The properties of tungsten powders may be seriously affected by the third element.
In production, these characteristics can be used to add third elements to control the particle size of tungsten powder to produce some special kinds of tungsten oxide. This method is called doping.
When doped, the tungsten oxide is added to the aqueous solution of the element compound. The most suitable for doping is not yellow tungsten nor purple tungsten, but blue tungsten. The commonly doped elements are sodium, lithium, chromium, and rare earth elements.
1. Sodium-doped: The addition of sodium compounds is a common method in preparing coarse tungsten powders, the amount of sodium is usually between 50-200 and m/g. At 1000°C under wet hydrogen conditions, tungsten powders with an average particle size of 10-25 um can be obtained according to the thickness of the material layer, high temperature ensures strong volatilization of sodium at the end of the reduction process, it is only necessary to reduce the residue by acid leaching or immersion in the case of special needs.
2. Lithium-doped: For a variety of tungsten powder applied to grain size, the addition of lithium is better than that of sodium. Due to lithium has a high boiling point, it can not be like sodium which has self volatilization in the reduction, therefore, after the reduction it must be washed with hydrochloric acid and water, and fine particles are separated by sieve.
3. When the ball milling process is adopted, a small amount of organic solvent, alcohol or gasoline can be added to make the tungsten powder and carbon black mixed evenly in the ball crusher.
4. Chrome-added: The addition of chromium compounds favors the production of fine tungsten powder, which produces chromium oxide during the reduction process, it acts as a grain refiner. This fine tungsten powder is then used to carbonize into ultrafine tungsten carbide, because the ultrafine metal powder has a strong spontaneous combustion, in the production process using inert gas protection.
5. The addition of rare earth elements: rare earth lanthanum or yttrium tungsten oxide thin film can be formed on the surface of La crystal lanthanum or yttrium Y thereby preventing gas hydrate tungsten oxide on the surface of tungsten particles precipitate or into the local solid phase chemical reaction. Thereby, the growth of tungsten powder particles is suppressed, and the preparation and reduction of ultrafine tungsten powder are greatly helpful.
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