protective glasses, protective clothing and masks, while it has a certain hazard for environment, so there is no relevant government departments permit can not discharged this product to surrounding environment. It can be sealed and stored in a cool and dry place. Cadmium tungstate can obtain in CdO-WO3 system by gradually high temperature reaction.

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Tungsten ore is usually mined underground. Scheelite and/or wolframite is frequently located in rather narrow veins which are slightly inclined and often widen with the depth. Open pit mines exist but are rare. Most tungsten ores contain less than 1.5% WO3 and ore dressing plants are always in close proximity to the mine. The ore is first crushed and milled to liberate the tungsten mineral crystals. Scheelite ore can be concentrated by gravimetric methods, often combined with froth flotation, whilst wolframite ore can be concentrated by gravity, sometimes in combination with magnetic separation.

Most tungsten ore flotation concentrates and some table concentrates are not of sufficiently high grade to meet the exacting specifications of the market. The principal diluents are phosphates and carbonates, which are difficult to depress sufficiently in flotation. The removal of these minerals is most frequently accomplished by leaching the concentrates with hydrochloric acid. Because of the relatively simple equipment required, the trend has been toward leaching at the flotation plant. However, a number of custom leaching plants are equipped to treat flotation concentrates from the smaller concentrators.

 

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